JP4744411B2 - MIMO antenna and communication apparatus - Google Patents

Description

  The present invention relates to a MIMO (Multiple Input Multiple Output) antenna.

  In recent years, a wireless local area network (LAN) technology that transmits and receives data with a plurality of antennas, that is, MIMO, has been applied to increase the speed of wireless LAN and the like. Under such circumstances, conventionally, a loop antenna has been disclosed in which feed points are arranged at intervals of ¼ wavelength in a conductor loop to realize antenna directivity like an omni pattern (for example, Patent Document 1). ).

US Pat. No. 6,515,632

However, the loop antenna disclosed in Patent Document 1 is not configured to ensure electromagnetic isolation between all feeding points. For this reason, when this antenna is used as a MIMO antenna, there is a problem that the SNR (Signal to Noise Ratio) characteristic is deteriorated due to the mutual coupling between the antenna elements, and the transmission capacity of the MIMO channel is also reduced.
Therefore, the present invention has been made under such circumstances, and an object thereof is to provide a MIMO antenna that realizes desired directivity.

In order to achieve the above object, the present invention includes a single loop element and a plurality of feeding points arranged at predetermined intervals at nodes of a standing wave current distribution generated in the loop element during feeding. including.
With this configuration, the standing wave current is zero at each feeding point, and the mutual coupling between the antenna elements is reduced.

  According to the present invention, desired directivity can be realized.

Embodiments 1 and 2 of the present invention will be described below with reference to the drawings.
(Embodiment 1)
FIG. 1 is a block diagram showing a configuration example of a radio communication apparatus 100 including a MIMO antenna 10 according to Embodiment 1 of the present invention.
In FIG. 1, a wireless communication apparatus (communication apparatus) 100 includes a MIMO antenna (hereinafter simply referred to as “antenna”) 10 and a transmission / reception module (transmission / reception unit) 20 connected to the antenna 10. . The wireless communication device 100 is, for example, a mobile phone, a notebook computer, a game machine, or the like.

  As shown in FIG. 1, the antenna 10 includes a single loop element 11 and a plurality of feeding points 12 that are connected to the loop element 11 and feed power. In the present embodiment, the loop element 11 is a linear conductor, for example, and is formed in a circular shape. However, the loop element 11 may be applied, for example, as a strip line that edges a loop-shaped pattern on a dielectric substrate. In this case, the loop element 11 can be reduced in size by using a dielectric substrate having a high relative dielectric constant.

The transmission / reception module 20 processes signals transmitted and received by the antenna 10 and is connected to a plurality of antenna terminals 21 and data terminals 22 as shown in FIG. Each of the antenna terminals 21 is connected to the feed point 12 via each feed line 30. The feeder line 30 is constituted by, for example, a coaxial cable or a strip line. Since the antenna terminals 21 are provided corresponding to the individual feeding points 12, there are as many as the number of feeding points 12.
The data terminal 22 inputs and outputs IP (Internet Protocol) packets, for example.

FIG. 2 is a block diagram illustrating a configuration example of the transmission / reception module 20.
In FIG. 2, the transmission / reception module 20 includes a plurality of antenna terminals 21, a data terminal 22, an RF (Radio Frequency) unit 23, a signal processing unit 24, and a REF generation unit 25.
In this case, when receiving the IP packet from the data terminal 22, the signal processing unit 24 converts the IP packet into a modulated signal D100 (for example, a baseband or IF band). Then, the signal processing unit 24 outputs the modulation signal D100 to the RF unit 23.

  When receiving the modulation signal D100 from the signal processing unit 24, the RF unit 23 upconverts the modulation signal D100 to the RF band. Then, the RF unit 23 outputs an RF band modulation signal to the antenna terminal 21. As a result, an RF band modulation signal is radiated from the antenna 10 through the antenna terminal 21 in the air. The up-conversion is performed using, for example, a known BPF (Band Pass Filter), LAN (Low Noise Amp), mixer, circulator, or the like.

  Note that a matching circuit (not shown) for impedance matching with the feeder line 30 is provided between the RF unit 23 and each feeding point 12.

The REF generator 25 generates a REF signal D102 that serves as a reference for the operation clock. Then, the REF generation unit 25 outputs the generated REF signal D102 to the signal processing unit 24. Thereby, the signal processor 24 operates in accordance with the REF signal D102. Further, the REF generation unit 25 generates a REF signal D103 that serves as a reference for the operating frequency. Then, the REF generation unit 25 outputs the generated REF signal D103 to the RF unit 23. As a result, the RF unit 23 operates according to the REF signal D103.
2 has been described independently, it may be incorporated into the RF unit 23 or the signal processing unit 24, for example.

Further, when receiving a modulation signal (for example, RF band) from the antenna terminal 21 (antenna 10), the RF unit 23 down-converts the modulation signal. Then, the RF unit 23 outputs the down-converted reception signal D101 (for example, baseband or IF band) to the signal processing unit 24. The down-conversion is performed using, for example, a known mixer.
When receiving the received signal D101 from the RF unit 23, the signal processing unit 24 converts the received signal D101 into an IP packet (predetermined data) and outputs it. As a result, the RF band modulation signal received via the antenna 10 is converted into, for example, an IP packet and output. Note that the conversion to the data such as the IP packet is performed after performing known detection processing, demodulation processing, decoding processing, error control processing (MAC layer), and the like.

As a MIMO communication scheme, for example, a scheme such as space-time coding, space division multiplexing, eigenmode, Spatial Spreading, CDD (Cyclic Delay Diversity) is applied.
As a MIMO detection method, for example, a method such as MRC (Maximum Ratio Combining), ZF (Zero Forcing), MMSE (Minimum Mean Square Error), or MLD (Maximum Likelihood Detection) at the time of space-time coding is applied.

FIG. 3 is a diagram illustrating an example of an arrangement pattern of the feeding points 12 in the present embodiment. In the present embodiment, a case will be described in which, for example, an odd number, that is, three feeding points 12 are provided.
In the antenna 10 of FIG. 3, three feeding points 12 are connected to positions of points P1, P2, and P3 on the loop element 11, respectively. That is, the three feeding points 12 are equally spaced on the loop element 11 in the first circumferential direction (clockwise direction) d1 or in the second circumferential direction (counterclockwise direction) d2. Is arranged in. In addition, these circumferential directions d1 and d2 are also simply referred to as a circumferential direction.
Specifically, the three feeding points 12 are arranged with a shift of 0.5λ. That is, the three feeding points 12 are arranged at predetermined intervals in the circumferential direction of the loop element 11. Note that λ means the wavelength of the use frequency (desired frequency) of the RF band of the antenna 10.
Further, the three feeding points 12 are arranged so as to be shifted by 120 degrees in the circumferential direction when viewed from the center point o of the loop element 11.

  By configuring in this way, for example, when viewed from the first circumferential direction (clockwise) d1, the path on the loop element 11 between the points P1 and P2 (also referred to as between the points P1 and P2) ( The distance) is 1.0λ. On the other hand, when viewed from the second circumferential direction d2, the path is 0.5λ. Therefore, the path difference between the points P1 and P2 when viewed from both the first and second circumferential directions d1 and d2 is 0.5λ.

Therefore, the traveling wave current generated from the feeding point 12 at the P1 point to the loop element 11 flows on the loop element 11 along the two circumferential directions d1 and d2, but cancels out at the position of the P2 point. That is, the standing wave current generated in the loop element 11 by the feeding point 12 at the P1 point becomes zero at the position of the P2 point.
From the above, it becomes possible to ensure electromagnetic isolation between the feeding points 12 and to compensate for SNR degradation due to mutual coupling.

  In the case of the arrangement pattern shown in FIG. 3, an unbalanced system (power is supplied by one line) is provided. Therefore, the power supply circuit (not shown) needs to be provided in consideration of the influence of the housing (ground surface) of the communication device 100. There is.

FIG. 4 is a graph showing a time change of the standing wave current generated in the loop element 11. 4A to 4C, the horizontal axis represents each point of P1 to P3, that is, the distance, and the vertical axis represents the standing wave current.
FIG. 4A is a graph showing a standing wave current distribution generated when power is supplied to the loop element 11 from the position of the point P1. In FIG. 4A, the positions of the points P2 and P3 are in the standing wave nodes.

  FIG. 4B is a graph showing a standing wave current distribution generated when power is supplied to the loop element 11 from the position of point P2. In FIG. 4B, the positions of the points P1 and P3 are in the standing wave nodes.

FIG. 4C is a graph showing a standing wave current distribution generated when power is supplied to the loop element 11 from the position of point P3. In FIG.4 (c), the position of P1 point and P3 point exists in the node of a standing wave.
Thus, since each point of P1, P2, and P3 exists in the position which corresponds to the node of a standing wave, at each point of P1, P2, and P3, a standing wave current becomes zero at the time of electric power feeding. Further, since the standing wave current becomes zero at each of the points P1, P2, and P3, the mutual electromagnetic coupling between these three points becomes small. Therefore, unlike the case of the loop antenna of Patent Document 1, the mutual coupling between the antenna elements (feeding points) can be reduced.

Next, the phase characteristics of the antenna 10 when the feeding points 12 at the respective points P1 to P3 are used as the feeding points 12 of the independent loop elements 11 will be described.
FIG. 5 is a graph showing the phase characteristics of the antenna 10 when each feeding point 12 is used independently. 5A to 5C, the horizontal axis represents the angle, and the vertical axis represents the phase. The phase of the vertical axis is shown as 0 when the standing wave current of FIG. 4 is zero.
FIG. 5A is a graph showing phase characteristics when the feeding point 12 at the point P1 is used independently. In FIG. 5A, the phase between 0 degrees to 120 degrees (P1-P2 point) and between 240 degrees to 360 degrees (P3-P1 point) is + π / 2, and the phase between P2-P3 points is −π / 2.

  FIG. 5B is a graph showing phase characteristics when the feeding point 12 at point P2 is used independently. In FIG. 5B, the phase between 0 degrees and 240 degrees (P1-P3 point) is + π / 2, and the phase between 240 degrees and 360 degrees (P3-P1 point) is −π / 2. .

FIG. 5C is a graph showing the phase characteristics when the feeding point 12 at point P3 is used independently. In FIG. 5C, the phase between 0 degrees and 120 degrees (P1-P2 point) is −π / 2, and the phase between 120 degrees and 360 degrees (P2-P1 point) is + π / 2. .
Thereby, for example, when the independent feeding point 12 moves from the point P1 (in the case of FIG. 5 (a)) to the point P2 (in the case of FIG. 5 (b)), from FIG. 10 shows that the phase is rotated 120 degrees. Similarly, for example, when the independent feeding point 12 moves from point P1 (in the case of FIG. 5A) to point P3 (in the case of FIG. 5C), FIG. ) Shows that the antenna 10 has a characteristic in which the phase is rotated by 240 degrees.

  Therefore, it can be seen that each of the feeding points 12 of P1 to P3 can be configured to ensure electromagnetic isolation. Further, unlike the case of the loop antenna disclosed in Patent Document 1, the feeding points 12 of P1 to P3 are arranged so as to rotate continuously 120 degrees on the loop element 11 in the second circumferential direction d2. You can see that Therefore, even if the antenna 10 is used as a MIMO antenna, the mutual coupling between the antenna elements can be reduced. This is based on the non-patent document “Takuuchi Takeuchi, 3 others,“ Directive diversity effect of antenna based on complex pattern ”, Shinsei Theory, vo1.J67-B, no.5, 1985”. In this non-patent document, it has been reported that the effect of directional diversity using two antenna terminals having different phase patterns is different from the spatial diversity using the antenna interval as a design parameter.

  As described above, the antenna 10 can secure the transmission capacity of the MIMO channel without degrading the SNR characteristic, and can realize desired directivity.

  In addition, the wireless communication device 100 can perform wireless communication on the MIMO channel using the antenna 10 having desired directivity. Therefore, for example, a modulated signal at the time of MIMO communication converted into space-time coding or eigenvector can be transmitted using a single loop element 11, which is useful.

In the first embodiment, the case where there are three feeding points 12 has been described. However, the number may be changed without departing from the gist of the present invention. For example, the number of feeding points 12 may be defined as 2n + 1 (n: a natural number of 1 or more). In this case, the interval (predetermined interval) between the feeding points 12 is set to an equal interval of 360 / (2n + 1) degrees in the second circumferential direction d2 of the loop element 11. In this way, not only the spatial position but also the pattern of complex directivity differs between the feeding points 12. Therefore, antenna correlation in a multipath propagation environment can be reduced.
In this case, the circumferential length of the loop element 11 is set to (2n + 1) × ½ wavelength, and the intervals between adjacent feeding points 12 are set to equal intervals of ½ wavelength.

(Embodiment 2)
The second embodiment is different from the first embodiment in which the odd number of feeding points 12 are provided in the loop element 11 in that the even number of feeding points 12 are provided in the loop element 11 as compared with the first embodiment. Different. Other configurations are the same as those in the first embodiment. Therefore, the following description will mainly focus on the differences from the first embodiment.

FIG. 6 is a block diagram showing a configuration example of the antenna 10 according to Embodiment 2 of the present invention. Note that in the second embodiment, the same portions as those in the first embodiment are denoted by the same reference numerals (including terms) as those in the first embodiment, and repeated description is appropriately omitted.
In the antenna 10 of FIG. 6, four feeding points 12 are connected to the positions of the points P1, P2, P3, and P4 on the loop element 11, respectively. That is, the four feeding points 12 are arranged on the loop element 11 at a predetermined interval in the second circumferential direction d2.
Specifically, the predetermined interval in the present embodiment is set so as to be shifted by 0.5λ continuously in the second circumferential direction d2. And the space | interval between P1-P4 points is 1.0 (lambda) seeing from the 1st circumferential direction d1.
Further, the four feeding points 12 are arranged so as to be shifted by 72 degrees continuously from the center point o of the loop element 11 in the second circumferential direction d2, which will be described later.

  By configuring in this way, for example, the path (distance) between the points P1 and P4 is 1.0λ when viewed from the first circumferential direction d1. On the other hand, when viewed from the second circumferential direction d2, the path between the points P1 and P4 is 1.5λ. Accordingly, the path difference between the points P1 and P4 when viewed from both the first and second circumferential directions d1 and d2 is 0.5λ. Therefore, the traveling wave currents generated from the feeding point 12 at the P1 point to the loop element 11 cancel each other out at the point P4 as in the case of the first embodiment. That is, the standing wave current generated in the loop element 11 by the feeding point 12 at the P1 point becomes zero at the position of the P4 point. From the above, it can be seen that the electromagnetic coupling between these four points becomes small as in the case of the first embodiment.

Next, the phase characteristics of the antenna 10 when the feed points 12 of the points P1 to P4 are used as the feed points 12 of the independent loop elements 11 will be described (similar to the case of the first embodiment in FIG. 5). ).
FIG. 7 is a graph showing the phase characteristics of the antenna 10 when each feeding point 12 is used independently.
FIG. 7A is a graph showing the phase characteristics when the feeding point 12 at the point P1 is used independently. In FIG. 7A, from 0 degree (P1) to 360 degrees (P1), the phase is alternately inverted at a period of 72 degrees to become −π / 2 or + π / 2.

  FIG. 7B is a graph showing the phase characteristics when the feeding point 12 at point P2 is used independently. In FIG. 7B, the phase is + π / 2 from 0 degree (P1) to 144 degrees (P3) out of 0 degree (P1) to 360 degrees (P1). In the range from 144 degrees (P3) to 360 degrees (P1), the phase is alternately inverted at a period of 72 degrees to become −π / 2 or + π / 2.

  FIG. 7C is a graph showing phase characteristics when the feeding point 12 at the point P3 is used independently. In FIG. 7C, the phase is −π / 2 between 0 degree (P1) and 72 degrees (P2), and the phase is + π between 72 degrees (P2) and 216 degrees (P4). / 2. The phase is −π / 2 between 216 degrees (P4) and 288 degrees, and the phase is + π / 2 between 288 degrees and 360 degrees (P1).

FIG. 7D is a graph showing phase characteristics when the feeding point 12 at the point P3 is used independently. In FIG. 7D, the phase is + π / 2 between 0 degree (P1) and 72 degrees (P2), and the phase is −π between 72 degrees (P2) and 144 degrees (P3). / 2. The phase is + π / 2 between 144 degrees (P3) and 288 degrees, and the phase is −π / 2 between 288 degrees and 360 degrees (P1).
Thereby, for example, it can be seen that the feeding points 12 of the points P1 to P4 are arranged on the loop element 11 so as to continuously rotate by 72 degrees in the second circumferential direction d2.

In the second embodiment, the case where the number of feeding points 12 is four has been described. However, the number may be changed without departing from the gist of the present invention. For example, the number of feeding points 12 may be defined as 2 m (m: a natural number of 2 or more). In this case, the interval (predetermined interval) between the feeding points 12 is set to an equal interval of 360 / (2n + 1) degrees in the second circumferential direction d2 of the loop element 11. In this way, not only the spatial position but also the pattern of complex directivity differs between the feeding points 12. Therefore, antenna correlation in a multipath propagation environment can be reduced.
Further, in this case, the circumferential length of the loop element 11 is set to (2m + 1) × ½ wavelength, and each feeding point 12 is continuously in the second circumferential direction d2 of the loop element 11, Set by shifting 1/2 wavelength.
That is, the path difference between the points P1 and P2 when viewed from both the first and second circumferential directions d1 and d2 is 0.5λ. Therefore, the traveling wave current generated from the feeding point 12 at the P1 point to the loop element 11 flows on the loop element 11 along the two circumferential directions d1 and d2, but cancels out at the position of the P2 point. That is, the standing wave current generated in the loop element 11 by the feeding point 12 at the P1 point becomes zero at the position of the P2 point.
From the above, it becomes possible to ensure electromagnetic isolation between the feeding points 12 and to compensate for SNR degradation due to mutual coupling.

(Example)
Next, for example, the antenna shown in FIG. 8 was prototyped, and the mutual coupling degree and VSWR (Voltage Standing Wave Ratio) of the system were measured. In the antenna of FIG. 8, the number of feeding points 12, that is, the number of antenna elements is set to 3 (similar to FIG. 3). And the space | interval of each feeding point 12 at this time was set to 60 nm along the circumferential direction of the loop element 11, respectively, and the loop element 11 used the copper wire whose diameter is 1 mm. Further, the three feeding points 12 were coaxially connected. In addition, the antenna of FIG. 8 was designed by the usage frequency of 2.4 GHz.

FIG. 9 is a graph showing the measurement results of the mutual coupling degree. Here, a network analyzer was connected to the three feeding points 12 in FIG. 8 and the S parameter was measured to obtain the mutual coupling degree. In the graph of FIG. 9, the horizontal axis indicates the frequency GHz, and the vertical axis indicates the mutual coupling degree dB.
Graph P12 represents the degree of mutual coupling between points P1-P2, and graph P23 represents the degree of mutual coupling between points P2-P3. And the graph P31 represents the mutual coupling degree between P3-P1 points. It can be seen from these graphs that the average value of the degree of mutual coupling is -16 dB or less. Therefore, it was confirmed that the antenna of FIG. 8 satisfies the antenna communication system setting standard (−15 dB or less).

  In addition, non-patent literature “3GPP,“ Spatial channel model for MIMO simulations ”, TR 25.996 V6.1.0, Sep. 2003”, “V. Erceg, et al.,“ TGn channel models ”, IEEE 802. 11-03 / 940r2, Jan. 2004. ”, a half-wave dipole is a general configuration of a MIMO channel, and when this dipole is arranged in series at intervals of a half-wave, The coupling is described to be approximately -15 dB (design basis). Non-patent literature “R. Janswamy,“ Effect of elemental mutual coupling on the capacity of fixed length linear arrays ”, IEEE Antenna and Wires ½, Prop. However, it is described that the transmission capacity of the MIMO channel deteriorates when the wavelength is ½ wavelength or less.

FIG. 10 is a graph showing the VSWR measurement results viewed from each feeding point. In addition, the horizontal axis of the graph of FIG. 10 shows frequency GHz, and a vertical axis | shaft shows VSRW.
The graph P11 represents the VSWR viewed from the feeding point 12 at the point P1, and the graph P22 represents the VSWR viewed from the feeding point 12 at the point P2. The graph P33 represents the VSWR viewed from the feeding point 12 at the point P3.

  The MIMO antenna of the present invention is useful for application to MIMO communication. In particular, the MIMO antenna is suitable for use in a communication device such as a mobile communication system such as a mobile phone or a wireless LAN. The MIMO antenna is also suitable for application to a television broadcast receiving apparatus (communication apparatus) to which diversity reception technology is applied.

1 is a block diagram showing a configuration example of a radio communication apparatus including a MIMO antenna according to Embodiment 1 of the present invention. The block diagram which shows the structural example of the transmission / reception module of FIG. FIG. 5 shows an example of an arrangement pattern of power supply points in the first embodiment. The graph which shows the standing wave current distribution which arises in the loop element of FIG. The graph which shows the phase characteristic of a MIMO antenna at the time of using each feeding point of FIG. 3 independently. The block diagram which shows the structural example of the MIMO antenna which concerns on Embodiment 2 of this invention. The graph which shows the phase characteristic of a MIMO antenna at the time of using each feeding point of FIG. 6 independently. The top view which shows an example of a prototype MIMO antenna. The graph which shows the measurement result of the mutual coupling degree of the MIMO antenna of FIG. The graph which shows the measurement result of VSWR seen from each feeding point of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 MIMO antenna 11 Loop element 12 Feeding point 20 Transmission / reception module 21 Antenna terminal 22 Data terminal 23 RF part 24 Signal processing part 25 REF production | generation part

Claims (6)

A single loop element formed of one conductor ;
A plurality of feeding points arranged at predetermined intervals at nodes of a standing wave current distribution generated in the loop element at the time of feeding ,
The path length on the conductor from the first feeding point to an arbitrary second feeding point differs by 1/2 × (2k−1) (k: natural number) wavelength between clockwise and counterclockwise.
MIMO antenna. When the loop element is configured in a circular shape and the feeding point has 2n + 1 (n: a natural number of 1 or more),
The predetermined interval is set to an equal interval of 360 / (2n + 1) degrees in the circumferential direction of the loop element.
The MIMO antenna according to claim 1. When the loop element is formed in a circular shape and the feeding point has 2m (m: natural number of 2 or more),
The at least two predetermined intervals are set to be shifted 360 / (2m + 1) degrees continuously in the circumferential direction of the loop element.
The MIMO antenna according to claim 1. The circumferential length of the loop element is set to (2n + 1) × 1/2 wavelength, and the predetermined interval is set to an equal interval of ½ wavelength.
The MIMO antenna according to claim 2. The circumferential length of the loop element is set to (2m + 1) × 1/2 wavelength, and at least two of the predetermined intervals are continuously shifted in the circumferential direction of the loop element by ½ wavelength. Set,
The MIMO antenna according to claim 3. The MIMO antenna according to any one of claims 1 to 5,
A transmission / reception unit for processing a signal transmitted / received by the MIMO antenna;
Including a communication device.

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