It may be receiver with two antennas for space diversity or transmitter with two antennas, or transceiver. The main idea is to improve the radio link. Mulipath is phenomenon when signals arrive to the same antenna with different phases due to different path lengths. It may be due to reflections or sometimes other phenomenons. If two antenna used and then signals combined with special way it is possible to mitigate the negative effect and get more stable radio link. It is not easy and requires channel modeling and other techniques. It also can be used to solve the coexistent problems. There are a lot of practical applications for antenna diversity techniques and it is hard to list all of them here. There is probably no one theory that describe all the situations and each practical case requires own analysis and solution. This problem become more and more serious today because a lot of radio devices deployed in close proximity and in adjacent spectra: wireless networking and Bluetooth, cell phones, wireless USB, UWB high speed home networking, satellite TV and so on. Another example is piconet transceiver and moving objects (people, animals, doors, etc) between antennas. This is when we use two antennas for both receiver and transmitter devices.
best regards,
RF-OM