Fading is the change in signal strength due to direct and reflected signals (multipath) interfering with each other (fast fading) and due to distance and terrain efffects (slow fading). Fast fading signal strength changes are due to relative motion and local scattering objects such as buildings, foliage, etc. and change rapidly over short distances. Slow fading is the change in the local mean signal strength as larger distances are covered. In a highly random (Rayleigh) environment fast fading will have a Gaussian distribution while slow fading will tend toward a log–normal distribution. Direct paths and larger bandwidths will result in Rician fast fading distributions.

Signals from multiple antennas, or "spatial diversity," can be used to reduce the effects of fast fading and improve received signal strength. Assuming that the signals from multiple antennas are independent, thus experiencing fades at different times and frequencies, the combined signal should have a lower probability of a given fade level than the individual signals. The more channels used, the better the fade reduction. Three common combining schemes used for Rayleigh fading channels are selection diversity, equal gain combining, and maximal ratio combining. Selection diversity chooses the strongest signal power, equal gain combines the cophased signal voltages with equal weights, and maximal ratio weights the cophased signal voltages relative to their signal to noise ratio. If the noise level is not be known a priori, the relative signal strength is used.

The ITS radio channel impulse response measurement system can be used to study diversity gain. The multiple channel capability allows different combing techniques to be examined via post–processing. Figure 1 shows the gain achieved in a suburban fast fading environment using signals measured with the ITS system from one to four antennas combined via maximal gain combining. Four signal bandwidths (19.6 kHz, 1.25 MHz, 5.0 MHz, and 10.0 MHz) were considered. We see that increasing the number of channel used increases the resulting spatial diversity gain while increasing the bandwidth decreases the gain. The ITS system can be used to quantify these diversity gains for a given propagation environment of interest.

References:
P. Wilson, P. Papazian, M. Cotton and Y. Lo, "Advanced antenna test bed characterization for wideband wireless communication systems", NTIA Report 99–369, Aug. 1999.

P. Wilson, P. Papazian, M. Cotton, and Y. Lo, "A Comparison of 1920 MHz Diversity Gain Using Horizontally and Vertically Spaced Antenna Elements", in Proc. 1999 IEEE Radio and Wireless Conference (Denver, CO), pp. 243–246, Aug. 1999

P. Papazian, P. Wilson, M. Cotton, and Y. Lo, "Advanced Antenna Test Bed Characterization For Wideband Wireless Communication Systems", to be presented at the 1999 Vehicular Technology Conference (Amsterdam, Netherlands), Sep. 1999.