Robust coordinated transmission for cooperative small cell networks

What is it about?

Within a macrocell with a large coverage area, multiple small cells are deployed such that each small cell base station (SBS) supports wireless service demands from user equipments (UEs). Each UE can be simultaneously served by multiple SBSs for quality of service (QoS) enhancement. When there exist hotspot areas with a number of UEs, the SBSs near the hotspot areas may experience a higher resource utilisation level than those outside of the hotspot areas, resulting in a shortage of available resources. The authors propose a robust resource-utilisation-based coordinated transmission for heterogeneous networks with a locally different level of traffic demands. In the utilisation-based coordinated transmission, low-utilisation SBSs with a small number of UEs are selected to serve a newly joining UE because they have more capacity to serve requests with bursty traffic demand. They further formulate the selection of cooperative SBSs as a robust optimisation problem in order to ensure that UEs have sufficiently high signal-to-interference-plus-noise ratios, even with channel estimation inaccuracy and strong interference from non-cooperative SBSs. The simulation results indicate that the proposed method guarantees robust and efficient service performance in heterogeneous small cell networks.

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Why is it important?

* We consider resource utilisation as a new performance metric for user scheduling in the optimisation of coordinate transmission. In previous studies, conventional coordinated transmission schemes allocated a set or cluster of coordinated transmission SBSs based on SINR performance, energy efficiency, and control overhead. To the best of our knowledge, this paper is the first study that exploits resource utilisation to overcome service performance degradation owing to overloaded SBSs in coordinated transmission scenarios, in which there exist several hotspot areas with high service demand. The proposed coordinated transmission can achieve a significant increase in the number of UEs serviced by the network because the SBSs with more available resources are cooperatively involved in the coordinated transmission. * In the case of incorrect channel estimation, we also derive robust resource allocation to avoid violating the minimum SINR even in the worst case. When we obtain the CSI for a coordinated transmission, the channel estimate can be inaccurate owing to interference from non-cooperative cells. In this case, UEs may fail to have high SINRs, which they are expected to obtain through coordinated transmission from cooperating SBSs. In this paper, a robust optimisation is considered for coordinated transmission in order to compensate for inaccurate channel estimates and to mitigate interference from non-cooperative cells. * We exploit user-centric clustering-based coordinated transmission to achieve a high level of coordination diversity and to enhance network throughput. The cooperating SBSs are determined based on possible clustering sets for each UE. We derive the outage probability for a user, and show that the proposed method provides robust and efficient service performance even with inaccurate CSI.

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