Patent GB-1347748-A (GB)
Title: Frame synchronization technique for satellite onboard switching  systems
IPC: H04B7/204
Filed: 1971-09-23
Abstract: 1347748 Satellite communication systems COMMUNICATIONS SATELLITE CORP 23 Sept 1971 [23 Sept 1970 26 March 1971] 44405/71 Headings H4L and H4K In a time division multiplexed space division multiple access communications satellite system in which the satellite carries switching means for providing a common loop-closure circuit of a predetermined time period as part of its switching processes, means are provided at an earth station for generating a synchronization unique word, for transmitting said unique word to the satellite, where its degree of coincidence with said time period will be dependent upon the synchronization condition of the earth station, and for detecting the degree of coincidence between said time period, as determined by the signal received from the satellite via the loopclosure circuit and the sync. unique word transmitted from the earth station, to alter the transmission time of the sync. unique word in response to the degree of coincidence detected. General description.-As shown in Fig. 1, the satellite 20 carries highly directional spot-beam aerials 21S 1  to 21S 3  receiving signals from ground stations S 1  to S 3  respectively. Aerials 22S 1  to 22S 3  transmit signals in the opposite direction and the system enables the use of a single carrier frequency without mutual interference. Each station S 1 , S 2 , S 3  &c. transmits during each frame period a sync. unique word AB, Fig. 2, followed by a plurality of time-divided signal bursts 1 to 8 destined for different earth stations and a switching matrix network 25 is programmed to provide the common loop-closure time slot for the sync. signal and to switch from origin grouping to destination grouping the information segments of a frame. The loopclosure is effected by connecting the receive aerial of a station with its associated transmit aerial on the satellite. Each information burst is preceded by a station identification code UW. Earth station, Fig. 3.-The sync. code AB generated at 35 is time division multiplexed with information signals fed in via shift registers 27 to 34 and with the code UW generated at 36 and supplied via PSK modulator 38 and frequency-changer 39 for transmission to the satellite. The transmitted frame is then distributed by the switching matrix on the satellite to first return the sync. word to the transmitting station followed by the information bursts destined for that station from other stations. At the earth station the received signals are frequency-changed at 41 and after demodulation at 43 the code UW received at the start of each burst is detected at 44 to supply a pulse to a gate pulse generator 45 distributing the signal bursts to their correct outputs via shift registers 46 to 53. The sync. code AB is detected at 54 and if the complete code is detected the station is frame synchronized and no further control is required. However if the part detected is A or B the detector 54 provides a signal via a gate control pulse generator 56 to advance or retard the transmitted burst as required until the complete code AB is received. Each part A, B of the code carries a short preamble for carrier and clock recovery. On-board T.D.M. switching matrix.-It is assumed that there are three earth stations and the satellite carries a 3 x 3 switching matrix 57, Fig. 4, with switches SW1 to SW9 having as input the corresponding row for each earth station 23S 1 , 23S 2 , 23S 3 . The outputs of all the switches in a column are combined to form a single output line and the switches are controlled by three 2-bit decoders at 59, one for each column. Assuming a frame of eight information time slots plus one loop-closure time slot, the time slot memory 62, Fig. 5, comprises three columns, each having eight two-bit registers, each register defining a particular switch SW1 &c. A clock generator 60 controls a decoder 61 to cyclically feed one pulse to a loop condition generator 63, providing the common loopclosure time slot and eight pulses to memory 62. At the start of each frame generator 63 is enabled whereby decoder 59 closes switches SW 1 , SW 5 , SW 9  to provide the loop-closure time slot, after which 8 pulses are supplied sequentially to time slot memory 62 with each pulse enabling one two-bit register in each column to close the appropriate switches in a pre-arranged sequence to provide the interstation connections shown in Fig. 2. This sequence is determined by the 2-bit coding stored in the registers and the number of repetitions thereof and may be controlled by information derived from a satellite command system through a command interface 64. This controls a 2-bit register 65 supplying a new routing pattern to control any selected switch S 1  to S 9 , a 2-bit register 66 which designates the column and a 3-bit register 67 which designates the row in time slot memory 62. Modifications.-The sync. signal AB may consist of two parts with part B frequencyshifted by about 5 MHz in relation to part A, occurring at the clock frequency and requiring no preamble. Two filters at the earth station are tuned to parts A and B respectively and when the outputs are equal the system is synchronized. Alternatively, the sync. signal may be a single PSK word preceded by a preamble and supplied to a correlation detector at the earth station which produces an output of amplitude determined by the number of correct bits detected and compares this output with a reference voltage, any error being used to control the clock generator. Modified system.-Each spot-beam aerial on the satellite may be arranged to irradiate a zone containing, for example, three earth stations, Fig. 7 (not shown), whose signals access the satellite in time division multiple access fashion. Each zone accesses the satellite in space division multiple access as previously described. The frame format of each zone is prearranged with respect to the on-board switching matrix so that when the respective frames from each zone are in frame synchronization there will be a correct routing of the information. Each earth station in a zone is assigned different frequencies for its sync. unique word and having acquired synchronization transmits its burst at a prearranged time in the frame. Alternatively, one of the stations in a zone may be designated a reference station for that zone and transmits a sync. signal which is received by the other stations in the zone during the loop-closure time slot.