On Usefulness of Multipath Diversity for Multiple Antenna OFDM Systems

摘要

Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) technology hold much promise in realizing broadband wireless systems which are both power efficient and bandwidth efficient. This review article focuses on MIMO-OFDM wireless systems, and the usefulness of multipath diversity in such systems are addressed. With increasing data rate requirements for wireless access, the technology challenges of the physical layer designers are moving towards wideband wireless communications. One of the key issue to be confronted in such systems is the frequency selective fading of the wireless channel due to multipath propagation. Orthogonal frequency division multiplexing helps in transforming this frequency selective channel into multiple narrowband flat sub-channels which facilitate a computationally efficient equalization. Usually multipath delay spread is considered as an annoyance factor. However, such channels are rich in multipath diversity, which could potentially be exploited to yield performance benefits. Using simple frequency domain processing in OFDM systems with closely spaced antennas, this multipath diversity could be exploited to achieve performance close to that of space diversity systems using well separated antennas. With such closely spaced antennas, it appears as though there is sampling in the spatial domain. Linear dispersion codes are capacity achieving space-time codes designed by optimizing the information-theoretic capacity function. Multipath diversity available as frequency diversity could be exploited by linear dispersion codes when the dispersion is over OFDM sub-channels in frequency and space. In this article, we also review such space-frequency linear dispersion codes which are particularly useful at low rate and for low complexity systems.