Antenna diversity
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Antenna diversity is a transmission technique in which the information-carrying signal is transmitted along different propagation paths. This can be achieved by using multiple receiver antennas (diversity reception) and/or by using multiple transmitting antennas (transmit diversity).
Even if the antennas are not more than a quarter of the wavelength apart, much of the achievable antenna gain is realized. A diversity combining circuit combines or selects the signals from the receiver antennas to constitute an improved quality signal.
A well-known practical application of diversity reception is in wireless microphones, and in similar electronic devices such as wireless guitar systems. A wireless microphone with a non-diversity receiver (a receiver having only one antenna) is prone to random drop-outs, fades, noise, or other interference, especially if the transmitter (the wireless microphone) is in motion. A wireless microphone or sound system using diversity reception will switch to the other antenna within microseconds if one antenna experiences noise, providing an improved quality signal with fewer drop-outs and noise. Ideally, no drop-outs or noise will occur in the received signal.
Another common usage is in Wi-Fi networking gear and cordless telephones to compensate for multipath interference. The base station will switch reception to one of two antennas depending on which is currently receiving a stronger signal. For best results, the antennas are usually placed one wavelength apart. For microwave bands, where the wavelengths are under 100cm, this can often be done with two antennas attached to the same hardware. For lower frequencies and longer wavelengths, the antennas must be multiple meters apart, making it much less reasonable.
Mobile phone towers also often take advantage of diversity -- each face of a tower will often have three antennas; one is transmitting, while the other two are performing Diversity reception.
The use of multiple antennas at both transmit and receive results in a multiple-input multiple-output (MIMO) system. The use of diversity techniques at both ends of the link is termed space–time coding.
References: Federal Standard 1037C and MIL-STD-188
