CDMA (over) OFDM

WINLAB, November 28, 2000

Andrej Domazetovic

Mainly relied on:

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

Objective

Richard Van Nee, Ramjee Prasad, “OFDM For Wireless Multimedia Communications”

Presentation Layout

• CDMA Reminder/Overview

• Multicarrier Modulation Schemes

• OFDM/CDMA

• Some results: DS-CDMA vs. MC-CDMA

CDMA Reminder

Classification of CDMACDMA

Pure CDMA Hybrid CDMA

DS

FH

TH

Wideband

Narrowband

Slow

Fast

Hybrid-pureCDMA

Hybrid-contentionless

CDMA

Hybrid-contentionCDMA

Hybrid-OFDM/CDMA

DS/FHFH/THDS/TH

DS/FH/TH

TDMA/CDMATDMA-FDMA/CDMA

CDMA/ISMACDMA/ALOHA

MC-CDMAMT-CDMA

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

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

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 -> 1/Gp percent of time (average)

• LPI: Low power, catch me!

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

Pure CDMA - Time Hopping

• Multiple access: One user at a time (FEC when not)

• Multipath interference: Signaling rate up -> dispersion -> no advantage

• Narrowband interference: 1/Gp percent of time, reduction by Gp

• LPI: Short time, catch me when, multiple users

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

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

Multicarrier Modulations

Conventional vs. Orthogonal

Ch.1 Ch.2 Ch.3 Ch.4 Ch.6Ch.5frequency

Conventional multicarrier

1/T

Ch.1 Ch.3 Ch.5frequency

Orthogonal multicarrier

1/T

Ch.6Ch.4Ch.2

Transmitter

IFFT

b (=RT)bit buffer

andEncoder

DAC1/T' = N/T

data stream R bps P/S

N QAMsymbols

2N timedomain

samples

LPF

Time-frequency occupancy

T’-symbol period; J symbols in parallel; T-OFDM symbol period (in practice T = J*T’ + Tg)

symbol 1

frequency

time

symbol J

T=JT'T'

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

OFDM / CDMA

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

Multicarrier CDMA flavors

• Multicarrier CDMA : MC - CDMA

• Multicarrier direct sequence CDMA : MC - DS - CDMA

• Multitone CDMA : MT - CDMA

MC - CDMA

X

Serialto

ParallelConver

ter

1/J

data stream

b k(t)T' - symbol period

b k,1(t)

T=J*T'

b k,j(t)

b k,J(t)

X

X X

C k1

C kM

cos(2pi f j,1 t)

cos(2pi f j,M t)SUM

SUM

IDFFT

User K; J BPSK (T’) symbols are grouped (T=J*T’); each spread by C=(Ck1,…,CkM) in frequency domain; separation between adjacent carriers = 1/T

Time-frequency occupancy

T’-symbol period; J symbols in parallel; T-OFDM symbol period (T = J*T’ + Tg); J*M total # of carriers

symbol 1 chip 1

frequency

time

symbol n chip M

o

symbol n chip 1

o

T=JT'T'

MC - DS - CDMA

X

Serialto

ParallelConver

ter

1/J

data stream

b k(t)T' - symbol period

b k,1(t)

b k,j(t)

b k,J(t)

X

X X

C k(t)

C k(t)

cos(2pi f j,1 t)

cos(2pi f j,M t)SUM

SUM

IDFFT

User K; J BPSK (T’) symbols are grouped (T=M*J*T’) 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 J*M

Time-frequency occupancy

T’-symbol period; J*M symbols in parallel; T-OFDM symbol period (T = M*J*T’ + Tg); J*M total # of carriers

symbol 1 sequence 1

frequency

time

symbol n sequence M

o

symbol n sequence 1

o

T=MJT'T'

Midenticalsymbols

MT - CDMA

User K; J BPSK (T’) symbols are grouped (T=J*T’); 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

X

Serialto

ParallelConver

ter

1/J

data stream

b k(t)T' - symbol period

b k,1(t)

T=J*T'

b k,j(t)

b k,J(t)

X

C k(t) cos(2pi f j t)

SUM

IDFFT

Time-frequency occupancy

T’-symbol period; J symbols in parallel; T-OFDM symbol period (T = J*T’ + Tg); J total # of carriers

frequency

time

T=JT'T'

symbol nspreadacrossseveralcarriers

‘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 (M*T’) (L chips per bit); all users across all carriers; total # of carriers M*L

X

Serial toParallel

Converter

1/LM

X

X

X

C1(t)

C N(t)

cos(2pi f1 t)

cos(2pi fLM t)

SUM

SUM

IDFFT

data stream - d1

data stream - dN

Interleav

er

Time-frequency occupancy

T’-symbol period; M*L symbols in parallel; T-OFDM symbol period (T = M*T’ + Tg); M*L total # of carriers

frequency

time

T=MT'T'

symbol nspread

across allcarriers

Remarks

• The M identical information bearing branches in MC-CDMA and MC-DS-CDMA is to increase frequency diversity

• Carrier separation big enough => 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

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

From the Prasad/Nee book

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)

Assumptions:

X

Serialto

ParallelConver

ter

1/J

data stream

b k(t)T' - symbol period

b k,1(t)

T=J*T'

b k,j(t)

b k,J(t)

X

X X

C k1

C kM

cos(2pi f j,1 t)

cos(2pi f j,M t)SUM

SUM

IDFFT

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 K=32• DS-CDMA - Gold K=31

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

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

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