0)+10n log(d/do)+x (dB) (9-3) The value of X is often derived empirically based on measurements. A typical value for o is 8 dB. The parameters that statistically describe path loss due to large-scale fading (macroscopic fading) for an 9.2 Characteristics of wireless channels arbitrary location with a specific transmitting-receiving antenna separation include the reference distance do, the path loss exponent n, and the standard deviation o of X. Microscopic fading refers to the rapid fluctuations of the received signal in time and frequency, and is caused by scattering objects between the transmitting and receiving antennas. When the received RF signal is a superposition of independent scattered components plus a line-of-sight (LoS) component, the envelope of the received signal r(t) has a Rician Probability Distribution Function (PDF) and is referred to as Ricean fading. As the magnitude of the LoS component approaches zero, the Ricean PDF approaches a Rayleigh PDF. Thus: (9-4) Where K and Io(r) denote the Ricean factor and zero-order modified Bessel function of the first kind, respectively. In the absence of an LoS path (K 0), the Ricean PDF reduces to Rayleigh distribution. Time-varying fading due to scattering objects or transmitter/receiver motion results in Doppler spread. The time spreading effect of small-scale or microscopic fading is manifested in the time- domain as multipath delay spread, and in the frequency-domain as channel coherence bandwidth. Similarly, the time-variation of the channel is characterized in the time-domain as channel coherence time and in the frequency-domain as Doppler spread. In a fading channel, the relationship between maximum excess delay time T'm and symbol time Ts can be viewed in terms of two different degradation effects, i.e., frequency-selective fading and frequency non-selective or flat-fading. A channel is said to exhibit frequency-selective fading if Tm T5. This condition occurs whenever the received multipath components of a symbol extend beyond the symbol's time dur