CA1332636C - Interactive television and data transmission system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A spread spectrum system provides bidirectional digital communication on a vacant television (TV) channel for simultaneous use by more than 75,000 subscribers using time and frequency division multiplex signals locked to horizontal and vertical sync pulses of an adjacent channel Host TV station. The system, whose operation is analogous to a radar system, comprises: (1) the Host TV station to send down-link sync and data pulses to subscribers during the horizontal blanking interval (HBI), (2) subscriber "transponders" which detect those signals and transmits up-link "echo" data pulses only during the HBI to eliminate interference to TV viewers, and (3) a central receiver which also uses the host TV sync pulses to trigger range gates to detect the up-link data pulses. In a preferred embodiment the central receiver employs directional antennas to determine direction to transponders and to define angular sectors partitioning the service are into pie-like "cells" which permit frequency re-use in non-contiguous sectors (like cellular radio).
The system thus operates like a radar to measure elapsed time between receipt of TV sync pulses and receipt of transponder response pulses and measures bearing to transponders to thereby determine the location of fixed or mobile subscribers as well as provide data links to them. Transponders may share user's existing TV antenna or may operate on cable TV and could be packaged as "RP modems" for personal computers, as transceivers for mobile or portable use, or they may be integrated with a TV
receiver to provide "interactive television".

Description

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1 3 ~2636 Background of . tbe Invent ion Thi~ flivisional of C~nadian Patent Application Ser~al No. 537,175 flled May 14, 1987.

Thi~ lnvention relatea to ~ new sy~tem referred ~o ~
5 ~T-NET~ ~hich provldes bidirectlonal communlc~tlon of digit~l lnformatlon to a plur~lity of fi~ed or moblle subscr~bers on ~
vacant TV channel adjacent to, ~nd cooper~tlng wlth ~n exi~ting ~Bost~ televls$on (~V) 6tatlon. The hor~zontal ~nd vertical ~ync pul6es of the host TV slgnal are URed ~fi ~ wlde-area clock to lo coordinate time and frequency divl~lon multiplexing of subscriber transponders ~nd to trigger up-llnk responses from the~ only durlng the horizont~l blanklng lnterval ~EBI) to prevent lnter-ference to televislon viewers. Down-llnk 8~9nal8 to ~ubscrlbers ~re ~160 6ent ~lthln the ~I. In a prefer~ed embodlment the typical 6u~scriber-to-cent~al receiver d~ta r~te i~ 300 or 1200 baud ~nd that 81gn~1~8 ~pectrum iB spread~ lnto subch~nnels 187.5 ~z wlde by vlrtue of ~odulatlng lt on a 6tream of 5 ffllcrosecond pul~e~. Th~rty-two of these ~ubchannels flt ln a 6tandard C ~z TV channel. More than 300 transponder~ can operate 6~multaneou~1y on e~ch subchannel. The 6ame ~ubchannels ~ay be u~ed f or up-l~nk and down-link commun$cat~ons, even simultaneously on the ~ame ~u~ch~nnel Means to multiplex ~nformatlon to TV recelver~ on ~n exi~tlnq TV ~lsnal durlng ltg horizont~l or ~ertlcal blanking 2s lnter~l are ~n u~e or have been contempl~ted ~e.g. present day ~Teletext~ owever, the advantage of us~ng TV horlzontal and/or vertlc~l ~ync pul~es to synchronlze both down-llnk ~nd up-llnk radlo slgn~ls~ on the ~ame or ~n adjacent TV channel, 80 they effect~vely exi~t only wlthln TV horizontal or vertlc~l blanking lnterv~ls, thu~ are invlsible to televlslon viewers, and 1 for the furtber purpo~e of enabling t~me ~nd frequency d~vision multipleYlng of many ~ignals, has not heretofore been d~scovered.
The present lnvention te~ches that technology.
A ma~or portion of the U.S. r~dio ~pectrum h~s been allocated to broadcast services and more specif$cAlly to television. A ~ubstanti~l number of televi6ion channels ~re unused in most cities because of phy~ical llmitations c~used by inadequate televis$on receiver selectivity~ As a consequence of ~his at lea~t one vacant channel exists between a~signed lo televislon ~tat~ons and tho~e channel6 have heretofore been unu~able. A~ ~ practlcal matter, intermodulation interference and other con~iderstions further llmit the number of usable television channel~ 80 that substantlally less than half the allocated TV channels are in u~e ln ~ given are~. Unus~ble channel8 are ~ometime~ referred to a~ taboo- frequencies. A
principal ob~ect of the instant invent$on i~ to make practical u6e of ~hls presently unusable ~pectrum.
De~cribed another way, typical televis~on receivers, partlcularly when operating at UH~ frequenc$es, have rel~tively poor frequency selectivity consequently rad~o transmis ion in channels ad~acent to ~ TV signal is prohibited because it would cause unacceptable interference. For example, even a low power conventional rAdio device which transmit~ one watt could easily cause un~cceptable interference to adjacent channel tele~ision ~iewer~ who live within ~ r~diu~ of ~everal city block~
surrounding it becau~e its power would overwhelm the TV ~ign~l.
Clearly, thousdnds of such conventional transmitters deployed throughout a city for the uses con~emplated bere ~ould generate unacceptable interference.

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Since television broadca~t channels are by qovernment regulatlon ~llocated to ~m~s media~ use, 1~ 18 lmpllc~t that such channels are not lntended for low capacity private radio communications such as a~60ciated with polnt-to-point or land moblle r~dio appllcations. Consequently applic~tion~ for the ~forementloned vacant TV ch~nnels, lf they could be used at ~11, would be expected to benefit the publ~c en mas~e as contemplated in the $n~tant in~ention for ~uch uses as future ho~e informatlon ~ystems, inter~ctive teleYi~ion, remote shopping, b~nking, lo ~lectronic mail, reservations mean~, security alarm communic~tions, and the like.
Ease of install~tion and ~implicity of operation are ~mportant con~der~tions ~or mass applications. Thus ~haring of the user's e~i~ting television antenna as taught here i~ an important feature. Integrating thi6 invent~on with a televislon receiver to provide interactive TV controllable from remote hand-held devices comparable to tho6e used today for ~emote TV channel switching are other features taught in this ~pecif$cation.
An object of the present invention $8 to provide ~eans to accurdtely partltion ~ub~cribers into geogr~phic ~r~dio cells~
within which speclfic subscriber transponder ~ubchannels m~y be assigned ~nd isol~ted from transponder~ in other cell8. This per~its re-use of ~ubchannel frequencies in non-contiguous cells to ~ignificantly exp~nd the number of users that can operate on one previously vacant TV channel ln ~ qiven clty. These desir~ble frequency re-use features are commonly ldentified today w$t~ ~Cellul~r ~adio~.

1 3 ~2~36 1 The instant lnventlon 18 al80 applicable to two-way cable TV
~ystem~ (CATV~ to provide improved lsolation of up-l~nk ~nd down-llnk slgnals compared to exi~ting methods.
A further ob~ect of the present ~nvention $~ to pro~lde l~proved means for locating and tracklng the poslt$on of moblle or portable Rubscriber tran6ponders to p~ovide economic~l service~ sometlmes referred to a8 ~utomatic ~ehicle location or automatic ~ehicle monitoring (AVM).
~ut~:atic ~h~nd-off~ of pre~ent day cellular radlo telephone lo fiubscribers a~ they move from cell-to-cell ln ~ clty i8 a problem ~ecau~e it ~B based on signal amplitude measurement~ and the e vary widely at dlfferent place~ ~nd at dlfferent times. An lndependent means such as T-NET to locate subscriber~ can form the bas~ for an ~lternat~ve hand-off method which could minimize or ~olve the exi~ting problem ~nd this constitutes another T-NET
application.
Yet ~nother ~pplication of tbe in~ention ~ B for 60-called video conferenclng~ ~hich uEually compri~e~ ~ dedicated TV
network connectlng ~ central office with many remote offices for such ~pplication~ ~s over-the-~ir teachlng, presentatlons by ~nagement, or even rv mon~toring of bank~ or other bu~inesses for security al~rm purpo~es. Such T-NET appl~cations would employ the down-link to send pictures ~video) and the ~p-link could either be d$gital or digitized ~low-voice~, all multipleYed ~imultaneously with the existing TV program.
It i8 clear that simultaneous synchronization of the T-NET
system wlth several TV stations fn ~ city as contemplated by the lnventor could be a problem. Thus a further ob~ect of the ~nvention 18 to teach an operating mean~ where~n the horizontal 1 3~26-~6 l sync pulses of ~e~er~l co-located television tran6mitter~ ~re locked togetber ln time ~o that 8ubscr~ber transponders working $n cooper~tion wlth one or ~ever~l such ~tation6 will ~lways transmlt w~thin the horlzontal blanking ~nterv~l ~H~I) of all the telev~slon gignal~ ~imult~neously, thu~ eliminating lnteference to ~lewers of ~ll of the~. TV transmitter co-location 18 a pr~ctice in ~Any larqe citie~ ~e.g. Loa Angeles and New York) to e~tabllsh a common ~ntenn~ direction for all TV viewers.
Alternatively, it i8 taught that if T~NET sub6cribers are lo located in a boundary ~ervice are~ between television st~tions not co-loc~ted (e.g. between TV tran~mitters ln adjacent c~ tie8), then those sub~criber transponders can be programmed to tr~n~mit only during the ~ertical blankin~ interval ~V~I, which i8 much longer ~n ti~e durat~on th~n th~ ~BI) and thus ~ill not ~nterfere with TV viewer~ of e~ther clty, provided tho~e telev~610n ~t~tion~ ~re synchronized to cause their vertic~l blank$ng interv~ls to overlap as taught $n thi~ invention.
Two new ~nd improved method~ ~re also taught for 6ending digit~l ~nformation to sub criber6 ~down-link) by eithers ~l) co-channel ~odul~tion of the Bost TV Rign~l ~n a non-interferring manner or (2) modulat~ng new ~out-of-channel~ subcarrier sideband6 $n ~djacent upper or lower (or both) TV channels.

Brief Summary of the Invention The lnventlon relate~ to a bidirectional radio commun$cat$on syQtem for use on presently vacant TV channels ln cooperation w~th a host televl~ion transmitter. In one embodiment the host provides down-link d$gital ~ignals to ~ plurality of Bubscr$ber . .. . .

t 332636 transponders using tbe ~mpro~ed methods here~n set fort~.
~ubscriber transponder devices detect these ~ign~ls ~nd transmit carefully synchronized up-link d$gital ~ignals to central receiving sites which are preferably located along the path between subscriberc and the host televi~ion tran~mitter. The lnventor call~ the sy~tem ~-NET~.
A network control center lNCC) interconnects the host televl~lon tran mitter and central receivers with Information Providers using convention~ trunk-line paths ~o ~8 to furnish lo the Information Provider~ with ~eAns to communicate with the$r ~ubscribers, or to provide virtual circuit~ for ~ubscribers to commun~cate with each otherO The Informat~on Provider~ m~y be organizations ~ucb a~ banks, retall stores, vehicle dispatc~ers, Data B~nks, entertalnDent sources such a~ pay TY, and the like.
The hor~zont~l ~nd vertlcal ~ynchronlz~ng pul es normally transmitted by the host televi~ion transmitter are employed ~n the lnvention A~ a clocking ~echani~m to coord$nate tlme ~nd frequency divi~on multiple~ing o~ sub~crlber receiver/tran~mitter device~ ~herein c~lled transponder~
Subscr$ber tr~nspondere are triggered by the host TV signal hori20ntal ~ync ~herein~fter called ~ ync~) pul~es 80 that they transmlt only during the borizontal blanking interv~l (HBI? or ~ertical blanking interv~l (VBI). ~;.e ~BI 1B typically eleven ~icroseconds in duration ~nd viewers living within ~ r~dius of 25 about one ~lle surround~ng ~ tr~n6ponder ~re 6imult~neously blanked out durinq this tlme period. Consequently they would not see the ~r~n~ponder'~ signal, thus will not be lnterfered by lt, provlding it~ tr~nsmisfiion dur~t$on 1B on the order of ~ few microseconds.

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1 33~36 1 ~he T-NET sy~tem 18 ~o~t easily described by eomparlng lt~
operation to ~ radar 6ystem. The Host TV B-~ync pulses ~re ~nalogous ~o the outgolng radar pul6es ~nd these trlgger tr~nsponder reply pul6e~ (~echos~). The reply-echos, each co~prl~ng one bit of lnformation, are recelved at ~ central ~ecel~er ~fter ~ ~rans~t del~y and that delJy le a ~ea~ur~ of the d~stance to t~e ~ubscr~ber. In the ~n~ted States the IV ~-~ync pulse recurrence frequency (called PRF ~n radar) ~B 15,73~ ~z and provides nn unambiguou~ radar range~ of ~bout ~ix ~lles because radio wa~e~ travel at about 10.7 micro~econds (two-way) per mile and the t~me between ~-6ync pul6es 18 63.555 mlcroseconds.

~ n one aspect, the present invention relates to a bldirectional wireless digital communlcation system comprising a television broadcast station for transmitting ordinary ~5 television programming including ver~ical sync and ~orizontal ~ync signalR and aqsociated blanking lntervals on a preass1gned television c~annel and associated video carrier;
broadcA~t ~ean6 ror controllably tran~mitting downllnk d1gltAl data Blgn~ls~
a plurality of flub6crlber recelver-trans~tter~ distr~butea about an ~re~ witbin the broadcast range o~ Baid tele~$sion broadcast ~tation and ~id ~roadcast ~eans, each sub~crlber recei~er-tran6~itter having a subscriber recelvlng means ~or receiving sald video s~gnal ~nd detectlng sync signAle, and for 2s recelving and detectlnq said downlink dlqit~l dAta slgnals, each ~ubscriber receiYer trans~itter having ~odulating me~ns ~or ~odulatlng uplinX dlgit~l data slgnnl~ on An upllnk carrier of freguency wlthln the ~regyency ~and o~ a tQlevislon channcl ~d~acent s~id preas6~gnad televiclon chann-l, each ~ubscriber tran~mitt$ng ~Qan~ coupled to ~aid subscrlbQr r~ceivlng ~eans for .
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1 tr~n~m~tti~g thQ ~odulated upl~nX digl~al d~ta ~4g.nal~ during at least ~o~e of the blanking lnterval6 o~ the recel~sd vldeo ignal; ~nd a plurality of central r~celver~, each central recelvQr S bQing located and h~vlng ~ dlrectional ~ntenna to predominately recelv~ the modulated upllnX dl~ital d~t~ slgnal6 ~rom ~
respectlve subarea w~thin ~ald area, e~ch sald c~ntr~l recelvar being ai ~ean~ ~or r~ceiv~ng ~nd detectlng ~ald uplink d~gltal data c~gnals transm~tted by ths respect$v~ sub6crlb~r transmltt~ng means within th~ respectlvo 6ubare~.

In a further aspect, the preQent invention relates to wireles~ digltal communlcation system comprising a broadcast ~ :
station for transmitting a video slgnal a wireles~ digltal communication sy~tem comprising:
15a broadcast station for transmitting ~ video 3ignal at least lncluding blanking intervals on a video carrier;
broadcast mean~ for controllably transmitting downlink signal~ on a second carrier; and a plurality of subscriber receiver-tran~mitters, each ~ubscriber receiver-transmltter haYing a subscriber recelving means for receiving said video s1gnal and detecting said blanking intervals, and for receiving and detecting said downlink signals, each subscriber receiver-transmitter ~180 having ~ subscriber transmltting means coupled to sald subscriber receiving means for transmitting uplink ~ignals only during at leaQt some of the blanking intervals of the received vldeo 8 ignal; and ~ e least one central receiver, each said central receiver being a mean~ for recelvlng dnd detecting sa1d upllnk s1gnal~
tran~mitted by ehch subscriber transmitting means.

1 3 ~2636 l In ~ st111 further aspect, the pre~ent lnvention relate~
to a cable televislon system comprl~ing:
a broadcast station for tran~mitting a video signal over a cable televi~lon channel; and broadcast means coupled to s~id broadcast stAtlon for transmltting a downllnk dlgltal dat~ slgnal 90 that .~ald broadcaQt statlon broadcast~ through said cable regular television programming and said downlink digital data, said downlink digital data being tran~mitted ~9 video information by adding ~aid downllnk dlgital data to flt least part of the video slgnal durlng one video frame ~nd by subtractlng ~aid downlink digital data from the corre~pondlng part of the video signal during the next video frame.

In a further a~pect the present lnventlon provide~ in an lnteractive televiQion system, a method of communicating information from a plurality of remo~e recelver location~, each connected to a cable television system, to a central location, the cable television ~ystem receiving ordinsry televi~ion programming over the air and providing the same over a cable and havlng amplifiers at various points along the cable, each amplifler serving a plurallty of remote location~, comprislng the steps of:
(i) at each remote location:
a) modulating the informatlon to be communicated from each remote locDtion onto a carrier having a frequency in the frequency band of an unused cable televl~ion channel;
b) tran~mitting the modulated information on the cable;

7b , . . , ~ . , y 1 3~32636 1 (ii) adjacent each cable amplifier:
c) tran~mitting the modulated informatlon recelved on the cable over the air only during ~t least some of the bl~nklng intervals of a first television channel S broadc~sting ordlnary televlsion programming over the Alr and ln the frequency band of ~ televi~ion channel adj~cent ~aid flr~t televislon channel; and (iii) ~t at least one central locatlon:
d) receiving the transmission~ of step (c~ and detecting the inform~tlon therein.

Further aspects of the inventlon reside in providing A
method of communicating information within an area served by a broadca~t ~tation transmitting on ~ broadcast statlon carrier Dnd wlthin a broadcast statlon frequency band comprlslng the ~teps of:
a) modulating the lnformatlon to be communicated by a carrier other than the broadcHst station carrier;
b) transmitting at least one side band of the modulation of step (a);
c) receiving at a remote location a signal containing the carrler of the broadcast station, and the at least one sideband tran3mitted in ~tep (b);
d) demodulating the signal received in step (c) u3ing the carrier of the broadcast ~tation a9 a reference to recover a signal corresponding to the signal tran.cmltted ln ~tep (b); and 9) demodulating the ~ignal recovered in ~tep (d) to recover the information to be communicated.

1 3~12636 1 Further a.Qpect~ of the invention reside in providing an apparatu~ for communicating informatlon within an area served by a commercial broadcaYt ~tation transmittlng over the alr on a broadca~t station carrier and within A broadcast statton frequency band compri 9 i ng:
modulatlon means for modul~ting the lnformation to be communicated by a carrler other than the broadca~t station carrier ~nd for providing an output reqponsive thereto;
tran~mitter means coupled to said modulation means for tran~mitting, from ~ transmitter other than the broadca~t station transmitter, the output of ~flid modulation mean~;
a receiver at a remote loc~tion with respect to ~ald transmitter means for receiving as a receiver signal a signal containing the carrler tran~mitted by the broadcast station and the ~lgn~l tran~mitted by 3aid transmitter means, said receiver means having;
D fir3t demodulator for demodulating said receiver signal u~ing the carrier of the broadca~t station a~ a reference to recover a ~ignal corresponding to the signal tran~mitted by said transmitter meanq, and;
a second demodulator coupled to ~aid fir~t demodul~tor for demodulating the sign~l recovered by the first demodulator to recover the information to be communicated.

7d 1 3~636 1 ~P~ ssYs?E~
A ~t~ndard ~.S. color TV frame consists of 525 horlzontal ~l~nes~ and 29.97 fr~mes ~re tr~n~mitted per ~econd to yleld 15,734 horizont~ nes wlth ~-sync pulses per ~econd. ~hu~
sub~criber tran ponder up-link reply-echos could be tr~qgered by the 5V horlzontal ~ync pulses at a rate up to 15,734 pul~e~ per ~econd~ ~owever, this dat~ rate 18 much f~te~ than typic~l transponders requlre because they are u~ually de~gned for a performan~e compar~ble ~o telephone modems (300 to 1200 bits per 0 8econd). In one embodiment of the lnventlon the tran~ponder~
tran~mlt ~ RF reply pulse to send a logic 1~ or no pul6e (no e~l6~0n~ for log$c ~0~ when lnterrogated by a IV B-~ync pul~e.
Con~equently, lf tran~ponder~ ~re deslgned to tran~mlt ~t 300 baud they wlll ~espond (1.~. provlde ~n echo) on e~ery 52nd TV
B-~ync pul~e (15,734/300 ~ 52).

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1 33~636 8e~eral transponder~ could therefore be ~cheduled to in~tlate tr~nsmission on different TV allnes~, ~hat i8, different B-sync pulses, and on every 52nd horizontal l$ne thereafter (~odulo 52). For example, one tran~ponder could u~e tbe 8-sync s pulse of TV horizontal llnes 1, 53, 105, . . . ~C9. Another tran~ponder at the same location could be progr~m~ed to transmit on horizontal lines 2, 54, 106, 158, . . . ~70, and 80 on. Thus up to 52 different sub~criber~ living ~t tbe same range could effect$vely transmit on the same subchannel ~.:om one range ~cell~
~l.e. one range gate) locatlon, but oper~tlng on 52 dif~erent TV
~-sync l~nes of the 525 horlzontal lines av~llable on each TV
frame. This permits each of them to 6end 10 bits on each TV
frame, wh$ch results in about 300 baud transmission rate each.
This multiplex Method i8 defined here$n a8 a ~coarse~ time lS ~v~sion multlplex proce~s to di~tingui~ $t from the ~fine~ tl~e di~lsion ~actually ~pace divi~$on) mult$plex proce 8 th~t occurs because different subscrlber~ llve at different di~tance~ from the TV transmltter, thus ~t d$fferent tran~it t$me (range gate) interval~.
For example, a system u~ing ~lx range gates, esch f$ve m$cro~econds wide, would provide ~n unamb~guous T~NET servlce area radius o~ about ~ix ~iles. Th$rty-t~o different radio frequency ~ubcharnels could be created $n one vacant 6 MHz wide TV channel. Consequently 9984 d$fferent subscribers (52 8-~ync 25 line~ ~ 6 range cells x 32 subchannels) could slmulataneously operate ~n one ~ngular sector w$thout signal ~cla~hes~ or range ambiqu$ty. A ~imil~r number could operate $n ~ 12, 18, or 24 m~le ~ervice radius us~ng a ~oftware routine to el$minate the -t 3 ~)2636 1 ~radar ran~e ambiqulty~ wh~ch ari~es wben more than one ~-~ync pulse i8 ln tran~it at one time between the TV tran6m~tter and the subscriber~.
~t the central recelving ~ite eacb basic timing proce~s S commences with each TV frame. Thls occurs upon recelpt of a vertical ~ynchron~zlng (V-~ync) pul6e from the televlsion ~tation ~nd the 52S B-sync pulses that follow it ~in the V.S. stand~rd 5 NTSC format). In ~ preferred embodiment the H-sync pulses each trigger the ~t~rt of ~ ~range address generator~ ~t e~-h central lo rece~ver wh$ch generates ~ ser~e3 of delayed receiver range gate~, each having a width of 5 m~crosecond~. This width iB
adjusted to ~atch the w$dth of the pulse ~lgn~l transmitted from e~ch subscriber. Five m$cro~econds i8 also the approximate width of the standard IV B-sync pul~es. The central receiver, which 15 has previously stored in lt~ memory the range to each subscriber transponder, opens up A range gate ~t the expected time of arri~l of each tr~nsponder digital bit pulse to thereby determine if the transponder has sent a logic ~ .e.
transmitted pulse) or a logic ~0~ ~no transmitted pulse).
T-NET system ~erving a 24-mile r~d~us would expect a 257 micro~econd maximum duration between receipt of ~ TV 8-cync pul~e and receipt of a delayed transponder reply pul~e if the TV
transmit'~r and the central receiver are co-located.
A computer at the central receiver collect~ all of the time interleaved ~0~ and 1~ responses from its many 6ubscr$ber~, sorts and groups them into separate packets and appends the appropriate subscriber address. One preferred packet structure which can be employed in the ~nvention is the ~o-called X.25 public packet ~witching protocol whlch is expected to be 1 3~636 1 unlver6ally accepted. The~e pzc~ets ~re then forwarded through conventional communica~ion trunk lines to a centrally located network control center (NCC) wbere the packets are further routed by conventional means to variou~ Information Provlders such as s data banks, electronic mall ~ervlces, f$nancial institutions, and tbe l$ke. ~heir replies are similarly routed back to each ~ubscriber a~ de~cribed below.
pOWN-LINR 5~5Y51~
P~esent aL~. In the instant invent$on, digital lo communications to transponder6 could be ~uperimpo~ed on a televl-sion signal using either conventional techniques known today (e.g. Telete~t) or the enhanced new methods d~closed ~n this specif~cation. In the present art, teletext d$gital sign~ls ~re tran~mitted on up to 8 of the 21 TV horizontal line6 that lie ~ithin th~ vertical blanking interval (VBI) of conventional television si~nal formats. One ~uch method ~8 called the North American Basic Teletext Specif$cation (NABTS) and this 6tandard permits approximately 28~ bit~ of information to be packed in ehch of eight of the twenty-one horizontal lines that lie within the TV vertical blanking interval. Since the VBI repeats at the rate of 60 times a second, tbis results in an ~veraqe down-link traffic capacity of approximately 138,000 bits per second ~288 b$ts x 8 lines x 60 Hz). We refer to these methods as co-channel technigues because they l$e within, and ~hare the Host TV'~ ~f 2s channel.
New and improved means are taught in this 6pecification for sending inform~tion down-link to 6ubscribers at greater 6peeds and more reli~bly. These are subdivided into two classes~

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those t~at opesate on the adjacent upper or lower channel (or both), and (2) co-channel technigues that share the ~ame channel as the IV Bo~t station. The lnventor's lmprovements ~re ~ummarized ~n the following paragraphs.
~J~S~D~ ~h~nn~ YncLiLk. A preferred embodiment of the present ~nventlon packs ~ bits of information on each of up to 32 time-gated subcarr$ers. These ~ubcarriers are tuned to a channel ad~acent to the ~ost TV ctation channel and are gated to e~ist only within itB hor~zontal bl~nking interval (~BI)~ S$nce the o BsI repeats at the rate of 15,734 times per second, thls provldes potentlal capacity of ~pproximately 2 million bits per ~econd ~15,734 ~z x ~ bits x 32 subchannels)5 a substantial impro~ement over exi~ting Teletext. Wbile th~s method require~ use of a vacant adjacent channel above or below (or both) the host IV
~tation, ome of the ~ame subchannel~ used for the T-NET up-link c~n be used for the down-link as well, even on the ~ame subchannel at the same tlme.
Co-channel Down-Link. ~he second improved method to increa~e ~he digital tr~ffic capacity of down-link data streams 8uperimposed on the TV transmission is taught here. It uses the same channel as the ~V signal ~co-channel) ~nd involves the sequential ~dding and subtracting of identical digital data streams to the existing video picture information at corresponding TV picture element6 of ~equential TV frame8. The process i~ as follows: the existlng TV video lnformation of each horizontal line of ~ fir~t frame i8 stored and compared to the video on the corresponding l$nes of the following frame to locate non-mo~ing (~frozen~) portions of each scene. The desired digital data i8 flrst ~dded then subtracted on corresponding 1 33~636 lines of the first ~nd second frame, preferably at only the frozen scene portions, For example, each dig~tal blt ~dded to each l$ne could be a pulse ~bout 185 nanosecond~ ln dur~tlon a8 ln existing Teletest to yelld 288 bit6 per llne. On the following frame the same d$gital data i8 inverted ~nd, in effect subtracted from the frozen v$deo of the previous correspond$ng horizontal lines and agaln transmitted. The result is that at any TY picture spot (pixtel) of the television vlewer's ~creen the digital lnformation which i8 fir~t added then ~ubsequently ~ubtracted, cancels And beco~es invis$ble. Each of the 525 l~nes per frame could c~rry data in thi~ manner. Note that each frame consist~ of 525 lines $n two interleaved fields~ of 261.5 lines e~ch ~n the ~.S. Stand~rd.
The invisibility of data ~ R prlmarily due to the known psychological canceling process of human vision, but ~lso because the phosphors of the telev$~on screen have a 61ight averaging effect th~t ~moothes out TV ~cenes. This cancelling effect can be optimally adjus~ed for data added/subtracted from fixed televised 6cenes as well as to minim$~e Ubeat~ effects caused by 20 $ts presence with the color TV chroma subcarsier. ~owever, for r~
televised scenes that include motion there i6 a slight difference in the video level from one frame to the next in the ~otion part of each scene and therefore s~perimpo~ed digital data, followed by inverted dat~, may not co~pletely c~ncel. Fortunately, lf the data i8 transmitted at the hiqh rate proposed here, one can capitalize on tbe fact that the fre~uency re~ponse of the human eye to thi~ ~high fidelity noise~ i8 masked by the motion in those portions of the moving scene. In other words, the human 1 33~6 ~6 1 eyes' resolutlon deteri~rates ~nd doe6 not 6ee hiqh frequency extr~neous component6 of a ~cene which i8 in motion and one could ~end data w~th motion scenes with some sacrlflce of picture quallty. Alternatively, d~ta tr~nsmi6sion could be lnhibited ln 5 Bcene Begments which contain motion a~ 6uggested ~bove.
The technique of ~dding and subtracting digital data ~ust described can be implemented using known d$gital TV ~cene store and ~orward technique~. Thl6 i8 rather simple on televi6ed black ind white programs. Ihe description of how the new process works o on color televi~ion transmissions i8 somewhat more complex, though essentially simll~r as w$11 now be described.
Color telev~slon basically transmit6 three different slgnals related to the pr~ary color~ ked, green, blue) and thege are generally refered to as the in-phase (~), guadrature (Q) and luminance ~M) co~ponent~. For reasons rel~ted to characteri~tics of human vision lt turn~ out that the frequency bandwidth requirements of the luminance component ~M~ ub~tant$ally greater than the other tws. The ~ nd ~Q~ components are in fact superimposed on ~ chroma subcarrier channel having a useful information bandwidth of only one-third to one-half that employed for the lumin~nce component. Consequently, the instant invention provides for the ~odulation of digital dat~ on the luminance component only and in such a ~anner that the digital data manifest freguency spectra well above the spectra of ~I~ and ~Q~
componentfi, thus invisible to them, ~nd in such a manner that ~uperimpo~ed digit~l data followed by lnverted data superlmposed on corregponding locations of a following frame visually cancel 1 3~26:~6 ~ubstAntially ~s de~cribed before. Thu~ regular IV video and p~ggyback data may be transm~tted ~imult~neou~ly on the 6ame TV
channel.
This process could be accompli5hed at a data rate representing the proper harmonic r~tio of the horlzontal ~ync r~te to optl~ize vlsual c~ncell~tion ln much the 6ame manner u6ed to select the proper TV chrom~ oscillator frequency in present day color TV receiver~O
Thi~ improved method of piqgyback down-link co-ch~nnel transmission 1~ partlcularly attract$ve for ~uch appllcation~ as ~ideo conferenc$ng where a speaker ln a central location may wish to addre~s ~ large number of remotely located offices and $n which he u~e~ a serie6 of chart~ and graphst thus ~ large part of the TV scene comprises low data content fi~ed video consi~tent with the c~pability of thi~ lnvention which i8 610wer than regular TV. Most of the mot~on i~ primarily ln the ~peaker's llp~. This point-to-multipoint video conferenclng mode ~uperimposed on regular TV i8 yet another attr~ctive application of the T-NET ~ystem and has the ~dditional benefit of having a return path so that the listeners can ~talk back~.
8Ds~lar ~ctor~. One preferred embodiment of the lnstant invention would employ directional ~ntennas at the central rece$vers, for example, each having a gain of approximately 20 dB
and beamwidth of about la degrees at ~F. Twenty such antennas would provide a full 360 degree omni-directional coverage lf all were located in one central locatlon. Central receiYers could be locsted near tbe ho~t television tran~mitter or they could be dispersed throughout a city area depending upon the local topography and coverage de~ired. In one preferred embodiment previously described, each ~ubscrlber transponder transmit~ for ~bout flve micro~econds when it 1~ lnterrogated by every 52nd B-~ync pulse. The radio frequency (RF) bandwidth requlred to carry such a slgnal 18 on the order of 187 ~z. Thu~ 32 d~fferent transponder ~subch~nnel~ could be assigned wlthin one typ$cal 6 M~z television channel. For example, sixteen even numbered tran6ponder ~ubchannel~ could be assigned to one directional receiving antenna sector while the ad~acent antenna sector~ could use the 16 odd numbered 6ubchannels. Such a plan o would permit the re-use of the even and odd numbered transponder subchannels many times within ~ city to ~ignificantly $ncrease the overall ~y~tem digital trafflc capacity.
Y~h~ Location. Because thl~ lnventlon operates in a ~anner analogou~ to a radar yctem, where$n the TV horizontal sync pulses are equivalent to the radar's outgolng transmission pulse, and where the transponder pul es triggered by it comprise ~ reply echo, it 1~ clear that distance to each subscriber can be accurately determined. Thi~ i8 u~ed to advantage in two ways:
for f$xed ~ubscribers the central receiver can accurately predict the time at which each subscriber transmission pulse will be received, consequently it can optimumly schedule subscriber responses in ~ space, time and frequency division manner to optimize the system traffic capacity. Alternatively, if the range to each subscriber i6 unknown, a8 or example in portable or vehicle mounted devices, then speclflc frequency subchannels can be dedicated to those mobile tran~ponder applicatlons 60 that one can measure the position of each vehicle and automatically keep track of its location ufiing target acguis$t~on and tr~cking techniques well known ln radar. Thls i8 done simultaneously with data tr~n~mission with the vehicle.
In the lllustratlve ~y6tem prev$ously descrlbed, the ~ngular 5 bearing mea~urement to the unknown vehlcle positlon would be rather crude becau6e of the relatively wlde beamwidth (i.e. 18 degrees). On the other hand, the range to trAn~ponders can be preci6ely mea~ured to the order of ~ hundred feet or BO.
Consequent!~j ~ T-NET system can be opt$mized to provlde much lo better vehicle location accuracy by us$ng two 6eparated central ~eceivers properly programmed ~o each measure~ range to each mobile transponder And thereby more accurately determ~nes vehicle r~t location tabou~ 300 feet accuracy iE ~ntlc$pated1 ~
~a~Y Ap~ tlon- Yet another ~pplication of the inYention 15 lles $n the ~re~ of c~ble television (CATV). The i~olation of signals to and from subscribers in present day CATV sy6te~s has been found to be a problem, partly because of the fact that when many subscriber~ transmitters ~re connected to the television cable they each contribute undesirable no$se. Since this noise 20 i~ additive in present day cont$nuous wave (Cw) techniques, the cumulative noise of all two-way CATV subscribers pose ~ serious problem; ~he instant invention solves th$s problem in essentially the same manner as described above for o~er-the-air ~pplications. The horizontal blanking lnterval of a cable TV
25 program i8 on the order of one mile a8 in the examples discussed before. In the instant lnvention the transponders' up and down-link emission~ e~ist only during the ~BI, hence are invisible to ~ubscribers living within ~ ~$1e of each other. One embod$ment of a cable TV application of thls invention would install T~NET

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1 3 ~26~6 1 ~master- (mult~ple~ed) repe~ters wlthln cArv ~mplifier boxes which typically ~re ~t lntérv41~ on the order of one mile along the TV ~able, and regular transponder~ at each subscrlber'~ home.
T~NET signals ~re collected at the master repeater ~nd relayed s ~over-the-ai~ to a q-NET central rece~ver and proce~sed ~n essentially the s~me manner prev~ously di~cussed.
The various techn~ques ~ust described comprise the essential building block~ from whlch variou6 system architectures may be devlsed to practlce this lnvention. For example, it 1B obviou6 lo each tr~nsponder could reply to all ~-~ync pul~efi to provide a 15,734 baud rate ~nd thereby ~bur6t~ it6 up-l~nk message much faster~ comb~ning thi~ with ~ different time-~haring arrangement between transponders provides yet another mode of operation.
Thu~ the variou~ appl~cations de~cribed herein and others will become evident to the Gkilled communications ~ystem designer upon careful study of the operating det~lls of these building blocks as hereinafter described.

1 3-~2h-~6 Further objects and advantages of the invention will become apparent fro~ the following ~pecification~ taken in connectlon with the ~ccompanying draw$ngs, whereln llke reference character~
~dentify parts of llke functlon~ throughout the different views thereof.
Figure 1 ~8 a block di~gram of the over~ll T~NET system configuration.
Figure 2 i6 a pictori~l of the ~nvention employing directional antennas to ~egment the service area into pie shaped sectors co~ering a c~ty area and illustrating possible locations ~or ~ receiver 6ubstation.
Figure 3 i~ block diagram and pictorial $11ustr~ting the manner in which TV horizontAl sync pul~es trigger subscriber transponder repl~es ~nd ~ pictorial illustrating the ~ppea~ance of the TV sync pulses followed by range del~yed fiubscr$ber replies, as in ~ radar ~A~ ~cope.
F$gure 4 is A plan vlew of one central receiver directional antenna coverage sector segmented into r~nge cells.
Fiqure 5 illustrates the manner in wh$ch the televi~ion horizontal blankinq lnterval (KsI) is superimposed on, and thus masks the ~ubscrlber transponder pulse transmissions.
Fiqure 6 i~ a graph illustr~t$ng the rapid drop-off ~attenuation) ~n the ~trength of the ~ubscriber transponder ~ignal pulses with propagation di~tance.
Fiqure 7 iB ~ top view of one typical communicat$on path between TY ~tation and ~ubscriber transFonder, illustrating the ~rea blacked out during the HBI.

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1 3 ~636 1 Figure 8 i~ anoth~r graph illustrating the typical signal level~ found in the EBI of a standard television waveform.
Figure 9 lllustrates t~e gated subcarrier down-link hd~acent channel embodiment.
~igure 10 illustrate5 two method~ of modulation which can be employ~d ln gated subcarrier down-l~nks.
Figure 11 iB ~n illustrat$on of the invention ~8 ~pplied to simultaneous up-lin~ ~nd down-link operation on the same ~ubchannel.
0 Figure 12 i8 A block d$agram of one embodiment of a typlcal subscriber transponder.
Figure 13 i8 a block diagram illustrating one way in which an antenna duplexer may be constructed to permit sharing of ~n eYisting TV antenna between the transponder and the exi6ting 1S televi~ion receiver.
~ifure 14 is ~ block d~aqram of a radio centr~l office.
Figure 15 is ~ block diagram of one embodiment of tbe central receiver.
F$gure 16 i~ a block diagram of one embodiment of the diqital interface clrcuit6 section of a central receiver.
Figure 17 iB ~ block diagram of the invention as applied to provide two-way cable televis$on.
Figure 18 i6 a bloc~ diagram of the invention as applied to ~utomatic vehicle location, $ncluding ~ digitized 610w voice-up-lin~.
Figure 19 illustrates the co-channel down-llnk digithl video tran~mi~sion technique.
Figure 20 illustrates the application to cellular radio.

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13~ hJ6 Reference now should be made to the draw~ngs ln which the same reference numbers ~re used throughout the vario~ figures to des~g~ate the ~ame or s~mllar component8.
Pigure 1 illustrate~ the ~ajor components of an ent~re ~ystem of the ~nvention for three applications serv~ng a large number of: mob~le tr2nsponders, r~dio modems ~3ed in con~unction with per60nal computers (PC), and two-way interactive television viewers having remote hand-held control means. The system of figure 1 is lntended to provide communications facilities for a plurality of host computers 4 who provide in~ormation to subscribers, or ~o that one or more ~osts, acting as switch centers, ~ay establish what are ~ometimes called virtual circuits tbat enable sub~cribers to communicate wlth each other. The principal device used by Qubcrlbers to eommunicate on the ~ystem of this ~nvent$on comprise receiver-transmitter devices usually referred to hereln a8 ~transponder~ but ometlmes c~lled ~radlo modem~ or ~RF ~odems~ when used wlth personal computers.
Referrlng to figure 1, the network control center 2 employs conventionai ~omputer h~rdware, ~oftware and trunk llnes 26 to recelve, tempo-arily ~tore, route, and forward digital messages between the host computers 4, the broadcast station lnterface unit 8, and radio centrAl offices 6. For example, a subcriber at a fixed location lS sitting at personal computer 20 may co~municate diqital lnformatlon packet~ through radio modem 14 vla radio ~$gnals transmitted through antenna 12 to a centrally located antenna 28 and radio central office C which detects and reformats these ~ess~ges into 5tandard packet8 and forwards the~

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:

1 33~636 1 ~o network control center 2. The network control center reads the destination address portion of these packets and forwards them to the ~ppropriate host computer 4, which is one of a plurality of hosts. If a reply iB required, the host computer 4 generates the reply message and sends it to the network control center 2 where it is reformatted and placed into ~ transmission queue where, at t~e appropriate time, broadcast station interface unit 8 transmits it over SV transmitte~ sthtion 10 where the message i8 radiated over the air ~nd detected by antenna 12, demodulated by radio modem 14 and sent to personal co~puter 20 to complete the messa~e loop.
The transponder device is functionally the same whether it is incorporated within a plurality of mobile subscriber packages 25, radio modems 14, or integrated in interactive televisions 16.
lS Device 25 could be a portable computer terminal or simply a ~Two-Way PagerR which has the added benefit of being able to acknowledge ~beeps~, or even send and receive alpha-numeric messages. If the subscriber transponder i5 integrated within television 16 then ~t may be conveniently operated through a remote hand-held device 18, which could communicate wlth televicion 16 using convent$onal wlreless techniques, such as infrared signallinq, thus providing ~Interactive TVn. The antennas 28 employed with radio central offices 6 may be directional in design so that each receives only from a specified direction and thereby partitions the service area into pie shaped sectors. The communication paths 26 connecting the various components of the common central equipment comprise conventional communication trunk lines ~uch as microwave links, dedicated phone lines, or other suitable means.

Figure 2 i~ ~ pictorial illustration of the lnvent~on u~ing a plurality of directional ~ntennas to co~er ~ city area from radio central offlce 6 ~nd a receiver ~ubstat~on 7, which substation i8 essentlally the ~ame as 6 but dl6placed from lt ~o ~ to extend coverage into areas that may not be acce6sable to radio centr~l office 6 because of mountains or other obstruct~ons. Fiqure 2 ~160 illustrates the definitl~n of a radio cell which, for purpose~ of thi6 specific~tion, i~
considered to comprl6e a qeographic area defined by the beamwidth o of each antenn~ 28 and a range gate interv~l fiuch a8 the distance between R2 and R3. It will be pointed out in subsequent dlscuss$on that the di6tance between R2 and R3 is proportional to the propagat~on di~tance covered during the 6ubscriber transponders pulse-width which lfi on the order of fiYe m~croseconds in a preferred embodiment. Consequently the dlstance between R2 and R3 i8 on the order o ~ mile.
Fi~ure 2 ~180 ~llustrates how network control center 2 may be provided with intercity communication means through use of ~atelllte commnication link 30. Messages may be communicated hetween clt~es by these and other well known methods.
Figure 3 i~ intended to facilltate the explanat$on of essçntial feature~ of the invention. For purposes of ~llustration, oper~tion of the invention i~ con~idered analogous to the oper~tion of ~ radar ~y6tem. A radar system typically 2s transmits brief radio pul~e~ which lmpinge on ~targets~ in its tran~mission path that reflect b2ck pulse energy ~called the echo ln radar) that is detected at ~ receivlng point after an elapsed time t. ~he el~p~ed time t i8 proportional to the pul~e propag~tion distance to and from the reflectlng targets and .... ~ . .

~ 33~636 con~equently distance to target~ ~subscriber6~ may be determlned by measuring t. W~en directional antenn~ ~re employed to either send or receive ~ignal~ ~or both) then the dlrection to the target may ~180 be determined.
Referrin~ to f~gure 3, TV tran~mitter 10 radi~te~ a conventional televi~ion signal ~ncluding horizontal sync pulse~
31 and diqital data whic~ ~re detected by the transponder antenna 12 and ~ent through antenna duple~er 32 to the receiver 34.
Receiver 3~ lock6 on to the TV signal and extract~ from $t the o ~orizontal and vertical synchronizing pulses which are subsequently employed to detect the T-NET dlgital down-link signal~ that are Eynchronized to ~nd accompany the TV signal and al o to coordinate the radio ~ode~'~ reply pul6e tran~mission ti~e slots 80 reply pul~es ex~st only in the ~BI. ~he digital lS information and synchronizing pulse~ are connected to microprocessor 36 where the address portion of the message packet i~ examined to determine if it i8 a ~ignal intended for that ~pecific subscriber. If it 18, it is forwarded to the companion personal co~puter 20. The l~nk between microproce~sor 36 and computer 20 could employ the well known RS-232 fitandard.
Computer 20 diqe6t6 that information and if a reply i8 nece~sary it will generate it ~nd transmit it back to microprocessor`36.
where lt i~ tempor~rily buffer-stored ~nd prepared for transmifision at appropriate t~me slot~ using transmitter 38.
Tran~mitter 38 generates an RF pulse in ~ynchronization with the horizontal sync ~ignal time ~lot ~eceived from microprocessor 36 and tran~mits that pulse through duplexer 32 and antenna 12 back to the central receiver ~ntenna 28. Antenna 28 may be one of a - -1 multiplicity of directional antenna~ to provlde the desired city coverage. Antenna 28 ~8 connected to the radio central off~ce 6 where the up-lin~ lnformation i6 detected, reform~tted, ~nd sent to the network control center 2 lllustrated ln flgure 1. Another antenna 27 nt the radlo central o~flce detects sync ~ignals from televi~lon station 10 and Gonnects them to the radio central office where they are employed to lnitiate the desired tim~ng processe~ based on the TV ~ignal's hor$zontal and vertical synchronizing pul6es.
o Referring ~gain to figure 3, the ~folded A scope~ ~eceiver monitor 6hown ln the center of the illustr~tion i~ lntended to facilitat~ the description of the T~NET ~y~te~ operation ln ~nalogy to a radar ~ystem. ~A~ scopes are commonly uRed in radar to display range to ~rious target~. In these e~amples the tr~ce 15 iB folded~ into many lines. The A~ ficope monitor ~hows a series of hor$zontal llne sweeps (like a TV raster sc~n) each of which ~tarts when ~t 1~ triggered by the horizontal sync pulse transmitted by TV ~tation lO. This 18 called the start pulse in that illustration and a short time later an echo pulfie from a subscriber sppears5 the time duration t between the ~tart pulse and the echo is ~ndicative of the range to that subscriber.
The first line of the folded ~A~ cope is called Bl, the second H2, and ~o on through H52 for a modulo 52 ~y~tem. Since e~ch line i~ triggered by 8-~ync pul~es from TV station lO, each 25 llne has a duration of 63.555 microseconds ln the U.S. TV
~tandard. The length of each llne thus corresponds to a di6tance of ~bout 6 mile~. This folded ~A~ scope monitor may also be vlewed as equivalent to ~ ~V screen rafitor scan which ~weeps out 52 lines then repeats ~t~elf.

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t 332636 r~dar jargon each horizont~l line 1~ called ~ ~An ~can but in this lllustration, h~vlng many horizontal ~weeps, we refer to lt as a folded ~A~ 8cope. The folded A scope monltor ~hown in figure 3 could be employed withln a r~d~o central offlce 6 for the purpose of monitoring the radio ~lgnal actlvlty, or perhapR for technical evaluatlon or trouble shooting and to ~how at a glance t~e ~trength of, and range to, various ~ubscriber~.
In ~ctu~l practlce the detection, storing, and routing of ~ignals i8 all done ~utomatically by computers and such ~ di~play ~ould 0 not be required for those functions.
Fi~ure 4 i8 a top vlew of one ~ngular ~ector of a radio central offlce ~erv$ce area. Antenna 28 provide~ receptlon of signal~ ~n ~n anqular sector approximately 18 degrees in width ~nd that sector i8 further partitioned lnto range cells numbered fro~ 1 through 6 in the flr~t range interval, and ~imilarly in the second ~nd third range interval. Each of these range cells ifi one mile in le~gth and thi~ corresponds to a time durat$on of 5 micro~econds, which dur~tion ~s also the width of each tran6ponder reply pulse.
It was po~nted out earlier in this specification that the unambiguous range of the T~NET 6yste~ i~ proportional to the ti~e duration between TV horizontal sync pulse6 ~nd thi~ turns out to be 63.555 microsecond~, about 6 mile~. This is called the first range interval. The ~econd range interval, also numbered 1 through 6 extends from 6 to 12 ~ile~ and t~e third range interval extends fro~ 12 to 18 miles. Obvlously, the number o~ range intervals required depends on the ~ize of the clty.

t ;~ '2636 1 S~nce each TY signal hor~zontal sync pul~e iB numbered from 1 to 525 st~rting from the first vertical ~ync pul~e which define~ a TV fr2me, lt i8 clear that the central receiver 6 as well a~ each ~ub~crlber transponder, is each capable of S un~mbiguously count~ng and keeping tr~ck of all 525 horizontal ~ync pul~e numbers. Con~eguently lt 18 clear that a softw~re ~lgor~thm c~n be devi6ed to remove any ambiguity t~at might eY$st as to wnether a subscriber lives in range interval 1, 2, 3, etc., and w~ich specific ~-~ync pulse they have been assigned to reply lo on.
It was al~o pointed out e~rlier ~hat ~bout 10,000 ~ubscribers could operate simultaneously ~t 300 baud each within one six-mlle range lnterval. That maximum number of subscr~bers would remain the ~ame even though more rznge interval~ might be lS employed to y~eld a 12, 18, or 24 miles ~ervice area. The number of 6ubscriber~ could be increased, however, by using more angul~r ~ectors or more subchannel~. ~or example, if 18-degree beamwidth antennas are u~ed to cover a 360-degree area, then a total of 20 antennas would result and thiE would 6ervice clo~e to 200,000 ~ubscriber~ simultaneously. ~owever, $n a practical world the subscriber~ are not uniformly distrlbuSed throughout a ~ervice area because of terra~n and service boundaries, therefore less than the optimum number of sub6cribers could be serviced simultaneously in a practical system. Obviously a much greater 2s number could be 5erved on a time-~hared basis because each ~ub~criber typically use the system only momentarily for a few minutes per day.

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1 Figure 5 lllustrateR in a ~eries of mlcro~econd time-step~
the manner ln wbich 6ubscr~ber tran6ponder pul6es are masked ~i.e. rendered invi61ble to TV v~ewers) by the horlzontal blanking interval of the HoBt televislon slgnsl. The telev~ Bion ~tat~on i3 afiBumed to be at the left side o~ figure 5 and lts ~gnal 1~ assumed ~o propagate from left to right. When the BoRt television 6~gnal B-~ync pulse ~mplnges on the sub~cr~ber' 8 televlsion antenna, lt trigger~ the generatlon of a tr~n~ponder reply pul~e ~echo)~ the leading edge of this reply pulse is o illustrated $n figure 5a ~8 a straight vertic~l line beginning at the leading edge of the 11 micro6econd 6quare pul~e labeled ~BI~. All this occurs at initial time t.
~ $gure Sb illustrates tbe almo~t fully developed reply pulse at a time t I ~.S microseconds. The ~ubscriber's reply pulse is lS ~hown in diagonally shaded lines (80 long as 1t i8 under the ~BI
pulse) and it i~ ~een propagating both to the lef~ and to the right, up~tream and downstream, respectively. Since r~dio waves tr~Yel ~t appro~lmately a thou~and feet per microsecond, the subscriber'6 reply pulse would have propagated approximately 0.9 miles with~n ~.5 micro6econds and, if this illustration was seen from the top v$ew, one would see that the 6ubscriber's reply - -pulse would represent a circle 1.8 mile6 ~n diameter centered on the subscriber's antenna. The 6quare pul6e labeled BBI above the ~haded ~ubscriber'~ pul~e $8 the ~o~t TV horizontal bl~nking ~ntervsl and it i8 seèn to propagate to the right at the same spèed a~ the subscriber's pulse and all of the energy of the ~ubscriber'~ pul~e traveling that direction i8 seen to exist within t~e horlzontal blanking lnterval. It will Always do this ln the downstream direction.

~ igure 5c lllustrates the t~me waveforms a~ they e~i6t at t ~ 8 micro~econd6. Slnce the 6ubscriber's pulse 1~ only 5 ~lcroseconds wide it i8 seen thst the subscriber'a transponder has cea~ed tran~mltting and the reply pulse trailing edge has 5 left the subscr$ber's antenna and i8 now propagating ln all directions. If thi~ illustratlon were ~iewed from the top, the reply pulse would appear as a douqhnut with ~n outside diameter of ~bout 3.2 mile~ and ~n lnner diameter (hole) slightly over 1 mile in diameter, all centered on the 6ubscr~ber's antenna. Note o the lmportant point that waves propagating to the rlght (downstream) still ex$st underneath the horizont~l blank~ng interval of the Host TV signal but the waves traveling to the left ~upstre~m toward the ~V ~tation) are no longer masked by the BBI5 ~n other words, TV viewers who live upstream more than about 1.2 miles ~rom the subscriber are not 6imultaneously blanked-out ~nd they could, if the subscriber'~ signal was strong enough, see the ~ubscriber' 8 response pulses. We will po~nt out shortly that the ~ub~cr~ber's pulse is guite weak by the time ~t reaches that distance.
Figure 5d shows the waveforms which exi6t at t ~ 18 microseconds at which point the ~ubscrlber's pulse waveform which propagates down6tream, shown in cros6-hatched 6hading, ~till rema~n~ under the BBI ~nd i8 thus ma6ked, but the pul~e propagating upstream (to the left) ~hown without cross hatching, 18 about 3.4 ~iles upstream and, ~ince it is out of the ~BI, could be seen by TV viewers, lf it were 6trong enough.
Figure 6 shows graphically ~ plot of the signal ~trength of the 6ubscriber tran~mi~6~0n pul~es a~ a function of distance from , 13~`~636 1 the ~ub~criber, It i~ well known $n radio wave propagation theory that in f ree ~p~ce the electric field $nten6$ty of propagatlng radio wave falls llnearly in proportion to the propagation d~gtance. The power ln that wave fall~ as the square s of the dl~tance and i8 plotted in figure 6~ the rad$o wave power drops very rapidly ~n the first few hundred feet ~fter leaving t~e radiating antenna and more slowly thereafter. By the time the radio wa~es reach a dl~tance of approximately 500 feet they have fallen ~n ~agnitude by about ? dB. The point of fiqure 6 18 to lo show that the ~trength of 6ubscriber's transmitting pulses drop 80 rapldly ~n the first few ~undred feet to a level which becomes insignificant in compar$son to the strength of TV r~dio wave~ and thus would not interfere with television ~ignal~.
Consequently televi6ion viewer~ watching programs sent by the ~o~t televlsion station must be protected only ~f they live within ~ few hundred feet of the subscriber'6 transponder antenna because thi~ i8 where the tranRponder s~gnal i~ ~tronq and '-potentially capable of $nterfering with the televi~ion program.
Fortunately, ~ shown ln flgure 5 illustrations, televi~ion vlewers living withln a few hundred feet of the transponder are ~imultaneou~ly blanked out by the horizontal blanking lnterval of the ~08t television ~ignal and consequently even though the ~ubscriber's transponder ~ignal i 6 relativ~ly strong and potentially capable of interfering with adjacent channel' 2s television viewer8, ~ll those viewers' televi~ion recelvers ~re blanked out and cannot #ee any video program at that instant.
~ y the ti~e the subscriber's signal propagates to a distance outside the horizontal blanking interval it i8 about 90 dB weaker (l.e. ~bout l billion ti~es vea~er) and will not interfere with 1 the Host televl6ion's ~ignal. Furthermore, since the ~ub~criber's tran6ponder operates on a channel adjacent to the Bost telev$6$0n slgnal, it 1B cuppressed further by radio frequency f$1ters which are tuned to the ~ost telev$sion slgnal rather than the transponders ~$gnal and thi~ suppression typically amounts to 35 dB or ~ore. In other ~ords, a televi~lon ~eceiver suppre~es adjacent channel ~ignalfi by about 35 dB or more. The combined effect of the signal propagation attenuAtion ~hown in f~gure 6 and the attenuation due to the televi~$on o receiver8 tuned circuits total over 115 dB. Therefore, for all practical purposes the subscriber'~ transponder ~ignal cannot lnterfere with television viewer~ tuned to the Host telev$sion ~$gnal.
In a pract$cal envlronment (not free space~ as plotted $n figu~e 6) the ~ub8criber'~ transponder pulse attenuates even more rapidly and ~o the signbl attenuation 1~ even greater than 115 d~. TV v$ewers tuned to channel~ further away than the f$rst adjacent channel ~uppres~ transponder pulses S0 dB or more because of their tuned circuits and tbey too are uneffected as has been demonstrated ln many field trials.
Figure 7 ~ a top vlew illu~tratlng the ~BI masking geometry and it corresponds to the s$de view shown in figure 5. The horizontal blanking pulse 31 i8 shown propagating outward as a circular wave centered on $V statlon 10. The cros~-hatched area 2s ~hown sround ~ubscriber 15 $g the area which $~ masked by the ~05t TV horizontal blanking interval ~BI) and all television viewers li~lng within that cro~-hatched area wo~ld not be able to see the subscrlber's transponder pulses because the ~creen of their televislon rece~vfr ~ bl~n~ed out by ~he ~sI at that ~oment. On the o~her hand, televiEion viewer~ living to the left of the 6ubscriber ln the area which ~8 not cros~-hatched would not be protected by the ~sI mask$ng but would, on the other hand, 5 be protected by the very low subscriber ~ignal strength which has ~lready been discussed ln relat~on to figure 6.
We have thus shown that transmi sions from ~ubscriber transponders would not cause interference to telev$6ion viewers looking at the ~ost telev$~ion s$gnal Lecause they are e$ther lo blanked out by $ts horizontal blanking interval ~f they live close to the subscr$ber, or they are too weak to interfere with the TV signal if they live ou~$de the horizontal blanking ~nterv~l since they would then be at least a mile away.
There i~ Another potent~al concern however and that i8 the question of whether the ~ubscr$ber's transponder 6ignal may ~omehow interfere ~ith certain televlsion receiver functions wh~ch must be accomplished during the horizontal blanking interval. We ~hall now addres~ that point.
Figure 8 show~ a standard television waveform def~ned by the NTSC ~National Television Standard Committee) during the horizontal blanking ~nterval. Time i5 assumed to start at the left and increase~ to the right. Thus the first feature one ~ees after the ~tart of the ~l i6 the front porch ~ust preceding the horizontal ~ync pulse. That front porch i8 used to define a 2s reference for the so-called black levelt signals weaker than that level l~e within the visible range of the TV screen and sign~l~
stronger th~n that level are black and cannot be seenO Thus the hor~zontal ~ync pulse, which i8 stronger than that reference level, cannot be seen. Since the subscriber'æ transponder ~.

1 commences transmi~ion with the beginning of the horlzontal ~ync pulse it i8 cle~r that ~t cannot ~nterfere w~th the u6e of that re~erence blac~ level because lt exists prior to the beginning of the horizontal sync pul6e. The TY hor~zontal sync pul~e ~tself trigqers the subscrlber's transponder as well as all of the circultR needed by telev~slon receiver~ at the subscriber's bome or in it~ neighborhoodO ~hus ~ ~ubscriber tran~ponder 61gnal occuring at th~s polnt would only appear like ~ regular horizontal sync pulse ~nd it does not intefere with the proper lo horizontal sync of any televisi~n recelvers in its vicinity.
Following the horizontal sync pulse ~figure 8) ~ ~chroma-burst~ waveform 1~ tr~nsmitted on the back porch in the case of ~-coloY TV cignals. That chroma-bur6t represent~ ~pproxi~ately 8 cycles of a chroma subcarrier oscill~tor operating at a frequency of about 3.57 M~z and ~ts purpose i6 to ~ynchronize a crystal-controlled oscillator within each televi~ion receiver which i6 used to demodulate the color signal~. Interference with that process could cause degradation in the color balance of colo~ TV
programs. ~elevision receivers un$ver~ally employ ~ crystal-controlled chrom~ oscillator t~ghtly locked to that chroma-bur~t and this acts as a very sharply tuned filter. The filter 16 in fact 60 narrow in bandwidth that the broadband energy den~ity of the sub6criber'~ transponder pul~e used in this lnvention ha6 minimal effect upon it. In other words, the ~pectral power density twatts per hertz) represented by the subscriber'~
transponder pulfies afi used ~n this invention i8 of 6uch low value th~t the very smal~ ~mount of energy which does e~ist withln the very n~rrow TV chroma bandpass f$1ter of televi6ion receivers is -;: ... . .

t 3;~2636 1 lnsufficient to interfere with it. Numerous e~perlment~
conducted by the inventor have ~hown that the tr~nsmissions contemplated ln this spec$fication have no effect on the color guallty of televi~ion programs, even ~f transponder~ and ~V
receivers share the ~ame antenna.
We have thus shown why tran~mi6sion~ by ~ubscriber transponders designed ~n the manner 6et forth hereln will have no deleter$ous effect on televi~ion viewers ~n the neighborhood of the subscriber or elsewhere, even if they share the ~ame dntenna - ;
o as the 6ub6criber's own television receiver.
One may question, howeYer, how weak the s~gnal muzt be ln order to not interfere with television s$gnal~. Those ~6sues ~nd related specificatlons are determined by the Pederal Communications Commisslon in the ~n~ted States and by slmilar agencies in other countries. At the present time, the FCC has 6tipulated that an adjacent channel 6ignal must be equal to, or weaker than, a television signal ~o as not to interfere with it~
Stated another way, the only protection afforded to that television signal i8 the protection provided by the TV receiver tuned filters which, a~ ~tated before, represents about a 35 dB
or more adjacent channel ~uppression. On the other hand, lf a potentially interfering signal lles w~thin the same channel as the t~_evision ~ignal, then ~t mu6t be weaker than the televi~ion signal by at lea~t 50 dB under existing ~CC rules and only 40 dB
under proposed new rules. ~sing existing FCC rules as ~
criteria, the inventor has found that ~ subscriber transponder may use a pulse power of approximately 2 watts peak and average power of a few milliwatts to meet the FCC criteria and th$s 18 al~o sufficient to provide a useable sign~l to a T-NET rad~o 1 central office at distance e~ceedlng 20 miles. Battery operated transponders appear practical because of the low average power of these sub~criber transponder~.
Thus f~r ~e have descri~ed the operation of the up-l$nk from ~ubscrlber to radio central office. We bave pointed out why ~ubscriber transmi~sions do not interfere with televi6ion viewer~ have also pointed out how the horizontal and vertical synchronizing pulses of the ~ost telev$sion signal coordinate the subscriber transponder trans~i6sions and permlt o many ~ubscriber~ to be multiplexed on different horizontal lines of the TV fr~me. We ~hall now de~cribe the theory and specific ~dvantages of the new and improved down-l$nk from the Host TV
station to ~ubscriber~.
Pigure 9a ~hows the spectrum of the ~ost televis$on signal and the 32 new down-link subcarrier of thi~ invention. In this lllustrat$on, they are ~hown to exi~t in the ~ost TV 6tation~8 ~:
lower adj~cent channel. It i8 well known that television Qignals employ what i~ referred to as upper ~ingle 6ideb~nd (SSB) modulation w~th a ~mall vest$gi~1 lower ~ideband. The ~08t televi~ion ~iqnal carrier frequency Fc i~ ~hown at the left ~ide of the TV 6 ~z signal channel and most of the video energy is ~hown in the upper sideband. That 8ignal energy comprises the video picture lnformation, ~ frequency modulated aud$o carrier at ~.5 MHz above Fc and ~ color subcarr$er at 3.57 MHz. There are also additlonal ~ubs$diary carr$er~ (SCA'~) which could e~i~t wlthin the TV channel for the purpose of providing stereophonic ~ound transm$s~ion and second audio programs but they are not shown in figure gA. The lower edge of the TV channel $~ 1.25 MHz 131~)f.)6 1 below the v~deo carrier Fc. Frequencles lower than 1.25 below Pc are considered to be in the ne~t TV ch~nnel, which 1E referred to ~8 the lower ~djacent chAnnel. As noted before, this has ~lways been ~acant. ~t ~8 ln this lower ad~acent cbannel where the 32 ~ubcarrlers of the lnstant lnvention are posltloned (they could also use upper adjacent channel). The bandwidth of each of these 32 subc~rrier~ i~ approximately 187.5 ~z and each of them i8 wide enough to carry independent up-link pul6e signal from subscriber transponders as well a8 fiepar2te down-link siqnals to o subscribers a8 will now be described.
Figure 9b illustra~es how ~ digital bits can be Modulated w~th$n tbe time interval of a horizont~l blank~ng lnterval which ~6 approxlmately 11 microseconds long (aboùt 2.~ microseconds per bit). Each of the 6ubcarriers ~hown in flgure 9b are gated 80 that they exi~t only during the ~BI and consequently they will not interfere with the video portion of the Bost tele~ision program. Within thi~ EBI lnterval ~ bit6 of informatlon are modulated on each ~ubcarrier. There are ~rious modulation methods which may be employed to accomplish this. One preferred method is a phase modulation technique wherein the phase of the ~ated subcarrier is ~dvanced 90 degrees and brought back to itB
starting phase within l blt interval when ever a logic ~l~ is to be transmitted. If more logic ~1' B~ are to be sent in success$on, the phase direction is reversed after each ~l~ bitt that is the 2s wave i~ advanced 90 degrees and brouqht back,to a starting phase within l bit interval and then retarded 90 degrees and brought back to ~ts starting phase within the 2nd bit interval. Tbis is done in a sine wave fashion 80 as to re~tr~in the 6ignal spectrum fi8 much a~ possible to keep most of itR energy within its ~ ~ .
. . . .

l assigned subch~nnel bandwldth. I~ a logic "o~ 1B to ba sent no phaso ~dvanca or retardation will occur. Thl~ procQs~ 1~ shown in figure 9b which lllustrates a 4-b~t Bequencs ~s lOll. slnce 4 blt~ are tran m~tted durlng ~ach HBI for each 6ubcarrler, and ~lnce the ~BI'~ occur at 15,734 Hz, this results in a down-link data rate o~ ~2,936 bps per ~ubchannel. Each subchannel could csrry lnformation lndependently.
Quadrature Am~lituda ~odulatlon (QAM). An alternative modulatlon ~e~hod 18 disclosed ln figure 9c which ~plits aach ~ubcarrier lnto guadrature components and each o~ thes~
component~ i~ independently nodulat~d to provide ~ moro narrow transmitted 8p2ctrum and thereby minimize~ lnterferenc~ to ad~acent subchannel ~ignals. It will be ~lear to tho6e E~illed ln the art that ~aid quadrature ~thod could employ ~ither binary (on-o~$) modulatlon o~ Qach quadrature term, or each term could take on ~ultlplR ~alues (~.g. guadraturo amplltude ~odulation:
QAM) to define ~ultlple 6ymbols for greater data rates per ass~gned subchannel.
Two dlfferent eguipment arrangements for transmitting the down-lin~ subcarriers will now ~e described. Flgure lOa shows one ~ethoa in which ~ eubcarrier oscillator 40 ~ollowed by a frequency ~ultiplier 42 generate~ th~ desired eubcarrler radio fre~uency which 1~ ~odulated by data ln 44, ampli~led ln 46 and radiated ~y antenna 48. Thi~ i~ one Or thlrty-two subcarrlers tuned to exi~t withi~ the lower TV ad~acent ch~nnel as ~hown ln figure 9a. Thosa ~ubcarrier~ ara algo gated to exist for only 11 ~icroseconds ln the ~BI. Figura lOa ~how~ an assembly o~ thirty-two such 6ubcarriers generator~. The output o~ all thes~
~ubcarrlers can ~e 6ummed together ln 50 ~nd amplified by :

1 amplifier 46 and radiated throu~h antenna 48. Antenna 48 and indeed the entire assembly of figure lOa, could be independent an~ distinct from the Host televlsion transmitter/antenna. This particular method has the ~dvantage that antenna 4B could be a direct~onal antenna. Several subcarrier assembl~es, each identical to figure lOa, and their associated antennas 4R could be provided to generate down-link transmissions into other angular sectors to thereby cover an entire city.
Fiqure lOb and lOc show two methods for transmitting down-lo link digital data. The RF subcarrier assembly of figure lOb, being already at the proper radio frequency, is ~added~ to the regular TV video carrier in 54 and radiates through the TV
transmitter antenna. This is a method to piggyback ~-NET data signals on an existing Host TV transmitter without interference because the T-NET subcarriers exist on an adjacent channel as previously explained. ~he ~ost TV transmitter may have to be retuned somewhat to permit this, however. In figure lOc the subcarriers are generated at baseband frequencies and they in turn mo~ulate the existing TV carrier in SSB modulators 11: the data carriers on the low sideband and video on the upper sideband (with slight vestigial low sideband).
The alternative quadrature modulation method for impressing four bits of data on each subcarrier during each horizontal blanking interval for down-link data transm$ssion wlll now be described. The gated subcarrier o~cillAtor 40 (figure 9c) is split into two quadrature components and each of these components is modulated with two bits of data during each HBI. This is in contrast to the method described above where$n a single subcarrier component ls modulated with four bits of data during :

the ~BI. The ~ethod of u~ing two quadrature ~ubc~rrier terms i8 attractiYe from t~e standpoint of minimizing the required radio 6pectrum bandwidtb. It i8 al60 very attractive bec~use low-co~t large integrated clrcuitff ~called IC's or ~chips~) now exi6t for ~color8 televis~on rece$vers that lncorporate within them all of the c~rcuits nece~sary to demodul~te the chroma subcarrier ~nd these can be adapted to demodulate the data ~ubcarrier lnstead, ~8 well a8 to detect the horizontal and ~ertical sync pulse6 and neces ary control 6ignals (AFT ~ AGC).
lo The manner in which the down-link 6ubcarriers are quadrature modulated at the down-link transm~tter end is relatively str-ight forward. Referrlng to figure 9c, the output of ~ubcarrier oscillator 40 is split lnto two quadr~ture components by 6hifting one sign~l path 9C degrees in ph~se ~hifter 41. The ln-phase and quadrature ~ignal i8 then ~mplitude ~odulated lndependently by U with 2 bits o~ data during each horizontal blanking interval. Of course subcarrier oscillator ~0 itself only ex$st during the horizontal blanking inter~al ~eleven microseconds) as explained before. Thirty-two oscillators identical to figure 9c could be provided for each antenna beam sector 8S previously explained.
It has been pointed out earlier ~n this specification that lt i~ possible to use the same ~ubchannel to transmit up-link ~8 well a8 down-link, even at the same time. The use of fieparate subchannels for elther up-link or down-link trfinsmission i~
fairly obviou~. ~owever, to understand the use of a s~ngle subchannel for both up-link and down-link transmission at the same time requires some explana~ion. It has already been pointed t 332636 l out that the gated ~ubcarrier~ u~ed ~n the down llnk ex~st only for approximately ll microsecond6 coinciding with each 00st TV
horizont~l ~BI. The time between ~ync pul6es 18 63.555 microsecond~ ~nd consequently the down-link subcarrier~ exist for S only 17.3~ of the total tlme (ll/63.55 - .173). Hence the down-lin~ ~ubchannel iE ~ctually Uoff~ and unused 82.73 of the time.
As noted before, the EBI time $nterval conta$ning down-llnk subcarriers propagates away from the television ~tation with the spe2d of light and sweeps across the country~ide to the maximum lo extent of the 6y6tem' 6 service area ~nd beyond as ~hown in figure 11 .
Since these gated subcarrler~ are only on for 17.3~ of the time this leaves U6 with ~pproximately 82.7~ of the time free to listen for reply ~echos~. These echos are in fact digital up-link data as pointed out in the prior di~cussion. In order to ~hare subchannel~ for ~imultaneous up-link and down-link transmission~ Gne must be careful to permit only certain fixed ~ubscriber locations to operate in this manner ~o as not to cau~e the receipt of a reply pulse from proh~bited locations (figure ll) st the 6ame instant a down-link 6ubcarrier transmission i8 occuring. It would be difficult, if not impossible, to detect the weak ~echos~ feom prohibited locations which arrive at the same time ~s one is qenerating a strong down-link transmission.
These prohibited areas ~re shown fn figure ll as cross-hatched annul~r rings. The width of these rings, about one ~ile, represents approximately 17.3% of the total service ranqe and occur every five miles. Subscribers located within these prohibited r~ngs would not be ~ble to use the same subchannel for simultaneous transmi~sion and reception, however, those ~ubscr~ber6 in prohiblted ~reas could u6e ~ dlfferent subchannel for transmi6~ion and reception as i~ customary ln radio transm~slon. Alternat~vely, ~ recelver substation 7 ~figure 2 could be positioned ~down-stream~ 80 ~ to effectively move its prohlbited area~ away from those of radio central office 6 and thereby pro~ide cont~nuous coverage.
Syne~3~ Q~ lQ~. A point of novelty in the instant invention should now be explained. It was pointed out in the discuQ~ion relating to figure 9 that thirty-two ~ubc~rriers are o po6itioned in the lower channel adjacent to the ~08t TV station signal (or alternatively on the upper adjacent channel). These subcarrier~ will in f~ct appear to the T-N~T transponder receiver ~figure 12b) ~8 if they were lower sideband~ of TY carrier Fc, even though they m~y have been independently generated, ~nd even though they ~ay be transmitted from a different locat$on than the TV transmitter. Another way of explaining thi6 is to point out that the ~beat~ freguencies which result when both the subcarriers and the ~V main carrier Fc exist within the bandpass of the transponder receiYer 3~ ~figure 12b) and are processed by detector 88; the result comprises envelop-~odulation, comparable to SSB ~odulation of carrier Fc by the eubcarriers. This envelope i8 demodulated by detector 88 as explained shortly.
S~nce this proce~s of effectively appending ~idebands to ~n exi~ting s~gnal ~i.e. the TV carrier) to explo$t its carrier energy and/or some of the modulation which it ~lready carrie~
~e.g ~ ~ Y 6ync slgnals) appears to be ~ unique concept, it has consequently been l~beled ~synergeti~ ~odulation~. Synergetic modulat$on i~ herein def~ned a~ follows: The creation of psuedo radio ideband~ on ~n existing r~dio ~ignal by means ~ndependent -l of the generator of that 61gnal wherein said me~n~ are located ~t the same or a remote location to thereby enhance tbe reliab~lity of the psuedo sideband tr~nsmi6sions and minim$ze mutual lnterferenc~
9~1im~n~ cesiqn 9~1Qn~. Specific I'NET equlpment and ~y~tem conflguratlons w$11 now be de~cribed in detall. It ~ill become evldent to skllled communlcation workers that ~ny variations of the ba6ic T-NET 6ystem design concept c~n be lmplemented for varlous appllcation6 and consequently the followlng circuits ~nd related lllutrhtion6 represent only one preferred embodiment.
Trans~onder/Tran5mlt~. F~gure 12 ~hows the RF 6ubsectlons of a typlcal radio modem (or transponder) which may be employed ln the $nstant lnventlon. Figure 12A, the trans~itter section, ~8 a relaSively convent~onal radio tr~nsmitter de6ign employlng lS fixed reference cry6tal 06clllator 60 and a ~ubchannel frequency synthesizer comprislng phase detector 62, low-pass filter 64, variable oscillator 66 ~nd programmable divider 745 all of the6e being combined in a clrcuit commonly called a phase-lock loop (PLL). The programmable divider 74 i6 controllable by the mlcroprocessor 36 previou61y 6hown in figure 3. Thu~ the subchannel frequency of the ~ransponder i5 controllable by that mlcroprocessor and ~t ~n turn may be controlled by the remote network control center 2 (figure 1) 60 a6 to asslgn transponder~
to different ~ubchannel frequencie6 dynam~cally at different times to optimize overall sy~tem traffic m~nagement.
The output of varlable o~clll~tor 66 16 amplified and frequency multipl$ed ~n C8 and pul6e modulated ln 70 by c06ine squared modulator 76. Hodulator 76 18 in fact a waveform 1 generator that provides a pulse waveform having a smooth attack ana decay ~hape (e.g. cosine ~quared) and this is done to optimize the spectral content of the transmitted pulses so that most of their radio energy falls within the desired subchannel s bandwidth. Alternatively, the output of 68 can be split into quadrature terms, each term be~ng modulated with one bit per HBI
(equivalent to the dow~-link QAM method previously described).
The pulsed output of modulator 70 is further amplified in 72 to a level of approximately 2 watts peak and connected through lo duplexer 32 to antenna 12 where it is radiated. Transmitted pulses are approximately 5 microseconds wide and the duty cycle is very low; the resulting average power of the transmitter is about l.S milliwatts at 300 baud. This is a very low avèrage power and is therefore attractive for battery-powered operation.
T-ansponder/Receive~. The transponder's receiver subsection is shown in figure 12b. It is intended to employ conventional integrated circuits designed for mass produced Ublack & white~
television receivers and consequently uses relatively inexpensive and reliable piece parts. An ~lternative, using ~color~ TV
circuits, is discussed later. Down-link signals are intercepted by antenna 12 and are connected to TV tuner 80 through duplexer 32. These signals are amplified in 82 and sent through interme~iate frequency (IF) bandpass filter~amplifier (~P~) assembly 84 and connected to TV receiver integrated circuit chip 86. Receiver 86 feeds back a control siqnal 87 to TV tuner 80 to provide automatic frequency tuning (AFT). These are all conventional TV components; for example, 84 could include ceramic ~ 33~636 1 IF filters used ln TV ~ece~vers. Detector 88 demodul~tee the down-link signals and removes the RF carrier to provide ~he TV
~ync and fiubcarrier baseband siqnal~ to both low pass filter 90 and hlgh pa~s fllter 92 connected ln par~llel.
The $ntermediate frequency ~IF) tuned circuit6 of the recelver in figure 12B are tuned 60 as to encompass ~11 thirty-two T-NET subcarriers as well a6 the television carrler Fc.
Slnce the television sisnal include~ lower vestigial ~ideband~
below fc, they are lncluded within the bandwidth of the receivee and are demodulated. Consequently, the output ~ignals from detector a8 include all of the thirty-two subcarriers as well as most of the ~05t TV horizontal ~nd vertical synchronizing pulse energy because that ~ynchronizing pul~e energy exi6t8 in the lower frequency components of the TV ~ignal and pas~es through low pass filter 90. It conseguently appear~ ~t the output of filter 90 ~s the H and V ~ync ~hown in figure 12B.
On the other hand, the data subcarriers exist between 1D25 and 7.25 M~z below the TV carr$er Fc ~nd they are filtered out by high pass filter 92 and ~ent to mixer 94 where a phase-lock loop arrangement provides for the ~election and demodulation of only one of the thirty-two Qubcarrier~. That phase-lock loop operates a8 follows. Frequency synthesizer 98, dynamically controlled by microproce~sor 36 ~figure 3), selects which of the thirty-two ~ubcarrier~ will be demodulated. Frequency synthesizer 98 may be 2s controlled by either companion devices such as a personal computer ~n the case where the receiver iB inside an RF modem, or by the ~y~tem network control center ~NCC) in the ~ame manner as ~t can control the progammable divider 74 of the transmitter ~3 - - ~
1 3 ')~636 1 6ection. In any event, ~re~uency synthesizer 98 control~ voltage controlled oscillator 96 to ~et it at ~ ~pecific freguency precisely equal to the ~ubcarrler frequency which i~ to be demodul~ted~ tbis process occurs in miser 94, low pass f$1ter 108, amplifier 110, fre~uency control varac~or 102, and cry tal 06cillator 100. Tbeir operation i~ identlcal to the operation of ~ co~mon phase-lock loop (PLL) which i8 well known. ~he re6ult 18 that VC0 96 i6 kept preci6ely ln tune witb, ~nd precisely in phase-lock wlth the average phase of the subcar~ier which i8 to lo be demodulated. Phase fluctuations ln the ~elected subcarrier will be smoothed out by low pass filter 108. 80wever~ fast phase fluctuat~ons, wh$ch will represent the des~red phase modulated digital dat~, are pas6ed through low pass filter 10~ ~nd amplifier lC6 and are ~ent to the microprocessor 36 ~shown in lS figure 3). It will be recalled thdt the ~ubcarr~ers are each gated to exist for only 11 micro~econds and coincide with the BB~
of the television s$gnal. Within this ~BI interval four bit of information $s phase-modul~ted $n the manner previously de&cribed in connection with figure 9. Thus the output of the receiver of figure 12B provides the B and V sync pulses of the Bost TV signal as well as the down-link digital data in ~ny one of the thirty-two 6ubcarriers of the down-link 6ubsystem.
The strength of the ~06t TV B-sync pulses coming out of filter 90 i~ lndicative of the radio path attenuation between the 2s Host TV and the subscriber. It i~ therefore a measure of the power required in the return (up-link) path. Based on the princ~pal of reciprocity, power level control 77 (figure 12) 1 3 ~636 provides ~ control s~gnal to modulator 76 which e&tabll~hes the deslrable output power level of the up-llnk transm~tter ~o as not to radiate e~rcess~ve power yet guarantee adequate levels.
ra~ nder/~yadrature g~~lvg~. Applic~tion of ~ TY color s ~chip~ lntegrated clrcuit 81 of figure 12c to ~-NET tranRponder~
w$11 be expl~ined in t~e following discu6sion to lllustr~te a practlcal economical design for detecting the alternative down-lin~ quadrature modulation (figure 9c) but lt should be emphasized that the essent~al feature of intere~t ln thl6 lo discussion has to do with the fact that this low-cost chip can be employed to demodulate both quadrature terms of the ~-NET down-link subcarrier because it ~ppears like the TV quadrature modulhted chroma signal.
A typical TV lntegrated circuit 81 includes ~n IF
preamplifier 83 and IF amplifier 85 and detector 89 that are relatively conventional in design and lnclude provlsion for automatic frequency tuning (AFT) circuit 87 and automatic gain control (AGC) circuit 91. It also lncludes horizontal ~nd vertical ~ync separation and detection circuits 93 and 95. It was pointed out earlier in this specification that in U.S. color television ~ystems the television ~ignal is coded and transmitted as three components: a monochrome luminence component ~M~ and two color component~ and ~Q~ that are uperimposed on ~ chr~a ~ubcarrier having a fi%ed precise frequency of 3.579545 MB2.
~hat chroma subcarrier i8 quadrature modulated with the I and Q
color ~ignals essentially in the same manner which can be employed for the alternative modul~tion of the T-NET gated ~ubcarriers (figure 9c).

1 3326~6 In the case ~f color televi~ion tr~nsmis~ion~, a brief chroma bur~t~ ~ync~ronizing Qi9n8l i8 transmitted by the tele~
vlsion transmltter (see figure 8) on the back porch~ of e~ch horizontal ~ync pul~e and itB purpo~e ~ to phase-lock voltage s controlled o~cillator 96 (fi~ure 12c) with all televi6ion receivers. Therefore phase-locked oscillator 96 can be used a~ a continuous phase reference to de~odulate the I and Q components of the transmitted color TV fi~gnal. In color TV recelver~ t~e chroma burst detector 103 accomplishe~ that ~ynchronizing p.oces~
lo by using a time gate derived from 8-~ync detector 93 to gate out.
the approximately 8 cycles of chroma burst; those 8 cycles are ~pplied to phase-lock loop circuit 101 which controls VC0 96 ~nd thereby keeps lt precisely ~n phase with the 8-cycle chroma burst. A precisely tuned guartz cryst~l at 3.579545 MHz i8 connected to terminal 105 and thi~ causes phase-lock loop 101 and VC0 96 to remain prec$sely ~n pha~e with the chroma burst oscill~tion even after the burst ceases. VC0 96 in effect ~coasts~ during the time interval between chroma bur~ts with neglig~ble drift.
The manner in which the color TV ~ntegrated circuit 81 can be adapted 80 that it can instead detect down-link quadrature modulated subcarrier digital signals of the in~tant invention will now be described. The objective is to use burst d~tector io3~ the I and Q detector 99, phase-lock loop 101 ~nd VC0 96 for this purpoRe. The ~ubcarrier frequency synthesizer 98 previously de~cribed ln regard to figure 12B is now connected ~n place of the chroma oscillato~ quartz crystal at lnput 105. ~t was pointed out earlier that the bandpas~ of the transponder receiver is tuned ~o as to pass only the thirty-two T-NET subcarrier~ and the ~ost TV

1 carrier frequency. ~hus it does not pas~ the 3.57954~ MBz chroma bur~t or the c~roma ~u~carrler. Conseguently the chroma bur~t detector, being gated to operate only during ~pecified portions of the bori20ntal blanking ~nterv~l, w~ll 6ee lnste2d a composite of many ~-NET dat~ subcarrier frequencies, depending upon whlch cubcarriers ~re being employed for down-link data tran~mis~ion.
Since frequency synthe~izer 98 i8 tuned to a specific ~ubcarrier ~nd i~ injecting signal ~nto the phase-lock loop 101, lt and VC0 96 can be forced to lock on to only that pecif~c down-llnk ~ubcarrier frequency. Consequently the reference frequency injected lnto I nnd Q detector 99 by YC0 9C cause~ detector 99 to demodul~te the in-phase and quadrature phase (I ~ Q1 digltal data components of that specific subcarrier only.
Consequently the read$1y available and inexpenslve ~color~
lS television lntegrated circuit 81 can be used to demodulate any one of the many quadrature modulated data ~ubcarrier~ used in the ln~t~nt invent~on. The output of detector 99 i8 connected to ~ntegr~te and dump circult 109 where a synchronizing ~lgnal based on the horizontal sync ~lgnal from 93 i5 used to accurately gate-out and optimumly detect the T-NET digital data of the ~ubcarrier which has been selected. ~he very powerful ~08t TV carrier component and ~ync cignal6 are consequently u~ed to effectively ~carry~ and thereby enhance ~ET transmis~ion reliabil~tyt i.e.
the 6ynergetic modulation advantage.
The transponder microproces~or 36 shown in figure 3 ~erYe~
the purpo6e of coordinating the timing of the transponder'~ rAdio ~ections as well as buffer ~tor$ng ~nd relsying mes6ages between it and the companion device (e.g. personal computer). It al80 4~

1 7)3~636 perform~ certain housekeeping funct~ons ~ucb ~ recognizing which incoming message~ it is to detect and pa6s on. It al~o helps the network cont~ol center coordinate overall T~NET ~ystem traffic by dynamically shiftinq to subch~nnel frequencies a~signed to lt to transm~ and receive on, either ~8 directed by the network control center or as a~signed by the companion device. The microrpocessor 18 conventional in lt6 design and it~ programming i8 relat~vely ~traight forward.
~ E~ Lr.PI~ he tr~nsponder duplexer 32 (figure 12) permlts ~haring of the subscriber's existing ~V antenna with the existing television receiver, the transponder'~ receiver section, ~nd its transmitter ~ection. It~ principal ~ob iE to igolate the transponder receiver and television rece~ver from the transmitter ~ection 80 that they wi~l not be damaged while it transmits. Figure 13 shows one possible duplexer design for lsolating these receivers from the transmitter. The subscriber's TV antenna i8 connected to the transponder receiver and television receiver through a one-guarter wave length coaxi~l cable 11~ which has ~t its output end ~ diode switch 116 that i8 controlled through a radio frequency choke 118 by microprocessor 36. When the transponder is required to transmit a data pulse, diode 116 i8 ~witched to a low impedance state by microprocessor 36 and this in effect ~hort circuits the output end of coaxial cable 11~ ~nd causes a reflected open circult impedance at ~t6 input end (the left side in fiqure 13). Consequently the RF
pulses generated by transmitter 38 ~ee an open circuit at the input to coax 11~ and the 6~gnals are consequently routed on to the subscriber ~V antenna and sadiated outward.

~8 -' 1 3 3~636 On the other ~and, w~en transmitter 38 i8 not transmitting, which is mo~t of tbe time, ~ignals coming into ~ub~cr~ber TV
~ntenna 12 pass through coax 114 and lnto slgnal ~plitter 120 where they are routed both to the exi6ting televi6ion rece~ver ~o th~t it ~ay receiYe convent~onal television programs ~nd ~l~o to recei~er 34, which ~ part of the transponder. ~nder these receiving conditions transmitter 38 represent~ ~n open circuit and it rejects the incoming received 6~9nal5.
~ransponder duplexer 32 of fiqure 13 i8 only one of ~everal ~ethods which can be used. For example, devlces referred to a8 microwave circulators compr$ e ~ three port passive network which can accompl~6h a comparable function ~nd have the ~dditional advantage of be~nq broadband.
We ~hall now describe the major components of a radio central off~ce and will emphasize the unique and novel circuits which have been devi~ed to pr~ctice ~he instant invention.
B~iQ Central Qff~ RC~ igure 14 i8 an overall block d~aqram of ~ typical radio central office. Antennas 28 represent one of a plural~ty of directional antennas, each connected exclus~vely to ~ separate sector receiver 122. Each sector receiver 122 covers the ent~re 6 M~z TV channel which ha~ been ~ssigned to the T-NET ~y~tem. For example, ten antennas 28, each hav~ng ~n 18-degree bea-~width, connected to ten rece$ver~ 122 will prov~de a 180-degree coverage. If each angular ~ector use~
~xteen o~ thirty-two ~ubchannels in ~ system where odd numbered subchannels are u~ed on one sector, even numbered used on the adjacent fiector~ ~nd the odd number again used on the next adjacent sector . . . etc., then the arrangement would be as ~hown in figure 14. In that case, each sector rece~ver 122 would .. , ,. , :
:-, - . . .. .

1 3 ~2636 regulre sixteen filters 12~ ~nd these are ghown a~ divided lnto two banks of eigh~ subchannel filter~ eaC~t the bank of filters shown ln the top row of figurQ 14 cover ~ubchannel~ 9 through 16 ~nd the lower row of e~ght fllters cover channels 1 through 8.
Each of the subchannel filters 12~ i8 connected to its ~eparate digital ~nterface c~rcuit card 126 and they are numbered in a corresponding ~anner.
. In one preferred embodiment ei~ht of these 6ubchannel f$1ter~ and ~ssociated d$gital interface circuits can be lo controlled from one sinqle-board computer 128 and this 1B the reason that figure 1~ 6hows two groups o~ eight subchannel fllter/diqital lnterface circu$ts connected to a ~B~ single-board computer 128, and 8 more subch~nnel filter/digit~l interface c~rcu$ts connected to ~ single-board co~puter 128. Thus ~ector ~1 receiver feeds ~ixteen ~ubchannel filter~ 124, sixteen digital interface circuit cards 126, ~nd two Eingle-board computers 128. If a T-NET ~ystem had ten ~ector antennas and ten associated sector receiver~, there would be a total of three hundred sixty subchannel ~ilters 124 and diqital ~nterface cards 126 and twenty single-board computers 128~
Protocol computer 130 (figure 14) collects the data fsom all single-board ~omputers 128 and reformats and buffer ~tores it a~
necessary ~nd tben transmits $t through trunkline 26 to network control center 2 (shown in fiqure 1). That trunkline may be ~ny one of ~everal ccmmonly used links, ~uch as a microwave link illustrated fiqure 14.
Di~play and I/0 133 shown in figure 14 is a computer monitor an~ input/output ~I/0) device which may be employed to input the t 3 ~2636 1 range address of the many subscriber~ who 6~gn up for this communication service. It ~ay ~l~o be used for overhead functlons such as ~on~tsr~ng the activlty of specif~c digital interface circuits 126, slngle bo~rd computer~ 128, or fog trouble ~ho~t$ng purposes.
BfCL~lQL Rec~iYer. Fiqure 15 shows the block diagram of typical sector recelver Eub~y~tem. Antenna 28 detect~ up-link ~ignals from subscriber transponder6 and connects them to ban~ass filter 134 and adjacent channel rejection fllter 136.
o The~e filter~ suppres~ most of the video components of the Host television signal and other interference to the level where they will not overwhelm the subscriber signalsO It should be pointed OUt that typical television transmitters have an effective radiated power (ERP) rang$ng from 25,000 to two mlllion watts or more and are consequently much more powerful than the subscriber signals. Incoming signals are further amplified in 138 and down converted in first mixer 140. Intermediate frequency (IF) filters 142 and amplifier 144 have approximately 6.0 MHz bandwidth and provide a sharp attenuation of all siqnals lyin~
outside its bandwidth.
The output of amplifier 144 is connected to a ~econd down converter mixer 146 and thls is followed by a second IF filter 1~8 a-~l amplifier 150. The second IF is 21 M~z and it also has 6.0 M~z bandwidth. All the thirty-two subchannels of a T-~ET
2s 8ystem are emcompassed within this bandwidth. The output of sector receiver 122 is connected in parallel to a bank of filter~
124~ one filter i8 required for each of the thirty-two subcarriers used in the ~ector. Included within these bandpass filters i8 ~ detector ~o that the output of each filter i8 the . . . .

su~carrier baseband with analog data, i.e. it i8 the ~um of 811 digltal pulses trAnsmitted by transponders in the ~ector. It has already bee~ noted that each of the bandpass filter/detector assemblie~ for a sector could consi~t of ~ixteen ~ubchannel~ 80 one ~ector would only operate on either odd ch~nnels or even ch~nnels in order to provide frequency re-use from ~ector to sector.
RÇQ~igi~al Interface. Each of the ~ubchannel filters 124 has connected to its output a digital interface circuit c~rd and a block diagram of that card i8 shown in figure 16. The purpose of the digital interface card i8 to create range gate~ ~t ~he proper tlme delay representing the distance to each of the many ~ubscribers operating on that 6ubchannel 80 as to detect pul~e from them and forward the data to its companion s$ngle board computer 128 ~figure 1~). We shall now explain the noYel ~spects of the block diagram in figure 16. All circuit~ of dig~tal interface card 126 are interconnected to other assemblies through ~ standard multi-bus 156. For example, it has already been pointed out that there will be eight dig~tal interface cards for each ~ingle board computer 128 and these w~ll be interconnected through multibus 156. Display and I/O device 133 may also be connected through that multibus.
In one mode of operation the display and I/0 device 133 ~figure 14) $s u~ed to input the range addres~ of a new subscriber based on known or computed range to that subscriber and ~hi~ information will be ~logged~ into subscriber range ~ddress memory 160 through channel address decoder 158 and data bus 157. At the same t~me, the strength of that ~ubscriber 1 3 , ~ 6 3 6 1 slgnal w~l~ be either ~easured or e~t~ated ~nd that information wlll be input to ~ubscriber ampl~tude ~ignature memory 16C $n l~ke manner. ~hus t~e range and ~mplitude of each ~ubscriber transpcnder will be held ~n memory 160 ~nd 166 re6pectively. The analog pul6es from ~ubchannel filter 124 representing incominq data from each transponder i8 connected to lntegrate ~nd dump analog circuit 16~.
The operation of digital interface card 126 18 repetitive ~nd triggered ,~to operation by the vert$cal and horizontal ~ync pulses of the ~ost TY station as detected by a ~eparate receiver using antenna 27 ~figure 3). These ~ync pulses are connected to address generators 17~ and 176. Upcn thl~ triggering, subscriber address counter 176 begins to count upward to qenerate addre~ses in a series of steps, each step being proportional to the di6tance the ~ sync pulses have propagated outward from the television ~tat~on as it sweeps acros6 the countryslde. In other words ~ubscriber address counter 176 will have developed a count which i~ equal to the distance from the television ~tation to the in~tant posit~on of the propagating Host TV horizontal blanking interval.
Subscriber address counter 176 i~ connected to the subscriber range address memory 160 and amplitude memory 166 and if those memory locations hold a subscriber, that fact ic caused to trigger gate generator 162 and A/D 168. Thus a comparison i8 constantly being made by range address gate generator 162 to see if any ~ubscriber llves at the currently developed ~ddress count~
if there i3, range gate address generator 162 generates a range gate which enables inteqrate and dump analog circuit 164 to accept and integrate the pulse fro~ that specific transponder ~t ~ 332636 l thAt ~pecific range addres6. ~t the end of a five microsecond integrate period the pul~e from that transponder iB
~nstantaneously compared in comparator 170 aga$nRt a threshold level ~h~ch has been establi~hed by digital-to-analog converter s 16~, which ln turn iE dependent upon the expected ~trength of that subscriber. Based on the result of an~log comparator 170 determination is made as to whether there i8 ~ logic ~l~
transmission or a logic ~0~ (no transmission) from the transponder ~t that ~pecific range addressD It can be lo appreciated that these comparisons are done on a microsecond by microsecond basis and in accordance with a prearranged schedule depending upon which subscribers are logged ln memory and what tbeir distance is from the ~ost telev$slon tran~mitter. It can be further ~ppreciated that pulses from many ~ubscriber6 ~re all time interleaved and must be sorted outt that 18 the job of double buffer demux l72.
The double buffer demux 172 circuit has connected to $t an address generator (counter) l74 which is triggered into operation by the ~ost TY vertical and horizontal 6ync pulses and it first generates ~ co~rse time divicion component of subscriber address (i.e. the ~pecific H-~ync pulses that the transponder has been assigned to operate on) and a ~econd fine time division address based on the range to each subscriber. Demux 172 also has as lnput the output of comparator 170 which comprises digital pul6es from each of the many subscribers assigned to that subchannel.
The double buffer demux 172 sort~ out these time interleaved tran~ponder pulses and reorganizes them ~nto data file~ in which the data from each transponder is grouped together with the 1 tranponder'~ ~ddress an~ placed into a buffer 6torage location.
That buffer storage is periodic~lly ~dumped~ ~nto the multibus for transfe~ to the single-board computer 128. The ~ingle-board computer 128 al80 receives like~data from seven other d~git~l lnterface c~rds a5 shown in f igure 14 . The 6ingle-board computer group~ ~11 of this information and forward~ $t to protocol computer 130 where it ifi properly ~ueued wit~ the output of many other slngle board computers and forwarded over ~ trunklink 26 to the notwork control center ~s ,reviously explained.
~ Y ~iQn. We ~hall now describe the application of the T-NET system ~o cable TV (CATV). cArv des~gn engineer6 ha~e ~ound that a problem exi~ts when many subscr$bers are connected to a coaxial cable for reverse transmlssions from subscribers to a central location. This problem i~ due to the fact that each of the ~ubscribers contributes a finite amount of noi~e and the cumulative effect of all of this noise seriously reduces each of their signal-~o-noise ratios, and perhaps al~o the down-l~nk TV
program. This problem i~ self defeating in that increasing the power of each subscriber does not offer any solution because that also increases their cumulati~e noise. The instant invention solves this problem because the T-NET transponders only transmit pulses and these pulses only exist at time intervalE which are distinct and separate for each transponder. Therefore their cumulative effect is negligible.
Figure 17 illustrates a cable TV application of the T-NET
system. A transponder 1~ operates ~ubstantlally ~n the same way as described in the precedinq sections of this specification.
TY and data signals from the coaxial cable are connected through transponder 1~ and CATV tuner 182 to TV receiver 112. The data , . .

137)?636 1 output of transponder 14 is connected directly to TV receiver 112 to provide interactive televi~ion operation~ The talk-back feature could be through hand-held remote controller 18~
Transponder 14 (figure 17) detects the down-link data and sync pulses of the ~08t TV station 10 signal injected at CATV
~Head-End~ 177 and rece~ves in the same ~anner already described.
The transponder 1~ up-link reply pulses are sent through coaxial cable 183 and are collected at a multiplexed repeater 180 which could be located within the existing cable TV amplifier boxes 181 which are typically ~paced at intervals less than one mile~ The multiplexed repeater 180 can be designed to function essentially as a multiplexed pulse transponder ~somewhat like 14) so as to collect and retransmit up-link signals detected by 1790 These are appropriately synchronized to the local TV signal and radiated through antenna 184. Usually there would not be more than a few dozen transponders 14 connected between cable TV
smplifier boxes. Their ~range address~ is determined by the ~ost TV-to-subscriber indirect di~tance which is the combined cable and over-the-air effective distance. The design of these multiplex transponders (repeaters) will be obvious to those 6killed in the art after studying the several drawings and discussion presented in this specification and duly observing the B-sync requirements on the cable TV signal because it is off-set from the over-the-air TV signal ~-~ync.
The radio central office detects and process these semi CATV
signals in much the same manner as it already processes trans-ponder replies from purely over-the-air subscribers. The output of multiplex repeater 180 would in fact appear like interleaved pulses from several dozen transponders. The fact that these particular CATV trasnponders operate partly through a coaxial cable would be transparent to the radio central office. A

~imilar arrangement could be u~ed in l~rge off~ce buildings which uæe a coaxial cable ~nd common antenna.
yeh~ oc~ti~n Design. We shall now discusfi a vehicle location ~pplicat~on of the T~NE~ 8y tem (figure 18). It ha~
been pointed out that the range to each fi~ed location subscriber represents itB ~range ~ddres~U and this information i~ kept ln memory in each radio central office. On the other hand, if the transponder i~ ~ portable device or in a vehicle, then lts range will initially be unknown. Specific subchannels cal. be dedicated to operate only wlth such moving transponders.
When a precise determination of the transponder location i8 desired, the T~NET system can be designed to provide for detection of up-link ~ignals by at least two central receivers labeled tl and t2 in figure 18. The position and di tance between these two central ~eceiver~ will be precisely ~urveyed to establish a fixed baseline from which the position of each transponder can be ~ccurately computed based on precise measurement of the range from the transponder to central receivers tl and t2. Such computations are well known and commonly employed in radio navigation. Figure 18 ~hows s~ch an operation in which vehicle 186 detect~ ~ost television data and sync 6ignal~ through mobile antenna 188 and connects those signals to ~F modem 14. The demodulated signals from mod~_ 14 are connected to vocoder 194 and to computer/monitor 196.
2s Up-lin~ data pulses from modem 1~ are detected by both central receiver tl and t2 through their antennas 128. Since the range to the transponder i~ unknown initially, a series of ~equential range gates i8 generated by central receiver tl and t2 and each of these gates i8 examined in ~equence to find where 1 pul~es are being received. When this ic determined, a pair of range gates, called an ~earlya ~nd ~late- ~ate ~n radar terminology, are positioned around the received pul~es ~o ~ to tr~ck i~ A8 the vehicle ~ove~. Such pulse ~cquis$tion ~nd tracking ~echniques are well known in ~he nrt of radar circuit design. The range information which i~ thus ~easured $8 communicated from central receiver ~2 to central receiver ~1 wbere ~ navigation computation algorith~ can be installed in A
conventional computer to solve the trlanqulation problem to precisely locate ~nd track Yehicle 186. That position information can be forwarded to the network control center ~nd/or to a Host such as ~ vehicle dispatcher. Indeed, the navigation computer could be $nstalled at the ~ost computer, if that were more convenient.
The vocoder 194 (fiqure 18) i8 intended to be a voice-to-digital and digit~l-to-voice converter which takes the output of microphone 190 and digitizes it ~o that it may be sent through ~F
modem 1~. Likewise, the digitized output of modem 1~ can be converted to voice signals and transmitted through ~peaker 192.
This provides a means of verbal communication through R~ modem 14. ~f RF modem 14 operates at 1200 baud, then ~t i8 too 810w for direct digitized voice transmissions, however! microprocessor 3C in RF modem 14 can buffer-store and thus time-stretch and compres~ the 1200 baud diqitized voice information in such ~
manner as to make it $ntellig~ble, although it may not permit effect~ve real-time dialog between two ~peakers because of the time del~y. This is referred to herein as ~slow-voice~ or ~voice messaging~. On the other hand, RF modem 14 could be designed to sa 1 transmit at a rate up to 15,734 baud Dnd thi~ ~ sufficiently fast to provide real-time volce transmiss~ons throuqh a vocoder 194, if this were desirable.
Dow~-llnk ~R~h~nn~ ~Q~ L. We ~hall now describe the manner ~n which digital dat~ ~ay be 5ent co-channel ~$multaneously with ~ regular television program without ~nterfering with it. This iR referred to as co-channel multiplexed dat~ And video in figure 19. lt has already been explained that the object of thi~ technique i~ to ~uperimpose o digital data onto the regular televislon video on each of the 525 lines of a TV frame and then, on the ~ucceeding frame, to superimpose the same digital dat~, but inverted, 80 that at each corresonding element (pixtel) of the TV picture the data is fir~t added and then subtracted ~o as to become invisible. It was pointed out that this could be done throughout the entire TV
picture at a s~crifice ~n picture quality ln motion segments, or it can be restrained to only those portion~ of the picture that convey fixed 6cenes.
Figure 19 ~hows the block d$agram of ~ system of this invention for ~ending digital data only in the fixed scene portion of TV pictures. The diqital data to be transmitted ~8 buffer~stored in diqital memory 200 and read out rom that storage device at prescribed times and bent through amplifier 202 to a 8pl it channel to provide data and inverted data into switch

2~6. One frame of 525 lines of regul~r TV video i~ connected to A/D convertcr 208 where ~t is diqit~zed and ~tored in memory 210.
The ~ideo from a following frame of 525 lines is compared pixtel-by-plxtel in 214 against the corresponding information stored from the previou~ frame in order to de~ect where differences (motion) e~i~t. Where the second and ~irst frame picture elements are ldentical, it 18 assumed to represen~ a fixed ~cene 6egment and in that event ~n enable command i6 sent by 214 to buffer ~tore 200 ~nd digital data i6 output from th~t buffer ~nd i8 connected to ~umming circuit 212. Switch 206 use rV V-sync to reverse its position every TV frame 80 that the dat~ 18 flrst added in one frame and subtracted in a subsequent frame~
~n effect comparator 21~ is constantly comparing the output of the current TV frame a~ainst the previous TV frame ln order to o find fixed ~cene locations so that ~t can transmit dat~ in those segments. The output Qf 214, being the current frame, is D/A
converted in 216 so ~6 to restore the original ~nalog video which i8 sent to summing circuit 212 where the data i~ added to it.
The output of summing circuit 212 represents video plus ~nd minus data and ~t i6 ~ent to the regular TV transmitter for transmicsion to TV viewers and to tran~ponder~ specif~cally designed to detect the data p~rtion of the TV si~nal. Such data receivers could operate essentially llke present day Teletext receivers but it would include circu~t6 which take advantage of the redundant transm~ssion ~i.e. Data ~ Data) for more reliable detection.
~ -NET~Cellular B~iQ ~ntear~iQn. The ~ehicle location capabilities of the ~-NET system can be used to advantage to initiate and coordinate the hand-off of cellular radio telephone ~ubscriber~. Pigure 20 illustr~tes such an applic~tion. TWQ
T-NET radio centr~l offices labeled RCO ~1 and RCO ~2 are located with respect to cellular system 218 so that they may determine the position of any vehicle 222. Although the hexagon shape 220 ~ 3~ 36 1 defining the various cellular limit6 ~re useful $n popular descr~ptions of the idea that cellul~r radio 18 partitioned into individual cells, it is clear in ~ctual practice the geometry of any specific cell may t~ke ~ny arbitr~ry ~hape 6uch as 224. ~h~s 5 i8 due ~o the fact that the only information avallable to the cellul~r radio 6ystem as to the position of vehicle 222 i8 its signal strength. Signal ~trength is not a reliable indicator of vehicle po~ition becau6e it varies from time to time and because of local reflections from buildings, other vehicles and for other physical reasons. On the other hand, using a relatively simple .
rRdio ~urvey, a geometr$c are~ such a~ 224 can be found where reliable transmission with ~ll vehicles in that ~rea can be established ~nd maintained. Many areas ~uch as 224 can be found so that complete coverage of the entire service ~rea 218 can be lS assured. In a practlcal world those areas 224 constitute the real cell~.
Conseguently if one has $ndependent means such as a T-NET
sy~tem to determine in which cell 224 ~ ~ubscriber 18 located, then the problem of handing-off vehicle 222 as it moves from cell-to-cell becomes a relatively Eimple computer function. ~hi~
i~ also a relatively si~ple process for the T-NET vehicle location mode to accomplish. It would also occupy very little of its traffic capabity. Furthermore, e specialized T-NET
transponder could be designed and built for th$s function alone to reduce its cost and increases $ts reliability in this - operating mode.
While the invention has been particularly shown and described with reference to preferred embodimentg thereof, tt will be understood by those ~killed in the art that varioug 6~

changes in form and deta~l~ may be made t~erein wit}~out departing f rom the 8pi rit ~nd scope of the ln~rent~onO

6~

Claims (57)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of communicating information within an area served by a commercial broadcast station, said commercial broadcast station having a broadcast station transmitter and a broadcast station modulator for transmitting over the air on a broadcast station carrier and within a broadcast station frequency band comprising the steps of:
a) using a modulator other than said broadcast station modulator, modulating the information to be communicated by a carrier other than the broadcast station carrier;
b) transmitting, from a transmitter other than the broadcast station transmitter, at least one sideband of the modulation of step (a);
c) receiving at a remote location a signal containing the carrier of the broadcast station and the at least one sideband transmitted from the transmitter other than the broadcast station transmitter;
d) demodulating at the remote location the signal received in step (c) using the carrier of the broadcast station as a reference to recover a signal corresponding to the signal transmitted in step (b), and;
e) demodulating at the remote location the signal recovered in step (d) to recover the information to be communicated.

2. The method of claim 1 wherein the demodulation of step (d) is an envelope detection.

3. The method of claim 1 or 2 wherein the carrier used in step (a) has a frequency in a frequency band adjacent the broadcast station frequency band.

4. The method of claim 3 wherein the carrier used in step (a) is generated independent of the broadcast station carrier frequency.

5. The method of claim 1 or 2 wherein the information to be communicated is digital information.

6. The method of claim 1 or 2 wherein the information to be communicated is analog information.

7. The method of claim 1 or 2 wherein the broadcast station is a television station broadcasting in a preassigned television channel frequency band, and the carrier used in step (3) has a frequency in the frequency band of an adjacent television channel.

8. A method of communicating information within an area served by a broadcast station transmitting on a broadcast station carrier and within a broadcast station frequency band comprising the steps of:
a) modulating the information to be communicated by a carrier other than the broadcast station carrier;
b) transmitting at least one side band of the modulation of step (a);
c) receiving at a remote location a signal containing the carrier of the broadcast station, and the at least one sideband transmitted in step (b);

d) demodulating the signal received in step (c) using the carrier of the broadcast station as a reference to recover a signal corresponding to the signal transmitted in step (b); and e) demodulating the signal recovered in step (d) to recover the information to be communicated.

9. The method of claim 8 wherein the demodulation of step (d) is an envelope detection.

10. The method of claim 8 or 9 wherein the carrier used in step (a) has a frequency in a frequency band adjacent the broadcast station frequency band.

11. The method of claim 10 wherein the carrier used in step (a) is generated independent of the broadcast station carrier frequency.

12. The method of claim 8 or 9 wherein the information to be communicated is digital information.

13. The method of claim 8 or 9 wherein the information to be communicated is analog information.

14. The method of claim 8 or 9 wherein the broadcast station is a television station broadcasting in a preassigned television channel frequency and, and the carrier used in step (a) has a frequency in the frequency band of an adjacent television channel.

15. The method of claim 8 or 9 wherein the broadcast station and the signal transmitted in step (b) are both transmitted over a cable.

16. The method of claim 14 wherein the broadcast station and the signal transmitted in step (b) are both transmitted over a cable.

17. The method of claim 8 or 9 wherein the broadcast station and the signal transmitted in step (b) are both transmitted over the air using the same antenna.

18. The method of claim 14 wherein the broadcast station and the signal transmitted in step (b) are both transmitted over the air using the same antenna.

lg. The method of claim 8 or 9 wherein the broadcast station and the signal transmitted in step (b) are both transmitted over the air using different antennas.

20. The method of claim 14 wherein the broadcast station and the signal transmitted in step (b) are both transmitted over the air using different antennas.

21. The method of claim 8 or 9 wherein said broadcast station has an antenna that is at the same location as an antenna used for transmitting said at least one sideband.

22. The method of claim 7 or 9 wherein said broadcast station has an antenna that is at the same location as an antenna for transmitting said at least one sideband.

23. The method of claim 8 or 9 wherein said broadcast station has an antenna that is remotely located with respect to an antenna for transmitting said at least one sideband.

24. The method of claim 14 wherein said broadcast station has an antenna that is remotely located with respect to an antenna for transmitting said at least one sideband.

25. A method of receiving and recovering information transmitted within an area served by a broadcast station transmitting on a broadcast station carrier and within a broadcast station frequency band, the information as transmitted being modulated by a carrier other than the broadcast station carrier to form a modulated information signal, comprising the steps of;
a) receiving at a remote location a signal containing the carrier of the broadcast station and the modulated information signal;
b) demodulating the signal received in step (a) using the carrier of the broadcast station as a reference to recover the modulated information signal, and;
c) demodulating the modulated information signal recovered in step (b) to recover the information.

26. The method of claim 25 wherein the demodulation of step (b) is an envelope detection.

27. The method of claim 25 or 26 wherein the carrier other than the broadcast station carrier has a frequency in a frequency band adjacent the broadcast station frequency band.

28. The method of claim 27 wherein the carrier other than the broadcast station carrier is generated independent of the broadcast station carrier frequency.

29. The method of claim 25 or 26 wherein the information is digital information.

30. The method of claim 25 or 26 wherein the information is analog information.

31. The method of claim 25 or 26 wherein the broadcast station is a television station broadcasting in a preassigned television channel frequency band, and the carrier other than the broadcast station carrier has a frequency in the frequency band of an adjacent television channel.

32. The method of claim 25 or 26 wherein in step (a), part of the signal received constituting the information transmitted as modulated by a carrier other than the broadcast station carrier is transmitted from a different location than the part of the signal containing the carrier of the broadcast station.

33. The method of claim 31 wherein in step (a), part of the signal received constituting the information transmitted as modulated by a carrier other than the broadcast station carrier is transmitted from a different location than the part of the signal containing the carrier of the broadcast station.

34. Apparatus for communicating information within an area served by a commercial broadcast station transmitting over the air on a broadcast station carrier and within a broadcast station frequency band comprising:
modulation means for modulating the information to be communicated by a carrier other than the broadcast station carrier and for providing an output responsive thereto;

transmitter means coupled to said modulation means for transmitting, from a transmitter other than the broadcast station transmitter, the output of said modulation means;
a receiver at a remote location with respect to said transmitter means for receiving as a receiver signal a signal containing the carrier transmitted by the broadcast station and the signal transmitted by said transmitter means, said receiver means having;
a first demodulator for demodulating said receiver signal using the carrier of the broadcast station as a reference to recover a signal corresponding to the signal transmitted by said transmitter means, and;
a second demodulator coupled to said first demodulator for demodulating the signal recovered by the first demodulator to recover the information to be communicated.

35. The apparatus of claim 34 wherein said first demodulator is an envelope detector.

36. The apparatus of claim 34 or 35 wherein the carrier of said modulation means has 2 frequency in a frequency band adjacent the broadcast station frequency band.

37. The apparatus of claim 36 wherein the carrier of said modulation means is generated independent of the broadcast station carrier frequency.

38. The apparatus of claim 34 or 35 wherein the information to be communicated is digital information.

39. The apparatus of claim 34 or 35 wherein the information to be communicated is analog information.

40. The apparatus of claim 34 or 35 wherein the broadcast station is a television station broadcasting in a preassigned television channel frequency band, and the carrier of the modulation means has a frequency in the frequency band of an adjacent television channel.

41. Apparatus for communicating information within an area served by a broadcast station transmitting on a broadcast station carrier and within a broadcast station frequency band comprising:
modulation means for modulating the information to be communicated by a carrier other than the broadcast station carrier and for providing an output responsive thereto;
transmitter means coupled to said modulation means for transmitting the output of said modulation means;
a receiver at a remote location with respect to said transmitter means for receiving as a receiver signal a signal containing the carrier transmitted by the broadcast station and the signal transmitted by said transmitter means, said receiver means having;
a first demodulator for demodulating said receiver signal using the carrier of the broadcast station as a reference to recover a signal corresponding to the signal transmitted by said transmitter means, and;
a second demodulator coupled to said first demodulator for demodulating the signal recovered by the first demodulator to recover the information to be communicated.

42. The apparatus of claim 41 wherein said first demodulator is an envelope detector.

43. The apparatus of claim 41 or 42 wherein the carrier of said modulation means has a frequency in a frequency band adjacent the broadcast station frequency band.

44. The apparatus of claim 43 wherein the carrier of said modulation means is generated independent of the broadcast station carrier frequency.

45. The apparatus of claim 41 or 42 wherein the information to be communicated is digital information.

46. The apparatus of claim 41 or 42 wherein the information to be communicated is analog information.

47. The apparatus of claim 41 or 42 wherein the broadcast station is a television station broadcasting in a preassigned television channel frequency band, and the carrier of said modulation means has a frequency in the frequency band of an adjacent television channel.

48. The apparatus of claim 41 or 42 wherein the broadcast station and said transmitter means both transmit over a cable.

49. The apparatus of claim 47 wherein the broadcast station and said transmitter means both transmit over a cable.

50. The apparatus of claim 41 or 42 wherein the broadcast station and said transmitter means both transmit over the air using the same antenna.

51. The apparatus of claim 47 wherein the broadcast station and said transmitter means both transmit over the air using the same antenna.

52. The apparatus of claim 41 or 42 wherein the broadcast station and said transmitter means both transmit over the air using different antennas.

53. The apparatus of claim 47 wherein the broadcast station and said transmitter means both transmit over the air using different antennas.

54. The apparatus of claim 41 or 42 wherein said broadcast station has an antenna at the same location as an antenna coupled to said transmitter means for transmitting the output of said transmitter means.

55. The apparatus of claim 47 wherein said broadcast station has an antenna at the same location as an antenna coupled to said transmitter means for transmitting the output of said transmitter means.

56. The apparatus of claim 41 or 42 wherein the antennas are located remotely to each other.

57. The apparatus of claim 47 wherein the antennas are located remotely to each other.