Patent GB-1337143-A (GB)
Title: Time division multiple access communication systesm
IPC: H04B7/212
Filed: 1970-10-14
Abstract: 1337143 Multiplex pulse signalling POST OFFICE 14 Oct 1970 [15 Oct 1969] 48866/70 Heading H4L A time division multiple access satellite communication system is provided with means for preventing failure of the system when the reference signal denoting the start of each frame is lost. It is assumed that the system includes three stations located in France, U.S.A. and U.K. respectively, that in France being the reference station, and appearing in a frame in that order as burst A, burst B, burst C, Fig. 1. Each burst of data must be synchronized so that the bursts do not overlap at the satellite and includes a preamble followed by P.C.M. speech signals in time division multiplex. The preamble comprises first a guard time to separate the bursts, followed by a carrier recovery portion consisting of unmodulated carrier. This is followed by a clock recovery portion consisting of ten bits followed by the twenty bit station address code (SAC). The SAC code is followed by channel, allocation and routing data (CARD) which consists of 32 bits submultiplexed over eight frames at 4 bits per frame. The CARD signal includes a 4 bit destination signal and a 9 bit signal indicating the channel number carrying the speech signal for that destination. As described in Specification 1,323,429 the destination slot and the channel number slot are also used to initiate a channel re-allocation process if additional unused channels become available. The code 0000 appearing in the destination slot informs all stations that reallocation is to take place and 1111 in the destination slot signifies that the number in the channel number slot indicates the highest numbered channel at that time in use by the sending station. The last part of the preamble is an 8-bit word called the access service circuit used as an interstation speech line. Two bits of the CARD signal are used to identify the reference station a one bit in the 20th bit position indicating that the transmitting station is the reference station, a zero in the 20th position indicating that the transmitting station has lost the reference station signal and a zero in the 21st position indicating that the reference station signal has not been lost. Each station includes logic circuits for detecting the data in CARD signals for all the stations, including its own, one of these logic circuits (for detecting the French CARD signal at the U.S. station) being described Fig. 3 (not shown). When the French SAC signal is detected a timing means is started and the received CARD signal is fed into a shift register, and after eight bursts the complete CARD signal is stored. The succeeding burst contains the complement of the SAC signal to indicate the start of a new 32-bit CARD signal. The circuit provides signals L or R indicating that the reference signal has or has not been lost and re-allocation information if present. Circuit details, Fig. 2.-Each station includes three of the logic circuits 50, 52, 54 referred to above, the re-allocation information being supplied to a logic circuit 56 described in the above-mentioned Specification. The station described is assumed to be the U.S. station. The information received from the satellite at 58 is supplied to a system 72, controlled by timing means 74, synchronized by the output of burst synchronizer 70, wherein the speech signals are decoded and supplied to the appropriate subscriber lines. Signals from the subscribers are coded at 72 and the burst preamble is assembled at 78 for transmission at 76 under the control of timing means 74. In the burst synchronizer 70, if the burst is correctly positioned the basic clock frequency is divided by N to initiate a burst from the local station every N clock pulses, there being N clock pulses per frame. If the burst synchronizer detects that the local station burst is too close to the reference burst the clock frequency is divided by N + 1 and if it is too far from the reference burst it is divided by N-1. This has the effect of delaying or advancing the burst one bit per frame respectively. The output of receiver 58 is also supplied to the CARD detector circuits 50, 52, 54 and the SAC detector 51. Detector 51 provides an output on the U.K., France or U.S. line in response to the corresponding address code and these signals also gate the respective CARD words into detectors 50, 52, 54. Assuming that France is the reference station, detector 52 will provide an R output every frame, which is supplied via OR gate 62 to a position detector 60, an integrating monostable 66, reallocate logic 56 and burst synchronizer 70. Position detector 60, Fig. 4 (not shown) provides an output if the U.S. station is in position B in the burst. The reference signal R operates monostable 66, which has a one second period, so that as long as the reference signals are received it remains operated. If the reference signal is lost the monostable releases and AND gate 82 is enabled via inverter 68 to cause a loss signal to be inserted at L in the preamble assembling means 78 which will be transmitted until a new reference station, i.e. station B, takes over. The L output from detectors 50, 52, 54 are supplied to a loss detector 64 which provides an output when at least two of the detectors indicate a loss of reference, since a single indication could be a failure at the local station. An output from detector 64 sets a bi-stable 94 which enables the divide by N gate 88 and inhibits gates 90, 92, resulting in no change in position of the local station burst. The signal from detector 64 is also supplied via gate 98 and gates 84, 86, to provide an indication in the reference slot of the CARD word that station B is now the reference station and to send out a re-allocation code and all stations now synchronize their bursts on the new reference burst. Delay 96 delays the resetting of bi-stable 94 for a period equal to approximately twice the round trip to the satellite to allow this to be achieved.