Department of Electrical and Computer Engineering
Ph.D. Thesis Abstract
Radio Resource Management in Power Controlled CDMA Systems
Ching Yao Huang
With an increasing demand for wireless communication services, better radio resource management is needed to improve system performance. Previous work decomposed the resource management problem into separate power control, base station assignment and channel assignment problems. In this thesis, we study resource management problems in power controlled Code Division Multiple Access (CDMA) systems. We propose an integrated approach to power control, base station assignment and call admission.
In a power controlled system, all users adjust their transmitter powers to maintain feasible connections. We formulate the minimum transmitted power (MTP) problem which minimizes the total transmitted power subject to meeting users' link quality requirements, as measured by the carrier to interference ratio (CIR). We find an iterative distributed algorithm, the minimum power assignment algorithm (MPA), that converges to a global optimal solution for any feasible instance of the MTP problem. In the presence of maximum power constraints, the constrained MPA algorithm is shown always to converge to a unique fixed point. When there is no feasible solution, the infeasibility can be detected at the fixed point solution which lies on the boundary of the maximum power set. In this case, some users will transmit at maximum power without achieving the desired CIR. Both MPA and constrained MPA are shown to converge both synchronously as well as totally asynchronously.
The rate of convergence of a generalization of the constrained MPA algorithm is also studied. This generalization includes both a fixed assignment of users to base stations as well as a form of soft handoff assignment. It is shown that iterative constrained power control will converge to a unique fixed point at a geometric rate. This is important for the maintenance of good service under rapid variations in channel quality and user mobility. Simulation results also indicate the rate of convergence of the constrained MPA algorithm is fairly practical compared to current CDMA standard. During the call set up stage, call admission control is required to improve system performance due to limited radio resources. In this work, a weighted combination of call blocking and dropping probabilities is used as a system performance measure. We consider two different approaches to call admission control. The Transmitter Power Call Admission Control (TPCAC) scheme protects existing users by blocking new calls when existing users can not maintain the target CIR. In the Received Power Call Admission Control (RPCAC) algorithm, each base station reserves capacity for handoff traffic by blocking new calls when the total received power at that base exceeds a threshold. The performance of proposed schemes are evaluated by simulation of a one dimensional system. It is found that the RPCAC scheme can provide significantly improved performance.
Ph.D. Dissertation Director: Professor Roy D. Yates
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