 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique for providing a plurality of
different frame rates for use in time-division multiple access (TDMA)
communication system and, more particularly, to a technique which uses the
well-known time-division multiple access mode while still providing a
plurality of different frame rates which is dependent on the traffic
demands between the various pairs of remote, spaced-apart, ground stations
making up the communication system to provide efficient access
therebetween.
2. Description of the Prior Art
In a time-division multiple access (TDMA) communication system, a
transmitting station or area normally communicates with a plurality of
remote, spaced-apart, receiving stations or areas by sequentially
accessing each of the receiving stations or areas before repeating the
sequence. The period of transmission to each receiving station or area is
generally known as a transmission burst and in general the sequence of
transmission bursts is synchronized in a TDMA frame period and each of the
transmitting station and receiving stations are synchronized to enable
reception of the associated transmission bursts. In this regard see, for
example, U.S. Pat. No. 3,772,475 issued to A. Loffreda on Nov. 13, 1973
and in particular FIG. 1 and the associated description therein.
It is generally found that the traffic demands between a transmitting
station or area and each of the receiving stations or areas is not equal
and to compensate for such diverse traffic demands prior art TDMA systems
have used longer or shorter transmission bursts in a direct relationship
to such traffic demands. In this regard see, for example, U.S. Pat. Nos.
3,711,855 issued to W. G. Schmidt et al on Jan. 16, 1973, and especially
FIGS. 4A and 4B thereof where two bursts are used for high traffic
stations; 3,778,715 issued to W. G. Schmidt et al on Dec. 11, 1973 and
especially FIG. 3A thereof; 3,789,142 issued to N. Shimasaki et al on Jan.
29, 1974, especially FIGS. 8A-8C; and Re. 28,577 issued to W. G. Schmidt
on Oct. 21, 1975, especially FIGS. 1 and 2.
The TDMA format is primarily used in multiple beam satellite communication
systems and in such systems it is desirable from a weight and reliability
consideration to have a single transponder associated with each beam which
is operating at the maximum channel bit rate. For example, at 12/14 GHz
the transponder bandwidth may be 500 MHz and can comfortably support 4
.phi. PSK modulation at 600 Mbs/sec (300 Mbauds/sec). For a channel
transmission at such a high rate which is to be accessed by perhaps
hundreds of stations in that beam, it is imperative to have an efficient
access scheme. Time-division-multiple access provides such a solution if
the number of accesses is small. When the number of accesses is large, the
overhead associated with each transmission burst starts to cut into the
system efficiency and longer frames must be used. Associated with the use
of longer frames is the problem that buffer (high speed) requirements
correspondingly increase. Thus, in the prior systems, the problem exists
that for system efficiency, longer frames should be used, yet for buffer
size considerations, shorter frames should be used.
BRIEF SUMMARY OF THE INVENTION
The foregoing problem has been solved in accordance with the present
invention which relates to a technique for providing a plurality of
different frame rates for the individual receiving stations or areas in a
TDMA communication system which is dependent on the traffic demands
between the various pairs of remote, spaced-apart, stations or areas
making up the communication system.
It is an aspect of the present invention to provide a TDMA architecture
which includes a plurality of different frame rates which is dependent on
the traffic requirements of each station. In operation, a standard TDMA
communication switching frame burst format is concurrently used for the
transmissions of each of a plurality of x remote spaced-apart ground
locations which permits each ground location to sequentially communicate
with every other ground location while assuming that no two ground
locations are concurrently accessing the same location. A sequence of a
plurality of s switching frames are used to form a super frame and each
communication burst of a switching frame is divided into a plurality of q
subbursts representative of q transmission channels so that a maximum of
(q.multidot.s) transmission channels are available for assignment in the
corresponding bursts of all the switching frames of the super frame. When
traffic demands require n.multidot.s channels between two ground stations,
then n subbursts in corresponding bursts in each switching frame will be
assigned thereto for an integer portion of n, while demands for less than
s channels will be spread as evenly as possible throughout the
corresponding bursts of the s switching frames of a super frame.
Other and further aspects of the present invention will become apparent
during the course of the following description and by reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings, in which like numerals represent like parts
in the several views:
FIG. 1 is a block diagram of a satellite repeater time-division
interconnection arrangement for interconnecting a plurality of fixed
up-link and down-link spot beams and an up-link and down-link scanning
spot beam;
FIG. 2 is a schematic diagram of a typical satellite time-division
switching matrix for use in the present satellite repeater of FIG. 1;
FIG. 3 illustrates an exemplary switching frame sequence format for
concurrently interconnecting 11 up-link fixed spot beams and one up-link
scanning spot beam with 11 down-link fixed spot beams and one down-link
scanning spot beam;
FIG. 4 illustrates exemplary burst assignments for transmission of signals
between the various ground areas and stations therein for the format of
FIG. 3 in accordance with the present invention.
DETAILED DESCRIPTION
The present invention is described hereinafter primarily in reference to a
multibeam satellite communication system. However, it will be understood
that such description is exemplary only and is for purposes of exposition
and not for purposes of limitation. It will be readily appreciated that
the inventive concept described is equally applicable to a single beam
TDMA communication system wherein one transmitting station or area desires
communication with a plurality of remote, spaced-apart, receiving stations
or areas where traffic demands between the transmitting station or area
and each receiving station or area may be different from that of the other
stations or areas.
A scanning beam satellite communication system can provide complete
coverage of a selected area, as, for example, the entire United States, on
a time-division multiple access (TDMA) basis. In such a system, the
scanning beam's instantaneous antenna pattern is concentrated to a small
area of the entire area to be covered. This results in, inter alia, a
realization of savings in RF power to permit the simultaneous use of a
scanning beam and a number of fixed spot beams. In this manner the
scanning beam can be used to serve many spaced-apart low traffic ground
station areas while each of the fixed spot beams can be used to serve a
separate high traffic ground station area such as a metropolitan area
located within the entire area to be served by the satellite system. In
this manner, a satellite system is provided having increased capacity over
the presently used area beam or spot beam system or the proposed scanning
beam systems. Additionally, such communication systems permit the fixed
and scanning spot beams, under normal operating conditions, to transmit
signals within the same frequency spectrum with virtually no interference
therebetween since the beams do not overlap each other at the associated
ground station areas as found in the proposed combined area coverage and
fixed spot beam systems which require interference rejection techniques to
be used when employing the same frequency spectrum for all signals.
However, to assure substantially no interference, it is preferred that the
scanning spot beams and the fixed spot beams utilize different
polarizations. Additionally, the use of different polarizations is
preferable since occasions can occur when the scanning beam may be
required to be directed either wholly or partially into a fixed beam
ground station area to overcome, for example, an overload condition
existing in a particular fixed beam ground station area or a malfunction
at the satellite of a particular fixed beam transmitter or receiver. Since
such communication system provides many variations of problems encountered
in systems using the TDMA concept for one or multiple beams of the same
kind, the present invention will be described for use with a multiple
fixed and scanning spot beam system for exemplary purposes only after such
system has been described to provide the proper background for
understanding the present invention.
Turning now to the drawings, FIG. 1 illustrates a basic satellite switching
repeater 10 configuration for concurrently transmitting and receiving a
plurality of fixed spot beams and one scanning spot beam. For illustrative
purposes only, 11 up-link and 11 down-link fixed spot beams designated
12a-12k and 13a-13k, respectively, and a scanning up-link and down-link
spot beam designated 14 and 15, respectively, are shown. Each of the
associated up-link and down-link fixed spot beams 12a, 13a; 12b, 12b; . .
. ; 12k, 13k are received from and transmitted to a separate fixed
geographical ground area (not shown) within the viewing area of satellite
switching repeater 10. The up-link and down-link scanning spot beams 14
and 15 are scanned independently among a plurality of spaced-apart
geographical ground areas (not shown) which do not form a part of the
geographical ground areas associated with the various fixed spot beams 12
and 13. For illustrative purposes only, the plurality of corresponding
spaced-apart geographical ground areas associated with scanning spot beams
14 and 15 will be considered to include one hundred separate areas. It is
also to be understood that each of the 11 fixed spot beam and 100 scanning
spot beam geographical ground areas can include one or more ground
stations desiring to communicate with any of the other ground stations in
the same or other geographical ground areas.
Each of the up-link fixed spot beams 12a-12k are shown being intercepted or
received at antennas 16a-16k, respectively, while each of the down-link
fixed spot beams 13a-13k, are launched by antennas 17a-17k, respectively.
It is to be understood that antennas 16a-16k and 17a-17k can comprise any
suitable antenna means capable of receiving or transmitting each of fixed
spot beams 12a-12k and 13a-13k as, for example, a single reflector having
the requisite aperture to cover all of the associated fixed spot beam
geographical ground areas and a separate feedhorn for each fixed spot beam
disposed on the focal plane of the reflector at a point where the
associated fixed spot beam is focused by the reflector.
The up-link and down-link scanning spot beams 14 and 15 are respectively
received and transmitted by any suitable antenna means which will permit
the individual beams to be scanned over all of the 100 exemplary scanning
spot beam geographical ground areas. Such antenna means can take the form
of, for example, a phased antenna array as shown in FIG. 1 where the
up-link receiving array and down-link transmitting array are each shown as
comprising a plurality of m elements 18.sub.1 -18.sub.m and 19.sub.1
-19.sub.m, respectively. The receiving antenna elements 18.sub.1 -18.sub.m
are connected to phase shifters 20.sub.1 -20.sub.m, respectively, which,
in turn, are connected to a summing circuit 21 for combining the output
signals from phase shifter circuits 20.sub.1 -20.sub.m into a single
combined output signal on lead 22. Similarly, the input terminal of the
down-link transmitting antenna elements 19.sub.1 -19.sub.m are connected
to the output terminals of phase shifters 24.sub.1 -24.sub.m,
respectively, with the input terminal of phase shifters 24.sub.1 -24.sub.m
being connected to the output terminals of a splitting circuit 25.
Splitting circuit 25 receives the instantaneous signal to be transmitted
via scanning spot beam 15 on lead 26 and divides the signal equally for
distribution to phase shifters 24.sub.1 -24.sub.m which alters the phase
of the associated input signal in a manner to cause elements 19.sub.1
-19.sub.m to transmit scanning spot beam 15 in the desired direction as is
well known in the art. It is to be understood that the corresponding
elements of the receiving and transmitting array such as elements
18.sub.1, 19.sub.1 ; 18.sub.2, 19.sub.2 ; . . . ; 18.sub.m, 19.sub.m can
comprise the same element but that under such condition separate paths for
the transmitting and receiving signals between each element and its
associated transmitting and receiving phase shifter may be required and
can be achieved by any suitable technique as, for example, the use of
known circulators in conjunction with a frequency difference between
signals. Additionally, each of phase shifters 20.sub.1 -20.sub.m and
24.sub.1 -24.sub.m, summing circuit 21, and splitting circuit 25 can
comprise may suitable circuit which is commercially available.
In the operation of each of scanning spot beams 14 and 15, an array
processor 28 transmits a sequence of control signals over a bus 29 to each
of the up-link phase shifters 20.sub.1 -20.sub.m using any suitable
technique well known in the art such as, for example, a memory for
sequentially storing the sequence of control signals similar to that shown
in FIG. 3b of U.S. Pat. No. 3,978,482 issued to F. C. Williams on Aug. 31,
1976 to cause the array to scan spot beam 14 over the associated scanning
spot beam geographical ground areas in synchronization with the expected
reception of signals from such ground areas. Concurrent therewith, the
array processor 28 similarly transmits a separate sequence of control
signals over a bus 30 to each of the down-link phase shifters 24.sub.1
-24.sub.m to cause the array to scan spot beam 15 over the associated
scanning spot beam geographical ground areas in synchronization with the
expected transmission of signals to such areas as will be more clearly
defined in conjunction with the discussion hereinafter relating to FIG. 3.
The up-link signals concurrently received via fixed spot beams 12a-12k and
scanning spot beam 14 form separate input signals on leads 34a-34k and 22,
respectively, to respective receivers 35a-35k and 39 and, in turn, to a
time-division switching matrix 32. These input signals are concurrently
and selectively switched by the space and time-division switching matrix
32, in response to control signals on bus 37 from a clock and switching
sequencer 36, to the appropriate output lead 38a-38k and 23 for
transmission via transmitters 31a-31k and 33 and, in turn, down-link fixed
spot beams 13a-13k and scanning spot beam 15, respectively.
Time-division switching matrix 32 can comprise any suitable switching
matrix which can provide high-speed switching with relatively low power
requirements. Exemplary switches which have the desired characteristics
are, for example, the well known microwave switches which include, inter
alia, the semiconductor diode (pin) switch and the magnetic latching
switch. An arrangement for switching matrix 32 is shown in FIG. 2 and is
typical of known arrangements. For the exemplary conditions of 11 fixed
up-link and down-link spot beams serving 11 spaced-apart high traffic
geographical ground areas and one up-link and down-link scanning spot beam
serving 100 spaced-apart low traffic geographical ground areas, the
switching matrix 32 comprises a 12.times.12 array of microwave switches 40
and their associated drivers (not shown). Each of the instantaneous
up-link signals on input leads 34a-34k and 22 are concurrently cross
connected via a separate one of switches 40 to the desired one of output
leads 38a-38k and 23 in response to control signals on bus 37 from clock
and switching sequencer 36. The dynamic switching of the satellite
switching matrix 32 is divided into a sequence of time intervals which are
combined into a frame interval as shown in FIG. 3.
Clock and switching sequencer 36 comprises a clock circuit 42, a switching
sequencer processor 44 and a memory section 46. The clock circuit 42 is
synchronized with all the system clocks at the remote ground areas via
telemetry signals on a two-way data link 48 from one or more ground
stations to permit effective reception and transmission of signals through
satellite repeater 10 via switching matrix 32. The clock pulses from
circuit 36 are also transmitted over lead 41 for use by array processor 28
for coordinating the control signals transmitted over buses 29 and 30 to
phase shifters 20.sub.1 -20.sub.m and 24.sub.1 -24.sub.m, respectively, to
direct scanning spot beams 14 and 15 at the appropriate ground area in
synchronization with (a) the expected arrival and transmission of signals
related to each ground area and (b) the simultaneous switching of the
received signals to the appropriate down-link beams by switching matrix
32. Synchronization of the system clocks can be achieved using any
suitable technique known in the art which, for example, can take the form
of synchronization pulses which are transmitted via round trip telemetry
signals through the satellite repeater 10 to the various ground stations.
The switching sequencer processor 44 of circuit 36 generates the necessary
control signals to interconnect the appropriate input and output leads
through switches 40 of matrix 32 during each frame interval in response to
the synchronized clock signals from clock 42 and the desired
interconnection sequence stored in the associated memory section 46. Any
suitable high speed switching sequencer and memory means which is
available can be used to generate the desired control signals.
In accordance with the present invention, it is to be understood that many
ground stations may be disposed within each of the receiving areas of
down-link fixed spot beams 13a-13k, and scanning spot beam 15 and that
these ground stations will have diverse circuit requirements ranging from
a few circuits for a small station to hundreds of circuits for a major
station. An additional difficulty is that although the ground stations
serviced by each of fixed spot beams 13a-13k are covered by that
particular beam all of the time, the ground stations serviced by scanning
spot beams 14 and 15 can only be intermittently covered. Therefore, each
scanning spot beam is individually steered so that various spaced-apart
geographical ground areas within the scanning range of the phased array
antenna 18.sub.1 -18.sub.m and 19.sub.1 -19.sub.m can be covered and a
TDMA configuration is perfectly suited therefor. To achieve total service,
it becomes necessary to scan both the transmit and receive scanning spot
beams 14 and 15, respectively, while coordinating their movements with
array processor 28 in accordance with the pair-wise traffic demands of the
system.
An architecture, in accordance with the present invention, which allows
efficient multiple access by the low traffic demand group stations while
still providing minimal buffer demands for the high traffic demand ground
stations is shown in FIGS. 3 and 4. For purposes of illustration, it will
be assumed that there is equal traffic among beams. Under such condition
the satellite time-division switching matrix 32 follows a cyclic pattern
and establishes connections among the various beams on a subframe basis
per each switching frame, where in FIG. 3, for exemplary purposes, the
frame is designated T and each of the sequential subframes therein has a
separate designation in the sequence t.sub.1 -t.sub.n, where n represents
the total number of down-link beams and equals 12 for the exemplary system
having 11 fixed-up link and down-link spot beams and one up-link and
down-link scanning spot beam. In FIG. 3, up-link fixed spot beam 12a is
shown sequentially connected by switching matrix 32 to down-link beams
A(13a), B(13b), . . . , K(13k), and S(15) once during each switching frame
period T. Concurrent therewith, up-link fixed spot beam 12b is
sequentially connected by switching matrix 32 to down-link beam B(13b),
C(13c), . . . , S(15), and A(13a) while up-link scanning spot beam 14 is
sequentially connected to down-link beams S(15), A(13a), . . . , J(13j)
and K(13k) during switching frame period T. Similarly all other up-link
fixed spot beams 12k-13k are concurrently connected to the various
down-link fixed spot beams 13a-13k and scanning spot beam 15 in a sequence
which assures that no two up-link beams are connected at any instant of
time to the same down-link beam. It is to be understood that the sequences
shown in FIG. 3 for the switching of signals between up-link beams 12a-12k
and 14 and down-link beams 13a-13k and 15 are merely illustrative of a
typical set of sequences and that any other set of sequences which do not
at any instant of time connect two up-link beam signals to the same
down-link beam can be substituted.
To implement a TDMA switching arrangement for the exemplary combination of
a plurality of up-link and down-link fixed spot beams and an up-link and a
down-link scanning spot beam presents various problems which must be
overcome for an operable system. For example, a minimum burst length for
communication between any two ground stations must be chosen to achieve
efficient access and then a subframe and a frame length determined to meet
the various traffic demands where each fixed and scanning spot beam ground
area can comprise one or more separate ground stations desiring access to
satellite repeater 10.
For purposes of illustration and not for purposes of limitation, the
arrangement of FIG. 1 will hereinafter be assumed to operate with voice
circuits at a 32 kb/sec. rate, a minimum burst length of 400 bauds with,
for example, a preamble of 67 bauds, and a frame T duration of 250
.mu.sec. With such conditions prescribed, a minimum burst length of 467
bauds would have a duration of 1.557 .mu.sec. at a bit rate of 600
mb/sec., and provide transmission for 100 voice circuits at the 32 kb/sec.
rate. The preamble for each burst generally provides the necessary
information for, inter alia, carrier and timing recovery, frame
synchronization, ground station identification, etc. In view of the format
of FIG. 3 and the above-mentioned assumptions, each of the 12 subframes
t.sub.1 -t.sub.12 of frame T has a capacity of 6250 bauds of information
which can include slightly more than 13 minimum bursts of 467 bauds. This
remainder above the 13 minimum bursts per subframe is used, for example,
with the scanning beams 14 and 15 as overhead which is a function of the
beam switching speed.
Therefore, during each subframe, t, of the format of FIG. 3, each of the
exemplary 11 up-link fixed spot beams 12a-12k and the up-link scanning
spot beam 14 can access no more than 13 ground stations within a fixed
spot beam receiving ground area or 13 scanning spot beam ground stations.
Since it was assumed that, in the exemplary system, scanning spot beams 14
and 15 were to be associated with 100 spaced-apart low traffic ground
areas each having one or more ground stations associated therewith, the
format of FIG. 3 must be expanded upon to permit each of the ground
stations associated with up-link fixed and scanning spot beams 12a-12k and
14 to communicate with all of the other ground stations via down-link
fixed and scanning spot beams 13a-13k and 15. Although preferably it is
desirable to space the required minimum burst evenly throughout the super
frame sequence to keep buffering requirements at a minimum where the voice
circuit (V.C.) requirement is less than the number of associated
subframes, e.g., 100 in the exemplary format, such required minimum
bursts, or subbursts can be arbitrarily spread randomly througout the
associated subframes where so desired. For example, for a 7 V.C.
requirement, 7 minimum bursts can be used in one subframe or spread
throughout 7 or less subframes. To accomplish this, the switching and
burst assignment format of FIG. 4 is used which applies a sequence of 100
frames, T, designated 1-100 to make up a super frame.
In the structure of FIG. 4, the sequence shown in FIG. 3 for each frame is
repeated for each of the up-link fixed spot beams 12a-12k and up-link
scanning spot beam 14 in each of the associated 100 frames 1-100. The
different traffic demands between the various pairs of ground stations of
the system is substantially met in accordance with the present invention
by the proper scheduling of an appropriate number of minimum bursts within
a subframe designated for communication between the ground areas wherein
the two stations are situated. For purposes of illustration only, an
exemplary sequence will be shown for the allocation of signalling time
between ground stations using fixed up-link spot beams 12a and, for
example, the ground stations in both fixed spot beam area K, served by
fixed down-link spot beams 13k, and scanning spot beams areas S.sub.1
-S.sub.100, served by scanning down-link spot beam 15. It is to be
understood that many other sequences can be used to fulfill the traffic
demands of the various pairs of ground stations and still fall within the
spirit and scope of the present invention and that such sequences can be
applied to TDMA systems using only one beam for communication purposes.
For purposes of illustration, it will be assumed that the ground area
served by up-link and down-link fixed spot beams 12a and 13a,
respectively, has four ground stations (A.sub.1 -A.sub.4) and that the
ground area served by up-link and down-link fixed spot beams 12k and 13k,
respectively, has seven ground stations (K.sub.1 -K.sub.7). It will be
further assumed that the traffic demands for communication from ground
stations A.sub.1 -A.sub.4 to each of the ground stations K.sub.1 -K.sub.7
require the following number of voice circuits: K.sub.1 =400 voice
circuits (V.C.); K.sub.2 =500 V.C.; K.sub.3 =200 V.C.; and K.sub.4
-K.sub.7 each require 50 V.C. It is to be understood that with the system
conditions previously assumed, only a maximum of 1300 voice circuits are
available between any two fixed beam ground areas since there are only
1300 minimum bursts available in a super frame between such ground
stations. More particularly, to provide 100 voice circuits between two
particular ground areas it is necessary to assign 100 minimum bursts per
super frame for such communication. This can be preferably accomplished by
assigning one corresponding burst in each frame of the super frame or
alternatively, for example, to assigning two minimum bursts in alternate
frames of the super frame to such intercommunication link.
In FIG. 4, each of the subframes K in switching frames 1 and 2 which
interconnect the signals in up-link fixed spot beam 12a to down-link fixed
spot beam 13k have been expanded to show the 13 possible sequential
minimum bursts therein and how such bursts can typically be assigned to
meet the traffic demands specified hereinabove. For example, the 400, 500
and 200 V.C. requirements for communication with ground stations K.sub.1,
K.sub.2 and K.sub.3, respectively, have been met by assigning these ground
stations respectively to minimum bursts 1-4, 5-9 and 10-11 in each of
switch frames 1-100. Since each of ground stations K.sub.4 -K.sub.7 only
require 50 V.C., these traffic demands are met by, for example, assigning
minimum bursts 12 and 13 of the odd numbered frames to ground stations
K.sub.4 and K.sub.5, respectively, and the bursts 12 and 13 of the even
numbered frames to ground stations K.sub.6 and K.sub. 7, respectively.
Therefore, any ground station having less than a 100 V.C. requirement uses
buffering means to store its signal until the assigned burst, at which
time the stored signals will be transmitted. For example, a ground station
having a 25 V.C. requirement might transmit its signals during a single
minimum burst once every 4th frame or a 10 V.C. requirement might use a
single minimum burst once every 10th frame. It is to be understood that a
minimum burst of 467 bauds may contain less than 400 bauds of information
where desired, but that such transmission would reduce the transmission
efficiency.
At the transmitting end in the area associated with up-link fixed spot beam
12a, ground stations A.sub.1 -A.sub.4 will be similarly scheduled to each
appropriately interleave their respective transmissions destined for
ground stations K.sub.1 -K.sub.7 in accordance with both their individual
traffic demands and the subframe sequence hereinbefore outlined in FIG. 4.
For example, if the traffic demands between each of ground stations
A.sub.1 -A.sub.4 and ground station K.sub.1 is 100 V.C., then during each
of frames 1-100 minimum bursts 1-4 can be assigned to ground stations
A.sub.1 -A.sub.4, respectively. Where the traffic demands between ground
stations A.sub.1 -A.sub.4 and ground station K.sub.1 are, for example,
A.sub.1 =200 V.C., A.sub.2 =100 V.C., and A.sub.3 and A.sub.4 each require
50 V.C. Then minimum bursts 1 and 2 can be assigned to ground station
A.sub.1 in each frame, burst 3 to ground station A.sub.2 in each frame and
burst 4 alternately shared by ground station A.sub.3 and A.sub.4 where
station A.sub.3 might be assigned burst 4 in the odd numbered frames while
station A.sub.4 is assigned burst 4 in the even numbered frames. A similar
technique is used with each of the other ground station areas and the
other fixed up-link spot beams 12b-12k.
With reference to the interconnection of up-link signals in fixed spot beam
12a to down-link scanning spot beam 15 during subframe S of each frame in
the super frame, a similar technique is used as outlined hereinbefore for
the communication of ground stations A.sub.1 -A.sub.4 with ground stations
K.sub.1 -K.sub.7. Since there are 100 exemplary scanning beam ground areas
each of which has one or more ground stations therein, it becomes
impossible to access them all during one frame when only 13 minimum bursts
are available during an associated subframe. Therefore, since the scanning
beam ground station areas and the associated ground stations therein are
of a low traffic type, assignment of the minimum bursts over the 100
subframes in a super frame is again accomplished in accordance with the
individual traffic demands existing between the various high and low
traffic ground station areas. For example, if the traffic demands between
ground stations A.sub.1 -A.sub.4 and scanning beam ground areas S.sub.1
and S.sub.2 are 15 and 11 voice circuits, respectively, then such traffic
demands can be met by respectively assigning bursts 1-13 of subframe S in
switch frame 1 and bursts 1-2 of subframe S in switch frame 2 to ground
area S.sub.1 and bursts 3-13 of subframe S in switch frame 2 to ground
area S.sub.2 as shown in FIG. 4. By combining adjacent bursts within a
single subframe or corresponding adjacent subframes to such traffic
demands instead of spreading them via single bursts over more frames
results in a reduction in the amount of movement of scanning beam 15.
Therefore, during each subframe S for fixed spot beams 12a-12k and
scanning spot beam 14 the down-link scanning beam is moved over from one
to 13 scanning beam ground areas in accordance with a schedule determined
from the various traffic requirements. It is to be understood that such
schedule for all beams is repeated every super frame.
While the down-link scanning spot beam 15 is moving between a maximum of
156 scanning beam ground areas (12 subframes.times.13 minimum bursts)
during each frame for all of the up-link beams, up-link scanning beam 14
is preferably moved in the following manner and as shown in FIG. 4. With
reference to the period of each frame devoted to the transmission of
signals via each of down-link fixed spot beam 12a-12k which originated at
one or more scanning beam ground areas, up-link scanning beam 14 is
d | | |
