Gsm advanced cell planning

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Gsm advanced cell planning

  1. 1.  Copyright 2002 AIRCOM International LtdAll rights reservedAIRCOM Training is committed to providing our customers with quality instructor ledTelecommunications Training.This documentation is protected by copyright. No part of the contents of thisdocumentation may be reproduced in any form, or by any means, without the prior writtenconsent of AIRCOM International.Document Number: P/TR/005/G103/2.0cThis manual prepared by: AIRCOM International Grosvenor House 65-71 London Road Redhill, Surrey RH1 1LQ ENGLAND Telephone: +44 (0) 1737 775700 Support Hotline: +44 (0) 1737 775777 Fax: +44 (0) 1737 775770 Web: http://www.aircom.co.uk ADVANCED GSM CELL PLANNING
  2. 2. Table of Contents1. Review of the GSM Air Interface 1.1 Introduction...................................................................................................... 1-1 1.2 Logical Channels.............................................................................................. 1-2 1.3 FDMA/TDMA Structures .............................................................................. 1-5 1.4 Timing Advance .............................................................................................. 1-7 1.5 Multiframes ...................................................................................................... 1-9 1.6 GSM Protocols................................................................................................ 1-12 Self Assessment Exercises............................................................................. 1-172. Traffic Theory and Channel Dimensioning 2.1 Introduction...................................................................................................... 2-1 2.2 Traffic measurement ....................................................................................... 2-2 2.3 Traffic Channel Dimensioning....................................................................... 2-5 2.4 Control Channel Dimensioning..................................................................... 2-6 2.5 Muli-Service Traffic Dimensioning............................................................. 2-16 2.6 Dimensioning Micro- and Picocells ............................................................ 2-27 Self-Assessment Exerecises .......................................................................... 2-313. Frequency Planning 3.1 Introduction ..................................................................................................... 3-1 3.2 Cellular Structures and Frequency Reuse Patterns. ................................... 3-2 3.3 Interference Calculations ................................................................................ 3-5 3.4 Cell Splittuing Techniques ............................................................................. 3-6 3.5 Multiple Reuse Patterns (MRPs) ................................................................. 3-12 3.6 Frequency Hopping....................................................................................... 3-15 Self-Assessment Exerecises .......................................................................... 3-194. Base Station Positioning 4.1 Introduction...................................................................................................... 4-1 4.2 BTS Positioning for Different Environments ............................................... 4-2 4.3 Microcell Positioning....................................................................................... 4-5 4.4 Picocell Arrangements .................................................................................... 4-6 4.5 Multilayer Cell Design .................................................................................... 4-7 4.6 Use of Repeaters............................................................................................. 4-12 Self-Assessment Exercises ............................................................................ 4-195. Base Station Engineering 5.1 Introduction...................................................................................................... 5-1 5.2 Site Suitability .................................................................................................. 5-2 5.3 Radio Property Testing ................................................................................... 5-3 5.4 Antenna Configurations ................................................................................. 5-8 5.5 Base Station Equipment ................................................................................ 5-15 5.6 Electrical Considerations .............................................................................. 5-21 5.7 Configuration Selection ................................................................................ 5-22 Self-Assessment Exercises ............................................................................ 5-29Advanced GSM Cell Planning© AIRCOM International 2002 0-1
  3. 3. 6. Network Operations 6.1 Introduction...................................................................................................... 6-1 6.2 Modes of MS Operation.................................................................................. 6-2 6.3 Operations in MS Idle Mode.......................................................................... 6-3 6.4 Operations in MS Dedicated Mode............................................................. 6-11 6.5 Handover Operations. .................................................................................. 6-13 6.6 Discontinuous Transmission (DTX)............................................................ 6-22 6.7 Extrending Cell Coverage. ........................................................................... 6-24 Self-Assessment Exercises ............................................................................ 6-277. System Optimisation 7.1 Introduction...................................................................................................... 7-1 7.2 The need for Optimisation ............................................................................. 7-2 7.3 The Optimisation Process............................................................................... 7-3 7.4 System Performance........................................................................................ 7-4 7.5 Test Mobile Survey Data ................................................................................ 7-6 7.6 Automated Analysis........................................................................................ 7-7 7.7 Remedial Action............................................................................................... 7-7Appendix A - GSM Spectrum AllocationAppendix B - Solutions to Self Assessment ExercisesAppendix C - Erlang B Tables Advanced GSM Cell Planning0-2 © AIRCOM International 2002
  4. 4. Course Objectives and Structure Course Objectives • Calculate dimensioning requirements for traffic and control channels • Understand the allocation of control channels on the multiframe structure • Understand the principals of frequency re-use planning including multiple re-use patterns (MRP) • Understand the principals of multi-layer cell design • Appreciate the considerations involved in positioning a base station • Describe the characteristics of base station equipment • Understand a range of network operations including: cell selection and re-selection, cell measurements, handover control • Appreciate the process of network optimisation Course Structure Day 1 Day 2 • Review of the GSM Air • Base Station Positioning Interface • Base Station Engineering • Traffic Theory and • Network Operations Channel Dimensioning • System Optimisation • Frequency PlanningAdvanced GSM Cell Planning© AIRCOM International 2002 0-3
  5. 5. Intentional Blank Page Advanced GSM Cell Planning0-4 © AIRCOM International 2002
  6. 6. 1. Review of the GSM Air Interface_____________________________________________________________________1.1 Introduction This section briefly reviews information relating to the GSM air interface. Areas covered include: • Logical Channels • FDMA/TDMA structures • Physical Channels • Multiframes • GSM Protocols Topics introduced for the first time in this section include: • The structure of data bursts in the GSM physical channels • Timing advance • Protocols for speech and signallingAdvanced GSM Cell Planning© AIRCOM International 2002 1-1
  7. 7. _____________________________________________________________________1.2 Logical Channels Section 1 – Review of Air Interface Logical Channels • GSM uses a set of logical channels to carry call traffic, signalling, system information, synchronisation etc. • These logical channels use physical channels (timeslots) defined by the FDMA/TDMA structure of the system Traffic Traffic Control Control TCH TCH BCH BCH CCCH CCCH DCCH DCCH FCCH FCCH PCH PCH SDCCH SDCCH TCH/F TCH/F SCH SCH RACH RACH SACCH SACCH TCH/H TCH/H BCCH BCCH AGCH AGCH FACCH FACCH CBCH CBCH NCH NCH TCH Traffic Channels TCH/F Traffic Channel (full rate) (U/D) TCH/H Traffic Channel (half rate) (U/D) BCH Broadcast Channels FCCH Frequency Correction Channel (D) SCH Synchronisation Channel (D) BCCH Broadcast Control Channel (D) CCCH Common Control Channels PCH Paging Channel (D) RACH Random Access Channel (U) AGCH Access Grant Channel (D) CBCH Cell Broadcast Channel (D) NCH Notification Channel (D) DCCH Dedicated Control Channels SDCCH Stand alone Dedicated Control Channel (U/D) SACCH Slow Associated Control Channel (U/D) FACCH Fast Associated Control Channel (U/D) Advanced GSM Cell Planning1-2 © AIRCOM International 2002
  8. 8. Section 1 – Review of Air Interface Traffic Channels (TCH) • TCH carries payload data - speech, fax, data • Connection may be: • Circuit Switched - voice or data or • Packet Switched – data • TCH may be: • Full Rate (TCH/F) • one channel per user • 13 kb/s voice, 9.6 kb/s data or • Half Rate (TCH/H) • one channel shared between two users • 6.5 kb/s voice, 4.8 kb/s data Section 1 – Review of Air Interface Broadcast Channels (BCH) BCH channels are all downlink and are allocated to timeslot zero Channels are: • FCCH: Frequency control channel sends the mobile a burst of all ‘0’ bits which allows it to fine tune to the downlink frequency • SCH: Synchronisation channel sends the absolute value of the frame number (FN), which is the internal clock of the BTS, together with the Base Station Identity Code (BSIC) • BCCH: Broadcast Control Channel sends radio resource management and control messages, Location Area Code and so on. Some messages go to all mobiles, others just to those that are in the idle state As the name suggests, the broadcast channels send information out to all mobiles in a cell. These channels are also important for mobiles in neighbouring cells which need to monitor power levels and identify the base stations.Advanced GSM Cell Planning© AIRCOM International 2002 1-3
  9. 9. Section 1 – Review of Air Interface Common Control Channels (CCCH) CCCH contains all point to multi-point downlink channels (BTS to several MSs) and the uplink Random Access Channel: • CBCH: Cell Broadcast Channel is an optional channel for general information such as road traffic reports sent in the form of SMS • PCH: Paging Channel sends paging signal to inform mobile of a call • RACH: Random Access Channel is sent by the MS to request a channel from the BTS or accept a handover to another BTS. A channel request is sent in response to a PCH message. • AGCH: Access Grant Channel allocates a dedicated channel (SDCCH) to the mobile • NCH: Notification Channel informs MS about incoming group or broadcast calls The main use of common control channels is to carry the information needed to set up a dedicated channel. Once a dedicated channel (SDCCH) is established, there is a point to point link between the base station and mobile. Associated control channels carry additional signalling to support dedicated channels. SACCH is associated with either SDCCH or TCH. FACCH is only associated with TCH. Section 1 – Review of Air Interface Dedicated Control Channels (DCCH) DCCH comprise the following bi-directional (uplink / downlink) point to point control channels: • SDCCH: Standalone Dedicated Control Channel is used for call set up, location updating and also SMS • SACCH: Slow Associated Control Channel is used for link measurements and signalling during a call • FACCH: Fast Associated Control Channel is used (when needed) for signalling during a call, mainly for delivering handover messages and for acknowledgement when a TCH is assigned Advanced GSM Cell Planning1-4 © AIRCOM International 2002
  10. 10. _____________________________________________________________________1.3 FDMA / TDMA Structures Section 1 – Review of Air Interface Multiple Access Schemes • Frequency Frequency Division Multiple Access (FDMA): User 1 User 2 User 3 User 4 User 5 • Time Division Multiple Access (TDMA): Time Frequency User 7 User 6 User 1 User 4 User 5 User 7 User 2 User 3 User 6 User 1 User 4 User 2 User 3 User 5 Time Frame Timeslot GSM systems use several spectrum allocations (P-GSM and E-GSM in the 900 MHz band and DCS and PCS in the 1800 MHz band). Details of these are given in Appendix A to these notes. Section 1 – Review of Air Interface Frequency Division (FDMA) 880 915 925 960 MHz Example: E-GSM 900 MHz band Uplink Downlink Duplex spacing = 45 MHz Guard Band Range of ARFCN for E-GSM: 0 - 124 975 - 1023 Channels (ARFCN) 200 kHz spacing For each GSM band: • Sub-bands are defined for uplink and downlink (Frequency Division Duplex) • Channels (carriers) 200 kHz bandwidth • ARFCN (Absolute Radio Frequency Carrier Number) identifies a channel (uplink / downlink pair)Advanced GSM Cell Planning© AIRCOM International 2002 1-5
  11. 11. Section 1 – Review of Air Interface Time Division (TDMA) • TDMA is used to provide a set of 8 physical channels (timeslots) on each carrier: 1 frame period 4.615 ms 0 1 2 3 4 5 6 7 Downlink 0 1 2 3 4 5 6 7 Delay Uplink 3 timeslots 0 1 2 3 4 5 6 7 1 burst period (timeslot) = 0.577 ms • One cycle of 8 timeslots forms the TDMA frame of 4.615 ms duration • Each timeslot lasts for 0.577 ms (156.25 bit periods) and can contain one of several types of data burst • Uplink and downlink frames are offset by 3 timeslots to allow the MS to switch between transmit and receive modes A timeslot is the basic physical resource (channel) in GSM, which is used to carry all forms of logical channel information, both user speech/data and control signalling. Different structures of data burst are used in the timeslot for different purposes. Section 1 – Review of Air Interface Types of Data Burst • The 156.25 bit periods of a timeslot can hold different types of data burst: Stealing flag bits Normal Burst 26 Training (Traffic and most control channels) 3 57 Data Bits 1 Bits 1 57 Data Bits 3 8.25 Frequency Correction Burst (FCCH) 3 142 fixed bits 3 8.25 Data and tail bits are all 0 Synchronisation Burst (SCH) 39 Data 64 Training Bits 39 Data 3 Sync Sequence 3 8.25 Data to synchronise MS with BTS Bits Bits Dummy Burst 26 Training Transmitted on BCCH carrier when there are no 3 Bits 3 8.25 other bursts - allows power level measurements Access Burst (RACH) 41 Training 8 36 Data Bits 3 68.25 Long guard period to avoid collisions Bits Tail bits Guard period Advanced GSM Cell Planning1-6 © AIRCOM International 2002
  12. 12. _____________________________________________________________________1.4 Timing Advance Section 1 – Review of Air Interface Timing Advance • Signal from MS1 takes longer to arrive at BTS than that from MS2 • Timeslots overlap - collision 1 2 3 1 2 3 MS1 - Timeslot 1 time 1 2 3 1 2 3 MS2 - Timeslot 2 time time • Timing Advance signal causes mobiles further from base station to transmit earlier - this compensates for extra propagation delay 1 2 3 1 2 3 MS1 - Timeslot 1 time 1 2 3 1 2 3 MS2 - Timeslot 2 time time Timing Advance Timing Advance is needed to compensate for different time delays in the transmission of radio signals from different mobiles. The maximum value of Timing Advance sets a limit on the size of the cell. The TA value to use is found initially from the position of the received RACH burst in the guard period and is adjusted during the call in response to subsequent normal burst positions.Advanced GSM Cell Planning© AIRCOM International 2002 1-7
  13. 13. Section 1 – Review of Air Interface Timing Advance • Timing Advance is calculated from delay of data bits in the access burst received by the base station - long guard period allows space for this delay Access burst data Guard Period delay Access burst data Guard Period • TA signal is transmitted on SACCH as a number between 0 and 63 in units of bit periods • TA value allows for ‘round trip’ from MS to BTS and back to MS • Each step in TA value corresponds to a MS to BTS distance of 550 metres • Maximum MS to BTS distance allowed by TA is 35 km Section 1 – Review of Air Interface Timing Advance • Timing Advance value reduces the 3 timeslot offset between downlink and uplink 0 1 2 3 4 5 6 7 Downlink Delay 3 timeslots Uplink 0 1 2 3 4 5 6 7 Timing Advance Uplink 0 1 2 3 4 5 6 7 Actual delay • The Timing Advance technique is known as adaptive frame alignment Advanced GSM Cell Planning1-8 © AIRCOM International 2002
  14. 14. _____________________________________________________________________1.5 Multiframes To provide all the logical channel operations with the physical resources (timeslots) available, an additional time frame structure is required in which the logical channels are multiplexed onto the timeslots. This is the concept of multiframes. Section 1 – Review of Air Interface Multiframes • Multiframes provide a way of mapping the logical channels on to the physical channels (timeslots) • A multiframe is a series of consecutive instances of a particular timeslot Time Frame 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 1 1 1 1 Multiframe • GSM uses multiframes of 26 and 51 timeslotsAdvanced GSM Cell Planning© AIRCOM International 2002 1-9
  15. 15. Section 1 – Review of Air Interface Traffic Channel Multiframe • The TCH multiframe consists of 26 timeslots. • This multiframe maps the following logical channels: •TCH •SACCH •FACCH • TCH Multiframe structure: T T T T T T T T T T T T S T T T T T T T T T T T T I 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 T = TCH S = SACCH I = Idle FACCH is not allocated slots in the multiframe. It steals TCH slots when required - indicated by the stealing flags in the normal burst. TCH is always allocated on the 26 frame multiframe structure shown above. Control channels may be allocated in several ways on the 51 frame structure. A basic BCCH multiframe is shown below. The main reason for other structures is the allocation of SDCCH/SACCH which is dealt with in Section 2. Section 1 – Review of Air Interface Control Channel Multiframe • The control channel multiframe is formed of 51 timeslots • CCH multiframe maps the following logical channels: Downlink: Uplink: •FCCH •RACH •SCH •BCCH •CCCH (combination of PCH and AGCH) Downlink F = FCCH S = SCH I = Idle F S BCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH CCCH I 0 1 2-5 6-9 10 11 12-15 16-19 20 21 22-25 26-29 30 31 32-35 36-39 40 41 42-45 46-49 50 RACH Uplink • Other multiframe structures (for SDCCH and CCCH) are described in Section 2 Advanced GSM Cell Planning1-10 © AIRCOM International 2002
  16. 16. Section 1 – Review of Air Interface Frame Hierarchy 1 timeslot = 0.577 ms 0 1 2 3 4 5 6 7 1 frame = 8 timeslots = 4.615 ms Multiframe: = 26 TCH Frames (= 120 ms) or 51 BCCH Frames (= 235 ms) Superframe: = 26 BCCH Multiframes (= 6.12s) or 51 TCH Multiframes (= 6.12s) = 2048 Superframes Hyperframe: (= 3 hr 28 min 53.76 s) The superframe provides a basic repeat period for both traffic and control multiframes. It is used as a reference period for reporting bit error rates. The timing of the hyperframe relates to the cycle of frame numbers transmitted on the synchronisation channel (SCH). After 26 x 51 x 2048 = 2715648 frames, the frame number (which consists of 22 bits) resets to zero.Advanced GSM Cell Planning© AIRCOM International 2002 1-11
  17. 17. _____________________________________________________________________1.6 GSM Protocols Section 1 – Review of Air Interface GSM Protocol Architecture • In the OSI Reference Model, the logical channels of the air interface are at the Service Access Point (SAP) of the Physical Layer (Layer 1) Control Plane User Plane • ISDN Reference Model divides the protocol plane into a Control Plane and a User Plane DCCH BCH CCH TCH • corresponds to the control and traffic channels of the logical channels • Some user data (notably SMS text Physical Layer messages) is carried by the control plane Section 1 – Review of Air Interface User Plane - Speech Transmission • Transport of speech across the Um and Abis interfaces: Voice TRAU ISDN format GSC GSC A-law ITU-T A-law FEC FEC MPX MPX Um A MS BSS MSC (BTS - Abis - BSC) Advanced GSM Cell Planning1-12 © AIRCOM International 2002
  18. 18. Speech Transmission The diagram above outlines a possible transport path for speech in GSM. The physical layer (FDMA / TDMA on the air interface) has been omitted for clarity. Speech is encoded at the MS by the GSM Speech Codec (GSC) using hybrid encoders to give a data rate of 13 kbps. Forward Error Correction (FEC) is applied using a concatenated combination of half rate convolutional coding and block coding. At the BSS the forward error correction and any encryption is decoded by the TRX and the data is converted to the ISDN format (ITU-T A-law) by a Transcoding and Rate Adaption Unit (TRAU). The A-law format carries data at 64 kbps across the fixed network. The TRAU may be part of the BTS or part of the BSC. If the TRAU is located at the BSC, then up to 4 speech channels may be multiplexed at the BTS (MPX in the diagram) onto an ISDN B channel which reduces the bandwidth required across the Abis interface. Locating the TRAU at the BSC allows the TRAU operation for all the BTSs to be combined in one unit. This however requires signalling on the Abis interface to control the TRAU function. One channel of 16 kbps is reserved on this link to allow in-band signalling (3 kbps) together with a speech channel (13 kbps).Advanced GSM Cell Planning© AIRCOM International 2002 1-13
  19. 19. Section 1 – Review of Air Interface GSM Signalling Protocols MS BTS BSC MSC CM CM MM MM ISDN UP BSSMAP BSSMAP MAP DTAP DTAP Layer 3 RR RR RR BTSM BTSM SCCP SCCP SCCP Layer 2 LAPDm LAPDm LAPD LAPD MTP’ MTP’ MTP G.703 G.703 TDMA/ TDMA/ Layer 1 FDMA FDMA G.705 G.705 G.732 G.732 Um Abis A GSM specific protocols SS7 based protocols Terminology of GSM Protocol Architecture CM Connection Management MM Mobility Management RR Radio Resources Management LAPD Link Access Procedure D LAPDm Link protocol adapted for air interface (Um) BTSM Base Transceiver Station Management BSSMAP Base Station System Management Application Part DTAP Direct Transfer Application Part SCCP Signalling Connection Control Part TCAP Transaction Capabilities Application Part MTP Message Transfer Part MAP Mobile Application Part UP User Part ITU-T G.703, G705, G.732: Protocols for digital transfer of signalling messages on the Abis and A interfaces at 2048 kb/s or 64 kb/s Advanced GSM Cell Planning1-14 © AIRCOM International 2002
  20. 20. AIR INTERFACE PROTOCOLS Layer 1 – Physical Layer On the air interface, the physical layer uses FDMA/TDMA, multiframe structure, channel coding etc. to implement the logical control channels. Services provided by layer 1 are: • Access capabilities – multiplexing logical onto physical channels • Error protection – error detection / correction coding mechanisms • Encryption Layer 2 – LAPDm – Link Access Procedure on Dm channels Data link protocol responsible for protected transfer of signalling messages between MS and BTS. LAPDm supports the transport of messages between protocol entities on Layer 3, in particular: BCCH, PCH, AGCH and SDCCH signalling. Layer 3 - Network Sub-layers: • Radio Resource Management (RR) • Mobility Management (MM) • Connection Management – 3 entities: • Call Control (CC) • Supplementary Services (SS) • Short Message Service (SMS) RR is responsible for: • Monitoring BCCH and PCH • Administering RACH • Requests for and assignments of data and signalling channels • Measurements of channel quality • MS power control and synchronisation • Handover • Synchronisation of data channel encryption and decryption MM is responsible for: • TMSI assignment • Location updating • Identification of MS (IMSI, IMEI) • Authentication of MS • IMSI attach and detach • Confidentiality of subscriber identityAdvanced GSM Cell Planning© AIRCOM International 2002 1-15
  21. 21. Within Connection Management, Call Control (CC) is responsible for: • Set up of normal calls (MS originated, MS terminated) • Set up of emergency calls (MS originated only) • Terminating calls • DTMF signalling • Call related supplementary services • Service modification during a call (e.g. speech/data, speech/fax) Section 1 – Review of Air Interface Summary • Logical Channels: TCH, BCH, CCCH, DCCH • FDMA / TDMA structure: multiple access schemes, spectrum allocations, timeslots • Physical Channels: Data bursts, timing advance • Multiframes: traffic and control multiframe structures, hierarchy • Protocols: speech channel, TRAU, signalling protocols, RR, MM, CC Advanced GSM Cell Planning1-16 © AIRCOM International 2002
  22. 22. Section 1 Self-Assessment ExercisesExercise 1.1 - Logical Channel UsageThe following tables list the operations involved in: a) Location Updating b) Mobile Terminated CallIn each case, state the logical channels used for the operationLocation Updating Operation Channel Used Channel request Channel assignment Request for location updating Authentication challenge and response Cipher mode request and acknowledgement Confirmation and acknowledgement of update Channel releaseMobile Terminated Call Operation Channel Used Paging of mobile Mobile requests a channel Channel assigned Mobile answers paging from network Authentication challenge and response Cipher mode request and acknowledgement Set up message and confirmation by mobile Traffic channel assigned Traffic channel acknowledged Alerting (phone rings) Connect and acceptance (user answers) Exchange of user data (speech)Advanced GSM Cell Planning© AIRCOM International 2002 1-17
  23. 23. Exercise 1.2 - Timing AdvanceTwo mobiles, A and B, are operating in cell. Mobile A is allocated TS2 and a TA of 50.Mobile B is in TS4 and has TA of 30.a) Find the distance of each mobile from the base station.b) Draw a timing diagram indicating the actual start times of the up and downlink bursts of each mobile. Mark time values in bit periods from the start of the frame, remembering that 1 timeslot (1 burst) = 156.25 bit periods. Advanced GSM Cell Planning1-18 © AIRCOM International 2002
  24. 24. Exsercise 1.3 - Multiframe Timings1. SACCH is used to report cell measurements made by the mobile back to the serving base station. It takes 4 bursts of SACCH to send one report. How often is a complete SACCH report received by the base station?2. When a mobile has measured the power level from a neighbouring base station, it also identifies the BTS by synchronising with its FCCH and SCH channels. This is done in the idle frame of the TCH multiframe. At a certain point in time, the start of the TCH and control channel multiframes are aligned: TCH CCH After various cycles of the TCH multiframe, the idle frame in TCH will align with the FCCH in the CCH multiframe. Find the first and second time this will occur (in terms of cycles of the TCH multiframe).Advanced GSM Cell Planning© AIRCOM International 2002 1-19
  25. 25. Intentional Blank Page Advanced GSM Cell Planning1-20 © AIRCOM International 2002
  26. 26. 2. Traffic Theory and Channel Dimensioning____________________________________________________________________2.1 Introduction In this section the following topics will be covered: • The theory of traffic measurement • Traffic channel dimensioning calculations • SDCCH dimensioning • CCCH configuration and dimensioning • Multi-service traffic dimensioningAdvanced GSM Cell Planning© AIRCOM International 2002 2-1
  27. 27. ____________________________________________________________________2.2 Traffic Measurement Section 2 – Traffic & Dimensioning Traffic Measurement • Unit of traffic measurement: erlang (E) • Traffic in erlangs is the number of call-hours per hour: A = C T / 3600 A = Traffic in Erlang C = number of calls during the hour T = mean holding time per call in seconds • One channel in continuous use is carrying a traffic of 1 erlang • Typical traffic per subscriber during the busy hour is 25 mE which corresponds to a mean call holding time of 90 s Another traffic unit, used mostly in the USA, is the Call Centum Second (CCS): 1 CCS = 100 call seconds per hour 1 Erlang = 3600 call seconds per hour 1 Erlang = 36 CCS Advanced GSM Cell Planning2-2 © AIRCOM International 2002
  28. 28. Section 2 – Traffic & Dimensioning Blocking Call Setup Offered Traffic Process Carried Traffic Blocked Traffic Offered Traffic = Carried Traffic + Blocked Traffic Offered Traffic : Total traffic offered to channel by all users Carried Traffic : Traffic successfully carried by the channel Blocked Traffic: Traffic which is blocked at call setup Section 2 – Traffic & Dimensioning Grade of Service (GoS) • Grade of Service is the fraction of incoming calls (offered traffic) allowed to be blocked due to congestion in the channel A Call A x (1 - GoS) Setup Offered Traffic Process Carried Traffic Blocked A x GoS Traffic • Typical Grade of Service is 0.02 (2%) • Grade of Service is also called blocking probability or loss probability A good grade of service is a low value. This implies low channel utilization. If a poorer grade of service is accepted, more traffic can be offered to the same number of traffic channels.Advanced GSM Cell Planning© AIRCOM International 2002 2-3
  29. 29. Section 2 – Traffic & Dimensioning Erlang Models of Traffic • Two commonly used models are Erlang B and Erlang C • Erlang B - blocked calls are lost or cleared • Erlang C - calls that cannot be handled are put in a queue until a channel becomes available A A(1-GoS) A (GoS) Queue Erlang C Erlang B • GSM uses the Erlang B model not Erlang C For GSM we are concerned with circuit switched voice traffic which must be handled in real time. Thus the Erlang B model with no queuing is appropriate. Section 2 – Traffic & Dimensioning Erlang B Calculations • Tables based on the Erlang B model allow calculations to be made relating: • Offered traffic • Grade of Service • Number of channels • Structure of Erlang B table: Grade of Service n 0.01 0.02 0.03 1 .01010 .02041 .03093 Number of 2 .15259 .22347 .28155 Offered traffic channels 3 .45549 .60221 .71513 • Example: at 2% blocking (0.02 GoS), 2 traffic channels can carry 0.22347 erlangs of traffic Advanced GSM Cell Planning2-4 © AIRCOM International 2002
  30. 30. ____________________________________________________________________2.3 Traffic Channel Dimensioning Section 2 – Traffic & Dimensioning Channel Dimensioning - Example • In GSM channel dimensioning, the number of channels must be related to the number of carriers (frequencies) available: • 8 channels (timeslots) per carrier • Some channels will be required for signalling • Example - in a particular cell: Mean call holding time = 90 seconds Grade of Service = 1 % Total number of available carriers = 4 3 timeslots allocated for signaling How many subscribers can this cell support ? This type of traffic calculation, using Erlang B tables, is fundamental to all dimensioning problems whether for user traffic (TCH) or control signalling (SDCCH). Section 2 – Traffic & Dimensioning Channel Dimensioning - Solution • Mean call holding time of 90 s implies the average traffic per subscriber is 25 mE • Number of channels available is given by: (carriers x 8) - signalling channels = 4 x 8 - 3 = 29 channels • Using Erlang B tables for GoS = 0.01 and n = 29 channels, gives traffic that can be offered as 19.487 E = 19487 mE • Number of subscribers that can be supported is: 19487 / 25 = 779Advanced GSM Cell Planning© AIRCOM International 2002 2-5
  31. 31. Section 2 – Traffic & Dimensioning Trunking Efficiency • Trunking efficiency or channel utilisation is given by: carried traffic / number of channels • In the Erlang B model: Trunking Efficiency = A (1- GoS) / n • Using the previous example: A = 19.487 E, GoS = 0.01, n = 29 • Trunking Efficiency = 19.487 (1 - 0.01) / 29 = 0.665 = 66.5 % Grade of service measures how well the network is performing from the user’s perspective. They are just concerned with being able to make their call. Trunking efficiency looks at the situation from the network operator’s point of view and asks how well are we using the network resources?____________________________________________________________________2.4 Control Channel Dimensioning So far we have considered the channels required for user traffic and taken a given figure for those needed for signalling. There are several signalling channel requirements, which we must allow for. The most demanding of these is SDCCH. This is the only other type of dedicated channel (apart from TCH) that may be allocated to a mobile. It is used for call set up as well as several other purposes. This means it is a ‘gateway’ to TCH and if it is incorrectly dimensioned, we could have calls blocked due to insufficient SDCCH when there is capacity available on TCH to carry them. Advanced GSM Cell Planning2-6 © AIRCOM International 2002
  32. 32. Section 2 – Traffic & Dimensioning SDCCH Dimensioning • A Standalone Dedicated Control Channel (SDCCH) is allocated to a user by the access grant channel (AGCH) in response to a random access (RACH) request for a channel SDCCH Activity Mean Holding Time (s) • SDCCH carries signalling between the MS and BTS while no traffic Call Set-up 2.5 channel (TCH) is active Location Updating (Automatic) 3.5 Location Updating (Periodic) 3.5 • The main activities on SDCCH IMSI Attach 3.5 IMSI Detach 3.0 and the mean holding times for SMS Message 6.5 these are shown here Supplementary Services 2.5 Section 2 – Traffic & Dimensioning SDCCH Grade of Service • The main function of SDCCH is to carry call setup signaling • Since access to a TCH is via SDCCH, the grade of service for SDCCH must be significantly better than for TCH - typically 2 to 4 times better - e.g. if TCH GoS = 2%, SDCCH GoS = 0.5% to 1% Services using Voice calls require only SDCCH SDCCH then TCH e.g. SMS TCH requests Carried traffic on TCH SDCCH requests Blocking BlockingAdvanced GSM Cell Planning© AIRCOM International 2002 2-7
  33. 33. Section 2 – Traffic & Dimensioning SDCCH Grade of Service • The carried traffic can be calculated by considering a two stage process - Erlang B blocking at SDCCH, then at TCH: (1 - GoS1) A’ Offered voice traffic = A (1 - GoS1) A (1 - GoS2)(1 - GoS1)A SDCCH TCH GoS1 GoS2 Offered SDCCH only traffic = A’ (GoS1) A GoS2 (1 - GoS1) A + (GoS1) A’ • The effective grade of service for the overall process is: GoS1 + (1 - GoS1)GoS2 5mE per subscriber is a good ‘rule of thumb’ for SDCCH traffic, just as we took 25mE per subscriber for TCH dimensioning. Section 2 – Traffic & Dimensioning SDCCH Example • Question: A cell is required is serve 500 subscribers SDDCH grade of service is set at 0.5% Typical SDCCH traffic in the busy hour is 5 mE How many SDCCH channels are required? • Solution: Total SDCCH traffic = 500 x 5 = 2500 mE = 2.5 E From Erlang B tables, using GoS = 0.005, this requires 8 channels • How are the required SDCCH channels to be allocated? Advanced GSM Cell Planning2-8 © AIRCOM International 2002
  34. 34. We must now be aware that an SDCCH channel is not simply allocated to a timeslot as is TCH, but to a set of instances of a timeslot within the multiframe structure. There are different ways of allocating SDCCH using combined or non-combined multiframes. Section 2 – Traffic & Dimensioning SDCCH Allocation • SDCCH is allocated on the control channel multiframe structure in blocks of 4 channels (SDCCH/4) or 8 channels (SDCCH/8) • SDCCH/4 is combined with other control channels on timeslot 0: SDCCH/4 allocation Downlink SDCCH SDCCH SDCCH SDCCH SACCH SACCH F S BCCH CCCH F S CCCH CCCH F S 0 1 F S 2 3 F S 0 1 I Uplink SDCCH SACCH SACCH SDCCH SDCCH SDCCH 3 R R 0 1 RACH 0 1 R R 2 Combined multiframe structure SACCH 2 and 3 are on the next multiframe • One SDCCH channel may be replaced by CBCH if required The combined multiframe (SDCCH/4) is only ever implemented on timeslot 0 of the BCCH carrier. Section 2 – Traffic & Dimensioning Non- Combined Multiframe SDCCH SDCCH/8 may be allocated on a non-combined multiframe: SDCCH/8 allocation Downlink SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SACCH SACCH SACCH SACCH 0 1 2 3 4 5 6 7 0 1 2 3 I I I Uplink SACCH SACCH SACCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SACCH 1 2 3 I I I 0 1 2 3 4 5 6 7 0 Non -combined multiframe structure Other SACCH channels are on the next multiframeAdvanced GSM Cell Planning© AIRCOM International 2002 2-9
  35. 35. SDCCH Allocation Depending on the SDCCH capacity required, SDCCH channels can be allocated in blocks of 4 or 8, as follows: 4 Channels SDCCH/4 8 Channels SDCCH/8 12 Channels SDCCH/4 + SDCCH/8 16 Channels SDCCH/8 + SDCCH/8 20 Channels SDCCH/4 + SDCCH/8 + SDCCH/8 Only one block of 4 channels is ever allocated since the combined multiframe used by SDCCH/4 is on the BCCH carrier. Cell broadcast (CBCH) will reduce the number of SDCCH channels by 1 if it is in use. The total amount of SDCCH traffic that must be accommodated will actually depend on the location of the cell, since other SDCCH activities such as location updating will only occur at certain cells. Section 2 – Traffic & Dimensioning Practical SDCCH Dimensioning • Certain locations make greater use of SDCCH and will require particular allocation, e.g. • Cells at the border between location areas where location updating occurs frequently Location area boundary cells • Airport: Passengers disembark in large numbers and switch on their mobiles imposing a lot of pressure on SDCCH for location updating Location updating may be prolonged for international roaming subscribers In addition to SDCCH, the multiframe structure also carries the common control channel (CCCH) which handles paging and access grant messages. We will now consider how CCCH is configured to provide PCH and AGCH and how to dimension this requirement. Advanced GSM Cell Planning2-10 © AIRCOM International 2002
  36. 36. Section 2 – Traffic & Dimensioning CCCH Configuration • On the downlink, CCCH consists of paging (PCH) and access grant (AGCH) messages • A combined multiframe has only 3 CCCH blocks to allow for SDCCH and SACCH: SDCCH SDCCH SDCCH SDCCH SACCH SACCH F S BCCH CCCH F S CCCH CCCH F S 0 1 F S 2 3 F S 0 1 I • A non-combined multiframe has 9 CCCH blocks on timeslot 0: F S BCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH CCCH I • A complete paging or access grant message takes four bursts (timeslots), i.e. one CCCH block. Section 2 – Traffic & Dimensioning CCCH Priority • CCCH blocks are allocated to either PCH or AGCH according to the following priority: High PCH Priority Immediate Assignment Message (AGCH) Immediate Assignment Reject Message (AGCH) Low • During periods of heavy paging, PCH could dominate, leaving no blocks for access grant messages • To avoid this, some blocks can be reserved for AGCH Paging will make more use of CCCH than the access grant messages, since paging is done across a location area. All cells in the location area are paged, whether the required mobile is in that cell or not. AGCH only takes place in the specific cell containing the mobile.Advanced GSM Cell Planning© AIRCOM International 2002 2-11
  37. 37. Section 2 – Traffic & Dimensioning Reserving AGCH Blocks on CCCH • In a non combined multiframe, up to 7 of the 9 blocks may be reserved for AGCH: F S BCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH CCCH F S CCCH CCCH I • In a combined multiframe, up to 2 of the 3 blocks may be reserved for AGCH: SDCCH SDCCH SDCCH SDCCH SACCH SACCH F S BCCH CCCH F S CCCH CCCH F S 0 1 F S 2 3 F S 0 1 I • Additional CCCH capacity can be provided on other timeslots (2,4 or 6) of the BCCH carrier if required • The number of AGCH blocks reserved is specified in the system information messages which the mobile reads on the BCCH CCCH Configuration Parameter - CCCH_CONF BCCH contains a number of system information messages, BCCH/SYS_INFO n. BCCH/SYS_INFO 3 carries a parameter, CCCH_CONF, which informs the mobile of the CCCH configuration to be used, including number of timeslots, combined or non-combined multiframes, reservation of AGCH blocks. CCCH_CONF Number of Timeslots Configuration for CCCH 0 1 TS0 non-combined 1 1 TS0 combined 2 2 TS0, TS2 non-combined 3 3 TS0, TS2, TS4 non-combined 4 4 TS0, TS2, TS4, TS6 non-combined Advanced GSM Cell Planning2-12 © AIRCOM International 2002
  38. 38. Section 2 – Traffic & Dimensioning Paging Capacity • Paging capacity is the number of mobiles that can be paged per second • This depends on: • CCCH configuration • AGCH blocks reservation • Type of paging message used • Paging message takes 4 bursts (1 CCCH block) • This can page up to 4 mobiles depending on the message type used Paging Message Types: Type 1: can address up to 2 mobiles using either IMSI or TMSI. Type 2: can address up to 3 mobiles, one by IMSI and the other 2 by TMSI. Type 3: can address up to 4 mobiles using the TMSI only. If the network does not use TMSI, only Type 1 can be used. Paging message for individual mobiles are sent to BSS which stores them temporarily until there are enough to make up a full type 1,2 or 3 message or until a configurable timer (set by the operator) expires. The message is then broadcast.Advanced GSM Cell Planning© AIRCOM International 2002 2-13
  39. 39. Section 2 – Traffic & Dimensioning Calculating Paging Capacity XY Paging Capacity = mobiles / second 0.235 X = number of mobiles paged per paging message (1 to 4) Y = number of possible paging messages per multiframe Duration of channel multiframe = 0.235 seconds (235 ms) • X depends on paging message type • Y depends on CCCH configuration in the multiframe (e.g. 3 or 9) and the number of AGCH blocks reserved Section 2 – Traffic & Dimensioning PCH Dimensioning Paging channel requirement in blocks per multiframe is given by: Calls x MT x PF x M PMF x 3600 x 4.25 Calls = Number of calls predicted for the location area during busy hour MT = Fraction of calls which are mobile terminated PF = Paging Factor = number of pages required per call M = safety margin PMF = Paging Message Factor = number of pages per message Number of control channel multiframes per second = 4.25 (1 / 0.235) Advanced GSM Cell Planning2-14 © AIRCOM International 2002
  40. 40. Section 2 – Traffic & Dimensioning PCH Dimensioning - Example • A particular location area contains 50 000 subscribers. It is predicted that 30% of these will receive a call during the busy hour. On average 2 pages are needed per call and only type 3 paging messages (TMSI) are used. This gives the following data: Calls = 50 000 PCH Requirement = Calls x MT x PF x M MT = 0.3 PMF x 3600 x 4.25 PF =2 PMF = 4 A typical safety margin for peak variations in number of calls is 1.2 • PCH requirement = 50000x 0.3 x 2 x 1.2 = 0.6 4 x 3600 x 4.25 • 1 PCH block per multiframe will be adequate Section 2 – Traffic & Dimensioning AGCH Dimensioning • AGCH requirement is found by adding up the activities which need an AGCH message during the busy hour • The following equation gives the number of AGCH blocks per multiframe: (Calls + LU + SMS + IA+ ID + SS) x M AGCH required = 3600 x 2 x 4.25 The terms in brackets are the predicted numbers during the busy hour for: Calls, Location Updates (LU), SMS, IMSI attaches (IA), IMSI detaches (ID), Supplementary Services (SS) M = safety margin (e.g. 1.2) The factor of 2 is because each AGCH block can carry 2 immediate assignment messagesAdvanced GSM Cell Planning© AIRCOM International 2002 2-15
  41. 41. Section 2 – Traffic & Dimensioning AGCH Dimensioning - Example • A cell has 1000 calls during the busy hour • Other AGCH activities are modelled as multiples of the calls figure. A possible model is: Activity Multiplier Total LU 2 2000 SMS 0.1 100 SS 0.2 200 IMSI attach 0.2 200 IMSI detach 0.1 100 • This gives the total activity (including Calls) as 3600 3600 x 1.2 AGCH required = = 0.14 AGCH blocks per 3600 x 2 x 4.25 multiframe The main conclusion to draw from these examples is that CCCH dimensioning is not such a serious issue as that for SDCCH. The additional allocation allowed by the CCCH_CONF settings is seldom required.____________________________________________________________________2.5 Multi-service Traffic Dimensioning GSM was originally a voice only system. Developments will include an increasing proportion of the traffic offered being in the form of High Speed Circuit Switched Data (HSCSD). Initially the HSCSD service will entail a user seizing two or three timeslots on a carrier. The network will have to accommodate both voice and HSCSD services. Unfortunately, the Erlang B formula is not appropriate for calculating the required number of timeslots when the resource is shared amongst services of different amplitude (“amplitude” is the name given to the unit of resource required by different services: for example, an HSCSD connection requiring two timeslots would have an amplitude of 2.). Advanced GSM Cell Planning2-16 © AIRCOM International 2002
  42. 42. Section 2 – Traffic & Dimensioning Accommodating a multi-service system • The Erlang B formula relies on the variance of the demand equalling the mean (a Poisson distribution). • If a particular service requires more than one “trunk” per connection, the demand is effectively linearly scaled and the variance no longer equals the mean. • Methods to investigate: • Equivalent Erlangs • Post Erlang-B • Campbell’s Theorem Section 2 – Traffic & Dimensioning Equivalent Erlangs • Combine the two traffic sources together by converting one to the bandwidth of the other Difference in capacity • The trunking efficiency will VARY with the bandwidth of required for equivalent Erlang that you choose! same GoS • Not suitable for use due to this property Low Bandwidth Equivalent + 2 Erlangs 1 Erlang of Low of High Bandwidth Bandwidth High Bandwidth EquivalentAdvanced GSM Cell Planning© AIRCOM International 2002 2-17
  43. 43. Section 2 – Traffic & Dimensioning Equivalent Erlangs Example • Consider 2 services sharing the same resource: • Service 1: uses 1 trunk per connection. 12 Erlangs of traffic. • Service 2, uses 3 trunks per connection. 6 Erlangs of traffic. • We could regard the above as equivalent to 30 Erlangs of service 1: • 30 Erlangs require 39 trunks for a 2% Blocking Probability • Alternatively, we could regard the above as equivalent to 10 Erlangs of service 2. • 10 Erlangs require 17 trunks, (equivalent to 51 “service 1 trunks”) for a 2% blocking probability • Prediction varies depending on what approach you choose. Section 2 – Traffic & Dimensioning Post Erlang-B • Consider 2 services sharing the same resource: • Service 1: uses 1 trunk per connection. 12 Erlangs of traffic. • Service 2: uses 3 trunks per connection. 6 Erlangs of traffic. • We could calculate the requirement separately • Service 1: 12 Erlangs require 19 trunks for a 2% Blocking Probability • Service 2: 6 Erlangs require 12 trunks (equivalent to 36 “service 1 trunks”). • Adding these together gives 55 trunks. • This method is known to over-estimate the number of trunks required as can be demonstrated by considering services requiring an equal number of trunks. Advanced GSM Cell Planning2-18 © AIRCOM International 2002

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