Cell breathing (or respiration) is a term used to reflect the fact that the coverage area of a base station in a CDMA system will become smaller if there are more subscribers.
While in conventional FDMA and TDMA systems, like GSM, each subscriber has the full transmission power of the base station, in CDMA systems (like UMTS and CDMA2000), the output power of the base station is divided among all active subscribers.
If, for example, a total power of 20 watts is available, the network can feed 10 watts to each of two subscribers, or, 2 watts to each of 10 subscribers. The maximum output power available for an individual subscriber depends on the number of active subscribers in each radio cell. The more subscribers logged on to the cell, the lower the power available for a individual subscriber and hence the lower its range.