United States Patent [191
`Witsaman et al.
`
`US005365569A
`Patent Number:
`[11]
`[45] Date of Patent:
`
`5,365,569
`Nov. 15, 1994
`
`[54]
`
`[75]
`
`[73]
`
`[21]
`[22]
`[51]
`[52]
`[53]
`
`[56]
`
`DIGITAL SIMULCAST TRANSMISSION
`SYSTEM
`
`Inventors: Mark L. Witsaman; Roger E. Benz;
`David W. Glessner; Joel R.
`Crowley-Dierks, all of Quincy, Ill.
`
`Assignee: Glenayre Electronics, Ltd.,
`Vancouver, Canada
`
`Appl. No.: 931,789
`
`Filed:
`
`Aug. 17, 1992
`
`Int. Cl.5 ..................... .. H04M 11/00; H04B 1/00
`U.S. Cl.
`.
`..
`............... .. 379/57; 455/51.2
`Field of Search ..................... .. 455/ 16, 51.2, 56.1,
`455/72, 12.1, 13.2, 13.1; 379/56, 57, 1, 9, 27,
`111, 279; IMO/825.44; 375/107, 38, 100;
`370/951, 95.3, 94.1, 94.2, 84
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,709,402 11/1987 Akerberg ......................... .. 455/5l.2
`4,849,993 7/1989 Johnson et a1.
`.... .. 375/108
`
`4,850,032 7/1989 Freeburg . . . . . . .
`
`. . . .. 455/5l.2
`
`340/82544
`4,968,966 11/1990 Jasinski et al. .
`. 379/59
`5,023,900 6/1991 Tayloe et a1. ..
`455/51.2
`5,060,240 10/1991 Erickson et al.
`345/107
`5,077,759 12/1991 Nakahara ......... ..
`375/4
`5,090,025 2/1992 Marshall et a1.
`375/38
`5,153,874 10/1992 Kohno ............. ..
`5,162,790 11/1992 Jasinski ....................... .. 455/16
`5,220,676 6/1993 LoGalbo et a1 .... ..
`455/ 13.2
`5,257,404 10/1993
`
`FOREIGN PATENT DOCUMENTS
`0228237 9/1989 Japan
`375/107
`0470027 3/1992 Japan ................................. .. 375/107
`Primary Examiner-Curtis Kuntz
`Assistant Examiner—Michael B. Chernoff
`Attorney, Agent, or Firm-—Christensen,O’Connor,
`Johnson & Kindness
`[57]
`ABSTRACT
`A simulcast system for broadcasting the same signal for
`a number of spaced-apart broadcast sites is disclosed.
`The system (20) of this invention includes a hub (28)
`adapted to receive the signal to be simulcast and a num
`ber of stations (30) that actually broadcast the signal.
`The hub places the digital signal packets referred to as
`PDBs (36). As part of the signal packetization process,
`the hub evaluates the rate at which the signals should be
`broadcast and assigns a start time at which the signals
`should be broadcast; this information is attached to the
`PDBs. After a PDB is created it is forwarded to the
`stations over a link channel. Each station includes a
`station controller (32) and a station transmitter (34). The
`station controller, upon receiving a PDB, forwards the
`signal therein to the transmitter at the start time indi
`cated and at the appropriate broadcast rate. All of the
`stations transmitting the signals contained within a sin
`gle PDB do so based on the start time and broadcast
`rate information contained in that PDB. Consequently,
`each station broadcasts the same signals at the same time
`and at the same rate so as to ensure their simultaneous
`transmission.
`
`47 Claims, 21 Drawing Sheets
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`U.S. Patent
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`US. Patent
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`Nov. 15, 1994
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`US. Patent
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`Nov. 15, 1994
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`US. Patent
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`Nov. 15, 1994
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`Nov. 15, 1994
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`U.S. Patent
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`Nov. 15, 1994
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`DIGITAL SIMULCAST TRANSMISSION SYSTEM
`
`FIELD OF THE INVENTION
`
`This invention relates generally to simulcast transmis-
`sion systems and, more particularly, to a simulcast trans-
`mission system for broadcasting signals that are for-
`warded to transmitting sites over one or more digital
`signal communication networks.
`
`BACKGROUND OF THE INVENTION
`
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`
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`Simulcasting is the practice of broadcasting a single
`radio-frequency signal from multiple locations at the
`same time. Simulcast transmission networks are estab-
`lished when it is desirable to maximize the area over
`which receivers tuned to the network’s broadcast fre-
`quency are able to pick up and process the broadcast
`signal. A paging system is one type of radio system that
`is operated as a simulcast system. In a paging system,
`system subscribers are provided with small radio re-
`ceivers, called pagers. The paging system further in-
`cludes one or more paging terminals and a number of
`transmitter sites. The paging terminals are connected to
`the publicly switched telephone network and receive
`calls for the individual system subscribers. In response
`to receiving an incoming call, a paging terminal will
`generate a message, a page. The page is forwarded to
`the transmitter sites, which broadcast the page for re-
`ceipt by the subscriber’s pager. When a page is received
`by a pager for the subscriber to whom the pager is
`assigned, an annunciator or display integral with the
`pager is actuated to inform the subscriber of the call.
`Paging systems are provided with multiple, spaced-
`apart transmitter sites to maximize the coverage area in
`which a pager can function.
`The individual transmitter sites of a paging system
`and of other simulcast networks must operate in concert
`so as to transmit the same signal at the exact same in-
`stant. This is important because pagers or other receiv-
`ers in areas where signals from two or more transmit-
`ting sites can be received will receive signals from each
`transmitter site. If the signals are out of phase, their sum
`produces a single signal that frequently cannot be pro-
`cessed by the receiver. Thus, paging systems and like
`broadcast systems are typically constructed so that each
`transmitter site broadcasts the same signal simulta-
`neously. This ensures that in overlap areas the signals
`from multiple transmitter sites will be in phase and
`combine to produce a single signal that can readily be
`processed by the intended receiver.
`Many simulcast systems have some type of analog
`signal link network between the central station, from
`which the signal to be broadcast originates, and the
`individual transmitter sites from which the signal is
`finally broadcast. A link network may take the form of
`a telephone or fiber-optic signal link between the cen-
`tral station and one or more particular transmitter sites.
`A link may alternatively take the form of a radio link
`between the central station and the transmitter site over
`a carrier frequency difierent from that over which the
`transmitter sites actually broadcast the simulcast signals
`The links to some transmitter sites may actually be a
`multi-link connection. For example, it is not uncommon
`for a central station to first forward the signal to be
`simulcast to a satellite transmitter. The signal is sent up
`to a satellite, which retransrnits it to one or more trans-
`mitter sites. In some of these systems each transmitter
`site includes a delay circuit that regulates when the
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`5,365,569
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`received signals are to be rebroadcast. Collectively, the
`individual delay circuits are set to ensure that the trans-
`mitter sites associated therewith all broadcast the same
`signal at the same time.
`While analog systems have proved useful for simul-
`cast applications, they are not without disadvantages.
`Some analog simulcast systems require significant
`amounts of air time to constantly send new delay rate
`instructions to the individual transmitter sites to ensure
`that they all transmit the same signal. This “overhead”
`air time may significantly depreciate the amount of air
`time that is available to forward signals containing use-
`ful data, such as paging signals, to the transmitter sites
`for rebroadcast. Furthermore, the actual transmission
`delay time for any transmit station is a function of the
`link propagation time, the time it takes the signals to
`travel from the central station to the transmit station. If
`this link should change, as may happen because of an
`intermediate component failure, or due to a change in
`the day-to-day economics of running particular links,
`then the link propagation time will change. For exam-
`ple, the satellite receiver at one transmitter station may
`be taken out of service for maintenance; in order to
`maintain the transmission of simulcast data to the site, a
`link over the publicly switched telephone network may
`be temporarily established. Until a new link propagation
`time is forwarded to a transmitter site, that site will
`broadcast the page at a time out of phase with those
`broadcast by the surrounding sites. Still another disad-
`vantage of many simulcast transmission systems is that
`they require maintenance receivers to continually moni-
`tor the delay between the time a signal is first forwarded
`by a central unit and the time it is finally broadcast by a
`transmitter site; this information is then used by the
`system’s control circuitry to regularly adjust the trans-
`mission delay times for the individual transmitting sites.
`A further limitation associated with analog linking
`networks is that in each step of the central station-to-
`transmitter site signal transfer, there is degradation of
`the signal. This happens as a result of the normal signal
`loss that occurs when an analog signal is processed by
`an amplifier and that occurs as a consequence of the
`broadcast and transmission of a radio signal. Usually,
`the longer the link between the central station and the
`transmitter site and/or the more times the signal is sub-
`jected to intermediate processing, the greater the degra-
`dation from the original signal. As a consequence of this
`degradation, the final signal that is received by the
`transmitter site for simultaneous broadcast may be so
`changed that the actual broadcast signal is unintelligible
`by the receivers for which it was intended.
`
`SUMMARY OF THE INVENTION
`
`This invention relates generally to a new simulcast
`system for linking remote transmitter sites to a central
`site and for ensuring that the transmitter sites all broad-
`cast the sa.me signal at the same time. More particularly,
`this invention is related to a simulcast transmission sys-
`tem that transmits data in digital format to the system
`transmitter sites and wherein integral with the data are
`instructions that direct all the transiriitter sites to broad-
`cast the data at a specific time and in a specific format.
`The simulcast transmission system of this invention
`includes a central unit, called a hub, that in a paging
`system environment is configured to accept the pages
`formed by one or more paging terminals. There are also
`a number of transmitter sites, called stations, that accept
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`pages from the hub and rebroadcast them for general
`reception by the pagers. The hub receives, or captures,
`paging signals generated by the paging terminals. Based
`on ancillary information generated with the paging
`signals, and on the characteristics of the paging signals
`themselves, the hub determines the particular format of
`the paging signals that are generated by the paging
`terminal. Specifically, the hub determines if the signals
`are in analog or digital format. If the signals are in digi-
`tal format, the hub determines the time period of each
`signal, which is referred to as its bit duration. Regard-
`less of the form of the pages upon leaving the paging
`terminal, the hub repackages the pages into a digital
`signal form wherein all of the signals have the same bit
`duration. The signals are placed in a packet called a
`paging data block (PDB). At the head of each PDB, the
`hub places a block of control instructions indicating the
`start time at which the signals therein should be broad-
`cast and data from which the rate of broadcast of the
`signals can be determined. The PDBs are then sent to
`the individual stations over one or more link channels,
`which serve as the hub-to-station signal pathways. The
`PDBs are sent over the link channels to the stations at a
`rate faster than the rate at which the signals therein are
`broadcast.
`The stations each include a station controller that is
`adapted to receive the PDBs over the link channels and
`a transmitter for broadcasting the signals contained in
`the PDBs. When a station receives a PDB, the station
`controller strips away the control information and for-
`wards the paging signals to the transmitter. Based on
`the control information, the station controller then reg-
`ulates the mode in which the signals are broadcast,
`when the transmitter broadcasts the paging signals. and
`the rate at which they are broadcast.
`In the simulcast system of this invention, each station
`controller receives the same PDB. Each station con-
`trols the broadcast of data from the associated transmit-
`ter based on a common start time and baud rate informa-
`tion. Consequently, all the stations will broadcast the
`same information at the same time. This ensures that
`receivers, the pagers, in areas where signals from multi-
`ple stations are picked up, will not receive overlapping
`signals that produce an unintelligible composite signal.
`There are a number of advantages to the simulcast
`system of this invention. The start time for each PDB is
`established by the hub and need be for only a time later
`than the time the last station will receive PDBs. Other
`than this requirement, the start time is independent of
`the time it takes the PDBs to propagate to the stations
`over the link channels. Consequently, a change in hub-
`to-station PDB propagation time for any single station
`will not affect that station's ability to broadcast informa-
`tion simultaneously with the other stations. Moreover,
`the PDBs are sent to the stations over the link channels
`at a rate faster than the rate at which the pages therein
`are to be broadcast. This makes it possible to use the link
`channels as a medium for other communication, such as
`the sending of commands and configuration information
`to the stations, without infringing on the time needed to
`forward the pages to the stations for broadcast.
`The system of this invention also operates indepen-
`dently of the characteristics of the signals it is used to
`broadcast. For example, whenever the data rate of the
`signals to be broadcast changes, or the nature changes
`from digital to analog format, the system automatically
`sends the signals in a PDB with appropriate transmis-
`sion control information. On receipt by the station con-
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`trollers, the data are automatically simulcast at the ap-
`propriate time, and in the appropriate format based on
`that control information. Thus, this system is well suited
`to simulcast signals from paging terminals and like units
`that generate signals having widely varying characteris-
`tics, such as changing baud rates, and/or that may
`change from analog to digital format.
`This simulcast system further allows PDBs to be
`forwarded to the individual stations over two or more
`link channels. Processing equipment at the individual
`station controllers can then selectively transmit the data
`from the first error-free PDB received, or from the
`packet that has the fewest errors. Link spreading can be
`employed to forward PDBs from the hub to a particular
`station so that the station will receive some PDBs over
`a first link channel and the remainder of its PDBs over
`one or more additional link channels. Link spreading is
`useful if, for example, economics or other factors make
`it difficult to send all the PDBs to a particular station
`over a single link channel. Still another advantage of
`this feature of the invention is that it allows the system
`to be configured so that there is always one or more
`secondary link channels over which PDBs can be for-
`warded to the individual stations in the event that the
`primary link channels fail. Should such failure occur,
`the station controllers will receive PDBs over the sec-
`ondary link channel to ensure the uninterrupted broad-
`cast of pages.
`Another feature of this invention is that it does not
`require the use of maintenance receivers to constantly
`monitor the signals broadcast by the various stations to
`ensure that the signals are broadcast in synchronization.
`Also, commands to the stations can be interleaved with
`the transmission of PDBs to the stations to keep the loss
`of link transmission time to a minimum.
`Furthermore, error correction material is added to
`the PDBs prior to their transmission over the link chan-
`nels. The station controllers use this error correction
`material to correct errors that develop in PDBs during
`the transmission to the stations. This ensures that the
`control information acted upon by the station controller
`and the paging signals that are broadcast by the trans-
`mitter are as similar as possible to the information and
`paging signals that were first generated by the hub.
`Still another feature of this invention is that signals
`other than signals for rebroadcast, the PDBs, can be
`broadcast over the link channels. The link channels can
`be used as the communications medium for broadcast-
`ing software instructions for downloading to the station
`controllers. This reduces the frequency with which
`personnel have to make site visits to the stations to
`perform hands-on maintenance or system-updating
`tasks. Also, some link channels may be configured as
`duplex links over which signals can be transmitted by
`the stations back to the hub. This allows the link chan-
`nels to be used not only as paths over which PDBs and
`station instructions are transmitted, but also as the me-
`dium through which the stations transmit information
`back to the hub regarding their operational status. This
`feature of the system serves to minimize the need to set
`up ancillary station-to-hub links to reduce the overall
`costs associated with operating a simulcast system.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`The invention is pointed out with particularity in the
`appended claims. The above and further advantages of
`the invention may be better understood by referring to
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`GLOSSARY
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`5
`the following description, taken in conjunction with the
`accompanying drawings, in which:
`FIG. 1 is a block diagram of the basic elements of the
`paging systemiof this invention;
`FIG. 2 is a block diagram illustrating how a plurality
`of local area groups of stations comprise a single wide
`area group of paging stations;
`FIG. 3 is a diagrammatic view of the publicly
`switched telephone net-work-to-station interconnec-
`tions established by the broadcast system of this inven-
`tion;
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`FIG. 4 is a block diagram View of the major compo-
`nents of the hub of the system of this invention;
`FIG. 5 depicts the elements of an HDLC frame;
`FIG. 6 illustrates in block diagram form a paging
`terminal interface;
`FIG. 7 illustrates a control-type paging data block;
`FIG. 8 illustrates a data-type paging data block;
`FIGS. 9A—9C represent in block diagram form the
`processing steps performed by the paging terminal in-
`terface to analyze, or capture, digital paging signals;
`FIG. 10A illustrates the profile of one particular
`digital paging signal captured by the paging terminal
`interface;
`FIGS. 10B and 10C illustrate the contents of the PDB
`buffer before and after the capture of the digital paging
`signal of FIG. 10A;
`FIG. 11A illustrates the profile of another digital
`paging signal captured by the paging terminal interface;
`FIGS. 11B—11D illustrate the contents of the PDB
`buffer before, during, and after the capture of the digital
`paging signal of FIG. 11A;
`FIG. 12A illustrates the profile of another digital
`paging signal captured by the paging terminal interface;
`FIGS. 12B—12D illustrate the contents of the PDB
`buffer before, during, and after the capture of the digital
`paging signal of FIG. 12A;
`FIG. 13 depicts in block diagram form the hub cen-
`tral processing unit;
`FIG. 14 depicts the link tables and link channel
`queues that are stored in the memory of the hub central
`processing unit;
`FIG. 15 depicts the structure of a station packet;
`FIG. 16 depicts in block diagram form a link modem;
`FIG. 17 depicts the structure of a link frame;
`FIG. 18 is a block diagram depicting the relationship
`between the redundancy controller and the other cen-
`tral-station elements of this invention, the main hub, the
`redundant hub, the paging terminals, and the link chan-
`nels;
`FIG. 19 depicts in block diagram form a transmitting
`station including the system controller through which
`paging signals are forwarded to a station transmitter for
`broadcast;
`FIG. 20 depicts in block diagram form primary com-
`ponents of a converter uplink repeater,
`FIG. 21 depicts the primary components of a mainte-
`nance operating point;
`FIG. 22 depicts how a system of this invention de-
`signed to simulcast multiple signals in a single geo-
`graphic area can be provided with a spare, or redun-
`dant, transmitting station;
`FIG. 23 illustrates an alternative form of a data—type
`paging block; and
`FIG. 24 depicts in block diagram form an alternative
`transmitting station including the system control that
`regenerates the signals for broadcast by the station’s
`transmitter.
`
`The following are definitions of frequently appearing
`terms and acronyms that appear in the Detailed De-
`scription. Terms that have a subscript suffix “X” are
`those wherein the system of this invention has multiple
`numbers of those units, which are distinguished from
`each other in the Detailed Description.
`CURE Converter Uplink Repeater. A unit in a hub-
`to-station link channel that serves as the interface
`between segments of the channel.
`GPIO General Purpose Input-Output board. An in-
`terface device connected between the hub, a sta-
`tion, a CURE or MOP, and the TNPP link.
`HDLC bus/frame High-level Data Link Control bus.
`The intra-hub bus over which signals are ex-
`changed between the hub central processor and the
`P'l‘Is and the LMs. Signals are transferred over the
`HDLC bus in packets known as HDLC frames.
`Hub The central unit of the system that receives
`paging signals from a paging terminal and that
`forward s the paging signals to the stations for
`broadcast by the stations.
`Link frame The packet in which station packets are
`sent over a link channel from the hub to the sta-
`tions.
`
`LAG Local Area Group. A subset of stations in a
`wide area group.
`Link channel, A communications network over
`which PDBs are transmitted from the hub to one or
`more stations.
`LMJ, Link Modem. The interface unit between the
`hub and a link channel over which PDBs are for-
`ward to the link channel. Each link modem serves
`as the interface to two separate link channels.
`MOP Maintenance Operating Point. A receiver posi-
`tioned to monitor the signals broadcast by one or
`more stations and that provides status reports on
`the operating state of the associated stations back
`to the hub on the basis of those signals.
`Paging terminal A unit that generates paging signals
`for simulcast by the system of this invention.
`PTI, Paging Terminal Interface. The interface unit of
`the hub that receives paging signals from the pag-
`ing terminal and converts them into PDBs.
`PDB Paging Data Block. The basic signal packet in
`which paging signals and the control information
`for regulating the broadcast of same are broadcast
`from the hub to the stations.
`Station A transmitter site distal from the hub from
`which signals are broadcast. Signals are broadcast
`from a number of stations simultaneously.
`Station packet A packet of data that is sent to the
`stations from the hub central processing unit
`through a link modem and a link channel. A. station
`packet may contain a paging data block or mate-
`rial, such as operating instructions, that the station
`should act upon.
`TNPP link Telocator Network Paging Protocol link.
`A communications network, separate from the link
`channels, over which the system state information
`is exchanged between the hub and the other ele-
`ments of the system, the stations, the MOPs, and
`the CUREs.
`
`WAG Wide Area Group. A large set of stations
`through which pages are broadcast.
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`I. System Overview
`FIG. 1 illustrates a simulcast broadcast system 20 of
`this invention. The system 20 broadcasts pages that are
`generated by a set of paging terminals 22 and 24 con-
`nected to a publicly switched telephone network
`(PSTN)26. The system 20 includes a hub 28 that re-
`ceives the pages from the paging terminals 22 and 24
`and a set of stations 30 that broadcast
`the pages
`throughout the area in which the system operates. The
`pages broadcast by the stations 30 are monitored by
`receivers, known as pagers 29, assigned to individual
`system subscribers. The hub 28 receives the pages from
`the paging terminals 22 and 24 in the form of paging
`signals and bundles the paging signals into packets re-
`ferred to as paging data blocks (PDBs) 36. Integral with
`each PDB 36 is control information that indicates the
`start time at which the pages therein should be broad-
`cast and the rate at which they should be broadcast. The
`hub 28 forwards the PDBs 36 to the stations 30 over any
`convenient communications link. Each station 30 in-
`cludes a station controller 32, that processes the infor-
`mation contained in the PDBs 3-6, and a transmitter 34
`capable of broadcasting the pages over a frequency on
`which they can be monitored by the pagers 29. The
`various copies of a PDB 36 sent to a number of stations
`30 all contain the same start time and baud rate informa-
`tion. Accordingly, the station controllers 32 integral
`with those stations/forward the pages contained in the
`PDB 36 to their associated transmitters 34 at the same
`time and at the same speed. Thus, the individual stations
`30 will all broadcast the same paging signal at the same
`time. Consequently, pagers 29 located in areas where
`paging signals from two or more stations 30 can be
`received, as represented by overlapping circles 35, will
`receive signals that are in phase and that can be pro-
`cessed as a single, coherent signal.
`The system 20 of this invention is capable of forward-
`ing pages for simulcast to the stations 30 located in one
`or more wide area groups (WAGs) 37 ofstations, one of
`which is illustrated in block form by FIG. 2. Each
`WAG 37 includes a number of stations located in a
`particular, relatively large geographic area. Depending
`on the availability of paging frequencies, there may be
`multiple WAGs 37 in a single geographic area. The
`stations 30 within each WAG 37 are further broken
`down into small geographic subgroups called Local
`Area Groups (LAGs) 38. Some individual stations 30
`may belong to two or more LAGs 38. Individual sta-
`tions 30, though, do not normally belong to multiple
`WAGs 37.
`Each station 30 is provided with at least one transmit-
`ter 34. When the system 20 of this invention is employed
`as a paging system, one type of transmitter that may be
`employed is a QT-7995 transmitter manufactured by
`Glenayre Electronics of Quincy, Ill., which broadcasts
`signals that have a carrier frequency of approximately
`900 MHz. Transmitter 34 is capable of broadcasting
`signals in any format in which they can be processed by
`the complementary pagers 29. For example, some trans-
`mitters are capable of broadcasting analog signals, two-
`level
`frequency shift digital signals, four-level
`fre-
`quency shift signals, and/or ERMES-format signals.
`Also, as represented diagrammatically by station 30a,
`some stations may have two or more transmitters 34.
`These stations 30a are able to transmit signals over two
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`distin