Multiple transmission point (TP) coordination is an effective tool for managing interference in heterogeneous networks. Using network-level MIMO coordination among TPs, multiple TPs can transmit independent data streams to a single user. We formulate our scheduling problem as a utility optimization problem over the set of user-TP pairs in each subframe, in which each user can be served by multiple TPs. We first prove that this optimization problem is NP-hard. Then, we propose a new scheme based on the difference-of-submodular-function optimization approach. We evaluate our scheme using detailed simulation and demonstrate that it performs on par with a more computationally demanding successive convex approximation optimization scheme. Moreover, the proposed scheme performs within a reasonable percentage of the optimal solution (Table IV). We also study its performance with varying network topology and parameters.

Deployment of low power pico basestations within cellular networks can potentially increase both capacity and coverage. However, such deployments require efficient frequency allocation schemes for managing interference from the pico and macro basestations that are located within each other's transmission range. Partitioning the available frequencies between the various basestations avoids the problem of interference, but can lead to inefficient spectrum usage. In this paper, we introduce a distributed frequency allocation scheme that shares frequencies between macro and pico basestations, and guarantees a minimum average throughput to users. The scheme seeks to minimize the total number of frequencies needed to honor the minimum throughput requirements. We evaluate our scheme using detailed simulations and show that it performs on par with the centralized optimum allocation. Moreover, our proposed scheme outperforms a static frequency reuse scheme and the centralized optimal partitioning between the macro and pico basestations.

We consider the problem of maximizing the alphafairness utility over the downlink of a heterogeneous wireless network (HetNet) by jointly optimizing the association of users to transmission points (TPs) and the activation fractions of all TPs. Activation fraction of each TP is the fraction of the frame duration for which it is active, and together these fractions influence the interference seen in the network. To address this joint optimization problem we adopt an approach wherein the activation fractions and the user associations are optimized in an alternating manner. The sub-problem of determining the optimal activation fractions is solved using an auxiliary function method that we show is provably convergent and is amenable to distributed implementation. On the other hand, the sub-problem of determining the user association is solved via a simple combinatorial algorithm. Meaningful performance guarantees are derived and a distributed variant offering identical guarantees is also proposed. The significant benefits of using the proposed algorithms are then demonstrated via realistic simulations.