Multiple-Symbol Differential Sphere Decoding Aided Cooperative Differential Space-Time Spreading for the Asynchronous CDMA Uplink

In this paper, we propose a Cooperative Differential Space-Time Spreading (CDSTS) scheme employing multiple relays, which eliminates the demanding requirement of channel estimation both at the relays and at the destination. More explicitly, the source node employs differential encoding during the first transmission interval, and the multiple relays perform Differential Space-Time Spreading (DSTS) based Amplify-and-Forward (AF) relaying in the second transmission interval. Eliminating high-complexity channel estimation is particularly important at the light-weight shirt-pocket-sized mobile handset based relays, since it is unrealistic to estimate the associated mobile-to-mobile channels, which would also pose a security threat and impose a Doppler-dependent pilot overhead. Finally, it would be prone to a channel-estimation-induced performance degradation, which might be close to the 3 dB non-coherent performance penalty. Loosely Synchronized (LS) CDMA spreading codes were adopted for the asynchronous CDMA uplink for the sake of achieving a near-single-user performance using a low-complexity single-user matched-filter detector. The potential performance degradation of the noncoherent receiver experienced in fast fading channels is mitigated by the proposed Multiple-Symbol Differential Sphere Decoding (MSDSD).