ation. Such multipath dispersion of the signal results in Inter-Symbol Interference (ISI) distortion. In the case of frequency-selective fading, mitigating the distortion is possible because many of the multipath components are separable by the receiver. A channel is said to exhibit frequency non-selective or flat- fading if Tm > T5. In this case, all the received multipath components of a symbol arrive within the symbol time duration; therefore, the components are not resolvable. In this case, there is no channel- induced ISI distortion, since the signal time spreading does not result in significant overlap among adjacent received symbols. There is still performance degradation because the unresolvable phasor components can add up destructively to yield a substantial reduction in signal-to-noise ratio (SNR). Also, signals that are classified as exhibiting flat-fading can sometimes experience frequency- selective distortion. Figure 9-3 illustrates a multipath-intensity profile A(t) versus delay T, where the term delay refers to the excess delay. It represents the signal's propagation delay that exceeds the delay of the first signal's arrival at the receiver. For a typical wireless communication channel, the received signal usually consists of several discrete multipath components. The received signals are composed of a continuum of multipath components in some channels, such as the tropospheric channel. In order to perform measurements of the multipath intensity profile, a wideband signal; i.e., a unit impulse or Dirac delta function, is used. For a single transmitted impulse, the time Tm between the first and last received component is defined as the maximum excess delay during which the multipath signal power typically falls to some level 10-20 dB below that of the strongest component. Note that for an ideal system with zero excess delay, the function A(T) would consist of an ideal impulse with weight equal 340 CHAPTER 9 The IEEE 802. .16m Physical Layer (Part I) Functions Vc Vmax Vc +Vmax Tm Maximum Excess Del