CA1310371C - Mechanism for deriving accurate frequency reference for satellite communications burst demodulator - Google Patents
Mechanism for deriving accurate frequency reference for satellite communications burst demodulatorInfo
- Publication number
- CA1310371C CA1310371C CA000608746A CA608746A CA1310371C CA 1310371 C CA1310371 C CA 1310371C CA 000608746 A CA000608746 A CA 000608746A CA 608746 A CA608746 A CA 608746A CA 1310371 C CA1310371 C CA 1310371C
- Authority
- CA
- Canada
- Prior art keywords
- frequency
- station
- burst
- carrier
- continuous
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
- H04L27/22—Demodulator circuits; Receiver circuits
- H04L27/227—Demodulator circuits; Receiver circuits using coherent demodulation
- H04L27/2271—Demodulator circuits; Receiver circuits using coherent demodulation wherein the carrier recovery circuit uses only the demodulated signals
Abstract
ABSTRACT OF THE DISCLOSURE
Accurate tuning of a satellite system's burst demodulator to a signal which is subject to frequency offset in the course of its transmission over the satellite link is achieved without the use of high precision oscillators at remote sites, or the transmission of a dedicated pilot tone. Instead, a dedicated high precision clock is used for the purpose of establishing both the outlink carrier and the return channel carrier. The modulation of the data on the outlink carrier is also derived from the precision clock source. At each remote station, the outlink channel is monitored to recover the high precision clock. This recovered clock is then used as a reference for generating the return channel carrier. The burst demodulator equipment at the master station monitors both the outlink equipment continuous carrier and burst mode transmissions from the remote stations. Each of the continuous and burst mode carriers, having been transmitted through the satellite, will undergo the same frequency offset or modification, so that the frequency difference between the outlink channel carrier and the return link channel carrier will always be constant. The outlink channel carrier, which is available on a continuous basis, is used to derive a local oscillator frequency reference for establishing the burst demodulator's local oscillator reference.
Accurate tuning of a satellite system's burst demodulator to a signal which is subject to frequency offset in the course of its transmission over the satellite link is achieved without the use of high precision oscillators at remote sites, or the transmission of a dedicated pilot tone. Instead, a dedicated high precision clock is used for the purpose of establishing both the outlink carrier and the return channel carrier. The modulation of the data on the outlink carrier is also derived from the precision clock source. At each remote station, the outlink channel is monitored to recover the high precision clock. This recovered clock is then used as a reference for generating the return channel carrier. The burst demodulator equipment at the master station monitors both the outlink equipment continuous carrier and burst mode transmissions from the remote stations. Each of the continuous and burst mode carriers, having been transmitted through the satellite, will undergo the same frequency offset or modification, so that the frequency difference between the outlink channel carrier and the return link channel carrier will always be constant. The outlink channel carrier, which is available on a continuous basis, is used to derive a local oscillator frequency reference for establishing the burst demodulator's local oscillator reference.
Description
13lo37l F ~HE I~V~NT~ON
~he pre~en~ inven~ion r~l~te~ in g~ne~ atPllit~.
commu~ication ~yate~ ~nd i~ p~rkiqularly dir~ct~d ~o a me~hanl~m f~r provlding an ~c~rate fxeguency réf~rence ~or enabling a burst d~modulator to r~cover in~orm~ti~n ~ign~l~
~rom a c~rrler ~h~t h~s been ~ub~e~ted ~o a ~req~ncy o~et duriny it3 transmi~ion over the sat~llite communic~tion ~ar~ ' The successful operation o~ satellite communication networks, such as dem~nd a~signment ~cont2ntion~ s~s~ems, depends upon the ~bill~ of the re~ei~er equlpmen~ a~ ~he respective sta~ions o~ the ne~work to b~ accurately ~uned t~ ~he incomin~ nal ~rom another ~ite. Xn ~ demand assignment communications scheme, me~sa~es from respective contention participant~ o~ th~ n~twork are tran~mi~ed ~n burst format, in whiah a respective. station's carrier is ~urn~d on ~or an a~reviated period, or tim~ sl~t, during whiclh ~ n~q~ (e~. d~ pa~ce~ tr~nsmi~ed and ~her i 20 turned off until th~t ~t~tion h~s a new me3~ag~ to ~end.: R~q~use o~ the lnkermittent nature of burst mode ~ommunioations and ~arrier dis~or~ion introdu~ing chara~eri~ o~ ~he ~elll~e qhannel, the ab~lity ~o ! provide an acaurate demodul~tion referenc~ ~re~u~nGy ~orsignal recovery ~ecome~ a 3igni~icant proble~ One way ~o i 341v~ the prohl~m i~ p~ovide eaah station with a high precision oscill~tor whi.ch monitor~ an ~feG~lve~y perfectly ~abl~ pilot fre~u~n~y~ ~hat ha~ h~n ~ra~smlt~ed fro~ a m~s~er site~ in order to d~t~r~tin~ ~r~u~ncy o~fs~t thro~g~
khe s~ellite ~ ink and tQ u~e t~is in~ormation to accurat~ly ~ control ~he ch~ractaris~ics o~ its bur~ ~arr~r, ~o ~ha~
~h~ me~s~ge, ~hen re~iY~d ~ the master ~it~, will be e~a~ively pre~orr~c~d, permit~ln~ demodulation a~d data recov~ry. The pr~lem wlth thi~ approach i3 ~wo-~old: on ! 35 the one h~nd, th~ a~t o f the equipment a~ ~ach bur~t-'` ~
J I U~ l 1 ~ouraing ~ite i~ inare~ed ~u~ n~ially by the provi5~ on o~ th~ high preci~ion r~ferenc~ osai~l~tor. In ~d~itlon, bec~u~e ~he ne~work u~e~ a pilo~ tvne *or the purpos~ o~
fre~uenay correction, the capacity o~ thQ satellite link (~n ex~rem~l~ prec~u~ re~our~e) ~ox d~a ~r~n~ml~ion i~
reduc~d.
One propo~al to eliminate p~rt o~ the problem, namely to reduc~ the expens~ oP th~ e~ipm~nt (high p~eci~lon osclllator) at th~ burst~sourcing si~e i~ d~çrihed in U.S.
Pat~n~ No~ 4,509,2~0 t~ ~uginbuhl et al, entitled "~atellite Telecommunic~tion~ System'l. Pursuant to the pAtented scheme, ~ high pre~l~ion pilot tone o~clllator i~
in~talled at the master or central station, thQ only purpo~2 o~ which is to measure ~requency o~et (~rift) through the ~telli~e. By monl~orin~ the pilot tone over a loop bac~
t~ it~el~, ~hQ ma~t~r 3tation i~ able ~o mea~ure ~hè offset throu~h the ~ellite, whiah, as pointed out above, must be ~orrect~d. ~ nal repre~en~ative o~ the value of ~his mea~ured o~set or error i~ ~hen transmi~ed a~ an ~0 ln~c.)rm~lon ~ignal for us~ at ~aah remo~ ~it~. ~he remote si~e, which doe~ t have the henefit o~ the high precision oscillator, ~xtra~t~ the dat~ to properly t~ne it~elf~ Thi~
operation presuppose~ ~hat the remote ~ite is prop~rly tuned to begin with, so~ethlncJ tha~ ~h~ aoar~e ~clllator used at ~5 ~he r~mote ~te ~amlo~ gu~rant~e. ~on~equently, the propos~d procedure i~ ~ues~ionable at hest. 0~ cour3e, due to the ~as~ that th~ patll~nted ~heme de~lcate~ part o~ the ~atelll~e link to the o~et-con~roll~ n~ pilot tone, the re.souxc~ oc~pancy p~obl~m ~till exist~
~
: In ac~ordance with th~ p~e~ent invention, the ability to ac~ura~el~ ~une ~ demodulator ~v an in~erml~nt~y transmitted ~bur~t~ ~ig~l, WhiCh is 3ubj~c~ ~.o ~requqn~y of~t ln the course o~ tr~nsmis~i~n over the ~atellite 3s lin~ chie~ wlthouk ~ither ~ ~he ~bove-ment~oned con~ehtic~ rti~i~e3, lncluding the in~ i on o~ hl~h preciE3ion o~ tor~ ~t the remot~ e~, or the ~ransmi~Z~îon ~ a ~eparate, dedlc~t~3d pilot tone Por t:h~
purpo~a o~ corre ~ n~ the prQ~lem o~ :erequency o~ e'c through the satellite lin3c.
Pux~uan~ to th~ present inventiorl, a 3ingle, d~ a~e~i high pr~cision clo~k i~ used Ic~r the purpo3e of e3tal~1i hing both the outlin~ ca:erieX fr~m the m~ster ~t~tion to e~ch o~
~he remote ~t~ions and tha ret~urn ch~nhe~ ier through which eac:h re~noke ~;t~tion kransmits hurst messayes ~o ~he ma~ter site over the ~atelli~e link~ In addition, ~he mo~ul~tion o~ ~he da~a on ~n~ outlink c:arrier is ~lerived ~rom ~he ~;~me pr~ci~ion clock source. At each remote ~:t~tion, th~3 ou~link channel is mc:nito~ed ~o r~over the hi~h preci~ion c~lo~k~ Thi~ recovered clock i8 than u~ed as : a re~erenc~ for gen~rating ~he re~uxn ahannel carxier.
Ra~he~ than attempt to correct or pr~modify aither the outlink chAnnçl contihuous ~axrler or the return link ~hannel bur~ aarrier, the ~ystem permits both channel6 to ~0 ` ~e 6ubjected to the drift or o~fs~t through the ~tellite.
The ~urst d~modul~r e~uipment at the master station mon~ors both the outlink channel ~ontinuou~ carrier ~ha~
ha~ been ~r~n~mitted throu~h the 6~t~111te and ha~ ~her~by been ~u~jec~ed ~o ~he ~atellite link o~e~) and the inaominy burst mo~e tran~mi~ion~ ~ro~ ~he re~ote station~.
Each o the contlnuous and ~ur~t mode carriers, having besn ~ran~mi~ed through the ~atellite, will undergo the ~ame frequency o~f~et or mo~ifiaa~lon. ~onsequently, th~
~re~uency dif~eren~e between th~ outlink ch~nel ~arrier and the r~turn linX chann~l ~ar~i~r will alway~ ~e r-on~tant.
~he ou~l$nk channel rarrier, which i~; available on a continuou~ b~;i6, $~ u~:ed to deriYa a lo~:al o~:ci~ tor fxe~u~ncy re~erenc~ for ~tabllGhin~ ~he ~urst ~lRmOdUla~Qr~
1OCA1 o~illa~or ~e~e~nae.~ A~ a Gon~e~uen~, reg~rdle~s o~ ~ny~ d~i~t throu~h th~ s~t~ e, the ~ur~t demodulator -- j f-iJ~ 7 i is always referenced to a local oscillator signal that tracks, precisely, any variation in the burst carrier.
S Pursuant to a further feature of the present invention, advantage is taken of the fact that the outlink channel car~ier is modulated with a timing signal that governs the occurrence of contention time slots that are used by the remote stations to send messages over the return link channel to the master station. The availability of this timing signal and the ability to adjust the operational frequency characteristics of the programmable frequency synthesizers for each of a continuous channel section (from which the 'continuous' local oscillator frequency reference is obtained) and a burst channel recovery section (from which the burst demodulator's reference is obtained) of the burst demodulator facilitates substitution of a redundant or backup unit in place of another unit.
In accordance with an embodiment of the invention, for use with a communication system having a first station and a second station with respect to the first station, the first and second stations communicating with one another over a communication link that subjects signals transmitted thereover to a modification of a characteristic thereof, the first station modulating information signals for delivery to the second station onto a first carrier frequency that is transmitted by the first station over the link on a continuous basis, the second station modulating information signals for delivery to the fir~st station onto a second carrier frequency that is transmitted over the link on a burst mode basis, a method of enabling information signals, modulated onto the second carrier ~requency which is transmitted on a burst mode basis from the second station over the link to the first station and thereby subjected to the characteristic modification, to be recovered at the first station is comprised o~ the steps of: at the second ~, ~ :'' ' "``
- 4a ~ 3 7 1 station, generating the second carrier freguency in accordance with a characteristic of the first carrier S frequency recsived thereby; and at the first station, receiving each of the first and second carrisr frequencies that have been transmitted over the communication link; and deriving, from the first and second carrier ~requencies, a filtered burst demodulation reference frequency for demo~ulating information signals that have been modulated onto the second, burst mode carrier frequency by the second station .
In accordance with another embodiment, an apparatus for generating a burst demodulation reference frequency, for enabliny a first station, within a communication system having first and second stations geographically separated from one another and communicating over a communication link that subjects signals transmitted thereover to a modification of a characteristic thereof, the first station modulating information signals for delivery to the second station onto a first carrier frequency that is transmitted by the first station over the link on a continuous basis, the second station modulating information signals for delivery to the first station onto a second carrier frequency that is transmitted over the link on a burst mode basis, to recover information signals that have modulated onto the second burst mode carrier frequency and transmitted from the second station over the link to the first station and thereby subjected to the characteristic modification, is comprised of at the second station, first apparatus for generating the second carrier frequency in accordance with a characteristic of the first carrier frequency received thereby; and at the first station, second apparatus for receiving each of the first and second carrier frequencies that have been transmitted over the communication link; and third apparatus, coupled to the 1 3 1 037 ~
-4b -second apparatus, for deriving, from the first and second carrier frequencies received thereby, a burst S demodulation reference frequency for demodulating information signals that have been modulated onto the second, burst mode carrier frequency by the second station.
In accordance with another embodiment, an apparatus for generating a burst demodulation reference frequency, for enabling a first station, within a communication system having first and second stations geographically separated from one another and communicating over a communication link that subjects signals transmitted thereover to a modification of a characteristic thereof, the first station modulating information signals for delivery to the second station onto a first carrier frequency that is transmitted by the first station over the link on a continuous basis, the second station modulating information signals for delivery to the first station onto a second carrier frequency that is transmitted over the link on a burst mode basis, to recover information signals that have modulated onto the second burst mode carrier frequency and transmitted from the second station over the link to the first station and thereby subjected to the characteristic modification, i9 comprised of at the second station, first apparatus for generating the second carrier frequency in accordance with a characteristic of the first carrier frequency received thereby and, at the first station, second apparatus for receiving each of the first and second carrier frequencies that have been transmitted over the communication link, a plurality of third apparatus, each of which is coupled to the second apparatus, for selectively deriving, from the first and second carrier frequencies received thereby, a burst demodulation reference frequency for demodulating information signals that have been modulated onto the second, burst mode carrier frequency by - 4c - 1 3~
the second station, and fourth apparatus, coupled to each of the third apparatus, for selectively defining prescribed operational characteristics of each of the third apparatus with respect of first and second carrier frequencies and enabling a selected third apparatus to derive the burst demodulation reference frequency in accordance with the selectively defined operational characteristics.
BRIEF DESCRIPT ON OF THE DRAWINGS
Figure 1 is a diagrammatic illustration of a satellite communications network employing the burst demodulator frequency reference derivation mechanism in accordance with the present invention;
Figure 2 is a diagrammatic illustration of the components of the burst frequency reference derivation unit employed in the system shown in Figure 1; and Figure 3 diagrammatically shows a master station having a plurality of transceiver units and a redundant burst demodulator section shown in Figure 2.
DETAILED DESCRIPTION
Before describing, in detail, the particular improved burst demodulator frequency reference derivation mechanism in accordance with the present invention, it should be observed that the invention resides primarily in a novel structural combination of conventional communication circuits and components and not in the particular detailed configurations thereof. Accordingly, the structure, control , . . , - ., ~ 3 ~ 037 ' and arrangement of these convsntional circuits and components have been illustrated in the drawings by readily understandable block diagrams which Rhow only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with structural details that will be readily apparent to those skilled in the art having the benefit of the description herein. Thus, the block diagram illustrations of the figures do not necessarily represent the mechanical structural arrangement of the exemplary system, but are primarily intended to illustrate the major structural components of the system in a convenient functional grouping, whereby the present invention may be more readily understood.
Re~erring now to Figure l, a diagrammatic illustration of a satellite communications network employing the burst demodulator frequency reference derivation mechanism in accordance with the present invention is illustrated as comprising a master or central station lO
which communicates by way of a satellite 20 with a plurality of remote stations, an individual one of which 30 i5 shown in the Figure. In effect, the network may be considered to be what is normally referred to as a star-configured satellite communications network, with the hub of the star corresponding to master station lO and the points of the star corresponding to the locations of the remote stations 30. For purposes of providing an illustrative example, the network will be assumed to be a Ru band system, the outlink channel (master-to-remote) carrier and return link channel (remote-to-master) carrier frequencies of which are on the order of 14 GHz up to the satellite and 12 GHz down from the satellite. Preferably, the communication~ control mechanism is of the type described in copending Canadian patent application 608l744 filed August l8th, 1989, entitled "Link Utiliæation Control Mechanism for Demand Assignment Satellite Communications Network'~ by E. Gerhardt et al to ~ 3 1 037 ~
which the reader is referred. It should be observed that the present invention is applicable to any communication S system employing burst communications that is subject to a frequency offset over the channel.
In the exemplary system, the outlink channel carrier, which is continuously transmitted from master station 10, is modulated with a network timing reference for establishing the occurrence of contention timeslots during which remote stations 30, each o~ which continuously monitors the outlink channel for messages directed to it from master station 10, transmit (burst) messages to the master station over the return link channel. Namely, each of remote stations 30 transmits messages to master station 10 in a demand assignment or contention burst-mode format through the satellite 20 by way of the dedicated remote-to-master return link.
As mentioned briefly above, a burst reference derivation equipment of the present invention obviates the need for installing a high precision oscillator at each of the remote sites and avoids the necessity of having to pre or post correct frequency offset through the satellite 20.
For this purpose, the network employs only a single precision reference oscillator, located at the master site, through which the outlink channel carrier and the return link channel carrier are generated.
More specifically, at the master station, a baseband clock signal (e.g. 5 MHz) from a precision source is applied over link 111 to the clock input of a (BPSK) digital data modulator 101 and to the reference input of an IF
translation tup-converter) unit 103. Modulator 101 contains conventional frequency reference converter circuitry (frequency multiplier/phase locked loop components) for 1 3 1 0~7 1 c:lockirly input clat~ on link 113 ~n aoc30rdarnc~ wi~h a pre~ribed I~AUd :~a~e ~ . g. 11~ for zLpplic~ n tc) X~
up~ conver'cer 10~, whiah i~ al~ con~rolled hy hlgh precieion 5 2~Hz ~lock ~rom link 111. Up converter 103 is o~
aonventional conflgur~tlon, multiplyin~ the hicJh pr~cision 5 MHæ ~lock to conver~ ~he ~ han~ si~n~l ~112 kb/~) to an lntermedi~L~e ~re~u~ncy on th~ order o~ 14 0 ~EIz . Thl~; ~Y
~ign~l is then ~pplied to PcF ~ansce~ver unik 145 WhiC~l tran~ es the IF ~ignal up to Ku b~n~ (14 GHZ) ~or ~pplicakion ~o RF antenna 107 and ~:ransmission over- the outlink ch~nnel ~hrough the satel:Lîte 20 to each ~f the remote s ite~ 3 0 .
E~h ~emote site 30 in~ludes an ~ntenna di3h 301 ~nd a~:soaiated RF transcei~rer unit 303 for receiving outlink ~ 15 channe.l ~e~sages and ~c~r transmlttlng return link buxs~
I mes~ages. Th~ ~e~eive ~utput o~ RF ~ran~ceiver unit 303 i~
I coupled to RF IF down-conve.~ter 305 which outp~t~ an IF
I si~nal (e.~ ~ an IF ~re~uency Q~ ~50-1450~Hz) to mQdsm 307.
Modem 3~7 includes a demo~ul~tor ~ection which ~perates o~
; 20 a 50 MX~ ource ~driving a phase lock loop) to I gene~a~e ~ re~erencs 11~ KH ~ox ~ecovering the incoming I ~ata s~ream. A~ me~lqned previously, r~th~ ~h~t employ an expe~ive preci~i~n o~illa~or to ~ener~k~ ~e modem ' referenae, the remot~ ~ite derive~ its r~eren~e ~rom th~
1 25 highly accu~-a~e aloa~ through which tne data i~ modulated and through which ~.he. outlink IF fre~ueno~ iE produced.
For ~.hlE purpose, mudem 307 con~ain3 con~en~ional ~
I ph~ ocked loop cl~k reaovery ~iroui~ for recovering the kHz c~o~ c~n~in~d wi~hin -~he lncoming dat~ ~tream. In addi~ion to using the r~coY~red clock ~or da~ demodul2tion, I thl~ sam~ r~coVere~ preclsi~n clo~k ~ f~ to ~h~ ~odulator ~ect~on o~ modem 307, wh~r~ it i~ u~ed a~ a r~ren~e to a I ~arrow ~andwi~h ph~e lock ~o~p ~t i~ drlven by an i o~h~rwi6e l~ss pre~l~e local os~illa~or ~e.g. 50 ~z). This ! 35 loaal oscilla~or is u~ed by ~he up-~onver6ion sacti~ of IF
tr~nsl~t~r unit 305 ~4r providiny ~ h~ghly aacJur~3 local o~alll~tor r~f!3r~n~ through whi~h dat~ modulation and ~re~uenay ~r~n~l~ti~n (Iro~ baE;cbancl to e~n IF frequency on the order of ~50-1450 MHæ) ;~e carried out, Thu~, because the mo~ula~ion r~erence ~r~qu~ncy i~ d~rived ~rom ~
pre~i~lon ~ource (locat~ ~t the master station), it i5 unnece~sary to in6~11 a ~p~rate high precision ~ei~erence fregu~ncy os~illator at each remo~e st~ation. The up-c:onverted ~ignal i~ output ~rom IF ~:~age 305 to txansceiver 303 fox tran~missi~n on the re~urn lirlk ~h~nnel (carri~ ~requency - 14GHz)~
It ~:hould b~ noted that t:he ~re~uency o~set through the ~t~l 1 it~ is not a problem ~o~ ~lat:a recovery ~t the remote ~tat~ons since the o~tlinlc ch~nnel carrier ~requent~y is con~inuou~ly tr~n~mitted an~ the high precision re~eren~e (5 MHz) Glock si~nal ~rom which the aarrier i~ der~ed i~
u~ed ~or modulating the data, which is un~f~ected by the ~r~c~ency o~f~t throu~h the ~atellit~. ~he pr~lem to whi~h ~he p~ent inven~ion 1~ di~e~ted, on the other hand, ls the faat that mes~ges fro~ t~e remote stG~tion~ to th~ master ~tation arQ bur3t m~, ra~her ~han ~ontinuoU~ ~ode tr~nsmi~ions, so that a dat~ recovery re~erence ~re~uency tha~ may b~ used ~y th~ ma~er ~atlon t~ ~emodulate reaeived burst traf~la i~ not continuou~ly availa~l~ from ~5 the remote S~A~ on~.
In accordanae with the pr~sent inven~ion, how~ver, advantage is taken o~ ~he ~a~t tha~ the outllnk channel aarxier i~ aontinuou~y Gavailable to ~he ma~er ~ation and u~de~g~ the ~me o~f~et ~hrou~h th~ ~t~llite ~ whiah bur~t mod~ tr~ns~ ion~ on the r~tuxn link channel ar~
: ~ub~eated~ BPcause burst mod~ transmi~ n the return link ch~nnel and c~n~lnuou~ mode tran~mi~ n~ on ~he ou~llnk channel und~r~o ~h~ ~ame of~et through th~
~at~llite, rega~dleG~ of the:~requency di~er~n~ial b~ween th~ outlin~ chann~l ~nd the r~turn link channel, the 1 3 1 ~37 1 di~P~re.nce ~tween thb two i5 ~lway~ cona~ant, ~g~rdl~s~
o* th~ magnitude of ~hc off~et through the ~a~ellike (~-7h~cA
wi.ll vary with ~ime1~ ~n a~cordanc~ with ~h~ prenent ~n~ention, thi~ constan~ dlf~er~ntial characteri~tic i~
~mpl~yed a~ tha mas~er ~i~e to d~rive a re~eren~e recove~y ~requency for ~ bur~ mode demodulator ~nd th~r~y ohvi~te I th~ neéd ~or ~ran~mit~in~ ~ ~epaxate pilot ~one ~or . ~orrecting the off~et through the Gatelli~e.
! ~ore particul~rly~ ~s ~hnwn generally in Figure 1 and lo in detail in ~igu~ ~, the mas~r ~ ion employ~ a burst I mode de~od~la or 121, coupled to a dual RF-IF down-conv~r~er I ~tage llS, wh~ch do~n-conver~ ~he outlink a~d r~turn lin~
I ~ha.nn~ls ~rom the 12 GHz band ~o the ~50 ~o 1450 MHz band.
~I The burst mode demodul~tor monitors the IF (~50-1450 MHz) 1 15 ou~put~ (burst and continuous mode outpu~) of transcei~er i 10~ and provide ~ pair of f~rther down-conver~d (e.g. 52-1 88 ~æ) output~ ~n llnk 123. For purpo~e~ of a~ illu~tratiYa ~ exAmpl~, wlthin tha p~ssband ~ inker~st a5 output, ~he : aontinuou~ mo~e ~ignal m~y have ~n IF re~ue~y of a 77.6 I ~o M~æ, while the bur3t mode ~l~n~l may have an I~ frequen~y o~ 77~4 M~z.
hown in ~'igur~ 2, de.~lodulator ~21 i~lalude~ ~
I ~ntinuou~ frequ~n~y r~er~llae recoværy ~e~ti~n 201 and a I ~us~t ~r~gu~lcy re~erence r~overy ~e~tion 211. Both 1 ~5 s~ctlon~ ~re coupled to link 123, so that each ~eotlon i reaeives the pair of XF frequen~ie~ ~77.4 ana 77,6 MH2) ~rom I ~F down~aonvert~r 115. ~ink 12~ i~ coupled t~ ~ir~t lnputs I of mi~r~ 204 and 214, ~eoond inputs o~ whi~h are I re~p~oti~ely ~.oupl~d ~o the ou~put~ o~ frPquency ~nthe~izer~ 203 arld : 213. Each o~ fr~quen~y ' synthe~lxer~; 203 and 21~ is ad~ustabl~ in lOO}CHZ ~t:eps ~ver a 3fi M~z b~ndwid~h and i~ drl~n by a loc~l clo~lc referenaP
~lpplled o~e~ k 112. Llnk 112 is ~oupled to a ~re~uenc:y I ~e~rence 206 vn ~he order o~ 12~ MH~, in r~pon~e ~o which 1 35 ~ynthesizçr 2a3 produ~e~ a preci~ion ou~put o~ 88.3 ~Xz, ~n~
1 3 1 [~37 ~
~yn~.he~izer 2].3 pro~uca~ ~ pre~i~lon ou~pu~ o* ~ Hz.
~or ~he ~bove d~cussed out~lnk and r~ur~ link ~hann~l ~ep~ra-~ion of ~.00 ~z (77~6 M~Iz - 77.~ ), e~rh o~
mixer~ 204 and 21~ will p~oduce an output of 10.7 ~HZ ln respon~e to the ~.3 ~Hz and 8~ z signals gener~ted by s~nthe3izer~ ~03 an~ ~13, re~peçt.i~ely. The ~ir~, COntillUOU~ reference ~xequena~ o~ 10.7 M~Iz produced a~ ~a output o~ mixer 20~ i~ couple.d through ~ C~t~s loop 20~, which produa~ ~n outpu~ ~re~enay on l~nk 2~7 th~t ef~e.ctivel~ tr~cks ~requency variation6 of tha ~ontinuou~
outlink ~hannel ~arrler~ Cost~ loop 20~ operate~ a~ a multiple of ~he 10.7 MHz produced ~y ~ixer ~04 ~e.y. four time~ the input 10.7 ~ ata at 42.~ M~z~. Output link ~0~
i5 a~uple~ to a ~ivid~-by-four divide~ 221 which controls the operatlon o~ a b~ndpa~ lter 217 within bu~s~ recover~
~ectl~n ~
Within the bur~ Xe~overy sec~i~n ~ll, thb ou~pu~ of mixe~ ~14 i~ connected ~o a ~ multiplier ~15. A~ in conven~ion~l ~odulator de~i~n, ths outp~ he X2 mul~iplier is ~ siyn~l a~ ~wi~e ~he IF ~requency whi~h i~
e~ecti~ely ~trippçd o~ the BPSK modulation on the IF
~ign~l . To ~e employed a~ a r~feren~e ~or the ~emodula~or, it i5 necessary ~ bandpa~ filter thi~ signal, and to then divid~ i~ hy ~Wo~ ha~ to the 10.7 ~H~ ~'r~quen~y where it is ~ hle to provide ~ r~renqe s~nal for d~modulatln~
the burst dat~ 3i~n~1. T~e output of X2 ~ul~iplier ~15 i~
~h~r~fore aoupled tc: th~ pu~ c~f a tracking b~ndpas~ lter 217~ Trackins b~ndp~s~ lter 217 in~ludes a splitter 223 ~u which re~peat.ive iTl-ph~ and quadra~ure (5~ hann~l filter ~ections are co~pl~d, The in-phas~3 fi Iter sec~ion include~2 a mixer ~25 ~o which the ou~put ~f ~plittç~r 2~3 and th~ in-ph~se ~omponent of ~he 21~4 MHz outpu~ o~ divlder ~21 i~ coupled . The output o~ mixer 2 2 5 i~: c!ouplsd to a low pa~ er 231, ~he ou~put o~ whic:h ~ oupl~d ~o a ~urth~r mlxe~ ~35 whic~h i~ dxlven l~y ~he ~1. 4 N}Iz output o~
dl~rldç~ 221. Th~ drature ohannel inolud~s ~ ~Qi,c~r 227, which i~ oupled ~o the qu~dra~ure output o~ 2Z3 and 1 driven by th~ quadratur~ ~ompon~nt of ~he ~1.4 ~Hz ~utpu~
o~ divider 221. ~he output o~ mi.xer 227 1 coupled through a lowpass filter 2~3 to a further qu~dr~ture ~ixe~ 237, which receives the 21.~ ~Hz ou~put o~ divide~ 2~ he outputs o~ mixers 235 ~nd 237 ~re ~ummed ~nd then ~oupled to a ~lvlde-~y-two divlder 241, the outpu~ o~ which is ~he ~atu~l 21.4 M~z xeference ~o ~e used ~or reaovery of ~urst channel data~ Each of low pa~ fil~er~ 231 ~nd 233 has a bandwidth that is ~s narrow as ~ossible, so a~ to recover ~he 10~7 MHz referon~e fox demodulatln~ t~e ~urst si~nal wi~h ~ ~igh a ~lgnal to noise ratlo as possible. I~ ~ixed low pass ~ilters were to be used ~or elements 231 and 23~, ~h~.lr handwid~h would have ~ ~e ~ le~ w~a~ ~he ~otal frequency o~set or dri~t thr~ugh the ~at~ e and down I conve~er. ~y using traakln~ ~iltexs ~or elemen~3 231 and! ~33, the po~ ility e~.ist~ o~ using ~ narrower fi~ter , ~andwidth, provid~d that the tracking ~ilter h~s a control 1 20 me~han~m t~ allow i~ t~ properly ~ra~ ~he inaomin~ bur~t3 s~nal. It i~ ~ox ~his reason that the re~erencQ ~igna~A ~rom I the ~utlink demodulator i~ ~enera~ed ~at 21.~ ~z). This erence s~n~l ~x~otly t~a~k~ any variation of the freq~len~y o~ th~ burs~ nal. The ~andwid~h 4~ ~h~ ~raokln~
i ~5 filter can there~ore be mad~ much narrower th~n the ! ~req~ency o~ or drlf~ ~hr~u~h ~lle satellite and ! downconverter. 1~ band.wi~h ~an then ~e ~reely cho~en toop~i~ize the data recoYery m~chani~m o~ the ~ur~t ! dem~dul~tor, (i~e. th~ b~ndwidth o~ the ~ilt~r m~y b~ mad~
just wide en~ugh so that ~ r tr~n~:len~s ~ tle out durin~
I ~he bur~ pxe~mble time.) The 21~ MHz ou~pu~ of ~r~clcin~I filk~r ~17 is dlvlded ~y ~w~, ~o that ~t may be u~ed in th~
d~ta recover~ pro~eg~.
: ~emodul~tion of ~ha d~ta i~ e~f~ted by coupliny ~he10.7 ~H re~erence ou~pu~ of divider 241 ~ orle input o~ a ~ ~ 1 OJ7 1 mlxer 243 the s~cond input o~ which is qoupled to recci~e ~hc ~ux~t modulation ~l~n~l pro~u~ed a~ ~he output o~ ~ixer 214. The output o~ mixer 243 i~ couplPd to a lo~pass ~ilter ~5 ~rom wh1ch the ~urst da~ recovered.
In opeX~t~on, ea~h return link channel ~U~5 tran~mi~ion from a remo~e ~atio~ tha~ pa~e~ tkrough ~h~
satellite and is th~rehy ~ubjected to it~ as30ciated fre~u~ncy o~`~set or drift is accompanied by an out~in~
. channel frequen~y th~t i~ being ~ontinuously tran~mitted and : 10 monitored ~y ~he ma~e~ ~t~tion'~ recelver e~uipment~, ~peci~ically, con~inu~u3 ~re~uency re~erenc~ r~covsry . se~-~io~ ~01 produce~ ~n ~utput frequency ~t twice the 10.
! MHZ r~erence ~requency, variations in which tas a re3ul~
I of ~he ~requenc~y of~et through the satellite repeat~r~ ar~
i 15 the sa~ç a~ tho~e ~ a ~s~ ~od~ ~lgnal on the return link ! channel who~e IF ~ignal~ are cou~lçd to burG~ frequ~ncy! referenae reaoYer~ ~ection ~ h~ center ~re~uenGy o~
! ba~dp~ ilter ~17 o~ hur~t s~ction i~ thereby controlled! by a 10.7 MHz reEerence ~hat ~X~ck~ ~he 10.7 MUIæ ao~ponent ! ~ 0 ~ ur~ nal, ~o ~ha~ an a~urate re~erence for ! recovering the data ~rom thR inc.oming hur~t IF ~i~n~l may be obtair.ed.
. A5 pointed out above, wi.t.h ~he ~atelll~e ~ unioatlons ! networ~ typica~ly servicing a multiplicity of u~ers, the ! 25 ma~ter ~tatlon will no~m~lly oon~in a plurality of ~r~ eive~ equi~ment~ ~o~ the type diacJrammatically shown ! in ~igure5 1 and 2), and, in a~ ion, ~on~aln one or more ! back-up or redundant unit~ ~o he ~U~ uted or ~witc~ed ! over in place ~f ~ pr~viou~ly on-lin~ uni~ in th~ ev~n~ of a m~l~u~a~ion or ~ilure. Conven~ionally, ~u~k~ ~u~i~n of redundant unit~ has ~ n aaoo~pli~hed hy a ~hard~ar~
in~en~iv~) ontro~l~d int~rconnect arran~en~ containin~
I ~uxll~ry ~wi~ching component~ a~d ~r~nsmi~sion lines hetween dedica~ r~dund~n~ unit~s) and ~h on~ e e~Uip~en~ Howev~r, pur~uant ~o a furth~r f~ur~ o~ th~
1 ;~ 1 037 1 pxesent inYen~lon, by virtue o~ the av~ila}~ility o~ a ~y~em timing ~ignal on t~he ou~link channel ~ar~ie~ an~ ~h~
ad~tls~2!bility ox programma}~ o~ ~re~uency characteri~ the bur~t demo~ulator, ~;ub~k~ ion o~
a new uni~ simp~y r~quire3 an appxoprla~e ~djus~menl; of the ~et~ing~ ~ï khe :Exe~uen~y synthe~iz~rs 203/213 (and a s~nthesizer ~or lF translator 305 which ~nay b~ physic~lly lnskalled withln the mo~m) in the re~und~nt demodulator unit ~ con~igured ~.n the ~anner shown in Figur~ nd ~hen placing th~t unit on line, ~hile d~s~bling the lmit t~ be taken out o~ r~rice, in acc:ordance with ~he s~stem ti~ai~g si~nal 'chat is m~dulated onto ~he ou~link channel oarrier.
More partioul~xly, a~: diagrammatic~ally ~hown in Figure 3, m~te~ station lo may ~on~'in a plurali~y o~ transce~ver Ullit~ TUl, T~2, 0.... ,~Un, e~h o~ wh~ch compris~ ths i continuous demodulator an~ hur~ demodula~or sections I de~ri~ed above with r~f~rence to Fi~ure3 1 an~ 2. I~
! a~dition, a~ redundan~ ~quipment, the master ~tation i ~nGludes one or more auxili~ry hurst demodulators AB~ o~ the ! 2 0 type ~h~wn in ~igure ~, ~oupled in ~andem with ~he bur demodulator in ~aah o~ ~hq ~ransaeiver uni~ TU~ Normally, th~ ~uxili~ry ~urs~ demodulator AB~ unit 1~ of~-llne ar in quiesGent state, ~owaver, in thq qvent o~ the need ~o mak~
~ an e~uipmen~ ~u~titution for an on-linR dem~dula~o~ e 1 2~ master station communicatio~ trol processor, through whiah the opera~ion of t~e ~a~ter station~ ~omm~nic~.~ion e~uipm~n~ ontro~led ~as de~cri~ed ln the ~ove-re~xen~ed copendin~ appli~ation3, ad~usts the ~raquenay I pa~meker~ ~f s~nth~iz~s ~10 ~nd 213 and ~hen pla~s ~h~
1 30 redun~ant demodula~r unit on-lin~ yn~hronl~m with ~ha &~s~em ~iming slgnal. ~ ~he ~ame time i~ ~isa~lss the uni~
ko b~ remov~d ~rom ser~i e~ In ok~er words, ~e~u~e of the av~ y ~ a network time ~lot-de~ nin~ timing siynal ~ ~n~ ~he ahility ko adju~ ~he ~requency ~ontrQl parameter~
1 35 o~ the freguen~y ~ynthe~izer o~ e~oh bur~t demodul~tor at 13~0~
kh~ ma~er ~tion, the ~d ~ compl~x inter~acc ~rnn3ml 3ion line3 and ~witch~ng ~l~cultry for e~eakiny a ; backup r~placem~nt o~ ~he demodulator 1~ avoidhd.
A~ ~111 be appreciated ~rom the ~oregoin~ d~criptlon, the ability ~o ~ ur~ely tune a bur~t dqmodulator to an lntermittPntly tran3mitt~d (burGk) ~ign~l, which is ~ubject to fre~uency off~et in the aou~e o~ its transmi~on over ~telli~e link, i~ achieved in a~rd~n~e with the present invelltion withou~ ~he use o~ conventional m~chanism, such a~ the ins~al~ation o high prsai~lon o~illators ~t th~
~ r~mote ~ites, or the tr~nsmi3siRn o~ ~ ~eparate, dedicated : : pilot ton~ for the p~rpo~ of corr~cting the problem of I fre~uency o~set thro~gh th~ satellite link, Pur~uant to the ! invelltioll, through the u~e Pf a ~ingl~, dedîcat~d hiyh pr~cislon ~ X ~or est~b1i~hing ~oth the ou~1ink cha~nel ! ~rrie~ and the ~e~uxn channQ1 carrier ~nd a novel dual ! continuou~ mode/burs~ mod~ d~mod~a~or con~iguration, it is ~ po~sible to providQ a bur~t re.cov~ry re~erenc~ s1~na~ that i is op~ima11y fi~t~r~d indepe.n~t o~ frequency offset 2~ throu~h the ~ate~lit~.
In addition, ~h~ availa~ility, at t~e ma3t~r s~a~ion, ! 0~ a sy~tem timing signal an~ ~he ahili~y ~o a~ust the ! operationa1 ~re~ue.ncy ~h~ teri~tias o~ ~ho burs~
demodu1ator's ~requency syn~h~izers ~r each o~ a 1 25 ao~tinuou~ ch~nnel s~tion, from which the 'con~ihu~u~' ! lo~al ~illator ~re~u~ncy re~eren~e is obtained, and a I bur~t channe1 recove;~y s~c~ion, from whi~h the burst I demodul~tor's re~e~nce is o~t~ined, ~acilitates I ~ub~kitution o~ a redund~nk ~r bacXup unit in pla~ of another uni~.
'~ ~hile we have ~hown a~d descrlb~d an Rmbodiment in I ~cordan~e w1~h ~h~ pre~ent inven~ion, i~ o ~
und~rs~ood ~hat the ~ame i~ not limi~d thereto l~ut is I suscepti~1e ~o numerou~ chan~es an~ ~odification~ ~5 known 1 35 to a peræon killed ln the art, and w~ ~herefo~a ~o not wish J~ I
tr~ be limlt~Pd to th~ de~hlls ~hown and de~crihed h~rein ~ut irlt~nd ~o c~ve~ all ~1lch ch~ng~s and mc~di~catlons ~ re obvlous t:o one ~ ordinaxy ~3cill in ~he ~r~.
,
~he pre~en~ inven~ion r~l~te~ in g~ne~ atPllit~.
commu~ication ~yate~ ~nd i~ p~rkiqularly dir~ct~d ~o a me~hanl~m f~r provlding an ~c~rate fxeguency réf~rence ~or enabling a burst d~modulator to r~cover in~orm~ti~n ~ign~l~
~rom a c~rrler ~h~t h~s been ~ub~e~ted ~o a ~req~ncy o~et duriny it3 transmi~ion over the sat~llite communic~tion ~ar~ ' The successful operation o~ satellite communication networks, such as dem~nd a~signment ~cont2ntion~ s~s~ems, depends upon the ~bill~ of the re~ei~er equlpmen~ a~ ~he respective sta~ions o~ the ne~work to b~ accurately ~uned t~ ~he incomin~ nal ~rom another ~ite. Xn ~ demand assignment communications scheme, me~sa~es from respective contention participant~ o~ th~ n~twork are tran~mi~ed ~n burst format, in whiah a respective. station's carrier is ~urn~d on ~or an a~reviated period, or tim~ sl~t, during whiclh ~ n~q~ (e~. d~ pa~ce~ tr~nsmi~ed and ~her i 20 turned off until th~t ~t~tion h~s a new me3~ag~ to ~end.: R~q~use o~ the lnkermittent nature of burst mode ~ommunioations and ~arrier dis~or~ion introdu~ing chara~eri~ o~ ~he ~elll~e qhannel, the ab~lity ~o ! provide an acaurate demodul~tion referenc~ ~re~u~nGy ~orsignal recovery ~ecome~ a 3igni~icant proble~ One way ~o i 341v~ the prohl~m i~ p~ovide eaah station with a high precision oscill~tor whi.ch monitor~ an ~feG~lve~y perfectly ~abl~ pilot fre~u~n~y~ ~hat ha~ h~n ~ra~smlt~ed fro~ a m~s~er site~ in order to d~t~r~tin~ ~r~u~ncy o~fs~t thro~g~
khe s~ellite ~ ink and tQ u~e t~is in~ormation to accurat~ly ~ control ~he ch~ractaris~ics o~ its bur~ ~arr~r, ~o ~ha~
~h~ me~s~ge, ~hen re~iY~d ~ the master ~it~, will be e~a~ively pre~orr~c~d, permit~ln~ demodulation a~d data recov~ry. The pr~lem wlth thi~ approach i3 ~wo-~old: on ! 35 the one h~nd, th~ a~t o f the equipment a~ ~ach bur~t-'` ~
J I U~ l 1 ~ouraing ~ite i~ inare~ed ~u~ n~ially by the provi5~ on o~ th~ high preci~ion r~ferenc~ osai~l~tor. In ~d~itlon, bec~u~e ~he ne~work u~e~ a pilo~ tvne *or the purpos~ o~
fre~uenay correction, the capacity o~ thQ satellite link (~n ex~rem~l~ prec~u~ re~our~e) ~ox d~a ~r~n~ml~ion i~
reduc~d.
One propo~al to eliminate p~rt o~ the problem, namely to reduc~ the expens~ oP th~ e~ipm~nt (high p~eci~lon osclllator) at th~ burst~sourcing si~e i~ d~çrihed in U.S.
Pat~n~ No~ 4,509,2~0 t~ ~uginbuhl et al, entitled "~atellite Telecommunic~tion~ System'l. Pursuant to the pAtented scheme, ~ high pre~l~ion pilot tone o~clllator i~
in~talled at the master or central station, thQ only purpo~2 o~ which is to measure ~requency o~et (~rift) through the ~telli~e. By monl~orin~ the pilot tone over a loop bac~
t~ it~el~, ~hQ ma~t~r 3tation i~ able ~o mea~ure ~hè offset throu~h the ~ellite, whiah, as pointed out above, must be ~orrect~d. ~ nal repre~en~ative o~ the value of ~his mea~ured o~set or error i~ ~hen transmi~ed a~ an ~0 ln~c.)rm~lon ~ignal for us~ at ~aah remo~ ~it~. ~he remote si~e, which doe~ t have the henefit o~ the high precision oscillator, ~xtra~t~ the dat~ to properly t~ne it~elf~ Thi~
operation presuppose~ ~hat the remote ~ite is prop~rly tuned to begin with, so~ethlncJ tha~ ~h~ aoar~e ~clllator used at ~5 ~he r~mote ~te ~amlo~ gu~rant~e. ~on~equently, the propos~d procedure i~ ~ues~ionable at hest. 0~ cour3e, due to the ~as~ that th~ patll~nted ~heme de~lcate~ part o~ the ~atelll~e link to the o~et-con~roll~ n~ pilot tone, the re.souxc~ oc~pancy p~obl~m ~till exist~
~
: In ac~ordance with th~ p~e~ent invention, the ability to ac~ura~el~ ~une ~ demodulator ~v an in~erml~nt~y transmitted ~bur~t~ ~ig~l, WhiCh is 3ubj~c~ ~.o ~requqn~y of~t ln the course o~ tr~nsmis~i~n over the ~atellite 3s lin~ chie~ wlthouk ~ither ~ ~he ~bove-ment~oned con~ehtic~ rti~i~e3, lncluding the in~ i on o~ hl~h preciE3ion o~ tor~ ~t the remot~ e~, or the ~ransmi~Z~îon ~ a ~eparate, dedlc~t~3d pilot tone Por t:h~
purpo~a o~ corre ~ n~ the prQ~lem o~ :erequency o~ e'c through the satellite lin3c.
Pux~uan~ to th~ present inventiorl, a 3ingle, d~ a~e~i high pr~cision clo~k i~ used Ic~r the purpo3e of e3tal~1i hing both the outlin~ ca:erieX fr~m the m~ster ~t~tion to e~ch o~
~he remote ~t~ions and tha ret~urn ch~nhe~ ier through which eac:h re~noke ~;t~tion kransmits hurst messayes ~o ~he ma~ter site over the ~atelli~e link~ In addition, ~he mo~ul~tion o~ ~he da~a on ~n~ outlink c:arrier is ~lerived ~rom ~he ~;~me pr~ci~ion clock source. At each remote ~:t~tion, th~3 ou~link channel is mc:nito~ed ~o r~over the hi~h preci~ion c~lo~k~ Thi~ recovered clock i8 than u~ed as : a re~erenc~ for gen~rating ~he re~uxn ahannel carxier.
Ra~he~ than attempt to correct or pr~modify aither the outlink chAnnçl contihuous ~axrler or the return link ~hannel bur~ aarrier, the ~ystem permits both channel6 to ~0 ` ~e 6ubjected to the drift or o~fs~t through the ~tellite.
The ~urst d~modul~r e~uipment at the master station mon~ors both the outlink channel ~ontinuou~ carrier ~ha~
ha~ been ~r~n~mitted throu~h the 6~t~111te and ha~ ~her~by been ~u~jec~ed ~o ~he ~atellite link o~e~) and the inaominy burst mo~e tran~mi~ion~ ~ro~ ~he re~ote station~.
Each o the contlnuous and ~ur~t mode carriers, having besn ~ran~mi~ed through the ~atellite, will undergo the ~ame frequency o~f~et or mo~ifiaa~lon. ~onsequently, th~
~re~uency dif~eren~e between th~ outlink ch~nel ~arrier and the r~turn linX chann~l ~ar~i~r will alway~ ~e r-on~tant.
~he ou~l$nk channel rarrier, which i~; available on a continuou~ b~;i6, $~ u~:ed to deriYa a lo~:al o~:ci~ tor fxe~u~ncy re~erenc~ for ~tabllGhin~ ~he ~urst ~lRmOdUla~Qr~
1OCA1 o~illa~or ~e~e~nae.~ A~ a Gon~e~uen~, reg~rdle~s o~ ~ny~ d~i~t throu~h th~ s~t~ e, the ~ur~t demodulator -- j f-iJ~ 7 i is always referenced to a local oscillator signal that tracks, precisely, any variation in the burst carrier.
S Pursuant to a further feature of the present invention, advantage is taken of the fact that the outlink channel car~ier is modulated with a timing signal that governs the occurrence of contention time slots that are used by the remote stations to send messages over the return link channel to the master station. The availability of this timing signal and the ability to adjust the operational frequency characteristics of the programmable frequency synthesizers for each of a continuous channel section (from which the 'continuous' local oscillator frequency reference is obtained) and a burst channel recovery section (from which the burst demodulator's reference is obtained) of the burst demodulator facilitates substitution of a redundant or backup unit in place of another unit.
In accordance with an embodiment of the invention, for use with a communication system having a first station and a second station with respect to the first station, the first and second stations communicating with one another over a communication link that subjects signals transmitted thereover to a modification of a characteristic thereof, the first station modulating information signals for delivery to the second station onto a first carrier frequency that is transmitted by the first station over the link on a continuous basis, the second station modulating information signals for delivery to the fir~st station onto a second carrier frequency that is transmitted over the link on a burst mode basis, a method of enabling information signals, modulated onto the second carrier ~requency which is transmitted on a burst mode basis from the second station over the link to the first station and thereby subjected to the characteristic modification, to be recovered at the first station is comprised o~ the steps of: at the second ~, ~ :'' ' "``
- 4a ~ 3 7 1 station, generating the second carrier freguency in accordance with a characteristic of the first carrier S frequency recsived thereby; and at the first station, receiving each of the first and second carrisr frequencies that have been transmitted over the communication link; and deriving, from the first and second carrier ~requencies, a filtered burst demodulation reference frequency for demo~ulating information signals that have been modulated onto the second, burst mode carrier frequency by the second station .
In accordance with another embodiment, an apparatus for generating a burst demodulation reference frequency, for enabliny a first station, within a communication system having first and second stations geographically separated from one another and communicating over a communication link that subjects signals transmitted thereover to a modification of a characteristic thereof, the first station modulating information signals for delivery to the second station onto a first carrier frequency that is transmitted by the first station over the link on a continuous basis, the second station modulating information signals for delivery to the first station onto a second carrier frequency that is transmitted over the link on a burst mode basis, to recover information signals that have modulated onto the second burst mode carrier frequency and transmitted from the second station over the link to the first station and thereby subjected to the characteristic modification, is comprised of at the second station, first apparatus for generating the second carrier frequency in accordance with a characteristic of the first carrier frequency received thereby; and at the first station, second apparatus for receiving each of the first and second carrier frequencies that have been transmitted over the communication link; and third apparatus, coupled to the 1 3 1 037 ~
-4b -second apparatus, for deriving, from the first and second carrier frequencies received thereby, a burst S demodulation reference frequency for demodulating information signals that have been modulated onto the second, burst mode carrier frequency by the second station.
In accordance with another embodiment, an apparatus for generating a burst demodulation reference frequency, for enabling a first station, within a communication system having first and second stations geographically separated from one another and communicating over a communication link that subjects signals transmitted thereover to a modification of a characteristic thereof, the first station modulating information signals for delivery to the second station onto a first carrier frequency that is transmitted by the first station over the link on a continuous basis, the second station modulating information signals for delivery to the first station onto a second carrier frequency that is transmitted over the link on a burst mode basis, to recover information signals that have modulated onto the second burst mode carrier frequency and transmitted from the second station over the link to the first station and thereby subjected to the characteristic modification, i9 comprised of at the second station, first apparatus for generating the second carrier frequency in accordance with a characteristic of the first carrier frequency received thereby and, at the first station, second apparatus for receiving each of the first and second carrier frequencies that have been transmitted over the communication link, a plurality of third apparatus, each of which is coupled to the second apparatus, for selectively deriving, from the first and second carrier frequencies received thereby, a burst demodulation reference frequency for demodulating information signals that have been modulated onto the second, burst mode carrier frequency by - 4c - 1 3~
the second station, and fourth apparatus, coupled to each of the third apparatus, for selectively defining prescribed operational characteristics of each of the third apparatus with respect of first and second carrier frequencies and enabling a selected third apparatus to derive the burst demodulation reference frequency in accordance with the selectively defined operational characteristics.
BRIEF DESCRIPT ON OF THE DRAWINGS
Figure 1 is a diagrammatic illustration of a satellite communications network employing the burst demodulator frequency reference derivation mechanism in accordance with the present invention;
Figure 2 is a diagrammatic illustration of the components of the burst frequency reference derivation unit employed in the system shown in Figure 1; and Figure 3 diagrammatically shows a master station having a plurality of transceiver units and a redundant burst demodulator section shown in Figure 2.
DETAILED DESCRIPTION
Before describing, in detail, the particular improved burst demodulator frequency reference derivation mechanism in accordance with the present invention, it should be observed that the invention resides primarily in a novel structural combination of conventional communication circuits and components and not in the particular detailed configurations thereof. Accordingly, the structure, control , . . , - ., ~ 3 ~ 037 ' and arrangement of these convsntional circuits and components have been illustrated in the drawings by readily understandable block diagrams which Rhow only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with structural details that will be readily apparent to those skilled in the art having the benefit of the description herein. Thus, the block diagram illustrations of the figures do not necessarily represent the mechanical structural arrangement of the exemplary system, but are primarily intended to illustrate the major structural components of the system in a convenient functional grouping, whereby the present invention may be more readily understood.
Re~erring now to Figure l, a diagrammatic illustration of a satellite communications network employing the burst demodulator frequency reference derivation mechanism in accordance with the present invention is illustrated as comprising a master or central station lO
which communicates by way of a satellite 20 with a plurality of remote stations, an individual one of which 30 i5 shown in the Figure. In effect, the network may be considered to be what is normally referred to as a star-configured satellite communications network, with the hub of the star corresponding to master station lO and the points of the star corresponding to the locations of the remote stations 30. For purposes of providing an illustrative example, the network will be assumed to be a Ru band system, the outlink channel (master-to-remote) carrier and return link channel (remote-to-master) carrier frequencies of which are on the order of 14 GHz up to the satellite and 12 GHz down from the satellite. Preferably, the communication~ control mechanism is of the type described in copending Canadian patent application 608l744 filed August l8th, 1989, entitled "Link Utiliæation Control Mechanism for Demand Assignment Satellite Communications Network'~ by E. Gerhardt et al to ~ 3 1 037 ~
which the reader is referred. It should be observed that the present invention is applicable to any communication S system employing burst communications that is subject to a frequency offset over the channel.
In the exemplary system, the outlink channel carrier, which is continuously transmitted from master station 10, is modulated with a network timing reference for establishing the occurrence of contention timeslots during which remote stations 30, each o~ which continuously monitors the outlink channel for messages directed to it from master station 10, transmit (burst) messages to the master station over the return link channel. Namely, each of remote stations 30 transmits messages to master station 10 in a demand assignment or contention burst-mode format through the satellite 20 by way of the dedicated remote-to-master return link.
As mentioned briefly above, a burst reference derivation equipment of the present invention obviates the need for installing a high precision oscillator at each of the remote sites and avoids the necessity of having to pre or post correct frequency offset through the satellite 20.
For this purpose, the network employs only a single precision reference oscillator, located at the master site, through which the outlink channel carrier and the return link channel carrier are generated.
More specifically, at the master station, a baseband clock signal (e.g. 5 MHz) from a precision source is applied over link 111 to the clock input of a (BPSK) digital data modulator 101 and to the reference input of an IF
translation tup-converter) unit 103. Modulator 101 contains conventional frequency reference converter circuitry (frequency multiplier/phase locked loop components) for 1 3 1 0~7 1 c:lockirly input clat~ on link 113 ~n aoc30rdarnc~ wi~h a pre~ribed I~AUd :~a~e ~ . g. 11~ for zLpplic~ n tc) X~
up~ conver'cer 10~, whiah i~ al~ con~rolled hy hlgh precieion 5 2~Hz ~lock ~rom link 111. Up converter 103 is o~
aonventional conflgur~tlon, multiplyin~ the hicJh pr~cision 5 MHæ ~lock to conver~ ~he ~ han~ si~n~l ~112 kb/~) to an lntermedi~L~e ~re~u~ncy on th~ order o~ 14 0 ~EIz . Thl~; ~Y
~ign~l is then ~pplied to PcF ~ansce~ver unik 145 WhiC~l tran~ es the IF ~ignal up to Ku b~n~ (14 GHZ) ~or ~pplicakion ~o RF antenna 107 and ~:ransmission over- the outlink ch~nnel ~hrough the satel:Lîte 20 to each ~f the remote s ite~ 3 0 .
E~h ~emote site 30 in~ludes an ~ntenna di3h 301 ~nd a~:soaiated RF transcei~rer unit 303 for receiving outlink ~ 15 channe.l ~e~sages and ~c~r transmlttlng return link buxs~
I mes~ages. Th~ ~e~eive ~utput o~ RF ~ran~ceiver unit 303 i~
I coupled to RF IF down-conve.~ter 305 which outp~t~ an IF
I si~nal (e.~ ~ an IF ~re~uency Q~ ~50-1450~Hz) to mQdsm 307.
Modem 3~7 includes a demo~ul~tor ~ection which ~perates o~
; 20 a 50 MX~ ource ~driving a phase lock loop) to I gene~a~e ~ re~erencs 11~ KH ~ox ~ecovering the incoming I ~ata s~ream. A~ me~lqned previously, r~th~ ~h~t employ an expe~ive preci~i~n o~illa~or to ~ener~k~ ~e modem ' referenae, the remot~ ~ite derive~ its r~eren~e ~rom th~
1 25 highly accu~-a~e aloa~ through which tne data i~ modulated and through which ~.he. outlink IF fre~ueno~ iE produced.
For ~.hlE purpose, mudem 307 con~ain3 con~en~ional ~
I ph~ ocked loop cl~k reaovery ~iroui~ for recovering the kHz c~o~ c~n~in~d wi~hin -~he lncoming dat~ ~tream. In addi~ion to using the r~coY~red clock ~or da~ demodul2tion, I thl~ sam~ r~coVere~ preclsi~n clo~k ~ f~ to ~h~ ~odulator ~ect~on o~ modem 307, wh~r~ it i~ u~ed a~ a r~ren~e to a I ~arrow ~andwi~h ph~e lock ~o~p ~t i~ drlven by an i o~h~rwi6e l~ss pre~l~e local os~illa~or ~e.g. 50 ~z). This ! 35 loaal oscilla~or is u~ed by ~he up-~onver6ion sacti~ of IF
tr~nsl~t~r unit 305 ~4r providiny ~ h~ghly aacJur~3 local o~alll~tor r~f!3r~n~ through whi~h dat~ modulation and ~re~uenay ~r~n~l~ti~n (Iro~ baE;cbancl to e~n IF frequency on the order of ~50-1450 MHæ) ;~e carried out, Thu~, because the mo~ula~ion r~erence ~r~qu~ncy i~ d~rived ~rom ~
pre~i~lon ~ource (locat~ ~t the master station), it i5 unnece~sary to in6~11 a ~p~rate high precision ~ei~erence fregu~ncy os~illator at each remo~e st~ation. The up-c:onverted ~ignal i~ output ~rom IF ~:~age 305 to txansceiver 303 fox tran~missi~n on the re~urn lirlk ~h~nnel (carri~ ~requency - 14GHz)~
It ~:hould b~ noted that t:he ~re~uency o~set through the ~t~l 1 it~ is not a problem ~o~ ~lat:a recovery ~t the remote ~tat~ons since the o~tlinlc ch~nnel carrier ~requent~y is con~inuou~ly tr~n~mitted an~ the high precision re~eren~e (5 MHz) Glock si~nal ~rom which the aarrier i~ der~ed i~
u~ed ~or modulating the data, which is un~f~ected by the ~r~c~ency o~f~t throu~h the ~atellit~. ~he pr~lem to whi~h ~he p~ent inven~ion 1~ di~e~ted, on the other hand, ls the faat that mes~ges fro~ t~e remote stG~tion~ to th~ master ~tation arQ bur3t m~, ra~her ~han ~ontinuoU~ ~ode tr~nsmi~ions, so that a dat~ recovery re~erence ~re~uency tha~ may b~ used ~y th~ ma~er ~atlon t~ ~emodulate reaeived burst traf~la i~ not continuou~ly availa~l~ from ~5 the remote S~A~ on~.
In accordanae with the pr~sent inven~ion, how~ver, advantage is taken o~ ~he ~a~t tha~ the outllnk channel aarxier i~ aontinuou~y Gavailable to ~he ma~er ~ation and u~de~g~ the ~me o~f~et ~hrou~h th~ ~t~llite ~ whiah bur~t mod~ tr~ns~ ion~ on the r~tuxn link channel ar~
: ~ub~eated~ BPcause burst mod~ transmi~ n the return link ch~nnel and c~n~lnuou~ mode tran~mi~ n~ on ~he ou~llnk channel und~r~o ~h~ ~ame of~et through th~
~at~llite, rega~dleG~ of the:~requency di~er~n~ial b~ween th~ outlin~ chann~l ~nd the r~turn link channel, the 1 3 1 ~37 1 di~P~re.nce ~tween thb two i5 ~lway~ cona~ant, ~g~rdl~s~
o* th~ magnitude of ~hc off~et through the ~a~ellike (~-7h~cA
wi.ll vary with ~ime1~ ~n a~cordanc~ with ~h~ prenent ~n~ention, thi~ constan~ dlf~er~ntial characteri~tic i~
~mpl~yed a~ tha mas~er ~i~e to d~rive a re~eren~e recove~y ~requency for ~ bur~ mode demodulator ~nd th~r~y ohvi~te I th~ neéd ~or ~ran~mit~in~ ~ ~epaxate pilot ~one ~or . ~orrecting the off~et through the Gatelli~e.
! ~ore particul~rly~ ~s ~hnwn generally in Figure 1 and lo in detail in ~igu~ ~, the mas~r ~ ion employ~ a burst I mode de~od~la or 121, coupled to a dual RF-IF down-conv~r~er I ~tage llS, wh~ch do~n-conver~ ~he outlink a~d r~turn lin~
I ~ha.nn~ls ~rom the 12 GHz band ~o the ~50 ~o 1450 MHz band.
~I The burst mode demodul~tor monitors the IF (~50-1450 MHz) 1 15 ou~put~ (burst and continuous mode outpu~) of transcei~er i 10~ and provide ~ pair of f~rther down-conver~d (e.g. 52-1 88 ~æ) output~ ~n llnk 123. For purpo~e~ of a~ illu~tratiYa ~ exAmpl~, wlthin tha p~ssband ~ inker~st a5 output, ~he : aontinuou~ mo~e ~ignal m~y have ~n IF re~ue~y of a 77.6 I ~o M~æ, while the bur3t mode ~l~n~l may have an I~ frequen~y o~ 77~4 M~z.
hown in ~'igur~ 2, de.~lodulator ~21 i~lalude~ ~
I ~ntinuou~ frequ~n~y r~er~llae recoværy ~e~ti~n 201 and a I ~us~t ~r~gu~lcy re~erence r~overy ~e~tion 211. Both 1 ~5 s~ctlon~ ~re coupled to link 123, so that each ~eotlon i reaeives the pair of XF frequen~ie~ ~77.4 ana 77,6 MH2) ~rom I ~F down~aonvert~r 115. ~ink 12~ i~ coupled t~ ~ir~t lnputs I of mi~r~ 204 and 214, ~eoond inputs o~ whi~h are I re~p~oti~ely ~.oupl~d ~o the ou~put~ o~ frPquency ~nthe~izer~ 203 arld : 213. Each o~ fr~quen~y ' synthe~lxer~; 203 and 21~ is ad~ustabl~ in lOO}CHZ ~t:eps ~ver a 3fi M~z b~ndwid~h and i~ drl~n by a loc~l clo~lc referenaP
~lpplled o~e~ k 112. Llnk 112 is ~oupled to a ~re~uenc:y I ~e~rence 206 vn ~he order o~ 12~ MH~, in r~pon~e ~o which 1 35 ~ynthesizçr 2a3 produ~e~ a preci~ion ou~put o~ 88.3 ~Xz, ~n~
1 3 1 [~37 ~
~yn~.he~izer 2].3 pro~uca~ ~ pre~i~lon ou~pu~ o* ~ Hz.
~or ~he ~bove d~cussed out~lnk and r~ur~ link ~hann~l ~ep~ra-~ion of ~.00 ~z (77~6 M~Iz - 77.~ ), e~rh o~
mixer~ 204 and 21~ will p~oduce an output of 10.7 ~HZ ln respon~e to the ~.3 ~Hz and 8~ z signals gener~ted by s~nthe3izer~ ~03 an~ ~13, re~peçt.i~ely. The ~ir~, COntillUOU~ reference ~xequena~ o~ 10.7 M~Iz produced a~ ~a output o~ mixer 20~ i~ couple.d through ~ C~t~s loop 20~, which produa~ ~n outpu~ ~re~enay on l~nk 2~7 th~t ef~e.ctivel~ tr~cks ~requency variation6 of tha ~ontinuou~
outlink ~hannel ~arrler~ Cost~ loop 20~ operate~ a~ a multiple of ~he 10.7 MHz produced ~y ~ixer ~04 ~e.y. four time~ the input 10.7 ~ ata at 42.~ M~z~. Output link ~0~
i5 a~uple~ to a ~ivid~-by-four divide~ 221 which controls the operatlon o~ a b~ndpa~ lter 217 within bu~s~ recover~
~ectl~n ~
Within the bur~ Xe~overy sec~i~n ~ll, thb ou~pu~ of mixe~ ~14 i~ connected ~o a ~ multiplier ~15. A~ in conven~ion~l ~odulator de~i~n, ths outp~ he X2 mul~iplier is ~ siyn~l a~ ~wi~e ~he IF ~requency whi~h i~
e~ecti~ely ~trippçd o~ the BPSK modulation on the IF
~ign~l . To ~e employed a~ a r~feren~e ~or the ~emodula~or, it i5 necessary ~ bandpa~ filter thi~ signal, and to then divid~ i~ hy ~Wo~ ha~ to the 10.7 ~H~ ~'r~quen~y where it is ~ hle to provide ~ r~renqe s~nal for d~modulatln~
the burst dat~ 3i~n~1. T~e output of X2 ~ul~iplier ~15 i~
~h~r~fore aoupled tc: th~ pu~ c~f a tracking b~ndpas~ lter 217~ Trackins b~ndp~s~ lter 217 in~ludes a splitter 223 ~u which re~peat.ive iTl-ph~ and quadra~ure (5~ hann~l filter ~ections are co~pl~d, The in-phas~3 fi Iter sec~ion include~2 a mixer ~25 ~o which the ou~put ~f ~plittç~r 2~3 and th~ in-ph~se ~omponent of ~he 21~4 MHz outpu~ o~ divlder ~21 i~ coupled . The output o~ mixer 2 2 5 i~: c!ouplsd to a low pa~ er 231, ~he ou~put o~ whic:h ~ oupl~d ~o a ~urth~r mlxe~ ~35 whic~h i~ dxlven l~y ~he ~1. 4 N}Iz output o~
dl~rldç~ 221. Th~ drature ohannel inolud~s ~ ~Qi,c~r 227, which i~ oupled ~o the qu~dra~ure output o~ 2Z3 and 1 driven by th~ quadratur~ ~ompon~nt of ~he ~1.4 ~Hz ~utpu~
o~ divider 221. ~he output o~ mi.xer 227 1 coupled through a lowpass filter 2~3 to a further qu~dr~ture ~ixe~ 237, which receives the 21.~ ~Hz ou~put o~ divide~ 2~ he outputs o~ mixers 235 ~nd 237 ~re ~ummed ~nd then ~oupled to a ~lvlde-~y-two divlder 241, the outpu~ o~ which is ~he ~atu~l 21.4 M~z xeference ~o ~e used ~or reaovery of ~urst channel data~ Each of low pa~ fil~er~ 231 ~nd 233 has a bandwidth that is ~s narrow as ~ossible, so a~ to recover ~he 10~7 MHz referon~e fox demodulatln~ t~e ~urst si~nal wi~h ~ ~igh a ~lgnal to noise ratlo as possible. I~ ~ixed low pass ~ilters were to be used ~or elements 231 and 23~, ~h~.lr handwid~h would have ~ ~e ~ le~ w~a~ ~he ~otal frequency o~set or dri~t thr~ugh the ~at~ e and down I conve~er. ~y using traakln~ ~iltexs ~or elemen~3 231 and! ~33, the po~ ility e~.ist~ o~ using ~ narrower fi~ter , ~andwidth, provid~d that the tracking ~ilter h~s a control 1 20 me~han~m t~ allow i~ t~ properly ~ra~ ~he inaomin~ bur~t3 s~nal. It i~ ~ox ~his reason that the re~erencQ ~igna~A ~rom I the ~utlink demodulator i~ ~enera~ed ~at 21.~ ~z). This erence s~n~l ~x~otly t~a~k~ any variation of the freq~len~y o~ th~ burs~ nal. The ~andwid~h 4~ ~h~ ~raokln~
i ~5 filter can there~ore be mad~ much narrower th~n the ! ~req~ency o~ or drlf~ ~hr~u~h ~lle satellite and ! downconverter. 1~ band.wi~h ~an then ~e ~reely cho~en toop~i~ize the data recoYery m~chani~m o~ the ~ur~t ! dem~dul~tor, (i~e. th~ b~ndwidth o~ the ~ilt~r m~y b~ mad~
just wide en~ugh so that ~ r tr~n~:len~s ~ tle out durin~
I ~he bur~ pxe~mble time.) The 21~ MHz ou~pu~ of ~r~clcin~I filk~r ~17 is dlvlded ~y ~w~, ~o that ~t may be u~ed in th~
d~ta recover~ pro~eg~.
: ~emodul~tion of ~ha d~ta i~ e~f~ted by coupliny ~he10.7 ~H re~erence ou~pu~ of divider 241 ~ orle input o~ a ~ ~ 1 OJ7 1 mlxer 243 the s~cond input o~ which is qoupled to recci~e ~hc ~ux~t modulation ~l~n~l pro~u~ed a~ ~he output o~ ~ixer 214. The output o~ mixer 243 i~ couplPd to a lo~pass ~ilter ~5 ~rom wh1ch the ~urst da~ recovered.
In opeX~t~on, ea~h return link channel ~U~5 tran~mi~ion from a remo~e ~atio~ tha~ pa~e~ tkrough ~h~
satellite and is th~rehy ~ubjected to it~ as30ciated fre~u~ncy o~`~set or drift is accompanied by an out~in~
. channel frequen~y th~t i~ being ~ontinuously tran~mitted and : 10 monitored ~y ~he ma~e~ ~t~tion'~ recelver e~uipment~, ~peci~ically, con~inu~u3 ~re~uency re~erenc~ r~covsry . se~-~io~ ~01 produce~ ~n ~utput frequency ~t twice the 10.
! MHZ r~erence ~requency, variations in which tas a re3ul~
I of ~he ~requenc~y of~et through the satellite repeat~r~ ar~
i 15 the sa~ç a~ tho~e ~ a ~s~ ~od~ ~lgnal on the return link ! channel who~e IF ~ignal~ are cou~lçd to burG~ frequ~ncy! referenae reaoYer~ ~ection ~ h~ center ~re~uenGy o~
! ba~dp~ ilter ~17 o~ hur~t s~ction i~ thereby controlled! by a 10.7 MHz reEerence ~hat ~X~ck~ ~he 10.7 MUIæ ao~ponent ! ~ 0 ~ ur~ nal, ~o ~ha~ an a~urate re~erence for ! recovering the data ~rom thR inc.oming hur~t IF ~i~n~l may be obtair.ed.
. A5 pointed out above, wi.t.h ~he ~atelll~e ~ unioatlons ! networ~ typica~ly servicing a multiplicity of u~ers, the ! 25 ma~ter ~tatlon will no~m~lly oon~in a plurality of ~r~ eive~ equi~ment~ ~o~ the type diacJrammatically shown ! in ~igure5 1 and 2), and, in a~ ion, ~on~aln one or more ! back-up or redundant unit~ ~o he ~U~ uted or ~witc~ed ! over in place ~f ~ pr~viou~ly on-lin~ uni~ in th~ ev~n~ of a m~l~u~a~ion or ~ilure. Conven~ionally, ~u~k~ ~u~i~n of redundant unit~ has ~ n aaoo~pli~hed hy a ~hard~ar~
in~en~iv~) ontro~l~d int~rconnect arran~en~ containin~
I ~uxll~ry ~wi~ching component~ a~d ~r~nsmi~sion lines hetween dedica~ r~dund~n~ unit~s) and ~h on~ e e~Uip~en~ Howev~r, pur~uant ~o a furth~r f~ur~ o~ th~
1 ;~ 1 037 1 pxesent inYen~lon, by virtue o~ the av~ila}~ility o~ a ~y~em timing ~ignal on t~he ou~link channel ~ar~ie~ an~ ~h~
ad~tls~2!bility ox programma}~ o~ ~re~uency characteri~ the bur~t demo~ulator, ~;ub~k~ ion o~
a new uni~ simp~y r~quire3 an appxoprla~e ~djus~menl; of the ~et~ing~ ~ï khe :Exe~uen~y synthe~iz~rs 203/213 (and a s~nthesizer ~or lF translator 305 which ~nay b~ physic~lly lnskalled withln the mo~m) in the re~und~nt demodulator unit ~ con~igured ~.n the ~anner shown in Figur~ nd ~hen placing th~t unit on line, ~hile d~s~bling the lmit t~ be taken out o~ r~rice, in acc:ordance with ~he s~stem ti~ai~g si~nal 'chat is m~dulated onto ~he ou~link channel oarrier.
More partioul~xly, a~: diagrammatic~ally ~hown in Figure 3, m~te~ station lo may ~on~'in a plurali~y o~ transce~ver Ullit~ TUl, T~2, 0.... ,~Un, e~h o~ wh~ch compris~ ths i continuous demodulator an~ hur~ demodula~or sections I de~ri~ed above with r~f~rence to Fi~ure3 1 an~ 2. I~
! a~dition, a~ redundan~ ~quipment, the master ~tation i ~nGludes one or more auxili~ry hurst demodulators AB~ o~ the ! 2 0 type ~h~wn in ~igure ~, ~oupled in ~andem with ~he bur demodulator in ~aah o~ ~hq ~ransaeiver uni~ TU~ Normally, th~ ~uxili~ry ~urs~ demodulator AB~ unit 1~ of~-llne ar in quiesGent state, ~owaver, in thq qvent o~ the need ~o mak~
~ an e~uipmen~ ~u~titution for an on-linR dem~dula~o~ e 1 2~ master station communicatio~ trol processor, through whiah the opera~ion of t~e ~a~ter station~ ~omm~nic~.~ion e~uipm~n~ ontro~led ~as de~cri~ed ln the ~ove-re~xen~ed copendin~ appli~ation3, ad~usts the ~raquenay I pa~meker~ ~f s~nth~iz~s ~10 ~nd 213 and ~hen pla~s ~h~
1 30 redun~ant demodula~r unit on-lin~ yn~hronl~m with ~ha &~s~em ~iming slgnal. ~ ~he ~ame time i~ ~isa~lss the uni~
ko b~ remov~d ~rom ser~i e~ In ok~er words, ~e~u~e of the av~ y ~ a network time ~lot-de~ nin~ timing siynal ~ ~n~ ~he ahility ko adju~ ~he ~requency ~ontrQl parameter~
1 35 o~ the freguen~y ~ynthe~izer o~ e~oh bur~t demodul~tor at 13~0~
kh~ ma~er ~tion, the ~d ~ compl~x inter~acc ~rnn3ml 3ion line3 and ~witch~ng ~l~cultry for e~eakiny a ; backup r~placem~nt o~ ~he demodulator 1~ avoidhd.
A~ ~111 be appreciated ~rom the ~oregoin~ d~criptlon, the ability ~o ~ ur~ely tune a bur~t dqmodulator to an lntermittPntly tran3mitt~d (burGk) ~ign~l, which is ~ubject to fre~uency off~et in the aou~e o~ its transmi~on over ~telli~e link, i~ achieved in a~rd~n~e with the present invelltion withou~ ~he use o~ conventional m~chanism, such a~ the ins~al~ation o high prsai~lon o~illators ~t th~
~ r~mote ~ites, or the tr~nsmi3siRn o~ ~ ~eparate, dedicated : : pilot ton~ for the p~rpo~ of corr~cting the problem of I fre~uency o~set thro~gh th~ satellite link, Pur~uant to the ! invelltioll, through the u~e Pf a ~ingl~, dedîcat~d hiyh pr~cislon ~ X ~or est~b1i~hing ~oth the ou~1ink cha~nel ! ~rrie~ and the ~e~uxn channQ1 carrier ~nd a novel dual ! continuou~ mode/burs~ mod~ d~mod~a~or con~iguration, it is ~ po~sible to providQ a bur~t re.cov~ry re~erenc~ s1~na~ that i is op~ima11y fi~t~r~d indepe.n~t o~ frequency offset 2~ throu~h the ~ate~lit~.
In addition, ~h~ availa~ility, at t~e ma3t~r s~a~ion, ! 0~ a sy~tem timing signal an~ ~he ahili~y ~o a~ust the ! operationa1 ~re~ue.ncy ~h~ teri~tias o~ ~ho burs~
demodu1ator's ~requency syn~h~izers ~r each o~ a 1 25 ao~tinuou~ ch~nnel s~tion, from which the 'con~ihu~u~' ! lo~al ~illator ~re~u~ncy re~eren~e is obtained, and a I bur~t channe1 recove;~y s~c~ion, from whi~h the burst I demodul~tor's re~e~nce is o~t~ined, ~acilitates I ~ub~kitution o~ a redund~nk ~r bacXup unit in pla~ of another uni~.
'~ ~hile we have ~hown a~d descrlb~d an Rmbodiment in I ~cordan~e w1~h ~h~ pre~ent inven~ion, i~ o ~
und~rs~ood ~hat the ~ame i~ not limi~d thereto l~ut is I suscepti~1e ~o numerou~ chan~es an~ ~odification~ ~5 known 1 35 to a peræon killed ln the art, and w~ ~herefo~a ~o not wish J~ I
tr~ be limlt~Pd to th~ de~hlls ~hown and de~crihed h~rein ~ut irlt~nd ~o c~ve~ all ~1lch ch~ng~s and mc~di~catlons ~ re obvlous t:o one ~ ordinaxy ~3cill in ~he ~r~.
,
Claims (26)
1. For use with a communication system having a first stations and a second station remote with respect to said first station, said first and second stations communicating with one another over a communication link that subjects signals transmitted thereover to a modification of a characteristic thereof, said first station modulating information signals for delivery to said second station onto a first carrier frequency that is transmitted by said first station over said link on a continuous basis, said second station modulating information signals for delivery to said first station onto a second carrier frequency that is transmitted over said link on a burst mode basis, a method of enabling information signals, modulated onto said second carrier frequency which is transmitted on a burst mode basis from said second station over said link to said first station and thereby subjected to said characteristic modification, to be recovered at said first station comprising the steps of:
at said second station, (a) generating said second carrier frequency in accordance with a characteristic of said first carrier frequency received thereby; and at said first station, (b) receiving each of said first and second carrier frequencies that have been transmitted over said communication link; and (c) deriving, from said first and second carrier frequencies, a filtered burst demodulation reference frequency for demodulating information signals that have been modulated onto said second, burst mode carrier frequency by said second station.
at said second station, (a) generating said second carrier frequency in accordance with a characteristic of said first carrier frequency received thereby; and at said first station, (b) receiving each of said first and second carrier frequencies that have been transmitted over said communication link; and (c) deriving, from said first and second carrier frequencies, a filtered burst demodulation reference frequency for demodulating information signals that have been modulated onto said second, burst mode carrier frequency by said second station.
2. A method according to claim 1, wherein step (a) comprises the steps of (a1) generating said first carrier frequency at said first station by means of an effectively precise oscillator signal, and (a2) generating said burst mode second carrier frequency at said second station in dependence upon the effectively precise oscillator signal through which said first carrier frequency received thereby has been generated at said first station.
3. A method according to claim 2, wherein said communication system comprises a satellite communication system and said communication link comprises a signal transmission path through said satellite.
4. A method according to claim 3, wherein said characteristic modification corresponds to an offset of the frequency of signals transmitted through said satellite.
5. A method according to claim 2, wherein step (c) comprises the steps of (c1) down-converting respective continuous mode and burst mode signals transmitted on said first and second carrier frequencies through said satellite link and received by said first station to a first, lower frequency continuous mode signals and a second, lower frequency burst mode signal, (c2) deriving, from said first, lower frequency continuous mode signal, a continuous reference frequency, and (c3) combining said continuous reference frequency with said second, lower frequency burst signal to produce said burst demodulation reference frequency.
6. A method according to claim 5, wherein step (c3) comprises the step of employing said continuous frequency reference signal to define the center frequency of a tracking filter which is used to narrowly bandpass filter the burst frequency reference signal and thereby allowing the bandwidth of the tracking filter to be much less than the bandwidth of the frequency uncertainty of both signals, while allowing the burst frequency reference signal to remain at or near the center frequency of the bandpass filter.
7. A method according to claim 1, further including the step of:
at said first station, (d) demodulating information signals that have been modulated onto said second carrier received thereby in accordance with said derived demodulation reference frequency.
at said first station, (d) demodulating information signals that have been modulated onto said second carrier received thereby in accordance with said derived demodulation reference frequency.
8. A method according to claim 1, wherein step (c) comprises the steps of (c1) down-converting respective continuous mode and burst mode signals transmitted on said first and second carrier frequencies through said satellite link and received by said first station to a first, lower frequency continuous mode signals and a second, lower frequency burst mode signal, (c2) deriving, from said first, lower frequency continuous mode signal, a continuous reference frequency, and (c3) combining said continuous reference frequency with said second, lower frequency burst signal to produce said burst demodulation reference frequency.
9. A method according to claim 8, wherein step (c3) comprises the step of employing said continuous frequency reference signal to define the center frequency of a tracking filter which is used to narrowly bandpass filter the burst frequency reference signal and thereby allowing the bandwidth of the tracking filter to be much less than the bandwidth of the frequency uncertainty of both signals, while allowing the burst frequency reference signals to remain at or near the center frequency of the bandpass filter.
10. An apparatus for generating a burst demodulation reference frequency, for enabling a first station, within a communication system having first and second stations geographically separated from one another and communicating over a communication link that subjects signals transmitted thereover to a modification of a characteristic thereof, said first station modulating information signals for delivery to said second station onto a first carrier frequency that is transmitted by said first station over said link on a continuous basis, said second station modulating information signals for delivery to said first station onto a second carrier frequency that is transmitted over said link on a burst mode basis, to recover information signals that have modulated onto said second burst mode carrier frequency and transmitted from said second station over said link to said first station and thereby subjected to said characteristic modification, comprising:
at said second station, first means for generating said second carrier frequency in accordance with a characteristic of said first carrier frequency received thereby; and at said first station, second means for receiving each of said first and second carrier frequencies that have been transmitted over said communication link; and third means, coupled to said second means, for deriving, from said first and second carrier frequencies received thereby, a burst demodulation reference frequency for demodulating information signals that have been modulated onto said second, burst mode carrier frequency by said second station.
at said second station, first means for generating said second carrier frequency in accordance with a characteristic of said first carrier frequency received thereby; and at said first station, second means for receiving each of said first and second carrier frequencies that have been transmitted over said communication link; and third means, coupled to said second means, for deriving, from said first and second carrier frequencies received thereby, a burst demodulation reference frequency for demodulating information signals that have been modulated onto said second, burst mode carrier frequency by said second station.
11. An apparatus according to claim 10, wherein said first station includes means for generating said first carrier frequency in accordance with an effectively precise oscillator signal, and wherein said first means includes means for generating said burst mode second carrier frequency at said second station in dependence upon the effectively precise oscillator signal through which said first carrier frequency received thereby has been generated at said first station.
12. An apparatus according to claim 11, wherein said communication system comprises a satellite communication system and said communication link comprises a signal transmission path through said satellite.
13. An apparatus according to claim 12, wherein said characteristic modification corresponds to an offset of the frequency of signals transmitted through said satellite.
14. An apparatus according to claim 13, wherein said third means includes down-conversion means for down-converting respective continuous mode and burst mode signals transmitted on said first and second carrier frequencies through said satellite link and received by said first station to a first, lower frequency continuous mode signals and a second, lower frequency burst mode signal, means for deriving, from said first, lower frequency continuous mode signal, a continuous reference frequency, and means for combining said continuous reference frequency with said second, lower frequency burst signal to produce said burst demodulation reference frequency.
15. An apparatus according to claim 14, wherein said combining means comprises a tracking filter coupled to receive said continuous frequency reference signal to define its center frequency, said tracking filter being coupled to narrowly bandpass filter the burst frequency reference signal and thereby allowing the bandwidth of the tracking filter to be much less than the bandwidth of the frequency uncertainty of both signals, while allowing the burst frequency reference signal to remain at or near the center frequency of the bandpass filter.
16. An apparatus according to claim 11, wherein said first station further includes means for demodulating information signals that have been modulated onto said second carrier in accordance with said derived demodulation reference frequency.
17. An apparatus according to claim 11, wherein said third means includes down-conversion means for down-converting respective continuous mode and burst mode signals transmitted on said first and second carrier frequencies through said satellite link and received by said first station to a first, lower frequency continuous mode signals and a second, lower frequency burst mode signal, means for deriving, from said first, lower frequency continuous mode signal, a continuous reference frequency, and means mode combining said continuous reference frequency with said second, lower frequency burst signal to produce said burst demodulation reference frequency.
18. An apparatus according to claim 17, wherein said combining means comprises a tracking filter coupled to receive said continuous frequency reference signal to define its center frequency, said tracking filter being coupled to narrowly bandpass filter the burst frequency reference signal and thereby allowing the bandwidth of the tracking filter to be much less than the bandwidth of the frequency uncertainty of both signals, while allowing the burst frequency reference signal to remain at or near the center frequency of the bandpass filter.
19. An apparatus for generating a burst demodulation reference frequency, for enabling a first station, within a communication system having first and second stations geographically separated from one another and communicating over a communication link that subjects signals transmitted thereover to a modification of a characteristic thereof, said first station modulating information signals for delivery to said second station onto a first carrier frequency that is transmitted by said first station over said link on a continuous basis, said second station modulating information signals for delivery to said first station onto a second carrier frequency that is transmitted over said link on a burst mode basis, to recover information signals that have modulated onto said second burst mode carrier frequency and transmitted from said second station over said link to said first station and thereby subjected to said characteristic modification, comprising:
at said second station, first means for generating said second carrier frequency in accordance with a characteristic of said first carrier frequency received thereby and, at said first station, second means for receiving each of said first and second carrier frequencies that have been transmitted over said communication link, a plurality of third means, each of which is coupled to said second means, for selectively deriving, from said first and second carrier frequencies received thereby, a burst demodulation reference frequency for demodulating information signals that have been modulated onto said second, burst mode carrier frequency by said second station, and fourth means, coupled to each of said third means, for selectively defining prescribed operational characteristics of each of said third means with respect to first and second carrier frequencies and enabling a selected third means to derive said burst demodulation reference frequency in accordance with said selectively defined operational characteristics.
at said second station, first means for generating said second carrier frequency in accordance with a characteristic of said first carrier frequency received thereby and, at said first station, second means for receiving each of said first and second carrier frequencies that have been transmitted over said communication link, a plurality of third means, each of which is coupled to said second means, for selectively deriving, from said first and second carrier frequencies received thereby, a burst demodulation reference frequency for demodulating information signals that have been modulated onto said second, burst mode carrier frequency by said second station, and fourth means, coupled to each of said third means, for selectively defining prescribed operational characteristics of each of said third means with respect to first and second carrier frequencies and enabling a selected third means to derive said burst demodulation reference frequency in accordance with said selectively defined operational characteristics.
20. An apparatus according to claim 19, wherein said first station includes means for generating a network timing signal that is modulated onto said first carrier frequency so as to define the times of occurrence of message transmission intervals for the transmission of messages from said second station to said first station and wherein said fourth means controllably enables said third means in accordance with said network timing signal.
21. An apparatus according to claim 20, wherein said first station further includes means for generating said first carrier in accordance with an effectively precise oscillator signal, and wherein said first means hncludes means for generating said burst mode second carrier frequency at said second station in dependence upon the effectively precise oscillator signal through which said first carrier frequency received thereby has been generated at said first station.
22. An apparatus according to claim 21, wherein said communication system comprises a satellite communication system and said communication link comprises a signal transmission path through said satellite.
23. An apparatus according to claim 22, wherein said characteristic modification corresponds to an offset of the frequency of signals transmitted through said satellite.
24. An apparatus according to claim 23, wherein said third means includes down-conversion means for down-converting respective continuous mode and burst mode signals transmitted on said first and second carrier frequencies through said satellite link and received by said first station to a first, lower frequency continuous mode signals and a second, lower frequency burst mode signal, means for deriving, from said first, lower frequency continuous mode signal, a continuous reference frequency, and means for combining said continuous reference frequency with said second, lower frequency burst signal to produce said burst demodulation reference frequency.
25. An apparatus according to claim 22, wherein said combining means comprises tracking filter coupled to receive said continuous frequency reference signal to define its center frequency, said tracking filter being coupled to narrowly bandpass filter the burst frequency reference signal and thereby allowing the bandwidth of the tracking filter to be much less than the bandwidth of the frequency uncertainty of both signals, while allowing the burst frequency reference signal to remain at or near the center frequency of the bandpass filter.
26. An apparatus according to claim 25, wherein said first station further includes means for demodulating information signals that have been modulated onto said second carrier in accordance with said derived demodulation reference frequency respective continuous mode and burst mode signals transmitted on said first and second carrier frequencies through said satellite link and received by said first station to a first, lower frequency continuous mode signals and a second, lower frequency burst mode signal, means for deriving, from said first, lower frequency continuous mode signal, a continuous reference frequency, and means for combining said continuous reference frequency with said second, lower frequency burst signal to produce said burst demodulation reference frequency.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/234,937 US4932070A (en) | 1988-08-22 | 1988-08-22 | Mechanism for deriving accurate frequency reference for satellite communications burst demodulator |
| US234,937 | 1988-08-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1310371C true CA1310371C (en) | 1992-11-17 |
Family
ID=22883401
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000608746A Expired - Fee Related CA1310371C (en) | 1988-08-22 | 1989-08-18 | Mechanism for deriving accurate frequency reference for satellite communications burst demodulator |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4932070A (en) |
| EP (1) | EP0383908B1 (en) |
| JP (1) | JPH03501677A (en) |
| AT (1) | ATE108964T1 (en) |
| CA (1) | CA1310371C (en) |
| DE (1) | DE68916935T2 (en) |
| WO (1) | WO1990002452A1 (en) |
Families Citing this family (18)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2599613B2 (en) * | 1988-03-24 | 1997-04-09 | 東北電力 株式会社 | Transmission line fault location system using artificial satellites |
| US5357380A (en) * | 1993-06-14 | 1994-10-18 | International Business Machines Corporation | Method and apparatus for indicating when each track is in burst in a multitrack tape device |
| US5697082A (en) * | 1993-10-01 | 1997-12-09 | Greer; Steven Craig | Self-calibrating frequency standard system |
| US5715235A (en) * | 1993-11-26 | 1998-02-03 | Ntt Mobile Communications Network Inc. | Communication system capable of performing FDMA transmission |
| US5454009A (en) * | 1994-01-13 | 1995-09-26 | Scientific-Atlanta, Inc. | Method and apparatus for providing energy dispersal using frequency diversity in a satellite communications system |
| US5506781A (en) * | 1994-06-03 | 1996-04-09 | Itt Corporation | RF link control of satellite clocks |
| US5790601A (en) * | 1995-02-21 | 1998-08-04 | Hughes Electronics | Low cost very small aperture satellite terminal |
| US5774788A (en) * | 1995-03-17 | 1998-06-30 | Hughes Electronics | Remote ground terminal having an outdoor unit with a frequency-multiplier |
| JP2929965B2 (en) * | 1995-03-31 | 1999-08-03 | 日本電気株式会社 | Wireless communication terminal |
| US5794119A (en) * | 1995-11-21 | 1998-08-11 | Stanford Telecommunications, Inc. | Subscriber frequency control system and method in point-to-multipoint RF communication system |
| US5923648A (en) * | 1996-09-30 | 1999-07-13 | Amsc Subsidiary Corporation | Methods of dynamically switching return channel transmissions of time-division multiple-access (TDMA) communication systems between signalling burst transmissions and message transmissions |
| US6041088A (en) * | 1996-10-23 | 2000-03-21 | Sicom, Inc. | Rapid synchronization for communication systems |
| US5844939A (en) * | 1997-02-14 | 1998-12-01 | Hewlett-Packard Company | Low-cost phaselocked local oscillator for millimeter wave transceivers |
| IL130559A (en) * | 1997-10-20 | 2003-06-24 | Comsat Corp | Method for generation of accurate doppler-free local clock in satellite/wireless networks |
| US6618435B1 (en) | 1997-10-20 | 2003-09-09 | Comsat Corporation | Method for measurement and reduction of frequency offsets in distributed satellite/wireless networks and corresponding communications system |
| US6684059B1 (en) * | 1999-08-27 | 2004-01-27 | Tachyon, Inc. | Frequency generation in a wireless communication system |
| US20020098799A1 (en) * | 2001-01-19 | 2002-07-25 | Struhsaker Paul F. | Apparatus and method for operating a subscriber interface in a fixed wireless system |
| WO2009155006A2 (en) * | 2008-05-27 | 2009-12-23 | Viasat, Inc . | Fault tolerant modem redundancy |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2524228A1 (en) * | 1982-03-26 | 1983-09-30 | Thomson Csf | SATELLITE TELECOMMUNICATIONS SYSTEM |
| FR2541533B1 (en) * | 1983-02-21 | 1987-10-30 | Nippon Telegraph & Telephone | OSCILLATOR VARIABLE IN PHASE AND IN FREQUENCY |
| JPS61120538A (en) * | 1984-11-15 | 1986-06-07 | Nec Corp | Multi-direction multiplex communication equipment |
| JPS6248819A (en) * | 1985-08-28 | 1987-03-03 | Kokusai Denshin Denwa Co Ltd <Kdd> | Satellite communication system |
-
1988
- 1988-08-22 US US07/234,937 patent/US4932070A/en not_active Expired - Lifetime
-
1989
- 1989-08-18 CA CA000608746A patent/CA1310371C/en not_active Expired - Fee Related
- 1989-08-22 DE DE68916935T patent/DE68916935T2/en not_active Expired - Fee Related
- 1989-08-22 JP JP1509823A patent/JPH03501677A/en active Pending
- 1989-08-22 AT AT89910573T patent/ATE108964T1/en not_active IP Right Cessation
- 1989-08-22 EP EP89910573A patent/EP0383908B1/en not_active Expired - Lifetime
- 1989-08-22 WO PCT/US1989/003591 patent/WO1990002452A1/en active IP Right Grant
Also Published As
| Publication number | Publication date |
|---|---|
| EP0383908A1 (en) | 1990-08-29 |
| JPH03501677A (en) | 1991-04-11 |
| WO1990002452A1 (en) | 1990-03-08 |
| ATE108964T1 (en) | 1994-08-15 |
| US4932070A (en) | 1990-06-05 |
| DE68916935D1 (en) | 1994-08-25 |
| DE68916935T2 (en) | 1995-01-19 |
| EP0383908B1 (en) | 1994-07-20 |
| EP0383908A4 (en) | 1991-10-30 |
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