Method for searching a control channel in a mobile station

Abstract: A control channel is searched for call waiting in a mobile station where the power supply for a receiving circuit is turned off during a break time of receiving level measurements, wherein in a first control channel selection mode, all the control channels are divided into a plurality of groups, the receiving levels of the control channels are measured group by group intermittently and sequentially. The intermittent measurement of these groups are repeated until a control channel having a receiving level equal to or greater than a first predetermined value is detected. When a receiving level equal to or greater than the first predetermined value is detected, the receiving levels of all the control channels are measured in a second control channel selection mode.

Description: The present invention relates to a method for searching a control channel in a mobile station for call waiting or cell switching, and particularly to such a search method which allows reduction of power consumption in the mobile station. In a mobile communication system, as shown in Fig. 1, a wide service area SA is divided into many cells (sometimes referred to as zones) CE1, CE2,.... Mobile stations MS (car telephones, portable telephones etc.) in each cell can be connected to an exchange office (not shown) via a base station BS in the cell. The size of each cell and a transmission power of the associated base station are determined so that receiving levels of control channels transmitted from the base stations of adjacent cells are substantially equal to each other on the border of the adjacent cells. A control channel and a speech channel are assigned to each cell and the frequencies of the channels are different between the adjacent cells. Information (control channel information) for identifying a base station in a cell and for identifying the base stations in the peripheral cells is always transmitted from the base station of the cell via a control channel allocated to the cell. The control channel is also used to send and receive information for processing the location registration of a mobile station, call-in and call-out. A communication channel is used by a mobile station for speech conversation. One or a plurality of speech channels are allocated to each cell dependent on the traffic volume of each cell. Since the present invention particularly relates to the control channel, only the control channel is explained hereinafter. A mobile station in each cell measures the receiving levels of the call waiting control channel of the cell and of the control channels of the peripheral cells. When the receiving level of the call waiting control channel becomes a predetermined level or more lower than the highest one of the receiving levels of the control channels of the peripheral cells, it is decided that the mobile station is moving into another cell, and a processing for call-waiting is performed with the control channel of the highest receiving level. Since the frequency band allocated to the entire communication system is limited, the total number N of the control channels obtainable at intervals of a constant band width ΔF is limited. Since different frequencies cannot be allocated to all the cells, the same set of channels, for example CH1-CHM, are repeatedly allocated to different groups of the cells sufficiently far from one another so that the interference can be ignored. Further, in order to efficiently utilize the frequency band allocated to mobile communications, control channels CH1'-CHM' shown in row B of Fig. 2 whose frequencies are displaced by a frequency smaller than the control channel interval (band width) ΔF, e.g. ΔF/2, respectively from the frequencies of the control channels CH1-CHM shown in row A of Fig. 2 in a interleaving manner may be allocated to the cells such that radio-waves of control channels whose frequencies are displaced by ΔF/2 do not interfere each other. Therefore, if M'=M, a maximum of 2M control channels can be used in the entire service area and the receiving circuit of each mobile station is constructed to be capable of selecting any one of these control channels for the communication. The number of total control channels is defined as N hereinafter. For a mobile station in any cell, the control channel being transmitted by a base station of the cell in which the mobile station is located is the call waiting control channel in that cell and a control channel whose frequency is displaced by ΔF/2 from the call waiting channel is unnecessary channel. Therefore, in an arbitrary cell where a mobile station is located, a control channel of another cell whose frequency is displaced by ΔF/2 from the control channel being transmitted by the base station of the arbitrary cell is referred to as an interleave channel hereinafter. After the power is turned on in a mobile station, in order to be ready for a call-in and a call-out, it is necessary to find out a control channel being transmitted by a base station of a cell where the mobile station is located and to be in the receiving state (call waiting state) of that control channel. It is also necessary to perform procedures for entering the call waiting state when a mobile station gets out of a tunnel or a mountain cove, where a control channel cannot be received, after a long stay therein or when enters the service area from the outside area. In such a case, a mobile station selects an appropriate control channel from all the control channels and waits for the receipt of a control signal related to the communication control from the selected control channel. In the conventional system, when a mobile station is powered on and the first call waiting process is performed, or when a mobile station gets out of incapable state of signal reception and the first call waiting process is performed, channels whose receiving levels are greater than a predetermined level defined by the system are selected as candidates and a decision on whether call waiting possible or not is performed starting from the highest receiving level channel in the descending order of the receiving level for all the candidate channels. That is, first, the mobile station selects a control channel candidate of the highest receiving level and tries to receive the control signal. If the control signal can normally be received and call waiting is deemed to be possible as a result of analysis of the contents of the received control signals, a call waiting is performed with the control channel. However, if the control signal cannot normally be received over the selected control channel, or if the analysis of the control signal indicates that the control channel cannot be used even though the control signal can normally be received, a decision on propriety for call waiting is tried with the next highest receiving level control channel (next candidate). The conditions by which call waiting is decided to be impossible are the cases where the signal received over the control channel indicates "out of service" due to under construction or under testing, or the received signal is not the control signal used in the communication system by which the mobile station is served, or the level of the received control signal is below the predetermined level, etc. The aforementioned decision on propriety for call waiting is performed until a control channel by which call waiting is possible is found in the candidate control channels. If all the candidate control channels are decided to be unable for call waiting, the receiving levels of all the control channels are measured again to obtain the candidate control channels. As mentioned above, in a mobile station, since the receiving level measurements are repeatedly performed until a control channel whose receiving level is equal to or greater than a predetermined value is found, the receiving levels of all the control channels are measured repeatedly and continuously even when the mobile station is located outside of the service area SA. Thus, a problem of an early battery exhaustion does exist in a portable mobile station etc.. In a mobile communication system where interleave channels are provided, as mentioned previously with reference to Fig. 2, with respect to each control channel there are two adjacent interleave control channels having frequencies within an interval (e.g., interval of ΔF/2) less than the control channel frequency band width ΔF, and the frequency band of each control channel party overlaps such two interleave control channels. Assuming that such two interleave control channels whose frequency bands are adjacent to a control channel CHa (row A of Fig.2) are CHb and CHc (row B of Fig.2), if the receiving level in the control channel CHa is equal to or greater than a predetermined value in a cell, it is possible that the receiving levels in the adjacent control channels CHb and CHc may also be equal to or greater than the predetermined value because of the partial band overlap with the control channel CHa. In a cell CEa (Fig.1), for example, there may be a case when a mobile station MS which is in the call waiting state with the control channel CHa enters a cove of buildings or a tunnel and is temporarily unable to receive the control signals, and then the mobile station selects the interleave channels CHb and CHc as the candidate control channels in order to search the call waiting channel again and decides propriety for call waiting. In addition, when the control signal received by the mobile station MS over the control channel CHa which is selected for call waiting in the cell CEa is the signal indicating "under construction" or "under test" of the base station and thus the control channel cannot be used, similar decision on propriety for call waiting may be performed on the interleave channel CHb or CHc as an alternative candidate control channel for CHa. In such a case, the receiving level in the candidate control channel CHb or CHc selected by the receiving circuit of the mobile station may not be the receiving level of the control signal from the base station of other cell CEb or CEc where the control channel CHb or CHc is allocated but may be the receiving level of the control signal of the control channel CHa whose frequency band is partially overlapped with that of CHb or CHc. In this case, even if the receiving level is sufficient to satisfy the predetermined level for call waiting, the decision of propriety for call waiting always fails because the control signal cannot normally be received and the contents cannot be analyzed. Since it usually takes several seconds to judge propriety for call waiting for one control channel, if there are many candidate control channels which need decisions for call waiting, it takes a long time to determine the call waiting channel and a large power is consumed. As described above, a mobile station possibly performs decision of propriety for call waiting on the interleave channels CHb and CHc which are unable to be call waiting and thus, it takes time to detect control channels in which the mobile station can be in call waiting state. In addition, when the mobile station is a portable mobile station, the battery consumption is accelerated. Once a mobile station enters call waiting state, the mobile station waits for a control signal such as a call-in signal from the base station over the control channel in the cell where the mobile station is located and also the mobile station always measures the receiving levels of the control channels of the peripheral cells surrounding the cell where the mobile station is located and the receiving level of the call waiting control channel continuously and repeatedly based on the identification information of peripheral cells' control channels contained in the received control signal. For example, as shown in Fig. 10A to be mentioned later, when a series of receiving levels of the call waiting control channel and the control channels of the peripheral cells are measured in each measuring time T M , the measurement of receiving levels of these control channels is immediately repeated again. For each constant time period T S (or constant number of repetitions), an average value of the measured receiving levels in the constant time period T S is obtained for each channel. The average receiving level of the call waiting control channel is compared with the average receiving level of each peripheral cell's control channel. If any one of the latter is equal to or greater than the sum of the call waiting channel's level and a predetermined value ΔL, the process shifts to the procedures for switching to the cell corresponding to the highest level control channel of those peripheral control channels. That is, for each time period T S , the process of the averaging the receiving levels is performed for each of the control channels and thus obtained average receiving levels are compared to decide whether processing for cell switching is necessary. In the conventional system, regardless of the fading period of the receiving level or the moving speed of a mobile station, the receiving levels of the call waiting control channel and the peripheral cells' control channels are measured in a constant period T P . When the measurement of all the receiving levels of the control channels to be measured is completed, it is usual to repeat, immediately, the measurement from the starting control channel. That is, the measurement repetition period T P is equal to the measuring time T M required for measuring, once, all the control channels to be measured. When the moving speed of the mobile station is high, rapid change in the receiving levels of the call-waiting channel and peripheral cell channels are likely to occur and, therefore, it is necessary to make the measuring period T P and call-switching decision period T S as short as possible. On the other hand, despite that rapid change might hardly occur in the receiving level when the moving speed is low, it has been conventional to repeat the receiving level measurement and the cell-switching decision with the same short periods T P and T S as those in the case of the high moving speed mentioned above, thus consuming undesirably large power. In addition, in the conventional system, since the period T S for deciding the cell switching is constant, the cell switching decision sometimes has to be made based on unreliable measurement results caused by temporary reduction in receiving level in a cove of buildings etc.. Hence, it has been frequent to perform processing for the cell switching, in vain. Therefore, much useless power has been consumed. The document FR-A-2 691 027 relates to power saving channel selection method, which includes steps of measuring levels of receiving control signals through all the control channels; selecting a first subset of control channels in which measured levels exceeded a predetermined level; checking if connection is possible through any of the control channels in the first subset; if connection is failed, storing measured levels and setting power-down for a predetermined period, and then resuming measurement of levels to select a second subset of control channels. This method has a drawback that for the starting, level measurement has to be performed on all the control channels, requiring a longer time. The document EP-A-0 589 280 relates to channel search method, wherein those channels in which receiving power levels exceed a predetermined level are selected as candidate channels which are successively checked if control signals are correctly received and if failures are counted for each channel. Those channels whose failure counts exceeded a predetermined value are excluded from the candidate channels, thereby to shortening the search time. SUMMARY OF THE INVENTION It is a first object of the present invention to provide a method for searching, with a less power consumption, control channels which can be candidates for a call-waiting control channel. It is a second object of the present invention to provide a method for searching, with less power consumption, a call-waiting control channel among the candidates for call-waiting control channels. These objects are achieved by method as claimed in claim 1 and its preferred embodiments as claimed in the dependent claims. According to a first embodiment of the present invention, when a mobile station selects candidate control channels for call waiting, in a first control channel selection mode, a plurality of control channels which are part of all the control channels stored in a mobile station are sequentially and intermittently measured until a control channel whose receiving level is equal to or greater than a predetermined value is detected. As the result, if any control channel whose receiving level is equal to or greater than the predetermined value is detected, then in a second control mode selection mode, the receiving levels of all the control channels stored in the mobile station are measured to select an optimum control channel in the cell. If the optimum control channel cannot be selected in the second control channel selection mode, the first control channel selection mode is performed again. Preferably, in the first control channel selection mode, a plurality of control channels are divided into a plurality of groups, receiving levels of the control channels are measured group by group intermittently and sequentially. According to a second embodiment of the present invention, in the method for selecting a control channel for call waiting, when the frequency interval between arbitrary two selected control channels is less than a channel frequency interval and the receiving levels of these control channels are equal to or greater than a predetermined value, only the control channel whose receiving level is higher is defined as the subject of selection for a call waiting control channel and the control channel whose receiving level is lower is excluded from the subject of selection for a call waiting control channel. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an outline of the service area in the conventional mobile communication system; Fig. 2 shows a frequency arrangement of all the control channels in a service area; Fig. 3 is a block diagram showing a basic configuration of a mobile station where the method in accordance with a first and second embodiment of the present invention is applied; Fig. 4 is a flow chart for implementing the control channel searching method in accordance with the first embodiment of the present invention; Fig. 5A is an operation timing chart in the conventional control channel searching method; Fig. 5B is an operation timing chart in the method of Fig. 4; Fig. 6 is a flow chart for implementing the control channel searching method in accordance with the second embodiment of the present invention; Fig. 7A is a first table TB1 generated in the method of Fig. 6; Fig. 7B is a second table TB2 generated in the method of Fig. 6; Fig. 7C is a table TB2' created by setting the receiving levels of the interleave channels to zero in the table TB2 of Fig. 7B; and Fig. 7D is a third table TB3 created by deleting channels whose receiving levels are zero from the table TB2' of Fig. 7C. DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS The embodiments of the present invention will be explained below referring to the drawings. Fig. 3 shows a basic configuration of a mobile station for implementing the method in accordance with a first and a second embodiment of the present invention which is to be explained below. A mobile station has a receiving circuit 11, a receiving level memory 12, a control part 13, a power supply part 14 and a transmission circuit 15. The control part 13 has a channel memory 13M internally and the frequencies corresponding to all the control channels usable in the service area are initially stored in the channel memory 13M. The receiving circuit 11 and the transmission circuit 15 are set to a desired channel by setting a frequency read out from the channel memory 13M to the receiving circuit 11 and the transmission circuit 15. The receiving circuit 11 gets into receiving state in the set control channel, measures the level R of the control signal being received, writes the level R into the receiving level memory 12 and gives the received control signal S to the control part 13. The control part 13 compares the receiving levels of the control channels read out from the receiving level memory 12 as required with a first predetermined value L 1 to select candidate control channels and to decide if the selected candidate control channels are equal to or greater than a second predetermined value L 2 . Also, the control part 13 decides if the received control signal S is a proper control signal and, based on these decision results, further decides if any one of the selected candidate control channels is the right control channel for call waiting the control part 13 further controls the power supply part 14 to turn off the power to the receiving circuit 11 while the measurement of receiving levels is not performed and to turn off the power to the transmission circuit 15 while transmission is not performed. First Embodiment For example, when a mobile station is located outside of the service area, if the receiving level of the control channel is less than the predetermined level, signals cannot be received normally. Therefore, continuous monitoring of all the control channels for searching a call waiting control channel is meaningless. On the other hand, when the mobile station is close to the service area, it is desired that a call waiting control channel can be found immediately. Therefore, in the first embodiment, after detecting that the mobile station is close to the service area in the first control channel selection mode SM1, the call waiting control channel searching is performed in the second control channel selection mode SM2 in the same manner as the conventional system. Even in the case when interleave channels are not used, the skirts of adjacent frequency bands of the control channels are overlapped as shown in row A of Fig. 2. For example, when a receiving level of either of the control channels (e. g., CH1 or CH3) adjacent to a control channel (e. g., CH2) of a certain cell is measured in that cell, a leaked power from the control channel CH2 is detected. Therefore, even if the receiving levels of all the control channels which have been installed in the mobile station are not measured, the signals of any one of the control channels from the service area can possibly be detected and it can possibly be detected that the mobile station is approaching to the service area. For example, in row A of Fig. 2, if the total number of control channels is 9 and those channels are CH1, CH2... CH9 in ascending order of frequency, it can be judged whether or not there is a control channel signal wave in any one of the channels CH1-CH9 by measuring the receiving levels of only three channels CH2, CH5 and CH8. From this reason, it is highly probable that the receiving level equal to or greater than the predetermined value L 1 can be detected in either case where a control channel of a cell in the approached service area matches the control channel which the mobile station selects or is a adjacent control channel of the control channel which the mobile station selects. According to the first embodiment of the invention, at least one group of plurality of control channels selected from all the control channels which a mobile station can set are predetermined. The measurements of receiving levels on the groups of control channels are repeatedly performed intermittently. When a receiving level equal to or greater than the predetermined value L 1 is detected in any of the control channels, it is judged that the mobile station is close to the service area. The power consumption in the receiving circuit can be reduced by such intermittent operations. Fig. 4 shows a flow chart for implementing the control channel searching method in accordance with the first embodiment of the present invention. According to the first embodiment, in the first control channel selection mode SM1, all the, e. g. N, control channels CH1, CH2... CHN pre-stored in the channel memory 13M of a mobile station are divided into a plurality of, e. g. G, groups. Each of the groups are intermittently and sequentially selected to measure the receiving levels. Dependent on the received level (whether or not the received level is equal to or greater than the predetermined value L 1 ), it is determined that the mobile station is approaching to the service area, i.e., to any one of the cells. Therefore, even if a control channel having a receiving level equal to or greater than the predetermined value L 1 is not detected in one group, it is highly possible that such a control channel can be detected in the other groups. Incidentally, an arbitrary grouping is possible, for example, as shown in following patterns 1, 2 and 3. Here, for explanation purposes, a case where the total number of control channels is 50 and the number of groups is 10 (G=10) is shown. Pattern 1: First group = CH1, CH2, ..., CH5 Second group = CH6, CH7, ..., CH10 : Tenth group = CH46, CH47, ..., CH50 Pattern 2: First group = CH1, CH11, ..., CH41 Second group = CH2, CH12, ..., CH42 : Tenth group = CH10, CH20, ..., CH50 Pattern 3: Channels are randomly allocated to First group - Tenth group. Incidentally, a method other than the above patterns 1-3 is of course possible. The grouping will be explained in accordance with the sequence shown in Fig. 4. In Fig. 4, G is the number of groups, g is the group number (g=1 through G), N is the number of control channels stored in a mobile station and n is the number of control channels per group (n=N/G). In Fig. 4, a variable g is set to 1 first in order to specify a group (step S1). Then in step S2, n control channels are selected in the gth group (first group) specified by this variable g and the receiving levels of those channels are measured (step S2). In the above pattern 1, the receiving levels of 5 waves of the control channels CH1, CH2, ..., CH5 are measured. As the result of this receiving level measurement, it is decided whether or not the control channel having receiving level equal to or greater than the first predetermined value L 1 does exist (step S3). The first predetermined value L 1 is set to relatively small value (e. g., 4-7 µ) so that it would be possible to detect, in a control channel, a leakage power from the adjacent rightous call-waiting control channel to thereby decide that the mobile station is located inside the service area. As the result of this judgment, if any control channel having a receiving level equal to or greater than the first predetermined value L 1 is not detected, the process proceeds to step S4 to turn off the power of the receiving circuit 11 for a predetermined time period T OFF . After the predetermined time period T OFF has passed, the variable g is incremented by one in step S5 and then g is checked to see if g=G+1 . If g is not G+1, the process returns to step S2 to select the control channels of the next group g+1 determined in step S5. In the current case, control channels of the second group are selected. In such a way, when the process returns to step S2 to select next group, as in the aforementioned manner, the receiving level of each control channel in the selected group g is measured and it is checked if a control channel having a receiving level equal to or greater than the predetermined value L 1 exists based on the received levels. The loop process of steps S2-S7 is repeated until a control channel having a receiving level equal to or greater than the first predetermined value L 1 is detected. Incidentally, if g is G+1 in step S6, the variable g is reset to 1 in step S7 to return to the first group. In step S3, if a control channel having a receiving level equal to or greater than the first predetermined value L 1 is detected as the result of the receiving level measurement mentioned above, it is judged that the mobile station is approaching to the service area. Then, the process proceeds to the second control channel selection mode SM2 described in step S8 and after. In the first step S8 in the second control channel selection mode SM2, the receiving levels of all the N control channels CH1-CHN prestored in the channel memory 13M of the mobile station are measured. Then, each of the measured levels of all the control channels is checked to see if any one of the levels is equal to or greater than the second predetermined value L 2 (step S9). If control channels having receiving levels equal to or greater than the second predetermined value L 2 are detected, each of these control channels is checked from the highest level control channel in descending order to see if any one of these control channels can be the call waiting control channel (step S10). The predetermined value L 2 is a reference value to be compared with to determine whether processing for call-waiting can be performed or not with the selected control channel, and is selected to be equal to or greater than L 1 , for example to 8-10 dBp. The decision on propriety for the call waiting is performed by receiving the control signal on the selected control channel and then by judging if the mobile station can receive and send a call over the control channel. The method of this decision is the same as mentioned above. If it is decided that call waiting is possible in this decision of propriety for call waiting, a call waiting or a call sending is performed on the control channel (step S11). When any control channel having the receiving level equal to or greater than the second predetermined value L 2 is not detected in the step S9, or when all the detected control channels are not qualified for call waiting in the decision of propriety for call waiting in step S10 even if the control channels having the receiving levels equal to or greater than the second predetermined value L 2 are detected, the process returns to the first step S1 to move to the first control channel selection mode SM1 again and a forecast for the service area is performed. Next, the effect of the first embodiment performing the intermittent operations will be explained referring to Figs. 5A and 5B. Fig. 5A is a timing diagram showing the operation of the conventional control channel searching method and Fig. 5B is a timing diagram showing the intermitted operation of the control channel searching method in accordance with the first embodiment of the present invention. Incidentally, in Figs. 5A and 5B, N is the number of all the control channels prestored in a mobile station, T OFF is the time duration where the power of the receiving circuit is off, n is the number of control channels per group (n=N/G), G is the number of groups (G=1 through N groups), t m is time required for measuring the receiving level per channel, I 1 is the consumed current value during the receiving level measurement and I 0 is the consumed current value when the power of the receiving circuit is off. As shown in Fig. 5A, in the conventional control channel searching method, the control channels having receiving levels equal to or greater than the predetermined value are detected for every receiving level measurement of all the control channels and then the decision of propriety for call waiting is performed on those detected control channels. Therefore, the power of the receiving circuit is always on. The average value I AV of the consumed current until a call waiting control channel is determined is I 1 mA. On the other hand, in the control channel searching method of the present invention, since the operation is intermittent in the first selection mode until the service area is detected as shown in Fig. 5B, if the time period of the second selection mode SM2 is shorter enough to be ignored than the time period of the first selection mode SM1, the average value I AV of the consumed current until a call waiting control channel is determined is expressed as below. I AV ={(n x t m ) x I 1 +T OFF x I 0 }/(n x t m +T OFF ) In order to further clarify the effect of the present invention, if I 0 =10 mA, I 1 =100 mA, T OFF is 1 second, n=5 (N=50, G=10) and t m =10 ms are assumed, the average consumed current in the conventional method is I AV =100 mA and the average consumed current in the method of the present invention is I AV =18 mA. Thus, it is appreciated that the consumed current in the method of the present invention is considerably reduced. In the embodiment mentioned above, although an example where a mobile station is approaching to the service area, in a place where the control channel cannot be received, such as a tunnel, under ground and a building, the first control channel selection mode SM1 is performed repeatedly under the control of the mobile station, and thus, the power supply to the receiving circuit is periodically turned off for a predetermined time period T OFF to reduce the consumed current. Second Embodiment Next, an second embodiment of the present invention will be explained below. As described above, in the case where a mobile station is entering the service area or a mobile station is exiting from a state incapable of receiving a control channel in the service area, a call waiting control channel is newly selected from all the control channels. In such a selection of a call waiting control channel, if interleave channels are used in the mobile communication system, in this embodiment, the interleave channels are excluded from the control channels whose receiving levels are equal to or greater than the predetermined value L 2 selected from all the control channels in accordance with the first embodiment described above. The decision of propriety for call waiting is performed only for the remaining control channels to reduce the number of measurements of the receiving levels and to reduce the power consumption. In this second embodiment of the present invention, when searching a call waiting control channel, a received signal on the selected control channel is checked to see if the selected control channel is interleave channel based on the frequency information of the control channel and the receiving level. The control channel which is judged as an interleave channel is not treated as a candidate control channel for call waiting and the propriety for call waiting is not decided on the control channel. The process flow is shown in Fig. 6 and is explained below referring to the specific examples in Figs. 7A-7D. However, the operation examples explained here are based on the conditions below. (a) Total number of the control channels which can be preset in a mobile station is 20 and those control channels are CH1-CH20. (b) Assuming that the frequency of channel CHn is Fn and δ is a positive value less than the channel frequency interval ΔF, a frequency interval between the adjacent channels in these 20 channels is less than the channel frequency interval ΔF as shown below. F2 = F1 + δ F3 = F2 + δ F4 = F3 + δ : F20 = F19 + δ (c) The predetermined value of the receiving level for allowing call waiting is 10 dbµ, for example. In this embodiment, step S10 in Fig. 4 comprises steps S10A to S10R in Figs. 6-a and 6-b. In step S10A, if it is decided in step S9 that there is at least one control channel having a receiving level equal to or greater than the predetermined value L 2 , the receiving levels equal to or greater than the predetermined value L 2 are extracted from the receiving levels of the control channels stored in the table TB1 and sorted in descending order. Each set of a control channel number and its receiving level obtained in such a way is stored in the candidate table TB2 (Fig. 7B) provided in the receiving level memory 12. In the candidate table TB2 of Fig. 7B, the receiving levels R equal to or greater than the predetermined value L 2 extracted from the table TB1 of Fig. 7A are sorted in descending order. Based on the candidate table TB2 created in such a manner, the relationship between the frequencies of the control channels is analyzed starting from the control channel of highest receiving level in descending order (i. e., starting from the highest receiving level control channel sequentially in the candidate table TB2). If the interval between compared frequencies of the two control channels is less than the channel frequency interval ΔF, the channel having lower receiving level is presumed as an interleave channel and deleted from the candidate control channels. The details of the deletion process are as follows. First, a variable m is set to 1 in step S10B. Then, referring to the mth (=1st) storage area TB2(m) of the candidate table TB2 in step S10C, the receiving level R(m) of the control channel stored in the storage area TB2(m) is read out to check to see if R(m)=0 . If R(m)=0 , then the process goes to step S10L skipping the processes S10D-S10K. If R(m) is not zero, the process proceeds to step S10D. In the example of TB2 of Fig. 7B, the receiving level R(1)=50 of channel CH5 is read out and then the process proceeds to step S10D. In step S10D, the control channel number CH(m) of the area TB2(m) is read out and the corresponding frequency F(m) is read out from the channel memory 13M (Fig. 3). In this example, the frequency F 5 of the channel number CH5 is read out. Then, in step S10E, a variable j=m+1 is set. In step S10F, a receiving level R(j) of the control channel of the area TB2(j) is read out to check to see if R(j)=0 . If R(j) is zero, the process immediately skips to step S10J. If R(j) is not zero, jth storage area TB2(j) in the candidate table TB2 is referred to read out the channel number CH(j) stored in this area TB2(j) and then the corresponding frequency F(j) is obtained from the channel memory 13M. In step S10H, the difference between the two frequencies F(m) and F(j) is checked to see if the difference is less than the control channel interval ΔF. If the difference is less than the control channel interval ΔF, in step S101, receiving level R(j) of channel CH(j) is compulsorily set to the value less than the predetermined value 10 µ, e. g. 0 (refer to table TB2' of Fig. 7C). If the difference is not less than ΔF, the process moves to step S10J. By this process, the frequency difference |F5-F6| between the channel CH5 and the channel CH6 having next largest level in the table TB2 is obtained. Since this difference is less than ΔF from aforementioned conditions, the process proceeds to step S10I and the receiving level R(6) of the channel CH6 is set to 0. In step S10J, j is checked to see if j is M. If j is not M, j is incremented by 1 in step S10K and the process returns to step S10F to repeat steps S10F-S10K until j becomes M. When j becomes M, in step S10L, m is checked to see if m is M. If m is not M, m is incremented by 1 in step S10M and the process returns to step S10C to repeat steps S10C-S10M until m becomes M. By this process, for example, regarding the channel CH14, the receiving levels R(13) and R(15) of the respective channels CH13 and CH15 whose frequency intervals relative to the channel CH14 are less than ΔF and whose receiving levels R(13) and R(15) are less than the channel CH14, are also set to 0. In such a way the table TB2' of Fig. 7C is obtained. When m becomes M a table TB3 (Fig. 7D) is obtained, in step S10N, by deleting all the channels having receiving levels of zero (R=0) from the table TB2'. In the table TB3, the data are also sorted in descending order starting from the highest receiving level. The number of control channels stored in the table TB3 at this time is K. In step S10O, a variable k is set to 1. In step S10P, the channel number CH(k) of the area TB3(k) of the table TB3 is read out and this control channel is set to the receiving circuit to perform the receiving operation and to decide the propriety for call waiting. That is, it is checked to see if the receiving level of the channel CH(k) is equal to or higher than the predetermined value, if the control signal can normally be received, if the analysis of the control signal indicates the contents defining conditions for call waiting and the mobile station can satisfy the conditions for call waiting, and if the control signal is not of a control signal inhibiting the use of the control channel. If call waiting is not possible, k is checked to see if k is M in step S10Q. If k is not M, k is incremented by 1 in step S10R and the process returns to step S10P to repeat the same processes. If the control channel set in step S10P is decided that the control channel can be a call waiting channel, the call waiting state is started (S11). If k is K in step S10Q, that is, none of the control channels in the table TB3 is qualified as a call waiting control channel, the process returns to step S1 to start the measurement of the receiving levels of all the control channels again. In such a way, the candidate control channels in the table TB3 of Fig. 7D are checked sequentially in descending order starting from the highest level control channel to see if these are capable for call waiting. As mentioned above, according to the second embodiment of the present invention, the number of decisions of propriety for call waiting can be reduced and the time up to the start of a call waiting can be shortened. Also, the battery consumption in a mobile station can be reduced by the reduced number of decisions of propriety for call waiting.

Claims: A control channel searching method in a mobile station (MS) wherein a power supply for a receiving circuit (11) is turned off during a break time (T OFF ) of receiving level measurements, comprising step of: (a) performing a first control channel selection mode by executing a receiving level measurement for measuring the receiving levels on control channels prestored in the mobile station (MS), turning off the power supply to the receiving circuit (11) at the end of the receiving level measurement for said break time (T OFF ), and subsequently repeating the receiving level measurement until at least any one of the control channels having a receiving level equal to or greater than a first predetermined value is detected; characterized in that step (a) comprises performing said receiving level measurement for measuring the receiving levels on the control channels of one of control channel groups (S2) each of which consisting of a plurality of control channels preselected from all the control channels prestored in the mobile station (MS), and stopping said repeating if at least any one of the control channels of at least one control channel group having a receiving level equal to or greater than a first predetermined value is detected; the method further comprising (b) performing a second control channel selection mode (SM2) for measuring receiving levels of all the control channels stored in said mobile station (MS) if a control channel having a receiving level equal to or greater than said first predetermined value is detected as the result of performing said first control channel selection mode; (c) selecting an optimum control channel based on the receiving levels measured in said second control channel selection mode; based on a comparison with a second predetermined value and (d) performing said steps (a), (b) and (c) again if an optimum control channel cannot be selected in said step (c). The method according to claim 1 further including a step of dividing all the control channels stored in said mobile station (MS) into a plurality of groups, step (a) performing said receiving level measurement sequentially for all groups by selecting another group each time the receiving level measurement is repeated. The method according to claim 1 or 2 wherein said step (c) comprises selecting control channels measured in said step (b) to have a receiving level equal to or greater than a second predetermined value as candidate control channels, deciding propriety for call waiting on said candidate control channels in descending order of the receiving levels and selecting a candidate control channel decided to be possible for call waiting as said optimum control channel. The method according claim 3 wherein said second predetermined value is selected equal to or greater than said first predetermined value. The method according to claim 1 or 2 wherein said step (c) includes following steps: (c-1) comparing each of said receiving levels measured in said second control channel selection mode with said second predetermined value and selecting control channels having a receiving level equal to or greater than said second predetermined value as candidate control channels; (c-2) checking to see if a frequency interval between arbitrary two candidate control channels in said candidate control channels is less than predetermined frequency width; (c-3) if the frequency interval between said two arbitrary candidate control channels is less than said frequency width, making one of said two control channels having higher receiving level to be one of the control channels for subject to selection for call waiting and excluding the other control channel having lower receiving level from subject to selection; and (c-4) selecting an optimum call waiting control channel out of said control channels for subject to selection.

Inventors: CHIBA KOUJI, SAWAYANAGI SHIGEHIDE, TOKUHIRO NORIHITO, YAZAKI HIDETOSHI, YUNOKI KAZUFUMI, CHIBA, KOUJI, SAWAYANAGI, SHIGEHIDE, TOKUHIRO, NORIHITO, YAZAKI, HIDETOSHI, YUNOKI, KAZUFUMI
Applicants: NIPPON TELEGRAPH & TELEPHONE, NTT MOBILE COMMUNICATIONS NETWORK INC.
Application year: 1995
Main classification: H04Q7
IPC codes: H04W52, H04W48