Interchannel Interference Considerations in Angle-Modulated Systems

The properties of frequency and phase modulation with respect to exchanging bandwidth for signal-to-noise ratio are well known, 1,2 but the type of noise considered is almost always limited to be random gaussian noise. In the design of any system, where the noise is likely to be interference limited, it is necessary to consider other kinds of disturbances such as co-channel and adjacent channel interference corrupting the desired received signal. Consider the following situation. In the frequency bands above 10 GHz where the signal attenuation resulting from rain could be very severe, close spacings of the repeaters are almost always mandatory for reliable communication from point-to-point and for all periods of time. 3,4 If low noise receivers are used in the system, it is possible 2333 2334 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1909 that the total interference power received by the system may be very much larger than the noise power in the system. For all practical purposes, the performance of such a system is determined by the interchannel interference. 3 ' 4 It is therefore desirable to evaluate the effect of co-channel and adjacent channel interference on the performance of any modulation system like FM or PM (or PCM) so that its advantages in combating interference can be determined, and any system parameters (such as rms phase deviation, channel separation, and so on) can be properly chosen to keep the baseband interference below a certain desired level. (It is possible to reduce adjacent channel interference by using suitable receiving filters, but co-channel interference occupies the same band as the signal.) The problem of interference in angle-modulated systems has been considered by many authors.