Abstract:
A communication device including a ground plane and an antenna system is provided. The antenna system includes at least two antennas, which are both located at a first edge of the ground plane and operate in at least a first band. The ground plane has at least one slit, and an open end of the slit is located at a second edge adjacent to the first edge. The open end of the slit has a distance of at least 0.2 wavelength of a frequency in the first band to the first edge. When the antenna system operates in the first band, the slit can attract excited surface currents on the ground plane, thereby causing weaker surface currents flowing along the first edge of the ground plane. The coupling between the at least two antennas in the antenna system is hence decreased.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of Taiwan Patent Application No. 101116785 filed on May 11, 2012, the entirety of which is incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The disclosure generally relates to a communication device, and more particularly, relates to a communication device comprising a MIMO (Multi-Input and Multi-Output) antenna system with high isolation. 
         [0004]    2. Description of the Related Art 
         [0005]    As people demand more and more data transmission, related communication standards are supporting higher and higher data transmission rates. For example, IEEE 802.11n can support MIMO technology to increase transmission rates. The related communication standards, such as LTE (Long Term Evolution), also support MIMO operations. As a matter of fact, it is a future trend to use multiple antennas in a mobile device. However, since multiple antennas are to be disposed in a limited space of a mobile device, the isolation between these antennas will be an important factor to be considered. 
         [0006]    Traditionally, the method for improving isolation and for reducing mutual coupling between MIMO antennas is to dispose an isolation element between two adjacent antennas, wherein the resonant frequency of the isolation element is approximately equal to that of the antennas so as to decrease the mutual coupling between the antennas. The drawbacks of the traditional method include decreased antenna efficiency and degraded radiation performance. In addition, if these antennas are operated in an LTE700 band (from 704 MHz to 787 MHz), the isolation element is required to become resonance at about 700 MHz and hence requires a large element size, which greatly increases the size of the whole antenna system. Integration of such an antenna system in the limited space inside the mobile device is a challenge for an antenna designer. 
         [0007]    Accordingly, there is a need to provide a new communication device which performs MIMO operations without any isolation element but has good isolation. The antenna efficiency of the antenna system in the communication device should not be affected, or should even be enhanced. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The invention is aimed to provide a communication device comprising an antenna system. The antenna system comprises at least two antennas and is located at an edge of a ground plane. The communication device of the invention has high isolation without any isolation element between the antennas in the antenna system, and the antenna efficiency is generally maintained. 
         [0009]    In an embodiment, the disclosure is directed to a communication device, comprising: a ground plane, having a first edge and a second edge, wherein the first edge and the second edge are adjacent edges of the ground plane; and an antenna system, comprising at least a first antenna and a second antenna, wherein the first antenna and the second antenna are both located at the first edge, each of the first antenna and the second antenna operates in at least a first band, and a plane on which the first antenna and the second antenna are disposed is substantially parallel to the ground plane, wherein a length of the first edge of the ground plane is greater than or equal to 0.3 wavelength of a first frequency in the first band, the ground plane has at least one slit, the slit has an open end located at the second edge, and a distance between the open end of the slit and the first edge of the ground plane is greater than or equal to 0.2 wavelength of a second frequency in the first band. Note that when the antenna system operates in the first band, the slit attracts surface currents on the ground plane, and causes the surface currents which flow along the first edge of the ground plane to be reduced such that the coupling between the first antenna and the second antenna is decreased. Accordingly, the invention can effectively improve the isolation between the first antenna and the second antenna. 
         [0010]    In an embodiment, the length of the slit is approximately equal to 0.25 wavelength of a frequency in the first band, and the distance between the open end of the slit and the first edge of the ground plane is greater than or equal to 0.2 wavelength of the second frequency in the first band. Furthermore, the slit is substantially parallel to the first edge and has a projection on the first edge, wherein the projection covers the first antenna. Under the circumstance, the slit can effectively attract surface currents on the ground plane, thereby reducing the surface currents which flow along the first edge of the ground plane. Since the surface currents which flow along the first edge of the ground plane significantly cause some coupling between two antennas, the presence of the slit can hence improve the isolation between the antennas. In addition, the slit generally does not affect the radiation performances of the first antenna and the second antenna, and the first antenna and the second antenna can maintain good antenna efficiency. In an embodiment, the distance between the open end of the slit and the first edge of the ground plane is smaller than 0.45 wavelength of a frequency in the first band. 
         [0011]    In an embodiment, the isolation of the antenna system in the first band may be improved by 7 dB or more, to be about −20 dB (S 21 ), but the radiation efficiency of the antenna system generally does not vary. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a diagram for illustrating a communication device according to a first embodiment; 
           [0014]      FIG. 2A  is a diagram for illustrating S parameters of the communication device according to the first embodiment; 
           [0015]      FIG. 2B  is a diagram for illustrating S parameters of the communication device in the first embodiment but without a first slit and a second slit; 
           [0016]      FIG. 3  is a diagram for illustrating a communication device according to a second embodiment; 
           [0017]      FIG. 4  is a diagram for illustrating a communication device according to a third embodiment; and 
           [0018]      FIG. 5  is a diagram for illustrating a communication device according to a fourth embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    In order to illustrate the foregoing and other purposes, features and advantages of the invention, the embodiments and figures thereof in the invention are shown in detail as follows. 
         [0020]      FIG. 1  is a diagram for illustrating a communication device  100  according to a first embodiment. In the embodiment, the communication device  100  comprises a ground plane  10  and an antenna system  150 . The ground plane  10  has a first edge  101 , a second edge  102 , and a third edge  103 , wherein the second edge  102  and the third edge  103  are both adjacent to the first edge  101 . The antenna system  150  comprises at least a first antenna  11  and a second antenna  12 . The first antenna  11  has a feeding end  111  and a shorted end  112 . A signal source  113  is configured as a feeding signal source of the first antenna  11 , and the signal source  113  is electrically coupled to the feeding end  111 . The shorted end  112  is electrically coupled to the ground plane  10 . Similarly, the second antenna  12  has a feeding end  121  and a shorted end  122 . A signal source  123  is configured as a feeding signal source of the second antenna  12 , and the signal source  123  is electrically coupled to the feeding end  121 . The shorted end  122  is electrically coupled to the ground plane  10 . The first antenna  11  and the second antenna  12  of the antenna system  150  are both substantially located at the first edge  101  of the ground plane  10 . The plane on which the first antenna  11  and the second antenna  12  are disposed is substantially parallel to the ground plane  10  and extends outwardly. Each of the first antenna  11  and the second antenna  12  operates in at least a first band. The length L of the first edge  101  of the ground plane  10  is greater than or equal to 0.3 wavelength of a first frequency in the first band. The first antenna  11  and the second antenna  12  are substantially close to two opposite corners of the first edge  101 , respectively. In an embodiment, the ground plane  10  may be a conductive supporting plate of the communication device  100  (e.g., a notebook computer). The ground plane  10  may have a first slit  13  and a second slit  14 . The first slit  13  has an open end  131  located at the second edge  102  of the ground plane  10 , and the second slit  14  has an open end  141  located at the third edge  103  of the ground plane  10 . The distance d between the open end  131  of the first slit  13  and the first edge  101  of the ground plane  10  is greater than or equal to 0.2 wavelength of a second frequency in the first band. Similarly, the distance d between the open end  141  of the second slit  14  and the first edge  101  of the ground plane  10  is greater than or equal to 0.2 wavelength of the second frequency in the first band. 
         [0021]    In some embodiments, the foregoing distance d is smaller than 0.45 wavelength of a frequency in the first band. 
         [0022]    In some embodiments, the length t 1  of the first slit  13  and the length t 2  of the second slit  14  are both approximately equal to 0.25 wavelength of a frequency in the first band. 
         [0023]    In some embodiments, both the first slit  13  and the second slit  14  are substantially parallel to the first edge  101  of the ground plane  10 . The first slit  13  has a projection on the first edge  101  of the ground plane  10 , and the projection covers the first antenna  11 . The second slit  14  has another projection on the first edge  101  of the ground plane  10 , and the projection covers the second antenna  12 . 
         [0024]      FIG. 2A  is a diagram for illustrating S parameters of the communication device  100  according to the first embodiment. In the embodiment, the ground plane  10  is a conductive supporting plate of an upper cover of a notebook computer. The length L of the conductive supporting plate is approximately equal to 260 mm. The length t 1  of the first slit  13  and the length t 2  of the second slit  14  are both approximately equal to 90 mm. The distance d between each slit and the first edge  101  is approximately equal to 150 mm. According to the criterion of 6 dB return loss (design specification widely used for the internal antennas in mobile communication devices), the reflection coefficient (S 11 ) curve  20  of the first antenna  11  of the antenna system  150  comprises a first band  201  and a second band  202 . In a preferred embodiment, the first band  201  covers the LTE700 band (about from 704 MHz to 787 MHz), and the second band  202  covers the LTE2300/2500 bands (about from 2300 MHz to 2400 MHz and from 2500 MHz to 2690 MHz). The reflection coefficient (S 22 ) curve of the second antenna  12  of the antenna system  150  is similar to the reflection coefficient (S 11 ) curve  20  of the first antenna  11 , and comprises at least the first band  201  and the second band  202 . The reflection coefficient (S 22 ) curve of the second antenna  12  will not be described again here. As shown in  FIG. 2A , the antenna system  150  in the first embodiment can be applied to MIMO operations of an LTE system, and the isolation (S 21 ) curve  21  which represents the isolation (S 21 ) between the first antenna  11  and the second antenna  12  is lower than −20 dB for frequencies over the operating bands. 
         [0025]      FIG. 2B  is a diagram for illustrating S parameters of the communication device  100  in the first embodiment but without the first slit  13  and the second slit  14 . According to the criterion of 6 dB return loss, the reflection coefficient (S 11 ) curve  22  of the first antenna  11  of the antenna system  150  also comprises a first band  221  and a second band  222 . The reflection coefficient (S 22 ) curve of the second antenna  12  of the antenna system  150  is similar to the reflection coefficient (S 11 ) curve  22  of the first antenna  11 , and comprises at least the first band  221  and the second band  222 . The reflection coefficient (S 22 ) curve of the second antenna  12  will not be described again here. As shown in  FIG. 2B , if the first slit  13  and the second slit  14  are not embedded in the ground plane  10 , the antenna system  150  will have the isolation (S 21 ) curve  23  of about −13 dB in the first band  221 . In comparison to  FIG. 2A , the invention has one or more slits formed in the ground plane  10  and improves the isolation of the antenna system  150  by 7 dB or more. Note that in the first embodiment, the isolation (S 21 ) in the first band  201  and the second band  202  is smaller than −20 dB, and the antenna efficiency of the first antenna  11  and the second antenna  12  is approximately from 40% to 60% in the first band  201  and approximately from 60% to 90% in the second band  202  (the antenna efficiency includes the mismatching losses). The antenna efficiency in the first band  201  of the invention is higher than the antenna efficiency in the first band  221  of  FIG. 2B , which has no slit in the ground plane  10 . 
         [0026]      FIG. 3  is a diagram for illustrating a communication device  300  according to a second embodiment. The communication device  300  in the second embodiment is similar to that in the first embodiment. The difference between them is that a ground plane  30  of the communication device  300  has only a single first slit  33 . The first slit  33  has an open end  331  located at a second edge  302  of the ground plane  30 . An antenna system  350  comprises at least a first antenna  31  and a second antenna  32 . The first antenna  31  has a feeding end  311  and a shorted end  312 . A signal source  313  is configured as a feeding signal source of the first antenna  31 , and the signal source  313  is electrically coupled to the feeding end  311 . Similarly, the second antenna  32  has a feeding end  321  and a shorted end  322 . A signal source  323  is configured as a feeding signal source of the second antenna  32 , and the signal source  323  is electrically coupled to the feeding end  321 . 
         [0027]      FIG. 4  is a diagram for illustrating a communication device  400  according to a third embodiment. The communication device  400  in the third embodiment is similar to that in the first embodiment. The difference between them is that a ground plane  40  of the communication device  400  has a first slit  43  and a second slit  44 , and each of the first slit  43  and the second slit  44  further has a bending portion at one end. The first slit  43  has an open end  431  located at a second edge  402  of the ground plane  40 , and the second slit  44  has an open end  441  located at a third edge  403  of the ground plane  40 . An antenna system  450  comprises at least a first antenna  41  and a second antenna  42 . The first antenna  41  has a feeding end  411  and a shorted end  412 . A signal source  413  is configured as a feeding signal source of the first antenna  41 , and the signal source  413  is electrically coupled to the feeding end  411 . Similarly, the second antenna  42  has a feeding end  421  and a shorted end  422 . A signal source  423  is configured as a feeding signal source of the second antenna  42 , and the signal source  423  is electrically coupled to the feeding end  421 . 
         [0028]      FIG. 5  is a diagram for illustrating a communication device  500  according to a fourth embodiment. The communication device  500  in the fourth embodiment is similar to that in the first embodiment. The difference between them is that an antenna system  550  of the communication device  500  further comprises a third antenna  55 . A first antenna  51 , a second antenna  52 , and the third antenna  55  are all located at a first edge  501  of a ground plane  50 . The first antenna  51  has a feeding end  511  and a shorted end  512 . A signal source  513  is configured as a feeding signal source of the first antenna  51 , and the signal source  513  is electrically coupled to the feeding end  511 . The second antenna  52  has a feeding end  521  and a shorted end  522 . A signal source  523  is configured as a feeding signal source of the second antenna  52 , and the signal source  523  is electrically coupled to the feeding end  521 . Similarly, the third antenna  55  has a feeding end  551  and a shorted end  552 . A signal source  553  is configured as a feeding signal source of the third antenna  55 , and the signal source  553  is electrically coupled to the feeding end  551 . 
         [0029]    For the invention, the communication device  300  in the second embodiment, the communication device  400  in the third embodiment, and the communication device  500  in the fourth embodiment are all similar to the communication device  100  in the first embodiment. Accordingly, the performance of the second, third, and fourth embodiments is similar to that of the first embodiment. 
         [0030]    Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. 
         [0031]    It will be apparent to those skilled in the art that various modifications and variations can be made in the invention. It is intended that the standard and examples be considered as exemplary only, with a true scope of the disclosed embodiments being indicated by the following claims and their equivalents.