BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to spread spectrum communication apparatus and method for receiving a wide-band spread spectrum signal. 
     FIG. 2 shows a spread spectrum communication apparatus. 
     A received spread signal is supplied from one of antennas  111  and  112  which is selected by an antenna switch  110 , to a low-noise amplifier  113  via a band-pass filter (BPF)  109 . Then, a frequency of the received signal is converted into a predetermined frequency by a frequency converter  114  to which a local oscillator  102  is connected. The received signal which has been subjected to the frequency conversion is further subjected to predetermined band restriction by a band-pass filter (BPF)  115 , and is then supplied to a variable amplifier  116 . After then, the received signal of which level has been set at a predetermined reception level is supplied to a demodulator  117  to be demodulated to data S 3 , and the data S 3  is subsequently supplied to a control unit  118 . 
     In this case, an output from the variable amplifier  116  is also supplied to a wave detector  119  to detect field intensity of the received signal. A voltage detected by the wave detector  119  is applied to an automatic gain control (AGC) voltage generator  120 , such that the applied voltage acts as a control voltage of the variable amplifier  116  to set the level of the received signal as a predetermined signal level. Also, an output from the AGC voltage generator  120  is supplied to an antenna switch signal generator  121  to switch or change the antenna in accordance with a signal S 7  in a case where a level of such output is equal to or smaller than a predetermined threshold level. 
     However, in the spread spectrum communication apparatus shown in FIG. 2, since wave detection is performed by the wave detector  119  for an entire band (i.e., a band of the BPF  115 ) of the received signal, if there are noise components, e.g., an unnecessary wave included in the band, a cross modulation wave generated in a system, and the like, voltage levels of these noise components are also detected, whereby it is difficult to detect a normal signal level. Especially, as the level of the received signal becomes lower, an amplification factor of the variable amplifier  116  becomes larger, whereby only noise power is further amplified. Therefore, the level of the received signal can not be accurately detected. As a result, it is difficult to switch the antenna normally. 
     Further, since the received signal is a wide-band spread signal, a fall of spectrum within the band has various forms because of multipath. In this case, the multipath represents a phenomenon in which there are a plurality of paths, e.g., a reception side receives a direct wave and a reflected wave or receives a plurality of reflected waves. 
     FIGS. 3A to  3 C respectively show states of the fall of spectrum of the received signal because of the multipath. 
     FIG. 3A shows the state that there is no fall of spectrum of the received signal because of the multipath, FIG. 3B shows the state that there is a fall of spectrum at a frequency adjacent to a central frequency f 0  (i.e., apart from the central frequency f 0  by f 1 ), and FIG. 3C shows the state that there is the fall of spectrum at the central frequency f 0 . In the case of FIGS. 3B and 3C, in the apparatus shown in FIG. 2, if received power is uniform within the band, the wave detector  119  detects the same-level voltage. 
     However, error generation probability in the case of the fall of spectrum shown in FIG. 3B tends to be higher than that in case of the fall of spectrum shown in FIG.  3 C. That is, as the fall of spectrum adjacent to a main lobe of the received signal is larger, the error generation probability tends to be higher. 
     Therefore, in the apparatus shown in FIG. 2, since the received signal is subjected to the wave detection for the entire band, differences of the fall of spectrum cannot be judged. Thus, there is a drawback that the antenna is switched only based on the field intensity of the received signal. 
     FIG. 7 shows the structure of a conventional spread spectrum communication apparatus which relates to transmission power control. 
     In FIG. 7, a code generator  7  generates a pseudo-noise (PN) code for use in de-spreading, and a multiplier  3  multiplies a received signal by an output of a local oscillator  10 . An automatic gain control (AGC) voltage generator  19  outputs a control signal to a variable gain amplifier on a reception side. Also, the AGC voltage generator  19  outputs the control signal to a variable gain amplifier  13  on a transmission side. 
     In this manner, if a correlation output is used for transmission power control, there is a drawback that a long time is required. 
     Such drawback also occurs in a case where the correlation output is used for switching the antenna. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to improve spread spectrum communication. 
     Another object of the present invention is to increase reliability of the spread spectrum communication. 
     Another object of the present invention is to quickly and accurately select a desired antenna from among a plurality of antennas which are used for the spread spectrum communication. 
     Another object of the present invention is to quickly and accurately control transmission power of a spread spectrum signal, to a desired value. 
     The above and other objects of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the structure of first and third spread spectrum communication apparatuses to which the present invention is applied; 
     FIG. 2 is a block diagram showing the structure of a spread spectrum communication apparatus; 
     FIGS. 3A to  3 C show states of a fall of spectrum of a received signal because of a multipath error in spread spectrum communication; 
     FIG. 4 is a flow chart showing a first example of antenna switching control in accordance with the present invention; 
     FIG. 5 is a block diagram showing the structure of second and fourth spread spectrum communication apparatuses to which the present invention is applied; 
     FIG. 6 is a flow chart showing a second example of antenna switching control in accordance with the present invention; and 
     FIG. 7 is a block diagram showing the structure of a conventional spread spectrum communication apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram showing a first spread spectrum communication apparatus to which the present invention is applied. 
     In the drawing, reference numeral  101  denotes a spread modulator which is composed of a local oscillator and a spread code generator (both not shown). The spread modulator  101  performs primary modulation on input data and then performs secondary modulation on the input data by using a spread code, to generate a spread signal having a predetermined band. Reference numeral  102  denotes a local oscillator, and reference numeral  103  denotes a switch. Supplying of an output from the local oscillator  102  is switched by the switch  103  from a transmission side to a reception side and vice versa at transmission and reception timings. Reference numeral  104  denotes a frequency converter which converts a frequency of the spread signal supplied from the spread modulator  101  into a desired frequency. The spread signal which has been subjected to the frequency conversion is amplified to have predetermined power via an amplifier  105  and a power amplifier  106 , and is then supplied to a transmission/reception switch  108 . Reference numeral  107  denotes a bias switch which operates to change an operation point of the power amplifier  106  in response to a bias switch signal S 6 . 
     Reference numeral  108  denotes the transmission/reception switch which performs switching between a transmission signal and a reception signal at the transmission and reception timings. In the case of transmission, the spread signal is supplied to a band-pass filter (BPF)  109  via the transmission/reception switch  108 . The output signal from the BPF  109  is then supplied to the antenna switch  110  as the spread signal which has been subjected to desired band restriction, and is thereafter radiated in the air via either one of antennas  111  and  112 . On the other hand, in the case of reception, the received spread signal is supplied from one of the antennas  111  and  112  selected by the antenna switch  110 , to a low-noise amplifier  113  via the BPF  109  and the transmission/reception switch  108 . Subsequently, the frequency of the supplied spread signal is converted into a predetermined frequency by a frequency converter  114 . 
     The received signal of which the frequency has been converted into the predetermined frequency by the frequency converter  114  is distributed to a band-pass filter (BPF)  115  and a narrow-band band-pass filter (BPF)  123 . The signal which has been subjected to the predetermined band restriction is supplied to a variable amplifier  116  and a second wave detector  122 . Then, a level of the signal which has passed through the variable amplifier  116  is set as a constant signal level, and such signal is supplied to a demodulator  117 . Subsequently, data S 3  which is output from the demodulator  117  is subjected to de-spreading demodulation in accordance with the spread code, and then the demodulated signal is supplied to a control unit  118 . 
     Further, field intensity of the received signal within the band of the BPF  115  is detected in response to the signal supplied to the second wave detector  122 . On the other hand, the signal of which the band has been restricted by the narrow-band BPF  123  to a frequency adjacent to a central frequency is supplied to a first wave detector  124 , and the field intensity of the signal is detected by the first wave detector  124 . In this case, the second wave detector  122  detects the intensity of the received signal having the wide reception band which signal has been subjected to spectrum spreading. Further, the first wave detector  124  detects intensity of the received signal having the band which has been restricted to be sufficiently narrower than the above-described wide reception band (i.e., such narrow band being adjacent to the central frequency of the reception band). 
     By using a signal S 1  supplied from the first wave detector  124  and a signal S 8  supplied from the second wave detector  122 , the control unit  118  performs the switching between the antennas  111  and  112  in accordance with a flow chart shown in FIG.  4 . That is, in a case where the signal S 1  becomes equal to or lower than a first threshold level (step S 10 ) or in a case where the signal S 2  becomes equal to or lower than a second threshold level (step S 20 ), the control unit  118  performs the switching of the antenna (step S 30 ). Further, in order to set the level of the received signal as the constant signal level, the control unit  118  also uses the signal S 8  detected by the second wave detector  122  as a control signal S 2  for the variable amplifier  116 . A control program of the control unit  118  which program is represented by the flow chart shown in FIG. 4 is stored in a memory  118 M. Such control program may be previously stored in the memory  118 M, or may be read from a disk memory (not shown) into the memory  118 M in response to power on. Further, such the control program stored in the memory  118 M may be rewritten in response to the received signal supplied from the antenna  111  or  112 . 
     FIG. 5 is a block diagram showing a second spread spectrum communication apparatus to which the present invention is applied. 
     In FIG. 5, the same components as those in FIGS. 1 and 2 are added with the same reference numerals, and thus the explanation thereof is omitted. 
     A received signal of which frequency has been converted into a predetermined frequency by a frequency converter  114  is distributed to a band-pass filter (BPF)  115 , a first narrow-band BPF  123 , a second narrow-band BPF  125  and a third narrow-band BPF  127 , respectively. The signal which has been subjected to predetermined band restriction by the BPF  115  is supplied to a variable amplifier  116  and a second wave detector  122 . Then, a level of the signal which has passed through the variable amplifier  116  is set as a constant reception level, and such signal is supplied to a demodulator  117 . Subsequently, a signal S 3  output from the demodulator  117  is subjected to despread demodulation and then supplied to a control unit  118 . 
     Further, field intensity of the received wide-band signal is detected on the basis of the signal supplied to the second wave detector  122 . On the other hand, reception field intensity of each signal which has been subjected to the band restriction by each of the first to third narrow-band BPFs  123 ,  125  and  127  to have the band narrower than a reception frequency band is detected by each of first, third and fourth wave detectors  124 ,  126  and  128 . These three narrow-band BPFs  123 ,  125  and  127  are allocated to respective frequency bands which are sufficiently apart from others, such that each band of these BPFs does not overlap with others within the reception frequency band. In this case, it is assumed that, within the reception frequency band, the narrow-band BPF  123  is set to be adjacent to a central frequency, the narrow-band BPF  125  is set to be adjacent to a lower-limit frequency, and the narrow-band BPF  127  is set to be adjacent to an upper-limit frequency. 
     In accordance with a flow chart shown in FIG. 6, the control unit  118  performs switching between antennas  111  and  112  in response to signals S 1 - 1  to S 1 - 3  respectively supplied from the first, third and fourth wave detectors  124 ,  126  and  128 , and a signal S 8  supplied from the second wave detector  122 . In a case where the signal S 8  is equal to or lower than a threshold level 1 (step S 20 ), in a case where the signal S 1 - 1  is equal to or lower than a threshold level 2 (step S 110 ), or in a case where one of the signals S 1 - 2  and S 1 - 3  and the signal S 1 - 1  are equal to or lower than a threshold level 3 (steps S 120  and S 130 ), the control unit  118  performs the switching of the antenna (step  530 ). In the flow chart shown in FIG. 6, the threshold level 1 corresponds to the signal S 8 , the threshold level 2 corresponds to the signal S 1 - 1  and the threshold level 3 corresponds to the signals S 1 - 1  to S 1 - 3 , respectively. Further, the threshold levels 2 and 3 satisfy relationship that the threshold level 2&lt; the threshold level 3. 
     Furthermore, the control unit  118  also uses the signal S 8  which has been detected by the second wave detector  122 , as a control signal S 2  of the variable amplifier  116 , to set a reception signal level as a constant signal level. A control program of the control unit  118  which program is represented by the flow chart shown in FIG. 6 is stored in a memory  118 M. Such control program may be previously stored in the memory  118 M, or may be read from a disk memory (not shown) into the memory  118 M in response to power on. Further, such control program stored in the memory  118 M may be rewritten in response to the received signal supplied from the antenna  111  or  112 . 
     Furthermore, in order to simplify the control, the process represented by the step S 20  may be omitted in the procedures shown in FIGS. 4 and 6. That is, the signal S 8  need not be used for switching the antenna. 
     In the above structure shown in FIGS. 1 and 5, the number of the antennas to be switched is two. However, the present invention is not limited to such structure. That is, even if the number of the antennas to be switched is three or more, the same effect as described above can be derived in the present invention. 
     Further, in the structure shown in FIG. 5, the number of the narrow-band BPFs is three. However, the present invention is not limited to such structure. That is, by observing only the specific frequency band using the two or more narrow-band BPFs, a propagation state within the reception band can be grasped. Therefore, more fine or smooth control becomes possible by increasing the number of the narrow-band BPFs. 
     Furthermore, in the structure shown in FIG. 5, the output threshold levels of the narrow-band BPF are two levels. However, the present invention is not limited to such structure. That is, by providing multiple threshold levels in accordance with the number of the narrow-band BPFs to be used, more fine or smooth control becomes possible. 
     In the above structure shown in FIGS. 1 and 5, the antenna switching control is performed in accordance with the level of the signal which has passed through the narrow-band BPF. In the third and fourth spread spectrum communication apparatuses to which the present invention is applied, the control unit  118  generates the signal S 6  and performs the transmission power control in the bi-directional (two-way) communication in response to the signal S 1  or the signals S 1 - 2  to S 1 - 3 . 
     Further, in the third and fourth spread spectrum communication apparatuses, the control unit  118  controls the bias switch  107  in response to the detection output signals S 1 , S 1 - 1 , S 1 - 2  and S 1 - 3 . However, the antenna switch  110  may be controlled or may not be controlled. 
     Furthermore, in the third and fourth spread spectrum communication apparatuses, the control programs of the control unit  118  are stored in the memory  118 M in the same manner as those of the control unit  118  in the first and second spread spectrum communication apparatuses. 
     Furthermore, in the case where the present invention is applied to the antenna switching, the present invention can be applied to an apparatus which is exclusively used for the reception and thus does not have any structure for the transmission. 
     Furthermore, in the first to third spread spectrum communication apparatuses, the control unit  118  detects magnitude of the reception signal supplied from the antenna which has been selected by the antenna switch  110 . However, by providing two pairs of the narrow-band BPF  123  and the first wave detector  124  or two pairs of the narrow-band BPFs  123 ,  125  and  127  and the wave detectors  124 ,  126  and  128  for each antenna, the antenna switching or the transmission power switching may be controlled in accordance with magnitude of the reception signals of the respective antennas. 
     Although the present invention has been described above with respect to the preferred embodiments, the present invention is not limited to the foregoing embodiments but many modifications and variations are possible with the spirit and scope of the appended claims.
There is provided a spread spectrum communication apparatus comprising a reception means for receiving a wide-band spread spectrum signal, a wave detection means for performing wave detection on a predetermined narrow-band signal within the reception signal received by the reception means, and a de-spread means for de-spreading the wide-band spread spectrum signal received by the reception means. Therefore, a desired one of a plurality of antennas to be used in a spread spectrum communication can be quickly and accurately selected, and also transmission power of the spread spectrum signal can be quickly and accurately controlled to have a desired value, whereby reliability of the spread spectrum communication can be increased.