Performance issues in two-tier cellular networks

A two-tier cellular network consists of a number of small microcells overlayed by larger macro (umbrella) cells. Such a network can support improved quality of service (QoS), higher capacity, and larger coverage area than existing networks. In this work, we study the channel allocation problem for a two-tier cellular network for two types of calls (voice and data), under different degrees of handoff overheads. We also compare the impact of two typical call admission algorithms on network performance. We observe that, when minimal handoff overheads are present, allocating more channels to the microcell layer results in lower blocking and dropout probabilities. Second, there is an optimal orthogonal assignment of channels between the two tiers that minimizes the blocking and the dropout probabilities, when handoff overheads are significant, suggesting a hierarchical approach for designing handoff controllers. Finally, on comparing two typical call admission algorithms, we show that an algorithm that treats all calls identically performs better than an algorithm which tries to assign calls to different tiers based on their holding times.