Patent Publication Number: US-2007123181-A1

Title: Antenna system for enabling diversity and MIMO

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to antenna systems and, more particularly, to antenna systems for mobile devices.  
      2. Background of the Invention  
      Mobile communication devices such as mobile telephones and personal digital assistants (PDAs) typically communicate via RF signals transmitted in the upper MHz (e.g. 900 MHz) or lower GHz (e.g. 1.8 GHz) frequency ranges. In multi-path environments, such as urban areas, RF signals propagated at these frequencies are especially susceptible to reflection and scattering caused by interaction of the signals with buildings and other structures. In consequence, mobile communication devices often receive multiple instances of the same RF signal with each instance following a different propagation path. For example, a device may receive a first instance of an RF signal that is reflected off of a first building and a second instance of the same RF signal that is reflected off of a second building. Different instances of the RF signal typically are received at different times, depending on the propagation path of each instance. Accordingly, the different signal instances are oftentimes referred to as uncorrelated signals.  
      Two types of antenna systems that have been developed for use in multi-path environments are diversity antennas systems and multiple-input/multiple-output (MIMO) antenna systems. A diversity antenna system includes multiple antenna elements to receive or transmit an RF signal and processes the signal from the element receiving the highest quality signal. Depending on their orientation, different antenna elements may receive different uncorrelated instances of the RF signal. A MIMO antenna system also includes multiple antenna elements. In contrast to diversity antenna systems, MIMO antenna systems simultaneously process uncorrelated signals. Both diversity and MIMO antenna systems can increase system capacity and improve reliability in comparison to antenna systems which use a single antenna element.  
     SUMMARY OF THE INVENTION  
      The present invention relates to an RF communication system. The RF communication system can include a first antenna having a first antenna element and at least a second antenna element, and a second antenna having a third antenna element and at least a fourth antenna element. The system also can include a first transmitter that applies a first outbound RF signal commonly to the first and second antenna elements, and a second transmitter that applies a second outbound RF signal differentially to the first and second antenna elements. In addition, the system can include a third transmitter that applies a third outbound RF signal commonly to the third and fourth antenna elements, and a fourth transmitter that applies a fourth outbound RF signal differentially to the third and fourth antenna elements.  
      The system further can include a printed circuit board. The printed circuit board can have a first edge portion and a second edge portion opposingly positioned with respect to the first edge portion. The first antenna element can be disposed proximate to the first edge portion and the second antenna element can be disposed proximate to the second edge portion. The printed circuit board also can have a third edge portion and a fourth edge portion opposingly positioned with respect to the third edge portion. The third antenna element can be disposed proximate to the third edge portion and the fourth antenna element can be disposed proximate to the fourth edge portion. An orientation of the first antenna can be perpendicular to an orientation of the second antenna.  
      The first transmitter and/or the second transmitter can be selectively operable in a transmit mode in which either of the first outbound RF signal and the second outbound RF signal that exhibits higher quality signal transmission characteristics in comparison to the other signal is exclusively transmitted from the first antenna. Similarly, the third transmitter and the fourth transmitter can be selectively operable in a transmit mode in which either of the third outbound RF signal and the fourth outbound RF signal that exhibits higher quality signal transmission characteristics in comparison to the other signal is exclusively transmitted from the second antenna.  
      The system also can include a first receiver that receives a first inbound RF signal commonly from the first and second antenna elements and a second receiver that receives a second inbound RF signal differentially from the first and second antenna elements. In addition, the system can include a third receiver that receives a third inbound RF signal commonly from the third and fourth antenna elements and a fourth receiver that receives a fourth inbound RF signal differentially from the third and fourth antenna elements.  
      The first receiver and/or the second receiver can be selectively operable in a receive mode in which either of the first inbound RF signal and the second inbound RF signal that exhibits higher quality signal reception characteristics in comparison to the other signal is exclusively received from the first antenna. Similarly, the third receiver and the fourth receiver can be selectively operable in a receive mode in which either of the third inbound RF signal and the fourth inbound RF signal that exhibits higher quality signal reception characteristics in comparison to the other signal is exclusively received from the second antenna.  
      The RF communication system also can include a first transmitter that applies a first outbound RF signal commonly to the first and second antenna elements in a first transmit mode, and applies the first outbound RF signal differentially to the first and second antenna elements in a second transmit mode. In addition, a second transmitter can be provided. The second transmitter can apply a second outbound RF signal commonly to the third and fourth antenna elements in a third transmit mode, and apply the second outbound RF signal differentially to the third and fourth antenna elements in a fourth transmit mode.  
      The RF communication system can be selectively operable in a plurality of system modes for transmitting RF signals. In a first system mode the first transmitter can operate in the first transmit mode and the second transmitter can operate in the third transmit mode. In a second system mode the first transmitter can operate in the first transmit mode and the second transmitter can operate in the fourth transmit mode. In a third system mode the first transmitter can operate in the second transmit mode and the second transmitter can operate in the third transmit mode. In a fourth system mode the first transmitter can operate in the second transmit mode and the second transmitter can operate in the fourth transmit mode.  
      The first transmitter can be selectively operable in the first transmit mode if the first outbound RF signal exhibits higher quality signal transmission characteristics in the first transmit mode in comparison to the second transmit mode, and the first transmitter can be selectively operable in the second transmit mode if the first outbound RF signal exhibits higher quality signal transmission characteristics in the second transmit mode in comparison to the first transmit mode. Likewise, the second transmitter can be selectively operable in the third transmit mode if the second outbound RF signal exhibits higher quality signal transmission characteristics in the third transmit mode in comparison to the fourth transmit mode, and the second transmitter can be selectively operable in the fourth transmit mode if the second outbound RF signal exhibits higher quality signal transmission characteristics in the fourth transmit mode in comparison to the third transmit mode.  
      The RF communication system also can include a first receiver and a second receiver. The first receiver can receive a first inbound RF signal commonly from the first and second antenna elements in a first receive mode, and can receive the first inbound RF signal differentially from the first and second antenna elements in a second receive mode. The second receiver can receive a second inbound RF signal commonly from the third and fourth antenna elements in a third receive mode, and can receive the second inbound RF signal differentially from the third and fourth antenna elements in a fourth receive mode.  
      The RF communication system can be selectively operable in a plurality of system modes for receiving inbound RF signals. In a first system mode the first receiver can operate in the first receive mode and the second receiver can operate in the third receive mode. In a second system mode the first receiver can operate in the first receive mode and the second receiver can operate in the fourth receive mode. In a third system mode the first receiver can operate in the second receive mode and the second receiver can operate in the third receive mode. In a fourth system mode the first receiver can operate in the second receive mode and the second receiver can operate in the fourth receive mode.  
      In addition, the first receiver can be selectively operable in the first receive mode if the first inbound RF signal exhibits higher quality signal receive characteristics in the first receive mode in comparison to the second receive mode, and the first receiver can be selectively operable in the second receive mode if the first inbound RF signal exhibits higher quality signal receive characteristics in the second receive mode in comparison to the first receive mode. Similarly, the second receiver can be selectively operable in the third receive mode if the second inbound RF signal exhibits higher quality signal receive characteristics in the third receive mode in comparison to the fourth receive mode, and the second receiver can be selectively operable in the fourth receive mode if the fourth inbound RF signal exhibits higher quality signal receive characteristics in the fourth receive mode in comparison to the third receive mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Preferred embodiments of the present invention will be described below in more detail, with reference to the accompanying drawings, in which:  
       FIG. 1  depicts an RF communication system useful for understanding the present invention.  
       FIG. 2  depicts another arrangement of the RF communication system.  
       FIG. 3  depicts yet another arrangement of the RF communication system.  
       FIGS. 4-7  depict examples of field patterns that can be generated by the RF communication system.  
       FIG. 8  depicts the RF communication system positioned proximate to the a user&#39;s head.  
       FIGS. 9-12  depict examples of field patterns that can be generated by the RF communication system when positioned as shown in  FIG. 8 .  
       FIG. 13  depicts the RF communication system positioned proximate to the a user&#39;s hand.  
       FIGS. 14-17  depict examples of field patterns that can be generated by the RF communication system when positioned as shown in  FIG. 13 . 
    
    
     DETAILED DESCRIPTION  
      While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the description in conjunction with the drawings. As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting but rather to provide an understandable description of the invention.  
      The present invention relates to an RF communication system that can simultaneously transmit and receive multiple uncorrelated signals. Because the signals are uncorrelated, the signals can be used as distinct communication channels, thereby providing high system capacity over a given bandwidth. In addition, the antenna system can simultaneously operate both as a diversity antenna system and as a multiple-input/multiple-output (MIMO) antenna system, thereby providing improved system performance in multi-path environments in comparison to systems of the prior art.  
       FIG. 1  depicts an RF communication system  100  that is useful for understanding the present invention. The RF communication system  100  can include a first antenna  110  and a second antenna  120 . The first antenna  110  can include a first antenna element  112  and at least a second antenna element  114 . Similarly, the second antenna  120  can include a third antenna element  122  and at least a fourth antenna element  124 . An orientation of the first antenna  110  can be approximately perpendicular to an orientation of the second antenna  120 . For instance, an alignment of the first antenna element  112  with respect to the second antenna element  114  can be perpendicular to an alignment of the third antenna element  122  with respect to the fourth antenna element  124 .  
      The RF communication system  100  also can include a printed circuit board  130  on which components of the system  100 , such as the antenna elements  112 ,  114 ,  122 ,  124 , are disposed. The printed circuit board  130  can be, for example, a printed circuit board for a mobile communication device, such as a handheld communication device.  
      The printed circuit board  130  can include a first edge portion  132  and a second edge portion  134 . The second edge portion  134  can be opposingly positioned with respect to the first edge portion  132 . The first antenna element  112  can be disposed proximate to the first edge portion  132  and the second antenna element  114  can be disposed proximate to the second edge portion  134 . The printed circuit board  130  further can include a third edge portion  136  and a fourth edge portion  138 . The fourth edge portion  138  can be opposingly positioned with respect to the third edge portion  136 . The third antenna element  122  can be disposed proximate to the third edge portion  136  and the fourth antenna element  124  can be disposed proximate to the fourth edge portion  138 .  
      In one arrangement, the antenna elements  112 ,  114 ,  122 ,  124  each can include an antenna feed  140  disposed proximate to a slot  142  defined within the printed circuit board  130 , thereby forming slot antenna elements. In another arrangement, the antenna elements  112 ,  114 ,  122 ,  124  each can include a patch antenna element or planar inverted-F antenna (PIFA) element realized proximate to the printed circuit board  130 . In yet another arrangement, the antenna elements  112 ,  114 ,  122 ,  124  each can include a monopole or folded monopole antenna element disposed approximately orthogonal to the respective edge portions  132 ,  134 ,  136 ,  138 . Slot antenna elements, patch antenna elements, PIFA antenna elements, and monopole and folded monopole antenna elements are known to the skilled artisan. Other antenna elements also are known to the skilled artisan and are within the scope of the present invention.  
      Referring again to  FIG. 1 , the RF communication system  100  also can include a first transceiver  150 , a second transceiver  152 , a third transceiver  154  and a fourth transceiver  156 . The first transceiver  150  can operate as a first transmitter and as a first receiver, the second transceiver  152  can operate as a second transmitter and as a second receiver, the third transceiver  154  can operate as a third transmitter and as a third receiver, and the fourth transceiver  156  can operate as a fourth transmitter and as a fourth receiver.  
      A first hybrid circuit  160  can be communicatively linked between the first and second transceivers  150 ,  152  and the first and second antenna elements  112 ,  114 . Similarly, a second hybrid  162  can be communicatively linked between the third and fourth transceivers  154 ,  156  and the third and fourth antenna elements  122 ,  124 . The hybrids  160 ,  162  can be multi-port devices which receive input signals and generate correlating output signals that are either in-phase with respect to the input signals and/or out-of-phase with respect to the input signals. Hybrids are reciprocal components that are known to the skilled artisan. Other reciprocal and non-reciprocal components which may perform the same functions as the hybrids  160 ,  162  also are known to those skilled in the art, and are within the scope of the present invention.  
      In operation, the first hybrid  160  can receive a first outbound RF signal—hereinafter “first signal S 1 ”—from the first transceiver  150  and propagate the first signal S 1  commonly, or in-phase, to the first and second antenna elements  112 ,  114 . An example of the field pattern  400  that can be produced by commonly applying the first signal S 1  to the first and second antenna elements  112 ,  114  is shown in  FIG. 4 . The first hybrid  160  also can receive a second outbound RF signal—hereinafter “second signal S 2 ”—from the second transceiver  152  and propagate the second signal S 2  differentially, or out-of-phase, to the first and second antenna elements  112 ,  114 . An example of the field pattern  500  that can be produced by differentially applying the second signal S 2  to the first and second antenna elements  112 ,  114  is shown in  FIG. 5 .  
      Notably, the degree of correlation between the radiated electromagnetic fields associated with the commonly applied first signal S 1  and the radiated electromagnetic fields associated with the differentially applied second signal S 2  can be very low. For example, the degree of correlation between the first and second signals S 1 , S 2  can be computed by the following equation:  
               ρ   12     ≈              E   ⁢     {         E   1     ⁡     (     θ   ,   ϕ     )       ·       E   2   *     ⁡     (     θ   ,   ϕ     )         }            2       E   ⁢     {              E   1     ⁡     (     θ   ,   ϕ     )            2     }     ⁢   E   ⁢     {              E   2     ⁡     (     θ   ,   ϕ     )            2     }                 (   1   )             
 
 where E{·} is the expected value operator and E i (θ,φ) (i=1, 2) is the radiated electric field pattern relative to the i-th transmit or receive mode. Applying this equation to the example RF communication system  100  of  FIG. 1 , assuming a uniform angular distribution for the multi-path components, an expected correlation coefficient of 7.7e−4 (−31 dB) can be computed. Such a level of correlation between the first and second signals S 1 , S 2  when the first and second signals are simultaneously transmitted is, for the most part, negligible. Accordingly, the level of interference to either of the signals S 1 , S 2  caused by their simultaneous transmission is negligible. 
 
      The second hybrid  162  can receive a third outbound RF signal—hereinafter “third signal S 3 ”—from the third transceiver  154  and propagate the third signal S 3  commonly to the third and fourth antenna elements  122 ,  124 . An example of the field pattern  600  that can be produced by commonly applying the third signal S 3  to the third and fourth antenna elements  122 ,  124  is shown in  FIG. 6 . The second hybrid  162  also can receive a fourth outbound RF signal—hereinafter “fourth signal S 4 ”—from the fourth transceiver  156  and propagate the fourth signal S 4  differentially to the third and fourth antenna elements  122 ,  124 . An example of the field pattern  700  that can be produced by differentially applying the fourth signal S 4  to the third and fourth antenna elements  122 ,  124  is shown in  FIG. 7 .  
      The field patterns  600 ,  700  produced by excitation of the third and fourth antenna elements  122 ,  124  can be non-symmetrical, as shown, by offsetting the third and fourth antenna elements  122 ,  124  with respect to a centerline  170  of the printed circuit board  130 . The invention is not limited in this regard, however. For example, the third and fourth antenna elements  122 ,  124  can be aligned on the centerline  170  to produce a symmetrical field pattern. Moreover, although the first and second antenna elements  112 ,  114  can be aligned with a centerline  172  of the printed circuit board  130 , as shown, the first and second antenna elements  112 ,  114  also can be offset from the centerline  172  to produce a non-symmetrical field pattern.  
      By way of example, equation (1) can be applied to the RF communication system  100  of  FIG. 1  to compute an expected correlation coefficient of 5.3e−9 (−83 dB) for the third and fourth signals S 3 , S 4 . Thus, the third and fourth signals S 3 , S 4  can be simultaneously transmitted from the second antenna  120  with negligible interference between the signals S 3 , S 4 .  
      In addition to providing a very small degree of correlation between signals applied commonly and differentially to a particular antenna, the present invention also provides for a very small degree of correlation between the signals S 1 , S 2  applied to the first antenna  110  and the signals S 3 , S 4  applied to the second antenna  120 . For example, applying equation (1) to the RF communication system  100  of  FIG. 1 , assuming a uniform angular distribution for the multi-path components, the following table of correlation coefficients can be predicted:  
                                                   First Antenna - Signal S 1     First Antenna - Signal S 2             Commonly Applied   Differentially Applied                                                        Second Antenna -   7.5e−9   (−81 dB)   1.3e−8   (−79 dB)       Signal S 3         Commonly Applied       Second Antenna -   6.0e−3   (−22 dB)   9.1e−4   (−30 dB)       Signal S 4         Differentially       Applied                  
 
 The values of the predicted correlation coefficients are very small. Accordingly, the RF communication system  100  can simultaneously transmit the signals, S 1 , S 2 , S 3 , S 4  with negligible signal degradation due to interference between signals. 
 
      The RF communication system  100  also can simultaneously receive multiple signals. For example, the first hybrid  160  can forward a first inbound RF signal that is received commonly on the first and second antenna elements  112 ,  114  to the first transceiver  150 , and forward a second inbound RF signal that is received differentially on the first and second antenna elements  112 ,  114  to the second transceiver  152 . Similarly, the second hybrid  162  can forward a third inbound RF signal that is received commonly on the third and fourth antenna elements  122 ,  124  to the third transceiver  154 , and forward a fourth inbound RF signal that is received differentially on the third and fourth antenna elements  122 ,  124  to the fourth transceiver  156 . Because of the reciprocal behavior of the structure, the degree of correlation between the inbound signals received at the first, second, third and fourth transceivers  150 ,  152 ,  154 ,  156  also can be predicted by equation (1).  
      Further, in addition to MIMO operation as described above, the RF communication system  100  can operate as a diversity antenna system. For example, during a communication session, inbound RF signals can include channel status information that represents the quality of the outbound RF signals. The channel status information can include, for instance, a bit error rate and/or a packet error rate of the transmitted signals. The channel status information can be extracted from the inbound RF signals and evaluated to determine whether to implement diversity for transmitting the outbound RF signals. If, for example, the quality of the first signal S 1  being transmitted by the first antenna  110  is low, the information contained in the first signal can be forwarded to the second transceiver  152  to be transmitted in the second signal S 2 . Similarly, if the quality of the second signal S 2  being transmitted by the first antenna  110  is low, the information contained in the second signal S 2  can be forwarded to the first transceiver  150  to be transmitted in the first signal S 1 .  
      In another arrangement, if the quality of the first and second signals S 1 , S 2  being transmitted by the first antenna  110  is low, the same first and second signals S 1 , S 2  can be forwarded to the third and fourth transceivers  154 ,  156  for transmission by the second antenna  120 . Likewise, if the quality of the third and fourth signals S 3 , S 4  being transmitted by the second antenna  120  is low, the same third and fourth signals S 3 , S 4  can be forwarded to the first and second transceivers  150 ,  152  for transmission by the first antenna  110 .  
      In another arrangement, the transceivers  150 ,  152  can select the first signal S 1  and the second signal S 2  so that their sum is equal to zero, i.e., S 2 =−S 1 . In this arrangement, the antenna element  114  can be excited while the antenna element  112  is not. Likewise, the first signal S 1  and the second signal S 2  can be selected so that their difference is equal to zero, which can result in the antenna element  112  being excited while the antenna element  114  is not. In a similar manner, the transceivers  154 ,  156  can select the third and fourth signals S 3  and S 4  to excite either the third antenna element  122  or the fourth antenna element  124 .  
      Furthermore, one or more of the transceivers  150 ,  152 ,  154 ,  156  can be selectively operable in a receive mode in which the transceiver processes signals that exhibit the highest quality reception characteristics. The reception characteristics can be determined, for example, by channel status information that includes the bit error rate and/or the packet error rate of the received signals. For example, the first transceiver  150  and/or the second transceiver  152  can be selectively operable in a receive mode in which either of the first inbound RF signal and the second inbound RF signal that exhibits higher quality signal reception characteristics in comparison to the other signal is exclusively received from the first antenna  110 . Similarly, the third transceiver  154  and the fourth transceiver  156  can be selectively operable in a receive mode in which either of the third inbound RF signal and the fourth inbound RF signal that exhibits higher quality signal reception characteristics in comparison to the other signal is exclusively received from the second antenna  120 .  
       FIG. 2  depicts another arrangement of the RF communication system  100  in which the RF communication system  100  is implemented using a single pair of transceivers. For instance, the RF communication system  100  can include a first transceiver  250  and a second transceiver  252 . In addition, a first switch  254  can be communicatively linked between the first transceiver  250  and the first hybrid  160 , and a second switch  256  can be communicatively linked between the second transceiver  252  and the second hybrid  160 . The first switch  254  and/or the first hybrid  160  can be components of the first transceiver  250 , or discrete components. Similarly, the second switch  256  and/or the second hybrid  162  can be components of the second transceiver  252 , or discrete components. Alternatively, the switches  254  and  256  can be replaced by respective functional blocks that combine the incoming signals according to different diversity schemes, such as Maximum Ratio Combining or Maximum Power Combining.  
      In a first transmit mode, the first switch  254  can receive the first signal S 1  from the first transceiver  250  and forward the first signal S 1  to a first input port  270  of the first hybrid  160 , which can cause the first signal S 1  to be commonly applied to the first and second antenna elements  112 ,  114 . In a second transmit mode, the first switch  254  can receive the first signal S 1  from the first transceiver  250  and forward the first signal S 1  to a second input port  272  of the first hybrid  160 , which can cause the first signal S 1  to be differentially applied to the first and second antenna elements  112 ,  114 .  
      Further, in a third transmit mode, the second switch  256  can receive the second signal S 2  from the second transceiver  252  and forward the second signal S 2  to a first input port  274  of the second hybrid  162 , which can cause the second signal S 2  to be commonly applied to the third and fourth antenna elements  122 ,  124 . In a fourth transmit mode, the second switch  256  can receive the second signal S 2  from the second transceiver  252  and forward the second signal S 2  to a second input port  276  of the second hybrid  162 , which can cause the second signal S 2  to be differentially applied to the third and fourth antenna elements  122 ,  124 .  
      Accordingly, the RF communication system  100  can be selectively operable in a plurality of system modes for transmitting RF signals. In a first system mode the first signal S 1  can be commonly applied to the first and second antenna elements  112 ,  114  while the second signal S 2  is commonly applied to the third and fourth antenna elements  122 ,  124 . In a second system mode the first signal S 1  can be commonly applied to the first and second antenna elements  112 ,  114  while the second signal S 2  is differentially applied to the third and fourth antenna elements  122 ,  124 . In a third system mode the first signal S 1  can be differentially applied to the first and second antenna elements  112 ,  114  while the second signal S 2  is commonly applied to the third and fourth antenna elements  122 ,  124 . In a fourth system mode the first signal S 1  can be differentially applied to the first and second antenna elements  112 ,  114  while the second signal S 2  is differentially applied to the third and fourth antenna elements  122 ,  124 .  
      The RF communication system  100  also can be selectively operable in a plurality of system modes for receiving RF signals. In a first of such system modes a first inbound RF signal can be commonly received from the first and second antenna elements  112 ,  114  while a second inbound RF signal is commonly received from the third and fourth antenna elements  122 ,  124 . In a second system mode the first inbound RF signal can be commonly received from the first and second antenna elements  112 ,  114  while the second inbound RF signal is differentially received from the third and fourth antenna elements  122 ,  124 . In a third system mode the first inbound RF signal can be differentially received from the first and second antenna elements  112 ,  114  while the second inbound RF signal is commonly received from the third and fourth antenna elements  122 ,  124 . In a fourth system mode the first inbound RF signal can be differentially received from the first and second antenna elements  112 ,  114  while the second inbound RF signal is differentially received from the third and fourth antenna elements  122 ,  124 . Provided all components are reciprocal, the degree of correlation between the inbound signals generated by the first and second transceivers  250 ,  252  also can be predicted by equation (1).  
      In an alternate arrangement shown in  FIG. 3 , in lieu of the switches and hybrids, the first transceiver  350  can include phase inverters  380 ,  382  and circulators  390 ,  392  that enable the first transceiver  350  to provide the first signal S 1  both commonly and differentially to the first and second antenna elements  112 ,  114 , and to receive signals both commonly and differentially from the first and second antenna elements  112 ,  114 . Similarly, the second transceiver  352  can include phase inverters  384 ,  386  and circulators  394 ,  396  that enable the second transceiver  352  to provide the second signal S 2  both commonly and differentially to the third and fourth antenna elements  122 ,  124 , and to receive signals both commonly and differentially from the third and fourth antenna elements  122 ,  124 . In an alternate arrangement, duplexers can be used in the respective transceivers  350 ,  352  in lieu of the circulators  390 ,  392 ,  394 ,  396 . The duplexers can separate the frequency bands on which the transceivers  350 ,  352  transmit and receive, which can virtually eliminate the amount of transmitted energy that is reflected back to a particular transceiver  350 ,  352  when that transceiver  350 ,  352  simultaneously transmits and receives.  
      In operation, a first control signal C 1  and a second control signal C 2  can selectively turn on and turn off the phase inverters  380 ,  382 , thereby controlling signal flow of signals to and from the first antenna element  112 . For instance, to provide the first signal S 1  commonly to the first and second antenna elements  112 ,  114 , the phase inverter  380  can be turned off thereby providing the first signal S 1  to the first antenna element  112  with a 0° phase shift. In this arrangement, the first signal S 1  also can be provided to the second antenna element  114  with a 0° phase shift. In order to provide the first signal S 1  differentially to the first and second antenna elements  112 ,  114 , the phase inverter  380  can be turned on, thereby providing the first signal S 1  to the first antenna element  112  with a 180° phase shift. The phase inverters  384 ,  386  and circulators  394 ,  396  can be operatively controlled by a third control signal C 3  and a fourth control signal C 4  in a similar manner to apply the second signal S 2  both commonly and differentially to the third and fourth antenna elements  122 ,  124 , and to receive the signals both commonly and differentially from the third and fourth antenna elements  122 ,  124 .  
      In this arrangement, each transceiver  350 ,  352  can simultaneously transmit and receive signals using different antenna modes. For instance, the transceiver  350  can transmit a signal by commonly applying the signal to antenna elements  112 ,  114  while simultaneously receiving a signal differentially from the antenna elements  112 ,  114 . Accordingly, the transceivers  350 ,  352  each can select an optimum transmit mode independent of an optimum receive mode that is selected.  
      Referring to  FIG. 8 , the RF communication system  100  can be operated next to a human head  800 , for example when used in a conventional mobile telephone. An example of the field pattern  900  that can be produced by common excitation of the first and second antenna elements  112 ,  114  during such operation is shown in  FIG. 9 . An example of the field pattern  1000  that can be produced by differential excitation of the first and second antenna elements  112 ,  114  when operated in accordance with  FIG. 8  is shown in  FIG. 10 . An example of the field pattern  1100  that can be produced by common excitation of the third and fourth antenna elements  122 ,  124  corresponding to such operation is shown in  FIG. 11 . An example of the field pattern  1200  that can be produced by differential excitation of the third and fourth antenna elements  122 ,  124  while the RF communication system  100  is operated next to the human head is shown in  FIG. 12 .  
      Referring to  FIG. 13 , the RF communication system  100  also can be operated while being held in a hand  1300 , for instance when used with a Bluetooth headset. An example of the field pattern  1400  that can be produced by common excitation of the first and second antenna elements  112 ,  114  during such operation is shown in  FIG. 14 . An example of the field pattern  1500  that can be produced by differential excitation of the first and second antenna elements  112 ,  114  when operated in accordance with  FIG. 13  is shown in  FIG. 15 . An example of the field pattern  1600  that can be produced by common excitation of the third and fourth antenna elements  122 ,  124  corresponding to such operation is shown in  FIG. 16 . Finally, an example of the field pattern  1700  that can be produced by differential excitation of the third and fourth antenna elements  122 ,  124  while the RF communication system  100  is operated while held in a human hand is shown in  FIG. 17 .  
      As used herein, numerical references such as “first,” “second,” “third,” “fourth,” etc. distinguish specific structures or steps from other structures or steps. Such numerical references do not, however, indicate any specific structural order or an order of steps performed in any process. The term “commonly applied,” as used herein, is defined as applying signals in-phase. Similarly, the term “commonly receive,” as used herein, is defined as receiving signals either with no applied phase adjustments, or with similar phase adjustments applied to each of the subject signals. The term “differentially applied,” as used herein, is defined as applying signals out-of-phase (e.g. with a phase difference of approximately 180°). The term “differentially receive,” as used herein, is defined as receiving signals out-of-phase.  
      The terms “a” and “an,” as used herein, are defined as one or more than one. The term “plurality”, as used herein, is defined as two or more than two. The term “another”, as used herein, is defined as at least a second or more. The terms “including” and/or “having”, as used herein, are defined as comprising (i.e., open language). The term “coupled”, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically, i.e. communicatively linked through a communication channel or pathway. The term “proximate to,” as used herein, is defined as at or near. For example, an antenna element proximate to an end portion of a printed circuit board can be at, or near, the end portion.  
      This invention can be embodied in other forms without departing from the spirit or essential attributes thereof. Accordingly, reference should be made to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.