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`
`
`
`1.1500591 5210A
`
`United States Patent
`Cameron et at.
`
`[19]
`
`[11] Patent Number:
`
`5,915,210
`
`[45] Date of Patent:
`
`Jun. 22, 1999
`
`[54] METHOD AND SYSTEM FOR PROVIDING
`MULTICARRIER SIMULCAST
`TRANSMISSION
`
`|75]
`
`Inventors: Dennis Wayne Cameron. Jackson.
`Miss; Walter Charles Rnehr. Jr.,
`Roston, VL; Jill 1’. Bhagn‘t. Jackson.
`Miss; Masnnd Garahi. Madison.
`Miss; William 1]. Hays, Jackson,
`Miss: David W. Ackerntan.
`Washington. D17.
`
`1731 Assignuc: Dcstincer Corporation, Jackson, Miss.
`
`|31] Appl.No.:
`
`081899376
`
`|22
`
`Filed:
`
`Jul. 24, 1997
`
`Related U.S. Application Data
`
`|63] Continuation of application No. “Stitmflfl Dec. (1. “3%.
`abandoned1 which is a continuation of application No.
`t't'?t9T3,9]S_. Nov. [2, 1992, Pat. No. 5-590.403.
`
`|51I
`[521
`[581
`
`[56 I
`
`Int. Cl.“ ....................................................... H1143 U50
`U.S. C1.
`.. 45559; 4553102; 455! 103
`
`Field of Search ..................................... 455.1502, 503.
`455,150? 509, 515 511% 5 | 7 524, 59, 60.
`(12. ('13 (17.1, 67.3 67.6 101,102, 103;
`375360. 267. 399; 370K343, 344
`
`References Cited
`1.1.5. PATENT [JOCUMENI‘S
`
`3.433.445
`3,914,554
`4,123,405
`4,244,047
`4,392,242
`
`U19?”
`[WWI-'5
`”£198“
`1f1981
`7:19.33
`
`Chang .
`Scidcl .
`Hattori ct al.
`Perkins.
`
`455,513
`
`45 5,34. 1
`
`45559
`
`12.1984 Kai el. al.
`4.490.830
`3t’1935 Lucas .
`4506:3434
`2ft F186 Tltm .......................................... 455359
`4.5T0.265
`1W1 98'?" Dunkct‘ton el al. _
`4,?l.ll,758
`4_,8:i(1,032 W108!)
`Trccburg .
`4.968.066
`lift 99!]
`Jasittski
`.................................. 455.51.:l
`5,128,934
`7tl‘193 Jasinski.
`5.163.181
`11t1993 Koontz .
`5,243,529
`art-m Wci
`......................................... 315nm
`5543,4091
`8tl904 Jasper ct a].
`.
`5,392,452
`2:1995 Davis.
`5,504,783
`4}“;th I‘omisato at al.
`FOREIGN PATENT DOCUMENTS
`
`455t’lfll
`
`W0 lN'ItTHfilsl
`WOUIr'IS-‘ifafi
`WO‘JZ’IIT’U?
`
`liu [Optflfl Pat. 011'.
`.
`+1199”
`................. 455,311.11
`1131991 W119i)
`
`it’ll”? WIPO
`45583.1
`Primary Examiner—Thanh Cong Le
`Attorney. Agent. or Fina—Finnegan, Henderson. Farahow,
`Garrett (St Dunner
`
`[57]
`
`ABSTRACT
`
`A two-way communication systcm for communication
`hctwcceu a system network and a mobile. unit. The system
`network. includes a plurality of base transmitlcrs and base
`rcccivcrs include.- in the network. The base transmitters art:
`divided into zonal assignmcnts and broadcast in simulcast
`using multi-carrior modulation techniques. Thc system not-
`work controls the bust: transmitters to broadcast in suimul—
`cast during both systcmwidc and soot;- boundaries to maxi—
`mize information throughout. The preferred mobile unit
`inlcudcs a noise detector circuit to prevent unwanted trans—
`missions. Thc system network Further provides an adaptive
`registration fcaturc for mobilc units which controls the
`registration operation by the mobile units to maximize
`infotTnalion throughout.
`
`19 Claims, 30 Drawing Sheets
`
`,_._.—._._._eijt1r3
`\( ~.__
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`615
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`

`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 1 of 30
`
`5,915,210
`
`F/ 6‘. /
`
`PRIOR ART
`
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`x"
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`

`

`US. Patent
`
`Jun.22,1999
`
`Sheetz ofSD
`
`5,915,210
`
`
`
`3
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`US. Patent
`
`Jun. 22, 1999
`
`Sheet 3 0f 30
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`US. Patent
`
`Jun. 22, 1999
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`5,915,210
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`F/6.5PRIORART
`
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`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 6 0f 30
`
`5,915,210
`
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`US. Patent
`
`Jun. 22. 1999
`
`Sheet 7 0130
`
`5,915,210
`
`FIG. 7
`
`T00
`
`
`
`
`Generating a system information
`signal which includes a plurality of
`
`blocks of information
`
`
`
`702
`
`
`
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`signal to the pluralityr of transmitters
`
`
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`705
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`
`information during the second
`time period
`
`
`information during the second
`
`
`
`
` time period
`
`8
`
`

`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 8 0f 30
`
`5,915,210
`
`FIG: 8
`
`800
`
`
`
`Transmitting a message signal by a
`base transmitter servicing a zone
`where the mobile transceiver was. last
`knoWn to be located
`
`802
`
`Transmitting a regional probe signal
`by a plurality of base transmitters
`servicing a plurality of zones if the
`mobile transceiver does not indicate
`
`
`
`receipt of the message signal from the
`base transmitter
`
`304
`
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`
`305
`
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`
`
`I
`
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`response to the received regional
`probe signal
`
`
`BUB
`
`
`
`Receiving the acknowledgment signal
`from the mobile transceiver by a base
`receiver
`
`3'0
`
` 3'
`
`Updating the data to reilect the zone
`of the base receiver that received the
`
`acknowledgment signal as the last
`known location of the mobile
`transceiver
`_________________
`
`3'2
`
`9
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`US. Patent
`
`Jun. 22, 1999
`
`Sheet 10 MM!
`
`5,915,210
`
`FIG”.
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`US. Patent
`
`Jun. 22, 1999
`
`Sheet 11 of 30
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`5,915,210
`
`FIG. 11
`
`FREQUENCY
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`US. Patent
`
`Jun. 22, 1999
`
`Sheet 12 um}
`
`5,915,210
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`”200
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`FIG. 12
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`13
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`

`

`US. Patent
`
`Jun.22,1999
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`US. Patent
`
`Jun. 22, 1999
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`Sheet 14 0f 30
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`5,915,210
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`U.S. Patent
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`Jun.22,1999
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`5,915,210
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`US. Patent
`
`Jun. 22, 1999
`
`Sheet 17 0130
`
`5,915,210
`
`FIG. /7
`
`Mobile Receiver
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`18
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`

`US. Patent
`
`Jun.22,1999
`
`Shect18 0f30
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`5,915,210
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`US. Patent
`
`Jun. 22, 1999
`
`Sheet 19 0f 30
`
`5,915,210
`
`F/ 6'.
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`US. Patent
`
`Jun.22,1999
`
`Sheet 20 0f 30
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`5,915,210
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`US. Patent
`
`Jun. 22, 1999
`
`Sheet 21 MB!)
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`5,915,210
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`22
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`22
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`

`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 22 0f 30
`
`5,915,210
`
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`US. Patent
`
`Jun. 22, 1999
`
`Sheet 23 of 30
`
`5,915,210
`
`FIG. 23
`
`Service Queue
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`3/300
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`

`

`US. Patent
`
`Jun.22,1999
`
`Sheet24 0f30
`
`5,915,210
`
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`

`US. Patent
`
`Jun.22,1999
`
`Shcct25 0f30
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`5,915,210
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`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 26 0f 30
`
`5,915,210
`
`FIG. 26
`
`
`
`
`
`Transmitting substantially
`
`
`simultaneously a first information
`
`mgnaland a secondinfonnahon
`
`signal, the first information signal
`
`being transmitted in simulcast by a
`first set of base transmitters
`
`assigned to a first zone, and the
`
`second information signal being
`
`transmitted in simulcast by a second
`
`
`
`
`
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`
`2602
`
`2604
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`2606
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`set of base transmitters assigned to a
`second zone
`
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`
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`of the base transmitters in the first set
`
`of base transmitters assigned to the
`first zone to the second set of base
`
`transmitters assigned to the second
`
`zone, thereby creating an updated
`first set of base transmitters and an
`
`updated second set of base
`transmitters
`
`
`
`
`l
`
`
`
`
`L
`
`Transmitting substantially
`
`simultaneously a third information
`
`signal and a fourth information signal,
`
`the third information signal being
`
`transmitted in simulcast by the
`
`updated first set of base transmitters,
`
`and the fourth information signal
`
`being transmitted in simulcast by the
`
`updated second set of base
`transmitters
`
`
`27
`
`27
`
`

`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 27 0f 30
`
`5,915,210
`
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`

`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 28 of 30
`
`5,915,210
`
`F/G. 2804/
`
`2800
`
`Send a message to disable the
`
`Store the number of probe
`
`signals sent and a number of
`
`
`registration feature
`
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`messages successfully
`
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`
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` Process the stored number of
`
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`
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`
`messages succesfully
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`
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`
`
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` 2806
`
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`the network to locate the mobile
`
`transceiver
`
`
`
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`
`unit to enable the mobile
`
`transceiver's capability to
`
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`
`2808
`
`
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`
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`
`transmit a registration signal it
`
`the likelihood exceeds a
`
`selected value
`
`29
`
`29
`
`

`

`US. Patent
`
`Jun. 22, 1999
`
`Sheet 29 of 30
`
`5,915,210
`
`F/6. 28/3}
`
`28l0
`
`registration feature
`
` Send a message to enable the
`
`
`
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`28|2
`
`of messages successfully
`
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`
`registration signals and
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`
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`evaluate a likelihood that a
`
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`
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`the network that will not be
`
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`
`
`
`
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`determine a set of base
`
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`
`transmit a message to the
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`
`28|8
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`
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`the likelihood exceeds a
`
`
`
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`
`30
`
`30
`
`

`

`US. Patent
`
`Jun.22,1999
`
`Sheet 30 0f 30
`
`5,915,210
`
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`

`5,915,210
`
`1
`METHOD AND SYSTEM FOR PROVIDING
`MUL'I‘ICARRIER SI MULCAST
`TRANSMISSION
`
`This application is a continuation of application Ser. No.
`(t8t7fi0,457, filed Dec. 6, 1996, now abandoned. which is a
`Rule 60 continuation of prior application Ser. No. UW‘JTB,
`918, filed Nov. 12. 1992, now US. Pat. No. 5,590,403.
`BACKGROUND 01“ THE INVENI'ION
`A. Field of the Invention
`
`5
`
`.10
`
`The present invention relates to methods and systems for
`providing two-way communication capability between a
`central network and a mobile unit over a relatively large
`area, and more particularly to such methods and systems
`which allow for rapid communication of large messages and
`efficient use of system resources.
`B. Description of the Related Art
`Conventional two-way portable-Mobile wireless messag-
`ing systems often provide a variety of services to subscrib-
`ers. Conventional messaging systems in particular provide
`one-way services using store and forward techniques to
`mobile rewivers carried by the subscriber. A fundamental
`goal of two—way messaging systems is to provide a network
`of interconnected transmitters and receivers which provides
`sufficient transmitted signal strength and receive capability
`to uniformly cover it geographic region. Some conventional
`messaging systems provide the message to the user on a
`small viewing screen on the mobile unit.
`However, such conventional systems often suffer from
`problems associated with low system throughput, evidenced
`by slow message delivery and message size limitations and
`do not provide an acknowledgment
`feature wherein the
`mobile unit
`transmits an acknowledgment signal
`to the
`system to acknowledge receipt of the message from the
`system. Generally, system throughput refers to overall com-
`munication capability of a system as defined by the total
`amount of message data from the system to the mobile units
`transferred by the system during a given period of time
`divided by the frequency bandwidth necessary to transmit
`the message data and may be measured in bits transferred
`per I12. Further. such conventional systems suffer from
`technical problems preventing consistent wide area cover-
`age and would require extremely wide portions of valuable
`frequency bandwidth to achieve acceptable system through-
`put rates.
`technology in communication systems was
`Simulcast
`originally developed to extend transmitter coverage beyond
`that which could be obtained from a single transmitter. Over
`time, however. simulcasting has evolved into a technique
`capable of providing continuous coverage to a large area.
`Generally, simulcast
`technology provides multiple
`transmitters, operating on substantially the same frequencies
`and transmitting the same information positioned to cover
`extended areas. As. shown in FIG. 1, transmitter 100 gener-
`ally provides coverage ovcr area A, I), and E, transmitter
`102 generally provides coverage over area B, I), and E, and
`transmitter 104 generally provides coverage over area C, E,
`and F. In some cases, the coverage area nt'a first transmitter
`may be entirely enclosed within the coverage area of another
`transmitter, such as in building interiors and valleys. In areas
`where one [and only one) transmitter dominates [e.g., areas
`A. B, and C in FIG. I), simulcast is effective because the
`other transmitters do not significantly afl'ect receivers in
`those areas.
`However, in "overlap" areas I), E, and F shown in FIG. 1,
`where the signals from two or more transmitters are approxi-
`
`30
`
`.‘S
`
`40
`
`50
`
`55
`
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`
`65
`
`2
`matcly equal. problems can arise because destructive inter-
`ference ofsig'ttals occurs in these overlap areas such as areas
`D, E, and F. Destructive interference occurs when the two
`signals are equal in magnitude and 180° out of phase and
`completely cancel each other. While there were some
`successes, reliable design procedures were not available.
`Attempting to precisely synchronize the carrier frequen-
`cies of at] simulcast transmitters does not overcome the
`pmblcm because points {i.c. nodes) at which destructive
`summing occurred persisted for long periods of time. At
`such points. a mobile receiver can not receive the simulcast
`signal.
`Deliberately offsetting the carrier frequencies of adjacent
`transmitters can ensure that destructive interference does not
`
`persist at one point for an extended period of time. The slight
`errors in frequency displayed by high quality reference
`oscillators (e.g., 20 herLtr. errors in 100 Mill signals or a few
`parts in 107]
`render deliberate offsetting unnecessary
`Further, merely offsetting the carrier frequencies could not
`guarantee acceptable quality demodulation because proper
`alignment ofthe modulating signals in time is also required.
`FIG. 2 displays the situation at, for example, point D in
`FIG. 1 when modulating waveforms are synchronized and
`includes coverage boundary 202 from a first transmitter and
`a second transmitter coverage boundary 204 from a second
`adjacent
`transmitter. An equi-signal boundary 200 exists
`where the signals from the first and second transmitters have
`approximately equal signal strengths. A more realistic equi-
`signal boundary would take into account natural and man-
`made topography and propagation conditions, and therefore
`would probably not be a straight line.
`FIGS. 3 and 4 generally illustrate various signals as they
`may occur at or near the cqui—signa] boundary 200 as shown
`in FIG. 2.
`In particular, FIGS. 3 and 4 illustrate various
`aspects of modulation synchronization and how altering
`transmission parameters may affect the synchronization. In
`general,
`there are at
`least three sources which cause the
`signals from the first transmitter and the second transmitter
`to be out of synchronization: (I) timing shifts in the delivery
`of the modulating waveform to each of the transmitters; (2)
`timing shifts internal to each transmitter; and (3) timing
`shifts caused by propagation distances and anomalies. From
`the perspective of a receiver located in an overlap area, these
`three sources of timing shifts combine to produce an overall
`timing shifts between the received signals from the first and
`second transmitters.
`In current commercial practice,
`the
`summation of these three components results in time shifts
`of about 200 microseconds. The timing shift present
`in
`simulcast systems disadvantageously limits the baud rate at
`which information may be transferred. In general, FIGS. 3
`and 4 will also illustrate how liming shifts prevents high
`baud rate transmissions.
`
`A time line representation of a signal 306 from a first
`transmitter is shown in tilt}. 3M) and a signal 308 from a
`second transmitter is shown in FIG. 3(8), both from the
`perspective of a receiver located in an overlap area. Vertical
`dashed lines 300 represent baud intervals on the time axis.
`As can be seen from FIGS. 3(A) and (B). the signals 306 and
`308 are frequency modulated between a high and a low
`frequency value and the signals 306 and 308 are exactly in
`phase. As will be appreciated,
`the tinting shift between
`signals 306 and 308 must be small when compared to the
`baud interval shown in FIGS. MA) and (B) since signals 306
`and 308 are in synchronization. Of course, as the baud
`interval decreases, the timing shifts will likely cause signals
`.306 and 308 to be out of synchronization.
`
`32
`
`32
`
`

`

`5,915,210
`
`3
`FIGS. 3(C), (D), and (E) show the summation of these
`two signals 306 and 303 at an equi~signal boundary, such as
`boundary 200 in FIG. 2. FIG. 3(C) shows a composite signal
`310 indicating that
`the frequency information remains
`unchanged, FIG. 3(D) shows a linear graph 312 of the
`relative phase difference caused by a slight carrier frequency
`difference between the signals from the first transmitter and
`the second transmitter. FIG. 305} shows a composite ampli-
`tude signal 314. A noise.
`threshold is indicated by the
`horizontal dashed line 304 in 1:10.3(li).
`Of interest, FIG. 3(13} shovtrs the composite amplitude
`signal 314 dipping below the noise threshold 304- at an
`anti-phase condition 302 (eg, when the relative phase angle
`is 1180“. as shown in FIG. 3U)”. As can be seen from FIG.
`3th), the anti—phase condition 302 caused by the slight phase
`shift between transmitter l and transmitter 2 will not cause
`any loss of data because the anti-phase condition persists for
`only a small portion of the baud interval.
`The slight olIset of the carrier frequencies between the
`first and second transmitters causes a slow drift of the
`relative phase of the two signals, as shown in FIG. 30)).
`When the signals are 2130" out of phase, the temporary dip
`in the amplitude signal may cause the loss of a few bits in
`the composite signal, at worst. These errors can be counter-
`acted with a conventional error correcting code, such as is
`commonly known.
`FIG. 4 shows a set of similar signals to those in 1316.3.
`but wherein the signal 402 from the first transmitter is offset
`from, or out of synchronization with, the signal 404 from the
`second transmitter by a full baud. In particular, signal 404
`tags signal 402 by one band interval. As previously
`discussed, the offset of signals 402 and 4-04 may be caused
`by various timing shifts in the delivery of both signals 402
`and 404 to a receiver in an overlap area. FIGS. duh) and (B)
`illustrate the extreme case where the sum of these timing
`shifts is equal to the baud interval shown by dashed lines
`400. As can be seen in FIG. 4(C), composite signal 406
`includes a period of indeterminate frequency which unde—
`sirably covers several entire baud intervals and, therefore.
`successful demodulation is impossible during those baud
`intcrvals. If the baud interval were increased to minimize the
`ctfcct of these timing shifts. data loss would be less likely.
`Therefore, it can be seen that the baud rate at which good
`data transfer can be accomplished is limited by the timing
`shifts between signals delivered to receivers in overlap
`areas.
`
`Through these examples, it can be seen that high degrees
`of modulation synchronization make it possible to obtain
`good data demodulation in a simulcast system. However, the
`baud rate limitation of simulcast systems is a significant
`drawback and limits. system throughput.
`An alternative to simulcast for wide area coverage is
`assignment of orthogonal, non-overlapping subdivisions of
`the available system capacity to adjacent areas. Subdivisions
`can be made in time (cg. broadcasting the information on
`the same frequency in dill'ercnl time slots to adjacent areas},
`or in frequency (cg, broadcasting the information simulta-
`neously on different Ircquencics in adjacent areas). There are
`several problems with such orthogonal systems. however.
`First. orthogonal assignments require tuning the receiver to
`the assigned frequency or time channel for the area in which
`the receiver currently resides. In the broadcast services every
`traveler has experienced the frustration of finding the correct
`channel for their favorite programs. Siniuleast operation
`avoids the need for scanning and re-tuning as the mobile unit
`moves between areas. Such scanning and rc-tuning also
`disadvantageously increase-s mobile unit power consump-
`tion.
`
`4
`the orthogonal assignment
`Second, and more serious,
`approach drastically reduces the system throughput capacity
`as measured in bits per Hz because anywhere from 3 to 7.
`or possibly more, orthogonal assignments are required to
`obtain continuous area coverage in most conventional
`orthogonal systems. This waste of capacity is somewhat
`recouped if the same- information is not needed throughout
`the service area because a given piece of information is sent
`only to those cells where it
`is needed.
`Conventional cellular radio service is a typical example of
`an orthogonal system. In cellular, the same frequencies are
`reused in spatially separated ceUs to allow different data to
`be transmitted to difl’erenl mobile units. An example of three
`cellular arrangements is shown in FIG. 5 where the number
`of cells [N] is equal to 3, 4, and 7. Each cell (i.e., A. B. C‘.
`.
`.
`. ) in conventional cellular service usually only includes
`a single transmitter and operates in a different frequency or
`time division within the communication protocol. As shown
`in FIG. 5, cellular service generally locates transmitters
`utilizing the same division (all
`the “A" transmitters} far
`enough apart
`to reduce the likelihood of interference
`between such transmitters. As the number of cells increases.
`the likelihood of interference decreases. For example, with
`N=3 as shown by arrangement 500 in FIG. 3, the distance
`between the coverage area of"A“ cells is about is cell width.
`with N='-l
`in arrangement 502,
`the distance between the
`cowrage areas of “A" cells is slightly larger, and with N=7
`in arrangement 504 the dista ncc between “A" cells. is larger
`than the width of one cell.
`
`However, as the number of cells increases. the length of
`the individual
`time intervals per cell decreases for time
`division multiplexed systems. thereby decreasing the- sys-
`tems total
`information transfer.
`ln frequency division
`systems, more cells undesirably increases the frequency
`bandwidth required. Therefore. system throughput in bits
`per It: is decreased as the number of cells increases.
`Furthermore, cellular systems often require an electronic
`"handshake“ between system and mobile unit to identify the
`specific cell (ie. transmitter} in which the mobile unit is
`located to allow capacity reuse.
`ll. SUMMARY OIi THE INVENTION
`
`The systems and methods of the present invention have a
`wide variety of objects and advantages. The systems and
`methods ofthe present invention have as a primary object to
`provide a communication system with wide area coverage
`and high message throughput while minimizing frequency
`bandwidth usage.
`It
`is an object of the invention to provide a simulcast
`communication system with a high data tra nsfer rate which
`does not exceed the baud rate limitations of simulcast
`transmission.
`
`It is a further object of the present invention to provide a
`communication system which provides for superior data
`communication integrity.
`Yet another object of the invention is to provide a mobile
`transceiver unit which prevents unnecessary RI’
`interference, particularly on commercial aircraft. Still
`further.
`it
`is alt object of the invention to provide a mono
`based communication system which may dynamically rede—
`fine zone tuiundaries to improve information throughput.
`Another object of the invention is to provide a zone based
`simulcast communication system which can effectively
`communicate with both mobile transceiver units located
`near the center of each zone as well as mobile transceiver
`uniLs located within the overlap areas between two or more
`zones.
`
`.10
`
`15
`
`35
`
`40
`
`5t]
`
`55
`
`6h
`
`65
`
`33
`
`33
`
`

`

`5,915,210
`
`5
`Additional objects and advantages of the invention will be
`set forth in part in the description which folloWs. and in part
`will be obvious from the description, or may be learned by
`practicing the it'lV'ention. The objects and advantages of the
`invention will he realized and attained by means of the
`elements and combinations particularly pointed out
`in the
`appended claims.
`To achieve the objects and in accordance with the purpose
`of the invention, as embodied and broadly described herein,
`the invention is directed to a method for information trans-
`mission by a plurality of transmitters to provide broad
`communication capability over a region of space, the infor-
`mation transmission occurring during at least both a first
`time period and a second time period and the plurality of
`transmitters being divided into at least a first and second set
`of transmitters,
`the method comprising the steps of (a)
`generating a system information signal which includes a
`plurality of blocks of information. (b)
`transmitting the
`system information signal to the plurality of transmitters, (c)
`transmitting by the lirst and second see; of transmitters a first
`block of information in simulcast during the first
`time
`period, {d} transmitting by the first set of transmitters a
`second block of information during the second time period,
`and (e) transmitting by the second set of transmitters a third
`block of information during the second time period.
`In another embodiment.
`the invention is directed to a
`math-carrier simulcast transmission system for transmitting
`in a desired frequency band a message contained in an
`information signal. the system comprising a first transmitter
`means for transmitting an information signal by generating
`a
`first plurality of carrier signals within the desired t're~
`quency hand and by modulating the first plurality of carrier
`signals to convey the information signal, and a second
`transmitter means, spatially separated from the first
`transmitter, for transmitting the information signal in simul-
`cast with the first transmitter by generating a second plu—
`rality of carrier signals at substantially the same frequencies
`as the first plurality oi'carrier signals and by modulating the
`second plurality of carrier signals to convey the information
`signal.
`the invention is directed to a
`In another embodiment,
`com municstion method implemented in a computer con-
`trolled communication network for locating a mobile trans-
`ceiver within a region of space. the region of space being
`divided into a plurality of zones with each zone serviced by
`at least one base transmitter and at least one base receiver,
`the network storing data corresponding to a zone where the
`mobile transceiver was last known to be located. the com-
`munication method comprising the steps of (a) transmitting
`a message signal by a base transmitter servicing a none
`where the mobile transceiver was last known to be located,
`(b) transmitting a systemwide probe signal by a plurality of
`base transmitters servicing a plurality of zones if the mobile
`transceiver does not indicate receipt of the message signal
`from the base transmitter, {c} receiving the regional probe
`signal by the mobile transceiver,
`(d)
`transmitting an
`acknowledgment signal by the mobile transceiver
`in
`response to the received regional probe signal, (e) receiving
`the acknowledgment signal from the