Joint optimization of user association and dynamic TDD for ultra-dense networks

Ultra-dense small cell networks will require sophisticated user association algorithms that consider (i) channel characteristics, (ii) base station load, and (iii) uplink/downlink

(UL/DL) traffic profiles. They will also be characterized by high spatio-temporal variability in UL/DL traffic demand, due to the fewer users per BS. In this direction, Dynamic TDD is a promising new technique to match BS resources to actual demand. While plenty of literature exists on the problem of user association, and some recent on dynamic TDD, most works consider these separately. In this paper, we argue that user association policies are strongly coupled with the allocation of resources between UL

and DL. We propose an algorithm that decomposes the problem into separate subproblems that can each be solved efficiently and in a distributed manner, and prove convergence to the global optimum. Simulation results suggest that our approach can improve UL and DL performance at the same time, with an aggregate improvement of more than 2, compared to user association under static TDD allocation.