Transmit Gain Optimization for Space Time Block Coding Wireless Systems with Co-channel Interference

The combination of Orthogonal Frequency Division Multiplexing and space-time block coding is a promising technique for wireless broadband transmission. In a scenario where other devices generate interference, we propose a scheme where the transmit gains of each OFDM subchannel are adaptively chosen. As a design criteria we consider both the minimization of the interference and maximization of the signal to interference plus noise ratio at the detection point. As a particular case we consider also the situation of varying only the amplitude or the phase of the gains. Indeed, it turns out that when interference is present, an important role is played by the phase of the transmit gains, and for the case of two transmit antennas we derive the optimum phase of the transmit gains, under the assumption of equal amplitudes. As performance measure we used the achievable bit rate of the various solutions for a broadband indoor system denoted Windflex (European Project). Performance was compared also with the system capacity obtained by a novel close form expression.