Abstract:
A transmission apparatus includes a filter, multiplier, and controller. The filter reduces leakage power outside the transmission signal band. In the filter, a first attenuation amount more than a predetermined amount or a second attenuation amount not more than the predetermined amount is selectively set in a range higher than the transmission signal band. The multiplier modulates the transmission signal output from the filter. The controller sets one of the first and second attenuation amounts in the filter in accordance with the use situation of a band adjacent to the transmission signal band.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a transmission apparatus in a mobile communication system and, more particularly, to a transmission apparatus in a mobile communication system using the CDMA (Code Division Multiple Access) scheme. 
   As shown in  FIG. 9 , a transmission apparatus in a conventional cellular mobile communication system comprises pulse shaping filters  201  and  202  for receiving transmission signals S′ 1  and S′ 2 , respectively, multipliers  203  and  204  for multiplying the outputs from the pulse shaping filters  201  and  202  by carrier waves, an adder  205  for adding the outputs from the multipliers  203  and  204 , a transmission amplifier  206  for amplifying the output from the adder  205 , a circulator  207  for receiving the output from the transmission amplifier  206 , and an antenna  208  for radiating the output from the circulator  207  as a radio wave. 
   In this arrangement, the transmission signals S′ 1  and S′ 2  are limited in their bands by the pulse shaping filters  201  and  202  and multiplied by carrier waves by the multipliers  203  and  204 , respectively. The output signals from the multipliers  203  and  204  are added by the adder  205  and then sent from the antenna  208  through the transmission amplifier  206  and circulator  207 . 
   Generally, the transmission power of a mobile communication system is designed to be small outside the band, as shown in  FIG. 10 . In the above-described conventional transmission apparatus, however, since the power is amplified by the transmission amplifier  206  after bands are limited by the pulse shaping filters  201  and  202 , sufficient attenuation is required in bands B 1  and B 2  outside a band B 0  in use. To obtain sufficient attenuation outside the band, the pulse shaping filters  201  and  202  must have steep characteristics or the transmission amplifier  206  must have high linearity. 
   However, steeper the characteristics on a pulse shaping filter and higher the linearity on a transmission amplifier are required, more the power consumption and the cost are necessary. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a transmission apparatus capable of decreasing power consumption and reducing leakage power outside the transmission band with a simple arrangement. 
   In order to achieve the above object, according to the present invention, there is provided a transmission apparatus comprising filter means for reducing leakage power outside a transmission signal band, the filter means having a first attenuation amount more than a predetermined amount or a second attenuation amount not more than the predetermined amount selectively set in a range higher than a transmission signal band, modulation means for modulating the transmission signal output from the filter means, and control means for setting one of the first and second attenuation amounts in the filter means in accordance with a use situation of a band adjacent to the transmission signal band. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing the arrangement of a transmission apparatus according to an embodiment of the present invention; 
       FIG. 2  is a block diagram showing the internal arrangement of a pulse shaping filter shown in  FIG. 1 ; 
       FIG. 3  is a graph showing the frequency characteristic of a low-pass filter having a small attenuation amount in  FIG. 2 ; 
       FIG. 4  is a graph showing the frequency characteristic of a low-pass filter having a large attenuation amount in  FIG. 2 ; 
       FIG. 5  is a view for explaining a state wherein adjacent cells use adjacent frequency bands; 
       FIG. 6  is a view for explaining a state wherein a large cell and a small cell contained in the large cell use adjacent frequency bands; 
       FIG. 7  is a block diagram showing another example of the pulse shaping filters  101  and  102  shown in  FIG. 1 ; 
       FIG. 8  is a view showing an example of a controller having a monitor device; 
       FIG. 9  is a block diagram showing the arrangement of a conventional transmission apparatus; and 
       FIG. 10  is a graph showing the frequency characteristic of a pulse shaping filter shown in  FIG. 9 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention will be described below in detail with reference to the accompanying drawings. 
     FIG. 1  shows the arrangement of a transmission apparatus according to an embodiment of the present invention. The transmission apparatus of this embodiment is arranged in a mobile station of a communication system using the CDMA scheme. 
   As shown in  FIG. 1 , the transmission apparatus according to this embodiment comprises pulse shaping filters  101  and  102  for reducing leakage power outside the transmission band from the transmission signals S 1  and S 2 , multipliers  103  and  104  serving as modulation means for modulating carrier waves by the outputs from the pulse shaping filters  101  and  102 , respectively, an adder  105  for adding the outputs from the multipliers  103  and  104 , a transmission amplifier  106  for amplifying the output from the adder  105 , a circulator  107  for receiving the output from the transmission amplifier  106 , an antenna  108  for radiating the output from the circulator  107  as a radio wave, and a controller  109  for selecting one of the pulse shaping filters  101  and  102  on the basis of a reception signal output from the circulator  107 . The controller  109  has an information extraction section  109   a  for extracting necessary information from a reception signal through the antenna  108  and circulator  107 . 
   As shown in  FIG. 2 , each of the pulse shaping filters  101  and  102  comprises low-pass filters  110  and  111  for commonly receiving a signal, and a switch  112  for selectively outputting one of the outputs from the low-pass filters  110  and  111 . The frequency characteristic of the low-pass filter  110  is designed such that the attenuation amount in the range higher than the use band has a predetermined or smaller value, as shown in  FIG. 3 . The frequency characteristic of the low-pass filter  111  is designed such that the attenuation amount in the range higher than the use band has a value larger than the predetermined value, as shown in  FIG. 4 . 
   In this arrangement, the low-pass filter  110  or  111  is selected by the controller  109  depending on whether an adjacent frequency band is used (assigned) in an adjacent system. More specifically, when the adjacent system uses the adjacent frequency band, the low-pass filter  111  is selected by a control signal output from the controller  109 . When no adjacent frequency band is used in the adjacent system, the low-pass filter  110  is selected by a control signal output from the controller  109 . At this time, power supply to the unselected filter is simultaneously stopped. 
   The operation of the transmission apparatus having the above arrangement will be described next. 
   The transmission signals S 1  and S 2  are limited in their bands by the pulse shaping filters  101  and  102  first. At this time, the information extraction section  109   a  of the controller  109  extracts, from a reception signal, use information that is transmitted from the base station (not shown) and indicates the use situation of an adjacent frequency band. The controller  109  outputs a control signal to the pulse shaping filters  101  and  102  on the basis of the received use information. In each of the pulse shaping filters  101  and  102 , the switch  112  is switched in accordance with the control signal from the controller  109  to select one of the low-pass filters  110  and  111 . In this case, the selected one of the low-pass filters  110  and  111  executes filter processing for the transmission signal. 
   The signals S 1  and S 2  whose bands are limited by the pulse shaping filters  101  and  102  are multiplied by carrier waves cos(ωt) and −sin(ωt) by the multipliers  103  and  104 , respectively, and added by the adder  105 . The output from the adder  105  is amplified by the transmission amplifier  106  and transmitted from the antenna  108  through the circulator  107 . 
   The transmission amplifier  106 , circulator  107 , and antenna  108  are devices for transmitting/receiving a signal through a radio channel. They are known by those who are skilled in the art well and are irrelevant to the present invention, so a detailed description of the arrangement and operation will be omitted. 
   Assume that a mobile station  113  which is communicating in a cell A is located near a base station  115  in a cell B adjacent to the cell A, as shown in  FIG. 5 . The cells A and B form different mobile communication systems. In this case, when the adjacent cell B uses an adjacent frequency band, the mobile station  113  can reduce the influence on the system of the adjacent cell B by selecting a filter having a large attenuation amount for transmission. On the other hand, when no adjacent frequency band is used in the adjacent cell B, power consumption can be reduced by lowering the attenuation amount to a predetermined value. 
   Next, assume that a small cell C is present in a large cell D, and that the mobile station  113  is communicating in the cell C, as shown in  FIG. 6 . In this case, when an adjacent frequency band is used in the superposing cell D, the mobile station  113  can reduce the influence on the system of the superposing cell D by selecting a filter having a large attenuation amount for transmission. On the other hand, when no adjacent frequency band is used in the superposing cell D, power consumption can be reduced by lowering the attenuation amount to a predetermined value. 
     FIG. 7  shows another example of the pulse shaping filters  101  and  102 . In this example, a filter having a small attenuation amount is implemented by part of a filter having a large attenuation amount. Referring to  FIG. 7 , the pulse shaping filter  101  or  102  is formed from an FIR (Finite Impulse Response) filter having a plurality of delay elements D 1  to D n  (n is a positive integer: n≧2) cascade-connected, a plurality of digital multipliers M 0  to M n  for weighting the outputs from the delay elements D 1  to D n , respectively, and accumulators A 1  and A 2  for dividing and cumulatively adding the outputs from the digital multipliers M 0  to M n . The pulse shaping filter  101  or  102  also has changeover switches  118 ,  119 , and  120  ON/OFF-controlled by a control signal from the controller  109 . The changeover switch  118  may be omitted. 
   In this case, the delay elements D 1  to D n , digital multipliers M 0  to M n , and accumulator A 1  correspond to the low-pass filter  110  shown in  FIG. 2 . The delay elements D 1  to D n , the plurality of digital multipliers M 0  to M n , and accumulators A 1  and A 2  correspond to the low-pass filter  111  shown in  FIG. 2 . The changeover switches  118 ,  119 , and  120  correspond to the switch  112  shown in  FIG. 2 . 
   In this arrangement, when the changeover switch  118  is turned off, and the accumulator A 2  is bypassed through a bypass path  121  by the changeover switches  119  and  120 , as shown in  FIG. 7 , a filter having a low attenuation amount can be formed from the delay elements D 1  to D m . When the changeover switch  118  is turned on, and the accumulator A 2  is cascade-connected to the accumulator A 1  by the changeover switches  119  and  120 , a filter having a large attenuation amount can be formed from the delay elements D 1  to D n . The number of delay elements D 1  to D m  or delay elements D 1  to D n , which determines the attenuation amount, is determined to an optimum number for the system. 
   According to this example, since neither pulse shaping filter with steep characteristics nor transmission amplifier with high linearity is required, and only one FIR filter suffices, the cost can be reduced. 
   A case wherein the base station transmits no use information representing the use situation of an adjacent frequency band will be described next. In this case, as show in  FIG. 8 , the controller  109  has a monitor device  109   b  for monitoring a reception signal. The controller  109  determines the use situation of an adjacent frequency band in accordance with the monitor result from the monitor device  109   b  and outputs a control signal. The processing of determining the use situation of an adjacent frequency band, including the monitor operation, is processing of determining on the basis of a reception signal whether an adjacent frequency band is being used in an adjacent system. This processing is known by a person skilled in the art in the communication system using the CDMA scheme. 
   In the above embodiment, the transmission apparatus is arranged in a mobile station. However, the present invention is not limited to this. For example, when the transmission apparatus is applied to a base station, the use frequency band of an adjacent system is fixed and known. For this reason, one of the low-pass filters  110  and  111  is prepared in each base station in accordance with whether the adjacent frequency band is used in the adjacent system. In this case, the controller  109  and switch  112  are unnecessary. 
   The present invention is not limited to the above-described embodiment, and various changes and modifications can be appropriately made without departing from the spirit and scope of the present invention.