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Description  |
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BACKGROUND OF THE INVENTION
This invention is directed to a communications channels evaluation system
and in particular to an evaluation system for point-to-point high
frequency communication systems.
Uncertainty exists in realizing the overall potential of high frequency
communications systems. Equipment parameters can be controlled by the
operating personnel but factors such as propagation conditions over the
transmission path, the presence of co-channel interference on a particular
channel and the ability to select the optimum traffic frequency at any
given time are normally beyond their control. Yet these factors are vital
in maintaining circuit reliability at its fullest potential.
The operational approach to high frequency communications has
conventionally been to assign a family of frequencies to a particular
circuit which are used at the discretion of the operating personnel, the
traffic frequency at any given time being selected either on the basis of
long term frequency predictions or operational experience. As high
frequency propagation is subject to the vagaries of the ionospheric
characteristics present at any given time over the propagation path, there
is no guarantee that the optimum channel is being used or that
communications can be successful using these conventional methods for
circuit control. Furthermore there is no means for the operating staff to
assess the relative performance of assigned channels without a lengthy
procedure of trial and error.
For a simplex communications system wherein the same frequency is used for
communication in both directions, the above problems have been obviated by
channel evaluation systems such as the one described in U.S. patent Ser.
No. 3,543,161 which issued on November 24, 1970 to Hatton et al. However,
this system is not suitable for point-to-point high frequency
communications systems wherein high traffic density is supported by
simultaneous transmission in both directions on two different frequencies,
each frequency being selected from a separate set of frequency
assignments.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide channel evaluation
apparatus for point-to-point communications systems.
It is a further object of this invention to provide apparatus which
indicates the best channel of communication in each direction for
point-to-point communications system.
These and other objects are achieved in a channel evaluation apparatus for
a point-to-point high frequency communications system which has a first
set of communication frequency channels for transmission between a first
terminal and a second terminal and a second set of communication frequency
channels for transmission between the second terminal and the first
terminal, and in which each of the first and second terminals has a
communication transmitter and a communications receiver. The evaluation
apparatus includes a first sounding transmitter located at the first
terminal for transmitting first sounding signals over the first set of
frequency channels to said second terminal and a second sounding
transmitter located at the second terminals for transmitting second
sounding signals over the second set of frequency channels to the first
terminal. The evaluation apparatus further includes a first sounding
receiver located at the second terminal which receives the first sounding
signals transmitted over the first set of frequency channels and
determines the quality of transmission over each of the frequency channels
in the first set, and a second sounding receiver located at the first
terminal which receives the second sounding signals transmitted over the
second set of frequency channels and determines the quality of
transmission over each of the frequency channels in the second set.
The sounding receivers include display devices for indicating channel
quality of the respective channel sets, and the sounding transmitters
include encoders for encoding channel selection information on the
sounding signals which are decoded and displayed by the sounding
receivers.
The apparatus may further include a first control circuit at the first
terminal synchronized with a second control circuit at the second
terminal. The first control circuit is coupled to the first sounding
transmitter for controlling its time-step sequence through the first set
of frequency channels and is further coupled to the second sounding
receiver for controlling its time-step sequence through the second set of
frequency channels. The second control circuit is coupled to the second
sounding transmitter means for controlling its time-step sequence through
the second set of frequency channels and is further coupled to the first
sounding receiver for controlling its time-step sequence through the first
set of frequency channels. The sounding signals may consist of a series of
repeated characters, a fixed number being transmitted over each channel
such that a counter in each of the first and second sounding receivers may
count the number of characters received correctly on each frequency
channel as a determination of channel quality.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 schematically illustrates a point-to-point communications system
with channel evaluation; and
FIG. 2 illustrates channel evaluation apparatus at one terminal.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A sounding assisted point-to-point high frequency communications system
having two identical terminals A and B is shown in FIG. 1. The
communications system is assigned a family of frequency channels which may
be any number of channels, such as 12 channels shown in this embodiment.
Six frequency channels F1-F6 are assigned to transmission link A to B and
the six other frequency channels transmissions F12 are assigned for
transmission link B to A.
Each terminal A or B includes a central communications center 1A, 1B for
receiving sounding and communications signals via an antenna 3A, 3B and a
remote transmitter site 2A, 2B for transmitting sounding and
communications signals via antennae 4A, 4B and 5A, 5B respectively. The
transmitter site 2A, 2B is remote from the communications center 1A, 1B to
avoid high level interference from local transmission which would
desensitize the receiving equipment and may be located at a distance of
two to twenty miles from the communications center 1A, 1B. The central
communications center 1A, 1B includes sounding receiving equipment 6A, 6B
and conventional communications receiving equipment 7A, 7B and, via
control lines 8A, 8B, controls the transmission site 2A, 2B which includes
sounding transmission equipment 9A, 9B and conventional transmission
equipment 10A, 10B. These control lines 8A, 8B may be telephone lines,
cable or a micro-wave link.
As illustrated in FIG. 1 the transmission equipment 2A at terminal A
transmits on the F1-F6 frequency channels and the receiving equipment 1A
receives on the F7-F12 frequency channel. In terminal B, the transmission
equipment 2B transmits on the F7-F12 frequency channels and receiving
equipment 1B receives on the F1-F6 channels.
In order to evaluate the communication channels in one link, for example
link A-B, sounding signals are produced by the sounding transmitter 9A,
these signals are transmitted to terminal B in time-step sequence through
frequency channels F1-F6. If propagation exists, sounding receiver 6B
receives the sounding signals and evaluates the received sounding signals
for each channel in order to determine the best transmission channel in
the A-B link. This information is used by the terminal B operator to
manually or automatically set the terminal B communications receiver 7B to
the proper channel and is also encoded in the sounding transmitter 9B and
transmitted on the sounding signals from terminal B to terminal A. This
information is usually transmitted on all of the frequencies to ensure
high reliability. Sounding receiver 6A, upon reception of the encoded
sounding signals, decodes the information on the sounding signals and
indicates the desired channel for link A-B to the operator of terminal A.
This indication is used by the operator to manually or automatically set
the communication transmitter 10A to the proper channel. For the second
link or link B-A, the encoded sounding signals which are transmitted from
terminal B in time-step sequence through frequency channels F7-F12, are in
a similar manner evaluated in the sounding receiver 6A. This information
is used by the terminal A operator to manually or automatically set the
terminal A communications receiver 7A to the proper channel and is also
encoded in the sounding transmitter 9A as the sounding signals which are
being transmitted to terminal B. In sounding receiver 6B, the sounding
signals are decoded to indicate the proper channel setting for the
terminal B communications transmitter 10B.
Sounding receiving apparatus 6A and sounding transmission apparatus 9A for
one terminal are shown in FIG. 2. This apparatus, being the same for
terminal A and B except for their different sets of frequency channels,
the description will be limited to one terminal, that of terminal A. This
apparatus may be operator controlled or programmed, and on the basis of
channel evaluation information the communications equipment may be made to
respond automatically to select and use the optimum traffic channel in
each direction.
In the communications center 1A, shown in FIG. 1, sounding receiver 6A
includes a master clock 11A, coupled to a program unit 12A and provides
precise time for the program unit 12A. The program unit 12A in turn
controls both the sounding receiver 6A and the sounding transmitter 9A,
under the control of the operator, enabling each terminal A and B to be
maintained in time and frequency synchronism. The program unit provides
(1) the times for initiating a sounding sequence, (2) frequency control
data for controlling the sounding transmitter 9A via the control line 8A,
and (3) the time-step sequence for the receiver circuit 13A through
channels F7-F12. The receiver circuit 13A detects the sounding
transmission received from terminal B by sequentially stepping through
each frequency channel F7-F12.
The receiver circuit 13A output is coupled to a signal evaluator and
decoder 14A wherein the quality of the sounding signal is determined. This
information is coupled to display 15A wherein channel quality is displayed
to the operator. This display 15A may consist of a number of lights or a
numerical indicator for each frequency channel such that the best
frequency channel or channels is obvious to the operator. He can then
adjust the program unit 12A to control the sounding transmitter 9A so as
to encode this information on the regular sounding signals. The regular
sounding signals received by receiver 13A may also be encoded to provide
information as to the best frequency channel for transmission from
communication transmitter 10A. These sounding signals are decoded by
decoder 14A and the channel number or change is displayed on display 15A.
The frequency channel for the communication transmitter 10A may then be
set either automatically or by the operator.
As mentioned above, the control unit 12A controls the sounding transmitter
9A via control lines 8A. The sounding transmitter 9A includes a frequency
agile transmitter circuit 16A which steps through the F1-F6 frequency
channels. Control data arriving over the control lines 8A from the program
unit 12A is decoded by transmitter control unit 17A which in turn provides
the stepping sequence for the frequency agile transmitter circuit 16A. The
transmitter circuit 16A may be modulated with a coded signal sequence
provided by the sounding transmission encoder 18A to include channel
information for terminal B.
Usually, it is desirable that the type of sounding signals used to evaluate
channel performance bears a relationship to the mode of transmission used
by the communications system. Normally this is frequency shift keyed
teletype (FSK). Furthermore, the sounding signal must be of sufficient
length to give a valid evaluation of a channel's performance, to allow for
the effect of short fades in signal strength. In the present system, a
series of characters such as a burst of 100 teletype characters, RYRYRY .
. . , may be transmitted on each channel, and to evaluate the channel, the
number of characters received correctly are counted.
These sequence of sounding signals through the set of frequency channels
may be continuously repeated or repeated at some interval such as 5 or 10
minutes. It has been found in practice, that up-dated channel evaluation
information every 10 minutes is adequate the majority of the time.
However, if there is a break-down in communications due to propagation or
communications equipment failure, sounding can be continuous in order to
re-establish contact as soon as possible. This feature and operational
procedure is particularly useful in establishing initial contact with a
terminal.
Another feature of the present evaluation apparatus is that the sounding
transmitter 9A, 9B can be operated at reduced power with respect to the
communication transmitter 10A, 10B since channel evaluations are relative.
In the channel evaluation apparatus described, the sounding transmissions
are made on all channels in parallel with the associated communications
transmission. Thus during each sounding sequence the sounding
transmissions will be received on the channel currently used for traffic
over the regular communications receiving system. This poses a source of
interference even though it is only a few seconds out of a 10 minute
period. Several methods can be used to minimize or avoid this problem,
such as: (a) sounding transmissions can be made at reduced power with
respect to the communications transmissions, (b) sounding on the "busy"
channel can be omitted, or (c) soundings can be made on a frequency
slightly off-set from the traffic channel. In a sounding system tested,
wherein sounding transmissions were down in the order of 14 db with
respect to those of the communications system and were off-set from the
communications channel by 1000 Hertz, satisfactory results were achieved.
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