Patent GB-1323429-A (GB)
Title: Time division multiple acess communication systems
IPC: H04J3/16
Filed: 1970-10-14
Abstract: 1323429 Multiplex pulse signalling; space satellite relay systems POST OFFICE 14 Oct 1970 [15 Oct 1969] 48865/70 Heading H4K and H4L A space-satellite time division multiple access system, in which each earth station has a burst position in the multiplex, periodically contracts and repositions the station bursts to withdraw channels idle in the bursts and then expands and repositions the bursts by distributing the withdrawn idle channels on an equitable basis over all stations. Each station burst comprises a preamble containing a station's own identity, transmitted and received back for synchronization, and containing a 4-bit portion of a 32-bit word spread over 8 frames for Channel Allocation and Routing Data and termed CARD. Speech channels comprised of 8-bit PCM codes occupy the remainder of the burst and are numbered in order of transmission, seizure of channels being according to the lowest numbered channel available so that busy chanels tend to bunch towards the front of the burst. Those channels, if any, beyond the highest numbered busy channel comprise a consecutive group of so-called slack channels liable to be called in for redistribution for the purpose of moving channels to those bursts carrying the most traffic. One station transmits the first burst of a multiplex as a reference in respect of which all other stations effect synchronization by appropriately setting their station burst transmission delay counter. To effect distribution of slack channels a station broadcasts over the satellite relay a "re-allocate" code as part of its preamble CARD code word. In response to receipt of this code all stations formulate a replay by way of their CARD codes in order to broadcast in respect of each station the number of the highest numbered busy channel in use at that time. Until such time as redistribution is complete stations are prohibited from seizing channels withdrawn for redistribution. As all stations receive all other stations' CARD codes and station identity codes, every station will be acquainted with and will store the number of the highest numbered busy channel in respect of every other station transmitting. As some stations may be off the air each station counts the number of bursts received in a multiplex frame period in order to determine the number of stations over which the distribution is to be made and in order to know when all stations have sent their slack channel data in a CARD code. When as many highest numbered busy channel codes have been received as there are station bursts, the stations each set about calculating their position in the frame of bursts on the understanding that the identified slack channels have been given up. Such a calculation merely adds together the highest busy channel numbers of all preceding station bursts and adds a fixed preamble time for each burst. In accordance with such calculation, burst transmission times are adjusted by resetting the burst synchronization delay counters, the burst following the reference burst being dealt with first with the other bursts being dealt with successively in the order of their position in the multiplex so as to crowd the contracted burst times towards the front of the multiplex. Once that is achieved each station calculates the nearest whole number of slack channels which constitute an equal share for each of the transmitting stations. If such a division results in a remainder it may be spread as far as it will go at one channel per station. Given a list of the shares under the redistribution for all stations, each station calculates its burst position by a reference to the proposals for stations whose bursts come before in the multiplex and burst transmission time is adjusted accordingly. As burst periods are in this second process expanded it is necessary in order to avoid overlapping of bursts to start the adjustment of burst times with the last burst of the multiplex and to progress burst by burst towards the reference burst for which no adjustment will be needed. Fig. 3, not shown, illustrates the logical circuits required at each station to conduct the computations and time adjustments of burst transmission times in odrer to effect the redistribution of slack channels. Such redistribution may occur at 3 minute intervals.