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. The delay spread is often characterized in terms of the RMS delay spread TRMS in which the average multi- path delay T is calculated as follows: An exact relationship between coherence bandwidth and 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 chan- nel's complex-valued frequency transfer function has a correlation of at least 0.9, the coher- ence bandwidth is approximately 2 1/(50TRMS). A common approximation of Afc corresponding to a frequency range over which the channel transfer function has a correla- tion of at least 0.5 is 1/(5TRMS). A channel is said to exhibit frequency-selective effects, if Afc <1/Ts where the 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 a signal's spectral components are not equally affected by the channel. This occurs whenever Afc < W. Frequency non-selective or flat fading degradation occurs when- ever Afc > W. Hence, all spectral components of the signal will be affected by the channel in a similar manner. Flat fading does not introduce channel-induced ISI distortion, but per- formance degradation can still be expected due to loss in SNR, whenever the signal New Radio Access Physical Layer Aspects (Part 1) 295 experiences fading. In order to avoid channel-induced ISI distortion, the channel is required to exhibit flat-fading by ensuring that Afc > W. Therefore, the channel coherence bandwidth Afc sets an upper limit on the transmission rate that can be used without incorporating an equalizer in the receiver [33]. 3.1.1.3 Doppler Spread Fig. 3.3 shows another function P(t) known as spaced-time correlation f