Digital radio communication system and primary and secondary station for use in such a system

Claim 1

A digital radio system comprising a primary station and a secondary station and wherein a succession of data symbols is exchanged between said stations via one or more time/frequency multiplex slots in each of successive multiplex frames, each multiplex slot accommodating a plurality of successive data symbols; handover from a present multiplex slot to a substitute multiplex slot being effected by transmitting in said substitute multiplex slot a copy of the data symbols concurrently being transmitted in the present multiplex slot; characterized in that either or both of said primary and secondary station comprises: a cyclic buffer having a first series of storage locations for storing at least the data symbols accommodated in a multiplex slot, and a parallel series of storage locations for storing a copy of the data symbols stored in the first series of storage locations; a high frequency buffer control stage for generating a first pointer (RFP) for controlling writing of successive data symbols from a received multiplex slot into said first series of storage locations in said buffer at a relatively high rate, commencing at an initial storage location determined by said first pointer; a low frequency buffer control stage for generating a second pointer (SSP) for controlling reading from said buffer at a relatively low data rate the data symbols which have been stored therein by said first pointer, reading from said buffer of all of the data symbols derived from a presently received multiplex slot being completed by said second pointer by the time data symbols from a next succeeding frame of said multiplex slot are written into said buffer by said first pointer; and a programmable communications controller for controlling both of said buffer control stages to effect handover from a present multiplex slot to a substitute multiplex slot, said communication controller being programmed to: i) cause first pointer to change said initial location in said first series of storage locations to a substitute location in said first series, the substitute location being displaced from the initial location by a number of storage locations corresponding to the time interval between said present multiplex slot and said substitute multiplex slot; ii) provide a substitute for said first pointer which relates to said substitute storage location instead of to said initial storage location; and iii) control said second pointer to read the copy data symbols from said parallel series of storage locations in said buffer until said second pointer reaches a location in said parallel series of storage locations which corresponds to said substitute storage location in said first series of storage locations, and to then switch to reading subsequent data symbols from said first series of storage locations; whereby handover from said present multiplex slot to said substitute multiplex slot is effected without any interruption of reading of data symbols by said second pointer.