GSM Cellular Standards: A look at the world’s most common digital cellular system

1. GSM Cellular Standards:A look at the world’s most common digital cellular system Kevin BoldingElectrical EngineeringSeattle Pacific University

2. GSM is the world’s most popular standard for cellular Digital, TDMA/WDMA H H H H H Base GSM • Details on • Voice data format, encoding • Error-control coding • Channel sharing (TDM) • Channel allocation (FDM)

3. Mobile Handset Voice Encoder Channel Encoder Modulator Noise Channel Base Station Channel Decoder Demodulator Wireless Channel Uplink Architecture

4. Voice Encoder 260 bits Voice Coding • Analog signal is sampled using PCM at 64kbps. • The signal is broken into 20 ms samples, which contain 1280 bits each • A Regular Pulse Excited - Linear Predictive Coder (RPE-LPC) is used to compress the 1280 bits into 260 bits • Compression into just over 1/5 the size • 64kbps voice signal compressed into 13kbps signal • Each 260 bit sample contains 20ms of speech data IA – 50 bits IB – 132 bits II – 78 bits Most critical Very Important Icing

5. Channel Encoder 456 bits Channel Coding - Blocks • The 260 bit (20ms) sample is divided into class IA, IB and II, based on how important the bits are in determining the sound quality. • IA uses a 3 bit CRC. If the CRC fails, the whole sample is thrown out One sample is 20ms of speech --> 456 bits --> 8 blocks • IA and IB together have a 4-bit trailer. Together, these are put into a ½-rate convolutional coder that doubles the number of bits One block is 2.5ms of speech --> 57 bits • II bits are appended unencoded, giving an overall sample of 456 bits IA – 50 IB – 132 bits IB – 132 bits II – 78 bits Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 • The 456 bit encoded sample is divided into 8 blocks of 57 bits each (each contains the equivalent of 2.5 ms of speech) – these are the basic units of transmission.

6. Channel Encoder First sample (20ms) Second sample (20ms) IA – 50 IA – 50 IB – 132 bits IB – 132 bits IB – 132 bits IB – 132 bits II – 78 bits II – 78 bits 156.25 bits Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 TDMA Bursts • Blocks are gathered together to form a TDMA burst • 2 separate speech sample blocks are gathered together • Interleaved to protect against burst errors • 26-bit training sequence • To characterize multipath and filter it out • 16.25 tail/guard bits One burst is two blocks --> Two 2.5ms samples of speechfrom same source • Total Burst is 156.25 bits TG Block - 57 T G Training - 26 T G Block - 57 T/G

7. Channel TG Block - 57 T G Training - 26 T G Block - 57 T/G TDMA Frame - 8 bursts - 8 x 2 x 2.5ms sample of speech - 1250 bits Sharing the channel – TDMA Frames • Eight bursts (from different sources) make up a TDMA frame One TDMA frame is eight bursts --> 8 sources x (2 x 2.5ms sample of speech) • This allows eight sources to share a channel Burst Burst Burst Burst Burst Burst Burst Burst Each burst comes from a different source (phone) Eight phones share a channel using TDM.

8. Channel F F F F F F F F F F F F F F F F F F F F F F F TDMA MultiFrame - 26 Frames - 24 x 8 x 2 x 2.5ms sample of speech - 32500 bits - 120ms Burst Burst Burst Burst Burst Burst Burst Burst Sharing the channel • 8 Bursts per TDMA frame (2 x 2.5ms sample each) • 26 TDMA frames make up one Multi-frame • 24 are for data (speech) • 1 is for control , 1 is unused One TDMA MultiFrame is 26 Frames (24 data) --> 24 Frames x 8 Bursts/Frame x 2 Blocks/Burst = 384 Blocks per MultiFrame --> For each of the 8 senders, there are 384/8 = 48 Blocks per MultiFrame --> For each sender, 48 blocks = 120 ms of speech (60 ms from 2 samples) F F F Each TDMA MultiFrame takes 120ms to transmit and contains 120ms of speech data from 8 sources  8 times as efficient as analog transmission

9. Modulator GSM: Modulation • Each Multiframe has 32500 bits and lasts 120ms • 270833 bps • Transmitted using a channel 200kHz wide • US: 890-915MHz band for cell uplink • Divided into 124 200kHz wide channels • Downlink from 935-960MHz • Cell towers arranged in a hexagonal grid, usually in groups of 7 – Requires 7 sets of independent channels • Each cell gets 124/7 = 17 channels • Cell capacity = 17 * 8 = 136 conversations

10. Modulator GSM: Modulation • GSM uses Gaussian-filtered Minimum Shift Keying (GMSK). • MSK is a minimum-shift form of FSK • Gaussian pre-filter reduces bandwidth • MSK gives the best spectral efficiency of any digital bandpass signal set. • FSK only has one amplitude level, allowing for a simpler amplifier in the handset

11. Discontinuous Transmission • Discontinuous transmission (DTX) allows for the transmitter to be turned off 60% of the time. • Saves power • Reduces the overall ambient noise in the cell sector. • DTX requires voice detection, so that the handset knows when to restart transmission. • DTX also requires a synch signal, so that the receiver can differentiate between silence and a dropped connection.

12. Dynamic Power Compensation • The power between the handset and the tower can be dynamically adjusted in response to the channel BER. • This allows the channel to start at a minimum power level, and only increase when the signal requires a greater SNR. • For CDMA, Dynamic Power Compensation is a necessity • All transmit on same band at the same time • Power must be adjusted so that all signals are received at the same strength • Otherwise, one channel would overpower all others