Abstract:
The present invention discloses an antenna diversity switch, used for receiving and transmitting RF signals for dual-mode co-existence wireless communication system. The antenna diversity switch comprises a first transmitting port; a second transmitting port; a receiving port; a first control unit, electrically connected between the first transmitting port and a first antenna; a second control unit, electrically connected between the first transmitting port and a second antenna; a third control unit, electrically connected between the second transmitting port and the first antenna; a forth control unit, electrically connected between the second transmitting port and the second antenna; a fifth control unit, electrically connected between the receiving port and the first antenna; a sixth control unit, electrically connected between the receiving port and the second antenna. The antenna diversity switch according to the present invention meets the requirement of IEEE 802.11b/g WLAN and Bluetooth co-existence operation with reduction of three or four antennas in the prior art to two antennas. Moreover, it provides antenna diversity for both WLAN and Bluetooth systems, where in the prior-art WLAN and Bluetooth co-existence system only WLAN system has the antenna diversity.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a monolithic microwave integrated circuit (MMIC), and more particularly to a microwave double-pole-three-throw switch, used for WLAN and Bluetooth dual-mode co-existence system.  
         [0003]     2. Description of the Related Art  
         [0004]     The rapid development of the co-existence operation of multi-standard wireless and mobile communication has been driving conventional radio frequency (RF) and baseband transceivers to have integrated multi-band and multi-functional characteristics, such as the multimode wireless local-area network (WLAN) IEEE802.11a/b/g card, the integrated WLAN/Bluetooth card, and the integrated GSM/WLAN handset. The IEEE802.11b standard operates in the frequency band of 2400-2483.5 MHz, which provides the transmission rate of 1-11 Mbps. The IEEE 802.11g standard, operating with the same 2400-2483.5-MHz band, has higher transmission rates, up to 54 Mbits per second. On the other hand, the wireless personal network (WPAN), such as Bluetooth and IEEE 802.15.4 standards, operates at the same 2400-2483.5 MHz and provides the feature of extremely low power consumption for low data transmission rates. For the performance and product-value enhancement of wireless products, such as notebook and PDA, the incorporation of WLAN and Bluetooth into the original product becomes crucial.  
         [0005]      FIG. 1  shows a conventional schematic of a IEEE802.11b/g WLAN and Bluetooth co-existence system. In general, the system employs three antennas, where two antennas are used for IEEE802.11b/g and the third antenna is for Bluetooth system. The two WLAN antennas are followed with a double-pole-double-throw switch and the Bluetooth antenna is followed with a single-pole-double-throw switch for transmit and receive operation. The WLAN antenna has an antenna diversity effect while the Buletooth antenna does not have the antenna diversity effect.  
         [0006]     On the aspect of circuit design, the field-effect transistors or diodes are used as the switching elements, which are arranged in various structures. First, the series-shunt architecture or their higher-order extensions, such as the series-shunt-series T-type structure or the shunt-series-shunt Pi-type structure. This structure has low RF power handling due to the voltage drop all across the transistor nodes, causing voltage breakdown. It also suffers poor insertion loss due to the turn-on resistance of the transistor and poor isolation due to the parasitic drain-source or collector-emitter capacitance. Second, the L-C resonant structure uses additional inductor to resonate the parasitic drain-source or collector-emitter capacitance and release the voltage drop across the transistor nodes. The paper, reported by Tokumitsu et al, entitled “Low Voltage, High Power T/R Switch MMIC Using LC Resonators,”  IEEE Microwave and Millimeter - Wave Monolithic Circuit Symposium,  pp. 27-30, June, 1993, provided a novel T/R switch for high-power/low-distortion operation at low control voltage. The LC-resonant switch structure, composed of inductors, capacitors, and switching FETs, is incorporated in the TX and RX circuit path for high power handling and low insertion loss. An LC-resonant T/R switch with total periphery of 2.88 mm exhibits the third-order inter-modulation ratio higher than 40 dB for input power up to 28 dBm when controlled at 0V and −2V.  
         [0007]     Another paper, reported by Tokumitsu et al, entitled “A low-voltage, high-power T/R-switch MMIC using LC resonators,” in  IEEE Transactions Microwave Theory and Techniques,  vol. 43, pp. 997-1003, May, 1995, provided a novel T/R switch for high-power/low-distortion operation at a low control voltage. A 1.9-GHz LC-resonant T/R switch MMIC with a total FET periphery of 3.36 mm exhibits third-order inter-modulation ratio higher than 40 dB for input power up to 31 dBm when controlled by a single-polarity voltage −2 V. This MMIC occupies an area less than 2×2 mm 2 , which makes it possible to implement advanced transmit/receive switches for applications in PCS and ISM frequencies below 5 GHz.  
         [0008]     U.S. Pat. No. 5,990,580 issued to Weigand et al, entitled “Single-pole-double throw switch”, discloses an electronic SPDT switch. The electronic SPDT switch has a series field-effect transistor (FET) in a first circuit arm between a common port and a first port, a shunt FET in a second circuit arm between the common port and a second port, the shunt FET being isolated 90 degrees or ¼ wavelength from the common port, a source applying pull-up voltage to sources of respective FETs and to the common port, to provide a connection of the common port with the second port, a source applying a first control voltage of opposite logic state to the gates, and the FETs being in the depletion mode that conduct at a zero sum of the bias voltage and the control voltage, and that conduct when DC power is interrupted, to provide a connection of the common port with the first port. However, in the above technologies, the single-pole-double-throw switch can not meet the requirement of the IEEE802.11b/g WLAN and Bluetooth dual-mode co-existence operation.  
         [0009]     According to the above problems, there is a need to provide an antenna diversity switch to overcome the above problems, meeting the requirement of IEEE 802.11b/g WLAN and Bluetooth co-existence operation with the reduction of three or four antennas in prior art to two antennas. Moreover, the antenna diversity for both WLAN and Bluetooth modes are also provided.  
       SUMMARY OF THE INVENTION  
       [0010]     It is an objective of the present invention to provide an antenna diversity switch, used for dual-mode co-existence communication system and implemented by the monolithic integrated circuit as a single-chip circuit.  
         [0011]     It is an objective of the present invention to provide an antenna diversity switch, which is used for the WLAN and Bluetooth co-existence system, and allows both WLAN and Bluetooth to have the antenna diversity.  
         [0012]     To obtain the above and other objectives, the present invention provides an antenna diversity switch, used for receiving and transmitting the RF signals of the WLAN and Bluetooth co-existence system. The antenna diversity switch comprises a first transmitting port, used for providing a first input signal of the antenna diversity switch; a second transmitting port, used for providing a second input signal of the antenna diversity switch; a receiving port, used for providing an output signal of the antenna diversity switch; a first control unit, electrically connected between the first transmitting port and a first antenna; a second control unit, electrically connected between the first transmitting port and a second antenna; a third control unit, electrically connected between the second transmitting port and the first antenna; a fourth control unit, electrically connected between the second transmitting port and the second antenna; a fifth control unit, electrically connected between the receiving port and the first antenna; and a sixth control unit, electrically connected between the receiving port and the second antenna.  
         [0013]     According to one aspect of the antenna diversity switch, the first RF signal is the RF signal of the WLAN system and the second RF signal is the RF signal of the Bluetooth system.  
         [0014]     According to one aspect of the antenna diversity switch, the first RF signal is the RF signal of the WLAN according to IEEE 802.11b/g standard.  
         [0015]     According to one aspect of the antenna diversity switch, the first control unit, the second control unit, the third control unit, the fourth control unit, the fifth control unit and the sixth control unit are all the field effect transistor, where each source node and each drain node are connected to the other devices and each gate is connected to the control voltage to determine the on/off state of each transistor.  
         [0016]     According to one aspect of the antenna diversity switch, the first antenna and the second antenna are the diversity antennas.  
         [0017]     According to the antenna diversity switch of present invention, it can be used for dual-mode co-existence system and reduce the numbers of the diversity antennas. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     All the objects, advantages, and novel features of the invention will become more apparent from the following detailed descriptions when taken in conjunction with the accompanying drawings.  
         [0019]      FIG. 1  shows a conventional schematic of a dual mode co-existence system;  
         [0020]      FIG. 2  shows a schematic of a dual mode co-existence system according to the embodiment of the present invention;  
         [0021]      FIG. 3  shows the circuit schematic of the antenna diversity switch according to the embodiment of the present invention;  
         [0022]      FIG. 4  shows the equivalent circuit schematic of the switching transistor in the control unit according to the embodiment of the present invention;  
         [0023]      FIG. 5  shows the simulated insertion loss and return loss of the first and second transmitting ports of the antenna diversity switch at the first operating mode;  
         [0024]      FIG. 6  shows the simulated isolation between each unit of the antenna diversity switch at the first operating mode;  
         [0025]      FIG. 7  shows the simulated P 1dB  compression point of the first and second transmitting ports of the antenna diversity switch at the first operating mode;  
         [0026]      FIG. 8  shows the simulated insertion loss and return loss of the receiving port of the antenna diversity switch at the second operating mode;  
         [0027]      FIG. 9  shows the simulated insertion loss and return loss of the first and second transmitting ports of the antenna diversity switch at the third operating mode;  
         [0028]      FIG. 10  shows the simulated isolation between each unit of the antenna diversity switch at the second and third operating mode;  
         [0029]      FIG. 11  shows the simulated P 1dB  compression point of the receiving port of the antenna diversity switch at the second operating mode;  
         [0030]      FIG. 12  shows the simulated P 1dB  compression point of the receiving port of the antenna diversity switch at the third operating mode;  
         [0031]      FIG. 13  shows the simulated insertion loss and return loss of the receiving port of the antenna diversity switch at the fourth operating mode;  
         [0032]      FIG. 14  shows the simulated isolation between each unit of the antenna diversity switch at the fourth operating mode; and  
         [0033]      FIG. 15  shows the simulated P 1dB  compression point of the receiving port of the antenna diversity switch at the fourth operating mode. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]     Although the invention has been explained in relation to several preferred embodiments, the accompanying drawings and the following detailed descriptions are the preferred embodiment of the present invention. It is to be understood that the following disclosed descriptions will be examples of present invention, and will not limit the present invention into the drawings and the special embodiment.  
         [0035]      FIG. 2  shows a schematic of a dual-mode co-existence system according to the embodiment of the present invention. The system uses two antennas, which are used for the transmission and reception of the IEEE802.11b/g system and Bluetooth system, and a switch  100  of the double-pole-three-throw configuration according to the present invention.  
         [0036]     According to the present invention, the antenna diversity switch collectively controls all the switching transistors to form the OFF or ON state such that it can be used as a transmit/receive switch for dual-mode co-existence system and therefore it reduces the areas of the RF modules. Now referring to the  FIG. 3 , it shows the circuit schematic of the antenna diversity switch  100  according to the embodiment of the present invention. The antenna diversity switch  100  comprises a first transmitting port  111 , a first antenna  121 ; a second transmitting port  112 ; a receiving port  130 ; a second antenna  122 ; a first control unit  141 , electrically connected between the first transmitting port  111  and the first antenna  121 ; a second control unit  142 , electrically connected between the first transmitting port  111  and a second antenna  122 ; a third control unit  143 , electrically connected between the second transmitting port  112  and the first antenna  121 ; a forth control unit  144 , electrically connected between the second transmitting port  112  and the second antenna  122 ; a fifth control unit  145 , electrically connected between the receiving port  130  and the first antenna  121 ; and a sixth control unit  146 , electrically connected between the receiving port  130  and the second antenna  122 .  
         [0037]     The first antenna  121  and the second antenna  122  can be the antennas of any type but preferably be the diversity antennas. In general, the actions of diversity antennas are classified into the predictive diversity and the true diversity. The predictive diversity uses two antennas located in different locations to receive the signal. It automatically selects the signal with higher intensity so that the antennas always receive the signal of higher intensity. The true diversity uses two receiving modules of the same performance, each connected to one antenna in different locations. The true diversity selects the baseband signal of higher intensity by the results of the comparator and switch circuits.  
         [0038]     The first transmitting port  111  and the second transmitting port  112  connected to the transmitting ports of an RF transceiver. As shown in  FIG. 3 , a first RF signal from the first transceiver of the dual-mode system can be fed to the first transmitting port  111  and then be radiated by the first antenna  121  selected by the first control unit  141  or be radiated by the second antenna  122  selected by the second control unit  142 . In the same way, a second RF signal from the second transceiver of the dual-mode system can be fed to the second transmitting port  112  and then be radiated by the first antenna  121  selected by the third control unit  143  or be t radiated by the second antenna  122  selected by the fourth control unit  144 .  
         [0039]     The first RF signal and the second RF signal are the signals processed by the RF transceiver, namely the first RF signal and the second RF signal are generated from the baseband signal up-converted by the RF module. The first RF signal and the second RF signal can be any microwave signals of different frequencies and systems, such as Advanced Mobile Phone System (AMPS); Code Division Multiple Access (CDMA); Wideband Code Division Multiple Access (WCDMA); Global System for Mobile Communications (GSM); Global Positioning System (GPS); Personal Handy-phone System (PHS); Industrial Scientific and Medical (ISM) devices; Wireless Local Area Networks (WLAN) and Bluetooth. In present invention, the first RF signal is preferably the RF signal of WLAN of IEEE802.11b/g standard and the second RF signal is preferably the RF signal of the Bluetooth system.  
         [0040]     The receiver  130  is the receiving port of a RF transceiver. As shown in  FIG. 3 , a first RF signal from the first antenna  121  will enter the receiving port  130  of the RF transceiver selected by the fifth control unit  145  to be further processed. In the same way, a second RF signal from the second antenna  122  will enter the receiving port  130  of the RF transceiver selected by the sixth control unit  146  to be further processed. A power divider connected to the receiving port  130  (not shown) is used for dividing the power of the RF signal from the receiving port  130  and then the RF signal will be further processed in several RF front-end circuits.  
         [0041]     The first control unit  141 , the second control unit  142 , the third control unit  143 , the forth control unit  144 , the fifth control unit  145  and the sixth control unit  146  all use the field effect transistor as the switching component, where each source node and each drain node are connected to the adjacent circuits and each gate is connected to the control voltage to determine the on/off state of each transistor. The types of the above transistors can be implemented include: Bipolar Junction Transistor (BJT), Heterojunction Bipolar Transistor (HBT), High Electronic Mobility Transistor (HEMT), Pseudomorphic HEMT (PHEMT), Complementary Metal Oxide Semiconductor Filed Effect Transistor (CMOS) and Laterally Diffused Metal Oxide Semiconductor Filed Effect Transistor (LDMOS). Preferably, PHEMT is suitable for the gain stage and power stage in the microwave to millimeter wave range. Semiconductor materials broadly applicable to the gain stage and power stage include: silicon, silicon-on-insulator (SOI), silicon-germanium (SiGe), gallium arsenide (GaAs), indium phosphide (InP) and silicon-germanium-carbon (SiGe—C).  
         [0042]     Across the source and drain nodes of each transistor, an inductor is connected to increase the isolation of the antenna diversity switch  100 .  FIG. 4  shows the equivalent circuit schematic of the switching transistor  200  in each control unit according to the embodiment of the present invention. The resistor  210  and the capacitor  220  are the equivalent circuit of each transistor of the control unit and it is noted that different transistor will have different resistance and the capacitance. The inductor  230  is employed to neutralize the parasitic capacitance  220  to increase the isolation of the antenna diversity switch in the OFF state. The inductor and capacitor form a shunt resonant circuit, which has a bandstop response, resulting in high isolation at the shunt resonance frequency. The angular frequency of the bandstop response is  
         ω   =     1       LC   OFF           ,       
 
 where ω=2πf, C OFF  is the parasitic capacitance of the turned-off transistor. In the ON state, the transistor is turned on and has a low turned-on resistance R ON  (typically a few Ω) such that the signal is passes through the switch transistor to the load with little insertion loss. By controlling the voltage on the gate of the transistor, the transistor of each control unit can pass or block the signal. 
 
         [0043]     Now refer to the  FIG. 3  for the description of the operating principle of the antenna diversity switch  100  of the present invention. Since the antenna diversity switch  100  is used for transmitting and receiving two RF signals of different wireless communication systems, the antenna diversity switch  100  has four different operation modes.  
         [0000]     (A) At the First Operating Mode:  
         [0044]     At the first operating mode, the antenna diversity switch  100  transmits two different RF signals. As the first control unit  141  is turned on; the second control unit  142  is turned off; the third control unit  143  is turned off; the forth control unit  144  is turned on; the fifth control unit  145  is turned off and the sixth control unit  146  is turned off, the first transmitting port  111  transmits the first RF signal to the first antenna  121  and the second transmitting port  112  transmits the second RF signal to the second antenna  122 . In addition, the first antenna  121  and the second antenna  122  have a diversity effect, so there is another operation condition as follows. As the first control unit  141  is turned off; the second control unit  142  is turned on; the third control unit  143  is turned on; the forth control unit  144  is turned off; the fifth control unit  145  is turned off and the sixth control unit  146  is turned off, the first transmitting port  111  transmits the first RF signal to the second antenna  122  and the second transmitting port  112  transmits the second RF signal to the first antenna  121 . It is noted that the above operating principle of the antenna diversity switch  100  transmits the two RF signals of the transceiver through two antennas  121  and  122 , respectively. Hence the two signal paths of the antenna diversity switch  100  are not overlapped.  
         [0045]      FIG. 5  shows the simulated insertion loss and return losses of the first transmitting port  111  and second transmitting port  112  of the antenna diversity switch at the first operating mode.  FIG. 6  shows the simulated isolation between each unit of the antenna diversity switch at the first operating mode.  FIG. 7  shows the simulated P 1dB  compression point of the first transmitting port  111  and second transmitting port  112  of the antenna diversity switch at the first operating mode. As shown in  FIG. 5 , the insertion loss is less than 0.68 dB from 1 to 4 GHz. The return losses are greater than 20 dB over a 600 MHz bandwidth centered at 2.4 GHz. As shown in  FIG. 6 , the isolation at 2.4 GHz between the first transmitting port  111  and the second transmitting port  112  is larger than 43 dB, the isolation between the first transmitting port  111  and the receiving port  130  is larger than 43 dB, and the isolation between the second transmitting port  112  and the receiving port  130  is larger than 43 dB. As shown in  FIG. 7 , the simulated P 1dB  compression point of the first transmitting port  111  and second transmitting port  112  is 34.5 dBm.  
         [0000]     (B) At the Second Operating Mode:  
         [0046]     At the second operating mode, one system is in the transmission state and the other system is in the receiving state. As the first control unit  141  is turned on, the second control unit  142  is turned off, the third control unit  143  is turned off, the forth control unit  144  is turned off, fifth control unit  145  is turned off and the sixth control unit  146  is turned on, the first transmitting port  111  transmits the first RF signal from the first antenna  121  and the second antenna  122  receives the second RF signal to the receiving port  130  so as the second RF signal can feed the transceiver. In addition, since the first antenna  121  and the second antenna  122  are the diversity pair, there is another situation at the second operating mode as follows. As the first control unit  141  is turned off; the second control unit  142  is turned on; the third control unit  143  is turned off; the forth control unit  144  is turned off; the fifth control unit  145  is turned on and the sixth control unit  146  is turned off, the first transmitting port  111  transmits the first RF signal from the second antenna  122  and the first antenna  121  receives the second RF signal to the receiving port  130  so as the second RF signal can feed the transceiver. It is noted that in above operating principle the first transmitting port  111  transmits the first RF signal from the one of the diversity antennas and another diversity antenna receives the second RF signal to the receiving port  130  so as the second RF signal can feed the transceiver. Due to the co-existence of receiving and transmitting, the antenna diversity  100  is in the state of the strong co-channel interference. The high isolation between switches reduces this co-channel interference.  
         [0000]     (C) At the Third Operating Mode:  
         [0047]     Since the configuration of the antenna diversity  100  is symmetry, the operating principle of the third operating mode is similar to the second operating mode. As the first control unit  141  is turned off; the second control unit  142  is turned off; the third control unit  143  is turned off; the forth control unit  144  is turned on, fifth control unit  145  is turned on and the sixth control unit  146  is turned off, the first antenna  121  receives the first RF signal to the receiving port  130  so as the first RF signal can feed the transceiver and the second transmitting port  112  transmits the second RF signal from the second antenna  122 . In addition, since the first antenna  121  and the second antenna  122  are the diversity pair, there is another situation at the third operating mode. As the first control unit  141  is turned off; the second control unit  142  is turned off; the third control unit  143  is turned on; the forth control unit  144  is turned off, fifth control unit  145  is turned off and the sixth control unit  146  is turned on, the second antenna  122  receives the first RF signal to the receiving port  130  so as the first RF signal can feed the transceiver and the second transmitting port  112  transmits the second RF signal from the first antenna  121 . It is noted that in above operating principle the second transmitting port  112  transmits the first RF signal from the one of the diversity antennas and another diversity antenna receives the second RF signal to the receiving port  130  so as the second RF signal can feed the transceiver. Due to the co-existence of receiving and transmitting, the antenna diversity  100  is also in the state of the strong co-channel interference. The high isolation between switches reduces this co-channel interference.  
         [0048]      FIG. 8  shows the simulated insertion loss and return losses of the receiving port  130  of the antenna diversity switch at the second operating mode.  FIG. 9  shows the simulated insertion loss and return losses of the first transmitting port  111  and second transmitting port  112  of the antenna diversity switch at the third operating mode.  FIG. 10  shows the simulated isolation between each unit of the antenna diversity switch at the second and third operating mode.  FIG. 11  shows the simulated P 1dB  compression point of the receiving port  130  of the antenna diversity switch at the second operating mode.  FIG. 12  shows the simulated P 1dB  compression point of the receiving port  130  of the antenna diversity switch at the third operating mode. As shown in  FIG. 8 , from 1 to 4 GHz, the insertion loss from the receiving port  130  to the transmitting port is less than 0.67 dB. At the center frequency of 2.4 GHz, the return loss of the receiving port is greater than 25 dB and the return loss of the transmitting port is larger than 23 dB. As shown in  FIG. 9 , from 1 to 4 GHz, the insertion loss from the receiving port  130  to the transmitting port is less than 0.76 dB. At the center frequency of 2.4 GHz, the return loss of the receiving port is greater than 35 dB and the return loss of the transmitting port is larger than 35 dB. As shown in  FIG. 10 , the isolation between the first transmitting port  111  and the second transmitting port  112  is larger than 43 dB, the isolation between the first transmitting port  111  and the receiving port  130  is larger than 43 dB, and the isolation between the second transmitting port  112  and the receiving port  130  is larger than 43 dB. As shown in  FIG. 11 , the simulated P 1dB  compression point of the receiving port  130  is 35 dBm. As shown in  FIG. 12 , the simulated P 1dB  compression point of the transmitting port is 35 dBm.  
         [0000]     (D) At the Fourth Operating Mode:  
         [0049]     At the fourth operating mode, the antenna diversity switch  100  receives two different RF signals. As the first control unit  141  is turned off; the second control unit  142  is turned off; the third control unit  143  is turned off; the forth control unit  144  is turned off; the fifth control unit  145  is turned on and the sixth control unit  146  is turned off, the first RF signal and the second RF signal are all received by the first antenna  121  and then propagate to the receiving port  130 . In addition, the first antenna  121  and the second antenna  122  are the diversity antennas, so there is another situation at the fourth operating mode. As the first control unit  141  is turned off; the second control unit  142  is turned off; the third control unit  143  is turned off; the forth control unit  144  is turned off; the fifth control unit  145  is turned off and the sixth control unit  146  is turned on, the first RF signal and the second RF signal are all received by the second antenna  122  and then propagate to the receiving port  130 . It is noted that in the above operating principle of the antenna diversity switch  100 , the two diversity antennas receive the two RF signals, which are then propagated to the receiving port  130 .  
         [0050]      FIG. 13  shows the simulated insertion loss and return losses of the receiving port  130  of the antenna diversity switch at the fourth operating mode.  FIG. 14  shows the simulated isolation between each unit of the antenna diversity switch at the fourth operating mode and  FIG. 15  shows the simulated P 1dB  compression point of the receiving port  130  of the antenna diversity switch at the fourth operating mode. As shown in  FIG. 13 , the insertion loss from the receiving port to the transmitting port is less than 0.76 dB from 1 to 4 GHz. The return loss of the receiving port  130  is larger than 25 dB and the return loss of the transmitting port is larger than 23 dB. As shown in  FIG. 14 , the isolation between the first transmitting port  111  and the second transmitting port  112  is larger than 43 dB, the isolation between the first transmitting port  111  and the receiving port  130  is larger than 43 dB, and the isolation between the second transmitting port  112  and the receiving port  130  is larger than 43 dB. As shown in  FIG. 15 , the simulated P 1dB  compression point of the receiving port  130  is 29 dBm.  
         [0051]     Although the invention has been explained in relation to its preferred embodiment, it is not used to limit the invention. It is to be understood that many other possible modifications and variations can be made by those skilled in the art without departing from the spirit and scope of the invention as hereinafter claimed.