An Optimum Filter for Mobile Digital Communication Systems

The conservation of bandwidth in multi channel mobile, digital communication systems for both civil and military services introduces stringent filtering requirements. To obtain security advantages or to aid the integration of a digital network with existing equipment, the multiplexing of digital channels is conventionally a hybrid system of time division multiplexing and frequency division multiplexing. With the allocation of channels on a frequency basis there is an inherent energy spillover between adjacent channels resulting in cross-talk effects. This degenerative effect is further accentuated in mobile systems due to the unpredictable nature of the physical position of particular transmitters and receivers in the communication complex. There is a likelihood of a situation such that in a given location a weak signal is being received and at the same time, from a short distance away, transmissions are taking place on an adjacent channel. This paper is concerned with the design of filters to effect adequate filtering of the signal before transmission whilst incurring a minimum degradation in system performance. An optimum filter is derived to satisfy typical performance criteria.