Why Open-loop power control mechanism does not solve "near-far problem"?

The Open-loop power control mechanism, used in CDMA based systems, requires the transmitiing entity (mobile station) to monitor the received signal strength and channel interference in the downlink and adjust its transmission power accordingly. Now uplink and downlink signals use different frequencies and there is large frequency separation of uplink and downlink bands in UMTS FDD mode (uplink frequency band is 1885–2025 MHz and downlink frequency band is 2110–2200 MHz). As such, uplink and downlink fast fading (on different frequency carriers) are not correlated. The downlink signal may suffer from different sets of diffractions and reflections that uplink signal may not encounter, thereby not giving a correct result. This is the reason that "open-loop power control" can not be used to solve "near-far problem". Usually this mechanism gives correct result only on average. Therefore open loop power control is used mainly to provide initial power setting for the initial access of system (RACH).

The open loop power control tolerance is ±9dB under normal conditions and ±12dB under extreme conditions.


Reference: 3GPP TS 25.101.