Abstract:
A spread spectrum communication system includes a plurality of cells which share common frequencies. Each base station managing each cell includes transmitters for transmitting a reception load state of the base station to other base stations managing other cells. Receivers for receiving reception load states of the other base stations and transmitters for transmitting a signal having control information interpolated therein to each mobile station within the cell under management are included in the system. The control information is used for controlling a transmission station of each mobile station. A second receiver for receiving a signal from each mobile station within the cell under management and a power control are included for determining the control information for each mobile station so as not to interfere with operations of the other base stations managing the other cells with reference to a reception level index of the signal from each mobile station and the reception load states of the other base stations managing the other cells.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a spread spectrum communication system divided into a plurality of cells, such as a CDMA (Code Division Multiple Access) system. 
     2. Description of the Prior Art 
     A conventional example of a power controller for use in the spread spectrum CDMA radio communication system varies a process gain in accordance with a location in a cell where a mobile station exists as disclosed in JPA 9-284212. 
     Process gain PG is derived from spread rate C (chip/sec)=1/Tc which is the reciprocal of 1 chip time Tc of Pseudo Noise series and data transmission rate D (bps), and is represented as PG=C/D=TcD (chip/bit). Increasing spread rate C of direct spread reduces a power spectral density and expands a spread bandwidth. If data transmission rate D is constant, process gain PG increases characteristically when spread rate C increases. 
     Therefore, if spread rate C is increased while sustaining a constant transmission rate of a mobile station that is located close to a certain base station relative to a mobile station that is located remotely, the process gain for the closely located mobile station increases and the power spectral density of the signal received at the base station from the closely located mobile station decreases so that the power spectral densities received at the base station from the respective mobile stations become substantially equal level. 
     On the other hand, if the data transmission rate is decreased while sustaining a constant chip rate of the mobile station that is located remotely from the base station relative to the mobile station that is located closely, the process gain increases and an S/N ratio (Signal-to-Noise Ratio) increases. 
     Thus, such distance problem existing among the mobile stations located close to the station and those located remotely from the base station can be solved, and in addition, interference to other stations can be suppressed. 
     FIG. 1 is a block diagram showing an arrangement of the above spread spectrum communication system. FIG. 1 shows a transmitter/receiver section, which includes SS (Spread Spectrum) transmitter  1  and SS receiver  2 , provided in a base station and each mobile station. In SS transmitter  1 , a signal containing audio data, information data and image data is primarily modulated by a data clock supplied from data clock generator  3  at information transmitter  4  so as to be another signal having a predetermined data transmission rate D. Thus generated signal is fed to next spreading modulator  5 . 
     PN clock is fed to PN generator  6  from PN clock generator  7 . PN generator  6  generates PN signal  8  with a predetermined spread rate C accordingly. The PN signal is fed to spreading modulator  5 . 
     The signal from information transmitter  4  is directly spread by PN signal  8  at spreading modulator  5 . The directly spread signal (hereinafter, referred to as an SS signal)  9  is converted into the SS signal having a radio frequency by frequency converter  10 , then amplified by power amplifier  11 , and transmitted from antenna  12 . 
     On the other hand, in SS receiver  2 , the SS signal received at antenna  13  is amplified by amplifier  14 , and then is converted into the SS signal having an intermediate frequency or baseband frequency. It is subsequently synchronized correlatively at correlation unit  16 , and then demodulated into the original signal by information demodulator  18 . 
     In the above configuration, data transmission rate D is controlled by varying a clock speed of data clock generator  3 , and spread rate C is controlled by varying a clock speed of PN clock generator  7 . Thus, the process gain can be controlled by controlling these clock speeds. 
     The above prior art, however, includes a disadvantage that increasing spread rate C of the closer mobile station widen a frequency bandwidth and reduces frequency usage efficiency. There is also a disadvantage that a circuit scale of controller for increasing spread rate C becomes large, and especially it becomes troublesome for the mobile station. 
     SUMMARY OF THE INVENTION 
     In order to overcome the aforementioned disadvantages, the present invention has been made and accordingly, has an object to provide a spread spectrum communication system divided into a plurality of cells (for example, the CDMA (Code Division Multiple Access) system) in which communications in one cell does not communications in adjacent cells. 
     According to an aspect of the present invention, there is provided a spread spectrum communication system comprising a plurality of cells which share common frequencies, wherein each base station managing each cell comprises: a first transmitting means for transmitting a reception load state of the own base station to other base stations managing other cells; a first receiving means for receiving reception load states of the other base stations from the other base stations managing the other cells; a second transmitting means for transmitting a signal having control information interpolated therein to each mobile station within the cell under management of the own base station, the control information being used for controlling a transmission state of the each mobile station; a second receiving means for receiving a signal from the each mobile station within the cell under management of own base station; and a power control means for determining the control information for the each mobile station so as not to interfere with operations of the other base stations managing the other cells with reference to a reception level index of the signal from the each mobile station and the reception load states of the other base stations managing the other cells. 
     In the spread spectrum communication system, the control information may be set to special information for lowering or cutting off a transmission power of the each mobile station if it is estimated that the transmission power of the each mobile station for gaining a desired value of the reception level index of the signal from the each mobile station interferes with the operations of the other base stations managing the other cells. 
     In the spread spectrum communication system, the estimation of interference may be executed with reference to the reception load states of the other base stations managing the other cells. 
     In the spread spectrum communication system, the estimation of interference may be executed in consideration of a relative relationship between a location of the each mobile station and locations of the other base stations managing the other cells, the location of the each mobile station being computed on the basis of the control information and the signal from each mobile station. 
     In the spread spectrum communication system, the special information may comprise such information as halts a transmission operation of the mobile station. 
     In the spread spectrum communication system, the special information may comprise such information as lowers the transmission power of the mobile station. 
     In the spread spectrum communication system, the special information may comprise such information as decreases a transmission rate of the mobile station. 
     In the spread spectrum communication system, contents of the control information sent to the each mobile station may be decided so as to gain a desired value of the reception level index of the signal from each mobile station if it is not estimated that the transmission power of the each mobile station for gaining the desired value of the reception level index of the signal from the each mobile station interferes with the operations of the the other base stations managing the other cells. 
     In the spread spectrum communication system, contents of the control information sent to the each mobile station may be decided so as to gain a desired value of the reception level index of the signal from each mobile station if a transmission power of the each mobile station for gaining the desired value of the reception level index of the signal from the each mobile station is less than a certain upper limit value. 
     In the spread spectrum communication system, the certain upper limit value may be determined in accordance with distances of the other base stations managing the other cells from the own base station. 
     In the spread spectrum communication system, the first transmitting means may transmit the reception load state of the own base station during the first receiving means is not receiving the reception load states from the other base stations managing the other cells. 
     In the spread spectrum communication system, the first transmitting means may transmit the reception load state of the own base station in time-division together with the first receiving means of the other base stations managing the other cells. 
     In the spread spectrum communication system, the power control means may determine the control information of the each mobile station dependently on every area formed by dividing the cell under management of the own base station. 
     Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood from the following detailed explanation with reference to the accompanying drawings in which: 
     FIG. 1 a block diagram showing a configuration of a transmitter and receiver at a base station and a mobile station in a conventional spread spectrum communication system; 
     FIG. 2A is a block diagram showing a configuration of a base station managing each cell in a spread spectrum communication system according to an embodiment of the present invention; 
     FIG. 2B is a block diagram showing a configuration of a mobile station in the spread spectrum communication system according to the embodiment of the present invention; 
     FIG. 3A is a flowchart explaining operations of the base station shown in FIG. 2A; 
     FIG. 3B is a flowchart explaining operations of the mobile station shown in FIG. 2B; 
     FIG. 4 is a flowchart explaining operations of a reception state measuring unit  25  of FIG. 2A; and 
     FIG. 5 is an arrangement example of a cell, a base station and mobile stations in the spread spectrum communication system according to the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be explained in detail with reference to the drawings. 
     Embodiment 1 
     Each cell in a spread spectrum communication system according to an embodiment of the present invention includes a base station shown in FIG.  2 A and mobile stations shown in FIG.  2 B. 
     Referring to FIG. 2A, antenna  21 , which serves as both a transmitting antenna and receiving antenna, transmits and receives a radio wave. 
     In the receiver side, an electric signal changed from the radio wave which is received at antenna  21  is amplified and frequency-converted into a baseband signal by receiver  22 , and then supplied to despreading circuits  23  prepared for respective mobile stations and despreading circuits  24  for other base station channels. Despreading circuit  23  despreads an input signal by a spread code to obtain received data  41 -i (i=1, 2, . . . , n) for each channel by multiplying the input signal by the same spread code as that in the transmitting side. Despreading circuit  24  also performs despreading similarly. The data after despread at despreading circuit  24  comprises base station information which is obtained from other base station. The base station information includes various information such as information relating to the received load state of other base station (information relating to a desired received signal power versus interference wave power ratio SIR (Signal-to-Interference Ratio) of the signal received from individual mobile stations, and whether the number of mobile stations in the cell is within a range capable of obtaining a certain SIR, for example). They also include information relating to the transmission power of the base station. Reception state measuring unit  25  controls the transmission power of each mobile station based on the power values of the received data and the received states of other base stations. 
     In the transmitter side, transmission data  42 -i (i=1, 2, . . . , n) for each channel is added with transmission state control information at transmission power controller  30  and is sent to spreading circuit  28 . Base station information  43  is added at any time, at transmission information generator  31 , with the reception load state which is obtained by reception state measuring unit  25 , and is sent to spreading circuit  29 . The data spread by each of spreading circuits  28  and  29  is synthesized by multiplexer  27  and is converted into a radio frequency at transmitter  26 . It is then transmitted from antenna  21  as a radio wave. 
     Referring to FIG. 2B, in the mobile station, antenna  32  serves as both a transmitting antenna and receiving antenna similarly to the base station. An electric signal changed from a radio wave that is received at antenna  32  is frequency-converted into a baseband signal by receiver  33  and supplied to despreading circuit  34 . Despreading circuit  34  performs despreading to obtain a received data by multiplying the frequency-converted baseband signal by the same spread code as was used at the time of spreading. The transmission state control information contained in the received data is extracted to determine the transmission power of the mobile station by transmission state control information processor  35 . In the transmitter at the mobile station, transmission power controller  38  determines the transmission power of a transmission data based on information obtained from transmission state control information processor  35 , and thereafter spreading circuit  37  performs spreading of the transmission data. After converted into a radio wave frequency at transmitter  36 , the transmission data is transmitted from antenna  32  as a radio wave. 
     Operations of the embodiment of the present invention will be explained next with reference to the drawings. 
     An outline of the operations will be explained first with reference to a flowchart of FIGS. 3A,  3 B. FIG. 3A is a flowchart showing operation of the base station and FIG. 3B the mobile station. For simplifying explanation, only two cells, cell #A and cell #B, are present as shown in FIG.  5 . Cell #A includes base station BS_A and cell #B includes base station BS_B. Base station BS_A is called as the concerned base station and base station BS_B is called as another base station. Mobile stations MS_A 1  and MS_A 2  belong to base station BS_A. Mobile station MS_A 2  is located apart from base station BS_B while mobile station MS_A 1  is located close to base station BS_B. 
     Cell #A and cell #B utilize a common frequency in FIG. 5 while communications between the base station and mobile stations are performed individually. 
     In the receiver side of base station BS_A (S 1 ), the following operation is executed. First, a signal from base station BS_B is searched by use of despreading circuit  24 . Each of base stations is identified with respective different spread code. However, it was difficult to receive signals from other stations using the common frequency because a pilot channel for transmitting a reference signal and a common control channel are always used for transmission in the prior art. This is solved in this embodiment by performing intermittent transmissions on these channels or by performing time-divisional transmissions together with other base stations. In the intermittent transmission, transmission to other base stations is preformed when signals from other stations are not received. In the time-divisional transmission, assigning an orthogonal code that has an identical length and exhibits a different value to each base station, respectively. This enables to receive the signals from other stations with the common frequency by transmitting when the value of the code is “0” and receiving when it is not “0”. Each base station establishes a channel for broadcasting it&#39;s station information to other base stations during a non-receiving duration between the searches, and transmits base station information including it&#39;s base station ID and reception load state constructed by transmission information generator  31 . The spread base station information data of base station BS_B which has been searched at step S 1  is demodulated at step S 2  by using despreading circuit  24  for despreading it. 
     SS signal received from each of mobile stations MS_A 1  and MS_A 2  which belong to base station BS_A in cell #A is demodulated by use of despreading circuit  23 , and a reception level index of the SS signal, which is sent from each of mobile stations MS_A 1  and MS_A 2  and received at base station BS_A, is measured at step S 3  simultaneously with steps S 1  and S 2 . Because the desired received signal power versus interference wave power ratio, SIR (Signal-to-Interference Ratio), can be detected for every mobile station, it is appropriate to used SIR as the reception level index. 
     Reception state measuring unit  25  computes the transmission power of each mobile station at step S 4  based on the reception load state of base station BS_B obtained at step S 2  and reception level index of each mobile station obtained at step S 3  so as to reduce the affection to base station BS_B as well as to maintain stable communicate with mobile stations MS_A 1  and MS_A 2  belonging to base station BS_A. It also generates the transmission state control information containing information related to the transmission power. This method will be explained in more detail later. 
     Transmission power controller  30  interpolates the transmission state control information computed at step S 4  into a down stream signal, which is transmitted to each mobile station from antenna  21  through multiplexer  27  and transmitter  26  (S 5 ). 
     In mobile stations MS_A 1  and MS_A 2 , antenna  32  converts the radio wave from base station BS_A into an electric signal and outputs it. Receiver  33  amplifies, frequency-converts this signal and outputs it. Despreading circuit  34  receives this signal and obtains the transmission state control information contained in the down stream signal by despreading (S 10 ). Despreading circuit  34  obtains received data  44  simultaneously and outputs it to a data processor not depicted. At step S 11 , transmission power controller  38  determines a power of the up stream signal including transmission data to base station BS_A by use of the transmission state control information processed at transmission state control information processor  35 . At step S 12 , the output of transmission power controller  38  becomes the SS signal by spreading at spreading circuit  37 , then is frequency-converted and amplified in transmitter  36 , and the output of transmitter  36  is transmitted from antenna  32 . 
     A power control method for each of mobile stations MS_A 1  and MS_A 2  performed in reception state measuring unit  25  of base station BS_A will be explained in detail next with reference to a flowchart of FIG.  4 . Each of base stations BS_A and BS_B transmits the base station information at an identical transmission power. Therefore, the distance between base station BS_A and station BS_B can be measured on the basis of the power attenuation of a signal received from base station BS_B in the adjacent cell by base station BS_A. If each base station synchronizes with each other, the distance can be also measured on the basis of a time delay. Base station BS_A decides the maximum control transmission power P_MAX for each mobile station, which does not give an affection that exceeds a certain degree to the system of base station BS_B, in consideration of the distance between base stations BA_A and BS_B (S 19 ). 
     At step S 20 , an SIR of a signal from a mobile station (for example, MS_A 1 ) is measured. Then, transmission power P_MS_A 1  of mobile station MS_A 1  which ensures the SIR of the signal received from mobile station MS_A 1  be a desired value based on the measured SIR. Thereafter, it is determined whether transmission power P_MS_A 1  is lower than the maximum control transmission power P_MAX. If the determined result is Yes, then the flow goes to step S 21 . If the determined result is No, then the flow goes to step S 22 . At step S 21 , difference ΔP_MS_A 1  of the transmission power of mobile station MS_A 1  is determined so that MS_A 1 _SIR, which is the present SIR of the signal received from mobile station MS_A 1 , becomes a required SIR. Difference ΔP_MS_A 1  is represented by: 
     
       
         ΔP_MS_A 1 =the required SIR−MS_A 1 _SIR 
       
     
     If a transmission power higher than P_MAX is required because mobile station MS_A 1  is located far apart from base station BS_A, the following control is performed. 
     At step S 22 , it is determined whether transmission power P_MS_A 1  of mobile station MS_A 1  affects the operation of base station BS_B. If the determined result is Yes, then the flow goes to step S 24 . If the determined result is No, then the flow goes to step S 23 . At step S 23 , difference ΔP_MS_A 1  of the transmission power of mobile station MS_A 1  is determined so that MS_A 1 _SIR, which is the present SIR of the signal received from mobile station MS_A 1 , becomes a required SIR similarly to step S 21 . 
     The reception load state of base station BS_B is considered in the determination at step S 22 . In addition, the distance between mobile station MS_A 1  and base station BS_B is considered. The distance therebetween is determined by both the location (specified by an orientation and distance) of mobile station, which is calculated on the basis of the transmission power of mobile station MS_A 1  instructed by base station BS_A and a level and incoming direction of the signal received from mobile station MS_A 1 , and the location of base station BS_B, which is calculated similarly. If there is little margin in the reception load state of base station BS_B, the result of determination at step S 22  becomes Yes because mobile station MS_A 1  is located close to base station BS_B. If there is some or more margin in the reception load state of base station BS_B, the result of determination at step S 22  becomes No though mobile station MS_A 1  is located close to base station BS_B. Because mobile station MS_A 2  is located far apart from base station BS_B as compared with mobile station MS_A 1 , even if the distance between mobile station MS_A 1  and base station BS_A 1  and the distance between mobile station MS_A 2  and base station BS_A 1  are the same and the transmission power necessary to obtain a required SIR are the same for both of them, the threshold level of the reception load state of base station BS_B, at which the determined result at step S 22  changes, is not the same for base station BS_A 1  and base station BS_A 2 . That is, the threshold level of the reception load state for base station BS_A 2  is located at a less margin position than that for base station BS_A 1 . 
     The following operations are executed at step S 24 : 
     (a) cutting off the transmission operation of mobile station MS_A 1 ; 
     (b) lowering the transmission power of mobile station MS_A 1 ; and 
     (c) reducing the transmission data rate of mobile station MS_A 1 . The transmission power of mobile station MS_A 1  may be gradually lowered with a certain step or may be instantaneously lowered to power P_MAX that has been determined so as not to affect station BS_B. Similarly, the bit rate may be gradually reduced with a certain step or may be instantaneously multiplied with P_MAX/P_MS_A 1 . These three methods (a), (b) and (c) may be used independently, in combination, or selectively in accordance with progress of control. 
     In a case where base station BS_A is surrounded by a plurality of other base stations, the process shown in FIG. 4 is performed in consideration of each of all other base stations and the results thereof are used to determine the final transmission power of the mobile station. The final transmission power of the mobile station may be, for example, the minimum among the transmission powers each determined in consideration of each of all other base stations. 
     Embodiment 2 
     A second embodiment of the present invention will be explained with reference to the drawings. 
     Referring to FIG. 5, a cell covered by a system of each base station has a form of circle in the first embodiment. In the second embodiment, a directivity of antenna  21  at base station BS_A of FIG. 2 is divided so as to provide plural directivities in a form of a multi-beam antenna so as to cover the cell with plural systems. Thus, dividing the cell of one base station into plural areas increases a capacity per area and, because of the directivity of antenna, reduces interference from other cells. Therefore, there is provided a system in which influence from the mobile stations to other base stations is reduced. 
     Though SIR is used as the index of the reception level at the mobile station in the above embodiment, an error rate of data transmission or RSSI (Receive Signal Strength Indicator) may also be used as the index. Combination of SIR, the error rate, and RSSI may be used as the index. 
     Although the present invention has been shown and explained with respect to the preferred mode embodiments thereof, it should be understood by those skilled in the art that the forgoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the present invention.