Efficient use of limited spectrum is emerging as a major issue in wireless systems. The proposed work focuses on technologies that provide better understanding of wireless networks and greatly enhance spectrum efficiency. with three coupled thrusts: (i) wireless network modeling; (ii) mechanisms for efficient spectrum sharing; and (iii) exploiting enhanced spectrum efficiency for wireless video communications. The project also includes an experimental research component in which the developed approaches will be implemented and tested on the cognitive radio testbed hosted at Virginia Tech and the cooperative networking testbed hosted at NYU-Poly.
The proposed research plans to systematically investigate several of the unique technical challenges and open problems in enhancing radio spectrum efficiency, and supporting emerging video services. This fundamental research will support the development of technologies that achieve new levels of efficiency and quality in wireless broadband services, and will help alleviate the wireless bandwidth limits now being experienced. The work is supported by the Industry Advisory Board as well as individual industry members of the center and has the potential to extend the centers portfolio. The PIs plan to disseminate the work to their industry members and the broader industry and academic community via open-source software as wel as introduce the content within their degree and outreach programs.
In the timeframe of five to seven years, cellular networks will face a shortage of spectrum. This situation is most likely to play out in densely populated urban environments. Spectrum sharing appears therefore as a sensible approach to cope with this limitation. Networks providing mobile telephony have traditionally relied on centralized approaches for design and operation. This was consistent with the current regime of exclusive licensing agreements for spectrum exploitation. However, in a future scenario of spectrum sharing, one can envision a situation in which resources from heterogeneous networks are opportunistically allocated and used in order to make the most of the available spectrum. In this first part of our work, we consider the downlink coordinated "beam-forming" problem in a cellular network. "Beam-forming" is a signal processing technique in which signals are transmitted (and/or received) at different angles in order to enable constructive interference. In our work, we assume the base stations are equipped with multiple antennas and each user is equipped with a single antenna. We have developed a decentralized interference pricing mechanism to identify an optimal beam form. The key idea is that each base station is penalized according to the interference it creates to its peers. Hence information sharing is only local. Numerical results show that the proposed iterative mechanism achieves high system throughput with reduced backhaul information exchange among the base stations. Important resource allocation problems in wireless networks can be modeled as graph coloring problems (e.g. channel allocation in frequency division multiplexing, time-slot allocation in time division multiplexing). However, this modeling approach fails to account for heterogeneity in user’s priority and/or the quality of resources. In other words, not all "colors" and "nodes" in the graph are alike. In the second part of our work, we consider the problem of optimizing over graph colorings by introducing an aggregate measure of efficiency. We have developed a distributed algorithm that is shown to converge to an approximately optimal graph coloring. Simulation results show that the algorithm is bound to converge to an approximately optimal solution in a relatively fast fashion.
