The very short transmission range for high data rate Ultra-Wideband (UWB) communication systems is an important factor that limits its potential applications. In the case when using Maximum Ratio Combiner (MRC) at receiver is expensive or impractical, gaining transmit diversity becomes critical. Transmitter beamforming without lowering its transmission power is no longer applicable to UWB due to the regulatory transmit Power Spectral Density (PSD) constraint by FCC. Utilizing Space-Time Coding (STC) will increase complexity and result in incompatibility to communicate with standard WiMedia devices. In this paper, a Space-Frequency Transmit Selection (SFTS) scheme is proposed for MB-OFDM UWB systems with multiple transmit antennas. It selects the best antenna for each subcarrier according to the calibrated equivalent channel response estimated from the reverse link. Further, RF mismatch calibration is incorporated into the transmit antenna selection process in consideration of the practical application scenario. The proposed technique is able to achieve the same transmit diversity as the optimal beamforming while retaining the total transmitting PSD under FCC mask. It is shown to be a promising technique due to its implementation simplicity and interoperability with current standard WiMedia devices.
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