GSM System. Global System for Mobile Communications

Transcription

1 GSM System Global System for Mobile Communications Introduced in Settings of standards under ETSI (European Telecommunication Standards Institute) Services - Telephone services - Data services - Short message paging System Architecture : three major subsystems. Network and Switching Subsystem (NSS) : 1. MSCs, Visitor Location Register (VLR), Home Location Register (HLR), Authentication Center (AUC) and Equipment Identity Register (EIR). 2. Switching of GSM calls between external networks and the BSCs. 3. Managing and providing external access to several customer databases, such as HLR : contains subscriber information (International Mobile Subscriber Identity -IMSI) and location information for each user who resides in the same city as the MSC. VLR : temporarily stores the IMSI and customer information for each roaming subscriber who is visiting the coverage area of a particular MSC. Once a roaming 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 184 Institute for Infocomm Research 185

2 4. BSC performs handover for MS under the control of same BSC. Operation Support Subsystem (OSS) 1. Support the operation and maintenance of GSM and allows system engineers to monitor, diagnose and troubleshoot all aspects of the GSM system. 2. Interacts with the other GSM subsystems. 3. Charging and billing. Frequency domain The frequency band for uplink (reverse) is MHz, downlink (forward) is MHz. Interface TDMA/FDMA/FDD - Uses both TDMA and FDMA to transmit and recover information. Systems are FDD. - The bandwidth for the GSM system is 25MHz, which provides 125 carriers uplink/downlink each having a bandwidth of 200 khz. The ARFCN (Absolute radio frequency channel numbers) denotes a forward and reverse channel pair which is separated in frequency by 45 MHz. Base-to-mobile : F u ( n) = ( n 1) MHz Mobile-to-base : Fd ( n) = Fu ( n) + 45 MHz - In practical implementations, a guard band of 100kHz is provided at the upper and lower end of the GSM spectrum, and only 124 (duplex) channels are implemented. Institute for Infocomm Research 186 Institute for Infocomm Research 187

3 There are a total of eight channels per carrier. Every eighth timeslot on a TDMA channel, the user transmits or receives his information. A second frequency band from MHz and MHz (three times as much as primary 900 MHz) are also specified in 1990, a total of 374 duplex channels DCS 1800 GSM has chosen the four-cell repeat pattern for the frequency reuse cell sets. In most cases, each cell is divided into 120-deg sectors, with three base transceiver subsystems in each cell. Each base transceiver has a 120-deg antenna. - These 12 sectors (called cells in GSM system) share the 124 channels. Institute for Infocomm Research 188 Institute for Infocomm Research 189

4 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 s), to support encryption with high security and frequency hopping. Physical Channel : specified by ARFCN and TN. Logical Channel : is mapped onto the physical channel. E.g, TCHs and control channels. Institute for Infocomm Research 190 Institute for Infocomm Research 191

5 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 TCH/F2.4 TCH/H4.8 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. Traffic or speech multiframe Control multiframe Institute for Infocomm Research 192 Institute for Infocomm Research 193

6 Control (Signaling) Channels : Three types : Broadcast CHannel (BCH) Common Control CHannel (CCCH) Dedicated Control CHannel (DCCH) Broadcast channel (BCH) site. Due to higher distance resulting in large propagation delay, the transmitted signal of mobile 1 during TS1 reaches the BS later. If no action is taken, this will overlap with other transmission slots. The only solution is that the MS advances its emission relative to its reception by a time corresponding to and from propagation delay. This 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 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 GSM systems. 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 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 Common control channel (CCCH) Occupies TS0 of every control frame not used by BCH or the Idle Frame. 3 different CCCH types Paging channel (PCH) 1. Provides paging signals from base to mobiles. 2. Notifies specific mobile of incoming call Random access channel (RACH) 1. Uplink channel 2. Used by mobiles to acknowledge page from PCH 3. Used by mobiles to originate a call Access grant channel (AGCH) 1. Provides forward link communication to mobile. 2. Specifies time slot, radio channel and dedicated control channel. 3. AGCH is final CCCH message before mobile is moved off the control channel. Institute for Infocomm Research 194 Institute for Infocomm Research 195

7 Dedicated control channel (DCCH) Bidirectional channels with same format and function on uplink and downlink. May exist in any time slot and on any radio channel except TS0 of the control radio channel. 3 different DCCH are specified. Stand-alone dedicated control channel (SDCCH) 1. Carries signaling data following the connection of the mobile with BS. 2. Intermediate and temporary channel which accepts a newly completed call from the BCH and holds the traffic while accepts a newly completed call from the BCH and holds the traffic while waiting for the BS to allocate a TCH channel. 3. Ensures that mobile and base remains connected during authentication and resource allocation. 4. Maybe assigned their own physical channel or may occupy TS0 of the BCH if there is low demand for BCH or CCCH traffic. Slow associated control channel (SACCH) 1. Always associated with a traffic channel 2. On downlink the SACCH carries power control and timing advance instruction 3. On uplink the SACCH carriers signal strength and quality information 4. SACCH is allocated every 13th frame of a traffic channel. 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 advance and power control info. - MS sends authentication and validation requests. - MS is instructed by the BTS over the SDCCH to retune to new radio channel and TS for TCH assignment. Time slot data bursts take on one of the 5 formats according to the logical 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 Institute for Infocomm Research 196 Institute for Infocomm Research 197

8 26 training bits for equalization. There are three cases of handover, Two stealing bits for FACCH (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. Mobile-Assisted handover There are four purposes for handover (1) rescue low-quality channel (2) recovering cochannel interference (3) traffic balancing among cells (avoid congestion or load balancing) directed handover. (4) recovering in the event of failure of a control channel (2) Asynchronous : this may lead to a longer interruption of communication during handover since MS has to re-initialize timing advance at the new cell. By repetitively monitoring the SCHs of all the surrounding cells between transmission and reception of traffic bursts, the MS can compute beforehand the timing advances required for all surrounding cells. The MS is then presynchronized with any such cell to which it may be handed. This speeds up the handover process. Institute for Infocomm Research 198 Institute for Infocomm Research 199

9 Speech coding 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. - The GSM speech coder is based on the Residually Excited Linear Predictor (RELP) - Enhanced by a long term predictor (LTP) - The coder provides 260 speech codec bits for each 20ms, ie. The speech codec bit rate is 13 kbps. - 40% average voice activity exploited by a discontinuous transmission mode. A voice activity detector (VAD) is used in the speech coder off the transmitter for power saving. - Half rate codec works at 6.5 kbps. Institute for Infocomm Research 200 Institute for Infocomm Research 201

10 For TCH/F.9.6, Channel coding control channel - GSM Control channel messages are defined to be 184 bits long, and are encoded using a shortened binary cyclic fire code, followed by a half-rate convolutional coder. - The fire codes uses the generator polynomial G5 ( x) = ( x + 1)( x + x + 1) = x + x + x + x + x + 1 which produces 184 message bits, followed by 40 parity bits. - Four tail bits are added to clear the convolutional coder which follows, yielding a 228 bits per block. - This block is applied to a half-rate K=5 convolutional code using the generator polynomials G 0( x) = 1+ x + x and G 1( x) = 1+ x + x + x (same as type Ia TCH). The resulting 456 encoded bits are interleaved onto eight consecutive frames in the same manner as TCH speech data. Institute for Infocomm Research 202 Institute for Infocomm Research 203

11 Modulation 0.3 GMSK. The channel data rate of GSM is kbps Interleaving - To minimize the effect of sudden fades on the received data, the total of 456 encoded bits within each 20 ms speech frame or control message frame are broken into eight 57 bit sub-blocks. These 8 subblocks which make up a single speech frame are spread over eight consecutive TCH time slots. - If a burst is lost due to interference or fading, interleaved data will help to spread the effect over a few error-correction-frames. Hopefully channel coding ensures that enough bits will still be received correctly. Institute for Infocomm Research 204 Institute for Infocomm Research 205

12 Frequency hopping Verify the following: - 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-byframe basis, thus hopping occurs at a maximum rate of hops per second (1/ frame rate). As many as 64 different channels may be used before a hopping sequence is repeated. Apparent bandwidth efficiency GSM bit rate = kbps, bandwidth = 200kHz Bandwidth efficiency = bs/hz The speech codec rate for each time slot = 456b/20ms (=22.8kbps). Each voice channel actually allocated for /8 = kbps. Number of bits/slot = x4.615 = bits ( 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/ 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 206 Institute for Infocomm Research 207