SYSTEM FOR JUDGING A SECTOR IN WHICH A MOBILE STATION EXISTS

Abstract: In the system of the invention, transmission means of base station transmission/reception means transmits signals of frequencies assigned to the sectors via a directional antenna. Reception means of mobile station transmission/reception means receives output signals from base station transmission means, and measuring means of the mobile station measures the reception levels of signals of frequencies inclusive of control channels and of signals of frequencies of each of the sectors. Determining means of the mobile station judges its own zone and its own sector based on the frequencies inclusive of control channels and frequencies of the sectors having large reception levels. Due to the above operation, the base station does not have to measure or compare the levels. Therefore, facility is simplified, and improvement is accomplished in regard to frequency utilization efficiency, sector judgement precision and signal reception reliability. In the base station, furthermore, the ratio of reception level of signals of the frequency in its own sector and reception level of signals of the frequency inclusive of control channel of its own sector, is used for detecting the motion among the sectors in its own zone, making it possible to reduce the frequency that is to be monitored. This enables the time for detecting the movement to be shortened and the intermittent time to be shortened.

Description: [Technical Field] This invention is applicable to mobile radio communication. This invention relates, more particularly, to a means of determining in which sector one mobile station stays in a mobile communication system where a radio zone covered by (referred to as a zone herein) one base station is divided into several sectors in operation. [Background Technology] In the prior art mobile communication system, a zone covered by a base station is divided into plural sectors and each sector is covered by an antenna having directivity. Such method is advantageous in that interferences between sectors can be reduced by the directivity of the antennas, and that the frequency utilization efficiency is improved as the distance between cells which are using the same frequency channels is shortened. In this type of mobile communication system, it is necessary for a mobile station to find out the serial numbers of the zone and of the sector in which the station stays at the time in order to set up a voice channel before the start of communication. Even in the prior art mobile communication system where a zone is not divided into sectors, a mobile station sometimes must decide in which zone to stay. In the system where different zones are assigned with different frequencies respectively, a mobile station scans control channels assigned respectively to the zones, and detects a zone in which the station stays from the frequency (the serial number) of which control channel the station could receive. Similarly, if a zone is divided into sectors, it is possible for a station to discriminate a sector by its frequency. In other words, sectors are assigned with control channels of different frequencies, and a mobile station decides in which sector it stays by detecting the frequency or channel it can receive or if a plurality of such channels exists, by measuring receiving levels of these frequencies, comparing them and discriminating that with the maximum reception level. However, in such a system of locating the sector in which one mobile station stays from its frequency, as sectors are assigned with a control channel of different frequencies, the number of frequencies for control channels is much larger than the number required for the system in which control channels are assigned to different zones. For instance, if the zones are assigned with control channels of different frequencies in the unit of nine zones, and the same frequency is repeatedly assigned to the next unit of nine zones or the 10th zone, then the number of necessary frequencies is nine. But if the sectors are assigned with different frequencies, a zone is divided into six sectors, and the zones are grouped in the unit of seven, 42 frequencies (6 x 7 = 42) will be required for control channels as the same frequency is reassigned only to the 8th zone. Even if the system has a comparatively simple structure as above,-the required number of control channels is as much as five times more. Moreover, the number of control channels which one mobile station receiver must scan for discriminating the sectors is large. Further, as it generally takes a certain amount of time to monitor the receiving level of a channel, the time required from activation of the power switch of a mobile station to when communication is ready with a set up control channel becomes inconveniently long. In the above conventional system, moreover, when one mobile station moves across the boundary of sectors, the number of frequencies it must scan in order to detect the next control channel is so large that the station takes much time for detection and control of movement between sectors to conduct switching between the control channels smoothly. As the mobile station moves further during the delay in detection and control, the receiving strength becomes lower than the marginal level at which a mobile station could receive control signals, and switching of control channels becomes occasionally not operable. Because one control channel can only cover a narrow area in such a system, the frequency of channel switching increases, and the possibility of not receiving the paging signals during such channel switching increases proportionally. Due to the above reasons, reliability of receiving the paging signals and channel switching control is deteriorated. In such a system, the same information is often transmitted to all the sectors such as in the case of paging. In such a case, efficiency in utilization of frequency is low as the sectors must be assigned with different frequencies. In order to overcome the defects of the system where sectors are assigned with different control channels, there has been proposed a practical system where each zone is assigned with control channels of the same frequency. However, the system cannot rely on the aforementioned method to locate a sector where a mobile station exists simply by scanning the frequencies of control channels and detecting the frequency with the maximum receiving level. Instead, a base station measures an upward control channel signal issued from a mobile station and compares with other signals to locate the relevant sector. (The signal generated by a mobile station for a base station is called an upward sIgnal while the one generated from the base station for mobile stations is called a downward signal). In these systems where locating of a sector should be done on the side of the base station, when a mobile station turns on the power source at the start, or when the zone where it stays needs to be located, the station first scans the frequencies of control channels assigned to dIfferent zones, measures and compares the receiving levels thereof, discriminates the control channel frequency which has the maximum receiving level, and uses the frequency of the upward control channel corresponding with the discriminated frequency. When a mobile station receives or originates a call, as the mobile station uses an upward control channel to transmit a signal to the base statIon, the base station receives the upward signal by using an individual receiver allocated to each sector. The base station measures and compares the levels at which each of the receivers receives the signal, detects the sector in which the relevant mobile station stays from the number of the individual receiver with the maximum level. In this system, the burden imposed on the base station becomes great as the individual receivers for the sectors of a zone should measure and compares the levels. When an upward signal is received to measure its level, if the length of the signals generated from a mobile station is too short, it cannot be measured sufficiently. Due to the changes in level by fading, sectors sometimes cannot be precisely discriminated. In the system where the bit transfer speed of signal is set at a high rate, the signal length generally becomes short to deteriorate precision in sector discrimination. Poor precision in the sector discrimination means a high possibility for connecting a wrong remote base station. In other words, the zones should be defined larger equivalently. This makes the distance which permits reassignment of the same frequency longer, the frequency utilization efficiency lower, and the capacity of subscribers of the whole system incapable of extension. The division of zones into a plurality of sectors in an area covered by a communication system accommodating a plurality of mobile stations is disclosed at pages 250-252 of a textbook entitled "Mobile Communication Systems" written by J. D. Parsons, D. Jardine and J. G. Gardiner and published by both Blackie and by Halsted Press. Japanese Patent Application No. 59-161139 discloses a communication system accommodating a plurality of mobile stations which are able to store information indicating which of a plurality of base stations provides the greatest received signal level and to use the stored information as the basis for selecting a base station through which to communicate when the time comes to communicate. This invention seeks to provide a system wherein a base station is relatively simple, the frequency utilization efficiency is high, there is high precision in sector discrimination and there is high reliability of paging signal reception in locating the sector where a mobile station lies. The invention provides a communication system including a plurality of mobile transmitter-receivers movable in an area divided into a plurality of contiguous zones each of which is divided into a plurality of sectors, each zone having assigned to it a unique control channel frequency, base transmitter-receiver means located in each zone capable of operating at the control channel frequency for the zone, characterised in that each base transmitter-receiver means is capable of transmitting signals directed at respective sectors of its zone at respective frequencies assigned to the sectors of its zone and each mobile transmitter-receiver includes means capable of measuring the received levels of the signals at the control channel frequencies along with the received levels of the signals at the sector frequencies and of determining, from those signal levels, its zone and sector location. Each mobile transmitter-receiver may include a detector which, in operation, compares the ratios of the received levels of the signals at the sector frequencies and the received levels of the signals at the control channel frequencies with a predetermined threshold, and based on the result of the comparison, detects the movement of the mobile transmitter-receiver within a zone. Each mobile transmitter-receiver may be capable, additionally, of determining its sector location by monitoring and comparing the strengths of a plurality of the sector signals which reach it with one another. A mobile transmitter-receiver for the communication system is capable of determining its position in the area covered by the communication system by monitoring and comparing the strengths of a plurality of the sector signals which reach it with the strength of the control signal that reaches it. A mobile transmitter-receiver for the communication system may be capable, additionally, of determining its position in the area covered by the communication system by monitoring and comparing the strengths of a plurality of the sector signals which reach it with one another. [Brief Description of the Drawings] Fig. 1 is a block diagram to show a base station transmitter/receiver of the first embodiment of the communication system for locating a sector of a mobile station according to this invention. Fig. 2 is a block diagram to show a mobile station transmitter/receiver of the first embodiment of the communication system for locating a sector of a mobile station according to this invention. Fig. 3 is a structural view of a control channel and sector individual frequency of the communication system for locating a sector of a mobile station according to this invention. Fig. 4 is a frame format of a control channel of the communication system for locating a sector of a mobile station according to this invention. Fig. 5 is a view to show the movement between zones of the communication system for locating a sector of a mobile station according to this invention. Fig. 6 is a view to show the movement between sectors within a zone of the communication system for locating a zone of a mobile station according to this invention. Fig. 7 is a view to show the sectors adjacent to the sector where a mobile station exists of the communication system for locating a sector of a mobile station according to this invention. Fig. 8 is a flow chart of the operation to detect the movement between sectors in a zone of the communication system for locating a sector of a mobile station according to this invention. Fig. 9 is a view to show the arrangement of zones and sectors of the communication system for locating a sector of a mobile station. Fig. 10 is a format of voice channel of the communication system for locating a sector of a mobile station according to this invention. Fig. 11 is a view to show another arrangement of zones and sectors of the communication system for locating a sector of a mobile station. Fig. 12 is a frame format of broadcasting information of a base station of the communication system for locating a sector of a mobile station according to this invention. Fig. 13 is a flow chart of the operation to obtain the control channels of the adjacent zones and individual frequencies of the adjacent sectors based on the broadcasting information from the base station of the communication system for locating a sector of a mobile station according to this invention. Fig. 14 is a frame format of broadcasting information of another base station of the communication system for locating a sector of a mobile station according to this invention. Fig. 15 is a chart to show correspondence between the order of identification numbers of sector individual frequencies broadcast from a base station of the communication system for locating a sector of a mobile station according to this invention. Fig. 16 is a flow chart of the operation which detects the individual frequencies of adjacent sectors from the broadcast sector individual frequency of the communication system for locating a sector of a mobile station according to this invention. Fig. 17 is a view to show an irregular zone/sector arrangement of the communication system for locating a sector of a mobile station according to this invention. Fig. 18 is a flow chart of the operation which detects adjacent sector individual frequencies of the other zones from the broadcast information of the communication system for locating a sector of a mobile station according to this invention. Fig. 19 is a flow chart to obtain zone control channels of adjacent sectors of another zone from broadcasting information of the communication system for locating a sector of a mobile station according to this invention. [Optimal Mode to Realize the Invention] This invention will now be described by referring to embodiments shown in attached drawings. Fig. 1 is a block diagram of an embodiment of the base station transmitter/receiver of the communication system for locating a sector of a mobile station according to this invention. Fig. 2 is a block diagram to show an embodiment of a mobile station transmitter/receiver of the communication system for locating a sector of a mobile station according to this invention. From Fig. 3, the relation between a zone and sectors will be easily understood. Referring now to Figs. 1 and 2, the system includes plural mobile stations having transmitter/receivers, a base station having a transmitter/receiver to communicate with said mobile stations, and zones each of which is further divided into sectors. The transmitter/receiver of the base station includes a first transmitter/receiver means 200 which transmits or receives signals of the frequency f₀ including a control channel which is assigned to each of the zones via directive antennas A₁ through A₆. The transmitter/receiver of each mobile station comprises a second transmitter/receiver means 100 which transmits/receives signals of the frequency f₀ including the control channel and an antenna 180. The second means 100 comprises a demodulator 110, a band pass filter 120, a frequency converter 130, a control unit 150 and a part of a synthesizer 170. This invention system is characterized in that said first transmitter/receiver means 200 includes transmitter means which transmits signals f₁ through f₆ of sector individual frequencies assigned to respective sectors via directive antennas A₁ through A₆, and said second transmitter/receiver means 100 includes a receiver means which receives signals f₁ through f₆ of said sector individual frequencies, a level measuring circuit 140 as a measuring means which measures the receiving levels of the signals of sector individual frequencies and of the signals of the frequencies of the control channels, a part of the control unit 150 and a frequency memory 160 as the means for deciding which discriminates the zone and sector for the station based on the result of the measurement by the circuit 140. The communication system for locating a sector of a mobile station will now be described in respect of the operation. Fig. 3 is a structural view to show the relation of control channels and sector individual frequencies of this invention system. Fig. 4 is a frame format of the invention system. Fig. 5 is a view to show the movement between zones of this invention system. Fig. 6 is a view to show the movement between sectors of a zone in which a mobile station exists. Fig. 7 is a view to show sectors adjacent to the sector in which a mobile station stays according to the present invention. Fig. 8 is a flow chart to show the operation for detecting movement of a station between sectors of a zone according to this invention system. Fig. 9 is a chart to show a zone/sector arrangement of this invention system. Fig. 10 is a format of voice channel of this invention system. Fig. 11 is a view to show another zone/sector arrangement of this invention system. Fig. 12 is a frame format of broadcasting information from the base station thereof, Fig. 13 is a flow chart of the operation for detecting the control channels of adjacent zones and adjacent sector individual frequencies based on the broadcasting information thereof, and Fig. 14 is a frame format of still another broadcasting information from the base station thereof. Fig. 15 is a chart to show correspondence between the order of identification numbers of the sector individual frequencies which is broadcast from the base station and the sector arrangements of the system, Fig. 16 is a flow chart for obtaining the individual frequencies of sectors adjacent to the sector where the mobile station stays out of the informed sector individual frequencies, Fig. 17 is a view to show the irregular zone and sector arrangement of this invention system, and Fig. 18 is a flow chart to obtain individual frequencies of adjacent sectors in other zones out of the broadcasting information of this invention system. Fig. 19 is a flow chart to obtain control channels of the adjacent sectors in other zones in the system thereof. Table 1 shows correspondence between the zones and the control channels of this invention system, Table 2 correspondence of the sectors and the sector individual frequencies, Table 3 the content of non-volatile memory of the mobile station transmitter/receiver, and Table 4 correspondence between the frequencies which a mobile station should store and the identification numbers thereof in a system where a base station broadcasts the sector individual frequencies of the system. Table 5 shows correspondence between the order of broadcasting from the base station and the identification numbers of the system, and Table 6 shows tables which the base station broadcasts to mobile stations in the system according to this invention. In Fig. 3, f₁₀, f₂₀, f₃₀, f₄₀, f₅₀, f₆₀, and f₇₀ denote frequencies of the control channels assigned to respective zones. The figures f₁₁ - f₁₆, f₂₁ - f₂₆, ..., f₇₁ - f₇₆ denote the sector individual frequencies which are constantly transmitted from the base station. Fig. 1 shows a structure where a zone is divided into six sectors wherein each of the antennas A₁ through A₆ is allocated to each sector. Antennas A₁ through A₆ are respectively connected to transmitter/receivers of the frequency F₀ of the control channel of the zone, the sector indivisual frequencies f₁ , f₂ , f₃ , f₄ , f₅ ,f₆ and voice channels of the sectors. Although the system shown in Fig. 1 is structured to have the transmitter/receivers for the frequencies f₀ of control channels in the number corresponding to that of sectors, signals from one transmitter/receiver may be distributed to the power supply sections of sectors. In Fig. 2, the level measuring circuit 140 is the circuit for measuring the receiving levels of the signal frequencies, the synthesizer 170 is controlled by the controller 150, and the frequencies are selected by the converter 130 and the band pass filter 120. When a power switch is turned on, a mobile station scans control channels assigned to zones to measure the receiving levels and compare them. In minimizing errors due to fading, it is more effective to switch to a high speed and repeat short measuring cycles for plural times per frequency rather than to measure levels of a frequency once. The scanning time may be shortened by omitting the frequencies with the levels which are formed extraordinarily low by the first measurement. From the result of scanning, the control channel with the strongest receiving level is judged as the zone in which the particular mobile station stays. The mobile station scans the sector individual frequencies of the zone when the receiving channel of the control channel is not used, measures the receiving levels and compares them in order to render sector judgement. It is quite obvious from Fig. 4(a) that a control channel has blank sections not used for reception. The letter X in Fig. 4(a) denotes a broadcasting section and Y a paging section. As the paging section is divided into sub-sections and a mobile station receives paging information only of a sub-section to which it belongs, the station does not have to receive control channels of the zone of the paging sections of other sub-sections. This produces blank time and permits receiving other frequencies. In the Fig. 4(b), shaded sections denote blank times of the paging section Y of the group of the mobile stations. Because there are blank times in the control channels for receiving, the mobile station can receive and at the same time measure the sector individual frequencies and judges that the sector having the sector individual frequency having the strongest receiving level is the sector in which the station exists. Operation is described for the zone/sector discrimination for the time of turning on the switch referring to the case of Fig. 4 where control channels are structured to repeat the same frequency in the unit of seven zones. It is assumed that the sector in which the particular mobile station stays is the sector having the individual frequency of f₇₃. The mobile station first scans the control channels f₁₀, f₂₀, ..., f₇₀. Then, it scans the sector individual frequencies f₇₁, f₇₂, ..., f₇₆ of the zone of which control channel is f₇₀ and judges that the sector where the station stays is f₇₃. After judging the sector for activation, the mobile station advances to the state of waiting for paging. While waiting, the mobile station is constantly moving, the movement thereof between the sectors and the zones are monitored. The mobile station can learn its movement between zones by monitoring control channels which are assigned to zones respectively in one-to-one relation, and learn movement between sectors in the zone by monitoring the sector individual frequencies. For instance, when a mobile station moves as shown in Fig. 5, as the control channel having the strongest receiving level moves from the zone Z1 to the zone Z2, the station can learn that it has moved from the zone Z1 to the zone Z2. In the case where a station moves from one sector to another sector of the same zone, as shown in Fig. 6, the strongest receiving level changes from the sector individual frequency of the sector S1 of the six sectors of the zone to that of the sector S2, and it becomes obvious that the station moves from the sector S1 to the sector S2. Although the movement between the cells is learned by monitoring the control channels in the above example, the movement between both sectors and zones in the waiting state may be learned by monitoring the sector individual frequencies alone. It may be learned by measuring and comparing the receiving levels of individual frequencies of sectors adjacent to the sector where the mobile station exists irrespective of zones. As shown in Fig 7, if it is assumed that the station stays in a sector S1, the station measures and compares the receiving levels of the individual frequencies of adjacent sectors, the sectors S2 through S6 of the same zone and the sectors S7 through S10 of other zones where the station possibly moves to. If the receiving levels are strong in the sectors S2 through S6, it is judged that the station moves within the zone. On the other hand, if they are strong in the sectors of S7 through S10, it is judged that the station moves to another zone. If the number of sector individual frequencies is reduced, it may shorten the detecting time of the movement. The reduction of the number of sector individual frequencies means an increase in time where no reception is being made, or an improvement of intermittent ratio. In other words, it saves consumption power of a mobile station, especially of a portable unit, prolongs the life of a battery, and improves the quantity of service. There is proposed a method to first scan only the sectors of higher probability to move to, and then, if necessary, to scan other sectors of lower possibility rather than to scan all of the adjacent sectors. For instance, in the case shown in Fig. 7, if the mobile station stays in the sector S1, the sectors of S2, S6, S8 and S9 which contact linearly with the sector S1 are scanned first. If any higher receiving level is not obtained even after scanning them, then other sectors are scanned as the station may have moved to others. In this method, the number of sectors which are monitored is 4. Compared to the method where all the sectors including those contacting the sector 1 with points are monitored, the number decreased from 10 to 4 or less than a half to thereby save time. The method of scanning from the sector of higher probability may be used for monitoring the control channels. The number of scanning may be reduced if the monitoring of a control channel of a zone having a higher probability in movement is preferentially scanned. The above mentioned monitoring of the movements between the zones and sectors is the method of scanning the frequencies of adjacent zones, comparing them and discriminating the frequency with the strongest receiving level as the frequency of the zone or sector where a particular mobile station exists. However, the movement between the sectors in a zone may be monitored by another method. Instead of constantly monitoring individual frequencies of the adjacent sectors of the zone, the method usually monitors only [the individual frequency of the sector where the station exists]/[control channel of the zone in which the station exists]. As the control channels are of the same frequency for all of the sectors of the zone, there is hardly any directivity in the level distribution thereof. But the sector individual frequency differs from one sector to another and has directivity; in other words, the receiving level may be strong in one sector but weak in another. As (the individual frequency of the sector where a station exists) / (control channel of the zone where the station exists) is close to 1 in the sector where the station stays and is close to 0 in other sectors, when a mobile station moves across the boundary between the two sectors, the ratio of the individual frequency of the sector as against the control channel of the zone where it stays decreases and the mobile station can learn from the decrease that it moves from one sector to another within the same zone. After moving out of the sector, the mobile station measures and compares the frequency of the sector adjacent to the former one or the one it moves into, and judges into which sector it moves. This method is advantageous as it can shorten the time for detecting and improve the intermittent ratio as it usually must monitor only the ratio of the individual frequency of its own sector against the control channel of the zone thereof. Fig. 8 shows the flow chart of the operation wherein the above method is applied to monitoring of the movement between the sectors of a zone. In the operation shown in Fig. 8, in addition to the aforementioned monitoring, the monitoring of control channels of adjacent zones is used in order to detect the movement between the zones. In Fig. 8, the relation holds as; Function L (f i )= (measured value of the receiving level of the frequency f i ) The symbols α and β denote constants. The zone/sector arrangement is shown in Fig. 9, and the control channels and the individual frequencies are assigned as shown in Table 1. The zone and the sector where the particular mobile station stays are Z0 and A respectively. The symbols used in relation to Fig. 9 such as Z0, Z1, Z2 or A, B, C, ... are relative symbols for the zone or sector where the station exists and will be changed when the station moves. As is obvious from Fig. 8, a mobile station measures the control channel of the zone f₀ where it stays and control channels f₀′ and f₀″ of the adjacent zones and detects the movement between the zones. When the station finds that it has not moved between the zones, it measures the sector individual frequency f A , detects the ratio of f A /f₀ and monitors the movement between the sectors. If the ratio f A /f₀ becomes lower than a certain value β, the station is likely to have moved from one sector to another. The receiving levels of the individual frequencies of sectors of the same zone, f B , f C , f D , f E , f F , are measured and compared, and the sector having the individual frequency with the highest receiving level is judged to be the sector to which the station has moved. If the value β is set high, as it scans the individual frequencies of sectors within the same zone faster when the station moved between sectors, the intermittent ratio decreases while judgement becomes easier. The value β may be set considering the tradeoff between the intermittent ratio and judgement precision. The foregoing explanation has been given to the case of judging a zone and a sector when the power is turned on or during waiting, but this system may be applied to the zone/sector decision during a voice communication. In such a case, a method which switches to another frequency in a time slot other than the communication slot by using TDMA (time division multiplex access method) or a method wherein a receiver for level monitoring is installed at a mobile station should be applied. The communication channels at TDMA may be structured as shown in Fig. 10 wherein the slot used for the communication is a part of the TDMA frames. A zone or sector may be judged as in the waiting time by using the blank time as such blank times are produced when not receiving the voice channels. The letters ST in Fig. 10(a) denote a slot used for the communication while the shaded portions in Fig. 10(b) denote blank times. As the above mentioned sector decision system according to this invention does not require measurement nor comparison of receiving levels on the side of the base station, the burden on the base station is not heavy since the control channels and the sector individual frequencies of a mobile station are measured and compared in the receiving levels. As the sector individual frequencies are constantly transmitted, a mobile station can receive signals of a sufficient length and measure the receiving levels for a sufficient time to thereby enable precise decision of a sector. As the control channels are assigned to zones rather than sectors, the frequency utilization efficiency of the channels is quite high. Compared to the case wherein control channels are assigned to sectors, the number of scans for the frequencies curing the time from switching on to the sector decision and the number of scans of the frequencies when movement is judged are smaller to thereby minimize the time required to judge the sector. For instance, if channels are assigned to sectors respectively and sectors are grouped in the unit of six and repeated in the unit of seven zones, the number of scans for the control channels is 6 x 7 = 42. According to this system wherein the same frequency is reassigned in the unit of nine zones, the control channels are scanned for nine times and sector individual frequencies are scanned for six times to total 15; (9 + 6 = 15). Therefore a sector can be decided within about one third of the conventionally required time. In the system according to this invention, the number of the frequencies which should be scanned before a control channel is detected and the state becomes ready for paging is nine. If control channels are assigned differently to sectors, the number of frequencies scanned is 42. This invention system becomes therefore "ready state" within about one fifth of the time conventionally needed. The time needed for turning on to the ready state is shortened, and the time needed from switching the control channel for the different zone and preparing for the ready state is also reduced. Further, as control channels are assigned to zones and as the same control channel is used within a zone, the control channels do not need to be switched from one to the other for movement between the sectors in a zone. This makes paging possible even if the decision of a sector is delayed or erroneously made. The first embodiment for a mobile station to learn the sector individual frequency is the method in which the sector individual freuency stored by the station in a non-volatile memory. Table 3 shows the structure of the contents stored in a memory. The letters f₁₀ - f₇₀ denote frequencies of the control channels assigned to cells. The letters f₁₁ - f₁₆ , ..., f₇₁ - f₇₆ denote sector individual frequecies. If the arrangement of the zones and sectors is regular, it is possible to learn the control channels and individual frequencies of adjacent zones and sectors out of the information stored in the non-volatile memory. The second embodiment for a mobile station to learn the sector individual frequencies is the method in which a base station informs the sector individual frequencies to mobile stations. Alternatively, this method may be used for the control channels so that the base station informs the control channels of the zones adjacent to the zone where the station stays. The method is advantageous as it can be flexibly used even if the arrangement of the zones/sectors is irregular when it should obtain the control channels of adjacent zones or the individual frequencies of the adjacent sectors. However, the amount of broadcasting information increases compared with the case where mobile stations are stored in memories, and it is preferable to reduce the amount of information. For this purpose, it is preferable to prepare a table which relates frequencies with identification numbers like Table 4 inside the mobile stations and broadcasting should be made with the ID numbers rather than frequencies. The content of broadcast information may be only the sector individual frequencies of a zone or the individual frequencies of the sectors of a zone where a mobile station exists and of the sectors adjacent thereto depending on the monitor method for the movement thereof. The number of control channels which are to be monitored may be reduced by informing not only the sector individual frequencies but also the control channels of the adjacent zones. As an example of the case wherein a base station broadcasts to mobile stations the frequencies to be monitored, there is cited a case where the base station informs the sector individual frequencies in the zone where it stays and the control channels of adjacent zones. Referring to Fig. 11 where the station stays in the zone Z0, the control channel of the zone informs the individual frequencies of the sectors A through F of the zone and the control channels of the zones which are adjacent to the sectors as shown in Fig. 12. In Fig 12, the parts marked with the letters A, ..., F are the parts in which the sector individual frequencies of the sectors A through F will be informed and those marked with Z1 through Z8 are the parts in which the control channels of the zones Z1 through Z8 will be informed. and denote the bits for identifying the sector individual frequencies from the zone control channels. If it is assumed that the sectors F, A, B, Z8, Z1, Z2, Z3 should be monitored while the mobile station stays within the sector A, the mobile station will obtain them from the broadcasting information in accordance with the operation flow shown in Fig. 13. It is assumed that the functions f and g have the values shown in Fig. 13. In Fig. 14 is shown another example of the broadcasting information structure where the sector individual frequencies of the zone wherein the mobile station stays and the control channels of adjacent zones should be informed. In Fig. 14, the letter V denotes parts in which the sector individual frequencies should be informed while W denote the parts in which the control channels of the zones adjacent thereto should be informed. For instance, if a mobile station stays within the sector A in Fig. 11, the mobile station receives the part of the sector A of the sector individual broadcasting part V to learn its own individual frequency. Then, the station reads out the part of the sector A of the part W the adjacent zone control channels' broadcasting information to learn the control channels of the zones adjacent to its own zone are for Z8, Z1, Z2 and Z3. This method can improve the intermittent ratio as the mobile station of the sector A must receive the broadcasting information aimed at the sector A alone. An example will be shown below for the case where both of the individual frequencies of the sectors of the zone in which the station exists and of the adjacent zones are broadcast. This method broadcasts to the mobile station the aforementioned information by arranging the identification numbers for the sector individual frequencies in an order shown in Fig. 15 and Table 5. Fig. 15 shows correspondence between the order of ID numbers of the sector individual frequencies to be informed and the sector arrangement while Table 5 shows correspondence between the order and the ID numbers. Fig. 16 shows in flow chart the method to obtain the sector in which a mobile station exists and its adjacent sectors by means of the table which is informed by the base station. For example, if the mobile station stays in the sector of the order 1, adjacent sectors of the other zones will be the sectors of the order 18, 7, 8 and 9 from Fig. 16. The method mentioned may be used for the case where the arrangement of zones and sectors is irregular. The method is exemplified below wherein the control channels of the adjacent zones is informed. Fig. 17 shows an irregular case wherein the sectors within the same zone are identified with letters A through F. In this case, the number of adjacent zones is seven and the number of adjacent sectors in the other zones which contact the zone B by line is six. This case is irregular compared to the case shown in Fig. 15 where the number of adjacent zones is six and the adjacent sectors of other zones is four. What the base station should inform are the ID number of the sector individual frequencies of the zone where the mobile station stays, the individual frequency ID numbers of the sectors in other zones on which the sectors of its own zone contact, and the control channels of the contacting sections of the other zones. The base station first of all informs the ID numbers of the sector individual frequencies of the zone where the mobile station stays from the sectors A to F as shown in the first table or Table 6(a) in the alphabetical order. Then, the adjacent sectors of the other zones correspond with the sectors of the zone where the mobile station stays. It prioritizes the sector which contacts with the other sector(s) most. For instance, as the sector S4 contacts only with the sector B, it is judged that it corresponds to the sector B. If a sector contacts with plural sectors like the sector S8 with the sectors B and C, the sector C is prioritized as it contacts with the sector S8 in a larger area, and judged to correspond with the sector S8. Then, the base station informs the number of adjacent sectors in other zones corresponding to the sectors A through F in the alphabetical order like Table 6(c). Then, it informs the ID number of the adjacent sector individual frequencies of other zones corresponding to the sectors A through F in the order of the number in Fig. 17 like sectors S1, S2, ..., S15 as shown in Table 6(b) starting from the sector corresponding to the sector A. The order is clockwise from the sector contacting with the Sector A. It also informs the control channels of the zones to which the sector S1 belongs in the clockwise order as shown under the column of the adjacent sector individual frequencies in Table 6(b). The order of notification is in the order of the zones numbered with I, II, ..., VII in Fig. 19. The data is read out from the Table 6(d) in order to learn the zones to which the adjacent sectors belong. The table groups the sectors in the unit of a zone and notifies the number of sectors of each zone starting from the sector S1. In this case, as they are grouped in (Sector S1), (S2, S3), (S4, S5, S6), (S7, S8), (S9), (S10, S11, S12), (S13, S14), (S15), it informs like , , , , , , , and . Figs. 18 and 19 show the method to obtain the individual frequencies of the adjacent sectors of other zones contacting with the sector in which a mobile station exists and the control channels of zones to which the sectors belong by means of the information of the broadcasting tables. In the figures, the function Fa is defined as follows. Fa(A) = 1, Fa(B) = 2, ..., Fa(F) = 6. The letter T₂(i) denotes the values which belong to the order i in the second table, T₃(i) the values which belong to the order (i) of the third table and T₄ the values which belong to the order (i) in the forth table. For special cases, An identifier of EOT (end of table), for instance the numeral 0, should be inserted in the end of the second table. The broadcasting method mentioned above has been described so far on assumption that the base station does not know in which sector a mobile station in question stays. The method is applicable to the time of switching on and in the waiting state. But after a commmunication is started and the base station has learned the sector where the mobile station exists, the base station has to inform only the individual frequencies of the adjacent sectors of the same zone and the control channels of the adjacent zones. During the waiting state, the mobile station learns these frequencies from the broadcasting information in accordance with a certain procedure, but during a communication, the base station informs simply the frequencies as they are. A part of the communication channels may be used for the channels through which the base station informs the mobile station of those frequencies during a communication by means of frequency division multiplex or time division multiplex. As communication channels are assigned to respective mobile stations during a communication, they can transmit information exclusive to each station. Taking advantage of the feature, the broadcasting method is practically feasible. Communication channels may be used for the sector individual frequencies by assigning a certain predetermined channel to each sector and using the channel as the individual frequency for identifying the sector. For the purpose, the communication channels do not need to have any special information in their content for sector identification but must constantly issue radio waves at their unique frequencies. If channels are preferentially assigned to communication, almost no demerit will be produced even if they have to constantly issue radio waves. Alternately, carriers without modulation may be used as the sector individual frequencies. They need not to be modulated as they do not have to transmit any information but are used simply to identify the receiving levels for identification of sectors. As carriers without modulation have almost no frequency band, they hardly impose extra burden on the other components even if their frequencies are utilized. The sector individual frequencies may be the control channels which transmit the control signals for respective sectors or the frequencies for user packets. This invention has been described referring to the case where a system is divided into sectors, but this method is applicable to decide the zones in the system where plural base stations are used as a unit in the control channel structure and assigned with individual frequencies respectively. The assignment of individual frequencies may be made in plural stages. More specifically, plural number of units to each of which is assigned the individual frequency is grouped and then the group is assigned with another individual frequency. By assigning them in two hierarchical stages, the number of scans for identification of the position of a mobile station may be simplified as the individual frequencies of the larger groups are first scanned and then those of downstream groups are sequentially scanned. [Industrial Use] As described in the foregoing statement, this invention achieves a high utilization efficiency of the frequencies as one control channel is assigned to one zone instead of one sector. It is advantageous as it needs not to measure the receiving levels at the base station and therefore can reduce the burden imposed on the base station. The system according to this invention can improve paging signal reception reliability and sector decision precision and shorten the time required from the activation to preparation of ready condition for paging. This invention system can reduce the number of frequencies which should be monitored, shorten the time for detecting movement, and improve the intermittent ratio as it uses the measurement and comparison of relative values of the control channels of the zone in which a mobile station stays and individual frequency of the sector it stays for detecting the movement within the zone. This invention can reduce the number of frequencies which should be monitored as the method for the base station informs mobile stations of the sector individual frequencies and the control channels of the zone may be used to the irregular arrangement of zones and sectors to learn the control channels of the adjacent zones and individual frequencies of the adjacent sectors.

Claims: A communication system including a plurality of mobile transmitter-receivers (100) movable in an area divided into a plurality of contiguous zones each of which is divided into a plurality of sectors, each zone having assigned to it a unique control channel frequency, base transmitter-receiver means (200) located in each zone capable of operating at the control channel frequency for the zone, characterised in that each base transmitter-receiver means (200) is capable of transmitting signals directed at respective sectors of its zone at respective frequencies assigned to the sectors of its zone and each mobile transmitter-receiver (100) includes means (140,150,160) capable of measuring the received levels of the signals at the control channel frequencies along with the received levels of the signals at the sector frequencies and of determining, from those signal levels, its zone and sector location. A communication system as claimed in claim 1, which is characterized in that each mobile transmitter-receiver (100) includes a detector (120,130,140,150,160) which, in operation, compares the ratios of the received levels of the signals at the sector frequencies and the received levels of the signals at the control channel frequencies with a predetermined threshold, and based on the result of the comparison, detects the movement of the mobile transmitter-receiver (100) within a zone. A communication system as claimed in claim 1, wherein each mobile transmitter-receiver (100) is capable of determining its sector location by monitoring and comparing the strengths of a plurality of the sector signals which reach it with one another. A mobile transmitter-receiver (100) for a communication system as claimed in claim 1 or claim 2, which is capable of determining its position in the area covered by the communication system by monitoring and comparing the strengths of a plurality of the sector signals which reach it with the strength of the control signal that reaches it. A mobile transmitter-receiver (100) for a communication system as claimed in claim 1 or claim 3, which is capable of determining its position in the area covered by the communication system by monitoring and comparing the strengths of a plurality of the sector signals which reach it with one another.

Inventors: ONOE SEIZO, UMEDA NARUMI, UTANO TAKANORI, YAMADA TOMOYUKI, ONOE, SEIZO, UMEDA, NARUMI, UTANO, TAKANORI, YAMADA, TOMOYUKI
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION, NTT MOBILE COMMUNICATIONS NETWORK INC.
Application year: 1990
Main classification: H04B7
IPC codes: H04W16, H04W64