Department of Electrical and Computer Engineering
Ph.D. Thesis Abstract
NETWORK COORDINATION FOR SPECTRALLY EFFICIENT COMMUNICATIONS IN CELLULAR SYSTEMS
Mehmet Kemal Karakayali
In conventional cellular systems, each base station transmits signals intended for users within its cell coverage. Depending on the users' channel conditions, interference caused by the neighboring cell transmissions can sharply degrade the received signal quality. Thus, the downlink capacity of cellular wireless networks is limited by inter-cell interference. Fortunately, since the base stations can be connected via a high-speed backbone, there is an opportunity to coordinate the base antenna transmissions so as to minimize the inter-cell interference, and hence to increase the downlink system capacity. In this thesis, we study various aspects of network coordination in cellular downlink systems.
In the first part of the study, we describe various coordination techniques, and conduct their performance analysis. The performance of each technique is given in terms of the max-min fair rate achievable subject to per-base power constraints. We compare the performance of coordinated transmissions to that of conventional cellular networks without coordination. It is shown that the coordinated base station transmissions can help to eliminate inter-cell interference, and result in a great capacity improvement on the downlink cellular networks.
In the second part of the study, we consider coordinated networks with multiple antennas. The great advantage of using multiple antennas is that, without increasing power or bandwidth, the capacity of a point-to-point link scales linearly with the minimum of the number of transmit or receive antennas deployed. The gain, in terms of the marginal increase in rate when an additional antenna is deployed, is especially large when the signal-to-noise ratio is high. We show that, without coordination, the link qualities can be very poor because of inter-cell interference. In this case, the network does not benefit significantly from multiple antennas. When the coordination is employed, the inter-cell interference is mitigated so that the links can operate in the high signal-to-noise ratio regime. This enables the cellular network to enjoy the great spectral efficiency improvement associated with using multiple antennas.
In the final part of the study, we investigate optimum linear beam-forming design with per-antenna power constraints. We show that the standard beam-forming techniques used mostly in the sum-power constrained systems are suboptimal when there are per-antenna power constrains. We formulate convex optimization problems finding the optimum zero-forcing and zero-forcing dirty paper coding beam-formers. We observe that optimizing the antenna outputs based on the per-antenna constraints may improve the rate considerably when the number of transmit antennas is much larger the number of receive antennas. The network coordination techniques assume the existence of a high-speed backhaul enabling communications between the base stations. In the last part of the study, we consider the design of such a backhaul in a cellular network. More specifically, we will investigate a mesh architecture providing wireless backhaul support in a cellular system.
Ph.D. Dissertation Directors: Professors Roy D. Yates, Gerard J. Foschini
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