LaNWiFi.NeTWorld's Wireless Technology Center // Wireless For Freedom.

          Physical layer techniques advance rapidly as communication theories, digital signal processing algorithms, RF technologies, and circuit design for wireless communications quickly evolve. These techniques mainly focus on three directions: increasing transmission rate, improving error resilience capability in a wireless environment, and enhancing reconfigurability and software controllability of radios. In order to increase the capacity of wireless networks, various high-speed physical techniques have been invented. For example, orthogonal frequency division multiplexing (OFDM) has significantly increased the speed of IEEE 802.11 from 11 Mbps to 54 Mbps. A much higher transmission rate can be achieved through Ultra-Wideband (UWB) techniques. However, UWB is only applicable to short-distance applications such as wireless personal area networks (WPANs).

          If a transmission speed as high as that ofUWB is desired in a wider area network such as WLANs or WMANs, other physical techniques, such as the multipleinput multiple-output (MIMO) mechanism, are needed. In order to further increase capacity and mitigate the impairment by fading, delay-spread, and co-channel interference, multipleantenna systems have been used for wireless communication. It should be noted that spectrum efficiency must be maintained as high as possible while a new physical layer technique is developed to increase the transmission rate. To improve error resilience, many channel coding schemes have been developed. Since channel condition varies, a fixed channel coding scheme is not efficient. Thus, an adaptive channel coding scheme is needed. For example, in 3G cellular networks and IEEE 802.11a, coding schemes must be varied as channel condition changes. In the third direction, physical layer techniques are developed such that they can be controlled by software. Such a capability brings many advantages to wireless communications. For example, physical layer techniques can be optimized adaptively according to the variable conditions in the environment, so that the research and development cycle can be dramatically shortened, radios can be reconfigured, and so on. When cognitive radios are considered, the scarce wireless spectrum can be much better utilized. In this chapter, advanced techniques along all three directions are discussed. In particular, techniques that have great potential for WMNs are studied. Examples include adaptive modulation and coding, multi-antenna systems, multichannel or multiradios, link adaptation techniques, software radios, and so on. In order to demonstrate how different physical layer techniques are integrated into the same system, the IEEE 802.11n physical layer is investigated. Physical layer techniques are usually only concerned with single-hop point-to-point communications. However, when they are applied toWMNs, they will experience new, challenging problems that degrade their performance in a wireless mesh networking environment. Thus, research problems involved in the physical layer of WMNs are pointed out in this chapter. Solutions to these problems are also discussed.