Satellite telecommunications system featuring multi-beam coverage and dynamically controlled allocation of the satellite transmission capacity

The invention relates to a satellite telecommunications system featuring multi-beam coverage and dynamically controlled allocation of the satellite transmission capacity, of the type where a number of earth stations are linked to a satellite on board which regeneration of the earth-to-satellite signal is brought about. The system also features a modular beam-switching pattern and utilizes asynchronous protocol procedures for the exchanges between earth traffic stations and master station relative to telephone conversations and videoconferences, and for remote control of an on-board switching matrix (2) which operates the system, that is, allocates the satellite's transmission capacity so as to serve all the earth stations.

BACKGROUND OF THE INVENTION 
The invention disclosed relates to a satellite telecommunications system 
featuring multi-beam coverage and dynamically controlled allocation of the 
satellite transmission capacity. 
An important facet of satellite communications is the method of link-up 
between the satellite and the various earth stations. A person skilled in 
the art will know that many satellite telecommunications systems developed 
thus far have been of a multiple access time-division type (TDMA and 
SS-TDMA) utilizing non-modular switching patterns. In such systems, earth 
stations can transmit special packages of data, known as bursts, relative 
to different kinds of media or different services. Only multi-destination 
bursts exist however, which are of varying length, and generally contain 
almost the total traffic handled by the emitting station and directed 
toward a given repeater. 
Briefly, with a non-modular switched pattern, each variation in capacity of 
a given earth station can affect the allocation of bursts emitted from 
other stations. As there is no beam-pattern password generated on board 
the satellite in such systems, recourse must be made to complex techniques 
for its diffusion, as well as to a number of tracking stations needed for 
continuous monitoring of an onboard clock which governs the beam switching 
matrix, and to stand-by protocol arrangements that must be implemented in 
the event of a malfunction occurring at one of the two stations linked. 
All communications satellites utilized hither to are therefore 
"transparent": that is, the signal received is amplified once and has its 
frequency changed, by a transponder, and is re-transmitted to earth; such 
satellites cannot therefore be regarded as genuine repeaters located in 
space. 
The system incorporates digital speech interpolation (DSI) which, when 
transmitting, permits doubling the gain of the transmission capacity. A 
DSI station transmits a multi-destination burst containing all the traffic 
between that station and corresponding stations of the same group. When 
receiving, the DSI station must be able to analyse a given quantity of 
bursts originating from different sources; the input capacity of a DSI 
station may be, for instance, 240 voice channels. The concept has since 
been introduced of allocation on demand within systems operating on DSI, 
that is, the facility of varying the number of satellite channels 
allocated to each DSI station. 
The introduction of such allocation-on-demand techniques necessarily 
dictates the use of special protocols, special procedures which must be 
adopted in allocation of available satellite transmission capacity to the 
various earth stations. In modern 4-6 GHz and 11-14 GHz systems, the lower 
frequencies utilized are such as to permit adopting protocols with a lower 
degree of protection than is necessary at 20-30 GHz, in view of the fact 
that the attenuating effect of the atmosphere is much reduced. These 
protocols are different from the procedures used in simple rearrangement 
of traffic; they can provide frequent re-allocation of transmission 
capacity to serve the various earth stations, as well as bring about such 
re-allocation in real time, whereas the latter type are unable to perform 
either function by reason of their periodic type of operation. 
With a non-modular beam-switching pattern it becomes necessary, generally 
speaking, to make a rearrangement of the entire system for integration 
even of the smallest variation. 
Protocols adopted for the systems mentioned thus far are exclusively of the 
synchronous type, inasmuch as a variation in the beam-pattern of TDMA 
systems will affect satellite and earth stations alike; one is therefore 
faced with limitations, as the system is rendered particularly inflexible. 
It will be observed moreover, that the method of access generally adopted 
in modern satellite systems (TDMA) is conditioned by the type of coverage 
selected. More exactly, one must ensure full interconnection between all 
antenna beams of a multi-beam coverage system, especially where the number 
of such beams is particularly great. This requirement has produced the 
passage from standard TDMA to SS-TDMA: satellite switched time-division, 
multiple access. 
A global coverage system, on the other hand, makes for difficult 
coordination with other systems and is characterized by low gain of the 
on-board antenna. 
There are also scanning-type systems, but these are extremely complex as a 
result of high transmission speeds, which call for equally complex 
apparatus. 
It is the object of the invention disclosed herein to eliminate the 
drawbacks thus outlined which beset systems currently in use, by design 
and embodiment of a satellite telecommunications system featuring 
multi-beam coverage and dynamically controlled allocation of the satellite 
transmission capacity, so as to enable optimum handling of signals 
transmitted from the various earth stations, wherein regeneration of such 
signals is brought about on board the satellite and wherein the system 
utilizes a modular beam switching pattern and asynchronous type protocol 
procedures. 
SUMMARY OF THE INVENTION 
The stated object, and other objects besides, can be realized according to 
the invention with a satellite telecommunications system which features 
multi-beam coverage and dynamically controlled allocation of satellite 
transmission capacity, of the type wherein a number of earth stations are 
linked to a satellite on board which regeneration of the 
earth-to-satellite signal is brought about. The system features a modular 
beam-switching pattern and utilizes asynchronous type protocol procedures 
in allocation of the satellite transmission capacity between the various 
earth stations, both for bursts exchanged between earth traffic stations 
carrying telephone conversations and videoconferences, and master station 
wherein recognition and recovery of bursts transmitted by a given DSI 
station can be effected by the DSI receiving station with control retained 
by the same DSI transmitting station, which enables attachment of a 
preamble to such bursts, and for the remote control of an on-board 
switching matrix controlling the overall system; the system thus arranged 
is controlled by microprocessors of requisite capacity.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the diagram, 1 denotes a microprocessor enabled for receipt and handling 
of a burst carrying remote control signals for an on-board switching 
matrix 2 for a satellite 2a. Handling of the remote control burst is 
accomplished by two memories interconnected by and operating in concert 
with the microprocessor 1--viz, a deferred instructions memory 3 for the 
storage of control data to be used at a future moment, and a program 
memory 4 governing operation of the microprocessor 1. 
Also connected to the microprocessor 1, one has a provisional priority data 
table 5 holding information used for validity check purposes, prior to the 
transfer of such data into a definitive priority table once the 
instruction to implement has been given; 6 denotes the definitive priority 
data table. 
7 denotes an operative memory with the time cycle to be implemented by the 
matrix; 8 is a standby memory by means of which to store modifications of 
the time cycle contained in the operative memory 7. 9 denotes a monitor 
system providing the exchange between operative and standby memories, 
needed for implementation of the on-board matrix remote control 
instructions; these memories are referred to in detail further on in the 
section regarding operation of the system. 
In a satellite telecommunications system according to the invention, use is 
made of asynchronous type protocol procedures: that is, implementation of 
the procedure is not instantaneous. More exactly, the protocol procedure 
consists of a set of operations to be carried out in sequence, wherein one 
operation cannot be brought about unless the operation preceding it has 
been successfully accomplished. In short, variations in the beam switching 
pattern are brought in frames whose identification need not necessarily be 
rigid. 
With this method of proceeding one obtains a dependability and security in 
implementation of the protocol required by the DSI stations, that cannot 
be guaranteed with a synchronous type procedure. It is clear that more 
time may be needed to make extensive rearrangements in traffic; 
nonetheless, such a drawback does not occasion any great delays in a 
system according to the invention, since operation in dynamic mode 
signifies that only adaptations are called for, as and when necessary, and 
not fundamental rearrangement of the time plan ; in this way one achieves 
optimized allocation of the entire satellite transmission capacity. 
The inclusion of a master station 2b with asynchronous protocol procedures 
amongst the other earth stations is necessary from the operation 
standpoint. In DSI telephony, this station 2b is informed from outside of 
each new network requirement, in terms of channel allocations to the 
various earth stations, and processes new time plan instructions which are 
duly transmitted to the traffic stations 2c involved. 
The system utilizes as many protocol procedures as there are types of 
traffic, i.e. four: two for DSI telephone traffic, one for non-DSI 
telephone traffic and a fourth for video broadcasts, each one with its own 
specific timing and steps (too detailed to be described adequately in this 
application). In the case of DSI traffic, the master station 2b supplies 
all the information regarding transmission, thereby functioning as an 
intelligent station in relation to the earth stations, whereas reception 
information is supplied directly from the corresponding traffic station 2c 
which labels the bursts one by one with a preamble characteristic of the 
DSI station transmitting, not of the master station 2b, in order to enable 
recognition and recovery, when receiving, of all the bursts emitted from 
the DSI station in question. This condition is necessary for correct 
operation of the system, most especially where strong atmospheric 
attenuation occurs. 
Messages which the master station 2b exchanges with the traffic station 2c 
may be numbered, 0 to 127, or otherwise. 
The adoption of a redundancy factor of 1/2 permits obtaining reduced error 
levels after decoding, whilst any residual error is shown up by a parity 
check. 
In order to avoid too fast an average message transfer rate, the 
transmission channel is supplied by the satellite 2a itself, though 
utilizing a signaling structure whereby selective rejection of messages is 
possible. There are two signaling channels, one of which is the DSI 
signaling channel containing the allocation message (earth 
channel-satellite channel) common to all digital speech interpolation 
systems; the other is the channel containing a message that permits 
delivering a burst to the receiving DSI station in the identical state to 
that emitted from the transmitting DSI station. 
The DSI maps which associate DSI Tx and DSI Rx are supplied by the master 
station 26 and distributed to the system using the station-to-satellite 
signaling channel. Maps are distributed to stations either reentering the 
frame or entering it for the first time, or whenever modifications are 
made to the maps themselves. 
In addition, there is a fifth protocol procedure, to be followed by the 
master station 26 when updating the monitor 9 which controls the switching 
matrix 2. This protocol procedure may be adopted both for immediate 
implementation of the remote control signal received from the master 
station 26 or from video-conferencing chairmen sources, and in cases where 
the signal must be re-transmitted to earth prior to its implementation. 
Security of the protocol procedure is thus ensured by re-transmission to 
earth before implementation, in the case of deferred instructions stored 
in the memory denoted 3, whereas in the case of real time operations, 
security is ensured by the fact that the instruction refers to areas of 
the beam-pattern that are accessible only to the party emitting the 
instruction. 
Instruction validity check data is stored initially in the provisional 
priority table 5 before being transferred to the definitive table 6. 
Where a considerable number of incoming channels, say 60, happen to be in 
use at a DSI station, the likelihood that more than half the subscribers 
will be speaking at any given moment is small, given that in the average 
telephone conversation neither subscriber will speak for more than 35 to 
40% of the total time connected. It becomes possible therefore, to use 
just half the satellite channels for transmission of signals originating 
from subscribers when actually speaking. 
The coverage technique adopted in the satellite communications system 
according to the invention is of a multi-beam type, covering the entire 
area served by way of say, 6 beams, each of which is coupled to a repeater 
rated 147,456 Mbps. 
The preferred method of access for this multi-beam type coverage is 
SS-TDMA, in which switching operations are performed by the satellite 2a 
itself so as to permit interconnecting all antenna beams within the 
specified coverage. In TDMA, multiple access is performed to the satellite 
on a time-division basis, whereby each station transmits its burst during 
an assigned state of a beam switching pattern repeated every M.times.125 
.mu.sec, where M is an integer. This system allows the significant 
advantage of having only one carrier at satellite level, which is utilized 
by all the earth stations in turn; thus one avoids problems connected with 
the effects of intermodulation, which are characteristic of other types of 
access. 
A further, functional advantage of a system featuring multi-beam coverage 
and dynamic control of the allocation of satellite transmission capacity 
is that the satellite 2a becomes a genuine repeater, with regeneration of 
the signal brought about on board; the switching matrix operates at base 
band, not at intermediate frequency. Also, one has the facility of 
on-board generation of the TDMA beam-pattern pass word, thus eliminating 
the need for additional earth stations and relative stand-by protocol 
procedures. 
There are other advantages offered by the system, such as the certainty of 
excluding multiple paths within the satellite, and the plus factors of 
better equalization and lower error rates. 
Finally, with a modular switched beam pattern, all earth stations can 
transmit different bursts for different media or different allocation 
techniques, namely: DSI telephony, in bursts known as `bricks` (4 channels 
at 32 Kbps plus preamble); non-DSI telephony utilizing `mini-bricks` (1 
channel at 32 Kpbs plus preamble) and video broadcast. There are also 
`super-bricks` of 30 channels at 64 Kbps, obtained by putting 15 bricks 
together. 
These sub-divisions permit creation of the modular beam pattern to which 
the invention relates, whereby parts of the switched pattern allocated to 
the different types of burst can be varied from transponder to 
transponder. By dividing the length of the two parts of the beam pattern 
(bricks and mini-bricks) and the type of traffic occupying it (say, 
DSI-telephony), the system is invested with complete flexibility, and one 
also has the bonus of a facility for transmission of, say, a digital 
television signal, obtained by maneuvering the confines between sections 
of the beam pattern relative to one transponder and leaving those of the 
remainder untouched. 
TMDA terminals are provided with common logic equipment (CLE) which is the 
section responsible for transmission of bursts according to the 
established time plan; synchronisation is thus maintained, all the bursts 
being transmitted in their allocated position within the beam pattern. 
The system, as described and integrated according to the foregoing 
specification, thus realises the stated objective by virtue of its 
providing optimum handling of the signals transmitted from earth stations 
to satellite.