Interference Aware Resource Allocation in Relay Enhanced Broadband Wireless Access Networks

Introduction

The communications landscape has been changing dramatically in recent years under the increasing pressure of rapid technological development and intense competition. Thus, wireless networks are becoming more pervasive, accelerated by new wireless communication technologies, inexpensive wireless equipment and broader Internet access availability. Broadband wireless access (BWA) networks are one such technology that is fast becoming a viable solution to provide ubiquitous communications.

BWA networks are designed to support fixed and mobile users with heterogeneous and high traffic rate requirements. In such networks, a single base station is deployed to cover a cellular area. In such a large area, users at the cell edge often experience bad channel conditions. Moreover, in urban regions, shadowing by various obstacles can degrade the signal quality in some areas. Emerging broadband wireless applications require increasingly high throughput and more stringent quality of service (QoS) requirements. As real-time applications (i.e., voice over IP and video streaming) rapidly grow, BWA networks are expected to achieve efficient communications. Increasing capacity along with coverage in conventional networks dictates the dense deployment of base stations. Increasing the number of base stations is an expensive solution and increasing the base station power only increases the intercell interference. To meet the goal of low cost network deployment for both short range and long range coverage, the use of relay nodes has been shown to be a promising solution (Pabst et al., 2004; Soldani & Dixit, 2008). Broadband cellular multihop networks consist of fixed infrastructure relay nodes whose sole priority is to forward data to and from the users to the base station. Deploying relays is a feasible solution since typical relays are cheaper than base stations and they do not need their own wired backhaul.

The use of relays to improve the performance of BWA networks has been the subject of intense research in recent years because of their several performance benefits. First, a relay works on behalf of the base station to increase the network coverage. While conventional cellular systems normally cover a diameter of 2-5km, a relay normally covers a region (subcell) with diameter 200-500m. If the density of relay stations is somewhat high, most user-terminals will be close to one or more relays than to a base station. This has two primary advantages: the radio propagation paths are shortened so that the pathloss is lowered, and the path essentially can be routed around obstacles to mitigate effects of shadowing (Pabst et al., 2004). This results in higher data rates on the links between relays and users, thereby increasing throughput. Also, from the point of view of the user, the relay acts like a base station and so by having intermediate points of traffic aggregation, the capacity per area element can be balanced (Walke, Mangold, & Berlemann, 2006). Second, because relay stations are closer to the individual user terminals, the transmit power required for a relay to transmit to a user and vice versa is significantly lower than for a base station, thereby allowing for energy saving. Thus, the practical rationale for the deployment of relay enhanced BWA networks is to ensure that the QoS of a user, in terms of data rate, delay, outage probability, etc. does not wholly depend on its location and distance from the base station.