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GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
Table of Contents
Chapter 1 GSM Principles and Call Flow.....
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
1.10.4 Immediate Assignment Failure..........................
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
Chapter 1 GSM Principles and Call Flow
1.1 GSM Frequency B...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
1.2 Multiple Access Technology and Logical Channel
1.2.1 G...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
1.2.2 TDMA Frame
The basic conception of GSM in terms of r...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
Physical channel combines frequency division multiple acce...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
Figure 1.3 Structure of TDMA frame
1.2.3 Burst
Burst is th...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
of each timeslot in transmission. Although timing advance ...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
 Normal burst
It has two 58-bit groups used in message fi...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
different burst type for different logical channel.
In GSM...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
IV. CCCH
 Paging Channel (PCH)
PCH is a downlink channel ...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
VI. Channel Combination
Logical channel is mapped to physi...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
has a strong time-varying characteristic. It has a high er...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
with 8 kHz, so each block has 160 samples. Each sample is ...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
channel, this block usually carries one piece of informati...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
coding only works for detection and correction of signal e...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
After internal interleaving, the 456 bits of a voice block...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
wave according to the rules. This feature is the data to t...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
Figure 1.6 Principle of dual timeslot extended cell
The pr...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
 In idle mode, system information 1– 4, 7, and 8 are tran...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
BA-PA-MFRMS, and T3212.
 Cell selection contains paramete...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
1.6.2 Cell Selection Process
To perform cell selection and...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
RXLEV_ACCESS_MIN: Minimum Relev that MS allows
MS_TXPWR_MA...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
includes several factors, such as RLA_C, cell restriction ...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
interference is to fully utilize the current spectrum, tim...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
I. Baseband Hopping
The system has multiple baseband and T...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
combination and the loss is greater in the link insertion ...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
FN
T2(0¡«25)
FN
T3(0¡«50)
MAI
(m0¡«mN-1)
MAIO
(0¡«N-1)
Rep...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
MAI, integer (0 ... N 1) : MAI = (FN + MAIO) modulo N (2-2...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
RFCHN=MA (MAI);
When HSN=0, S equals the frame number, in ...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
Statistics shows that frequency hopping gain is related to...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
the received flow. The hopping gain is obtained only when ...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
AGCH in 51 multiframe), and BS_PA_MFRMS (the number of 51 ...
GSM Radio Network Planning and Optimization
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parameter be configured as little as possible in order to ...
GSM Radio Network Planning and Optimization
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start DTX function based on this information.
DTX can be u...
GSM Radio Network Planning and Optimization
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measurement and sub measurement.
Global measurement is the...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
and initial adjustment stage. Stable adjustment is the com...
GSM Radio Network Planning and Optimization
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Figure 1.11 Execution of power control command
The purpose...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
system, which specifies the maximum output power (suppose ...
GSM Radio Network Planning and Optimization
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ensure the algorithm stability.
Figure 1.13 Measurement re...
GSM Radio Network Planning and Optimization
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Improve TP Improve TP Improve TP
Improve TP No action Impr...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
assigns proper channels for the granted calls. This kind o...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
message contains the assignment information of one MS. Acc...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
level, encryption algorithm, short message capacity, and f...
GSM Radio Network Planning and Optimization
第 2 章 GSM 系统原理及呼叫流程
and no re-assignment is initiated, the immediate assignmen...
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  1. 1. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 Table of Contents Chapter 1 GSM Principles and Call Flow....................................................................................3 1.1 GSM Frequency Band Allocation .....................................................................................3 1.2 Multiple Access Technology and Logical Channel.............................................................4 1.2.1 GSM Multiple Access Technology...........................................................................4 1.2.2 TDMA Frame..........................................................................................................5 1.2.3 Burst.......................................................................................................................7 1.2.4 Logical Channel......................................................................................................9 1.3 Data Transmission..........................................................................................................12 1.3.1 Voice Coding........................................................................................................13 1.3.2 Channel Coding....................................................................................................14 1.3.3 Interleaving ..........................................................................................................15 1.3.4 Encryption ............................................................................................................17 1.3.5 Modulation and Demodulation..............................................................................17 1.4 Timing advance...............................................................................................................18 1.5 System Information.........................................................................................................19 1.6 Cell Selection and Re-Selection......................................................................................21 1.6.1 Cell Selection........................................................................................................21 1.6.2 Cell Selection Process .........................................................................................22 1.6.3 Down Link Failure ...........................................................................................23 1.6.4 Cell Re-Selection Process....................................................................................23 1.7 Frequency Hopping ........................................................................................................24 1.7.1 Types of Frequency Hopping................................................................................25 1.7.2 Frequency Hopping Algorithm..............................................................................27 1.7.3 Benefits of Frequency Hopping.............................................................................30 1.8 Discontinuous Reception and Discontinuous Transmission............................................32 1.8.1 Discontinuous Reception and Paging Channel.....................................................32 1.8.2 DTX......................................................................................................................34 1.9 Power Control.................................................................................................................36 1.9.1 Power Control Overview ......................................................................................36 1.9.2 MS Power Control.................................................................................................36 1.9.3 BTS Power Control...............................................................................................38 1.9.4 Power Control Processing....................................................................................39 1.10 Immediate Assignment Procedure................................................................................41 1.10.1 Network Access License and Random Access Request.....................................41 1.10.2 Initial Immediate Assignment..............................................................................42 1.10.3 Initial Message....................................................................................................43 1
  2. 2. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 1.10.4 Immediate Assignment Failure............................................................................44 1.11 Authentication and Encryption ......................................................................................45 1.11.1 Authentication ....................................................................................................45 1.11.2 Encryption ..........................................................................................................48 1.11.3 TMSI Reallocation ..............................................................................................49 1.11.4 Exceptional Situations.........................................................................................50 1.12 Location Update............................................................................................................51 1.12.1 Generic Location Update (Inter-LA Location Update).........................................51 1.12.2 Periodic Location updating.................................................................................53 1.12.3 IMSI Attach and Detach......................................................................................54 1.12.4 Exceptional Situations........................................................................................55 1.13 MS Originating Call Flow...............................................................................................57 1.13.1 Called Number Analysis .....................................................................................58 1.13.2 Voice Channel Assignment (Follow-up Assignment)...........................................58 1.13.3 Call Connection .................................................................................................62 1.13.4 Call Release.......................................................................................................62 1.13.5 Exceptional Situations........................................................................................64 1.14 MS Originated Call Flow...............................................................................................66 1.14.1 Enquiry...............................................................................................................66 1.14.2 Paging ...............................................................................................................67 1.14.3 Call Establishment for the Called Party..............................................................68 1.14.4 The Influence of Call Transfer to Routing............................................................69 1.14.5 Exceptional Situations........................................................................................70 1.15 HO.................................................................................................................................72 1.15.1 HO Preparation...................................................................................................73 1.15.2 HO Types............................................................................................................76 1.15.3 HO Process Analysis..........................................................................................78 1.15.4 Exceptional Situations........................................................................................87 1.16 Call Re-Establishment .................................................................................................88 1.16.1 Introduction.........................................................................................................88 1.16.2 Call Re-Establishment Procedure.......................................................................89 1.16.3 Exceptional Situations........................................................................................90 1.16.4 SM Procedure.....................................................................................................91 1.16.5 Short Message Procedure on SDCCH When MS is calling ...............................91 1.16.6 Short Message Procedure on SDCCH When MS is called ................................92 1.16.7 Short Message Procedure on SACCH When MS is calling................................93 1.16.8 Short Message Procedure on SACCH when MS is called..................................94 1.17 CBS...............................................................................................................................94 1.17.1 CBS Mechanism ................................................................................................95 1.17.2 BSC-BTS Message Transmission Mode.............................................................96 2
  3. 3. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 Chapter 1 GSM Principles and Call Flow 1.1 GSM Frequency Band Allocation GSM cellular system can be divided into GSM900M and DCS1800M according to frequency band, with carrier frequency interval of 200 KHz and up and down frequencies as follows: Table 1.1 GSM frequency allocation Frequency band(MHz) Bandwidth( MHz) Frequency number Carrier frequency number (pair) GSM900 Up 890–915 Down 935–960 25 1–124 124 DCS1800 Up 1710–1785 Down 1805–1880 75 512–885 374 “Up” and “down” are classified according to base station. Base station transmitting - mobile station receiving is “down”; mobile station transmitting - base station receiving is up. With the expanding services, GSM protocol adds EGSM(expanded GSM frequency band) and RGSM (expanded GSM frequency band including railway service) to the original GSM900 frequency band. The frequency band allocation is as follows: Table 1.2 EGSM/RGSM frequency allocation Frequency band(MHz) Bandwidth (MHz) Frequency number Carrier frequency number (pair) EGSM Up 880–915 Down 925–960 35 0–124 975–1023 174 RGSM Up 876–915 Down 921–960 40 0–124 955–1023 199 3
  4. 4. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 1.2 Multiple Access Technology and Logical Channel 1.2.1 GSM Multiple Access Technology In cellular mobile communications system, since many mobiles stations communicate with other mobiles stations through one base station, it is necessary to distinguish the signals from different mobile stations and base stations for them to identify their own signals. The way to this problem is called multiple access technology. There are now five kinds of Multiple access technology, namely: Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Space Division Multiple Access (SDMA), and polar division multiple access (PDMA). GSM multiple access technology focuses on TDMA, and takes FDMA as complement. The following only introduces FDMA and TDMA technologies. I. FDMA FDMA divides the whole frequency band into many single radio channels (transmitting and receiving carrier frequency pairs). Each channel transmits one path of speech or control information. Any subscriber has access to one of these channels under the control of the system. Analog cellular system is a typical example of FDMA application. Digital cellular system also uses FDMA, but not the pure frequency allocation. For example, GSM takes FDMA technology. II. TDMA TDMA divides a broadband radio carrier into several time division channels according to time (or timeslot). Each subscriber takes one timeslot and sends or receives signals only in the specified timeslot. TDMA is applied in digital cellular system and GSM. GSM adopts a technology combined with FDMA and TDMA. 4
  5. 5. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 1.2.2 TDMA Frame The basic conception of GSM in terms of radio path is burst. Burst is a transmission unit consists of over one hundred of modulation bits. It has a duration limit and takes a limited radio frequency. They are exported in time and frequency window which is called slot. To be specific, in system frequency band, central frequency of slot is set in every 200 KHz (in FDMA). Slot occurs periodically in each 15/26 ms, which is about 0.577 ms (in TDMA).The interval between two slots is called timeslot. Its duration is used as time unit, called burst period (BP). Time/frequency map illustrates the concept of slot. Each slot is expressed as one little rectangle with 15/26ms length and 200 KHz width. See 1.2.2. Similarly, the 200 KHz bandwidth in GSM is called frequency slot, equal to radio frequency channel in GSM protocol. Burst represents different meaning in different situation. Sometimes it concerns time – frequency “rectangle” unit, and sometimes not. Similarly, timeslot sometimes concerns time value, and sometimes means using one of every eight slots periodically. Using a given channel means transmitting burst with a particular frequency at particular time, that is, a particular slot. Generally, the slot of a channel is not continuous in time. Figure 1.2 Timeslot 5 Frequency 200kHz BP 15/26ms Slot Time
  6. 6. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 Physical channel combines frequency division multiple access and time division multiple access together. It consists of timeslot flow that connects base station (BS) and mobile station (MS).The position of these timeslots in TDMA frame is fixed. 1.2.2 shows the complete structure of TDMA frame, including timeslot and burst. TDMA frame is a repetitive “physical” frame in radio link. One TDMA frame consists of eight basic timeslots, about 60/13≈4.615ms in total. Each timeslot is a basic physical channel with 156.25 elements, coving 15/26≈0.557ms. There are two kinds of multiframes, consisting of 26 and 51 continuous TDMA frames respectively. Multiframes are applied when different logical channels are multiple used in one physical channel. The 26 multiframe, with a period of 120 ms, is used in traffic channel and associated control channel. Among the 26 bursts, 24 are used in traffic and 2 are used in signaling. The 51 multiframe, with a period of 3060/13≈235.385 ms, is specially used in control channel. Many multiframes together form a super frame. Super frame is a continuous 51×26TDMA frame, that is to say, a super frame consists of fifty-one 26 TDMA multiframes or twenty-six 51 TOMA multiframes. The period of super frame is 1,326 TDMA frames, or 6.12 s. Many super frames together form a hyper frame. A hyper frame consists of 2,048 super frames with a period of 12,533.7s, or 3 hours and 28’ 53’’ 760’’’. It is used in encrypted voice and data. Each period of hyper frame consists of 2,715,648 TDMA frames numbered from 0 to 2,715,648. The frame number is transmitted in sync channel. The structure of GSM frame is shown in 1.2.2. 6 0 1 2 3 2044 2045 2046 2047 0 1 2 3 48 49 5047 0 1 24 25 0 1 24 25 1 49 500 0 1 4 5 762 3 TB 3 TB 3 GP 8.25 TB£ ºtail bits TB 3 TB 3 GP 8.25 GP£ ºguard period TB 3 TB 3 GP 8.25 TB 3 TB 3 GP 68.25 58 information bits26 training sequency58 information bits constant bits 142 information bits 39extended training sequency64information bits 39 synchronization sequence 41information bits 36 Normal burst£ ¨NB£ © Frequency correction burst£ ¨FB£ © synchronized burst£ ¨SB£ © Access burst£ ¨AB£ © 1 Hyper frame =2018 Super frames =2715648 TDMA frames (3Ð ¡Ê ±28· Ö53Ã ë760º ÁÃ ë) 1 Super frame =1326 TDMA frames £ ¨6.12 s£ © 1 Multiframe =26TDMA frames£ ¨120 ms£ © 1 Multiframe =51 TDMA frames£ ¨3060/13ms£ © 1 TDMA frame =8 time slots£ ¨120/26=4.615ms£ © 1 time slot =156.25 bits duration£ ¨15/26=0.557ms£ © £ ¨1bit duration£ º48/13=3.68us£ © BCCH CCCH SDCCH TCH SACCH/T FACCH
  7. 7. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 Figure 1.3 Structure of TDMA frame 1.2.3 Burst Burst is the message layout of a timeslot in TDMA channel, which means each burst is sent to a timeslot of TDMA frame. Different message in the burst determines its layout. There are five kinds of bursts:  Normal burst: used to carry messages in TCH, FACCH, SACCH, SDCCH, BCCH, PCH and AGCH channels  Access burst: used to carry message in RACH channel  Frequency correction burst: used to carry message in FCCH channel  Synchronization burst: used to carry message in SCH channel  Dummy burst: transmitted when no specific message transmission request from system (In cells, standard frequency sends message continuously) Each kind of burst includes the following elements:  Tail bits: Its value is always 0 to help equalizer judge start bit and stop bit to avoid lost synchronization.  Information bits: It is used to describe traffic and signaling information, except idle burst and frequency correction burst.  Training sequence: It is a known sequence, used for equalizer to generate channel model (a way to eliminate dispersion). Training sequence is known by both transmitter and receiver. It can be used to identify the location of other bits from the same burst and roughly estimate the interference situation of transmission channel when the receiver gets this sequence. Training sequence can be divided into eight categories in normal burst. It usually has the same BCC setting with cells, but when accessed to burst and synchronization bust, training sequence is fixed and does not change with cells. For example, in access burst, training sequence is fixed (occupying 41 bits). The 36-bit message digit of the random access burst includes BSIC information of the cell. BSIC settings of the same BCCH should be different, in order to avoid mis-decoding of random access burst from neighboring cells into local access.  Guard period: It is a blank space. Since each carrier frequency can carry a maximum of eight subscribers, it is necessary to guarantee the non-overlapping 7
  8. 8. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 of each timeslot in transmission. Although timing advance technology (introduced later) is used, bursts from different mobile stations still show little slips; therefore, protection interval is adopted to allow transmitter to fluctuate in a proper range in GSM. On the other hand, GSM requires protection bits to keep constant transmission amplitude of the effective burst (except protection bits) and properly attenuate the transmission amplitude of mobile station. The amplitude attenuation of two sequential bursts as well as proper modulation bit stream can reduce the interference to other RF channels. The following is a detailed introduction to the structure and content of burst:  Access burst It is used for random access (channel request from network and switchover access). It is the first burst that the base station needs in uplink modulation. Access burst includes a 41-bit training sequence, 36-information bit, and its protection interval is 68.25 bits. There is only one kind of training sequence in access burst. Since the possibility of interference is rather little, it is unnecessary to add extra kinds of training sequences. Both training sequence and protection interval are longer than normal bursts in order to offset the bug of timing advance ignorance in the first access of mobile station (or switch over to another BTS) and improve demodulation ability of the system.  Frequency correction burst It is used for frequency synchronization in mobile station, equal to an unmodulated carrier. This sequence has 142 constant bits for frequency synchronization. Its structure is pretty simple with all constant bits being 0. After modulated, it becomes a pure sine wave. It is used in FCCH channel for mobile station to find and modulate synchronization burst of the same cell. When mobile station gets the frequency through this burst, it can read the information of following bursts (such as SCH and BCCH) in the same physical channel. Protection interval and tail bit are the same with that of normal burst.  Synchronization burst With a 64-bit training sequence and two 39-bit information fields, synchronization burst is used for time synchronization of mobile station in SCH channel. It belongs to downlink. Since it is the first burst required to be modulated by mobile station, its training sequence is relatively long and easy to be detected. 8
  9. 9. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程  Normal burst It has two 58-bit groups used in message field. To be more specific, two 58-bit groups are used to transmit subscriber data or voice together with two stealing flags. Normal burst is used to describe whether the transmitted is traffic information or signaling information. For example, to distinguish TCH and FACCH (when TCH channel is used as FACCH channel to transmit signaling, the stealing flag of the 8 half bursts should be set to 1. It has no other use in channels except in TCH channel, but can be regarded as the extension of training sequence and always set to 1.Normal burst also includes two 3-bit tails and a protection interval of 8.25 bits. The only bug is that the receiver has to store the preceding part of burst before modulation. Normal burst has a total of 26 bits, 16 of which are information bits. In order to get 26 bits, it copies the first five bits to the end of the training sequence and the last five bits to the head of the training sequence. There are eight kinds of such training sequence (these eight sequences have the least relevancy with each other). They correspond to different base station color code (BCC, 3 bits) respectively to distinguish the two cells using the same frequency.  Dummy burst This kind of bust is sometimes sent by BTS without carrying any information. Its format is the same with normal burst. The encrypted bits are changed into mixed bits with certain bit model. 1.2.4 Logical Channel In real networking, each cell has several carrier frequencies and each frequency has eight timeslots, proving eight basic physical channels. Logical channel carries out time multiplexing in one physical channel. It is classified according to the type of information in physical channel. Different logical channel transmits different type of information between BS and MS, such as signaling and data service. GSM defines 9
  10. 10. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 different burst type for different logical channel. In GSM, logical channel is divided into dedicated channel (DCH) and common channel (CCH), or traffic channel (TCH) and control channel (CCH) sometimes. I. TCH TCH carries coded voice or subscriber data. It is divided into full rate TCH (TCH/F) and half rate TCH (TCH/H) with 22.8 bit/s information and 11.4 Kbit/s information respectively. Using half of the timeslots in TCH/F can get TCH/H. A carrier frequency can provide eight kinds of TCH/F or sixteen kinds of TCH/H. Voice channel types are as follows:  Enhanced full rate speech TCH (TCH/EFS)  Full rate speech TCH (TCH/EFS)  Full rate 9.6 Kbit/s TCH (TCH/F9.6)  Full rate 4.8 Kbit/s TCH (TCH/F4.8)  Full rate ≤2.4 Kbit/s TCH (TCH/F2.4) II. CCH CCH is used to transmit signaling or synchronous data. It mainly consists of broadcast channel (BCCH), common control channel (CCCH), and dedicated control channel (DCCH). III. BCCH  Frequency Correction Channel (FCCH) It carries the information for frequency correction in mobile station. Through FCCH, mobile station can locate a cell and demodulate other information in the same cell, and recognize whether this carrier frequency is BCCH or not.  Sync Channel (SCH) After FCCH decoding, mobile station has to decode SCH information. This information contains mobile station frame synchronization and base station identification. Base station identification code (BSIC) occupies six bits, three of which are PLMN color codes ranging from zero to seven, and the other three are base station color codes (BCCs) ranging from zero to seven. Reduced TDMA frame (RFN) occupies 22 bits.  BCCH Generally, each BTS has a transceiver containing BCCH in order to broadcast system information to mobile station. System information enables mobile station to work efficiently in null state. 10
  11. 11. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 IV. CCCH  Paging Channel (PCH) PCH is a downlink channel used to page mobile station. When the network wants to communicate with a certain mobile station, it sends paging information marked as TMSI or IMSI through PCH to all the cells in LAC area according to the current LAC registered in mobile station.  Access Grant Channel (AGCH) AGCH is a downlink channel used for base station to respond the network access request of mobile station, that is, to allocate a SDCCH or TCH directly. AGCH and PCH share the same radio resource. Keep a fixed number of blocks for AGCH or just borrow PCH when AGCH requires without keeping special AGCH block (AGB).  Random Access Channel (RACH) RACH is an uplink channel used for mobile station to request SDCCH allocation in random network access application. The request includes the reason to build 3-bit (call request, paging response, location update request and short message request) and 5-bit reference random number for mobile station to identify its own access grant message. V. DCCH  Stand-alone Dedicated Control Channel (SDCCH) SDCCH is a bi-directional dedicated channel used to transmit information of signaling, location update, short message, authentication, encrypted command, channel allocation, and complementary services. It can be divided into SD/8 and SD/4.  Slow Associated Control Channel (SACCH) SACCH works with traffic channel or SDCCH to transmit subscriber information and some specific information at the same time. Uplink mainly transmits radio measurement report and the first layer head information; downlink mainly transmits part system information and the first layer head information. The information includes quality of communications, LAI, CELL ID, BCCH signal strength in neighboring cells, NCC limit, cell options, TA, and power control level.  Fast Associated Control Channel (FACCH) FACCH works with TCH to provide signaling information with a rate and timeliness much higher than that provided by SACCH. There is another control channel called cell broadcast channel (CBCH) besides the three control channels mentioned above. It is used in downlink and carries short message service cell broadcast (SMSCB) information. CBCH uses a physical channel same as SDCCH. 11
  12. 12. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 VI. Channel Combination Logical channel is mapped to physical channel according to certain rules. The channe l combinations specified in GSM protocol are as follows:  TCH/F + FACCH/F + SACCH/TF  TCH/H(0,1) + FACCH/H(0,1) + SACCH/TH(0,1)  TCH/H(0,0) + FACCH/H(0,1) + SACCH/TH(0,1) + TCH/H(1,1)  FCCH + SCH + BCCH + CCCH (main BCCH)  FCCH + SCH + BCCH + CCCH + SDCCH/4(0..3) + SACCH/C4(0..3)(BCCH combination)  BCCH + CCCH(BCCH extension)  SDCCH/8(0. .7) + SACCH/C8(0. .7) VII. Uncombined BCCH/SDCCH and Combined BCCH/SDCCH Paging information transmits in the timeslot 0 of BCCH. Timeslot 0 has the following s ub channels:  Broadcast channel (BCH): FCCH, SCH, BCCH  CCCH: PCH, AGCH  DCCH (combined BCCH/SDCCH): SDCCH, SACCH, CBCH ( if using cell broadcast) Physical channel timeslot 0 is made of multiframes logically. Each multiframe is 235.4 ms in length. Multiframe has different channel configurations, such as combined BCCH/SDCCH and uncombined BCCH/SDCCH. Different configuration has different paging capacity.  Uncombined BCCH/SDCCH Each frame of Uncombined BCCH/SDCCH can have nine paging blocks. The timeslot 0 of BCCH carrier frequency does not have SDCCH channel or CBCH channel.  Combined BCCH/SDCCH Each multiframe of combined BCCH/SDCCH can have three paging blocks. The timeslot 0 of BCCH carrier frequency contains four SDCCH subchannels (no CBCH) or three SDCCH and one CBCH subchannel. The configuration of combined BCCH/SDCCH has a great influence on paging capacity. Each multiframe has only three paging blocks instead of nine in uncombined BCCH/SDCCH, which means the paging capacity of cells with combined BCCH/SDCCH is only one third of that of cells with uncombined BCCH/SDCCH. 1.3 Data Transmission Radio channel has totally different characteristics from wired channel. Radio channel 12
  13. 13. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 has a strong time-varying characteristic. It has a high error rate when the signal is influenced by interferences, multipath fading, or shadow fading. In order to solve these problems, it is necessary to protect the signals through a series of transformation and inverse transformation from original subscriber data or signaling data to the information carried by radio wave and then to subscriber data or signaling data. These transformations include channel coding and decoding, interleaving and de-interleaving, burst formatting, encryption and decryption, modulation and demodulation. See 1.3 Figure 1.4 Forward and reverse data transmission process 1.3.1 Voice Coding Modern digital communication system usually uses voice compression technology. GSM takes tone and noise from human throat as well as the mouth and tongue filter effect of acoustics as voice encoder to establish a model. The model parameters transmit through TCH channel. Voice encoder is based on residual excited linear prediction encoder (REIP) and its compression effect is strengthened through long term predictor (LTP). LTP improves residual data encoding by removing the vowel part of voice. Voice encoder divides voice into several 20 ms voice blocks and samples each block 13
  14. 14. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 with 8 kHz, so each block has 160 samples. Each sample is quantified through frequency A 13 bits (frequency μ 14 bits). Since the compression rates of frequency A and frequency μ are different, add three and two “0” bits to the quantification values respectively, and then each sample gets 16 bits quantification value. Therefore, 128 Kbit/s data flow is obtained after digitizing but before encoding. This data flow is too fast to transmit in radio path and has to be compressed in encoder. With full speed encoder, each voice block is encoded into 260 bits to form a 13 Kbit/s source coding rate. Next is channel coding. With 20 ms as a unit, 260 bits are output after compression encoding, so the encoding rate is 13Kbit /s. Compared with the direct coding transmission of voice in traditional PCM channel, the 13kbps voice rate of GSM is much lower. More advance voice encoder can reduce the rate to 6.5kbps (half rate encoding). 1.3.2 Channel Coding Channel coding is used to improve transmission quality and remove the influence of interferential factors on signals at the price of increasing bits and information. The basic way of coding is adding some redundant information to the original data. The added data is calculated on the basis of original data with certain rules. The decoding process of receiving end is judging and correcting errors with this redundant bit. If the redundant bit of received data calculated with the same way is different from the received redundant bit, errors must have occurred in transmission. Different code is used in different transmission mode. In practice, several coding schemes are always combined together. Common coding schemes include block convolutional code, error correcting cyclic code and parity code. In GSM, each logical channel has its own coding and interleaving mode, but the principle is trying to form a unified coding structure.  Encode information bit into a unified block code consisting of information bits and parity check bits.  Encode block code into convolutional code and form coding bits (usually 456 bits).  Reassemble and interleave coding bits and add a stealing flag to form interleaving bits. All these operations are based on block. The block size depends on channel type. After channel coding, all channels (except RACH and SCH) are made of 464-bit block, that is, 456 coded information bits plus 8-bit header (header is used to distinguish TCH and FACCH). Then these blocks are reinterleaved (concerning channel). In TCH/F voice service; this block carries one speech frame of information. In control 14
  15. 15. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 channel, this block usually carries one piece of information. In TCH/H voice service, speech information is transmitted by a block of 228 coded bits block. For FACCH, each block of 456 coded information bits is divided into eight sub blocks. The first four sub blocks are transmitted by even bits of the four timeslots borrowed from the continuous frames of TCH, and the rest four sub blocks borrows odd bits of the four timeslots from the four continuous frames delayed for two or four frames after the first frame. Each 456 coded bit block has a stealing flag (8 bits), indicating whether the block belongs to TCH or to FACCH. In the case of SACCH, BCCH or CCCH, this stealing flag is dummy. The synchronous information in Downlink SCH and the random access information in uplink use short coded bit blocks transmitted in the same timeslot. In TCH/F, a 20ms speech frame is encoded into 456-bit code sequence. The 260 bits of the 13 Kbit/s 20ms speech frame can be divided into three categories: 50 most import bits, 132 important bits and 78 unimportant bits. Add 3 parity check bits to the 50 most important bits, and these 53 bits together with 132 important bits and 4 tail bits are convolutionally encoded ( with 1/2 convolutional coding rate ) into 378 bits, plus the 78 unimportant bits, and the 456 bits code sequence is obtained. In BCCH, PCH, AGCH, SDCCH, FACCH and SACCH, data is transmitted by Link Access Procedure on the Dm channel (LAPDm). Each LAPDm frame has 184 bits, together with 40 bits error correcting cyclic code and 4 tail bits, through 1/2 convolutional coding rate, and the 456 bits code sequence is obtained. Each SCH contains 25-bit message field. Among them, 19 bits are frame number and 6 bits are BSC number. These 25 bits plus 10 parity check bits and 4 tail bits are 39 bits. Through 1/2 rate convolutional coding, 78 bits are obtained, which occupy an entire SCH burst. . RACH message only has 8 bits, including 3-bit setup cause message and 5-bit discrimination symbol. On the basis of these 8 bits, add 6 bits of color code (obtained through the MOD 2 of the 6-bit BSIC and 6-bit parity check code), plus 4 tail bits to get 18 bits. Through 1/2 rate convolutional coding, 36 bits are obtained, which occupy an entire RACH burst. 。 1.3.3 Interleaving If speech signal is modulated and transmitted directly after channel coding, due to parametric variation of mobile communication channel, the long trough of deep feeding will affect the succeeding bits, leading to error bit strings. That is to say, after coding, speech signal turns into sequential frames, while in transmission, error bits usually occur suddenly, which will affect the accuracy of continuous frames. Channel 15
  16. 16. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 coding only works for detection and correction of signal error or short error string. Therefore, it is hoped to find a way to separate the continuous bits in a message, that is, to transmit the continuous bits in a discontinuous mode so as to change the error channel into discrete channel. Therefore, even if an error occurs, it is only about a single or very short bit stream and will not interrupt the decoding of the entire burst or even the entire information block. Channel coding will correct the error bit under such circumstances. This method is called interleaving technology. Interleaving technology is the most effective code grouping method to separate error codes. The essence of interleaving is to disperse the b bits into n bursts in order to change the adjacent relationship between bits. Greater n value leads to better transmission performance but longer transmission delay. Therefore, these two factors must be considered in interleaving. Interleaving is always related to the use of channel. GSM adopts secondary interleaving method. After channel coding, The 456 bits are divided into eight groups; each group contains 57 bits. This is the first interleaving, also called internal interleaving. After first interleaving, the continuity of information in a group is broken. As one burst contains two groups of 57-bit voice information, if the two-group 57 bits of a 20 ms voice block after first interleaving are inserted to the same burst, the loss of this burst will lead to 25% loss of bits for this 20 ms voice block. Channel coding cannot restore so much loss. Therefore, a secondary interleaving, also called inter-block interleaving, is required between two voice blocks. The entire interleaving process is shown in 1.3.3. Figure 1.5 Interleaving process 16
  17. 17. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 After internal interleaving, the 456 bits of a voice block B are divided into eight groups. Interleave the first four groups of voice block B (B0, B1, B2, and B3) with the last four groups of voice block A (A4, A5, A6, and A6), and then (BO, A4), (B1, A5), (B2, A6), and (B3, A7) form four bursts. In order to break the consistency of bits, put block A at even position and block B at odd position of bursts, that is, to put B0 at odd position and A4 at even position. Similarly, interleave the last four groups of block B with the first four groups of block C. Therefore, a 20 ms speech frame is inserted into eight normal bursts after secondary interleaving. Theses eight bursts are transmitted one by one, so the loss of one burst only affects 12.5% voice bits. In addition, as these bursts have no relations with each other, they can be corrected by channel coding. The secondary interleaving of control channel (SACCH, FACCH, SDCCH, BCCH, PCH, or AGCH) is different from voice interleaving which requires three voice blocks. The 456-bit voice block is divided into eight groups after internal interleaving (the same as that of voice block), and then the first four groups are interleaved with the last four groups (the same interleaving method as that of voice block) to get four bursts. Interleaving is an effective way to avoid interference, but it has a long delay. In the transmission of a 20 ms voice block, the delay period is (9*8)-7=65 bursts (SACCH occupying one burst), which is 37.5 ms. Therefore, MS and trunk circuit have echo cancellers added to remove the echo due to delay. 1.3.4 Encryption Security is a very important feature in digital transmission system. GSM provides high security through transmission encryption. This kind of encryption can be used in voice, user data, and signaling. It is used for normal burst only and has nothing to do with data type. Encryption is achieved by XOR operation of poison random sequence (generated through A5 algorithm of encryption key Kc and frame number) and the 114 information bits of normal burst. The same poison random sequence generated at receiving end and the received encryption sequence together produce the required data after XOR operation 1.3.5 Modulation and Demodulation Modulation and demodulation is the last step of signal processing. GSM modulation adopts GMSK technology with BT being 0.3 at the speed of 270.833 Kbit/s and Viterbi algorithm. The function of modulation is to add a certain feature to electromagnetic 17
  18. 18. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 wave according to the rules. This feature is the data to transmit. In GSM, the phase of electromagnetic field bears the information. The function of demodulation is to receive signals and restore the data in a modulated electromagnetic wave. A binary numeral has to be changed into a low-frequency modulated signal first, and then into an electromagnetic wave. Demodulation is the reverse process of modulation. 1.4 Timing advance Signal transmission has a delay. If the MS moves away from BTS during calling, the signal from BTS to MS will be delayed, so will the signal from MS to BTS. If the delay is too long, the signal in one timeslot from MS cannot be correctly decoded, and this timeslot may even overlap with the timeslot of the next signal from other MS, leading to inter-timeslot interference. Therefore, the report header carries the delay value measured by MS. BTS monitors the arrive time of call and send command to MS with the frequency of 480 ms, prompting MS the timing advance (TA) value. The range of this value is 0–63(0–233 us), and the maximum coverage area is 35km. The calculation is as follows: 1/2×3.7us/bit×63bit*c=35km 3.7us/bit is the duration per bit (156/577); 63bit is the maximum bit for time coordination; c is light velocity (transmission rate of signal); 1/2 is related to the round-trip of signal. According to the preceding description, 1bit to 554 m, due to the influence of multi- path transmission and the accuracy of MS synchronization, TA error may be about 3 bits (1.6km). Sometimes a greater coverage area is required, such as in coastal areas. Therefore, the number of channels that each TRX contains must be reduced. The method is to bind odd and even timeslots, so there are only four channels (0/1, 2/3, 4/5, and 6/7) for each TDMA frame in extended cell. Allocate channels 0, 2, 4, and 6 to MS. Within 35 KM around BTS, the TA value of MS is in the normal range 0-63; for the area beyond 35 KM, TA value stays at 63. This technology is called extended cell technology. The maximum value of TA in BTS measurement report is 63+156.25=219.25 bit, so the maximum radius of coverage area is: 1/2×3.7us× (63+156.25) ×3×108m/s=120km 18
  19. 19. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 Figure 1.6 Principle of dual timeslot extended cell The principle of dual timeslot extended cell is shown in 1.4. In real scheme, in order to improve the utilization of TRX, both common TRXs and dual timeslot TRXs can be included. BCCH must be in dual timeslot TRX to receive random access from any area. The calls within 35 km are allocated to common TRX; the calls within 35 km– 120 km and the switched in calls are allocated to dual timeslot TRX. If the system detects the switched in call is within 35km, it will switch over this call to common TRX. If the MS in conversation goes beyond 35 km, an intra-cell switchover will be carried out. Therefore, both the capacity requirement for remote areas and the coverage requirement for local areas can be satisfied. 1.5 System Information System information is sent to MS from network in broadcast form. It informs all the MSs within the coverage area of location area, cell selection and re-selection, neighbor cell information, channel allocation and random access control. By receiving system information, MS can quickly and accurately locate network resources and make full use of all kinds of services that network provides. There are 16 types of system information: type1, 2, 2bis, 2ter, 3, 4, 5, 5bis, 5ter, 6, 7, 8, and 13. System information is transmitted on BCCH or SACCH. MS receives system information in different mode from different logic channel. 19
  20. 20. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程  In idle mode, system information 1– 4, 7, and 8 are transmitted on BCCH ;  In communication mode, system information 5 and 6 are transmitted on SACCH; The content of system information is as follows:  System information 1 : cell channel description + RACH control parameter, transmitted on BCCH  System information 2: frequency description of neighbor cell + RACH control information + network color code (NCC) permitted, transmitted on BCCH, used for cell re-selection  System information 2bis: Extended neighbor cell BCCH frequency description + RACH control information, transmitted on BCCH, used for cell re-selection.  System information 2ter: Extended neighbor cell BCCH frequency description, transmitted on BCCH, used for cell re-selection.  System information 3 : Cell identity + location area identity (LAI) + control channel description + cell selection + cell selection parameter + RACH control parameter, transmitted on BCCH.  System information 4 : LAI + cell selection parameter + RACH control parameter + CBCH channel description + CBCH mobile configuration, transmitted on BCCH.  System information 5: Neighbor cell BCCH frequency description, transmitted on SACCH channel, used for cell handover.  System information 5bis: Extended neighbor cell BCCH frequency description, transmitted on SACCH channel, used for cell handover.  System information 5ter: Extended neighbor cell BCCH frequency description, transmitted on SACCH channel, used for cell handover.  System information 6 : Cell Global Identification (CGI) + cell option + NCC Permitted, transmitted on SACCH.  System information 7: cell re-selection parameter  System information 8: cell re-selection parameter BCCH is a low-capacity channel, every 51 multiframes ((235 ms) have only four frames (one information block) to transmit a 23 byte LAPDm message. Each information unit contains:  Cell channel description contains all the frequencies used in this cell.  RACH control information contains parameters such as Max Retrans, TX_integer, CBA, RE, EC, and AC CN.  Neighbor cell BCCH frequency description contains the BCCH frequency that the neighbor cell uses.  Allowed PLMN is used to provide NCC Permitted that MS monitors on BCCH TRX.  Control channel description contains parameters such as MS ATTACH/DEATTACH allowed Indicator ATT, BS-AG-BLKS-RES, CCCH-CONF, 20
  21. 21. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 BA-PA-MFRMS, and T3212.  Cell selection contains parameters such as power control (PWRC) indication, discontinuous Transmission (DTX) indication, and RADIO-LINK-TIMEOUT.  Cell selection parameter contains parameters such as cell re-selection hysteresis, MS-TXPWR-MAX-CCH, and RXLEV-ACCESS-MIN.  CBCH channel description contains channel type and TDMA deviation (the combination mode of dedicated channel), timeslot number (TN), training sequence code (TSC), hopping frequency channel indication H, mobile allocation index offset (MAIO), hopping frequency sequence number (HSN) and absolute radio frequency channel number ( ARFCN).  CBCH mobile configuration contains the relationship between hopping channel sequence and cell channel description.  Cell re-selection parameter contains CELLRESELIND, cell bar qualify (CBQ), cell reselection offset (CRO), temporary offset (TO), and penalty time (PT). 1.6 Cell Selection and Re-Selection 1.6.1 Cell Selection When a MS is switched on, it tries to contact GSM PLMN that the SIM permits and select a proper cell to extract control channel parameters and other system information. This process is called cell selection. The priority levels of cells include normal, low, and barred. Low priority level cell is selected when there is no proper normal cell. A proper cell means:  The cell belongs to the selected network;  The cell is not barred;  The cell is not in the national prohibited roaming location area;  The path loss between MS and BTS is under the limit set by network. The priority level of a cell is determined by CELL_BAR_QUALIFY (CBQ) and CELL_BAR_ACCESS (CBA). Table 6.1 Cell priority level CBQ CBA Cell priority level Cell re-selection status 0 0 Normal Normal 1 1 Barred Barred 0 0 Low Normal 1 1 Low Normal 21
  22. 22. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 1.6.2 Cell Selection Process To perform cell selection and re-selection, MS requires all the frequencies monitored to stay at the unweighted average value of Relev RLA_C. I. Cell Selection When MS Storing No BCCH Information MS searches all RF channels (at least 30 channels for 900 M, 40 for 1800 M, and 40 for PSC1900) in the system to obtain the Relev of each RF channel, and calculate the RLA_C based on at least five samples in three to five seconds, and then arrange these levels in descending order to select the proper BCCH. MS selects the cells with normal priority first. If the proper cells have low priority, MS will select the cell with the highest Relev. MS has already decoded and identified all these frequencies by now. If there is no proper cell, MS will keep on searching. It takes a maximum of 0.5 s to synchronize a BCCH TRX and 1.9 s to read the synchronized BCCH TRX data, except that it takes n*1.9s(n>1)to obtain the system information. II. Cell Selection When MS Storing BCCH Information If MS stores the BCCH frequency list of the former selected networks, MS will perform measurement sampling procedure (only for the stored BCCH TRX) according to this list. If the cell selection within this list fails, common cell selection will be performed. If all the cells have low priority level, MS will select the cell with the highest Relev. MS has already decoded and identified all these frequencies by now. When a 900 M MS enters the 900/1800 network, MS will probably choose 900 M network and ignore the priority level, because the MS stores all the 900 M frequency information in BCCH frequency list. III. Cell Selection Criteria Parameter C1 is the path loss criteria for cell selection, C1 of the service cell must exceed 0, the formula is as follows: C1= RLA_C - RXLEV_ACCESS_MIN- MAX ((MS_TXPWR_MAX_CCH- P), 0) (2-1) For DCS 1800 cells: C1 = RLA_C - RXLEV_ACCESS_MIN- MAX ((MS_TXPWR_MAX_CCH + POWER OFFSET- P), 0) In the formula: RLA_C: Average value of Relev 22
  23. 23. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 RXLEV_ACCESS_MIN: Minimum Relev that MS allows MS_TXPWR_MAX_CCH: Maximum transmit power on control channel P: Maximum transmit power of MS POWER OFFSET : Power offset related to MS_TXPWR_MAX_CCH used by DCS1800 cells. 1.6.3 Down Link Failure Downlink failure criteria are based on DSC. When a mobile phone stays in a cell, DSC is initialized to an integer most close to 90/N ( N is BS_PA_MFRMS, range value: 2–9). Each time when mobile phone successfully decodes a message on its paging subchannel, DSC increases by 1, but DSC cannot exceed the initial value; when decoding fails, DSC decreases by 4. When DSC<=0, downlink failure occurs. Down signaling link failure will lead to cell re-selection. 1.6.4 Cell Re-Selection Process In cell re-selection, mobile phone will synchronize and read the information from six BCCH TRXs (in BA list) with strongest signals outside the service area. For multi- frequency mobile phones, the TRXs with strongest signals may be in different frequency bands. In idle mode, mobile phone monitors all the BCCH TRXs in BA list and averages each Relev from BCCH TRX within 5 s to Max {5, ((5 * N + 6) DIV 7) * BS_PA_MFRMS / 4} s. N is the number of BCCH TRXs outside service area in BA list. Each RLA_C requires at least five level measurement samples and has to be updated from time to time. Service area samples the Relev at least once for each paging block to mobile. RLA_C is calculated by averaging the level samples received from 5s to Max {5s, five consecutive paging blocks of that MS}. Each RLA_C update is followed by the update of the six BCCH TRXs outside the service area in BA list. And the latter update may be even faster. Mobile phone decodes all the BCCH data in a service cell every other 30 s and the BCCH data blocks related to cell re-selection parameters of the six BCCH TRXs with strongest signals every other five minutes. When the mobile phone detects that a new BCCH TRX becomes one of the six TRXs with strongest signals, this BCCH TRX data should be decoded within 30 s. Mobile phone checks the BSICs of the six BCCH TRXs with strongest signals to make sure they are in the same cell. If the BSIC of a TRX is changed, the MS will regard the TRX as new TRX and reread the BCCH data. MS will re-select a neighbor cell as service cell under certain condition. This condition 23
  24. 24. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 includes several factors, such as RLA_C, cell restriction (decided by cell_bar and cell_bar_qualify), and access state of the neighbor cell. Cell re-selection adopts C2 algorithm. The calculation formula is as follows:  When PENALTY TIME is not 11111 C2=C1+CELL_RESELECT_OFFSET–TEMPORARY_OFFSET*H (PENALTY_TIME– T);  When PENALTY_TIME is 11111 C2=C1-CELL_RESELECT_OFFSET. When X>0, function H(x) =0; when X≤O, function H(x) =1. T is a timer; its initial value is 0. When a cell is included in the six neighbor cells with strongest signals by MS, the timer T of this cell begins to time; when a cell is excluded from the six neighbor cells with strongest signals by MS, T will be reset. CELL_RESELECT_OFFSET adjusts the value of C2. After T starts, TEMPORARY_OFFSET will modify the C2 algorithm according to the defined value before the penalty time in order to avoid a micro cell or a cell with small coverage area is selected by a fast moving MS. If the defined penalty time is out, the temporary offset will be ignored. Penalty time can avoid the frequent cell re-selection in those coverage areas like express highway. These parameters in C2 algorithm works only when CELL_RESELECTION_INDICATION is activated. Otherwise, MS will ignore the setting of CELL_RESELECT_OFFSET, TEMPORARY_OFFSET, and PENALTY_TIME, under such circumstances, C2=C1. Cell re-selection will be triggered under the following conditions:  The C2 value of a certain cell (belonging to the same location area with the current cell) exceeds that of the current cell by 5 seconds successively;  The C2 value of a certain cell (belonging to different location area from the current cell) exceeds the sum of the C2 value of the current service cell and cell selection hysteresis value by 5 seconds successively;  The current service cell is barred;  MS detects downlink failure;  The C1 value of the service cell is less than 0 for 5 seconds successively. 1.7 Frequency Hopping With the ever growing traffic volume and the limited frequency resource, frequency reuse is more and more aggressive. Therefore, the problem of how to reduce frequency interference becomes more and more remarkable. The essence of anti- 24
  25. 25. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 interference is to fully utilize the current spectrum, time domain, and space resources. The key measures include frequency hopping, discontinuous transmission (DTX), and power control. Frequency hopping also can effectively reduce the influence of fast fading. 1.7.1 Types of Frequency Hopping GSM radio interface uses slow frequency hopping (SFH) technology. The difference between slow frequency hopping and fast frequency hopping is that the frequency of latter changes faster than frequency modulation. In GSM, the frequency remains the same during burst transmission. Therefore, GSM frequency hopping belongs to slow frequency hopping. In frequency hopping, the carrier frequency is controlled by a sequence and hops with time. This sequence is frequency hopping sequence. Frequency hopping sequence is a sequence of frequencies decided by hopping sequence number (HSN), mobile allocation index offset (MAIO) and frame number (FN) through a certain algorithm in the mobile allocation containing N frequencies. The N channels of different timeslots can use the same hopping sequence. The different channels of the same timeslot in the same cell adopt different MAIO. Frequency hopping can be divided into frame hopping and timeslot hopping according to time domain and RF hoping and baseband hopping according to implementation mode.  Frame hopping: the hopping frequency changes once in each TDMA frame period. Each TRX can be regarded as a channel. The TCH of BCCH TRX cannot join in the frequency hopping in a cell. The hopping TRX should have a different MAIO. Frame hopping is an exception of timeslot hopping.  Timeslot hopping: the timeslot frequency of each TDMA frame changes once. The TCH of BCCH TRX can join in the frequency hopping, which happens in baseband hopping.  RF hopping: both transmission and reception of TRX join in the frequency hopping. The number hopping frequencies can exceed the number of TRXs in the cell.  Baseband hopping: each transceiver works at a fixed frequency. TX does not join in frequency hopping. Frequency hopping is performed through the handover of banseband signal. Therefore, the number of hopping frequencies cannot exceed the number of TRXs in the cell. The two frequency hopping modes above are based on BTS. As for MS, since each MS has only one TRX unit, RF hopping is the only mode. 25
  26. 26. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 I. Baseband Hopping The system has multiple baseband and TRX processing unit. Each TRX processing unit has a fixed working frequency; each baseband processing unit processes one line of service information and sends the processed information to the TRX unit with bus topology in time sequence according to frequency hopping rule. This kind of frequency hopping is called “baseband hopping”. In baseband hopping, each transceiver works with a fixed frequency. The bursts on the same speech path are sent to each transceiver. Baseband hopping is based on the handover of baseband signals. Since the transceiver of each BTS has a fixed working frequency, both broadband combiner and cavity combiner can be adopted. The number of TRXs decides the maximum number of frequency hopping. The problem for baseband hopping is that if one TRX board fails, the corresponding code word will be lost, thus affecting all the calls under hopping mode in the cell. Figure 1.7 Baseband hopping II. RF Hopping Under this mode, each line of service information is processed by fixed baseband unit and frequency band unit. The working frequency of frequency band unit is provided by frequency combiner. Under the control of control unit, frequency can be changed according to certain rules. In RF hopping, the frequencies used by a TRX to handle all the bursts of a call come from the frequency change of combiner, instead of the handover of baseband signals. The number of TRXs is not limited by carrier frequency. As the working frequency of TRX changes, which means the frequency of the input port to combiner changes, only broadband combiner can be adopted. This kind of broadband combiner leads to about 3dB insertion loss in two-in-one 26
  27. 27. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 combination and the loss is greater in the link insertion of multi-combiner. GSM protocol does not specify which kind of frequency hopping is used in GSM BTS. The mode of frequency hopping can be decided by operators according to the equipments. Figure 1.8 RF hopping 1.7.2 Frequency Hopping Algorithm The parameters related to frequency hopping algorithm are as follows:  CA: cell allocation, the collection of frequencies used by a cell  FN: TDMA frame number, broadcasted on sync channel. FN (0–2715647) synchronizes BTS with MS  MA: mobile allocation, the collection of radio frequencies used for MS frequency hopping. It is a subset of CA. MA contains N frequencies, 1≤N≤64.  MAIO: mobile allocation index offset, (0–N-1). During communication, the radio frequency at air interface is an element of MA. Mobile allocation index (MAI, 0– N-1) is used to determine the element of MA. That is to say, the actual frequency used is decided by MAI. MAIO is the initial offset of MAI and it is used to avoid the contention of frequency by several channels at the same time.  HSN: hopping sequence number (0–63). It determines that the hopping sequence with concentrated frequencies is adopted in frequency hopping. When HSN=0, the hopping is cyclic hopping; when HSN≠0, the hopping is random hopping. The proper setting of parameters is based on the understanding of the use of each parameter in hopping algorithm and the hopping theory. The proper setting ensures the healthy working state of the system. 1.7.2 is the flow chart of frequency hopping algorithm. 27
  28. 28. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 FN T2(0¡«25) FN T3(0¡«50) MAI (m0¡«mN-1) MAIO (0¡«N-1) Represent in 7 bits T1R= T1 MOD 64 Exclusive OR FN T1(0¡«2047) HSN (0¡«63) Addition Look-up table Addition M'=M mod 2^NBIN T=T3mod 2^NBIN M'<N S=M'S=(M'+T) mod N MAI=(S+MAIO) mod N RFCN=MA£¨MAI£© 7bits 5bits11bits 6bits 6bits 7bits 7bits 8bits 6bits6bitsNBIN bits NBIN bits YN NBIN bits NBIN bits NBIN bits Figure 1.9 Frequency hopping algorithm In 1.7.2: Mod: modular arithmetic ^: power arithmetic NBIN: number of bits required to represent N = INTEGER (log2 (N) +1) According to GSM protocol 0502: For cyclic hopping (HSN = 0): 28
  29. 29. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 MAI, integer (0 ... N 1) : MAI = (FN + MAIO) modulo N (2-2) Otherwise, see 1.7.2: M, integer (0 ... 152) : M = T2 + RNTABLE((HSN xor T1R) + T3) S, integer (0 ... N 1) : M' = M modulo (2 ^ NBIN) T' = T3 modulo (2 ^ NBIN) If M' < N: S = M' Otherwise: S = (M'+T') modulo N MAI, integer (0 ... N 1) : MAI = (S + MAIO) modulo N (2-3) Remarks: For the cyclic hopping in discontinuous transmission (DTX), the number of hopping frequencies should avoid N mod 13 = 0, because under such condition, the probability of transmission and measurement of SACCH frame at the same frequency is rather high, and the harms are obvious. See the description of DTX in section 1.8 RNTABLE is a function with the parameters from integer 0 to 113, GSM protocol defines its values as shown in 1.7.2: Table 9.1 RNTABLE(X) The following conclusion can be used in the rough estimate of whether inter- frequency or intra-frequency collision exists: MAI=(S+MAIO) MOD N 29
  30. 30. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 RFCHN=MA (MAI); When HSN=0, S equals the frame number, in other cases, S is only related to frame number and frequency hopping number. When HSN is fixed and frame number is the same, S must be the same. Therefore, as the TRXs of each sync cell have the same frame number, different hopping groups in sync cells can adopt the same HSN. A proper configuration of MAIO can avoid the inter-cell or intra-cell frequency collision within the same BTS. The aggressive frequency reuse adopts this theory. 1.7.3 Benefits of Frequency Hopping In GSM, frequency hopping has two benefits: frequency diversity and interference averaging. I. Frequency Diversity Frequency hopping can reduce the influence of signal strength change due to multipath transmission. This effect equals that of frequency diversity. In mobile communications, Rayleigh fading leads to the great change of radio signal in a short time. This kind of change is related to frequency: a more independent fading accompanies a greater frequency difference. The 200 KHz interval generally ensures the independence of inter-frequency fading, while the 1 MHz interval can fully guarantee this kind of independence. Through frequency hopping, all the bursts containing the code word of the same speech frame are protected from the damage of Rayleigh fading in the same way. See I. Figure 1.10 Fading 30
  31. 31. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 Statistics shows that frequency hopping gain is related to environmental factors, especially to the moving speed of MS. When the MS moves at a high speed, the location difference between two bursts on the same channel is also affected by other kinds of fading. The higher the speed is, the lower the gain will be. Frequency diversity benefits a lot to a large number of MSs moving at low speed. Frequency hopping gain is also related to the number of frequencies. When the number of frequencies decreases, the hopping gain falls. The relationship between the number of frequencies and hopping gain can be explained in this way: frequency hopping is pseudo spectrum spread, and the hopping gain is the processing gain after transmission frequency band spread. The basic way to test frequency hopping gain is to calculate the differences between different C/I at different hopping frequencies under the same FER. These C/I differences are the frequency hopping gain. The relationship between the number of frequencies and frequency hopping gain is shown in I. (The actual gain may be affected by environment) Table 10.1 The relationship between the number of frequencies and frequency hopping gain Number of TRXs in frequency hopping Gain of frequency diversity(dB) 〈=1 0 2 3 3 4 4 5 5 5.5 6 6 7 6.3 8 6.5 9 6.8 10 6.9 >=11 7 II. Interference Averaging Frequency hopping provides the diversity of interference on transmission channel, so that all the bursts containing the code word of the same speech frame are protected from the damage of interference in the same way. Through error correction coding and interleaving of the system, the original data can be restored from the rest part of 31
  32. 32. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 the received flow. The hopping gain is obtained only when the interference is in narrowband distribution. If the interference is in broadband distribution, all the bursts will be destroyed and the original data cannot be restored. Therefore, no hopping gain is obtained. The common interference after frequency hopping can be regarded in narrowband distribution. In frequency hopping, error rate tends to increase in the test, but we feel the conversation quality improves. It is because although the error rate increases, the influence of interference is homogenized in frequency hopping, the speech restoring ability improves because of the interleaving and de-interleaving before. In GPRS data services, frequency hopping can be harmful when the data rate is rather high (CS4). 1.8 Discontinuous Reception and Discontinuous Transmission 1.8.1 Discontinuous Reception and Paging Channel In idle mode, if MS selects a cell as its service cell, it begins to receive the paging information from this cell. But in order to reduce power consumption, discontinuous reception (DRX) is introduced in GSM. Each user (IMSI) belongs to a paging group and each paging group corresponds to a paging subchannel. MS can calculate which group it belongs to based on the last three digits of its IMSI and the configuration of paging channel in this location area, and then locate the paging subchannel of this paging group. In fact, in idle mode, MS just listens to the paging information from the system on its subchannel (MS also monitors the Relev of BCCH carrier frequency in non-service area during this period of time) and ignores the information on other paging subchannels. Some of the hardware equipments are even switched off to save the power of MS. But MS must complete the required task of network information measurement within a specified time. Through DRX, MS can receive the broadcast short messages that the users want to know with less power consumption, thus extending the service time. BSC has to send scheduling messages to support DRX at MS. One scheduling message contains lots of broadcast short messages to be sent soon. The time that all broadcast short messages of a scheduling information takes is a scheduling cycle. Scheduling information contains the description of all short messages to be broadcast in order and also indicates the position of the messages in scheduling cycle. Through scheduling messages, MS can find the broadcast short messages it wants quickly so as to reduce its power consumption. The number of paging subchannels of each cell can be calculated based on the configuration type of CCCH, BS_AG_BLKS_RES (the number of blocks belonging to 32
  33. 33. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 AGCH in 51 multiframe), and BS_PA_MFRMS (the number of 51 multiframes used as one paging subchannel cycle). When there are three CCCHs in a 51 multiframe, the number of paging subchannels is (3- BS_AG_BLKS_RES) ×BS_PA_MFRMS When there are nine CCCHs in a 51 multiframe, the number of paging subchannels is (9- BS_AG_BLKS_RES)×BS_PA_MFRMS In addition, the configuration of CCCH parameters has the following principles:  The greater the parameter BS_PA_MFRMS, the more the paging subchannels, and the less the users of each paging subchannel, but the total capacity of the system remains the same, because the average delay of the paging information on radio channel increases. When the ratio of retransmission waiting is relatively high, BS_PA_MFRMS should be improved to increase the paging subchannels; when the ratio of retransmission waiting is relatively low, BS_PA_MFRMS should be reduced to shorten the paging delay.  The capacities of paging subchannels of all cells in a location area should be the same, because the paging message of a location area must be sent in all the cells of this location area at the same time.  The longer the cycle of paging channel, the less power the MS in this service area takes. For example, in cities, this cycle can be defined as 2, which means MS listens to paging messages once for every 102 frames. In rural areas, this cycle can be defined as 4 or 6. The MS with the paging channel cycle of 6 consumes 18% less power than the MS with the paging channel cycle of 2. After measuring the system information, MS enters the rest state and listens to the paging information in the specified paging blocks only and measures the Relev of BCCH of neighbor cells at the same time. After 30 s, MS will listen to system information again to judge the cell re-selection process.  In GSM, CCCH mainly includes AGCH and PCH. Its primary function is to transmit immediate assignment messages and paging messages. CCCH can be one or several physical channels and it can also share a physical channel with SDCCH. The combination mode of CCCH depends on the parameter CCCH_CONF. The configuration of CCCH_CONF must be consistent with the actual configuration. It is recommended that when there is only one TRX in a cell, the configuration of CCCH can be a physical channel shared with SDCCH (3 CCCH information blocks).  When the traffic volume is extremely large, in case one physical timeslot is not enough, GSM specification allows the configuration of multiple CCCH channels on the TRX besides BCCH, but these channels must be used in timeslot 0, 2, 4, and 6.  When CCCH_CONF is confirmed, parameter BS_AG_BLKS_RES actually decides the ratio of AGCH and PCH on CCCH. It is recommended that this 33
  34. 34. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 parameter be configured as little as possible in order to reduce the response time of MS to paging. 1.8.2 DTX I. DTX Overview During communication, only 40% time is used for conversation; no useful information is transmitted during the rest 60% time. If all the information is transmitted to network, many of the system resources will be wasted, in addition, the interference will aggravate. In order to solve this problem, GSM adopts DTX technology to stop signal transmission when there is no voice signal. Therefore, the interference level is reduced and the system efficiency is improved. There are two kinds of transmission modes in GSM: normal mode and discontinuous transmission (DTX) mode. In normal mode, noise and voice have the same transmission quality. In DTX mode, the transmission of unuseful messages is prohibited. MS only sends man-made noise signals that are tolerable, which means this noise will not annoy the listeners nor affect the conversation. This kind of noise is called comfort noise. In DTX mode, 260-bit code is transmitted in every 480 ms; in normal mode, 260-bit code is transmitted in every 20 ms. Whether the downlink DTX is adopted or not is controlled by network operators of the exchange part. This kind of control is based on BSC. The control information is transmitted to baseband processing part through dedicated signaling channel, and then arrives at TC through the inband signaling of TRAU frame to indicate whether downlink DTX is adopted. For some vendors, the downlink DTX can be configured on the basis of cell. Uplink DTX is configured by network operators of the radio part. The parameter DTX in system information consists of 2 bits. Its coding scheme is shown in I: Table 10.2 Value range of DTX DTX Meaning 00 MS can use DTX 01 MS must use DTX 10 MS is not allowed to use DTX 11 Reserve Parameter DTX is contained in “cell option” of information unit and transmitted periodically in the system information of each cell broadcast. MS decides whether to 34
  35. 35. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 start DTX function based on this information. DTX can be used for voice signal transmission and nontransparent data transmission. BCCH TRX does not use this technology. The benefits of DTX are listed below:  Uplink DTX can save MS batteries and reduce interference.  Downlink DTX can save BTS power consumption and reduce interference and intra-BTS intermodulation.  Uplink DTX and downlink DTX used together can improve the intra-frequency ratio of the system. This kind of improvement, when used in aggressive- frequency-reuse cell planning, especially when used with frequency hopping, can greatly expand the system capacity. II. Voice Activity Detection For voice activity detection (VAD), the source must indicate when the transmission is required. When DTX mode is activated, the encoder must detect the signal is voice or noise. Therefore, the VAD is required. VAD can differentiate voice from noise through calculating some signal parameters and threshold values. This kind of differentiation is based on an energy rule: the energy of noise is always lower than that of voice. VAD generates a group of threshold value in every 20 ms to judge whether the next 20ms block is voice or noise. When the background noise is too loud, the noise signal will be regarded as voice signal to transmit. III. Silence Indicator The coding procedure of noise is the same as that of voice. After sampling and quantification, a noise block will be produce by encoder in every 20ms. Like voice block, the coded noise block also contains 260 bits, which forms a SID frame. The SID frame will go through channel coding, interleaving, encryption and modulation and finally be sent by eight continuous bursts. On TCH, a complete SACCH information block has four 26 muliframe cycles (480 ms). In order to differentiate voice frame and SID frame, these eight continuous bursts are arranged at the beginning of the third multiframe. During other time of the 480 ms, no information is transmitted except SACCH timeslot. The SID frame made from the 20 ms noise block is interleaved with the preceding frame and the following frame; the first SID frame is interleaved with the preceding voice frame and the following SID frame. IV. Measurement Uplink DTX and downlink DTX are two irrelevant procedures that are activated by system parameters respectively. There are two kinds of measurement in GSM: full 35
  36. 36. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 measurement and sub measurement. Global measurement is the average of the level and quality of the 104 timeslots in a measurement cycle (four 26 multiframes); local measurement is the average of level and quality of 12 timeslots, including eight continuous TCH bursts (for TCH/F, 0-103 TDMA frames as a cycle. The frame numbers of these eight bursts are 52, 53, 54, 55, 56, 57, 58, and 59. when no voice or signaling is transmitted, the descriptor of comfort noise they contain is called SID) and four SACCH bursts (0-103 TDMA frames as a cycle, for timeslot 0, the frame numbers of these four bursts are 12, 38, 64, and 90; for timeslot 1, the frame number is that of timeslot 0 plus 13. similarly, the frame numbers that the eight timeslots correspond to can be obtained in this way). In order to achieve uniformity, no matter the uplink DTX or downlink DTX is activated or not, BTS and MS must complete these two kinds of measurement. Each SACCH measurement report of BTS and MS indicates whether DTX is used in last measurement report time. BSC choose one of the two kinds of measurement based on this indication. 1.9 Power Control 1.9.1 Power Control Overview Power control is to change the transmission power of MS or BTS (or both) in radio mode within certain area. Power control can reduce the system interference and improve the spectrum utilization and prolong the service time of MS battery. When the Relev and quality is good, the transmission power of the peer end can be reduced to lower the interference to other calls. In GSM, power control can be used in uplink and downlink respectively. The power control range for uplink MS is 20 dB–30dB. Based on the power class of MS (most MSs belongs to class 4, which means the maximum transmission power is 33 dbm), each step can change 2 dB. The downlink power control range is decided by equipment manufacturer. Although whether to adopt uplink or downlink power control function is decided by network operators, all MSs and BTS equipments must support this function. BSS manages the power control in the two directions. To facilitate BCCH frequency pull-in and the measurement of Relev (including the Relev of neighbor cell BCCH frequency), GSM protocol specifies that no power control is allowed for the timeslots in the downlink of BCCH TRX. 1.9.2 MS Power Control The power control of MS includes two adjustment stages: stable adjustment stage 36
  37. 37. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 and initial adjustment stage. Stable adjustment is the common way to implement power control algorithm. Initial adjustment is used at the beginning of call connection. When a connection occurs, MS sends signals with nominal power (before receiving power adjustment commend, the nominal transmission power of MS is the maximum transmission power on BCCH of the cell. If MS does not support this power level, it will adopt other power level most close to this level, such as the maximum power level supported by the classmark of MS in indication message establishment). Therefore, MS accesses to network through RACH with the maximum power broadcast on BCCH. When MS power is lower than this value, it will transmit with its maximum transmission power. The system specifies that the power level of the first message that MS sends on DCH is also this value. The system control begins after MS receives the power control command in SACCH information block from SDCCH or TCH. Since BTS can support multi-call at the same time, the Rxlev should be quickly reduced in the new connection. Otherwise, other calls supported by this BTS will deteriorate and the calls in other cells will also be affected. The purpose of initial adjustment stage is to quickly reduce the transmission power of MS to get the stable MR, so MS can be adjusted according to stable power control algorithm. The required parameters in uplink power control, the expected uplink Rxlev, and the uplink received quality can be adjusted according to the situation of the cell. After receiving a certain number of uplink MRs, the system compares the actual uplink Rxlev and received quality obtained by interpolation, filtering, and other methods with the expected values and calculate the power level that the MS should be adjusted to through power control algorithm. If the calculated power level differs from the output power level of MS and meets certain limit conditions (such as step limit of power adjustment and range limit of MS output power), the system will send power adjustment command. The command of changing MS power and the required time advance will be sent to MS in the layer 1 header of each downlink SACCH information block. MS will configure the power level it uses now in its uplink SACCH information block and send it to BTS in measurement report. This level is the power level of the last burst in the previous SACCH measurement cycle. When MS receives the power control information in SACCH information block from DCH, it will transmit with this power level. One power control message does not make the MS switch to the required level immediately. The maximum change rate of MS power is 2 dB for every 60 ms. For 12 dB, before MS receives the next power control message, it will not end as one SACCH measurement cycle takes 480 ms. In addition, it takes three measurement cycles to send power control message and execute the command. Therefore, the power control cycle should not be too short in order to ensure its accuracy. See 1.9.2. 37
  38. 38. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 Figure 1.11 Execution of power control command The purpose of uplink power control adjustment is to minimize the difference between the actual uplink Rxlev and received quality and the expected uplink Rxlev and received quality. The purpose of interpolation and filtering is to process the lost measurement reports and remove temporary nature to ensure the stability of power control algorithm. The difference between initial adjustment and stable adjustment is that the expected uplink Relev and received quality and the length of filter in initial adjustment are different from that of stable adjustment, and the initial adjustment only has downlink adjustment. 1.9.3 BTS Power Control BTS power control is an optional function. It is similar to MS power control, but it only uses stable power control algorithm. The required parameters are Rxlev threshold (lower limit), and the maximum transmission level can be received (upper limit). The Relev is divided into 64 levels ranging from 0 to 63. Level 0 is the lowest Rxlev; level 63 is the highest Rxlev. BTS power control is divided into static power control and dynamic power control. Dynamic power control is the fine tuning based on static power control. There are six steps (2 dB/step) of static power control according to Protocol 0505. If the maximum output power is 46 dBm (40W), the step 6 is 34 dBm. Static power control step is defined in the cell distributes list of data management 38
  39. 39. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 system, which specifies the maximum output power (suppose this value is Pn) of static power control. For step 15 of dynamic power control, the corresponding value range is Pn dB–Pn-30dB. When the maximum power control still cannot satisfy the requirement, adjust static power control step to improve the maximum output power of dynamic power control Pn. 1.9.4 Power Control Processing I. Measurement Report Interpolation Each measurement report has a sequence number. If network detects incontinuous sequence numbers, it means some of the measurement reports are missing. The network will complete the reports based on interpolation algorithm. As shown in I, the network receives measurement reports n and n+4. It detects the sequence numbers are not continuous, so it uses an algorithm to add n+1, n+2, and n+3 (yellow) to complete the reports. The purpose of measurement report interpolation is to avoid call loss when the power is too low. Figure 1.12 Measurement report interpolation II. Measurement Report Filtering Network will not judge the state of MS based on only one measurement result, because that is too incomprehensive, in addition, the MS may be fluctuating. Therefore, filtering is required. Filtering combines several continuous measurement results together to determine the state of MS during this period of time. In II, the network uses four measurement reports (yellow). TA has filters for Rxlev and received quality of uplink and downlink The purpose of measurement report filtering is to remove temporary nature and 39
  40. 40. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 ensure the algorithm stability. Figure 1.13 Measurement report filtering III. Power Control Adjustment Calculate the power adjustment value based on the difference between the Rxlev and the expected value.  Power control adjustment based on Rxlev Power control module compares the estimate value of Rxlev obtained through pre- processing of measurement report with the expected value, and calculates the step length of adjustment. In power control algorithm, variable step is often used for quick power control.  Power control adjustment based on received quality Power control module compares the estimate value of received quality obtained through pre-processing of measurement report with the expected value, and calculates the step length of adjustment. When the received quality is bad, improve the transmit power; when the received quality is good, reduce the transmit power. This kind of power control adopts fixed step.  Comprehensive decision for power control Consider both Rxlev and received quality and adopt different power control strategies in different conditions to keep the stability and efficiency of power control algorithm. Table 13.1 Comprehensive decision for power control Relev power control adjustment Received quality power control adjustment Comprehensive power control adjustment Reduce TP Reduce TP Reduce transmit power Reduce TP Improve TP No action Reduce TP No action Reduce TP Improve TP Reduce TP Improve TP 40
  41. 41. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 Improve TP Improve TP Improve TP Improve TP No action Improve TP No action Reduce TP Reduce TP No action Improve TP Improve TP No action No action No action Note: TP = transmit power III shows how comprehensive decision for power control works. When the received quality requires the improving of transmit power while the Rxlev requires the reducing of it, the system will make a comprehensive decision to perform no power control adjustment, because bad received quality and good Rxlev represent strong network interference. Under such circumstances, improving transmit power will further increase the interference. 1.10 Immediate Assignment Procedure The purpose of immediate assignment is to establish a radio connection (RR connection) between MS and system at Um interface. 1.10.1 Network Access License and Random Access Request The request of MS for channel assignment is controlled by its own access level and the access grant level broadcast in cell. Each MS has one access level of the ten levels from 0 to 9. In addition, it may also have one or several levels of the five special access levels from l1 to 15. Access level is stored in SIM card. BCCH system information broadcasts access levels and special access levels that the network grants and the information that whether all MSs allow emergency call or allow special access levels only. If the mobile originated call is not emergency call, the MS can access to network only when it belongs to the granted access level or granted special access level. If the mobile originated call is emergency call, the MS can access to network only when all the MSs in the cell allow emergency call or it belongs to the granted special access level. When an MS wants to establish connection with the network, it sends a channel request to network through RACH channel. Channel request information contains 8- bit useful signaling information, among which 3 bits–6 bits are used as the minimal indicator of access cause. The system processes different channel requests based on this rough indication. It differentiates the granted calls from the denied calls and 41
  42. 42. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 assigns proper channels for the granted calls. This kind of process is especially useful when the network is overload and the flow control is required. Since the channel capacity is limited, this indicator cannot transfer all the information from MS, such as the detailed cause of channel request, user identity and the features of mobile equipment. These kinds of information are sent in the following SABM messages. The 8-bit information also contains the random discriminator sent by the MS and the immediate assignment command (it contains information about the assigned channel). Immediate assignment command carries the discriminator sent by the previous MS. MS compares this discriminator with its own discriminator and judges whether it is the message for itself from network. Since there are at most 5 bits in the 8 bits information carrying discriminator, only 32 MSs can be differentiated at the same time. Further discrimination of the MSs requires the response information at Um interface. Channel request information belongs to internal information of BSS. In GSM, RACH is a kind of ALOH. In order to reduce the collision on RACH during MS access to network and improve the efficiency of RACH channel and MS access. GSM specifies the required access algorithm for MS. This kind of algorithm defines three parameters: Tx_interger T, the maximum retransmission times RET, and parameter S related to T and channel combination. T represents the number of timeslots between two transmissions when continuous channel requests are sent. S is an intermediate variable depends on T and the configuration of CCCH. See the description of this parameter in Chapter 7. RET is the MS maximum retransmission times allowed in order to avoid access collision. Each time after MS sends access request, T3120 is to receive (or reject) immediate assignment message. MS will retransmit access request for the messages that are not received or rejected when T3120 times out under the premise that RET is not exceeded and restart the T3120. When the retransmission times reaches RET and T3120 times out, T3126 will be started to receive (or reject) immediate assignment message. When T3126 times out, cell re-selection will be initiated. 1.10.2 Initial Immediate Assignment After decoding the channel request information, BTS sends a channel required message to BSC. This message contains important additional information and the estimation of TA by BTS. After receiving this message, BSC selects a proper channel for this request and activates the land resources by sending a channel active message to BTS. BTS returns a channel active acknowledge message to BSC. If BSC receives this message, BTS will send an immediate assignment command or immediate assignment extended message on CCCH. In order to improve channel efficiency, GSM introduces the message layout of immediate assignment extended that contains the assignment information of two MSs. The immediate assignment 42
  43. 43. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 message contains the assignment information of one MS. According to GSM specifications, MS must identity the immediate assignment (extended) information for the last three channel requests. If there is no channel to activate, BSC will send an immediate assignment reject or immediate assignment extended reject message to MS. After receiving the reject message, MS stops T3120 based on one of the last three channel requests and starts T3122. During the specified time of T3122, MS has no access to network and turns into idle mode. Before T3122 times out, MS cannot initiate connection attempt except emergency call within the same cell. After receiving immediate assignment message, MS compares the received assignment command with the information stored in its channel request and judges whether this message is for itself. If this message matches one of its last three channel requests, MS will stop T3120 or T3126 and switch to the assigned channel. Then it starts to establish the signaling link by using Set Asynchronous Balanced Mode (SABM) command. 1.10.3 Initial Message After receiving immediate assignment message and decoding it, MS adjusts its configuration of transmission and reception to the assigned channel and transmits signaling according to the TA value specified by BSS and the initial maximum transmission power broadcast in BCCH system information (see the description of msTxPwrMaxCCH). MS sends an SABM frame on assigned SDCCH/TCH to establish the asynchronous balanced mode (SAPI=0) that is used to establish signaling message link layer connection under acknowledgement mode. According to GSM protocol, SABM carries an initial message that contains layer 3 service request information. When two MSs send the same channel requests (which is possible in high traffic volume area), the two MSs may respond to the same dedicated channel. in order to save this problem, after receiving SABM frame, BTS makes no modification but sends a UA frame (no frame number acknowledgement) containing the same information as that of initial message. If the information of UA frame is different from that of SABM frame, MS will abandon this channel and start reaccess process. Only the right MS can stay on this channel. SABM frame carries four kinds of initial messages: CM service request (such as call setup, short message, and supplementary service), location updating request (generic location updating, periodic location updating, and IMSI attach), IMSI detach, and paging response. All these messages contain the identity of MS, detailed access cause, and MS classmark (indicating some key features such as transmission power 43
  44. 44. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 level, encryption algorithm, short message capacity, and frequency capacity). After receiving the initial message, BTS sends an establish indication message to BSC. BSC receives this message and sends complete layer 3 information to MSC to request SCCP connection to MSC. Layer 3 information carries the causes for CM service request, which includes mobile originated call, emergency call, location updating, and short message service. This information also carries cipher key sequence number, MS identification number, and some physical information of the MS such as transmit power level, ciphering algorithm, pseudo-synchronization, and short message. After receiving this information, MSC sends connection confirmed message to BSC (if the connection cannot be established, MSC will send SCCP refused message) to indicate that the signaling link between MS and MSC has been established. By this time, MSC can control the transmission properties of RR management; BSS monitors the transmission quality and prepares for handover. Then the MM connection begins. Authentication or encryption is triggered when required in the following processing. The process of immediate assignment is shown in 1.10.3. Figure 1.14 Immediate assignment In the immediate assignment process, T3101 starts when BSC sends channel active message to BTS and ends when the establish indication is received. If T3101 times out before signaling channel is established, the activated channel will be released. 1.10.4 Immediate Assignment Failure  If a failure occurs to the underlaying MS on the new channel before the establishment of signaling link, the network releases the assigned channel of MS. The following processing depends on the failure type and previous actions. If the failure is caused by the mismatch of message field in decision contention 44
  45. 45. GSM Radio Network Planning and Optimization 第 2 章 GSM 系统原理及呼叫流程 and no re-assignment is initiated, the immediate assignment is restarted. If the failure is caused by other reasons or if the re-assignment triggered by the mismatch of message field in decision contention is carried out and the assignment still fails, MS turns into idle mode and triggers cell re-selection.  If the available information is not sufficient to define a channel after the MS receives immediate assignment message, RR connection fails.  If the assigned frequencies of MS belong to two or more than two frequency bands, RR connection fails. If the assigned frequency of MS is not consistent with the requested frequency but supported by MS, MS accesses the channel with the frequency used in channel request. If MS does not support the assigned frequency, RR connection fails.  If T3101 times out before the signaling channel is established, network releases the assigned channel. Network cannot tell whether MS resends the access attempt or not. 1.11 Authentication and Encryption GSM takes lots of measures to protect the safety of system, such as using Temporary Mobile Subscriber Identity (TMSI) to protect IMSI, using Personal Identification Number (PIN) to protect SIM card, authentication through authentication center (AUC) for network access, encryption, and equipment identity register. Authentication and encryption require a group of three parameters that generated in AUC. Each client is assigned a Mobile Station International ISDN Number (MSISDN) and IMSI when registers in GSM network. IMSI is preserved onto SIM card through SIM printer and SIM printer will generate a corresponding client authentication value Ki that is stored in SIM card and AUC as permanent information. AUC has a pseudo number generator used to generate a random number RAND. GSM defines algorithm A3, A8, and A5 that are used for authentication and encryption. In AUC, RAND and Ki together produce a response number SRES through A3 authentication algorithm and a Kc through A8 encryption algorithm. RAND, Kc, and SRES form a three-parameter group of client. This group is stored in the data base of this client in HLR. Generally, AUC transfers five groups of parameters to HLR for automatic storage. HLR can save ten groups of such parameters. When MSC/VLR requests for three-parameter group transfer, HLR sends five groups at the same time for MSC/VLR to use one by one. When there are two groups left, MSC/VLR will request for transfer again. 1.11.1 Authentication Authentication is the process that GSM network checks whether the IMSI or TMSI from MS at radio interface is valid or not. The purpose of authentication is to avoid unauthorized access to GSM network and the theft of private information by illegal 45

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