Cross-layer Optimization in Wireless Multihop Networks

T he interest in wireless communications has grown constantly for the past decades, leading to an enormous number of applications and services embraced by billions of users. In order to meet the increasing demand for mobile Internet access, several high data-rate radio networking technologies have been proposed to offer wide area high-speed wireless communications, eventually replacing fixed (wired) networks for many applications. This thesis considers cross-layer optimization of multi-hop radio networks where the system performance can be improved if the traditionally separated network layers are jointly optimized. The networks we consider have links with variable transmission rates, influenced by the allocation of transmission opportunities and channels, modulation and coding schemes and transmit powers. First, we formulate the optimal network operation as the solution to a network utility maximization problem and review decomposition methods from mathematical programming that allow to translate a centralized network optimization problem into distributed mechanisms and protocols. Second, particular focus is given to networks employing spatial-reuse TDMA, where we develop detailed distributed solutions for joint end-to-end communication rate selection, multiple time-slot transmission scheduling and power allocation which achieve the optimal network utility. In the process, we introduce a novel decomposition method for convex optimization, establish its convergence and demonstrate how it suggests a distributed solution based on flow control optimization and incremental updates of the transmission schedule. We develop a two-step procedure for distributed maximization of computing the schedule updates (maximizing congestion-weighted throughput) and suggest two schemes for distributed channel reservation and power control under realistic interference models. Third, investigate the advantages of employing multi-user detectors within a CDMA/TDMA framework. We demonstrate how column generation techniques can be combined with resource allocation schemes for the multi-access channel into a very efficient computational method. Fourth, we investigate the benefits and challenges of using the emerging OFDMA modulation scheme within our framework. Specifically, we consider the problem of assigning sub-carriers to wireless links in multi-hop mesh networks. Since the underlying mathematical programming problem is computationally hard, we develop a specialized algorithm that computes optimal near-optimal solutions in a reasonable time and suggest a heuristic for improving computation at the price of relatively modest performance losses.