United States Patent
`Cooper et al.
`
`9
`
`3,906,166
`( 1,
`(45 Sept. 16, 1975
`
`54 RADIO TELEPHONE SYSTEM
`75 Inventors: Martin Cooper, Glencoe, Richard
`W. Dronsuth, Westchester, Albert J.
`Mikulski, Chicago, Charles N. Lynk,
`Jr., Arlington Heights; James J.
`Mikulski, Deerfield; John F.
`Mitchell, Elmhurst; Roy A.
`Richardson, Skokie, John H.
`Sangster, Hoffman Estates, all of Ill.
`73 Assignee: Motorola, Inc., Chicago, Ill.
`(22
`Filed:
`Oct. 17, 1973
`21
`Appl. No.: 403,725
`
`U.S. Cl.................................. 179/41 A; 325/16
`52
`Int. Cl. ............................................. H04q 7/00
`(5
`58) Field of Search ......... 179/4 A; 325/16, 55, 64
`
`56
`
`References Cited
`UNITED STATES PATENTS
`35 | 735 6/1970 Malm................................ 17914. A
`3,586,978
`6/1971
`Van Gorder,..................... 17914. A
`3,663,762
`5, 1972 Joc, Jr. ............................ 17914. A
`3,745,462
`7, 1973 Trimble ................................ 325/55
`
`Primary Examiner-Kathleen H. Claffy
`Assistant Examiner-Gerald L. Brigance
`Attorney, Agent, or Firn-Eugene A. Parsons; James
`W. Gillman
`
`ABSTRACT
`57 )
`A portable duplex radio telephone system includes at
`
`east one base station transmitter having a predeter
`mined base transmission range, and a plurality of por
`table or nobile units each having a predetermined
`portable maximum transmission range predetermi
`nately shorter than the base transmission range. Satel
`lite receivers are deployed about the base station
`within the base station transmission range for receiv
`ing transmissions from the portable units. The base
`station transmitter transmits signals on a signalling
`channel and on at least one communications channel.
`Each transmitter signalling and communications chan
`nel has a frequency that is paired or associated with a
`receiving frequency of the satellite receivers. In a nul
`tiple base station system, the portable receiver has
`means for scanning the base station transmitter signal
`ling frequencies and for tuning the portable transmit
`ter to the signalling frequency associated with the fre
`quency of the strongest signalling signal received from
`the base transmitter. When communication is initi
`ated, the portable transmitter and receiver are auto
`matically retuned to one of the communications chan
`nels as determined by the strongest signalling fre
`quency received by the portable receiver and by chan
`nel availability. Means are also provided in the system
`to continuously locate a portable unit and switch the
`operating frequency thereof as the portable unit
`moves between base station transmitter coverage ar
`eas. Further means are provided to automatically re
`duce the output power of each portable transmitter to
`the minimum level required for satisfactory communi
`cations in Order to reduce battery drain and the inter
`ference caused by the portable transmitters.
`31 Claims, 10 Drawing Figures
`
`/29
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`FORD EX. 1016, p. 5
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`FORD EX. 1016, p. 6
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`

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`PATENTED SEP 6975
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`FORD EX. 1016, p. 8
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`
`
`

`

`3,906,166
`
`RADIO TELEPHONE SYSTEM
`
`BACKGROUND FELD OF INVENTION
`This invention relates generally to communications
`systems, and more particularly to organized radio tele
`phone systems having a plurality of base station and
`portable units, each having a predetermined coverage
`area, and means for adjusting the operating frequencies
`of the portable units to provide the optimum communi
`cations path.
`
`5
`
`()
`
`2
`of the radio frequency spectrum than systems hereto
`fore developed.
`It is yet another object of the invention to provide a
`fully automatic portable telephone system.
`In accordance with a preferred embodiment of the
`invention, the geographic area over which communica
`tions is to be provided is divided into a series of base
`station cells, and each station cell is further subdivided
`into a series of sub-cells. A base station transmitter is
`located within each cell and transmits to portable re
`ceivers within the cell. The transmission range of the
`portable transmitters is deliberately reduced to allow
`less precise location of the portable units without caus
`ing interference between the portable units. A network
`of satellite base station receivers, one base station re
`ceiver being located in each sub-cell, is employed to re
`ceive signals from the portable transmitters. A different
`sct of incoming and outgoing frequencies are employed
`in each cell to avoid interference between units in adja
`cent or closely spaced cells. The same frequencies may
`be reused in cells that are sufficiently geographically
`separated from each other to prevent interference
`there between.
`Each base transmitting station radiates at least one
`out-going signalling frequency to the sub-cells within its
`coverage area. The receiver in each of the portable
`units scans the signalling frequencies of all of the base
`station transmitters within its area of operation and
`stores an indication of which of the received signalling
`signals is the strongest to determine the base station
`transmitter that will provide the best communications
`link there with. Transmissions by the portable unit are
`made on an incoming signalling frequency that is
`paired or associated with the strongest outgoing signal
`ling frequency received. The transmission from the
`portable unit is received by the receivers in the nearest
`sub-cells and a comparison is made between the signal
`strength received by the various satellite receivers to
`determine which satellite receiver provides the best
`communications with the portable unit. After the opti
`mum base station transmitter and satellite receiver
`have been determined, the base station transmitter sig
`nals the portable unit, on the outgoing signalling fre
`quency, to retune to a communications channel com
`prising a pair of frequencies assigned to the selected
`base station transmitter and stellite receiver
`which communication will be estahlished.
`Other scanning base station receivers are employed
`to monitor all active connunications ch:nnels, and
`means are provided to compare the signal strengths re
`ceived by each of the scanning receivers. Automatic
`Switching circuitry is provided to cause the portable
`unit to change operating frequency ind to make the
`necessary wire line switching as a portable proceeds
`from one cell to anothet.
`Because the range of each portable unit is less than
`the range of a base station transmitter, the frequencies
`at which the portable unit operates may be chosen to
`assure that the portable unit is receiving the best signal.
`regardless of whether it is actually operating within the
`particular cell to which those frequencies have been as
`signed, without causing interference to the rest of the
`system. The aforementioned feature assures that the
`best possible communications link is provided, elimi
`nates the need for precise geographic location of each
`individual portable unit and makes more efficient use
`of the radio frequency spectrum.
`
`PRIOR ART
`Organized communications systems are known, one
`variety of which is commonly known as a cell system.
`In such a system, the geographic area to be covered is
`divided into a group of cells, each cell having a base
`station transmitter and a base station receiver. The
`ranges of the base and portable or mobile units are
`made substantially equal, and the mobile unit covers
`the entire geographic area covered by the base station
`transmitter. The base and mobile frequencies of adjoin
`ing cells are selected to be different to avoid interfer
`ence between cells, and the same frequencies may be
`reused in cells that are sufficently spaced so as to pre
`vent interference there between. Location means are
`provided to determine the cell in which the portable
`unit is operating, and to adjust the operating frequency
`thereof to the frequency designated for the cell in
`which the portable is located. The location function
`may be accomplished by base station receivers located
`in the corners of the cell which have directional anten
`nas looking inwardly into the cell and a computer con
`nected to the base receivers for determining the
`strength of the signal received from the portable unit by
`the corner located receivers.
`Whereas this technique provides a way to achieve
`reasonably good communications, because the trans
`mission range of a portable or mobile unit is equal to
`the coverage range of a base station, the location of the
`4)
`portable unit must be determined very accurately, and
`the assignment of the operating frequency of the porta
`ble must be based on the geographic location of the
`unit to avoid interference with portables in other cells
`operating on the same frequency. The aforementioned
`requiremcnt requires complex and expensive location
`equipment, does not provide optimum spectrum utili
`zation, and does not assure that the portable unit is re
`ceiving the best signal since the assignment of operat
`ing frequency is based on location and not on the
`strength of the signal received thereby. Furthermore,
`the fixed, relatively high power of the portable unit
`causes interference to other units in the system when
`the portable unit is operated at a high location, such as
`the upper floors of a high rise building. This occurs be
`55
`cause the increased coverage arca resulting from the
`improved propagation characteristics of a high antenna
`cause the portable unit to radiate into areas in which
`other portable units may be operating on the same fre
`quency.
`
`SUMMARY
`It is an object of the present invention to provide an
`improved organized communications system that pro
`vides improved communications and reduced interfer
`ence between units operating on the same frequency.
`It is a further object of this invention to provide a
`communications system that makes more efficient use
`
`(5.
`
`w
`
`FORD EX. 1016, p. 9
`
`

`

`3,906, 166
`
`25
`
`3
`To further improve the interference protection be
`tween closely spaced cells, and to reduce the portable
`unit battery drain, an automatic output control system.
`is provided within each portable transmitter to main
`tain the transmitter output power at the minimum level 5
`required for reliable communications. The automatic
`output control system further provides the portable
`unit with vertical mobility by automatically reducing
`the output power thereof when its coverage area in
`creases as a result of operation from a high location,
`thereby preventing interference with other portable
`units opcrating on the same frequency. In addition, fre
`quency offsets may be provided between cells reusing
`the same frequencies to provide additional co-channel
`protection without reducing the frequency separation
`bctween channels used in adjacent cells.
`DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a plan view of the organization of the radio
`telephone system according to the invention, assuming
`uniform propagation, and showing the allocation of fre
`quencies to the various cells,
`FIG 1a is a more detailed plan view of some of the
`cells of the system of FIG. 1 showing the division of the
`cells into sub-cells and the location of base station and
`receiver sites therein;
`FIG. 2 is a partial block diagram of the portable radio
`telephone system showing the operation thereof;
`FIG. 3 is a plan view of the organization of a practical to
`radio telephone system according to the invention
`showing the variation in spacing between base stations
`and receiver sites encountered in a typical practical
`mix cd urban and rural area,
`FIG. 4 is a sequence diagram showing the typical Se- 35
`quence of events occurring in the system according to
`the invention when a call is initiated by a land base tele
`phone,
`FIG. 5 is a sequence diagram showing the sequence
`of events occurring during a portable unit initiated call; 40
`FIG. 6 is a block diagram of one of the remote re
`ceiver sites, indicated by crosscs in FIGS. 1 and 3, of
`the system according to the invention;
`FIG. 7 is a block diagram of one of the base stations
`indicated by circles in FIGS. 1 and 3;
`FIG. 8 is a more cletailed block diagram of the central
`control center 30 of FIG. 2; and
`FIG. 9 is a detailed block cliagram of one of the porta
`ble units usable with the system according to the inven
`t1O.
`
`4.
`In FIG. , each of the cells 10a, 20a, 30a, 40a, 50a,
`60a and 70a has a base station transmitter and at least
`one base station receiver located therein. Each base
`station transmitter is allocated at least one outgoing sig
`nalling frequency and at least one outgoing communi
`cations frequency, while each base station receiver is
`allocated at least one incoming signalling frequency
`and one incoming communications frequency, each in
`coming frequency being paired with an outgoing fre
`quency to provide a full duplex channel. The duplex
`channel sets allocated to each of the cells 10a, 20a,
`30a, 40a, 50a, 60a, and 70a are denoted as F1A-F7A,
`respectively. In a typical system, employing frequency
`modulation and 5 KHZ deviation, a 25 KHz separa
`tion between frequencies used within a cell group has
`been found to provide adequate protection from adja
`cent channel interference.
`In a cell system of the type illustrated in FIG. 1, the
`frequencies F1A-F7A may be reused in other cell
`groups that have sufficient geographic scparation
`therebetween to substantially climinate co-channel in
`terference. For example, the frequencies F1A-F7A
`may be reused in the cell group comprising cells 10b,
`2Ob, 30b, 40b, 5Ob, 60b, and 70b, respectively, and in
`the group comprising cells 10c, 20c, 3Oc, 40c, 50c', 60c
`and 70c, the cells having the same numerical prefixes
`being assigned the same group of frequencies. How
`ever, prior art systems employing groups of seven cells
`each and reusing the frequencies in each seven cell
`group have been found to provide marginal co-channel
`interference protection. Accordingly, systems have
`been designed using larger cell groups, such as, for cx
`ample, twenty-one cells per group, and allocating dif
`ferent frequencies to cach of the twenty-one cells in the
`group. Unfortunately, the allocation of twenty-one dif
`ferent frequency sets is wasteful of the radio frequency
`spectrum, a twenty-one cell group requiring three times
`the spectrum of a seven cell group.
`The frequency allocation concept of the present in
`vention has recognized the fact that cells that are not
`adjacent to each other geographically, such as cells
`10a, Ob and 10c do not require a 25 KHz separation
`between frequencies assigned thereto because of the
`geographic spacing therebetween. Accordingly, fre
`quencies assigned to cells having similar numeric pre
`fixes in FIG. 1 may be assigned channels that are
`spaced much less than 25 KHZ apart while maintaining
`adequate interference protection.
`For example, in the system of FIG. 1, each of the fre
`quency sets F1 B-F7B, assigned to cells 10b, 2Ob, 3Ob,
`40b, 50b, 60b and 70b may be spaced only 8.33 KHz
`from one of the frequency sets F1A-F7A, respectively.
`Similarly, the frequency sets F1C-F7C assigned to the
`cells Oc, 20c, 30, 40C, 5 Oc, 6Oc and 70 need be
`spaced only 8.33 KHz from the frequency sets F1A
`F7A and FIB-F7B, respectively. The above described
`interleaved frequency allocation system provides in
`proved co-channel interference protection over that
`provided by a normal seven cell system while maintain
`ing the spectrum economy of a seven cell system. The
`offset system may be adapted to any cell group having
`any number of cells, and the criteria for determining
`the frequency offset between cell groups of such a sys
`tem is described later in the application.
`Referring to FIG. a, there is shown a more detailed
`drawing of the cell structure of FIG. 1. Although the
`frequency allocation scheme of FIG. 1 may be used in
`
`45
`
`S)
`
`DETALED DESCRIPTION
`Referring to FIG. 1, there is shown a plan view of a
`frequency allocation scheme, according to the inven
`tion, usable with mobile or portable radio or radio tele
`phone systems. The geographic area to be covered is
`divided into a plurality of cell groups, each group con
`taining a predetermined number of cells. The number
`of cells in each group is determined by the following
`equation:
`
`55
`
`N = i + i + i.
`where N represents the number of cells in each cell
`group and i and j may be any integers. In the system
`shown in FIG. 1, i is equal to 2 and is equal to 1 to pro
`vide a seven cell group, however, other values of i and
`j may be selected to provide different patterns.
`
`65
`
`FORD EX. 1016, p. 10
`
`

`

`5
`systems employing a single base station transmitter and
`receiver per cell, and a mobile unit having the same
`range as a base station, in a preferred embodiment, the
`system according to the invention uses a base station
`transmitter having a coverage range which covers the
`entire cell, a portable unit having a coverage area
`smaller than that of the base station transmitter and a
`plurality of receiver sites deployed within each cell.
`In FIG. 1a, the receiver sites are denoted by crosses
`and the combination receiver-transmitter base station
`sites are denoted by circles. The radially extending
`lines about the circles denote directional antennas for
`portable unit locating receivers, the function of which
`will be explained in a subsequent portion of this appli
`cation. Each of the cells is divided into a group of sub
`cells, for example, the cell 10a is divided into the sub
`cells 1 1a- 17a, the cell 20a into sub-cells 2a-27c, etc.
`Each base station site transmits and receives on duplex
`channels assigned to the cell in which the base station
`is located. For example, the base station site in the cell
`0a transmits and receives on the frequencies in the set
`F1A, the base station site in the cell 20a transmits and
`receives on thc channels in the set F2A and the base
`station site located in the cell 30a transmits and re
`ceives on the channels in the set F3A.
`Because the range of a portable unit is intentionally
`made smaller than the range of the base station trans
`mitter, receiver sites in addition to the receiver located
`in the base station must be deployed within each cell to
`receive transmissions from portable units. The receiver
`sites are denoted by crosses, and are connected to the
`base station sites by means of wire telephone lines or
`other voice grade interconnections. Each receiver site,
`in the present embodiment, is located near the edge of
`the cell and receives signals from portable units in two
`adjoining cells. The coverage area of each of the re
`ceiver sites is indicated in FIG. la by a hexagonal
`dashed line sub-cell about cach receiver site. Each cell
`is divided into seven sub-cells, one about the base sta
`tion site, and six about the six receiver sites. For cxam
`ple, the cell 10a is divided into sub-cells 11a-17a, the
`cell 20a into Sub-cells 21-27a and the cell 30, into
`sub-cells 31G-37a. Of the aforementioned sub-cells,
`only the sub-cells 11a, 2 la and 31a are contained en
`tirely within their respective cells. The remaining sub
`cells overlap two cells. For example, the sub-cell 13a of
`cell 10a overlaps the sub-cell 36a of cell 30a. Accord
`ingly, the receiver site located at the boundary of cells
`10a and 30a must be capable of receiving signals on all
`of the frequencies FA and F3A assigned to cells 10a
`and 30a, respectively. Similarly, each of the receiver
`sites located at a cell boundary must be capable of re
`ceiving signals on frequencies assigned to both cells ad
`joining the boundary. The base station sites need only
`transmit and receive on frequencies assigned to the
`cells in which they are located for purpose of communi
`cation, however, directional antennas and receivers for
`monitoring all active communications channels are em
`ployed at the base station sites for monitoring the activ
`6)
`ity of the portable units and for reassigning communi
`cations channels and land lines, as necessary, as the
`portable units move between cells and sub-cells.
`Referring to FIG. 2, there is shown a block diagram
`showing the interconnections between the base station
`transmitter and receiver sites and the portable units
`which communicate with the System. Three base sta
`tions 102, 104 and 106 are shown. Each of the base sta
`
`65
`
`30
`
`O
`
`3,906, 166
`
`O
`
`15
`
`6
`tions 102, 104 and 106 contains a transmitter and a re
`ceiver and corresponds to one of the transmitter
`receiver sites denoted by circles in FIG. 1a, such as, for
`example, the circles shown in cells 11, 21 and 31. Only
`three base stations are shown for purposes of simplic
`ity, however any number may be used depending on the
`size of the area to be covered. The base station 102 has
`three receiver sites
`10, 1 2 and
`4 connected
`thereto. Similarly, receiver sites 1 16, 1 18 and 120 are
`connected to the base station 104, and the receiver
`sites 22, 24 and 126 are connected to the base sta
`tion 106. The receiver sites correspond to the crosses
`shown in FIG. a. The number of receiver sites con
`nected to each base station is determined by the num
`ber of sub-cells in each cell, and six receiver sites would
`be required for each base station for a seven cell group
`such as the one shown in FIG. 1a, however, only three
`receiver sites have been shown in FIG. 2 to avoid un
`necessarily complicating the drawing.
`Each of the base stations 102, 104 and 06 is furthcr
`connccted to a central control center 130 which is also
`connected to a standard wire line telephone network
`via lines 131. The lines 131 provide a connection to a
`plurality of fixed telephones 127 via a telephone cen
`tral 129. Three portable units 132, 134 and 136, each
`containing a transmitter and a receiver for communi
`cating with the base station and receiver site network
`arc shown. Whereas only three portable units arc
`shown, the actual number which may be used in a prac
`tical system is limited only by the number of base sta
`tion and receiver sites in the system, and the number of
`frequencies allocated to the system.
`In operation, outgoing messages are transmitted from
`a base Station, such as the base Station 102, to a porta
`ble unit, such as the unit 132. Incoming messages from
`the portable unit 132 are received by a receiver site
`such as the receiver site 112 and routed to the base sta
`tion 102 and the central control center 130. The cen
`tral control center 130 connects the base station 02 to
`either the wire line telephone network or to another
`base station, such as base station 106, depending upon
`whether communication with a fixed or portable tele
`phone is desired.
`ln the system of the instant invention, the transmis
`sion range of the base station is intentionally made
`greater than the transmission range of a portable unit.
`To provide two-way communications, the base station
`transmitter transmits directly to the receiver in the por
`table unit, and the portable unit transmitter transmits
`to the base station receive or to one of the receiver
`sites deployed within the coverage area of the base sta
`tion. The transmission range of the portable unit is in
`tentionally limited because, unlike a base station, a por
`table may move between areas and interfere with other
`portable transmissions in areas using the same fre
`quency.
`Prior art systems, in which the range of the base and
`protable units were fixed and equal, sought to control
`the portable interference problem by accurately locat
`ing the portable within a given cell and assigning a
`transmission frequency to the portable based on its
`geographic location. The assignment of a portable
`transmission frequency based upon geographic area re
`duces portable interference to an acceptable level,
`however, it does not provide the portable unit with ver
`tical mobility, and it does not assure that the best com
`munications channel is provided, because due to ter
`
`FORD EX. 1016, p. 11
`
`

`

`3,906, 66
`
`7
`rain and other factors, the best communication often
`occurs with a base station located outside of the cell in
`which the portable is located. Furthermore, the loca
`tion equipment necessary to locate a portable accu
`rately enough to avoid interference is rather costly, and
`optimum spectrum utilization is not achieved.
`By limiting the transmission range of a portable unit
`to less than the transmission range of a base station,
`and by deploying receiver sites about each base station
`to receive transmissions from the portable unit, the out
`put power of the portable unit may be sufficiently re
`duced to allow less accurate location of the unit with
`out causing interference with other portables operating
`at the same frequency.
`The signal to interference ratio between units operat
`ing on the same frequency is expressed by the following
`cquation:
`
`()
`
`S
`
`= K log
`
`(
`R -
`
`)
`
`35
`
`45
`
`8
`information indicating which of the signalling channels
`is the strongest. The strongest signalling channel is gen
`erally the signalling channel assigned to the base trans
`mitter that is nearest the portable unit. For example, if
`the portable unit were located in the sub-cell 23a of
`FIG. 1a, the strongest signalling channel would be the
`signalling channel of the transmitter located in sub-cell
`21a, however, due to shadowing or interference, the
`strongest received signalling channel received could
`also be one transmitted by a transmitter in sub-cell 61c.
`or sub-cell 3 a.
`When transmission is initiated by the portable unit.
`logic within the portable unit tunes the transmitter
`thereof to the incoming signalling frequency that is
`paired with the strongest received outgoing signalling
`frequency. The transmission from the portable unit is
`received by one or more receivers located in a base sta
`tion or receiver site, and the signal strength of the in
`coming signal is monitored by the system to determine
`which fixed receiver is receiving the strongest signal. In
`the aforementioned example, for a portable located
`within the sub-cell 23a, the strongest incoming signal
`would most likely be received by the receiver site lo
`cated in sub-cell 23a, however, due to transmission ir
`regularities, it is also possible that the strongest signal
`would be received by a receiver in one of the adjoining
`cells, such as sub-cell 22a.
`If the receiver in sub-ccl. 2.3a receives the strongest
`signal, the central control center 130 causes the base
`station transmitter in sub-cell 21a to transmit a signal
`on an outgoing signalling frequency assigned to the cell
`20a to the portable unit to cause the portable unit to
`automatically retune its transmitter and receiver to a
`frequency pair selected from the group of frequencics
`F2A assigned to cell 20a. At the same time a land com
`munications link would be established between the
`hase station in sub-cell 21a and the receiver site in sub
`cell 23a. If the strongest signal had been received by the
`receiver located in sub-cell 22a, the portable unit would
`have been assigned the same pair of frequencies from
`the group F2A but the signal received by the receiver
`site in Suh-cell 22a would be relayed to the base station
`in sub-cell 2ia even though the portable unit is physi
`cally located within sub-cell 23a to assure that the best
`communication channel is provided.
`If the portable unit located within the sub-cell 23a
`had received the strongest signalling channel signal
`from the base station transmitter located in sub-cell
`6C, the operating frequency of the portable unit would
`have been tuned to one of the frequencies F6C as
`signed to the cell 60C. A land communications link
`would be established between the base station transmit
`ter located in the sub-cell 61c and the receiver site lo
`cated in sub-cell 66C (assuming that the receiver site in
`sub-cell 66c receives the strongest signal from the por
`table unit). Since the coverage area of a portable unit
`is approximately equal to the size of one sub-cell, and
`since the nearest reuse of any frequency used in the cell
`60c is in the cells 60'c and 60'c' (see FIG. 1) the assign
`ment of a cell 60 frequency to a portable unit operat
`ing in cell 20a will not cause interference to any porta
`ble unit operating elsewhere on the same frequency,
`such as in cell 60'c' or 60'c.
`Once the initial voice frequency pair has been as
`signed to a portable unit, the location of the unit must
`be continuously monitored in order that new communi
`cations channel frequencies may be assigned thereto as
`
`where S/l is the signal to interference ratio, D is the dis
`tance between stations operating on the same fre
`quency, and K is a constant. From the above equation,
`it can be seen that reducing the range of a portable unit
`reduces R, thereby improving the signal to interference
`ratio and allowing portable units operating on the same
`channel to operate closer together. Because the porta
`ble units may now be allowed to operate more closely
`together without causing excessive interference, the
`transmission frequency of each portable unit can be as
`signed to provide the best communications link rather
`than being arbitrarily assigned on a geographic basis.
`Following is a description of the steps involved in cle
`termining the best transmission and reception fre
`quency for a portable unit. Each base station within :
`predetermined