(Raj) Pawate Mansoor Chishtie Digital Signal Processing Applications S

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Digital Cellular Phone: Functional Analysis
(Raj) Pawate Mansoor Chishtie Digital Signal Processing Applications Semiconductor Group
SPRA134 October 1994
Printed Recycled Paper
IMPORTANT NOTICE Texas Instruments (TI) reserves right make changes products discontinue semiconductor product service without notice, advises customers obtain latest version relevant information verify, before placing orders, that information being relied current. warrants performance semiconductor products related software specifications applicable time sale accordance with TI's standard warranty. Testing other quality control techniques utilized extent deems necessary support this warranty. Specific testing parameters each device necessarily performed, except those mandated government requirements. Certain applications using semiconductor products involve potential risks death, personal injury, severe property environmental damage ("Critical Applications"). SEMICONDUCTOR PRODUCTS DESIGNED, INTENDED, AUTHORIZED, WARRANTED SUITABLE LIFE-SUPPORT APPLICATIONS, DEVICES SYSTEMS OTHER CRITICAL APPLICATIONS. Inclusion products such applications understood fully risk customer. products such applications requires written approval appropriate officer. Questions concerning potential risk applications should directed through local sales office. order minimize risks associated with customer's applications, adequate design operating safeguards should provided customer minimize inherent procedural hazards. assumes liability applications assistance, customer product design, software performance, infringement patents services described herein. does warrant represent that license, either express implied, granted under patent right, copyright, mask work right, other intellectual property right covering relating combination, machine, process which such semiconductor products services might used.
Copyright 1996, Texas Instruments Incorporated
Introduction
This document presents functional components dual-mode cellular phone specified CTIA IS-54 standard. each functional component, relevant algorithm, data structures, any, implementation details given. Functional View Dual-Mode Cellular Phone shown Figure dual-mode cellular phone consists following:
Transmitter Receiver Coordinator
Antenna assembly Control panel
dual-mode phone capable operating analog-only cell dual-mode cell. Both transmitter receiver support both analog digital time division multiple access (TDMA) schemes. Digital transmission preferred, when cellular system digital capability, mobile unit assigned digital channel first. digital channels available, cellular system will assign analog channel. transmitter converts audio signal radio frequency (RF), receiver converts signal audio signal. antenna focuses converts energy reception transmission into free space. control panel serves input/output mechanism user; supports keypad, display, microphone, speaker. coordinator synchronizes transmission receive functions mobile unit. Figure Functional Components Dual-Mode (IS-54) Cellular Phone
Transmitter
Analog-to-Digital Converter
Coder
Amplifier Phase Modulator
Transmit Audio Signal Processing
Modulator
Amplifier
Display
Control
Coordinator
Duplexer
Keyboard Antenna Assembly Digital-to-Analog Converter Decoder Amplifier Demodulator
Receive Audio Signal Processing Control Panel
Demodulator
Amplifier
Receiver
Figure shows functional components digital portion dual-mode cellular phone. Figure Functional Blocks Digital Portion Dual-Mode Phone
Speech Coder Channel Coder DQPSK Modulator Bandpass Filter Isolator
FACCH CDVCC
SACCH
Phase Shift
Detector 824-849 Control Bandpass Filter Bandpass Filter
Coordinator
Detector Phase Shift
864-904 CDVCC FACCH SACCH
Speech Decoder Channel Decoder Equalizer DQPSK Demodulator
CDVCC coded digital verification color code DQPSK differential quaternary phase-shift keying FACCH fast associated control channel SACCH slow associated control channel
Transmitter
transmitter converts low-level audio signals from microphone digitally coded signals audio processing, digital signal processing, modulation, amplification. transmitter converts 64-kbps pulse code modulation (PCM) data lower data rate, multiplexes control information, error-protects data, then passes data stream section modulation, amplification, transmission. coordinator inserts system control messages. Transmit Front-End Processing Speech signals from microphone first amplified, passed through antiliasing filter, sampled rate create digitized µ-law 64-kbps stream. Typically, pre-emphasis applied. Figure shows functional blocks front-end analog section. standard does propose specific echo canceler; however, recommends implementing one. front-end processing includes following:
amplifier. gain specified produce average signal energy, during frame, which down from full scale. bandpass filter avoid antialiasing. analog-to-digital converter. standard recommends that either directly convert analog signal uniform format with minimum resolution bits convert analog signal 8-bit µ-law codec sample.
Figure Front-End Analog Section Converts Audio 64-kbps Data Stream
Amplifier
Filter
kbps
Either linear with bits resolution 8-bit µ-law codec sampled
Speech Coder speech coder further reduces data rate compressing 64-kbps data stream input create 7.950-kbps data stream. IS-54 standard accepts full-rate speech coder called vector excited linear prediction (VSELP). This algorithm belongs class speech coders known code excited linear predictive coders (CELP). This class uses code books vector quantize excitation (residual) signal. VSELP variation CELP. incoming kbps data grouped into frames frame rate frames second. Hence, each frame contains samples represents duration Each frame coded into bits. Hence, rate conversions 7950 bps, shown Figure
Figure Full-Rate Speech Coder (VSELP) Reduces 64-kbps Data Stream 8-kbps Data Stream
Speech Coder MIPS
64-kbps
7.950-kbps
speech decoder utilizes separate code books. Each code book independent gain. code-book excitations each multiplied their corresponding gains summed create combined code-book excitation. basic parameters shown Table
Parameter Notation Specification Sampling rate Frame length samples samples Subframe length Short-term predictor order Number taps long-term predictor Number bits code word (number basis vectors) Number bits code word (number basis vectors) bits bits
NOTE: Within frame, bits allocated shown Table detailed allocations shown Table
Table Basic Parameters VSELP Speech Coder
Gains beta, gamma1, gamma2 Code words, Lag, Frame energy, Short-term filter coefficients Parameter {GS, code fourth subframe {GS, code third subframe {GS, code second subframe {GS, code first subframe code book, fourth subframe code book, third subframe code book, second subframe code book, first subframe code book, third subframe code book, second subframe code book, first subframe fourth subframe third subframe second subframe first subframe 10th reflection coefficient reflection coefficient reflection coefficient reflection coefficient reflection coefficient reflection coefficient reflection coefficient reflection coefficient reflection coefficient reflection coefficient Frame energy Parameter
Table Detailed Allocations Parameters Within Frame
Table Allocations Within Frame Speech
Bits Allocated
GSP0_4 GSP0_3 GSP0_2 GSP0_1 LAG_4 LAG_3 LAG_2 LAG_1 LPC10 LPC9 LPC8 LPC7 LPC6 LPC5 LPC4 LPC3 LPC2 LPC1
CODE2_4
CODE2_3
CODE2_2
CODE2_1
CODE1_3
CODE1_2
CODE1_1
Parameter Name
Bits Allocated
Channel Coder main function channel coder protect data stream against noise fading that inherent radio channel. coder accomplishes this adding extra redundant bits. greater number redundant bits, higher immunity interference lower bit-error rate. tradeoff increased data rate. channel coder protects data stream four stages: Convolutional coding Cyclic redundancy check (CRC) generation Interleaving Burst generation first mathematical operations, whereas last heuristic approaches. receiver performs inverse operation determine whether errors have occurred during propagation. radio propagation, been found that fading occurs localized instances time space. result, interleaving spreads information data stream across frames, because unlikely that clustered error would occur successive frames. Finally, data propagated bursts. Between interleaving burst generation, channel coder multiplexes control information. Figure shows functional components channel coder. Figure Channel Coder Functional Components With Associated Data Rates
7.950-kbps Data Stream Channel Coder 48.6-kbps Data Burst
7.950 kbps
Error Protection
Interleaving kbps kbps
Control Signal Multiplexing
16.2 kbps
Burst Generator
48.6 kbps
Convolutional Coding Convolutional coding provides error-correction capability adding redundancy transmitted sequence. Convolutional encoding implemented linear feed-forward shift registers. convolutional coder described rate which data enters coder rate which data leaves coder. example, rate-1/2 convolutional coder implies that every data entering coder, bits leave coder. smaller ratio, greater redundancy. This improves error-protection capability. reduce rate, bits frame error-protected. Only these bits, called class bits, error-protected. remaining bits, called class bits, error-protected. This shown Figure
Figure Error Protection Convolutional Coding Computation
Most Perceptually Significant Bits 7-Bit Calc. Speech Coder Class Bits
Tail Bits Coded Class Bits Voice Cipher 2-Slot Interleaver
Rate Convolutional Encoder
Class Bits
Speech Frames
Speech Frames
Cyclic Redundancy Check bits that error-protected, been found that only perceptually significant. Hence these protected using 7-bit cyclic redundancy computation before they input convolutional coder. 7-bit computed dividing data specified constant transmitting remainder with data. receiver detects errors comparing received remainder with what calculated. following generator polynomial used CRC: gCRC(X)
parity polynomial, b(X), remainder division input polynomial generator polynomial shown below: a(X)*X7 gCRC(X) q(X) b(X)/gCRC(X)
where q(X) quotient division b(x) remainder. quotient discarded, only parity bits identified b(X) encoded transmission. facilitate convolutional coder, these parity bits placed into array class bits. Figure Error Protection Adds Extra Bits Speech Frame
Error Protection 7.950 kbps kbps
Error Protection Adds Bits/20
short, shown Figure error protection adds bits every additional 5050 bps.
Table shows data interleaved when current frame previous frame. Note that speech data entered into interleaving array columns.
explained earlier, data from each frame divided spread across transmit slots. This done because fading might destroy frame, unlikely that will destroy frames succession. result, bits from speech frame lost slot. Figure shows data interleaved when three speech frames succession.
Interleaving
Table Interleaving Adjacent Speech Frames,
Figure Interleaving Adjacent Frames Error Protection
Speech Frames
y103 x102
y129 x128 y117 x116 x106 y105 x104
Speech Frames
y155 x154 y143 x142 x132 y131 x130
y181 x180 y169 x168 x158 y157 x156
y207 x206 y195 x194 x184 y183 x182
y233 x232 y221 x220 x210 y209 x208
y259 x258 y247 x246 x236 y235 x234
bits from speech frame classified class class bits; data placed into interleaving array such that class bits intermixed with class bits. Class bits sequentially placed into array occupy following numbered locations:
Control Signal Multiplexing
Control signal information added interleaved data. Control information includes
Slow associated control channel (SACCH) Fast associated control channel (FACCH) Digital verification color code (DVCC) Synchronization word (SYNC) Figure Control-Signal Multiplexing
Figure shows this control information multiplexed.
FACCH Data
Speech Data SACCH Data
Slow associated control channel (SACCH) signaling channel parallel with speech path used transmission control supervisory messages between base station mobile unit. SACCH messages continuously mixed with channel data; bits allocated SACCH. Fast associated control channel (FACCH) signaling channel transmission control supervisory messages between base station mobile unit. FACCH messages mixed with user information bits; they replace user information block whenever necessary.
through 130, 156, 182, through
DVCC/SYNC Data kbps
16.2 kbps
Combined Data
Digital verification color code (DVCC) 8-bit code that sent base station mobile unit used generate coded digital verification color code (CDVCC). CDVCC 12-bit field that includes 8-bit DVCC; CDVCC sent each slot from base station mobile unit vice versa. CDVCC used receiver distinguish current traffic channel from traffic cochannels. Synchronization word (SYNC) 14-symbol field that used slot synchronization, equalizer training, time slot identification. Mobile Assisted Handoff Mobile Assisted Handoff (MAHO) feature IS-54. base station command mobile unit perform signal quality measurements current forward channel other forward channels. mobile unit measure quantities: Received signal strength indicator (RSSI), which measure signal strength expressed error rate (BER), which estimate error information obtained measuring correctness data stream input mobile unit's channel decoder.
These channel quality measurements (RSSI BER) sent base station assist handoff. This reduces overhead base station. RSSI usually sent SACCH, although they could sent FACCH during discontinuous transmission (DTX). mode operation which mobile unit transmitter autonomously switches between transmitter power levels while mobile unit conversation state analog voice channel digital traffic channel. Burst Generator After data been compressed error-protected, stream compressed time only) into burst format. Burst timing offsets applied facilitate dynamic time alignment. Figure shows data compressed time-aligned allow data sent using one-third 48.6-kbps channel. Figure Burst Generator
16.2 kbps Speech FACCH SACCH Temporary Storage 48.6 kbps Burst Modulator
6.67-ms Pulse 48.6 kbps Delay 29-44 Symbols Receive Burst
Transmitter DQPSK Modulator Amplifier 48.6-kbps data input differential quaternary phase-shift keying (DQPSK) modulator. This phase modulator groups bits time create symbol. This results four levels modulation, shown Figure Hence, name quaternary. term differential used because symbols transmitted relative phase changes, rather than absolute phase values.
Figure 4-Level Modulator Groups Bits Form Symbol
cosct (0,1)
(-1,0)
(1,0)
sinct
(0,-1)
Figure shows that certain transitions, origin will have crossed. This implies that power envelope decoder will when origin crossed; this have undesired impact filters. alleviate this, scheme used. This shown Figure transitions this scheme either +/-45 degrees +/-135 degrees, origin never traversed transition from state another. This results eight points circle, shown Figure Figure Differential Quaternary Modulator States
Figure shows input serial data presented 2-bit parallel data supplied multipliers after digital-to-analog conversion. Since digital-to-analog converters (DACs) needed, they sometimes referred dual DACs. Binary signals vary phase-shifted signals multipliers. Filters limit impulse response binary signals ensure that carrier occupies allocated bandwidth. signals then summed together form final phase-shifted carrier. conversion from baseband (that frequency translation modulated carrier) typically carried several stages order reach 800-MHz range.
Figure DQPSK Modulator
Multiplier
cosct
Phase Shift
48.6 kbps
Multiplier
sinct
Amplifier amplifier boosts RF-modulated signal output levels, specified base station. Unlike analog transmission, which uses amplifier DQPSK carrier must linear. class push-pull nonlinear amplifiers used amplification purposes. These nonlinear amplifiers efficient (about 50%) order conserve power. However, nonlinear amplifiers cannot used DQPSK, because they would cause phase distortion. Linear amplifiers used DQPSK less efficient (30%). Figure shows amplifier. Figure Linear Amplifiers Needed IS-54 Cellular Phone
Linear Amplifier Efficient
Receiver
Switch
While duplexer required analog section dual-mode phone, required digital portion, because this case transmitter receiver operate simultaneously. simple switch enough isolate receiver from transmitter, allowing duplexer removed from digital portion. Removing duplexer added benefits: when DQPSK signals passed through
duplexer, phase distortion occurs because group delay; addition, there some power loss, which, turn, requires higher-rated power amplifier. Hence, removing duplexer reduces rating power amplifier, which extends battery life mobile unit.
Receiver
receiver functions following order: Amplifies received radio signal Superheterodynes signal lower workable frequency range Demodulates signal Equalizes compensates mitigate effects distortions introduced radio channel Detects errors Decodes speech signal Converts back into analog form eventually feeds speaker receiver consists several functional components:
Receiver amplifier Mixer section Demodulator Channel decoder Speech decoder
Receiver Amplifier This section receiver amplifies low-level DQPSK carrier, which could weak picowatts (116 dBm). amplifier increases this weak signal workable range before feeding mixer section. receiver amplifier broadband amplifier, which variable gain controlled automatic gain controller (AGC). compensates large dynamic range received signal, which approximately also reduces gain sensitive amplifier that input signal increases, distortions overdriving receiver occur. Figure shows portion receiver. Figure Portion Receiver Section Dual-Mode Cellular Phone
48.6-kbps Burst
Amplifier
DQPSK Demodulator
Equalizer
Mixer frequency received carrier range 869-894 MHz. cost-effective directly demodulate this signal this frequency range. Typically, received signal stepped down lower
frequency, called intermediate frequency (IF), mixing with local oscillator (refer Figure oscillator source varied that constant frequency, which simplifies amplifier design. Typically, second mixer superheterodynes first with another oscillator source produce much lower frequency than first lower frequency enables design narrow-band filters. Demodulator DQPSK demodulator extracts data from signal. Typically, local oscillator with 90-degree phase-shifted signal used. demodulator determines which decision point phase moved then determines which symbol transmitted calculating difference between current phase last phase (note that transmitter differential modulator). Once symbol been identified, next step decode bits. However, noise, Doppler effects, Rayleigh fading, signal must compensated equalized. Fading occurs when same signal arrives receiver different times because multiple paths caused reflections. Doppler effect caused motion transmitter relative received signal. Doppler effect causes received frequency vary proportion speed which mobile unit moving; this implies that equalizer section personal communication systems (PCS) unit need complex when traveling pedestrian speeds when travels higher vehicular speeds. Equalizer equalizer effectively inverse filter channel distortion. Since channel constant wireline channel assumed be), necessary track adapt changing channel. Hence name adaptive equalizer. IS-54 specification does recommend specific equalizer algorithm. present, classes equalizers popular:
decision feedback equalizer (DFE) maximum likelihood sequence estimator (MLSE)
Figure shows example MLSE adaptive equalizer [4]. operates adaptively training mode beginning each burst, well tracking mode during message detection. includes matched filter modified Viterbi processor. equalizer Figure used European system similar ones used North America.
Figure MLSE Adaptive Equalizer
cosct r(t) y(t) x(t) Matched Filter Phase Delay Time Delay Viterbi Processor
sinct
Phase Adaptation Signal Reconstruct
Coefficient Adaptation
Viterbi Adaptation
After demodulation low-pass filtering received signal, components x(t) y(t) sampled converted, with sampling frequency equal rate. Then signal samples filtered through digital N-tap transversal filter, which approximates matched filter (MF) shown. Theoretically, makes receiver insensitive carrier clock phases used demodulate sample received signal, provided that coefficients properly adjusted time span long enough include channel impulse responses. this end, must choose number taps, comply with maximum number echo delays that expect observe operational environment. Note that modulator output pulses spread over three periods. Typically, seems suffice. output samples finally processed according modified Viterbi processor, which operates number states complexity Viterbi processor varies exponentially with respect Channel Decoder channel decoder detects errors stream, demultiplexes control data, feeds data speech decoder. This shown Figure errors detected, masking strategy, explained Frame-Masking Strategy page applied.
Figure Channel Decoding Speech Decoding
48.6 kbps kbps 7.950 kbps 64-kbps
Control Signal Multiplexer CDVCC SACCH SYNC
Error Detection Discarded Speech Data FACCH Decoder
VSELP Decoder
Channel Decoder
FACCH Message
channel decoder works following stages: Control signal demultiplexer Error detector Control Signal Demultiplexer Speech, SACCH, FACCH, DVCC data signals from demodulator demultiplexed separate various signaling information. SACCH DVCC data simply demultiplexed directing dedicated bits from each burst their control-processing locations. Speech FACCH demultiplexing however, more challenging. Since FACCH data replace speech data time, FACCH data extracted first attempting detect errors speech data. appears correct decoded speech slot, data routed speech codec section. When error, data then decoded FACCH message. appears correct, this FACCH message routed call-processing location.
Error Detector DVCC words error-detected, compared assigned DVCC determine cochannel interference, sent transmit section echoed back base station. channel decoder provides information RSSI when commanded base station. This feature called MAHO, which discussed Mobile Assisted Handoff section page Frame-Masking Strategy frame-masking strategy based 6-state machine. every decode speech frame, state machine change states. State occurs most often implies that comparison successful. State implies that there were least consecutive frames that failed check. action taken each these states varies well. state action taken. States simple frame repeats. States repeat attenuate speech. State completely mutes speech. detailed description action corresponding each state follows:
State error detected. received decoded speech data used. State error detected. Parameter values R(0) bits from last frame that state repeated. remaining decoded bits frame passed speech decoder without modification. State Identical action state State Similar action state except that value R(0) modified. 4-dB attenuation applied R(0) parameter: that R(0) last state frame greater than then R(0) decremented repeated this lower level. State Similar state further attenuation applied R(0) that level much from original value R(0). State Similar R(0) further attenuated State frame repeated; this time R(0) cleared totally muting output speech. Alternatively, comfort noise could inserted place speech signal.
Speech Decoder speech decoder, VSELP, converts 7950-bps input data stream into 64-kbps data. poor radio conditions, performance VSELP been shown superior analog cellular. This primarily error-protection error-detection capabilities that made possible digital techniques. When speech frames lost because errors correctable, speech coder repeats previous frame information. number consecutive lost speech frames increases, gradual muting applied. Thus, gaps filled using characteristics human ear. When user data speech, computer facsimile data, then speech decoder bypassed. Adaptive Spectral Postfilter perceptual quality synthetic speech enhanced using adaptive spectral postfilter final processing step. form postfilter
Coefficient synthesis filter
Audio Interface output speech coder, 64-kbps stream, input audio interface, which consists following stages: Digital-to-analog conversion Reconstruction filter Receive-level adjustment reconstruction filter minimizes step transients caused converter. receive-level sensitivity defined that value field, frame energy, causes acoustic level least transducer when measured artificial ear. equal represents average frame energy during frame, which down from full scale.
Summary
This report presents brief functional overview digital cellular mobile station. Emphasis given algorithmic description implementation aspects each function. main purpose this paper provide general introduction various functional blocks. Refer other papers this book detailed implementation description individual functions.
References
Cellular System: Dual-Mode Mobile Station Base Station Compatibility Standard, IS-54 Project Number 2215, Electronics Industries Association, December 1989. Pawate, B.I., "Wireless Communication: Systems Perspective", Texas Instruments (internal document), 1992. Lin, al., Error Control Coding, Prentice-Hall, 1983. Avella, R.D., al., Adaptive MLSE Receiver TDMA Digital Mobile Radio", IEEE Journal Selected Areas Communications, Vol. 122-129, January 1989.

(Raj) Pawate Mansoor Chishtie Digital Signal Processing Applications S

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