WO1996020543A1 - 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

SIMULCAST RESYNCHRONIZATION IMPROVEMENT USING GLOBAL POSITIONING SYSTEM

CROSS-REFERENCES TO RELATED APPLICATIONS ANP PATENTS

This application is somewhat related to commonly-assigned U.S. Patent No. 5,172,396 to Rose et al., issued on December 15, 1992, entitled "Public Service Trunking Simulcast System," and U.S. Patent No. 4,903,321 to Hall et al., issued on February 20, 1990, entitled "Radio Trunking Fault Detection System". This application is also somewhat related to the following commonly-assigned copending applications: serial number 07/824,123 of Brown et al. entitled "Self Correction Of PST Simulcast System Timing", filed 22 January 1992 (Attorney Docket Number 46-444; Client Reference No. 45-MR-664) and serial number 07/906,438 of Thomas A. Brown entitled "Control Channel Timing Detection and Self Correction For Digitally Trunked Simulcast Radio Communication System", filed 30 June 1992 (Attorney Docket Number 46-594; Client Reference No. 45-MR-711). The disclosures of each of the above patents and applications are incorporated by reference as if expressly set forth herein.

FIELD OF THE INVENTION

This invention relates to radio frequency (RF) signal transmission systems, and in particular to "simulcasting" systems for providing the simultaneous transmission of the same information by two or more separately located RF transmitters. More particularly, the invention relates to an improved method and apparatus for generating simulcast timing "resynch" (resynchronization) reference signals at each transmitter site to maintain coherency of transmissions. 96/20543 PCΪYUS95/ 16847

2 BACKGROUND AND SUMMARY OF THE INVENTION

As is well known, due to FCC power limitations, geographical and/or other factors, it is sometimes not possible for a single RF transmitting site to provide adequate coverage to a large desired coverage area. For example, government entities commonly use land-mobile radio communications systems to provide communications between a headquarters and various mobile and portable radio users that rove throughout the jurisdiction of the governmental entity. In some cases the geographical area of jurisdiction is so large that it is not possible for a single land-based RF transmitting site to cover it. Even if the effective radiated power of the single transmission site was sufficiently great to cover the entire area, users in outlying or fringe areas might receive only spotty service because of the "line-of-site" nature of VHF transmissions and/or due to geographical obstructions (e.g., hills, bridges, buildings, and the curvature of the earth) interposed between the single transmitter site and various fringe locations within the coverage area.

One known way to expand the coverage area is to provide multiple, "simulcasting" transmitting sites. In order to simplify mobile radio operation and conserve radio frequency spectrum, such "simulcasting" RF transmitting sites all transmit substantially identical signals at substantially identical times on substantially identical radio frequencies. Such "simulcasting" eliminates control overhead and other complexities 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" digitally trunked RF repeater systems are generally known. The following is a listing (which is by no means exhaustive) of prior documents that describe various aspects of RF transmission simulcasting and related issues:

U.S. Patent No. 5,172,396 to Rose et al.;

U.S. Patent No. 4,903,321 to Hall et al.;

U.S. Patent No. 4,696,052 to Breeden; U.S. Patent No. 4,696,051 to Breeden; U.S. Patent No. 5,245,634 to Averbuch; U.S. Patent No. 5,287,550 to Fennell et al; U.S. Patent No. 4,782,499 to Clendening; U.S. Patent No. 5,052,028 to Zwack; U.S. Patent No. 4,570,265 to Thro; U.S. Patent No. 4,516,269 to Krinock; U.S. Patent No. 4,475,246 to Batlivala et al.; U.S. Patent No. 4,317,220 to Martin; U.S. Patent No. 4,972,410 to Cohen et al.; U.S. Patent No. 4,608,699 to Batlivala et al.; U.S. Patent No. 4,918,437 to Jasinski et al.; U.S. Patent No. 4,578,815 to Persinotti; U.S. Patent No. 5,003,617 to Epsom et al.; U.S. Patent No. 4,939,746 to Childress; U.S. Patent No. 4,903,262 to Dissosway et al.; U.S. Patent No. 4,926,496 to Cole et al.; U.S. Patent No. 4,968,966 to Jasinski et al; U.S. Patent No. 3,902,161 to Kiowaski et al; U.S. Patent No. 4,218,654 to Ogawa et al; U.S. Patent No. 4,255,815 to Osborn; U.S. Patent No. 4,411,007 to Rodman et al; U.S. Patent No. 4,414,661 to Karlstrom; U.S. Patent No. 4,472,802 to Pin et al.; U.S. Patent No. 4,597,105 to Freeburg; and Japanese Patent Disclosure No. 61-107826.

U.S. Patent No. 5,172,396, issued December 15, 1992 to Rose et al., entitled "Public Service Trunking Simulcast System", discloses a trunked radio simulcast system having control site and remote site architectures that include RF transmission timing synchronization features that are relevant to the presently preferred exemplary embodiment. In addition, U.S. Patent No. 4,903,321, issued February 20, 1990 to Hall et al., entitled "Radio Trunking Fault Detection System," discloses a trunked radio repeater system having a radio frequency repeater site architecture that includes fault and call testing and failure detection features that are somewhat relevant to the present invention. These patents are both commonly assigned to the assignee of the present invention and are both incorporated by reference herein.

While simulcasting thus provides various advantages as compared to other techniques for expanding coverage area, it also introduces its own particular set of complexities that must be dealt with. By way of illustration, please refer to FIGURE 1 ~ which is a schematic diagram of an exemplary three-site simulcasting digitally trunked land-mobile RF communications system 10. System 10 includes three simulcasting transmitting sites, SI, S2 and S3. The transmissions of site SI cover the coverage area Al, and similarly, the transmissions of sites S2 and S3 cover respective coverage areas A2, A3. A central control point C coupled to each of sites S I, S2 and S3 via respective communication links (L1-L3) delivers, in real time, substantially identical signalling (including digital control channel signalling and associated timing information) for transmission by the various sites.

Each RF channel at all sites is modulated with 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 system arrangement (e.g., 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 grade, synchronous multiplex used in conjunction with radio, microwave or fiber optic back¬ bone distribution paths most effectively do provide the needed communications circuits and stabilitv for simulcast. Exemplary system 10 is preferably a digitally trunked simulcast communications system of the type marketed by Ericsson-GE Mobile Communications Inc. (EGE) under the trade name EDACS. This system provides a digital RF control channel and plural RF working channels. In such a digitally trunked system, an exemplary mobile radio unit M within one (or more) of coverage areas A1-A3 continuously monitors an "outbound" digital control channel when it is not actually engaged in active communications on a working channel with other units. Mobile M may request communications by transmitting a channel assignment request message on an "inbound" control channel. Upon receipt of such channel assignment request (and presuming that at least one working channel is available for temporary assignment to mobile unit M and other units with which mobile unit M wishes to communicate), control point C responds by causing a control channel assignment message to be transmitted by each site S1-S3 over the outbound control channel. In simulcast system 10, this channel assignment message is transmitted simultaneously by each of transmitting sites SI -S3 over the same outbound control channel frequency (such that mobile unit M and other mobile units "called" by the channel assignment message will receive the message regardless within which coverage areas A1-A3 they may happen to be located). Mobile unit M (and other called mobile units) respond to the received outbound trunking control channel assignment message by changing frequency to an RF working channel and conducting communications on the working channel. Once the working channel communications are concluded, the mobile unit M (and other called mobile units) return to monitoring the outbound control channel for additional messages directed to them.

Referring once again to FIGURE 1, suppose mobile unit M is located within an overlap area X wherein coverage areas A2 and A3 overlap one another. Within this overlap area X, mobile unit M will receive (perhaps at approximately equal signal 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 desianed such that such outbound control channel transmissions from sites S2 and S3 are on substantially the same RF frequency so that no heterodyning or other interference occurs. Similarly, control point C sends, over links L1-L3, substantially identical outbound control channel messages for transmission by each of sites SI -S3.

However, a problem can arise if the outbound control 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 52 microseconds for 9600 baud operation) could end up losing bits and/or temporarily losing synchronization, bit recovery and error checking capabilities.

Delays due to the limited speed at which electromagnetic waves propagate must be taken into account in systems simulcasting data at high data transmission rates (an RF signal 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 delay times across an overlap region due to the different RF path lengths between the site and a receiver within the overlap region is minimized. Even with this optimization, 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 ±5 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 52 microseconds of one another.

Fortunately, it is typically possible to minimize time delay differences to on the order of a microsecond through various known techniques. For example, it is well known in the art to introduce adjustable delay networks (and phase equalization networks) in line with some or all of inter-site links L1-L3 to compensate for inherent differential link delay times (see U.S. patent 4,516,269 to Krinock, and U.S. patent O 96/20543 PC17US95/16847

numbers 4,696,051 and 4,696,052 to Breeden, for example). Conventional microwave and fiberoptic link channels exhibit amplitude, phase and delay characteristics that are extremely stable over long periods of time (e.g., many months), so that such additional delays, once adjusted, guarantee that a signal input into all of the inter-site links L1-L3 at the same time will arrive at the other ends of the links at almost exactly the same time. The same or additional delays can be used to compensate for different, constant delay times introduced by signal processing equipment at the sites SI -S3 to provide simultaneous coherent transmission of the signals by the different sites. For example, the above-identified Rose et al. patent application describes a technique wherein additional frequency and timing information is provided to each site over one or more particular inter-site link channels so as to eliminate timing ambiguities that may result from the use of conventional multi-level, multi-phase protocol-type modems. In this manner, the above mentioned simulcast system forces coherence at the start of data transmission 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 transmission system of the type described in accordance with the above mentioned Rose et al. patent, a "master" resynch (resynchronization signal) circuit 100 located at control point site C produces reference edges/tones, e.g., at 2400 Hz and 300 Hz, that are sent to each transmit site (S1-S2) on a dedicated channel over the inter-site communication links (L1-L2). Digital and voice data aligned to these reference signals is also sent via the communication links (L1-L2) between control point C and the transmit sites (S1-S2). 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 clocking frequency reference. Each transmit site (S1-S2) 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-aligns the broadcast data received via the inter-site links to these reference edges. Consequently, as previously mentioned above, it is required 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 transmitter 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, 123 to Brown et al., filed January 22, 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 performed working channel "test call" (e.g., as described in U.S. Patent 4,903,321 to Hall et al.) fails.

With respect to the above mentioned EGE simulcast systems, applicant has disclosed in the instant 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 providing a global 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 navigational purposes, is a series of satellites synchronized in time and continuously transmitting, inter alia, time, date and positioning information. Although, cellular radiotelephone system sites using the GPS for providing absolute timing is known, such systems admit to numerous inherent problems which render these systems expensive and unreliable (see, for example, U.S. Patent 5, 245,634 to Averbuch). In accordance with the present invention, an improved multiple site RF simulcast system is achieved using GPS system broadcast signals in conjunction with a particular resynchronization circuitry arrangement at each simulcast site which overcomes many of the drawbacks of previous GPS synchronized 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 (LI, L2, etc.). Instead, each simulcast site includes a "universal" (generic) EGE simulcast system "resynch" circuit and simulcast data resynchronization is periodically performed on a routine basis by each system site separately and independently. In a preferred exemplary embodiment, a stable, precise 9600 bps data clock reference and the lower frequency "gating" signal (e.g., 300Hz as described above) are derived separately at each simulcast system site from a global positioning satellite (GPS) broadcast transmission acquired by using 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 links are no longer needed for distributing "resynch" reference signal tones. In addition, because the EGE "resynch" operation will force alignment to the correct simulcast system timing, any variation in site interconnect link latency is automatically corrected whenever a "resynch" operation is performed (so long as the latency variation is within the "gating" signal 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 latency change and/or storage of link timing parameters. Moreover, employing a GPS receiver in combination with an EGE universal "resynch" circuit in an arrangement to generate "resynch" reference tones for the resynch circuit and autonomously aligning the data broadcast at each site in accordance with the present invention further improves simulcast operations by reducing timing jitter and eliminating the need for alignment checks on reference tone polarity. Because fine tuning a particular multiple site simulcast system arrangement often requires specific adjustments of the broadcast timing at one or more sites off of the "nominal" equal timing, a method of producing such timing offsets must provided. In accordance with a further aspect provided by the present invention, the GPS receiver utilizes a delay unit that provides adjustable signal delays of ±125μs in incremental 0.5μs steps and which is accessible via an RS-232 port. Accordingly, an additional advantage is achieved by the present invention in that simulcast system fine tuning timing adjustments can be made remotely via an RS-232 link.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will become more completely understood by referring to the following detailed description of presently preferred exemplary embodiments in conjunction with the FIGURES in which like reference numerals refer to like elements throughout:

FIGURE 1 is a general schematic illustration of a simplified exemplary multiple site RF communication simulcast system;

FIGURE 2 is a general schematic block diagram of the central control point C and remote transceiver sites S 1 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 general schematic block diagram of an exemplary arrangement of a modified multiple site RF simulcast communication system using GPS receivers to improve the resynchronization reference signal generation at every site.

DETAILED DESCRIPTION OF THE DRAWINGS In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well known methods and programming 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 system as described above is shown in FIGURES 1 and 2 — that is, it includes a central control point C and plural transmitting sites Sl...Sn. Although only two (remote) transmitting sites S1-S2 are shown in FIGURES 2 and 3, it will be appreciated by one skilled in the art that numerous remote sites participating in simulcasting are likewise in communication with control point C via identical microwave, fiber-optic, cable or land-line communication path links Ll-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 transmission system, data provided via the inter-site communication links (LI, L2, etc.) from control point C to the RF transmitter sites (SI, S2, etc.) exhibits random time delay skew because multi¬ phase link modems (MC, Ml, M2, 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, M2, etc. associated with the modem at each site. Timing information provided to each site from control point C via link channels initially sets the memory buffer output timing at each site to eliminate transmission timing ambiguities. On a continual basis, resynch circuitry at the sites periodically resynchronizes memory buffer output timing in accordance with a pair of reference frequency tones continuously 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 communication simulcast system in accordance with the present invention is discussed. GPS receivers 301, 301a and 301b at all sites (including control point C) provide a reference signal acquired from a common GPS timing signal broadcast via satellite 300. Tone generator circuits 302, 302a, 302b at each site utilize the received GPS timing reference signal to generate a high frequency 9600 bps data clocking reference tone and a low frequency data "gating" (timing) tone that are used by universal resynch circuits 303, 303a, 303b for performing the periodic resynch operations. In accordance with the present invention, a gating signal frequency much lower than, for example, 300Hz may be used as long as the frequency chosen is an integral submultiple of the data'stream frame timing (i.e., the data frame period divided by the gating frequency period) and a submultiple of 9600 (e.g., using 100Hz provides a gating period of 10 ms; 60Hz provides a gating period of 16.6 ms; etc). Gating frequency is preferably selected based on expected system link latency variations.

Resynchronization reference tones are not sent to transmission sites via the inter-site communication links from a designated control point site as in the exemplary EGE simulcast systems discussed above. Consequently, additional phase-stable, delay- compensated channels for resynch signals are not required. Instead, a resynch operation is periodically performed on a continuing basis by universal resynch circuits 303, 303a, 303b separately at each simulcast system site using the resynch reference tones derived from the received GPS signal by tone generator circuits 302, 302a, 302b. Retimed (realigned) data is thereby provided on a continuing basis to channel transmitter(s) 304a, 304b at each site for simultaneous RF transmission. In addition, because a resynch operation forces data stream alignment to the correct simulcast system timing, any inter-site interconnection link latency variation (within the "gating" signal timing window) will be automatically corrected whenever the resynch operation is performed. Moreover, this modified arrangement for providing resynchronization reduces timing jitter and eliminates the need for alignment checks on reference tone polarity.

As also depicted in FIGURE 3, GPS receivers 301, 301a, 301b include a delay unit that provides adjustable signal delays of ±125μs in incremental steps of 0.5μs and which is controllable via an RS-232 port. Accordingly, an additional advantage is achieved by the present invention in that simulcast system fine tuning timing adjustments can be made remotely via an RS-232 link.

While 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 intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

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 2 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.