TABLE OF CONTENTS A primary design objective for any commercial or military mobile-radio system is to conserve the available spectrum by reusing allocated frequency channels in areas that are geographically located as close to each other as possible. The limitation in distance for reusing frequency channels can be determined by the amount of cochannel interference. The separation between adjacent channels and the assignment of frequency channels within specified geographic areas is limited by the parameters for avoidance of adjacent-channel interference. To achieve a satisfactory frequency channel-assignment plan it is necessary to fully understand the effects of cochannel and adjacent-channel interference on mobile-radio reception. The following paragraphs discuss cochannel interference and are followed by a discussion of adjacent-channel interference for different situations involving FM reception. At the present time, most mobile-radio systems in use are of the FM type.
Three other types of interference will also be described in this chapter. The problems are near-to-far ratio, intermodulation, and intersymbol interference. The near-to-far ratio interference problem is inherent in the nature of mobile communications; e.g., at times the mobile unit may be too close to an undesired transmitter and too far away from the desired transmitter. The intermodulation (IM) interference problem is usually experienced in multichannel, frequency-division multiplex applications. Finally, the problem of intersymbol interference because of digital transmission is also described.
Assuming that s( t) is the desired signal and i( t) is the interfering signal, the relationship between them can be expressed...
