Downlink Steered Space-Time Spreading For Multi-Carrier Transmission Over Frequency Selective Channels

This paper presents a novel amalgam of steered space-time spreading (SSTS) and orthogonal frequency division multiple access (OFDMA) designed for attaining both spatial diversity gain and beamforming gain for transmission over OFDM-symbol-invariant frequency selective channels. We propose a flexible technique for increasing the number of users beyond the number of chips in the spreading sequence employed by the SSTS scheme with the aid of the multiple carriers of OFDMA, which requires an extended bandwidth. However, employing a separate low-complexity SSTS detector combined with another separate OFDMA detector is potentially less complex than a single detector designed for detecting all the users supported in a single domain, regardless whether the single- domain Multi-User Detector (MUD) is an SSTS or OFDMA MUD. This is because the MUD's complexity tends to increase exponentially with the number of users detected. The SSTS-OFDMA system is benchmarked against its counterpart using S-depth Frequency Domain (FD) repetition, which increases the FD diversity order at the cost of reducing the overall throughput by a factor of S. Fortunately, a similar FD diversity gain may be achieved without a factor-S reduction in the throughput, when using FD spreading and assigning all the superimposed FD spreading codes to the same user.