Resource allocation issues in broadband wireless networks with OFDM signaling

Title of Dissertation: RESOURCE ALLOCATION ISSUES IN BROADBAND WIRELESS NETWORKS WITH OFDM SIGNALING Iordanis Koutsopoulos, Doctor of Philosophy, 2002 Dissertation directed by: Professor Leandros Tassiulas Department of Electrical and Computer Engineering Wireless broadband technologies are anticipated to flourish in the next few years, due to the increasing demand for wireless connectivity and the need to support enhanced services and applications in localor wide-area environments. The primary goal in a communications system is Quality of service (QoS) provisioning to users, which depends on procedures that span several communication layers. Although independent consideration of different layers simplifies system design, it often turns out to be insufficient for wireless networks. Cochannel interference between users that reuse the limited spectrum and the resulting impact of local adaptation actions on overall network performance impose layer interactions in wireless systems. The purpose of this work is to identify and study some of the issues that arise from the synergy between the physical and the MAC layer in the context of multiple access schemes with orthogonal channels. Using the essential feature of channel orthogonality as a baseline, our approach places emphasis on Orthogonal Frequency Division Multiplexing (OFDM), which is an emerging multiple access and signaling method for future wireless broadband networks. In OFDM, the broadband spectrum is divided into orthogonal, narrowband subcarriers and user symbols are split into subsymbols, which are transmitted in parallel over those variable-quality subcarriers. OFDM transmission reduces the effective symbol transmission rate, simplifies equalization at the receiver and provides high immunity to inter-symbol interference and delay spread. Furthermore, it defines a framework for flexible adaptation to varying channel conditions, by allowing transmission parameter control for each subcarrier. We first address the joint problem of channel allocation with simultaneous adaptation of modulation level and transmission power in a multi-cell OFDM network. We study the impact of those parameters on cochannel interference and channel reuse and present two classes of centralized heuristic algorithms to perform the allocation. Next, we focus on a single-cell multi-user system with modulation control and study the problem of subcarrier assignment to users subject to time resource constraints. We study and compare integral and fractional user assignment, whereby a user is assigned to one subcarrier or can be partially assigned to multiple subcarriers. In addition, we consider the synergy between link-layer ARQ protocols and physical layer parameter adaptation. We consider a simple channel monitoring method which is based on counting received ACKs and NACKs. For a single subcarrier, we show that the adaptation policy which maximizes long-term average throughput per unit time is of threshold type. We also expand our policy to the multiple-subcarrier case with similar or different channel qualities. In the sequel, we study the impact of smart antennas and Space Division Multiple Access (SDMA) on MAC layer channel allocation for a single-cell multi-user system. Our approach encompasses multiple access schemes with orthogonal channels, such as OFDM. We first consider the case of unlimited transceiver resources, where a separate beam can be formed for each user of a spatially separable cochannel user set in a subcarrier. We present heuristic algorithms to allocate subcarriers to users and adjust down-link beam patterns, transmission powers and rates with the objective to increase total achievable system rate and provide QoS to users in the form of minimum rate guarantees. Then, we consider the allocation problem for limited transceiver resources, which arises whenever certain reasons impose limitations on the number of beams that can be formed. We propose meaningful heuristic algorithms to jointly form beams from corresponding transceivers and assign subcarriers and transceivers to users, such that the total achievable system rate is increased. RESOURCE ALLOCATION ISSUES IN BROADBAND WIRELESS NETWORKS WITH OFDM SIGNALING