On determining the transmission power of a digital radio link system, not only the so-called constant power dissipations caused by the link span but also random signal fades caused by multi-path propagation, for instance, have to be taken into account. Therefore the transmission power of the transmitter is often adjusted to a value considerably above the minimum transmission power which in normal conditions would suffice for interference-free reception at the other end of the link span. Excessive transmission power provides the link span with a power margin which ensures interference-free reception even in difficult conditions. On the other hand, however, high transmission power increases interference in adjacent channels or systems. This hampers the realization of dense radio link networks, efficient utilization of frequency bands and the reuse of the same frequency within the same area.
U.S. Pat. No. 4,004,224 discloses a method in which transmission power within a certain link span is increased automatically if fading is observed within this specific span, so that a constant reception power is maintained at the receiver. The receiver produces a signal representing the level of the received signal, and this signal is sent through a return channel to the transmitter for the adjustment of the transmission power.
In U.S. Pat. No. 4,309,771, the bit error rate of the received digital signal is monitored in place of the received signal level, and the transmission power is adjusted through the return channel in such a way that the bit error rate of the received signal does not exceed a predetermined threshold value.
In French Patent 2,592,256 the range of adjustment is, in principle, divided into two portions. At low reception levels the transmitter is instructed through the return channel to increase power to compensate for fading. At high levels the error rate too is monitored. If the error rate is high irrespective of the sufficient level, the transmission power is increased temporarily so that it exceeds the normal backoff of 6 dB (such as a backoff of 3 dB).
The above-described methods aim at adjusting the transmission power so as to decrease the transmission power required in normal conditions in order to decrease the interference caused in other link spans or channels while enabling sufficient quality of data transmission even during the fading periods. These methods, however, are not sufficiently effective in all situations.