MC-CDMA

					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(2f1t )



                                 Ck 2       cos(2f 2t )
 Data
 Stream
              Copier                    
                                                            
                                 CkN        cos(2f 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
   [1] T. Sälzer, ”Design of a multi-carrier CDMA downlink with different transmit
    antenna array strategies,” in VTC-Fall 2005.
   [2] 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.
   [3] 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.
   [4] 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.
   [5] 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.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:7
posted:9/21/2011
language:English
pages:15