Non-linear pre-coding for MIMO multi-user downlink transmissions with different QoS requirements

An efficient non-linear pre-filtering technique based on Tomlinson-Harashima pre-coding (THP) has recently been proposed by Liu and Krzymien for multiple antenna multi-user systems. The algorithm is based on the Zero-Forcing (ZF) criterion and assumes a number of transmit antennas equals to the number of active users. In contrast to other methods, it ensures a fair treatment of the active users providing them the same signal-to-noise ratio. In multimedia applications, however, several types of information with different quality-of-service (QoS) must be supported. Motivated by the above problem, in the present work we design a ZF THP-based pre-filtering algorithm for multiple antenna multi-user networks in which the base station allocates the transmit power according to the QoS requirement of each active user. In doing so, we consider a system in which the number of active users may be less than the number of transmit antennas. As we will see, in such a case there exists an infinite number of solutions satisfying the ZF criterion. We address the problem of finding the best using as optimality criterion the maximization of the signal-to-noise ratios at all mobile terminals.