A General Pre-FFT Criterion for MIMO-OFDM Beamforming

• Published in: 2010 IEEE International Conference on Communications pp. 1 - 5
• Main Authors: Vía, Javier, Santamaría, Ignacio, Elvira, Victor, Eickhoff, Ralf
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01.05.2010
• Subjects: Array signal processing; Bit error rate; Circuit synthesis; Communications Society; Computational efficiency; Cost function; Design engineering; MIMO; OFDM; Receiving antennas
• ISBN: 1424464021; 9781424464029
• ISSN: 1550-3607; 1938-1883
• DOI: 10.1109/ICC.2010.5502040

In this paper, we propose a general beamforming criterion for pre-FFT processing in orthogonal frequency division multiplexing (OFDM) systems with multiple transmit and receive antennas. The proposed criterion depends on a single parameter α, which establishes a tradeoff between the energy of the equivalent SISO channel (after Tx-Rx beamforming) and its spectral flatness. The proposed cost function embraces most reasonable criteria for designing Tx-Rx pre-FFT beamformers. Hence, for particular values of α the proposed criterion reduces to the minimization of the mean square error (MSE), the maximization of the system capacity, or the maximization of the received signal-to-noise ratio (SNR). In general, the proposed criterion results in a non convex optimization problem. However, we show that the problem can be approximately solved by semidefinite relaxation (SDR) techniques. Additionally, since the computational cost of SDR for this problem is rather high, we propose a simple yet efficient gradient search algorithm which provides satisfactory solutions with a moderate computational cost for OFDM-based WLAN standards such as 802.11a. Finally, the good performance of the proposed technique is illustrated by means of some numerical results.