1. GSM Cellular No. 1  Seattle Pacific University GSM Cellular Standards: A look at the world’s most common digital cellular system Kevin Bolding Electrical Engineering Seattle Pacific University

2. GSM Cellular No. 2  Seattle Pacific University GSM GSM is the world’s most popular standard for cellular Digital, TDMA/WDMA H Base H H H H Details on Voice data format, encoding Error-control coding Channel sharing (TDM) Channel allocation (FDM)

3. GSM Cellular No. 3  Seattle Pacific University Base Station Mobile Handset Wireless Channel Uplink Architecture Voice Encoder Modulator Channel Encoder Demodulator Channel Decoder Channel Noise

4. GSM Cellular No. 4  Seattle Pacific University Voice Coding  64kbps voice signal compressed into 13kbps signal  Each 260 bit sample contains 20ms of speech data Analog signal is sampled using PCM at 64kbps. The signal is broken into 20 ms samples, which contain 1280 bits each Voice Encoder 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 IA – 50 bitsIB – 132 bitsII – 78 bits 260 bits Most criticalVery ImportantIcing

5. GSM Cellular No. 5  Seattle Pacific University 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. Channel Encoder IA uses a 3 bit CRC. If the CRC fails, the whole sample is thrown out 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 II bits are appended unencoded, giving an overall sample of 456 bits IA – 50 IB – 132 bitsII – 78 bits 456 bits IB – 132 bits 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. One sample is 20ms of speech --> 456 bits --> 8 blocks One block is 2.5ms of speech --> 57 bits

6. GSM Cellular No. 6  Seattle Pacific University 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 interference and filter it out 16.25 tail/guard bits Block - 57 Training - 26 TGTG TGTG TGTG T/G 156.25 bits IA – 50 IB – 132 bitsII – 78 bitsIB – 132 bits Block - 57 First sample (20ms) IA – 50 IB – 132 bitsII – 78 bitsIB – 132 bits Block - 57 Second sample (20ms) Total Burst is 156.25 bits One burst is two blocks --> Two 2.5ms samples of speech from same source Channel Encoder

7. GSM Cellular No. 7  Seattle Pacific University 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) Channel This allows eight sources to share a channel Block - 57 Training - 26 TGTG TGTG TGTG T/G Burst TDMA Frame - 8 bursts - 8 x 2 x 2.5ms sample of speech - 1250 bits Each burst comes from a different source (phone) Eight phones share a channel using TDM.

8. GSM Cellular No. 8  Seattle Pacific University Sharing the channel 26 TDMA frames make up one Multi-frame 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) Channel 24 are for data (speech) Burst TDMA MultiFrame - 26 Frames - 24 x 8 x 2 x 2.5ms sample of speech - 32500 bits - 120ms FFF FFFFFFFFFFFFFFFFFFFFFFF Each TDMA MultiFrame takes 120ms to transmit and contains 120ms of speech data from 8 sources  8 times as efficient as analog transmission 1 is for control, 1 is unused 8 Bursts per TDMA frame (2 x 2.5ms sample each)

9. GSM Cellular No. 9  Seattle Pacific University GSM: Modulation Each Multiframe has 32500 bits and lasts 120ms 270833 bps Modulator 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. GSM Cellular No. 10  Seattle Pacific University GSM: Modulation GSM uses Gaussian-filtered Minimum Shift Keying (GMSK). MSK is a minimum-shift form of FSK Gaussian pre-filter reduces bandwidth Modulator 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. GSM Cellular No. 11  Seattle Pacific University 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. GSM Cellular No. 12  Seattle Pacific University 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