Optimal Downlink and Uplink Fractional Frequency Reuse in Cellular Wireless Networks

Densely deployed cellular wireless networks, which employ small cell technology, are being widely implemented. Mitigating the impact of inter- and intracell signal interferences induced by the operations of these networks is a challenging yet essential task. In this paper, we consider adaptive rate scheduling for a transmitting node, regardless of whether it is a base station (BS) or a mobile user. We aim to maximize the system's throughput through the employment of fractional frequency reuse (FFR) schemes. Each BS employs either an omnidirectional or a directional antenna system. We derive the optimal configuration of the FFR scheme and evaluate the ensuing system's performance behavior under absolute and proportional fairness requirements. To maximize the attained throughput by mobiles, we determine the best method to use to classify cell users into interior and edge mobiles. The bandwidth levels allocated for serving interior and edge mobiles are optimized. We derive approximate closed-form mathematical expressions for calculating the probability distributions of the interference signal levels measured at the destined receivers. We account for the impact of the classification process on intercell interference power levels. Under an absolute fairness requirement, we show that optimally configured FFR schemes lead to much enhanced performance. We show that the optimally configured directional-FFR schemes increase the throughput capacity by a factor of about 60% relative to that obtained by using optimal omnidirectional-FFR schemes. The analyses and simulation results presented in this paper serve to characterize the performance behavior attainable by using such small cell deployed cellular wireless network systems when employing the underlying configurations.

I. Introduction

Efficient scheduling and resource allocation for packet transmissions over dense wireless cellular network systems, using small cell deployments, are tasks of primary importance. Fractional frequency reuse (FFR) schemes (see [1]) serve as effective methods for mitigating the impact of intercell interference signals on the reception of messages targeted to and originating by mobiles that reside near the edge of a cell. FFR schemes are readily implementable, requiring no base station (BS) or mobile modifications, in that they employ control procedures that are embedded with current cellular system standards based implementations. Such operations have been made part of the intercell interference coordination scheme that has been introduced in Third-Generation Partnership Project (3GPP) release 8 [2]. Under an FFR scheme, each cell is partitioned into two zones, identified as interior and exterior regions. Mobiles located in an interior (or exterior) region are scheduled by using a reuse-1 (or reuse-3) scheme. For a dynamic FFR scheme, FFR parameters are controlled dynamically by monitoring the cell's channel quality and traffic loading conditions [3]– [5]. In turn, under a static FFR scheme, system parameters are adjusted in accordance with longer term statistics collected by a system's management station [6].