Patent GB-1300960-A (GB)
Title: System for controlling the transmit time of stations which are in communication with one another via a satellite
IPC: H04B7/212
Filed: 1970-11-05
Abstract: 1300960 Satellite communication systems INTERNATIONAL STANDARD ELECTRIC CORP 5 Nov 1970 [12 Nov 1969] 52590/70 Heading H4L  Means are provided for varying the length of the time slots allotted to the respective stations communicating via a satellite repeater station in a time division multiple access system using P.C.M. in accordance with traffic requirements so that the full frame is always in use even if some stations are not participating, one of the ground stations acting as a master station. In the system described each station transmits its information compressed into a pulse burst in such manner that the bursts of all the stations arrive at the satellite in a predetermined sequence without overlap, timing correction being applied at the ground stations in known manner. Each burst includes a preamble including a sequence of bits for effecting carrier and bit synchronization, a codeword for identifying precisely the beginning of a burst address codes and other supervisory codes as required. The master station determines the position of the time slot of each station within a frame and informs all the stations via a service channel which thereupon store this information. Each station at the start of each received frame starts a time measuring arrangement whose reading at the commencement of a new burst is compared with the stored information any difference being used to control the transmission of the stations own burst and also being stored to provide an updated reference for the following comparisons. Details of the control system. Incoming signals Inf. are supplied to a drift register SR and when the information stored in section SRB includes a starting code for a message burst from any of the stations this is detected by a correlation receiver KE and provides a pulse a, Fig. 2. At the same time the corresponding address signal stored in section SRA is checked by devices AAE, AAL, and passed to an address store ASP under the control of pulses a and if it is the station's own address (device AAE) a pulse c is generated, or the master station's (device AAL) a pulse b is generated which denotes the start of a frame. Pulses b synchronize the station receiver clock generator TE which generates channel clock pulses d and comparator V compares the phase of pulses c with the nearest pulse d to control the clock generator TS for the transmitter to reduce the difference to zero. A channel counter ZS is controlled by generator TS. Pulses a also control a clock generator DKT for the service channel. The master station continuously transmits the station addresses together with the allotted time slots and burst durations and information regarding the last time slot alteration. This data is received at each station by the service channel receiver DKE and transferred to a store RSP the information relating to the associated station being contained in row EA which contains the address Adr. the last difference of the beginning of a burst DB, the beginning of a burst period B, the last difference of a burst duration DL and the duration of the burst L. Information L is passed to the counter ZS to effect disconnection of the carrier at the correct time. A channel counter ZE is returned to zero at the start of a frame by pulse b and counts the pulses d, each pulse a causing the counter reading to be transferred to a store ZSP at the same time that the corresponding address is stored at ASP. A control signal from ASP causes the value stored at B (from EA) to be compared at Diff. with the value which has just been stored at ZSP and if there is any difference the channel counter ZS for the transmitter is adjusted accordingly via a control store KSP which prevents a difference signal from having an immediate effect on counter ZS. If a burst difference for another station is detected the output from Diff. together with the address Adp from the store RSP cause a generator WS to transmit a warning signal via channel SK to the affected station. The difference detected at Diff. is also compared at ST with the signal DB stored in row EA of store RSP and when these values agree it assumed that a master station controlled operation is involved. Should they not agree a fault is assumed and a signal ST is transmitted.