ay Spectral Broadening Multipath Intensity Profile Doppler Power Spectrum Fourier Fourier Transforms Transforms Functions Spaced-Frequency Correlation Spaced-Time Correlation Function Function Ste - 1 Coherence Coherence Bandwidth T x 1 Coheren Coherence Time FIGURE 9-3 An illustration of the duality principle in time-and frequency-domains [3] to the total average received signal power. In the literature, the Fourier transform of A(T) is referred to as the spaced-frequency correlation function (v). The spaced-frequency correlation function (v) is the channel's response to a pair of sinusoidal signals separated in frequency by U. The coherence bandwidth A fc is a measure of the frequency range over which spectral components have a strong likelihood of amplitude correlation. In other words, a signal's spectral components over this range are affected by the channel in a similar manner. Note that Afc and Tm are inversely proportional, i.e., Afc 8 1/Tm. The maximum excess delay Tm is not the best indicator of how any given wireless system will perform over a communication channel, because different channels with the same value of Tm can exhibit different variations of signal intensity over the delay span. Thus the delay spread is often characterized in terms of the Root Mean Square (RMS) delay spread TRMS, where: TRMS = (9-5) 9.2 Characteristics of wireless channels where (9-6) A closed-form relationship between coherence bandwidth and RMS delay spread does not exist and must be derived from signal analysis of actual signal dispersion measurements in specific channels. If coherence bandwidth is defined as the frequency interval over which the channel's complex frequency transfer function has a correlation of at least 0.9, the coherence bandwidth is approximately fc = 1/(50TRMS). A common approximation of Afc corresponding to a frequency range over which the channel transfer function has a correlation of at least 0.5 is Afc 11 1/(5TRMS). A channel is said to exhibit frequency-selective effects, if Afc <1/Ts, where the 