WO2001041494A1

WO2001041494A1 - System and method for private communications system selection

Info

Publication number: WO2001041494A1
Application number: PCT/US2000/030290
Authority: WO
Inventors: Kenneth Wallstedt
Original assignee: Ericsson Inc.
Priority date: 1999-12-03
Filing date: 2000-11-02
Publication date: 2001-06-07
Also published as: GB2377590A; GB2377590B; GB0212439D0; AU1458201A; AR032751A1

Abstract

A system and method for determining at a cellular station a dedicated control channel (DCCH) frequency of a digital private communications system including determining a DCCH frequency set which is less than all frequencies designated for DCCHs of cellular communications systems. The select DCCH frequency set is stored in a private base station memory of the digital private communications system. A digital private communications system DCCH is operated using only the frequencies within the select DCCH frequency set, and the select DCCH frequency set is scanned at the cellular station to locate the digital private communications system DCCH. An indication such as a 'flag' is provided at a public base station, indicating the presence of the digital private communications system. The flag is transmitted from the public base station to the mobile cellular station, where the mobile cellular station scans the select DCCH frequency set only where the indication indicates the presence of the digital private communications system. A cellular station timer for measuring a predetermined time period triggers automatic scanning of the select DCCH frequency set by the mobile cellular station at intervals of the predetermined time period.

Description

SYSTEM AND METHOD FOR PRIVATE COMMUNICATIONS SYSTEM SELECTION

BACKGROUND OF THE INVENTION

The present invention is directed toward digital private communications systems, and more particularly toward digital private communications system selection when a mobile station is roaming in a public communications system.

A public cellular communications system, for example a public cellular network, provides cellular services to mobile cellular stations. The public cellular network is divided into cells, where a mobile cellular station within a particular cell is serviced by a public base station within the cell. Depending on the capabilities of the mobile cellular station and the public base stations, the cellular service provided may be analog or digital. Where the public base station and mobile cellular station are in accordance with the IS 1 36 standard, the public base station communicates with the mobile cellular station using a dedicated control channel (DCCH) according to the IS 1 36 standard, on which operating information is transmitted to the mobile cellular station using, for example, logical channels of a time-division multiple access (TDMA) frame. A set of frequencies on which the control frequencies may be located is limited by the standard, as, for example, AMPS.

Most of the examples present are for IS 1 36 standard. Digital private communications systems are commonly used by corporations, hospitals and universities to provide cellular service to employees. Digital private communications systems exist within the public cellular network, and provide cellular service to mobile cellular stations specially programmed for accessing the digital private communications system. The special programming includes a private system identity code (PSID), a system identity code (SID), a system operator code (SOC), and a mobile country code (MCC). As in the public network, a private base station of the digital private communications system communicates with the specially programmed mobile cellular station using a DCCH, on which operating information including the PSID, SID, SOC and MCC are transmitted to the mobile cellular station, using for example a logical channel of a TDMA frame. The specially programmed mobile cellular stations store an operating frequency for a current DCCH on which the private communications system is operating.

A digital private communications system operates within a public cell, and typically has a coverage area of just a small portion of that covered by the public cell. The digital private communications system uses the same band of frequencies allocated to the public base stations within the public cell.

However, the actual frequencies being used by the private base station at any one time are different than those used by base stations close by to prevent interference between the private and public base stations. Accordingly, the digital private communications system must continually determine which of the allocated frequencies are being used by the public base stations to insure that the same frequencies are not used by the private base station.

At times, the public base station replans, that is, changes operating frequencies. Replanning may occur as more frequencies are allocated for use by the public base station, or as the cell coverage area is divided to provide coverage for a greater number of mobile cellular stations.

If the private base station has detected that the public base station has replanned and is operating on frequencies currently being used by the private base station, the private base station will change its operating frequencies to prevent interference between the private and public base stations. However, mobile cellular stations temporarily out of the private communications system coverage area, or powered-down when the private system operating frequencies changed will not be updated with the new digital private communications system DCCH operating frequencies. Accordingly, these mobile cellular stations will be unable to find the private base station, and coverage must be provided to the mobile cellular stations by the public base station.

To re-establish communication with the private base station, a DCCH wide-band scan of the allocated frequencies for the DCCH must be performed at the mobile cellular station. Potentially all of the frequency bands, both the 1 900 MHz bands and the 850 MHz cellular bands, will have to be scanned to find the private base station. The DCCH wide-band scan takes several minutes for the mobile cellular station to complete, tying-up the receiver in the mobile station which could result in missed pages, and consuming a large amount of battery power.

Additionally, when the specially programmed cellular stations lose communications with the digital private communications system, a user of the specially programmed cellular station must initiate the DCCH scan using onscreen menus activated with the cellular telephone keypad to re-establish communications with the digital private communications system. However, the user of the specially programmed cellular station may not notice that service is being provided by the public base station, resulting in a significant period of time before the DCCH scan is initiated. As the service provided by the public base station is generally more expensive and of lesser quality than that provided by the digital private communications system, this delay in initiating the DCCH scan could result in increased operating costs for the specially programmed cellular station, and reduced service quality.

The present invention is directed to overcoming one or more of the problems discussed above.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method for determining at a cellular station a dedicated control channel (DCCH) frequency of a digital private communications system includes determining a select DCCH frequency set which is less than all frequencies designated for DCCHs of cellular communications system, and storing the select DCCH frequency set in a private base station memory of the digital private communications system. A digital private communications system DCCH is operated using only the frequencies within the select DCCH frequency set, and the select DCCH frequency set is scanned at the cellular station to locate the digital private communications system DCCH.

In one form of this aspect of the present invention, the step of determining the select DCCH frequency set includes entering the select DCCH frequency set less than all frequencies at a public base station, and communicating the select DCCH frequency set to the digital private communications system.

In another form of this aspect of the present invention, the step of communicating the select DCCH frequency set includes the step of transmitting the select DCCH frequency set from the public base station to the digital private communications system.

A further form of this aspect of the present invention includes the step of transmitting the select DCCH frequency set from the public base station to the cellular station. In a further form, the step of transmitting the select DCCH frequency set to the cellular station includes transmitting the select DCCH frequency set over a broadcast control channel.

In a further form of this aspect of the present invention, the step of determining the select DCCH frequency set includes assigning at a controller of the public base station frequencies comprising the select DCCH frequency set, and transmitting the select DCCH frequency set to the digital private communications system. A further form includes the step of transmitting the select DCCH frequency set from the public base station to the cellular station. In a further form, the step of transmitting the select DCCH frequency set to the cellular station includes transmitting the select DCCH frequency set over a broadcast control channel. In another form of this aspect of the present invention, the cellular station includes a scan enabler for enabling scanning of the select DCCH frequency set, and further includes generating an indication at a public base station, where the indication indicates the presence of the digital private communications system, and transmitting the indication from the public base station to the cellular station. The step of scanning includes receiving the indication at the cellular station, and activating the scan enabler where the indication indicates the presence of the digital private communications system. In a further form, the indication is a digital bit, and the step of transmitting the indication includes transmitting the digital bit in an indication location of a data field on a broadcast control channel.

A further form of this aspect of the present invention includes programming a cellular station memory with the select DCCH frequency set.

In a further form of this aspect of the present invention, the step of scanning includes setting a cellular station timer to measure a predetermined time value, and scanning the select DCCH frequency set only after the timer has measured the predetermined time value.

In another form of this aspect of the present invention, the step of determining the select DCCH frequency set includes receiving the select DCCH frequency set from a private communications system programmer in communication with the digital private communications system.

It is another aspect of the present invention to provide a communications system for allowing a DCCH operating frequency of a digital private communications system to be determined by a cellular station performing a narrow band scan, including a private base station having a private base station memory for storing a select DCCH frequency set which is less than all frequencies designated for DCCHs of cellular communications systems, the private base station operating a private base station DCCH using only the frequencies within the select DCCH frequency set. A scanner within the cellular station performs the narrow band scan of the select DCCH frequency set to determine the DCCH frequency of the digital private communications system.

In another form of this aspect of the present invention, a public base station in communication with the cellular station is provided having a public base station controller for generating an indication indicating the presence of the digital private communications system. A public base station transmitter is coupled to the public base station controller for transmitting the indication to the cellular station, and a scanner-enabler is coupled to the scanner for enabling the narrow band scan of the select DCCH frequency set responsive to the indication. In a further form of the present invention, the public base station transmitter transmits the indication as a digital indication bit in an indication location of a data field on a broadcast control channel.

In another form of this aspect of the present invention, the cellular station is a mobile cellular station. In another form of this aspect of the present invention, the communications system includes a public base station in communication with the cellular station having a public base station controller for determining the select DCCH frequency set, and a public base station memory coupled to the public base station controller for storing the select DCCH frequency set. A public base station transmitter coupled to the public base station memory transmits the select DCCH frequency set to the private base station. In a further form, the transmitter transmits the select DCCH frequency set to the cellular station using a data field of a broadcast control channel. In a further form, the public base station transmitter transmits the select DCCH frequency set to the cellular station. In yet a further form, the communications system includes an input device coupled to the controller for entering the select DCCH frequency set, where the controller determines the select DCCH frequency set by receiving the select DCCH frequency set from the input device.

In another form of this aspect of the present invention, the cellular station includes a cellular station memory for storing the select DCCH frequency set. In a further form of the present invention, the communications system further includes a cellular station programmer in communication with the cellular station memory for programming the select DCCH frequency set in the cellular station memory. In another form of this aspect of the present invention, the communications system includes a private base station programmer in communication with the private base station memory for providing in the private base station memory the select DCCH frequency set.

In yet another form of this aspect of the present invention, the communications system includes a timer for measuring a predetermined time value, and a scanner-enabler coupled to the scanner and in communication with the timer for enabling the narrow band scan of the select DCCH frequency set after the timer has measured the predetermined time value.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a drawing showing a cell of the public cellular network including a private communications system;

Figure 2 is a block diagram of the private base station in accordance with an embodiment of the present invention; Figure 3 is a block diagram of the specially-programmed mobile station in accordance with an embodiment of the present invention;

Figure 4 is a block diagram of the public base station in accordance with an embodiment of the present invention;

Figure 5 is a flow chart describing a general embodiment of the present invention;

Figure 6 is a flow chart describing operation of two embodiments of the present invention;

Figure 7 is a drawing showing how BCCH information is transmitted on the DCCH, in accordance with embodiments of the present invention; and Figure 8 is a flow chart describing operation of two embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a public cellular network cell 1 00 including a public base station 1 1 0 and a digital private communications system 1 20.

The digital private communications system 1 20 includes a private base station 1 30, in communication with the public base station 1 1 0 via landline 1 70 using the Public Land Mobile Network (not shown) as is known by one skilled in the art. The digital private communications system 1 20 further includes a first and a second specially-programmed mobile cellular station 1 40 and 1 50.

The public base station 1 1 0 provides cellular services bounded by the cell 1 00. The private base station 1 30 provides cellular services bounded by the region designated as 1 60. The specially-programmed mobile cellular station (PMCS) 1 40 and 1 50 are generally provided cellular service by the private base station 1 30. PMCS 1 40, being within the region 1 60, receives cellular service directly from the private base station 1 30. However, if PMCS 1 40 is not in communication with the private base station 1 30 when the digital private communications system replans, for example, the PMCS 1 40 is deactivated (turned off), the PMCS 1 40 is not able to regain communication with the private base station 1 30 when reactivated, and cellular service is provided to the PMCS 1 40 by the public base station 1 1 0. The PMCS 1 50, being outside the region 1 60, receives cellular service through the public base station 1 1 0.

Figure 2 shows the private base station (PRBS) 1 30 in accordance with an embodiment of the invention. A PRBS antenna 200 is coupled to a PRBS transceiver 21 0 for transmitting or receiving information using the PRBS antenna 200. A PRBS controller 220, coupled to the PRBS transceiver 21 0, generates the information to be transmitted by the PRBS transceiver 21 0, or compiles the information received by the PRBS transceiver 21 0. A PRBS dedicated control channel (DCCH) memory 230, coupled to the PRBS controller 220, is suitable for storing a select DCCH frequency set. Operation of the PRBS 1 30 is described in relation to Figs. 5-8.

Figure 3 shows a block diagram of a specially-programmed mobile cellular station, such as PMCS 140 or 1 50. A PMCS antenna 300 is coupled to a PMCS transceiver 31 0, for transmitting and receiving information using the PMCS antenna 300. A PMCS controller 320 is coupled to the PMCS transceiver 31 0, and generates the information to be transmitted by the PMCS transceiver 31 0, or compiles information received by the PMCS transceiver 31 0. A PMCS DCCH memory 330, coupled to the PMCS controller 320, is suitable for storing the select DCCH frequency set. Operation of the PMCS 1 40 is described in relation to Figs. 5-8.

Figure 4 is a block diagram illustrating the public base station (PUBS) 1 1 0 in accordance with an embodiment of the invention. A PUBS antenna 400 is coupled to a PUBS transmitter 41 0, for transmitting information from the PUBS 1 1 0 using the PUBS antenna 400. A PUBS controller 420 is coupled to the PUBS transmitter 41 0 and provides the PUBS transmitter 41 0 with information to be transmitted. A PUBS DCCH memory 430 is coupled to the PUBS controller 420 and is suitable for storing the select DCCH frequency set.

Figure 5 is a flow chart describing operation of a general embodiment of the present invention. Operation begins in step 500 where the select DCCH frequency set is determined. The select DCCH frequency set is a set of frequencies less than all DCCH frequencies allocated for the public cellular network. The select DCCH frequency set may be, for example, 20 of the 500 or more frequencies allocated for the DCCH of the public cellular network. The select DCCH frequency set may be determined in a number of ways, as further described in Figures 6 and 8. Once determined, the select DCCH frequency set is stored in the PRBS DCCH memory 230, as shown in step 51 0. The PRBS controller 220 is suitably programmed to use only frequencies present within the select DCCH frequency set for the DCCH of the private base station as shown in step 520. The PMCS 1 40 scans only the select DCCH frequency set to locate the DCCH currently used by the private base station 1 30 when the PMCS 140 loses communication with the private base station 1 30, as shown in step 530.

Thus, when the public base station 1 1 0 replans its network by altering the frequencies on which it is operating, causing the private base station 1 30 to replan its network, users of PMCSs which have lost communications with the private base station prior to the replanning need not perform a DCCH wideband scan to reestablish communications. Because the private base station 1 30 operates using only the select DCCH frequency set, a narrow band scan of the select DCCH frequency set may be performed. The narrow band scan takes much less time for the PMCS to perform as there are less frequencies to be scanned, tying up the receiver less time and thereby reducing the chance of missed pages, and consuming much less battery power than the DCCH wideband scan.

Figure 6 is a flow chart showing a more-detailed embodiment of the present invention, designated by the solid flow lines, where the private base station 1 30 includes a PRBS programmer 240 (see Fig. 2) suitable for programming the PRBS 1 30, and the PMCS 1 40 includes a PMCS programmer 340 (see Fig. 3) suitable for programming the PMCS 140. The PRBS programmer 240 is used to program the private base station 1 30 with the select DCCH frequency set, as shown in step 600. Such programming typically occurs when the private base station 1 30 is brought on-line for the first time, but may occur at any time software within the PRBS 1 30 is upgraded. The PRBS programmer 240 provides the PRBS controller 220 with the select DCCH frequency set. The PRBS controller 220 stores the select DCCH frequency set in the PRBS DCCH memory 230, as shown in step 61 0. The PRBS controller 220 limits the frequency on which the DCCH is operated to only those frequencies within the select DCCH frequency set, shown in step 620. The PMCS programmer 340 programs the PMCS 1 40 with the select DCCH frequency sat (step 630) . This is accomplished by the PMCS programmer 340 providing the select DCCH frequency set to the PMCS controller 320, and the PMCS controller 320 storing the select DCCH frequency set in the PMCS DCCH memory 330, shown in step 640. When scanning for the DCCH of the private communications system, the PMCS controller 320 is suitably programmed to limit the frequencies scanned by the PMCS transceiver 31 0 to those of the select DCCH frequency set. Thus, where the PMCS 1 40 has lost communication with the private base station 1 30, and the private base station 1 30 has subsequently replanned its network, the PMCS 1 40 need only perform a narrow band scan of the select DCCH frequency set to locate a current DCCH frequency on which the private base station 1 30 is operating, shown at step 650. A DCCH wideband scan is not necessary, reducing the time necessary to determine the DCCH operating frequency for the private base station 1 30, and tying up the receiver less time thereby reducing the chance of missed pages to the PMCS 140. Additionally, battery power is conserved in the PMCS 1 40 as less frequencies need to be scanned.

Programming the PMCS 1 40 as shown in step 630 is typically done at subscription set-up, but may be done at any time. The PMCS programmer 340 may be connected to the PMCS controller 320 using a PMCS systems port 350, located on mobile cellular stations and used for programming the mobile cellular station.

Further shown in Figure 6 is a flow chart describing operation for an alternate embodiment of the present invention, designated by the dashed flow lines, where step 630 is replaced by steps 660 and 670.

Once the private base station 1 30 is operating using only the select DCCH frequency set, step 620, the select DCCH frequency set is communicated to the public base station 1 10 as shown in step 660. For example, a public base station programmer 440, coupled to the PUBS controller 420 and suitable for programming the PUBS 1 1 0 with the select DCCH frequency set, provides the PUBS controller 420 with the select DCCH frequency set. The PUBS controller 420 stores the select DCCH frequency set in the PUBS DCCH memory 430. The select DCCH frequency set may be communicated as a block of frequencies, where a first frequency in the block of frequencies and a last frequency in the block of frequencies is provided. Alternatively, the select DCCH frequency set may be communicated as a list of frequencies. An information element may be added to the communication indicating the number of DCCH frequencies being communicated, or a predetermined number of DCCH frequencies may be communicated at a time. Once communicated to the public base station 1 1 0, the select DCCH frequency set is transmitted to the PMCS 1 40 as shown in step 670. Fig. 7 is a drawing showing how the select DCCH frequency set may be transmitted using a forward DCCH (F- DCCH) channel.

The select DCCH frequency set may be transmitted from the public base station 1 1 0 to the PMCS 1 40 by adding an information element to the BCCH of an F-DCCH channel 700, shown in Figure 7. The F-DCCH 700 includes two data fields, a data field 705 and a data field 71 0, in which information including BCCH information is transmitted. The select DCCH frequency set is transmitted on the E-BCCH channel in an L3 information field 720 of a layer 2 message 730, as a block of frequencies, or as a list of frequencies. Using the E-BCCH to send information is known by one skilled in the art and will not be discussed in detail. The select DCCH frequency set is received by the PMCS 140, where a receiver section of the PMCS transceiver

31 0 delivers the received information to the PMCS controller 320. The PMSC controller 320 is suitably programmed to extract the select DCCH frequency set from the E-BCCH channel received at the PMCS transceiver 31 0, and stores the select DCCH frequency set in the PMCS DCCH memory 330, shown in step 640. Operation continues as previously described. Although step 660 has been described with the PUBS programmer 440 communicating the select DCCH frequency set to the PUBS 1 10, the select DCCH frequency set may be communicated to the public base station 1 1 0 by adding information elements to communications between the public base station 1 1 0 and the private base station 1 30 via the PLMN, as would be apparent to one skilled in the art. Alternatively, the select DCCH frequency set may be communicated to the public base station 1 1 0 by the private base station 1 30 transmitting the select DCCH frequency set to the public base station 1 1 0 using an extended broadcast control channel (E- BCCH), a logical channel of the DCCH, as would be known by one skilled in the art.

Utilizing the BCCH of the DCCH to transmit the select DCCH frequency set to the PMCS allows the select DCCH frequency set to be communicated in a more convenient manner. When the frequencies within the select DCCH frequency set are altered, the PMCS need not be attached to the PMCS programmer 340 to be updated with the new select DCCH frequency set. The new select DCCH frequency set is communicated to the PMCS 1 40 over the DCCH, allowing the PMCS to be updated so long as the PMCS 1 40 is in communication with the public base station 1 1 0. Figure 8 is a flow chart describing operation of an additional embodiment of the present invention. In step 800, the select DCCH frequency set is entered at the public base station 1 1 0, using for example an input device shown as a terminal 450 with suitable keyboard 452, coupled to the PUBS controller 420. The select DCCH frequency set is stored by the PUBS controller 420 in the PUBS DCCH memory 430. The select DCCH frequency set is communicated to the private base station 1 30 and transmitted to the PMCS 1 40, shown in step 81 0. The select DCCH frequency set may be communicated to the private base station 1 30 using the PLMN in a similar fashion as described with respect to communicating the select DCCH frequency set to the public base station 1 1 0 in step 660 (fig. 6) . Altematively, the select DCCH frequency set may be transmitted to the private base station using the E-BCCH of the F-DCCH, in a similar fashion as described regarding transmitting the select DCCH frequency set to the PMCS 1 40 in step 670. Transmitting the select DCCH frequency set to the PMCS 140 may be performed as described with respect to step 670 (Fig. 6) .

The select DCCH frequency set is received at the private base station 1 30 as shown in step 820. The select DCCH frequency set may be received via the PLMN as would be apparent to one skilled in the art, where the select DCCH frequency set may be communicated as a block of frequencies, or as a list of frequencies over the PLMN. The select DCCH frequency set is communicated to the PRBS controller 220. If the select DCCH frequency set is transmitted to the PRBS 1 30, the select DCCH frequency set is received at the PRBS antenna 200, where a receiver section of the PRBS transceiver 21 0 delivers the received information to the PRBS controller 220. The PRBS controller 220 identifies the select DCCH frequency set transmitted on the E-BCCH. Whether the select DCCH frequency set is communicated via the PLMN or transmitted over the DCCH, the PRBS controller 220 stores the select DCCH frequency set in the PRBS DCCH memory 230, shown in step 830. The private base station DCCH is then operated using only the select DCCH frequency set, as shown in step 840. The PMCS 1 40 receives the select DCCH frequency set, step 850, in a similar fashion as decsribed with respect to step 670 (Fig. 6) . The PMCS 1 40 scans only frequencies in the select DCCH frequency set to locate the operating frequency for the digital private communications system DCCH, shown in step 860.

The select DCCH frequency set may be entered at the public base station by a system operator, thus allowing the select DCCH frequency set to be changed as deemed necessary by the system operator, for example as new frequencies become available for use as DCCH frequencies. Alternatively, the system operator may change the frequencies of the select DCCH frequency set pursuant to an agreement between the public base station and the digital private communications system.

Further shown in Figure 8 is a flow chart describing operation of an alternative embodiment of the present invention, shown by dashed flow lines. Step 800 is replaced by step 870, where the select DCCH frequency set is determined at the PUBS controller 420. When the public base station 1 1 0 replans its network, it determines the select DCCH frequency set by assigning the select DCCH frequency set on which the private base station DCCH 1 30 is to operate. This is possible as the PUBS controller 420 is programmed with the frequencies allocated for the DCCH, and currently knows which DCCH frequencies are being used by the public base station 1 1 0. After replanning, the PUBS controller 420 assigns frequencies for the select DCCH frequency set that are not then in use by the public base station 1 1 0. The select DCCH frequency set is transmitted to the private base station 1 30 and the PMCS 140, as shown in step 81 0. Steps 81 0 through

860 are performed as previously described.

Having the system where the PUBS controller 420 assigns frequencies for the select DCCH frequency set allows the select DCCH frequency set to be altered at any time, without intervention of the system operator. Further, as the PUBS controller 420 knows the frequencies on which the PUBS 1 1 0 is operating when it is assigning the select DCCH frequency set, the PRBS 1 30 need not determine whether the select DCCH frequency set overlaps the frequencies in use by the PUBS 1 1 0, thereby freeing-up PRBS controller resources. In another embodiment of the present invention, an indication, such as a "flag", is transmitted by the public base station, and indicates whether a private communications system such as the private communications system 1 20 is present within the cell. The indication is received by the PMCS 1 40, where the PMCS 140 performs a scan of the select DCCH frequency set only if the indication indicates the presence of the digital private communications system 1 20. Where the indication indicates that no private communications system is present, the PMCS does not expend battery power or controller computation time to scan the select DCCH frequency set, thereby conserving the PMCS battery. For example, where the user of the PMCS initiates a scan to locate the DCCH operating frequency for the private communications system, and the flag transmitted by the PUBS 1 10 indicates that no private communications systems are present, no scan is performed. A message such as "No private communications present" may appear on the display of the PMCS. The "flag" may be transmitted as a digital bit located in an indication location (not shown) on the E-BCCH channel within the L3 information field 720 of the 2nd Layer message 730, as would be apparent to one skilled in the art. The PMCS controller 320 is suitably programmed to locate the indication location, and determine whether the flag indicates the presence or not of the private communications system 1 20. For example, where the flag in the indication location has a value of " 1 ", presence of the private communications system 1 20 is indicated and the select DCCH frequency set is scanned by the PMCS. However, where the flag has a value of "0", it is indicated that no private communications system is present within the cell 1 00, and battery power and controller resources are not expended on a scan of the select DCCH frequency set.

The public base station 1 1 0 may be programmed with the flag at system power-up or at any other time, using the PUBS programmer 440. Alternatively, the flag value may be entered at the input terminal 450. The PUBS controller 420 is suitably programmed to receive the flag information, and to incorporate the flag information into the indication location within the E-BCCH.

Using a flag to indicate the presence of the private communication system 1 20 allows the PMCS 1 40 to conserve battery power and controller resources as a scan of the select DCCH frequency set is performed only where the flag indicates the presence of the private communications system 1 20.

In another embodiment of the invention, a PMCS timer 360 is provided, coupled to the PMCS controller 320, for measuring a pre-determined time period at which the PMCS 1 40 scans the select DCCH frequency set.

Once the PMCS timer 360 has measured the pre-determined time, the PMCS controller causes a scan of the select DCCH frequency set to be performed. The PMCS timer 360 thus allows the select DCCH frequency set to be scanned automatically by the PMCS 140, at intervals of the pre-determined time period. It is unnecessary for a user of the PMCS 1 40 to initiate the scan for the dedicated control channel frequency of the digital private communications system, resulting in reduced operating costs and improved service for the mobile cellular station where communications with the private base station 1 30 are lost. The pre-determined time may be, for example, a time period of 5 minutes.

One skilled in the art would realize that the PMCS timer 360 may be an integrated circuit for performing timer functions, having a suitable on- chip memory for storing the pre-determined time value. Alternatively, one skilled in the art would realize that the function performed by the PMCS timer 360 may be incorporated within the PMCS controller 320. Further the functions performed by the PMCS timer 360 may alternatively be incorporated in the PUBS 1 1 0, using a PUBS timer (not shown) or the PUBS controller 420. The PUBS 1 1 0 transmits an indication to the PMCS 1 40 to perform the scan of the select DCCH frequency set after the predetermined time period has elapsed. The indication could be in the form of a flag, transmitted in a timer location of the E-BCCH in a similar fashion as described with respect to the indication location. For example, a "1 " may be transmitted in the timer location for a suitable amount of time after the predetermined time period has elapsed, for the PMCS to initiate the scan of the select DCCH frequency set. The timer indication is then changed to "0" indicating no further scan is to be performed by the PMCS. After the predetermined time period elapses, the value in the timer location is once again set to " 1 " .

Although the PRBS DCCH memory 230, the PMCS DCCH memory 330, and the PUBS DCCH memory 430 have been described as distinct memories, one skilled in the art would realize that the DCCH memories may be a part of a larger memory element used by each of the PRBS 1 30, the PMCS 1 40 and the PUBS 1 1 0. Additionally, although the system and method has been described with respect to a cell with a single base station and a private communications system with a single base station, one skilled in the art would realize that the teachings could be applied to cells having more than one public base station, and private communications systems which have more than one private base station.

The communications system of the present invention thus allows the DCCH frequency of the digital private communications system to be determined using a narrow band scan of the select DCCH frequency sets where the PMCS loses communications with the digital private communications system and the digital private communications system replans its network. Performing the narrow band scan of the select DCCH frequency set reduces the time for determining the frequency on which the

DCCH of the digital private communication system is operating. The receiver is not tied up, thereby reducing the chance of missed pages to the PMCS, and battery power within the PMCS is conserved. Further, an indication, such as a flag, may be provided by a public base station indicating the presence of a private communications system. Thus, the PMCS scans for the DCCH frequency of the private communications system only where the flag indicates the presence of the digital private communications system. Further, a timer may be provided at the PMCS allowing the select DCCH frequency set to be scanned automatically by the PMCS at intervals of the predetermined time period. Thus, a user of the PMCS need not manually initiate the narrow band scan each time the DCCH of the private communications system is to be located, reducing operating costs and improving service for the PMCS.

Further, as described herein in the context of the Background Art, it should be recognized by those skilled in the art that the present invention could be used not only with systems using the IS 1 36 standard, but also with systems using still other standards.

Still other aspects, objects, and advantages of the present invention can be obtained from a study of the specification, the drawings, and the appended claims. It should be understood, however, that the present invention could be used in alternate forms where less than all of the objects and advantages of the present invention and preferred embodiments as described above would be obtained.

Claims

1 . A method for determining at a cellular station a dedicated control channel (DCCH) frequency of a digital private communications system, the method comprising the steps of: determining a select DCCH frequency set from a plurality of

DCCH designated frequencies, wherein the select DCCH frequency set is less than all frequencies designated for DCCHs of cellular communications systems; storing the select DCCH frequency set in a private base station of the digital private communications system; operating a digital private communications system DCCH using only the frequencies within the select DCCH frequency set; and scanning at the cellular station the select DCCH frequency set to locate the digital private communications system DCCH.

2. The method of claim 1 wherein the step of determining a select DCCH frequency set from a plurality of DCCH designated frequencies includes entering the select DCCH frequency set at a public base station, and communicating the select DCCH frequency set to the digital private communications system.

3. The method of claim 2 wherein the step of communicating the select DCCH frequency set to the digital private communications system includes the step of transmitting the select DCCH frequency set from the public base station to the digital private communications system.

4. The method of claim 2 further including the step of transmitting the select DCCH frequency set from the public base station to the cellular station.

5. The method of claim 4 wherein the step of transmitting theselect DCCH frequency set from the public base station to the cellular station includes transmitting the select DCCH frequency set over a broadcast control channel.

6. The method of claim 1 wherein the step of determining a select DCCH frequency set from a plurality of DCCH designated frequencies includes assigning at a controller of a public base station frequencies comprising the select DCCH frequency set, and transmitting the select DCCH frequency set to the digital private communications system.

7. The method of claim 6 further including the step of transmitting the select DCCH frequency set from the public base station to the cellular station.

8. The method of claim 7 wherein the step of transmitting the select DCCH frequency set from the public base station to the cellular station includes transmitting the select DCCH frequency set over a broadcast control channel.

9. The method of claim 1 wherein the cellular station includes a scan enabler that enables scanning of the select DCCH frequency set, and further including the steps of: generating an indication at a public base station, the indication indicating the presence of the digital private communications system; and transmitting the indication from the public base station to the cellular station; wherein the step of scanning at the cellular station the select DCCH frequency set to locate the digital private communications system

DCCH includes receiving the indication at the cellular station, and activating the scan enabler where the indication indicates the presence of the digital private communications system.

1 0. The method of claim 9 wherein the indication is a digital bit, and the step of transmitting the indication from the public base station to the cellular station includes transmitting the digital bit in an indication location of a data field on a broadcast control channel.

1 1 . The method of claim 1 further comprising the step of programming a cellular station with the select DCCH frequency set.

1 2. The method of claim 1 wherein the step of scanning at the cellular station the select DCCH frequency set to locate the digital private communications system DCCH includes: setting a cellular station timer to measure a predetermined time value; and scanning the select DCCH frequency set only after the timer has measured the predetermined time value.

1 3. The method of claim 1 wherein the step of determining the select DCCH frequency set from a plurality of DCCH designated frequencies includes the step of receiving the select DCCH frequency set from a private communications programmer in communication with the digital private communications system.

1 4. A communications system comprising: a private base station of a digital private communications system, the private base station having a private base station memory storing a select dedicated control channel (DCCH) frequency set from a plurality of DCCH designated frequencies, wherein the select DCCH frequency set is less than all frequencies designated for DCCH's of cellular communications systems, the private base station operating at a private base station DCCH frequency using only the frequencies within the select DCCH frequency set; and a cellular station having a scanner performing a narrow band scan of the select DCCH frequency set to determine the private base station DCCH frequency of the digital private communications system.

1 5. The communications system of claim 14 further comprising: a public base station in communication with the cellular station, the public station comprising: a public base station controller generating an indication indicating the presence of the digital private communications system, and a public base station transmitter coupled to the public base station controller, the public base station transmitter transmitting the indication to the cellular station; and wherein the cellular station further comprises a scanner enabler coupled to the scanner, the scanner enabler enabling the narrow band scan of the select DCCH frequency set responsive to the indication.

1 6. The communications system of claim 1 5 wherein the public base station transmitter transmits the indication as a digital indication bit in an indication location of a data field on a broadcast control channel.

1 7. The communications system of claim 1 4 wherein the cellular station is a mobile cellular station.

1 8. The communications system of claim 1 4 further comprising: a public base station in communication with the cellular station, the public base station comprising: a public base station controller determining the select DCCH frequency set, a public base station memory coupled to the public base station controller, the public base station memory storing the select DCCH frequency set, and a public base station transmitter coupled to the public base station memory, the public base station transmitter transmitting the select DCCH frequency set to the private base station.

1 9. The communications system of claim 1 8 wherein the public base station transmitter transmits the select DCCH frequency set to the cellular station using a data field of a broadcast control channel.

20. The communications system of claim 1 8 wherein the public base station transmitter transmits the select DCCH frequency set to the cellular station.

21 . The communications system of claim 1 8 further comprising an input device coupled to the public base station controller, the input device entering the select DCCH frequency set, and wherein the controller receives the select DCCH frequency set from the input device.

22. The communications system of claim 1 4 wherein the cellular station includes a cellular station memory storing the select DCCH frequency set.

23. The communications system of claim 22 further comprising a cellular station programmer in communication with the cellular station memory, the cellular station programmer programming the select DCCH frequency set in the cellular station memory.

24. The communications system of claim 1 4 further comprising a private base station programmer in communication with the private base station memory, the private base station programmer programming the select

DCCH frequency set in the base station memory.

25. The communications system of claim 1 4 further comprising: a timer measuring a predetermined time value; and a scanner enabler coupled to the scanner and in communication with the timer, the scanner enabler enabling the narrow band scan of the select DCCH frequency set after the timer has measured the predetermined time value.