CA1042989A - Radio telephone system - Google Patents

Abstract

ABSTRACT

A portable duplex radio telephone system includes base station transmitters having a predetermined base transmission range, and a plurality of portable or mobile units each having a predetermined maximum transmission range shorter than the base transmission range. Satellite receivers are deployed about each base station for receiving transmissions from the portable units. The base station transmitters transmit signals on a signalling channel and on at least one communications channel. The portable receivers have means for scanning the base station transmitter signalling frequencies and for tuning to the signalling frequency associated with the strongest signalling signal received from the base transmitter.
When communication is initiated, the portable transmitter and receiver are automatically retuned to one of the communications channels as determined by the strongest received signalling frequency and by channel availability. Means are also pro vided to continuously locate a portable unit and switch the operating frequency thereof as the portable unit moves between base station transmitter coverage areas. Further means are provided to reduce the output power of each portable transmitter to the minimum satisfactory level in order to reduce battery drain and interference.

Description

~ CM-73537 ~, .
~2~89 BACKGROUND
FIELD OF INVENTION

This invention relates generally to communications systems, -and more particularly to organized radio telephone 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 optLmum communications path.

PRIOR ART

Organized communications systems are ~nown, 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 trans-mitter. The base and mobile frequencies of adjoining cells are selected to be different to avoid interference between cells, and the same frequencies may be reused in cells that are sufficently spaced so as to prevent interference therebetween. Location means are provided to determine the cell in which the portable unit is operating, and to adjust the operating frequency thereo 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 antennas looking inwardly into the cell and a computer connected to ~he base receivers for determining the strength of the signal received from the portable unit by the corner located receivers.

. . . i C~~73537 .

~(~4Z989 Whereas this technique provides a way to achieve reasonably good communications, because the transmission range of a portable or mobile unit is equal to the coverage range of a base station, the location of the portable unit must be determined very accurately, and the assignment of the operat-ing frequency of the portable must be based on the geographic !location of the unit to avoid interference with portables in other cells operating on the same Erequency. The aforementioned requirement requires complex and expensive location equipment, does not provide optimum spectrum utilization, and does not ; assure that the portable unit is receiving the best signal since the assignment of operating 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 because the increased coverage area 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 frequency.

SUMMARY

It is an object of the present invention to provide an improved organized communications system that provides improved communications and reduced interference between units operating on the same frequencyO
It is a further object of this invention to provide a communications system that makes more efficient use of the radio frequency spectrum than systems heretofore developed.
It is yet another object of the invention to provide a fully automatic portable telephone system.

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In accordance with a preferred embodiment o~ the invention, the geographic area over which communications is to be provided is divided into a series of base station cells, and each station cell is further subdi~ided into a series of sub-cells. A base station transmitter is located within each cell and transmits to portable receivers within the cell. The transmission range of the portable transmitters is deliberately reduced to allow less precise location of the portable units without causing inter-ference between the portable units. A network of satellite base station receivers, one base station receiver being located in each sub-cell, is employed to receive signals from the portable transmitters. A different set of incoming and outgoing fre-quencies are employed in each cell to avoid interference between units in adjacent or closely spaced cells. The same frequencies may be reused in cells that are sufficiently geographically separated from each other to prevent interference therebetween.
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 com-munications link therewith. Transmissions by the portable unit are made on an incoming signalling frequency that is paired or associated with the strongest outgoing signalling 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 satel-lite receivers to determine which satellite receiver provides thebest com~unications with the portable unit. After the optimum CM- ï3537 ,' ~ 0429~39 base station transmitter and satellite receiver have been determined, the base station transmitter signals the portable unit, on the outgoing signalling frequency, to retune to a communications channel comprising a pair of frequencies assigned to the selected base station transmitter and satellite receiver over which communication will be established.
Other scanning base station receivers are employed to monitor all active communications channels, and means are provided to compare the signal strengths received by each of the scanning receiversO Automatic switching circuitry is pro-vided to cause the portable unit to change operating frequency and to make the necessary wire line switching as a portable pxoceeds from one cell to another.
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 assigned, without causing interference to the rest of the system The aforementioned feature assures that the best possible communications link is provided, eliminates the need for precise geographic location of each individual portable unit and makes more efficient use of the radio frequency spectr~n.
To further improve the interference protection between closely spaced cells, and to reduce the portable unit battery drain, an automatic output control system is provided within each portabl_ transmitter to maintain the transmitter output power at the minimum level required for reliable communications.
The automatic output control system further provides the portable .;' Z91~9 unit with vertical mobility by automaticall~.reducing the output power thereof when its coverage area increases as a result of operation from a high location, thereby preventing interference with other portable units operating on the same frequency. In addition, frequency offsets may be provided between cells ~`
;~ reusing the same frequencies to provide additional co-channel protection without reducing the frequency separation between channels used in adjacent cells.
More particularly there is provided a communications system comprising:
;. a first ~ase station site located in a first pre-determined geographic area and including means for receiving and transmitting signals on a plurality of first radio channels, eacH of said first radio channels having a predetermined carrier requency, the carrier frequencies of individual ones of said first radio channels being separated by at least a first pre-determined frequency separation;
a second base station site located in a second pre-determined geographic area adjacent said first predetermined geographic area, said second base station site including means for receiving and transmitting signals on a plurality of second radio channels r each second radio channel having a predetermined carrier frequency different from the carrier frequencies of said first radio channels~the carrier frequencies of the in-dividual ones of said second channels being separated by at least said first predetermined frequency separation, the carrier - frequencies of each of said second channels being further separated from the carrier frequencies of each of said first c~annels by at least said first frequency separation;
a third base station site located in a third pre-determined geographic area non-adjacent to said first geographic area, said third base station site including means for receiving and transmitting signals on a plurality of third radio channels, ..~
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each.of said third ra,dio ~hannels ha~ing a predetermined carrier frequency dif~erent from the carrier frequencies of said first and second radio channels, the carrier frequencies of individual ones of said third channels being separated by at least said ~irst predetermined frequency separation, the carrier frequencies ~, of each one of said third channels being separated from the - carrier frequency of one of said first channels by a second pre-determined frequency separation, said second predetermined frequency separation being less than said means connected to said firs:t, 3econd and third base-station sites for transferring signals between said base station sites;
a plurality of first receiver sites each including means for receiving signals on said plurality of first radio channels, each of said first receiver sites being located in a predetermined geographic area smaller than said first pre-determined geographic area wherein the geographic area of each of said first receiver sites overlaps said first geographic area;
a plurality of second receiver sites each including means for receiving signals on said plurality of second radio channels, each of said second receiver sites being located in a predetermlned geographic area smaller than said second yeo-~ra~ia ~rsa wh~in ~e g~ograpl~c ax~a ~. aaa~ o ~aid ~a~n~
1 receiver sites overla~s said second geographic area, one of said second receiver sites including means for receiving signals on said plurality of first radio channels, the geographic area of said one of sa~d second receiver sites overlapping both ~aid fi~3~ co~ d~ter~e~.g~grap~ic a~ea~
a plurality of third receiver sites each including means for receiv~ng s'ignals on said plurality of third radio channels, each of said third receiver sites being located in apredetermined geographic area smaller than said third pre-determined geographic area wherein the geographic area of each - 6 a ~ Z9~39 ::
of sa~d third receiver s~tes overlaps said third geographic ; area; and means for connecting each of said first, second and third receiver sites to said first, second and third base station~sites, respectively.
DESCRIPTION OF THE DRAWINGS
Fig, I is a plan view of the organization of the radio telephone system according to the invention, assuming uniform propagation, and showing the allocation of frequencies to the various cells;
P~g, la is a more detailed plan view of some of the cells of the system of Fig. 1 showing the division of the cells ~nto 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 radio telephone system according to the invention showing the variation in spacing between base stations and receiver sites encountered in a typical practical mixed urban and rural area;
Fig. 4 is a sequence diagram showing the typical sequence of events occurring in the system according to the invention when a call is initiated by a land base telephone;
Fig. 5 is a sequence diagram showing the sequence of events occurring during a portable unit initiated call;

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- 6 b -~ ~04Z98g Fig. ~ is a block diagram of one of the remote receiver sites, indicated by crossas in Figs. l 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 detailed block diagram of the central ~-; control center 130 of Fig. 2; and Fig. 9 is a detailed block diagram of one of the portable units usable with the system according to the invention.

DETAILED DESCRIPTION

Referring to Fig. 1, there is shown a plan view of a fre-quency allocation scheme, according to the invention, usable with mobi.le or portable radio or radio telephone systems. The geo-graphic area to be covered is divided into a plurality of cell groups, each group containing a predetermined number of cells.
The number of cells in each group is determined by the following equation:
N - i2 ~ j2 + ij 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. l, i is equal to 2 and j is equal to l to provide a seven cell group, however, other values of i and j may be selected to provide different patterns.
In Fig. l, each of the cells lOa, 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 signalling frequency and at least one outgoing communications frequency, while each base station receiver is allocated at least one incoming signalling frequency and one incoming communications frequency, each incoming frequency being paired with an outgoing frequency to .

~ r~ t ;29~
provide a full duplex channel. The duplex channel sets allocated to each of the cells lOa, 20a, 30a, 40a, 50a, 60a, and 70a are denoted as FlA-~7A, respectively. In a typical system, employing frequency modulation and + 5 KHz deviation, a 25 KHz separation between frequencies used within a cell group has been found to provide adequate protection from adjacent channel interference.
In a cell system of the type illustrated in Fig. 1, the frequencies FlA-F7A may be reused in other cell groups that have sufficient geographic separation therebetween to substantially eliminate co-channel interference. For example, the frequencies FlA-F7A may be reused in the cell group com-prising cells lOb, 20b, 30b, 40b, 50b, 60b, and 70b, respec-tively, and in the group comprising cells lOc, 20c, 30c, 40c, 50c, 60c and 70c, the cells having the same numerical prefixes being assigned the same group of frequencies. However, 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. Accord-ingly, systems have been designed using larger cell groups,such as, for example, twenty-one cells per group, and allocating different frequencies to each of the twenty-one cells in the group. Unortunately, the allocation of twenty-one different 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 invention has recognized the fact that cells that are not adjacent to each other geographically, such as cells lOa, lOb and lOc do not require a 25 KHz separation between frequencies assigned CM~73537 l~Z9l39 thereto because of the geographic spacing therebetween.
Accordingly, frequencies assigned to cells having similar numeric prefixes 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 frequency sets FlB-F7B, assigned to cells lOb, 20b, 30b, i4~b, 50b, 60b and 70b may be spaced only 8.33 KHz from one of the frequency sets FlA-F7A, respectively. Similarly, the fre~uency sets FlC-F7C assigned to the cells lOc, 20c, 30c, 40c, 50c, 60c and 70c need be spaced only 8.33 XHz from the frequency sets FlA-F7A and FlB-F7B, respectively. The above described interleaved rrequency allocation system pro-vides improved co-channel interference protection over that provided by a normal seven cell system-while maintaining 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 of~set between cell groups of such a system is described later in the application.
Referring to Fig. la, there is shown a more detailed drawing of the cell structure of Fig. 1. Although the fre-quency allocation scheme of Fig. 1 may be used in 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.

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C~-73537 In Fig~ la, the rec~elv~ s81~es 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 application. Each of the cells is divided into a group of sub-cells, for example, the cell lOa is divided into the sub-cells lla-17a, the cell 20a into sub-cells 21a-27a, 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 lOa transmits and receives on the frequencies in the set FlA, the base station site in the cell 20a transmits and receives on the channels in the set F2A and the base station site located in the cell 30a transmits and receives 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 transmitter, receiver sites in add.ition 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 o~ the receiver sites is indicated in Fig. la by a hexagonal dashed line sub-cell about each receiver site. Each cell is divided into seven sub-cells, one about the base station site, and six about the six receiver sites. For example, the cell lOa is divided into sub-cells lla-17a, the cell 20a into sub-cells 21a-27a and the cell 30a into sub-cells 31a-37a. Of the aforementioned sub-cells, only the sub-cells lla, 21a and 31a are contained entirely with-~429~
in their respective cells. The remaining sub-cells overlap two cells. For example, the sub-cell 13a of cell 10a overlaps the sub-cell 36a o~ cell 30a. Accordingly, the receiver site located at the boundary of cells 10a and 30a must be capable of receiviny signals on all of the fre~uencies FlA and F3A
assigned to cells 10a and 30a, respectively. Similarly, each of the receiver sites located at a cell boundary must be capable of receiving signals on frequencies assigned to both cells adjoining 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 communication, however, directional antennas and receivers for monitoring all active communications channels are employed at the base station sites for monitoring the activity of the portable units and for reassigning communications 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 t~e interconnections between the base station transmitter and receiver sites and the portable units which communicate with the sy~tem. Three base stations 102, 104 and 106 are shown. Each of the base stations 102, 104 and 106 contains a transmitter and a receiver and corresponds to one of the transmitter-receiver sites denoted by circles in Fig. la, such as, for example, the circles shown in cells 11, 21 and 3I. Only three base stations ara shown for purposes of simplicity, however any number may be used depending on the size of the area io be covered. The base station 102 has three receiver sites 110, 112 and 114 connected thereto. Similarly, receiver sites 116, 118 and 120 are connected to the base station 104, and the recaiver sites 122, 124 and 126 are connected to the base j C~73537 .
~ 298~
station 106. The receiver sites correspond to the crosses shown in Fig. la. The number of receiver sites connected to each base station is determined by the number 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. la, however, only three receiver sites have been shownin Fig. 2 to avoid unnecessarily complicating the drawing.
~ach of the base stations 102, 104 and 106 is further con-nected 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 central 129. Three portable units 132, 134 and 136, each containing a transmitter and a -~
receiver for communicating with the base station and receiver site network are shown. Whereas only three portable units are shown, the actual number which may be used in a practical system is limited only by the numbex of base station and receiver sites in the system, and the number of fre~uencies allocated to the system.
In operation, outgoing messages are transmitted from a base station, such as the base station 102, to a portable 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 station 102 and the central control center 130. The central control center 130 connects the base station 102 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 telephone is desired.
In the system of the instant invention, the transmission 7 :~ 5 ~ 7 , ~ , range of the base station~l~s l~ntentionally 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 portable unit, and the port-able unit transmitter transmits to the base station receiver or to one of the receiver sites deployed within the coverage area of the base station. The transmission range of the portable unit is intentionally limited because, unlike a base station, a portable may move between areas and interfere with other portable transmissions in areas using the same frequency.
Prior art systems, in which the range of the base and protable units were fixed and equal, sought to control the portable intererence problem by accurately locating 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 ar~a reduces portable interference to an acceptable level, however, it does not provide the portable unit with vertical mobility, and it does not assure that the best communications channel is provided, because due to terrain 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 location equipment necessary to locate a portable accurately enough to avoi.d`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 output power of the portable unit may be sufficiently reduced to allow less !' !',- ' ~L!al429~9 accurate location of the unit without causing interference with other portables operating at the same frequency.
1.:
The signal to interference ratio between units operating ; on,the same frequency is Pxpressed by the following equation:

, S = K log (D - 1) I R
where S/I is the signal to interference ratio, D is the ~istance between stations operating on the same frequency, R is the distance between the station and the unit tPortable or mobile) with which it is communicating and'~ 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 portable units may now be allowèd to operate more closely together without-causing excessive inter-ference, the transmission frequency of each portable-unit can-be assigned to provide the best communications link rather ' than being arbitrarily assigned on a geographic basis.

Following is a description of the steps involved in determining the best transmission and reception frequency for a portable unit. Each base station within a predetermined geographic'area wherein co-channel interference may occur trans-mits a signal on a different outgoing signalling frequency.
Each base station transmitter also is capable of transmitting signals on different voice channels, also commonly referred ; to as information or communication channels. The receiver in each portable unit is automatically tunable to receive signals on any one of the signalling or voice channels transmitted by any of the base stations in the area. Each portable unit is also capable of transmitting a signal on different incoming signalling and voice channels, each incoming channel being . CM-73537 , 3L~4Z91~9 paired or associated with one of the outgoing channels, but having a different frequency than the outgoing channel to allow duplex operation. The receivers located in the base station and in the receiver sites are capable of receiving signals on the signalling channel that is paired with the outgoing signalling channel of the base station transmitter in the cell in which the receivers are located. Each of the receivers is also capable of receiving signals on each of the incoming voice channel ~requencies paired with the outgoing voice channel frequencies assigned to the base station trans-mitters associatad with the particular receiver site.
Referring to Figs. la and 2, in operation, each of the base station transmitters continuously sendsall signalling information on its signalling channel. The receiver in each portable unit continuously scans the outgoing signalling channels, measures the strength of the signal received on each of the signalling channels, and stores information indicating which of the signalling channels is the strongest.
'rhe strongest signalling channel is generally the signalling channel assigned to the base transmitter that is nearest the portable unit. For example, if the portable unit were located in the sub-cell 23a of Fig. la, the strongest signalling channel would be the signalling channel of the transmitter located in s~b-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 31a.
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 - ~ , i CM-73537 . .
9~
strongest received ou~going signalling ~requency. The transmission from the portable unit is received by one or more receivers located in a base station or receiver site, and the signal strength of the incoming 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 located in sub-cell 23a, however, due to transmission irregularities, 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~cell 23a 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 frequencies F2A assigned to cell 20a. At the same time a land communications link would be established between the base 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 fre-quencies from the group F2A but the signal received by the receiver site in sub-cell 22a would be relayed to the base station in sub-cell 21a even though the portable unit is physically located within sub-cell 23a to assure that the best co~nunication channel is provided.
If the portable unit located within the sub-cell 23a .

C~-73537 , had received the strongest signalling channel signal from the base station transmitter located in sub-cell 61c, the operating frequency of the portable unit would have been tuned to one of the frequencies F6C assigned to the cell 60c. A
land communications link would be established between the base station transmitter located in the sub-cell 61c and the receiver site located in sub-cell 66c (assuming that the receiver site in sub-cell 66c receives the strongest signal from the portable unit~. Since the coverage area o a portable unit is approx-imately 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 assignment of a cell 60c fre-quency to a portable unit operating in cell 20a will not cause interference to any portable unit operating elsewhere on the same frequency, such as in cell 60'c or 60"c.
Once the initial voice frequency pair has been assigned to a portable unit, the location of the unit must be continuously monitored in order that new communications channel frequencies ;
may be assigned thereto as required when the portable unit moves bet~een cells. The location function is provided by a group of receivers located at the base station sites which monitor all of the active voice or communications channels. Directional antennas may be employed at each base station site in order that the direction from which the strongest signal is being received may be ascertained. For example, the base station in the cell 30a of Fig. la employs an antenna array (denoted by the six radially extending lines) which has six lobes, each lobe covering a portion of the sub-cell 31a and one of the outer sub-cells 32a-37a. The other cells also utilize similar antenna arrays, each lobe covering a portion of the central sub-cell and one of the outer sub-cells.

, 10~2989 Each directional antenna is connected to either a plurality of receivers or to a single scanning receiver that may be rapidly tuned to any incoming voice frequency assigned to any nearby cell. Each receiver includes means for deter-mining the strength of the signal received, and is connected, either directly or indirectly to a central control center, such as the central control center 130. The control center idetermines the location of each portable unit based on the signal strength received by the location receivers, and initiates a reassignment of the portable communication channel as the portable unit moves from one cell to another.
In operation, assume that the unit had been located in the cell lOa when the call was initiated, and had been assigned a voice channel from the frequency group FlA. The voice channel assigned to the portable unit from the group FlA now becomes an active voice channel and is scanned by the location receivers located in cells lOa, 20a, 30a, 40a, 50a, 60a, and 70a. If the portable unit moves from cell lOa, towards cell 20a, the signals received by the antennas covering cell lOa will decrease and the signals received by the antennas covering the cell 20a will increase. The strength of the signals is compared by the central control unit 130, and when the signal received by an antenna covering the cell 20a exceeds the signal received by the antenna covering cell lOa by a predetermined amount, the base station located in cell lOa transmits a command (on the voice channel) to the portable unit to assign a new voice channel from the group F2A thereto. The central control unit also automatically switches the wire land lines from the base station transmitter and receiver site located in cell lOa to the base station transmitter and the receiver site located in cell 20a that is receivina the strongest signal.

CM~73537 9~39 In a similar fashion, had the portabla unit moved from cell lOa to cell 30a, the signal received by the antenna co~ering cell 30a whould have increased, and a voice channel from the group F3A would have been assigned. Had the unit only moved between sub-cells within a cell, such as between sub-cell 22a and 23a, there would be no frequency reassign-;; !ment, but only a switching of the wire land lines ~rom the receiver site in sub-cell 22a to the receiver site in sub-cell 23a. As in the case of the initial location and freauency assignment, due to the limited power of the portable unit, the location need not be precise and a portable unit operating in one cell may be assigned a frequency from an adjoining cell without causing interference to the rest of the system.
In order to provide for further interference protection and to reduce the battery drain of the portable unit, an automatic output control feature is also provided. The automatic output control eature also provides the portable unit with vertical mobility by reducing its output power when its transmission range increases as a result of being operated at a high point such as the upper stories of a high rise building.
To provide the automatic output control feature, each base receiver in the system is equipped with circuitry for monitoring the absolute level of the incoming signals received from the portable units. If the signal received by any receiver e~ceeds ~ a predetermined level which has been determined to be adequate ; to provide good communications, the base station transmitter sends a command to the portable unit to cause the portable unit to reduce its power until the signal received by the receiver is reduced to the minimum required for satisfactory communications.

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. CM-73537 , ~(~4Z989 The automatic output control may be provided in a variety of ways, for example, the transmitter transmitting a tone to the portable unit when the power is excessive, and the portable unit being responsive to the tone ~o gradually reduce the power to an acceptable level, at which point the transmission of the tone is terminated. A dynamic system may be provided by providing circuitry within the por~able unit which gradually increases the output power when a tone is absent and gradually decreases the output power in the presence of a tone, thereby assuring that the output power is always maintained at an optimum level.
The organization of the system of the present invention provides for a considerable saving in the amount of radio spectrum used. It has been found that in a normal unorganized FM two-way radio system such as the type used by police and business services, a 25 KHz spacing between channels provides adequate adjacent channel interference protection. The afore-mçntioned 25 KHz channel separation has been designed to provide adjacent channel interference protection to a receiver located near an adjacent channel transmitter, and attempting to receive signals from a distant transmitter on its channel, this being a worst case condition that does not occur in organized systems. ~-However, prior art cell systems have used channel spacings :.,.
that have been designed for uncontrolled systems thereby result-ing in an excessive spacing between channels and a consequent waste of the radio spectrum. The applicants have recognized that in an organized system, the situation of a portable unit being located close to an adjacent channel transmitter while attempting to receive a signal from a distant on-channel transmitter never occurs because of the geographic organization of the system, and as a result of the protection provided by C~-73537 ~ 0429f~39 the geographic organization of the system, the amount of protection that need be provided by frequency separation can be reduced.
The aforementioned concepts may be specifically implemented in the system of Fig. 1 as follows. The channel separation between adjacent cells of each group need be no more than 25 KHz to provide a total spectrum 175 KHz for a requirement of 7 x 25 XHz or a basic channel set in each seven cell group.
A basic channel set is defined as one channel out of each frequency set from each cell within a cell group, such as, for example, one channel from each of the frequency sets FlA-F7A from the cell group comprising cells lOa, 20a, 30a, 40a, 50a, 60a and 70a in Fig. 1. No more than 25 K~z separation between channels is necessary because, even though the cells are geographically adjacent to each other, the situation in which a portable unit is located near a strong adjacent channel trans-mitter while trying to receive signals from a distant on-channel transmitter never arises. Consequently, the spacing may even be somewhat less than 25 KHz. The spacing between adjacent frequency channels in each individual cell also need not be more than 25 KHz, however, in practical systems it may be more than 25 KHz because the 25 KHz spaced channel will generally be used in an adjacent cell. Because the geographic separation between cells in different cell groups provides additional interference protection, the frequency spacing between channels in cells of different cell groups need not be 25 KHz but may be considerably less. For example, where three different groups of frequencies FA, FB and FC are used, each containing frequencies FlA-F7A, FlB-F7B and FlC-F7C, respectively, the frequency separation need be only one third of 25 KHz, or 8.33 KXz. Hence, ~_ L'~-- / J ~
.
,0~ 9 a channel in a particular cell in one cell group, such as cell 10a, is separated from a corresponding channel in a corresponding cell, such as cell 10b, of a different cell group by only 8.33 KHz. The remaining interference protection is provided by the geographic separation between the cells of the different groups. As a result, twenty-one frequencies for twenty-one different cells are provided by the basic 175 KHz spectrum.
The same basic idea may be applied to any number of cells. This is accomplished by first determining the number of cells in each cell group and the amount of spectrum to be allocated to a basic channel set, and dividing the spectrum by the number of cells in each group to provide the channel spacing between cells in a group. Since co-channel inter-ference between cells in different cell groups is the limiting case in practical systems, the number of cell groups using different frequencies must be determined. This can be done using propagation measurements and calculations. Once the number of different cell groups has been determined, the spacing between frequencies in adjacent cell groups can be determined by dividing the basic channel spectrum by the total number of cells in all of the different cell groups.
In the example illustrated in Fig. 1, the basic channel set requires 175 KHz of spectrum, and the frequency separation between cells in a given cell group is 175 KHz divided by seven (for a seven cell pattern) or 25 KHz. The separation between frequencies in cells from different groups is 175 KHz divided by twenty-one (three groups of seven cells each) or 8.33 KHz.
The twenty-one cell pattern has been found to work well, however, other patterns are also possible.

_~

. CM-73537 .
Z98~
The discussion up to this point showing the layout of the system has used hexagonally shaped cells to illustrate the concepts of the invention; however, such regularly shaped cells would only be used in an ideal environment having uniform transmission characteristics and a lack of interference from other sources of electromagnetic radiation. In a practical system, the coverage provided by each base station and receiver site varies drastically depending on the environment, and the system would be tailored to provide base stations and receiver sites wherever necessary as determined by the environment.
Fig. 3 shows the layout of a typical practical system according to the invention. The areas 150, 152, 154, 156 and 158 indicate urban areas, the rest of the area being rural or suburban. Highways 160, 162, 164, 166 and 168 interconnect the various urban areas. The urban area 152 is the largest and ;~
most densely populated area of Fig. 3, and accordingly has the highest concen~ration of base stations and receiver sites, denoted by circles and crosses, respectively, as in Fig. 1.
The spacing between the base stations and receiver sites is small due to the large number of users and the shadowing effects o tall buildings generally present in large urban areas. The spacing between sites in the non-urban areas and in small urban ;-areas such as area 156 is considerably greater due to the improved propagation characteristics compared to those of a densely populated urban area, and the lower p3pulation density which allows less frequent frequency re-use~ Furthermore, as the number of users in an area, such as, for example, area 156, expands additional sites may be added where necessary to pro-vide the required communications. Communications is also provided along highways, the highways 162 and 168 beinq ' '.

1[)~29~39 served by base stations and receiver sites constructed nearby, and the highways 160 and 164 being served by extensions of the networ~ covering urban areas 152 and 150, respectively.
Fig. 4 shows the operation of the system, and shows, in detail, the sequence of events that happens when a call to a portable unit is initiated by a land based telephone. The telephone number dialed by the land based telephone is received by the central control center 130 which generates a portable address which corresponds to the address of the portable being called. Because, in general, the system has no way of knowing where the particular portable unit being called is located, the address of the portable unit being called is transmitted ; by all of the base station transmitters in the system on their respective outgoing signalling channels. Followiny the address of the portable, instructions are relayed to the portable unit requesting the portable unit to reply. The portable ~nit automatically selects the incoming signalling channel that is paired with the strongest outgoing signalling channel being received on which to reply. The last mentioned sequence of events is shown on line A Of ~ig. 4. As shown on line Br the portable then replies by transmitting its address and a "ready"
message on the incoming signalling channel corresponding to the strongest outgoing signalling channel received. The reply is received by the system, which then determines which receiver site has received the strongest signal. Based on this information, the system can determine in what area the portable is located and transmits instructions on the outgoing signalling channel assigned to that area to the portable to switch to a voice channel assigned to that area. This action is shown on line C.
` 30 The portable unit acknowledges receipt of the command ;~by trans-mitting its address and a "command executed" signal on the _ _ _ _ . CM~73537 9~9 assigned incoming voice channel as shown on line D. Upon receipt of the"command executed"signal, a ringing signal (line E~ is sent to the portable unit on the assigned voice channel to initiate ringing. Raising the portable receiver off hook generates a signal consistlng of the portable address and an"off hook"signal, which is transmitted to the system to terminate the ringing, as shown on line F.
The sequence of events for a portable initiated call is shown in Fig. 5. The sequence is less complex because in a portable initiated call, there is no need to transmit signals over the entire area to locate the portable. The sequence begins at line A when the portable unit goes off hook and transmits its address and a message requesting channel assignment on the incoming signalling channel paired with the strongest outgoing signalling channel it has monitored.
The request for a channel assignment is received by the system, which determines which site is receiving the strongest signal and assigns a voice channel (line B~ used in the area associated with that site and the signalling channel to the portable unit.
The channel assignment is acknowledged by the portable, which ~~
transmits its address and a request for a dial tone on the assigned voice channel, as shown on line C. The ~ase station ;
then responds on the voice channel by supplying a dial tone (line D~., whereupon the system is ready to accept dialing information. The dialing information is sent by pushing buttons on the portable unit to generate the standard Bell System tone signalling frequencies. The tones are received by the land lines ; network and processed in a fashion similar to the processing of normal land initiated dialing signals. Based upon the 30 particular number dialed, the receiver site and base s~ation communicating with the portable unit are connected to either , ~ 429~9 a land based telephone or to another base station and receiver site to provide com~unications with another portable unit.
Figs. 6-9 are block diagrams showing the structure of the base and portable sites, and the interconnections and logic therebetween. Referring to Fig. 6, there is shown a block diagram o one of the remote receiver sites, such as, for e~ample, the receiver site 110 in Fig. 2. A master oscillator 200 generates a stable frequency reference for a plurality of synthesizers 202. Each of the synthesizers generates a local oscillator signal for one of a plurality of receivers 204 connected thereto, each receiver being tuned to receive signals on the signalling and voice channels assigned to the cell in which the receiver site is located. The signals are xeceived by an antenna 206 and applied to a multi-coupler amplifier 208 which applies the received signal to each of the receivers 204. The outputs of the receivers 204 are connected to a switching con-trol unit 210 which applies the output signals from the receivers 204 to wire lines 209 interconnecting the receiver sites and the base stations. A signal strength detector and encoder 212 re-ceives information from each o~ the receivers 204 indicative ofthe strength of the signals received thereby, and encodes the signal strength information to provide a signal strength indica-tive signal having a bandwidth that is compatible with the band-width of a telephone line. The outputs of the signal strength detector and encoder 212 are connected to the switching control unit 210 which applies the signal strength indicative signals to a data line 211 for transmission to a base station site.
Peferring to Fig. 7, there is shown a block diagram of one of the base stations in the system, such as, for example, the base station 102 of Fig. 2. The base station site contains a plurality of receivers similar to the receivers located in the .

' , ~)4Z5~89 remote sites of Fig. 6. The receivers are indicated by the blocks 200a, 202a, 204a and 208a, which provide functions analogous to the functions provided by the blocks 200, 202, 204 and 208, respectively, of Fig. 6. In addition to providing local oscillator signals to the receivers 204a, the synthe-sizers 202a also provide reference signal for a plurality of exciters 214 connected thereto. Each local oscillator signal applied to one of the receivers 204a has a companion signal paired therewi~h applied to one of the exciters 214 to pro-vide a full duplex channel. The outpu~s from the exciters214 are applied to a common power amplifier 216 which amplifies each of the exciter signals to a level suitable for transmission.
Because of the nature of the overall system, wherein each poxtable receiver is assured of receiving the strongest signal in its area, a common power amplifier is p~actical because the intermodulation components generated thereby will always be smaller than the ~agnitude of the desired signal being received.
In prior art systems wherein voice channels are assigned on the basis o~ geographic location rather than signal strength, the portable unit is not assured of receiving the strongest com-munications channel, and separate power amplifiers must be used to prevent the intermodulation components generated by a single power amplifier from exceeding the level of the signals being received by the portable units.
The output of the common power amplifier 216 is coupled to a diplexer 218 which applies the amplified signal to an antenna 220 for transmission thereby. The diplexer 218 is also connected to the multi-coupler amplifier 208a for coupling signals received by the antenna 220 to ihe multi coupler amplifier 208a.
The output of the multi-coupler amplifier 208a is also ~ LO~Z91!39 connected to a scanning receiver 222, the purpose of which is to scan all active voice channels to provide location information concerning the location of active portables, as previously described. The scanning receiver 222 is tuned by a synthesizer 224 connected thereto which provides local oscillator signals to the scanning receiver. A scanning control circuit 226 periodically changes the output frequency of the synthesizer 224 to cause the scanning receiver 222 to -~ scan all active voice channels. The channels scanned are determined by signals received from the switching control circuit 228 based on a signal received from the central control center 130, which monitors the active voice channels. An out-put signal, such as, for example, a limiter current or squelch signal is applied to a logarithmic amplifier 230 connected thereto.
The output of amplifier 230 is connected to the switching control

2~8 which applies the signal strength indicative signal from the logarithmic amplifier 230 to the central control unit 130 for determination of the location of the active portable units.
Signals indicative of the strength of the signal received by the receivers 204a are applied to the signal strength -~
detector 232, which also receivës signal strength information ~rom the satellite receiver sites. The signal strength detector 232 detects the levels of the signals received by the various receivers located in the base station and receiver sites and generates a tone for application to the exciters 214 connected thereto to modulate the exciter corresponding to a received channel having an excessive received power level. The tone is transmitted on the outgoing channel corresponding to the incoming channel having the excessive power, and causes the offending portable unit to reduce its output power to an acceptable level.

.

CM~73537 .

Referring to Fig. 8 ~ ~ ~e9 81s shown a general block diagram of the central control center 130. Incoming wire lines 131 from a normal telephone network are connected to a switching network 232 which is also connected to a computer 234. The computer translates incoming dial pulses or tones from the wire lines 131 to corresponding portable addresses based upon the information stored in the memory 236, The stored information includes the addresses of all portable units in the area, plus the addresses of units from other areas or "roamers" which are currently operating in the area.
The addresse~ are transmitted to the various base stations via data lines and modem 238 to allow a portable unit to be paged.
Information from the base stations including signal strength data from the receiver sites and base station receivers, and address and signalling information transmitted b~ the portable units is received from the data lines 240 via the modem 238.
The received informa~ion is applied to the main computer 234, which controls the switching network 232 to cause the switching network to connect ~he incoming wire lines 131 to the appro-priate voice lines 242 connected to the base station sites. An operator's console 244 is provided to control the overall system, to insert and remove the addresses of "roamers" into the memory as the "roamers'l enter and leave the area, and to override the computer as necessary.
Referring ~o Fig. 9, there is shown a block diagram of a portable unit, such as, for example, the portable unit 132, for use with the system according to the invention r The receiver portion of the portable unit is a dual conversion receiver con-taining several blocks which are of conventional design includ-ing an RF amplifier 250, a first mixer 252, a first ~F amplifier 254, a second mixer and second local oscillator 256 and 258, .

respectively, a second intermediate frequency ampliier 260, a discriminator 262, an audio amplifier 264 and an earpiece 266, all of which operate in a conventional manner. The trans-mitter portion also contains several conventional blocks in-cluding a power amplifier 268, a driver 270, a doubler 272 and a tripler 274. An antenna 276 is connected to a diplexer 278, which is in turn connected to the RF amplifier 250 and the power amplifier 268 for applying signals from the antenna 276 to the RF amplifier 250 and for transmitting power from the power amplifier 268 to the antenna 275.
A signal strength detector 280 is connected to the second IF amplifier 260 of the receiver for detecting the strength of the received signals when the receiver is scanning the signalling channels. The signal strength indications from the detector 280 are applied to a supervisory unit 282 and stored therein.
A frequency synthesizer 284 is connected to the supervisory unit 282 and to the tripler 274 of the transmitter. The frequency synthesizer 284 is also connected to the first mixer 252 by means of a multiplier 286 for providing local oscillator injection for the receiver. The supervisory unit 282 causes the frequency synthesizer to change frequency cause the receiver to scan the various signalling frequencies, and upon appropriate command, as described in previous sections of this disclosure, to retune the frequency of the transmitter and receiver to the incoming siqnal-ling frequency or voice frequency associated with the strongest received outgoing signalling frequency.
The supervisory unit 282 is also connected to the discxim-inator 262 and receives tones transmitted by the base stations indicative of excessive portable power being received by the base stations or remote receiver sites. Upon receipt of an excessive ~-7353l power tone from the discriminator 262, the supervisory unit applies a signal to an automatic output control 290, which gradually reduces the power output of the driver 270 until transmission of the excessive power tone has terminated.
Upon termination of the excessive power tone, the automatic output control 290 again gradually increases the power output of the driver 270 until excess power is again detected where-upon the power reduction sequence is repeated.
; A microphone 292 is connected to an audio amplifier and instantaneous deviation control circuit 294 which is in turn controlled to a voice operated transmitter control 296. The voice operated transmitter detecto~ detects the output of the amplifier 294 for the presence of signals from the microphone 292 or tones from the tone generator 298 and renders the trans-; mitter operative only in the presence thereo, thereby turning off the transmitter to save battery power during pauses in speech.
The supervisory unit 282 is also connected to the audioamplifiers 264 and 294 for rendering the latter inoperative except upon the receipt or initiation of a call as indicated ; by a signal from the discriminator 262 or the off hook button 300, respectively. The off hook button 300 serves the same function as the cradle buttons in a normal telephone and renders the transmitter operative to transmit its address, as prevlous described, when a call is being initiated by the portable unit.
Although the invention has been described with reference to particular circuits and embodiments, other embodiments employing the teachings of the foregoing disclosure are deemed to lie within the purview of the invention.
.

Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A communications system comprising:
a first base station site located in a first pre-determined geogpaphic area and including means for receiving and transmitting signals on a plurality of first radio channels, each of said first radio channels having a predetermined carrier frequency, the carrier frequencies of individual ones of said first radio channels being separated by at least a first pre-determined frequency separation;
a second base station site located in a second pre-determined geographic area adjacent said first predetermined geographic area, said second base station site including means for receiving and transmitting signals on a plurality of second radio channels, each second radio channel having a predetermined carrier frequency different from the carrier frequencies of said first radio channels, the carrier frequencies of the individual ones of said second channels being separated by at least said first predetermined frequency separation, the carrier frequencies of each of said second channels being further separated from the carrier frequencies of each of said first channels by at least said first frequency separation;
a third base station site located in a third pre-determined geographic area non-adjacent to said first geographic area, said third base station site including means for receiving and transmitting signals on a plurality of third radio channels, each of said third radio channels having a predetermined carrier frequency different from the carrier frequencies of said first and second radio channels, the carrier frequencies of in-dividual ones of said third channels being separated by at least said first predetermined frequency separation, the carrier frequencies of each one of said third channels being separated from the carrier frequency of one of said first channels by a second predetermined frequency separation, said second pre-determined frequency separation being less than said means connected to said first, second and third base station sites for transferring signals between said base station sites;
a plurality of first receiver sites each including means for receiving signals on said plurality of first radio channels, each of said first receiver sites being located in a predetermined geographic area smaller than said first predeter-mined geographic area wherein the geographic area of each of said first receiver sites overlaps said first geographic area;
a plurality of second receiver sites each including means for receiving signals on said plurality of second radio channels, each of said second receiver sites being located in a predetermined geographic area smaller than said second geographic area wherein the geographic area of each of said second receiver sites overlaps said second geographic area, one of said second receiver sites including means for receiving signals on said plurality of first radio channels, the geographic area of said one of said second receiver sites overlapping both said first and second predetermined geographic areas;
a plurality of third receiver sites each including means for receiving signals on said plurality of third radio channels, each of said third receiver sites being located in a predetermined geographic area smaller than said third pre-determined geographic area wherein the geographic area of each of said third receiver sites overlaps said third geographic area; and means for connecting each of said first, second and third receiver sites to said first, second and third base station sites, respectively.

2. A communications system as recited in claim 1 further including; a fourth base station site located in a fourth predetermined geographic area non-adjacent to said first predetermined geographic area, said fourth predetermined geographic area being separated from said first predetermined geographic area by a predetermined geographic separation greater than the separation between said first and third predetermined geographic areas, said fourth base station site including means for receiving and transmitting signals on at least one of said first radio channels.