(12) United States Patent
`H0 et al.
`
`USOO6282428B1
`(10) Patent No.:
`US 6,282,428 B1
`(45) Date of Patent:
`Aug. 28, 2001
`
`(54) TWO-WAY PAGING SYSTEM HAVING
`RANDOM REVERSE CHANNEL SLOT
`SCHEDULING
`
`(75) Inventors: Derek Ho, Vancouver; Barry
`Buternowsky, Burnaby, both of (CA)
`
`(73) Assignee: Glenayre Electronics, Inc., Charlotte,
`NC (US)
`-
`0
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`- - -
`(*) Notice:
`
`(21) Appl. No.: 09/191,368
`(22) Filed:
`Nov. 12, 1998
`7
`(51) Int. Cl." ........................................................ H04. 3/16
`(52) U.S. Cl. ..................
`455/458; 455/466; 340/825.44
`(58) Field of Search .................................. 455/31.1, 31.2,
`455/31.3, 458; 370/336; 340/825.44
`References Cited
`U.S. PATENT DOCUMENTS
`
`(56)
`
`3/1985 Kume.
`4,506,361
`11/1989 Siwiak.
`4,882.579
`7/1995 Kou.
`5,434,847
`1/1996 Godoroja.
`5,485.463
`5,638,369 * 6/1997 Ayerst et al. ........................ 370/346
`5,663,715
`9/1997 Godoria.
`5,689,503
`11/1997 Wada et al..
`
`11/1997 Guntin.
`5,691,709
`5,748,100 * 5/1998 Gutman et al. ................. 340/825.44
`5,751,702
`5/1998 Evans et al. .
`5,799,012 * 8/1998 Ayerst et al. ........................ 370/336
`5,805.576
`9/1998 Worley, III et al. ................. 370/337
`:
`5,883,886
`3/1999 Eaton et al. ......................... 370/314
`* cited by examiner
`Primary Examiner Daniel Hunter
`Assistant Examiner Thuan T. Nguyen
`(74) Attorney, Agent, or Firm-Christensen O'Connor
`Johnson Kindness PLLC
`
`
`
`ABSTRACT
`(57)
`In a two-way paging System including a paging terminal, a
`plurality of paging transmitters, a plurality of paging
`receivers, and a plurality of two-way paging units, the
`plurality of two-way paging units transmitting reverse chan
`nel messages to the paging receivers in a time division
`multiplexed protocol, the protocol including a frame with a
`plurality of reverse channel time slots, a method of assigning
`reverse channel time slots to the paging units is disclosed.
`The method comprises: (a) determining a set of transmitting
`paging units from the plurality of two-way paging units, the
`Set of transmitting paging units being those of the plurality
`of two-way paging units that are to transmit a reverse
`channel message; and (b) randomly assigning each of the
`transmitting paging units in the Set of transmitting paging
`units one of the reverse channel time slots in the frame.
`
`10 Claims, 3 Drawing Sheets
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`ERICSSON v. UNILOC
`Ex. 1020 / Page 1 of 8
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`U.S. Patent
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`Aug. 28, 2001
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`US 6,282,428 B1
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`ERICSSON v. UNILOC
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`U.S. Patent
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`Aug. 28, 2001
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`(PRIOR ART)
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`ERICSSON v. UNILOC
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`U.S. Patent
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`Aug. 28, 2001
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`Sheet 3 of 3
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`US 6,282,428 B1
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`
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`DETERMINE NUMBER OF
`REWERSE CHANNEL MESSACES
`TO BE SCHEDULED
`
`RANDOMILY ASSICN THE
`PACINC, UNITS TO TIME SLOTS
`IN THE FRAME
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`ERICSSON v. UNILOC
`Ex. 1020 / Page 4 of 8
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`US 6,282,428 B1
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`1
`TWO-WAY PAGING SYSTEM HAVING
`RANDOM REVERSE CHANNEL SLOT
`SCHEDULING
`
`FIELD OF THE INVENTION
`The present invention relates to two-way paging Systems,
`and more particularly, to a two-way paging System that
`randomly Schedules reverse channel transmissions to mini
`mize co-channel, time slot, and adjacent channel interfer
`CCC.
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`15
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`2
`Typically, in the prior art, reverse channel transmissions
`are Scheduled to occur Sequentially at the beginning of each
`frame, for as many time slots as are required in order for all
`paging units that need to transmit have the opportunity to do
`So. Further, in most commercially operated Systems, not all
`of the time slots in a given frame are required to fully Service
`the paging units.
`For a multitude of reasons, it is not always possible for all
`of the paging units to be perfectly Synchronized in time to
`each other and to the paging System. This results in Some
`paging units transmitting outside of their assigned time slot
`and possibly Spilling over into other adjacent time slots.
`Because the paging units are assigned time slots in a
`Sequential manner in the prior art, this will result in what is
`termed “adjacent time slot interference”. For example, if two
`paging units transmitting in adjacent time slots have timing
`errorS Such that a first transmission is late and the following
`transmission is early, the transmissions may interfere with
`each other.
`Furthermore, the transmitters located in the paging units
`used to transmit the back channel messages, for reasons of
`economy, are generally spectrally imperfect. This results in
`reverse channel messages not only being transmitted in the
`desired frequency channel, but also in nearby frequency
`channels. These reverse channel messages in nearby fre
`quency channels are referred to as "image' messages. This
`results in what is termed adjacent channel interference.
`Adjacent channel interference is undesirable in locations
`where multiple two-way paging Systems are operating. For
`example, in large metropolitan areas, two or more Separate
`two-way paging Systems in operation are not uncommon. In
`addition, it is typical for the paging Systems to be assigned
`reverse channel frequency bands that are close to or adjacent
`to each other. Thus, the image messages transmitted by a
`paging unit for a first paging System may interfere with the
`authentic reverse channel messages transmitted on a Second
`paging System.
`In addition, because radio frequency channels are Scarce
`resources, the same radio frequency channel may be re-used
`in a cellular System. Even though these cellular Systems are
`designed Such that units using the same radio frequency
`channel are geographically Separate, the units may still
`interfere with each other. This is termed co-channel inter
`ference.
`Therefore, there is a need for implementing reverse chan
`nel transmissions So as to minimize interference. The present
`invention addresses these and other problems of the prior art.
`SUMMARY OF THE INVENTION
`In a two-way paging System including a paging terminal,
`a plurality of paging transmitters, a plurality of paging
`receivers, and a plurality of two-way paging units, Said
`plurality of two-way paging units transmitting reverse chan
`nel messages to Said paging receivers in a time division
`multiplexed protocol, Said protocol including a frame with a
`plurality of reverse channel time slots, a method of assigning
`reverse channel time slots to Said paging units is disclosed.
`The method comprises: (a) determining a set of transmitting
`paging units from Said plurality of two-way paging units,
`Said Set of transmitting paging units being those of Said
`plurality of two-way paging units that are to transmit a
`reverse channel message; and (b) randomly assigning each
`of the transmitting paging units in Said Set of transmitting
`paging units one of Said reverse channel time slots in Said
`frame.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The foregoing aspects and many of the attendant advan
`tages of this invention will become more readily appreciated
`
`25
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`BACKGROUND OF THE INVENTION
`A paging System might Support millions of pagers using
`a network of thousands of paging transmitters in fixed
`locations nationwide. The paging transmitters are also Sup
`ported by a control System and at least one paging terminal.
`The paging terminal operates to Supply the pages to be
`broadcast to each of the paging transmitters. The combina
`tion of paging transmitters, control Systems, and paging
`terminals, is collectively known as the paging infrastructure
`of the paging System.
`For modern two-way paging Systems where the pagers
`can Send as well as receive information, the paging infra
`Structure will also have a large number of paging receivers
`in fixed locations. The frequency band at which the pagers
`transmit to the paging System is referred to as the “reverse
`channel’. Similarly, messages from the pagers to the paging
`System are referred to as “reverse channel messages'.
`In a two-way paging System, the paging infrastructure
`tracks the location of the pagers in order to deliver messages
`to the pagers using only a Subset of the fixed location
`transmitters, thus allowing reuse of the frequency in geo
`graphically Separate locations. One prior art method of
`35
`tracking pagers requires the pagers to transmit a registration
`message when the pager enters a new geographic "area'. An
`"area' is specified by the paging System and indicated to the
`pagers by periodically broadcasted System wide informa
`tional messages.
`The registration message, and indeed all reverse channel
`messages, are transmitted by the pager on a predefined or
`otherwise dictated frequency. In order to prevent two paging
`units from transmitting on the reverse channel
`Simultaneously, the paging System provides an indication to
`a paging unit of the time period at which to transmit the
`reverse channel message. Thus, the reverse channel mes
`Sages are transmitted in a time division multiplexed manner.
`More Specifically, in most prior art two-way paging
`Systems, the reverse channel is divided into discrete time
`intervals referred to as frames. The frames are further
`divided into individual time slots that accommodate the
`reverse channel messages broadcast by the paging units.
`Additionally, other portions of the frames are reserved for
`Synchronization and other "overhead’ requirements. An
`55
`example of Such a two-way paging protocol is the
`ReFLEXTM family of paging protocols that is widely used in
`the industry.
`Because of the time division multiplexed protocol used in
`reverse channel messages, it is crucial for the paging units
`to be Synchronized with a master clocking Scheme, Such as
`the Global Positioning System (GPS). For further informa
`tion regarding a two-way paging System, the reader is
`directed to U.S. Pat. No. 5,663,715 entitled “Synchronized
`Paging System' to Godoroja assigned to the same assignee
`as the present invention and herein incorporated by refer
`CCC.
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`3
`as the Same becomes better understood by reference to the
`following detailed description, when taken in conjunction
`with the accompanying drawings, wherein:
`FIG. 1 is a Schematic diagram of a typical paging System;
`FIG. 2 is a Schematic diagram of a frame used in Struc
`turing reverse channel transmissions,
`FIG. 3 is a Schematic diagram illustrating a prior art
`technique of Scheduling reverse channel transmissions,
`FIG. 4 is a Schematic diagram illustrating a technique of
`Scheduling reverse channel transmissions in accordance
`with the present invention; and
`FIG. 5 is a flow diagram illustrating the method of
`Scheduling reverse channel transmissions in accordance
`with the present invention.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`FIG. 1 shows a paging System 101 that includes paging
`stations 103, paging terminals 105, public Switched tele
`phone network (PSTN) 107, paging receivers 109 and pager
`111. It can be appreciated by those skilled in the art that
`while only two paging terminals 105 are shown, a paging
`System may include many paging terminals. Likewise, the
`number of other components of the paging System 101 are
`merely illustrative. Indeed, paging Systems can be grown or
`shrunk to meet consumer demand.
`The paging Stations 103 are also referred to as base
`Stations or paging transmitters. In operation, callers who
`wish to page a subscriber use the PSTN 107 to call the
`Subscriber's pager telephone number. Alternatively, pages
`can be originated through a computer network directly
`connected to a paging terminal. The call is routed, perhaps
`through a plurality of paging terminals, from the PSTN 107
`to paging terminal 105 which formulates a page. The page
`is distributed over a communications network to each of the
`paging Stations 103. The paging Stations 103 in turn transmit
`the page throughout the geographic coverage area of the
`paging System 101.
`The pager 111 receives the page and processes the mes
`Sage embedded within the page for display to the Subscriber.
`In the case of a two-way paging System, the pager 111
`broadcasts back to paging receivers 109 an acknowledgment
`Signal that informs the paging terminal 105 that the page has
`been Successfully received. Typically, the timing for broad
`cast of the acknowledgment Signal is dictated by the paging
`System, for example, by embedding in the original received
`page the timing information. The pager 111 may also initiate
`other types of messages that are unsolicited and are not
`acknowledgments for received messages. In any event, the
`Signals transmitted by a pager to a paging System will be
`referred to as reverse channel messages.
`Turning next to FIG. 2, a Schematic illustration of a
`typical reverse channel frame 201 is shown. The frame 201
`may be an arbitrary length of time, but in one actual
`embodiment, the frame is 1.875 seconds long. The frame
`201 is divided into a plurality of time slots 203. The time
`slots 203 are discrete segments of time that are intended for
`use by paging units to transmit their respective reverse
`channel messages. In the example of FIG. 2, the frame 201
`has N time slots numbered from 1 to N. It can be appreciated
`that the frame 201 typically also includes other reserved
`time slots used for other purposes not germane to the present
`invention and that are not shown for clarity.
`AS noted above, the assigned time slot for the reverse
`channel message is determined by the paging System. In
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`particular, the paging System transmits a message to the
`paging unit informing the paging unit of the assigned time
`slot 203 upon which to transmit. In the case of an acknowl
`edge back request, this message is typically included with
`the originating message.
`In the case of the paging unit wishing to originate a
`message, the paging unit will transmit a request to the
`paging System for a time Slot assignment. The paging unit
`will transmit the request on a predetermined "hailing time
`Slot that is reserved for unscheduled reverse channel mes
`Sages. The hailing time slot is not shown in FIG. 2 and it can
`be appreciated that there may be more than one hailing time
`slot. The number of hailing time slots in a frame is generally
`determined by the number of paging units in the System. The
`greater the number of paging units, the greater the number
`of necessary hailing time slots in order to avoid collisions.
`The pager is informed by the forward channel, in every
`frame, the location of the boundary between scheduled and
`unscheduled reverse channel messages. The pager randomly
`decides when to transmit in the unscheduled area.
`AS noted above, in the prior art, the paging System
`assigned time slots Sequentially from the beginning of the
`frame. However, in the present invention, the paging System
`randomly assigns time slots 203 for use uniformly through
`out the frame 201. This can be seen in FIGS. 3 and 4 which
`shows a prior art assignment of time slots and assignment of
`time slots in accordance with the present invention, respec
`tively.
`In FIG. 3A, the frame 201 includes nine time slots 203.
`ASSume for example that four paging units are to transmit
`reverse channel messages. In the prior art, the four paging
`units would be assigned the first four time slots in the frame
`201. Thus, paging unit one PU1 would transmit in the first
`time slot, paging unit two PU2 would transmit in the Second
`time slot, paging unit three PU3 would transmit in the third
`time slot, and paging unit four PU4 would transmit in the
`fourth time slot. While simple to schedule, this method
`increases the chance that adjacent time-slot interference
`would take place. In addition, because all of the reverse
`channel transmissions are packed together Sequentially at
`the beginning of a frame, an increased probability of adja
`cent channel and co-channel interference arises. This is
`because, most commercial two-way paging Systems operate
`with the same protocol (ReFLEXTM) which typically sets the
`frame length and the absolute GPS time at which the first
`frame Starts.
`Turning now to FIG. 4, like FIG. 3, the frame 201 also
`includes nine time slots 203. ASSume again that four paging
`units are to transmit reverse channel messages. In accor
`dance with the present invention, the four paging units are
`randomly assigned time slots 203 within the frame 201.
`Thus, instead of being assigned Sequentially from the begin
`ning of a frame 201 as in the prior art, the paging unit
`transmissions are spaced randomly throughout the frame
`201. In the particular example shown in FIG. 4, paging unit
`one PU1 has been assigned to transmit in the fifth time slot,
`paging unit two PU2 has been assigned to transmit in the
`third time slot, paging unit three PU3 has been assigned to
`transmit in the ninth time slot, and paging unit four has been
`assigned to transmit in the Second time slot.
`Turning next to FIG. 5, a flow diagram illustrating the
`method of Scheduling the reverse channel messages is
`shown. These StepS are typically performed by the paging
`infrastructure, and more likely, the paging terminal of the
`paging System. First, at a box 501, a determination is made
`as to how many reverse channel messages (or paging units)
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`are to be scheduled. Next, at box 503, the paging units are
`then randomly assigned to time slots in the frame. Note, of
`course, that only one paging unit can be assigned to one time
`slot. Finally, at box 505, the assignment of time slots is
`transmitted as paging messages to each of the paging units.
`The random assignment of time slots to the paging units
`can be accomplished in any number of ways that are
`apparent to one of ordinary skill in the art, Such as by a
`random number generator. This random placement of the
`reverse channel messages in the frame helps to lessen
`interference. For example, in the scheme of FIG. 4, only
`paging unit four PU4 and paging unit two PU2 are capable
`of having adjacent time slot interference, Since these are the
`only two time slots 203 that are being used and adjacent to
`each other. Furthermore, by randomizing the reverse channel
`messages, co-channel and adjacent channel interference is
`reduced. Moreover, with the teachings of the present
`invention, it can be appreciated by those skilled in the art
`that the assignment of the time slots may be further ran
`domized to include assignment into multiple frames.
`While such a technique of the present invention would not
`provide maximum advantage for Systems that are fully
`loaded, i.e., all of the reverse channel time slots being used
`at all times, the present invention has been found to reduce
`adjacent channel, co-channel, and adjacent Slot interference
`in non-fully loaded systems. Note that the more heavily
`loaded the reverse channel, the less the advantageous effect
`of the present invention.
`Moreover, even in a fully loaded System, there are situ
`ations when it is advantageous to randomly Schedule reverse
`channel messages. For example, Suppose there is a pager at
`the periphery of the System which is being received very
`weakly at the System base receiver. If at each instance the
`pager is trying to transmit, a “strong pager also transmits,
`the weak pager may not be able to get through to the base
`receiver. Randomly assigning the slots may still help the
`weaker pager to get through Since without random
`assignment, the weak pager may be guaranteed through
`deterministic Scheduling, to be transmitting at the same time
`or adjacent in time to a Strong pager transmission.
`While the preferred embodiment has been illustrated and
`described, it will be apparent that various changes can be
`made therein without departing from the Spirit and Scope of
`the invention.
`The embodiments of the invention in which an exclusive
`property or privilege is claimed are defined as follows:
`1. In a two-way paging System including a paging
`terminal, a plurality of paging transmitters, a plurality of
`paging receivers, and a plurality of two-way paging units,
`Said plurality of two-way paging units transmitting reverse
`channel messages to Said paging receivers in a time division
`multiplexed protocol, Said protocol including a frame with a
`plurality of reverse channel time slots, a method of Sched
`uling Said reverse channel messages by assigning reverse
`channel time slots to Said two-way paging units comprising:
`(a) determining a set of transmitting paging units from
`Said plurality of two-way paging units, Said Set of
`transmitting paging units being those of Said plurality
`of two-way paging units that are to transmit a reverse
`channel message, and
`(b) Scheduling said reverse channel messages by ran
`domly assigning each of the transmitting paging units
`in Said Set of transmitting paging units one or more of
`Said reverse channel time slots in Said frame, Such that
`after at least Some of the time slots within the frame
`have been scheduled, but before the transmission of the
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`frame begins, Said Scheduled time slots will have been
`randomly interspersed with unscheduled time slots
`within the frame.
`2. The method of claim 1 further including the step of
`using Said paging transmitters to inform each of the trans
`mitting paging units in Said set of transmitting paging units
`their assigned one of Said reverse channel time slots in Said
`frame.
`3. A two-way paging System comprising:
`(a) a plurality of two-way pagers, said plurality of two
`way pagerS operative to formulate a reverse channel
`message for transmission on a reverse channel fre
`quency in a time division multiplexed protocol, Said
`protocol including a frame with a plurality of reverse
`channel time slots,
`(b) a paging terminal for formulating paging messages for
`transmission to Said plurality of two-way pagers,
`(c) a plurality of paging transmitters for transmitting said
`paging messages to Said plurality of two-way pagers,
`and
`(d) a plurality of paging receivers for receiving said
`reverse channel message transmitted by Said pager,
`wherein Said paging terminal Schedules Said reverse chan
`nel messages by assigning reverse channel time slots to
`Said two-way paging units by: (i) determining a set of
`transmitting paging units from Said plurality of two
`Way paging units, said Set of transmitting paging units
`being those of Said plurality of two-way paging units
`that are to transmit a reverse channel message; and (ii)
`Scheduling Said reverse channel messages by randomly
`assigning each of the transmitting paging units in Said
`Set of transmitting paging units one or more of Said
`reverse channel time slots in Said frame, Such that after
`at least Some of the time slots within the frame have
`been scheduled, but before the transmission of the
`frame begins, Said Scheduled time slots will have been
`randomly interspersed with unscheduled time slots
`within the frame.
`4. The two-way paging System of claim 3 further wherein
`Said paging transmitters transmit a message to each of the
`transmitting paging units in Said Set of transmitting paging
`units indicating the assigned one or more of Said reverse
`channel time slots in Said frame.
`5. In a wireleSS communication System including a central
`controller, one or more receivers, and a plurality of paging
`units, Said paging units utilizing one or more reverse chan
`nels to transmit reverse channel messages to Said receivers
`in a time division multiplexed protocol, Said protocol includ
`ing a frame with a plurality of reverse channel time slots, a
`method of Scheduling Said reverse channel messages by
`assigning reverse channel time slots to Said paging units,
`comprising:
`(a) determining a Subset of paging units from said plu
`rality of paging units, Said Subset of paging units
`including paging units that have requested one or more
`reverse channel time slots for transmitting reverse
`channel messages, and
`(b) Scheduling said reverse channel messages by assign
`ing each of the paging units in Said Subset of paging
`units one or more of Said reverse channel time slots in
`Said frame on a randomized basis, Such that after at
`least Some of the time slots within the frame have been
`Scheduled, but before the transmission of the frame
`begins, Said Scheduled time slots will have been ran
`domly interspersed with unscheduled time slots within
`the frame.
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`US 6,282,428 B1
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`7
`6. The method of claim 5, wherein said wireless commu
`nication System further includes transmitters, Said transmit
`ters being used to inform each paging unit in Said Subset of
`paging units of their assigned one or more reverse channel
`time slots.
`7. The method of claim 5, wherein the paging units are
`two-way Subscriber pagers.
`8. A wireleSS communication System comprising:
`(a) a plurality of paging units, said plurality of paging
`units transmitting reverse channel messages in reverse
`channel time slots, and
`(b) one or more receivers for receiving said reverse
`channel messages transmitted by Said paging units,
`wherein Said reverse channel messages are Scheduled by:
`(i) determining a set of paging units that have reverse
`channel messages to transmit; and (ii) assigning each of
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`Said paging units in Said Set of paging units one or more
`of Said reverse channel time slots on a randomized
`basis, Such that after at least Some of the time slots
`within the frame have been Scheduled, but before the
`transmission of the frame begins, Said Scheduled time
`slots will have been randomly interspersed with
`unscheduled time slots within the frame.
`9. The wireless communication system of claim 8,
`wherein Said wireleSS communication System further com
`prises transmitters that are used to inform each paging unit
`in Said Set of paging units of their assigned one or more
`reverse channel time slots.
`10. The wireless communication system of claim 8,
`wherein the paging units are two-way Subscriber pagers.
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