Sequence synchronization

DETAILED DESCRIPTION We claim: 1.
Apparatus for transmitting a data sequence from a transmitter to a receiver, including means at the transmitter for scrambling the data sequence by modulo addition thereto of a pseudo random sequence (PRS) output from a PRS generator at the transmitter, and means at the receiver for unscrambling the received scrambled data sequence by modulo subtraction therefrom of the same PRS, the PRS at the receiver being generated by a respective PRS generator threat, the receiver PRS generator being synchronised to the transmitter PRS generator by means of samples taken from the transmitter PRS and conveyed to synchronisation means at the receiver, and wherein the transmitter includes a PRS sample mapping block and the receiver includes a functionally complementary sample mapping block and the time of transmission of the conveyed samples is decoupled from the points in the PRS for which they were selected, the conveyed samples being skewed with respect to the PRS, and wherein the transmitter sample mapping block is such that a plurality of linearly independent samples are selected from the PRS, each sample being selected at a regular interval mutually prime with respect to the PRS cycle length.
2.
Apparatus as claimed in claim 1 wherein the plurality of samples are conveyed to the receiver at selected positions in the transmitted data sequence and wherein at the receiver their position is located and they are applied to the receiver synchronisation means, said synchronisation means having knowledge of their original relative positions and absolute positions in respect of the scrambled data sequence.
3.
Apparatus as claimed in claim 1 wherein the PRS is an nth order pseudo random binary sequence (PRBS).
4.
Apparatus as claimed in claim 1 wherein the linearly independent samples taken regularly from the transmitter PRS are stored by the transmitter sample mapping block and conveyed in a group to the receiver where they are stored in the receiver sample mapping block and including means for injecting the receiver stored samples into the receiver synchronisation means at the same interval as they were sampled