MXPA00011319A - Method of selectively directing a mobile station to retry system access in a radio telecommunication system - Google Patents

Abstract

A method of reducing co-channel interference in a cellular telecommunication system having a congested serving cell, a plurality of neighbor cells controlled by a mobile switching center (MSC), and an accessing mobile station attempting to access the cellular system in the serving cell. The MSC fetches (16) signal strength information from mobile stations camped on the serving cell's digital control channel (DCCH). The signal strength information includes received signal strengths at the camped mobile stations from the serving cell and the neighbor cells. The MSC then fetches (27) signal strength information from busy mobile stations involved in on-going calls in the serving cell. For each busy and camped mobile station, the expected downlink signal strength from each neighbor cell is subtracted from the maximum downlink signal strength from the serving cell (28). The mobile station/neighbor cell combination with the numerically least difference causes the least co-channel interference when the identified mobile station is moved into the identified neighbor cell. If the identified mobile station is the accessing mobile station (34), the MSC redirects the accessing mobile station to access the cellular system in the identified neighbor cell. If the identified mobile station is a busy mobile station involved in a call, the MSC forces a handoff of the identified mobile station to the identified neighbor cell.

Description

METHOD FOR GIVING INSTRUCTIONS IN A SELECTIVE MANNER TO A MOBILE STATION FOR REINTENTING ACCESS IN A SYSTEM OF RADIOTELECOMMUNICATION BACKGROUND OF THE INVENTION TECHNICAL FIELD OF THE INVENTION This invention relates to telecommunications systems and, more particularly, to a method for selectively instructing a mobile station to retry access to systems in a radio telecommunication system. DESCRIPTION OF THE RELATED TECHNIQUE The existing cellular radio telecommunications systems perform a function known as directed retry. If a particular cell is congested (ie all cell traffic channels are occupied) when a mobile station (MS) tries to access the system, the mobile service switching center (MS) can instruct the station mobile to retry access in a neighboring cell. When a directed retry is performed, there may be a negative impact on the level of co-channel interference when the mobile station begins to operate in a cell other than the cell with the best signal strength. This, in turn, can cause a decrease in the overall voice quality achieved in the system. Even when there are no teachings of the prior art Known from a solution to the aforementioned deficiency and limitation as disclosed herein, U.S. Patent Nos. 5,509,051 to Barnett et al. (Barnett); 5,287.5 5 by Kallin (Kallin); , and 5, 97,504 of Acampora et al. (Acampora); and U.S. Patent Application GB 2,287,614A to Ueno et al. (Ueno) comment on a subject that has some relationship with the subject of this document. Barnett discloses a method to prioritize neighboring cells for transfer in a cellular telecommunication system. In Figure 3, the method is shown that includes the comparison of the signal strength in the service cell with the signal strength in each neighboring cell, and the establishment of a graph with a transfer region. Neighboring cells are then sorted in order of priority according to the strength of the signal, and cells falling on the right side of the plotted transfer region have a higher priority for transfer. However, Barnett does not teach or suggest a method within a mobile switching center (MSC) that collects signal strength measurements (from the service cell and neighboring cells) of both the mobile stations involved in calls and stations. mobiles that are currently in the Digital Control Channel (DCCH) waiting for access to the system. In the same way Barnett does not teach or suggests a method that uses this measurement information either to transfer mobile stations currently involved in calls or to redirect access from mobile stations located in target cells in such a way as to minimize co-channel interference in the system cell phone. Kallin discloses a method and apparatus for advanced directed retries in which a directed retry message is sent to a mobile station that attempts to access a cellular system in a first congested cell. The mobile station then tries to access the system in a second cell but if a traffic channel becomes available in the first cell, the call is established in the first cell, thereby reducing interference in the cell. system. However, Kallin does not teach or suggest a method Acampora discloses a system and method to control the admission of new calls to a cellular telecommunication system. New calls are accepted or rejected based on various factors such as call classes, number of calls in each class in each group of cells, traffic characteristics, quality of service requirements for each class, and programming policies in each station. base. However, Acampora does not teach nor does it suggest a method that picks up signal strength measurements from both mobile stations involved in calls and mobile stations currently present from DCCH while waiting for system access. In the same way, Acampora does not teach or suggest a method to calculate later if a mobile station currently involved in a call is transferred or if a mobile station in the process of accessing a white neighboring cell is redirected in such a way that interference is minimized of co-channel in the cellular system. Ueno discloses a method that allows a mobile station user to perform a transfer in a cellular telecommunication network. A signal transmitted from the mobile station to the network causes the network to switch or preserve the voice channel as the case may be. Nevertheless, Ueno does not teach or suggest a method that collects signal strength measurements from both mobile stations involved in calls and mobile stations presently present in the DCCH at waiting for system access. In addition, Ueno does not teach or suggest a method to calculate later if a mobile station currently involved in a call is transferred or if a mobile station in the process of accessing a white neighbor cell is redirected in such a way as to minimize the interference of a mobile station. channel in the cellular system. The review of each of the above references does not show any disclosure or suggestion of a system or method such as the system or method described and claimed herein. In order to overcome the disadvantage of existing solutions, it would be advantageous to have a method for collecting signal strength measurements from mobile stations involved in calls and from mobile stations currently present in the DCCH while waiting for system access. In addition, the method will be calculated later if a mobile station currently involved in a call is transferred or if a mobile station in the process of accessing a white neighbor cell is redirected in such a way as to minimize the co-channel interference in the cellular system. The present invention offers such a method. SUMMARY OF THE INVENTION In one aspect, the present invention is a method for reducing co-channel interference in a cellular telecommunication system having a service cell congested, several neighboring cells controlled by a mobile switching center (MCS), and a mobile station in the access process trying to access the cellular system in the service cell. The invention can be applied to any cellular system that can measure the strength of the downlink signal from neighboring cells in both the control channel and the traffic channel. The preferred embodiment is described in terms of systems operating under IS-136. The method starts by placing the mobile station in the access process in the digital control channel of the service cell (DCCH), and looking for signal strength information from the mobile station in the access process, the signal strength information it includes the strength of the signal received in the mobile station in access process from the service signal and the neighboring cells. This is followed by the search for signal strength information of several mobile stations occupied in calls in progress in the service cell, the signal strength information includes the signal strength received in each mobile station occupied from the cell of service and the neighboring cells. This is followed by the identification of a combination of mobile station / white cell that causes the least co-channel interference when an identified mobile station is moved in an identified white cell. The MSC then determines whether the mobile station identified is the mobile station in process of access, and if it is the case, it redirects the mobile station identified for access to the cellular system in the identified target cell. If the MSC of the identified mobile station is a busy mobile station involved in a call, the method requires a transfer of the identified mobile station to the identified target cell. The combination mobile station / white cell that causes the least co-channel interference can be identified by finding the difference, for each mobile station occupied and located in the network, between the maximum downlink signal strength coming from the cell of service, and the expected downlink signal strength of each neighboring cell. The method then identifies the mobile station / white cell combination for which the difference in signal strength is numerically smaller. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and its numerous objects and advantages will be more apparent to those skilled in the art by reference to the following drawings, in combination with the appended specification, wherein: The figure (prior art) is a Illustrative drawing of a network of cells in a typical plane of cells in a cellular radiotelecommunication system; Figure 2 is an illustrative drawing of a row of cells contiguous in the coverage area of a cellular radio telecommunication system suitable for the implementation of the method of the present invention; Figure 3 is an illustrative drawing of the row of contiguous cells of Figure 2 when one of the cells is in a congested state; Figure 4 is a flow diagram illustrating the steps in the preferred embodiment of the present invention during an originating access by a mobile station; and Figure 5 is a flow diagram illustrating the steps in a preferred embodiment of the present invention in a determination access by a mobile station. DETAILED DESCRIPTION OF MODALITIES Figure 1 is an illustrative drawing of a cell network in a typical cell plan in a cellular radiotelecommunication system. Each cell is marked with a frequency designation, thus generating a frequency reuse plan 7/21. In order to minimize co-channel interference, a reuse distance is defined for the network, and base stations using the same frequencies are separated by the reuse distance. In figure 1, it can be seen for example that the cells that used the frequency Cl are separated by the distance of reuse. Figure 2 is an illustrative drawing of a row of cells contiguous 1-6 in the coverage area of a radio telecommunication system suitable for the implementation of the method of the present invention. Base stations are illustrated as being in the center of each cell. In order to minimize co-channel interference, cells operating on the same frequency are separated by intermediate cells operating at different frequencies. In the example illustrated in Figure 2, three frequencies are employed. Cells 1 and 4 operate in the frequency Fl; cells 2 and 5 operate at frequency F2; and cells 3 and 6 operate at frequency F3. Thus, the co-channel base stations are separated by three cell widths between them. Figure 3 is an illustrative drawing of the row of contiguous cells 2-6 of Figure 2 wherein one of the cells (cell 4) is congested. For exemplary purposes, we will consider that cell 4 operating on frequency Fl has 3 digital traffic channels (DTCs), and there are four mobile stations in cell 4: MS1-MS4. MSI, MS2 and MS3 are busy in calls. MS4 tries to access the system but there is a congestion due to the fact that all three channels are busy. In existing systems with directed retry capability, a directed number of retry cells is defined for service cell 4 (e.g., cell 3 and cell 5). When to the mobile station 4 is denied access due to a congestion in cell 4, a list of directed retry cells is sent to MS2. However, MS4 is illustrated very close to the base station for cell 4, 5 and regardless of which addressed retry cell MS4 has access to, the reuse distance will be substantially reduced. For example, if MS4 has access to cell 5, then MS4 operates on a frequency F2 much closer to cell 2, the co-channel cell operating in the frequency F2. Instead of a separation of three cells, we would obtain only a separation of two cells. This could lead to increased co-channel interference and a ^ M-aÉBBBaliiiMfa-f 1 i fflriiTr f- ^ Mii If MS4 is redirected to cell 3 or cell 5, then the reuse distance will be substantially reduced. This could cause more co-channel interference and lower voice quality. Accordingly, it is better to transfer MS2 to cell 5, for example, and to provide access to MS4 in cell 4. MSI is shown operating in cell 4 near the edge with cell 3. Thus, an alternate solution is to bind a cell. transfer of MSI to cell 3, and use the channel released in cell 4 to provide access to MS4. The co-channel interference will not be affected to a large extent. In the same way, MS3 could be transferred to cell 5 and the channel released in cell 4 could be used to provide access to MS4. The co-channel interference would only be slightly affected. The present invention normally selects the MS / cell combination that offers the least interference situation. For the case in which the impacts on voice quality are identical for more than a mobile station / cell combination, it is preferable to retain an ongoing call on your existing channel and redirect a mobile station in the process of accessing another cell with a signal strength equivalent to the signal strength in the cell to which it is accessed. In order to make decisions regarding the potential effect on co-channel interference, the MSC requires J ^ fe ^^ = ^ of signal strength information from mobile stations involved in calls as well as mobile stations that are in the Digital Control Channel (DCCH) waiting for access. In a procedure known as Mobile Assisted Transfer (MAHO), mobile stations involved in calls report signal strengths received from their service cell and neighboring cells in order to assist in the transfer decision. In a procedure known as Mobile Assisted Channel Assignment (MACA), the mobile stations in DCCH measure signal strengths from their service cell and neighboring cells and report the measurements to the MSC. In the situation illustrated in Figure 3, the MSC uses MAHO information from MSI, MS2 and MS3, and MACA information from MS4 to determine potential interference levels and make a decision as to which MS to transfer or redirect. Figure 4 is a flow chart illustrating the steps in the preferred embodiment of the present invention during an originating access by a mobile station. The column on the left is what is obtained in a mobile station 11 that is making an originating access in the cellular system, and the column on the right illustrates the corresponding steps carried out in the service and / or base station MSC ( MSC / BS) 12. In step 13, a user does an origin An origin begins at the mobile station in the access process at 14, and a source request message 15 is sent through the base station to the MSC. At 16, the MSC / BS starts an originating access and looks for the MACA signal strength information from the mobile station in access process 11. In step 18, MSC / BS determines if there is congestion in the cell where the mobile station is located in the process of access. If this is not the case, the mobile station is given access at 19. However, if there is a congestion, MSC / BS begins a queue update process at -21 and sends a queue update message 22 to the mobile station in the process of access that informs the mobile station that there is currently no available channel. In an implementation of IS-136, the mobile station in the access process, which has been in the state of origin procedure at 3, receives the queue update message at 24, and then passes to the presence state at DCCH at 25 and wait for additional information. In step 26, if there is a cell reselection, whatever the reason, while the mobile station is present, the mobile station automatically reoriginates the call in step 14. For example, while it is present, the mobile station measures continuously out the signal from neighboring cells, and if a better service cell is found, the station mobile reselects the best cell and automatically originates another call through the new service cell. While the mobile station 11 in the process of access is present, the procedure moves to step 27 where MSC searches from the base station for the MAHO information for all mobile stations involved in calls in the congested cell. MSC / BS then performs selective directed retry calculations at 28 using the MAHO information and the MACA information from the mobile station 11 in the access process. For all mobile stations, the strength of the expected downlink signal of each neighboring cell is subtracted from the maximum downlink signal strength of the service cell. The MS / neighbor cell combinations are classified from the most negative differences to the most positive differences. The MS / neighbor cell combination with the numerically lower signal strength difference generally has the least impact on interference in the cellular system. From this list, the combination with an expected downlink signal strength of the neighboring cell is removed less than a predefined threshold. If it is obtained in the same result for a mobile station in the process of accessing the system and a mobile station in an ongoing call, it is preferable to keep an ongoing call in its existing channel and redirect the mobile station in ------..- »-.« ----. > - «- - '¡f ^ f¡ ^ -' -riMt- access process to another cell with a signal strength equivalent to the signal strength in the cell to which it is being accessed. The selective directed retry calculations are described in more detail below. MSC / BS then determines in step 29 whether the mobile station 11 in the access process is still in the congested cell. The mobile station is not already in the congested cell if it has performed a cell reselection and, therefore, an automatic origin in a new cell. If the mobile station is no longer in the congested cell, the process stops at 30. If the mobile station in the access process is still present in the congested cell, the processes proceed to step 31 where the MS list is scanned. / neighbor cell, starting from the top, to identify the first cell where a channel is available to service the call. In step 32, it is determined if an available channel is found. If no available channel can be found, the process stops at 33. Otherwise, the process proceeds to step 34 where it is determined whether the mobile station identified by the selective targeted retry calculation is the mobile station in the process of access 11. If the mobile station identified is not the mobile station in access process 11, if not a mobile station already involved in a call, the process continues to step where MSC / BS 12 transfers the identified mobile station to the target cell. The process then proceeds to step 36. In the same way, if it is determined in step 34 that the mobile station identified by the selective directed retry calculations is the mobile station 11 in the process of accessing, in the process it proceeds to step 36, wherein MSC / BS 12 sends a warning message to the mobile station in access process 11 instructing the mobile station to move to a state in which I could receive a DTC assignment message. The mobile station in the process of access receives the warning message at 37 and waits for the order status at 38. MCS / BS then sends to the accessing mobile station a DTC assignment message at 39 which is received at 41. The DTC assignment message assigns the mobile station in process of accessing the channel that produces the lowest level of interference in the cellular system. This channel can be in the disservice cell or in a neighboring cell. Figure 5 is a flow diagram illustrating the steps in the preferred embodiment of the present invention during a termination access by a mobile station. The column on the left is what happens in a mobile station 11 that performs a termination access in the cell system, and the column on the right illustrates the corresponding steps carried out in service MSC / BS 12. In the step 51, MSC / BS 12 radio locates the mobile station 11. A radio location signal 52 is sent through the base station at the air interface and is received at the mobile station at 53. The mobile station responds to the radio 5 location at 54, and a radio location response 55 is transmitted at the air interface through the base station to the MSC. At 56, MCS / BS receives the radio location response and searches for the MACA signal strength information from the mobile station in access process 11.

In step 57, MCS / BS determines whether there is a congestion in the cell where the mobile station is located in the access process. If this is not the case, the mobile station is given access in 58. However, if there is a congestion MCS / BS, a process of queue update at 59 and sends a queue update message 60 to the mobile station in the access process that informs the mobile station that there is currently no available channel. In an implementation of IS-136 the mobile station in process access, which has been waiting for the order status at 61, receives the queue update message at 62, and then passes to the presence state at DCCH at 73 and waits for more information. In step 64, if there is a cell reselection for whatever reason while the mobile station is present in DCCH, the preferred mode ^ Hü ¡^^^ --- ia a- = A ...---- ^ - ^^ M ^. it implements an auto location radio response in the mobile station where the mobile station automatically returns to step 54 and sends another radio location response to the new service cell. This allows the cellular system to track the mobile station at the cell level in a manner similar to the auto-origin procedure. While the mobile station 11 in the process of access is present, the process proceeds to step 65 where the MSC 12 searches from the base station for MAHO information for all mobile stations involved in calls in the congested cell. MCS / BS then performs selective directed retry calculations at 66 using the MAHO information and the MACA information for the mobile station in access process 11. As previously described, for all mobile stations, the strength of the link signal The expected downlink of each neighboring cell is subtracted from the maximum downlink signal strength of the service cell. Combinations of MS / neighboring cell are classified from the most negative results to the most positive results. The MS / neighbor cell combination at the top of the classified list has, as expected, the least impact on the interference in the system. From this list the combination with an expected downlink signal strength of the neighboring cell less than a predefined threshold is removed. If you get the same result from a mobile station that has access to the system and a mobile station in an ongoing call, it is preferable to keep an ongoing call on its existing channel and redirect the mobile station in process of access to another cell with a signal strength equivalent to the signal strength in the cell to which it is being accessed. The selective directed retry calculations are described in more detail below. MSC / BS then determines in step 67 whether the mobile station in access process 11 is still present in the congested cell. The mobile station will no longer be in the congested cell if it has performed a cell reselection and, therefore, an auto localization radio response. If the MS is no longer in the congested cell, the process stops at 68. If the mobile station in the access process is still in the congested cell, the process proceeds to step 69 where the MS / cell list is scanned. neighbor, starting from the top, to identify the first cell where a channel is available to service the call. In step 70, it is determined whether an available channel is found. If no channel can be found, the process stops at 71. Otherwise, the process proceeds to step 72 where it is determined whether the mobile station identified by the selective directed retry calculations is the mobile station 11 in process of access. If the mobile station identified is not the mobile station 11 in the process of accessing, if not a mobile station that is already involved in a call, the process proceeds to step 73 where MSC / BS 5 12 transfers the identified mobile station to the white cell. The process then proceeds to step 74. In the same way, if it is determined in step 72 that the mobile station identified by the selective directed retry calculations is the mobile station 11 in the process of access, the process proceeds to step 74, where MSC / BS sends a warning message to the mobile station 11 in the process of access, instructing the mobile station to go to a state in which it can receive an assignment message of DTC. The mobile station in process of access receives the warning message at 75 and waits for the order status at 76. MCS / BS then sends a DTC assignment message to the mobile station in the process of access.

(MaxServSS) of all mobile stations in the cell using the equation: MaxServSS, = SsservL + ATT_ - B0CC-DTC, where: (a) Ssseri is the downlink signal strength 5 measured in the service channel for MSX ( units = dBm). This factor is obtained from the MAHO information for mobile stations connected to a DTC, and for the MACA information for mobile stations that are located in DCCH. 10 (b) ATTj is the power attenuation relative to the maximum transmitted power in the DTC for MS_. This factor takes into account the attenuation from any downlink power control algorithm (units = dB). Note: ATT, = 0 15 for a mobile station in the process of accessing the cellular system, (c) BOCC-DTC is the difference between the maximum transmitted power in the control channel and the maximum transmitted power in the DTC. (Units = dB). 20 Note: B0CC-DTC = 0 for mobile stations connected to a DTC. "Transmitted power" refers to the downlink power measured at the base station antenna terminal. Each mobile station in the cell is identified by a number and each neighboring cell is identified by another number.

Tj-nrritMi-iifii IÍÍÍ - Í-ÍI- i - • • ** "t * - * - ** - **** The expected downlink signal strength (ExpNeighSS) for each mobile station for each neighboring cell is calculated using the equation: ExpNeighSS-, = SSNeigh-, - BOCC-DTC ,, where 5 (a) i = the mobile station number . (b) j = the neighbor cell number. (c) SSNeighi} is the downlink signal strength measured for MS, of cells, neighbors (units = dBm). This factor is obtained from information from MAHO for mobile stations connected to a DCT, and from MACA information for mobile stations we are going to consider that MS- and MS2 are in a service cell (ie, i = 1, 2), and there are two neighboring cells, neighboring cell and neighboring cell2 (ie, j = 1, 2). There are four possible MS / neighbor cell combinations, and the calculation of Diffl: can result in differences of -10, -5, -3, and +10 for the four possible MS / neighbor cell combinations. The list will then be classified as follows, and the combination MS / neighboring cell?, J = 1.2 is identified as causing the least interference in the cellular system. MSi neighbor cell? Diff?, 2 = -10 MS2 cell neighbor Diff2 ,? = -5 MS3 cell vecmai Diff_ ,? = -3 MS4 neighbor cell? Diff2,2 = +10 It is therefore believed that the operation and construction of the present invention will be apparent from the foregoing description. While the illustrated and described method has been characterized as preferred, it will be readily apparent that various changes and modifications may be made without departing from the scope of the present invention as defined in the appended claims.

Claims (1)

1. CLAIMS A method for reducing co-channel interference in a cellular telecommunication system that has a congested service cell, several neighboring cells controlled by a mobile switching center (MSC), and a mobile station in the process of access that attempts to have access to the cellular system in the service cell, said method comprises the steps of: placing the mobile station in the access process in the service cell digital control channel (DCCH); looking for signal strength information from the mobile station in the access process, said signal strength information includes the signal strength received in the mobile station in access process from the service cell and neighboring cells; Searching for signal strength information from several occupied mobile stations involved in calls in progress in the service cell, said signal strength information includes the signal strength received in each mobile station occupied from the service cell and the neighboring cells, identifying a mobile station / neighbor cell combination that causes the least co-channel interference when an identified mobile station moves in an identified neighbor cell; determining in the MSC whether the mobile station identified is the mobile station in the process of access; redirecting the mobile station identified so that it has access to the cellular system in the identified neighbor cell, by determining that the mobile station identified is the mobile station in the process of access; And compelling a transfer of the identified mobile station to the identified neighbor cell, upon determining that the identified mobile station is a busy mobile station involved in a call. The method for reducing the co-channel interference of claim 1, wherein said step of identifying a mobile station / neighbor cell combination causing the least co-channel interference includes the steps of: finding a difference, for each mobile station in the network, between the maximum downlink signal strength of the service cell and the expected downlink signal strength of each neighboring cell; identify a mobile station / neighbor cell combination for which the difference between the maximum signal strength of the service cell and the expected signal strength of the neighboring cell is numerically smaller, the combination identifying the mobile station identified and the neighbor cell identified . The method for reducing co-channel interference according to claim 1, further comprising following the step of identifying a mobile station / neighbor cell combination that causes the least co-channel interference, the steps of: determining whether the station Mobile in process of access continues in the service cell; and stop the process by determining that the mobile station in the access process is no longer in the service cell. The method for reducing the co-channel interference of claim 3, wherein the step of determining whether the mobile station in the access process is still in the service cell includes the determination that the mobile station in the access process has made a reselection of cell and an automatic origin in one of the neighboring cells. The method for reducing the co-channel interference of claim 3, wherein the step of determining whether the mobile station in the access process is still in the service cell includes the determination that the mobile station in the access process has made a reselection of cell and an automatic localization response in one of the neighboring cells. l Co-channel interference reduction method • "- • * ¿* - ** J * ia" of claim 1, further comprising, after the step of seeking the signal strength information from the mobile station in the access process, the steps of: determining whether the service cell is still congested; and provide access to the mobile station in the process of access by determining that the service cell is no longer congested. A method for minimizing co-channel interference in a cellular telecommunication system having a congested service cell, a plurality of neighboring cells controlled by a mobile switching center (MSC), a plurality of busy mobile stations in which they make calls , and a plurality of mobile stations in the process of access that attempt to access the cellular system in the service cells, said method comprises the steps of: searching for signal strength measurements from each of the several mobile stations in the process of access, said signal strength measurements include the signal strength received at each of the mobile access stations, from the service cell and neighboring cells; look for signal strength measurements from each one of the several mobile stations occupied, said signal strength measurements include the signal strength received in each of the mobile stations from the service cell and the neighboring cells; and minimizing co-channel interference in the cellular system, said minimization case comprises the steps of: identifying a mobile station / neighbor cell combination for which a difference between the maximum downlink signal strength of the service cell and the expected downlink signal strength of each neighboring cell is the numerically smallest of all possible mobile station / neighbor cell combinations; and moving the identified mobile station to the identified neighbor cell. The method for minimizing the co-channel interference of claim 7, wherein the step of moving the identified mobile station to the identified neighbor cell includes the steps of: determining in the MSC whether the identified mobile station is one of several mobile stations in access process; redirect the mobile station identified for access to the cellular system in the identified neighbor cell, determine that the mobile station identified is one of several mobile stations in the process of access; and obliging a transfer of the identified mobile station to the identified neighbor cell, on determining that the identified mobile station is one of the several mobile stations occupied. The method for minimizing the co-channel interference of claim 7, wherein the step of minimizing co-channel interference in a cellular system includes 10 the calculation of the maximum downlink signal strength of the service cell (MaxServSS using the equation: MaxServSS. = SsserVj. + ATTa - B0CC-DTC, where SSser i is the link signal strength 15 downstream measured in the service channel for a mobile access (MS_), ATTt is the attenuation of power in the service channel in relation to the maximum power transmitted in the digital traffic channel (DTC) for MS_, and BOCC-DTC is the difference between the power 20 transmitted maximum in the digital control channel (DCCH) and the maximum transmitted power in DTC. The method for minimizing the co-channel interference of claim 7, wherein the step of minimizing co-channel interference in the cellular system includes 25 the calculation of the downlink signal strength - * «*» «" - tftí'itirft-r "- ... s ^ .--- * -.---! --------- expected from each neighboring cell (ExpNeighSS-,) using the equation: ExpNeighSSi, = SSNeigh_, - BOCC-DTCJ, where SSNeighij is the measured downstream signal strength for neighboring cell MS_, and BOCC-DTC, is the difference between the power maximum transmitted on the digital control channel (DCCH) of the neighboring cell, and the digital traffic channels (DTCs) of the neighboring cell,. SUMMARY OF THE INVENTION A method for reducing co-channel interference in a cellular telecommunication system having a congested service cell, several neighboring cells controlled by a mobile switching center (MSC), and a mobile station attempting to reduce co-channel interference is presented. have access to the cellular system in the service cell. The mobile switching center seeks (16) signal strength information from mobile stations that are located in the digital control channel of the service cell (DCCH). The signal strength information includes signal strengths received in the mobile stations from the service cell and neighboring cells. The mobile switching center then seeks (27) signal strength information from busy mobile stations involved in calls in progress in the service cell. For each mobile station occupied and present, the expected downlink signal strength from each neighboring cell is subtracted from the maximum downlink signal strength of the service cell (28). The combination of mobile station / neighbor cell with the smallest numerical difference causes the least co-channel interference when the identified mobile station is moved in the identified neighbor cell. If the mobile station identified is the mobile access station (34), the mobile switching center instructs the mobile access station to have access to the cellular system in the neighboring cell identified. If the identified mobile station is a busy mobile station involved in a call, the mobile switching center imposes a transfer from the identified mobile station to the identified neighbor cell.