Abstract:
Provided is a spread spectrum communication system in which the data rate can be varied through a relatively simple constitution. A slave station estimates the distance to a master station by using the I and Q signals generated by carrier demodulation, selects the code of a code length associated with the estimated distance from the previously held codes, performs spreading/despreading using the same, and performs transmission/reception. The master station performs spreading/despreading using a code having the same code length as that of the code selected by the slave station.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National-Stage entry under 35 U.S.C. § 371 based on International Application No. PCT/JP2005/020046, filed Oct. 26, 2005, which was published under PCT Article 21(2) and which claims priority to Japanese Application No. JP 2004-317046, filed Oct. 29, 2004. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a spread spectrum communication system and its components. In particular, the invention relates to a system that makes the data rate variable through a simple method. 
       BACKGROUND 
       [0003]    Many of new automobile models manufactured in recent years are equipped with remote keyless entry (RKE) systems. Remote keyless entry systems are systems for unlocking/locking a vehicle&#39;s door or trunk without the driver inserting the key to the keyhole. Some of remote keyless entry systems have a function of starting the engine through remote control and locating where the vehicle is parked. 
         [0004]    While laws allow remote keyless entry systems to use only limited frequency bands, people wish to combine various applications with remote keyless entry systems. One possible way to fulfill them both is to employ a spread spectrum communication system for communications between a master station and a slave station in a remote keyless entry system. 
         [0005]    However, conventional spread spectrum communication systems have been developed with a certain application in mind which assigns a relatively large service area to a master station and makes the master station transmit/receive audio signals carrying a large amount of information in the service area. Unless some modifications are made, conventional spread spectrum communication systems are not suitable for remote keyless entry systems and other applications in which the distance between a master station and a slave station is relatively short and the data amount is relatively small. 
         [0006]    In addition, slave stations in remote keyless entry systems take the form of a key fob type or card type remote controller, which puts a size limitation on the slave stations and accordingly limits the battery capacity to a certain degree. Therefore, reduction of power consumption constitutes a large proportion of requests made to the systems. These problems are not confined to remote keyless entry systems, but are common to any systems that transmit/receive low-power radio waves. 
         [0007]    In many various applications, it is considered that a slave station located close to a master station demands the master station to respond immediately whereas an immediate response is not often requested when a slave station is distant from a master station. For example, in a remote keyless entry system that incorporates an engine starter function as an additional application in a key fob type remote controller, it is unlikely that the master station&#39;s not-so-immediate response to the operation of the slave station raises a problem when the driver operates the controller from some distance from the vehicle to start the engine or to unlock/lock a door, whereas the master station is required to respond immediately when the driver operates the controller to unlock/lock a door at a short distance from the vehicle. 
         [0008]    In the case where a master station is required to respond immediately, the master station needs to be on stand-by while building a link at a relatively quick data rate. When there is no demand for immediate response, the master station can afford to slow down the data rate in the stand-by state which is the foundation of immediate response, and the power consumption is thus reduced. On the other hand, the mechanism of making the data rate variable must not be too complicated since it is not practical to mount such an intricate mechanism to a slave station. 
         [0009]    An object of the present invention made in view of the above is therefore to provide a spread spectrum communication system capable of varying the data rate with a simpler mechanism and components of the system. 
       SUMMARY 
       [0010]    The present invention provides a wireless unit for use in a spread spectrum communication system, including: a reception processing section which performs reception processing on a signal sent from a counterpart wireless unit (or an other-end wireless unit) to create reception data; a variable spread code creating section which monitors the reception processing in the reception processing section to identify a distance between this wireless unit and the counterpart wireless unit, and which creates a spread code variably in accordance with the identified distance; and a transmission processing section which performs transmission processing including spreading processing that uses the spread code created by the variable spread code creating section, in which the variable spread code creating section creates the spread code variably from one code chosen in accordance with the distance out of multiple codes having different code lengths, and from a fixed chip rate clock. 
         [0011]    The present invention provides a unique effect in that a spread spectrum communication system having a wireless unit as a slave station can change the data rate with a simple mechanism in which the slave station chooses a code whose code length is associated with the distance from a master station and the chosen code is converted into a spread code based on a chip rate clock of fixed rate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
           [0013]      FIG. 1  is a schematic block diagram showing the configuration of a slave station wireless unit in a spread spectrum communication system according to an embodiment of the present invention. 
           [0014]      FIG. 2  is a schematic block diagram showing the configuration of a master station wireless unit in the spread spectrum communication system according to the embodiment of the present invention. 
           [0015]      FIG. 3  is a table showing the relation between received signal levels, data rates, and spread codes. 
           [0016]      FIG. 4  is a state flow chart showing states the slave station wireless unit and the master station wireless unit in the spread spectrum communication system according to the embodiment of the present invention can be in, and transitions between the states. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
         [0018]    A spread spectrum communication system according to this embodiment has a wireless unit that is shown in  FIG. 1  as a slave station and a wireless unit that is shown in  FIG. 2  as a master station. Hereinafter, the wireless unit that is shown in  FIG. 1  and the wireless unit that is shown in  FIG. 2  are referred to as a slave station wireless unit  100  and a master station wireless unit  200 , respectively. 
         [0019]    For better understanding of items unique to the embodiment of the present invention, elements that are usually included in spread spectrum communication but are not directly relevant to processing unique to the embodiment of the present invention are omitted from  FIGS. 1 and 2 , which show the slave station wireless unit  100  and the master station wireless unit  200 , respectively, and only abbreviated descriptions will be given on such elements. 
         [0020]    The slave station wireless unit  100  has a reception processing unit  110 , which processes a received signal to create reception data, a transmission processing unit  120 , which performs transmission processing including spreading processing on transmission data to create and send a transmission signal, and a variable spread code creating section  130 , which creates a spread code that is used for the spreading processing in the transmission processing section  120  and that has a data rate associated with the distance between the master station wireless unit  200  and the slave station wireless unit  100 . 
         [0021]    The reception processing section  110  performs, among other types of processing, wireless processing in which received signals are filtered if necessary and a reception IF signal is created with the use of a frequency oscillated by a local oscillator, A/D conversion processing in which the reception IF signal is put through A/D conversion, carrier demodulation processing  111  in which carrier demodulation is performed with the use of carrier data, inverse spreading processing  112  in which an I signal (real part) and a Q signal (imaginary component), which are obtained through the carrier demodulation, are each spread inversely, and primary demodulation processing in which the signals obtained through the inverse spreading receive primary demodulation. BPSK demodulation is given as an example of primary demodulation processing. Needless to say, the signals obtained through the inverse spreading are detected when BPSK demodulation is employed. 
         [0022]    The transmission processing section  120  performs, among other types of processing, primary modulation processing in which primary modulation is performed on transmission data, spreading processing  122  in which the transmission data that has received primary modulation is spread with the use of a spread code, carrier modulation processing  123  in which carrier modulation is performed with the use of carrier data, D/A conversion processing in which the data that has received the carrier modulation is put through D/A conversion to create a transmission IF signal, and wireless processing in which a transmission signal is created by mixing the transmission IF signal with a local oscillation frequency through mixing processing or amplifying the transmission IF signal through signal amplification, and then sent to the master station wireless unit  200 . The primary modulation processing is in concert with the primary demodulation processing. For instance, when the employed primary demodulation processing is BPSK demodulation, it is basically BPSK modulation that is employed as the primary modulation processing. The primary modulation processing may be executed after the spreading processing  122 . 
         [0023]    The variable spread code creating section  130  in this embodiment has a distance calculating section  131 , which calculates/identifies the distance between the master station wireless unit  200  and the slave station wireless unit  100 . A characteristic of the distance calculating section  131  in this embodiment is that, instead of calculating/identifying the actual distance, the section  131  calculates, as the signal level of a received signal, the vectorial sum of an I signal and a Q signal which are obtained as a result of the carrier demodulation processing  111  in the reception processing section  110  (√(I2+Q2): I/Q amplitude). The received signal level (I/Q amplitude) is smaller when the distance is longer and larger when the distance is shorter. In effect, the distance can be estimated with precision by calculating the received signal level. 
         [0024]    The variable spread code creating section  130  has a data rate table  132 , which holds a distance (received signal level) and a data rate in association with each other. More specifically, for instance, the data rate table  132  holds, as shown in  FIG. 3 , a data rate in association with a threshold value (a 1 , a 2 , a 3  in  FIG. 3 ) bordering between ranges that are obtained by partitioning the I/Q amplitude.  FIG. 3  shows four types of data rate: an ultra high-speed mode, which is associated with a very short distance, a high-speed mode, which is associated with a short distance, an intermediate mode, which is associated with an intermediate distance, and a slow mode, which is associated with a long distance. A characteristic of the data rate table  132  in this embodiment is that threshold values of the I/Q amplitude are associated with, instead of the actual data rates, codes (α, β, γ, δ in  FIG. 3 ) of code lengths associated with the respective data rates. In short, the data rate table  132  of this embodiment indirectly associates a distance with a data rate. The code α has the longest code length, and the code lengths of the codes β, γ, and δ are progressively shorter in the order stated. The data rate table  132  thus associates, albeit indirectly, a distance with a data rate in a manner that makes the data rate faster as the distance becomes shorter. 
         [0025]    The variable spread code creating section  130  also has a data rate determining section  133  and a spread code creating section  134 . The data rate determining section  133  searches the data rate table  132  using a received signal level (I/Q amplitude) which is calculated in the distance calculating section  131 , and thus determines an appropriate data rate. Due to the data configuration of the data rate table  132 , the data rate determining section  133  in this embodiment decides on one of codes (α, β, γ, and δ in  FIG. 3 ) that has a code length associated with the appropriate data rate instead of determining an actual data rate. More specifically, the data rate determining section  133  sequentially compares a received signal level which is calculated in the distance calculating section  131  against I/Q amplitude threshold values stored in the data rate table  132  (a 1 , a 2 , and a 3  in  FIG. 3 ), thereby determining which data rate (code) to choose. 
         [0026]    The spread code creating section  134  creates a spread code that is used in the spreading processing  122  from a data rate (code) determined in the data rate determining section  133  and a chip rate (fixed length). The spread code creating section  134  in this embodiment is a shift register constituted of a code that the data rate determining section  133  outputs and a chip rate clock. 
         [0027]    In this manner, the variable spread code creating section  130  creates variable spread codes based on codes that have code lengths each associated with a specific distance and fixed chip rate clocks. 
         [0028]    Furthermore, a command creating section  121 , which monitors outputs of the data rate determining section  133 , is provided in the transmission processing section  120  in this embodiment. When the data rate is to be changed in the data rate determining section  133 , the command creating section  121  creates a data rate changing command with which the master station wireless unit  200  is requested to switch to a new data rate. The created command receives the same processing that is used to process normal transmission data, including modulation, before sent to the master station wireless unit  200 . The transmission processing section  120  containing the command creating section  121  operates, in this sense, as a data rate notifying measure for notifying the master station wireless unit  200  of a data rate that is determined in the data rate determining section  133 . 
         [0029]    The master station wireless unit  200  has, as shown in  FIG. 2 , a reception processing section  210 , which processes a received signal to create reception data, a transmission processing section  220 , which performs transmission processing including spreading processing on transmission data to create and send a transmission signal, and a variable spread code creating section  230 , which creates a spread code that is used for the spreading processing in the transmission processing section  220  and that has a data rate determined by the slave station wireless unit  100 . 
         [0030]    The reception processing section  210  is similar to the reception processing section  110  of the slave station wireless unit  100  and performs, among other types of processing, wireless processing in which received signals are filtered if necessary and a reception IF signal is created with the use of a frequency oscillated by a local oscillator, A/D conversion processing in which the reception IF signal is put through A/D conversion, carrier demodulation processing  211  in which carrier demodulation is performed with the use of carrier data, inverse spreading processing  212  in which an I signal (real part component) and a Q signal (imaginary component), which are obtained through the carrier demodulation, are each spread inversely, and reception data creating processing  213  in which the signals obtained through the inverse spreading receive primary demodulation to create reception data. 
         [0031]    The transmission processing section  220  is similar to the transmission processing section  120  of the slave station wireless unit  100  and performs, among other types of processing, primary modulation processing in which primary modulation is performed on transmission data, spreading processing  221  in which the transmission data that has received primary modulation is spread with the use of a spread code, carrier modulation processing  222  in which carrier modulation is performed with the use of carrier data, D/A conversion processing in which the data that has received the carrier modulation is put through D/A conversion to create a transmission IF signal, and wireless processing in which a transmission signal is created by mixing the transmission IF signal with a local oscillation frequency through mixing processing or amplifying the transmission IF signal through signal amplification, and then sent to the master station wireless unit  200 . 
         [0032]    The variable spread code creating section  230  creates a spread code of a data rate corresponding to a data rate notified from the slave station wireless unit  100 , and outputs the spread code to the spreading processing  221 . 
         [0033]    To give a more detailed description, the variable spread code creating section  230  has a command detecting section  231 , which monitors reception data as an output of the reception data creating processing  213  and, when the reception data contains a command, detects the command. A characteristic of the command detecting section  231  in this embodiment is that the command detecting section  231  detects a data rate changing command created in the command creating section  121  of the slave station wireless unit  100  and outputs a new data rate specified in the data rate changing command to a code selecting section  232 . 
         [0034]    The variable spread code creating section  230  has a data rate table  233  similar to the data rate table  132  of the slave station wireless unit  100 . Specifically, the data rate table  233  holds different data rates in association with different codes. To take  FIG. 3  as an example, the data rate table  233  holds four types of data rate, an ultra high-speed mode, which is associated with a very short distance, a high-speed mode, which is associated with a short distance, an intermediate mode, which is associated with an intermediate distance, and a slow mode, which is associated with a long distance, in association with codes (A, B, Γ, Δ), respectively, having different code lengths. As to the codes (A, B, Γ, Δ) stored in the data rate table  233 , the contents of those codes substantially differ from those of the codes (α, β, γ, δ) stored in the data table  132 , however, the codes (A, B, Γ, Δ) have the same code lengths as the codes α, β, γ, δ, respectively. Accordingly, the code A has the longest code length and the code lengths of the codes B, Γ, and Δ are progressively shorter in the order stated. 
         [0035]    The code selecting section  232  receives a new data rate from the command detecting section  231 , and consults the data rate table  233  to choose one of the master station side codes (A, B, Γ, Δ) that is associated with the new data rate. The code selecting section  232  outputs the chosen code to the spread code creating section  234 , which creates a spread code in the same way as the spread code creating section  134  of the slave station wireless unit  100  does, and delivers the spread code to the spreading processing  221  of the transmission processing section  220 . The master station wireless unit  200  thus forms a link at a data rate consistent with a data rate in the slave station wireless unit  100 . 
         [0036]    Now, the spread spectrum communication system according to this embodiment is described with reference to a state flow shown in  FIG. 4 . 
         [0037]    When the system in a power off state (S 1 ) is powered on, synchronization establishing processing is executed (S 2 ). Specifically, the slave station wireless unit  100  intermittently sends a pilot signal that is associated with the code α for the slow mode to the master station wireless unit  200 , and at the same time waits for a call signal from the master station wireless unit  200  by voluntarily opening a reception window. The master station wireless unit  200  enters a successive reception state when the power is turned on in order to capture the pilot signals from the slave station wireless unit  100 . Receiving the pilot signals from the slave station wireless unit  100 , the master station wireless unit  200  sends a call signal that is associated with the code B to the slave station wireless unit  100 . The call signal sent by the master station wireless unit  200  is associated with the code B, which has a shorter code length, instead of the code A, which has the same code length as the code α, because, in remote keyless entry systems, which are one of possible applications of this system, and the like, the antenna sensitivity in the slave station wireless unit  100  is generally likely to be inferior to the antenna sensitivity in the master station wireless unit  200 . This system takes the difference in antenna sensitivity into consideration and makes the master station wireless unit  200  recognize the slave station wireless unit  100  at an early point, thereby enabling the master station wireless unit  200  to properly manage the subsequent processing. 
         [0038]    Reception of the call signal associated with the code B by the slave station wireless unit  100  establishes the synchronization, and the system proceeds to the next state, handshake processing (S 3 ). Note that, while the synchronization establishing processing (S 2 ) is accomplished upon reception of the call signal, the slave station wireless unit  100  cannot judge at this point whether the master station wireless unit  200  that has sent the call signal is its own master or not. In other words, the slave station wireless unit  100  accepts a call signal from any wireless unit and establishes synchronization once. 
         [0039]    Receiving the call signal, the slave station wireless unit  100  uses the code β to send its own ID to the master station wireless unit  200 , and waits for the master station wireless unit  200  to send an ID in response. The master station wireless unit  200  checks the ID received from the slave station wireless unit  100 . After confirming the slave station wireless unit  100  as its own slave, the master station wireless unit  200  uses the code B, which corresponds to the code β, to send its own ID (the ID of the master station wireless unit) to the slave station wireless unit  100 , and then waits for the slave station wireless unit  100  to notify a data rate. 
         [0040]    In the case where the wireless unit to which the slave station wireless unit  100  has sent its own ID is not the master of this slave station wireless unit  100 , the slave station wireless unit  100  does not receive an ID from the master station wireless unit  200  in response. The system anticipates such a case and makes the slave station wireless unit  100  wait for a given period of time for an ID from the master station wireless unit  200 , and thereafter, in the case where there is no ID sent in response from the master station wireless unit  200 , proceeds to the synchronization establishing processing (S 2 ). Similarly, in the case where an ID received from the master station wireless unit  200  in response is not the ID of the master of the slave station wireless unit  100 , the system moves to the synchronization establishing processing (S 2 ). 
         [0041]    On the other hand, in the case where the master station wireless unit  200  is judged from the received ID as the master of the slave station wireless unit  100 , the slave station wireless unit  100  calculates the received signal level to estimate the distance, determines an appropriate data rate, and chooses a code that is associated with the estimated distance. The determined data rate is subjected to spreading processing with the use of the code β, and then sent to the master station wireless unit  200  by the data rate notification measure described above, which completes the handshake processing on the side of the slave station wireless unit  100 . 
         [0042]    In the case where the slave station wireless unit  100  from which an ID has been received and to which the master station wireless unit  200  has sent its own ID in response is the slave of the master station wireless unit  200 , the master station wireless unit  200  can receive a data rate notification via the data rate notifying measure of the slave station wireless unit  100 . The reception of the data rate notification completes the handshake processing on the side of the master station wireless unit  200 . If a given period of time passes since the master station wireless unit  200  has sent its own ID to the slave station wireless unit  100  without the master station wireless unit  200  receiving a data rate notification, the master station wireless unit  200  determines that this slave station wireless unit  100  is not its own slave and moves to the synchronization establishing processing (S 2 ). 
         [0043]    After the handshake processing (S 3 ) is completed, the master station wireless unit  200  moves on to a standby state in accordance with the data rate determined by the slave station wireless unit  100  (S 4 -S 7 ). Standby states shown in  FIG. 4  correspond to the data rates shown in  FIG. 3 . In a very short distance standby state (S 4 ), a pair consisting of the code Δ and the code δ is chosen. In a short distance standby state (S 5 ), a pair consisting of the code Γ and the code γ is chosen. A pair consisting of the code B and the code β is chosen in an intermediate distance standby state (S 6 ). A pair consisting of the code A and the code α is chosen in a long distance standby state (S 7 ). In each of the standby states (S 4 -S 7 ), a link is maintained by sending and receiving pilot signals with the use of the chosen code pair. 
         [0044]    The slave station wireless unit  100  performs distance estimation and data rate determining processing through transmission and reception of pilot signals in the respective standby states (S 4 -S 7 ). If a change in location of the slave station wireless unit  100  during the respective standby states (S 4 -S 7 ) causes changes in the distance between the slave station wireless unit  100  and the master station wireless unit  200  with the result that a threshold value as those shown in  FIG. 3  is crossed, the data rate may have to be changed. In such cases, the slave station wireless unit  100  notifies a new data rate to the master station wireless unit  200  using the existing code. Since a link is thus maintained with the use of pilot signals before and after a standby state is entered, communications can be started at the optimum data rate in transmitting/receiving data (S 8 ). During the data transmission/reception, the slave station wireless unit  100  continues distance estimation and data rate determining processing with the use of transmitted/received data signals, and therefore suspends transmission/reception of pilot signals. Also, since the system does not stop changing the data rate in accordance with the distance during the data transmission/reception, the system does not always move back from the data transmission/reception state (S 8 ) to the same standby state as the previous standby state (S 4 -S 7 ). 
         [0045]    A state to which the system moves from one of the standby states (S 4 -S 7 ) is another of the standby states (S 4 -S 7 ) that is immediately next to the current standby state in the case where the distance between the slave station wireless unit  100  and the master station wireless unit  200  changes gradually. The system moves from one of the standby states (S 4 -S 7 ) to the data transmission/reception state (S 8 ) when communications are actually started. There is also a case in which the link between the master station wireless unit  200  and the slave station wireless unit  100  is disconnected forcibly when, for example, a user carrying the slave station wireless unit  100  steps into an elevator. In such cases, the system moves from one of the standby states (S 4 -S 7 ) to a sleep mode (S 9 ). When it is from the long distance standby state (S 7 ) that the system moves to the sleep mode (S 9 ), transmission/reception of pilot signals is continued for a given period of time since the exit of the slave station wireless unit  100  from the area of the master station wireless unit  200  may be merely temporary. This way, when the slave station wireless unit  100  reenters the area of the master station wireless unit  200 , the system can return to the long distance standby state (S 7 ). If the system fails to return to the long distance standby state (S 7 ) after the given period of time, the same procedure as when the system moves to the sleep mode (S 9 ) from other standby states (S 4 -S 6 ) is employed as will be described below. 
         [0046]    Once entering the sleep mode (S 9 ), the system does not return to the standby states (S 4 -S 7 ) directly, except for a case where it is from the long distance standby state (S 7 ) that the system has moved to the sleep mode (S 9 ) as described above. To return to the standby states, the system again performs the synchronization establishing processing in S 2 . The synchronization timing may be held for a given period of time in the sleep mode (S 9 ) for smooth execution of the synchronization establishing processing (S 2 ). As shown in  FIG. 4 , the system may automatically move to a power save mode (S 10 ) after staying in the sleep mode (S 9 ) for a given period of time. The power save mode (S 10 ) corresponds to, for example, in a remote keyless entry system that employs this system, a case where a driver does not drive the car (does not come near the car) for days. In this state (S 10 ), the master station wireless unit  200  suspends reception whereas the slave station wireless unit  100  suspends transmission. The transition from the power save mode (S 10 ) to the synchronization establishing processing (S 2 ) may be induced by, for example, resetting the hardware. 
         [0047]    The present invention is applicable not only to remote keyless entry systems but also to systems that transmit/receive low power radio waves such as short-distance wireless control systems and telemetering systems. 
         [0048]    While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims and their legal equivalents.