Note: Descriptions are shown in the official language in which they were submitted.
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BACKGICOUND OF THE INVENTION
This invention is directed to a communications
channels evaluation system and in particular to an
evaluation system for point-t~~point high frequency
.
communication systems.
' Uncer-tainty 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
. . .
~` 10 the transmission path, the presence of co-channel inter-
ference 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 it~ fullest potential.
The operational approach to high frecluency
communications has conventionally been to assign a ~clmily
of ~requencies to a particular circuit which are used at
the discretion of the operating personnel, the traffic
,~ ' frequency at any yiven time being selected either on the
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~'~ 20 basis of long term frequency predictions or operational
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experience. As high frequellcy propagation is subject to the
vagaries of the ionospheric characteristics present at an,y
given t.ime over the propagation path~ there is no ~uarantee
that the optimum channel is being used or that communications
can be ~successful usiny these conventional methods Eor
circuit control. Furthermore there is no means for the
operating ,staff to assess the relative performance of
assicJned channels without a length,y procedure of trial and
error.
For a simplex communications system wherein the same
frequency is used for communication in hokh d:irections, the
above problems have been obviated by channc-!l eva'luation
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systems such as -the one described in Uni-ted States Patent
serial no. 3,543,161 which issued on November 2~, 1970 to
Hat-ton et al. However, thi.s system is not suitable for
~: point-to-point high ~re~uency communications systems wherei.n
~ high traffic density is supported by simultaneous transmission
,~ in both directions on two different frequencies, each
freguency being selected from a separate set o~ ~requenc~
' assignments.
,-',- SUMMARY OF THE INVENTION
,' 10 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 Eor point-to-point cornmunications system.
These and other objects are achieved in a channel
,,'l evaluation apparatus ~or a point-to~point high frequencycommunications system which has a first set of communication
~ frequency channels .~or transmission between a first terminal
,~ 20 and a second te.rminal and a second set of communication.,~- freciuency channels ~o.r transmission hetween the secondh'~ termlnal and the first terminal, and in which each of thefirsk and second terminals has a cornmun:ication transmittex
and a communications receiver. The evaluation apparatus
~, includes a first sounding transmitter located at the first
terminal for transmittiny Eirst soundi.ng,signals over the
fi.rst set oE :Erequency channels to said seconcl terminal and
a second so~mding transmitter located at the ~econd te.rminals
~or transmi-t-t:ing second soundiny siynals over the second se-t
of frequency channels to the ~irst terminal. The evaluation
apparatlls further includes a f rst sounclin~ receiver located , ,~
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- at the second terminal which receives the first sounding
signals transmitted over the first set of freguency channels
and determines the quality o:E transmissi.on over each of the
frequency channels in the first set, and a second sounding
receiver located at the firsl 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.
~he sounding receivers include display devices fo.r
indicatiny channel quality of the respective channel sets,
.;~ and the sounding transmitters include encoders for encodiny
channel selection information on the sounding signals which
are decoded and displayed by the sounding receivers.
The apparatus may ~urther include a first control
circuit at the ~irst terminal synchroni~ed 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
:j! frequency channels and is further coupled to the second
o ~ sounding receiver ~or controlling its time-step sequence
I through the second set of frequenc~ channels~ The second
;',!~ control circui-t is coupled to the second sounding transmitter
'~! means for controlliny its time-step sequence through the
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second set o~ ~requency channel.s and is further coupled to
the first soundiny receiver for controlling its time-step
sequence through the first set of frequency channels. The ,
;~ soundiny siynals may consist of a series of repeated
characters, a fixed number beiny transmitted over each
channel such that a counter in each of the firs-t and second
J~:~ 30 sounding receivers may count the numbex of characters re-
Z~ ceived correctly on each frequency channel as a determina-tion
~Z~ of channeZl quality.
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BRIEF DESCRIPTION OF TE~E DRAWINGS
In the drawings:
. Figure 1 schematically illustrakes a point-to-
: point communications sys-tem with channel evaluation; and
Fi.gure 2 illustrates channel evalua-tion
:: apparatus at one terminal.
DESCRIPTION OF THE P:REFERRFD EMBODIM:ENTS
:~ A sounding assisted point-to point high frequency
communications system having two identical termlnals A and
:~ 10 B is shown i.n figure 1. The communications system is
assigned a famil.y o~ frequency channels which may b~ any
number vf channels, such as 12 channels shown in *his
. , .
`. embodiment. Six frequency chann~ls Fl - F6 are assigned to
.-. transmission link A to .B and the six other frequency channels
`.-. F7 - Fl~ are assigned ~or txansmission link B to A~
Each terminal A or B includes a central communi-
: ca~ions center lA, lB ~or receiving sounding and :~
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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, SB
respectively. The transmittex site 2A, 2B is remote from
~,' the communications center lA, lB to avoid high level inter-
!'~, ference from local transmissions which would desensitize
the receiving equipment and may be located at a distance
J13~ o~ two to twenty miles from the communication~ center lA,
~:~ lB. The central co~m~nications center lAr lB includes
sounding receiving equipment 6A, 6B and conventional
:,Jj' ~ ~ communica tiOIls recPiving equipment 7A, 7B and, via control
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lines 8A, 8B~ controls the transmission site 2A~ 2B which ,~
~ 30 includes sound.ing transmiss.ion equipment 9A, 9B and conven-
.', tional transmission e~uipment lOA, :LOB. ~hese control lines
~ 8A, 8B may be telephone lines, cable or a m.icro~wave link.
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As illustra-ted in Eiyure 1 the transmi.ssion ',
equipment 2A at terminal A t:ransmits on the Fl - F6 fre~uency
channels and the receiv:ing equipment lA receives on the
,~ F7 - F12 frequency channel. In terminal B, the transmission
~ equipment 2B transmits on the F7 - F12 frequency channels
. , .
and the recei~ing equipment lB receives on -the Fl ~ F6
,~ channels.
In order to evaluate the communication channels
. in one link, or example link A-B, sounding siynals are
.' 10 produced by the sounding transmitter 9A~ these signals are
transmitted to terminal B in time-step sequence through
~-~, frequency channels Fl - F6. If propac3ation e~ists, soundiny -,,
: receiver 6B receives the sounding signals and evaluates the
:,; recei~ed sounding signals for each channel in order to
, . . .
~,. determine the best tranmission channel in the A-B link. This
,`, information is used by the terminal B operator to manually
, or automatically set -~he 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.,; .
;, 20 terminal B to terminal A. This information is usuall.,y
'~'5'',' transmitted.on,all of the frequenci.es to ensure high
-,: reliability. Sounding receiver 6A, upon recept.ion of the
encoded sounding signals, decodes the informat:ion on the
sounding si.gnals and indicates the desired channel for link
~! A-B to the operator of terminal A. This indicati.on is used
:':J by the opera;to.r to manually or automatically set the
1,l communication transmitter lOA to the proper channel. For
'1'1 the second link or link s~A~ the encoded sound:ing signals
~1
j which are transrnitted from terminal B .in time~step sequence
.,
~ 30 through frequency channels F7 - F12, are in a similar manner
evaluated in the soundinc3 receiver 6A~ This in:Eo:rmation :is
used by the terminal A ope:rator to manua:lly o.r automl-ticcll:ly
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set the te.rminal A communicati.ons receiver 7A to the proper
channel and is also encodecl in the sounding transmitter 9A
as the sounding signals which are beiny transmi-t-ted to
terminal B. In sounding receiver 6B, the soundlng siynals
are decoded to indicate the proper channel sett.ing for -the
terminal B communications transmitter lOB.
Soundiny receiving apparatus 6A and sounding
: transmission apparatus 9A for one terminal are shown in
~ figure 2. This apparatus, being the same or terminal A
:; 10 and B except for their dif~erent sets o~ ~requency channels,
. the description w.ill be limited to vne terminal, that of :
terminal A. This apparatus may be operator controlled or
`. programmed, and on the basis of channel evaluation
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:, information the communications equipment may be made to
~ respond automatically to select and use the optimum traffic
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channel in each direction.
.~ In the communications center lAr shown in Eigure 1,
f~`' sounding receiver 6A includes a master clock llA, coupled
.to a program unit 12A and provides precise time for the
~; 20 program unit 12A. The p.rogram unit 12A in turn con-trols bo-th
the sounding receiver 6A and the sounding transmitter 3A,
~i under the control of the operator, enabling each te.rminal A
: and B to be maintained in time and Erequency synchronism.
i;! The program unit provides 1~ the times Eor i.ni-tia-ting a .
sounding sequence, 2) ~requency control data for controlling
the soundi~g transmitter 9A via the control line 8A, and
3) the time-step sequence ~or the receiver circuit 13A through
channels F7 - F12. The receiver circuit 13A de-tects the
sounding transmiss.ion .received :Erom -terminal B by
i~ 30 sequent:ially stepping through each Erequency channel F7 - F12.
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The recelver circuit 13A output is coupled to a
signal evaluator and decoder ].4A wherein the qwality of the
sounding signal is determined, This inforrna-tion is coupled
~ to display lSA wherein channel quality is displayed to the
. operator. This display 15A may consist of a number of ligh-ts
,~ or a numerical indicator for each frequency channel such that
.. the best frequency channel ox channels is obvious to the
:~ operator. He can then ad~ust the program unit 12A to control
.'~ the sounding transmitter 9A so as to encode this information
~o on the regular sounding sigrlals. The regular souncling siy~als
"' received by reeeLver 13A may also be encod~d to provide
;`,' in~ormation as to the best frequency channel for transmission
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~. ~rom communication transmitter lOA~ These sounding signals
. ;
are decoded by decoder 14A and the channel number or change .,
.,; is displayed on display 15Ao ~he frequency channel for the
communication transmitter lOA may then be set either au~o-
" - matically or by the operator.
s mentioned above, the control unit l~A controls
;i:
~ the sounding transmitter 9A via control lines 8A. The
-.~i 20 sounding transmitter 9A includes a frequency agile trans-
:
mitter circuit 16A which steps throuyh the Fl - F6 frequency
~1: ehannels. Control data arri~ing over the cont:rol lines 8A
.-
rom the program unit 12A is decoded by transm:Ltter control
~ unit 17~ which in turn provides the stepping sequence for the
'~ frequency agile transmitter circuit 16A. The transmitter
~: circuit 16A may be modu.lated w.ith a coded signal sequence
provicled by the sounding transmiss:ion encoder.l8A to include
channel information for terminal B. ,.
Usually, it is desirable that the type of sounding -
signals used to evaluate channel, performance bears a
, i ~ ,relationship to the mocle of transmission used by the "~ ,
communications s~stem. Normally this .is :Erequency shiE-t
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keyed teletype (FS~). Fur-thermore, the soundiny signal must
be of sufficient length to give a valid ev~luation of a
channel's performance, to allow for the effect of short fades
in s.ignal strength~ In the presen-t system, a series of
characters such as a burst oE 100 teletype characters,
RYRYRY...., may be transmitted on each channel, and tc>
evaluate the channel~ the number of characters received
~: correctly axe counted.
,. : . .
~ The sequence of sounding ~ignal5 thxouyh the set
.~ lO 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, i ~
;j sounding can be continuous in order to re-establish con-tact ~- -
as soon as possible. This feature and operational procedure .
~-J~ is particularly useful in establishing initial contact with
~,,
~ . a terminal.
', 20 Another *eature oE the present evaluation apparatus
is that the sounding transmitter 9A, 9B can be operated at
~. reduced power with respect to the communication transmitter
:. lOA, lOB since channel evaluations are relative.
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In the channel evaluation apparatus described, the
sounding transmissions are made on all channels in parallel
with the associated communicat.ions trarlsmisslon. Thus duriny
~YI each soundiny sequence the sounding transIllissions will be
~ received on the channel currently used ~or tra:E~ic over the
;~ regular co~nunications receiv.iny system. This poses a source
,~ 30 of inter:Eerence even though it is only a *ew seconds out of a
lO minute perio~. Several me-thods can be used to min:imize or
avoid this problem, such as: (a) soundincJ -transm:issions can
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be made at reduced power with respect to the communications
transmissions, (b) sounding on the "busy" channel can be
omit-ted, or (c) soundings can be made on a frequeIlcy
slightly off-set from the traffic channel. In a sounding
; system tested, wherein sound:ing transmissions were down in the
order of 14 db with respect to those of the communications
system and were off se-t from the communications channel by
1000 Hertz~ satisfactory results were achieved.
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