JP2007184797A - Multi-antenna system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multi-antenna system that improves a place rate of a communication system of a MIMO system. <P>SOLUTION: The multi-antenna system has a first radio device having M (M: more than one) antenna elements and communicates with a second radio device having N (N: one or more) antenna elements. At least one of half-value widths of in-horizontal-plane radiation directivities of the M antenna elements is different from others. Alternatively, when the M antennas are represented as A<SB>1</SB>, A<SB>2</SB>, ..., A<SB>M</SB>and the angle that directions where some adjacent antenna elements A<SB>l</SB>and A<SB>l+1</SB>(where 1≤l≤M) have maximum radiation electric power on horizontal planes contain is denoted as ∠A<SB>l</SB>A<SB>l+1</SB>, the antenna elements are prepared such that at least one combination of difference angles ∠A<SB>i</SB>A<SB>i+1</SB>and ∠A<SB>j</SB>A<SB>j+1</SB>(where 1≤i<M, 1≤j<M, and i≠j) is present. Beam patterns of the individual antenna elements may be uni-directional. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a multi-antenna apparatus used in a MIMO (Multi Input Multi Output) communication system.

  The MIMO scheme can improve transmission capacity or reliability by using a plurality of antennas for transmission and reception and transmitting and receiving signals independently for each antenna. In general, the former is called a MIMO multiplexing method, and the latter is called a MIMO diversity method. A conventional MIMO communication system using a multi-antenna is described in Patent Document 1, for example. In this conventional technique, an array (or sub-array) is formed using multiple antennas, and the array directivity is formed adaptively to the environment.

  On the other hand, Non-Patent Document 1 describes a technique for reducing a spatial correlation between streams and improving transmission characteristics by providing a directional antenna as a transmission antenna in a MIMO system. In this technique, 2 × 2 MIMO communication is performed using a four-directional directivity switching antenna as a transmission antenna. A comparison is made between the case where the directivity of the antenna element alone is omni directivity and the case where it has a sharper directivity, and it is shown that the channel capacity and BER characteristics are improved when the half-value angle is 120 degrees. Yes.

Although not based on the MIMO system, there is a method of shifting the antenna directivity direction from the diagonal line of the room in order to eliminate the problem of multipath fading in indoor wireless communication (see, for example, Patent Document 2).
JP 2005-136492 A JP-A-8-84367 Naoto Ito and Hiroyuki Arai, "Research on MIMO Transmission Characteristics Considering Transmitting Antenna Directivity", 2005 IEICE Communication Society Conference, B-1-218

  In order to explain the problem to be solved by the invention, first, a transmitting antenna model as shown in Non-Patent Document 1 is used.

FIG. 1 shows an example of such a transmit antenna model. In the illustrated example, a multi-antenna device having four antenna elements is prepared, and this multi-antenna device communicates with a terminal (not shown). The terminal may be a mobile station or a fixed terminal. Each antenna element has a radiation directivity showing a strong power density in a specific direction, and four beams are evenly directed to the area around the base station. The four beams have equal directivity half-value angles (half-value width or beam width). That is, in this transmission antenna model, all elements are uniform and have an equally divided directivity direction. In the example shown in the figure, the four elements all have a uniform half-value angle θ _H , and the directing directions are different from each other by 90 degrees so that they are equally divided in the horizontal direction.

  In order to obtain the characteristics when the antenna of FIG. 1 is used, a simulation using ray tracing was performed by the present inventors. FIG. 2 shows a state in which the antenna device shown in FIG. 1 is installed in a room having a length of 6 m, a width of 10 m, and a height of 3 m. The antenna device is provided with four uniform antenna elements, and each antenna element has an evenly divided directivity direction (having equal division directivity). This antenna device is installed in the center of the room. The height of the room was 3 m, but the installation height of the antenna was 2.7 m.

  FIG. 3 shows the specifications of the simulation. The communication method is MIMO-SDM (Space Division Multiplexing). It is assumed that two antenna elements are selected from the four antenna elements of the antenna device so that the BER characteristics are optimal, and 2 × 2 MIMO communication is performed with the mobile station (receiving terminal). The carrier frequency is 5 GHz, the modulation method is 16QAM, the synchronization / channel estimation is an 8th order M sequence 50 [symbol / ch], the noise power is -85 dBm, and the reception processing is performed by channel estimation and zero forcing. It is assumed that the number of reflections of radio waves is five.

FIG. 4 shows the cumulative probability of the average error rate obtained by simulation. In the figure, the horizontal axis represents the average error rate (BER), and the vertical axis represents the cumulative probability. In the simulation, the full width at half maximum θ _H which is one of the beam directivity parameters is changed from 30 degrees to 180 degrees (more specifically, θ _H = 30 °, 60 °, 90 °, 120 °, 180 °). °) For comparison, the numerical value in the case of omni directivity is also plotted. In the simulation, an arbitrary place or coordinate in the indoor space is specified based on a random number, an average error rate is calculated when a mobile station exists at that place, and such a BER is calculated for many places in the indoor space. It was calculated and the graph as shown in the figure was obtained. In either case, it can be seen that the BER characteristics are better than the omni directivity. In particular, when the BER is 10 −3, the half-value width θ _H is best when it is 120 degrees, and the cumulative probability is about 45%. It can be seen that the cumulative probability is about 25% when the average error rate is 10 −4. However, this level of BER characteristics is not sufficient in future communication systems where high-speed and large-capacity communication is expected, and further improvement is required.

FIG. 5 shows the simulation result of the cumulative probability of channel capacity or transmission capacity. As in the case of FIG. 4 showing the cumulative probability of transmission capacity, in the simulation, the full width at half maximum θ _H of the beam directivity is changed from 30 degrees to 180 degrees, and a graph in the case of omni directivity is also plotted. By preparing an appropriate half-value width θ _H as shown in the figure, the directional antenna can increase the capacity compared to the case of omni directivity. In the illustrated example, it can be seen that the capacity increases most when the half-value width is 120 degrees.

  An object of the present invention is to provide a multi-antenna device that further improves the location rate in a MIMO communication system.

The present invention includes a first wireless device having M (where M is a plurality) antenna elements, and communicates with a second wireless device having N (where N is one or more) antenna elements. A multi-antenna device that performs At least one of the half widths of the radiation directivity in the horizontal plane of each of the M antenna elements is different from the others. Alternatively, the M antenna elements are A ₁ , A ₂ ,. . . , A _M , and the angle between the directions in which the radiated power in the horizontal plane of a certain adjacent antenna element A _l , A _{l + 1} (where 1 ≦ l <M) is maximized is ∠A _l A _{l + 1} In addition, there exists at least one combination in which angles of ∠A _i A _{i + 1} and ∠A _j A _{j + 1} (where 1 ≦ i <M, 1 ≦ j <M, i ≠ j) are different.

  According to the present invention, the location rate in a MIMO communication system can be further improved.

  In one embodiment of the present invention, a multi-antenna device includes a first wireless device including M (where M is a plurality) antenna elements, and includes N (where N is one or more) antenna elements. Communicate with the second wireless device. Each of the plurality of antenna elements has a predetermined directivity such as unidirectionality. Which antenna element is actually used for communication with each terminal is appropriately determined during communication based on some criteria such as BER characteristics, capacity or eigenvalue. Thereby, it is possible to perform SDM-MIMO communication suitable for the communication status of the terminal. The directivity of the antenna element is prepared so that at least one of the half widths of the radiation directivity in the horizontal plane of each of the M antenna elements is different from the others. In the prior art, it has been assumed that the directional antenna has the same full width at half maximum for each element. However, in this embodiment, a non-uniform half width is prepared. The beam pattern of each antenna element may exhibit unidirectionality or may exhibit directivity having a plurality of main lobes.

In one form of the invention, the M antenna elements are A ₁ , A ₂ ,. . . , A _M , the angle formed by the direction in which the radiated power in the horizontal plane of a certain adjacent antenna element A _l , A _{l + 1} (where 1 ≦ l <M) is maximized is given as ∠A _l A _{l + 1} Then, there is at least one combination with different angles of ∠A _i A _{i + 1} and ∠A _j A _{j + 1} (where 1 ≦ i <M, 1 ≦ j <M, i ≠ j).

  In the prior art, the beam directing directions of each of the four antennas differ by 90 degrees, and the maximum direction of each antenna is set along the wall of a rectangular room so that the reflection of radio waves is suppressed. It was. However, in one embodiment of the present invention, the directivity of the beam is intentionally directed to the wall, and the directivity is adjusted so that the radio wave is reflected.

  According to the present invention, the space around the multi-antenna device is non-uniformly covered with a plurality of directional beams, thereby expanding the diversity of space (becomes multipath rich) and maintaining a large channel capacity. It is possible to reduce the error rate and improve the location rate.

  In the embodiment described below, it is assumed that 2 × 2 MIMO communication is performed between the base station and the terminal. However, according to the research results of the present inventors, characteristics such as an average error rate are also obtained when 1 × 2 SIMO communication is performed using one antenna element of a base station and two antenna elements of a mobile station. The improvement effect has been confirmed. Therefore, the present invention selects K1 (where 2 ≦ K1 ≦ M) from the M antenna elements connected to the first radio apparatus, and K2 from the N antenna elements connected to the second radio apparatus. (However, 1 ≦ K2 ≦ N) is selected, and the method can be widely applied when a K1 × K2 space division multiplex transmission scheme is performed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 6A shows a directivity pattern of the multi-antenna apparatus according to the first embodiment of the present invention. The multi-antenna device is provided in the center of the ceiling, for example. FIG. 6B shows a functional block diagram of the multi-antenna apparatus. The multi-antenna apparatus has M antenna elements 11-1 to 11 -M, an antenna element selector 12, a beam controller 13, and a MIMO transmitter / receiver 14. The multi-antenna device is typically used as a radio base station for an indoor communication system. The antenna element selector 12 examines the signal quality (measured by the bit error rate (BER) in this embodiment) for each of the M antenna elements, and determines a plurality of antenna elements suitable for communication with each mobile station. Decide which one. The beam controller controls the directivity of the beam emitted from each antenna element. The directivity pattern is characterized by a directivity direction and a half-value width (or half-value angle) as shown in FIG. 6C. The directing direction or main beam direction is the direction in which the main lobe faces. The half-value width or half-value angle is an angle (an angle seen from the antenna element side) from the maximum power value to a power value deteriorated by 3 dB. In general, the directivity includes the directivity in the vertical plane in addition to the directivity in the horizontal plane. In this embodiment, the directivity in the horizontal plane is mainly handled. In other embodiments, horizontal and vertical in-plane directivities may be used. For convenience of explanation, in this embodiment, the directivity of each antenna element can be adjusted as appropriate by the beam controller, but it is not essential to the present invention that the directivity of the antenna element is variable. An antenna element having a predetermined directivity that is not variable may be used. In any case, it is only necessary that an antenna element having a predetermined directivity can be prepared in the multi-antenna device. This is true not only for this embodiment but also for other embodiments. The MIMO transmitter / receiver 14 performs signal processing for transmitting and receiving signals in the MIMO scheme.

In this embodiment, four antenna elements are arranged in a substantially square shape, and each antenna element has a directivity pattern that is narrower than the omni directivity. As shown in FIG. 6A, in this embodiment, unlike the conventional case, two types of half-value widths θ _H1 and θ _H2 are prepared, and a beam of half-value width θ _{H1 and} a beam of half-value width θ _H2 are alternately arranged. It has been. However, the directivity directions of the four beams are 90 degrees apart as in the prior art.

  In order to see the effect of the present embodiment, simulation was performed on the BER characteristics and capacity. In the simulation, the antenna of FIG. 6A is installed at the center (2.7 m in height) of the ceiling of a room having a length of 6 m, a width of 10 m, and a height of 3 m, as in the conventional technique of FIG. In addition, ray tracing was used for the simulation, and the specifications shown in FIG. 3 were also used. That is, of the four antenna elements installed in the transmitting antenna, two antenna elements with high BER characteristics are selected, and the two antenna elements and the two antenna elements on the mobile station side are 2 × 2 SDM-MIMO systems. It is assumed that communication is performed.

FIG. 7 shows the simulation results for the first embodiment of the present invention. In the figure, the horizontal axis indicates the average error rate BER, and the vertical axis indicates the cumulative probability. Individual graphs show what was obtained for various combinations of half-value angles. The combinations of half-value angles are (θ _1H , θ _2H ) = (60 °, 120 °), (60 °, 180 °), (90 °, 120 °), (90 °, 180 °). For comparison, a graph (the best graph in FIG. 4) obtained when the antenna element is omnidirectional (omnidirectional) and when the half-value angles are all equally 120 degrees are also shown. As shown in FIG. 7, when (θ _1H , θ _2H ) = (60 °, 120 °) or (90 °, 120 °), the place ratio is greatly improved as compared with the prior art. I understand that. In particular, when (θ _1H , θ _2H ) = (60 °, 120 °), the cumulative probability that an average error rate of 10 −4 or higher is secured is 50%, and the conventional uniform directivity is used. Compared to when it was, the improvement effect more than twice can be confirmed.

FIG. 8 also shows simulation results for the first embodiment of the present invention. In the figure, the horizontal axis represents channel capacity, and the vertical axis represents cumulative probability. Individual graphs show what was obtained for various combinations of half-value angles. The combinations of half-value angles are (θ _1H , θ _2H ) = (60 °, 120 °), (60 °, 180 °), (90 °, 120 °), (90 °, 180 °). For comparison, a graph (the best graph in FIG. 5) obtained when the antenna element is omni-directional (omnidirectional) and when the half-value angles are all equally 120 degrees are also shown. It can be seen that the channel capacities for the combinations of (60 °, 120 °) and (90 °, 120 °) that showed good values in FIG. 7 are equal to or higher than the conventional best values. In the above embodiment, two types of half-value angles θ _H are prepared, but more than two half-value angles (θ _1H , θ _2H , θ _3H ,...) _May be prepared. Further, the total number of antenna elements may be greater than 4, and the number of antennas used for notification with each mobile station may be greater than 2. In any case, according to the first embodiment of the present invention, by using the non-uniform directivity, the signal quality can be greatly improved while ensuring the maximum capacity when the uniform directivity is used.

FIG. 9 shows a directivity pattern of the multi-antenna apparatus according to the second embodiment of the present invention. In this embodiment, the half width θ _H of each beam is maintained constant among the four elements, but the directing direction is set unevenly. In the illustrated example, the beam directions are set so that any two adjacent directing directions of the four beams form 60 ° or 150 ° instead of 90 ° with each other.

FIG. 10 shows the simulation results for this example. Similar to FIG. 7, the horizontal axis represents the average error rate BER, and the vertical axis represents the cumulative probability. Individual graphs show what was obtained for various half-value angles. The half-value angles are θ _H = 30 °, 60 °, 90 °, 120 °, and 180 °. For comparison, a graph (the best graph in FIG. 4) obtained when the antenna element is omnidirectional (omnidirectional) and when the half-value angles are all equally 120 degrees are also shown. As shown in FIG. 10, it can be seen that when the half-value angle is 90 ° or 120 °, the place ratio is greatly improved as compared with the prior art.

FIG. 11 also shows simulation results for the second embodiment of the present invention. In the figure, the horizontal axis indicates the capacity, and the vertical axis indicates the cumulative probability. As in FIG. 8, the horizontal axis represents the average error rate BER, and the vertical axis represents the cumulative probability. Individual graphs show what was obtained for various half-value angles. The half-value angles are θ _H = 30 °, 60 °, 90 °, 120 °, and 180 °. For comparison, a graph (the best graph in FIG. 5) obtained when the antenna element is omni-directional (omnidirectional) and when the half-value angles are all equally 120 degrees are also shown. According to FIG. 11, the channel capacity is high when the half-value angle is 120 ° or 90 °, and particularly the channel capacity when the half-value angle is 120 ° is almost the same as the best value of the conventional equally divided directivity. I understand that. In the above-described embodiment, two types of half-value angles θ _H having a directivity direction interval of 60 ° or 150 ° are prepared. However, more than two types of intervals (angle differences) may be prepared. Further, the total number of antenna elements may be greater than 4, and the number of antennas used for communication with individual mobile stations may be greater than 2. In any case, according to the second embodiment of the present invention, the signal quality can be greatly improved by using the unequal division directivity while securing the same capacity as that of the conventional division directivity. .

  FIG. 12 shows a directivity pattern of the multi-antenna apparatus according to the third embodiment of the present invention. In this embodiment, a combination of half widths (60 °, 120 °), which was excellent in both BER characteristics and channel capacity in the first embodiment, is used. The multi-antenna device is installed not on the center of the ceiling but on the edge (end) of the ceiling. Further, the directing directions of the four beams are rotated by an angle α with respect to the wall. The average error rate and channel capacity characteristics at this time were simulated in the same manner as in the first and second embodiments.

  FIG. 13 shows the simulation results regarding the average error rate and the cumulative probability. Each graph in the figure is a graph in the case where the parameters representing the tilt angle or the rotation angle α are 0 °, 45 °, 90 °, and 135 °. For comparison, a graph in which the beam directivity is omni directivity is also shown. As shown in FIG. 13, it can be seen that the average error rate when tilted by α = 45 ° and 135 ° is improved as compared to α = 0 when tilted.

  FIG. 14 shows simulation results regarding channel capacity and cumulative probability. Each graph in the figure is a graph in the case where the parameters representing the tilt angle or the rotation angle α are 0 °, 45 °, 90 °, and 135 °. For comparison, a graph in which the beam directivity is omni directivity is also shown. Compared with the improvement effect of the average error rate in FIG. 13, the difference due to α is small, but the channel capacity is slightly higher when α is 45 ° or 135 ° than when α is 0 °. I understand that. It can be seen that BER characteristics can be improved while maintaining a high channel capacity by installing a multi-antenna having non-uniform directivity at the end of the room and rotating it by an angle α.

FIG. 15 shows the directivity pattern of the multi-antenna apparatus according to the fourth embodiment of the present invention. The multi-antenna apparatus according to the present embodiment is the same as the example of FIG. 9, but the positional relationship or direction between the wall and the beam directing direction is different. In the illustrated example, the beam directions are set so that any two adjacent directing directions of the four beams form 150 ° or 60 ° instead of 90 ° with each other.
The simulation result of the average error rate characteristic at this time is shown in FIG. As shown in the figure, when the half-value width is 30 °, a high improvement effect is shown as compared with the average error rate in the conventional equal division.

  Each embodiment described above may be used alone, or a technique described in one or more embodiments may be combined.

It is a figure which shows the conventional multi-antenna apparatus. It is a figure which shows a mode that the multi-antenna apparatus of FIG. 1 is used for the indoor communication system. It is a figure which shows the parameter of the parameter used by simulation. It is a figure which shows the simulation result of the average error rate at the time of using the conventional multi-antenna apparatus and a cumulative probability. It is a figure which shows the simulation result of the channel capacity at the time of using the conventional multi-antenna apparatus and a cumulative probability. It is a figure which shows a mode that the multi-antenna apparatus by one Example of this invention is used for the indoor communication system. It is a figure which shows the schematic block diagram of the multi-antenna apparatus by one Example of this invention. It is a figure which shows the relationship between a half value angle and a directivity direction. FIG. 6B is a diagram showing simulation results of average error rate and cumulative probability when the multi-antenna apparatus shown in FIG. 6A is used. FIG. 6B is a diagram showing simulation results of channel capacity and cumulative probability when the multi-antenna apparatus shown in FIG. 6A is used. It is a figure which shows a mode that the multi-antenna apparatus by one Example of this invention is used for the indoor communication system. FIG. 10 is a diagram illustrating simulation results of average error rate and cumulative probability when the multi-antenna apparatus shown in FIG. 9 is used. FIG. 10 is a diagram showing simulation results of channel capacity and cumulative probability when the multi-antenna apparatus shown in FIG. 9 is used. It is a figure which shows a mode that the multi-antenna apparatus by one Example of this invention is used for the indoor communication system. It is a figure which shows the simulation result of the average error rate and cumulative probability at the time of using the multi-antenna apparatus shown in FIG. FIG. 13 is a diagram illustrating simulation results of channel capacity and cumulative probability when the multi-antenna apparatus shown in FIG. 12 is used. It is a figure which shows a mode that the multi-antenna apparatus by one Example of this invention is used for the indoor communication system. FIG. 16 is a diagram illustrating simulation results of average error rate and cumulative probability when the multi-antenna apparatus shown in FIG. 15 is used.

Explanation of symbols

11 Antenna Element 12 Antenna Element Selector 13 Beam Controller 14 MIMO Transmitter / Receiver

Claims (8)

A multi-antenna having a first radio apparatus having M (where M is a plurality) antenna elements and communicating with a second radio apparatus having N (where N is one or more) antenna elements A device,
Of the M antenna elements, at least one of the half-value widths of radiation directivities in a horizontal plane is different from the others. A multi-antenna having a first radio apparatus having M (where M is a plurality) antenna elements and communicating with a second radio apparatus having N (where N is one or more) antenna elements A device,
The M antenna elements are A ₁ , A ₂ ,. . . , A _M , and the angle between the directions in which the radiated power in the horizontal plane of a certain adjacent antenna element A _l , A _{l + 1} (where 1 ≦ l <M) is maximized is denoted by ∠A _l A _{l + 1} In addition,
A multi-antenna apparatus characterized in that there is at least one combination having different angles of ｉA _i Ai _{+ 1} and ｊA _j Aj _{+ 1} (where 1 ≦ i <M, 1 ≦ j <M, i ≠ j). The multi-antenna apparatus according to claim 2, wherein at least one of the half-widths of radiation directivities in the horizontal plane of the M antenna elements is different from the others. 3. The multi-antenna apparatus according to claim 2, wherein each of the plurality of antenna elements has an equal half-value width of radiation directivity in a horizontal plane, and each antenna element has a directivity half-value width smaller than 360 / M degrees. Of the M antenna elements connected to the first wireless device, K1 (2 ≦ K1 ≦ M) is selected for wireless communication by a predetermined means, and K2 (however, 1 ≦ K2 ≦ N) antenna elements are selected for wireless communication,
5. The multi-antenna apparatus according to claim 1, wherein K1 × K2 space division multiplex transmission (multi-antenna transmission) is performed with the second radio apparatus. The multi-antenna apparatus according to claim 5, wherein the maximum radiation direction in the horizontal plane of the M antenna elements is not perpendicular to an indoor wall or is directed to a corner other than a corner of a room. 7. The apparatus according to claim 1, further comprising means for rotating the entire M antenna elements in a horizontal plane so that the directivity directions of all M antenna elements are changed by the same amount. Multi-antenna device described in The multi-antenna apparatus according to claim 1, further comprising a beam controller that adjusts the directivity of each of the one or more antenna elements.

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