GSM System


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GSM System

  1. 1. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications – Network and Switching Subsystem (NSS) : GSM System 1. MSCs, Visitor Location Register (VLR), Home Location Register (HLR), Authentication Center (AUC) and Global System for Mobile Communications Equipment Identity Register (EIR). 2. Switching of GSM calls between external networks and Introduced in 1991. Settings of standards under ETSI the BSCs. (European Telecommunication Standards Institute) 3. Managing and providing external access to several customer databases, such as Services HLR : contains subscriber information (International Mobile Subscriber Identity -IMSI) and location - Telephone services information for each user who resides in the same city as the MSC. - Data services VLR : temporarily stores the IMSI and customer - Short message paging information for each roaming subscriber who is visiting the coverage area of a particular MSC. Once a roaming System Architecture : three major subsystems. mobile is logged in the VLR, the MSC sends the necessary information to the visiting subscriber’s HLR so that calls to the roaming mobile can be appropriately routed over the PSTN by the roaming user’s HLR. AUC : Strongly protected database which handles the authentication and encryption keys for every single subscriber in the HLR and VLR. – Radio Subsystem (Base Station Subsystem BSS): 1. Mobile stations (MS), Base Transceiver Station (BTS) and the Base Station Controller (BSC) 2. Provides and manages radio transmission paths between the MS and MSC. 3. One BSC controls up to several hundred BTSs. Institute for Infocomm Research 188 National University of Singapore Institute for Infocomm Research 189 National University of Singapore
  2. 2. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications 4. BSC performs handover for MS under the control of same BSC. Location Management – Operation Support Subsystem (OSS) 1. Support the operation and maintenance of GSM and In cellular wireless networks, location management is allows system engineers to monitor, diagnose and an important mechanism that enables the network to troubleshoot all aspects of the GSM system. discover the current attachment point of a MS so as to 2. Interacts with the other GSM subsystems. facilitate successful information delivery. 3. Charging and billing. Interface For GSM networks, location information about MSs is maintained by registration, whereby MSs update their registration area(s) information with HLR and VLR, in the event of changes in the registration areas(s). Radio bandwidth normally needs to be reserved for carrying location management traffic, which comprises of two major components: paging and location update. Paging is the process of searching for a MS in the vicinity of its last-known location, typically in an area of several cell sizes which is also known as a location area (LA), so as to connect an incoming call. TDMA/FDMA/FDD – Uses both TDMA and FDMA to transmit and recover Location update is the process undertaken by a MS to information. inform the network of any changes in its current – Systems are FDD. location. Location updates typically occur each time a MS crosses an LA boundary. Institute for Infocomm Research 190 National University of Singapore Institute for Infocomm Research 191 National University of Singapore
  3. 3. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications It is also to be noted that both the location update 1 traffic and paging traffic are inversely proportional to PSTN HLR MSC/ MSC/ VLR one another with respect to the size of the LA, i.e. VLR 2 BSC1 BSC2 BSC3 location update load decreases whilst paging load Pa gin g increases for increasing sizes of LA, and vice versa. g pdate Pagin Paging has to be made at every cell of a LA. tion U Pag ing Loca LA 1 LA 3 Legends : LA 2 10 10 10 MSC/VLR Mobile-services Switching Center/ Visitor Location Register; / Moving/Still MSs; PSTN Public Switched Telephone Network; BSC Base Station Controller; (10 + 10 + 10 + 10) × 4 10 HLR Home Location Register; Base Transceiver Station; = 160 Current GSM Location Management Scheme. A service area comprises of 9 cells are (20 + 20) × 2 partitioned into 3 static LA, each having 4, 2 and 3 cells, respectively. 20 = 80 20 The location management scheme of GSM Mobile is Note: the number indicates the nomalized paging traffic of the cell or cells cluster. given in IS-41. It is based on partitioning the cells into static registration areas, known as location areas Avoid setting the LA boundary at places when cell (LAs). crossing rate is high. Location updates are to be performed when MSs cross the partitioning of LA so that up-to-date LA1 LA2 location information for each MS on is stored in the B C λB = 10 µ BC = 20 λC = 10 respective HLR and VLR. µCB = 20 µ BA = 30 Since the location information of each MS is tracked µ BD = 20 µCD = 10 µCE = 15 on a resolution that is dependent on the size of the µ DB = 20 µ = 10 µ AB = 30 DC µ EC = 15 LA, to facilitate a successful data delivery, all the µ AD = 20 µ DE = 20 µ ED = 20 cells within the LA have to be paged concurrently. λ A = 20 µ DA = 20 λD = 20 λF = 10 A D E Institute for Infocomm Research 192 National University of Singapore Institute for Infocomm Research 193 National University of Singapore
  4. 4. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications Frequency domain There are a total of eight channels per carrier. Every eighth timeslot on a TDMA channel, the user transmits The frequency band for uplink (reverse) is 890-915 MHz, or receives his information. downlink (forward) is 935-960 MHz. - The bandwidth for the GSM system is 25MHz, which provides 125 carriers uplink/downlink each having a A second frequency band from 1710-1785 MHz and bandwidth of 200 kHz. The ARFCN (Absolute radio 1805-1880 MHz (three times as much as primary 900 frequency channel numbers) denotes a forward and MHz) are also specified in 1990, a total of 374 duplex reverse channel pair which is separated in frequency by 45 MHz. channels – DCS 1800 Base-to-mobile : Fu (n) = 890.2 + 0.2(n − 1) MHz GSM has chosen the four-cell repeat pattern for the Mobile-to-base : Fd (n) = Fu (n) + 45 MHz frequency reuse cell sets. In most cases, each cell is - In practical implementations, a guard band of 100kHz is divided into 120-deg sectors, with three base transceiver provided at the upper and lower end of the GSM spectrum, and only 124 (duplex) channels are subsystems in each cell. Each base transceiver has a 120- implemented. deg antenna. - These 12 sectors (called cells in GSM system) share the 124 channels. Institute for Infocomm Research 194 National University of Singapore Institute for Infocomm Research 195 National University of Singapore
  5. 5. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications Time domain RF carrier channel is time division multiple accessed by users at different locations within a cell site. - Frame duration is 4.615ms, and each frame consists of 8 time-slots. - Each of the time-slot is a traffic channel having time duration 0.577ms (3/5200s). - The start of an uplink TDMA frame is delayed with respect to downlink by three timeslots. Staggering TDMA frames allows the same timeslot number (TN) to be used in both directions. Multiframe – 26 frames (traffic or speech ~): Traffic CHannel (TCH), Slow Associated Control CHannel (SACCH), Fast Associated Control CHannel (FACCH). – 51 frames (control ~) : Broadcast Common Control (BCC), Stand Alone Dedicated Control Channels,.. – Superframe : 51 traffic multiframes or 26 control multiframes. – Hyperframe : 2048 superframes (3 hrs 28 min 52.76 s), to support encryption with high security and frequency hopping. Institute for Infocomm Research 196 National University of Singapore Institute for Infocomm Research 197 National University of Singapore
  6. 6. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications Traffic Channel (TCH) : - Traffic channels carry digitally encoded user speech or user data. Have identical functions and formats on both the forward and reverse link. - Full rate : user data is contained within one TS per frame. Half rate : user data is mapped onto the same time slot, but is sent in alternate frames. • TCH/FS : Full rate speech, raw data sent at 13kbps, with GSM channel coding 22.8kbps. • TCH/HS : Half rate speech, raw data sent at 6.5kbps, with GSM channel coding 11.4kbps. • TCH/F9.6 : Full raw data rate 9.6kbps, send at 22.8kbps. • TCH/F4.8 Physical Channel : specified by ARFCN and TN. • TCH/F2.4 Logical Channel : is mapped onto the physical channel. E.g, • TCH/H4.8 TCHs and control channels. • TCH/H2.4 - TCH data may not be sent in TS0 within a TDMA frame on certain ARFCNs which serve as the broadcast station (control burst) for every frame. Furthermore, frames of TCH data are broken up every thirteenth frame by SACCH or idle frame. Institute for Infocomm Research 198 National University of Singapore Institute for Infocomm Research 199 National University of Singapore
  7. 7. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications Traffic or speech multiframe Control (Signaling) Channels : Three types : – Broadcast CHannel (BCH) – Common Control CHannel (CCCH) – Dedicated Control CHannel (DCCH) Broadcast channel (BCH) T11 – Only on downlink – Three types of BCH – Broadcast control channel (BCCH) 1. Broadcast cell and network information 2. Cell and network identity 3. List of channels in use Control multiframe – Frequency correction channel (FCCH) 1. Occupies TS0 of first frame 2. Repeated every 10th frame within a control channel multiframe 3. Synchronization of local oscillator (radio frequency) to base station oscillator. – Synchronization channel (SCH) 1. Broadcast in TS0 immediately following a FCCH frame. 2. Allows for frame synchronization 3. Base station issues timing advancement commands. Assume that the mobile 2 was allocated TS2 and he is close to the BS and that mobile 1 has been allocated TS1 and is at the boundary of the cell site. Due to higher distance resulting in large propagation delay, the transmitted Institute for Infocomm Research 200 National University of Singapore Institute for Infocomm Research 201 National University of Singapore
  8. 8. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications signal of mobile 1 during TS1 reaches the BS later. If no action is taken, this will overlap with other transmission slots. Dedicated control channel (DCCH) The only solution is that the MS advances its emission relative to its reception by a time corresponding to and from propagation delay. This – Bidirectional channels with same format and function value is call the timing advance. The timing advance value can be computed by the BTS and is then provided to the MS through signaling. The on uplink and downlink. maximum timing advance of 233us (6 bits data) is adequate for MS to be up to 35 km from the BS, which is the maximum allowable cell radius of the – May exist in any time slot and on any radio channel GSM systems. except TS0 of the control radio channel. Common control channel (CCCH) – 3 different DCCH are specified. – Stand-alone dedicated control channel (SDCCH) – Occupies TS0 of every control frame not used by BCH 1. Carries signaling data following the connection of or the Idle Frame. the mobile with BS. – 3 different CCCH types 2. Intermediate and temporary channel for mobiles – Paging channel (PCH) while waiting for the BS to allocate a TCH channel. 1. Provides paging signals from base to mobiles. 3. Ensures that mobile and base remains connected 2. Notifies specific mobile of incoming call during authentication and resource allocation. 4. Maybe assigned their own physical channel or may – Random access channel (RACH) occupy TS0 of the BCH if there is low demand for 1. Uplink channel BCH or CCCH traffic. 2. Used by mobiles to acknowledge page from PCH – Slow associated control channel (SACCH) 3. Used by mobiles to originate a call 1. Always associated with a traffic channel – Access grant channel (AGCH) 2. On downlink the SACCH carries power control and 1. Provides forward link communication to mobile. timing advance instruction 2. Specifies time slot, radio channel and dedicated 3. On uplink the SACCH carriers signal strength and control channel. quality information 3. AGCH is final CCCH message before mobile is 4. SACCH is allocated every 13th frame of a traffic moved off the control channel. channel. Institute for Infocomm Research 202 National University of Singapore Institute for Infocomm Research 203 National University of Singapore
  9. 9. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications – Fast associated control channel (FACCH) 1. Carries urgent messages to the mobile, for example handover. 2. FACCH gains access by stealing frames from TCH (e.g. data transmission slot are stolen). Example : Mobile to PSTN GSM Set-up - Synchronize to nearest BTS by monitoring BCH. - Dial intended number - Burst of RACH data - BTS responds with AGCH message on CCCH and assigns the mobile a new channel for SDCCH connection. - The MS listens to the SACCH frame to get timing Mobile-Assisted handover advance and power control info. – There are four purposes for handover - MS sends authentication and validation requests. (1) rescue low-quality channel - MS is instructed by the BTS over the SDCCH to retune (2) recovering cochannel interference to new radio channel and TS for TCH assignment. (3) traffic balancing among cells (avoid congestion or Time slot data bursts take on one of the 5 formats load balancing) – directed handover. according to the logical channel. (4) recovering in the event of failure of a control channel – A normal burst consists of 148 bits – Guardtime of 8.25 bits to avoid frame overlap. – Two batched of 57 bits are information bits – 26 training bits for equalization. – Two stealing bits for FACCH Institute for Infocomm Research 204 National University of Singapore Institute for Infocomm Research 205 National University of Singapore
  10. 10. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications – There are three cases of handover, Speech coding (1) From one radio channel to another of the same BSC. (2) Between channel of different BSCs under the control of the same MSC. (3) Between different MSCs in the same licensed operator. Between different licensed operators there are no procedure. – There are two modes of handover : (1) Synchronous : the old and new cells are synchronized so that their TDMA timeslots start at exactly the same time. - The GSM speech coder is based on the Residually (2) Asynchronous : this may lead to a longer Excited Linear Predictor (RELP) interruption of communication during handover since MS has to re-initialize timing advance at the - Enhanced by a long term predictor (LTP) new cell. - The coder provides 260 speech codec bits for each 20ms, ie. The speech codec bit rate is 13 kbps. By repetitively monitoring the SCHs of all the - 40% average voice activity exploited by a surrounding cells between transmission and reception of discontinuous transmission mode. A voice activity traffic bursts, the MS can compute beforehand the timing detector (VAD) is used in the speech coder – off the advances required for all surrounding cells. The MS is transmitter for power saving. then presynchronized with any such cell to which it may - Half rate codec works at 6.5 kbps. be handed. This speeds up the handover process. Institute for Infocomm Research 206 National University of Singapore Institute for Infocomm Research 207 National University of Singapore
  11. 11. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications – For TCH/F.9.6, Channel coding – data channels – Full rate 22.8kbps : • The output bits of the speech coder are ordered into groups for error protection, based upon their significance in contributing to speech quality. Out of the total 260 bits in a frame, • The most important 50 bits, called type Ia bits, have 3 parity check (CRC) bits added to them. • The next 132 bits along with the first 53 are reordered and appended by 4 trailing zero bits, and then encoded for error protection using a rate ½ convolutional encoder with constraint length K=5. • The least important 78 bits do not have any error protection. Institute for Infocomm Research 208 National University of Singapore Institute for Infocomm Research 209 National University of Singapore
  12. 12. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications Modulation Channel coding – control channel – 0.3 GMSK. - GSM Control channel messages are defined to be 184 bits long, and are encoded using a shortened binary cyclic fire – The channel data rate of GSM is 270.833333 kbps code, followed by a half-rate convolutional coder. Interleaving - The fire codes uses the generator polynomial - To minimize the effect of sudden fades on the received G5 ( x) = ( x 23 + 1)( x17 + x 3 + 1) = x 40 + x 26 + x 23 + x17 + x 3 + 1 w data, the total of 456 encoded bits within each 20 ms hich produces 184 message bits, followed by 40 parity speech frame or control message frame are broken bits. into eight 57 bit sub-blocks. These 8 subblocks which - Four tail bits are added to clear the convolutional coder make up a single speech frame are spread over eight which follows, yielding a 228 bits per block. consecutive TCH time slots. - This block is applied to a half-rate K=5 convolutional - If a burst is lost due to interference or fading, code using the generator polynomials G0 ( x) = 1 + x3 + x 4 interleaved data will help to spread the effect over a few error-correction-frames. Hopefully channel coding and G1( x) = 1 + x + x3 + x 4 (same as type Ia TCH). The ensures that enough bits will still be received resulting 456 encoded bits are interleaved onto eight correctly. consecutive frames in the same manner as TCH speech data. Institute for Infocomm Research 210 National University of Singapore Institute for Infocomm Research 211 National University of Singapore
  13. 13. EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications Frequency hopping - Under normal conditions, each data burst belonging to a particular physical channel is transmitted using the same carrier frequency. - If users in a particular cell have severe multipath problems, the cell may be defined as a hopping cell by the network operator. - Frequency hopping is carried out on a frame-by-frame basis, thus hopping occurs at a maximum rate of 216.7 hops per second (1/0.004615 – frame rate). As many as 64 different channels may be used before a hopping sequence is repeated. Institute for Infocomm Research 212 National University of Singapore Institute for Infocomm Research 213 National University of Singapore
  14. 14. EE5401 Cellular Mobile Communications Verify the following: – Apparent bandwidth efficiency GSM bit rate = 270.83 kbps, bandwidth = 200kHz Bandwidth efficiency = 1.354 bs/Hz – The speech codec rate for each time slot = 456b/20ms (=22.8kbps). – Each voice channel actually allocated for 270.83/8 = 33.854kbps. – Number of bits/slot = 33.854x4.615 = 156.25 bits (148+8.25 guard time) – For each TDMA slot in each frame, 114 bits are transmitted, and only 24 data frames per 26 frames are transmitting, therefore the vocoder output rate = 114/0.004615 x 24/26 = 22.8kbps – However, of the 114 data bits in a slot, only 65 are raw speech codec bits Raw data rate = 22.8*65/114=13 kbps Or 65 bits in 20ms = 13 kbps Institute for Infocomm Research 214 National University of Singapore