Patent GB-1487689-A (GB)
Title: Time division multiple access satellite communications system
IPC: H04B7/212
Filed: 1975-02-11
Abstract: 1487689 TDMA satellite communication system DIGITAL COMMUNICATIONS CORP 11 Feb 1975 [11 Feb 1974] 5819/75 Heading H4M In a TDMA satellite communication system, the need to separately measure the propagation delay between each ground station and the satellite, so that the ground station transmission bursts can be multiplexed at the satellite in non- overlapping relationship, is avoided. A reference marker identifying the start of the satellite multiplex frame is transmitted from the satellite to each station. Detection of this at the station initiates a delay corresponding to the position of the time slot for the ground station. At the end of the delay the ground station transmission is started, and ended after a predetermined time less than the time slot duration, so that a guard time is provided. The guard time is selected to be at least equal to twice the propagation delay variation among all ground station-satellite transmissions. At the ground station, a satellite signal is received at 39 and demodulated at 40, demodulated data passing to output circuits not shown. The synchronizing code at the beginning of the satellite frame is detected in reference unique word detector 21 and used to reset a counter 23 which is counting scaled clock pulses provided by a clock in the ground station. The output counts from the counter 23 pass to a decoder logic circuit 25 which is set to respond to a given count, identifying the time slot appropriate to the ground station. When the decoder responds, it triggers a flip-flop 32 whose output acts as a window to enable data from the ground station to pass into and through the buffer 35 and modulator 36 to the ground station transmitter 37. The output from the decoder 25 also resets a burst duration counter, also receiving the clock pulses. The count outputs pass to a decoder 38 which after a time which is the same for all ground stations, resets the flip-flop 32 and thus closes the burst window. The duration of the window as set by the decoder 38, is arranged to be less than the duration of a time slot by at least twice the overall propagation delay variation among all ground station-satellite transmissions. Thus the remainder of the time slot provides a guard such that transmissions from different ground stations don't overlap. The output from the counter 23 is applied to a second decoder 30 which responds to a count corresponding to the satellite frame length to reset the counter in the event that the unique sync word is not detected. Associated circuitry may be provided to shut down the ground station after a certain number of unique words have failed to appear, in case sync with the satellite is being lost. Should the multiplex frame duration for the ground stations data be longer than the satellite frame, so that data from a particular ground station is to be inserted only in say every eighth satellite frame, it is necessary to detect the super frame repetition for the satellite transmission at 26 and use similar logic to that for establishing the proper time slot to establish also the proper frame (counter 28 and decoder 29). The output of the decoder 29 is applied to the decoder 25 such that the burst window opens only in the correct frame.