Sphere Decoder for a MIMO Multi-User MC-CDMA Uplink in Time-Varying Channels

We focus on iterative detection for the uplink of a multi-carrier (MC) code division multiple access (CDMA) system based on orthogonal frequency division multiplexing (OFDM). The multiple-input multiple-output (MIMO) channel between each of the K users and the receiver is time-varying. At the receiver soft-symbol feedback is utilized for iterative linear minimum mean square error (LMMSE) channel estimation and multi-user detection after parallel interference cancelation. For multi-user detection we compare sphere decoding with an LMMSE detector. We show that a sphere decoder is more robust to channel estimation errors and less complex than an LMMSE detector while achieving better performance in terms of bit error rate versus signal to noise ratio.