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Top1. Introduction
Wireless Mesh Networks (WMNs) provide alternative technologies for last-mile broadband Internet access and high speed connectivity with cost-effectiveness. WMNs have emerged as a key technology for the next generation of wireless networking. Instead of being another type of ad-hoc networking, WMNs diversify the capabilities of ad-hoc networks by integrating additional routing function to support wireless networks such as cellular wireless sensors (Akyildiz & Wang, 2005; Pathak & Dutta, 2011). WMNs have the advantages of being self-organizing, self-configuring, and offering increased reliability. Nodes in WMNs automatically form an ad-hoc network and maintain mesh connectivity. These features bring further advantages to WMNs, such as low up-front cost, easy network maintenance, robustness, reliable service coverage, etc (Akyildiz & Wang, 2005; Pathak & Dutta, 2011). WMNs are comprised of two types of nodes: mesh routers and mesh clients. In the WMN architecture, mesh routers form an infrastructure for various types of client (Figure 1), where dashes indicate wireless and solid lines indicate wired links on the WMN. Various wireless devices (such as laptops, PDAs, cellular networks) equipped with wireless cards can connect to a WMN through a mesh router with gateway/bridge capabilities. The gateway/bridge integrates various exiting wireless networks such as cellular, wireless sensors, Wireless-Fidelity (Wi-Fi), LTE-Advanced and Worldwide Interoperability for Microwave access (WiMAX) (Akyildiz, Wang, & Wang, 2005). As WMNs are self-organized, self-configured with wireless mesh routers, and automatically establish and maintain wireless mesh connectivity (effectively, creating an ad-hoc network), they can provide wireless transport services to data travelling from other users, access points or base stations (access points/base stations are special wireless routers with a high-bandwidth wired connection to the Internet backbone) (Figure 1).
Figure 1. Wireless mesh infrastructure
Currently, WMNs are going through rapid commercialization in several application scenarios such as broadband home networking, community networking, building automation, high speed metropolitan area
networks, and enterprise networking (Akyildiz & Wang, 2005; Pathak & Dutta, 2011). This is due to the fact that WMNs can be relatively easily established because all the required components are already available in the form of ad-hoc network routing protocols, IEEE 802.11 MAC protocols, Wired Equivalent Privacy (WEP) security, and so on
Although WMNs could be established straightforwardly, obtaining high data rates in WMNs is still a big challenge since bandwidth in wireless is limited. One way to achieve higher data rates is the use of a well-designed modulation format. The performance of each modulation scheme is measured by its capability to precisely maintain the encoded data, which is represented by the low Bit Error Rate (BER). Variation in the BER is directly related to the received Signal-to-Noise Ratio (SNR) (Figure 2). BER and SNR are inversely related that when the SNR is decreased the BER increases. Therefore, when the SNR is lowered, it is more difficult for the modulation scheme to decode the received signal as the BER is too high. The relationship between the BER and the SNR for various modulation schemes is illustrated in the references (Holland, Vaidya, & Bahl, 2001, 2000). Generally, when data rate is increased, the BER also increases consequently. A logical question arises would be “Are there any ways to attain high data rates while maintaining lower BER?” One of the effective approaches for that is adaptation of different modulation schemes (Holland et al., 2001, 2000). This as a result, will lead to improvements in the performance of a wireless device.
Figure 2. General performance of BER versus SNR