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 singledomain 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 SSTSOFDMA 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

Asymptotic Performance of Transmit Diversity via OFDM for Multipath Channels

Conference paperMany wireless systems exploit transmit diversity for more reliable detection of signals at the receiver. To accomplish this, coding is spread across multiple transmit antennas. An example of this is the well known "Alamouti transmit diversity", where a very simple coding scheme across multiple transmit antennas allows systems to attain performance similar to systems with multiple receive antennas. The major drawback is that this system only works when a "flat-fading" model for the channel is assumed; when used in a multipath environment, the system breaks down. Here we show that when the Alamouti code is placed within an OFDM structure, using adjacent frequency bands rather than consecutive symbol intervals, it can asymptotically achieve the same performance in multipath fading as the Alamouti code in flat-fading