Abstract:
A Time Division Duplex (TDD) switch of a TDD wireless communication system is provided. By its unique arrangement, the TDD switch protects a receiver of the wireless system. The TDD switch includes an isolator connected to an output port of a transmitter, a first transmission line stub, connected between the isolator and a circulator, for transmitting a transmission signal received from the isolator when in a transmission mode, and for isolating a transmission path when in a reception mode, a first transmission line connected to the circulator, for isolating a reception path when in the transmission mode, and for supplying to a receiver a reception signal provided from an antenna connected to the circulator when in the reception mode, and a second transmission line stub connected between the first transmission line and an input port of the receiver in a stub form, for controlling the first transmission line to isolate the reception path when in the transmission mode, and for supplying the reception signal to the receiver when in the reception mode.

Description:
PRIORITY 
       [0001]    This application claims the benefit under 35 U.S.C. § 119(a) to a Korean patent application filed in the Korean Intellectual Property Office on Apr. 25, 2006 and assigned Serial No. 2006-37259, 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 incorrectly introduced into the receiver due to erroneous operations such as a malfunction of the TDD switch. 
         [0004]    2. Description of the Related Art 
         [0005]    In a Time Division Duplex (TDD) wireless communication system, a TDD switch is generally used for mode changes between a transmission mode and a reception mode. The TDD switch operates in response to a TDD control signal of the wireless communication system. 
         [0006]      FIG. 1  is a diagram illustrating a conventional location of a 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 a transmission mode, a signal from transmitter  101  is amplified to a high-power signal through the PA  103  and is then radiated through the antenna  111  via a transmit port  105  and an antenna port  109 . 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 signal of the transmitter  101 . 
         [0009]    When the wireless communication system operates in the reception mode, the power signal sent from the antenna  111  is received through the antenna port  109  and a receive port  113 . The TDD switch  107  operates in the reception mode and thus enables the received power signal to be sent to the receive port  113 . The received power signal has significantly low power due to attenuation and noise. Therefore, the power signal is amplified by the LNA  115  which amplifies a signal while minimizing noise. The amplified power signal is received by the receiver  117 . 
         [0010]      FIG. 2  is a diagram illustrating a conventional TDD switch. 
         [0011]    Referring to  FIG. 2 , the conventional TDD switch includes an isolator  203 , a circulator  205 , a λ/4 transmission line  209 , a pin diode  211 , and so on. The λ/4 transmission line  209  and the pin diode  211  are interconnected between a receive port  213  and the circulator  205 . 
         [0012]    In the conventional TDD switch shown in  FIG. 2 , the λ/4 transmission line  209  and the pin diode  211  are connected in a three connection configuration. The number of connection configurations of the λ/4 transmission line  209  and the pin diode  211  may be determined through simulation or theoretical calculation. In addition, the number of connection configurations may vary depending on the extent of isolation. 
         [0013]    In the TDD wireless communication system, a transmitter including a PA may be connected to a transmit port  201 . A receiver including an LNA may be connected to the receive port  213 . Furthermore, an antenna may be connected to an antenna port  207  of the TDD switch. 
         [0014]    The isolator  203  transmits a power signal only in one direction and is located between the transmit port  201  and the circulator  205 . The isolator  203  is designed to pass only the power signal transmitted from the transmit port  201 . Furthermore, the isolator  203  acts as a terminator for an external power signal that is reflected and returned. For example, when the power signal is not successfully radiated from the antenna and is thus reversely introduced, the circuit of the transmit port  201  may be damaged by the reflected power signal. Therefore, the isolator  203  protects the circuit of the transmit port  201 . 
         [0015]    The circulator  205  is a 3-port circuit element for branching the power signal. A resonance plate and a magnetic substance (e.g., ferrite) are placed inside the circulator  205  having a shape in which three ports are arranged by 120 degrees. The circulator  205  incurs an approximately 0.3 dB path loss when passing the power signal in a direction from the isolator  203  to the antenna port  207 . Also, the circulator  205  isolates the power signal by a specific level (about 20 dB) in another direction from the circulator  205  to the receiver port  213 . For example, when the TDD control signal operates in the transmission mode, the power signal amplified by the transmitter exhibits an approximately 0.3 dB path loss while passing through the circulator  205  and is then radiated through the antenna via the antenna port  207 . In the direction from the circulator  205  to the receiver port  213 , the power signal is attenuated by a certain level (about 20 dB). Although the power signal is attenuated by the specific level (about 20 dB), the receive port  213  may be damaged when the attenuated signal is transmitted to the receive port  213 . 
         [0016]    The TDD control signal is used to control the transmitter and the receiver of the TDD wireless communication system. In response to the TDD control signal, the transmitter amplifies a power signal to be transmitted and then radiates the amplified power signal to the antenna. In addition, the TDD control signal is used to control a bias circuit  221  which regulates a Direct Current (DC) bias supplied to the pin diode  211 . The DC bias is supplied to the pin diode  211  through a transmission line, but this does not affect wireless communication characteristics. The pin diode  211  acts as a part of the TDD switch according to the DC bias. A capacitor (not shown) is provided to block the DC bias. Although not shown, it will be assumed that the capacitor for blocking the DC bias exists throughout  FIGS. 2 to 6 . 
         [0017]    According to the transmission line theory, when an output port of a transmission line is open to ground, the impedance of the input port of the transmission line is expressed as Z=−jZo cot βl. 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 tan βl. 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 known, waves have the same amplitudes at λ/4, 3λ/4, 5λ/4, 7λ/4, and so on. Hence, the λ/4 transmission line  209  may be generalized as a (λ/4)*(2m+1) transmission line [m=0,1,2,3, . . . ]. The λ/4 transmission line  209  corresponds to a (λ/4)*(2m+1) transmission line [m=0,1,2,3, . . . ], where m is 0. 
         [0018]    The pin diode  211  and the 50 ohm transmission line (receiver) are connected in parallel to the output port of the λ/4 transmission line  209 . The pin diode  211  acts as a part of the TDD switch according to the DC bias. When the impedance of the pin diode  211  becomes nearly 0 (short-circuited), the parallel impedance between the pin diode  211  and the 50 ohm transmission line becomes nearly 0 (short-circuited). On the other hand, when the impedance of the pin diode  211  becomes nearly infinite (open-circuited), the parallel impedance between the pin diode  211  and the 50 ohm transmission line becomes nearly 50 ohm. Therefore, impedance changes in the pin diode  211  according to the DC bias allow the output port of the λ/4 transmission line  209  to become substantially shorted to ground or substantially connected only to the 50 ohm transmission line. 
         [0019]    When the pin diode  211  is substantially open to ground, the output port of the λ/4 transmission line  209  is nearly connected only to the 50 ohm transmission line. Thus, according to the above expression of Zo=50 ohm, the impedance Z of the input port of the λ/4 transmission line  209  becomes nearly 50 ohm. 
         [0020]    When the output port of the λ/4 transmission line  209  is substantially shorted to ground, according to the above expression of Z=−jZo tan βl where β=2π/λ, and l=(λ/4)*(2m+1)[m=0,1,2,3, . . . ], the impedance Z of the input port of the λ/4 transmission line  209  becomes nearly infinite (open-circuited). 
         [0021]    In the transmission mode, when the TDD control signal is transmitted to the bias circuit  221 , the bias circuit  221  supplies a forward DC bias to the pin diode  211 . The forward DC bias allows the impedance of the pin diode  211  to become nearly 0 (short-circuited). Since the output port of the λ/4 transmission line  209  is connected to the pin diode  211 , the impedance of the output port of the λ/4 transmission line  209  also becomes nearly 0 (short-circuited). Thus, the output port of the λ/4 transmission line  209  becomes substantially shorted to ground. According to the characteristic of the λ/4 transmission line  209 , the impedance of the input port of the λ/4 transmission line  209  (a port nearest to the circulator  205 ) changes to be opposite to the impedance of the output port of the λ/4 transmission line  209  and thus becomes nearly infinite (open-circuited). Hence, the receive port  213  can be protected against the power signal while the TDD control signal operates in the transmission mode. 
         [0022]    In the reception mode, when the TDD control signal is transmitted to the bias circuit  221 , the bias circuit  221  supplies a reverse DC bias to the pin diode  211 . The reverse DC bias allows the impedance of the pin diode  211  to become nearly infinite (open-circuited). Since the output port of the λ/4 transmission line  209  is connected to the pin diode  211  and the 50 ohm transmission line (receiver), when the impedance of the pin diode  211  becomes nearly infinite (open-circuited), the impedance of the output port of the λ/4 transmission line  209  becomes 50 ohm, and the impedance of the input port of the λ/4 transmission line  209  also becomes 50 ohm. Therefore, a path that spans from the antenna port  207  to the receive port  213  via the circulator  205  is not affected. Accordingly, most of the power signal received through the antenna can be input to the receive port  213 . 
         [0023]    The TDD wireless communication system may operate correctly without any problem. However, when the TDD switch incorrectly 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 power signal may not be completely isolated by the circulator  205  and thus may be introduced to the receiver, which may lead to damage in the circuit of the receiver. 
         [0024]    Furthermore, a cable connected to the antenna port  207  may be open when the TDD switch is turned off, or a high-power signal may be reflected when a Voltage Standing Wave Ratio (VSWR) of the circuit of the transmitter increases due to impedance mismatching. In this case, most of the reflected high-power signal is introduced into the receiver, which may damage the circuit of the receiver. Impedance matching is used to reduce performance degradation caused by an impedance difference between two separate connection ports. The VSWR represents a reflection amount of the power signal transmitted to the antenna port  207 . 
         [0025]    The DC bias of the bias circuit  221  cannot be supplied to the pin diode  211  when the TDD switch is turned off. This is similar to the case where the reverse DC bias is supplied to the pin diode  211 . Thus, the TDD switch operates in the reception mode. 
         [0026]    The conventional TDD wireless communication system cannot operate correctly when the aforementioned problems occur mostly because the introduction of the power signal into the receiver may damage the circuit of the receiver. Accordingly, there is a need for a TDD switch that can protect the receiver even when the TDD wireless communication system operates incorrectly. 
       SUMMARY OF THE INVENTION 
       [0027]    The present invention has been made to address the above-mentioned problems and/or disadvantages. Accordingly, an object of the present invention is to provide a Time Division Duplex (TDD) switch that protects a receiver in a wireless communication system. 
         [0028]    Another object of the present invention is to provide a TDD switch that protects the receiver even when the TDD wireless communication system operates incorrectly due to errors. 
         [0029]    According to one aspect of the present invention, an apparatus for protecting a receiver in a wireless communication system is provided. The apparatus includes a TDD switch in a wireless communication system comprising an isolator connected to an output port of a transmitter, a first transmission line stub which is connected between the isolator and a circulator, transmits a transmission signal received from the isolator when in a transmission mode, and isolates a transmission path when in a reception mode, a first transmission line which is connected to the circulator, isolates a reception path when in the transmission mode, and supplies to a receiver a reception signal provided from a antenna feed line connected to the circulator when in the reception mode and a second transmission line stub which is connected between the first transmission line and an input port of the receiver in a stub form, controls the first transmission line to isolate the reception path when in the transmission mode, and supplies the reception signal to the receiver when in the reception mode. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
           [0031]      FIG. 1  is a diagram illustrating a conventional location of a Time Division Duplex (TDD) switch in a TDD wireless communication system; 
           [0032]      FIG. 2  is a diagram illustrating a conventional TDD switch; 
           [0033]      FIG. 3  is a diagram illustrating a TDD switch according to an exemplary embodiment of the present invention; 
           [0034]      FIG. 4  is a diagram illustrating a TDD switch without a λ/2 transmission line stub according to an exemplary embodiment of the present invention; 
           [0035]      FIG. 5  is a diagram illustrating a TDD switch having three connection configurations which include a λ/4 transmission line, a λ/4 transmission line stub, and a λ/2 transmission line stub according to an exemplary embodiment of the present invention; and 
           [0036]      FIG. 6  is a diagram illustrating an exemplary TDD switch having three connection configurations each of which includes a λ/4 transmission line, a λ/4 transmission line stub, and a pin diode, where a λ/2 transmission line stub is absent. 
       
    
    
       [0037]    Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures. 
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0038]    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, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
         [0039]    The present invention relates to a Time Division Duplex (TDD) switch for protecting a receiver of a TDD wireless communication system when operating not only correctly but also incorrectly due to errors. 
         [0040]      FIG. 3  is a diagram illustrating a TDD switch according to an exemplary embodiment of the present invention. 
         [0041]    Referring to  FIG. 3 , the TDD switch includes an isolator  303 , a circulator  305 , pin diodes  311  and  317 , a λ/4 transmission line  313 , a λ/4 transmission line stub  315 , and a λ/2 transmission line stub  309 . The λ/2 transmission line stub  309  and the pin diode  311  are connected between the isolator  303  and the circulator  305 , thereby forming a TDD switch. The λ/4 transmission line  313 , the λ/4 transmission line stub  315 , and the pin diode  317  are connected between the circulator  305  and a receive port  319 , thereby forming another TDD switch. A plurality of TDD switches may be present, and the number of TDD switches may be determined through simulation or theoretical calculation. This will be described below with reference to  FIGS. 5 and 6 . 
         [0042]    A transmission line is provided to transmit a high frequency signal. In a wireless communication system, the use of the high frequency signal may allow the transmission line itself to have a characteristic of a specific circuit element. In general, a transmission line stub having a specific length is perpendicularly attached to the 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 stub has the same characteristic as a λ/4 transmission line. 
         [0043]    According to 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 cot βl. 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 tan βl. 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  309  may be generalized as a (λ/2)*m transmission line stub [m=0,1,2,3, . . . ]. The λ/2 transmission line stub  309  corresponds to a (λ/2)*m transmission line stub [m=0,1,2,3, . . . ], where m is 1. 
         [0044]    The output port of the λ/2 transmission line stub  309  is connected to the pin diode  311 . According to a DC bias of a bias circuit  321 , the pin diode  311  acts as a part of a TDD switch. Therefore, impedance changes in the pin diode  311  according to the DC bias allow the output port of the λ/2 transmission line stub  309  to become nearly shorted to or open to ground. 
         [0045]    When the output port of the λ/2 transmission line stub  309  becomes substantially open to ground, according to the above expressions of Z=−jZo cot βl, 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  309  becomes nearly infinite (open-circuited). Since the input port of the λ/2 transmission line stub  309  and a 50 ohm transmission line are connected in parallel to the isolator  303 , when the impedance Z of the input port of the λ/2 transmission line stub  309  becomes nearly infinite (open-circuited), an input impedance viewed from the isolator  303  towards the circulator  305  becomes 50 ohm. 
         [0046]    On the other hand, when the output port of the λ/2 transmission line stub  309  becomes substantially shorted to ground, according to the above expressions Z=−jZo cot ⊕l, 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  309  becomes nearly 0 (short-circuited). 
         [0047]    In the transmission mode, when the TDD control signal is transmitted to the bias circuit  321 , the bias circuit  321  supplies a reverse DC bias to the pin diodes  311  and  317 . The reverse DC bias allows each of the impedances of the pin diodes  311  and  317  to become nearly infinite (open-circuited). Since the pin diode  311  located between the isolator  303  and the circulator  305  is connected to the output port of the λ/2 transmission line stub  309 , the impedance of the output port of the λ/2 transmission line stub  309  also becomes nearly infinite (open-circuited). Hence, the output port of the λ/2 transmission line stub  309  becomes substantially open to ground (open-circuited). Similar to the impedance of the output port of the λ/2 transmission line stub  309 , according to the characteristic of the λ/2 transmission line stub  309 , the impedance of the input port (a port nearest to the isolator  303 ) of the λ/2 transmission line stub  309  becomes nearly infinite (open-circuited). Accordingly, an input impedance viewed from the isolator  303  towards the circulator  305  becomes 50 ohm. 
         [0048]    When the TDD control signal operates in the transmission mode, the reverse DC bias allows the impedance of the pin diode  317  located between the circulator  305  and the receive port  319  to become nearly infinite (open-circuited). Since the pin diode  317  is connected to the output port of the λ/4 transmission line stub  315 , the impedance of the output port of the λ/4 transmission line stub  315  also becomes nearly infinite (open-circuited). Hence, the output port of the λ/4 transmission line stub  315  becomes substantially open to ground (open-circuited). The impedance of the input port of the λ/4 transmission line stub  315  is nearly 0 (short-circuited), similar to that of the λ/4 transmission line  209  of  FIG. 2 . The impedance of the output port of the λ/4 transmission line  313  becomes nearly 0 (short-circuited) since it is a parallel impedance between the impedance of the input port of the λ/4 transmission line stub  315  and the impedance of the 50 ohm transmission line. The impedance of the input port (a port nearest to the circulator  305 ) of the λ/4 transmission line  313  becomes nearly infinite (open-circuited) according to the characteristic of the λ/4 transmission line  313 . Therefore, it is possible to isolate most of the power signal to be supplied from the circulator  305  from the receive port  319  while the wireless communication system operates in the transmission mode, thereby protecting the circuit of the receiver. 
         [0049]    Consequently, when the wireless communication system operates in the transmission mode, the λ/2 transmission line stub  309  and the pin diode  311  operate as if they do not exist, and the receive port  319  is protected by the λ/4 transmission line  313 , the λ/4 transmission line stub  315 , and the pin diode  317 . Therefore, the wireless communication system can perform a transmission operation without damaging the receiver. 
         [0050]    In the reception mode, when the TDD control signal is transmitted to the bias circuit  321 , the bias circuit  321  supplies a forward DC bias to the pin diodes  311  and  317 . The forward DC bias allows each of the impedances of the pin diodes  311  and  317  to become nearly 0 (short-circuited). Since the pin diode  311  located between the isolator  303  and the circulator  305  is connected to the output port of the λ/2 transmission line stub  309 , the impedance of the output port of the λ/2 transmission line stub  309  also becomes nearly 0 (short-circuited). Hence, the output port of the λ/2 transmission line stub  309  becomes substantially shorted to ground (short-circuited). Similar to the impedance of the output port of the λ/2 transmission line stub  309 , according to the characteristic of the λ/2 transmission line stub  309 , the impedance of the input port (a port nearest to the isolator  303 ) of the λ/2 transmission line stub  309  becomes nearly 0 (short-circuited). Since the input port of the λ/2 transmission line stub  309  and the 50 ohm transmission line are connected in parallel to the isolator  303 , when the impedance Z of the input port of the λ/2 transmission line stub  309  becomes nearly 0 (short-circuited), the input impedance viewed from the isolator  303  towards the circulator  305  becomes nearly 0 (short-circuited). If the TDD switch incorrectly 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 high-power signal amplified by the transmitter and transmitted by transmission port  301  is reflected by the pin diode  311  and is returned to the isolator  303 , thereby being terminated. Therefore, the circuit of the receiver can be protected. 
         [0051]    When the TDD control signal operates in the reception mode, the forward DC bias also allows the impedance of the pin diode  317  located between the circulator  305  and the receive port  319  to become nearly 0 (short-circuited). Since the pin diode  317  is connected to the output port of the λ/4 transmission line stub  315 , the impedance of the output port of the λ/4 transmission line stub  315  also becomes nearly 0 (short-circuited). Therefore, the output port of the λ/4 transmission line stub  315  becomes substantially shorted to ground (short-circuited). According to the characteristic of the λ/4 transmission line stub  315 , the impedance of the input port of the λ/4 transmission line stub  315  changes to be opposite to the impedance of the output port of the λ/4 transmission line stub  315  and thus becomes nearly infinite (open-circuited). Since the input port of the λ/4 transmission line stub  315  and the 50 ohm transmission line are connected in parallel to the output port of the λ/4 transmission line  313 , the impedance of the output port of the λ/4 transmission line  313  becomes 50 ohm. As a result, the impedance of the input port (a port nearest to the circulator  305 ) of the λ/4 transmission line  313  becomes 50 ohm according to the characteristic of the λ/4 transmission line  313 . 
         [0052]    Consequently, when the wireless communication system operates in the reception mode, according to the operations of the λ/2 transmission line stub  309  and the pin diode  311 , an output of the isolator  303  is reflected, and the reflected output is returned to the isolator  303 , thereby being terminated. Therefore, even if an abnormal output is produced from the isolator  303 , the receive port  319  can be protected. In addition, the λ/4 transmission line  313 , the λ/4 transmission line stub  315 , and the pin diode  317  enable the receive port  319  to receive a signal received through the antenna port  307 . 
         [0053]    If the TDD switch operates incorrectly, the TDD wireless communication system may operate in the transmission mode while the TDD switch operates in the reception mode. In this case, the high-power transmission signal is reflected by the operations of the pin diode  311  and the λ/2 transmission line stub  309  and is thus returned to the isolator  303 , thereby being terminated. Therefore, the circuit of the receiver can be protected. 
         [0054]    A cable connected to the antenna port  307  may be open when the TDD switch is turned off, or a high-power signal may be reflected when a Voltage Standing Wave Ratio (VSWR) of the circuit of the antenna increases due to impedance mismatching. Even in this case, the pin diode  317 , the λ/4 transmission line stub  315 , and the λ/4 transmission line  313  operate in the same state as when the TDD switch operates in the transmission mode. Therefore, most of the power signal to be introduced into the receive port  319  is isolated, thereby protecting the circuit of the receiver. 
         [0055]      FIG. 4  is a diagram illustrating a TDD switch without a λ/2 transmission line stub according to an exemplary embodiment of the present invention. Referring to  FIG. 4 , the TDD switch includes an isolator  403 , a circulator  405 , pin diodes  411  and  417 , a λ/4 transmission line  413  and a λ/4 transmission line stub  415 . The λ/4 transmission line  413 , the λ/4 transmission line stub  415 , and the pin diode  417  are connected between the circulator  405  and a receive port  419 . The exemplary embodiment illustrated in  FIG. 4  also includes a transmission port  401 , an antenna port  407  and a bias circuit  421 . 
         [0056]    As illustrated in  FIG. 4 , an exemplary embodiment includes a generalized (λ/2)*m transmission line stub [m=0,1,2,3, . . . ] when m is 0. According to transmission line theory, the transmission line stub has a specific length, and waves have the same amplitudes at 0, λ/2, λ, 3λ/2, 2λ, and so on. Therefore, the λ/2 transmission line stub may be generalized as a (λ/2)*m transmission line stub [m=0,1,2,3, . . . ]. As a result, a high frequency system such as the TDD wireless communication system has the same characteristic regardless of whether the λ/2 transmission line stub is absent or whether the transmission line stub has the length of λ/2, λ, 3λ/2, 2λ, and so on. 
         [0057]    Accordingly, an exemplary TDD switch without the λ/2 transmission line stub as shown in  FIG. 4  operates in the same manner as the TDD switch with the λ/2 transmission line stub  309  as illustrated in  FIG. 3 . 
         [0058]      FIG. 5  is a diagram illustrating a TDD switch having three connection configurations each of which includes a λ/4 transmission line, a λ/4 transmission line stub, and a λ/2 transmission line stub according to an exemplary embodiment of the present invention. 
         [0059]    Referring to  FIG. 5 , a connection configuration A  516  includes a λ/4 transmission line  513 , a λ/4 transmission line stub  514 , and a pin diode  515 . In the connection configuration A  516 , the function of the λ/4 transmission line  513  is similar to that of the λ/4 transmission line  313  of  FIG. 3 , and the function of the λ/4 transmission line stub  514  is similar to that of the λ/4 transmission line stub  315 . In addition, the function of the pin diode  515  is similar to that of the pin diode  317  of  FIG. 3 . Likewise, connection configurations B  517  and C  518  each includes a λ/4 transmission line, a λ/4 transmission line stub, and a pin diode each having the same functions as those in the connection configuration A  516 . As further illustrated in  FIG. 5 , an exemplary embodiment may also include a transmission port  501 , an isolator  503 , an antenna port  507 , a λ/2 transmission line stub  509 , a PIN diode  511  and a bias circuit  521 . 
         [0060]    The number of connection configurations may vary depending on the extent of isolation between a circulator  505  and a receive port  519 . Further, the number of connection configurations may be determined through simulation or theoretical calculation. 
         [0061]      FIG. 6  is a diagram illustrating an exemplary TDD switch having three connection configurations each of which includes a λ/4 transmission line  613 , a λ/4 transmission line stub  615 , and a pin diode  617 . However, in the exemplary embodiment illustrated in  FIG. 6 , a λ/2 transmission line stub is absent. 
         [0062]    In  FIG. 6 , the configuration of a circuit located between a circulator  605  and a receive port  619  is the same as that of  FIG. 5 . Similar to  FIG. 4 , the TDD switch of  FIG. 6  does not have the λ/2 transmission line stub. As further illustrated in  FIG. 6 , the exemplary embodiment also includes a transmission port  601 , an isolator  603 , an antenna port  607 , a PIN diode  611  and a bias circuit  621 . 
         [0063]    For example, the TDD switch for protecting the receive port  619  may employ the configuration shown in  FIG. 4 , which is the same as the configuration shown in  FIG. 3  except that the λ/2 transmission line stub is absent, when the TDD switch operates in the reception mode while the TDD wireless communication system operates in the transmission mode. Further, the TDD switch for protecting the receive port  619  may employ the configuration shown in  FIG. 5 , when the TDD switch is turned off. 
         [0064]    Accordingly, when the TDD switch operates in the reception mode while the TDD wireless communication system operates in the transmission mode, or when the switch of the TDD wireless communication system is turned off, the circuit of the receiver can be protected. 
         [0065]    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 and their equivalents.