Synchronisation of cyclic coset codes for railway track circuit data

Abstract

The data capacity of railway track circuits can be increased by the use of binary block codes. The application of standard block coding techniques is limited because of the fail safe requirement and because each code must be independently synchronisable. Also, the hostile electrical environment suggests that error detection/correction should be employed. The two problems of error detection/correction and synchronisation are simultaneously addressed by cyclic coset codes, without incorporating any special signals or bit patterns for 'start of message' or 'end of message' indicators. Codes suitable for track circuit data are analysed in terms or their synchronisation error performance and a new measure of the comma free properties of these codes, the overlap probability distribution, is introduced. This provides a better criterion for the choice of coset codes than is given by the index of comma freedom alone.