Determining the path loss is the key problem. Certainly it is possible in
a duplex link for the remote user to measure the power received from the base-station site
and hence calculate the path loss in the 'downlink'; however, unless the user is
stationary and operating on the same frequency for transmit and receive, this does not
necessarily translate to the path loss in the 'uplink' direction. For example, the remote
user could be receiving in a frequency selective fade, in which case the unit would
overestimate the path loss involved. Alternatively the uplink could be subject to a
frequency selective fade and the unit not generate sufficient transmit power. A solution
to this problem is to operate a closed loop power control
system whereby the base-station unit monitors the signal power from each remote unit and
issues commands to increase or decrease TX power accordingly. This can, however, incur a
high signalling overhead in the system. It turns out that power control is very critical
in CDMA and to a lesser extent TDMA operation, and closed loop power control is common in
CDMA applications. |