Joint TDD Backhaul and Access Optimization in Dense Small-Cell Networks

This paper addresses the problem of joint backhaul (BH) and access link optimization in dense small-cell networks with a special focus on time-division duplexing (TDD) mode of operation in BH and access link transmission. Here, we propose a framework for joint radio resource management, where we systematically decompose the problem in BH and access links. To simplify the analysis, the procedure is tackled in two stages. At the first stage, the joint optimization problem is formulated for a point-to-point scenario where each small cell is simply associated with a single user. It is shown that the optimization can be decomposed into separate power and subchannel allocation in both BH and access links, where a set of rate-balancing parameters in conjunction with duration of transmission governs the coupling across both links. Moreover, a novel algorithm is proposed based on grouping the cells to achieve rate balancing in different small cells. Next, in the second stage, the problem is generalized for multiaccess small cells. Here, each small cell is associated with multiple users to provide the service. The optimization is similarly decomposed into separate subchannel and power allocation by employing auxiliary slicing variables. It is shown that similar algorithms, as in the previous stage, are applicable by a slight change with the aid of slicing variables. Additionally, for the special case of line-of-sight BH links, simplified expressions for subchannel and power allocation are presented. The developed concepts are evaluated by extensive simulations in different case studies from full orthogonalization to dynamic clustering and full reuse in the downlink, and it is shown that the proposed framework provides significant improvement over the benchmark cases.