CA1061412A - Mobile communication system and method employing directional service area coverage - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A communication method and system for providing two-way radio communication links between fixed stations and mobile units, particularly suitable for use in mobile telephone communications. A geographical service area is sectionalized through the use of omnidirectional and directional antennas resulting in increased call capacity for a given number of available communication channels. An orderly, cost-effective manner for accommodating service area expansion, a dynamic channel assignment technique for accommodating variations in user density and a technique for maintaining an established call as a mobile unit moves throughout the sectionalized service area are also disclosed.

Description

BACKGROUND OF THE INVENTION

The present invention relates to mobile radio communication systems and, in particular, to a mobile radiotelephone method and system in which a geographical area is provided with mobile radiotelephone service by dividing the geographical area into smaller geographical areas through the use of directional and omnidirectional antennas.
Approximately twelve communication channels, each including two distinct frequencies for two-way communica-tions (an up-link and a down-link), are typically available for use in present day mobile telephone systems. In one known system, usually referred to as Improved Mobile Telephone Service (IMTS), communication is established over the available channels from a wire line telephone system to mobile units through the use of transmitters which transmit omnidirectionally throughout a large geo-; graphical area or zone from an antenna approximately centered in the area. A plurality of satellite receivers are spaced throughout the geographical area to receive transmissions from the mobile units and relay the trans-missions to the central location. Calls are established through seizure of a marked idle channel by a mobile unit and by placing or receiving a call over the seized marked idle channel.

~1 - 3 -With only twelve channels available for use in this type of system, only twelve simultaneous conversa-tions are possible and, not only is the total number of subscribers in a service area nece~csarily limited, but also the subscribers who do obtain service tend to find circuits busy a very high percentage of the time when attempting to place a call. It can thus be seen that 12-channel systems based upon a large zone coverage from a central location have been unable to meet present user demands, let alone ~(~6~41Z
future requirements for mobile radiotelephone service. As a result, several different arrangements have been empioyed in an attempt to increase the capacity o a systemO
One attempt to improve channel availability is to provide freq~ency reuse of the existing channels by changing radio coverage philosophy from that of high power transmission, long range reception to low power trans-mission, short range reception. ~his type of radio coverage is typicaily referred to as a small zone concept in which small zones of reception and transmission are defined by spaced base stations. In a small zone system, channel - euse is permissible where sufficient spacing exists behveen two ~mall zones.
For example, increased capacity may be attaincd by- dividing a large service area or zone into small service areas or zo~es with a base station at the center of each small zone. ~ransmitters and receivers a~ each base station together provide mobile telephone coverage of the large geographical area. In such an arrangement, channels used in one small zone may be reused in another small zone if sufficient spacing exists between the two zones. ~hus, in a large metropolitan area, for example, available channels may be reused in several separated small zones with a minimum of irterfcrence.

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i~61412 The equipment requirements in a small zone system may greatly exceed that required in a large zone system but the increased system capacity may more than balance this increased expense. Another drawback in a typical small zone system is that the mobility of a mobile telephone unit may also be somewhat decreased due to the movement of a mobile unit from one small zone into another and the resultant loss of the signal channel over which a call is established.
A further problem in known mobile telephone systems is the problem of increasing area coverage as the outer limits of the service area expand. Relatively facile expansion of the mobile telephone system as a service area grows in size is extremely important both in efficiently employing the limited channels presently allocated to mobile telephone service and in making proper use of the vastly increased number of channels which may become available in the UHF spectrum.
It is an object of the present invention to provide a novel method and mobile communication system which obviates many of the problems associated with known mobile telephone systems.
STATEMENT OF THE INVENTION
Accordingly, the invention includes a method for pro-viding broadcast service in a mobile communication system service area. The method includes the steps of providing at least one base station in the system service area and directionally broadcasting wave energy from the one base station in a plurality of sectors of the service area. The wave energy is broadcast so as to define directionally separate broadcast sectors of the service area. These sectors extend radially outwardly from the base section. The directionally broadcast wave energy is broad-cast in adjacent of the sectors at different frequencies and in ~ _5_ ,'~' J 0ti~412 at least some of the sectors at the same frequency.
In the present invention, the method may comprise the further step of omnidirectionally broadcasting wave energy in the respective zones surrounding each of the base stations, the plurality of sectors extending radially from each of the base stations beyond a radially outward limit of the associated zone.
An idle channel marker signal may be omnidirectionally broad-cast from locations spaced radially outwardly from each of the base stations, the locations being generally central of each of the sectors.
The instant invention also embraces a system for providing broadcast service between fixed stations and mobile units within a service area in a mobile communications system. The system includes a first means at at least one base station in the service area having a plurality of directional transmitters for directionally braodcasting wave energy in a plurality of directionally defined sectors of the service area. The sectors extend radially outwardly from the base station in different directions so as to define directionally separate broadcast sectors of the service area. The wave energy being broadcast by the base station is directionally broadcast in adjacent of the sectors at differing frequencies and in at least some of the sectors at the same frequency.
The system may include first, second and third base stations each including a plurality of transmitters, each one of the base stations being spaced approximately equidistantly from each of the other base stations. A plurality of directional antennas may be provided at each of the base stations. Each directional antenna at each base station may be operatively associated with at least one of the plurality of transmitters to broadcast wave energy in a sector of the service area. One ~ -5a-I0614~;~
of the plurality of directional antennas at each base station may be operable to broadcast wave energy in substantially the same sector of the service area thereby providing broadcast coverage of the same sector from each of the first, second and third base stations. The remaining ones of the plurality of directional antennas at each of the base stations may be oriented to provide broadcast coverage of other sectors of the service area from each of the first, second and third base stations. The directional characteristics of the directional antennas may be such that immediately adjacent of the sectors overlap in area by an amount less than the area of any one of the sectors.
The system may also include second means at each of the base stations for omnidirectionally broadcasting wave energy in a zone surrounding each base station. In such a case the sectors extend beyond a radially outward limit of the respective zones.
In the system of the claimed invention immediately adjacent of the sectors may be served by different ones of the communication channels assigned to the system. The same communication channel serving at least some of the sectors may be spaced by at least one other sector, whereby the same communication channel may be simultaneously used in more than one sector of the service area.
The system may further include a plurality of satellite receivers adapted to receive transmissions from mobile telephones of communications units, the satellite receivers being located remote from the directional antennas. At least one satellite receiver may be located approximately centrally of each of the sectors served by the directional antennas.
The system may also include means for establishing a call between a fixed communication unit and a mobile communication unit over an available one of the two way communication channels ~ -5b-lQ6141Z
through one of the base stations. A means may be operatively connected to the plurality of directional antennas at at least one of the base stations for monitoring the signal strength of transmissions from the mobile unit engaged in the established call received over a first one of the directional antennas defining a first one of the sectors designated as a home sector.
A further means, operatively connected to the monitoring means, may be provided for detecting a drop in the monitored signal strength received over the first one of the directional antennas.
A drop in monitored signal strength received over the first one of the directional antennas below a predetermined threshold may be determined by a means operatively connected to the monitoring means. The detection means may, in turn, be coupled to a means for monitoring and comparing the signal strength of the trans-missions from a mobile unit engaged in the established call received over only three of the directional antennas defining three monitoring sectors adjacent to and surrounding the first one of the sectors, two of the three directional antennas defining the three monitoring sectors being at the one of the base stations and the third of the three directional antennas being at another of the base stations. Finally, a means may be provided responsive to the comparing means for designating one of the three monitoring sectors as a new home sector.

-5c-BRTEF ~ 'SCI~IPTION OF TH~ D~AWINGS

Figure 1 is a functional block diagram of one embodiment of a mobile telephone system according to the present invention;
Figure 2 is a plan view of a. nobile teLephone service area illus-5 . trating the communication coverage pattern of the system of Figure 1;
Figure 3 is a functional block diagram of one embodiment of thecentral control unit of Figure 1;
Figure 4 is a functiona.l block diagram of one embodiment of the base station of Figure 1;
Figure 5 is a functional hlock diagram of one embodiment of a dynamic c:hannel assignment unit which may k,e employed in conjunction with the system of F igure 1 at the base station of Figure ~;
Figure 6 is a functional blo~k diagram of another embodiment of a mobile telephone system according to the present invention;
Figure 7 Ls a plan view of a mobile telepllone service area illus-trating the cornmunication coverage p attern of the system of Figure 6;
Figures "8~, 8B, 8C, 9 and 10" are p.lan view of mobile te'ephone service areas i.llustrating various system e~pansion tec~miques employing various directi.onal coveragc patterns according to the prcsent invention;

Figures 11 and 12 are pictorial views of alter-native embodiments of directional antenna arrays for use in the system of the present invention;
Figure llA is a plan view of a mobile telephone service area illustrating the directional coverage obtained with the directional antennas of Figures 11 and 12;
` Figure 13 is a functional block diagram illustrating one embodiment of a base station for accommodating sector change in conjunction with the Figure 1 and Figure 6 embodiments of the invention;
Figure 14 is a plan view of a mobile telephone service area illustrating one embodiment of a directional : approach for accommodating sector change in both known small zone communication systems and in expanded area coverage systems employing directional coverage as in Figures 8-10; and, Figures 15 and 16 are functional block diagrams illustrating base station equipment which may be employed in the directional antenna approach for accommodating sector change in accordance with the invention.

DETAILED DESCRIPTION
Embodiments of the present invention and various features thereor are described hereinafter in the environ-ment of a mobile telephone system as set forth in the following table of contents:

~l ~ _ 7 _ ~0~412 I. General Description of One System Embodiment (Figures 1 and 2) A. Central Control Unit (Figure 3 B. Base Station (Figure 4) C. Adaptive Channel Assignment Technique (Fiyure 4) D. Fixed Frequency Dynamic Channel Assignment Unit (Figure 5) ~06141Z

II. Description o~ Another System Embodiment (Figures 6 and 7) - III. Service Area Expansion ~Figures 8A, 8B, 8C, 9 and 10) IV. Directional Antenna Array (Figures 11 and 12) V. Sector Change Techniques ~Figures 13, 14, 15 and 16~

I. General Description of One System Embodiment One embodiment of the present invention as employed in a large zone mobile telephone system is illustrated in Figures 1-5. The embodiment of Figures 1-5 is compatible with existing IMTS equipment, and in particular, existing IMTS mobile units operable to seize a marked idle channel.
The mobile units and other equipment specified hereinafter as being conventional will thus not be described in detail.
Referring now to Figures 1 and 2 wherein one embodi-ment of the system is generally illustrated, a centralcontrol unit 20 may be conventionally interfaced with the incoming or input trunk lines IT and the outgoing or output - trunk lines OT of a commercially installed wire line telephone system 22. The central control unit 20 may con-trol the selective connection of the incoming trunks IT to a base station 24 via pairs of voice grade transmission lines

2~, hereinafter referred to by the designation VOICE (TX).
The central Control unit 20 may also transmit control signals CONTROL by way of transmission lines 28 to provide control of the base station operation.
As will hereinafter be described in greater detail, the base station 24 may include a plurality of transmitters (Figure 4) and the output signals from certain of the transmitters may be applied to a transmitting portion of ~ _ g _ an omnidirectional antenna 30 located at or in proximity to the base station 24 at approximately the center of the service area 38 illustrated in Figure 2. The output signals from other transmitters at the base station 24 may be applied to directional sections or segments of a directional antenna array 32 also located at or in proximity to the base station 24 at approximately ~0614~2 the center of the service area 38, The omnidirectional antenna 30 provides mobile telephone service to a sector or zone Cl of a mobile telephone service area surrounding the base station 24 over predetermined communication channels as will subsequently be described in detail.
Each section of the directional antenna array 32 prGvides service to a different one of a plurality of sectors Al-A3 and Bl-B3 extending radially outwardly from the antenna array 32 into the mobile telephone service area, as will also subsequently be described in detail.
For example, as is generally indicated in Figure 1, a group of transmitter output signals designated Cl may be applied to the omnidirectional antenna 30 serving the correspondingly designated sector Cl of Figure 2. Groups of transmitter output signals Al-A3 and Bl-B3 may be applied to selected sections or segments of the directional antenna array 32 serving the correspondingly designated sectors Al-A3 and Bl-B3 of Figure 2, The system employing this combination of directional and omnidirectional ser-vice area coverage is hereinafter referred to as an omni-sectional array ~OSA) system.
In addition, the base station 24 may include a plurality of receivers (Figure 4~ and a signal ORCV from a receiving portion of the omnidirectional antenna 30 may 1~

11~61~12 be applied to these receivers at the base station.
Signals received from the mobile units by the receivers at the base station 24 over the directional antenna array 32 and the omnidirectional antenna 30 may be ; 5 supplied from the base station 24 to the central control unit 20 by way of voice grade transmission lines 27 desig-nated VOICE (RCV) hereinafter.
In prov~ding broadcast coverage of a large area, e.g., an area having a radius of about 15-20 miles, reception of signalq oy way of the , .

~06141Z

directional antenna array 32 may result in low quality voice communications. To improve reception from the mobile units, a plurality of satellite receivers 34 (designated SRl-SR6 to facilitate the description) may be connected to associated omnidirectional antennas 36.
The satellite receivers 34 may be spaced throughout a mobile telephone service area in accordance with a pre-determined pattern as will hereinafter be described in greater detail to receive signals from mobile units in the service area. The signals received from the mobile units by the satellite receivers 34 may be trans-mitted to the base station 24 by way of voice grade transmission lines as the SRCV signals and may be trans-mitted from the base station 24 to the central control unit 20 by way of the VOICE (RCV) lines 27.
With continued reference to Figures 1 and 2, the system of the present invention may service a large zone or service area 38 and provide telephone communications between the wire line telephone system 22 and mobile units 40 within the service area 38. The omnidirectional antenna 30 at the base station 24 may define by its propagation pattern one small omnidirectionally defined service zone or sector within the service area 38 with the base station 24 at the center of the small sector or zone as is generally indicated by the circle 42. Within the zone 42, communi-cations between the wire line telephone system 22 and ~061~12 mobile units 40 may thus be established in a conventional manner through the use of the omnidirectional antenna 30 and the equipment at the base station 24 under the control of the central control unit 20.
The directional antenna array 32 may be structured to define a plurality of directionally defined service zones or sectors Al-A3 and Bl-B3 extending radially out-wardly from the base station 24 and together providing coverage of the entire large zone or service area 38. In the example - llA -illustrated in Figure 2, six sectors are defined by the individual directional antennas or segments of the direc-tional antenna array 32 (e.g., Figures 11, lla and 12) although it should be understood that a fewer or greater number of sectors may be established by the directional antenna array 32 as desired. The six sectors may overlap slightly and together provide 360 of broadcast (and reception) coverage.
In each of the sectors Al-A3 and Bl-B3, one or more associated satellite receivers SRl-SR6 may be pro-vided to omnidirectionally receive signals transmitted from the mobile units 40 within essentially omnidirectional receiving zones surrounding the satellite receivers as is generally indicated by the circles 44. The signals received by the satellite receivers SRl-SR6 may be transmitted to the central control unit 20 as was previously described to provide the return portion of the two-way link between the wire line telephone system 22 and the mobile units 40, transmission to the mobile units located in the zones 44 being provided from the directional antenna array 32 as was previously described.
With continued reference to Figures 1 and 2, twelve two-way communications channels are typically available for use in present mobile telephone systems ~ - 12 -and, in accordance with the present invention, may be distributed for use in the zones or sectors Al-A3, Bl-B3 and Cl as follows:

TABLE
Zone Channel*
Al 1-4 Bl 5-8 Cl 9-12 *Numbers herein do not represent frequency.

As can be seen from Table 1, the twelve avai.lable ct)mmL1nication channels may be reused in the sectors A1-A3 and B1-B3 as long as there exists at least one sector OI separation between those sectors in the servic~
area 38 using the same channels. For example, channels 1-4 may a.Ll be utilized in each of the sectors A1, A2, and A3 in that these sectors are separated by at least one of the sectors B1, B2 and B3. Since the sectors B1-B3 are separated by at least one of the sectors A1, A2, and A3, the cha~nels 5-8 may be reused in sectors B1-B3. Of course, it should be understood that other distributions o~channe.1s to the various ~ones is possible to meet the particular requirements of a specific installation.
Moreover, it shou.ld be understood that channels may be dynamically assigned to various zones to meet varying user density requirernents as iong as the assignment of a channe.l to a zone is accomplished on a non-interfering basis, i. e., sufficient separation is provided between zones emp.loying the same channel. If dynamic channel assignment is employed, remotely tunable transrnitters may be used and the satellite receivers mc.y-be remotcly t~mab.Le or standby receivers may b~ provided. One technique which may be employed for dynaJnical.ly assigning channels in response to variations in user density is described in Canad;an patent application Serial No. 199, 234 for "Method and System for Multiple Zone Communica-tion" by J. D. WelLs et al, filed on May 7, 1974, concurrently herewith, and assigned to the assignee of the present invention. The disclosure of the Wells ct al application is hereby incorporatecl herein by reference.
Typical.ly, the front-to-back ratio of the sections of the directional antellna array 32 at the l~ase stal.ion 2-1 may be such that the antenna sectionp. rc,viding service in tlle sector ~91 may rad~ le at a reduced power ~.cvel Jnthe opposite directic~n, i. e., into t.lle secto{ 132, In adcZiiion, a mobiLe IU~it - 1~--,-~

~06141Z

40 traveling in the vicinity of the base station 24 may rapidly move from one sector defined by the directional antenna array 32 into another sector. In an IMTS type system, the same channel must be available for use in the adjacent sector into which th~ mobile unit 40 moves to prevent a loss of communication. To alleviate these problems, small zone Cl served by the omnidirectional antenna 30 is established and assigned channels preferably differing from all other assigned channels to service those mobile units in the immediate vicinity of the base station 24.
To achieve compatibility with existing IMTS mobile units, one of the free or idle channels in each of the sectors Al-A3, Bl-B3 and Cl may be marked with a tone hereinafter referred to as an "idle channel marker" by modulating the signal transmitted at the idle channel frequency with the marker tone. In accordance with the embodiment of the invention illustrated in Figures 1 and 2, the transmitter transmitting the idle channel marker in each of the sectors Al-A3 and Bl-B3 may transmit at a reduced power level relative to the normal power level employed for communicating after a call is established.
Transmission of all idle channel markers in sectors Al-A3 and Bl-B3 at a reduced power level, e.g., six dB below the level of the signals from those transmitters engaged ~ - 14 -~(~61~1Z

in calls, increases the probability of proper idle channel seizure by the mobile units operating within the various sectors of the service area 38.
For example, channel 1 may be available in sector Al and may be designated as the idle channel by transmitting the idle channel marker or tone on channel 1 in section Al. A mobile unit 40 entering sector Al will search for a channel having an idle channel marker and will lock onto that marked channel. Since the marked channel is directionally broadcast in 10614~Z

sector Al at a reduced signal level, a mobile unit in sector Al is much more likely to lock onto and track the idle channel broadcast in section Al by the direc-tional antenna section serving the Al service sector than it is to lock onto the marked idle channel in either of the adjacent sectors Bl or B3. This primarily results from the capture characteristics of the typical FM
receivers utilized in the mobile units. Some ambiguity may persist at the periphery of zone Cl, but the back-to-front antenna ratio problem and interzone movement problemsare less serious at the periphery of zone Cl and an overall improvement thus results.
In placing a call from the wire line telephone system 22 of Figure 1 to a mobile unit 40 shown in Figure 2, the central control unit 20 is accessed by a subscriber to the wire line telephone system 22 via the incoming trunk lines IT. The mobile subscribers address, e.g., in the form of a four-digit telephone number following the initial three-digit access code, may then be broadcast by the base station 24 under the control of the central control unit 20 on the idle channel in each of the seven sectors Al-A3, Bl-B3 and Cl.
Upon receipt of and proper decoding of the broad-cast mobile unit address over the idle channel, the addressed mobile unit 40 may return a supervisory signal ~ - 15 -106~4~2 followed by an appropriate identifying code resulting in seizure of the idle channel by the called mobile unit.
Since the mobile unit transmitters are typically less powerful than the base station transmitters, the response from the mobile unit 40 may be relayed to the central control unit 20 via the satellite receiver 34 tuned to the return link, i.e., the uplink, of the seized idle channel in the appropriate sector.
The central control unit 20 thereafter designates and marks a new idle channel in that sector in which the idle channel is seized by the mobile 10614~2 unit 40. Moreover, the central control unit 20 commands the appropriate transmitter at the base station 24 to increase its output power level to normal operating power, e.g., increases the transmitter output power level by 6dB.
After the idle channel has been seized by the called mobile unit, the central control unit 20 performs the necessary switching to connect the appropriate in-coming trunk line IT to the appropriate transmitter in the base station 24 by way of the voice lines VOICE (TX).
Similarly, the CONTROL signal from the central control unit 20 effects the connection of the appropriate receiver of the satellite receivers 34 tuned to the seized idle channel to the appropriate outgoing trunk line OT by way of the voice lines VOICE (RCV).
In placing a call from a mobile unit 40 to a fixed telephone in the wire line system 22, the operator of the mobile unit 40 may effect the transmission of an off-hook signal and/or other appropriate supervisory control signals to the central control unit 20 over the idle channel or to which the mobile unit is locked resulting in seizure of the idle channel by the mobile unit. The central control unit 20 may thereafter seize one of the available outgoing trunks OT and connect the mobile unit 40 to the seized outgoing trunk OT by way of the seized communication channel. Dialing pulses or tones may thereafter be ~ - 16 -transmitted directly from the mobile unit 40 to the seized outgoing trunk over the seized communication channel and the appropriate switching and signaling will be accomplished by the wire line telephone system 22 to connect the mobile unit to the desired wire line telephone system subscriber or to another mobile unit.

~061412 A. Central Control Unit One embodiment of the central control unit 20 of Figure 1 employed in the OSA system is illustrated in greater detail in Figure 3 to facilitate an under-standing of the operation of the system of the presentinvention.
Referring now to Figure 3, the incoming trunk lines IT and the outgoing trunk lines OT may be connected through a suitable conventional trunk interface unit 50 to a suitable conventional switching unit 52 such as a square or rectangular single stage switch matrix or a multistage switch matrix. Detectors in the trunk inter-face unit 50 may provide access signals ACCS such as telephone system supervisory and status signals and address signals identifying the called mobile unit.
The detected ACCS signals may be applied to a central processing unit 54 and various supervisory control and status signals SUPC, e.g., on-hook and off-hook signals, may be generated by the central processing unit 54 and supplied to the trunk interface unit 50 for supervision of incoming and outgoing calls in a conventional manner.
Although not shown in Figure 3, direct operator lines may be provided between a switchboard and the trunk interface unit 50. With such an arrangement, an operator may be contacted to place calls to mobile units not ~ - 17 -~06~41Z

ordinarily assigned to the system and to perform other call intercept, information or control services which may be required.
The central processing unit 54 may generate various switch control signals for application to the switching unit 52 to selectively connect the interfaced incoming and outgoing trunk lines IT and OT, as well as various tone signals generated by conventional tone generators 56, to a plurality of two-wire lines generally indicated at 58. Each of the two-wire lines 58 may be connected to a two-wire/four-wire hybrid circuit 62.
The two-wire 10614~2 four-wire hybrid circuit 62 may separate the signals of the two-wire lines 58 into incoming and outgoing signals for transmission to and from the system trans-mitters and receivers by way of voice grade line pairs 64 and 66, respectively.
Signals received over the voice lines VOICE (RCV) via the line pairs 66 may include supervisory signals indicating, for example, an off-hook condition of a mobile unit or the like. These signals from the mobile units may be detected by a suitable conventional signal detector 60 associated with each receive voice line pair 66 and applied to the central processing unit 54 as the detected supervisory control signals DSC. For example, the seizure of a marked idle channel by a mobile unit subscriber wishing to place a call may be indicated by an appropriate signal or guard tone transmitted over the seized channel by the mobile unit. The signal detector 60 connected to the lines designated for service of the receiver serving the seized channel may detect the guard tone indicating seizure of the channel and provide an appropriate indica-tion in the form of the DSC signal. In response to this DSC signal, the central processing unit 54 may seize an available outgoing trunk OT through the generation of an appropriate supervisory control signal SUPC and may generate an appropriate base station control signal CONTROL. The CONTROL signal may be applied from the ~ - 18 -10614~2 central processing unit 54 to the base station 24 via a suitable conventional modem 68 and the control signaling lines 28.
The central processing unit 54 may also generate billing and traffic signals from the various calling information supplied thereto and these signals may be applied to a suitable billing and traffic recorder 70 to provide a record of the billing and traffic data. More-over, a suitable conventional input/output unit 72 such as a teletype unit may be provided to permit communication between an operator and the central control unit ~06~41Z

20 and the central processing unit 54. Communication between the input/output unit 72 and the central processing unit 54 may be desirable, for example, when new subscribers are added to the system or when other changes in the system make-up are required.
The operation of the central control unit 20 of Figure 3 may be more clearly understood through a dis-cussion of the operation thereof in placing calls to and from the wire line telephone system 22 and the mobile units 40.
When a call is placed from the wire line telephone ; system 22, the appropriate signaling-is initiated over one of the incoming trunks IT connected to the central control unit 20. The trunk interface unit 50 detects the request for service on the seized incoming trunk and connects the incoming trunk to the appropriate lines 58 through the switching unit 52. The address of the called mobile unit may thereafter be transmitted through the switching unit 50 and to the voice lines VOICE (TX) for transmission to the transmitters at the base station 24 over which the idle channel marker is presently being transmitted.
Since the switching of the incoming call to the VOICE (TX) lines 26 is controlled by the central processing unit 54, the central processing unit 54 may simultaneously remove the idle channel marker from the transmitters at ~ - 19 -1061d.~2 the base station 24 which were previously designated as the idle channel transmitters. For example, the idle channel marker may be one of the tone signals TNE
generated by the tone generator 56 and applied to the appropriate transmitters through the switching unit 52 in response to the switch control signals from the ; central processing unit 54. When a call is to be placed to a mobile unit, the central processing unit 54 may block the - l9A -idle channel marker from the tone generators 56 and connect the seized incoming trunk to the idle channel transmitters while simultaneously increasing the output power level of the idle channel transmitters through the application of the appropriate CONTROL signals to the base station 24 by way of the signaling lines 28.
When the called mobile unit responds with an appropriate signal, the response signal is transmitted to the central control unit 20 over the VOICE (RCV) lines 27 and is detected by an appropriate one of the signal detectors 60 as the DSC signal. The central processing unit 54 thereafter generates the appropriate supervisory control signals SUPC and switch control signals in response to the detected signal DSC returned from the called mobile unit and connects the receiver with which the called mobile unit is communicating to an available one of the outgoing..trunks OT. Thereafter, the central processing unit 54 may designate another available channel in the sector in which the previously idle channel was seized as the new idle channel and apply the appropriate idle channel marker to the transmitter serving that idle channel. Likewise, the central processing unit 5~ may apply the marker tone to the transmitters serving the idle channel in each of the other sectors at any time after the called mobile unit has responded.

~ - 20 -~06141Z

In placing a call from a mobile unit to a tele-phone of the wire line telephone system or another mobile, the mobile unit is initially locked onto the idle channel serving the sector in which the mobile unit is located.
The mobile unit seizes the idle channel by transmitting an appropriate signal, e.g., a guard tone, over the idle channel and the guard tone is detected by an appropriate one of the signal detectors 60. The central processing unit 54 generates the appropriate switch control signals and supervisory control signals SUPC in response to the detected signal DSC
from the mobile unit and effects a connection between an available outgoining trunk OT and the appropriate VOICE (RCV) line 27 through the switching unit 52.
Either simultaneously with the seizure of an outgoing trunk or sequentially thereafter, the central processing unit 54 increases the output power level of the trans-mitter serving the seized idle channel. The mobile unit may thereafter receive an appropriate signal indicating that it is connected to an available outgoing trunk OT and may supply the dialing signals required by the wire line telephone system 22 to effect a connection to the desired wire line telephone system subscriber.
A new idle channel may be designated and marked in the zone in which the idle channel was seized as was pre-viously described.
For billing purposes, the central processing unit 54 may detect completed calls and time the duration of all such calls. The time information, together with information identifying the subscriber using the system, may be recorded by the billing and traffic recorders 70.
In addition, user density throughout each day or during peak periods may be recorded by the billing and traffic recorder 70 to provide information as to future system requirements.

~ - 21 -106~41Z

B. Base Station One embodiment of the base station 24 of Figure 1 employed in the OSA system is illustrated in greater detail in Figure 4 to facilitate an understanding of the invention.
Referring now to Figure 4, the signals from the central control unit 20 may be applied by way of the two-wire VOICE (TX) transmission lines 26 to the base station 44 for transmission to the mobile units. Each two-wire pair of the VOICE (TX) lines 26 may be connected to an associated one of a plurality of transmitters designated TXl - TX28 so that the central processing unit 54 may selectively connect the incoming trunk lines IT and/or the tone generators 56 to each and any one of the transmitters 80 as was previously described in connec-tion with Figure 3. In a 12-channel system with a fixed channel assignment in each of the sectors Al-A3, Bl-B3 and Cl, four channels and thus four transmitters 80 may be assigned for transmission in each sector. For example, channels 1-4 (CHl-CH4) may be assigned to zones Al, A2 and A3. Thus, transmitters TXl-TX4 tuned to the respec-tive channels CHl-CH4 may be provided for transmission in zone Al. The signals from the transmitters TXl-TX4 may be combined in any suitable conventional manner by a combiner 82 and applied to the section of the directional antenna array 32 defining and serving the sector Al to provide transmission over channels CHl-CH4 in sector Al.
Similarly, the voice lines VOICE (TX) for the channels assigned to zone Bl may be connected to the transmitters TX5-TX8 (not shown) which may be tuned to the appropriate ones of the channels 5-8 (CH5-CH8). The output signals from the transmitters TX5-TX8 may be applied to a conventional combiner 84 to provide a combined output signal to the appropriate sector of the directional antenna array 32 for coverage of the zone Bl.

~ - 22 -In the illustrated fixed channel assignment system of Figure 4, the channels CH9-CH12 may be assigned to the sector Cl surrounding the base station 24 (see Figure 2). For this purpose, four transmitters TX9-TX12 tuned to the respective channels CH9-CH12 may be provided.
The signals from the transmitters TX9-TX12 may be com-bined through a suitable conventional combiner 86 and the combined output signal from the combiner ~06141Z

86 may be applied to the omnidirectional antenna 30 at the base station 24 for coverage of the sector Cl of the service area 38 of Figure 2.
The remaining 16 transmitters TX13-TX28 may pro-vide coverage of the zones A2, B2, A3 and B3 in the same manner as described above. For example, the groups of transmitters TX13-TX16 and TX17-TX20 may be tuned to channels CHl-CH4 and may be combined to provide coverage of sectors A2 and A3 via the appropriate section of the directional antenna 32 defining and serving these sectors.
Channels CH5-CH8 may likewise be provided within zones B2 and B3 from the groups of transmitters TX21-TX24 and TX25-TX28 via appropriate sections of the directional antenna 32.
The transmitters TXl-TX28 may be any suitable conventional transmitters such as FM transmitters having output power levels of sufficient magnitude to provide wave energy signals of a desired level throughout the sectors served by the transmitters. Of course, at least those transmitters serving the zones Al A3 and Bl-B3 must be variable in output power level between two discrete values in response to the control signals TXC as was previously described and is hereinafter dis-cussed. Further, a third power level (Fo) consisting of an unmodulated carrier at low power, perhaps .5W, suffi-cient to effectively swamp intermods and false marked idle signals, may be radiated from transmitters not in use for calls.

~ - 23 -1~61412 Control of the transmitters 80 may be effected by the CONTROL signals supplied to the base station 24 via the lines 28. The CONTROL signals may be applied to a suitable transmitter control unit 90 including a suitable modem, and various decoded transmitter con-trol signals TXC generated by the transmitter control - unit 90 in response to the CONTROL signals may be applied to the transmitters 80. For example, the particular transmitter 80 serving each sector in which the idle channel marker is being ~061412 transmitted at any particular time may be controlled by the TXC signal to turn on and transmit at a reduced power level as was previously described. When the idle channel is seized for conversation, the TXC signal may command the appropriate transmitter 80 to increase its output signal power level to the normal communication power level for two-way conversation. At the end of the call, the TXC signal from the transmitter control unit 90 may command the appropriate transmitter 80 to reduce to the power level Fo or to turn off.
Moreover, the TXC signal may include channel assignment signals for effecting a dynamic channel-to-zone assignment in connection with a dynamic or adaptive channel assignment algorithm for systems having remotely tunable (or combinations of fixed and remotely tunable) transmitters and receivers. For fixed channel systems with few channels, a dynamic channel assignment unit such as that illustrated in and described hereinafter with reference to Figure 5 may be controlled by the TXC
signal to adaptively assign channels to the sectors. In the event that dynamic channel assignment is employed, the TXC signal may also be supplied to the system receivers to effect receiver channel changes as channels are dynamically assigned to the various sectors.

/ ( - 24 -To complete the two-way communication path between the mobile units 40 of Figure 2 and the wire line tele-phone system 22 of Figure 1, the signals ORCV detected by the receiving portion of the omnidirectional antenna 30 at the base station 24 may be applied to a plurality of receivers 92 corresponding in number to the number of channels assigned to the sectors Al-A3, Bl-B3, and zone Cl in the embodiment of Figures 1-5. The signals SRCV from the satellite receivers SRl-SR6 of Figures 1 and 2 1061~1Z
may also be transmitted to the base station 24 in a suitable manner, e.g., over commercially installed transmission lines. In the event that the directional antenna array 32 is employed to receive signals from the mobile units, the signals from each directional antenna segment may be applied to the receivers 92 as is illustrated in phantom. The output signals from the receivers 92 and the signals SRCV from the satellite receivers SRl-SR6 may then be transmitted to the central control unit 22 via the voice grade transmission lines VOICE (RCV) for selective application to the appropriate outgoining trunks OT of the wire line telephone system as determined by the central processing unit 54 of Figure 3.

~061412 C. Dynamic Channel Assignment As an alternative to fixed channel assignments described in connection with Figure 4, a dynamic channel assignment technique (also referred to as an adaptive channel assignment technique) may be used where the frequency of the remotely tuned transceivers at the OSA
base stations is controlled by the central processor.
This allows each transceiver connected to any one of the sectors to be selectively tuned to any one of the authorized frequencies. Thus, channel frequencies may be assigned to any sector simply by a command from the central processor. Further, by providing additional transceivers at each sector, channels may be shifted from sector to sector as needed to accommodate heavier traffic loading in busier sectors by diverting channels from the lightly loaded sectors. An algorithm designed to minimize interference may control the channel assign-ments.
For example, four channels may be available for use in each of the sectors Al-A3 and Bl-B3 and in the central zone Cl of Figure 2 under uniform user density conditions. Assuming that four mobile units 40 are simultaneously engaged in calls in sector Al, a call cannot be established with another mobile unit in sector Al since there are no available channels.

~ - 26 -The central processor at the control unit 20 may determine that sector Al is fully loaded, i~e., that there are no more available channels in section Al, through the number of established connections therethrough to the sector Al of the base station 24. If a channel is available in one of the other sectors Bl-B3 or the central zone Cl, this available channel may be temporarily shifted to sector Al to accommodate the heavier traffic loading in sector Al. Thus, for example, channel 12 may be borrowed from zone Cl and assigned for use in sector Al.
A transmitter-receiver pair (e.g., a transceiver) con-nected to the directional antenna segment ~26A--1~614~Z
defining sector Al (or connectable thereto) may then be tuned to the channel 12 frequency and channel 12 may become the marked idle channel in sector Al.
In the above example, channel 12 may also be employed in others of the sectors A2, A3, and Bl-B3 on a noninterfering basis. However, the use of channel 12 in zone Cl would be proscribed because of potential inter-ference problems. Thus, for example, channel 12 may be assigned for use in sector B3 or in each of the sectors A2 and A3 in addition to sector Al. Sector Al and any other sectors to which channel 12 is assigned in this manner would then have a five-channel (and five call) capability while zone Cl would have a three-channel (and three call~ capability at any-one instant.

D. Fixed Frequency D~namic Channel Assignment Unit In systems where only a few channels are available, e.g., four channels, and the transmitter frequencies are fixed, then alternative methods may be provided to allow communications via the antenna sectors. Such a dynamic channel assignment unit 94, illustrated in Figure 5, may be utilized in connection with the system of the present invention. Referring now to Figure 5, there is illus-trated a system for combining channels in a four channel system so that all possible combinations are made available ~ - 27 -~0614~Z
for transmission in the various zones. It should be understood that any number of channels may be combined in this manner and selected for transmission within a desired zone in response to the CONTROL signals from the central processing unit 54 of Figure 3 as decoded by the transmitter control unit 90 of Figure 4. The illus-trated embodiment of Figure 5 has been limited to four channels only to facilitate an understanding thereof.

106141;~

As can be seen in Figure 5, four transmitters TXl-TX4 may be connected to transmit voice and control signals received over the VOICE (TX) lines in response to the transmitter control signals TXC. The trans-mitters TXl-TX4 may be tuned to transmit at the channel carrier frequencies of channels 1-4, respectively, or may be selectively tunable under the control of the central processing unit 54 of Figure 3.
The output signals from each of the transmitters TXl-TX4 may be applied through respective directional couplers 96~102 to respective selector switches 104-110.
The selector switches 104-110 as well as the other selector switches hereinafter described in connection with the Figure S embodiment of the dynamic channel assignment unit may be any suitable conventional signal controllable mechanical or electronic switches. These switches may be controlled by appropriate decoded control signals TXC from the transmitter control unit 90 of Figure 4 in response to CONTROL signals from the central control unit 22 of Figures 1 and 3.
Each of the selector switches 104-110 is operable in response to the TXC signals and in conjunction with conventional hybrid combiners 112-130 and similar selector switches 132-146 to provide any individual channel or any ~ - 28 -cornbination of the four channe,ls at the input terminals of a conventional switch matrix 148. The switch matrix 148 may selectively apply the input sign,als corresponding to the selected channels or combinations thereof to the appropriate directional antennas as the signals Al-A3 and Bl-B3 to serve the correspondingly designated sectors of the mobile telephone service area. The selection of channels by the switch matrix 148 may be controlled by' the TXC signals in coordination with the operation of the selector switches 104-110 and 132-146. Moreover, each of the individual channels or combinations ~06141Z

thereof may be made available to the omnidirectional antenna as the Cl signal in coordination with the selector switches and switch matrix as is illustrated.
In operation, and assuming that a larger number of transmitters than the four illustrated in Figure 5 are available for dynamic assignment to the various zones, it may be assumed that where user density is equal in all sectors, Al-A3, Bl-B3 and Cl, the twelve available channels may be assigned as was previously described in connection with Figure 4 so that four, channels are available in each sector. In-the event that only two channels are being utilized in sector Cl, e.g., channels Ch9 and ChlO, and all channels become busy in one of the other sectors, e.g., sector Al, either channel 11 or channel 12 and one or both of the transmitters ordinarily used to transmit over channels 11 and 12 in sector CI may be switched by the dynamic channel assignment unit 94 of Figure 5 so as to provide one or more additional communication channels in sector Al. This may also be accomplished by the preferred method of adaptive channél assignment by remotely tuning the transmitters (Figure 4) with no switching required.
During the time in which the channel borrowed from sector Cl is utilized in sector Al, sector Cl may have only two or three available channels and sector Al may have five or six available channels thereby accommodating variations in user density.

~ - 29 -Since all of the information as to which channels in the various zones are busy at any one time is available at the central control unit 22 of Figure 3, the adaptive assignment of channels to the various zones as user density varies may be accomplished through the trans-mission of appropriate frequency control signals to the base station in any suitable manner. The only requirement in adaptively assigning channels to the various sectors 1~6141Z
in this manner is that all assignments be made on a noninterfering basis. In other words, a channel assigned for communication in sector Al could not be also assigned for communication in sector Bl. Similarly, a channel assigned for communication in sector Cl could not ordinarily be assigned for communication in any of the sectors Al-A3 or Bl-B3. It can thus be seen that an adaptive or dynamic channel assignment unit of the type exemplified in Figure 5 may be provided at the base sta-tion 24 of Figure 4 in lieu of the fixed combiners 82-88 to more fully utilize the available channels as user density varies within the sectors of the mobile telephone service area.

II. Description of Another System Embodiment As was previously described in connection with Figures 1 and 2, one of the available channels in each of the zones A1-A3, Bl-B3 and Cl may be designated as the idle channel and marked with an idle channel marker tone. The idle channel information may be broadcast in each zone at a decreased power level by one of the trans-mitters available for transmission in that zone and when the idle channel is seized the power level of the idle channel transmitter may be increased so that the trans-mitter may be employed to completely establish and service the call over the seized channel.

~ - 30 -1~61412 In accordance with another embodiment of the invention illustrated in Figures 6 and 7 wherein like numerical and letter deisgnations have been utilized to designate elements of the system previously discussed, the marked idle channel may be broadcast from marker beacons provided in each of the zones Al-A3 and Bl-B3 as is hereinafter described in detail. Referring now to Figures 6 and 7, the central control unit 20 and the base station 24 may operate essentially as was described in connection with Figures l-5 to selectively connect subscribers of the wire line telephone system 22 and at a plurality of satellite stations 160 remote from the base station 24.
Each of the satellite stations 160 may include a plurality of satellite receivers 34 designated SRl-SR6 and previously described in connection with Figures l and 2. In addition, each of the satellite stations 160 may include one marker beacon transmitter 162 each deslg-nated MBl-MB6 to facilitate the description.
The service area or sector Cl in Figure 7 may be served by the transmitters and receivers connected to the omnidirectional antenna 30 as was previously described in connection with Figures 1-5. Transmission from the base station 24 to the mobile units 40 outside the sector Cl may be provided by the transmitters at the base station 24 connected to the various sections of the directional antenna 32 as in the embodiment of Figures 1-5. In addition, reception of signals transmitted by the mobile units 40 in the zones Al-A3 and Bl-B3 may be provided by the satellite receivers at each of the satellite stations 160 as was previously described in connection with the embodiment of Figures 1-5.

~ - 31 -However, to minimize marked channel ambiguity between the sectors Al-A3, Bl-B3 and Cl, the marked idle channels may be established in each of the zones Al-A3 and Bl-B3 through the use of the marker beacon transmitters 162. With continued reference to Figure 6, the control signal CONTROL from the central control unit 20 designating the idle channel to be marked in each of the zones Al-A3 and Bl-B3 may be decoded by the base station 24 as was previously described to provide the transmitter ~061~
control signals TXC. The transmitter control signals TXC
may be applied to the marker beacon transmitters 162 and the output signal from each of the transmitters 162 may be applied to a transmitting portion of the antennas 36 at each of the satellite stations 160.
Each of the marker beacon transmitters 162 may be any suitable conventional FM transmitters capable of being remotely tuned in response to an appropriate elec-trical signal. In a fixed channel assignment system, each marker beacon transmitter 162 need only be capable of being tuned over the range of frequencies of the channels assigned to the zone in which the beacon transmitter is located. In a system employing dynamic channel assignment, the marker beacon transmitters 162 must be tunable over the entire frequency range of the communication channels available for use in the system.
The operation of the system of Figure 6 may be similar to that of Figure 1 with the exception of the idle channel marking technique. In accordance with the system of Figures 6 and 7, the marker beacon transmitters 162 are tuned to the frequency of an available or idle channel in the zone in which the particular marker beacon transmitter is located in response to the transmitter control signal TXC from the base station 24. When tuned to the appropriate carrier frequency by the TXC signal each ~ -32-marker beacon transmitter 162 may be modulated by the idle channel marker or tone either generated at the satellite station or transmitted thereto and the marker modulated carrier signals from the marker beacon trans-mitters MB1-MB6 may be broadcast from the respective antennas 36 associated with each satellite station 160.
The antennas 36 are preferably omnidirectional antennas and the propagation pattern of each of the marked carrier signals transmitted from 106~41Z
each of the marker beacon transmitters 162 may generally correspond to the relatively circular areas designated 44 in Figure 7. A mobile unit entering one of the areas 44 (which areas may also generally define the reception pattern of each of the satellite receivers centrally located therein) will search for and lock onto the marked idle channel in that area. The initial communication required for establishing a call either to or from a mobile unit 40 may occur over the marked idle channel and after a marked idle channel has been seized for a call, actual voice communication may occur over the seized marked idle channel by way of the satellite receivers 34 and the transmitters at the base station 24 connected to the directional antenna array 32. In this latter respect, the system of Figure 6 may operate identically to the system of Figure 1 after an idle channel has been seized and a call has been established.
As an alternative to signaling over the marked idle channel by way of each of the marker beacon trans-mitters MBl-MB6 at the satellite stations 160, the initial signaling required for establishing a call may be accom-plished by way of the appropriate section of the directional antenna 32 and a transmitter at the base station 24 tuned or tunable to the idle channel. For example, the marker 25 beacon transmitters in each of the sectors Al-A3 and Bl-B3 , ~ - 33 -1061~12 may merely operate to transmit the appropriate marker signals in each of these sectors so that all mobile units are initially tuned to the idle channel available in the sectors in which they are located. When the control unit 20 receives a call for a mobile unit the TXC signal may command each of the marker beacon transmitters to stop transmitting the marker tone. Simultaneously therewith or very shortly thereafter, a seize tone may be placed on the idle channel. The seize tone holds the mobile units on the idle channel and the central control unit 20 may then command a transmitter associated with ~0614~Z
each of the sectors Al-A3 and Bl-B3 defined by the pattern of the directional antenna array 32, as well as a transmitter associated with the sector Cl defined by the pattern omnidirectional antenna 30, to initiate a call-up sequence over the previously designated and marked idle channel in each of the seven sectors.
When the mobile unit being called receives and decodes the appropriate signal during the call-up sequence, the mobile unit responds and the response is received by a satellite receiver at the satellite station 160 from which the marker signal was previously transmitted to the mobile unit. The marker beacon transmitters in the re-maining sectors may then be commanded to transmit the marker signals at the appropriate idle channel frequencies and the established call is serviced by the transmitter-satellite receiver pair which was used for call placement signaling by way of the directional antenna array 32 at the base station 24 and the antenna 36 at the satellite station 160 servicing the call. If another channel is available in the sector in which the call is established, the central control unit 20 may thereafter command the marker beacon transmitter in that sector to the available channel frequency and command the marker beacon transmitter to transmit the marker signal at that carrier frequency.
In placing a call from a mobile unit to the wire line telephone system 22, a mobile unit locked onto a ~ - 34 -channel designated idle by the appropriate one of the marker beacon transmitters MBl-MB6 in the sectors Al-A3 and Bl-B3 or by the appropriate base station transmitter in the sector Cl may transmit a predetermined tone, e.g., a guard tone, indicating that service is requested. The guard tone is received by the satellite receiver at the appropriate satellite station 160 (or by the appropriate receiver at the base station 24 if the mobile unit is in the sector Cl)and an available transmitter trans-mits a response to the request for service in the appropriate sector at the marked idle channel frequency either from the appropriate directional antenna section or from the omnidirectional antenna 30. At the same time, the marker beacon transmitter in the zone in which service is requested may be commanded by the central control unit 20 to turn off the marker tone and the remaining processing of the call to establish communication with a wire line tele-phone systems subscriber may be continued as was previously described in connection with Figure 1. If another channel is available for use in the sector in which the call is established, the central control unit 20 may command the marker beacon in that sector to the frequency of the available channel and the marker signal may be transmitted at that available channel frequency.

~0614~2 III. Service Area Expansion The growth capability of an OSA system implemented in accordance with the techniques of the present invention permits an OSA system to be first used to replace a standard omnidirectional large zone system resulting in up to three times more frequency reuse for the same area coverage. Secondly, large zone OSA base stations can be located closer together than omnidirectional systems with further increase in frequency reuse per area covered.
Thirdly, OSA base stations can be located with contiguous coverage patterns throughout an area with co-channel inter-ference minimized by proper orientation of the sectors and assignment of channels to those sectors. Fourthly, base station to base station sector overlap is possible which allows the OSA system technique to provide a multicellular system having greater channel reuse with a superior channel distribution over the service area, requiring fewer base station sites, and less equipment than a small cell system utilizing omnidirectional antennas to cover the equivalent service area.
As a first step in employing the OSA techniques of the present invention to replace large zone, omni-directional systems, an OSA base station 24 may be provided generally centrally of each of a plurality of preexisting, separated large zones, as is illustrated ~ - 36 -in Figure 8A. The preexisting large zones will typically be separated sufficiently to provide about a 12 dB C/N
ratio between any two base stations. The spacing between base stations serving areas approximately 28 miles in diameter will thus be on the order of 42 miles and the central, omnidirectionally served zones C may be approxi-mately 9.3 miles in diameter.
The directionally served sectors A and B may be oriented so that sectors served by the same communication channels do not face one another directly. Thus, as an alternative to placing the base stations 24 at preexisting sites, the base sta-tions 24 may be placed somewhat closer together than is permissible in an omnidirectionally served large zone system. The spacing between base stations 24 should be approximately 28 miles in a large zone on the directionally served system.
Referring now to Figure 8B, as mobile telephone communication is required in the areas intermediate the spaced zones of Figure 8A, OSA base sta-tions 24 may be added as required as illustrated. In Figure 8B, each of the letters A, B, C, and D indicates a sector served by three communication channels. The sector A served by one of the base stations 24 may employ the same channels as the sector A served by another of the base stations 24.
Similarly, the same channels may be employed in like desig-nated sectors through the service area. With the embodiment illustrated in Figure 8B, the frequency reuse factor is approximately 1.75 times that of large zone omnidirectionally served systems and additionally, 85% more area may be covered.
As will hereinafter become apparent to one skilled in the art to which this invention pertains, the area 38 may be served in a manner which meets present requirements ~ - 37 -and which permits expansion to meet future needs in a cost effective and orderly manner as is illustrated by the service area coverage patterns of Figures 8A, 8B, 8C, 9 and 10.
Referring now to Figure 8C, as a service area grows from one which can be served by a single base station 24A to one requiring additional coverage, addi-tional base stations 24B-24G having overlapping coverage may be provided as illustrated. In the plural base station embodiment illustrated in Figure 8, the base stations may be spaced approximately equidistantly in such manner that any three adjacent base stations, e.g., 24A, 24B and 24D, are located at the corners of an equilateral triangle. The directional antennas 32 at each of the base stations 24A-24G are oriented directionally so that each sector of a large geographical service area 200, designated by the alpha-numeric descriptors Sl-S24, is served by one or more directional antenna sec-tions and at least some of the sectors are served by two and as many as three directional antenna sections each located at different base stations.
In the illustrated embodiment of Figure 8C, sectors Sl-S12 are each covered by one directional antenna section.
The remaining sectors S13-S24 are served by two or more directional antenna sections. Thus, with this arrangement of directional antennas and base stations, at least one-half of the overall service area sectors Sl-S24 are served by more than one base station. With arrangements having fewer base stations, a smaller percentage of overlapping coverage may result. However, even with arrangements employing fewer base stations, overlapping sector coverage from plural base stations is provided centrally of the over-all service area where increased capacity is typically needed as a system expands.
Assuming that twelve channels are available for use in the mobile telephone system, various combinations of channels may be assigned to each of the sectors Sl-S24 on a noninterfering basis. One example of a channel assignment scheme may be as follows:

Sectors Channels Base Stations Sl 1, 3 24A

S3 12, 1 24B
S4 6, 3 24E
S5 8, 4 24E
S6 3, 11 24G
S7 10, 5 24G
S8 1, 8 24F
S9 9, 11 24E
S10 5, 8 24C
Sll 1, 6 24C
S12 11, 10 24A
S13 6, 2 24A, 24B
S14 10, 7 24B, 24E
S15 5, 1 24E, 24G
S16 4, 7 24G, 24F
S17 2, 12 24F, 24C
S18 4, 12 24C, 24A
Sl9 8, 5, 1 24A, 24B, 24D
S20 3, 4, 11 24B, 24D, 24E
S21 12, 2, 9 24D, 24E, 24G
S22 6, 5, 8 24D, 24F, 24G
S23 11, 3/ 10 24C, 24D, 24F
S24 2, 9, 7 24A, 24C, 24D

As can be seen from the above exemplary channel assignment scheme, sector Sl9 may be served by channel 8 from base station 24A, by channel 5 from base station 24B
and by channel 1 from base station 24D. Sectors S20-S24 may also each be served by three channels, one channel from each of the three adjacent base stations at the periphery of each of these sectors. The sectors S13-S18 may each be served by two channels each with the above exemplary channel assignment scheme. Each of the sectors Sl-S12 may also be served by two channels but are not served from two different base stations but rather are served by one section of the directional antenna array at a base station with two transmitter-receiver pairs assigned to each of these peripheral sectors Sl-S12.
Of course, other noninterfering channel assignment schemes may be implemented using the basic base station configura-tion and directional antenna coverage pattern illustratedin Figure 8. Thus, in areas of high user density, additional channels may be made available and in areas of lower user density as few as one channel may be made available. Moreover, channels may be assigned to various sectors dynamically as the need arises as was previously described.
To insure effective coverage of the mobile tele-phone service area 200, omnidirectionally served sectorsslightly overlapping the sectors Sl-S24 may be provided around each of the base stations 24A-24G. Moreoverr marker beacon transmitters and satellite receivers may be provided in each of the sectors Sl--S24 to further enhance the operation of the system of Figure 8C.
Of course, it should be noted that continued growth is readily accommodated in accordance with the invention through the addition of base station equipment at any of the base station sites illustrated in phantom at 24H-24M. For example, the base station 24H may be added as the service area expands outwardly at its periphery. The base station 24H not only provides V

~ ~ - 40 -~' ~;061412 additional area coverage but also provides additional or back-up coverage in the preexisting sectors S9, S10 and S17. It can readily be seen from the foregoing description of Figure 8C that the system can be continuously expanded in any direction and as far as required to cover any desired geographical area.
Moreover, it can be seen with reference to Figures 2 and 8C that the system of the present invention provides orderly growth from an existing system serving a relatively small area 38 to a large area multiple zone 106~4~Z
system serving a large area 200 in both an orderly and cost effective manner. For example, as system expan-sion is required, the required call capacity in outlying or fringe areas will ordinarily be low as compared to the required call capacity at the center of the service area. This may be more readily seen by noting that the center of the service area will typically be a large urban area with high call capacity requirements and as the system expands outwardly into suburban areas, the initial call capacity will ordinarily be much lower than that of the urban area. Moreover, as the suburban area grows around the urban area, increased call capacity is typically required in the urban area. As can be seen from Figure 8C, each time the system is expanded to provide service in peripheral areas, the installed base station equipment also provides additional call capacity in the existing service area.
of course, in the event that any of the sectors served by the added base station equipment, e.g., at the base station 24H, is insufficiently served by a single base station, i.e., by nonoverlapping coverage, addi-tional transmitting and receiving equipment may be readily added to the base station 24H to provide more channel availability. As an alternative or an adjunct to the addition of equipment to the existing base station .`

~ - 41 -24H, another base station initially employing only one or two of its available directional antennas may be pro-vided. In this latter respect, only selected ones of the available directional antennas at the base station 24I may be initially used where additional call capacity is required in sectors S12 and S18 and wherein future system growth is contemplated beyond the sectors S12, S18 and Sll. It should be noted that this type of orderly system growth cannot be accommodated with known mobile telephone multiple zone systems where only a small number of channels (e.g., 12) are available.

10614~Z

In accordance with another embodiment of the present invention, coverage of a zone or sector from up to six different base stations with overlapping directional antenna-defined sectors may be provided.
Referring now to Figure 9 which illustrates another growth pattern for accommodating service area expansion in an orderly and cost effective manner, the pattern previously described in connection with Figures 2 and 7, with the exception of the service area Cl, served by the omnidirectional antenna 30 at the base station 34 is illustrated in phantom. In Figures 2 and 7, satellite receiving stations were provided in each of the zones Al-A3 and Bl-B3 served by the base station 24 so as to serve the geographical area 38. In accordance with the embodiment of the invention illustrated in Figure 9, orderly growth of the service area 38 may be accommodated by adding transmitters at any one or more of the six satellite recei~er stations to form one or more of the additional stations 24N-24T.
The base stations 24N-24T formed in this manner may accomplish all transmitting and receiving (or at least all transmitting) by way of a directional antenna array as opposed to the omnidirectional receiving antennas previously employed at the satellite stations. As a result, the sector Al served by the base station 24 serves ~ - 42 -the area around the base station 24N and the sector Nl served by the base station 24N serves an area around the base station 24. With six base stations 24N-24T located as illustrated, the area around the base station 24 may be served by six sectors Nl, Pl, Ql, Rl, Sl and Tl each associated with a different one of the respective base stations 24N-24T, thus forming one sector SA served by up to six base stations. In addition to serving the area around the base station 24, the base station 24N may also serve new sectors or areas N2 and N3 as well as provide additional call capacity in existing sector Bl through the provision of the sector N4 served by the base station 24N. Satellite receiving stations SRN2 and SRN3 may be provided in the respective peripheral sectors N2 and N3 if omnidirectional receiving is desired.
While transmission to each of the sectors from the base stations is accomplished directionally using directional antenna arrays as was previously described, the receivers in each of the sectors may operate through omnidirectional antennas at each of the base stations 24 and 24N-24T and at the satellite receiving stations SRN2 and SRN3. Moreover, to improve performance by insuring that the mobile units lock onto the channel designated as the idle channel in each of the sectors SA, Al-A3, Bl-B3, N2 and N3, an omnidirectional marker beacon transmitting unit may be provided in each of the sectors at or in the vicinity of the base stations 24 and 24N-24T
as well as the satellite receiving stations SRN2 and SRN3 as was previously described in connection with Figures 6 and 7.
As the system illustrated in Figure 9 is even further expanded, a service area coverage pattern from the directional antennas at each of the base stations substantially as illustrated in Figure 10 may result.
Referring to Figure 10, the sectors designated S30-S35 are each served by one base station and may be assigned ~/
~ - 43 -1~6~4~Z
one or more channels. The sectors designated S36-S41 are each served by two base stations and may be assigned one or more channels from each of the two base stations. The sectors designated S42~S47 are each served by three base stations and may be assigned one or more channels from each of the three base stations. The central sector S48 is served by six base stations and may be assigned one or more channels from each of the six surrounding base stations.
Additional base stations may be provided gen-erally centrally of one or more of the peripheral sectors S30-S41 to increase the overall geographical size of the system and/or to increase call capacity with existing 10614~;2 sectors. The additional base stations may be formed by the addition of transmitting equipment and directional antenna arrays at the satellite receiving stations in the peripheral S30-S41.
As with the embodiment of the expanded system of the present invention illustrated in Figure 8C, the sectors S30-S48 of the embodiment of Figure 10 may be assigned channels served by base stations in adjacent sectors in the following exemplary manner:
Sector Channels Base Stations S30 2, 4 S42 S31 1, 3 S43 S32 2, 6 S44 S33 1, 5 S45 S34 4, 6 S46 S35 3, 5 S47 S36 6, 1 S47, S42 S37 5, 6 S42, S43 S38 4, 5 S43, S44 S39 3, 4 S44, S45 S40 2, 3 S45, S46 S41 1, 2 S46, S47 S42 8, 10, 12 S43, S47, S48 S43 11, 9, 7 S42, S44, S48 S44 12, 10, 8 S43, S45, S48 S45 7, 11, 9 S44, S46, S48 S46 8, 12, 10 S44, S47, S48 S47 7, 9, 11 S42, S46, S48 S48 6, 5, 4,3, 2,1 S42-S47 As an example of the operation of the system of Figures 9 and 10, the sector S48 of Figure 10 may be served by channels 1-6 each provided from a different one of the base stations i.n the sectors S42-S47. Channel 6 ~ - 44 -~(~6~4'1Z

may be initially designated the idle channel and the marker beacon transmitter at the base station in sector S48 may transmit the idle marker at the frequency selected for channel 6. Thus, all mobile units entering sector S48 will lock onto the marked idle channel, channel 6.

1~6~412 When the idle channel in sector S48 is seized to establish a call, transmission of the idle channel marker by the marker beacon transmitter may be dis-continued and the transmitter and directional antenna at the base station serving sector S48 on the idle channel may be energized to complete the calling process as was previously described. Thus, in the foregoing example, marked idle channel 6 may be seized and the marker beacon transmitter in sector S48 inhibited.
Assuming that channel 6 is served from the base station in sector S42, the call would thereafter be serviced by the base station 42 over channel 6. A new channel, e.g., channel 2, may thereafter be designated as the idle channel to be marked and the marker beacon transmitter in sector S48 may then come back on the air to transmit the idle channel marker at the appropriate freqùency.

IV. Directional Antenna Array The directional antenna arrays 32 described briefly in connection with Figures 1-10 and utilized in the disclosed embodiments of the invention may be any suitable conventional array of directional antennas oriented to provide the desired sector coverage. For example, one suitable directional antenna array may com-prise a plurality of commercially available Yagi antennas having the desired directional characteristics and oriented to provide sector coverage of a service area. Another antenna arrangement may comprise a plurality of corner reflector antennas having the desired directional charac-teristics and oriented to provide the desired sector coverage.
~ directional antenna array utilizing commercially available Yagi antennas for use in the presently available 450 MHz band is illustrated in Figure 11. Referring to Figure 11, the directional antenna array 32 employed in conjunction with a six sector pattern such as that of Figures 1 and 6 may comprise six Yagi antennas 210 each having an angular beam-width of about 60 and each secured to an antenna mast 212 at angular intervals approximately equal to the sum of one-half the angular beamwidth of two adjacent antennas 210, i.e., 60 in the embodiment ~ - 46 -X

1~6141Z

of Figure 11. The angular beamwidth of each antenna 210 may be defined by the approximate angle between the lines defining the half power points of the antenna pattern or in any other conventional manner.
In the 450 MHz band, the directive gain resulting from each directional antenna 210 with a half power angular beamwidth of 60 will approximate 10 dB above the gain of the dipole. Since high gain is not a require-ment and is difficult to achieve in the 150 MHz band within desirable weight and size limitations, a wider beamwidth on the order of 76 and a gain of approximately 7 dB may be utilized in this lower frequency band. With the 76 beamwidth, only five directional antennas oriented 72 apart may be required in a 150 MHz band system.
Moreover, it should be understood that other beamwidths and antenna orientations may be employed depending upon system requirements. For example four 90 beamwidth antennas spaced at 90 intervals about the mast 212 may be employed. In addition, it should be understood that some overlap between the sectors defined by adjacent directional antennas, e.g., the sectors Al and Bl and Al and B3 in Figure 2, will occur and may, in fact, be desirable. As long as this area of overlap is fairly well defined by the directional characteristics of the adjacent directional antennas and the power level of the wave energy broadcast in the area of overlap is significantly below that broadcast centrally of the sectors, the capture characteristics of the mobile unit receivers alleviate any problems.
With the antenna illustrated in Figure 11, coverage of the sectors Al-A3 and Bl-B3 of Figure 2 may be achieved by orienting the longitudinal axes 214 of each of the Yagi antennas 210 along the longitudinal axes of the respective sectors Al-A3 and Bl-B3 with which the antennas are associated. Thus, in the plan view of Figure llA, the longitudinal axes of the Yagi antennas 214 coincide with ~ - 47 -X

~06~4~2 the longitudinal axes of the sectors Al-A3 and Bl-B3 in a 60, six sector pattern provide a full 360 degrees of coverage.
A suitable antenna array 32 comprising several corner reflector antennas is illustrated in Figure 12.
Referring to Figure 12, the directional antenna array 32 employing corner reflectors may include a vertically dis-posed, generally cylindrical housing 216 constructed from a suitable electrically nonconductive material such as fiberglass and a plurality of corner reflectors 218 within the housing 216. The corner reflectors 218 may be constructed from a suitable metallic material such as a metal screen and may each present vertically disposed walls or reflectors 220 which intersect along a common axis 222 coaxial with the longitudinal axis of the cylindrical housing. Each adjacent pair of the vertical walls 220 formed by the metal screening material intersect at a predetermined angle determined by the angular beamwidth required to provide coverage of a sector. When used in conjunction with a six sector, 60 coverage pattern such as that illustrated in Figure 2, the angle of intersection of all of the walls or reflectors 220 may be approximately 60.
With continued reference to Eigure 12, each of the sectors of the directional antenna array 32 formed by the walls or reflectors may include colinearly disposed dipole antennas, generally indicated at 224 and 226 adjacent the corners thereof. One of the dipole antennas 224 and 226 may be the transmitting antenna and may be connected to the appropriate transmitters at the base station 24 as was previously described in connection with Figures 1 and 6. With the colinear dipole arrangement illustrated and with appropriate isolating means there-between an extremely high degree of isolation, e.g., 50 to 70 dB, exists between the transmitting and receiving antennas.
The size of the antenna array 32 of Figure 12 will, of course, vary with the frequency band of the system with which the antenna array is employed. For example, ~ - 48 -~061412 at approximately 800-900 MHz, the diameter and length of the housing 216 may each be on the order of one foot.
In the 152-162 MHz -and, the housing 216 of the antenna array 32 of Figure 12 may be on the order of seven feet in diameter and three feet in height.
The orientation of the directional antenna array 32 of Figure 12 in providing coverage of the sectors Al-A3 and Bl-B3 of Figure 2 is illustrated for these sectors in Figure llA. As is illustrated in Figure llA wherein a view from above the directional antenna array 32 is provided, the walls or reflectors 220 defining each of the sectors may extend upwardly substantially coplanarly with the lines 228 defining the approximate edges of the sectors.
It should be noted that under some conditions the corner reflector arrangement of Figure 12 may be more desirable than the Yagi antenna arrangement of Figure 11 since the corner reflector arrangement reduces back and side lobes somewhat more than the Yagi arrangement.

- 49 ~

1(~6141Z

V. Sector Change Techniques As a mobile unit operating within the mobile telephone service is served in accordance with the coverage techniques of the present invention moves from one sector or zone into an adjacent sector, the mobile unit automatically searches for a new idle channel if not engaged in a call. If, however, the mobile unit is engaged in a call as it moves from one sector to another, the established communication path through the base station serving the sector in which the call was established is broken.
When this occurs in large zone systems such as known systems, employing IMTS techniques, the call must be reestablished by one of the two parties in order to continue the conversation in the new area (i.e., if mobile telephone service is available in the new area).
To minimize the complexity of the system of the present invention, the same approach may be employed as sector changes occur while a mobile unit is engaged in a call.
Thus~ with reference to Figure 2, a mobile unit 40 in sector Al which moves into one of the adjacent sectors Bl, B3 or Cl after a call is established would at some point suffer a loss of communication and be required to reestablish the call in the new sector.

~ - 50 -If, however, continuity of communication is desired as sector changes occur, the call must be reestablished in the new sector without subscriber intervention. In the system of the present invention previously described in connection with Figures 1-7, continuity of communication may be provided as is illus-trated in Figure 13.
Referring now to Figure 13 wherein previously described elements have been given like numerical desig-nations, each of the receivers associated with the largearea coverage pattern of Figure 2, i.e., the receivers 92 associated with the omnidirectional antenna 30 at the base station 24 and the ~06141Z

remote satellite receivers 34, may supply signals OR~
and SRM indicative of received signal strength to a receiver monitoring circuit 230 at the base station 24.
The receiver monitoring circuit 230 may supply receiver monitoring signals RMS to the central processing unit 54 (Figure 3) at the central control unit 20 by way of the control signaling lines 28.
The CONTROL signals from the central control unit 20 may be supplied to the transmitting equipment 80-90 at the base station 24 as was previously described in connec-tion with Figure 4 and may also be applied to a receiver control unit 92A. The receiver control unit 92A may decode the CONTROL signal and provide receiver control signals RCC
in response thereto. The receiver control signals RCC may be applied to the receivers 92 and to each of the satellite receivers 34 by way of suitable transmission lines.
In operation and with reference to both Figures 2 and 13, a mobile unit 40 in a particular sector Al-A3, Bl-B3 and Cl will communicate to the central control unit 20 by way of either the receivers 92 associated with the omnidirectional antenna 30, the directional antenna array 32, or by way of the satellite receivers 34. The strength ~ - 51 -1~)614~2 or signal level of the signals received from each mobile unit engaged in a call may be indicated by the ORM and SR~1 signals which are monitored by the receiver monitoring circuit 230. If the singal level of any of the signals received from mobile units engaged in calls drops below a predetermined threshold level slightly above the minimum level acceptable for conversation (hereinafter referred to as the vote threshold) the receiver monitoring circuit 230 may detect this condition and indicate this condition to the central control unit 20 through an appropriate RMS signal. In addition, the receiver monitoring circuit 230 may indicate to the central control unit 20 the identity of the sector in which the received signal has dropped below the vote threshold as well as the identity of the channel serving the call in that sector. The central control unit 20 may thereafter initiate a vote in adjacent sectors to ascertain into which adjacent sector the mobile unit 40 is moving.
For example, a mobile unit 40 may become engaged in a call in sector Al of Figure 2. The mobile unit 40 may there-after move into sector B3 and the satellite receiver 34 ser-ving the call in sector Al may experience a drop in level of the signal received from the mobile unit. When the received signal level drops below the vote threshold, the receiver monitoring circuit 230 of Figure 13 may supply an appropriate RMS signal to the central control unit 20 indicating, for ex-ample, that the mobile unit engaged in a call on channel 1 in section Al is moving out of sector Al.
When the central control unit 20 receives this informa-tion from the receiver monitoring circuit 230, the central control unit may generate appropriate control signals for transmission to the receiver control unit g2A at the base station 24. The CONTROL signals applied to the receiver control unit 92A may be decoded as appropriate receiver con-trol signals RCC and may command an available one of the re-ceivers 92 and 34 in each of the adjacent sectors Bl, B3 and Cl to the channel frequency over which the mobile unit is communicating, i.e., channel 1 in the present example.
Of course, separate monitoring receivers may be provided and may be tuned to the appropriate channel under the control of the central control unit 20 in this manner. However, use of the available equipment decreases system cost with little or no sacrifice in efficiency since, in the event that no equipment is available in a particular sector to provide the monitoring required for voting, there will be no equipment available in that sector to service the call should ~ -52-`` 1061412 the mobile unit be entering that sector.

-52a-Continuing with the above example, when each of the three available receivers in the respective sectors Bl, B3 and Cl is tuned to the appropriate channel, the receiver monitoring circuit 230 may compare the relative signal levels received by the three receivers to thereby determine the one of the three sectors into which the mobile unit is moving. Alter-natively, the receiver monitoring circuit 230 may sample the signal levels of the signals received by the three re-ceivers in the three adjacent sectors and may transmit the sampled signal level information to the central control unit 20 for a determination of the new sector.
After it has been determined that the mobile unit is moving into a particular sector, a channel must be assigned to the established call in that sector to avoid loss of com-munication. If the mobile unit is an IMTS type unit, thesame channel over which the call was initially established must be made available in the new sector if this is possible on a noninterfering basis. The central control unit 20 may make such a determination since information as to the assignment of channels to the various sectors and the cur-rently engaged channels in each sector is available at the central control unit 22. In the previous example wherein the call was established on channel 1 in sector Al and the mobile unit thereafter moves into sector B3, chan-nel 1 may be utilized in sector B3 if it is not being~utl-lized in sector A3 and if the system is provided with dyna-mic channel assignment capabilities as was previously described.
- Of course, if the mobile units are of the more advan-ced type capable of being remotely tuned to a different channel, any available channel in the new sector may be assigned to the established call. The assignment of a new channel may be effected by connecting the call from X ~53~

l(~ lZ

the wire line telephone system through the appropriate transmitting equipment and directional -53a-1~61~Z
\

antenna at the base station 24 and by remotely commanding the mobile unit transmitting and receiving equipment to the new channel. Of course, appropriate switching must be accomplished at the central control unit 20 to connect the receiver serving the new channel in the new sector to the appropriate outgoing trunk line.
A directional receiving approach may be utilized to accommodate zone or sector changes in small zone systems such as that disclosed in the previously referenced Wells et al application and in conjunction with the present in-vention. Various techniques have been proposed for auto-matically establishing a new communication path to serve an established call as a mobile unit changes zones in a small zone system. The proposed techniques vary depending upon a number of factors including the type of mobile units being employed in the system. For example, an IMTS mobile unit of the type presently in wide use does not have remotely controllable tuning capabilities. Thus, as an IMTS mobile unit moves from one sector into another when engaged in a call on a particular channel, the same channel must be available for use in the new sector in order to have con-tinuity of communication. On the other hand, a mobile unit which can be remotely commanded to a new channel while engaged in a call can be assigned an available channel in a new sector, whether or not the available channel differs from the channel in use in the old sector. This may be rapidly accomplished without substantial interruption of service by commanding the mobile unit to the available channel as the mobile unit enters the new sector engaged in a call.
Irrespective of the type of mobile unit being utilized in connection with the mobile telephone system, a sector change by a mobile unit engaged in a call must be detected ~ -54-~06~41Z

so that either the same channel may be made -54a-1~61~1Z
available in the new sector or a new channel may be assigned to the established call in the new sector. Detection of sector or zone change by a mobile unit may be accomplished as is described in the referenced Wells et al application or in any suitable conventional manner. Moreover, the directional characteristics of the coverage afforded in accordance with the present invention may be employed to permit the use of a simplified technique for accommodating sector or zone change in conjunction with the previously described embodiments of this invention or with a small zone system such as the Wells et al system.
In Figure 14, for example, there is illustrated a typical small zone communication system of the type dis-closed in the referenced Wells et al application in which sector change is accommodated through a directional re-ceiving technique. Referring now to Figure 14, the direc-tional antennas employed in conjunction with the present invention as applied to a small or large zone system se-parate the small or large zone about each base station 24W and 24X into a plurality of discrete sectors, e.g., six sectors. A mobile unit MU within the communication system may become engaged in a call in a home sector designated SH . As the mobile unit communicates over the channel assigned to the call, transmissions from the mobile unit may be monitored by monitoring equipment at the base station 24W serving the sector SH .
When the signal received from the mobile unit by the monitoring equipment for the sector SH drops below a pre-determined minimum signal level, the central control unit may initiate voting action to determine into which sector the mobile unit is moving. Because of the directional characteristics of the coverage afforded by the directional antenna arrays at each of the base stations 24W and 24X, ~ -55-~6~4:12 the sector change determination -55a-iO~14~Z
may be made by monitoring only three sectors. These three sectors designated S S and S in Figure 14 are those Ml, M2 M3 sectors adjacent to and surrounding the home sector S , i.e., the sectors defining the area 233. In other words, only the two adjacent sectors SMl and SM3 served by the base station 24W which serves the home sector SH and the adjacent sector SM2 served by the base station 24X need be monitored to determine into which sector the mobile unit has moved.
For example, if a mobile unit leaves its home sector SH, the most reasonable assumption would be that the mobile unit has entered either sectors S S or S as is Ml, M2 M3 generally indicated by the three arrows 235 indicating directions of movement of the mobile unit MU in Figure 14.
As the mobile unit progresses farther from the home sec-tor SH, the receiver at the base station 24W serving the mobile unit in the home sector S will indicate a drop in received signal strength below the predetermined mini-mum, thus indicating a requirement for a vote of receivers serving the sectors SMl SM2 and SM3 As will hereinafter be described in greater detail, the central control unit may initiate a vote of receivers serving the adjacent and surrounding sectors SMl - S 3 by tuning these receivers to the channel serving the call. The relative signal strengths of the signals received over the channel serving the call may be compared and the call may be assigned to a channel in that sector in which the strongest received signal is detected.
Assuming, for example, that a vote is initiated by the central control unit in response to a drop in received signal level below the vote threshold level in home sec-tor SH and further that the received signal in the adjacent and surrounding sectors SMl - SM3 is strongest in sector ~ -56-10~1412 S available equipment at the base station 24X serving the sector SM2 may be assigned -56a-lO~ 2 to service the call previously established between the mobile unit and another subscriber in the home section SH.
The sector SM2 may thereafter be designated a new home sector for the mobile unit.
of course, in initiating the previously described voting action, the mobile unit may have traveled along a path generally corresponding to borders between the adjacent and surrounding sectors S 1 ~ S 3 to a point 232 in a sector adjacent the sector SM2. In this situation, the vote of the adjacent and surrounding sectors SMl - SM3 may result in the designation of the sector SM2 as the new home sector and, instead of getting stronger, the sig-nal received in the new home sector SM2 may actually get weaker and initiate a sector change request, i.e., a re-quest for a vote of the three sectors adjacent and sur-rounding the new home sector SM2. Again, the normal three vote sequence will occur and since the point 232 toward which the mobile unit has traveled is in one of the sec-tors SM4 S M5 and S~ adjacent and surrounding the new home sector SM2 the adjacent sector SM4 will be designa-ted the new home sector as a result of the vote.
It can thus be seen that the three-sector vote de-scribed above can accommodate sector changes along any line from the home sector to or through the adjacent and surrounding sectors. In the event that the assignment of a new home sector is in error due to borderline move-ment of the mobile unit, the immediate occurrence of a new three-sector vote in the sectors adjacent and sur-rounding the new home sector automatically corrects for the mistake.
The actual monitoring of signal level by the home sector (to initiate a vote) and by the adjacent and sur-~ -57-~06~41Z
rounding sectors (to determine a new home sector) may be accomplished in accordance with the monitoring tech-niques described in detail in the referenced Wells et al application.

-57a-~061412 For example, the monitoring function in the home sector (vote initiation) may beprovided by the receiver serving the established call since established calls to the mobile unit MU may be served directionally by way of the directional antennas at the base stations in either known small zone systems or in the previously described embodi-ments of this invention. The monitoring function for determination of the new home sector may be provided by available receiving equipment at each base station, i.e., receiving equipment not in use for handling a call, or by separate monitoring receivers. If available receiving equipment is employed, the receivers must be selectively tunable. Moreover, if the directional antenna arrange-ments at the base stations are employed in a small zone or other system solely for zone change determinations, one or more separate monitoring receivers may be provided at each base station to provide all monitoring functions.
Referring to Figure 15, for example, the section 234 of the directional antenna array 32 at the base-station 24W

may serve the home sector SH of Figure 13. The receiver 236 receiving transmissions from the mobile unit MY by way of a receiving portion 238 of the directional antenna section 234 may supply a signal RV indicative of received signal strength to a vote decision circuit 240 for deter-mination of a vote requirement. The vote decision cir-cuit 240 may supply a signal VOTE to the central control unit 20 of Figure 1 on the control signaling lines 28 to initiate a vote.

If the receiver 236 is part of the communication link between the mobile unit and the wire line telephone system, the signal received from the mobile unit may be supplied to the central control unit 20 by way of the VOICE

~ -58-106i~1Z

(RCV) lines 27 previously described in connection with Figure 1. If normal call reception is not accomplished by way of the directional antenna array 32, the receiver 236 may be a monitoring receiver which scans each -58a-~61412 channel in each of the sectors served by the base station 24W to detect requirements for voting, i.e., to detect the sensor change requirements.
The vote decision circuit 240 may indicate to the central control unit 20 that a mobile unit engaged in a call on a particular channel in the home sector SH requires a sector change and the central control unit 20 may then initiate a vote in the sectors adjacent and surrounding the home sector SH. One technique for voting in the adja-cent and surrounding sectors and thereby establishing anew home sector is illustrated in Figure 16.
Referring now to Figure 16, the signals received by each segment of each of the directional antenna arrays 32 at the base stations 24W and 24X may be applied to a suitable conventional RF coupler or combiner 242 and an output signal from the coupler may be applied to a monitor-ing receiver 244 at each base station. The output signal from each of the monitoring receivers 244 may be applied to a suitable conventional sample and hold circuit 246 and the receiver monitoring signals RMS indicative of moni-tored signal strength may be applied from the sample and hold circuit 246 to the central processing unit at the central control unit 20 over the signaling lines 28 as was previously described in connection with Figure 13.

The CONTROL signal from the central control unit 20 of Figure 13 may be applied to a suitable receiver control unit 248 at each of the base stations and a receiver con-trol signal RCC from each receiver control unit 248 may be applied to each associated monitoring receiver 244.

If established calls are to be served by way of the direc-tional antenna array 32 both on the receiving and trans-mitting communication links, the coupler 242 may be re-~4 ~ -59-1~36~ z placed by or may additionally function as a suitable con-ventional signal splitter so that the signals received by the directional antenna array 32 may be coupled both to the monitoring receiver 244 and to the receivers 92 -59a--` ~0ti14~Z

which serve the communication link from the mobile units as is illustrated in phantom at the base station 24W
in Figure 16.
In operation, the central control unit 20 of Figure 13 initiates a vote in response to the detection of a drop in signal level below the vote threshold as was pre-viously described. The central control unit may trans-mit the appropriate CONTROL signal to the receiver control unit 248 at each of the base stations affected by the vote request. The monitoring receivers 244 at the affec-ted base stations may be tuned to the appropriate channel by the receiver control signal RCC from the control unit 248 and then sequentially connected to the directional antennas serving the sectors adjacent to and surrounding the home sector in which the vote request is received.
In Figure 16, for example, the home sector SH is adjacent to and surrounded by sectors SMl S M2 and SM3. The monitoring receiver 244 at the base station 24W may thus be tuned to the appropriate channel serving the established call which initiated the vote in the home sector SH
and then would be sequentially coupled to the directional antennas serving the sectors SMl and SM3. Similarly, the receiver control unit 248 at the base station 24X may tune the monitoring receiver 244 to this same channel and connect the monitoring receiver 244 to the directional antenna serving the sector SM2.
The signal indicative of the received signal level in each of the adjacent and surrounding sectors SMl - SM3 is sampled and held for a predetermined, relatively short period of time by the sample and hold circuit 246 and trans-mitted to the central control unit 20 as the RMS signal.
The relative signal levels received in the adjacent and ~ -60-surrounding sectors S 1 ~ S are compared and the largest signal indicates the new home sector. If available, a channel is thereafter assigned in the new home sector to the established call as was previously described.

-60a-

Claims (33)

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

1. A method for providing broadcast service in a mobile communi-cation system service area, the method comprising the steps of:
providing at least one base station in the system service area; and, directionally broadcasting wave energy from the one base station in a plurality of sectors of the service area so as to define directionally separate broadcast sectors of the service area, said sectors extending radially out-wardly from said base station, said directionally broadcast wave energy being broadcast in adjacent of said sectors at differing frequencies and in at least some of said sectors at the same frequency.

2. The method of claim 1, wherein several base stations are provided, each base station including a plurality of transmitters, and wherein said ..
wave energy is broadcast into at least some of said plurality of sectors from a plurality of said base stations.

3. A method as defined in claim 1 and further comprising the steps of:
generating an idle channel marker signal; and, modulating said directionally broadcast wave energy said idle channel marker signal.

4. A method as defined in claim 3 wherein the output power level of said wave energy is reduced from first predetermined level to a second predetermined level during said modulation.

5. A method as defined in claim 1 further comprising the step of omnidirectionally broadcasting wave energy in respective zones surrounding each said base stations, said plurality of sectors extending radially from each said base stations beyond a radially outward limit of the associated zone.

6. A method as defined in claim 1 further comprising the steps of:
generating an idle channel marker signal; and, omnidirectionally broadcasting the idle channel marker signed in each of said sectors from locations spaced radially outwardly from each said base stations, said locations being generally central of each said sectors.

7. A method as defined in claim 6 wherein said omnidirectional wave energy is broadcast in said zone or zones at a first frequency while said directional wave energy is broadcast in said sectors at frequencies differing from said first frequency.

8. A method as defined in claim 6 wherein transmissions from mobile units are received in the respective ones of said sectors generally at said radially outwardly spaced locations.

9. A communication system for providing broadcast service between fixed stations and mobile units within a service area when performing the method as defined in claim 1, the system being CHARACTERIZED by comprising first means at at Least one base station in the service area, said first means including a plurality of directional transmitters for direction-ally broadcasting wave energy in a plurality of directionally defined sectors of the service area, the sectors extending radially outwardly from the at least one base station in different directions so as to define directionally separate broadcast sectors of the service area, said wave energy being directionally broadcast in adjacent of said sectors at differing frequencies and in at least some of said sectors at the same frequency.

10. A system as defined in claim 9 and further CHARACTERIZED in THAT it comprises:
first, second and third base stations each including a plurality of transmitters, each one of the base stations being spaced approximately equidistantly from each of the other base stations, and a plurality of directional antennas at each of said base stations, each directional antenna at each base station being operatively associated with at least one of said plurality of transmitters to broadcast wave energy in a sector of the service area, one of said plurality of direc-tional antennas at each base station being operable to broadcast wave energy in substantially the same sector of the service are to thereby provide broadcast coverage of said same sector from each of said first, second and third base stations, the remaining ones of said plurality of directional antennas at each of the base stations being oriented to provide broadcast coverage of other sectors of the service area from each of said first, second and third base stations.

11. A system as defined in claim 10 and further CHARACTERIZED in THAT said same sector of the service area is covered from each of said first, second and third base stations at different frequencies whereby at least three transmitters each at a different base station can simultan-eously broadcast in said same sector.

12. A system as defined in claim 10 and further CHARACTERIZED by including:
a fourth base station including a plurality of trans-mitters, said fourth base station being spaced approximately equidistantly from each of said first, second and third base station and generally centrally of said same sector covered from each of said first, second and third base stations, and a plurality of directional antennas at said fourth base station each operatively associated with at least one of said plurality of transmitters to each broadcast wave energy in a sector of the service area, the directional antennas at said fourth base station being oriented to provide broadcast coverage of sectors surrounding said first, second and third base stations.

13. A system as defined in claim 12 and further CHARACTERIZED by comprising:
fifth, sixth and seventh base stations each including a plurality of transmitters and each being equidistantly spaced radially outwardly from said fourth base station and two of said first, second and third base stations;
a plurality of directional antennas at each of said fifth, sixth and seventh base stations, each directional antenna at each of said fifth, sixth and seventh base sta-tions being operatively associated with at least one of said plurality of transmitters thereat to broadcast wave energy in a sector of the service area, one of said plurality of directional antennas at each of said fifth, sixth and seventh base stations being operable to broadcast wave energy in said same sector of the service area covered by said first, second and third base stations to thereby pro-vide coverage of said same sector from each of said first, second, third, fifth, sixth and seventh base stations.

14. A system as defined in claim 13 and further CHARACTERIZED in THAT said wave energy is broadcast by said fifth, sixth and seventh base stations in said same sector at frequencies differing from each other and from the fre-quencies of the wave energy broadcast from said first, second and third base stations to thereby provide coverage of said same sector from each of said, first, second, third, fifth, sixth and seventh base stations at six different frequencies.

15. A system as defined in claim 9 and further CHARACTERIZED in THAT said first means at each of said base stations comprises a plurality of directional antennas each operatively connected to at least one of said plurality of transmitters at each base station and each oriented to broadcast wave energy from said transmitters in a different sector.

16. A system as defined in claim 15 and further CHARACTERIZED in THAT said sectors are defined by the direc-tional characteristics of said directional antennas and THAT immediately adjacent of said sectors overlap in area by an amount less than the area of any one of said sectors.

17. A system as defined in claim 9 and further CHARACTERIZED by comprising second means at each of said base stations for omnidirectionally broadcasting wave energy in a zone surrounding each base station and THAT said sec-tors extend beyond a radially outward limit of the respec-tive zones.

18. A system as defined in claim 17 and further CHARACTERIZED in THAT said directional antennas at each base station are located centrally of said zone, and THAT
each directional antenna has a directional characteristic including an angular beam width defining one of said pre-determined sectors, the sum of the angular beam widths of said directional antennas being at least 360°.

19. A system as defined in claim 18 and further CHARACTERIZED in THAT adjacent of said directional antennas in each base station are oriented angularly relative to each other by an angle approximately equal to the sum of one-half of each angular beam width thereof.

20. A system as defined in claim 18 and further CHARACTERIZED in THAT immediately adjacent of said sectors are served by different ones of the communication channels assigned to the system, the same communication channels serving at least some of the sectors spaced by at least one other sector whereby the same communication channel may be simultaneously used in more than one sector of the ser-vice area.

21. A system as defined in claim 9 and further CHARACTERIZED in THAT said wave energy is omnidirectionally broadcast in said zone at each base station at a first fre-quency and that said wave energy is directionally broadcast in said sectors at frequencies differing from said first frequency.

22. A system as defined in claim 9'and further CHARACTERIZED by comprising means for generating an idle channel marker signal, and means radially outward spaced from each of said base stations generally centrally of each of or at least certain of said sectors for omnidirectionally broad-casting said idle channel marker signal in said sector in which positioned.

23. A system as defined in claim 22, CHARACTERIZED in THAT
said means for omnidirectionally broadcasting said idle channel signal include a plurality of beacon transmitters, at least one of said beacon transmitters being positioned approximately centrally with respect to each of said sectors and being operable to omnidirectionally broadcast wave energy modulated by a marker signal in the sector in which positioned.

24. A system as defined in claim 17 and further CHARACTERIZED
in THAT said beacon transmitters are located radially remote from said central zone surrounding each of said base stations.

25. A system as defined in claim 15 and wherein the output level of each of said transmitters operatively connected to said directional antennas is selectively controllable, the system being further CHARACTERIZED by comprising means for generating an idle channel marker signal, means for generating a control signal, and circuit means for selectively applying said idle channel marker signal and said control signal to said transmitters operatively connected to said directional antennas, said idle channel marker signal modulating said wave energy transmitted in said sectors and said control signal reducing the output power level of said transmitters from a first predetermined level to a second predetermined level during the modu-lation of said wave energy by said idle channel marker signal.

26. A system as defined in claim 13 and FURTHER CHARACTERIZED in THAT a marker beacon transmitter is located generally centrally of at least said same sector covered from said first, second and third base stations or from said first,second, third, fifth, sixth and seventh base stations, said marker beacon transmitter being operable to omnidirectionally broadcast wave energy in said same sector at a selected one of a plurality of predetermined frequencies and THAT the system includes means for modulating the wave energy broadcast by said marker beacon transmitter with a marker signal.

27. A system as defined in claim 9 and further including a plurality of satellite receivers adapted to receive transmissions from mobile telephones or communication units, the system being CHARACTERIZED in THAT
said satellite receivers are located remote from said directional antennas and THAT at least one satellite receiver is located approximately centrally of each of said sectors served by said directional antennas.

28. A system as defined in claim 17 and further CHARACTERIZED in THAT each of said satellite receivers is located remote from said central zone surrounding each of said base stations.

29. A system as defined in claim 26 and further CHARACTERIZED in THAT at least one satellite receiver is located generally centrally of said sector covered from said first, second and third base stations of from said first, second, third, fifth, sixth and seventh base stations.

30. A system as defined in claim 29 and further CHARACTERIZED by including a central control unit operatively connected to each of said base stations, said satellite receivers, said marker beacon transmitter and to a wire line telephone system for selectively controlling the broadcast of wave energy by said base stations and marker beacon transmitters and for selectively connecting said satellite receivers and said transmitters at said base stations to said wire line telephone system.

31. A system as defined in claim 9 and further CHARACTERIZED by including:
means for establishing a call between a fixed com-munication unit and a mobile communication unit over an available one of said two-way communication channels through one of said base stations, means operatively connected to said plurality of directional antennas at said one of said base stations for monitoring the signal strength of transmissions from said mobile unit engaged in said established call received over a first one of said directional antennas defining a first one of said sectors designated as a home sector;
means operatively connected to said monitoring means for detecting a drop in monitored signal strength received over said first one of said direction antennas, means operatively connected to said monitoring means for detecting a drop in monitored signal strength received over said first one of said directional antennas below a predetermined threshold, means responsive to said detecting means for monitoring and comparing the signal strength of said transmissions from said mobile unit engaged in said established call received over only three of said directional antennas de-fining three monitoring sectors adjacent to and surrounding said first one of said sectors, two of said three directional antennas defining said three monitoring sectors being at said one of said base stations and the third of said three directional antennas being at another of said base stations;
and, means responsive to said comparing means for designa-ting one of said three monitoring sectors as a new home sector.

32. A system as defined in claim 31, CHARACTERIZED
by further comprising means responsive to said designating means for reestablishing said established call through another of said plurality of base stations.

33. A system as defined in claim 32, and further CHARACTERIZED in THAT said established call is reestablished over the same one of said dual frequency communication channels.