6.5 Countermeasures to interference 6.5.1 Automatic transmit power control (ATPC)
Automatic regulation of the transmitted power of digital fixed wireless systems can be used, in particular, to reduce interference between channels on the same route and between channels radiating from the same system node. This reduction of interference is of value in helping to achieve optimum use of digital fixed wireless systems in a telecommunications network, particularly by:
– increasing the capacity of a system node; a reduced interference power permits a reduction in angular separation between adjacent radial routes;
– reducing interference from a digital channel into an adjacent analogue channel thus easing compatibility problems in shared bands;
– reducing the digital-to-digital interference between hops which re-use the same frequency.
ATPC is a technique in which the output power of a microwave transmitter is automatically varied in accordance with path conditions. In normal path conditions, the ATPC maintains the transmitter output power at a reduced level. Fades are detected by the far-end receiver and the up-stream transmitter is commanded to increase power via overhead bits.
The full advantages of ATPC are only realized if the power-control signal is closely attuned to error performance and if fading characteristics are taken into account. This means that attention must be paid to possible increases in bit errors as power is increased which causes transmitter linearity to degrade and spill-over into adjacent channels to increase. Attention must also be paid to the partial correlation between AGC signal level, digital signal distortion and interference power-density distribution during multipath fading events.
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