Abstract:
It is an object of the present invention to implement stabilized wireless communications by establishing suitable channel switching conditions for a digital wireless communication device that performs wireless communications with a single base station. In order to achieve this object, the cordless phone child device according to the present invention comprises a wireless unit that sends and receives a wireless signal to and from the base station; a frame extractor that extracts frames from the wireless signal; a frame error rate computation unit that computes the frame error rate; a counter that counts the number of frames in which a frame error occurs and for which the received signal intensity exceeds a predetermined threshold value; and a channel switching judgment unit that judges that the channel switching conditions have been fulfilled when the frame error rate computed by the frame error rate computation unit is equal to or more than a predetermined threshold value and the number of frames counted by the counter is equal to or more than a predetermined threshold value.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a channel switching technology for a digital wireless communication device that performs wireless communications with a single base station.  
         [0003]     2. Description of the Related Art  
         [0004]     A PHS (Personal Handy phone System) is a system for developing digital cordless phones in which wireless interfaces between a PHS terminal that corresponds to a child device and a base station that corresponds to a parent device are standardized as the ‘Second-generation cordless phone system standard (RCR STD-28)&#39;. As a countermeasure for interference during a call, this standard provides regulation such that the quality of the wireless line is measured by means of the FER (Frame Error Rate) and channel switching or other processing is then performed according to the degree of quality. Further, regulation is such that channel switching is executed if the RSSI (Received Signal Strength Indicator) level is equal to or less than a threshold value in a weak electrical field environment. On the other hand, in the case of a cordless phone that adopts the 2.4-GHz band DSSS (Direct Sequence Spread Spectrum) communication method, radio wave interference is sometimes received from a variety of interference sources such as a wireless LAN, microwave oven, and so forth that utilize the ISM (Industry Science Medical) band. Therefore, the channel must be suitably switched. As an example of related technology, Japanese Patent Application Laid Open No. 2000-92559 proposes a technology in which a base station performs channel switching because a mobile wireless terminal communicates used channel reception state information to the base station.  
         [0005]     However, in the case of a cordless phone, there is normally a single base station (parent device), and therefore channel switching renders reconnection impossible in a weak electrical field and might bring about deterioration in the conditions. Hence, channel switching has not been possible under the conditions of STD-28.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is accordingly tasked with implementing stabilized wireless communications by establishing suitable channel switching conditions for a digital wireless communication device that performs wireless communications with a single base station.  
         [0007]     In order to achieve this object, the digital wireless communication device according to the present invention is a digital wireless communication device that performs wireless communications with a single base station and performs channel switching when pre-established channel switching conditions are fulfilled, comprising: a wireless unit that sends and receives a wireless signal to and from the base station; a frame extractor that extracts frames from the wireless signal; a frame error rate computation unit that computes the frame error rate of the frames extracted by the frame extractor; a counter that counts the number of frames in which a frame error occurs among the frames extracted by the frame extractor and for which the received signal intensity exceeds a predetermined threshold value; and a channel switching judgment unit that judges that the channel switching conditions have been fulfilled when the frame error rate computed by the frame error rate computation unit is equal to or more than a predetermined threshold value and the number of frames counted by the counter is equal to or more than a predetermined threshold value. When there is a single base station, channel switching is not executed merely because the wireless environment is a weak electric field. Suitable channel switching can be performed by making a distinction as to whether the cause of a frame error generated under the condition that the received signal intensity should be equal to or more than a predetermined threshold value is due to interfering waves or generated unexpectedly as in the case of phasing, frequency hopping, or the like.  
         [0008]     The digital wireless communication device preferably further comprises a control unit that, when the channel switching conditions are fulfilled, switches the channel in sync with the base station on the basis of channel switching information that is shared beforehand with the base station. It is therefore possible to perform muteless channel switching.  
         [0009]     The digital wireless communication device desirably further comprises a measurement unit that measures the received signal intensity in a channel that is not being used for a connection with the base station among a plurality of channels, wherein the control unit sets reconnection channel priority levels in order starting with the highest priority level for the lowest received signal intensity measured by the measurement unit. Because the existence of interfering waves can be distinguished by means of the received signal intensity, it is possible to raise the call quality by setting reconnection channel priority levels in order starting with the highest priority level for the lowest received signal intensity. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  shows the constitution of the circuit of the cordless phone child device of this embodiment;  
         [0011]      FIG. 2  is a flowchart in which the channel switching judgment routine is described; and  
         [0012]      FIG. 3  is an explanatory view of the channel switching conditions.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     This embodiment will be described hereinbelow with reference to the drawings.  
         [0014]      FIG. 3  shows the channel switching conditions of this embodiment. In this figure, the RSSI-FER curve shows the relationship between the FER and RSSI in a wireless environment without interference. The channel switching conditions prescribed by the above-mentioned STD-28 are the locations denoted by area  1  and area  2 . Area  1  is independent of FER and prescribes channel switching under the condition that the RSSI should be equal to or less than a threshold value S 1 . Area  2  prescribes channel switching under the condition that the RSSI should be equal to or more than the threshold value S 1  and that the FER be equal to or more than the threshold value S 2 . In a digital wireless communication system that performs wireless communications via a single base station as in the case of a cordless phone, the following conditions must be fulfilled as channel switching conditions:  
         [0015]     (1) Channel switching should not be executed in a weak electric field;  
         [0016]     (2) Channel switching should not be executed by phasing;  
         [0017]     (3) Channel switching should not be executed with respect to frequency hopping interfering waves.  
         [0018]     For this reason, reconnection is difficult when channel switching is performed in a weak electric field. Further, this is because performing channel switching by means of phasing brings about an increase in the unnecessary mute interval, which is unpleasant for the caller. Further, the frequency of a frequency hopping interfering waves varies at very short intervals and the effect on the cordless phone is small. There is therefore no need to perform channel switching because of these interfering waves.  
         [0019]     In order to satisfy conditions (1) to (3), channel switching is required in area  2 . In order to execute channel switching in area  2 , it is necessary to distinguish whether or not the wireless environment is a weak electric field. A suitable distinction must also be made of whether the cause of the frame error is interfering waves or whether the frame error was generated unexpectedly as is true for phasing, frequency hopping, and so forth. The constitution of a digital wireless communication device for performing channel switching in area  2  (a cordless phone child device is illustrated here) will be described in detail hereinbelow.  
         [0020]      FIG. 1  shows the receiver circuit of a digital cordless phone child device  10  that performs wireless communications with a single parent device (base station). The child device  10  is constituted comprising a speech signal decoding system (an antenna  11 , wireless unit  12 , demodulator  13 , frame extractor  14 , and speech decoder  15 ); a frame error rate computation system (error detection unit  16 , error counter  17 , frame counter  18 , and FER computation unit  19 ) for computing the frame error rate; a RSSI count system (RSSI level judgment unit  20  and RSSI counter  21 ) for counting the number of frames for which a frame error is generated with an RSSI at or above a certain fixed level; a channel switching judgment unit  22  that judges channel switching; a control unit  23  that controls channel switching; and a storage unit  24  for sharing channel switching information with a parent device.  
         [0021]     After being amplified by the wireless unit  12 , the signal received via the antenna  11  is demodulated by the demodulator  13 . The demodulated signal undergoes signal processing by the frame extractor  14  and frames are extracted. After being subjected to speech signal processing to raise the acoustic characteristic above the output of the error detection unit  16  that performs frame error detection, the extracted frames are decoded in the speech decoder  15  and outputted as a speech signal. In addition, the frames extracted by means of the frame extractor  14  are checked for the presence of code errors by the error detection unit  16 . The error counter  17  counts the code errors when code errors are detected by the error detection unit  16 . The frame counter  18  counts the number of frames based on a frame interrupt signal. When the number of received frames reaches a prescribed number (several hundred frames, for example), the FER computation unit  19  computes the frame error rate on the basis of the number of errors counted by the error counter  17 .  
         [0022]     When a frame error is detected by the error detection unit  16 , same outputs an error detection signal to the RSSI level judgment unit  20 . The RSSI level judgment unit  20  judges whether the RSSI of the frame in which the frame error was detected is equal to or more than the threshold value S 1 . The threshold value S 1  is desirably set at an RSSI value at which no frame error occurs in a wireless environment in which no interfering waves exist. The RSSI counter  21  counts the number of frames in which a frame error is detected and for which the RSSI exceeds the threshold value S 1 . It can be concluded that frames for which the RSSI is equal to or more than the threshold value S 1  and in which a frame error has been generated are frames in which a frame error has occurred due to the effects of interfering waves. When such frames are continuously detected at fixed intervals linking the frames (frame number NE, for example), it may be considered that the cordless phone does not generate frame errors due to frequency hopping interfering waves or unexpected phasing but instead generates frame errors due to an interference source such as a wireless LAN system or microwave oven, or the like.  
         [0023]     When the frame error rate computed by the FER computation unit  19  is equal to or more than a threshold value S 2  and the number of frames counted by the RSSI counter  21  is equal to or more than the threshold value NE, the channel switching judgment unit  22  judges that the channel switching conditions have been fulfilled. The threshold value NE is desirably set at a number of frames that is aimed at judging whether the radio interference is caused by phasing, frequency hopping, or the like.  
         [0024]     When the channel switching conditions are fulfilled, interfering waves are thought to exist in the wireless environment and hence the control unit  23  controls the wireless unit  12  to execute channel switching.  
         [0025]     Information on channels for a new connection at the time of channel switching (channel switching information) is stored in the storage unit  24 . Information that is the same as this channel switching information is also stored in the parent device and constituted to allow the child device  10  and parent device to change channels in sync with each other. For example, the channel switching information may be shared via a control channel or the like between the child device  10  and parent device, and the channel switching order may be established beforehand. The control unit  23  selects the channel that is to be connected next on the basis of the channel switching information stored in the storage unit  24  and changes channel with substantially the same timing as the timing at which the parent device changes channel. It is therefore possible to perform muteless channel switching.  
         [0026]     Further, where the priority level for the channel switching is concerned, the level is desirably set such that the priority level is higher for call channels in which no interfering waves are present wherever possible. In order to set the call channel priority level in this manner, the RSSI may be measured by allocating call channels that are not used for a connection with the base station as channels for control at fixed intervals and the existence of interfering waves may be checked from the RSSI level, for example. The RSSI of each channel can be measured by the RSSI level judgment unit  20 , for example. The RSSI level judgment unit  20  functions as a measurement unit for measuring the RSSI. The result of the RSSI measurement of each channel is output to the control unit  23  by the RSSI level judgment unit  20 . The control unit  23  establishes the channel switching priority level from the RSSI measurement result for each call channel that is not used for a connection with the base station, and stores the channel switching priority level in the storage unit  24 .  
         [0027]      FIG. 2  shows a channel switching judgment routine. When the above channel switching judgment is restated based on this judgment routine, the routine runs as follows. Upon receiving the frame interrupt signal from the frame extractor  14  (step S 1 ; YES), the frame counter  18  increments the frame count number by one (step S 2 ). Then, using the error detection unit  16 , a check is made of whether a frame error has been generated (step S 3 ). When a frame error is detected (step S 3 ; YES), the error counter  17  increments the count value by one (step S 4 ). The RSSI level judgment unit  20  checks whether the RSSI level of the frame in which the frame error was detected is equal to or more than the threshold value S 1  (step S 5 ). When the RSSI level of the frame in which the frame error was detected is equal to or more than the threshold value S 1 , the RSSI counter  21  increments the counter value by one (step S 6 ). When a frame error is not detected (step S 3 ; NO) or when the RSSI level of the frame in which the frame error was detected is less than the threshold value S 1  (step S 5 ; NO), processing jumps to step S 7 .  
         [0028]     In step S 7 , a check is made of whether the number of received frames has reached the prescribed number. When the number of received frames has not reached the prescribed number (step S 7 ; NO), steps S 1  to S 6  are repeated until the prescribed number is reached. If the number of received frames has reached the prescribed number (step S 7 ; YES), a check is made of whether the frame error rate computed on the basis of the count value of the error counter  17  is equal to or more than the threshold value S 2  (step S 8 ). If the frame error rate is equal to or more than the threshold value S 2  (step S 8 ; YES), a check is made of whether the counter value of the RSSI counter  21  is equal to or more than the threshold value NE (step S 9 ). If the counter value of the RSSI counter  21  is equal to or more than the threshold value NE (step S 9 ; YES), the channel switching judgment unit  22  judges that the channel switching conditions have been fulfilled and the control unit  23  performs channel switching processing (step S 10 ). However, when the frame error rate is less than the threshold value S 2  (step S 8 ; NO), or when the counter value of the RSSI counter  21  is less than the threshold value NE (step S 9 ; NO), it is judged that the channel switching conditions are not fulfilled and channel switching processing is not executed.  
         [0029]     With this embodiment, when there is a single base station, channel switching is not executed merely because the wireless environment is a weak electric field. Suitable channel switching can be performed by making a distinction as to whether the cause of a frame error generated under the condition that the RSSI should be equal to or more than the threshold value S 1  is due to interfering waves or generated unexpectedly as in the case of phasing, frequency hopping, or the like.  
         [0030]     This embodiment was described by taking the example of a cordless phone. However, the present invention is not limited to or by a cordless phone, being equally applicable to a cellular terminal, a wireless LAN system or Bluetooth, or the like as long as such as a device is a digital wireless communication device that performs wireless communications with a single base station.