A Generalized Zero-Forcing Precoder with Successive Dirty-Paper Coding in MISO Broadcast Channels

In this paper, we consider precoder designs for multiuser multiple-input-single-output (MISO) broadcasting channels. Instead of using a traditional linear zero-forcing (ZF) precoder, we propose a generalized ZF (GZF) precoder in conjunction with successive dirty-paper coding (DPC) for datatransmissions, namely, the GZF-DP precoder, where the suffix ‘DP’ stands for ‘dirty-paper’. The GZF-DP precoder is designed to generate a band-shaped and lower-triangular effective channel F such that only the entries along the main diagonal and the v first lower-diagonals can take non-zero values. Utilizing the successive DPC, the known non-causal inter-user interferences from the other (up to) users are canceled through successive encoding. We analyze optimal GZF-DP precoder designs both for sum-rate and minimum user-rate maximizations. Utilizing Lagrange multipliers, the optimal precoders for both cases are solved in closed-forms in relation to optimal power allocations. For the sum-rate maximization, the optimal power allocation can be found through water-filling, but with modified water-levels depending on the parameter . While for the minimum user-rate maximization that measures the quality of the service (QoS), the optimal power allocation is directly solved in closed-form which also depends on v. Moreover, we propose two low-complexity user-ordering algorithms for the GZF-DP precoderdesigns for both maximizations, respectively. We show through numerical results that, the proposed GZF-DP precoder with a smallv (<=3) renders significant rate increments compared to the previous precoder designs such as the linear ZF and user-grouping based DPC (UG-DP) precoders.