End-to-end design principles for broadband cellular mesh networks

Abstract

The cellular industry is currently positioned for a transition from traditional voice networks to high-rate data networks that will enable ubiquitous internet connectivity and support of a variety of new wireless applications. The rapid deployment of broadband wireless access networks over large coverage areas (e.g., wide-area networks (WANs)) calls for the investigation of low-cost and high-performance infrastructure technologies. One major source of cost in cellular networks is the wireline backhaul that connects infrastructure devices (e.g., base stations, access points etc.) to the core internet. These wired backhaul connections are often electrical or fiber-optic and incur significant recurring costs of deployment, leasing and maintenance for service providers. Therefore, technologies that enable the wireless backhaul between the core network and infrastructure devices are of great interest from a cost savings perspective. The demands and constraints on future wireless networks outlined above lead to a multihop cellular mesh architecture [1]- [2], an example of which is depicted in Fig. 3.1. The role of the additional infrastructure deployment points is to serve as relay terminals for the data to be routed between the wired infrastructure devices (labeled as BS, i.e. base station) and end users (labeled as MS, i.e. mobile station) and thereby to enhance the quality of end-to-end communication. Depending on the size of their coverage area, these fixed radio relay nodes are referred to as "micro" or "pico" relay stations (RS) (e.g., nodes 102-110 in Fig. 3.1 each cover their respective shaded hexagonal micro cells) and are generally much smaller in size and less expensive than the wired infrastructure devices. These relay deployments will serve toward various objectives, such as enhancing data rate coverage and enabling range extension over cellular networks. With this motivation, there has recently been growing interest from both academia and industry in the concept of relaying in infrastructure based wireless networks such as next generation cellular networks (B3G, 4G), wireless local area networks (WLANs) (IEEE 802.11, WiFi, HyperLAN) and broadband fixed wireless networks (IEEE 802.16, WiMax, HyperMAN). Chapter Overview. End-to-end optimization of certain quality of service (QoS) measures such as throughput, reliability and latency plays a key role in designing novel algorithms and architectures for next generation relay-assisted broadband cellular mesh networks. Toward this end, the development of performance characterization methodologies as a function of the physical channel conditions and system parameters is essential to manage end-to-end QoS requirements. Motivated by the observation that both base stations and relay stations are stationary (fixed network topology) and are expected to enjoy slow-varying channel conditions which allows for rate-adaptive relaying over each hop, we first perform an information-theoretic capacity analysis to propose end-to-end throughput and latency measures for multihop routing as a function of the physical channel and system parameters in broadband cellular mesh networks employing orthogonal frequency division multiplexing (OFDM). Several centralized functionalities coordinated by the base station (e.g. scheduling algorithms, routing algorithms, network entry and handoff, latency management, other MAC and higher layer functions) can benefit from the knowledge of such end-to-end quality of service (QoS) measures; examples of which are provided in later sections. We have previously reported our research results in earlier publications [2]- [7] and standard contributions [8]- [9]. This chapter is organized as follows: Section 3.2 introduces a broadband fading physical channel model [3] to study multihop communication protocols over cellular mesh networks and discusses our assumptions regarding channel statistics and terminal transmission/reception capabilities. Section 3.3 characterizes the end-to-end capacity and throughput in a broadband cellular mesh network in the presence of a multihop routing protocol that employs rate-adaptive relaying and OFDM-based codeword transmissions over each hop [3, 5], accounting for transmission errors due to decoding failures and retransmissions until successful message reception based on an automatic repeat request (ARQ) mechanism. Section 3.4 shows that end-to-end throughput maximization in a broadband cellular mesh network is equivalent to a minimum-cost routing problem [3] by defining the multihop routing metric as the reciprocal of the per-hop throughput, which is also known as the expected transmission time (ETT) [10], and the objective of route selection is to dynamically minimize end-to-end latency. Section 3.5 discusses the use of end-to-end metrics toward the design of novel resource allocation, scheduling and multihop routing algorithms. In particular, we propose orthogonal frequency division multihop multiple access (OFDM2A) resource allocation policy [4] for relay-assisted broadband cellular mesh networks, which ensures interference-free multi-user communication. This policy allows for the design of low-complexity centralized opportunistic scheduling algorithms by separating the problems of subcarrier allocation and multihop route selection. Section 3.6 illustrates another use of end-to-end metrics for network entry and handoff; under the assumption that relay stations have capabilities very similar to base stations, i.e. they can perform association, authentication, time/frequency resource allocation. In particular, we propose novel algorithms for network entry and handoff in cellular mesh networks that yield enhanced link performance while maintaining the backward compatibility of the end users. Section 3.7 briefly summarizes ongoing standardization activities in IEEE 802.16j Relay Task Group. © 2007 Springer Science+Business Media, LLC. All rights reserved.