Multi-carrier CDMA Outline Introduction System Model Types Applications References Introduction Introduction OFDM benefits: – Robustness against multipath propagation channels – Low-complex technique – Bandwidth-efficient technique CDMA benefis – Enables multiple access – Provide frequency diversity – Mitigates multipath interference OFDM Signal in Frequency Domain Simple Transmitter Structure ck (t ) ck 1 c k 2 Ck 1 cos(2f1t ) Ck 2 cos(2f 2t ) Data Stream Copier CkN cos(2f N t ) Transmitter Structure Receiver Structure Main advantage of the MC-CDMA – receiver employ all the received signal energy Receiver stages – Coherently detection with DFT – Despreading – Using combining techniques (EGC, MRC, …) Single-user detection: prefiltering method Multi-user detection (MUD) – Linear: MMSE, – Nonlinear: PIC, SIC – Turbo MUD Advanced Variations MIMO-MC-CDMA – Prefiltering CSI at tx Single user detection – Space-time codes (CSI at rx) – Layered space-time Beamforming – Smart antenna Low data rate in low number of users Lower complexity Transmit array vs. Receive array Outdoor application BD-MC-CDMA – Transmitters select carriers (frequency bands) which are under good condition according to feedback information from the receivers and can decrease transmission power for each receiver. Applications Beyond 3G and 4G – High data rates: 100 Mbps for DL 20 Mbps for UL – High spectral efficiency in multi-cell environment – Open issues: slot/frame acquisition, channel estimation: MIMO and UL, UL synchronization – Competition with evolution of existing DS-CDMA systems, OFDMA and solution from NTTDoCoMo VSF-OFCDM (variable spreading factor Orthogonal Frequency and Code Division Multiplexing) for the DL. Applications Power Line Communications (PLC) – Cellular system: resistance to inter-cell interference – High data rates (100 Mbps) – Compliant with authorized spectrum mask – Competition with OFDMA Cognitive radio – Adaptive to unused frequency bands MC-CDMA vs. OFDMA OFDMA – Assignment of one or several sub-carriers to each user – User-data symbols are allocated directly to channel resources and therefore offers no diversity without channel coding MC-CDMA – Transmits in parallel chips of a spread data symbol on different sub-carriers – Spreads the user data symbol energy over all channel resources and therefore offers diversity. – Trade-off between the negative impact of MAI and the positive effect of frequency diversity MC-CDMA vs. OFDMA Single user scenario: – Comparison depends on coding rates, constellation size and receiver structure – MC-CDMA outperforms COFDM for small constellation (QPSK) Cellular environment: – Resource allocation for OFDMA system – MC-CDMA can outperform OFDMA in the case of varying resource loads – OFDMA performance in the peripheral cell area surpasses the MC-CDMA References  T. Sälzer, ”Design of a multi-carrier CDMA downlink with different transmit antenna array strategies,” in VTC-Fall 2005.  Z. Li, M. Vehkaperä, D. Tujkovic, M. Juntti, M. Latva-aho, and S. Hara, “Performance evaluation of space-frequency coded MIMO MC-CDMA system,” in Proc. IST Mobile & Wireless Telecommun. Summit (IST), Aveiro, Portugal, June 15–18, 2004.  S.Hara, R.Prasad, “Overview of Multi-Carrier CDMA,” IEEE 4th International Symposium on Spread Spectrum Techniques and Applications Proceedings 1996, Vol. 1, pp. 22-25, Sep. 1996.  H. Tao, N. Arumugam, and G. H. Krishna, “Performance of space-time coded MC-CDMA over time and frequency selective fading channel,” in Proc. IEEE Conf. Mobile and Wireless Networks (MWCN), Stockholm, Sweden, Sept. 9–11, 2002, pp. 419–423.  Z. Li, M. Vehkaperä, D. Tujkovic, M. Juntti, “Receiver Design for Spatially Layered Downlink MC-CDMA System,” IEEE Trans. Commun., vol.54, pp.1471- 1477, May 2005.
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