Spread spectrum communication system and overload control method therefor

Abstract: In an overload control method for a spread spectrum communication system, when the number of mobile stations in the process of communication with a base station in a self-cell is equal to or more than a predetermined number, the ratio of increase instructions to reverse link transmission power control bits contained in radio frames transmitted from the mobile stations in the process of communication to the base station and indicating increases/decreases in transmission power of the mobile stations is measured. When the measured ratio of increase instructions is equal to or more than a predetermined ratio, it is determined that the reverse link interference amount between communications performed between the base station and the mobile stations is equal to or more than an allowable value. When it is determined that the reverse link interference amount between the communications is equal to or more than the allowable value, the reverse link interference amount between the communications is reduced by performing at least one of transmission power restriction and origination/termination restriction. A spread spectrum communication system is also disclosed.

Description: Background of the Invention The present invention relates to a spread spectrum communication system and an overload control method for a case wherein the interference amount of a reverse link signal in communication between a mobile station and a base station in a cell (communication from the mobile station to the base station) is equal to or larger than an allowable value in the spread spectrum communication system. In a spread spectrum communication system, a plurality of communications can be performed by using the same frequency band. This is because each communication is spread-modulated with codes having orthogonality on the transmission side, and each communication can be specified by spread-demodulation (despreading) with the same codes on the reception side. This orthogonality is made imperfect by propagation delay differences due to geographical and weather conditions and the like and time deviations due to multipath in the propagation path between the mobile station and the base station, and multipath associated with irrelevant codes, i.e., irrelevant communication, and multipath associated with relevant codes, i.e., relevant communication, have correlation components in some case. These correlation components become interference components in the relevant communication, resulting in a deterioration in communication quality. Since interference components are generated by such a factor, interference components increase as the number of communications increases. In a radio communication system, a predetermined signal/noise ratio is generally required to ensure high communication quality. In a spread spectrum communication system, since each communication is performed by using the same frequency band, a predetermined signal/(noise + interference) ratio is required to ensure high communication quality. This ratio is generally expressed as a signal/interference ratio = Eb/IO ratio (Eb: desired reception wave power, and IO: interference wave power). In addition, the Eb/IO required to ensure predetermined communication quality is expressed as a specified Eb/IO ratio. In general, communication quality is determined on the basis of a frame error rate (FER). The Eb/IO ratio required to obtain a given FER changes owing to the influences of fading frequencies and Eb/IO ratio measurement accuracy, and hence needs to be always corrected. For this reason, a base station changes the Eb/IO ratio specified to obtain the FER required to guarantee communication quality in accordance with the measured FER of a received radio wave such that the FER required to guarantee the communication quality remains constant. This function is called outer loop control. In general, as the number of communications increases, since the interference wave (IO) increases, the Eb/IO ratio decreases. In this case, a base station increases the transmission output of each mobile station to increase the signal level (Eb) so as to ensure the specified Eb/IO ratio in each communication. In EP 682 418 A2, for example, there is disclosed a transmission power control method in a CDMA (Code Division Multiple Access) system which performs multiple access using a spread spectrum technique in mobile communications, and a communication system using this method, in order to prevent the transmission power in accordance with the ratio of the received signal level of a desired wave to the power from other communicators. According to an aspect of this citation, the first predetermined maximum transmission power may be determined on the basis of a maximum capacity in terms of number of subscribers in a cell, a radius of the cell, and an outage probability of the cell. In U.S. Pat. No. 5,604,730 a power control process in a CDMA communication system is described, using a comparison threshold to determine whether the base station should instruct the mobile radios to increase or decrease their transmit power. The total received Eb/IO ratios for all the mobile radios communicating with a particular base station is compared to a maximum threshold for the reverse channel. Hence, according to this citation, the base station communicates with mobile radios over a forward channel and the mobile radios communicate with the base station over a reverse channel. A base station uses hereby this power control process over the forward packet channel, to dynamically adjust the transmission power of one or more mobile radios transmitting on the reverse packet channel. By using above methods, interference with the neighboring base stations may occurs and as a result, the communication capacities of the neighboring base stations decrease. Furthermore, the conventional methods are rather time-consuming since it takes relatively much time to detect a deterioration in communication quality. In a spread spectrum communication system, however, since excessive signal energy causes interference in irrelevant and relevant communications, the Eb/IO ratios in the respective communications in the base station must be made almost equal. The base station therefore controls the transmission power of each mobile station to make the Eb/IO ratios in the respective communications almost equal. This control is called high-speed closed loop control because the control operation is performed at high speed by feedback control between a mobile station and a base station to absorb level variations such as fading. An Eb/IO ratio as a reference in high-speed closed loop control will be referred to as a reference Eb/IO ratio. In general, this reference Eb/IO is set to be equal to the specified Eb/IO. That is, the Eb/IO ratio specified to obtain an FER that guarantees communication quality and obtained by outer loop control is set to the reference Eb/IO ratio in high-speed closed loop control, and each mobile station is instructed to increase/decrease the transmission power such that the Eb/IO ratio obtained by reception power from each mobile station is set to this reference Eb/IO ratio in the base station. A combination of this outer loop control and high-speed closed loop control will be referred to as transmission power control. As shown in Fig. 10A, however, when the communication amount becomes a predetermined amount or more, even if the signal level (Eb) is increased, the interference amount increases accordingly. As a result, as shown in Fig. 10B, the specified Eb/IO cannot be obtained. In this case, if the communication amount further increases, all communications are disabled. This state is a state in which the interference amount is equal to or larger than the allowable value. Another problem is that the base station continuously increases the transmission output of each mobile station to obtain the specified Eb/IO ratio in each communication, and unnecessarily large interference is caused in neighboring base stations as well. As a result, the communication capacities of the neighboring base stations decrease. In the worst case, communication failures occur in the neighboring base stations. Furthermore, a chain reaction occurs in such a manner that communication quality deteriorates or communication failures occur in a wide range. In a conventional technique (IS-95 system), that the reverse link interference amount between communications in a cell is equal to or larger than an allowable value is detected in accordance with a decrease in FER of reverse link radio frames owing to a failure to obtain the specified Eb/IO ratio. On the basis of this detection result, origination/termination control is performed for the base station in which the reverse link interference amount is equal to or larger than the allowable value and mobile stations belonging to the neighboring stations, thus reducing a deterioration in communication quality. To increase the FER measurement accuracy, however, many samples are required, and hence it takes much time to detect such a deterioration. For this reason, the transmission power of each mobile station belonging to the base station in which the reverse link interference amount between communications in the cell is equal to or larger than the allowable value is further increased until a deterioration is detected. Meanwhile, excess interference occurs in the neighboring base stations. In the above conventional spread spectrum communication system, it takes much time to detect that the interference amount between communications in a cell is equal to or larger than the allowable value, because this detection is based on detection of a decrease in FER. Summary of the Invention It is an object of the present invention to provide a spread spectrum communication system which can detect in a short time that the reverse link interference amount between communications in a cell is equal to or larger than an allowable value. In order to achieve the above object, according to the present invention, there is provided an overload control method for a spread spectrum communication system, comprising the steps of, when the number of mobile stations in the process of communication with a base station in a self-cell is not less than a predetermined number, measuring a ratio of increase instructions to the sum of increase and decrease instructions, which comprises reverse link transmission power control bits contained in radio frames transmitted to the mobile stations from the base station and indicating increases/decreases in transmission power of the mobile stations, when the measured ratio is not less than a predetermined ratio, determining that a reverse link interference amount between the base station and the mobile stations is not less than an allowable value, and when it is determined that the reverse link interference amount between the communications is not less than the allowable value, reducing the reverse link interference amount between the communications by performing at least one of transmission output restriction and origination/termination restriction. Brief Description of the Drawings Fig. 1 is a view showing the arrangement of a spread spectrum communication system according to the first embodiment of the present invention; Fig. 2 is a view showing the structure of transmission data from a base station to each mobile station in Fig. 1; Fig. 3 is a view showing the operation of a base station in the spread spectrum communication system in Fig. 1 in a case wherein the ratio of transmission power increase instructions is almost equal to the ratio of transmission power decrease instructions; Fig. 4 is a view showing the operation of the base station in the spread spectrum communication system in Fig. 1 in a case wherein the ratio of transmission power increase instructions is larger; Fig. 5 is a flow chart showing an outline of the operation of the spread spectrum communication system in Fig. 1; Fig. 6 is a block diagram showing the detailed arrangements of a base station and mobile station in Fig. 1; Fig. 7 is a flow chart showing the operation of the spread spectrum communication system in Figs. 1 and 6; Fig. 8 is a flow chart showing the detailed processing in step S12 in the flow chart of Fig. 7; Fig. 9 is a flow chart for explaining the operation of a spread spectrum communication system according to the second embodiment of the present invention; and Figs. 10A and 10B are graphs showing changes in power values and Eb/IO ratio as a function of the number of mobile stations. Description of the Preferred Embodiments The present invention will be described in detail below with reference to the accompanying drawings. (First Embodiment) Fig. 1 shows the arrangement of a spread spectrum communication system according to the first embodiment of the present invention. Referring to Fig. 1, the spread spectrum communication system of this embodiment is comprised of a mobile communication network 40 constituted by a mobile communication switching center (not shown) and the like, a plurality of base stations 30 1 to 30 3 connected to the mobile communication network 40, and a plurality of mobile stations 20 that move in cells 38 1 to 38 3 formed by the base stations 30 1 to 30 3 . In spread spectrum communication, the radio waves transmitted from the respective mobile stations 20 must reach the base stations 30 1 to 30 3 with almost constant reception power. For this purpose, the base stations 30 1 to 30 3 control the transmission power of the respective mobile stations 20. If the power received by each of the base stations 30 1 to 30 3 is higher than a specified Eb/IO ratio, each base station outputs a transmission power decrease instruction to the corresponding mobile station 20. If the received power is lower than the specified Eb/IO, each of the base stations 30 1 to 30 3 outputs a transmission power increase instruction to the corresponding mobile station 20. These increase and decrease instructions are expressed by TPC (Transmission Power Control) bits contained in data transmitted from the base stations 30 1 to 30 3 to the respective mobile stations 20. Fig. 2 shows the structure of data transmitted from each of the base stations 30 1 to 30 3 to the corresponding mobile station 20. Referring to Fig. 2, the data consists of a plurality of channels 130. The respective channels 130 are called super-frames that are arranged at 640-ms intervals. Each channel 130 consists of 64 radio frames 131 1 to 131 64 . Each of the radio frames 131 1 to 131 64 consists of 16 time slots 132 1 to 132 16 . In each channel 130 as a normal traffic channel, each of the time slots 132 1 to 132 16 is made up of a pilot symbol 133, a TPC bit 134, and user data 135. As described above, the TPC bits 134 are set in the respective time slots 132 1 to 132 16 , and hence are transmitted from each base station to the corresponding mobile station at 0.625-ms intervals. For this reason, each mobile station 20 changes its transmission output at 0.625-ms intervals. Assume that in each of the base stations 30 1 to 30 3 , the number of mobile stations 20 in the process of communication is equal to a predetermined number or more, and the reverse link interference amount between the communications in each cell is smaller than an allowable value. In this case, since the mobile stations 20 independently operate, the ratio of increase instructions for transmission power control to the ratio of decrease instructions become almost equal as a whole in each base station. In this case, the reason why the number of mobile stations 20 is limited to the predetermined number or more is that if, for example, the number of mobile stations 20 is one, and the mobile station 20 moves away from a corresponding one of the base stations 30 1 to 30 3 , the ratio of increase instructions for transmission power control becomes 100%. As described above, when the number of mobile stations 20 is small, the ratios of increase and decrease instructions greatly change in accordance with the operation of each mobile station 20. If, however, the number of mobile stations 20 is the predetermined number or more, the portion of increase instructions becomes almost equal to that of decrease instructions in terms of probability. Since each of the base stations 30 1 to 30 3 manages the number of radio channel resources that can be used in the self-cell, the number of mobile stations 20 that are currently present in the self-cell can be known from the use state (allocation state) of these radio channel resources. Fig. 3 shows the operation of the base station 30 1 in the system in Fig. 1 in a case wherein the portion of transmission power increase instructions is almost equal to that of decrease instructions. Referring to Fig. 3, six mobile stations 20 1 to 20 6 are present in the cell 38 1 . If the reverse link interference amount between the respective communications in the cell does not exceed the allowable value, and the normal radio communication quality is maintained, increase and decrease instructions represented by TPC bits as bits that are used by the base station 30 1 to output reverse link transmission power instructions to the mobile stations 20 1 to 20 6 are contained in data in almost equal ratios. Once the reverse link interference amount in the cell 38 1 exceeds the allowable value, the base station 30 1 outputs transmission power increase instructions to all the mobile stations 20 1 to 20 6 in the cell 38 1 to obtain the specified Eb/IO ratio. As a result, the portion of increase instructions becomes larger as a whole in the cell 38 1 . Fig. 4 shows this state. In this embodiment, therefore, that the reverse link interference amount exceeds the allowable value is detected by checking whether the ratio of increase instruction to TPC bits is large. The operation of the spread spectrum communication system of this embodiment will be briefly described next with reference to the flow chart of Fig. 5. First of all, in a state wherein the radio channel quality is normal, and the ratio of increase instructions to TPC bits in a cell is not large but normal (step S100), it is checked whether the reverse link interference amount between the respective communications in the cell is equal to or more than the allowable value (step S101). If the reverse link interference amount is less than the allowable value, the current state is maintained. When the number of mobile stations 20 1 to 20 6 in the cell is equal to or more than the predetermined number, and the ratio of increase instructions to the TPC bits in the cell becomes equal to or more than a predetermined ratio, it is determined in step S101 that the reverse link interference amount in the cell is equal to or more than the allowable value. As a result, the transmission power of each of the mobile stations 20 1 to 20 6 in the cell is decreased, and origination/termination with respect to the mobile stations 20 1 to 20 6 in the cell formed by an adjacent base station is restricted (step S102). With this control, the reverse link interference amount is decreased to less than the allowable value. In this manner, origination/termination restriction and transmission power restriction are flexibly performed until the reverse link interference amount becomes less than the allowable value (step S103), thereby optimally maintaining the communication amount and service quality. In this state, it is checked whether the reverse link interference amount becomes less than the allowable value (step S104). If it is determined that the reverse link interference amount becomes less than the allowable value, the origination/termination restriction and transmission power restriction are canceled (step S105). After this cancel operation, the flow returns to step S100 in which the original radio channel is normal. Fig. 6 is a block diagram showing the arrangements of a base station and mobile station in Fig. 1. For the sake of descriptive convenience, Fig. 6 shows only the base station 30 1 connected to the mobile communication network 40 and the mobile station 20 1 that performs radio communication. The mobile station 20 1 is made up of a reception section 21, a reverse link transmission power control section 23, a transmission section 24, and a control section 22. The reception section 21 receives and decodes a signal from the base station 30 1 and transfers the decoded signal to the control section 22. The reception section 21 also outputs a TPC bit contained in the decoded data to the reverse link transmission power control section 23. The reverse link transmission power control section 23 changes the transmission power of the transmission section 24 in accordance with the TPC bit from the reception section 21. The transmission section 24 performs transmission with the transmission power instructed by the reverse link transmission power control section 23. The control section 22 inhibits origination in accordance with an origination restriction signal, from the mobile communication network 40, which is contained in the data transferred from the reception section 21, and cancels origination restriction in accordance with an origination restriction cancel signal. As will be described later, the origination restriction signal is sent from the mobile communication network 40 to the mobile station 20 1 through the base station 30 1 , and the mobile station 20 1 having the mobile station identification number designated by the origination restriction signal (for example, designated by the units digit) inhibits new origination. The base station 30 1 is made up of a reception section 33, a reverse link transmission power control bit determination section 32, a transmission section 31, a control section 34, and a reverse link transmission power outer loop control section 35. The reception section 33 receives and decodes a signal from the mobile station 20 1 , performs radio frame error detection, and measures the Eb/IO (the ratio of desired reception wave power to interference wave power) ratio. The reverse link transmission power outer loop control section 35 measures the frame error rate (FER) of reverse link radio frames received by the reception section 33, and sets a specified Eb/IO ratio to set an FER corresponding to predetermined service quality. This specified Eb/IO is set as a reference Eb/IO as a determination criterion in the reverse link transmission power control bit determination section 32. If the Eb/IO ratio of reverse link radio frames, which is measured by the reception section 33, is equal to or higher than the reference Eb/IO ratio set by the reverse link transmission power outer loop control section 35, the reverse link transmission power control bit determination section 32 generates a TPC bit indicating an instruction to decrease the transmission output of the mobile station. If the Eb/IO ratio of the reverse link radio frames, which is measured by the reception section 33, is lower than the reference Eb/IO ratio, the reverse link transmission power control bit determination section 32 generates and outputs a TPC bit indicating an instruction to increase the transmission output of the mobile station. The transmission section 31 inserts the TPC bit generated by the reverse link transmission power control bit determination section 32 into a transmission frame, and transmits the resultant frame as transmission data upon reception of an origination restriction signal and origination restriction cancel signal, which is sent from the mobile communication network 40 to the mobile station 20 1 , through the control section 34. The control section 34 obtains the ratio of increase instructions to the TPC bits to the mobile station 20 1 in the process of communication. If the number of mobile stations in the process of communication is equal to or larger than the predetermined number, and the ratio of increase instructions to the TPC bits is equal to or higher than the predetermined ratio, the control section 34 determines that the reverse link interference amount in the cell is equal to or larger than the allowable value. As a result, the control section 34 outputs to the reverse link transmission power control bit determination section 32 an instruction to invalidate the currently set reference Eb/IO ratio, and stores the reference Eb/IO ratio. The control section 34 notifies the mobile communication network 40 that the reverse link interference amount in the cell is equal to or more than the allowable value. In addition, upon reception of an origination/termination restriction signal from the mobile communication network 40, the control section 34 outputs the origination/termination restriction signal to the transmission section 31. The control section 34 keeps outputting the instruction to decrease the reference Eb/IO ratio to the reverse link transmission power control bit determination section 32 until the increase instruction ratio to the TPC bits becomes the predetermined ratio or less. Upon detecting that the increase instruction ratio to the TPC bits is lower than the predetermined ratio, the control section 34 determines that the reverse link interference amount in the cell 38 1 has fallen within the allowable value, and gradually restores the reference Eb/IO ratio to the reference Eb/IO ratio immediately preceding the origination/termination restriction. The control section 34 notifies the mobile communication network 40 that the reverse link interference amount in the cell 38 1 has fallen within the allowable value (this notification is indicated by "A" in Fig. 1). With regard to a new originating/termination call generated during origination/termination restriction control (e.g., an originating call from a mobile station other than those subjected to origination restriction or a call associated with a mobile station that has moved into the cell upon hand over), the control section 34 can instruct the reverse link transmission power control bit determination section 32 to set the initial reference Eb/IO ratio as the reference Eb/IO ratio immediately preceding origination/termination restriction so as to maintain the communication quality. Notified by the base station 30 1 that the reverse link interference amount in the cell 38 1 is equal to or more than the allowable value, the mobile communication network 40 outputs origination/termination restriction instructions to the base stations 30 2 and 30 3 (Fig. 1) adjacent to the base station 30 1 . Notified by the base station 30 1 that the reverse link interference amount has fallen within the allowable value, the mobile communication network 40 outputs origination/termination restriction cancel instructions to the base station 30 1 and the base stations 30 2 and 30 3 (Fig. 1) adjacent to the base station 30 1 (this notification is indicated by "B" in Fig. 1). The operation of the spread spectrum communication system having the above arrangement will be described in detail next with reference to Figs. 7, 8, and 9. Fig. 7 is a flow chart for explaining the operation of the spread spectrum communication system in Fig. 6, and more specifically, overload monitoring operation in the base station 30 1 . The base station 30 1 performs no operation when the number of mobile stations 20 in the process of communication in the cell 38 1 is smaller than the predetermined number, but performs overload monitoring operation only when the number of mobile stations 20 is equal to or more than the predetermined number (step S1). The control section 34 measures the ratio of increase instructions to all the transmission power control (TPC) bits transmitted to the mobile station 20 in the cell 38 1 (step S2) and checks whether the ratio is equal to or higher than the predetermined ratio (step S3). If the ratio is equal to or higher than the predetermined ratio, the control section 34 determines that the reverse link interference amount in the cell 38 1 is equal to or more than the allowable value (step S4). For example, this predetermined ratio is set to 60%. Subsequently, the control section 34 performs reverse link transmission power outer loop control to store the reference Eb/IO ratio set in the reverse link transmission power control bit determination section 32 and invalidates the reference Eb/IO ratio (step S5). The control section 34 notifies the mobile communication network 40 that the reverse link interference amount in the cell 38 1 is equal to or more than the allowable value (step S6). Notified that the reverse link interference amount in the cell 38 1 is equal to or more than the allowable value, the mobile communication network 40 performs origination/termination restriction with respect the base station 30 1 exhibiting the reverse link interference amount equal to or more than the allowable value and the adjacent base stations 30 2 and 30 3 (step S7). As described above, in this case, the mobile communication network 40 designates each mobile station to be subjected to restriction with the units digit of the identification number or an even/odd number. The control section 34 then outputs to the reverse link transmission power control bit determination section 32 an instruction to decrease the reference Eb/IO ratio (step S8). The control section 34 measures the ratio of increase instructions to the TPC bits again (step S9) and checks whether the ratio is equal to or more than the predetermined ratio (step S10). If the ratio is equal to or more than the predetermined ratio, the control section 34 determines that the reverse link interference amount in the cell 38 1 does not fall within the allowable value, and instructs the reverse link transmission power control bit determination section 32 to further decrease the reference Eb/IO ratio (step S10). If it is determined in step S10 that the ratio of increase instructions to the TPC bits in the cell 38 1 is lower than the predetermined ratio, the control section 34 determines that the reverse link interference amount in the cell 38 1 is smaller than the allowable value (step S11), and gradually cancels control of the reference Eb/IO ratio with respect to the reverse link transmission power control bit determination section 32, thereby restoring it to the original reference Eb/IO ratio (step S12). The base station 30 1 then notifies the mobile communication network 40 that the reverse link interference amount in the cell 38 1 is smaller than the allowable value (step S13). Notified that the reverse link interference amount in the cell 38 1 is smaller than the allowable value, the mobile communication network 40 gradually cancels the origination/termination restriction on the base station 30 1 and the adjacent base stations 30 2 and 30 3 (step S14). This operation is performed by gradually decreasing the number of units digits of the identification numbers of mobile stations designated as mobile stations subjected to restriction so as to gradually decrease the number of mobile stations to be subjected to restriction. An example of how the reference Eb/IO ratio is gradually restored to the reference value before restriction by reverse link transmission power outer loop control in step S12 in the flow chart of Fig. 7 will be described in detail next with reference to Fig. 8. After it is determined in step S11 that the reverse link interference amount in the cell 38 1 has fallen within the allowable value, the control section 34 increases the reference Eb/IO ratio set for the reverse link transmission power control bit determination section 32 to make the ratio approach the reference Eb/IO ratio before transmission power restriction (step S12-1). The control section 34 then measures the ratio of increase instructions to all the TPC bits in the cell 38 1 (step S12-2) and checks whether the ratio is lower than the predetermined ratio (step S12-3). If it is determined that the ratio is lower than the predetermined ratio, the control section 34 keeps outputting to the reverse link transmission power control bit determination section 32 an instruction to decrease the reference Eb/IO ratio until the ratio of increase instructions to all the TPC bits in the cell 38 1 becomes equal to or more than the predetermined ratio (step S12-4). If it is determined in step S12-3 that the ratio of increase instructions to the TPC bits is lower than the predetermined ratio, the control section 34 determines that the reverse link interference amount in the cell 38 1 falls within the allowable value (step S12-5). The control section 34 compares the reference Eb/IO ratio currently designated for the reverse link transmission power control bit determination section 32 with the reference Eb/IO ratio before transmission power restriction (step S12-6). If they do not coincide with each other, the control section 34 determines that the specified Eb/IO ratio that satisfies predetermined communication quality has not been obtained. The flow then returns to step S12-1. Subsequently, the processing in steps S12-2 to S12-6 is repeated until the reference Eb/IO ratio currently designated for the reverse link transmission power control bit determination section 32 becomes equal to the reference Eb/IO ratio before transmission power restriction to make the reverse link interference amount in the cell 38 1 fall within the allowable value. If it is determined in step S12-6 that the reference Eb/IO ratio currently designated for the reverse link transmission power control bit determination section 32 is equal to the reference Eb/IO ratio before transmission power restriction, the control section 34 determines that the required communication quality is ensured, and the reverse link interference amount in the cell 38 1 has fallen within allowable value (step S12-7). As a result, the control section 34 notifies the mobile communication network 40 that the reverse link interference amount in the cell 38 1 has fallen within the allowable value (step S13). In the spread spectrum communication system of this embodiment, since it is determined on the basis of the ratio of increase instructions to TPC bits whether the reverse link interference amount is equal to or more than the allowable value, whether the reverse link interference amount becomes equal to or more than the allowable value can be determined more accurately and faster than in the prior art. This minimizes interference with neighboring base stations. In addition, according to the spread spectrum communication system of this embodiment, after the reverse link interference amount exceeding the allowable value is decreased to the allowable value, the communication amount and service quality can be kept optimal near the allowable value until the reverse link interference amount becomes smaller than the allowable value. This is because origination/termination restriction and transmission power restriction can be gradually canceled. If origination/termination restriction and transmission power restriction are canceled at once, the communication amount increases, and the reverse link interference amount greatly increases again, posing a problem in terms of the stability of the mobile communication system. (Second Embodiment) Fig. 9 is a flow chart for explaining the operation of a spread spectrum communication system according to the second embodiment of the present invention. Since the arrangement of the second embodiment is the same as that of the first embodiment, a description thereof will be omitted. The operation of the spread spectrum communication system of this embodiment differs from that of the first embodiment only in that the processing in the flow chart of Fig. 9 replaces the processing in steps S12 and S14 in the flow chart of Fig. 7. The operation of the spread spectrum communication system of this embodiment will be described in detail next by taking, for example, a case wherein a mobile communication network 40 gradually cancels origination/termination restriction on a base station 30 1 exhibiting a reverse link interference amount within an allowable value in a cell 38 1 and adjacent base stations 30 2 and 30 3 . Notified by the base station 30 1 that the reverse link interference amount in the cell 38 1 has fallen within the allowable value (step S13), the mobile communication network 40 checks whether origination/termination restriction on the base station 30 1 and the adjacent base stations 30 2 and 30 3 is canceled (step S14-1). If the restriction is not canceled, the mobile communication network 40 relaxes all the origination/termination restrictions (step S14-2). In this case, to relax the origination/termination restrictions is to change, for example, a state wherein connection is permitted for only one mobile station out of ten mobile stations that have generated connection requests to a state wherein connection is permitted for three mobile stations out of ten mobile stations. If it is determined in step S14-1 that all the origination/termination restrictions are canceled, the flow returns to step S1. The base station 30 1 measures the ratio of increase instructions to all the TPC bits in the cell 38 1 (step S12-12) and checks whether the ratio of increase instructions to the TPC bits is equal to or higher than a predetermined ratio (step S12-13). If the ratio of increase instructions exceeds the predetermined ratio, the base station 30 1 performs control to decrease the reference Eb/IO ratio for reverse link transmission power control (step S12-14). Subsequently, the base station 30 1 keeps performing control to decrease the reference Eb/IO ratio for reverse link transmission power control until the ratio of increase instructions to all the TPC bits becomes almost equal to the ratio of decrease instructions (steps S12-12 to S12-14). In this case, the base station 30 1 checks whether the reference Eb/IO ratio for reverse link transmission power control is equal to or less than a predetermined lower reference Eb/IO ratio threshold (e.g., the Eb/IO ratio before origination/termination restriction, which is a value for guaranteeing the minimum service quality determined by a person in charge of the mobile communication system) (step S12-18). If the reference Eb/IO ratio is equal to or less than the lower threshold, the base station 30 1 notifies the mobile communication network 40 that the reverse link interference amount is equal to or more than the allowable value (step S6). As a result, the mobile communication network 40 performs origination/termination restriction again on the base station 30 1 exhibiting the reverse link interference amount equal to or more than the allowable value and the adjacent base stations 30 2 and 30 3 (step S7 in Fig. 7). If it is determined in step S12-13 that the ratio of increase instructions is less than the predetermined ratio, the same processing as that in steps S12-3 to S12-6 in Fig. 8 is performed in steps S12-15, S12-16, and S12-17, and the flow advances to step S13. According to this embodiment, even if the ratio of increase instructions does not become lower than the predetermined ratio when the reference Eb/IO ratio is decreased, since the reference Eb/IO ratio is kept higher than the lower threshold, the minimum communication quality can be maintained. According to the first and second embodiments, the mobile communication network 40 sets the reverse link interference amount in the cell 38 1 of the base station 30 1 to be less than the allowable value. However, the mobile communication network 40 performs similar control on the base stations 30 2 and 30 3 . In addition, according to the first and second embodiments, each of base stations 30 1 to 30 3 includes the reverse link transmission power outer loop control section 35. However, the mobile communication network 40 may include the reverse link transmission power outer loop control section 35. Furthermore, according to the first and second embodiments, whether the reverse link interference amount is equal to or more than the allowable value is determined by checking whether the ratio of increase instructions is equal to or more than the predetermined ratio. However, this predetermined ratio may change depending on the number of mobile stations. For example, no transmission power restriction and no origination/termination restriction are performed when the number of mobile stations is 0 to 4; the predetermined ratio is set to 80% when the number of mobile stations is 5 to 10; and the predetermined ration is set to 60% when the number of mobile stations is 11 or more. As has been described above, according to the present invention, when the reverse link interference amount in a cell becomes equal to or more than an allowable value, appropriate transmission power restriction and origination/termination restriction can be performed more accurately and faster than in the conventional scheme. Therefore, interference with the neighboring base stations can be minimized. This can improve the stability of the spread spectrum communication system.

Claims: An overload control method for a spread spectrum communication system, characterized by comprising the steps of: measuring a ratio of increase instructions to the sum of increase and decrease instructions, which comprises reverse link transmission power control bits contained in radio frames transmitted to mobile stations (20, 201 - 206) from a base station (301' - 303) and indicating increases/decreases in transmission power of said mobile stations (20, 201 - 206), in case that the number of mobile stations (20, 201 - 206) in the process of communication with a base station (301' - 303) in a self-cell is not less than a predetermined number; determining that a reverse link interference amount between said base station (301' - 303) and said mobile stations (20, 201 - 206) is not less than an allowable value in case that the measured ratio is not less than a predetermined ratio; and reducing the reverse link interference amount by performing at least one of transmission power restriction and origination/termination restriction in case that it is determined that the reverse link interference amount is not less than the allowable value. A method according to claim 1, characterized in that the predetermined ratio is variably set in accordance with the number of mobile stations in the process of communication in the cell. A method according to claim 1, characterized in that the step of reducing the reverse link interference amount comprises the steps of: starting transmission power restriction to decrease a reference Eb/IO (desired reception wave power/interference wave power) ratio as a criterion for high-speed closed loop control on the transmission power of said mobile stations in case that it is determined that the reverse link interference amount is not less than the allowable value; and gradually making control on the reference Eb/IO ratio effective in case that the ratio of increase instructions to the reverse link transmission power control bits becomes less than the predetermined ratio. A method according to claim 3, characterized in that the step of making control effective comprises the steps of: gradually decreasing the reference Eb/IO ratio within a range in which the radio of increase instructions to the reverse link transmission power control bits is less than the predetermined ratio; and determining that the reverse link interference amount has fallen within the allowable value in case that the reference Eb/IO ratio reaches the reference Eb/IO ratio before transmission power restriction, and the radio of increase instructions to the transmission power control bits becomes less than the predetermined ratio. A method according to claim 1, characterized in that the step of reducing the reverse link interference amount comprises the steps of: performing alarm notification to notify a mobile communication network (40) that the reverse link interference amount is not less than the allowable value; performing origination/termination restriction on said mobile stations in accordance with an origination/termination restriction instruction from said mobile communication network that responds to the notification; starting transmission output restriction to decrease the reference Eb/IO (desired reception wave power/interference wave power) ratio as a criterion for high-speed closed loop control on the transmission power of said mobile stations; gradually decreasing the reference Eb/IO ratio in a range in which the ration of increase instructions to the reverse link transmission power control bits is less than the predetermined ratio in case that the ratio of increase instructions to the reverse link transmission power control bits becomes less than the predetermined ratio; determining that the reverse link interference amount has fallen within the allowable value in case that the reference Eb/IO ratio reaches the reference Eb/IO ratio before transmission output restriction, and the ratio of increase instructions to the reverse link transmission power control bits becomes less than the predetermined ratio; performing alarm cancel notification to notify said mobile communication network that the reverse link interference amount has fallen within the allowable value; and canceling origination/termination restriction on said mobile stations in accordance with an origination/termination restriction cancel instruction from said mobile communication network that responds to the alarm cancel notification. A method according to claim 5, characterized in that the method further comprises the step of determining whether the reference Eb/IO ratio as the reference for high-speed closed loop control is not more than a predetermined lower limit, and alarm notification is performed to notify said mobile communication network that the reverse link interference amount is less than the allowable value in case that the reference Eb/IO ratio is not more than the lower limit. A spread spectrum communication system characterized by comprising: a plurality of mobile stations (20, 201 - 206) each of which is connected to a mobile communication network (40) and controls transmission power in accordance with a reverse link transmission power control bit contained in a received frame; and a base station (301 - 303) for determining that the reverse link interference amount between said mobile stations (20, 201 - 206) in a self-cell is not less than an allowable value, and performing at least one of transmission output restriction and origination/termination restriction on said mobile stations, in case that the number of mobile stations (20, 201 - 206) in the process of communication is not less than a predetermined number, and a ratio of increase instructions to the sum of increase and decrease instructions, which comprises reverse link transmission power control bits contained in radio frames transmitted to said mobile stations (20, 201 - 206) and indicating increases/decreases in the transmission power of said mobile station is not less than a predetermined ratio. A system according to claim 7, characterized in that said base station (301' - 303) performs alarm notification to notify said mobile communication network that the reverse link interference amount is not less than the allowable value in case that it is determined that the reverse link interference amount is not less than the allowable value, and performs origination/termination restriction on said mobile stations in accordance with an origination/termination restriction instruction from said mobile communication network that responds to the alarm notification. A system according to claim 8, characterized in that said base station performs alarm cancel notification to notify said mobile communication network that the reverse link interference amount has fallen within the allowable value, and cancels origination/termination restriction on said mobile stations in accordance with an origination/termination restriction cancel instruction from said mobile communication network in response to the alarm cancel notification, and said mobile communication network outputs an origination/termination restriction instruction to said base station from which the alarm notification is performed and an adjacent base station in accordance with the alarm notification from said base station, and outputs an origination/termination restriction cancel instruction to said base station from which the alarm cancel notification is performed and said adjacent base stations in accordance with the alarm cancel notification from said base station. A system according to claim 7, characterized in that said base station starts transmission output restriction to decrease the reference Eb/IO (desired reception wave power/interference wave power) ratio as a reference for high-speed closed loop control on the transmission power of said mobile stations when determining that the reverse link interference amount is not less than the allowable value, gradually increases the reference Eb/IO ratio within a range in which a ratio of increase instructions to reverse link transmission power control bits is less than a predetermined ratio when the ratio of increase instructions to the reverse link transmission power control bits becomes less than the predetermined ratio, and determines that the reverse link interference amount has fallen within the allowable value when the reference Eb/IO ratio reaches the reference Eb/IO ratio preceding transmission power restriction, and the ratio of increase instructions to the reverse link transmission power control bits becomes less that the predetermined ratio. A system according to claim 7, characterized in that each of said mobile stations comprises: reception means (21) for receiving a signal transmitted from said base station and outputting a reverse link transmission power control bit contained in decoded data; reverse link transmission power control means (23) for designating a change in transmission power in accordance with the reverse link transmission power control bit from said reception means; transmission means (24) for performing transmission to said base station with the transmission power designated by said reverse link transmission power control means; and control means (22) for restricting and canceling origination/termination in accordance with an origination/termination restriction signal and origination/termination restriction cancel signal contained in the data received by said reception means. A system according to claim 7, characterized in that said base station comprises: reception means (33) for receiving and decoding a signal from said mobile station, detecting an error in a radio frame, and measuring an Eb/IO (desired reception wave power/interference wave power) ratio; reverse link transmission power outer loop control means (35) for measuring a frame error rate of reverse link radio frames received by said reception means, and setting a reference Eb/IO ratio serving as a frame error rate for obtaining predetermined service quality; reverse link transmission power control bit determination means (32) for generating a reverse link transmission power control bit representing an instruction to decrease a transmission output of said mobile station when a measured Eb/IO ratio of radio frame is not less than the set reference Eb/IO ratio, and generating a reverse link transmission power control bit representing an instruction to increase the transmission output of said mobile station when a measured Eb/IO ratio of a radio frame is less than the set reference Eb/IO ratio; transmission means (31) for inserting a reverse link transmission power control bit output from said reverse link transmission power control bit determination means into a transmission frame; and control means (34) for determining that a reverse link interference amount between communications is not less than the allowable value, outputting to said reverse link transmission power control bit determination means an instruction to invalidate the reference Eb/IO ratio, in case that the number of mobile stations in the process of communication is not less than a predetermined number, and a ratio of increase instructions to reverse link transmission power control bits to said mobile stations in the process of communication is not less than a predetermined ratio, performing alarm notification to notify said mobile communication network that the reverse link interference amount is not less than the allowable value, and outputting an origination/termination restriction signal to said transmission means in accordance with an origination/termination restriction instruction from said mobile communication network that responds to the alarm notification, and said transmission means transmits data containing an origination/termination restriction signal from said control means to said mobile station. A system according to claim 12, characterized in that said control means instructs said reverse link transmission power control bit determination means to decrease the reference Eb/IO ration until a ratio of increase instructions to reverse link transmission power control bits becomes less than the predetermined ratio when the ratio of instructions to the reverse link transmission power control bits is not less than the predetermined ratio, gradually increases the reference Eb/IO ratio in a range in which the ratio of increase instructions to the reverse link transmission power control bits is less than the predetermined ratio when the ratio of increase instructions to the reverse link transmission power control bits becomes less than the predetermined ratio, and determines that the reverse link interference amount has fallen within the allowable value when the reference Eb/IO ratio reaches the reference Eb/IO ratio preceding transmission output restriction, and the ratio of increase instructions to the reverse link transmission power control bits becomes less than the predetermined ratio. A system according to claim 13, characterized in that said mobile communication network outputs an origination/termination restriction instruction to said base station from which the alarm notification is performed and an adjacent base station in accordance with the alarm notification from said base station, and outputs an origination/termination restriction cancel instruction to said base station from which the alarm cancel notification is performed and said adjacent base stations in accordance with the alarm cancel notification from said base station, said transmission means transmits an origination restriction signal and an origination restriction cancel signal to said mobile station in accordance with instructions from said mobile communication network, and said control means outputs an origination/termination restriction cancel signal to said transmission means in accordance with the origination/termination restriction cancel instruction from said mobile communication network, and performs alarm cancel notification to said mobile communication network when determining that the reverse link interference amount has fallen within the allowable value.

Inventors: KONDO TAKAYUKI, KONDO, TAKAYUKI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1999
Main classification: H04B7
IPC codes: H04B7, H04J13, H04Q7