JPL's Wireless Communication Reference Website

Chapter: Wireless Channels

Reciprocity and differences between uplink and downlink

Two-way communication requires facilities for 'inbound', i.e., mobile-to-fixed, as well as 'outbound', i.e., fixed-to-mobile communication. In circuit-switched mobile communication, such as cellular telephony, the inbound and outbound channel are also called the 'uplink' and 'downlink', respectively. The propagation aspects described on other pages are valid for inbound and outbound channels. This is understood from the reciprocity theorem:
If, in a radio communication link, the role of the receive and transmit antenna are functionally interchanged, the instantaneous transfer characteristics of the radio channel remain unchanged.

In mobile multi-user networks with fading channels, the reciprocity theorem does not imply that the inbound channel behaves identically as the outbound channel. Particular differences occur for a number of link aspects:

JPL's Wireless Communication Reference Website Jean-Paul M.G. Linnartz, 1993, 1995.