The effect of various modulation schemes on digital fixed links

Fixed links are planned and assigned frequencies on the basis of adequate signal to noise ratio and freedom from unwanted interference. For digital links this is based on the bit error (or symbol error) ratio achieved for a set proportion of the time. Planning utilises known BER/signal to noise ratio (or Eb/No) graphs for the various modulation types. Historically these curves have been based on white or Gaussian noise and frequency planning has assumed a noise limited environment. It is now being recognised that an interference limited environment may provide a more optimum assignment strategy. The characteristics of the interference from other transmissions differs from that of Gaussian noise and this raises the question of the continued validity of Eb/No curves based on Gaussian noise. There are two assignment benefits to be gained in changing to an interference limited strategy. If levels are increased such that the noise becomes a negligible fraction of the unwanted energy then the increase in allowable interference is greater than the increase in the wanted signal. For example if the receiver reference level is increased from 1 dB above the noise to 11 dB above, the interference can increase by 16.5 dB for the same carrier to interference plus noise ratio, C/(I+N). This is an improvement of 6.5 dB. However a further benefit is also obtained because of the changed characteristics of the interference plus noise component. The paper compares, by computer simulation, the effect on the symbol error probability of pure Gaussian noise (as a reference) and interfering signal plus noise for a variety of test cases