CA2208697A1 - Simulcast resynchronisation improvement using global positioning system - Google Patents

Abstract

In a multiple site radio frequency simulcasting R.F. transmission system, data provided via inter-site communications links (L1, L2, etc.) from a control point to the R.F. transmitter sites exhibits random time delay skew because multi-phase modems recover clock signals from an arbitrary one of multiple phases. The output data streams of the modems are temporarily stored at the sites in memory buffers for temporal alignment for simultaneous R.F. transmission. The memory buffers at each site are periodically resynchronised on a continual basis by a universal "resynch" circuit to maintain optimum simulcast system performance. A 9600 bps data clocking reference and a low frequency data "gating" (timing) signal required for performing a periodic "resynch" operation are derived separately from a common satellite reference signal acquired by using a GPS (global positioning satellite) receiver at each simulcast system site. A resynch operation is periodically performed on a continuing basis separately by each simulcast system site and the distributed multi-site broadcast data stream is aligned using the resynch reference tones derived from the received GPS signal.

Description

. CA 02208697 1997-06-24 ., wo 96120s43 ~ U~SI16847 SIMULCAST RESYNCHRONIZATION IMPROVEMENT USING GLOBAL POSITIONING SYSTEM

CRoss-RE~ERENcFs TO
Rl~LATED APPLICATIQNS AND PATENTS

This application is somewhat related to commonly-assi ~ned U.S. Palent No. ~,1?7,396 to Rose et al.. issued on December lj. 199~. entitled "Public Ser~ice Trunking Simulcast System," and U.S. Patent i~o. 4,90~,37 l to Hall et al.. issued on February 20, 1990, entitled "Radio Trunking Fault Detection System". This application is also somewhat related to the followin~J commonly-assigned copendino applications:
serial number 07/~'J4, 1~3 of Brown et al. entitled "Self Correction Of PST Simulcast System Timin~", filed '~ January 19g~ (Attorney Docket Number ~6 444; Client Reference No. 45-~ 64) and serial number 07/906,4, 3 of Thomas A. Brown entitled "Control Channel Tirninc~ Detection and Self Correction For Dic~itally Trunked Simulcast Radio Cornmunication System". filed 30 June 1997 (Attorney Docket Number 46-59~. Client Reference ~o. 4~-~-711). The disclosures of each of the above patents and applications are incorporated b,v reference as iI' e~pressly se~ forth herein.

FIELl) OF TElE ~N~v~NTION

This invention relates to radio frequency (RF) si~nal transmission systems, and in particular to "sirnulcastinc" systems for providin~ the simultaneous transmission of the same information b,v two or more separatelv located RF transmitters. ~lore particularly, the invemion relales to an improved method and apparatus for C~enerating simulcast timin~ "resynch" Iresvnchronization~ reference si~nals at each transmitter site to maintain coherency of transmissions.

wa 96/205~3 PCT~US95116847 BACKC.RO~D AlND SUM~L~RY OF THE ~NrrloN

As is well known, due to FCC power l;mit~tions, geo;,raphical and/or other factors, it is sometimes r ot possible for a single RF tr~ncmittin~ site to provide adequate covera~e to a large desired coverage area. For e~ample, government entities commonly use land-mobile radio communications systems to provide communications between aheadquarters and various mobile and portable radio users that rove throu._hout the jurisdiction of the governmental entity. In some cases the _eographical area of jurisdiction is so laroe that it is not possible for a single land-based RF tr~n.~mi~;n ~ site to cover it. Even if the effective radiated power of the single tr~n.cmi.csion site was sufficiently _reat to cover the entire area, users in outlying or fringe areas might receive only spottv service because of the "line-of-site" nature of VHF tr~n.~mic~ions and/or due to geographical obstructions (e.g., hills, bridges~ buildincTs, and the curvature of the earth) interposed between the single tr~n~mitt~r site and various fringe locations within the covera=e area.

One known way to expand the coverage area is to provide multiple, "simulcasting" tr~n~mitlin~ sites. In order to simplify mobile radio operation and conserve radio frequency spectrum, such "simulcasting" RF tr~n~mit~in g sites all transmit substantially identical signals at substantialIy identical times on substantially identical radio frequencies. Such "simulcasting" e~imin~tes control overhead and other comple~ities associated with performing "hand offs" from one RF transmitting site coverage area to another as is common, for example, in cellular and "multi-site" RF
communications systems. So-called "simulcasting" dic~itally trunked RF repeater systems are generally known. The followinc~ is a listing (which is by no means e~haustive) of prior documents that describe various aspects of R~ transmission simulcasting and related issues:
U.S. Patent No. ~,17~,396 to Rose et al.;
U.S. Patent No. 4,903,,~1 to Hall et al., I T S. Patent l~io. 4,696,05' to Breeden;

W O 96no543 PCTrUS95/16847 U.S. Patent No. 4,696,~51 to Breeden;
U.S. PatentNo. 5,~45,63 l to Averbuch;
U.S. Patenl: No. 5,287,550 to Fennell et al;
U.S. Patenl: No. 4,782,499 to Clendening;
U.S. Patenl:No. 5,052,028 to Zwack;
U.S. Paten~ No. 4,570,265 to Thro;
U.S. Patent No. 4,516,269 to Krinock;
U.S. Patemt No. 4,475,246 to Batlivala et al.;
U.S. Patent No. 4,~17,220 to Martin;
U.S. PatentNo. 4,97~,410 to Cohen et al.;
U.S. Patent No. 4,608,699 to Batlivala et al.;
U.S. PatentNo. 4,918,437 to Jasinski et al.;
U.S. Patent No. 4,578,515 to Persinotti;
U.S. PatentNo. 5,003,617 to Epsom et al.;
U.S. PatentNo. 4,939,746 to Childress;
U.S. Patent No. 4,903,26~ to Dissosway et al.;
U.S. Patent No. 4.~6,496 to Cole et al.;
U.S. Patent ~io. 4,96~,966 to Jasins~;i et al;
U.S. Patent~o. 3,90'?,161 to Kiowaski et al;
U.S. PatentNo. 4,'~18,65~ to Ogawa et al;
U.S. PatentNo. 4,~55,815 to Osborn;
U.S. Patent No. 4,~11,007 to Rodman et al;
U.S. Patent No. 4,414,661 to Karlstrom;
U.S. Patent No. 4,47~,80' to Pin et al.;
U.S. Patenti~o. 4,597.105 to Freebur~; and Japanese E'a2ent Disclosure No. 61-1078 76.

U.S. Patent No. 5,17~,396, issued December 15, 199~ to Rose et al., entitled "Public Service Trunl;ing Simulcast Svstem", discloses a trunked radio simulcast svstem havin~ control site and remote site architectures that include R~

W 096/20543 PC~US9~/168t7 tr~n.~mi~.~ion timing synchrnni7~tinn fea~ures that are relevant to the presently preferred exemplary embodiment. In addition, U.S. Patent No. 4,903,3~1, issued Februa~ 20,1990 to Hall et al., entitled "Radio Trunkin;, Fault Detection System," discloses a trunked radio repeater system having a radio frequency repeater site architecture that includP~ fault and caIl testing and failure detection features that are somewhat relevant to the present inventiorl. These patents are both cr mmonly assigned to the assignee of the present invention ;and are both incorporated by reference herein.

While simulcastin~ thus provides various advantages as compared to other techniques for e~pandin_ covera~e area, it also in~roduces its own particular set of comple~ities that must be dealt with. By way of illustration, please refer to FIGURE
1 -- which is a schematic diagram of an e~emplary three-site simulcasting digitally trunked land-mobiIe RF cornmunications system 10. System 10 includes three simulcastiIlg transmitting sites, S1, S and S3. The tr~n~mi~sions of site S1 cover the coverage area Al, and similarly, the tr~n~mi~ions of sites S~ and S3 cover respective coverage areas A~, AJ . A centraI control point C coupled to each of sites S 1, S2 and S3 via respective communication links (L1-L3) delivers, in real time, substantially identical si~n~llin~. (including digital control charmel sign~lling and associated timing information) for tr~n~micsion by the various sites.

Each RF cha~nel at all sites is modulated w ith amplitude, phase and time delay corrected information. To accomplish this, time, phase and amplitude stable communication links must be provided between a main control point site and all other simulcast transmit sites by means of a high quality phase-stable back-bone communication systern arran~ement (e.~J., radio. microwave or fiber optic). In this regard, commercial wire-common-carriers do not provide the degree of stability required for simulcast; whereas, dedicated, user controlled, voice/data ~rade, synchronous multiple~ used in conjunction with radio, microwave or fiber optic back-bone distribution paths rnost effectively do provide the needed communications circuits and stabilit~ for simulcast.

W 096/205~3 PCTrUS95/16847 E~emplary system 10 is preferably a digitally trunked simulcast communications systerm of the type m~rket. d by Ericsson-GE Mobile Communications Inc. (ECiE) under the ~rade name EDACS. This system provides a digital RF control channel and plural RF workin~, channels. In such a digitally trunked system, an exemplary mobile radio unit M within one (or more) of covera~e areas Al-A3 continuously monitors an "outbound" di~ital control channel when it is not actually enga~ed in active communications on a workin~ channel with other units. Mobile Mmay request co~ u"ications by transmitting a channel acsi~nment request messa~Je on an "inbound" control charmel. Upon receipt of such channel assignment request (and presumin~ that at least one working channel is available for temporary assignment to mobile unit ~1 and other units with which mobile unit ~I wishes to communicate),control point C responds by causing a control channel assi_nment messa~e to be tr~ncmitted by each site S1-S3 over the outbound control channel. In simulcast system 10, this channel ~c~ignment message is trancmitt~d simultaneously by each of tr~n~mittin~ sites S l-S3 over the sarne outbound control channel frequency (such that mobile unit M and other mobile units "called" by the channel assi~nment messa~e will receive the message regardless within which coverage areas Al-A3 they may happento be Located). Mobile unit ~vI (and other called mobile units) respond to the received outbound trunl;ing control channel assignment message by ~h~nging frequencv to an RF
wor~ing channel and conducting communications on the working channel. Once the wor~ing channel communications are concluded, the mobile unit ~ (and other called mobile units) retum to monitoring the outbound control channel for additional messages directed to them.

Referrinc once again to FIGU~E 1, suppose mc,bile unit M is located within an overlap area X wherein covera~e areas A~ and .~3 overlap one another.
Within this overlap area X mobile unit ~L will receive (perhaps at approximately equal si~nal strength levels) the outbound control channel transmission of site S2 and also the outbound control channel transmission of site S3 . Simulcast system 10 is appropriately desi~ned such that such outbound control channel transmissions from sites S~ and S3 W 096/20543 PCTrUS9S/16847 are on substantially the same RF frequency so that no heterodyning or other interference occurs. Similarly, control point C sends, over links Ll-L3, substantially identical outbound control çh~nnel messa~Jes for tr~n.~mis.~ion by each of sites S1-S~.

However, a problem can arise if the outbound controI channels are not precisely synchronized to one another. A transceiver located within overlap region X
that receives outbound control channel synchronization signals delayed with respect to one another by even a small time period (e.g., more than a one-half bit period, or about ~ microseconds for 9600 baud operation) could end up losing bits and~or temporarily losing synchronization, bit recovery and error checkin_ capabilities.

Delays due to the limited speed at which electromagnetic waves propagate must be taken into account in systems simulcasting data at high data tr~n~micsion rates (an RF si_nal travels "only" about 300 meters in one microsecond). It is possible (and usually necessary) to adjust the relative effective radiated power levels of the site transmitters so that the distances across the overlap regions X are kept less than a desired maximum distance--and thus, the difference in the RF propagation delav times across an overlap recrion due to the different RF path len~ths berween the site and a receiver within the overlap region is minimi7Pd Even with this optim i7~tion, however, it has been found that (due to the additional differential delay caused by the different RF path lengths) a maximum system differential delay stability of ' S microseconds must be observed to guarantee that the transceiver in any arbitrary location within a typical overlap region X will receive the corresponding digital signal bit edges within 5~ microseconds of one another.

Fortunately, it is typicallv possible to minimi7e time delay differences to on the order of a microsecond throu~h various known techniques. For exarnple, it is well known in the art to introduce adjustable delay networks (and phase equalization networks) in line with some or all of in~er-site links L l-L3 to compensa._ for inherent difrerential link del~y ~imes (see l~.S. patent 4~516,'~69 to Krinock, and ~.S. patent W 096/20543 PCTrUS95/16847 numbers 4,696,051 and ~,696,052 to Breeden, for e~ample). Conventional microwaveand fiberoptic link chamlels e~hibit arnplitude, phase and delay characteristics that are extremely stable over long periods of time (e. ~., many months), so tnal such additional delays, once ad~usted, guarantee that a signal input into all of the inter-site links L l-L~
at the same ~ime will arrive at the other ends of the links at almost exactIy the sarne time. The same or additional delays can be used to compensate for differeM, constant delay times introduced by signal processin~ e4uipment at the sites S l-S3 to provide .~imlllt~n~ous coherent tr~n.~mi.~.cion of the signals by the different sites. For e~ample, the above-identified Rose et al. patent application describes a techni~ue wherein ~n'~iti~n~l frequency and timina information is provided to each site over one or more particular inter-site link channels so as to elimin~te timin~ ambiguities that may result from the use of conven-tional multi-level, multi-phase protocol-type modems. In this manner, the above mentioned simulcast system forces coherence at the start of data tr~n~mission on a particular established communications path, thus correcting for any multi-bit ambiguity created by the inter-site communication link modem.

Briefly, referring now to FIGURE 2 which generally depicts an Ericsson-GE (EGE) multiple site simulcast tr~ncmics;on system of the type described in accordance with the above mentioned Rose et al. patent, a "master!' resynch (resynchroni~tion signal) circuit 100 located at control point site C produces reference edges/tones, e.g., at 740~ Hz and 300 ~, t~nat are sent to each transrnit site ~S 1~ 7) a dedicated channel over the inter-site communication links (L l-L2). Digital and voice data aligned to these reference signals is also sent via the communication links (L 1 -I,2) between control point C and the transmit sites (S1-S~). The lower (300 Hz) tone is used as a "gating" reference (for read-out timing of a broadcast data buffer at the transmit sites) and the higher (2400 Hz) tone is used as a data clockin~ frequency reference.
Each transmit site (S l-SX) in the simulcast system includes a "universal" (i. e., common hardware) resynchronization circuit for recovering reference edges from the tones. By performance of a periodic "resynch" operation the universal resynch circuit at each simulcast system site re-ali~ns the broadcast data received via the inter-site links to . . CA 02208697 1997-06-24 W O 96/20543 PCT~US9SJ16847 these reference edges. Consequently, as previouslv mentioned above, it is reguired that the signal paths for these reference tones (conventionally provided via the inter-site links) be of high quality and very phase-stable as any variation or noise in these signals will have an adverse affect on overall simulcast system performance.

Thus, it is known to resynchronize the control channel at each tr,~n~mi1ter site periodically on a routine basis in order to correct any control channel timing errors that may arise in simulcast system 10. Moreover, the above-identified U.S. patent application serial no. 07/824,1~3 to Brown et al., filed Januarv 2~, 1992, describes additional techniques (which have been in public use for more than a year and are therefore prior art to the present application) for periodically "kicking" a site modem in order to ensure that the modem uses a distributed common clocking signal; and for "retraining" a communications link and associated site modems for a simulcast system working channel if a routinely pe;fo;rned working channel "test call" (e.g., as described in U.S. Patent 4,903,3'~1 to Hall et al.) fails.

With respect to the above mentioned EGE simulcast svstems, applicant has disclosed in the instar~t specification an improved method and apparatus for generating resynchronization tones at each transmit site that increases the reliability of synchronized timing throughout a simulcast system and greatly simplifies the procedure for simulcast system alignment. More specifically, an improvement in system performance is accomplished by providinc~ a c~lobal positioning satellite (GPS) receiver at each transmitting site to serve as the source for generating the precise, stable frequency reference tones needed for periodic "resynch" operations. The GPS system, traditionally used for navi ,~ti~n~,l pulposes, is a series of satellites svnchronized in time and continuousl~ transmitting, inter alia, time, date and positioning information.
Althou_h, cellular radiotelephone system sites using the GPS for providing absolute timing is kno~vn, such systems admit to numerous inherent problems which render these systems expensive and unreliable (see, for example, U.S. Pa~ent 5, Z45,634 to Averbuch). In accordance with the present in~rention. an improved multiple site RF

W 096/20S43 PCTrUS951168~7 simulcast system is achieved using GPS system broadcast si~nals in conjunction with a p2rticular resynchronization circuitry arrangeme~t at each simulcast site which overcomes many of the drawbacks of previous GPS synchroni~ed systems.

In accordance with the present invention, resynchronization reference signals are not sent to transmit sites along with the broadcast signals via the site interconnect links (L1, L~, etc.). Illstead, each simulcast site includes a "universal"
(_eneric) EGE ~imlllr~.~t system "resynch" circuit and simulcast data resynchronization is periodically perforrned on a routine basis by each system site separately andindependently. In a preferred e~emplary embodiment, a stable, precise 9600 bps data clock reference and the lower frequency ",~ating" sianal (e.g., 300Hz as described above) are derived separately at each simulcast system site from a global positionina satellite (GPS) broadcast transmission ac~uired by USiIl_ a GPS receiver at each site.
The control point site (C) includes a universal EGE "resynch" circuit and it is no longer employed as a "master" circuit. The "resynch" circuit at each site utilizes the GPS
derived reference signal tones to align the RF broadcasting of simulcast data.
Consequently, dedicated stabilized channels on the site interconnect linL;s are no longer needed for distributina "resynch" reference sianal tones. In addition, because the EGE
"resvnch" operation will force alignment to the correct simulcast system timina. any variation in site interconnect link latency is automatically corrected whenever a "res,vnch" operation is performed (so long as the latency variation is within the "gating"
sianal timing window) without any link latency measurement or correction.
Conventional non-EGE commercial simulcast systems if similarly outfitted with GPS
receivers would require an arithmetic calculation of iatency change and/or stora~e of lirl~ timing parameters. Moreover, employing a GPS receiver in combination with an EGE universal "resynch" circuit in an arranoement to generate "res,vnch" reference tones for the resynch circuit and autonomously aliuriing the data broadcast at each site in accordance with the present invention further improves simulcast operations by reducing timing jitter and eliminatin,g the need for alignment checl~s on reference tone po larit~ .

_ _ _ _ . _ . ... . . . .. ... ... _ _ W 0~6/20543 PC~rUS95/16847 Because fine tuning a par~icular mlllt;ple site simulcast system a~angement often requires specifIc ad3ustments of the broadcast timin;, at one or more sites off of the "nomin~l" equal ~ming, a method of producing such timing offsets must provided.
In accordance with a fi~her aspect provided by the present invention, the ~PS receiver utilizes a delay unit that provides adjustable sign~1 delays of +12511s in incremental 0.~1s steps and which is accessible via an RS-232 port. AccordingIy, an additional advanta=e is achieved by the present invention in that simulcast system fine tuning timing adjus~nents can be made remotely via an RS-232 link.

BRIEF DESCRIPTION OF TEIE DRAWINGS

These and other features and advantages of the present invention wilI
~ecome more cc~mrJIe~1y understood by referring to the following detailed description of presently pre~erred exemplary embodiments in conjunction with the FIGURES in which Iike reference numerals refer to like elements throughout:

FIGIJ~RE 1 is a general schematic illustration of a simplified e~emplary=
multiple site RF comrnunication simulcast system;

FIGURE 2 is a general schematic block dia~ram of the central control point C and remote transceiver sites S I and S2 of an Ericsson-GE multiple site RF simulcast communication system of a type on which operation of the present invention may be particularly suited;

FIGURE 3 is a oeneral schematic block diagram of an e~emplaly arran~ement of a modified multiple site RF simulcast communication system usin_ GPS
receivers to improve the resynchronization reference si(Jnal Feneration at every site.

DETAIlLED DESCRlPTTON OF TT~E DR~,VINGS

.

. CA 02208697 1997-06-24 wo 96/20~3 PCrlUS95/16847 In the following description, for purposes of e~cplanation and not limitation, specific details are set forth, such as particular circuits, circuit components, irlterfaces, techniques, etc. in order to provide a ~orough understanding of the present invention.
However, it will be a~ L to one skilled in the art that the present invention may be practiced in other e:mbodiment~s that depart from these specific details. In other instances, detailed descriptions of we~l known methods and progr~mming procedures, devices, and circuits are omitted so not to obscure the description of the present invention with unnecessary detail.

The basic: architecture of an Ericsson-GE simulcast syslem as described above is shown in FIGURES 1 and 2 -- that is, it includes a central control point C and plural tr~n~mitting sites S 1. .. Sn. Althou~h only two (remote) tr~n.sm ittin g sites S I -S'' are shown in FIGURES 2 and 3, it will be appreciated by one skilled in ~e art that numerous remote sites participating in siml~lc~sting are likewise in communication with control point C via identical microwave, fiber-optic, cable or land-line communication path links L l -Ln. Moreover, the present invention is not limited to use solely with a microwave or land-line link but may be used with any other type of appropriate communication link such as radio wave.

In a multiple site radio frequency simulcasting RF tr~ncmi.csion system, data provided via the inter-site communication links (L1, L2, etc.) from control point C to the RF transmitter sites (S 1, S2, e~c.) e~hibits random time delav skew because multi-phase link modems (MC, M1, ~, etc.) at each site recover clock signals from an arbitrary one of multiple phases. The data stream outputs of modems are temporarily stored at the sites in memory buffers Ml, ~, etc. associated with the modem at each site. Timing information provided to each site from control point C via linL; channels initially sets the memoly buffer output timing at each site to elimin~te transmission timing ambiguities. On a continual basis, resynch circuitr,v at the sites periodically resynchronizes memory buffer output timin~ in accordance ~hith a pair of reference _ _ ,, , . . .. ... .. . . ... ... . . . . . _ .. .. .. . _ . CA 02208697 1997-06-24 W 096/20543 PCTrUS9SI16847 frequency tones sontimlously provided to each site over the dedicated link channels from control point C as previously discussed above.

Referring now to FIGURE 3, an exemplary embodiment of a modified multiple site RF commllnication simulcast system in accordance with the present invention is discussed. GPS re~eiv~ 301,301a and 301b at all sites (including control point C) provide a reference si_nal ac~?uired from a common GPS timing si~nal broadcast via satellite 300. Tone generator circuits 302, 30'~a, 302b at each site utilize the received GPS timing reference signal to generate a hi ,h frequency 9600 bps data clocking reference tone and a low frequency data "gating" (timing) tone that are used by universal resynch circuits 30,, 303a, 303b for performing the periodic resynch operations. In accordance with the present invention, a ~ating signal frequency much lower than, for example, 300Hz may be used as long as the frequency chosen is anintegral submultiple of the data stream frarne timin_ (i.e., the data frame period divided by the gating frequency period) and a submultiple of 9600 (e.g., using lOOHz provides a gating period of 10 ms; 60Hz provides a gating period of 16.6 ms; etc). Gatingfrequency is preferably selected based on e~pected system link latency variations.

Resyn~ i~L,on reference tones are not sent to tr~n~mi~cion sites via the inter-site communication links from a riesign~t~d control point site as in the e~emplary EGE simulcast systems discussed above. Consequently, additional phase-stable, delay-compensated ch~nnels ~or resynch signals are not required. Instead, a resynch operation is periodically performed on a contim~inG basis by universal resynch circuits 303, 303a, 303b separately at each ~iml~lc~t system site using the resynch reference tones derived from the received GPS signal by tone ~enerator circui~s 302, 302a, 302b. Retimed(realigned) data is thereby provided on a continuin~ basis to channel transmitter(s) 30~a, 304b at each site for simultaneous RF tr~n~micsion. In addition, because aresynch operation forces data strearn alignment to the correct simulcast system timin~g, any inter-site intercom1ection link latency variation (within the "gating" si_nal timing ~indow) will be automatically corrected whenever the resvnch operation is performed.

CA 02208697 l997-06-24 W O 96/Z0543 PCT~US95/16847 ~ oreover, this modified arrangement for providing resynchronization reduces timing jitter and el;min~t~ the need for alignment checks on reference tone polarity.
. ~

As also d~ picted in FIGURE 3, GPS receivers 301, 301a, 301b include a delay unit that provides adjustable si~nal delays of +12511s in incremental steps of 0.5~1S
and which is controllable via an RS-232 port. Accordin~ly, an additional advanta_e is achieved by the present invention in that simulcast system fine tuning timing adjustments can be made remotely via an RS-~32 link.

W~ile the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is int~n~led to cover various modifications and equivalent arrangements included within the spirit and scope of the appended clain~s.

Claims (8)

WHAT IS CLAIMED IS;

1. In a simulcasting radio frequency (RF) communications system of the type having a central site providing a stream of high speed data signals comprising timed frames of data to plural RF transmitter sites, and each of said transmitter sites including a multi-phase modem and data buffer means for receiving and storing said stream of high speed data signals, said data signals received at each said transmitter site having time ambiguities with respect to said data signals received at another said transmitter site, each of said transmitter sites further including means for periodically resynchronizing and coherently simulcasting information from said high speed data signal streams over a common RF channel comprising:
a global positioning satellite (GPS) timing reference broadcast signal receiver;
a resynch tone generator, said tone generator providing a pair of reference frequency tones synchronized to a broadcast GPS timing reference signal received by said GPS receiver; and a universal resynch circuit responsive to said pair of reference tones, said universal resynch circuit synchronizing a read out from said buffer means of said received and stored high speed data signal stream for effecting a simultaneous transmission by said plural RF sites.

2 The simulcasting communications system of claim 1 wherein said resynch tone generator produces a 9600 Hz data clocking reference signal and a low frequency data gating tone.

3. The simulcasting communications system according to claim 7 wherein said low frequency data gating tone is an integral submultiple of said simulcasting system data stream frame timing and a submultiple of 9600.

4. The simulcasting communications system according to claim 2 wherein said low frequency data gating tone is 300 Hz.

5. In a simulcasting radio frequency (RF) communications system of the type having a central site providing a stream of high speed data signals to plural RF
transmitter sites, said data signals received at each said transmitter site having time ambiguities with respect to said data signals received at another said transmitter site, an improved means at each of said transmitter sites for synchronizing and coherently simulcasting over a common RF channel information from said high speed data signal streams comprising:
receiver means for receiving and extracting a timing reference signal from a GPS system broadcast transmission;
resynch tone generating means for generating a pair of resynchronization reference frequency tones synchronized to said GPS signal;
memory buffer means for receiving and storing said stream of high speed data signals; and universal resynch circuit means responsive to said pair of reference tones for periodically resynchronizing a reading-out of said high speed data signal stream on a continuing basis with at least one of said resynchronization reference frequency tones.

6. In a simulcast radio frequency communications system having plural spatially separated RF transmitters transmitting substantially the same radio signal at substantially the same radio frequency, each of said RF transmitters having an associated GPS receiver, a method of periodically resynchronizing radio signals transmitted from each of said RF transmitters, including the steps of:
(a) temporarily storing radio signal data to be transmitted in a buffer at each of said RF transmitters;
(b) generating a pair of resynchronization reference tones synchronized to said GPS signal;

(c) periodically resynchronizing, on a continuing basis at each of said plural transmitters, the reading of stored radio signal data to be transmitted out of said buffer at times and at a rate responsive to said pair of reference tones.

7. In a radio frequency (RF) simulcasting system of the type including a control point connected by data links to plural spatially separated radio frequency transmitters, a clocking signal synchronized to a received global positioning satellite (GPS) signal broadcast being available at said control point, an improved method of providing substantially simultaneous high speed digital data transmissions from said plural transmitters, said improved method including the steps of:
distributing said high speed digital data from said control point to said plural transmitters over said data links at timings responsive to said clocking signal available at said control point, said distributing of said high speed data including the steps of receiving and storing said high speed data in a buffer at each of said plural transmitters;
receiving a timing reference signal from a GPS broadcast at each of said transmitters;
generating timing/frequency signals at each of said transmitters synchronized to said received GPS reference signal;
generating a further clocking signal at each of said transmitters in response to said generated timing/frequency signals;
synchronizing, at each of said plural transmitters, the reading of said stored high speed digital data out of said buffer at times and at a rate responsive to said timing/frequency signals and said further clocking signal; and transmitting said read out synchronized data over RF channels.

8. A method as in claim 7 wherein said synchronizing step includes the step of periodically resynchronizing on a continuing basis said reading of said stored high speed digital data out of said buffer at a rate responsive to said timing/frequency signals.