The re-engineering of VHF mobile radio services in the UK
This thesis covers the work and considerations necessary for the design of VHF mobile radio systems, and is directed particularly at the requirements of the emergency services of the United Kingdom. It was occasioned by an international regulatory decision to re-allocate part of their existing operating frequencies. This work provides both a structure for such a design, and a record of events. It considers the various elements of design, initiallv from a theoretical base which is developed to the extent where practical trials and external constraints overtake the theoretical analysis. The constraints can take the form of compatibility with the eXisting system, cost. timescale. and political considerations. The concept of a design flow chart is shown to lead to many circular arguments of the chicken-and-eg~ variety to the extent that the resultant diagram is termed in interaction diagram since its main function is to show how the design elements interface with each other. It consists of many feed-back loops; and design completion is achieved when a traverse of all paths of the interaction diaram can be made without meeting discontinuities at the interfaces. The topic of area coverage is examined in depth and the multiple transmitter common channel (Quasi synchronous) mode is shown to be efficient from both the time and the spectrum economy viewpoints. There are benefits also from operational considerations. The section shows that a reuse factor of 3 should be practical, which is of interest to mobile radio users in general. and cellular radio telephony systems in particular. The choice of type of modulation is quickly shown to lie between FM, AM and SSB. All three are analysed from single station and quasi synchronous aspects: the theoretical predictions of the superiority of AM, for the system under study, is confirmed by subjective trials from which design parameter limits are obtained. Examination of equipment hardware. mobile and base station, shows that system design aspects are severely constrained by intermodulation products due to passive site non linearities. Measurements indicate that these will be troublesome up to at least the 11th order. A frequency assignment plan covering mobile and fixed services for the country is derived and the dominating influence of the intermods is shown to result in a workable, but unconventional version.