WO1992013417A1

WO1992013417A1 - Simulcast transmission system having predetermined launch times

Info

Publication number: WO1992013417A1
Application number: PCT/US1992/000497
Authority: WO
Inventors: Bradley M. Hiben; Paul M. Bocci
Original assignee: Motorola, Inc.
Priority date: 1991-01-28
Filing date: 1992-01-21
Publication date: 1992-08-06
Also published as: AU1342892A

Abstract

A simulcast radio transmission system (250) is disclosed for transmitting message signals (318), originating from a message source (202), to a geographical area (115). The simulcast system includes a controller (204) coupled between the message source (202) and a plurality of remote transmitter sites (208, 210) for exchanging message signals (318) therebetween. Connecting the controller (204) to the plurality of transmitter sites (208, 210) are a plurality of interconnect links (224, 226), each having bounded propagation time delay characteristics. The simulcast system (250) employs a precision reference timing signal (314) for calculating a launch time, which calculation also uses a predetermined propagation time delay for determining the launch time. The controller (204) then combines (508) the message signals (318) with the launch time to establish a message bundle and sends the message bundle to at least one of the plurality of transmitters (208, 210), via at least one of the plurality of interconnect links (224, 226).

Description

SIMULCAST TRANSMISSION SYSTEM HAVING PREDETERMINED LAUNCH TIMES

Field of the Invention

The present invention relates generally to simulcast (i.e., simultaneous broadcast) radio transmission systems, and particularly to such systems which launch message signals at a predetermined time. This launch time is based on inputs from a precision timing source, such as a global positioning system (GPS) timing reference signal, and a predetermined propagation time delay within the system.

Background of the Invention

The use of simulcast transmission to increase the effective coverage area of land-mobile radio systems is well known in the art. In simulcast transmission, two or more transmitters, broadcasting identical information simultaneously on the same frequency, are located such that contiguous coverage is available over a larger area than can be covered by the transmitters acting alone. Simulcast transmission systems require that the base band signals be transmitted at a precisely controlled time. If the signal is transmitted by the various transmitters at the wrong time, distortion occurs in the area where signals from both transmitters are received with similar signal strengths. This distortion effect is present when the various signals arrive at the receiving end with even slight phase, or timing, differences with respect to each other. FIG. 1 shows a simplified graphical representation of a typical simulcast radio transmission system 100. The system comprises two base stations, or remote site transmitters 101, 103. Remote site transmitter 101 has an accompanying coverage area 111 , within which subscriber units 105, 107 are able to receive transmitted messages via transmissions 119, and 117, respectively. Similarly, remote transmitter site 103 has an accompanying coverage area 113, within which subscriber units 107, 109 are able to receive transmitted messages via transmissions 118, and 121 , respectively. Note that coverage areas 111 and 113 have, by design, an overlapping coverage area 115, within which a subscriber unit 107 receives transmissions from both transmitters. It is this overlapping coverage area 115 that incorporates simulcast technology in order to enhance the respective coverage areas of the transmitter sites involved. Accordingly, these transmissions 117, 118 are perceived by subscriber unit 107 as a single signal. FIG. 2A shows a simplified block diagram of a typical simulcast radio transmission system 200. A typical transmission sequence begins when a message source 202 (e.g., console, radio, key management center (KMC), etc.) sends a message signal to be transmitted to one or more coverage areas. Upon receipt of the message signal, the controller 204 distributes the message signal to one or more remote site transmitters (e.g. 208, 210). This distribution is typically done through an expensive microwave distribution system 206 which comprises, for example, interconnect links 214, 216. When the message signal has been received at the remote site transmitter, for example transmitter 208, the message is transmitted to an awaiting mobile subscriber unit within the coverage area for that site. Because of the critical timing requirements of simulcast transmission, the interconnect links must be precisely calibrated, or netted, using a variable delay within the receiving modem at each remote site transmitter. Such calibration maintenance is required to insure that the total propogation delays are identical across all interconnect links. Microwave distribution is used because other methods, such as telephone lines, do not maintain constant propagation delays over time, thus demanding more frequent maintenance cycles. Additionally, custom-built modems are required in order to maintain the required frequency response tracking and provide needed features. These are very costly when compared with off-the-shelf modems that are readily available. Furthermore, the system must be re-netted occasionally, which is usually performed from a central site at which signals from the transmitter site can be received and the signal delay measured. Test signals and expensive measurement equipment are used to determine signal delays for each site, while a data network is used to adjust these measured delays. Simpler, less expensive calibration systems do exist, but these do not generally perform well on systems using high speed data rates, such as those in a typical simulcast environment operating at or beyond 9600 bits/second.

Accordingly, there exists a dire need for a simulcast radio transmission system which is both capable of meeting the critical timing requirements of simulcast transmission and is not constrained by the aforementioned cost problems.

Summary of the Invention

The present invention encompasses a simulcast radio transmission system for transmitting message signals, originating from a message source, to a geographical area. The simulcast system includes a central controller coupled between the message source and a plurality of remote site transmitters for exchanging message signals therebetween. Connecting the controller to the plurality of transmitter sites are a plurality of interconnect links, each of which have bounded propagation time delay characteristics. The simulcast system employs a precision reference timing signal for calculating a launch time, which calculation also uses a predetermined propagation time delay for determining the launch time. The controller then combines the message signals with the launch time to establish a message bundle and sends the message bundle to at least one of the plurality of transmitters, via at least one of the plurality of interconnect links.

Brief Description of the Drawings

FIG. 1 is a graphical representation of a partial simulcast radio transmission system, known in the art, which shows the respective coverage areas for two remote site transmitters.

FIG. 2A is a simplified block diagram of a typical simulcast radio transmission system which is known in the art.

FIG. 2B is a simplified block diagram of a simulcast radio transmission system which employs a GPS timing reference signal scheme, in accordance with the present invention.

FIG. 3 is a simplified block diagram of the hardware components which make up a simulcast radio transmission system controller, in accordance with the present invention.

FIG. 4 is a simplified block diagram of the hardware components which make up a simulcast radio transmission system remote site transmitter, in accordance with the present invention.

FIG. 5 is a flow diagram detailing the operation of the simulcast radio transmission controller shown in FIG. 3, in accordance with the present invention.

FIG. 6 is a flow diagram detailing the operation of the simulcast radio transmission remote site transmitter shown in FIG. 4, in accordance with the present invention.

Detailed Description of a Preferred Embodiment

FIG. 2B shows a simplified block diagram of a simulcast radio transmission system 250, in accordance with the present invention. Message source 202 sends a message signal to controller 204. Upon receipt, controller 204 converts, if necessary, the received message signal, which may be in either analog or digital form, into digital form before passing it to the respective transmitters, for example transmitters 208, 210. Additionally, the present invention employs a precision timing reference signal receiver 219, 221 in order to establish the critical timing features of the simulcast radio transmission system. In the preferred embodiment of the present invention, an earth-orbiting vessel 201 is used to transmit a precision timing signal to each of the respective antennas 203, 207, 209, 211 within the system. These antenna/receiver combinations, for example antenna 203 in combination with receiver 219, are used to establish an exact, or absolute, time reference for the controller and remote site transmitters within the simulcast system. At the sending end, controller 204 calculates a "launch" time (i.e., a predetermined, exact, future time at which the buffered message data is to be sent to the transmitting end of the remote site transmitter). In the preferred embodiment of the present invention, this launch time is based on the precision timing reference signal and a predetermined propagation delay, generally slightly more than the expected maximum delay for the interconnect links in the distribution system 205. This launch time is then combined with the digital representation of the original message signal to form a message bundle, and sent, for example via interconnect links 224, 226, to one or more of the remote site transmitters 208, 210 within the simulcast radio transmission system. Similarly, antenna 207, in combination with receiver 221 , receives a timing reference signal which is then used by the remote site transmitter 208 as an exact, or absolute, timing reference. Upon receipt of the message bundle, transmitter 208, for example, strips off the launch time data and reconstructs the original message signal using the digital representation which is sent in the message bundle. A comparison is then made between the current absolute time provided by the timing reference signal receiver 221 and the launch time received from the controller. When these respective times are identical, antenna 212 then transmits the reconstructed message signal to the coverage area for that transmitter, thus ensuring substantially identical transmission times among all the active remote sites. The timing signals received from the GPS are typically phase-synchronized to within 100 nanoseconds. However, the phase difference between the resident phase-locked-loops (PLLs) at the various remote sites may add up to 325 nanoseconds, resulting in a worst- case timing difference of approximately 425 nanoseconds.

In the preferred embodiment of the present invention, a highly accurate and precise timing source is used, such as that found in a global positioning system (GPS). Since this absolute time reference is independently sent to both the sending (controller 204) and receiving

(remote site transmitters 208, 210) ends, the interconnect links 224, 226 which make up distribution system 205, may include inexpensive, time-variant medium, for example public switched telephone network (PSTN) lines.

Also, inexpensive, off-the-shelf modems may be used for establishing sending and receiving end protocols. In addition to saving hardware costs, use of such a distribution system 205 provides a system which does not require the costly maintenance of a typical microwave distribution system.

FIG. 3 shows a simplified block diagram of the internal components of a controller 204. The GPS antenna 203 is used to receive precision timing reference signals from an earth-orbiting vessel 201. Frequency reference signal 312 (F_ref) and timing reference signal 314 (T_ref) are then generated by the GPS timing receiver 219. The F_ref signal is then used as an input to a typical PLL circuit 302 in order to generate a clock input signal 316, which is then used as a timing input to clock generator 304. Clock generator 304 uses the clock input signal 316 and the T_ref signal 314 to update the internal absolute time clock, which serves to synchronize the clocks within the various remote sites, generally to within 425 nanoseconds. Furthermore, clock generator 304 sends a master synchronization input signal 320 to the combiner 306 which is used, along with T_ret signal 314, to produce a time stamp to be combined with the message signal 318. Also, clock input 316 is used as an input to a divide-by-n circuit 310 which then establishes the data rate at which the message bundle is transferred onto the distribution system 205. In the preferred embodiment of the present invention, the frequency of the clock input signal 316 is 3.072 MHz which, by using a divide-by-320 circuit, provides a 9.6 kHz data rate on the distribution system 205. Of course, any combination of frequencies and circuitry may be used to produce the desired results for a given distribution system. Furthermore, this data rate clocking scheme may be provided by the distribution system itself.

FIG. 4 details one hardware implementation of a remote site transmitter 208, in accordance with the present invention. Similarly to the controller embodiment, the GPS receiver 221 , upon receipt of the GPS precision timing reference signal, generates the frequency reference signal 405, and the timing reference signal 407. The frequency reference signal 405 is used, in combination with a typical PLL circuit 425, to generate the site frequency reference for the transmitter 208, which may, as in the preferred embodiment, operate at 14.4 MHz. As in the controller, the frequency reference signal 405 is also used, in combination with a PLL circuit 427, to produce a clock input signal 409 for the clock generator 403. Such a clock generator produces a master synchronization input 411 , a convert clock input signal 413, and a receive data clock signal 415 which, in the preferred embodiment, are used to provide synchronizing and clock data to the signal processing hardware; processor 417 and D/A converter 419 (e.g.,Motorola codec Model No. MC145402).

Upon arrival of the message bundle at the remote site transmitter 208, processor 417 separates the message bundle into its sub-components; digital message data representing the message signal and launch time data.

Note that the message bundle may also include additional information, such as control data, diagnostic data, etc.

The digital message data is then placed into a data buffer

401 , which may be a first-in-first-out (FIFO) buffer. The message data is held in this data buffer until the current time of day, provided by manipulation of the timing reference signal 407, matches exactly with the launch time provided in the message bundle. When this condition exists, the message data is passed to the D/A converter 419 to reconstruct the original message signal. The reconstructed message signal is then sent, via transmitter

421 , out to the coverage area defined by the location of that remote site transmitter. FIG. 5 shows a flow diagram 500 outlining the operation of the controller 204. The sequence begins when a message signal is gathered at 502. Using an established timing reference signal as an absolute time reference, and the maximum propagation delay possible over a given interconnect link, the controller then calculates at 506 a launch time, in part, by summing these parameters. The controller then combines the message signal with the launch time at 508 to generate a message bundle to be sent out on the distribution system. The message bundle is then sent at 510, via at least one interconnect link, to one or more of the remote site transmitters, before the routine is exited at 512. In an alternate embodiment of the present invention, the launch time may be sent via a common signalling channel to the remote site transmitters, for example via an over-the-air transmission between controller 204 and transmitter 208 through antenna 212. This type of transmission may be done either in addition to the message bundle described herein, or in place of having the launch time embedded in a message bundle.

The operation of the remote site transmitter is shown in FIG. 6 as flow diagram 600. Upon receipt of the message bundle, the remote site transmitter separates at 601 the bundle into launch time data and digital message data, and removes all other data which is unnecessary to the simulcast transmission (e.g., dignostics, control information, etc.). The digital message data is then stored at 602, which storing may be accomplished, for example, using a first-in-first-out (FIFO) buffer. The launch time data is then recorded at 603. The current absolute time is determined at 607 using the precision timing reference signal before the routine goes to decision 609, where it is determined whether or not the current absolute time is the same as the launch time. If not, the remote site transmitter routine returns to process 607 to get an updated current time, after which it returns to decision 609. If the absolute time is equivalent to the launch time, the message signal is reconstructed at 605 to an 5 equivalent form using the digital message data. The remote site transmitter then transmits at 613 the reconstructed equivalent of the message signal, before the routine is exited at 615.

10 What is claimed is:

Claims

1 ) A simulcast radio transmission system for transmitting message signals, originating from a message source, to a geographical area, the simulcast radio transmission system comprising:

a controller coupled to the message source and a plurality of remote site transmitters for exchanging message signals therebetween;

a plurality of interconnect links, coupled between said controller and said plurality of remote site transmitters, having bounded propagation time delay characteristics;

means for receiving a precision reference timing signal;

means for calculating a launch time based on said precision reference timing signal and a predetermined propagation time delay;

means, coupled to said controller, for gathering the message signals and combining said gathered message signals with said launch time to establish a message bundle; and

means for sending said message bundle to at least one of said plurality of remote site transmitters, via at least one of the plurality of interconnect links.

2) A simulcast radio transmission system in accordance with claim 1 , wherein said predetermined propagation time delay is substantially equal to a maximum propogation time delay associated with said plurality of interconnect links.

3) A simulcast radio transmission system in accordance with claim 1, wherein said means for receiving a precision reference timing signal comprises a suitable navigation receiver such as a Global Positioning System (GPS) receiver.

4) A controller for use in a simulcast radio transmission system, which controller is linked to a message source and coupled, via a plurality of interconnect links having bounded propagation time delay characteristics, to a plurality of transmitters for transmitting message signals to a geographical area, the controller comprising:

means for gathering the message signals from the message source;

means for receiving a precision reference timing signal;

means for calculating a launch time based, at least in part, on said precision reference timing signal and a predetermined propagation time delay;

means for combining said gathered message signals with said launch time to establish a message bundle; and

means for sending said message bundle to at least one of the plurality of remote site transmitters, via at least one of the plurality of interconnect links.

5) A controller in accordance with claim 4, wherein said means for receiving a precision reference timing signal comprises a suitable navigation receiver such as a Global Positioning System (GPS) receiver. 5

6) A remote site transmitter for use in a simulcast radio transmission system, which remote site transmitter is coupled to a central controller which receives from a message source a message signal to be transmitted to a geographical area, the remote site transmitter receiving from the central controller a message bundle, via an interconnect link, the remote site transmitter comprising:

means for separating the message bundle into digital mesage data and a launch time;

means for storing said digital message data;

means for recording said launch time;

means for reconstructing an equivalent of the message signal using said digital message data;

means for receiving a precision reference timing signal;

means for determining a current time using said precision reference timing signal;

means for comparing said current time with said launch time; and

means, coupled to said means for comparing, for transmitting said reconstructed equivalent of the message signal to the geographical area.

7) A remote site transmitter in accordance with claim 6, wherein said means for receiving a precision reference timing signal comprises a suitable navigation receiver such as a global positioning system (GPS) receiver.

8) A method of sending message signals from a controller in a simulcast radio transmission system, which controller is coupled to a message source and connected, via a plurality of interconnect links having bounded propagation time delay characteristics, to a plurality of remote site transmitters, each transmitter sited for transmitting message signals to a particular geographical area, the method comprising the steps of:

gathering the message signals from the message source;

calculating a launch time based, at least in part, on a received precision reference timing signal and a predetermined propagation time delay;

combining said gathered message signals with said launch time to establish a message bundle; and

sending said message bundle to at least one of the plurality of transmitters, via at least one of the plurality of interconnect links.

9) A method of transmitting message signals from a remote site transmitter in a simulcast radio transmission system, which transmitter is connected, via an interconnect link, to a controller which receives a message signal from a message source and sends a message bundle to said remote site transmitter, via said interconnect link, the method comprising the steps of:

separating the message bundle into digital message data and a launch time;

storing said digital message data;

recording said launch time;

reconstructing an equivalent of the message signal using said digital message data;

determining a current time based, at least in part, on a received precision reference timing signal;

comparing said current time with said launch time; and

transmitting, responsive to said step of comparing, said reconstructed equivalent of the message signal to the geographical area.

10) A method of sending message signals from a controller in a simulcast radio transmission system, which controller is coupled to a message source and connected, via a plurality of interconnect links having bounded propagation time delay characteristics, to a plurality of remote site transmitters, the controller further being coupled to a common communication resource, the method comprising the steps of:

gathering the message signals from the message source;

sending said launch time to at least one of the plurality of remote site transmitters, via the common communication resource; and

sending said gathered message signals to at least one of the plurality of r emote site transmitters, via at least one of the plurality of interconnect links.