inverse symbol rate is approximately equal to the signal bandwidth W. In practice, W may differ from 1/Ts due to filtering or data modulation. Frequency-selective fading effects arise whenever the signal's spectral components are not affected equally by the channel. This occurs whenever Afc < W. Frequency non-selective or flat-fading degradation occurs whenever Afc > W. Hence, all of the signal's spectral components will be affected by the channel in a similar manner. Flat-fading does not introduce channel-induced ISI distortion, but performance degradation can still be expected due to a loss in SNR whenever the signal is fading. In order to avoid channel-induced ISI distortion, the channel is required to exhibit flat-fading by ensuring that Afc > W. Hence, the channel coherence bandwidth Af sets an upper limit on the transmission rate that can be used without incorporating an equalizer in the receiver. Figure 9-3 shows function P(t), which is known as the spaced-time correlation function. It is the autocorrelation function of the channel's response to a sinusoid. This function specifies the extent which there is correlation between the channel's response to a sinusoid sent at time t1 and the response to a similar sinusoid sent at time t2, where At=t1-t2. The coherence time is a measure of the expected time duration over which the channel's response is essentially invariant. To estimate P(t), a sinusoidal signal transmitted through the channel and the autocorrelation function of the channel output is calculated. The function (t) and the parameter T provide us with information about the speed of fading channel variation. Note that for an ideal time-invariant channel, the channel's response would be highly correlated for all values of At and P(t) would be a constant function. If one ideally assumes uniformly distributed scattering around a mobile station with linearly-polarized antennas, then the Doppler power spectrum (i.e., the inverse Fourier transform of the spaced-time correlation function) A(v) has a U-shaped