A Transmit Beamforming and Nulling Approach with Distributed Scheduling to Improve Cell Edge Throughput

We propose a transmit scheme for WiMAX systems, where multiple base stations (BSs) employ downlink transmit beamforming and nulling for interference mitigation, with minimal coordination amongst BSs. This scheme improves system throughput and robustness, by increasing cell edge and overall cell throughputs by 68% and 19%, respectively, and by delivering improvement for mobile speed up to 60？km/h. First, cell edge users suffering from severe interferences are identified. Next, the RRM unit allocates resource to serving cell edge users only. BSs will schedule to serve their cell edge users independently using the allocated resources by the RRM. A special uplink sounding region is designed for BSs to learn the interference environment and form proper beams and nulls. The nulls formed towards users served by other BSs reduced interference from a BS towards these users and is the basic building block of our algorithm. 1. Introduction In a cellular network with frequency reuse one, downlink (DL) performance is limited by cochannel interference. In the downlink of a cellular system, it is well known that BS with multiple transmit antennas can improve the desired signal power by transmit beamforming [1–3]. However, the performance gain generated by the nulls from the multiple antennas is much less studied. In Figure 1, BS helps BS by forming a null towards MS and BS helps BS by forming a null towards MS . Hence, the SINR at MS and MS is increased by higher signal power from the beam of its own serving BS and reduced interference from the null of a nearby interfering BS. This is referred to as beamforming and nulling (BFaN) from hereon. All BSs with cell edge MSs must enable BFaN simultaneously in order to achieve throughput improvement for all cell edge MSs. For example, if BS enables BFaN while BS does not, only MS benefits from the reduced interference. Note that MS denotes the cell edge MS that is currently being scheduled to be served by BS . Hence, in the next frame, another cell edge MS may be scheduled and MS will refer to a different cell edge MS. However, we keep the same MS index to simplify our notations. In addition, BFaN at the BS is attractive since it moves the implementation and computation complexity from MSs (which is more cost sensitive and power limited) to BSs. Figure 1: A simple 2 cell deployment showing BFaN. This paper studies the benefits of BFaN and has made two key contributions. First, a simple and effective BFaN scheme is proposed for cellular systems with multiple BS antennas. Unlike conventional downlink beamforming with nulling