Patent Publication Number: US-2010120380-A1

Title: Method for communicating with other radio apparatuses and radio apparatus using the method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2008-233951, filed on Sep. 11, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a communication technology and, in particular, to a method for performing communications with other radio apparatuses and a radio apparatus utilizing the communication method. 
     2. Description of the Related Art 
     JPEG (Joint Photographic Experts Group) is one of international standards for compression technology for compressing still images. In JPEG, original data (hereinafter referred to as “original frames”) are subjected to DCT (Discrete Cosine Transform), quantization and entropy coding. The format of data compressed under JPEG is such that marker (hereinafter referred to as “header marker”), header, image data and marker (hereinafter referred to as “end marker”) are assigned in this order starting from the header position. Note that the image data correspond to data where the original frames have been compressed. 
     An on-vehicle camera is installed in a vehicle to avoid the collision of vehicles such as automobiles and improve the driving safety, and images picked up by the on-vehicle camera are displayed on an on-vehicle monitor. A driver grasps surrounding circumstances of the vehicle through not only checking but also the images displayed on the on-vehicle monitor. In general, the on-vehicle monitor is installed near a driver&#39;s seat and the on-vehicle camera is installed in a front and/or rear part of the vehicle, so that the on-vehicle monitor and the on-vehicle cameras are remotely positioned. Accordingly, images picked up by the on-vehicle camera need to be transmitted to the on-vehicle monitor. To save time for installation, the use of a wireless communication system such as wireless LAN (Local Area Network) is preferable over that of wired cables. In consideration of transmission capacity in the wireless LAN, images to be transmitted are subjected to the aforementioned compression technology. 
     When two or more vehicles to which such on-vehicle cameras are installed move closer to one another and when wireless LAN access points provided outside the vehicles get closer to the vehicles, interference may occur in wireless LAN. In such a case, the communications may not be performed normally. To avoid this, it is preferred that mutually different frequency channels be used. However, since the number of frequency channels is limited, the interference may occur as the vehicles move, for instance. Depending on a frequency channel used, interference may occur between a vehicle and radar. When the interference occurs and the images cannot be transmitted normally, the safety may be jeopardized, which is not desirable at all. Therefore, the frequency channel must be appropriately switched depending on the circumstances where the radio communication is being performed. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the foregoing circumstances, and a purpose thereof is to provide a technology for suitably selecting a frequency channel in accordance with circumstances where radio communication is performed. 
     In order to resolve the above problems, a radio apparatus according to one embodiment of the present invention comprises: an input unit which receives input of image data sequentially; a specifying unit which specifies a synchronization period for the image data sequentially inputted by the input unit; a measurement unit which measures the characteristics of a radio channel over the synchronization period specified by the specifying unit; and a transmitter which stores the characteristics of a radio channel measured by the measurement unit in a radio packet and transmits the radio packet storing the measured characteristics thereof to another radio apparatus and which further stores the image data sequentially inputted by the input unit in the radio packet and transmits the radio packet storing said image data to the another radio apparatus. 
     Another embodiment of the present invention relates to a communication method. This method comprises: receiving input of image data sequentially; specifying a synchronization period for the image data inputted sequentially; measuring the characteristics of a radio channel over the specified synchronization period; and storing the measured characteristics of a radio channel in a radio packet, transmitting the radio packet storing the measured characteristics thereof to another radio apparatus, further storing the sequentially inputted image data in the radio packet and transmitting the radio packet storing said image data to the another radio apparatus. 
     Optional combinations of the aforementioned constituting elements, and implementations of the invention in the form of methods, apparatuses, systems, recording mediums, computer programs and so forth may also be practiced as additional modes of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
         FIG. 1  shows a structure of a communication system according to an exemplary embodiment of the present invention; 
         FIG. 2  shows a structure of an on-vehicle camera apparatus shown in  FIG. 1 ; 
         FIGS. 3A to 3C  show operation timings in the on-vehicle camera apparatus shown in  FIG. 2 ; 
         FIG. 4  shows a data structure of measurement conditions stored in a storage shown in  FIG. 2 ; 
         FIG. 5  shows a structure of an on-vehicle monitor apparatus shown in  FIG. 1 ; 
         FIG. 6  shows a data structure of a table, containing measurement results, stored in a storage shown in  FIG. 5 ; 
         FIG. 7  shows a data structure of a threshold value, with which to determine a measurement period, stored in a measurement unit shown in  FIG. 5 ; 
         FIG. 8  is a sequence diagram showing a procedure for updating a table in the communication system shown in  FIG. 1 : 
         FIG. 9  is a sequence diagram showing a procedure for switching a frequency channel in the communication system shown in  FIG. 1 ; 
         FIG. 10  is a flowchart showing a procedure for measuring the quality of a frequency channel by the on-vehicle camera apparatus shown in  FIG. 2 ; 
         FIG. 11  is a flowchart showing a receiving procedure performed by the on-vehicle monitor apparatus shown in  FIG. 5 ; 
         FIG. 12  is a flowchart showing a procedure for switching a frequency channel by the on-vehicle monitor apparatus shown in  FIG. 5 ; and 
         FIG. 13  is a flowchart showing a procedure for determining the quality of a frequency channel by the on-vehicle monitor apparatus shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
     The present invention will now be outlined before it is described in detail. Exemplary embodiments of the present invention relate to a communication system. In this communication system, an on-vehicle camera apparatus generates image data by compressing the captured original frames and then transmits the thus generated image data, and an on-vehicle monitor apparatus, which receives the generated image data from the on-vehicle camera, reproduces image data. Here, the on-vehicle camera apparatus and the on-vehicle monitor apparatus are installed on a vehicle. Wireless LAN is used to transmit the image data to the on-vehicle monitor apparatus from the on-vehicle camera apparatus; packet signals are used in the wireless LAN. A plurality of frequency channels are defined in the wireless LAN, and it is desirable that a frequency channel having less interference is used. However, since the vehicle can move freely, such a frequency channel having less interference also varies as the surrounding circumstances vary. Here, the effect of interference varies depending on the circumstances where the radio communications are being performed. For instance, if the vehicle moves slow, the vehicle will suffer interference for a longer period of time; if the vehicle moves fast, the vehicle will suffer interference for a shorter period of time. Also, it is desirable that transmission interruption due to the switching of frequency channels be shorter. In order to cope with these, a communication system according to an exemplary embodiment carries out the following processings. 
     The radio apparatus included in the on-vehicle camera apparatus receives the image data and also receives vertical synchronization signals which are synchronized with the image data. The vertical synchronization signal contains periodically a vertical blanking interval (hereinafter referred to as “V blanking interval”) and no image data is contained in this V blanking interval. Using the V blanking interval, a radio apparatus measures the characteristics for a plurality of frequency channels, respectively. That is, the radio apparatus also measures the characteristics of frequency channels not used for the transmission, while the image data are being sent. Also, prior to the switching of frequency channels, the radio apparatus transmits beforehand a measurement result to the on-vehicle monitor apparatus. A radio apparatus included in the on-vehicle monitor apparatus receives the image data and also receives the measurement result. Based on the measurement result, the radio apparatus generates a table where the respective characteristics for a plurality of frequency channels are aggregated. The radio apparatus measures the quality of image data over a measurement period. Here, the measurement period is set such that faster the moving velocity is, longer the measurement period becomes. In other words, if the radio apparatus determines the switching of frequency channels because of deterioration in the quality of image data, the radio apparatus will determine a new frequency channel by referencing the table. 
       FIG. 1  shows a structure of a communication system  100  according to an exemplary embodiment of the present invention. The communication system  100  includes an on-vehicle camera apparatus  10  and an on-vehicle monitor apparatus  12 . The communication system  100  is installed in a not-shown vehicle. The on-vehicle camera apparatus  10  picks up moving images or still images (hereinafter generically referred to as “images”) and transmits data of the picked-up images (hereinafter referred to as “image data”) to the on-vehicle monitor apparatus  12 . Here, the image data are compressed using JPEG. As described above, wireless LAN is used for a radio network between the on-vehicle camera apparatus  10  and the on-vehicle monitor apparatus  12 . Accordingly, a plurality of frequency channels are defined, and the on-vehicle camera apparatus  10  and the on-vehicle monitor apparatus  12  select and use a common frequency channel. If another vehicle (not shown), which is compatible with the communication system  100 , approaches the vehicle and the same frequency channel is being used for each of the vehicles, interference occurs. A part of the plurality of frequency channels is also used for radar. In other words, interference with the radar may occur in a part of the frequency channels. 
     The on-vehicle monitor apparatus  12  receives image data from the on-vehicle camera apparatus  10  and displays the images on a monitor. During the transmission of the image data, the on-vehicle camera apparatus  10  measures the characteristics of a plurality of frequency channels, respectively, and transmits the thus measured characteristics to the on-vehicle monitor apparatus  12 . The on-vehicle monitor apparatus  12  measures the quality of image data, and determines the switching of the current frequency channel to another frequency channel if the quality thereof deteriorates. In so doing, the on-vehicle monitor apparatus  12  selects a frequency channel, based on the measurement result sent from the on-vehicle camera apparatus  10 . 
       FIG. 2  shows a structure of an on-vehicle camera apparatus  10 . The on-vehicle camera apparatus  10  includes a radio unit  50 , a modem unit  52 , a processing unit  54 , a control unit  56 , a coding unit  58 , an image pickup unit  60 , a storage  62 , a specifying unit  64 , and a measurement unit  66 . 
     The image pickup unit  60 , which is a CCD (Charge Coupled Device) image sensor or the like, picks up the images of original image frames. As described above, an original image frame corresponds to an image on which no compression is performed. In what is to follow, no distinction will be made between images per se and digital data, and the term “original image frame” will be used in general. Images are taken by the image pickup unit  60  on a periodic basis, for instance. The image pickup unit  60  outputs sequentially the captured original frames to the coding unit  58 . 
     The coding unit  58  sequentially receives the input of original image frames from the image pickup unit  60 . The coding unit  58  compresses and codes the original image frames so as to generate image data. For example, a motion JPEG (Joint Photographic Experts Group) scheme is used as a compression scheme. If, for instance, interlaces are used, each image data is comprised of odd-numbered field data (this will also be hereinafter referred to as “odd-numbered field”) and even-numbered field data (this will also be hereinafter referred to as “even-numbered field”). 
     The coding unit  58  appends a JPEG header to a position anterior to each odd-numbered field, and appends markers to the header and the tail, respectively. Here, the marker appended to the header is “SOI (Start of Image)”, whereas the marker appended to the tail is “EOI (End of Image)”. The JPEG header, SOI and EOI are also appended to each even-numbered field in the similar manner. Hereinafter, each odd-numbered field and even-numbered field to which the JPEG header, SOI and EOI are appended will also be referred to as “odd-number field” and “even-numbered field”. Note that the odd-numbered fields and the even-numbered fields are generically referred to as “image data” also. Further, the coding unit  58  generates vertical synchronization signals which are synchronized with a plurality of image data, respectively. The vertical synchronization signals may be those generated using any known technology, so that the description thereof is omitted here. The coding unit  58  outputs the image data to the processing unit  54  and outputs the vertical synchronization signals to the specifying unit  64 . 
     As a transmit processing, the processing unit  54  receives sequentially the input of image data from the coding unit  58 . The processing unit  54  stores the sequentially inputted image data in packet signals. If the size of a single piece of image data is larger than that of a packet signal, the processing unit  54  will divide the image data into a plurality of parts so that the image data can be stored in packet signals. That is, the processing unit  54  stores such a single piece of image data in a plurality of packet signals. The processing unit  54  outputs the packet signals to the modem unit  52 . 
     As a receive processing, the processing unit  54  receives the input of decoding results from the modem unit  52 . The processing unit  54  carries out processings according to the decoding results. One example of the decoding results is a request for the switching of frequency channels made from the on-vehicle monitor apparatus  12 . In such a case, the request contains information on a new frequency channel. In accordance with this request, the processing unit  54  instructs the radio unit  50  to switch the current frequency channel to the new frequency channel. After instructing the radio unit  50  to switch the frequency channel, the processing unit  54  performs switching processing with the on-vehicle monitor apparatus  12  over the new frequency channel, via the modem unit  52  and the radio unit  50 . A detailed description of switching processing is omitted here. Further, the processing unit  54  performs digital signal processing on the packet signals. One example of the digital signal processing is error correction coding as a transmit processing and error correction decoding as a receive processing. Note that the digital signal processing is not limited thereto. 
     As a transmit processing, the modem unit  52  modulates the packet signals outputted from the processing unit  54 . Any modulation scheme may be used here. Further, the modem unit  52  outputs the modulated packet signals to the radio unit  50  as baseband packet signals. As a receive processing, the modem unit  52  demodulates the baseband packet signals outputted from the radio unit  50 . Further, the modem unit  52  outputs the demodulation results to processing unit  54 . If the communication system  100  is compatible with an OFDM modulation scheme (e.g., the IEEE 802.11a standard), the modem unit  52  will also perform FFT as a receive processing and also perform IFFT as a transmit processing. If the communication system  100  is compatible with a spread spectrum scheme (e.g., IEEE 802.11b), the modem unit  52  will also perform despreading as a receive processing and also perform spreading as a transmit processing. If the communication system  100  is compatible with a MIMO scheme (e.g., IEEE 802.11n), the modem unit  52  will also perform adaptive array signal processing as a receive processing and also perform distribute data streams to multiple streams as a transmit processing. 
     The radio unit  50  communicates wirelessly with the not-shown on-vehicle monitor apparatus  12 . Following the instructions given by the processing unit  54 , the radio unit  50  sets a frequency channel which is to be used for the radio communication, as described earlier. That is, the radio unit  50  uses any one of a plurality of frequency channels. As a transmit processing, the radio unit  50  receives the input of baseband packet signals from the modem unit  22 . The radio unit  50  performs quadrature modulation on the baseband packet signals so as generate packet signals with intermediate frequency band. Further, the radio unit  50  generates radiofrequency-band packet signals by frequency-converting intermediate-frequency-band packet signals. The radiofrequency band corresponds to the frequency channel. After amplifying the radiofrequency-band packet signals, the radio unit  50  transmits the amplified radiofrequency-band packet signals via an antenna. A PA (Power Amplifier), a mixer and a D-A conversion unit are also included in the radio unit  50 . 
     As a receive processing, the radio unit  50  generates intermediate-frequency-band packet signals by frequency-converting the radiofrequency-band packet signals received via the antenna. The radio unit  50  performs quadrature detection on the intermediate-frequency-band packet signals so as to generate baseband packet signals. The radio unit  50  outputs the baseband packet signals to the modem unit  52 . The baseband packet signal, which is composed of in-phase components and quadrature components, shall generally be indicated by two signal lines. For the clarity of Figures, the baseband signal is presented here by a single signal line only. An LNA (Low Noise Amplifier), a mixer, an AGC (Automatic Gain Control) unit and an A-D conversion unit are also included in the radio unit  50 . 
     The specifying unit  64  receives the input of vertical synchronization signals from the coding unit  58 . Here, a vertical synchronization signal is a signal that indicates a scanning frequency and a starting point of a scanning in a vertical scanning. That is, the vertical synchronization signal is a signal synchronized with the image data outputted from the coding unit  58 .  FIGS. 3A to 3C  show operation timings in the on-vehicle camera apparatus  10 . In each of  FIGS. 3A to 3C , the horizontal axis represents time.  FIG. 3A  shows image data from the coding unit  58 . “SOI” and “EOI” are appended to the beginning of each odd-numbered field  250  and the tail end thereof, respectively. Similarly, “SOI” and “EOI” are appended to the beginning of each even-numbered field  252  and the tail end thereof, respectively. Pairs of odd-numbered fields  250  and even-numbered fields  252  are continuously assigned. 
       FIG. 3B  shows a vertical synchronization signal. As shown in  FIG. 3B , the vertical synchronization signal is a signal that goes “high” in an interval during which the odd-numbered field  250  and the even-numbered field  252  are contiguous, and goes “low” between the end timing of the even-numbered field  252  and the next odd-numbered field  250 . Here, the interval during which the vertical synchronization signal goes “low” corresponds to a V blanking interval  254 .  FIG. 3C  will be discussed later. Now refer back to  FIG. 2 . The specifying unit  64  specifies the V blanking interval  254  in the vertical synchronization signal, as the synchronization period for the image data inputted sequentially. The specifying unit  64  outputs the thus specified V blanking interval  254  to the measurement unit  66 . 
     The measurement unit  66  receives the input of the V blanking interval  254  from the specifying unit  64 . The measurement unit  66  measures the characteristics of a radio channel over the V blanking interval  254 , via the radio unit  50 . A detailed description is further given here of measurement processing. Prior to the actual measurement, the measurement unit  66  acquires measurement conditions stored in the storage  62 .  FIG. 4  shows a data structure of measurement conditions stored in the storage  62 . As shown in  FIG. 4 , the measurement conditions include a frequency channel number space  200 , a presence-of-radar space  202 , and a number-of-measurements space  204 . The frequency channel number space  200  indicates numbers by which to identify a plurality of frequency channels, respectively (hereinafter referred to as “frequency channel numbers”). Note that each frequency channel number is associated with a frequency value. 
     The presence-of-radar space  202  indicates whether radar is allocated to each frequency channel or not. When “yes” is indicated in the presence-of-radar space  202 , radar is allocated; when “no”, no radar is allocated. The number-of-measurements space  204  indicates the number of measurements required for the measurement of each frequency channel. Though, as described earlier, the measurement unit  66  carries out measurement in the V blanking interval  254 , the V blanking interval  254  is, for instance, several msecs or so and is generally shorter than a period required for the measurement in each frequency channel. In order to cope with this, the measurement unit  66  carries out measurement a plurality of times in the V blanking interval  254  for each frequency channel. 
     The measurement unit  66  accumulates a plurality of measurement results so as to derive a measurement result for a frequency channel. That is, in order to measure the characteristics of radio channel for a frequency channel, the measurement unit  66  uses a plurality of V blanking intervals  254 . The number of V blanking intervals  254  used in the measurement is indicated in the number-of-measurements space  204 . In the case of  FIG. 4 , the number of measurements for a frequency channel to which no radar is allocated is “A”, whereas the number of measurements for a frequency channel to which radar is allocated is “B”. Assume that B is greater than A. That is, the number of V blanking intervals  254  to be used for the measurement is varied according to each frequency channel. Now refer back to  FIG. 2   
     Based on the measurement condition as shown in  FIG. 4 , the measurement unit  66  not only specifies the number of measurements for a frequency channel but also measures the electric power of signal received via the radio unit  50  during the V blanking interval  254 . The measurement unit  66  repeats the measurement until the specified number of measurements is met, and the measurement unit  66  accumulates the received powers when the number of measurements is met. Further, the measurement unit  66  repeats the similar measurement for another frequency channel, based on a condition. The measurement unit  66  outputs the electric power for each frequency to the processing unit  54  as a measurement result. 
     The processing unit  54  also receives the input of a measurement result from the measurement unit  66 . The processing unit  54  stores the measurement results in the packet signals. Here, the processing unit  54  also receives the input of field synchronization signals from the coding unit  58 .  FIG. 3C  is now referred to for the explanation of field synchronization signal. The field synchronization signal is a signal that goes “high” over each odd-numbered field  250  interval, and goes “low” for the remaining intervals, so that the field synchronization signal may serve to distinguish the odd-numbered fields  250  from the even-numbered fields  252  and vice versa. 
     Based on the field synchronization signal, the processing unit  54  specifies an inter-field blanking interval as a period between an odd-numbered field  250  and an even-numbered field  252 . In other words, the processing unit  54  specifies a predetermined interval after a field synchronization signal is switched from high to low. In the specified interval, the processing unit  54  transmits the packet signals via the modem unit  52  and the radio unit  50 . That is, the packet signals in which the measurement results are stored are transmitted in the inter-field blanking intervals  256  which are intervals other than the odd-numbered fields  250 , the even-numbered fields  252  and the V blanking intervals  254 . The control unit  56  controls the whole operation of the on-vehicle camera apparatus  10 . 
     The structure as described above may be achieved hardwarewise by elements such as a CPU, memory and other LSIs of an arbitrary computer, and softwarewise by memory-loaded programs having communication functions or the like. Depicted herein are functional blocks implemented by cooperation of hardware and software. Therefore, it will be obvious to those skilled in the art that the functional blocks may be implemented by a variety of manners including hardware only, software only or a combination of both. 
       FIG. 5  shows a structure of an on-vehicle monitor apparatus  12 . The on-vehicle monitor apparatus  12  includes a radio unit  20 , a modem unit  22 , a processing unit  24 , a control unit  26 , a decoding unit  28 , a display unit  30 , a storage  36 , an acquisition unit  38 , a measurement unit  40 , a decision unit  42 , and a correlation unit  44 . Since the radio unit  20 , the modem unit  22  and the processing unit  24  are of the same type as the radio unit  50 , the modem unit  52  and the processing unit  54  shown in  FIG. 2 , respectively, a description is given hereinbelow of the on-vehicle monitor apparatus  12  centering around the difference. 
     The processing unit  24  receives the input of image data, which are obtained as a result of decoding by the modem unit  22 , from a on-vehicle camera apparatus  10  (not shown). The processing unit  24  performs digital signal processing, such as error correction decoding, on the image data. The processing unit  24  outputs the image data which have been subjected to the digital signal processing (hereinafter also referred to as “image data”) to the decoding unit  28 . The decoding unit  28  receives the input of the image data from the processing unit  24 . Since the image data are compressed in compliance with JPEG, the decoding unit  28  decodes the image data. Any known technique may be used for the decoding, so that the description thereof is omitted here. The decoding unit  28  outputs image frames, which are obtained as a result of decoding, to the display unit  30 . The image frames may be identical to the original image frames. The display unit  30  is structured by an LCD (Liquid Crystal Display) and the like. The display unit  30  receives the image frames from the decoding unit  28  and displays the image frames. 
     The processing unit  24  receives a measurement result as appropriate from the not-shown on-vehicle camera apparatus  10  via the radio unit and the modem unit  22 . Upon receipt of the measurement result, the processing unit  24  updates the table, containing the measurement result, which is stored in the storage  36 .  FIG. 6  shows a data structure of the table, containing measurement results, which is stored in the storage  36 . As shown in  FIG. 6 , the table contains a priority level space  210 , a frequency channel number space  212 , a received power space, and a presence-of-radar space  216 . Here, the frequency channel number space  212  and the presence-of-radar space  202  correspond respectively to the frequency channel number space  200  and the presence-of-radar space  202  of  FIG. 4 . The received power contained in the measurement results is shown in the received power space  214 . 
     The processing unit  24  extracts a frequency channel number and a received power from the measurement results, and specifies a corresponding row from the frequency channel number space  212  based on a frequency channel. The processing unit  24  enters the received power into the specified row in the received power space  214 . Further, the processing unit  24  compares the received powers with one another and updates the data in the table in such a manner that a higher priority level, namely, a smaller number in priority level is given to a frequency channel number having a lower received power. The priority level for each frequency channel is indicated in the priority level space  210 . Since the received power corresponds to the interference power, a higher priority level is given to a frequency channel having a lower interference power. Now refer back to  FIG. 5 . 
     The acquisition unit  38  is connected to a not-shown speed sensor of a vehicle installing the on-vehicle monitor apparatus  12  therein, and acquires the travelling speed of the vehicle using the speed sensor. For instance, the acquisition unit  38  acquires the travelling speed thereof at predetermined intervals. Since any known technique may be used to realize the speed sensor, the description thereof is omitted here. The acquisition unit  38  outputs information on the travelling speed to the measurement unit  40 . The measurement unit  40  receives successively the information on the travelling speed from the acquisition unit  38 . The measurement unit  40  determines a threshold value for the travelling speed. The threshold value therefor is used to determine the measurement period. 
       FIG. 7  shows a data structure of a threshold value, with which to determine the measurement period, stored in the measurement unit  40 . As shown in  FIG. 7 , the data structure includes a condition space  220  and a measurement period space  222 . The condition space  220  indicates the conditions for the travelling speeds, and the measurement period space  222  indicates the measurement periods that meet the conditions on the left, respectively. Here, the travelling speeds are set such that V 1 &gt;V 2 &gt; . . . &gt;VX, and the measurement periods are set such that T 1 &gt;T 2 &gt; . . . &gt;TX&gt;T(X+1). In other words, the travelling speed and the measurement period are defined such that the faster the travelling speed, the longer the measurement period will be. The measurement unit  40  sequentially extracts conditions starting from the top of the condition space  220 , and compares the extracted condition with the travelling speed. If the travelling speed meets the condition, the measurement unit  40  will specify a measurement period associated with the condition in question. Now refer back to  FIG. 5 . 
     The measurement unit  40  measures the characteristics of a frequency channel used by the radio unit  20  for a specified measurement period. Specifically, the measurement unit  40  measures the error rate of packet signals, received by the processing unit  24 , via the radio unit  20  and the modem unit  22 . Here, the packet signals are transmitted from the not-shown on-vehicle camera apparatus  10 . Since any known technique may be used for the measurement of the error rate, the description thereof is omitted here. If the frequency channel used by the radio unit  20  is not allocated to radar, namely, if used is the frequency channel where “no” is indicated in the presence-of-radar space  216  of  FIG. 6 , the measurement unit  40  will measure only the error rate as the characteristics of the frequency channel. The measurement unit  40  outputs the measured error rate to the decision unit  42 . 
     On the other hand, if the frequency channel used by the radio unit  20  is also allocated to radar, namely, if used is the frequency channel where “yes” is indicated in the presence-of-radar space  216  of  FIG. 6 , the measurement unit  40  will input not only the error rate but also a correlation value sent from the correlation unit  44 , as the characteristics of the frequency channel. As the correlation unit  44  receives a received signal from the radio unit  20 , the correlation unit  44  converts the received signal from the time domain into the frequency domain. This conversion may be done using FFT. The correlation unit  44  stores beforehand the frequency characteristics of radar and calculates a value of correlation between the frequency characteristics of radar and the received signal in the frequency domain. The correlation unit  44  outputs the calculated correlation value to the measurement unit  40 . Here, a period over which the correlation value is calculated differs from the aforementioned measurement period, and is defined as a fixed period irrespective of the travelling speed. Note that calculating the correlation value corresponds to measuring the interference amount of radar. The measurement unit  40  outputs the correlation value to the decision unit  42 . 
     The decision unit  42  receives the error rate from the measurement unit  40 . The decision unit  42  compares the error rate against a threshold value. If the error rate deteriorates exceeding the threshold value, the decision unit  42  will determine that the quality of a frequency channel is deteriorated. If, on the other hand, the error rate does not deteriorate, the decision unit  42  will determine that the quality of a frequency channel is not deteriorated. There may be cases where the decision unit  42  receives the correlation value from the measurement unit  40 . The decision unit  42  compares the correlation value against another threshold value. If the correlation value is larger than the threshold value, the decision unit  42  will determine that the Quality of a frequency channel is deteriorated. If, on the other hand, the correlation value is not larger than the threshold value, the decision unit  42  will determine that the quality of a frequency channel is not deteriorated. 
     If the decision unit  42  determines that the quality of a frequency channel is deteriorated, the decision unit  42  will determine the switching from the frequency channel being used by the radio unit  20  to another frequency channel among a plurality of frequency channels. In so doing, the decision unit  42  references the table stored in the storage  36  and selects a frequency channel whose priority level is the highest, namely a frequency channel having the smallest number in priority level. Note that the frequency channel currently in use is excluded in this selection. The decision unit  42  instructs the radio unit  20  to switch it to the selected frequency channel. The radio unit  20  switches the frequency channel by following the instructions given from the decision unit  42 . Further, the decision unit  42  transmits a signal requesting the switching of the frequency channel, to the not-shown on-vehicle camera apparatus  10  via the processing unit  24 , the modem unit  22  and the radio unit  20 . The control unit  26  controls the whole operation of the on-vehicle monitor apparatus  12 . 
     An operation of the communication system  100  structured as above will now be described.  FIG. 8  is a sequence diagram showing a procedure for updating a table in the communication system  100 . The on-vehicle camera apparatus  10  and the on-vehicle monitor apparatus  12  perform connection processings (S 10 ). The on-vehicle camera apparatus  10  transmits image data to the on-vehicle monitor apparatus  12  (S 12 ). The on-vehicle camera apparatus  10  measures the characteristics of a radio channel at timing during which the image data are not being transmitted (S 14 ). Then the on-vehicle camera apparatus  10  transmits the image data to the on-vehicle monitor apparatus  12  (S 16 ). 
     Further, the on-vehicle camera apparatus  10  measures the characteristics of a radio channel for the frequency channel at timing during which the image data are not being transmitted (S 18 ). Then the on-vehicle camera apparatus  10  transmits the image data to the on-vehicle monitor apparatus  12  (S 20 ). Also, the on-vehicle camera apparatus  10  transmits a measurement result to the on-vehicle monitor apparatus  12  (S 22 ). For simplicity of explanation, assume herein that the aforementioned number of measurements is “2”. The on-vehicle monitor apparatus  12  updates the table, based on the measurement result (S 24 ). The processings by Step  14 , Step  18  and Step  22  are repeated by varying the frequency channel. 
       FIG. 9  is a sequence diagram showing a procedure for switching a frequency channel in the communication system  100 . The on-vehicle camera apparatus  10  transmits image data to the on-vehicle monitor apparatus  12  (S 40 ). The on-vehicle monitor apparatus  12  measures the error rate of the image data (S 42 ). The on-vehicle monitor apparatus  12  determines the switching of the frequency channel, based on the error rate (S 44 ). The on-vehicle monitor apparatus  12  transmits a switching request to the on-vehicle camera apparatus  10  (S 46 ). The on-vehicle camera apparatus  10  and the on-vehicle monitor apparatus  12  carry out switching processings (S 48 ). The on-vehicle camera apparatus  10  transmits the image data to the on-vehicle monitor apparatus  12  over a new frequency channel (S 50 ). 
       FIG. 10  is a flowchart showing a procedure for measuring the quality of a frequency channel by the on-vehicle camera apparatus  10 . The measurement unit  66  selects a predetermined frequency channel which is to be measured (S 70 ). The processing unit  54 , the modem unit  52  and the radio unit  50  transmit the image data sent from the coding unit  58  (S 72 ). As the specifying unit  64  specifies a V blanking interval (Y of S 74 ), the measurement unit  66  measures the characteristics of a radio channel (S 76 ). If the number of measurements has not been met (N of S 78 ), return to Step  72 . If, on the other hand, the number of measurements is met (Y of S 78 ), the processing unit  54  will generate a measurement result (S 84 ). 
     If there is any frequency channel which has not been measured (Y of S 86 ), the measurement unit  66  will change the frequency channel (S 88 ) and the processing will be returned to Step  72 . If there is no frequency channel which has not been measured (N of S 86 ), the processing will be terminated. If the specifying unit  64  does not specify any V blanking interval (N of S 74 ) and there is any measurement result which has not been transmitted (Y of S 80 ), the processing unit  54  will transmit the measurement result via the modem unit  52  and the radio unit  50  (S 82 ) and the processing will be returned to Step  72 . If there is no measurement result which has not been transmitted (N of S 80 ), return to Step  72 . 
       FIG. 11  is a flowchart showing a receiving procedure performed by the on-vehicle monitor apparatus  12 . The processing unit  24  receives the image data via the radio unit  20  and the modem unit  22  (S 100 ). If the processing unit  24  receives the measurement result via the radio unit  20  and the modem unit  22  (Y of S 102 ), the processing unit  24  will update the table stored in the storage  36  (S 104 ). If, on the other hand, the processing unit  24  does not receive the measurement result via the radio unit  20  and the modem unit  22  (N of S 102 ), the processing unit  24  will skip Step  104 . If the image data are not completed (N of S 106 ), return to Step  100 . If the image data are completed (Y of S 106 ), the processing will be terminated. 
       FIG. 12  is a flowchart showing a procedure for switching a frequency channel by the on-vehicle monitor apparatus  12 . The acquisition unit  38  acquires travelling speed (S 120 ). The measurement unit  40  specifies a measurement period (S 122 ) and measures the quality (S 124 ). If radar is allocated to the frequency channel in use (Y of S 126 ), the correlation unit  44  will acquire a correlation value (S 128 ). If radar is not allocated to the frequency channel in use (N of S 126 ), Step  128  will be skipped. If the quality is deteriorated (Y of S 130 ), the decision unit  42  will determine the switching of the frequency channel in use to another frequency channel (S 132 ). If the quality is not deteriorated (N of S 130 ), Step  132  will be skipped. 
       FIG. 13  is a flowchart showing a procedure for determining the quality of a frequency channel by the on-vehicle monitor apparatus  12 .  FIG. 13  corresponds to Step  130  of  FIG. 12 . If the error rate deteriorates exceeding a threshold value (Y of S 150 ), the decision unit  42  will determine that the quality of a frequency channel is deteriorated ( 154 ). If, on the other hand, the error rate does not deteriorate exceeding the threshold value (N of S 150 ) and the correlation value is larger than another threshold value (Y of S 152 ), the decision unit  42  will determine that the quality of a frequency channel is deteriorated (S 154 ). If the correlation value is not larger than the another threshold value (N of S 152 ), the decision unit  42  will determine that the quality of a frequency channel is not deteriorated (S 156 ). 
     By employing the present exemplary embodiments, the characteristics of a radio channel are measured while the image data are being transmitted, so that the characteristics of a radio channel can be acquired beforehand. Since the characteristics of a radio channel is acquired beforehand, information can be provided which is used to select a frequency channel appropriately according to the circumstances where the radio communication is being performed. Since the information used to select a frequency channel appropriately is provided, the frequency channel can be selected suitably. Since the characteristics of a radio channel are measured during a vertical blanking interval, the effect of image data on the transmission can be minimized. 
     Though each measurement period is comparatively short, a plurality of numbers of measurement results are merged into a single measurement result and therefore the deterioration of accuracy in the measurement result can be suppressed. Since the number of measurements is adjusted according as the radar is allocated or not, the measurement according to the frequency channel can be made. Also, since the number of measurements is adjusted according as the radar is allocated or not, both the accuracy in the measurement result and the improvement in the measurement efficiency can be achieved. Since the measurement result is transmitted in a period during which the image data are not being transmitted and the measurement is not being made, the effect of the image data on the transmission and measurement can be minimized. 
     Also, the measurement period according to the travelling speed is set, so that the accuracy in measurement can be enhanced. Since the measurement period according to the travelling speed is set, the switching of a frequency channel can be appropriately determined according to the circumstance where the radio communication is being performed. As the travelling speed increases, the measurement period is made longer, which can check the recovery of the quality of a frequency channel in the event that the quality thereof gets deteriorated. Thus the unnecessary switching of a frequency channel can be avoided. Since the unnecessary switching of a frequency channel is avoided, the stability of the communication system can be maintained. 
     As the travelling speed decreases, the measurement period is made shorter, which can instantly switch the frequency channel. Since a new frequency channel is predetermined, the switching processing time can be reduced. Where radar is allocated to a frequency channel in use, the value of correlation between the frequency characteristics of radar and the received signal are examined, so that the signal from the radar can be detected. Since the signal from the radar is detected, the interference with the radar can be reduced. 
     The present invention has been described based on the exemplary embodiments. The exemplary embodiments are intended to be illustrative only, and it is understood by those skilled in the art that various modifications to constituting elements and processes could be developed and that such modifications are also within the scope of the present invention. 
     In the present exemplary embodiments described above, the specifying unit  64  specifies a V blanking interval as the synchronization period for the image data. However, this should not be considered as limiting and, for example, the specifying unit  64  may specify the period from when the EOI of image data is first detected until when the SOI of the next image data is detected. In particular, if progressive method is employed, said period and the V blanking interval are associated with each other. According to this modification, the synchronization period is specified using the image data alone, so that the processing can be simplified. 
     In the exemplary embodiments described above, JPEG is used to compress the original image frames. However, this should not be considered as limiting and, for example, the on-vehicle camera apparatus  10  and the on-vehicle monitor apparatus  12  may use other compression techniques than JPEG. Not only still images but also moving images may be compressed. For example, MPEG (Moving Picture Experts Group) is used to compress the moving images. According to this modification, the exemplary embodiments of the present invention and their modifications are applicable to various types of compression techniques. 
     In the present exemplary embodiments described above, the on-vehicle camera apparatus  10  transmits the packet signals containing the measurement result in a period other than the periods during which the packet signals containing the image data are being transmitted and each frequency channel is being measured. However, this is not limited thereto. For example, the on-vehicle camera apparatus  10  may transmit the packet signals containing the measurement during an H blanking interval, and the on-vehicle camera apparatus  10  may transmit the packet signals in such a manner that the measurement result is inserted into an unused part of the header of packet signal containing the image data. Or the on-vehicle camera apparatus  10  may use such periods in combination. According to this modification, the measurement result can be transmitted immediately. 
     In the present exemplary embodiments described above, the acquisition unit  38  acquires the travelling speed via the speed sensor. However, this is not limited thereto and, for example, the acquisition unit  38  may estimate the travelling speed based on the variation in RSSI (Received Signal Strength Indicator) in the radio unit  20  and the variation in the error rate in the processing unit  24 . According to this modification, the processing can be accomplished using the on-vehicle monitor apparatus  12  only, so that the on-vehicle monitor apparatus  12  can be mounted on a vehicle with ease. 
     In this modification as well as the above-described exemplary embodiments, the measurement unit  40  measure the quality of a frequency channel and the error rate. However, this should not be considered as limiting and, for example, the measurement unit  40  may measure EVM (Error Vector Magnitude) as the quality of a frequency. According to this modification, a variety of parameters can be used to measure the quality of a channel. 
     While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be further made without departing from the spirit or scope of the appended claims.