| Wireless communications is one of the most active areas of research over the past and the current decades. In fact, the demand for wireless services has been changing from the regular voice telephony services to mixed voice, data, and multimedia services over the wireless media. Since the mid-1990s, the wireless industry has been advancing at an incredible speed. For example, the 2G cellular systems (such as GSM, D-AMPS, and IS-95) have significantly improved the spectral efficiency and network capacity to support wireless telephony services. Fueled by the explosion of demands for applications over the fixedline and broadband Internet access, there is a parallel development in the wireless domains to support high-quality and high-speed data and multimedia services. For example, we have the development of 3G systems (CDMA2000, UMTS), 3.5G systems (HSDPA,EV-DO,EV-DV), B3G systems (Beyond 3G), wireless LAN (IEEE 802.11a/b/g), ultrawideband (UWB) systems, and Wi- MAX (IEEE 802.16) as well as Wi-MAN (IEEE 802.20) systems. These technologies have spurred a lot of research in the signal processing and cross-layer design for wireless communications.
Realizing reliable and efficient communications over the wireless channel has been a very challenging topic for over 50 years. This is attributed to the hostile nature of the wireless channel in the form of rapid time variation, extreme fading, and multipath. For instance, the transmission of signals over the wireless channels is affected by time-varying channel attenuation, called fading. The received signal strength can fluctuate over a wide range of 80dB in the order of milliseconds. On one occasion, the transmission may experience good fading and the transmission error probability will be low. On the other hand, the transmission may experience bad fading on other occasions and the error probability will be high. Hence, in general, the fading effects of wireless channels impose additional challenges for signal transmissions besides the regular channel noise.Two very promising more recent approaches address the challenging problems of wireless transmissions: the multipleantenna technologies and the cross-layer transmitter adaptation designs. |