Abstract:
A cell phone antenna switching circuit includes a plurality of built-in antennas which perform radio transmission/reception. This circuit includes reception level measuring sections and an antenna switching section. The reception level measuring sections measure the reception levels of the corresponding built-in antennas. The antenna switching section always automatically selects an antenna whose reception level is high during a standby period, and selects an antenna with good antenna characteristics depending on a function during speech communication. A cell phone antenna switching method is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a cell phone antenna switching circuit and antenna switching method and, more particularly, to a cell phone antenna switching circuit and switching method which use two built-in antennas in a cell phone which requires continuous transmission/reception as in the CDMA (Code Division Multiple Access) scheme or the like and selectively use the antennas depending on the function of the cell phone which is to be used. 
     In general, with increasing reductions in the size and thickness of a cell phone, it has excellent portability. The cell phone is placed near the user during speech communication, and hence the influence of the human body on an antenna is a factor that causes a deterioration in reception characteristics and the like. 
     Of the recent cell phones, cell phones capable of continuous transmission/reception as in the CDMA (Code Division Multiple Access) scheme or the like which realizes good communication quality have been widely used. 
     Cell phones using the 2-GHz band and frequencies higher than the 2-GHz band, such as cell phones based on the CDMA scheme, allow reductions in antenna length because of short wavelengths they use. For this reason, compact built-in antennas are often applied to such cell phones. 
     When a built-in antenna is applied to a cell phone, the antenna is mounted in a place where the human body has the smallest influence. In general, since the antenna is greatly influenced by the head of the human body during speech communication, the antenna is often mounted in a place free from the influence of the head during speech communication. 
     There is a recent trend toward a remarkable increase in the number of users of cell phones who connect to the Internet when executing the mail function using packet communication. That is, some users mainly perform key operation for mail communication with their hands independently of the influences of their heads. Conventional cell phones have been mainly so designed as to avoid the influences of the head of the human body. From now on, cell phones need to be designed in consideration of antenna characteristics (radiation efficiency) when they are mainly used with key operation. In order to realize this, two antennas and a conventional diversity function are used. In this case, the reception electric field levels of the two antennas which have received radio waves are measured, and one of them which has a higher reception electric field level is selected. 
     In a cell phone designed to perform continuous transmission/reception as in the CDMA scheme or the like, once a selected antenna is determined, the antenna in the use cannot be switched to another one by antenna diversity during continuous transmission/reception in speech communication or packet communication. If an antenna is placed at a portion where the movement of the hand exerts influences, an unfavorable antenna may be erroneously selected during key operation due to unstable reception electric field levels during the key operation. For this reason, demands have arisen for a cell phone having a built-in antenna switching function of automatically selecting an optimal antenna depending on the function. 
     A conventional cell phone antenna switching circuit is a portable radio apparatus which has two or more antennas capable of transmission/reception and selects a transmission antenna by comparing the reception electric field strengths of the antennas in the diversity reception mode (see, for example, Japanese Patent Laid-Open No. 2002-261665 (pp. 2-4; FIG. 2)). 
     Some cell phone is designed to obtain good reception characteristics even during speech communication by manually switching built-in antennas even if the cell phone is unfolded into upper and lower housings during speech communication, and the reception characteristics deteriorate when the ear of the user is brought into contact with the speaker in the upper housing and a built-in antenna is located close to the ear (see, for example, Japanese Patent Laid-Open No. 11-177485 (pp. 2-3; FIG. 2)). 
     In a cell phone designed to perform continuous transmission/reception as in the CDMA scheme or the like, the conventional cell phone antenna switching circuit described above cannot switch an antenna in use to the other antenna by antenna diversity during continuous transmission/reception in speech communication or packet communication after the selected antenna is determined. 
     In addition, since the cell phone comes into contact with part of the human body during speech communication, the antenna characteristics deteriorate. 
     If an antenna is placed in a portion where the movement of the hand exerts influences, in particular, since the reception electric field levels become unstable during key operation, an unfavorable antenna may be erroneously selected during the key operation. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a cell phone antenna switching circuit and antenna switching method which obtain good reception characteristics by automatically switching built-in antennas even when a built-in antenna in a cell phone comes close to the human body during speech communication, and select an optimal antenna during continuous transmission/reception in speech communication or packet communication after an antenna is selected in a cell phone designed to perform continuous transmission/reception as in the CDMA scheme or the like. 
     In order to achieve the above object, according to the present invention, there is provided a cell phone antenna switching circuit including a plurality of built-in antennas which perform radio transmission/reception, comprising a plurality of reception level measuring means for measuring reception levels of the corresponding built-in antennas, and antenna selection means for always automatically selecting an antenna whose reception level is high during a standby period, and selecting an antenna with good antenna characteristics depending on a function during speech communication. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing the outer appearance of a cell phone antenna switching circuit according to an embodiment; 
         FIG. 2  is a block diagram showing the cell phone antenna switching circuit in  FIG. 1 ; and 
         FIG. 3  is a flow chart for explaining the operation of the cell phone antenna switching circuit according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will be described next with reference to the accompanying drawings. 
       FIG. 1  shows a cell phone antenna switching circuit according to an embodiment of the present invention. 
     This embodiment shown in  FIG. 1  is comprised of a speaker  11  and microphone  12  incorporated in a cell phone  10 , an antenna  1  mounted in an area  13 , and an antenna  2  mounted in an area  14 . 
       FIG. 1  shows the positions of the two built-in antennas of the cell phone of the present invention. In this case, the shape and material of each antenna are irrelevant to the present invention. The speaker  11  and microphone  12  of the cell phone are described to mark the places where the two built-in antennas are mounted. However, the functions of these components are irrelevant to the present invention but they serve as only marks in explaining the positions of the antennas. In this case, the two built-in antennas will be referred to as the antenna  1 , which is located on the speaker  11  side in the area  13  of the cell phone  10 , and the antenna  2 , which is located on the microphone  12  side in the area  14 , respectively. 
     The antenna  1  is located on the speaker  11  side and mainly used when key operation is performed. The antenna  2  is located on the microphone  12  side and mainly used when the user performs speech communication while his/her ear is in contact with the speaker. 
     Although the antenna  1  is located on the speaker  11 , there is no special reason why it is located near the speaker  11 . The antenna  1  can be placed in any place as long as it exerts no influence on antenna radiation efficiency during key operation. 
     Likewise, although the antenna  2  is located on the microphone  12  side, the antenna can be placed in any place free from the influences of the human body in speech communication. 
       FIG. 2  shows an antenna switching circuit for the cell phone in  FIG. 1 . 
     Note that the same reference numerals and symbols as in  FIG. 1  denote the same constituent elements in  FIG. 2 , and a description thereof will be omitted. 
     Referring to  FIG. 2 , this circuit is comprised of the antennas  1  and  2  which are used for radio transmission/reception, an antenna switch  3  which switches the antennas  1  and  2 , a transmission circuit  4  which modulates speech and packet data and transmits the modulated data, a reception circuit  5  which receives and demodulates the modulated waves of speech and packet data, a duplexer  6  which passes the transmission wave output from the transmission circuit  4  to the antenna  2  side (passes only waves in the transmission band) and passes the reception wave received by the antenna  1  to the reception circuit  5  side (passes only waves in the reception band) but does not pass transmission waves from the transmission circuit  4  side to the reception circuit  5  side, i.e., prevents interference, a CPU (Central Processing Unit)  7  which controls the antenna switch  3  functioning as a control means, a reception level measuring section  8  which measures the reception level of the antenna  1 , and a reception level measuring section  9  which measures the reception level of the antenna  2 . 
     The operation of this embodiment will be described in detail next with reference to  FIGS. 2 and 3 .  FIG. 3  explains the operation of the antenna switching circuit of the cell phone according to the present invention. Note that  FIG. 3  shows the operation of the embodiment based on the CDMA scheme as an example. 
     When the power switch of the apparatus is turned on (step S 1 ), a cell search is executed (step S 2 ). At this time, the antenna stored when the power switch was turned off is temporarily selected. It is checked in the next step whether the cell search is complete, i.e., the cell search has succeeded (step S 3 ). If the cell search is not complete and has failed, the antenna is switched to the other antenna, and the flow returns to step S 2  (step S 4 ) to execute a cell search again. 
     After the cell search is complete, the flow shifts to a position registration sequence for a base station to register the position of the cell phone (step S 5 ). It is checked in the next step whether or not the position registration has succeeded (step S 6 ). In this case as well, if the position registration has failed, the antenna is switched to the other antenna, and the flow returns to step S 5  (step S 7 ) to execute position registration again. 
     If it is determined in step S 6  that the position registration has succeeded, the position registration is complete, and the flow shifts to a standby state to wait for an outgoing or incoming call for speech communication (step S 8 ). In this standby state, one of the antennas which has a higher reception level is always selected. In this case, the reception levels of the respective antennas are measured by the reception level measuring sections  8  and  9  to determine whether the reception level of the current antenna is higher than that of the other antenna (step S 9 ). If the reception level of the current antenna is not higher than that of the other antenna, the current antenna is switched to the other antenna, and the flow returns to the standby state in step S 8  (step S 10 ). In steps  8  and  9 , one of the antennas which has a higher reception level is always selected, and the flow advances to the next step. 
     The next step is speech communication. When the flow advances to this step, one of the antennas which has a higher reception level is finally determined as the current antenna. 
     When an outgoing call for speech communication is to be generated or an incoming call is to be received in the standby state (step S 11 ), the antenna  2  is selected by the antenna switch  3  to improve the radiation efficiency affected by the human body (ear and the like) in speech communication (except for speech communication through an earphone/microphone). That is, the CPU  7  instructs the antenna switch  3  to connect the switch to the A side so as to select the antenna  2  (step S 12 ). If the antenna  1  is selected during speech communication, since the antenna is located near the head of the human body, the antenna radiation efficiency deteriorates. If the antenna  2  is selected, since the microphone is separated from the mouth of the human body to a certain degree, the radiation efficiency can be improved as compared with the antenna  1 . 
     When Internet connection (packet communication) is started by key operation in the standby state (step S 13 ), since the user mainly performs key pressing operation during packet communication, one of the antennas which is less influenced by the hand must be selected. If the antenna  2  is used in this case, the reception level becomes unstable because the antenna  2  is covered with the hand. Using the antenna  1  will therefore improve the antenna radiation efficiency. For this reason, when packet transmission is to be performed, the CPU  7  instructs the antenna switch  3  to connect the antenna switch  3  to the B side so as to select the antenna  1  (step S 14 ). 
     In either case, the flow advances to the next step to always check whether speech communication is being performed (step S 15 ). If no speech communication is being performed, on-hook operation is performed, and the flow returns to the standby state in step S 8  again (step S 16 ). 
     As described above, when a cell phone which requires continuous transmission/reception as in the CDMA scheme or the like is to use built-in antennas, a sufficient radiation efficiency can be obtained by selectively using two built-in antennas in accordance with the function of the cell phone, e.g., the speech communication function or the packet communication function. 
     That is, since the antenna switching circuit of the cell phone can maximize the antenna radiation efficiency in both speech communication and packet communication, the SIR (Signal to Interference power Ratio) on the base station side can be increased by using an optimal antenna. In addition, a reduction in speech communication current can be achieved by reducing the transmission power by TPC (Transmission Power Control). 
     Note that the number of antennas incorporated in the cell phone is not limited to two. For example, a cell phone having n (n is an integer equal to or more than 2) built-in antennas can also be designed such that the reception levels of the n built-in antennas are measured during a standby period to always automatically select an antenna with a high reception level, and an antenna having good antenna characteristics (radiation efficiency) is reliably selected in accordance with the function during speech communication. 
     In addition, if the above antenna switching circuit of the cell phone is formed into a hybrid IC, and the IC is applied to the cell phone, reductions in the size and weight of the cell phone can be attained. 
     As has been described above, the antenna switching circuit of the cell phone according to the present invention can obtain good reception characteristics by automatically switching the built-in antennas even if the antenna incorporated in the cell phone is brought close to the human body during speech communication. 
     In addition, even with regard to a cell phone designed to perform continuous transmission/reception as in the CDMA scheme or the like, after a selected antenna is determined, an optimal antenna can be selected during continuous transmission/reception in speech communication or packet communication.