METHOD FOR CONTROLLING CHANNEL RE-SELECT BACKGROUND OF THE INVENTION The present invention is directed to a method for controlling re-selection of control channel by a mobile communication station. More particularly, the invention relates to allowing the re-selection process to be performed only with respect to those candidate control channels in which the mobile station is eligible to be established. In the wireless communications environment, it is known that as a mobile station moves through an area, it is likely to find different cells that are served by different base stations. An example of a multi-cell wireless environment is illustrated in Figure 1. There are a plurality of cells (Cx to C12) and each is served by a separate base station (BS2 to BS12). As a mobile station moves from one cell to another cell, it is served by a different base station. Typically base stations operate with different control channels, voice channels or data channels. It is known that a communication lasts, such as a call, as a mobile station passes from one cell to another is transferred from one base station to another according to a certain protocol. However, in an activated state when a call is not in process, while the mobile station is in a cell, it is with a control channel, i.e. in essence
REF: 26266 is tuned to that channel and ready to dock in communications of control information on that channel. The mobile station must execute a process to choose another control channel as the mobile station moves from cell to cell. This is called a re-selection process. In the wireless environment, a known standard referred to as the IS-136 standard defines protocols for digital wireless communications involving digital mobile stations. That standard defines a process by which the mobile station can re-elect a control channel as it moves between the cells. In that process, the mobile station is assumed to be "established" with a first control channel. That control channel then transmits a list of the neighboring control channels, this is the list of the control channels for the neighboring cells (or service providers). The mobile station receives the list and stores it. Then according to a predetermined algorithm, the mobile station determines which of the control channels in the list constitutes the primary candidate for the re-selection. This determination is made based on such things as reading the RF (radio frequency) level of the various control channels in the list and determining which is the control channel that the mobile station will have to try to choose next. For example, if the mobile station was in cell C2 moving to cell C? / It could detect that the next strongest control channel among the neighbors of the
cell C2 originates from the base station BS-L and therefore will consider that control channel as the primary candidate for re-selection. In addition, according to the algorithm described in the IS-136 standard, once a primary candidate has been determined, the mobile station tries to re-select for that control channel. If that re-selection attempt is successful, then the mobile station is established with the new control channel, receives a new list of the neighborhood control channels and starts the process again with the new neighbor list. If the attempt is unsuccessful, then the mobile station either chooses the next appropriate candidate channel (this is a secondary candidate channel) or repeats the analysis of the entire list of candidate channels trying to determine another primary candidate channel for re-selection. A problem arises since when the mobile station receives the list of neighbors, it can include control channels with which the mobile station is simply incompatible. For example, the list may include one or more channels that relate to private systems and a mobile station may not be granted access to those private systems. Alternatively, the mobile station can search for voice service and the control channels can be directed to data or fax services. However, based on the parameters used in the analysis algorithm, these control channels with which the mobile station can not really interact are included in the analysis operation and one of these incompatible control channels can be chosen as the channel of primary candidate control. Naturally when this happens, the attempt to re-elect the control channel will fail. In this way, the execution of the process has wasted time and resources towards an attempt to re-select an incompatible control channel. Furthermore, under the known algorithms, once the attempt to re-select the incompatible control channel has failed, this channel is still included in the next cycle of the re-selection process and can again be selected as the candidate control channel primary. It is conceivable that the mobile station may be trapped in a prolonged loop to determine an incompatible control channel as the primary candidate control channel and make a failed attempt to re-select that control channel. In view of the disadvantages of the known selection process, it is convenient to provide greater controls in the re-selection process to avoid the attempt to re-select incompatible control channels. It is also convenient to avoid second attempts to re-select control channels that are determined to be incompatible with the mobile station during a re-selection attempt. SUMMARY OF THE INVENTION In accordance with the present invention, a mobile station performs a method for controlling the re-selection of a control channel. In that method the mobile station before performing any algorithm to choose a primary candidate control channel, marks as ineligible any of the control channels identified in the list of neighbors that are incompatible with the mobile station. The selection of a primary control channel then proceeds only with respect to those control channels that are determined to be eligible. Once a primary candidate control channel is chosen, the mobile station tries to re-select this primary candidate control channel. If the attempt to re-select is successful, then the mobile station is established with this control channel. However, if the attempt fails, the mobile station determines why the attempt failed. If the failure arises from an incompatibility between the mobile station and the control channel, then that control channel is marked as not eligible in the neighbor list and when the re-selection process is repeated with the same list of neighbors and that the control channel has been marked as ineligible will not be a factor in the selection of a primary candidate control channel Examples of incompatibility are as follows: The mobile station may not have access privileges with respect to private systems. In this case, the mobile station will scan the list of neighbors and mark as ineligible any control channel identified as being related to a private system since the mobile station will not be allowed access to these systems. can have access to only a private system, in which case, then if a primary candidate control channel is a private system channel, and the mobile station is not determined t With access to this particular private system, then the attempt to re-select the private system will fail and the list of neighbors will be modified to reflect that fact that this channel is not eligible for analysis in a subsequent re-selection process. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates in a high-level schematic form a wireless system in which the present invention can be employed. Figure 2 illustrates an information table used by a mobile station in the environment of Figure 1 to implement the present invention. Figure 3 illustrates a process flow for re-selecting a control channel in the wireless channel of Figure 1, according to one embodiment of the present invention. DETAILED DESCRIPTION As described above, the mobile station employing the present invention can be used in a wireless mode which is schematically illustrated in Figure 1. A cellular telephone providing voice service can constitute said station. A plurality of cells Cx to C12 are served by respective base stations BSX to BS12. When the mobile station is in cell C1 # it can be established with the control channel from the base station ßS ^ The control channel BSj ^ will transmit a list of neighboring control channels. The list will include control channels associated with the base station BS2, base station BS5 and base stations serving other cells adjacent to the Cx cells. In addition, the list of neighborhood channels can include a list of control channels that relate to fax services or data services. In addition, the list may include a control channel associated with a private system as illustrated in cell Cx. Typically, this private system is superimposed by the public system, so that the area covered by the private system is covered by both the public and private systems, but access to the private system itself, for example a wireless PBX in the facilities of a part, is not allowed except by members of that private system. When the me / il station receives the list of neighbors from the control channel in - .. that is established, it stores that list of neighbors in memory. The mobile station can be a cell phone that complies with IS-136 standards. This mobile communications device includes processing capabilities and memory of both read-only memory (ROM) and random access memory (RAM). The ROM stores control programs to operate the device, while the RAM stores dynamic information that can be updated over time, such as the list of neighbors transmitted from a control channel in which the station is established. In this memory, the mobile station will follow up on the identification of the control channels included in the list. In addition, in compliance with the IS-136 standard, the neighbor list will also provide certain parameter information that is related to the characteristics of the control channel. For example, the parameter information will indicate whether a control channel is associated with a private, public or semi-private system. The identification of the particular system to which the control channel is associated will not necessarily be presented in the neighbor list. On the contrary, only the type of system that the control channel is associated with, will be supplied. Similarly, the parameter information may indicate whether the control channel is designed to provide voice service, data service or fax service. Other parameter information that defines the characteristics of the control channel can also be supplied with a list of neighbors (for example, cell type, protocol version, cell synchronization, etc.). An example of a correlation of the information presented by the control channel in which the mobile is established, is illustrated in Figure 2. In this tabular representation of the data that can be stored in the mobile station, a first control channel in the list has an ID of "XXXXX". The parameter information indicates that this control channel is associated with a private system and provides voice services. The control channel identified by "YYYYY" by contrast is associated with the public system that still provides voice services. The control channel "ZZZZZ" is also associated with the public system, but providing fax services. Similar information will be provided for each control channel identified in the neighbor list that is provided by the control channel in which the mobile station is established. This parameter information can then be used to modify or control the process by which an alternate control channel can be chosen. A flow chart, which illustrates the process for controlling re-selection using the received neighbor list, is illustrated in Figure 3. In step 300, the mobile station receives the neighbor list from the control channel in which it is currently established. In step 301, the mobile station processor scans the list of neighbors according to a stored control program and examines the entries in the list, to determine if any of the control channels (the candidate control channels) are not eligible due to the lack of compatibility between the mobile station and the control channel. As an example, incompatibility may arise when the mobile station does not have access to any private network or system. The mobile station memory will store system IDS for those systems with which the mobile station is allowed to communicate. You can also store a flag that indicates whether the mobile station has access to any private system. Under those circumstances, when there is access to private systems, any control channel that is related to a private system is incompatible with the mobile station. Similarly, if the mobile station seeks voice services, then any control channel associated with fax or data services would be incompatible with the mobile station. Once a candidate control channel is detected or recognized as ineligible in step 302, then each of those ineligible control channels is marked as not eligible in the neighbor list. In particular, the processor in the mobile station modifies the neighbor list to somehow mark a control channel as ineligible. One way to do this is to include an eligibility flag in the neighbors list. All control channels in the neighbors list will initially have their eligibility flags set indicating an ineligible control channel. Then, when a control channel is marked as ineligible, the flag will be readjusted to an ineligible state. Alternatively, it is possible that other steps may be performed to effectively remove the ineligible control channels of subsequent consideration in the re-election process. In step 304, a process known in the prior art is initiated to study or analyze candidate channels 3 to try to choose the optimal candidate channel for re-selection. This process is referred to in Figure 1 as "analyzing candidate control channels." This stage of analysis is limited to those control channels that are considered eligible. F, to is, if the list of eligibility reflected in the list of ecines in step 303 indicates that a channel of CJ trol is not eligible, then the control channel will not be included in the operation, to analysis beyond that point. In essence, it will be ignored. It will not appear in the calculations to determine the best candidate control channel for the re-selection process. In the analysis operation, each of the selected control channels 3 will be tested for certain criteria, such as RF level. j once each of the eligible control channels is tested. , a primary candidate channel is chosen (step 305) according to the J and the JES identifier which of the eligible control channels is the best candidate for the re-selection, based on the test results obtained during the analysis of fr >; pa 304. This may arise under the circumstance where the? The processor will run tests on each of the readable control channels. As an example, a result of the tests on things such as the RF level in step 304, the processor can determine that one or more of the candidate control channels satisfy certain criteria to be eligible by the mobile station. Then, in step 305, the processor will pass through this sub-set of eligible candidate control channels and choose the optimal or primary candidate control channel based on the parameters associated with that control channel. In this manner, a primary candidate channel may be as in step 305. Once this primary candidate channel is identified, the mobile station attempts to re-elect the primary candidate channel in step 306. If the attempt to re-select is successful as detected in step 307, then the mobile station is established on the primary candidate channel in step 308, and receives a new neighbor list of the primary candidate control channel identifying neighbors associated with that control channel. However, if the attempt to re-elect is unsuccessful, then the processor can make a determination as to how the re-selection attempt failed. If the failure is due to a mismatch of certain predetermined criteria as referred to in step 309, then the channel identified as the primary candidate control channel may then be treated as a non-eligible candidate channel. The eligibility status within the neighbor list will be modified to reflect this state change and the processor can then resume the reselection process from step 304, where the processor can start again the analysis of the candidate control channels by focusing only in those that remain as eligible control channels. The system will then choose another primary candidate and try to re-elect that second primary candidate channel. This process will continue until the mobile station is established on an alternate control channel. As described above, a candidate channel may not be eligible because the control channel is associated with a private system, while the mobile station is not affiliated with any private system. In addition, a control channel can be considered ineligible due to the type of service it provides and the lack of compatibility between that service and the service of the mobile station. The predetermined criteria referred in connection with step 309 may be any condition that causes the failure of a reselection attempt. Typically, such an event would be if the primary candidate channel is associated with a private system and the mobile station is also associated with a private system. In that circumstance, then the primary candidate channel will not be marked as ineligible in step 303. Then, during the reselection attempt the private system identifier associated with the primary candidate channel will be provided to the mobile station. If the mobile station private system identifier stored in the memory does not correspond to the private system ID received from the primary candidate channel during the re-elect attempt, then the mobile station will not gain access to the private system associated with that primary candidate channel . Therefore, there is an incompatibility between the mobile station and the system associated with that primary candidate channel. In view of this incompatibility, it is consistent with the present invention to mark this primary control channel as ineligible as in step 310. Then, if necessary, analyze the neighbor list again (step 304) to find a candidate channel for re-selection, the newly designated non-eligible control channel will not be analyzed. In accordance with the present invention, other parameter information associated with a control channel may be used to determine whether the control channel is eligible for reselection by this particular mobile station. furtherModifications to the process of Figure 3 are also possible. For example, in one variation, steps 301, 302 and 303 will be eliminated, that is, the processor will not do a preliminary examination of the neighbor list to determine if channels are not eligible. candidates. On the contrary, the system will simply rely on marking primary candidate channels as ineligible for subsequent re-selection attempts. Alternatively, the control method of the present invention can be simply based on the determination of non-eligibility based on an attempt to re-elect a particular candidate channel to be successful. In this way, steps 309 and 310 can be eliminated and still benefits can be obtained from the remaining process. Since the present invention resides in a method by which the mobile station chooses a candidate for appropriate re-screening and since it is implemented using software running on a processor within the mobile station, it should be recognized that variations in the order in which the that certain steps are performed and the specific techniques or parameters involved in the process can be modified while still falling within the spirit of the present invention. For example, it is conceivable that in one variation, the station microprocessor can scan the entire neighbor list and adjust the eligibility status when appropriate for all ineligible control channels before proceeding with the analysis of the eligible control channels . In an alternate mode, the first time through the neighbor list the processor can first determine if a particular control channel is eligible and then, if it is eligible, perform the analysis with respect to that control channel. Then, the microprocessor can go to the next control channel in the list, determine if it should be considered eligible and if so, perform the evaluation with respect to that control channel and so on. In this second configuration, the processor does not scan the entire list before beginning the evaluation process. On the contrary, it combines the stages of marking and evaluation. Other modifications may include other techniques to mark a control channel as ineligible. In yet another embodiment, it is conceivable that the mobile station may choose for analysis a subset of the control channels from the neighbor list; The subset is based on any one or combination of the parameters. In accordance with the present invention, a mobile station can optimally control the process by which it chooses alternate control channels. It provides the mobile station with a way to focus only on those candidate control channels, with which the mobile station can communicate effectively. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following: