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 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

Transmitter

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

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 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

MC - DS - CDMA 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

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

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

‘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

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

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:

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