gsm cellular no. 1 seattle pacific university gsm cellular standards: a look at the world’s most...
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GSM Cellular No. 1Seattle Pacific University
GSM Cellular Standards:A look at the world’s most common digital cellular
system
Kevin BoldingElectrical Engineering
Seattle Pacific University
GSM Cellular No. 2Seattle Pacific University
GSM
• GSM is the world’s most popular standard for cellular
• Digital, TDMA/WDMA
HBase
H
H
H
H
• Details on
• Voice data format, encoding
• Error-control coding
• Channel sharing (TDM)
• Channel allocation (FDM)
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Base Station
Mobile Handset
Wireless Channel Uplink Architecture
Voice Encoder Modulator
ChannelEncoder
DemodulatorChannelDecoder
Channel
Noise
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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 eachVoice
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 bits IB – 132 bits II – 78 bits
260 bits
Most critical Very Important Icing
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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.
ChannelEncoder
• 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 bits II – 78 bits
456 bits
IB – 132 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.
One sample is 20ms of speech--> 456 bits--> 8 blocks
One block is 2.5ms of speech--> 57 bits
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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 Block - 57Training - 26TG
TG
TG
T/G
156.25 bits
IA – 50 IB – 132 bits II – 78 bitsIB – 132 bits
Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57
First sample (20ms)IA – 50 IB – 132 bits II – 78 bitsIB – 132 bits
Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57 Block - 57
Second sample (20ms)
• Total Burst is 156.25 bits
One burst is two blocks--> Two 2.5ms samples of speechfrom same source
ChannelEncoder
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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 Block - 57Training - 26TG
TG
TG
T/G
Burst Burst Burst Burst Burst Burst Burst 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.
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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 Burst Burst Burst Burst Burst Burst Burst
TDMA MultiFrame - 26 Frames - 24 x 8 x 2 x 2.5ms sample of speech - 32500 bits - 120ms
F F FF F F F F F F F F F F F F F F F F F F F 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
• 1 is for control , 1 is unused
• 8 Bursts per TDMA frame (2 x 2.5ms sample each)
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GSM: Modulation• Each Multiframe has 32500 bits and lasts 120ms
• 270833 bpsModulator
• 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
GSM Cellular No. 10Seattle Pacific University
GSM: Modulation• GSM uses Gaussian-filtered Minimum Shift Keying (GMSK).
• MSK is a minimum-shift form of FSK
• Gaussian pre-filter reduces bandwidthModulator
• 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
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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.
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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