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## CDMA (over) OFDM

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**CDMA (over) OFDM**WINLAB, November 28, 2000 Andrej Domazetovic**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”**Presentation Layout**• CDMA Reminder/Overview • Multicarrier Modulation Schemes • OFDM/CDMA • Some results: DS-CDMA vs. MC-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**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 - Direct Sequence**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!**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 - Frequency Hopping**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**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 Pure CDMA - Time Hopping**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 Hybrid CDMA**Time-frequency occupancy**T’-symbol period; J symbols in parallel; T-OFDM symbol period (in practice T = J*T’ + Tg)**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**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