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Hybrid automatic repeat request (HARQ) plays an important role in improving the transmission efficiency and the robustness of wireless networks. Considering K-tier heterogeneous networks (HetNets) and modelling the locations of the base stations (BSs) as a homogeneous Poisson point process (PPP), this paper investigates the performance of HetNets implementing HARQ. We give closed-form expressions for the quality of service (QoS) coverage probability which is defined in terms of whether the received signal quality is above a predetermined threshold, and the per-user throughput with HARQ. We show that using HARQ can indeed improve the QoS coverage probability. However, depending on the channel conditions, the per-user throughput of the HetNets may decrease by the implementation of HARQ. Furthermore, we show that the small cell density has negligible effect on the QoS coverage probability and the peruser throughput, and the per-user throughput may increase with the small cell path loss.

Clustering is a promising interference mitigation scheme in dynamic TDD systems. However, most previous works just took large-scale path loss or coupling loss as criteria of the clustering schemes, thus the throughput performance would be limited by the varying traffic requirements among different small cells within one cluster. In this paper, a novel dynamic clusterbased Interference Mitigation and Traffic Adaptation (IMTA) scheme is proposed and evaluated with dense deployment of small cells (SCs). Firstly, a new clustering criterion named Differentiating Metric (DM) is defined. Based on the defined DM value, a DM matrix is formed and further presented by a clustering graph. In the clustering graph, the dynamic clustering strategy is mapped to a MAX K-CUT problem, which is addressed in polynomial time by a proposed heuristic clustering algorithm. Furthermore, the system level simulation results demonstrate a promising improvement on uplink traffic throughput (UTP) in our proposed scheme compared with traditional clustering schemes.

In LTE-Advanced (LTE-A) and beyond networks, deploying complementary small cells on an existing macro layer is recognized as an attractive solution to improve the network capacity and provide seamless broadband services in local areas. However, existing cell discovery mechanism is tailored for homogeneous networks (macro only). User Equipment (UE) can't energy-efficiently detect the small cells on a dedicated carrier or maximally exploit the offloading opportunities provided by such heterogeneous deployments. In this paper, we propose a Predictive Connection time based Inter-frequency Measurement (PCIM) solution to cope with the problems. With the aid of the positioning results, the small cell connection time is derived, both user velocity and moving direction are taken into account. Using 3rd Generation Partnership Project (3GPP) LTE-A Heterogeneous Network (HetNet) mobility evaluation methodology, the proposed PCIM scheme is compared with some currently standardized techniques. Simulation results provide insights on the small cell discovery schemes in terms of energy efficiency and small cell usage efficiency, and demonstrate the benefits of the proposed PCIM scheme over the other alternatives.