CA1064579A - Channel evaluation apparatus for point-to-point communications systems - Google Patents

Abstract

TITLE

CHANNEL, EVALUATION APPARATUS FOR
POINT-TO-POINT COMMUNICATIONS SYSTEMS

INVENTOR

Everett E. Stevens ABSTRACT OF THE DISCLOSURE
A point-to-point high frequency communications system has a first set of frequency channels for transmission between a first terminal and a second terminal, and a second set of frequency channels for transmission between the second terminal and the first terminal. The channel evaluation apparatus includes a sounding transmitter and receiver at each terminal. These equipments operate in parallel with the regular communications system and provide the facility for real-time evaluation of all assigned channels using a sampling technique. Each sounding transmitter periodically transmits sounding signals in time-step sequence through its respective frequency channels to the sounding receiver at the other terminal. The sounding signals for each channel are evaluated for transmission quality and this information is displayed.
The operator at each terminal may manually or automatically determine the desired channel of reception and this information which is then encoded on the sounding signals transmitted to the other terminal, is used by the operator at the other terminal to set the communication channel.

Description

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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 .i :
~'~ 20 basis of long term frequency predictions or operational ."
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 ~6~

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 .,Z
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 .~
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 l .
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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 ., .
:, 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 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 .,"~ :
~. ~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 7 _ ,!

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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.
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~ 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 r' : ~

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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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Claims (6)

CLAIMS:

1. Apparatus for evaluating 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, wherein said first and second terminals have a communications transmitter and a communications receiver, the apparatus comprising:
- first sounding transmitter means located at said first terminal for transmitting first sounding signals over said first set of frequency channels to said second terminal, said first sounding transmitter means includes first encoder means for encoding channel selection information for communication transmissions between said second terminal and said first terminal on said first sounding signals;
- second sounding transmitter means located at said second terminal for transmitting second sounding signals over said second set of frequency channels to said first terminal, said second sounding transmitter means includes second encoder means for encoding channel selection information for communication transmission between said first terminal and said second terminal on said second sounding signals;
- first sounding receiver means located at said second terminal for receiving the first sounding signals transmitted over said first set of frequency channels and for determining the quality of transmission over each of the frequency channels in said first set; and - second sounding receiver means located at said first terminal for receiving the second sounding signals transmitted over said second set of frequency channels and for determining the quality of transmission over each of the frequency channels in said second set.

2. Apparatus as claimed in claim 1 wherein said first sounding receiver means includes first display means for indicating channel quality at said second terminal, and said second sounding receiver means includes second display means for indicating channel quality at said first terminal.

3. Apparatus as claimed in claim 2 wherein said first sounding receiver means includes first decoder means for decoding the first sounding signals and for providing the channel selection information to the first display means, and wherein the second sounding receiver means includes second decoder means for decoding the second sounding signals and for providing the channel selection information to the second display means.

4. Apparatus as claimed in claim 3 which further includes first control means at said first terminal syn-chronized with second control means at said second terminal, said first control means coupled to said first sounding transmitter for controlling a first time-step sequence through the first set of frequency channels for said first sounding transmitter and further coupled to said second sounding receiver means for controlling a second time-step sequence through the second set of frequency channels for said second sound receiver, said second control means coupled to said second sounding transmitter means for controlling a time-step sequence identical to the second time-step sequence through the second set of frequency channels for said second sounding transmitter and further coupled to said first sounding receiver means for controlling a time-step sequence identical to the first-step sequence through the first set of frequency channels for said first sounding receiver means.

5. Apparatus as claimed in claim 4 wherein the sounding signals consist of a series of repeated characters, a fixed number being transmitted over each channel.

6. Apparatus as claimed in claim 5 wherein said first and second sounding receiver means each include counting means for counting the number of characters received correctly on each frequency channel as a determination of channel quality.