WiMAX (Worldwide Interoperability for Microwave Access) is a standard-based technology that, for the first deployments, is expected to provide fixed last mile broadband wireless access, intended as a cost-effective alternative to existing wired technologies such as cable and Digital Subscriber Line (DSL). In this research project we considered WiMAX multi-cell systems conforming to the IEEE 802.16-2004 standard, which prescribes the use of OFDM modulation and supports multiple antennas and adaptive modulation/coding. In the uplink of these systems, a major source of impairment is the degradation of the signal-to-interference-noise ratio (SINR) generated by multipath fading and out-of-cell interference (due to frequency reuse). The use of multiple antennas at the base station (BS) is known to reduce such effects by providing either diversity or beamforming gains, depending on the inter-element spacing of the array. The optimal spacing is the one providing the best trade-off between diversity gain and interference rejection capability, depending on the specific propagation environment.
We considered the optimization of the array deployment, formulated as the maximization of the average (or outage) system performance. Assuming the optimal array deployment at the BS, we evaluated the average throughput for the overall cell and the corresponding gain with respect to conventional (non-optimized) array receivers. Furthermore, we developed an adaptive technique for interference mitigation based on Minimum Variance Distortionless Response (MVDR) beamforming. This method is designed to cope with time-varying interference due to the asynchronous access of users in the neighboring cells.
The research activity is now focused on the IEEE 802.16e standard for mobile subscriber stations.