Patent GB-1397979-A (GB)
Title: Time division multiple access satellite communications system having special reference bursts
IPC: H04B7/15
Filed: 1972-08-10
Abstract: 1397979 Pulse multiplex signalling KOKUSAI DENSHIN DENWA CO Ltd and POST OFFICE 10 Aug 1972 [11 Aug 1971] 37416/72 Heading H4L All earth stations in a TDMA satellite repeater system transmit a respective "burst" of communication signal, all "bursts" containing an identifying "unique" word in addition to data and signalling words, and one station acts as reference by additionally sending a burst which is similar but which contains no data and signalling and whose unique word is different from the rest. This enables any earth station to assume the reference function in the event of failure. The bursts are sent in t.d.m. and each may be at the same frequency in which case each station synchronizes its own unique word Fig. 2, to the reference unique word. Alternatively each burst may contain several frequencies occurring at separate times, to be handled by separate repeaters on the satellite, in which case reference unique words are sent by the reference station at some or all of these frequencies, but in non-overlapping sequence in order to limit power requirements. Other stations then synchronize their respective frequencies with these reference bursts (Figs. 3, 3a, not shown). The slave stations unique words are all the same. Initial synchronization.-A local station sends a low power pulse, compares the time of return of this pulse with the time of reception of the reference unique word, and stores the difference (1232, Fig. 12, not shown). The start pulse which initiates burst transmission is then shifted stepwise until this difference is removed, whereupon the local station burst is in the correct position within the TDMA frame. The PSK modulator 408 (Fig. 4, not shown) is disabled during this operation. Final synchronization involves generating an aperture gate pulse (710, Fig. 7, 900, Fig. 9, not shown) which is seven symbols wide and "looks for" the reference unique word, a reference pulse being generated upon recognition thereof. Lock is confirmed only when five successive reference pulses occur, and then non-reference aperture gates (936) are generated to look for the unique words in local stations' bursts, the timing of these gates being modified by a memory 940 which knows approximately where to look. Prior to initial synchronizing procedures, particularly in the multifrequency embodiment, the complement of the reference sync unique word which occurs in each 32nd frame, is detected and used to generate a 4-second acquisition window (1312, 1314, Fig. 13, not shown) which prevents two earth stations trying to simultaneously acquire a frame position. Summary of other circuit features.-The transmission section of each station includes a multiplexer and pseudo random code type scrambler, and a PSK modulator; and of the component sections of a burst (Fig. 2), the data comes from interface modules (412, Fig. 4, not shown), the station identification and "housekeeping" signals from a control unit (410), and the carrier and symbol timing recovery and the unique word from a preamble generator (402). The latter responds to a code select signal from the multiplex unit (400, also 616, Fig. 6, not shown) to alter the unique word to the reference sync unique, in response to a received signal, when the station is to assume the reference function. The receiver section (Fig. 7, not shown) includes demultiplexing (Fig. 10, not shown) preamble detector (Fig. 8, not shown) and aperture generator and locking (Fig. 9, not shown). Pulse stuffing, and stuffing codes, may be employed once each frame, or every eighth frame (Figs. 16-21, not shown), and involve distributed pulse stuffing codes using compression and expansion techniques.