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Spatial diversity techniques are now used in most wireless communications systems to improve robustness to signal fading. On the downlink of cellular systems, low-cost requirements of the user terminal tend to favor transmit diversity over receive diversity, and Alamouti's transmit diversity technique appears today in most wireless communications systems standards. But in terms of the total transmit power, Alamouti's technique loses 3 dB with respect to optimum transmit diversity which requires perfect Channel State Information (CSI) at the transmitter. In this paper, we introduce a simple transmit diversity technique that relies on simple power measurements at the receiver which comes very close to optimum transmit diversity performance when assuming perfect CSI is available at the transmitter while relaxing the transmit power requirements by 3 dB. In this technique, the same power is transmitted from the two antennas and the phase difference of the signals received from the two channels is constrained to be smaller than π/N, where N is a predefined parameter. Its performance is studied in narrowband fading channels, in OFDM systems operating on frequency-selective channels, and in the presence of channel estimation errors.
Date of Publication: January 2011