Comparison of generalized Tomlinson-Harashima precoding strategies for the broadcast channel

We study Tomlinson-Harashima precoding (THP) for the broadcast scenario, i.e., transmission from a central base station to a number of scattered, non-cooperating mobile stations. In the linear description of THP, given the data symbol, a suited signal representation is selected from an effective signal constellation. In this paper, we assess how to choose the effective signal constellation. It is shown that conventional THP is optimal with respect to minimizing transmit power under the constraint of preserving the minimum distance of the initial signal constellation, but suffers from an increased number of nearest neighbors. Especially when using 4QAM signaling (the worst case constellation in THP) an alternating periodic extension is attractive as it enables a tradeoff between an increased transmit power and the number of nearest neighbors.