Abstract:
Methods and systems are provided for tracking of a private communication system allowing operation of a mobile terminal with both a public communication system and the private communication system even when the private communication system retunes its control channel while not in communication with the mobile terminal. This automatic selection of the new control channel for the private communication system is provided by communication of both the current control channel frequency and one or more candidate control channel frequencies which will be used in turn by the private radio communication system in retunes. These candidate frequencies are originally provided to the mobile terminal, for example, when it first establishes a connection with the private communication system. The candidate control channel frequency information is updated during periods when the mobile terminal is connected to the private communication system. When the mobile terminal subsequently leaves the coverage area of the private communication system and then returns to find that the control channel previously used is no longer in use, it attempts to establish a connection using the candidate control channel frequencies. Accordingly, the mobile terminal is able to respond to retunes occurring while it is not in communication with the private communication system without the need for user intervention.

Description:
FIELD OF THE INVENTION 
     This invention relates to communication systems and more particularly to acquisition of communication systems by mobile terminals. 
     BACKGROUND OF THE INVENTION 
     Public wireless radiotelephone systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990&#39;s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in  The Mobile Communications Handbook , edited by Gibson and published by CRC Press (1996). 
     FIG. 1 illustrates a conventional terrestrial wireless public communication system  20  that may implement one of the aforementioned wireless communication standards. The wireless system may include one or more wireless mobile terminals  22  that communicate within a plurality of cells  24  served by base stations  26  and a mobile telephone switching office (MTSO)  28 . Although only three cells  24  are shown in FIG. 1, a typical cellular radiotelephone network may comprise hundreds of cells, and may include more than one MTSO  28  and may serve thousands of wireless mobile terminals  22 . 
     The cells  24  generally serve as nodes in the communication system  20 , from which links are established between wireless mobile terminals  22  and a MTSO  28 , by way of the base stations  26  servicing the cells  24 . Each cell  24  will have allocated to it one or more dedicated control channels and one or more traffic channels. Through the communication system  20 , a duplex radio communication link  30  may be effected between two wireless mobile terminals  22  or between a wireless mobile terminal  22  and a landline telephone user  32  via a public switched telephone network (PSTN)  34 . The function of the base station  26  is commonly to handle the radio communications within the cell  24  to and from the wireless mobile terminal  22 . In this capacity, the base station  26  functions chiefly as a relay station for data and voice signals. 
     As mobile terminals in cellular radio communication systems may cross geographic regions served by different base stations, it is known in the prior art that means should be provided to select the appropriate base station to service a mobile terminal. In the U.S. AMPS system, the mobile terminal would typically use signal strength measurements (such as RSSI) to determine the strongest base station to which it should listen for calls during standby (idle) mode. This determination of which base station to communicate with may be referred to as site scanning (reselection) as periodically the radiotelephone determines the RSSI for other base station sites. Also in the U.S. AMPS system, base stations belonging to the cellular network typically listen to the signal strengths received from mobile telephones that are actively transmitting during calls, and the network uses its measurements to determine an optimum base station for handling a call in progress. When a call in progress is switched from one base station to another, it is commonly known as “handover” or “handoff.” 
     Cellular telephones using a Time Division Multiple Access (TDMA) method conforming to the TDMA cellular standards, for example, those known respectively as GSM, D-AMPS, IS54, IS136 or PCS1900, may use spare time between transmit and receive timeslots to change frequency and monitor the signal strengths of other base stations. The mobile telephone may make measurements of the signal strengths received from surrounding base stations. Mobile Assisted Handover (MAHO) may be implemented using these measurements. The measurements are typically reported to the currently serving base station, which determines if a handoff or reselection should be made to another base station. 
     The control channel as described above is a dedicated channel used for transmitting cell identification and paging information often referred to as a broadcast control channel (BCCH). Identification information in, for example, an IS-136 system may be referred to as a Public Service Profile (PSP) and may include frequency information on various channels as well as the cell&#39;s associated Digital Voice Color Code (DVCC), System Identification (SID), System Operator Code (SOC) and Mobile Country Code (MCC). The traffic channels carry the voice and data information. The dedicated control channels are generally broadcast using a different radio frequency in each cell to allow frequency reuse for greater system capacity while limiting the problems caused by interference. The traffic channels are also typically allocated among a plurality of radio frequencies to reduce and preferably eliminate interference using conventional frequency allocation techniques that are well known to those having skill in the art. However, capacity problems may still be encountered, such as in high population density areas. 
     Capacity limitations and cost and service considerations have been addressed by using private wireless communication systems, which use low power cellular transmissions that provide coverage over a smaller area. These private communication systems may be located within the coverage area of a public communication system such as one of the cellular systems described above. An indoor cellular communication system typically includes one or more mobile stations or units, one or more base stations, a radio control interface, and a mobile switching center. A typical indoor cellular network may have one or more cells. 
     It is desirable for mobile terminals having access rights to both public and private communication systems to determine the availability of both systems when in operation. Accordingly, it is known, for example under the IS-136, Rev. A standard, to provide mobile terminals which monitor not only for alternate base stations within the public network but also to monitor for transmissions from private communication systems. It is also known to provide transmission of identification information from private communication systems for use by the mobile terminal in its monitoring operations. 
     Under the IS-136 standard, the private communication system transmits identification information referred to as Private Operating Frequencies (POF). The POFs retained by a mobile terminal are a set of private communication system control frequencies previously identified by the mobile terminal as being actively in use by a private communication system within the vicinity of a public cell (associated by the mobile terminal with a specific control channel frequency) for which the mobile terminal has retained a PSP. Upon acquisition of a particular control channel (CCH) transmission, the mobile terminal may recognize the control channel as corresponding to a PSP retained by the mobile terminal and declare a PSP match. When a PSP match is declared, any stored associated POFs are treated as reselection candidates for the mobile terminal. If the mobile terminal then finds a satisfactory POF candidate (for example, with acceptable signal strength), the mobile terminal typically registers for service on the private communication network (which has an associated private system identification or PSID) and switches to a non-public mode of operation. 
     A mobile terminal supporting PSP/POF pursuant to the IS-136 Rev. A. standard typically stores at least 4 PSPs and 4 POFs per private communication system (PSID) as shown in Table 1 below. Each private system has an associated identifier (PSID). The PSP/POF information for each PSID is typically updated by information transmitted by the private system on its control channel. Accordingly, the mobile terminal generally must be locked on to the private system&#39;s control channel to be updated if the POF information changes. Furthermore, under IS-136, one of the POFs is the currently used control channels for the private system. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 PSID 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 PSP#1 
                 POF#1 
               
               
                   
                 PSP#2 
                 POF#2 
               
               
                   
                 PSP#3 
                 POF#3 
               
               
                   
                 PSP#4 
                 POF#4 
               
               
                   
                   
               
             
          
         
       
     
     One problem with this approach to integrating private and public systems is that private communication systems may operate within the same frequency spectrum as the surrounding public system and/or adjacent private communication systems. Accordingly, to overcome interference problems, private communication systems may be provided with adaptive channel allocation (ACA) or other automated frequency reallocation means. Therefore, private communication systems may retune on occasion, i.e., change the frequency used for the control channel (or channels in some cases) transmission by the private communication system. 
     After a retune, the PSP/POF typically must be updated in the mobile terminals. However, for mobile terminals that are not receiving transmissions at the time of the retune, the change will not be received and the mobile terminal will not have a correct POF for the PSP match (i.e. for the public region in which the private system is located). Therefore, the mobile terminal will not be expected to find and camp on the private system automatically. Under the IS-136 standard, this problem is addressed by requiring the user of the mobile terminal to force a non-public search by the mobile terminal to find the private communication system. 
     In light of the above discussion, improvements are needed in integration of private and public communication systems to provide improved user perception of the ease of accessing either type of system. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an objective of the present invention to resolve the problem of integration of public communication systems with communication systems subject to retuning. 
     It is a further objective of the present invention to provide systems and methods which provide users with mobile terminals which automatically track retuning of communication systems without requiring user intervention. 
     These and other objects are provided, according to the invention, by providing methods and systems for tracking of a communication system allowing operation of a mobile terminal with both a public communication system and the private communication system even when the private communication system retunes its control channel whether or not the mobile terminal is in communication with the private system at that time. This automatic selection of the new control channel for the private communication system is provided by communication of both the current control channel frequency and one or more candidate control channel frequencies which will be used in turn by the private radio communication system in retunes. These candidate frequencies are originally provided to the mobile terminal, for example, when it first establishes a connection with the private communication system. The candidate control channel frequency information is updated during periods when the mobile terminal is connected to the private communication system. When the mobile terminal subsequently leaves the coverage area of the private communication system and then returns to find that the control channel previously used is no longer in use, it attempts to establish a connection using the candidate control channel frequencies. Accordingly, the mobile terminal is able to respond to retunes occurring while it is not in communication with the private communication system without the need for user intervention. 
     In particular, a method for tracking of a private communication system is provided. A first control channel frequency for the private communication system is selected as well as a candidate control channel frequency. Control information is transmitted on the first control channel frequency which includes an identification of the candidate control channel frequency and an identification of the private communication system. The private communication system may be a TDMA system such as an IS-136 compliant system. The private communication system is preferably within a cell of a public communication system and the control information then also includes identification information for the public cell. In one embodiment of the present invention, the identification information for the cell is a PSP for the cell. 
     In another embodiment of the methods of the present invention, after the control information is transmitted at the first control channel frequency, the private communication system is retuned to transmit control information on the candidate control channel frequency instead of the first control channel frequency. The control information is then transmitted on the candidate control channel frequency. The control information then includes an identification of a second selected candidate control channel frequency and an identification of the private communication system. Preferably, retuning is performed using an adaptive channel allocation scheme having a determined sequence of alternate candidate carriers for retuning. 
     In a further embodiment of the methods of the present invention, a plurality of candidate control channel frequencies are selected for the private communication system. Identifications of the plurality of candidate control channel frequencies are transmitted in the control information. 
     In another aspect of the methods of the present invention, the control information is received by a mobile terminal on the first control channel frequency from the private communication system. The mobile terminal subsequently receives control information from the private communication system on the candidate control channel frequency if control information from the private communication system is absent from the first control channel frequency. The private communication system is retuned to transmit control information on the candidate control channel frequency instead of the first control channel frequency. The control information transmitted on the candidate control channel frequency includes an identification of a second selected candidate control channel frequency and an identification of the private communication system. The mobile terminal then receives control information from the private communication system on the second selected candidate control channel frequency if control information from the private communication system is not being received on either the first control channel frequency or the candidate control channel frequency. 
     In a system aspect of the present invention, a private communication system is provided. The system includes a means for selecting a first control channel frequency for a control channel and at least one candidate control channel frequency. The system further includes a means coupled to the means for selecting for changing a frequency on which the private communication system transmits the control channel from the first control channel frequency to a selected one of the at least one candidate control channel frequency. The control information is transmitted by a means coupled to the means for changing a frequency on which the private communication system transmits for transmitting control information on the control channel. The control information includes an identification of the at least one candidate control channel frequency and an identification of the private communication system. The private communication system may be an IS-136 compliant system. The private communication system may be within a cell of a public communication system and the control information may then include identification information for the cell such as a public service profile for the cell. 
     In a further aspect of the present invention, a mobile terminal is provided for use with a private communication system which transmits control information on one of a plurality of control channel frequencies. The mobile terminal includes a means for receiving the control information including an identification of a candidate control channel frequency selected from the plurality of control channel frequencies on a first control channel frequency selected from the plurality of control channel frequencies. The mobile terminal further includes means for receiving control information from the private communication system on the candidate control channel frequency if control information from the private communication system is not being received on the first control channel frequency. 
     Accordingly, the present invention provides systems and methods allowing operation of a mobile terminal with both a public communication system and a private communication system even when the private communication system retunes its control channel while not in communication with the mobile terminal. This automatic selection of the new control channel for the private communication system is provided by transmitting to mobile terminals not only the control channel frequency (or frequencies) currently in use by the private communication system but also the candidate frequency (or frequencies) that the system will use for subsequent retunes. Accordingly, the mobile terminal is able to respond to retunes occurring while it is not in communication with the private communication system without the need for user intervention by using the saved candidate frequencies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates a portion of a public wide area cellular network including a plurality of base stations servicing different geographic areas; 
     FIG. 2 schematically illustrates a portion of a public wide area cellular network including a private communication system within the coverage area of the public network and a mobile terminal for tracking both according to an embodiment of the present invention; 
     FIG. 3 is a block diagram of a mobile terminal according to an embodiment of the present invention; 
     FIG. 3 a  is a block diagram of a private communication system according to an embodiment of the present invention; 
     FIG. 4 is a flowchart illustrating operations for transmitting and retuning a control channel in a private radio communication system according to an embodiment of the present invention; and 
     FIG. 5 is a flowchart illustrating operations for a mobile terminal tracking a private communication system within the coverage area of a public communication system according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. As will be appreciated by one of skill in the art, the present invention may be embodied as methods or devices. Accordingly, the present invention may take the form of a hardware embodiment, a software embodiment or an embodiment combining software and hardware aspects. 
     Operations of the present invention will be described primarily herein with reference to mobile terminals which will operate in an IS-136 standard public communication system. However, the benefits of the present invention may also be realized with mobile terminals which operate in other communication systems including support for interconnection with both the public system and a private system within the coverage area of the public system. The present invention will also be described with reference to private systems. However, the present invention may also be utilized with overlapping public systems. 
     FIG. 2 illustrates an embodiment of the systems and methods for tracking a private communication system subject to retuning according to the present invention. As illustrated in the embodiment of FIG. 2, a mobile terminal  60  according to the present invention operates within a cell  24  of public communication system  20 . For purposes of the discussion herein, public system  20  will be described as an IS-136 standard cellular telephone system. Also shown in FIG. 2 is a private communication system  80  according to the present invention. Private communication system  80  includes a wireless transmission coverage area overlapping several cells  24  of public communication system  20 . Accordingly, pursuant to the present invention, mobile terminal  60  will attempt to acquire communication with private communication system  80  when it is located within any cell  24  of public communication system  20  in which the respective base station  26  transmits a PSP having an associated POF list for private communication system  80 . Public communication system  20  otherwise operates as described with respect to FIG.  1 . 
     In the illustrated embodiment of the present invention shown in FIG. 2, private communication system  80  transmits on a control channel its POF information as well as PSP information for the home public cell or cell identifiers covering the region in which private communication system  80  is located. The POF list transmitted by private communication system  80  includes the private system&#39;s currently active control channel frequency and at least one, and preferably three, additional candidate control channel frequencies available for use by private communication system  80 . As is known with the aforementioned standard IS-136, each cell  24  of the public communications network  20  as well as private communication system  80  include associated identifying information. Preferably, this identifying information is also transmitted along with the POF and PSP information. 
     Private communication system  80 , according to the present invention, periodically retunes its control channel frequencies to reduce problems associated with interference as would be known to those of ordinary skill in the art. A commonly used form of retuning is referred to as an adaptive channel allocation method (ACA) in which the private communication system  80  knows in advance what frequencies it will allocate if it initiates a retune to replace its currently allocated frequency (or frequencies). Note that while it is known to provide private communication systems having more than one active control channel frequency it is preferred, according to the present invention, that only a single one of the allocated control channel frequencies be actively used by private communication system  80  at any given time. 
     As will be described herein with reference to the flowcharts, according to the present invention, private communication system  80  includes in its transmitted POF list both the currently active control channel frequency and the candidate frequency list specifying the frequencies that will be selected as new control channel frequencies when the existing frequency is replaced. This information is provided and stored by mobile terminal  60  so that terminals  60  may automatically retune to private communication system  80  without requiring user intervention. It is intended that such automatic tuning will enhance the end user&#39;s perception of private system support in a POF/PSP system. 
     According to the present invention, the majority of the retunes of the private communication system  80  will be expected to pass unnoticed to the end user. In fact, retunes may occur during high traffic times as camping mobile terminals, while they will lose the control channel from private communication system  80 , will preferably automatically rescan the search list including the candidate control channel frequencies from the POF and lock on the new control channel frequency. Alternatively, the terminal will jump out of the private system and then automatically reacquire the private system pursuant to the methods of the present invention. 
     Pursuant to the IS-136 standard, PSP information stored by mobile terminal  60 , typically for each of a plurality of neighboring adjacent cells, would include the frequency of the active control channel, DVCC and SID/SOC/MCC for each cell. Mobile terminal  60  also stores the PSID and, optionally, name, for private communication systems  80  within the coverage area of one of the stored public cell&#39;s PSPs. The private system operating control channel frequency (or frequencies) and one or more candidate control channel frequencies are stored as the POF list. 
     Referring now to the embodiment illustrated in FIG. 3, the user (mobile) terminal  60  is a transceiver device capable of communicating with the public communications system  20 . The user terminal  60  may operate as a cellular radiotelephone. The user terminal  60  includes an antenna  62  and transceiver  64  or other transceiver means for transmitting and receiving communications to/from a public and/or private communication network (system). The processor  66  accepts and provides information to and from the transceiver  64  and selects a control channel for the user terminal  60  to communicate with the public communication system  20  and/or private communication system  80 . The user terminal memory  68  stores information related to the communication systems  20 ,  80 . The user interface  70  may include a key pad by which the user may control the operation of the user terminal  60 . User terminal  60  also includes a tracking circuit  72  or other means for tracking a private communication system  80  from mobile terminal  60  by selection of candidate control channel frequencies according to the present invention. 
     User interface  70  provides both an input and output means. Interface  70 , according to one embodiment of the present invention, includes display means for notifying a user, for example, that tracking with a private communication system  80  has been lost. Interface  70  further includes an input means, such as a keyboard, for receiving input from the user, for example, specifying available or preferred private communication systems for initial acquisition. Processor  66 , in this embodiment, includes means for controlling the transceiver  64  so as to communicate over the public system  20  and/or the private system  80 . Memory  68  preferably contains a list of candidate carriers as stored POF information associated with a PSP as well as a PSP associated with the stored POFs in a format such as illustrated for IS-136 in Table 1. 
     Referring now to the embodiment illustrated in FIG. 3 a , the private communication system  80  is illustrated as a single base station. However, it is to be understood that the system would typically be expected to include a plurality of stations as illustrated interconnected through a common server. Private communication system  80  is a transceiver device capable of communicating with the mobile terminals  60 . The system  80  includes an antenna  82  and transceiver  84  or other transceiver means for transmitting and receiving communications to/from a mobile terminal. The processor  86  accepts and provides information to and from the transceiver  84  and selects a control channel frequency for the private communication system  80  to communicate control information to mobile terminal  60 . The memory  88  stores information related to the communication systems  20 ,  80 . The user interface  90  may include a key pad by which the user may control the operation of the private communication system  80 . Private communication system  80  also includes a retuning circuit  92  or other means for adaptively changing the control channel frequency of private communication system  80  to respond to potential interference problems. As was described with reference to FIG. 3, an active and a number of alternate candidate control channel frequencies are maintained by private communication system  80 . 
     As will be appreciated by those of skill in this art, the above-described aspects of the present invention in FIGS. 3 and 3 a  may be provided by hardware, software, or a combination of the above. While various components of mobile terminal  60  and the private communication system  80  have been illustrated in FIGS. 3 and 3 a , in part, as discrete elements, they may, in practice, be implemented by a microcontroller including input and output ports and running software code, by custom or hybrid chips, by discrete components or by a combination of the above. For example, memory  68  may be contained within processor  66 . Similarly, various of the operations of tracking system  72  may be implemented as code executing on processor  66  as shown in the illustrated embodiment. 
     Operations of the present invention will now be described with respect to the flowcharts of FIGS. 4 and 5. It will be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by computer program instructions. These program instructions may be provided to a processor to produce a machine, such that the instructions which execute on the processor create means for implementing the functions specified in the flowchart block or blocks. The computer program instructions may be executed by a processor, such as processor  66  of FIG. 3, to cause a series of operational steps to be performed by the processor to produce a computer implemented process such that the instructions which execute on the processor provide steps for implementing the functions specified in the flowchart block or blocks. 
     Accordingly, blocks of the flowchart illustrations support combinations of means for performing the specified functions, combinations of steps for performing the specified functions and program instruction means for performing the specified functions. It will also be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by special purpose hardware-based systems which perform the specified functions or steps, or combinations of special purpose hardware and computer instructions. 
     Referring now to FIG. 4, operations for selecting and transmitting control information by a private communication system  80  according to an embodiment of the present invention will now be described. At block  100 , private radio communication system  80  selects a first control channel frequency for use in transmitting control information. At block  102 , private communication system  80  selects one or more candidate control channel frequencies for the private communication system. These candidate control channel frequencies are preferably maintained in a sequential order reflecting the sequence which will be used by private communication system  80  in selecting frequencies for retuning under an adaptive channel allocation methodology. 
     At block  104 , private communication system  80  transmits control information on the first control channel frequency for reception by a mobile terminal in the coverage area of private communication system  80 . In addition to information related to normal control operations supporting communication traffic within private communication system  80 , the control information includes an identification of the candidate control channel frequencies and an identification of private communication system  80 . Preferably, the transmission also includes PSP information for the home cell or cells of a public communication system  20  in which private communication system  80  resides, thereby providing identification information for the cell. 
     At block  106 , private communication system  80  determines if control channel frequency retuning is required. This determination is preferably accomplished using an adaptive channel allocation methodology such as those which are known to persons of ordinary skill in the art. Accordingly, the adaptive channel allocation methodology will not be further described herein. If retuning is required, at block  108  private communication system  80  retunes to the next sequential candidate control channel frequency from the sequences of POFs established at block  102 . Operations then return to block  102  to update the candidate control channel frequency list as one of the candidate frequencies has now been selected and taken the place of the first control channel frequency so the candidate list is preferably updated after each retuning to transmit a full list of candidate carriers to mobile terminals  60 . 
     Operations then proceed again to block  104  with private communication system  80  transmitting the control information on the newly selected candidate control channel frequency including the updated POF list of candidate control channel frequencies. For example, if only a single candidate control channel frequency is maintained, a second selected candidate control channel frequency is chosen at block  102  after operations move from block  108 . The second selected candidate control channel frequency is included in the control information transmitted subsequently at block  104 . Note that frequencies may be reused, in which case the control channel frequency in active use before the retune may be added to the list of candidate control channel frequencies after retuning at block  108 . 
     While it is possible to obtain the benefits of the present invention with only a single alternate candidate carrier for retuning, it is preferred that a plurality of candidate control channel frequencies for the private radio communication system be selected at block  102  and that identification of all of the plurality of candidate control channel frequencies in a selection order be transmitted at block  104 . 
     Referring now to FIG. 5, operations according to the present invention for locating/tracking a private radio communication system according to the present invention will now be further described. At block  120 , operations begin with mobile terminal  60  acquiring initial PSP/POF information for private communication system  80 . This may occur, for example, by a user initiated manual system search when a user moves to a new geographic region, or this initial information may be stored in mobile terminal  60  by a service provider at the time mobile terminal  60  is activated and sold to a user. 
     At block  122 , mobile terminal  60  receives control information transmitted by a base station  26  in a local cell  24  of public communication network  20 . At block  124 , mobile terminal  60  determines if the PSP received in the control information matches a PSP stored in mobile terminal  60 . If no PSP match is found at block  124 , mobile terminal  60  contains no associated POF information for that geographic region as indicated at block  126  and remains camped on public communication system  20  with operations returning to block  122 . 
     If a PSP match is found at block  124 , the associated POFs for private communication system  80  are included in mobile terminal  60  reselection options at block  128 . Pursuant to the IS-136 standard, mobile terminal  60  then considers these associated POFs as well as PSPs in mobile assisted handover and reselection operations. At block  130 , mobile terminal  60  determines if an acceptable POF has been identified during reselection operations. If not, operations return to block  126  and communications continue through public communication system  20  as no private system is available. 
     If an acceptable POF is found and mobile terminal  60  is configured to prefer operations on a private communication system  80  over public system  20  when the private system is available, mobile terminal  60  camps on private system  80  at block  132 . While connected to private system  80  at block  132 , mobile terminal  60  is further able to update POF information as such information is contained in the control information transmitted by private communication system  80 . 
     If a system retune by private communication system  80  is detected at block  134 , mobile terminal  60  attempts to retune to stay on private system  80  at block  136 . Alternatively, mobile terminal  60  may first return to the public system and then reacquire the private system as shown by the dotted line in FIG.  5 . Mobile terminal  60  detects a private system retune at block  134  by detecting an absence of transmitted control information from private communication system  80  on the active control channel frequency of the private system. Mobile terminal  60  then retunes at block  136  to receive control information from the private communication system  80  on a candidate control channel frequency when control information is not being transmitted on the original active control channel frequency. Regardless of whether a retune is detected at block  134  and retuning occurs at block  136  or not, operations move back to block  130  where, so long as the control information is still being received from private communication system  80 , mobile terminal  60  continues to camp on the private system as illustrated at block  132 . 
     Automatic retuning to a private communication system after a mobile terminal is moved out of the region of the private communication system is also shown in FIG.  5 . If mobile terminal  60  moves out of the range of private communication system  80 , it will no longer receive control channel information from private system  80  and, as indicated at block  130 , operations will move back to receiving control information from the local cell of the public system  20  as indicated at block  122 . If the mobile terminal  60  has moved to a different cell, then mobile terminal  60  will detect this move when it receives the control information from the new public cell at block  122  and will test for a PSP match on the new cell at block  124 . Operations may then include acquisition of a different private system associated with the new PSP or continued operations through the public system. 
     On return to the original public cell region in which private communication system  80  resides, the appropriate PSP will be received at block  122  and mobile terminal  60  will detect a match to that PSP at block  124  and add its previously saved POFs associated with private communication system  80  to its reselection options at block  128 . Assuming that mobile terminal  60  has not been isolated from private communication system  80  for a sufficient period for the adaptive channel allocation methodology of private system  80  to make all of the previously saved candidate control channel frequencies invalid, mobile terminal  60  will successfully detect an acceptable POF reflecting the current control channel frequency in use by private communication system  80  at block  130 . Mobile terminal  60  will then recamp on private communication system  80  at block  132  and update its associated POF list to the current list of candidate control channel frequencies for private communication system  80 . Accordingly, using the methods and systems of the present invention, mobile terminal  60  is able to leave the region of private communication system  80  and return after a retuning by private communication system  80  and still reacquire the system without user intervention. 
     The present invention also provides private communication systems and mobile terminals supporting the benefits of the present invention. A private communication system  80  according to the present invention may include a means for selecting a first control channel frequency for the frequency at which the control channel is transmitted by the private communication system and at least one alternate candidate control channel frequency (and preferably a plurality of candidate control channel frequencies)(processor  86 ). Means coupled to the means for selecting is provided for changing the frequency on which the private radio communication system  80  transmits the control channel from the first control channel frequency to a selected one of the at least one alternate candidate control channel frequencies, preferably using an adaptive channel allocation system (retuning system  92 ). Means are also provided coupled to the means for changing the frequency for transmitting the control information at the frequency on which the private communication system transmits the control channel for receipt by a mobile terminal  60  in the coverage area of the private communication system  80  which control information includes an identification of the at least one alternate candidate control channel frequency and an identification of private communication system  80  (transceiver  84  through antenna  82  responsive to processor  86 ). 
     A mobile terminal for use with a private radio communication system which transmits control information on one of a plurality of control channel frequencies according to the present invention (see FIG. 3) includes means for receiving the control information (transceiver  64  through antenna  62 ). The received control information includes an identification of a candidate control channel frequency as well as the active control channel frequency on which the transmission is received. Mobile terminal  60  further includes means for detecting an absence of received transmitted control information from the private communication system on the first (or original) active control channel frequency and for receiving control information from the private communication system on a candidate control channel frequency if control information from the private communication system is not being received on the first control channel frequency (transceiver  64  responsive to tracking system  72  of processor  66 ). 
     Preferably, mobile terminal search operations according to the present invention further provide for an opportunity for the user of the mobile terminal to interfere and force attempts to obtain initial acquisition of a specified private communication system. This may be provided based upon a menu interface of user interface  70 . The user may be provided an express right to select a specific private communication system to attempt to acquire from an available list. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.