In heterogeneous cellular scenarios with macrocells, femtocells or picocells users may suffer from significant co-channel cross-tier interference. To manage this interference 3GPP introduced almost blank subframe (ABSF), a subframe in which the interferer tier is not allowed to transmit data. Vulnerable users thus get a chance to be scheduled in ABSFs with reduced cross-tier interference. We analyze downlink scenarios using stochastic geometry and formulate a condition for the required number of ABSFs based on base station placement statistics and user throughput requirement. The result is a semi-analytical formula that serves as a good initial estimate and offers an easy way to analyze impact of network parameters. We show that while in macro/femto scenario the residue ABSF interference can be well managed, in macro/pico scenario it affects the number of required ABSFs strongly. The effect of ABSFs is subsequently demonstrated via user throughput simulations. Especially in the macro/pico scenario, we find that using ABSFs is advantageous for the system since victim users no longer suffer from poor performance for the price of relatively small drop in higher throughput percentiles.
Index TermsAlmost blank subframe, interference management, heterogeneous network, HetNet;
We develop and analyze a time-domain bi-level power control scheme using Almost Blank Subframe (ABS) for downlink cross-tier interference mitigation. Even for only one femto cell and macro cell, the optimization problem given rate constraint is nonconvex. We prove that by relaxing the integer requirement for subframe division, the optimal strategy is using either power control or ABS individually, and a joint scheme is not needed in the optimal solution. Imposing the integer subframe number requirement, this conclusion is no longer true. However, we show that the optimal solution also assumes simple structure by choosing from one of three feasible solutions. Simulation results show that the joint power control and ABS scheme outperforms traditional single-level power control scheme for downlink cross-tier interference mitigation with low computational complexity.
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