CDMA (over) OFDM

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CDMA (over) OFDM

• WINLAB, November 28, 2000
• Andrej Domazetovic

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Mainly relied on
Objective

• To present the idea behind combining DS-CDMA
  systems with OFDM

Richard Van Nee, Ramjee Prasad, OFDM For
Wireless Multimedia Communications
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Presentation Layout

• CDMA Reminder/Overview
• Multicarrier Modulation Schemes
• OFDM/CDMA
• Some results DS-CDMA vs. MC-CDMA

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CDMA Reminder
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Classification of CDMA
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Pure CDMA - Direct Sequence

• Multiple access Coherent detection,
  cross-correlation among codes small
• Multipath interference If ideal code sequence,
  zero out of -Tc, Tc
• Narrowband interference Coherent detection,
  spread the interferer
• LPI Whole spectrum, low power per Hz

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Pure CDMA - Direct Sequence

• PROs
• Coded signals implemented by multiplication
• Simple carrier generator
• No synchronization among users necessary

• CONs
• Difficult to acquire and maintain synchronization
  (fraction of the chip time)
• Bandwidth limited to 10 to 20 MHz
• Near-far problem - power control needed

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Pure CDMA - Frequency Hopping

• Multiple access One user at one frequency band
  (FEC when not)
• Multipath interference Responses at different
  hop. freqs are averaged (noncoherent combining)
• Narrowband interference Gp hopping freqs -gt 1/Gp
  percent of time (average)
• LPI Low power, catch me!

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Pure CDMA - Frequency Hopping

• PROs
• Synchronization easier than DS (fraction of the
  hop time)
• Larger bandwidth (need not be contiguous)
• Better near-far performance
• Higher possible reduction of narrowband
  interference

• CONs
• Sophisticated frequency synthesizer needed
• Abrupt changes lead to wider occupied spectrum
• Coherent demodulation difficult

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Pure CDMA - Time Hopping

• Multiple access One user at a time (FEC when
  not)
• Multipath interference Signaling rate up -gt
  dispersion -gt no advantage
• Narrowband interference 1/Gp percent of time,
  reduction by Gp
• LPI Short time, catch me when, multiple users

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Pure CDMA - Time Hopping

• PROs
• Simple implementation
• Useful when transmitter avg. power limitted, but
  not peak
• Near-far is not a problem

• CONs
• Long time until synchronized
• Good FEC code and data interleaving needed

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Hybrid CDMA

• The goal is to combine two or more of
  spread-spectrum modulation techniques in order to
  improve the overall system performance by
  combining their advantages
• 1. Combination of Pure CDMAs lead to 4 hybrids
• 2. Combination with TDMA
• 3. Combination with multicarrier modulation

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Multicarrier Modulations
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Conventional vs. Orthogonal
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Transmitter
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Time-frequency occupancy
T-symbol period J symbols in parallel T-OFDM
symbol period (in practice T JT Tg)
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OFDM

• PROs
• Efficient way to deal with multipath
• Possibility to enhance the capacity
• Robust against narrowband interference
• Single-frequency networks possible

• CONs
• More sensitive to frequency offset and phase
  noise
• Large PAPR

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OFDM / CDMA
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Why Multicarrier CDMA ?

• Robust to frequency-selective fading (OFDM)
• Robust to frequency offsets and nonlinear
  distortion (DS-CDMA)
• Fast FFT/IFFT devices
• Good frequency use efficiency
• OFDM/CDMA can lower the symbol rate in each
  subcarrier, so longer symbol duration makes
  quasisynchronization easier

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Multicarrier CDMA flavors

• Multicarrier CDMA MC - CDMA
• Multicarrier direct sequence CDMA MC - DS -
  CDMA
• Multitone CDMA MT - CDMA

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MC - CDMA
User K J BPSK (T) symbols are grouped (TJT)
each spread by C(Ck1,,CkM) in frequency domain
separation between adjacent carriers 1/T
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Time-frequency occupancy
T-symbol period J symbols in parallel T-OFDM
symbol period (T JT Tg) JM total of
carriers
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MC - DS - CDMA
User K J BPSK (T) symbols are grouped
(TMJT) M times longer M identical branches
of each symbol are spread by Ck(t)(Ck1,,CkN) in
time domain N-processing gain separation
between adjacent carriers N/T total of
carriers is JM
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Time-frequency occupancy
T-symbol period JM symbols in parallel T-OFDM
symbol period (T MJT Tg) JM total of
carriers
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MT - CDMA
User K J BPSK (T) symbols are grouped (TJT)
each spread by signature waveform
Ck(t)(Ck1,,CkN) in time domain separation
among carriers 1/T prior to spreading! - after
spreading spectrum overlaps more densely
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Time-frequency occupancy
T-symbol period J symbols in parallel T-OFDM
symbol period (T JT Tg) J total of
carriers
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MT - CDMA
BPSK(T) streams N users each spread by its own
signature Ck(t)(Ck1,,CkL) in time domain
orthogonal M user bits per OFDM symbol to
transmit (MT) (L chips per bit) all users
across all carriers total of carriers ML
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Time-frequency occupancy
T-symbol period ML symbols in parallel T-OFDM
symbol period (T MT Tg) ML total of
carriers
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Remarks

• The M identical information bearing branches in
  MC-CDMA and MC-DS-CDMA is to increase frequency
  diversity
• Carrier separation big enough gt uncorrelated
  fading
• J must be large enough to insure that each
  subchannel be frequency non-selective
• MC-CDMA needs reliable carrier and phase recovery
  - coherent modulation
• MC-DS-CDMA and MT-CDMA better with non-coherent
• MT-CDMA has much denser spectrum, more
  susceptible to MAI and ICI

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DS-CDMA vs. MC-CDMA-BER performance-

• From the Prasad/Nee book

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Assumptions

• fast Rayleigh fading channel (WSSUS)
• L received paths
• Synchronous downlink channel quasisynchronous
  uplink
• Perfect synchronization, no frequency offset, no
  nonlinear distortion, perfect phase estimation
  (OFDM)
• Perfect path gain estimation and carrier sync.
  (DS-CDMA)

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Assumptions
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Assumptions

• Numerical values used in simulations
• Delay spread 20ns
• Doppler power spectrum with max fd 10Hz
• Transmission rate R 3Msyb/sec (BPSK)
• MC-CDMA - Walsh Hadamard K32
• DS-CDMA - Gold K31

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Conclusions

• It can be shown that as long as we use the same
  frequency-selective fading channel, the BER lower
  bound is the same for both DS-CDMA and MC-CDMA
• MC-CDMA has no major advantage in terms of signal
  bandwidth, as compared with DS-CDMA (although
  when Nyquist filters are used within DS-CDMA,
  RAKE may wrongly combine paths)
• Also, the number of users in the system depends
  on the combining strategy for MC-CDMA and on RAKE
  finger number for DS-CDMA

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Downlink

• It may be difficult for DS-CDMA RAKE to employ
  all the received signal energy scattered in time
  domain, whereas MC-CDMA receiver can effectively
  combine all the received signal energy scattered
  in the frequency domain
• MMSEC based MC-CDMA - Minimum Mean Square Error
  Combining (error in the estimated data symbols
  must be orthogonal to the baseband components of
  the received subcarriers)
• MMSEC MC-CDMA is promising although noise power
  estimation and subcarrier references are required

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Uplink

• As compared with the DS-CDMA scheme, MMSEC
  MC-CDMA performs well only for the single user
  case (code orthogonality among users is totally
  distorted by the instantaneous frequency
  response)
• Multiuser detection scheme is required which
  jointly detects the signals to mitigate the
  nonorthogonal properties