Abstract:
A Time Division Duplex (TDD) wireless communication system includes a switch connected to an output port of a transmitter, a first transmission line for transmitting a transmission signal and for isolating a transmission path according to a mode, a first transmission line stub connected between the isolator and the first transmission line for reflecting a transmission signal transmitted from the isolator, a second transmission line connected between an output port of the first transmission line and an input port of a receiver for isolating a reception path in the transmission mode and for providing a reception signal received from the antenna feed line to the receiver, and a second transmission line stub connected in a stub form between the second transmission line and the input port of the receiver, for controlling the second transmission line to isolate the reception path.

Description:
PRIORITY 
       [0001]    This application claims the benefit under 35 U.S.C. § 119(a) of a Korean patent application filed in the Korean Intellectual Property Office on May 25, 2006 and assigned Serial No. 2006-47000, the entire disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a Time Division Duplex (TDD) switch of a TDD wireless communication system. More particularly, the present invention relates to an apparatus for protecting a receiver when a high-power transmission signal is introduced to the receiver out of sync due to erroneous operations such as a malfunction of the TDD switch or when power of the TDD switch is blocked. In addition, the present invention relates to a TDD switch using a transmission line and a transmission line stub without having to use a conventional circulator. 
         [0004]    2. Description of the Related Art 
         [0005]    In general, a Time Division Duplex (TDD) wireless communication system uses a TDD switch to change its mode between a transmission mode and a reception mode. Such a mode change allows a transmission path to be separated from a reception path, so that a receiver can be protected when in the transmission mode. The TDD switch operates in response to a TDD control signal of the wireless communication system. The TDD switch is generally located as will now be described. 
         [0006]      FIG. 1  illustrates a conventional TDD switch in a TDD wireless communication system. 
         [0007]    Referring to  FIG. 1 , a TDD switch  107  is connected to a Power Amplifier (PA)  103 , an antenna  111 , and a Low Noise Amplifier (LNA)  115 . 
         [0008]    When the wireless communication system operates in the transmission mode, a transmission signal transmitted from a transmitter  101  is amplified to a high-power transmission signal by the PA  103  and is then radiated through the antenna  111  via a transmit port  105  and an antenna port  109 . In this cases the TDD switch  107  operates in the transmission mode and thus isolates the transmitter  101  from a receiver  117 . Therefore, the receiver  117  can be protected against the high-power transmission signal from the transmitter  101 . 
         [0009]    When the wireless communication system operates in the reception mode, a reception signal received through the antenna  111  is transmitted to a receive port  113  via the antenna port  109 . In this case, the TDD switch  107  operates in the reception mode and thus enables the reception signal to be transmitted to the receive port  113 . The reception signal itself has a significantly low power level due to attenuation and noise. Therefore, the reception signal is amplified by the LNA  115  in which a signal is amplified with minimum noise. The amplified reception signal is transmitted to the receiver  117 . 
         [0010]      FIGS. 2A and 2B  illustrate a conventional TDD switch having a circulator and a λ/4 transmission line. 
         [0011]      FIG. 2A  illustrates a conventional TDD switch having a circulator  201  and a λ/4 transmission line  202 . In  FIG. 2A , the λ/4 transmission line  202  and a pin diode  203  are connected in three connection configurations. About 20 dB of signal attenuation can be prevented per each connection configuration. Thus, the three connection configurations shown in  FIG. 2A  can prevent about 60 dB of signal attenuation. The connection configurations are located between a receive port  206  and the circulator  201 . 
         [0012]    In the TDD communication transmission system, it will be assumed hereinafter that a transmit port  204  includes a PA, and the receive port  206  includes an LNA. An antenna is connected to an antenna port  205  of the TDD switch. 
         [0013]    An isolator  207  transmits a signal only in one direction and is located between the transmit port  204  and the circulator  201 . The isolator  207  is designed to pass only a transmission signal transmitted from the transmit port  204 . Furthermore, the isolator  207  acts as a terminator for an introduced signal. 
         [0014]    For example, when the transmission signal is not successfully radiated through the antenna and is thus reversely introduced, the isolator  207  terminates the introduced signal. Thus, the circuit of the transmit port  204  is protected. 
         [0015]    The circulator  201  is a 3-port circuit element for branching signals. A resonance plate and a magnetic substance (e.g., ferrite) are placed inside the circulator  201  having a shape in which three ports are arranged by 120 degrees. The circulator  201  leads to an approximately 0.3 dB path loss when a power signal is transmitted in a direction from the isolator  207  to the antenna port  205 . Also, the circulator  201  attenuates the power signal by a specific level (about 20 dB) in another direction from the circulator  201  to the receive port  206 . 
         [0016]    For example, when a TDD control signal operates in the transmission mode, the transmission signal amplified through the transmit port  204  exhibits an approximately 0.3 dB path loss while passing through the circulator  201  and is then radiated through the antenna via the antenna port  205 . In the direction from the isolator  207  to the pin diode  203 , the transmission signal is attenuated by a certain level (about 20 dB). The receive port  206  may be damaged when the attenuated transmission signal is introduced to the receive port  206 . 
         [0017]    The TDD control signal is used to control the transmit port  204  and the receive port  206  of the TDD wireless communication system. In response to the TDD control signal, the transmit port  204  amplifies the transmission signal and then radiates the amplified power signal to the antenna. 
         [0018]    In addition, the TDD control signal is used to control a bias circuit  209  which regulates a Direct Current (DC) bias supplied to the pin diode  203 . The DC bias is supplied to the pin diode  203  and is independent from wireless communication characteristics. The pin diode  203  acts as a part of the TDD switch according to the DC bias. 
         [0019]    Although not shown in  FIG. 2A , a capacitor is disposed between the circulator  201  and the λ/4 transmission line  202  so as to block the DC bias. Thus, the DC bias is prevented from being introduced to other circuits. Hereinafter, the capacitor for blocking the DC bias is assumed to be present throughout the figures. 
         [0020]    According to transmission line theory, when the output port of the transmission line is open to ground, the impedance of the input port of the transmission line is expressed as Z=−jZo cotβl. When the output port of the transmission line is shorted to ground, the impedance of the input port of the transmission line is expressed as Z=−jZo tanβl. When the output port of the transmission line is connected to a 50 ohm transmission line, the impedance of the input port of the transmission line is expressed as Z=Zo=50 ohm. Here, β=2π/λ, and l is the length of the transmission line. As is known, waves have the same amplitudes at λ/4, 3λ/4, 5λ/4, 7λ/4, and so on. Hence, the λ/4 transmission line  202  may be generalized as a (λ/4)*(2m+1) transmission line [m=0,1,2,3, . . . ]. The λ/4 transmission line  202  corresponds to a (λ/4)*(2m+1) transmission line [m=0,1,2,3, . . . ], where m is 0. 
         [0021]    The λ/4 transmission line  202  nearest to the receive port  206  is connected to the receive port  206 . A nominal impedance of the λ/4 transmission line  202  is 50 ohm. 
         [0022]    When the forward DC bias is supplied to the pin diode  203 , the impedance of the pin diode  203  decreases. Thus, the impedance viewed from the λ/4 transmission line  202  towards the pin diode  203  becomes similar to a state of being connected to ground. In addition, when the impedance of one end of the λ/4 transmission line  202  decreases, according to the above expression of Z=−jZo tanβ           
         [0000]    where β=2π/λ, and l=(λ/4)*(2m+1)[m=0,1,2,3, . . . ], the impedance Z of the other end of the λ/4 transmission line  202  becomes nearly infinite (open-circuited). 
         [0023]    Conversely, when the reverse DC bias is supplied to the pin diode  203 , the impedance of the pin diode  203  increases. As a result, the impedance viewed from the λ/4 transmission line  202  towards the pin diode  203  becomes nearly 50 ohm since it is a parallel impedance between an infinite impedance (open-circuited) and a 50 ohm impedance. Specifically, when the impedance of the pin diode  203  increases, the λ/4 transmission line  202  is substantially connected only to the 50 ohm transmission line. Thus, according to the above expression of Zo=50 ohm, the impedance Z of the other end of the λ/4 transmission line  202  becomes nearly 50 ohm. This is similar to the case when the circulator  201  is directly connected to the receive port  206 . 
         [0024]    Consequently, impedance changes in the pin diode  203  in response to the DC bias allow the output port of the λ/4 transmission line  202  to become substantially shorted to ground or substantially connected only to the 50 ohm transmission line. 
         [0025]    In the transmission mode, when the TDD control signal is transmitted to the bias circuit  209 , the bias circuit  209  supplies a forward DC bias to the pin diode  203 . The forward DC bias allows the impedance of the pin diode  203  to become nearly 0 (short-circuited). Therefore, the λ/4 transmission line  202  is substantially connected to ground. According to the characteristic of the λ/4 transmission line  202 , the impedance of an input port of the λ/4 transmission line  202  changes to be opposite to the impedance of an output port of the λ/4 transmission line  202  and thus becomes nearly infinite (open-circuited). 
         [0026]    Accordingly, the transmission signal transmitted from the isolator  207  to the circulator  201  is reflected, thereby protecting the receive port  206  against the transmission signal. 
         [0027]    In the reception mode, when the TDD control signal is transmitted to the bias circuit  209 , the bias circuit  209  supplies a reverse DC bias to the pin diode  203 . The reverse DC bias allows the impedance of the pin diode  203  to become nearly infinite (open-circuited). Therefore, the λ/4 transmission line  202  is substantially directly connected to the receive port  206 . In this case, the impedance of the output port of the λ/4 transmission line  202  becomes 50 ohm, and the impedance of the input port of the λ/4 transmission line  202  also becomes 50 ohm. Accordingly, most of the transmission signal can be transmitted from the antenna port  205  to the receive port  205  via the circulator  201 . 
         [0028]      FIG. 2B  illustrates a conventional TDD switch having a circulator  211  and a λ/4 transmission line  212 . In  FIG. 2B , the λ/4 transmission line  212  and a pin diode  213  are connected in two connection configurations. About 20 dB of signal attenuation can be prevented per each connection configuration. Thus, the two connection configurations shown in  FIG. 2B  can prevent about 40 dB of signal attenuation. The connection configurations are located between a receive port  216  and the circulator  211 . The operation of the TDD switch of  FIG. 2B  is the same as that of  FIG. 2A . Also illustrated in  FIG. 2B  are elements similar to those in  FIG. 2A  such as a transmit port  214 , an antenna port  215 , an isolator  217  and a bias circuit  219 . 
         [0029]      FIGS. 3A to 3C  illustrate a conventional TDD switch having a circulator, a λ/4 transmission line, a λ/4 transmission line stub, and a λ/2 transmission line stub. 
         [0030]    Referring to  FIG. 3A , the TDD switch includes an isolator  307 , a circulator  301 , pin diodes  306  and  304 , a λ/4 transmission line  302 , a λ/4 transmission line stub  303 , and a λ/2 transmission line stub  305 . The λ/2 transmission line stub  305  and the pin diode  306  are connected between the isolator  307  and the circulator  301  so as to act as a part of the TDD switch. In addition, the λ/4 transmission line  302 , the λ/4 transmission line stub  303 , and the pin diode  304  are connected between the circulator  301  and a receive port  310 , so as to act as a part of the TDD switch. Also illustrated are a transmit port  308 , an antenna port  309  and a bias circuit  311 . 
         [0031]    In general, a transmission line stub has a specific length and is perpendicularly attached to a transmission line. According to a connection state between the transmission line stub and ground, the transmission line stub may be either an open stub or a shorted stub. Similar to the transmission line, when used in a high frequency circuit, the transmission line stub may also have a characteristic of a specific circuit element. In addition, a λ/4 transmission line has the same characteristic as a λ/4 transmission line stub. 
         [0032]    According to the transmission line theory, when the output port of the transmission line stub is not connected to ground (i.e., open stub), the impedance of the input port of the transmission line stub is expressed as Z=−jZo cotβl. Further, when the output port of the transmission line stub is connected to ground (i.e., shorted stub), the impedance of the input port of the transmission line stub is expressed as Z=−jZo tanβl. Here, β=2π/λ, and l is the length of the transmission line stub. As known, waves have the same amplitudes at 0, λ/2, λ, 3λ/2, 2λ, and so on. Hence, the λ/2 transmission line stub  305  may be generalized as a (λ/2)*m transmission line stub [m=0,1,2,3, . . . ]. The λ/2 transmission line stub  305  corresponds to a (λ/2)*m transmission line stub [m=0,1,2,3, . . . ], where m is 1. 
         [0033]    The output port of the λ/2 transmission line stub  305  is connected to the pin diode  306 . Impedance changes in the pin diode  306  in response to the DC bias allow the output port of the λ/2 transmission line stub  305  to become nearly shorted or open to ground. 
         [0034]    When the output port of the λ/2 transmission line stub  305  becomes substantially open to ground, according to the above expressions of Z=−jZo cotβl, β=2π/λ, and l=(λ/2)*m transmission line stub [m=0,1,2,3, . . . ], the impedance Z of the input port of the λ/2 transmission line stub  305  becomes nearly infinite (open-circuited). Since the input port of the λ/2 transmission line stub  305  and a 50 ohm transmission line are connected in parallel to the isolator  307 , when the impedance Z of the input port of the λ/2 transmission line stub  305  becomes nearly infinite (open-circuited), the input impedance viewed from the isolator  307  towards the circulator  301  becomes 50 ohm. This is the similar to the case when the λ/2 transmission line stub  305  and the pin diode  306  are not present. 
         [0035]    On the other hand, when the output port of the λ/2 transmission line stub  305  becomes substantially shorted to ground, according to the above expressions Z=−jZo tanβl, β=2π/λ, and l=(λ/2)*m transmission line stub [m=0,1,2,3, . . . ], the impedance Z of the input port of the λ/2 transmission line stub  305  becomes nearly 0 (short-circuited). 
         [0036]    In the transmission mode, when the TDD control signal is transmitted to the bias circuit  311 , the bias circuit  311  supplies a reverse DC bias to the pin diodes  306  and  304 . The reverse DC bias allows each of the impedances of the pin diodes  306  and  304  to become nearly infinite (open-circuited). 
         [0037]    Since the output port of the λ/2 transmission line stub  305  (nearest to the pin diode  306 ) located between the isolator  307  and the circulator  301  is connected to the pin diode  306 , the impedance of the output port of the λ/2 transmission line stub  305  also becomes nearly infinite. Hence, the output port of the λ/2 transmission line stub  305  becomes substantially open to ground (open-circuited). 
         [0038]    Similar to the impedance of the output port of the λ/2 transmission line stub  305 , according to the characteristic of the λ/2 transmission line stub  305 , the impedance of the input port (nearest to the isolator  307 ) of the λ/2 transmission line stub  305  becomes nearly infinite (open-circuited). Accordingly, the input impedance viewed from the isolator  307  towards the circulator  301  becomes 50 ohm. 
         [0039]    When the TDD control signal operates in the transmission mode, the reverse DC bias allows the impedance of the pin diode  304  located between the circulator  301  and the receive port  310  to become nearly infinite. Since the pin diode  304  is connected to the output port of the λ/4 transmission line stub  303 , the impedance of the output port of the λ/4 transmission line stub  303  also becomes nearly infinite (open-circuited). Hence, the output port of the λ/4 transmission line stub  315  becomes substantially open to ground. 
         [0040]    The impedance of the input port of the λ/4 transmission line stub  303  is nearly 0, similar to that of the λ/4 transmission line  212  of  FIG. 21 . The impedance of the output port of the λ/4 transmission line  302  becomes nearly 0 since it is a parallel impedance between 0 and 50 ohm. 
         [0041]    The impedance of the input port (nearest to the circulator  301 ) of the λ/4 transmission line  302  becomes nearly infinite according to the characteristic of the λ/4 transmission line  302 . Therefore, it is possible to isolate most of the power signal to be supplied from the circulator  301  to the receive port  310  while the wireless communication system operates in the transmission mode. 
         [0042]    Consequently, when the wireless communication system operates in the transmission mode, the λ/2 transmission line stub  305  and the pin diode  306  operate as if they do not exist, and the receive port  310  is protected by the λ/4 transmission line  302 , the λ/4 transmission line stub  303 , and the pin diode  304 . Therefore, the wireless communication system can perform a transmission operation without damaging the receive port  310 . 
         [0043]    In the reception mode, when the TDD control signal is transmitted to the bias circuit  311 , the bias circuit  311  supplies a forward DC bias to the pin diodes  306  and  304 . The forward DC bias allows each of the impedances of the pin diodes  306  and  304  to become nearly 0. Since the pin diode  306  located between the isolator  307  and the circulator  301  is connected to the output port of the λ/2 transmission line stub  305 , the impedance of the output port of the λ/2 transmission line stub  305  also becomes nearly 0. Hence, the output port of the λ/2 transmission line stub  305  becomes substantially shorted to ground (short-circuited). 
         [0044]    Similar to the impedance of the output port of the λ/2 transmission line stub  305 , according to the characteristic of the λ/2 transmission line stub  305 , the impedance of the input port (nearest to the isolator  307 ) of the λ/2 transmission line stub  305  becomes nearly 0. 
         [0045]    Since the input port of the λ/2 transmission line stub  305  and the 50 ohm transmission line are connected in parallel to the isolator  307 , when the impedance Z of the input port of the λ/2 transmission line stub  305  becomes nearly 0, the input impedance viewed from the isolator  307  towards the circulator  301  becomes nearly 0. 
         [0046]    When the TDD switch abnormally operates, the TDD wireless communication system may operate in the transmission mode while the TDD switch operates in the reception mode. In this case, the transmission signal amplified through a transmit port  308  is reflected due to impedance changes in the pin diode  306  and is returned to the isolator  307 , resulting in termination. Therefore, the circuit of the receive port  310  can be protected. 
         [0047]    When the TDD control signal operates in the reception mode, the forward DC bias also allows the impedance of the pin diode  304  located between the circulator  301  and the receive port  310  to become nearly 0. Since the pin diode  304  is connected to the output port of the λ/4 transmission line stub  303 , the impedance of the output port of the λ/4 transmission line stub  303  also becomes nearly 0. Therefore, the output port of the λ/4 transmission line stub  303  becomes substantially shorted to ground. 
         [0048]    According to the characteristic of the λ/4 transmission line stub  303 , the impedance of the input port of the λ/4 transmission line stub  303  changes to be opposite to the impedance of the output port of the λ/4 transmission line stub  303  and thus becomes nearly infinite. 
         [0049]    Since the output port of the λ/4 transmission line  302  and the 50 ohm transmission line are connected in parallel to the input port of the λ/4 transmission line stub  303 , the impedance of the output port of the λ/4 transmission line  302  becomes 50 ohm. As a result, the impedance of the input port (nearest to the circulator  301 ) of the λ/4 transmission line  302  becomes 50 ohm according to the characteristic of the λ/4 transmission line  302 . This is similar to the case when the circulator  301  is directly connected to the receive port  310 . 
         [0050]    Consequently, when the wireless communication system operates in the reception mode, according to the operations of the λ/2 transmission line stub  305  and the pin diode  306 , the output of the isolator  307  is reflected, and the reflected output is returned to the isolator  307 , resulting in termination. Therefore, even if an abnormal output is produced from the isolator  307 , the receive port  310  can be protected. In addition, the λ/4 transmission line  302 , the λ/4 transmission line stub  303 , and the pin diode  304  enable the receive port  310  to smoothly receive a signal transmitted through an antenna port  309 . 
         [0051]      FIG. 3B  illustrates a TDD switch without the λ/2 transmission line stub  305  of  FIG. 3A . This is equivalent to the case when m is 0 in a generalized (λ/2)*m transmission line stub [m=0,1,2,3, . . . ]. Other components and operations of the TDD switch of  FIG. 3B  are the same as those of  FIG. 3A . For example, similar to that illustrated in  FIG. 3A ,  FIG. 3B  includes an isolator  326 , a circulator  321 , pin diodes  324  and  325 , a λ/4 transmission line  322  and a λ/4 transmission line stub  323 . The λ/4 transmission line  322 , the λ/4 transmission line stub  323 , and the pin diode  324  are connected between the circulator  321  and a receive port  329 . Also illustrated are a transmit port  327 , an antenna port  328  and a bias circuit  331 . 
         [0052]      FIG. 3C  illustrates the same TDD switch as shown in  FIG. 3B  except that a λ/4 transmission line  322 , a λ/4 transmission line stub  323 , and a pin diode  324  of  FIG. 3B  are connected in two connection configurations. More specifically, the TDD switch of  FIG. 3C  includes a λ/4 transmission line  352 , a λ/4 transmission line stub  353 , and pin diodes  354  and  360 . About 20 dB of signal attenuation can be prevented per each connection configuration. Thus, the two connection configurations shown in  FIG. 3C  can prevent about 40 dB of signal attenuation. Other components and operations of the TDD switch of  FIG. 3C  are the same as those of  FIG. 3B . For example, similar to that illustrated in  FIG. 3B ,  FIG. 3C  includes an isolator  356 , a circulator  351 , and pin diode  355 . Also illustrated are a transmit port  357 , an antenna port  358 , a receive port  359  and a bias circuit  361 . 
         [0053]    As described above, the TDD switch of  FIGS. 2A and 2B  has a problem in that it cannot be separated by the circulator when errors occur in the antenna port (i.e., a signal is not properly radiated through the antenna). Moreover, when the TDD wireless communication system operates in the transmission mode while the TDD switch operates in the reception mode, a transmission signal may be introduced to the receive port without being blocked by the TDD switch, thereby damaging the circuit of the receive port. 
         [0054]    Furthermore, in the TDD switch of  FIGS. 2A ,  2 B,  3 A,  3 B and  3 C, about 0.3 dB of signal attenuation is produced while a signal is transmitted or received, due to the use of the circulator. In addition, the circulator is provided as an additional hardware, thereby increasing a size of the TDD. Accordingly, there is a demand for a TDD switch that can protect the receive port without having to use the circulator when the TDD switch abnormally operates. 
       SUMMARY OF THE INVENTION 
       [0055]    An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a Time Division Duplex (TDD) switch that protects a receive port in a wireless communication system. 
         [0056]    Another aspect of the present invention is to provide a TDD switch that protects the receive port even when the TDD wireless communication system abnormally operates due to errors. 
         [0057]    Yet a further aspect of the present invention is to provide a TDD switch that protects the receive port without having to use a circulator in a TDD wireless communication system. 
         [0058]    According to one aspect of the present invention, a TDD switch in a wireless communication system is provided. The switch includes an isolator connected to an output port of a transmitter, a first transmission line which is connected between the isolator and an antenna feed line, for transmitting a transmission signal received from the isolator to the antenna feed line when in a transmission mode and for isolating a transmission path when in a reception mode, a first transmission line stub which is connected in a stub form between the isolator and the first transmission line for reflecting the transmission signal transmitted from the isolator in the transmission mode and for changing an impedance of the first transmission line in the reception mode, a second transmission line connected between an output port of the first transmission line and an input port of a receiver for isolating a reception path in the transmission mode and for providing a reception signal received from the antenna feed line to the receiver in the reception mode and a second transmission line stub connected in a stub form between the second transmission line and the input port of the receiver, for controlling the second transmission line to isolate the reception path when in the transmission mode and for supplying the reception signal provided from the antenna feed line to the receiver when in the reception mode. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0059]    The above and other aspects, features and advantages of certain exemplary embodiments will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
           [0060]      FIG. 1  illustrates a conventional TDD switch in a TDD wireless communication system; 
           [0061]      FIGS. 2A and 2B  illustrate a conventional TDD switch having a circulator and a λ/4 transmission line; 
           [0062]      FIGS. 3A to 3C  illustrate a conventional TDD switch having a circulator, a λ/4 transmission line, a λ/4 transmission line stub, and a λ/2 transmission line stub; 
           [0063]      FIG. 4  illustrates a TDD switch having a λ/4 transmission line, a λ/4 transmission line stub, and a λ/2 transmission line stub and not having a circulator according to an exemplary embodiment of the present invention; 
           [0064]      FIG. 5  illustrates a TDD switch when a λ/2 transmission line stub of  FIG. 4  is removed according to an exemplary embodiment of the present invention; 
           [0065]      FIG. 6  illustrates a TDD switch having a λ/4 transmission line and a λ/4 transmission line stub and not having a circulator according to an exemplary embodiment of the present invention; 
           [0066]      FIG. 7  illustrates a TDD switch equivalent to that shown in  FIG. 6  except that a λ/2 transmission line stub is added according to an exemplary embodiment of the present invention; 
           [0067]      FIG. 8  illustrates a TDD switch having a plurality of connection configurations each illustrated in  FIG. 4  according to an exemplary embodiment of the present invention; 
           [0068]      FIG. 9  illustrates a TDD switch having a plurality of connection configurations each illustrated in  FIG. 5  according to an exemplary embodiment of the present invention; 
           [0069]      FIG. 10  illustrates a TDD switch having a plurality of connection configurations each illustrated in  FIG. 6  according to an exemplary embodiment of the present invention; 
           [0070]      FIG. 11  illustrates a TDD switch having a plurality of connection configurations each illustrated in  FIG. 7  according to an exemplary embodiment of the present invention; 
           [0071]      FIGS. 12A and 12B  illustrate graphs showing a performance of the conventional TDD switch of  FIG. 2B , which is obtained through a simulation test; 
           [0072]      FIGS. 13A and 13B  illustrate graphs showing a performance of the conventional TDD switch of  FIG. 3C , which is obtained through a simulation test; and 
           [0073]      FIGS. 14A and 14B  illustrate graphs showing a performance of a TDD switch as illustrated in  FIG. 9  according to an exemplary embodiment of the present invention, which is obtained through a simulation test. 
       
    
    
       [0074]    Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features and structures. 
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0075]    The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, in the following description, well-known functions or constructions are omitted for clarity and conciseness. 
         [0076]    The present invention relates to a Time Division Duplex (TDD) switch for protecting a receive port without having to use a circulator in a TDD wireless communication system when operating abnormally due to errors. 
         [0077]    An exemplary TDD switch is illustrated throughout  FIGS. 4 to 11  as will be described below.  FIGS. 8 to 11  are exploded views for showing the structures depicted in  FIGS. 4 to 7 , respectively. 
         [0078]    In  FIGS. 4 to 11 , λ/4 transmission lines  401 ,  402 ,  501 ,  502 ,  601 ,  602 ,  701 , and  702  are examples of a (λ/4)*(2m+1) transmission line [m=0, 1, 2, 3, . . . ]. Further, λ/4 transmission line stubs  403 ,  503 ,  604 , and  705  are examples of a (λ/4)*(2m+1) transmission line stub [m=0, 1, 2, 3, . . . ]. Furthermore, λ/2 transmission line stubs  405  and  703  are examples of a (λ/2)*m transmission line stub [m=0, 1, 2, 3, . . . ]. These elements have the same characteristics when m is an integer equal to or greater than 0. Therefore, the structure of  FIG. 4  is similar to that of  FIG. 5 , and the structure of  FIG. 6  is similar to that of  FIG. 7 . 
         [0079]      FIG. 4  illustrates a TDD switch having a λ/4 transmission line, a λ/4 transmission line stub, and a λ/2 transmission line stub and not having a circulator according to the present invention. The TDD switch of  FIG. 4  includes an isolator  407 , the (λ/2)*m transmission line stub (where m=1, hereinafter referred to as a λ/2 transmission line stub)  405 , pin diodes  406  and  404 , the (λ/4)*(2m+1) transmission line stub (where m=0, hereinafter referred to as a 214 transmission line stub  403 ), and the (λ/4)*(2m+1) transmission lines (where m=0, hereinafter referred to as λ/4 transmission lines  401  and  402 ). A transmission line  411  disposed near an antenna port  409  has an arbitrary length, and may be referred to as an antenna feed line. The transmission line  411  is connected to the λ/4 transmission lines  401  and  402 . The isolator  407  terminates a signal introduced after being reflected, and protects the output port of a power amplifier. In the transmission mode, a TDD control signal is supplied to a bias circuit  421 , and the bias circuit  421  then supplies a reverse bias to the pin diodes  406  and  404 . As a result, the impedance of the pin diode  406  becomes nearly infinite, and thus the impedance viewed from a transmission path (in a direction from the isolator  407  to the λ/4 transmission line  401 ) to the pin diode  406  also becomes nearly infinite according to the characteristics of the λ/2 transmission line stub  405 . The impedance of the output port of the λ/2 transmission line stub  405  becomes nearly infinite, and according to the transmission line theory, the impedance of the input port of the λ/2 transmission line stub  405  also becomes nearly infinite. Thus, the λ/2 transmission line stub  405  becomes substantially open to the transmission path. Accordingly, a transmission signal transmitted from a transmit port  408  is transmitted to the λ/4 transmission line  401  without loss. 
         [0080]    The reverse bias is also supplied to the pin diode  404  connected to the λ/4 transmission line stub  403 . Thus, the impedance of the pin diode  404  also becomes nearly infinite. As a result, the impedance of the output port of the λ/4 transmission line stub  403  also becomes nearly infinite, and according to the transmission line theory, the impedance of the input port of the λ/4 transmission line stub  403  becomes close to 0. In other words, according to characteristics of the λ/4 transmission line stub  403 , the impedance viewed from the intersection of the λ/4 transmission line  402  and the λ/4 transmission line stub  403  towards the pin diode  404  becomes nearly 0. 
         [0081]    Since the output port of the λ/4 transmission line  402  is connected to the input port of the λ/4 transmission line stub  403 , the impedance of the output port of the λ/4 transmission line  402  is nearly 0. Thus, according to the transmission line theory, the impedance of the input port of the λ/4 transmission line  402  becomes nearly infinite. In other words, the impedance viewed from the intersection of the λ/4 transmission line  402 , the λ/4 transmission line  401 , and the transmission line  411  towards a receive port  410  becomes nearly infinite. 
         [0082]    Consequently, without having to use the circulator, the transmission signal transmitted from the transmit port  408  passes through the λ/4 transmission line  401  without loss and is totally reflected from the λ/4 transmission line  402  rather than being introduced to the λ/4 transmission line  402 . The transmission signal is then transmitted to the antenna port  409  and is radiated through an antenna. Accordingly, the receive port  410  is isolated from the transmit port  408 , and thus the receive port  410  can be protected against the large-power transmission signal transmitted from the transmit port  408 . 
         [0083]    In the reception mode, a TDD control signal is supplied to the bias circuit  421 , and the bias circuit  421  then supplies a forward bias to the pin diodes  406  and  404 . As a result, the impedance of the pin diode  406  becomes nearly 0, and thus the impedance viewed from a transmission path (in a direction from the isolator  407  to the λ/4 transmission line  401 ) to the pin diode  406  also becomes nearly 0 due to the λ/2 transmission line stub  405 . The impedance of the output port of the λ/2 transmission line stub  405  becomes nearly 0, and according to the transmission line theory, the impedance of the input port of the λ/2 transmission line stub  405  also becomes nearly 0. Thus, the transmission path is substantially short-circuited. Accordingly, a transmission signal transmitted from the transmit port  408  is totally reflected so that the isolator  407  is isolated from the λ/4 transmission line  401  with about 20 dB isolation. The transmit port  408  maintains this isolation with respect to the receive port  410 . 
         [0084]    According to the connection configuration between the λ/2 transmission line stub  405  and the pin diode  406 , the impedance viewed from the intersection of the λ/4 transmission line  401  and the λ/2 transmission line stub  405  towards the transmit port  408  becomes nearly 0. In addition, according to the characteristics of the λ/4 transmission line  401 , the impedance viewed from the intersection of the λ/4 transmission line  401 , the transmission line  411 , and the λ/4 transmission line  402  towards the transmit port  408  becomes nearly infinite. Therefore, the signal received through the antenna port  409  is not introduced towards the transmit port  408 . 
         [0085]    Meanwhile, the impedance of the pin diode  404  connected to the λ/4 transmission line stub  403  becomes nearly 0, and according to the characteristics of the λ/4 transmission line stub  403 , the impedance viewed from the intersection between the λ/4 transmission line  402  and the λ/4 transmission line stub  403  becomes nearly infinite. As a result, the impedance viewed from the intersection of the λ/4 transmission line  401 , the transmission line  411 , and the λ/4 transmission line  402  towards the receive port  410  is similar to the impedance measured when the λ/4 transmission line stub  403  and the pin diode  404  are not present. Therefore, the signal received through the antenna port  409  is readily transmitted towards the receive port  410 . 
         [0086]    Consequently, in the TDD switch not having the circulator according to the present invention, even when the TDD communication system operates in the transmission mode in a state that the TDD control signal is provided out of sync, it is possible to protect the receive port  410  due to the connection configuration of the λ/4 transmission line  401 , the λ/2 transmission line stub  405 , and the pin diode  406 . 
         [0087]    Furthermore, the antenna feed line may have a trouble (e.g., a cable connected to the antenna port  409  is broken), or a transmission signal may be reflected when a Voltage Standing Wave Ratio (VSWR) increases due to impedance mismatching. Even in this case, according to the configuration structure of the λ/4 transmission line  402 , the λ/4 transmission line stub  403 , and the pin diode  404 , the receive port  410  can be protected. When power supplied to the TDD switch is blocked, the impedances of the pin diodes  404  and  406  become nearly infinite, which is the same as the case when operating in the transmission mode. Therefore, the receive port  410  can be protected. 
         [0088]    The number of connection configurations of the λ/4 transmission line  402 , the λ/4 transmission line stub  403 , and the pin diode  404  or the number of connection configurations of the λ/2 transmission line stub  405 , the λ/4 transmission line  401 , and the pin diode  406  may vary according to which standard is adopted. In general, isolation can be secured by about 20 dB per each connection configuration. 
         [0089]      FIG. 5  illustrates a TDD switch when the λ/2 transmission line stub  405  of  FIG. 4  is removed according to an exemplary embodiment of the present invention. 
         [0090]    Referring to  FIG. 5 , the λ/2 transmission line stub  405  of  FIG. 4  is not present. This is equivalent to the case when m is 0 in a generalized (λ/2)*m transmission line stub [m=0,1,2,3, . . . ]. Other components and operations of the TDD switch of  FIG. 5  are the same as those of  FIG. 4 . For example, the TDD switch of  FIG. 5  includes a transmit port  507 , an isolator  506 , pin diodes  504  and  505 , transmission line  510 , antenna port  508 , receive port  509  and bias circuit  521 . 
         [0091]      FIG. 6  illustrates a TDD switch having a λ/4 transmission line  601  and a λ/4 transmission line stub  604  and not having a circulator according to an exemplary embodiment of the present invention. 
         [0092]    The TDD switch of  FIG. 6  has the same structure as that of  FIG. 7  to be described below except that the λ/2 transmission line stub  703  is not present. This is equivalent to the case when m is 0 in a generalized (λ/2)*m transmission line stub [m=0,1,2,3, . . . ]. Other components and operations of the TDD switch of  FIG. 6  are the same as those of  FIG. 7  as will be described below. 
         [0093]      FIG. 7  illustrates a TDD switch substantially equivalent to that shown in  FIG. 6  except that a  212  transmission line stub is added according to an exemplary embodiment of the present invention. 
         [0094]    The TDD switch of  FIG. 7  includes an isolator  707 , the (λ/4)+(2m+1) transmission line stub (where m=0, hereinafter referred to as a  214  transmission line stub  705 ), pin diodes  706  and  704 , the (λ/4)*(2m+1) transmission line (where m=0, hereinafter referred to λ/4 transmission lines  701  and  702 ), and the (λ/2)*m transmission line stub (where m=1, hereinafter referred to as a λ/2 transmission line stub  703 ). 
         [0095]    A transmission line  711  disposed near an antenna port  709  has an arbitrary length, and may be referred to as an antenna feed line. The transmission line  711  is connected to the λ/4 transmission lines  701  and  702 . The isolator  707  terminates a signal introduced after being reflected, and protects the output port of a power amplifier. 
         [0096]    In the transmission mode, a TDD control signal is supplied to a bias circuit  721 , and the bias circuit  721  then supplies a forward bias to the pin diodes  706  and  704 . As a result, the impedance of the pin diode  706  becomes nearly 0, and thus the impedance viewed from a transmission path (in a direction from the isolator  707  to the λ/4 transmission line  701 ) to the pin diode  706  becomes nearly infinite according to the characteristics of the λ/4 transmission line stub  705 . Thus, the λ/4 transmission line stub  705  becomes substantially open to the transmission line. Accordingly, a transmission signal transmitted from a transmit port  708  is transmitted to the λ/4 transmission line  701  without loss. 
         [0097]    The forward bias is also supplied to the pin diode  704  connected to the λ/2 transmission line stub  703 . Thus, the impedance of the pin diode  704  also becomes nearly 0. As a result, the impedance of the output port of the λ/2 transmission line stub  703  also becomes nearly 0, and according to the transmission line theory, the impedance of the input port of the λ/2 transmission line stub  703  becomes close to 0. In other words, according to characteristics of the λ/2 transmission line stub  703 , the impedance viewed from the intersection of the λ/4 transmission line  702  and the λ/2 transmission line stub  703  towards the pin diode  704  becomes nearly 0. 
         [0098]    Since the output port of the λ/4 transmission line  702  is connected to the input port of the λ/2 transmission line stub  703 , the impedance of the output port of the λ/4 transmission line  702  is nearly 0. Thus, according to the transmission line theory, the impedance of the input port of the λ/4 transmission line  702  becomes nearly infinite. In other words, the impedance viewed from the intersection of the λ/4 transmission line  702 , the λ/4 transmission line  701 , and the transmission line  711  towards a receive port  710  becomes nearly infinite. 
         [0099]    Consequently, in the TDD switch not having the circulator, the transmission signal transmitted from the transmit port  708  passes through the λ/4 transmission line  701  without loss and is reflected from the λ/4 transmission line  702  rather than being introduced to the λ/4 transmission line  702 . The transmission signal is then transmitted to the antenna port  709  and is radiated through an antenna. Accordingly, the receive port  710  is isolated from the transmit port  708 , and thus the receive port  710  can be protected against the large-power transmission signal transmitted from the transmit port  708 . 
         [0100]    In the reception mode, the TDD control signal is supplied to the bias circuit  721 , and the bias circuit  721  then supplies a reverse bias to the pin diodes  706  and  704 . As a result, the impedance of the pin diode  706  becomes nearly infinite, and thus the impedance viewed from a transmission path (in a direction from the isolator  707  to the λ/4 transmission line  701 ) to the pin diode  706  becomes nearly 0 according to the characteristics of the λ/4 transmission line stub  705 . The impedance of the output port of the λ/4 transmission line stub  705  becomes substantially shorted to the transmission path. Accordingly, the transmission signal transmitted from the transmit port  708  is totally reflected so that the isolator  707  is isolated from the λ/4 transmission line  701  with about 20 dB isolation. The transmit port  708  maintains this isolation with respect to the receive port  710 . According to the connection configuration of the λ/4 transmission line stub  705  and the pin diode  706 , the impedance viewed from the intersection of the λ/4 transmission line  701  and the λ/4 transmission line stub  705  towards the transmit port  708  becomes nearly 0. In addition, according to the characteristics of the λ/4 transmission line  701 , the impedance viewed from the intersection of the λ/4 transmission line  701 , the transmission line  711 , and the λ/4 transmission line  702  towards the transmit port  708  becomes nearly infinite. Therefore, the signal received through the antenna port  709  is not introduced towards the transmit port  708 . 
         [0101]    Meanwhile, the impedance of the pin diode  704  connected to the λ/2 transmission line stub  703  becomes nearly infinite, and according to the characteristics of the λ/2 transmission line stub  703 , the impedance viewed from the intersection between the λ/4 transmission line  702  and the λ/2 transmission line stub  703  becomes nearly infinite. As a result, the impedance viewed from the intersection of the λ/4 transmission line  701 , the transmission line  711 , and the λ/4 transmission line  702  towards the receive port  710  is similar to the impedance measured when the λ/2 transmission line stub  703  and the pin diode  704  are not present. Therefore, the signal received through the antenna port  709  is readily transmitted towards the receive port  710 . 
         [0102]    Consequently, in the TDD switch not having the circulator according to an exemplary embodiment of the present invention, even when the TDD communication system operates in the transmission mode in a state that the TDD control signal is provided out of sync, it is possible to protect the receive port  710  due to the connection configuration of the λ/4 transmission line  701 , the λ/4 transmission line stub  705 , and the pin diode  706 . 
         [0103]    Furthermore, the antenna feed line may have a trouble (e.g., a cable connected to the antenna port  709  is broken), or a transmission signal may be reflected when a VSWR increases due to impedance mismatching. Even in this case, according to the exemplary configuration structure of the λ/4 transmission line  702 , the λ/2 transmission line stub  703 , and the pin diode  704 , the receive port  710  can be protected. Power supplied to the TDD switch may be blocked. Since this is similar to the case when the reverse bias is provided, the impedances of the pin diodes  704  and  706  become nearly infinite. In addition, isolation can be secured according to the exemplary connection configuration of the λ/4 transmission line  701 , the λ/4 transmission line stub  705 , and the pin diode  706 , thereby protecting the receive port  710 . 
         [0104]    The number of connection configurations of the λ/4 transmission line  702 , the λ/2 transmission line stub  703 , and the pin diode  704  or the number of connection configurations of the λ/4 transmission line stub  705 , the λ/4 transmission line  701 , and the pin diode  706  may vary according to which standard is adopted. In general, isolation can be secured by about 20 dB per each connection configuration. 
         [0105]    With reference again to  FIG. 6 , the TDD switch of  FIG. 6  has the same structure as that of  FIG. 7  except that the λ/2 transmission line stub  703  is not present. This is equivalent to the case when m is 0 in a generalized (λ/2)*m transmission line stub [m=0,1,2,3, . . . ]. Other components and operations of the TDD switch of  FIG. 6  are the substantially the same as those of  FIG. 7 . For example, an exemplary embodiment shown in  FIG. 6  includes a transmit port  607 , an isolator  606 , a transmission line  610 , an antenna port  608 , a receive port  609 , pin diodes  603  and  605  and a bias circuit  621 . 
         [0106]      FIG. 8  illustrates a TDD switch having a plurality of connection configurations similar to that illustrated in  FIG. 4  according to an exemplary embodiment of the present invention. 
         [0107]    Referring to  FIG. 8 , the TDD switch has three connection configurations (first to third connection configurations)  881 ,  882 , and  883  in which, similar to that shown in  FIG. 4 , the λ/4 transmission line  402  (here  802 ), the λ/4 transmission line stub  403  (here  803 ), and the pin diode  404  (here  804 ) included in the TDD of  FIG. 4  are connected, and two connection configurations (fourth and fifth connection configurations)  884  and  885  in which, similar to that shown in  FIG. 4 , the λ/2 transmission line stub  405  (here  805 ), the λ/4 transmission line  401  (here  801 ), and the pin diode  406  are connected (here  806 ). In general, isolation can be secured by about 20 dB per each connection configuration. Components and operations of the TDD switch of  FIG. 8  are the same as those of  FIG. 4 . For example, as shown in  FIG. 8 , an exemplary embodiment further includes a transmit port  808 , an isolator  807 , a transmission line  811 , an antenna port  809 , a receive port  810  and a bias circuit  821 . 
         [0108]      FIG. 9  illustrates a TDD switch having a plurality of connection configurations similar to that illustrated in  FIG. 5  according to an exemplary embodiment of the present invention. 
         [0109]    Referring to  FIG. 9 , the TDD switch has three connection configurations (first to third connection configurations)  981 ,  982 , and  983  in which, similar to that shown in  FIG. 5 , the λ/4 transmission line  502  (here  902 ), the λ/4 transmission line stub  503  (ere  903 ), and the pin diode  504  (ere  904 ) included in the TDD of  FIG. 5  are connected, and two connection configurations (fourth and fifth connection configurations)  984  and  985  in which, similar to that shown in  FIG. 5 , the λ/4 transmission line  501  (here  901 ), and the pin diode  505  (here  905 ) are connected. Similar to that shown in  FIG. 5 , in the TDD switch of  FIG. 9 , the λ/2 transmission line stub is not present. This is equivalent to the case when m is 0 in a generalized (λ/2)*m transmission line stub [m=0,1,2,3, . . . ]. In general, isolation can be secured by about 20 dB per each connection configuration. Components and operations of the TDD switch of  FIG. 9  are the same as those of  FIG. 5 . For example, the TDD switch of  FIG. 9  includes a transmit port  907 , an isolator  906 , transmission line  910 , an antenna port  908 , a receive port  909  and a bias circuit  921 . 
         [0110]      FIG. 10  illustrates a TDD switch having a plurality of connection configurations similar to that illustrated in  FIG. 6  according to an exemplary embodiment of the present invention. 
         [0111]    Referring to  FIG. 10 , the TDD switch has three connection configurations (first to third connection configurations)  1081 ,  1082 , and  1083  in which, similar to that shown in  FIG. 6 , the λ/4 transmission line  602  (here  1002 ) and the pin diode  603  (here  1003 ) are connected, and two connection configurations (fourth and fifth connection configurations)  1084  and  1085  in which, similar to that shown in  FIG. 6 , the λ/4 transmission line stub  604  (here  1004 ), the λ/4 transmission line  601  (here  1001 ), and the pin diode  605  (here  1005 ) are connected. In general, isolation can be secured by about 20 dB per each connection configuration. 
         [0112]    Components and operations of the TDD switch of  FIG. 10  are the substantially the same as those of  FIG. 6 . For example, the TDD switch of  FIG. 10  includes a transmit port  1007 , an isolator  1006 , transmission line  1010 , an antenna port  1008 , a receive port  1009  and a bias circuit  1021 . 
         [0113]      FIG. 11  illustrates a TDD switch having a plurality of connection configurations similar to that illustrated in  FIG. 7  according to an exemplary embodiment of the present invention. 
         [0114]    Referring to  FIG. 11 , the TDD switch has three connection configurations (first to third connection configurations)  1181 ,  1182 , and  1183  in which, similar to that shown in  FIG. 7 , the λ/4 transmission line  702  (here  1102 ), the λ/2 transmission line stub  703  (here  1103 ), and the pin diode  704  (here  1104 ) of  FIG. 7  are connected, and two connection configurations (fourth and fifth connection configurations)  1184  and  1185  in which, similar to that shown in  FIG. 7 , the λ/4 transmission line stub  705  (here  1105 ), the λ/4 transmission line  701  (here  1101 ), and the pin diode  706  (here  1106 ) are connected. In general, isolation can be secured by about 20 dB per each connection configuration. 
         [0115]    Components and operations of the TDD switch of  FIG. 11  are substantially the same as those of  FIG. 7 . For example, the TDD switch of  FIG. 11  includes a transmit port  1108 , an isolator  1107 , transmission line  1111 , an antenna port  1109 , a receive port  1110  and a bias circuit  1121 . 
         [0116]    Now, an exemplary TDD switch of the present invention will be described in terms of its performance with reference to a simulation test result as follows. 
         [0117]      FIGS. 12A and 12B  illustrate graphs showing a performance of the conventional TDD switch of  FIG. 2B , which is obtained through a simulation test. 
         [0118]      FIG. 12A  shows a transmission loss, and  FIG. 12B  shows a reception loss. According to the graphs, the TDD of  FIG. 2B  has a performance in which a signal is attenuated by about 0.172 dB in the transmission mode and about 0.222 dB in the reception mode at a frequency range of 2300˜2400 MHz. Isolation is 60.325 dB in the transmission mode and 25.022 dB in the reception mode. 
         [0119]      FIGS. 13A and 13B  illustrate graphs showing a performance of the conventional TDD switch of  FIG. 3C , which is obtained through a simulation test. 
         [0120]      FIG. 13A  shows a transmission loss, and  FIG. 13B  shows a reception loss. According to the graphs, the TDD of  FIG. 3C  has a performance in which a signal is attenuated by about 0.175 dB in the transmission mode and about 0.252 dB in the reception mode at a frequency range of 2300˜2400 MHz. Isolation is 71.106 dB in the transmission mode and 40.033 dB in the reception mode. 
         [0121]      FIGS. 14A and 14B  illustrate graphs showing a performance of the TDD switch of  FIG. 9  according to an exemplary embodiment of the present invention, which is obtained through a simulation test. 
         [0122]      FIG. 14A  shows a transmission loss, and  FIG. 14B  shows a reception loss. According to the graphs, the TDD of  FIG. 9  has a performance in which a signal is attenuated by about 0.04 dB in the transmission mode and about 0.11 dB in the reception mode at a frequency range of 2300˜2400 MHz. Isolation is 78.736 dB in the transmission mode and 41.739 dB in the reception mode. 
         [0123]    The simulation test results of  FIGS. 12A ,  12 B,  13 A,  13 B,  14 A and  14 B show that the exemplary TDD switch of the present invention has a better performance without having to use the circulator as compared with the conventional TDD switch using the circulator. 
         [0124]    According to the exemplary TDD switch of the present invention, a receiver can be protected even when errors occur in the TDD communication system. In addition, since a circulator is not required, an exemplary TDD switch can be realized while requiring a smaller mounting area and fewer components. 
         [0125]    While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims and their equivalents.