Department of Electrical and Computer Engineering
M.S. Thesis Abstract
RESOURCE MANAGEMENT FOR DOWNLINK WIRELESS SYSTEMS
M. Kemal Karakayali
In this thesis, we investigate radio resource management for the downlink of multimedia
wireless networks. Our target is to develop simple practical algorithms utilizing
scarce and expensive radio resources in the most efficient way, while meeting quality of
service (QoS) requirements of various service classes.
In the first part of the thesis, we investigate optimum rate scheduling on the downlink
of a multirate CDMA wireless network. Systems employing orthogonal variable
spreading factor codes as well as systems using multiple codes have been studied. Our objective is to maximize the network throughput under constraints on total transmit power, total bandwidth and individual QoS requirements in terms of minimum rates. First, users are ordered based on their transmit energy per bit requirements to achieve
the target received energy per bit to interference power spectral density ratio at the receivers. Based on the initial ordering, we prove that for systems employing multiple codes, the greedy rate scheduling is optimal, and therefore it yields maximum network throughput. For systems employing OVSF codes, the greedy rate scheduling is optimal only if the minimum rate requirement of a user is larger than or equal to the minimum rate requirement of any other user with a larger transmit energy per bit requirement. Simulation results show that the greedy algorithm, even when it is suboptimal, is a good heuristic yielding average throughput which is very close to the optimal achievable throughput in OVSF-CDMA systems. In the second part of the thesis, we investigate joint power control and orthogonal code selection (rate control) in frequency selective multipath channels. We show that the standard power control framework can be extended to include a form of rate control as well. Using this framework, we prove that a joint power and rate control algorithm converges to optimum assignments of multiaccess resources (time slots for TDMA, spreading codes for CDMA, subcarriers for OFDM etc.) to users, and to optimum transmit power levels, such that the total transmit power is minimized while each transmitted bit can be decoded with sufficient SINR. Specifically, for a CDMA wireless network, we observe that the optimal solution is achieved when each user selects those spreading codes from the Walsh set whose frequency domain responses match to the channel response of the user. Finally, we show how to apply combinatorial network flow models in wireless resource management problems. Network flows are a well-known subject with many applications in various fields of computer science, engineering, management and operations research. The minimum cost flow problem, a fundamental problem of network flows, deals with determining a least cost shipment of a commodity through a network in
order to satisfy demands at certain nodes from available supplies at other nodes. Here, we model the communication through a wireless network as a network flow, and we aim to minimize the cost of information flow through the network under constraints on demands (minimum rate) of mobiles and supply (bandwidth) of the base station. This model helps us to deal with practical discrete system constraints, and to improve fairness by enforcing minimum rate constraints on each mobile.
Thesis Director: Professor Roy Yates