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Module-3 : Transmission Lecture-9 (18/5/00)

Module-3 : Transmission Lecture-9 (18/5/00). Marc Moonen Dept. E.E./ESAT, K.U.Leuven marc.moonen@esat.kuleuven.ac.be www.esat.kuleuven.ac.be/sista/~moonen/. Lecture 9 : CDMA -Overview. Multiple Access Techniques TDMA FDMA

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Module-3 : Transmission Lecture-9 (18/5/00)

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  1. Module-3 : TransmissionLecture-9 (18/5/00) Marc Moonen Dept. E.E./ESAT, K.U.Leuven marc.moonen@esat.kuleuven.ac.be www.esat.kuleuven.ac.be/sista/~moonen/ Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven/ESAT-SISTA

  2. Lecture 9 : CDMA -Overview • Multiple Access Techniques TDMA FDMA CDMA : Frequency hopping vs. direct sequence CDMA SDMA : See also Lecture-10 • DS-CDMA Single-user (matched filter) receiver, RAKE receiver Code design, synchronization, near-far problem,.. • Multi-user detection based CDMA Receivers • CDMA/DMT combined schemes Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  3. CDMA Introduction • Pioneer CDMA era: 1949-… : first contributions by Pierce, Shannon,… 1956 : `RAKE’ patent (Price & Green) 1970-… : military CDMA systems • Narrow-band CDMA era: 1978-… : CDMA for cellular applications 1993 : IS-95 standard (Qualcomm) 1996-… : IS-95 commercial operation (US, Korea, …) • Wide-band CDMA era: 1995-… : Research programs in Europe, USA, Japan 2000-… : Commercial operation UMTS/IMT2000 Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  4. CDMA Introduction • 1st generation cellular/mobile: -analog transmission -speech services -AMPS (US), TACS (UK), NMT (Scandinavia), NTT (Japan) • 2nd generation cellular/mobile: -digital transmission -speech & data services -IS-136/D-AMPS (US), GSM, PDC, IS-95 • 3rd generation cellular/mobile: -higher bit-rates, multiple services, etc. -wide-band CDMA air interface -UMTS (ETSI), IMT2000 (ITU) Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  5. Multiple Access Techniques • Multiple Access Schemes : allow different users to access/share the same communication channel. • Underlying principle = `orthogonality’ sent signal for user j Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  6. Multiple Access Techniques Multiple Access Schemes : • Frequency Division (FDMA) • Time Division (TDMA) • Code Division (CDMA) - Frequency Hopping (FH-CDMA) - Direct Sequence (DS-CDMA) • Space Division (SDMA) see Lecture-10 Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  7. user-1 out-1 CHANNEL user-2 out-2 : : out-M user-M Multiple Access Techniques Frequency Division Multiple Access (FDMA) • Different users use passband modulation (Lecture-3) with different carrier frequencies • Different frequency bands separated by guard bands (-> `orthogonality’) • PS: Compare with DMT/OFDM (Lecture-7/8). • PPS: DMT/OFDM sometimes also used as MA technique. Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  8. Multiple Access Techniques Time Division Multiple Access (TDMA) • Different users use different time slots in one and the same communication channel • Different time slots separated by guard times (->`orthogonality’) user-1 out-1 CHANNEL user-2 out-2 : : user-M out-M Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  9. FDMA frequency TDMA frequency time time Multiple Access Techniques TDMA/FDMA • Example GSM : - 125 frequency channels (in 900MHz band) - 8 users time-multiplexed in each channel - neighboring cells use different frequency bands to avoid inter-cell interference (hence actual number of frequency bands in a cell 7 times lower…) Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  10. Multiple Access Techniques Code Division Multiple Access (CDMA) • Frequency Hopping (FH-CDMA) : - Carrier frequency for user-i is changed for each time slot, based on periodic pseudo-random code sequence for user-i. - `Fast’ versus `slow’ frequency hopped systems `Fast’ = several hops over one symbol period `Slow’ =several symbols transmitted during one hop • Time Hopping (TH-CDMA) : - Periodic pseudo-random code sequence for user-i defines transmission moment for user-i. FH-CDMA and TH-CDMA not further addressed here... Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  11. CDMA frequency code time Multiple Access Techniques Code Division Multiple Access (CDMA) • Direct Sequence CDMA (DS-CDMA) : • Relevance : Third Generation wide-band CDMA proposals Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  12. Multiple Access Techniques Code Division Multiple Access (CDMA) • Direct Sequence CDMA (DS-CDMA) : -Each user-i is assigned a periodic (period N) pseudo-random code sequence -For each symbol (k-th symbol for user-i), a `chip’ sequence is transmitted -Mostly binary codes ( ) with BPSK/QPSK symbols -Multiple access based on code-orthogonality (see below) Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  13. Multiple Access Techniques Direct Sequence CDMA (DS-CDMA) • Transmission : Example : transmitted symbols +1……… -1……… -1……… +1……… code sequence +1,+1,-1 transmitted chips +1,+1,-1, -1,-1,+1, -1,-1,+1, +1,+1,-1 • N = `spreading factor’ = # chips per symbol • Symbol rate = fs, `chip rate’ = fc = N.fs Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  14. Multiple Access Techniques Direct Sequence CDMA (DS-CDMA) • Transmission block scheme: • code-multiplication may be viewed as (digital) filtering operation, with FIR transmit filter • Chip sequence is fed into (analog) transmit filter p(t) for transmission (not shown) (see Lecture-3) N C(z) N-fold upsampling Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  15. Multiple Access Techniques Direct Sequence CDMA (DS-CDMA) • Reception : if received signal = transmitted chip sequence, multiply chips with (synchronized) code sequence + sum. • Example : transmitted symbols +1……… -1……… -1……… +1……… code sequence +1,+1,-1 transmitted chips +1,+1,-1, -1,-1,+1, -1,-1,+1, +1,+1,-1 received chips +1,+1,-1, -1,-1,+1, -1,-1,+1, +1,+1,-1 +1,+1,-1 received symbols +1……… -1……… -1……… +1……… Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  16. Multiple Access Techniques Direct Sequence CDMA (DS-CDMA) • Reception block scheme: basic operation is `matched filter’ + downsampling (=symbol-rate sampling) (see Lecture-3) N-fold downsampling N C(1/z) Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  17. Multiple Access Techniques Direct Sequence CDMA (DS-CDMA) • Multiple access based on code orthogonality N-fold downsampling N C(1/z) chip sequence user-j code sequence user-i Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  18. Multiple Access Techniques Direct Sequence CDMA (DS-CDMA) • Need for code synchronization (acquisition+tracking) procedure : not addressed here. • Reception for -Asynchronous CDMA -Dispersive channel (hence inter-symbol and/or inter- chip-interference (ISI/ICI), multi-user-interference (MUI)) needs more refined techniques…….see below • PS : here we only consider `short codes’ 1 code period for each symbol in practice also `long codes’ (`scrambling codes’) code period >> symbol period Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  19. Multiple Access Techniques CDMA • PS : Direct sequence spreading and frequency hopping are `Spread Spectrum’ (SS) techniques, where transmission bandwidth for every signal is much larger than information bandwidth. SS Techniques originally developed for military radar and communication systems because of robustness against (narrow band) jamming. • PS: CDMA application examples : - IS-95 cellular telephony (Qualcomm) - IMT2000-UMTS (`wideband CDMA’) - IEEE 802.11 wireless LANs - GPS - cable modems, power line comms, …. Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  20. Multiple Access Techniques Space Division Multiple Access (SDMA) - Different users are separated based on spatial properties - Hence different users can use the same time slots and/or frequencies, or the same codes (= SDMA on top of TDMA/FDMA or CDMA) Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  21. Multiple Access Techniques Space Division Multiple Access (SDMA) - Signal separation based on -Beamforming techniques (`1G’) -Advanced (multi-path) channel modeling & signal processing techniques - May be viewed as `dynamic sectorization’ (currently only `static sectorization’ used (e.g. GSM), based on directional antennas) - See Lecture-10 Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  22. noise channel N N DS-CDMA General CDMA setup, with several users + noise : • Code orthogonality is • Receiver structure a la page 13 (with code orthogonality) sufficient/optimal ? received signal Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  23. channel N N N DS-CDMA : Single-User Receiver `Single-user’ (`matched filter’) receiver : • Aim is to suppress inter-symbol-interference (ISI for user-of-interest i) and multi-user interference (from other users) noise Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  24. noise channel N N N DS-CDMA : Single-User Receiver `Single-user’ (`matched filter’) receiver : case-1 • if Hi(z)=Hj(z)=1 (no channel dispersion & synchronous) - MUI=0 because of code-orthogonality - ISI=0 because no channel dispersion Hence optimal receiver structure (in the sense of Lecture-4) Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  25. DS-CDMA : Single-User Receiver `Single-user’ (`matched filter’) receiver : case-2 • if Hi(z)=1, Hj(z)=pure delay (e.g asynchronous CDMA) - MUI=0 only if code cross-correlations are zero : Hence need for proper code design (or need for other receiver) noise channel N N N Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  26. channel N N N DS-CDMA : Single-User Receiver `Single-user’ (`matched filter’) receiver : case-2 (ctnd) • if but , then MUI may still ruin performance (=`near-far problem’). • Solutions: - Power control - `Near-far’ resistant receiver. noise Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  27. channel N N N DS-CDMA : Single-User Receiver `Single-user’ (`matched filter’) receiver : case-3 • if Hj(z)=0 (hence no MUI), but Hi(z)=multipath propagation - ISI=0 only if code auto-correlations are zero : Hence need for proper code design (or need for other receiver) noise Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  28. DS-CDMA : Single-User Receiver `Single-user’ (`matched filter’) receiver : case-3 • alternative structure is a `true’ matched filter receiver: • This is the `RAKE receiver’ (mostly used in practice) noise channel N N N Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  29. noise channel N N N N DS-CDMA : Single-User Receiver `Single-user’ (`matched filter’) receiver : case-3 • RAKE receiver example : • One `finger’ for each channel tap. Each `finger’ draws energy from one channel component (reflection). Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  30. DS-CDMA : Single-User Receiver Conclusion : • Single-user receivers o.k. for synchronous DS-CDMA with mild multi-path effects • Requires proper code design, with suitably small auto-correlations & cross-correlations (see below) • Near-far problem • More general receivers : Multi-user detection (MUD) based receivers (see below) Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  31. DS-CDMA : Code Design • `Pseudo-random’ codes : deterministic sequences with noise-like (statistical) properties • Example : Maximum length codes -linear shift register : binary & length n -EXOR operation on selected bits -code repetition period is output=code 1 2 3 4 5 example: 5-stage, [2,3] Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  32. state 1000 output 0 0100 0 0010 0 1001 1 1100 0 0110 0 1011 1 0101 1 1010 0 1101 1 1110 0 1111 1 0111 1 0011 1 0001 1 1 2 3 4 DS-CDMA : Code Design • Example : Maximum length codes, 4-stage, [3,4] Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  33. DS-CDMA : Code Design • Example : Maximum length codes, 4-stage, [3,4] properties: - # 1-outputs (8) = # 0-outputs (7) + 1 - 4 runs of length 1 2 runs of length 2 1 run of length 3 1 run of length 4 properties always hold for max.length codes Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  34. DS-CDMA : Code Design • Example: Maximum length codes auto-correlation : (cyclic) cross-correlation : worst-case cross-correlation in a `family’ of codes (same n), does not go to zero for large n ps: for this, replace {0,1} by {-1,+1} (BPSK) Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  35. DS-CDMA : Code Design • Other (improved) codes : Gold Codes Kasami Codes Barker Codes Walsh-Hadamard Codes …. Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  36. DS-CDMA : Code Design • Example : IS-95 (CDMA/TDMA) - Carrier spacing (bandwidth) 1.25 MHz - Chip rate 1.2288 Mchips/sec - QPSK/O-QPSK modulation - Walsh codes (length 64) to separate physical channels - Long code of length (2^42-1) for baseband data-scrambling - Long code of length (2^15-1) for quadrature spreading (one for I, one for Q) - Soft handover - Power control Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  37. noise channel N N received signal DS-CDMA : Multi-user detection • Problem Statement : optimal receiver structure for the general case (asynchronous and/or multipath and/or near-far…) ?? • Basic principles are those of Lecture-4 : minimum-distance receiver, matched filter front- ends and/or Nyquist-rate sampling front-end, etc.. Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  38. noise channel N N MU-detection N N DS-CDMA : Multi-user detection • Nyquist-rate sampling is chip-rate sampling (if excess bandwidth sufficiently small) • Matched filter front-end consists of a bank of matched filters, one for each active user. -> Need to know all the user-codes : only @ base station Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  39. DS-CDMA : Multi-user detection • Minimum-distance receiver : possible, but complexity is major impediment (exponential in number of users !) • Other : Zero-forcing (ISI/MUI) MMSE SIC (`serial interference cancellation’) PIC (`parallel interference cancellation’) = Active research area, vast recent literature & considered for W-CDMA... Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  40. CDMA/DMT Combined Schemes Several schemes have been presented that combine CDMA features with DMT features: • Example-1 : multicarrier-CDMA (MC-CDMA) first spreading, then different chips on different carriers • Example-2 : MC-DS-CDMA frequences for different carriers spread by 1 and the same spreading sequence • Example-3 : MT-CDMA first DMT, then time domain spreading Pros & Cons : see literature... Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  41. CDMA/DMT Combined Schemes Example-1 : multicarrier-CDMA (MC-CDMA) [Yee, Linnartz en Fettweis] [Fazel en Papke] Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  42. CDMA/DMT Combined Schemes • Example-2 : MC-DS-CDMA [DaSilva en Sousa] Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  43. CDMA/DMT Combined Schemes • Example-3 : MT-CDMA [Vandendorpe] Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  44. Other... • Block Spreading (`chip interleaving’) : [Cirpan en Tsatsanis] for a code sequence transmit a block of symbols multiplied by , then the same block multiplied by , etc… Leads to simpler (block) channel models. • DMT-CDMA with block spreading • etc. Conclusion : Active research area, more to come... Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

  45. Conclusions • Multiple Access : -TDMA/FDMA/CDMA/SDMA • DS-CDMA: - Single-user receiver structures (RAKE) : mostly used in practice - Code design - Multi-user detection receivers - CDMA/DMT Module-3 Transmission Marc Moonen Lecture-9 CDMA K.U.Leuven-ESAT/SISTA

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