TABLE OF CONTENTS Other technologies have been identified as fundamental for allowing the development of future 4G. We now list some other important techniques and technologies not considered in this chapter: adaptive modulation and coding, techniques for seamless vertical and horizontal handovers, cross-layer design and optimization, multidimensional scheduling (time, frequency, space), techniques for reducing the PAPR problem typical of multicarrier systems, sensor networks, battery technology, and access techniques specifically tailored for short-range communications [171]. The latter techniques have received considerable attention lately. These are basically ultra-wideband (UWB) and optical wireless communication techniques. Interested readers are referred to [172, 173] and the references therein for a comprehensive introduction to UWB. The role of optical wireless systems within future 4G communications is discussed in detail in [174]. Optical wireless communications is in fact a very attractive complementary technology that has been extensively considered recently due to its unmatched advantages [175].
