ion in terrestrial channels Terrestrial Broadband Wireless Telecommunications Signal Fading In contrast to line-of-sight satellite channels which are largely time-invariant, terrestrial communications channels are time-variant. That is, the received power level changes rap- idly over time. This phenomenon is referred to as signal fading, as illustrated below. Fading occurs because radio communications channels are characterised by signal arrival by multiple paths. A received signal is usually composed of many reflections of the transmitted signal - reflections from natural and man-made objects such as mountains, buildings, cars, and the ground, as illustrated below. Vector sum Each received signal ray travels through a different path and has a different propagation delay time, amplitude and phase. As a result each arrives with a certain amplitude and phase as a function of distance travelled and efficiency of reflection. When combined, the sum of the received signal rays (vectors) may combine constructively or destructively, resulting in the signal power characteristics shown above. The range of received power variation can be as much as 60 dB. Fading can cause significant degradation of system performance: for example a system operating in a fading environment requires nearly 100 times more transmission power to achieve the same quality of communications compared with a non-fading environment. Broadband Telecommunications Technologies and Management Example 10.1 Compare the path loss between a line-of-sight and terrestrial channel over a distance of 1 km at the operating frequency band of 2.3 GHz. Answer Free space: L (FreeSpace) =32.4-20log(d)+201og(f L = 99.6 dB Urban (Hata): L = 140.9 dB (higher than line-of-sight by a factor of more than 13 000 times, over a dis- tance of only 1 km) The large path loss presents two problems to the telecommunications technologist. One is the required strong transmission power for compensating the path loss, which may not be practical or even possible in small hand-held device