The Overlapping Problem and Performance Degradation of Mobile Digital Communication Systems

The expansion in the use of land mobile communications have led to a state of congestion in that portion of the radio spectrum allocated to the land mobile service. Digital communication is one of the methods that can be applied to improve spectrum utilization. In a typical digital communication system, messages are not queued and all overlapped messages have to be retransmitted. That overlapping phenomenon has a negative impact on the performance of the system. The behavior of various determinants of system performance (like probability distribution of waiting time and the load that saturates the channel capacity) are studied in detail. A simulation model was developed to simulate four basic design approaches. It was found that a combination of randomizer (for automatic retransmission) and an inhibit signal has no significant payoff and that each one of these features is superior to a commonly used method of automatic retransmission with constant delay. The relative merits of each design is studied as a function of the load on the channel.