Pre-DFT processing for coded OFDM systems with receive space diversity

Coded orthogonal division multiplexing (OFDM) is an enabling technology for broadband wireless communications. Multiple receive antennas can be employed in OFDM systems to achieve space diversity and thus significantly improve system performance. Conventionally, sub-carrier based space combining is used in such systems and the complexity can be huge because multiple DFT blocks, each for one receive antenna, are needed. In this paper, a pre-DFT processing technique based upon eigen-analysis, which can be employed in coded OFDM systems to effectively trade off system performance and complexity, is presented. For coherent modulation and without performance loss compared with the conventional scheme, it is shown that the number of DFT blocks required is the minimum of the number of receive antennas and the number of distinct paths in the channel. For differential modulation and with a smaller number of DFT blocks, simulation results show that the performance can even be better than that of the conventional scheme.