Multiuser diversity is a form of diversity inherent in a wireless network, provided by independent time-varying channels across the different users. The diversity benefit is exploited by tracking the channel fluctuations of the users and scheduling transmissions to users when their instantaneous channel quality is near the peak. The diversity gain increases with the dynamic range of the fluctuations and is thus limited in environments with little scattering and/or slow fading. In such environments, we propose the use of multiple transmit antennas to induce large and fast channel fluctuations so that multiuser diversity can still be exploited. The scheme can be interpreted as opportunistic beamforming and we show that true beamforming gains can be achieved when there are sufficient users, even though very limited channel feedback is needed. Furthermore, in a cellular system, the scheme plays an additional role of opportunistic nulling of the interference created on users of adjacent cells. We discuss the design implications of implementing this scheme in a complete wireless system.

Abstract — The Open Spectrum approach to spectrum access can achieve near-optimal spectrum utilization by letting users sense and utilize available spectrum opportunistically. However, naive spectrum assignment can lead to significant interference. We propose a network controlled spectrum access scheme where users behave collaboratively to optimize spectrum allocation for the entire network. We develop a graph-theoretical model to characterize the spectrum access problem under a number of different optimization functions, and devise rules for users to utilize available spectrum while avoiding interference with its neighbors. Experimental results confirm that user collaboration yields significant benefits (as much as 50 % improvement) in opportunistic spectrum access. I.

Abstract — Current and next-generation wireless networks rely on multi-user diversity and scheduling techniques (such as the commonly used Proportional Fair (PF) algorithm) to achieve greater system throughput and higher efficiencies for wireless data applications over a timevarying wireless channel. In this paper, we show that the variability of the inter-scheduling intervals, as introduced by the PF scheduling algorithm, can have adverse effects on TCP and its congestion control mechanism and lead to spurious timeouts and unnecessarily low throughput. We propose an enhanced scheduling algorithm that is explicitly tuned towards throughput performance at the TCP layer. However this algorithm does not use any explicit information from the TCP layer and solely relies on information readily available at the link layer at which the scheduler resides. The performance of this improved algorithm is assessed through extensive simulations to show an average TCP throughput improvement of 12 % compared to PF. In addition, the TCP-level fairness across all users is increased as is the individual user throughput.

The Open Spectrum approach to spectrum access can achieve near-optimal spectrum utilization by letting users sense and utilize available spectrum opportunistically. However, naive spectrum assignment can lead to significant interference. We propose a network controlled spectrum access scheme where users behave collaboratively to optimize spectrum allocation for the entire network. We develop a graph-theoretical model to characterize the spectrum access problem under a number of different optimization functions, and devise rules for users to utilize available spectrum while avoiding interference with its neighbors. Experimental results confirm that user collaboration yields significant benefits (as much as 50% improvement) in opportunistic spectrum access.