Patent Publication Number: US-2015071331-A1

Title: Transmission device and bandwidth adjustment method

Description:
TECHNICAL FIELD 
     The present invention relates to a transmitting apparatus capable of performing communication at a plurality of different transmission rates and also to a bandwidth adjusting method. 
     BACKGROUND ART 
     PTL 1 is known as a radio transmission scheme that makes a conventional transmission rate variable. 
     In the radio transmission scheme of PTL 1, a transmitting apparatus transmits radio signals compliant with a plurality of specifications differing in the occupied bandwidth, baseband filter band and bit rate or the like. That is, the transmitting apparatus transmits radio signals of a plurality of different transmission rates. 
     A receiving apparatus receives radio signals transmitted from the above-described transmitting apparatus and converts the radio signals arriving at an antenna to intermediate frequency signals in an RF receiving section. The intermediate frequency signals are demodulated by a demodulator, caused to pass through a band pass filter, and converted to digital signals by an A/D converter. The band pass filter is a filter used for the purpose of removing unnecessary noise by allowing a necessary detection signal to pass through the filter, and configured to allow a signal having the widest bandwidth among signals compliant with various specifications to be received to pass through the filter. That is, the band pass filter has a passband that matches the occupied bandwidth of a signal having a high transmission rate. 
     CITATION LIST 
     Patent Literature 
     PTL 1 
     
         
         Japanese Patent Application Laid-Open No. 2010-278741 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, according to the radio transmission scheme in PTL 1, when the band pass filter removes a noise component of the signal having a narrow occupied bandwidth and a low transmission rate, the power of the signal components is reduced so that there arises a problem in that an S/N ratio deteriorates. On the other hand, when a band pass filter having a passband that matches the occupied bandwidth of a low transmission rate signal is used to improve the S/N ratio of the low transmission rate signal, there is a problem in that the frequency component of the high transmission rate signal is cut and cannot be demodulated. Moreover, when band pass filters having different passbands corresponding to the signals of respective transmission rates are used, there is a problem in that the manufacturing cost increases. 
     An object of the present invention is to provide a transmitting apparatus and a bandwidth adjusting method that adjust an occupied bandwidth for each transmission rate, and thus can prevent deterioration of an S/N ratio while preventing a situation in which frequency components are cut and cannot be demodulated, without increasing the manufacturing cost. 
     Solution to Problem 
     A transmitting apparatus according to an aspect of the present invention is an apparatus that performs communication at a plurality of different transmission rates, the apparatus including: an adjusting section that modulates transmission data to generate a transmission signal, and that adjusts, for each of the transmission rates, an occupied bandwidth in a predetermined band of the transmission signal such that the occupied bandwidth approximates to a bandwidth of the predetermined band; and a transmitting section that transmits the transmission signal whose occupied bandwidth has been adjusted by the adjusting section. 
     A bandwidth adjusting method according to an aspect of the present invention is a method in a transmitting apparatus that performs communication at a plurality of different transmission rates, the method including: modulating transmission data to generate a transmission signal; and adjusting, for each of the transmission rates, an occupied bandwidth in a predetermined band of the transmission signal such that the occupied bandwidth approximates to a bandwidth of the predetermined band. 
     Advantageous Effects of Invention 
     The present invention adjusts an occupied bandwidth for each transmission rate, and thus can prevent deterioration of an S/N ratio while preventing a situation in which a frequency component is cut and cannot be demodulated, without increasing the manufacturing cost. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration of a radio transmission system according to Embodiment 1 of the present invention; 
         FIG. 2  is a block diagram illustrating a configuration of an occupied bandwidth control section according to Embodiment 1 of the present invention; 
         FIG. 3  is a block diagram illustrating a configuration of an RF receiving section according to Embodiment 1 of the present invention; 
         FIG. 4  is a block diagram illustrating a configuration of a rate determining section according to Embodiment 1 of the present invention; 
         FIG. 5  is a block diagram illustrating a configuration of a frequency analysis section according to Embodiment 1 of the present invention; 
         FIGS. 6A and 6B  illustrate occupied bandwidths of modulated signals having a high transmission rate and a low transmission rate according to Embodiment 1 of the present invention; 
         FIG. 7  is a block diagram illustrating a configuration of a frequency analysis section according to Embodiment 2 of the present invention; 
         FIG. 8  is a block diagram illustrating a configuration of a frequency analysis section according to Embodiment 3 of the present invention; 
         FIG. 9  is a block diagram illustrating a configuration of a modulation section according to Embodiment 4 of the present invention; 
         FIG. 10  is a block diagram illustrating a configuration of a radio transmission system according to Embodiment 5 of the present invention; and 
         FIG. 11  is a block diagram illustrating a configuration of an occupied bandwidth control section according to Embodiment 5 of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     Embodiment 1 
     Configuration of Radio Transmission System 
     A configuration of radio transmission system  100  according to Embodiment 1 of the present invention will be described with reference to  FIG. 1 . 
     Radio transmission system  100  mainly includes transmitting apparatus  150  and receiving apparatus  160 . 
     Transmitting apparatus  150  and receiving apparatus  160  can communicate with each other at a plurality of different transmission rates. 
     When a mutual distance is small, transmitting apparatus  150  and receiving apparatus  160  perform large-volume communication at a high transmission rate and in a short time. On the other hand, when a mutual distance is large, transmitting apparatus  150  and receiving apparatus  160  set a low transmission rate and secure a wide link budget with high reception sensitivity. This allows radio transmission system  100  to be used for a wide variety of purposes. 
     &lt;Configuration of Transmitting Apparatus&gt; 
     A configuration of transmitting apparatus  150  according to Embodiment 1 of the present invention will be described with reference to  FIG. 1 . 
     Transmitting apparatus  150  mainly includes modulation section  101 , occupied bandwidth control section  102 , RF transmitting section  103 , and antenna  104 . Modulation section  101  and occupied bandwidth control section  102  constitute an adjusting section. 
     Modulation section  101  modulates inputted transmission data based on bandwidth setting information inputted from occupied bandwidth control section  102  and generates a modulated signal resulting from adjusting for each transmission rate, an occupied bandwidth in a passband of channel selection filter  305  (see  FIG. 3 ) which will be described later of receiving apparatus  160 . Modulation section  101  outputs the generated modulated signal to RF transmitting section  103 . Here, the bandwidth setting information is, for example, an adjustment coefficient. 
     Occupied bandwidth control section  102  outputs to modulation section  101 , bandwidth setting information that causes a bandwidth to approximate to the bandwidth of a passband of channel selection filter  305  based on the inputted transmission rate setting information. Here, the transmission rate setting information is information indicating a transmission rate predetermined for every transmission data. Details of the configuration of occupied bandwidth control section  102  and the method of adjusting an occupied bandwidth will be described later. 
     RF transmitting section  103  applies predetermined radio processing to the modulated signal inputted from modulation section  101 . RF transmitting section  103  transmits the signal subjected to the radio processing via antenna  104 . 
     &lt;Configuration of Receiving Apparatus&gt; 
     A configuration of receiving apparatus  160  according to Embodiment 1 of the present invention will be described with reference to  FIG. 1 . 
     Receiving apparatus  160  mainly includes antenna  111 , RF receiving section  112 , rate determining section  113 , demodulation section  114  and clock reproducing section  115 . Data acquisition processing section  120  includes demodulation section  114  and clock reproducing section  115 . Data acquisition processing section  120  performs processing of acquiring received data from a baseband signal. 
     RF receiving section  112  applies predetermined radio processing to a signal received via antenna  111  and generates a baseband signal. RF receiving section  112  outputs the generated baseband signal to rate determining section  113  and demodulation section  114 . The baseband signal outputted from RF receiving section  112  is inputted to rate determining section  113  and demodulation section  114  in parallel. Details of the configuration of RF receiving section  112  will be described later. 
     Rate determining section  113  analyzes a frequency component of the baseband signal inputted from RF receiving section  112  and determines a transmission rate. When the determined transmission rate is not a predetermined transmission rate, rate determining section  113  sets a parameter corresponding to the determined transmission rate. Rate determining section  113  outputs the set parameter to demodulation section  114  and clock reproducing section  115 . When the determined transmission rate is the predetermined transmission rate, rate determining section  113  does not set any parameter. Here, the parameter is, for example, the frequency of a reference clock used to perform digital signal processing, sampling frequency of signal processing, factor of a digital filter, or the number of taps of the digital filter. Details of the configuration of rate determining section  113  will be described later. 
     Demodulation section  114  includes a preset parameter of a predetermined transmission rate and demodulates, when no parameter is inputted from rate determining section  113 , the baseband signal inputted from RF receiving section  112  based on the preset parameter and generates a demodulated signal. When a parameter is inputted from rate determining section  113 , demodulation section  114  demodulates the baseband signal inputted from RF receiving section  112  based on the inputted parameter and generates a demodulated signal. Demodulation section  114  outputs the generated demodulated signal to clock reproducing section  115 . 
     Clock reproducing section  115  includes a preset parameter of a predetermined transmission rate and reproduces, when no parameter is inputted from rate determining section  113 , a clock from the demodulated signal inputted demodulation section  114  based on the preset parameter and converts the clock to bit data. When a parameter is inputted from rate determining section  113 , clock reproducing section  115  reproduces a clock from the demodulated signal inputted from demodulation section  114  based on the inputted parameter and converts the clock to bit data. Clock reproducing section  115  outputs the bit data as received data. 
     &lt;Configuration of Occupied Bandwidth Control Section&gt; 
     A configuration of occupied bandwidth control section  102  according to Embodiment 1 of the present invention will be described with reference to  FIG. 2 . A case will be described in  FIG. 2  where an adjustment coefficient is used as bandwidth setting information as an example. 
     Occupied bandwidth control section  102  mainly includes storage section  201 , storage section  202 , and rate difference calculation section  203 . 
     Storage section  201  stores the fastest transmission rate among transmission rates that can be transmitted from transmitting apparatus  150 . 
     Storage section  202  stores a table that lists rate differences associated with adjustment coefficients. 
     Upon receiving transmission rate setting information, rate difference calculation section  203  calculates a rate difference between the transmission rate of the transmission rate setting information and the transmission rate stored in storage section  201 . With reference to the table stored in storage section  202 , rate difference calculation section  203  selects an adjustment coefficient associated with the calculated rate difference. Rate difference calculation section  203  outputs the selected adjustment coefficient to modulation section  101 . 
     When the above-described rate difference is “0,” rate difference calculation section  203  does not select any adjustment coefficient. The case where the above-described rate difference is “0” is a case where it is not necessary to adjust the occupied bandwidth because the transmission rate of the transmission data to be transmitted from now on is fastest. Rate difference calculation section  203  may also be configured not to select any adjustment coefficient when the above-described rate difference is less than a threshold. This is applicable to a case where, at a transmission rate slightly lower than the fastest transmission rate, the signal has a relatively broad occupied bandwidth, and the occupied bandwidth thereby need not be adjusted. 
     For example, when fastest transmission rate x n [bps] having occupied bandwidth y n [Hz] is stored in storage section  201 , and when transmission rate setting information of low transmission rate xm having occupied bandwidth y m  is inputted, rate difference calculation section  203  multiplies occupied bandwidth y m  by α (α&gt;1) and outputs adjustment coefficient α to cause occupied bandwidth y m  to approximate to occupied bandwidth y n  to modulation section  101 . 
     &lt;Configuration of RF Receiving Section&gt; 
     A configuration of RF receiving section  112  according to Embodiment 1 of the present invention will be described with reference to  FIG. 3 . 
     RF receiving section  112  mainly includes low noise amplifier  301 , frequency synthesizer  302 , mixer  303 , intermediate frequency amplifier  304 , channel selection filter  305 , and A/D converter  306 . 
     Low noise amplifier  301  amplifies a signal received at antenna  111  and outputs the amplified signal to mixer  303 . 
     Frequency synthesizer  302  generates a reference signal of a predetermined frequency and outputs the reference signal to mixer  303 . 
     Mixer  303  mixes the signal inputted from low noise amplifier  301  and a reference signal inputted from frequency synthesizer  302  and generates an intermediate frequency signal. Mixer  303  outputs the generated intermediate frequency signal to intermediate frequency amplifier  304 . 
     Intermediate frequency amplifier  304  amplifies the intermediate frequency signal inputted from mixer  303  and outputs the amplified intermediate frequency signal to channel selection filter  305 . 
     Channel selection filter  305  is provided to remove a noise component. Channel selection filter  305  allows a predetermined passband of the intermediate frequency signal inputted from intermediate frequency amplifier  304  to pass through the filter and prevents bands other than the passband from passing through the filter. 
     A/D converter  306  converts the intermediate frequency signal of the passband inputted from channel selection filter  305  from an analog signal format to a digital signal format and outputs the digital signal to rate determining section  113  and demodulation section  114  as a baseband signal. 
     &lt;Configuration of Rate Determining Section&gt; 
     A configuration of rate determining section  113  according to Embodiment 1 of the present invention will be described with reference to  FIG. 4 . 
     Rate determining section  113  includes frequency analysis section  401  and storage section  402 . 
     Frequency analysis section  401  analyzes a frequency component of the baseband signal inputted from RF receiving section  112  and determines a transmission rate. Frequency analysis section  401  selects and sets a parameter associated with the determined transmission rate with reference to a table stored in storage section  402  and outputs the set parameter to demodulation section  114  and clock reproducing section  115 . When there is no parameter associated with the determined transmission rate, frequency analysis section  401  outputs nothing. 
     Storage section  402  stores a table that lists transmission rates associated with parameters. The table dose not store any parameter preset in demodulation section  114  and clock reproducing section  115 . 
     Here, the synchronization frame detection section of PTL 1 determines the transmission rate after demodulating the modulated signal into bit data, and therefore it has a harmful effect that the header portion of a frame becomes longer and a frame for determining a transmission rate is necessary. On the other hand, the present embodiment determines the transmission rate by analyzing a frequency component of a modulated signal, and thus can determine the transmission rate before demodulating the modulated signal into bit data. As a result, the present embodiment can solve the above-described harmful effect. 
     &lt;Configuration of Frequency Analysis Section&gt; 
     A configuration of frequency analysis section  401  according to Embodiment 1 of the present invention will be described with reference to  FIG. 5 . 
     Frequency analysis section  401  mainly includes first filter  501 , second filter  502 , third filter  503 , and selection section  504 . 
     First filter  501  allows only a frequency f1 component of the baseband signal inputted from RF receiving section  112  to pass through the filter. 
     Second filter  502  allows only a frequency f2 component of the baseband signal inputted from RF receiving section  112  to pass through the filter. 
     Third filter  503  allows only a frequency f3 component of the baseband signal inputted from RF receiving section  112  to pass through the filter. 
     Selection section  504  analyzes the frequency component from the baseband signal inputted after passing through first filter  501 , second filter  502  or third filter  503 . 
     Selection section  504  determines a transmission rate from the analyzed frequency component. With reference to the table stored in storage section  402 , selection section  504  selects and sets a parameter associated with the determined transmission rate. Selection section  504  outputs the set parameter to demodulation section  114  and clock reproducing section  115 . When there is no parameter associated with the determined transmission rate, selection section  504  outputs nothing. 
     &lt;Method of Adjusting Occupied Bandwidth&gt; 
     The method of adjusting an occupied bandwidth according to Embodiment 1 of the present invention will be described with reference to  FIG. 6 . In  FIGS. 6A and 6B ,  FIG. 6A  illustrates an occupied bandwidth of a modulated signal having a high transmission rate and  FIG. 6B  illustrates an occupied bandwidth of a modulated signal having a low transmission rate. A case will be described in  FIG. 6  where an adjustment coefficient is used as bandwidth setting information as an example. 
     Channel selection filter  305  is generally adjusted so as to minimize noise power, with passband #601 that will not cut the occupied bandwidth of a modulated signal having a high transmission rate set therein. 
     In the case of a high transmission rate, since occupied bandwidth H1 is sufficiently large compared to bandwidth H0 of passband #601 of channel selection filter  305  as shown in  FIG. 6A , occupied bandwidth control section  102  leaves occupied bandwidth H1 as is without any adjustment. 
     On the other hand, occupied bandwidth H2 of the modulated signal having a low transmission rate is narrower than bandwidth H0 of passband #601 of channel selection filter  305  as shown in  FIG. 6B . As a result, signal power decreases and an S/N ratio deteriorates in the modulated signal having a low transmission rate. 
     Therefore, in the present embodiment, occupied bandwidth control section  102  outputs adjustment coefficient α for adjusting occupied bandwidth H2 to modulation section  101 . When generating a modulated signal, modulation section  101  multiplies occupied bandwidth H2 by adjustment coefficient α (α&gt;1) to expand occupied bandwidth H2 to occupied bandwidth H3 (H3=H2×α). That is, modulation section  101  makes an adjustment so that the occupied bandwidth in the case of a low transmission rate approximates to bandwidth H0 of passband #601 of channel selection filter  305 . 
     Assuming that noise power per Hz is N0, the bandwidth of passband of channel selection filter  305  is BW, power of a modulated signal at a conventional low transmission rate is S_Low0, and power of a modulated signal at a low transmission rate of the present embodiment is S_Low1, the S/N ratio at the low transmission rate according to the present embodiment is as shown in Equation 1. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     It is understandable from Equation 1 that the S/N ratio at the low transmission rate according to the present embodiment is multiplied by a and improved compared to the S/N ratio at the conventional low transmission rate. 
     Occupied bandwidth H3 after adjustment may be equal to occupied bandwidth H1 (H1=H3) or may be greater than occupied bandwidth H1 (H1&lt;H3) or may be smaller than occupied bandwidth H1 (H1&gt;H3) if it falls within passband #601. 
     For example, when the low transmission rate is 1 kbps and the high transmission rate is 1 Mbps, the rate difference becomes 1000-fold. Therefore, occupied bandwidth control section  102  selects adjustment coefficient α such that the occupied bandwidth approximates to an occupied bandwidth value which is 1000 times the occupied bandwidth of the inputted low transmission rate of 1 kbps and adjusts the occupied bandwidth. 
     As described above, it is possible to prevent deterioration of an S/N ratio by expanding the occupied bandwidth in the case of a low transmission rate. The passband of channel selection filter  305  is set according to the occupied bandwidth of the high transmission rate. As a result, the frequency component of a signal having a high transmission rate is never cut in receiving apparatus  160 . 
     Effects of Embodiment 1 
     According to the present embodiment, adjusting the occupied bandwidth for each transmission rate makes it possible to prevent the S/N ratio from deteriorating while preventing a situation in which a frequency component is cut and cannot be demodulated, without increasing cost. 
     According to the present embodiment, the frequency component is analyzed with the first filter, second filter and third filter connected in parallel, so that it is possible to perform the processing of selecting a parameter fast. 
     Variation of Embodiment 1 
     Although an adjustment coefficient is used as bandwidth setting information in the present embodiment, it is also possible to adjust the occupied bandwidth using optional parameters or information other than the adjustment coefficient. 
     Embodiment 2 
     Configuration of Frequency Analysis Section 
     A configuration of frequency analysis section  700  according to Embodiment 2 of the present invention will be described with reference to  FIG. 7 . The configuration of the rate determining section of the present embodiment has the same configuration as that in  FIG. 4  except in that frequency analysis section  700  is provided instead of frequency analysis section  401 , and therefore the description thereof will be omitted. Moreover, the configuration other than the rate determining section and the method of adjusting an occupied bandwidth are the same as those of Embodiment 1 above, and therefore the description thereof will be omitted. 
     Frequency analysis section  700  mainly includes timer  701 , cut-off frequency setting section  702 , filter  703 , and selection section  704 . 
     Timer  701  measures a time according to an instruction from cut-off frequency setting section  702  and outputs the measurement result to cut-off frequency setting section  702 . 
     When a baseband signal is inputted from RF receiving section  112 , cut-off frequency setting section  702  causes timer  701  to start time measurement. Cut-off frequency setting section  702  performs control of changing a cut-off frequency of filter  703  in a predetermined cycle based on the time measurement result inputted from timer  701 . 
     Filter  703  changes the cut-off frequency according to the control of cut-off frequency setting section  702 . Filter  703  prevents a cut-off frequency of the baseband signal inputted from RF receiving section  112  from passing through the filter and allows frequencies other than the cut-off frequency to pass through the filter. 
     Selection section  704  analyzes the frequency component from the baseband signal inputted after passing through filter  703 . Selection section  704  determines the transmission rate from the analyzed frequency component. Selection section  704  selects and sets a parameter associated with the determined transmission rate with reference to the table stored in storage section  402 , and outputs the set parameter to demodulation section  114  and clock reproducing section  115 . When there is no parameter associated with the determined transmission rate, selection section  704  outputs nothing. 
     Effects of Embodiment 2 
     According to the present embodiment, adjusting the occupied bandwidth for each transmission rate makes it possible to prevent deterioration of an S/N ratio while preventing a situation in which a frequency component is cut and cannot be demodulated, without increasing the manufacturing cost. 
     The present embodiment analyzes the frequency component while changing the cut-off frequency using a single filter and selects a parameter, and thus can reduce the circuit scale and the occupied area of the circuit on the substrate on which the receiving apparatus is mounted. 
     Variation of Embodiment 2 
     The adjustment coefficient is used as bandwidth setting information in the present embodiment, but it is possible to use optional parameters or information other than the adjustment coefficient to adjust the occupied bandwidth. 
     Embodiment 3 
     Configuration of Frequency Analysis Section 
     A configuration of frequency analysis section  800  according to Embodiment 3 of the present invention will be described with reference to  FIG. 8 . The configuration of the rate determining section according to the present embodiment is the same as the configuration in  FIG. 4  except in that frequency analysis section  800  is provided instead of frequency analysis section  401 , and therefore the description thereof will be omitted. Moreover, the configuration other than the rate determining section and the method of adjusting an occupied bandwidth are the same as those of Embodiment 1 above, and therefore the description thereof will be omitted. 
     Frequency analysis section  800  mainly includes FFT processing section  801  and selection section  802 . 
     FFT processing section  801  performs FFT processing on a baseband signal inputted from RF receiving section  112  to transform the baseband signal from a time-domain signal into a frequency-domain signal and outputs the frequency-domain signal to selection section  802 . 
     Selection section  802  analyzes a frequency component from the frequency-domain signal after the FFT processing inputted from FFT processing section  801 . Selection section  802  determines a transmission rate from the analyzed frequency component. With reference to the table stored in storage section  402 , selection section  802  selects and sets a parameter associated with the determined transmission rate and outputs the set parameter to demodulation section  114  and clock reproducing section  115 . When there is no parameter associated with the determined transmission rate, selection section  802  outputs nothing. 
     Effects of Embodiment 3 
     The present invention adjusts an occupied bandwidth for each transmission rate, and thus can prevent deterioration of an S/N ratio while preventing a situation in which a frequency component is cut and cannot be demodulated, without increasing the manufacturing cost. 
     In addition, the present embodiment analyzes a frequency component through FFT processing, and thus can perform a frequency analysis with high accuracy, select an optimum parameter and perform processing such as AGC (Automatic Gain Control) or the like in parallel. 
     Variation of Embodiment 3 
     Although an adjustment coefficient is used as bandwidth setting information in the present embodiment, it is also possible to adjust the occupied bandwidth using optional parameters or information other than the adjustment coefficient. 
     Embodiment 4 
     Configuration of Modulation Section 
     A configuration of modulation section  900  according to Embodiment 4 of the present invention will be described with reference to  FIG. 9 . The configuration of a transmitting apparatus according to the present embodiment has the same configuration as that in  FIG. 1  except in that modulation section  900  is provided instead of modulation section  101 , and therefore the description thereof will be omitted. Moreover, the configuration other than the transmitting apparatus of the present embodiment is the same as that of Embodiment 1 above, and therefore the description thereof will be omitted. 
     Modulation section  900  mainly includes digital filter section  901  and converter  902 . 
     Digital filter section  901  allows a predetermined band of inputted transmission data to pass through the filter. 
     Converter  902  frequency-modulates the transmission data that has passed through digital filter section  901  while changing a control voltage at predetermined resolution which is bandwidth setting information inputted from occupied bandwidth control section  102  and thereby generates an FSK (frequency-shift keying) modulated signal. Converter  902  outputs the generated FSK modulated signal to RF transmitting section  103 . Converter  902  is, for example, a voltage-controlled oscillator (VCO). 
     For example, when the low transmission rate is 1 kbs and the high transmission rate is 1 Mbps, the rate difference becomes 1000-fold. Therefore, converter  902  can broaden the occupied bandwidth of a modulated signal when a low transmission rate is used by adjusting the resolution so as to approximate to 1000-fold according to control of occupied bandwidth control section  102 . 
     When a Gaussian filter is used for digital filter section  901 , modulation section  900  can generate a GFSK modulated signal. 
     The method of adjusting an occupied bandwidth according to the present embodiment is the same as that of Embodiment 1 above except in that resolution is used instead of an adjustment coefficient, and therefore the description thereof will be omitted. 
     Effects of Embodiment 4 
     The present invention adjusts an occupied bandwidth for each transmission rate, and thus can prevent deterioration of an S/N ratio while preventing a situation in which a frequency component is cut and cannot be demodulated. 
     Moreover, the present embodiment performs modulation using an FSK modulation scheme and adjusts an occupied bandwidth by adjusting the resolution in frequency conversion, and thus can adjust the occupied bandwidth using a simple method. 
     Embodiment 5 
     Configuration of Radio Transmission System 
     A configuration of radio transmission system  1000  according to Embodiment 5 of the present invention will be described with reference to  FIG. 10 . 
     Radio transmission system  1000  mainly includes transmitting apparatus  1050  and receiving apparatus  1060 . 
     Transmitting apparatus  1050  and receiving apparatus  1060  can communicate with each other at a plurality of different transmission rates. 
     When the mutual distance is small, transmitting apparatus  1050  and receiving apparatus  1060  perform large-volume communication at a high transmission rate and in a short time. On the other hand, when the mutual distance is large, transmitting apparatus  1050  and receiving apparatus  1060  set a low transmission rate and secure a wide link budget at high reception sensitivity. This allows radio transmission system  100  to be used for a wide variety of purposes. 
     &lt;Configuration of Transmitting Apparatus&gt; 
     A configuration of transmitting apparatus  1050  according to Embodiment 5 of the present invention will be described with reference to  FIG. 10 . 
     Compared to transmitting apparatus  150  according to Embodiment 1 shown in  FIG. 1 , transmitting apparatus  1050  shown in  FIG. 10  is additionally provided with switch section  1001 , spectrum spreading section  1003  and switch section  1004 , and includes modulation section  1002  instead of modulation section  101  and occupied bandwidth control section  1005  instead of occupied bandwidth control section  102 . Parts in  FIG. 10  having the same configuration as that in  FIG. 1  are assigned the same reference numerals and the description thereof will be omitted. 
     Transmitting apparatus  1050  mainly includes RF transmitting section  103 , antenna  104 , switch section  1001 , modulation section  1002 , spectrum spreading section  1003 , switch section  1004 , and occupied bandwidth control section  1005 . 
     Switch section  1001  switches between the output of inputted transmission data to modulation section  1002  and the output of inputted transmission data to spectrum spreading section  1003  according to the control of occupied bandwidth control section  1005 . 
     Modulation section  1002  modulates transmission data inputted via switch section  1001  and generates a modulated signal. Modulation section  1002  outputs the generated modulated signal to RF transmitting section  103  via switch section  1004 . 
     Spectrum spreading section  1003  modulates the transmission data inputted via switch section  1001  and generates a modulated signal. Spectrum spreading section  1003  performs spreading processing on the modulated signal using a spreading code which is bandwidth setting information inputted from occupied bandwidth control section  1005  using a predetermined spreading factor and generates a spread signal obtained by adjusting the occupied bandwidth in a passband of channel selection filter  305  of receiving apparatus  1060  for each transmission rate. Spectrum spreading section  1003  outputs the generated spread signal to RF transmitting section  103  via switch section  1004 . 
     Switch section  1004  switches between the output of the modulated signal inputted from modulation section  1002  to RF transmitting section  103  and the output of the spread signal inputted from spectrum spreading section  1003  to RF transmitting section  103  according to the control of occupied bandwidth control section  1005 . 
     RF transmitting section  103  applies predetermined radio processing to the modulated signal or spread signal inputted from modulation section  1002  or spectrum spreading section  1003  via switch section  1004 . RF transmitting section  103  transmits the signal subjected to the radio processing via antenna  104 . 
     Occupied bandwidth control section  1005  outputs a spreading code of a spreading factor that allows approximation to the bandwidth of the passband of channel selection filter  305  to spectrum spreading section  1003  based on the inputted transmission rate setting information. Occupied bandwidth control section  1005  makes the sequence length of a spreading code variable and thereby makes the spreading factor variable. Details of the configuration of occupied bandwidth control section  1005  will be described later. 
     &lt;Configuration of Receiving Apparatus&gt; 
     A configuration of the receiving apparatus according to Embodiment 5 of the present invention will be described with reference to  FIG. 10 . 
     Compared to receiving apparatus  160  according to Embodiment 1 shown in  FIG. 1 , receiving apparatus  1060  shown in  FIG. 10  is additionally provided with switch section  1011 , spectrum despreading section  1013  and switch section  1014  and includes rate determining section  1012  instead of rate determining section  113 . Parts in  FIG. 10  having the same configuration as that in  FIG. 1  are assigned the same reference numerals and the description thereof will be omitted. 
     Receiving apparatus  1060  mainly includes antenna  111 , RF receiving section  112 , demodulation section  114 , clock reproducing section  115 , switch section  1011 , rate determining section  1012 , spectrum despreading section  1013 , and switch section  1014 . Data acquisition processing section  1020  includes demodulation section  114 , clock reproducing section  115 , switch section  1011 , spectrum despreading section  1013 , and switch section  1014 . Data acquisition processing section  1020  performs processing of acquiring received data from a baseband signal. 
     RF receiving section  112  applies predetermined radio processing to a signal received via antenna  111  and generates a baseband signal. RF receiving section  112  outputs the generated baseband signal to demodulation section  114  via rate determining section  1012  and switch section  1011 . The baseband signal outputted from RF receiving section  112  is inputted to rate determining section  1012  and data acquisition processing section  1020  in parallel. 
     Switch section  1011  switches between the output of the modulated signal inputted from RF receiving section  112  to demodulation section  114  and the output of the spread signal inputted from RF receiving section  112  to spectrum despreading section  1013  according to the control of rate determining section  1012 . 
     Rate determining section  1012  performs despreading processing on the baseband signal inputted from RF receiving section  112 , analyzes the frequency component and determines a transmission rate. When the determined transmission rate is not a predetermined transmission rate, rate determining section  1012  sets a parameter corresponding to the determined transmission rate. Rate determining section  1012  outputs the set parameter to demodulation section  114  and clock reproducing section  115 . When the determined transmission rate is a predetermined transmission rate, rate determining section  1012  does not set any parameter. 
     Rate determining section  1012  stores beforehand a table that lists transmission rates associated with spreading codes. Rate determining section  1012  selects a spreading code associated with the determined transmission rate with reference to the table and outputs the spreading code to spectrum despreading section  1013 . 
     Rate determining section  1012  controls the switching by switch section  1011  and switch section  1014  according to the determined transmission rate. 
     A parameter of a predetermined transmission rate is set beforehand in demodulation section  114  and if no parameter is inputted from rate determining section  1012 , demodulation section  114  demodulates the baseband signal inputted from RF receiving section  112  based on the preset parameter and generates a demodulated signal. When a parameter is inputted from rate determining section  1012 , demodulation section  114  demodulates the baseband signal inputted from RF receiving section  112  based on the inputted parameter and generates a demodulated signal. Demodulation section  114  outputs the generated demodulated signal to clock reproducing section  115  via switch section  1014 . 
     Spectrum despreading section  1013  despreads the baseband signal inputted from RF receiving section  112  via switch section  1011  using the spreading code inputted from rate determining section  1012 . Spectrum despreading section  1013  demodulates the despread signal and generates a demodulated signal. Spectrum despreading section  1013  outputs the demodulated signal to clock reproducing section  115  via switch section  1014 . Here, the spreading code inputted from rate determining section  1012  to spectrum despreading section  1013  is identical to the spreading code inputted from occupied bandwidth control section  1005  to spectrum spreading section  1003 . 
     Switch section  1014  switches between the output of the demodulated signal inputted from demodulation section  114  to clock reproducing section  115  and the output of the demodulated signal inputted from spectrum despreading section  1013  to clock reproducing section  115  according to the control of rate determining section  1012 . 
     A parameter of a predetermined transmission rate is set beforehand in clock reproducing section  115  and when no parameter is inputted from rate determining section  1012 , clock reproducing section  115  reproduces a clock from the demodulated signal inputted from demodulation section  114  or spectrum despreading section  1013  via switch section  1014  based on the preset parameter and converts the clock to bit data. When a parameter is inputted from rate determining section  1012 , clock reproducing section  115  reproduces a clock from the demodulated signal inputted from demodulation section  114  or spectrum despreading section  1013  via switch section  1014  based on the inputted parameter and converts the clock to bit data. Clock reproducing section  115  outputs the bit data as received data. 
     &lt;Configuration of Occupied Bandwidth Control Section&gt; 
     A configuration of occupied bandwidth control section  1005  according to Embodiment 5 of the present invention will be described with reference to  FIG. 11 . 
     Occupied bandwidth control section  1005  mainly includes storage section  1101 , storage section  1102 , and rate difference calculation section  1103 . 
     Storage section  1101  stores the fastest transmission rate among transmission rates that can be transmitted from transmitting apparatus  1050 . 
     Storage section  1102  stores a table that lists rate differences associated with spreading codes of different spreading factors. 
     When transmission rate setting information is inputted, rate difference calculation section  1103  calculates a rate difference between the transmission rate of the transmission rate setting information and the transmission rate stored in storage section  1101 . Rate difference calculation section  1103  selects a spreading code associated with the calculated rate difference with reference to the table stored in storage section  1102 . Rate difference calculation section  1103  outputs the selected spreading code to spectrum spreading section  1003 . 
     When the rate difference is other than “0,” rate difference calculation section  1103  controls the switching by switch section  1001  so that transmission data is inputted to spectrum spreading section  1003  and controls the switching by switch section  1004  so that spectrum spreading section  1003  and RF transmitting section  103  are connected. When the calculated rate difference is “0,” rate difference calculation section  1103  controls the switching by switch section  1001  so that transmission data is inputted to modulation section  1002  and controls the switching by switch section  1004  so that modulation section  1002  and RF transmitting section  103  are connected. The case where the above-described rate difference is “0” is a case where the occupied bandwidth need not be adjusted because the transmission rate of transmission data to be transmitted from now is fastest. 
     When the above-described rate difference is equal to or above a threshold, rate difference calculation section  1103  may control the switching by switch section  1001  so that transmission data is inputted to spectrum spreading section  1003  and may control the switching by switch section  1004  so that spectrum spreading section  1003  and RF transmitting section  103  are connected. On the other hand, when the above-described rate difference is less than a threshold, rate difference calculation section  1103  may control the switching by switch section  1001  so that transmission data is inputted to modulation section  1002  and may control the switching by switch section  1004  so that modulation section  1002  and RF transmitting section  103  are connected. This is applicable to a case where it is not necessary to adjust the occupied bandwidth because a relatively wide occupied bandwidth is provided at a slightly lower transmission rate than the fastest transmission rate. 
     The method of adjusting an occupied bandwidth according to the present embodiment is the same as that of Embodiment 1 above except using a spreading code instead of an adjustment coefficient, and therefore the description thereof will be omitted. 
     Effects of Embodiment 5 
     According to the present embodiment, by adjusting an occupied bandwidth for each transmission rate, it is possible to prevent deterioration of an S/N ratio while preventing a situation in which a frequency component is cut and cannot be demodulated. 
     According to the present embodiment, transmission data is spread and then transmitted, and it is thereby possible to perform communication with high confidentiality and also improve use efficiency of resources. 
     Variation Common to all Embodiments 
     In Embodiment 1 to Embodiment 5 above, the occupied bandwidth of a signal having a low transmission rate has been extended, but the occupied bandwidth of a signal having a high transmission rate may also be reduced. 
     The disclosure of the specification, drawings, and abstract in Japanese Patent Application No. 2013-051626 filed on Mar. 14, 2013, is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     The present invention is suitable for use in a transmitting apparatus that can perform communication at a plurality of different transmission rates and a bandwidth adjusting method. 
     REFERENCE SIGNS LIST 
     
         
           100  Radio transmission system 
           101  Modulation section 
           102  Occupied bandwidth control section 
           103  RF transmitting section 
           104 ,  111  Antenna 
           112  RF receiving section 
           113  Rate determining section 
           114  Demodulation section 
           115  Clock reproducing section 
           120  Data acquisition processing section 
           150  Transmitting apparatus 
           160  Receiving apparatus