Antenna Diversity

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Using antenna arrays for diversity reception is one of the most straightforward uses of antenna arrays. Because the power level of a received signal can vary significantly with small changes in distance, a diversity array simply uses a set of antennas and combines the signals to obtain the maximum signal. Consider the example of Figure 1. Someone is talking on their cell phone, and a hypothetical (though reasonable) power is shown in the areas around the user.

example received level as a function of spatial diversity

Figure 1. Example of signal level versus position.

As Figure 1 illustrates, the received power can change by 30 dB (a factor of 1000) by moving the cell-phone a relatively small distance. This strong variation in signal power is caused by motion of surrounding objects (the user, cars, windy trees, etc) and multipath.

To combat this effect (known as fading), an array of antennas can be used. For instance, if 3 antennas are placed in the above situation, as shown in Figure 2, the antenna with the maximum signal can be selected and used.

multiple antennas used for diversity reception to mitigate fading

Figure 2. Three antennas used in a diversity array.

In the case shown in Figure 2, the signal at antenna number 1 has the largest signal, and can therefore be used. In this manner, the effects of fading are greatly reduced, and the probability of having a signal too small to work with decreases as the number of antennas increases.

Finally, diversity reception can occur for two antennas not separated, but receiving orthogonal polarizations. If one antenna receives vertically polarized waves, a second antenna can be placed near the first that receives horizontally polarized waves (which in a fading environment, are not strongly correlated). In this manner, diversity can be achieved.

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