the performance of wireless communication systems is mainly governed by the wireless channel environment, As opposed to the typically static and predictable characteristics of a wired channel,the wireless channel is rather dynamic and unpredictable,which makes an exact analysis of the wireless communication system often difficult . in recent years, optimization of the wireless communication system has becom critical with the rapid growth of mobile communication services and emerging broadband mobile internet access services. in face, the understanding of wireless channels will lay the foundationi for the development of high performance and bandwidth-efficient wireless transmission technology.
in wireless communication,radio propagation refers to the behavior of radio waves when they are propagated from transmitter to receiver. in the course of propagation,radio waves are mainly affected by three different modes of physical phenomena:reflection,diffraction,and scattering.
1.reflection is the physical phenomenon that occurs when a propagating electromagnetic wave impinges upon an object with very large dimensions compared to the wavelength,for example,surface of the earth and building. it forces the transmit signal power to be reflected back to its origin rather than being passed all the way along the path to the receiver.
2.diffraction refers to various phenomena that occur when the radio path between the tranmitter and receiver is obstructed by a surface with sharp irregularities or small openings. it appears as a bending of waves around the small obstacles and spreading out of waves past small openings. the secondary waves generated by diffraction are useful for establishing a path between the transmitter and receiver, even when a line-of-sight path is not present.
3.scattering is the physical phenomenon that forces the radiation of an electromagnetic wave to deviate from a straight path by one or more local obstacles, with small dimensions compared to the wavelength. those obstacles that induce scattering,such as foliage,street signs,and lamp posts,are referred to as the scatters.
a unique characteristic in a wireless channel is a phenomenon called 'fading' the variation of the signal amptitude over time and frequency. in contrast with the additive noise as the most common source of signal degradation , fading is another source of signal degradation that is characterized as a non-additive signal disturbance in the wireless channel. fading may either be due to multipath propagation,fererred to as multi-path fading,or to shadowing from obstacles that affect the propagation of a radio wave,referred to as shadow fading.
the fading phenomenon in the wireless communication channel was initially modeled for HF(high frequency,3~30MHz), UHF(Ultra HF,300~3000GHz),and SHF(Super HF,3~30Ghz)bands in the 1950s and 1960s, currently ,the most popular wireless channel models have been established for 800MHz to 2.5GHz by extensive channel measurements in the field.These include the ITU-R standard channel models specialized for a single-antenna communication system,typically