The radio environment on electrically small platforms is changing rapidly. Until recently one radio was used in isolation and was usually connected to only one antenna. The situation today is very different: there is usually more than one radio used at once for example a handset may have 4 cellular bands, GPS and BluetoothTM. Sometimes WLAN radios are also present. This means that more RF filtering of signals is necessary. It is also becoming common for each radio to use more than one antenna in order to create diversity or for MIMO applications.
Antenna diversity is already used with WLAN radio in order to counter multipath, reduce outages and improve the quality and reliability of the communications link. Generally three types of diversity are used, two antennas can be deployed as far apart as possible to create some spatial diversity, they can be oriented orthogonally to give polarisation diversity or they can have different beams patterns. Diversity in current WLAN systems is usually restricted to two antennas for each radio as this is enough to ensure that if one antenna is in an RF null, the other is generally not, thereby providing better performance in multipath environments. Only one radio is present and so the receiver listens to one antenna at a time and a RF switch is used to select the antenna giving the best signal.
Reference:
Antenova, "Antenna Designs for MIMO Systems", Queen Mary University of London 2004.
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