A constrained approach to pre-compensation for TDD OFDM systems

Pre-compensation techniques can be used to enhance the bit error rate (BER) performance of time division duplex (TDD) orthogonal frequency division multiplexing (OFDM) communication links. With pre-compensation at the transmitter, the power assigned to each subcarrier is varied according to the channel state information (CSI). This paper derives a pre-compensation algorithm for OFDM systems that use coherent quadrature phase shift keying (QPSK) modulation on each subcarrier. The algorithm attempts to minimize the received BER subject to a constraint on the transmitted signal power. In order to yield a closed-form solution, the minimization is based on an approximation of the BER function, and is sub-optimal. The performance of the algorithm is compared with the fully optimal solution. In addition, the possible exploitation of the technique within a wireless local area network (WLAN) is discussed