Patent GB-1210403-A (GB)
Title: Synchronizer for time division multiple access satellite communication system
IPC: H04B7/15
Filed: 1967-11-13
Abstract: 1,210,403. Radio signalling. COMMUNICATIONS SATELLITE CORP. 13 Nov., 1967 [16 Nov., 1966], No. 51496/67. Heading H4L. A synchronizing arrangement for a time division multiplex communication system wherein a plurality of earth stations communicate via an orbiting satellite relay station, each earth station being allotted a time slot in the satellite time frame, each station transmission including a synchronizing signal and one earth station being designated a master station, comprises means at each slave station for detecting the master synchronizing signals and said slave stations synchronizing signals returned from the satellite stations, means for comparing said signals in the same frame time and means responsive to the comparison for controlling the transmission timing of said slave stations. The arrangement compensates for variations in propagation time and ensures that each ground station signal burst occupies its allotted time slot in a frame at the satellite station. Slave station, Fig. 2.-When a non-transmitting station initially requires access to the satellite a preamble signal burst including a unique code word for that station is transmitted using a computer predicted time position which is set via a control circuit 18 adjusting a counter 12 controlled by bit rate clock pulses from a generator 10. The pulses for the various timing operations are selected by a decoder 14. The master station reference signal unique word and the slave station unique word are detected initially by means disclosed in Specification 1,210,406. The counter 12 controls the decoder 14 to start the station transmitter to transmit the signal burst, and the received local station unique word and the reference station unique word provide master and slave sync. pulses which after being delayed at 26 by an amount representing the number of time slots separating the master station and the local station, are supplied to a phase comparator 28. The output of comparator 28 has a polarity dependent on the direction of the error and this determines which of the gates 32, 34 is opened to advance or retard counter 12 via a control circuit 20 by one bit per frame until the error is reduced to zero. The correction rate circuit 24 selects the associated master and slave sync. pulses every (2Td + a) seconds, where 2Td is the time of one roundtrip via the satellite and a is a margin to provide for movement of the satellite &c. and if these signals are not found a sync.-loss detection circuit 36 switches the circuit 24 to a "search" mode in which it looks for the sync. pulses in each frame, and after a predetermined time disables the transmitter if the search is unsuccessful. When associated master and slave sync. pulses are found either during normal operation or during a "search" mode, the phase comparator 28 supplies a signal to an error storage input of a counter 22 which counts up or down in accordance with the amount and direction of the error and on receipt of a confirmation signal from circuit 36 acts to change the position of counter 12 in accordance with the number of bits stored. A second signal from circuit 36 prevents action being taken by counter 22 on any false phase comparisons which may have occurred during loss of synchronization. The logic circuitry is described in some detail.