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Wideband Communications . Lecture 20-23: MC-CDMA Aliazam Abbasfar. Outline. IS-95/ WCDMA MC-DS-CDMA MC-CDMA OFDMA. IS-95. DSSS 1.2288 Mchips /sec ( BW = 1.25 MHz) FDD Downlink Synchronous CDMA Quadrature spreading 2 Extended M-seq. (2 15 -chip long)

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wideband communications

Wideband Communications

Lecture 20-23:

MC-CDMA

Aliazam Abbasfar

outline
Outline
  • IS-95/ WCDMA
  • MC-DS-CDMA
  • MC-CDMA
  • OFDMA
is 95
IS-95
  • DSSS
  • 1.2288 Mchips/sec ( BW = 1.25 MHz)
  • FDD
  • Downlink
    • Synchronous CDMA
    • Quadrature spreading
      • 2 Extended M-seq. (215-chip long)
      • Offsets identify BTS (synched)
    • 64 orthogonal Walsh codes
      • Codes 0 : pilot, 1 : sync, 2: paging
      • max. 61 data channel (users)
    • Coherent reception
  • Uplink
    • Asynchronous CDMA
    • No pilot channel (Non-coherent reception)
    • Long M-seq. (242-1 long) to distinguish users
    • Walsh coded symbols
    • OQPSK
  • Fast power control
wcdma
WCDMA
  • DSSS
  • Pulse shape : SRC with roll off 0.22
  • 3.84 Mcps (BW = 3.88*1.22 = 4.68 MHz)
  • FDD (UL: 1920–1980, DL: 2110–2170 MHz) and TDD
  • Downlink
    • Synchronous CDMA
    • Complex spreading
      • 2 Gold codes (M-seq of 218-1)
      • BTS are unsynched
    • OVSF codes
      • SF:4-512
      • Codes 0 : pilot, 1 : sync
      • max. 255 data channel (users)
    • Coherent QPSK
  • Uplink
    • Asynchronous CDMA
    • Complex spreading
      • 2 Gold codes
    • Coherent QPSK
      • Send pilots
  • Fast power control
multi carrier cdma
Multi-carrier CDMA
  • Multi-carrier modulation is very good at combating ISI
    • Low complexity equalizer
    • High performance
  • combine Multi-carrier modulations with CDMA
  • Types:
    • MC-DS-CDMA ( synch/asynchronous)
      • Spreading in Time
    • MC-CDMA ( synchronous)
      • Spreading in Frequency
    • Hybrid approaches
      • 2D spreading
      • Spreading in time and frequency
mc ds cdma
MC-DS-CDMA
  • Multiple DS-CDMA with different carriers
  • Parallel sub-channels each with DSSS
    • Sub-channels have narrower band (reduced ISI)
      • RAKE or equalizer receiver for each
    • Few sub-channels makes the PAPR small
  • Very narrowband sub-channels can be implemented efficiently by OFDM/DMT
    • Spreading is done over OFDM/DMT symbols
mc cdma
MC-CDMA
  • Signature waveforms are all between 0-T
    • xk(t) = Xksk(t)
    • Xk : kth user data symbol (bk bits)
    • Rk = bk / T
    • orthonormal bases : fn (t)=rect(t/T) ej2pf0 t
    • sk(t) = Sck,nfn (t) ; n=0,…,N-1
    • Vector representation : sk = [ck,0 ck,1 … ck,N-1]T : codes
    • Each user uses all the tones
      • The tone coefficient is determined by its code
  • Signature waveforms at RX
    • rk(t) = sk(t)*hk(t) = Sck,nHk,nfn (t) ; n=0,…,N-1
    • rk = [Hk,0ck,0 Hk,1ck,1 … Hk,N-1ck,N-1]T = Hksk : RX codes
    • r(t) = SXkrk(t) + n(t) = SXkSck,nHk,nfn (t) + n(t)
    • r = A X + n ; A=[r1 r2 … rK]
  • High spectral efficiency
mc cdma codes
MC-CDMA codes
  • Walsh/Hadamard codes
    • Orthogonal at TX
    • NOT orthogonal at RX when ISI
    • Golay/Zadoff-Chu
    • Codes with good PAPR
  • Pseudo noise sequences
    • M-sequence/Gold/Kassami codes
    • Simple to generate
    • Good cross-correlation property
  • Low rate convolutional codes
    • Coding gain in addition to processing gain
    • Bad MAI
hybrid methods
Hybrid methods
  • Two dimensional spreading
    • Easy to implement in OFDM-CDMA
  • # of dimensions = N x # data symbols
    • # of Time slots x # of tones
  • r(t) = SSXk,irk,i(t) + n(t)
    • r = S Ai Xi + n
    • Ai=[r1 r2 … rK]
  • Ai are orthogonal
    • Parallel channels
  • Separate dimensions
    • ri = Ai Xi + ni
    • Ai=[r1 r2 … rK]
single user detection
Single user detection
  • Only one user channel coefficients and code are available
    • r(t) = SXkrk(t) + n(t) = SXkSck,nHk,nfn (t) + n(t)
    • rk = [Hk,0ck,0 Hk,1ck,1 … Hk,N-1ck,N-1]T = Hksk
    • r = A X + n ; A=[r1 r2 … rK]
    • Matched filter (Correlator) receiver
    • Equalizer + despreader
        • zk = sk* G r
    • MRC: G = Hk*
    • EGC: G = Hk* / |Hk|
    • ZF: G = (Hk*Hk)-1Hk*
    • MMSE: G = (Hk*Hk+ s2I)-1Hk*
multi user detection
Multi-user detection
  • All user channel coefficients and codes are available
    • r = A X + n ; A=[r1 r2 … rK]
    • rk = [Hk,0ck,0 Hk,1ck,1 … Hk,N-1ck,N-1]T = Hksk
    • Optimum: min |r – A X|2
  • De-correlator : z = (A*A)-1 A* r
  • MMSE : z = (A*A + s2I)-1 A* r
  • Interference cancellation
    • Successive (SIC)
    • Parallel (PIC)
pre equalization
Pre-equalization
  • Channel is known at TX (e.g. TDD)
    • Spreader + pre-equalizer
    • xk(t) = Xksk(t)
    • sk(t) = Sck,nGk,nfn (t) ; n=0,…,N-1
    • Codes : sk = [Gk,0ck,0 Gk,1ck,1 … Gk,N-1ck,N-1]T
    • r(t) = SXkrk(t) + n(t) = SXkSck,nHk,nGk,nfn (t) + n(t)
    • rk = [Hk,0Gk,0ck,0 Hk,1Gk,1ck,1 … Hk,N-1Gk,N-1ck,N-1]T = Hk Gksk
    • r = A X + n ; A=[r1 r2 … rK]
  • Pre-equalizers:
    • MRC: G = Hk*
    • EGC: G = Hk* / |Hk|
    • ZF: G = (Hk*Hk)-1Hk*
    • MMSE*: G = (Hk*Hk+ s2I)-1Hk*
  • Power constraint  scale the G coefficients
new multiple access schemes
New Multiple Access schemes
  • MC-TDMA or OFDM-TDMA
    • Time slots are assigned to users
    • OFDM modulation for users (OFDM symbols in time slots)
    • In many wireless standard, e.g. WiFi ( 802.11a/g)
  • OFDMA
    • Assign sub-carrier frequencies to users
    • Synchronous users
      • DL : TX sends an OFDM symbol for all users
      • UL: RX receives an OFDM symbol from all users
      • Very sensitive to synchronization (ICI)
        • Time, clock, and Freq
        • Users are synchronized by BTS
  • OFDMA + Code division multiplexing (CDM)
    • Users are assigned L sub-carriers
    • L Codes are used for each user
      • Frequency diversity (per user)
frequency hopping ofdma
Frequency hopping OFDMA
  • Time varying sub-carrier frequency assignment
    • Each user is given a hopping pattern
      • Frequency diversity
      • Orthogonal patterns
        • Independent of channels
  • Latin square
    • Nc orthogonal patterns = a family
      • Using all freqs only once
    • Nc-1 families with maximum

one collision per pair

      • Cross-correlation = 1/Nc
    • If Nc is prime  simple formula
reading
Reading
  • Fazel & Kaiser 2.1, 3.1, 3.2, and 3.3