Abstract:
The invention provides a radio reception apparatus by which a delay by initialization of an equalizer is minimized and an equalization process with a minimum preamble signal can be performed normally to assure a high transmission efficiency and suppress an increase of the power consumption. First and second RF sections convert the frequencies of reception data from first and second antennae. A comparison section compares reception sensitivities of outputs of the first and second RF sections to select the output having a higher reception sensitivity.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a reception apparatus for a radio communication system which utilizes a minimum preamble signal to assure a high efficiency in high speed transmission in a multi-path environment, and more particularly to a technique for high speed equalization processing and a technique for reduction of the power consumption. 
   2. Description of the Related Art 
   In data transmission of a high speed radio ATM (Asynchronous Transfer Mode) system which is one of multimedia mobile communication systems of 20 to 30 Mbps (megabits/second: unit of the transmission rate) and uses the 5.2 GHz band, in order to prevent quality deterioration of data by multi-path fading, an equalization function is used, and in order to allow high speed processing of an equalizer, a minimum preamble signal is used. 
   As an apparatus of the type described, for example, a radio data communication terminal of the narrow-band modulation type is disclosed in Japanese Patent Laid-Open No. 308158/1999 which can use a minimum preamble to determine a frequency offset value for operating a phase rotating element and then set a tap coefficient to be used by an equalizer. More particularly, the radio data communication terminal determines a frequency offset value for operating an automatic frequency control circuit within a preamble period of one frame period in accordance with a narrow-band modulation system wherein NRZ (Non-Return to Zero) codes of the opposite polarities of the GMSK (Gaussian filtered MSK (Minimum Shift Keying)) are passed through a low-pass filter of the Gaussian type and then inputted to a phase-continuous FSK (Frequency Shift Keying) modulator of a modulation index of 0.5 to modulate the codes, estimates the transmission line characteristic, determines a tap coefficient necessary for an equalizer, sets the tap coefficient to the equalizer and then performs equalization of a reception signal by means of the equalizer. 
     FIG. 4  shows an example of a configuration of an equation function of a conventional radio reception apparatus which uses a minimum preamble to determine a frequency offset value for operating a phase rotating element and then sets a tap coefficient to be used by an equalizer. 
   Referring to  FIG. 4 , the radio reception apparatus shown includes two first and second antennae  8   a  and  8   b , a radio frequency (RF) section  9 , a carrier detection section  10 , an equalization processing section  11 . The equalization processing section  11  includes a memory section  12 , a phase rotating section  13 , a phase difference detection section  14 , an average value detection section  15 , an integration circuit  16 , a vector conversion circuit  17 , a transmission line characteristic estimation section  18 , a tap coefficient setting section  19 , and an equalizer  20 . 
   Transmission data from a base station are received by the two antennae  8   a  and  8   b . The RF section  9  receives the reception data from the antennae  8   a  and  8   b , performs a frequency conversion process and outputs the reception data of the converted frequency (quadrature demodulated I and Q signals) to the equalization processing section  11 . The RF section  9  outputs a received signal strength indicator (RSSI) signal Q to the carrier detection section  10 . 
   The carrier detection section  10  discriminates presence/absence of a carrier based on the RSSI signal Q from the RF section  9 , and outputs, to the equalization processing section  11 , a carrier sense signal S which exhibits an active state when a start of receive data is detected. Further, the carrier detection section  10  receives a demodulation data end signal R of a one-pulse signal representative of an end of demodulation data supplied thereto and outputs, to the equalization processing section  11 , the carrier sense signal S serving as a control signal for stopping the outputting of demodulation data from the equalization processing section  11 . 
   The memory section  12  fetches a reception data signal P (quadrature demodulation signal after conversion into a digital signal by an A/D conversion section not shown) for an arbitrary period of time and controls the outputting. 
   The phase rotating section  13  rotates the phase of the output signal of the memory section  12  by a necessary amount. The phase difference detection section  14  determines an angle at present and another angle after one period of a PN (Pseudo Noise) code string and determines a difference between the angles. 
   The average value detection section  15  integrates the value of the angle difference determined by the phase difference detection section  14  for a predetermined number of times and then divides the integrated value by the number of times to determine an average value (frequency offset value) of an average phase difference per one symbol. The integration circuit  16  integrates the frequency offset value determined by the average value detection section  15  in a unit of a symbol. The vector conversion circuit  17  converts a signal outputted from the integration circuit  16  into a real part amplitude value and an imaginary part amplitude value and outputs the real part amplitude value and the imaginary part amplitude value to the phase rotating section  13 . The transmission line characteristic estimation section  18  uses the signal after the phase rotation by the phase rotating section  13  to determine a transmission line characteristic for one period of the PN code string within the preamble period. The tap coefficient setting section  19  determines a tap coefficient necessary for the equalizer  20  from the transmission line characteristic determined by the transmission line characteristic estimation section  18  and sets the tap coefficient to the equalizer  20 . The equalizer  20  equalizes the output of the phase rotating section  13  by means of a filter having the tap coefficient set by the tap coefficient setting section  19  and outputs a demodulation data signal T to effect a reception process. 
     FIG. 5  illustrates an example of operation timings in an equalization function process of the radio reception apparatus shown in FIG.  4 . Referring to  FIG. 5 , within an antenna changeover selection period δ of a preamble signal period γ positioned preceding to an information data period, the integration is performed on the antenna  8   a  side for a certain fixed period for each one frame, and then the antenna to be used is changed over to the antenna  8   b . After the changeover, the integration is performed on the antenna  8   b  side for another certain fixed period. The integration output values integrated for the first antenna  8   a  side and the second antenna  8   b  side are compared with each other to select the antenna which exhibits a higher reception level. Then, the antenna to be used is fixed to the selected antenna, and burst reception (reception of information data) is performed using the selected antenna. 
   Within the preamble signal period γ, the carrier detection section  10  discriminates presence/absence of a carrier to detect a start of reception data, and then automatic gain control (AGC) and automatic frequency control (AFC) by an automatic frequency control circuit (not shown) for dealing with amplitude and phase variations in demodulation processing are performed. Further, the equalization processing section  11  performs detection of a frequency offset, estimation of a transmission line characteristic and setting of a tap coefficient. 
     FIG. 6  illustrates an example of reception timings of the conventional radio reception apparatus shown in  FIG. 4  when an idle time is comparatively long. The reception data signal P successively received from the RF section  9  is composed of a preamble signal used for various kinds of training and information data. Within a preamble period placed before an information data period within one frame period, the same PN string is transmitted repetitively. The carrier detection section  10  discriminates presence/absence of a carrier based on the RSSI signal Q from the RF section  9  to detect a start of reception data, and after a start of reception data is detected, that is, after the carrier sense signal S outputted from the carrier detection section  10  changes into an active state, the equalization processing section  11  performs detection of a frequency offset, estimation of a transmission line characteristic and setting of a tap coefficient. 
   In the initialization of the equalizer  20 , a preamble signal which includes repetitions of a PN code is stored into the memory section  12  and processed for a certain fixed time, and therefore, a delay appears as much. The demodulation data signal T is outputted after the initialization of the equalizer  20 . As seen from  FIG. 6 , a delay corresponding to the initialization period of the equalizer  20  occurs at the equalization processing section  11 , and the transmission efficiency is deteriorated because the idle period is long. 
     FIG. 7  illustrates an example of reception timings of the conventional radio reception apparatus shown in  FIG. 4  when the idle period is short. The reception data signal P successively received from the RF section  9  is composed of a preamble signal used for various kinds of training and information data. After presence/absence of a carrier is discriminated based on the RSSI signal Q from the RF section  9  to detect a start of reception data, the equalization processing section  11  performs detection of a frequency offset, estimation of a transmission line characteristic and setting of a tap coefficient. In the initialization of the equalizer  20 , a preamble signal which exhibits repetitions of a PN code is stored into the memory section  12  and processed for a certain fixed period of time. Therefore, a delay for approximately 200 symbols in the maximum occurs. The demodulation data signal T is outputted after the initialization of the equalizer  20  is performed. 
   If the idle period is shorter than the delay and a next frame is received within a carrier sense period ε, then the carrier sense signal ζ at a rising edge cannot be detected due to a collision of the frames. Reception data for one frame within which the carrier sense signal ζ is not successfully detected at a rising edge cannot be received normally, and a miss of one frame occurs with the demodulation data signal. 
   The conventional reception apparatus shown in  FIG. 4  cannot perform high speed reception since the preamble signal is longer by the antenna changeover selection period δ of the preamble signal period γ. On the other hand, where the idle period from the end of information data to the start of the preamble period of the next frame is short, reception data cannot be received normally. 
   In the conventional radio reception apparatus described with reference to  FIGS. 4  to  6 , integration output values integrated for the antenna  8   a  side and the antenna  8   b  side are compared with each other to select that one of the antennae which exhibit a higher reception level (reception sensitivity) within the antenna changeover selection period δ within the preamble signal period γ within which various kinds of training are performed, and then AGC and AFC as well as initialization necessary for the equalizer are performed. Therefore, the preamble signal becomes longer by the antenna changeover selection period δ, and this deteriorates the transmission efficiency. Then, where the idle period is comparatively long, the transmission efficiency is deteriorated. On the other hand, where the idle period is comparatively short, if the idle period is shorter than a delay time and a next frame is received within a processing period of demodulation data, then a carrier sense signal cannot be detected. Consequently, such a problem occurs that reception data are received but abnormally for every other frame. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a radio reception apparatus and a high speed equalization process therefor by which a delay caused by initialization of an equalizer is minimized. 
   It is another object of the present invention to provide a radio reception apparatus and a high speed equalization process therefor wherein an equalization process with a minimum preamble signal can be performed normally to assure a high transmission efficiency and suppress an increase of the power consumption. 
   In order to attain the object described above, according to an aspect of the present invention, there is provided a radio reception apparatus, comprising first and second antennae for receiving data from a transmission side, a first radio frequency (hereinafter referred to as RF) section for performing a frequency conversion process of the reception data from the first antenna, a second RF section for performing a frequency conversion process of the reception data from the second antenna, first and second carrier detection sections for discriminating presence/absence of a carrier based on received signal strength indicator (hereinafter referred to as RSSI) signals from the first and second RF sections, respectively, a comparison section for comparing reception levels of the RSSI signals from the first and second RF sections with each other to select that one of the RSSI signals which has a higher reception level and supplying a signal representative of the selected reception level to that one of the first and second carrier detection sections which corresponds to that one of the first and second RF sections from which the selected signal has been outputted, an equalization processing section including an equalizer, and control means for supplying, based on carrier sense signals outputted from the first and second carrier detection sections, the reception data signal outputted from that one of the first and second RF sections which has a higher reception sensitivity to the equalization processing section and controlling the equalization processing section to operate based on the reception data signal. 
   The control means may include first and second logic circuits provided corresponding to the first and second carrier detection sections, respectively, each for controlling so that a reception data signal outputted from a corresponding one of the first and second RF sections corresponding to the first and second carrier detection sections may pass therethrough and be outputted within a period within which the carrier sense signal outputted from a corresponding one of the first and second carrier detection sections is active, a third logic circuit for logically ORing the outputs of the first and second logic circuits and outputting a result of the logical ORing as a reception data signal to the equalization processing section, and a fourth logic circuit for logically ORing the carrier sense signals outputted from the first and second carrier detection sections and outputting a result of the logical ORing as a signal for controlling operation of the equalization processing section to the equalization processing section. 
   The comparison means may supply, to that one of the first and second carrier detection sections which corresponds to that one of the first and second RF sections which has a higher reception sensitivity, a signal indicative of the reception level from the RF section, but supply, to the other one of the first and second carrier detection sections which corresponds to the other RF section which has a lower reception sensitivity, a signal of a level for rendering inactive the carrier sense signal to be outputted from the carrier detection section. 
   The equalization processing section may perform detection of a frequency offset, estimation of a transmission line characteristic and setting of a tap coefficient based on the reception data signal from the third logic circuit and the signal from the fourth logic circuit, and, after necessary initialization for the equalizer, output a demodulation data signal to perform a reception process. 
   Each of the first and second carrier detection sections may receive a demodulation data end signal, which is inputted commonly to the first and second carrier detection sections, and render inactive the carrier sense signal to be outputted. 
   According to another aspect of the present invention, there is provided a radio reception apparatus, comprising first and second antennae for receiving data from a transmission side, a first RF section for performing a frequency conversion process of the reception data from the first antenna, a second RF section for performing a frequency conversion process of the reception data from the second antenna, a comparison section for comparing a reception sensitivity of an output signal of the first RF section and a reception sensitive of an output signal of the second RF section with each other to select that one of the output signals which has a higher reception sensitivity, first and second carrier detection sections for outputting an output signal of an active state when a start of reception data is detected based on an output signal of the comparison section but rendering the output signal inactive when a demodulation data end signal is received, a first OR circuit for logically ORing the output signal of the first carrier detection section and the output signal of the second carrier detection section and outputting a result of the logical ORing, a first AND circuit for receiving the output signal of the first RF section and the output signal of the first carrier detection section and outputting the output signal of the first RF section within a period within which the output signal of the first carrier detection section is active, a second AND circuit for receiving the output signal of the second RF section and the output signal of the second carrier detection section and outputting the output signal of the second RF section within a period within which the output signal of the second carrier detection section is active, a second OR circuit for logically ORing the output signal of the first AND circuit and the output signal of the second AND circuit and outputting a result of the logical ORing, and an equalization processing section for receiving the reception data signal outputted from the first OR circuit and performing an equalization process for the received reception data based on the output signal of the second OR circuit. 
   The radio reception apparatus are advantageous in that, since the two systems of the RF sections are provided in parallel, a delay caused by initialization of the equalizer can be reduced. The radio reception apparatus are advantageous also in that, since that one of the antennae which has a higher reception sensitivity is selected for each one frame and also equalization processing with a minimum preamble signal is processed normally, a high transmission efficiency can be achieved. Further, the radio reception apparatus are advantageous in that, since only output reception data of that one of the RF sections which has a higher reception sensitivity are selectively used, the power consumption can be reduced. 
   The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference symbols. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a radio reception apparatus to which the present invention is applied; 
       FIG. 2  is a diagrammatic view illustrating operation timings of the radio reception apparatus of  FIG. 1 ; 
       FIG. 3  is a waveform diagram illustrating reception timings of the radio reception apparatus of  FIG. 1 ; 
       FIG. 4  is a block diagram showing an equalization processing section of a conventional radio reception apparatus; 
       FIG. 5  is a diagrammatic view illustrating operation timings of the equalization processing section shown in  FIG. 4 ; 
       FIG. 6  is a waveform diagram illustrating operation timings of the conventional radio reception apparatus of  FIG. 4  when the idle period is comparatively long; and 
       FIG. 7  is a waveform diagram illustrating operation timings of the conventional radio reception apparatus of  FIG. 4  when the idle period is comparatively short. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , there is shown a radio reception apparatus to which the present invention is applied. The radio reception apparatus shown includes first and second antennae  1   a  and  1   b  for receiving data from the transmission side, first and second RF sections  2   a  and  2   b  for converting the frequency of reception data from the first and second antennae  1   a  and  1   b , respectively, a comparison section  3  for comparing the magnitudes of the reception sensitivity α of an output signal of the first RF section  2   a  and the reception sensitivity β of an output signal of the second RF section  2   b  with each other to select that one of the output signals which has a higher reception sensitivity, first and second carrier detection sections  4   a  and  4   b  for detecting a start of reception data based on an output signal of the comparison section  3  and outputting an output signal of an active state and for receiving a demodulation data end signal and placing the output signal into an inactive state, a first OR circuit  5   a  for outputting a result of logical ORing of output signals of the first carrier detection section  4   a  and the second carrier detection section  4   b , a first AND circuit  6   a  for receiving an output signal of the first RF section  2   a  and an output signal of the first carrier detection section  4   a  and outputting the output signal of the first antenna  1   a  within a period within which the output signal of the first carrier detection section  4   a  is in an active state, a second AND circuit  6   b  for receiving the output signal of the second RF section  2   b  and the output signal of the second carrier detection section  4   b  and outputting the output signal of the second RF section  2   b  within a period within which the output signal of the second carrier detection section  4   b  is in an active state, a second OR circuit  5   b  for outputting a result of logical ORing of output signals of the first AND circuit  6   a  and the second AND circuit  6   b , and an equalization processing section  7  for receiving a reception data signal outputted from the second OR circuit  5   b  and equalizing the received reception data based on the output signal of the first OR circuit  5   a.    
   The equalization processing section  7  receives a reception data signal L from the second OR circuit  5   b , performs detection of a frequency offset, estimation of a transmission line characteristic and setting of a tap coefficient based on a carrier sense signal H from the first OR circuit  5   a , and outputs, after the setting of the equalizer, a demodulation data signal M to perform a reception process. 
   Transmission data from a base station are received by the two first and second antennae  1   a  and  1   b . The first RF section  2   a  performs frequency conversion processing of reception data from the first antenna  1   a  and outputs an RSSI signal A representative of a start of reception data to the comparison section  3 . The second RF section  2   b  performs frequency conversion processing of reception data from the second antenna  1   b  and outputs an RSSI signal B representative a start of reception data to the comparison section  3 . 
   The comparison section  3  compares the RSSI signal A from the first RF section  2   a  and the RSSI signal B from the second RF section  2   b  with each other to select that one of the two signals which has a higher reception level. More particularly, the comparison section  3  compares the reception sensitivity α of the RSSI signal A and the reception sensitivity β of the RSSI signal B with each other, and when the reception sensitivity α is equal to or higher than the reception sensitivity β, the comparison section  3  outputs an RSSI signal C (=RSSI signal A) to the first carrier detection section  4   a  and outputs an RSSI signal D of a low level signal to the second carrier detection section  4   b . On the other hand, when the reception sensitivity α is lower than the reception sensitivity β, the comparison section  3  outputs the RSSI signal D (=RSSI signal B) to the second carrier detection section  4   b  and outputs the RSSI signal C of a low level signal to the first carrier detection section  4   a.    
   The first carrier detection section  4   a  discriminates presence/absence of a carrier from the RSSI signal C from the comparison section  3 . If the first carrier detection section  4   a  detects a start of reception data, then it outputs a carrier sense signal F of an active state (of the high level). Then, when a demodulation data end signal E of a one-pulse signal representative of an end of demodulation data is received, the first carrier detection section  4   a  places the carrier sense signal F into an inactive state and supplies it to the first OR circuit  5   a  and the first AND circuit  6   a.    
   The second carrier detection section  4   b  discriminates presence/absence of a carrier from the RSSI signal D from the comparison section  3 . If the second carrier detection section  4   b  detects a start of reception data, then it outputs a carrier sense signal G of an active state (of the high level). Then, when the demodulation data end signal E of a one-pulse signal representative of an end of demodulation data is received, the second carrier detection section  4   b  places the carrier sense signal G into an inactive state and supplies it to the first OR circuit  5   a  and the second AND circuit  6   b.    
   The first OR circuit  5   a  logically ORs the carrier sense signal F from the first carrier detection section  4   a  and the carrier sense signal G from the second carrier detection section  4   b  and outputs a result of the logical ORing as a carrier sense signal H to the equalization processing section  7 . In other words, the first OR circuit  5   a  selectively outputs one of the carrier sense signal G and the carrier sense signal H which has a higher reception sensitivity to the equalization processing section  7 . 
   The first AND circuit  6   a  logically ANDs a reception data signal J from the first RF section  2   a  and the carrier sense signal F from the first carrier detection section  4   a  and outputs the reception data signal J to the second OR circuit  5   b  only within a period within which the carrier sense signal F is active. 
   The second AND circuit  6   b  logically ANDs a reception data signal K from the second RF section  2   b  and the carrier sense signal G from the second carrier detection section  4   b  and outputs the reception data signal K only within a period within which the carrier sense signal G is active. 
   The second OR circuit  5   b  logically ORs the outputs of the first AND circuit  6   a  and the second AND circuit  6   b  and outputs a result of the logical ORing as a reception data signal L to the equalization processing section  7 . In other words, that one of the reception data signals which has a higher reception sensitivity is selectively supplied as a reception data signal L to the equalization processing section  7 . 
   The equalization processing section  7  performs detection of a frequency offset, estimation of a transmission line characteristic and setting of a tap coefficient based on the reception data signal L from the second OR circuit  5   b  and the carrier sense signal H from the first OR circuit  5   a . After the initialization of the equalizer, the equalization processing section  7  outputs a demodulation data signal M to perform a reception process. In the radio reception apparatus of  FIG. 1 , the equalization processing section  7  may have, for example, a similar configuration to that described hereinabove with reference to FIG.  4  and include a memory section  12 , a phase rotating section  13 , a phase difference detection section  14 , an average value detection section  15 , an integration circuit  16 , a vector conversion circuit  17 , a transmission line characteristic estimation section  18 , a tap coefficient setting section  19  and an equalizer  20  as seen in FIG.  4 . Operation of the components is similar to that described in the description of the related art hereinabove, and therefore, overlapping description of the operation is omitted here to avoid redundancy. 
     FIG. 2  illustrates operation timings of the radio reception apparatus of FIG.  1 . Referring to  FIG. 2 , in the radio reception apparatus of  FIG. 1 , the carrier sense signal F of the first antenna  1   a  side and the carrier sense signal G of the second antenna  1   b  side are normally outputted within a one-frame period. 
   For each one frame, that one of the first antenna  1   a  side and the second antenna  1   b  side which exhibits a higher reception level is selected, and while the antenna to be used is fixed to the selected antenna, burst reception is performed. 
   Within the preamble signal period γ, presence/absence of a carrier is discriminated. After a start of reception data is detected, automatic gain control (AGC) and automatic frequency control (AFC) for dealing with amplitude and phase variations in a demodulation process are performed. Further, detection of a frequency offset, estimation of a transmission line characteristic and setting of a tap coefficient (filter coefficient for a transversal filter which forms the equalizer) are performed. 
   Use of the two parallel RF sections allows further shortening of the preamble signal. In particular, when compared with a case wherein, within an antenna changeover selection period δ of the preamble signal period γ, integration is performed on the antenna  8   a  side for a certain fixed period for each one frame and the antenna to be used is changed over to the antenna  8   b  and then, after the changeover, the integration is performed on the antenna  8   b  side for another certain fixed period, whereafter the integration output values are compared with each other to select the antenna which exhibits a higher reception level and then the antenna to be used is fixed to the selected antenna and burst reception is performed using the selected antenna, in the radio reception apparatus, that one of the first antenna  1   a  and the second antenna  1   b  which outputs a higher one of reception levels outputted in parallel from them is selected. Consequently, the preamble period is reduced. 
     FIG. 3  illustrates reception timings of the radio reception apparatus of FIG.  1 . Referring to  FIG. 3 , the reception data signal L successively received from the second OR circuit  5   b  includes a preamble signal to be used for various kinds of training and information data. The preamble signal includes repetitions of a PN code for a fixed period of time. 
   After the carrier sense signal H rises, detection of a frequency offset value, estimation of a transmission line characteristic and setting of a tap coefficient are performed with a signal of the PN code for one period. 
   Initialization of the equalizer suffers from a delay corresponding to 96 symbols in the maximum because the signal is processed after it is stored into the memory (memory section  12  of  FIG. 4 ) provided in the equalization processing section  7 . In other words, when compared with the conventional radio reception apparatus described hereinabove with reference to  FIG. 4 , the delay is reduced approximately to one half, and consequently, the carrier sense signal can be detected. 
   Simultaneously with an end of demodulation data, the first and second carrier detection sections  4   a  and  4   b  place the carrier sense signal F and carrier sense signal G to the low level, respectively, and consequently, the carrier sense signal H is placed into an inactive state (the low level). 
   The comparison section  3  selectively outputs that one of the RSSI signals which exhibits a higher reception sensitivity, but does not select the other RSSI signal having a lower reception sensitivity and changes it into a low level signal. 
   Due to the parallel circuit configuration of the first RF section  2   a  and the second RF section  2   b , even if an equalization process is performed with a minimum preamble signal, a reception process can be performed normally. Further, since that one of the first antenna  1   a  and the second antenna  1   b  which has a higher reception sensitivity is selectively used, even if a delay is caused by initialization of the equalizer, a normal reception process can be achieved. 
   In the radio reception apparatus of  FIG. 1 , since output reception data of the RF section of one of the systems of the first RF section  2   a  and the second RF section  2   b  which has a higher reception sensitivity is selectively rendered operative (activated), the power consumption can be reduced. 
   While a preferred embodiment of the present invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.