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Andrew J. and Erna Viterbi Family Archives
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Viterbi: Presentations
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Andrew J. Viterbi, "CDMA Waveform Design, Rationale and Performance."
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Andrew J. Viterbi, "CDMA Waveform Design, Rationale and Performance."
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COMA Waveform Design, Rationale and Performance A.J. Viterbi ~..._-~.® Summary • Basic Techniques and Rationale • Waveform and Primary Parameters • Multipath Characteristics • Forward Link Processing • Reverse Link Processing • Power Control • Reverse Link Performance with Power Control ~..._ ...~® COMA WAVEFORM DESIGN • Direct Sequence Pseudo-Noise • QPSK by filtered binary pseudo-noise (PN) sequence • FIR digital filter • 1.2288 MHz chip rate • Filter bandwidth ~1.25 MHz • Time resolution '"1 J.lsec IMPULSE G MOOULAJOn OuadralLre D Binary Energy/Impulse & ( Wave- lr~nler ENCODER CJ I~t Random sh~lng Sequence Impuse I r am Period • 'c fillers ttJlllpliers IMPULSE MOOULAIOn tUf) Modulalor for Jlh User .. tIC') SIGNAL OualJra'Lre Yl n ) I I Receiver Waveform SC:JrlJI'U1lJ tJ /IIary Ma'clled Per IUlJ lIafl(Jom V((OV[U .. V'gllal f tilers I Sequence output ( f1Jlllplters PUO((SSOU .. tI( f ) Oernodulalor ror jlh User Analysis Based on Random Binary Spreading Sequences But, Pseudorandom sequence generated by Maximum LengtJl Linear Feedback Shift register of r stages )1as same basic three Randomness Properties r In every period of length P =2 -1 •... 1. Number of +IS and _IS (or zeros and ones) is nearly equal 2. 1/2 the runs (of +IS or _IS) 11ave lengtl1 1 k 1/4 have length 2, l/B length 3, ... 1/2 lengt)l k p 3. Sequence correIation c('t ) = ~ L 3 n 3 n -t-t >= 8('t'), Kronecler della n=1 Thus indistinguishable from pure random wIlen P is large Note, 110wever, wIlen correlation is over mucil sl10rter span, eft} is a random variable witil distribution siluilar to ..~ of binary i.i.d. variables just as for random s~uence --~------ --~-------~~---- _ -~-~ _ --------------------- ~-~- -------- -- Interference Model for Direct Sequence - CDMA with M Users M Var (y) = NO J 1H(f) 1 2 df + I (Bc./T c ) J 1H(f) 1 4 df j=2 J M N O + L B c . JIHU)1 4 dflT c P rl.:..,. Interference Energy ;=2 J er~ S' I E = ----=------- 18 na nergy E Cl Assuming bandlimited filters, ~n ~n 2 J IH(f)l4df J 1df >[ JIHtf)12. 1 df] =1 ~JIHU)l4 df >1/W -W/2' -W/2 J/HU)1 4 dflTe > I For bandlimited filter equality if and only if H (f) is constant ~..__.... ® CAUTION, BEWARE THE STANDARD MISCONCEPTION!! (Pursley, 1977; Turin, 1980 and 1984; Pickholtz et ai, 1991) all of whom consider only~uare Pulse Signals h(t) = -V ~e [u(t + Te/2) -u(t-Te/2)] ---7 H(f) = -V Te sin (nfTe)/nfTe then ~e f IH(f)1 4 df = f[f h(t)h(t-t)dt] 2 dt/Tc 1 Tc 2 [E =- J (1- Itl/Tc) dt =+ Tc -Tc seems more optimistic, but note that I Main Lobe Bandwidth = 2fTe I With ideallowpass H(f), on the other hand, ~e J IH(f)14 df = 1 and Bandwidth = 1rrc ~-~... ® MULTIPATH vs DISTANCE FROM CELL Distance from Cell Q) -c :J ..... .- - Q. E <C ....... "." -:::::;:;:: . .......;.: :.:-:..;.::•.•..... -:-: ;..;.::;.;::-::;:.:;;.;.; ," ... - - '" . •.................:.::;:;:;.;. . . . . ....... ~.~~ . . , . Time Delay (microsec.) MULTIPATH CHANNEL MODEL x(t) Pilot Signal provides capability to track delay, amplitude and phase of each delay element t k ' A k (Rayleigh) and Ok (uniform), independent random variables. Thus overall sjgnal fjlter FORWARD LINK Thus Signal output E(y) - Ef /HW/ 2 df = f /HOW/ 2 df t A k 2 , while Noise Variance proportional to Var(y) - JlH(f)/4df = JIH o (f)1 4 df[(t Ak 2 r- t( Ak 2 J] , <JI H oWI 4 df~ A k 2 ) · Hence Multipath does not change SNR, only distribution of signal. If Ak 's m are i.i.d Rayleigh, I Ak 2 is chi-squared k=l and square root is Nakagami - m Forward Link uses coherent BPSK with rate 1/2, K =9 convolutional code. CELL MODULATOR ARCHITECTURE Paging Channel~ Encoderl Interleaver Sync Channel Traffic Channel • • • Add'i Traffic Channels Pilot Channel Encoder/ Interleaver Address Encoder/ Interleaver Address PN Generator Walsh Generator • c o .- ~ ctI E E :J en FORWARD LINK WAVEFORM RATIONALE f1bQI (llNMODULATED PN SEQUENCE) Included in each sector of each cell site VALUABLE I FOR: 1) Cell site acquisition and tracking of each multipath component 2) Accurate estimation (tracking) of time delay, phase and amplitude of each multipath component even during moderately fast fading This justifies dedicating excess power to pilot (between 4 and 6 dB above each traffic channel - cost for 40 users < 0.5 dB) FORWARD LINK WAVEFORM (CONTINUED) ADDlIlQNAL FEATURE: Make all traffic channels orthogonal by using 64 Walsh functions to cover (BPSK Modulate) each traffic channel But Note: Orthogonality maintained at receiver only on each multipath component; different multipath components of different traffic channels are not orthogonal Reduction of at least 3 dB (on average) in same cell interference COHERENT RECEIVER IN MOBILE CONSISTS OF: a) Pilot tracker, including "EARLY-LATE Gate" delay-lock loop for each of 3 multipath components, as well as phase-lock loop for each component b) Coherent reception of each of 3 components using pilot tracking output c) Coherent combining of all 3 received components (form inner product of 3-component pilot signal vector with 3-component traffic signal vector) [Use less components, if less than 3 multipath components are detected by pilot receiver] All the above plus an additional multipath component "searcher" are contained in one "MOBILE DEMODULATOR" VLSI chip. REVERSE LINK WAVEFORM RATIONALE • Pilot Impractical since 1 pilot/traffic channel would be required • Fading and time varying character of each multipath component causes phase and amplitude to vary, but not more rapidly than fraction of a millisec. Hence, each component is relatively stable for duration of a few bit times. (1 bit time @ 10 Kb/s = 0.1 msec.) • Use M-ary Orthogonal Modulation Preceded by rate r < 1 binary code such that r.log2 M = small integer Specifically we use r = 1/3 and M = 64 Hence, r .log2M = 2 bit times of coherent energy [Note with DPSK, coherence maintained over only 2r bit times] Thus 64-ary Orthogonal Walsh Functions are used for modulation - Not traffic channel separation, as is done in forward link ----- - REVERSE LINK WAVEFORM (CONTINUED) ACQUISITION and TRACKING of each multipath required just as in forward link but 1) Only 1ime need be tracked, not amplitude or phase 2) Must be performed on modulated channel (except during call initiation when unmodulated signal is sent) This requires MORE SEARCHERS (Twice as many searches as cell demodulators are provided, in contrast with only 1/3 as many for pilot-aided forward link) REVERSE LINK RECEIVER PROCESSING M-ary ORTHOGONAL MODULATION with NONCOHERENT RECEIVE PROCESSOR 1. Track each path delay tk for significant multipath components (employs twice as many searchers as trackers) 2. Perform 1 2 + 0 2 (energy) estimate for each of M signal hypotheses for each component and sum component energies for each of M 3. Choose Maximum over M for Hard Decisions and use Energy value of Maximum after Nonlinear (Log Likelihood) transformation for Soft Decisions (Note, AGe normalizes noise level to unity) 4. De-Interleave 5. Decode with Conventional Viterbi Decoder Reverse Link uses M = 64, rate 1/3 K =9 convolutional code Binary Convoluttonal Encoder Crate r) Interleaver Encoder-Modulator M-ary Orthogonal Waverorm Generator M Orthogonal Waverorm Envelope-Correlators y, ,y~.....y Select Maximum yand Calculate Metric IndexCy) Ley) Delnterleaver Sort -Decision Decoder Demodulator-Decoder Figure 3: Block Diagram of Modulation-Coding Signal Processors for Noncoherent Receptfon Power Control FORWARD LINK: Slow, weak control suffices, Useful primarily in fringe (handoff) areas REVERSE LIMK: Critical to solve "Near-Far" problem (on the order of 80 dB or more) Two Loosely Coupled Techniques: 1) Open Loop (coarse) power control • Operates with AGC circuit of mobile receiver • Corrects to within ~ 10 dB (asymmetry of forward/reverse links) • Response very rapid Power Control (Continued) 2) Closed loop (fine) power control • Eb/NO estimation measurement made by cell-site receiver every 1.25 msec. (combine metrics of 6 successive Walsh Symbols) • Measurement compared with threshold 0 • Commands sent by cell-site to mobile; one bit every 1.25 msec. (800 bps) as follows: If Y >0, C(Y) = - ~ Y <0, C(Y) = +~ where +~ increase/decrease in mobile transmitted power (in dB) Power control commands sent on forward link by "stealing" 1 symbol every 1.25 msec. on each traffic channel; No coding on command telemetry since would require intolerable delay. In this manner, delay held to 2.5 msec. Hence, command link suffers high error rate (~ .05), but errors have minimal effect on control loop performance. Reverse Link Frame Error Rate: Single Path, Full Rate, 5 Levels ...... .......... .............. .............. ~ ~ \ ~ & \ ~ ---.. '\ ~\ ~ ~ '\ "' '" \ " '\ ........ \ " \ r-........ \ "- \ ""'- \ f\... \ "- \ ""-\ \ "'-\ \ "i ~ \ \ , " 1.0E-01 Pf 1.0E-02 1.0E-03 5.4 5.6 5.8 6 6.2 6.4 6.6 6.8 • Simulation - PC (Finite IL) --D-- Simulation - Fixed Energy --.-- Upper Bound - PC (Infinite IL) --<>--- Upper Bound - Fixed Energy Eb/No (dB) Figure 1: Steady-state Energy Probabilities wllh Power Control (Full Simulation) Pr 0.09 0.072 0.054 0.036 0.018 o • Threshold a 14, Mean a 5.6025 dB, Sid. Dev. a 1.17 dB 0- Threshold = 16, Mean I: 6.208 dB, Sid. Dev. a 1.15 dB -.- Threshold c: 18, Mean a 6.7433 dB, Sid. Dev. I: 1.13 dB o 1 2 3 456 Avg. Eb/No (dB) 7 8 9 10
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Core Title
Andrew J. Viterbi, "CDMA Waveform Design, Rationale and Performance."
Publisher
University of Southern California. Libraries
(digital)
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OAI-PMH Harvest
Permanent Link (DOI)
https://doi.org/10.25549/vit-m2709
Unique identifier
UC1957426
Identifier
VIT-001397 (filename),Cost 231 (folder),Box 24, Folder 435 (identifying number),vit-m1 (legacy collection record id),vit-c117-1632 (legacy record id),vit-m2709 (legacy record id)
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VIT-001397.pdf
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1632
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There are materials within the archives that are marked confidential or proprietary, or that contain information that is obviously confidential. Examples of the latter include letters of references and recommendations for employment, promotions, and awards; nominations for awards and honors; resumes of colleagues of Dr. Viterbi; and grade reports of students in Dr. Viterbi's classes at the University of California, Los Angeles, and the University of California, San Diego.; These restricted items were not scanned and, therefore, are not included in the USC Digital Archive.; Researchers wishing to see any of the restricted materials should consult with the USC Libraries Special Collections staff.
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Andrew J. and Erna Viterbi Family Archives
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University of Southern California
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Viterbi: Presentations
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There are materials within the archives that are marked confidential or proprietary, or that contain information that is obviously confidential. Examples of the latter include letters of referenc...
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USC Libraries Special Collections
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Doheny Memorial Library 206, 3550 Trousdale Parkway, Los Angeles, California,90089-0189, 213-740-4035, specol@usc.edu
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Title
Viterbi: Presentations