Diversity Reception To reduce fading effects, diversity reception techniques are used. Diversity means the provision of two or more uncorrelated (independent) fading paths from transmitter to receiver These uncorrelated signals are combined in a special way, exploiting the fact that it is unlikely that all the paths are poor at the same time. The probability of outage is thus reduced. Uncorrelated paths are created using polarization, space, frequency, and time diversity

Space Diversity A method of transmission or reception, or both, in which the effects of fading are minimized by the simultaneous use of two or more physically separated antennas Antennas must be spatially distant so that no correlation exists between fading phases (one or more wavelengths). Power Height

Polarization Diversity Diversity transmission and reception wherein the same information signal is transmitted and received simultaneously on orthogonally polarized waves Power (Horizontal) Time Power (Vertical)

Frequency Diversity Transmission and reception in which the same information signal is transmitted and received simultaneously on two or more carrier frequencies. The frequency difference must be high enough so there no correlation exists between their fading phases

Diversity Combining - Switching The current branch remains selected until a metric fails a certain threshold, the Received Signal Strength Indicator (RSSI). The next branch is then blindly selected. An adaptive threshold removes unnecessary switching. When the signal fades relative to the mean, switching occurs Cheap and simple. RF switch IF Comparator Fixed Threshold RSSI F C + F IF To Detector Sync RF switch IF Comparator RSSI F C + F IF To Detector Sync Average

Diversity Combining - Selection Uses M receivers- branches, compares signals and selects the best one. Slight performance advantage over switch diversity The system is expensive Using RSSI as indication is unduly affected by interference Rx RSSI Selection Logic Rx RSSI Rx RSSI Detector

Diversity Combining – EGC Equal Gain Combining All branches are merely co-phased and summed Co-phasing & Summing Detector 111

Diversity Combining – MRC Maximal Ratio Combining Each branch is weighted before summationin proportion to its own signal-to-noise ratio Slightly better performance than EGC but more complex Co-phasing & Summing Detector a1 a2a2 aMaM