Optimal resource allocation in downlink CDMA wireless networks

Abstract

This thesis presents a full analytical characterization of the optimal joint downlink rate and power assignment for maximal total system throughput in a multi cell CDMA network. In Chapter 2, we analyze the feasibility of downlink power assignment in a linear model of two CDMA cell, under the assumption that all downlink users in the system receive the same rate. We have obtained an explicit decomposition of system and user characteristics. Although the obtained relation is non-linear, it basically provides an effective interference characterisation of downlink feasibility for a fast evaluation of outage and blocking probabilities, and enable a quick evaluation of feasibility. We have numerically investigated blocking probabilities and have found for the downlink that it is best to allocate all calls to a single cell. Moreover, this chapter has also provided a model for determining an optimal cell border in CDMA networks. We have combined downlink and uplink feasibility model to determine cell borders for which the system throughput, expressed in terms of downlink rates, is maximized. In Chapter 3, we have considered the two cell linear model where the coverage area was divided into small segments. Previously, we have assumed that all users in the cell are using the same rate, regardless their location. In this chapter, we have differentiated rate allocation based on their location. We have assumed that users in the same segment receive the same rate which is chosen from a discrete set. The goal is to assign rates to users in each segment, such that the utility of the system is maximized. In this chapter, we design an algorithm that is actually a fully polynomial time approximation scheme (FPTAS) for the rate optimization problem. The model in this chapter indicates that the optimal downlink rate allocation can be obtained in a distributed way: the allocation in each cell can be optimized independently, interference being incorporated in a single parameter $t$. In Chapter 4, we have analyzed the two cell model under the assumption that the rates are continuous and may be chosen from a given interval. Moreover, we also taken into account the downlink limited transmit power. First, we developed a model for the joint rate and power allocation problem. Despite its non-convexity, the optimal solution in this chapter can be very well characterized. Second, we analyzed several properties of the optimal solutions. We have proved that the optimal rate allocations are monotonic as a function of the path loss. Based on this property, we have showed that in the optimal rate allocation, in each cell, only three rates are given to users. Finally, we have proposed a polynomial time algorithm in the number of users that solves optimally the joint rate and power allocation problem. The results can be extended to non-decreasing utility functions. In Chapter 5, we have extended the model of the previous chapter to a multi-cell setting. We have presented a full analytical characterization of the optimal joint downlink rate and power assignment for maximal total system throughput in a multi cell CDMA network. Moreover, the cell model is a planar model. Chapter 5 has three main contributions. First, we provide an explicit and exact characterization of the structure of the optimal rate and power assignment. Second, we give a characterization of the optimal rate assignment in each cell. Third, based on these results, we give an exact algorithm for solving the rate and power assignment problem and a fast and accurate heuristic algorithm for power and rate assignment to achieve maximal downlink throughput in a multi cell CDMA system.