Multiple antenna techniques for wireless mesh networks

Abstract

Wireless mesh networks (WMNs) is a relatively new and promising key technology for next generation wireless networking that have recently attracted both the academic and industrial interest. Mesh networks are expected gradually to partially substitute the wired network infrastructure functionality by being able to provide a cheap, quick and efficient solution for wireless data networking in urban, suburban and even rural environments. Their popularity comes from the fact that they are self-organized, self-configurable and easily adaptable to different traffic requirements and network changes. Mesh networks are composed of static wireless nodes that have ample energy supply. Each node operates not only as an conventional access point (AP)/gateway to the internet but also as a wireless router (Fig. 11.1) able to relay packets from other nodes without direct access to their destinations [1] [2]. The destination can be an internet gateway or a mobile user served by another AP in the same mesh network. Moreover, some nodes may only have the backhauling functionality, meaning that they do not serve any mobile user directly but their purpose is to forward other APs' packets. For wide area access, the access points (or base stations) are typically located at high towers or at the rooftop of buildings. However, as the capacity demands increase, the AP is moving closer to the user and it could be placed at below-therooftop heights. In this way it can provide better signal reception and higher spatial frequency reuse factor. For a cellular network point of view this means that the cell size has to shrink in order to satisfy the increased capacity demands. Therefore, the whole network topology in terms of base station location has to be revised and additional base stations have to be installed by the network operator. In case where fiber is not readily available, the cost of backhauling by using e.g. E3 lines may be prohibitive for the operator. Wireless mesh networks can be proven an appealing alternative backhauling solution in this case since they do not require wired connection, they are easy to deploy fast and without extensive network planning requirements since they are self configurable and adaptable to network changes and demands. For instance, Multi-Element Multihop Backhaul Reconfigurable Antenna Network (MEMBRANE) [3] is an IST-funded project that aims to bring an efficient wireless backhaul design as an alternative technology to serve wireless broadband networks in cases where a wired backhaul would be more costly to access and/or would take longer to deploy. Moreover, wireless mesh network can enhance the presence of broadband in rural and remote zones, thus helping combat the "digital divide" between these areas and the big urban centers, caused mainly by the inadequacy (or even absence) of wired network infrastructure. It is not only doubtful that the required investment for bringing cable and/or fiber will ever pay for itself in such remote zones and communities but, more importantly, such an undertaking will probably take several years. Wireless mesh networks can provide a quick and economically affordable solution in this case. Technology Enablers: Mesh networks must meet a number of technical requirements. First of all, they must meet the high capacity needs of the access nodes which have to forward the accumulated traffic of their underling users. Furthermore, they have to cope with the delay and other strict quality-of-service (QoS) requirements of the end user applications. Finally they must provide a large enough effective communication range to ensure that no APs (or groups of APs) are isolated from the Internet gateways. In order to satisfy the above requirements, a range of novel techniques has to be exploited. Such technology enablers include but not limited to multi-hopping, various multiple antennas techniques and novel medium access control (MAC) and routing algorithms. Multi-hopping, i.e. the use of multiple relays (or forwarding nodes) between the end user and the Internet gateway, is primarily motivated by the low power and the low heights of the access and relay nodes. Clearly, in low power transmissions, multihopping helps increase the range. Moreover, since low height access points are likely to be surrounded by several obstacles (buildings, cars, etc.), their line-of-site (LOS) will be typically obstructed, affecting in this way the one-hop node connectivity to a gateway. Multiple antenna techniques (also known as smart antennas) constitute another enabling technology that is highly beneficial to the wireless mesh network architectures. These techniques include fixed beam antennas, adaptive antennas and multipleinput multiple-output (MIMO) coding [4] [5]. Depending on the used technique, multiple antennas can provide power, diversity and multiplexing gain and therefore increase the transmission range, reduce the transmitting power, mitigate interference, increase channel reliability and increase data throughout. Each of these techniques is more appropriate in different types of propagation scenarios; for example beamforming is well suited to cases with narrow angle spread, such as in high towers; whereas MIMO is more appropriate in cases of rich electromagnetic scattering, such as in low-height links without strong LOS component. Given the different types of propagation environments that are expected in wireless mesh networks, smart antenna techniques are expected to boost throughput performance and reduce interference and delay, thus improving overall end-to-end performance. However, multiple antenna techniques have been extensively analyzed only for single-link communications. The combination of multi-hopping with multiple smart antennas in a wireless mesh network environment is a field that has not received much research attention. This combination is expected to boost network capacity and achieve the target QoS. Novel medium access control and routing algorithms that are able to exploit the benefits of multiple antenna usage on the wireless mesh access points is another enabling technology of paramount importance. Employment of smart antennas techniques without thorough understanding and consideration of their interaction with layer two and three algorithms can be proven counter productive for the mesh network functionality. Deafness, hidden and exposed terminals and multi-stream interference are some of the problems that have to be addressed by novel MAC and routing schemes since they can highly affect not only the individual links but also the overall network performance. The main aim of this chapter is to give an insight into both the improvements and various challenges generated by the deployment of multiple antennas in wireless mesh networks and how these can be addressed by layer two and three algorithms. In the rest of this chapter the wireless mesh network and channel characteristics are discussed in Section 11.2. A thorough analysis of the various smart antenna techniques and theirmain advantages and disadvantages follows in Section 11.3. In Section 11.4, the challenges that smart antenna techniques will impose to medium access control and network layers are introduced together with some possible solutions. Finally, Sections 11.5 and 11.6 discus several scheduling and routing schemes, respectively, with smart antenna considerations. © 2007 Springer Science+Business Media, LLC. All rights reserved.