Patent GB-1477614-A (GB)
Title: Transmitting station and receiving station for operating with a systematic recurrent code
IPC: H04L1/00
Filed: 1975-02-14
Abstract: 1477614 Data transmission; error correction THOMSON-CSF 14 Feb 1975 [19 Feb 1974] 6491/75 Heading H4P In a system combining the use of systematic self-correcting code with repetition on request comprises an L stage coding shift register 9, at least the first and last stages having outputs, and mod-2 adders each coupled to different stages of the register delivering a parity bit associated with that in the first stage, with a selecting means coupled to a clock pulse circuit for supplying to the register input (a) L message bits, or (b) L message bits immediately previously stored, and a controller for selecting (a) or (b) whereby (a) continuation bit trains + parity bits associated therewith or (b) repetition bit trains + parity bits associated therewith are passed to a transmitter, and particularly a radio transmitter. At the transmitter. Fig. 1, a three stage register 80 receiving continuation (101) or repetition (010) prefixes forms part of register 9 through which the prefix is clocked followed by (a) data from source E, clocked by 12 through AND 129 or (b) recirculated data through inverted input AND 28, dependent on a selecting signal 16, gates 28, 129 being commoned through OR 20. Register 9 may be duplicated. At the same time as prefix + message bits are stepped through 9 they are applied through AND 27 to a radio transmitter 2 together with parity bits through AND 26 which are both transmitted to a remote station preferably through parallel channels. After transmission of a 22 bit block (plus prefix) transmission ceases until receiver 4 receives either a continuation request (1111) when further data is passed, or repetition request (0000) hence (010) is placed in register 80, gate 28 being enabled and previous character is re-sent, the bits in register 9 being recycled. Circuit 10 has a counter (201, Fig. 3, not shown) which records repetition requests and where a determined value, e.g. 3 is reached denoting that transmission quality is bad, this arranges that two continuation requests must be detected before resumption of normal transmission. At the receiver 5, Fig. 4, 25 bit blocks serially issued on line 51 have their prefix detected at 7 which synchronizes a local generator, line 51 feeding a 22 stage register 61 and corresponding parity bits on line 52 feed a similar register 62 which are coupled through mod-2 adders 24 to computer 6 which performs operations according to equations given in the Specification, output being to a 25 stage register 67 providing a "replica" which is equal to the original data if transmission is error free. If transmission is not error free a decision is made from a number of "replicas" produced in 6 and the most likely correct one is passed to register 67, the first 3 stages of which contain the prefix which is identified at 100, also compared with a corresponding prefix, derived from a different degree of evaluation and placed in a register 63. A signal 102 to control 8 determines whether continuation or repetition is required. Fig. 6 (not shown) gives details of computer 6 together with a method of performing calculation to produce the "replicas". Various modifications are mentioned in the Specification including the provision of two or more parity bits for each information bit; use of prefixes and associated circuity are advantageous but not essential.