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Description  |
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FIELD OF THE INVENTION
The present invention relates in general to communication systems and is
particularly directed to a scheme for optimizing the connectivity of an HF
network by employing a multiplicity of simultaneously engaged relay
stations communicating over the same frequency.
BACKGROUND OF THE INVENTION
One of the most challenging tasks presently facing the communications
industry is providing reliable data and voice transmission over an HF
ionospheric channel. The characteristics of the channel itself, which vary
with both time and external conditions that affect the ionospheric
propagation medium, as well as the presence of both natural and man-made
interference, have made the HF channel relatively unreliable for analog
voice communications. In addition, the multipath and rapid phase
variations of such a channel make its use for reliable digital
communications particularly difficult.
A number of proposals for improving HF communication reliability have
involved the proper selection of frequencies which provide favorable
propagation conditions for a particular transmission. Still, even though
such "frequency management" approaches offer improvement, they usually
achieve increased reliability by occupying additional spectral space, a
commodity that is already very precious within the limited HF band. Other
proposals that have sought to avoid the bandwidth crowding problem of
frequency allocation have employed time-path diversity. The drawback with
the latter approach has been the need for instantaneous status relative to
the connectivity of the network.
SUMMARY OF THE INVENTION
In accordance with the present invention the reliability of the
connectivity of an HF communication channel is substantially improved, as
compared with the above-referenced proposals, by a communication scheme
that uses relay techniques to achieve path diversity without the need for
additional frequency allocation or the requirement of instantaneous status
information. To achieve this objective the relay scheme of the present
invention essentially has only two fundamental requirements--the
availability of network timing and the use of digital modems that have the
capability of reception over multiple receive paths. Both of these
requirements are readily incorporated in what is termed an "avalanche"
relay communication network.
Pursuant to the present invention this network is configured of a plurality
of transceiver stations spread out over a geographic area of interest to
establish multipath communication diversity among the stations. The
transceiver equipment at each station of the network enjoys the capability
of simultaneous transmission over the same frequency through a "common
knowledge" network timing scheme such as TDMA (time division multiple
access) commonly employed in present day communication-relay systems,
(e.g. satellite transmission networks). With the use of a TDMA timing
scheme, digital signalling transmission is accomplished through packet
switching. The modem employed in each transceiver may be the type
described in U.S. Pat. No. 4,365,338 issued Dec. 21, 1982 to Daniel D.
McRae et al, entitled "Technique for High Rate Digital Transmission Over a
Dynamic Dispersive Channel" and assigned to the Assignee of the present
application. Essentially, that type of modem has the capability of taking
advantage of all received multipath signals with an arrival time spread
plus the status time inaccuracy plus the propagation time uncertainty
associated with receiving from unknown stations in the network.
Within the above network, communications between an originating station and
an intended recipient station are achieved by an "avalanche" relay scheme
initiated by the originating station. In accordance with this "avalanche"
scheme, the originating station initiates a message transmission by
modulating onto an HF carrier, during a preassigned TDMA time slot, a
digital packet formatted to contain, inter alia, a control segment
specifying the number of times the message is to be repeated and a means
of establishing the quality of the received message (e.g. CRC or parity
bits). All stations which have correctly received the packet (based upon
the established quality criteria) repeat that same received message
(except for a modification of the control segment) at the same
preestablished future absolute time on the same carrier frequency. The
modification of the control segment is to reduce the number of times that
the message is to be repeated, and essentially involves decrementing the
repeat number by one. (Thus, when a station receives a message with a
repeat number equal to zero it does not repeat the message.) The number of
repeats prescribed by the originating station's initial transmission may
be selected for a particular network of interest, on the basis of the
number of multipaths available and the number of anticipated multiple
transmission (among the various stations) per simultaneous message repeat,
so that connectivity availability essentially ensures that the intended
recipient will, at some time during the avalanche sequence (from initial
transmission to last repeat), receive the message.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a layout illustration of the distribution of a plurality of
stations making up an avalanche-relay communication network;
FIG. 2 shows a block diagram of an avalanche-relay network station;
FIG. 3 is an exemplary timing format diagram showing the arrangement of a
control slot and data frame for TDMA formatted avalanche signalling; and
FIG. 4 is a pictorial layout illustration of the distribution of a
plurality of mobile ground stations communicating with a remote earth
station via a satellite relay link.
DETAILED DESCRIPTION
Referring now to FIG. 1, an exemplary distributed layout for an
avalanche-relay communication network is shown as comprising six stations
A-F distributed over a prescribed geographical area. The make-up of an
individual station may be represented by the simplified block diagram of
FIG. 2 and essentially comprises a transceiver-modem 10 coupled to an HF
antenna 12 and a voice/data I/O interface. Also associated with the
transceiver modem is a counter 11, which is shown as a separate element
simply for the purposes for the explanation to follow. It should be
understood at this point that the make-up of the transceiver modem may be
selected from any number of communication equipments that provide the
capability of taking advantage of all received multipath signals having an
arrival time spread which is equal to the channel multipath spread, plus
the station absolute timing accuracy, plus the propagation time
uncertainty associated with receiving from unknown stations in the
network. As mentioned briefly above, one type of HF modem which satisfies
this criterion is that embodied in the communication scheme described in
U.S. Pat. No. 4,365,338 issued Dec. 21, 1982 to Daniel D. McRae et al
entitled "Technique for High Rate Digital Transmission Over a Dynamic
Dispersive Channel", assigned to the Assignee of the present application.
This type of modem is especially designed to solve the problem of
multipath, intersymbol interference and phase transitions which have been
a substantial source of degradation in HF communication schemes. With
recent advances in sophisticated HF modems, it is now possible to
successfully address these problems and to provide voice and data
signalling successfully over an HF ionospheric channel. Since an
understanding of the details of the transceiver-modem 10 are unnecessary
for an understanding of the present invention, no further description will
be provided here. Instead, reference may be made to the above reference
patent or other current literature in which sophisticated HF modems having
the above capability are described.
In addition to the need for a modem with the above capability, the present
invention also requires that the network stations A-F of the network shown
in FIG. 1 know absolute time (within a few milliseconds) and that the
digital traffic being transmitted over the network be packetized, with
each data packet consisting of a specified number of bits with a
prescribed format. Advantageously, timing for the network and access to
the network among the various stations for message transmission uses a
time division multiple access (TDMA) scheme, such as those commonly
employed in satellite communications, through which each station of the
network is assigned a prescribed time slot within which to have
communication access to the network for transmission and the data format
is time division multiplexed, having a format such as that illustrated in
FIG. 3.
With reference to FIG. 3, one exemplary TDMA frame format is illustrated.
Each frame is shown as consisting of a control slot of a prescribed time
length or number of data bits, followed by a data slot. For providing
robustness against jamming, it is often the case that the control slot and
subdivisions of the data slot are randomly interspersed among one another
through a suitable spreading or set of spreading sequences.
In the signalling format shown in FIG. 3, each control slot is illustrated
as occurring at the beginning portion of a frame, since it is within this
control slot that important information, especially the repeat control
information, pursuant to which the relay avalanche technique is carried,
out is provided. Thus, in accordance with the preferred iteration of the
TDMA format, the control slots occupy the beginning of a message frame.
As is common practice in the art, control slots include a variety of
information such as transmission requests, priority, access granting, etc.
and, pursuant to the present invention, a message repeat number. It should
be noted that the message format to be used in accordance with the
principles of the present invention is not limited to that shown in FIG.
3. The TDMA format illustrated in the Figure is simply for purpose of
providing an exemplary illustration of a preferred embodiment. Moreover,
in terms of an understanding of the present invention, what is important
is that there be an established timing synchronization among all the
stations on the network and the use of some portion of a message packet to
identify the number of repeats for the transmission. Thus, in conjunction
with the use of an HF modem and communication signalling scheme as
described in the above referenced patent each frame-i of FIG. 3
corresponds to an information frame between which PN sequences are
inserted. Also, the frame includes reception quality bits, such as parity
or a cylic redundancy check sequence appended to a sequence of data bits
that the receiver user in the customary manner to establish signal quality
(i.e. a message has been correctly received). In the transceiver-modem an
acceptable received message is buffered for preparation for subsequent
retransmission at a preestablished time slot in accordance with the
synchronized timing signals of the TDMA clock shared by the stations of
the network.
In order to gain a full appreciation of the invention, reference is again
directed to FIG. 1 wherein an originating station A is desirous of
communicating with the intended recipient station B. Of course, also
included within the network are a number of additional stations C-F
capable of communicating with one another and with stations A and B.
With originating station A being desirous of communicating with station B
and having gained access to the communication channel via its assigned
TDMA time slot, a digital packet is assembled at station A, which packet
contains a number (in the present example the number 3) indicating the
number of times the message is to be repeated. Also, as mentioned above in
the digital packet there is included some means of establishing the
quality of reception at a receiving station, such as parity or cyclic
redundancy check (CRC) bits which form part of the message packet. In the
example shown, the reference numerals above each of the stations indicate
the number of times that those particular stations will be instructed to
(re)transmit the received message. For its initial attempt, station A has
placed in a counter 11 associated with its modem 10 (see FIG. 2) the
number three which will be included as part of the control slot in the
message packet. The number 3 indicates that station A will transmit the
message a total of three times, once initially, followed by two repeats.
Each time the message is transmitted, the contents of counter 11 are
decremented by one. In the course of assembling a message for
transmission, transceiver modem 10 reads the contents of counter 11 and
continues to transmit the message during a preestablished time slot as
governed by the TDMA time slot assignment structure until the contents of
counter 11 have been decremented to zero.
For purposes of the present description, let it be assumed that station A
succeeds in conveying the message only to stations C and E during the time
of the original transmission. These successful transmissions are
illustrated in FIG. 1 as corresponding to communication channels AC and
channels AE, the first letter indicating the originating station and the
second letter indicating the terminating station for the channel.
For the original message transmission, each of stations C and E receives
the data packet, decodes the control slot and processor any data that may
be addressed to it. In the present example, it is assumed that the address
of the intended recipient is station B so that neither station C nor
station E processed any data but simply buffers the message for
retransmission. However, control information is processed, specifically
the number of times of repeat transmission. During the transmission of the
initial message from station A, the counter at station A was set at three
and decremented upon the first transmission. This means that station A
will transmit the message three times, one originating transmission,
followed by two repeats. The control information that is assembled in the
message packet to be transmitted to other stations in the network causes a
lesser number of repeats (by stations C and E) so that the control slots
of the retransmitted message packets will contain the number two as an
indicator of the number of repeats for the stations receiving the initial
transmission. Thus, each of stations C and E (the only stations that
received the message on the initial transmission from station A) will
repeat the message (along with station A) during the next two subsequent
time slots, corresponding to time slots i+1 and i+2 shown in FIG. 3.
When a station receives a message and reads the contents of the control
slot indicating the number of repeats, it loads its counter (counter 11 as
shown in FIG. 2) with that number and decrements the counter on a
subsequent transmission.
On the next transmission, namely the first repeat or second transmission of
the message from each of stations A, C and E, it will be assumed that the
transmission from station C reaches stations D and F over communication
channels CD and CF and the transmission from station E reaches stations D,
F and B over communications channels ED, EF and EB, respectively. Thus, on
the first repeat or second transmission, the original message from station
A that has been repeated from stations A, C and E has reached every
station except for station G. With station B being the intended recipient
station, it has received the communication on the second repeat.
In the transceiver-modem equipments at stations A, C and E, counters 11
will have been decremented at this point to the number one. The number one
will also have been inserted into the counters at stations D, F and B so
that, during the next or third time slot, each station in the network,
except for station G, will repeat the message. Station G is shown in FIG.
1 as receiving the message on third try (with a repeat count of zero), so
that is will not repeat the message at all, since its counter will be
loaded with the number zero, indicating no transmission.
In the foregoing example, it has been assumed that the connectivity of
paths among stations does not change in the time between repeats. This is
a worst case assumption, but is approximately true for many networks if
the packets are short and the specified repeat times sequential.
As will be appreciated from the foregoing description of the
avalanche-relay communication technique in accordance with the present
invention, all stations within the network receive the message correctly
even though individual link connectivity is extremely poor. Since all
stations receive the message, the destination of any particular station or
stations for whom the message is intended may be included in a header in
the digital packet.
As contrasted with conventional frequency diversity or continuous status
availability schemes, the present invention offers a number of advantages.
First of all, if any relay path or paths from the originating station to
the intended recipient or recipients exist at any instant of transmission,
the recipient will receive the message. Moreover, if the transceiver-modem
is capable of taking advantage of the extra power from the multiple
stations that are transmitting, such as the modems at stations D, F and G
which receive the repeated message over a plurality of paths, there will
be cases where the avalanche technique will succeed when the best possible
relay with the same number of repeats will not succeed.
Furthermore, the achieved connectivity will be the best available for
whatever network status exists, even though the originating station was
not aware of that status. Thus, if one or more of the stations become
inoperative, the message would be relayed automatically by whatever paths
remain.
The avalanche communication technique according to the present invention is
particularly powerful when broadcast messages are required which would
normally demand connectivity between the originating station and all other
stations in the network. Furthermore, for ground wave networks such as
might be employed in military tactical HF networks, the avalanche scheme
according to the present invention represents a method of extending
conductivity beyond that normally available.
Finally, the avalanche approach of the present invention may not normally
include radio silent stations in a network as automatic repeaters.
However, the increase in reliability versus required transmission power
coupled with the fact that multiple locations are normally transmitting
during repeat times would make the location of individual repeater
stations difficult.
As an illustration of some numerical examples of connectivity of networks
using the avalanche scheme according to the present invention, tabulated
below is a comparison to the connectivity of using only a single
transmission and the connectivity if the same number repeats is used
without avalanche, as compared to using it with avalanche. In all cases,
the total number of transmission is equal to three and, for purposes of
simplification, the probability of correct transmission of a packet is the
same for all links in the network. In the tabulated comparison below, two
link conditions are analyzed. In the first case, each transmission on a
link is totally dependent upon the outcome of the previous transmission on
that link. Thus, if the initial transmission from the sender to the
recipient fails, the next two transmission will also fail. The second
condition is for the case of total independence between transmissions at
different times on the same link. In reality, transmission conditions will
fall somewhere between these two analyzed cases.
TABLE 1
______________________________________
Time Dependent - Point to Point Transmissions
Three
Pro- dB dB
Link posal 10-Station 20-Station
Gain Gain
Miss Miss Avalanche Avalanche
10-Sta-
20-Sta-
Prob. Prob. Miss Prob. Miss Prob.
tions tions
______________________________________
.1 .1 7.98 .times. 10.sup.-8
1.91 .times. 10.sup.-15
61.2 dB
103.2 dB
.2 .2 1.40 .times. 10.sup.-5
8.94 .times. 10.sup.-11
42.0 dB
93.9 dB
.3 .3 2.09 .times. 10.sup.-4
2.35 .times. 10.sup.-8
32.3 dB
71.8 dB
.4 .4 1.30 .times. 10.sup.-3
9.29 .times. 10.sup.-7
25.9 dB
57.3 dB
.5 .5 6.33 .times. 10.sup.-3
1.60 .times. 10.sup.-4
20.4 dB
46.4 dB
.6 .6 3.20 .times. 10.sup.-2
2.16 .times. 10.sup.-4
14.5 dB
36.3 dB
.7 .7 .141 4.55 .times. 10.sup.-3
9.0 dB
24.2 dB
.8 .8 .421 9.21 .times. 10.sup.-2
4.7 dB
12.1 dB
.9 .9 .791 .587 1.7 dB
4.1 dB
______________________________________
Table 1 set forth above, shows the results of connectivity between a single
originating station and a single recipient station. In all cases, the
probabilities shown are the probability of not getting the correct packet
to the intended recipient. Column 1 of Table 1 shows the assumed
probability of incorrect transmission on an individual link. Columns 2, 3
and 4 show the probability of failure assuming three transmissions (an
initial transmission followed by two repeats), without avalanche, with
avalanche in a ten station network and with avalanche in a twenty station
network. The data set forth assumes total dependence among paths.
For purposes of simplification, it is assumed that the transceiver-modem
requires some prescribed signal-to-noise power ratiol p, in order to
correctly receive the message (i.e. any greater power results in a correct
reception, any less power results in an incorrect reception). Furthermore,
it is assumed that the channel of interest is Rayleigh fading, resulting
in an exponential distribution of power signal-to-noise ratio, i.e.:
##EQU1##
where: p.sub.a equals the average received power signal-to-noise ratio.
These assumptions permit the calculation of the signal-to-noise ratio to
support the specified link reliability illustrated in Table 1. Columns 5
and 6 of the table show the reduction in required signal-to-noise ratio in
dB as a result of avalanche for the message failure probabilities with
avalanche than with the same number of repeats without it. As can be seen
from Table 1, a significant advantage in power consumption is afforded.
Specifically, a network using a 400 watt transmitter achieving this
reliability without using the avalanche scheme of the present invention
could be converted to only a 4 watt network is avalanche is employed.
TABLE 2
__________________________________________________________________________
Time Independent - Point to Point Transmissions
Link
Three 10-Stations
20-Stations
dB dB
Miss
Repeat Avalanche
Avalanche
Gain Gain
Prob
Miss Prob
Miss Prob.
Miss Prob.
10-Stations
20-Stations
__________________________________________________________________________
.1 1.00 .times. 10.sup.-3
0 0 -- --
.2 8.00 .times. 10.sup.-3
5.78 .times. 10.sup.-12
0 92.0 dB
--
.3 2.70 .times. 10.sup.-2
8.28 .times. 10.sup.-9
0 65.2 dB
--
.4 6.40 .times. 10.sup.-2
1.34 .times. 10.sup.-6
1.43 .times. 10.sup.-12
47.7 dB
101.2 dB
.5 .125 7.10 .times. 10.sup.-5
2.82 .times. 10.sup.-9
32.7 dB
76.8 dB
.6 .216 1.81 .times. 10.sup.-3
1.34 .times. 10.sup.-6
21.3 dB
52.6 dB
.7 .343 2.38 .times. 10.sup.-2
2.89 .times. 10.sup.-4
12.4 dB
31.6 dB
.8 .512 .160 2.16 .times. 10.sup.-2
6.1 dB
15.2 dB
.9 .729 .554 .350 2.1 dB
4.7 dB
__________________________________________________________________________
Table 2 above, provides the same point-to-point information above but for a
situation where the time transmissions are independent. It will be
observed that even under this condition, the avalanche communication
technique according to the present invention provides a 21.3dB improvement
over a three repeat system when 99.8% reliability is required. This is a
28.2dB gain over a single transmission.
TABLE 3
__________________________________________________________________________
Time Dependent - Broadcast Transmissions
Link
10-Station
20-Station
10-Station
20-Station
dB dB
Miss
3-Repeat
3-Repeat
Avalanche
Avalanche
Gain Gain
Prob
Miss Prob.
Miss Prob.
Miss Prob.
Miss Prob.
10 Stations
20-Stations
__________________________________________________________________________
.1 .612 .865 9.88 .times. 10.sup.-7
0 54.2 dB
--
.2 .866 .986 2.04 .times. 10.sup.-4
1.45 .times. 10.sup.-8
37.1 dB
83.3 dB
.3 .960 .998 2.97 .times. 10.sup.-3
1.04 .times. 10.sup.-6
28.6 dB
67.3 dB
.4 .990 1.0 1.48 .times. 10.sup.-2
3.14 .times. 10.sup.-5
23.6 dB
54.3 dB
.5 .998 1.0 9.92 .times. 10.sup.-2
3.79 .times. 10.sup.-4
20.1 dB
45.0 dB
.6 1.0 1.0 .176 3.39 .times. 10.sup.-3
15.6 dB
36.8 dB
.7 1.0 1.0 .530 6.71 .times. 10.sup.-2
10.1 dB
24.9 dB
.8 1.0 1.0 .907 .640 7.5 --
.9 1.0 1.0 .999 .998 4.7 dB
--
__________________________________________________________________________
TABLE 4
__________________________________________________________________________
Time Independent - Broadcast Transmissions
Link
10-Station
20-Station
10 Stations
20 Stations
DB DB
Miss
3-Repeat
3-Repeat
Avalanche
Avalanche
Gain Gain
Prob.
Miss Prob.
Miss Prob.
Miss Prob.
Miss Prob.
10 Stations
20-Stations
__________________________________________________________________________
.1 .230 .647 0 0 -- --
.2 .649 .957 0 0 -- --
.3 .884 .996 6.99 .times. 10.sup.-8
0 74.8 dB
--
.4 .970 1.0 1.05 .times. 10.sup.-5
0 55.2 .infin.
--
.5 .994 1.0 5.37 .times. 10.sup.-4
4.96 .times. 10.sup.-8
39.8 dB
83.8 dB
.6 .999 1.0 1.39 .times. 10.sup.-2
2.56 .times. 10.sup.-5
27.0 dB
58.0 dB
.7 1.0 1.0 .157 6.52 .times. 10.sup.-3
17.5 dB
35.2 dB
.8 1.0 1.0 .646 .282 11.0 dB
--
.9 1.0 1.0 .987 .983 6.6 dB
--
__________________________________________________________________________
Tables 3 and 4 above, provide the same data corresponding to Tables 1 and 2
above, except for the case where a broadcast is made to all of the
stations in the network, as opposed to point-to-point transmission. The
probabilities shown are those for not correctly reaching all stations, as
opposed to not getting the message to a particularly identified intended
recipient. As can be seen from Tables 3 and 4, in this situation, the
gains afford from the avalanche technique according to the present
invention are even more substantial than those associated with a single
recipient. The probability of failure through avalanche is increased by a
approximately an order of magnitude over that for reaching a single
station. Without avalanche, the probability of failure approaches one.
As will be appreciated from the foregoing description and illustrative
contrasts between the avalanche scheme according to the present invention
and communication techniques not employing the multiple transmission relay
scheme embodied in the avalanche approach of the present invention,
significant improvement over conventional schemes is provided. It should
also be noted that the avalanche communication technique according to the
present invention may be employed in conjunction with most frequency
management and/or frequency-hoped techniques and can provide substantial
additional performance gains over those available from the frequency
management approach alone.
In the foregoing description, the exemplary embodiment employed is one
having a plurality of relay stations distributed over a prescribed
geographical area. By virtue of the fact that each relay station contains
a modem that is capable of taking advantage of multipath signals, the
probability of success of completing a transmission from an originating
station to an attended recipient station is optimized. The avalanche
scheme described in conjunction with the embodiment illustrated in FIG. 1
involves the controlled repetition of the same message at prescribed
simultaneous transmission times for all stations in the network receiving
the message in order to substantially guarantee the probability of success
of message throughput from originator to intended recipient. A scheme
somewhat similar to that employed in the embodiment of FIG. 1, but not
involving an interated sequence of simultaneous transmissions of the same
message from a plurality of stations making up the network, may be
employed for optimizing or essentially guaranteeing communications between
a ground station and a remote earth station via a satellite link. This
scheme is particularly attractive for mobile military operations where the
failure (e.g. elimination) of a single mobile station will not prevent
completion of the transmission from an originating site to a remote
station.
FIG. 4 illustrates a multiple mobile transmitter layout comprised of a
plurality of mobile (e.g. truck-mounted) earth stations MES.sub.1 . . .
MES.sub.4, each of which is coupled to a command station CS via a
dedicated control link 21. With mobile earth stations MES.sub.1 . . .
MES.sub.4 being dispursed over a prescribed (e.g. battlefield) terrain,
and being equipped to communicate with a remote earth station RES via
respective satellite links 31-34 which are relayed onto earth station RES
from satellite SAT over down link 41, it can be seen that was is
effectively received by the earth station RES is the equivalent of a
multipath signal from the mobile earth stations over a multipath links
31-41, 32-41, 33-41 and 34-41. The use of a plurality of mobile earth
stations MES.sub.i is required to essentially guarantee that the command
station CS is assured a communication link to earth station RES via
satellite SAT in the invent of a (catostrphic) failure or elimination of
one or more (not all) of the mobile earth stations MES.sub.i.
In accordance with the present embodiment, the avalanching of the
communications from the command stations CS to the remote earth station
RES is achieved by the simultaneous transmission of the same message from
the plurality of mobile stations MES.sub.i under the control of the
dedicated link 21. Because remote earth station RES is equipped with a
modem that has the capability of taking advantage of all received
multipath signals, such as that described in the above mentioned patent,
then messages originating at command station CS are essentially guaranteed
to be received by remote earth station RES, again assuming integrity of
the satellite link for any of the mobile earth stations.
In the present embodiment, successive iterations or repetitions of the same
messages over a network are not carried out, as the configuration of the
network is different than that in the embodiment shown in FIG. 1. However,
there is an avalanching of the message by effecting a plurality of
simultaneous transmissions of the same message on the same frequency from
a plurality of spaced apart transmitter sources which effectively creates
a multipath communication situation. Thus, like the embodiment described
in connection with FIG. 1, the avalanche scheme of the communication
configuration employing mobile earth stations in a satellite link in the
embodiment of FIG. 4 takes advantage of multipath, as contrasted to
dealing with multipath as a problem as in the prior art.
Again, as in the case with the embodiment of FIG. 1, each of mobile earth
stations MES.sub.i transmits on the same frequency so that band allocation
does not become a problem. Moreover, with each of the earth stations being
coupled to the command station | | |
