JP2023041153A - Calibration method and calibration system - Google Patents

Abstract

To perform calibration of an array antenna regardless of an installation environment of the array antenna and without adding a device dedicated to calibration to the array antenna.SOLUTION: A flying object such as a drone is used for calibration of an array antenna having a plurality of antenna element parts. A radio wave is transmitted from one to the other of the array antenna and the flying object, and a K-factor is calculated based on a result of reception of the radio wave on the reception side. The altitude of the flying object is adjusted so that the K-factor becomes equal to or more than a threshold. Subsequently, a calibration signal is transmitted from one to the other of each of the plurality of antenna element parts and the flying object. A calibration value of a transmitter or a receiver between the plurality of antenna element parts is calculated based on a result of reception of the calibration signal.SELECTED DRAWING: Figure 3

Description

The present invention relates to calibration of array antennas.

With the spread of smartphones and the like, it has become possible to perform high-speed data communication using small terminals. As a key technology for improving transmission speed in a limited frequency band, Multiple Input Multiple Output (MIMO) transmission technology that realizes high-speed transmission using a plurality of antennas for transmission and reception has been introduced. LTE-Advanced and IEEE802.11ac employ multi-user MIMO (MU-MIMO) that allows MIMO technology to be applied to a plurality of users. A concept called Massive MIMO has been proposed for the fifth generation mobile communication system (5G) (see Non-Patent Document 1). Massive MIMO improves the service area and solves the problem of interference in an environment where the number of terminals increases by increasing the number of antenna elements in base stations.

MU-MIMO, which is scheduled to be introduced in IEEE802.11ac and LTE-Advanced, supports up to eight antennas in a base station. It is known that in such a situation, when the total number of user antennas becomes equal to the number of base station antennas, the transmission rate per user drops sharply. Massive MIMO, on the other hand, assumes that the number of antenna elements in the base station is sufficiently larger than the total number of user antennas, so it has the advantage that even if the number of users increases, the transmission rate does not decrease significantly.

In order to implement Massive MIMO, similarly to single-user MIMO (SU-MIMO) and MU-MIMO, propagation channel response, ie, Channel State Information (CSI) information is required. Conventional standardization in wireless LAN and cellular systems employs a method of feeding back CSI from the terminal side to the base station side. However, in the case of Massive MIMO, if the CSI feedback from the terminal side is applied, the number of elements to be used becomes enormous, resulting in a serious problem of reduction in transmission efficiency.

Therefore, in the case of Massive MIMO, it is conceivable to transmit a signal from a terminal to a base station and perform CSI estimation in the base station. FIG. 1 schematically illustrates this approach. In FIG. 1, the array antenna has a plurality of antenna element units (M is an integer of 2 or more). Each antenna element section includes a transmitter Tx, a receiver Rx, and an antenna element.

What must be noted in the method shown in FIG. 1 is the “individual difference” of transmitters Tx and receivers Rx among a plurality of antenna element units. Typically, there are variations in the electrical characteristics of the transmitter Tx and the receiver Rx among the plurality of antenna element units. Due to such individual differences, amplitude and phase errors occur between a plurality of antenna element units. If there is an amplitude/phase error, the null will rise. FIG. 2 shows, as an example, a calculation example when amplitude and phase errors are given to a three-element array transmitter. It can be seen that the null rises as the error increases.

Therefore, it is necessary to correctly recognize and correct amplitude/phase errors between a plurality of antenna element portions. That is, it is necessary to calibrate a plurality of antenna element parts of the array antenna.

Far-field/near-field calibration methods and in-apparatus feedback calibration methods are known as conventional techniques related to calibration (see Non-Patent Document 2 and Non-Patent Document 3).

F. Rusek, D. Persson, B. K. Lau, E. G. Larsson, T. L. Marzetta, O. Edfors, and F. Tufvesson, "Scaling Up MIMO -- Opportunities and challenges with very large arrays --," IEEE Signal Processing Magazine, pp. 40-60, Jan. 2013. Kiyoshi Mano, Takashi Katagi, "Element Amplitude Phase Measurement Method for Phased Array Antenna - Element Electric Field Vector Rotation Method -," Transactions of the Institute of Electronics and Communication Engineers Vol.J65-B, No. 5, pp.555-560, May 1982. Tsoulos, J. McGeehan and M. Beach, "Space division multiple access (SDMA) field trials. Part 2: Calibration and linearity issues," IEE Proc. Radar, Sonar Navig., Vol.145, No.1, Feb. 1998 .

Problems with the far-field/near-field calibration method include the following. First, in far-field measurement, it is necessary that there is a line-of-sight environment between the reference base station and the base station and that there are no reflected waves. Therefore, the environment in which calibration can be realized is limited. In addition, since an anechoic chamber or the like is required, calibration cannot be performed during the actual transmission of radio waves. In the case of near-field measurements, post-processing becomes complicated for large-scale arrays.

The in-device feedback calibration method achieves calibration within the base station without using any external device. However, for that purpose, it is necessary to add a dedicated calibration device in the base station.

One object of the present invention is to provide a technique capable of calibrating an array antenna regardless of the installation environment of the array antenna and without adding a dedicated calibration device to the array antenna.

A first aspect relates to a calibration method applied to an array antenna having a plurality of antenna element units.
The calibration method is
K factor calculation processing for transmitting radio waves from one of the array antenna and the flying object to the other and calculating the K factor based on the reception result of the radio waves on the receiving side;
Altitude adjustment processing for adjusting the altitude of the flying object so that the K factor is greater than or equal to the threshold;
After the altitude adjustment process, a calibration signal is transmitted from one of each of the plurality of antenna element units and the flying object to the other, and based on the reception result of the calibration signal, the transmitter or receiver between the plurality of antenna element units and a calibration value calculation process for calculating a calibration value.

A second aspect relates to a calibration method applied to an array antenna having a plurality of antenna element units.
The calibration method is
a process of transmitting radio waves from one of the array antenna and the flying object to the other;
altitude adjustment processing for adjusting the altitude of the flying object until the radio waves received by the other of the array antenna and the flying object can be regarded as direct waves only;
After the altitude adjustment process, a calibration signal is transmitted from one of each of the plurality of antenna element units and the flying object to the other, and based on the reception result of the calibration signal, the transmitter or receiver between the plurality of antenna element units and a calibration value calculation process for calculating a calibration value.

A third aspect relates to the calibration system.
The calibration system is
an array antenna having a plurality of antenna element parts;
Includes projectiles and
One of the flying object and the array antenna receives radio waves transmitted from the other of the flying object and the array antenna, and calculates the K factor based on the result of receiving the radio waves.
The flying object performs altitude adjustment processing for adjusting the altitude of the flying object so that the K factor is equal to or greater than the threshold.
After the altitude adjustment process, each of the plurality of antenna element units and one transmitting side of the flying object transmits a calibration signal to each of the plurality of antenna element units and the receiving side of the flying object.
The receiving side calculates calibration values for transmitters or receivers between a plurality of antenna elements based on the reception result of the calibration signal.

According to the present invention, the array antenna can be calibrated regardless of the installation environment of the array antenna and without adding a dedicated calibration device to the array antenna.

It is a conceptual diagram for explaining a conventional technology. It is a graph diagram for explaining a conventional technology. FIG. 4 is a conceptual diagram for explaining an overview of calibration processing according to the embodiment of the present invention; It is a graph chart for explaining K factor. 1 is a schematic diagram showing a configuration example of a calibration system according to an embodiment of the present invention; FIG. 1 is a block diagram showing a configuration example of a flying object according to an embodiment of the present invention; FIG. 4 is a flow chart showing an example of calibration processing according to the embodiment of the present invention; FIG. 4 is a conceptual diagram for explaining calibration value calculation processing (step S400) according to the embodiment of the present invention; 4 is a flowchart showing an example of calibration value calculation processing (step S400) according to the embodiment of the present invention; 9 is a flowchart showing another example of calibration value calculation processing (step S400) according to the embodiment of the present invention; FIG. 4 is a conceptual diagram for explaining a specific example of calibration processing according to the embodiment of the present invention; FIG. 5 is a graph diagram for explaining a specific example of calibration processing according to the embodiment of the present invention; It is a graph diagram for explaining the effect of the calibration process according to the embodiment of the present invention. It is a graph diagram for explaining the effect of the calibration process according to the embodiment of the present invention.

Embodiments of the present invention will be described with reference to the accompanying drawings.

1. Outline FIG. 3 is a conceptual diagram for explaining the outline of the calibration process according to the present embodiment. A base station BS comprises an array antenna 10 having a plurality of antenna element units. This array antenna 10 is the object of calibration.

Among the plurality of antenna element units of the array antenna 10, there may be individual differences (eg, variation in electrical characteristics) between transmitters and receivers. Due to such individual differences, amplitude and phase errors occur between the plurality of antenna element units. Therefore, it is necessary to correctly recognize and correct amplitude/phase errors between a plurality of antenna elements. However, in a multipath environment where signals arrive from multiple directions, it is not possible to correctly estimate amplitude/phase errors between multiple antenna elements.

For example, in FIG. 3, radio waves transmitted from the array antenna 10 are reflected by surrounding buildings. As a result, radio waves arrive at the position P1 from multiple directions. Therefore, even if the receiver is placed at position P1, it is not possible to correctly estimate the amplitude/phase error between the multiple antenna elements. On the other hand, almost only direct waves arrive at position P2, which is sufficiently higher than surrounding buildings. In other words, at position P2, the direct wave is sufficiently dominant compared to the reflected wave. Therefore, if the receiver is placed at position P2, it is possible to correctly estimate the amplitude/phase error between the plurality of antenna elements.

Therefore, in the present embodiment, the "flying object 100" is used for calibration. A drone or the like is exemplified as the flying object 100 . This flying object 100 is equipped with a communication device and can communicate with the array antenna 10 . Multipath can be avoided by raising the flying object 100 to a position P2 sufficiently higher than surrounding buildings. By exchanging signals between the flying object 100 and the array antenna 10 in this state, it becomes possible to correctly estimate the amplitude/phase error between the plurality of antenna element portions. Based on the estimated amplitude/phase errors, calibration of the plurality of antenna elements of the array antenna 10 is performed.

In this way, by using the flying object 100, the array antenna 10 can be calibrated regardless of the installation environment of the array antenna 10 and without adding a dedicated calibration device to the array antenna 10. becomes.

2. K Factor In the present embodiment, it is necessary to determine whether direct waves are dominant between array antenna 10 and flying object 100 (whether multipath can be avoided). For this purpose, it is conceivable to use a technique for estimating the direction of arrival of radio waves, but in that case, signal processing may become complicated. Therefore, as another example, the use of the "K factor" that represents the power ratio between the direct wave and the reflected wave is proposed.

In the case of Nakagami-Rice fading, the received signal h _Rice is expressed by the following equation.

h _LOS is a line-of-sight signal and h _NLOS is a non-line-of-sight signal. L-1 is the number of non-line-of-sight signals. r and θ are the amplitude and phase, respectively. The K factor is represented by the following formula.

Thus, the K-factor represents the power ratio between direct and reflected waves. It can be said that the larger the K factor, the more dominant the direct wave is in the environment. Then, when the K factor is equal to or greater than the threshold, it can be considered that only direct waves have arrived at the receiving side.

FIG. 4 is a graphical representation of a cumulative distribution function (CDF) of relative received power as a function of K-factor. The horizontal axis represents the relative received power [dB], and the vertical axis represents the cumulative probability (probability of being equal to or less than the value on the horizontal axis). As shown in FIG. 4, the cumulative probability distribution of relative received power fluctuates according to the K factor. Conversely, if the cumulative probability distribution of the relative received power can be obtained, the corresponding K factor can be calculated. That is, the K factor can be calculated based on the received power and its fluctuation value.

In this embodiment, when the K factor is equal to or greater than the threshold, it is determined that only direct waves have arrived at the receiving side. The threshold is set to 20 dB, for example. However, in practice, if the K factor increases to about 10 to 15 dB, it can be regarded as a substantially direct wave.

3. Configuration Example and Processing Flow Example Hereinafter, the calibration processing related to the present embodiment will be described in more detail.

3-1. Configuration Example FIG. 5 is a schematic diagram showing a configuration example of the calibration system 1 according to the present embodiment. A calibration system 1 includes a base station BS and a flying object 100 .

A base station BS includes an array antenna 10 . The array antenna 10 has a plurality of antenna element sections 20-1 to 20-M. Here, M is an integer of 2 or more. Each antenna element section 20-i (i=1 to M) includes a transmitter 21-i, a receiver 22-i, and an antenna element 23-i.

The base station BS further includes a controller 30 . The control device 30 performs various signal processing, controls the array antenna 10 , and transmits and receives signals through the array antenna 10 . Typically, controller 30 is implemented by a computer including a processor and memory.

FIG. 6 is a block diagram showing a configuration example of the flying object 100 according to this embodiment. The flying object 100 includes an actuator 110 , a communication device 120 and a control device 130 .

Actuator 110 is a power source for causing flying object 100 to fly. For example, if the flying object 100 is a drone, the actuator 110 includes a motor that rotates a rotor.

The communication device 120 communicates with the array antenna 10 (base station BS). Communication device 120 includes transmitter 121 , receiver 122 and antenna 123 .

The control device 130 controls the flying object 100 . The control device 130 performs various information processing. The controller 130 also controls the flight of the flying object 100 by controlling the actuator 110 . Furthermore, the control device 130 transmits and receives signals via the communication device 120 . Typically, controller 130 is implemented by a computer including a processor and memory.

3-2. Example of Processing Flow FIG. 7 is a flowchart showing an example of calibration processing according to the present embodiment.

3-2-1. K factor calculation process (step S100)
In step S100, a "K factor calculation process" is executed for transmitting radio waves from one of the array antenna 10 and the flying object 100 to the other, and calculating the K factor based on the reception result of the radio waves on the receiving side. As an example, consider a case where radio waves are transmitted from the array antenna 10 to the flying object 100 .

In step S110, the control device 30 of the base station BS controls the array antenna 10 to transmit radio waves. The communication device 120 of the flying object 100 receives radio waves transmitted from the array antenna 10 .

In step S120, the communication device 120 of the flying object 100 measures the received power of the radio wave and its fluctuation amount. At this time, it is not always necessary to measure the reception direction of radio waves.

In step S130, the control device 130 of the flying object 100 calculates the K factor based on the radio wave reception result. For example, reference information indicating the cumulative probability distribution of relative received power shown in FIG. 4 is generated in advance and stored in the storage device of control device 130 . The reference information indicates the cumulative probability distribution of relative received power for different K-factors. Control device 130 calculates (estimates) the K factor based on the received power and the amount of variation measured in step S120 and the reference information. For example, control device 130 calculates (estimates) the K factor by comparing the cumulative probability distribution obtained from the received power and variation measured in step S120 with the reference information.

As another example, radio waves may be transmitted from the flying object 100 to the array antenna 10 . In this case, the array antenna 10 receives radio waves, and the control device 30 calculates the K factor based on the radio wave reception results (received power and variation) and reference information. Then, the control device 30 transmits information on the calculated K factor to the flying object 100 .

3-2-2. Judgment processing (step S200)
In step S200, the control device 130 of the flying object 100 compares the K factor with a threshold. For example, the threshold is 20 dB. If the K factor is less than the threshold (step S200; No), the process proceeds to step S300. On the other hand, if the K factor is greater than or equal to the threshold (step S200; Yes), the process proceeds to step S400.

3-2-3. Altitude adjustment process (step S300)
In step S<b>300 , the control device 130 of the flying object 100 controls the actuator 110 to raise the flying object 100 . The process then returns to step S100.

In this way, the altitude adjustment process adjusts the altitude of the flying object 100 so that the K factor is greater than or equal to the threshold. In other words, the altitude of the flying object 100 is adjusted until the radio waves received by the receiving side in step S100 can be considered to be only direct waves.

3-2-4. Calibration value calculation process (step S400)
In step S400, calibration values are calculated. FIG. 8 is a conceptual diagram for explaining this calibration value calculation process.

Here, the first antenna element section 20-1 and the second antenna element section 20-k (k=2 to M) are considered. The transfer functions of the transmitter 21-1 and the receiver 22-1 of the first antenna element section 20-1 are T ₁ and R ₁ , respectively. The transfer functions of the transmitter 21-k and the receiver 22-k of the second antenna element section 20-k are T _k and R _k respectively. Each transfer function is complex amplitude and includes a phase component and an amplitude component. A calibration value for correcting an error in the transfer function between the transmitter 21-1 of the first antenna element section 20-1 and the transmitter 21-k of the second antenna element section 20-k is T _k / _T1 . Similarly, the calibration value for correcting the error in the transfer function between the receiver 22-1 of the first antenna element section 20-1 and the receiver 22-k of the second antenna element section 20-k is _Rk / _R1 .

FIG. 9 is a flow chart showing calibration value calculation processing for the transmitter 21 .

In step S410, the control device 30 of the base station BS transmits the calibration signal s to the flying object 100 via the first antenna element section 20-1. The calibration signal s transmitted from the transmitter 21-1 of the first antenna element section 20-1 is T ₁ ·s.

In step S420, the communication device 120 of the flying object 100 receives the calibration signal transmitted from the first antenna element section 20-1 as the first reception signal _T1 ·s.

In step S430, the controller 30 of the base station BS transmits the calibration signal s to the flying object 100 via the second antenna element section 20-k. The calibration signal s transmitted from the transmitter 21-k of the second antenna element section 20-k is T _k ·s.

In step S440, the communication device 120 of the flying object 100 receives the calibration signal transmitted from the first antenna element section 20-1 as the second received signal _Tk ·s.

In step S450, the controller 130 of the flying object 100 calculates calibration values for the transmitters 21-1 and 21-k based on the comparison between the first received signal T ₁ ·s and the second received signal T _k ·s. Calculate T _k /T ₁ .

FIG. 10 is a flow chart showing calibration value calculation processing for the receiver 22 .

In step S460, the control device 130 of the flying object 100 transmits the calibration signal s to the array antenna 10 (base station BS) via the communication device 120. FIG.

In step S 470 , the receiver 22 - 1 of the first antenna element section 20 - 1 receives the calibration signal s transmitted from the flying object 100 . The calibration signal s received by the receiver 22-1 becomes the first received signal R ₁ ·s.

In step S480, the receiver 22-k of the second antenna element section 20-k receives the calibration signal s transmitted from the flying object 100. FIG. The calibration signal s received by receiver 22-k becomes the second received signal R _k ·s.

In step S490, the controller 30 of the base station BS calculates calibration values for the receivers 22-1, 22-k based on the comparison between the first received signal R ₁ s and the second received signal R _k s. Calculate R _k /R ₁ .

Thus, the calibration signal s is transmitted from one of each antenna element section 20-i and the flying object 100 to the other. Then, based on the reception result of the calibration signal s, the calibration value of the transmitter 21 or the receiver 22 between the plurality of antenna element units 20-1 to 20-M is calculated.

4. Effects As described above, according to the present embodiment, array antenna 10 is calibrated by using flying object 100 . As a result, regardless of the installation environment of the array antenna 10, it is possible to avoid multipaths and appropriately perform calibration.

Further, according to the present embodiment, it is not necessary to add a dedicated calibration device to array antenna 10 . This is preferable from a cost point of view.

Furthermore, according to this embodiment, it is possible to implement calibration while the base station BS is transmitting and receiving signals.

5. Specific Example A specific example of the effect of the calibration process according to the present embodiment will be described below.

A calibration process was performed in the environment shown in FIG. The array antenna 10 is a 16-element half-wave circular array, and the flying object 100 is a drone. In FIG. 11, Tx represents the horizontal position of the array antenna, and Rx represents the horizontal position of the drone. Assume that the frequency is 5.12 GHz, the transmission power is 1 W, the thermal noise power is -90 dBm, and the band is 100 MHz. Assume that the number of reflections is five and the number of diffractions is two.

FIG. 12 shows the relationship between the drone altitude h and the radio wave arrival direction. The vertical axis represents received power, and the horizontal axis represents the radio wave reception direction. It can be seen that if the drone altitude h is increased, the main paths converge in the direction of the direct wave.

The altitude of the drone was changed until the K-factor was 20 dB. Then, calibration values were calculated by the above-described method, and calibration was performed. 13 and 14 are diagrams for explaining the effects of calibration.

FIG. 13 shows the phase error between the antenna element units 20 before and after calibration. The horizontal axis represents the identifiers of the 16 antenna element units 20-1 to 20-16. The vertical axis represents the phase error with reference to the antenna element section 20-1. Before calibration, the phase error ranged from -60° to 150°. After calibration, the phase error converged to a range of -1° to 1°. That is, the phase error was significantly reduced.

FIG. 14 shows an example of the radiation pattern of the array antenna 10. As shown in FIG. The horizontal axis represents angle and the vertical axis represents relative power. The array factor was calculated taking into account the phase error. Each graph shows a radiation pattern before and after calibration and an ideal radiation pattern (theoretical value) without phase error. In the radiation pattern before calibration, the side lobes have increased, deviating from the ideal radiation pattern. On the other hand, it can be seen that an ideal radiation pattern, that is, an ideal array directivity is achieved after calibration.

1 Calibration System 10 Array Antenna 20 Antenna Element Part 21 Transmitter 22 Receiver 23 Antenna Element 30 Control Device 100 Flying Object 110 Actuator 120 Communication Device 121 Transmitter 122 Receiver 123 Antenna 130 Control Device BS Base Station

Claims (7)

A calibration method applied to an array antenna having a plurality of antenna element units,
K factor calculation processing for transmitting radio waves from one of the array antenna and the flying object to the other and calculating the K factor based on the reception result of the radio waves on the receiving side;
altitude adjustment processing for adjusting the altitude of the flying object so that the K factor is equal to or greater than a threshold;
After the altitude adjustment processing, a calibration signal is transmitted from one of each of the plurality of antenna element units and the flying object to the other, and transmission between the plurality of antenna element units is performed based on a reception result of the calibration signal. a calibration value calculation process for calculating a calibration value of a device or a receiver; and a calibration method. A calibration method according to claim 1,
The plurality of antenna element units includes a first antenna element unit and a second antenna element unit,
The calibration value calculation process includes:
a process of transmitting the calibration signal from the first antenna element unit to the flying object;
a process of receiving, in the flying object, the calibration signal transmitted from the first antenna element unit as a first received signal;
a process of transmitting the calibration signal from the second antenna element unit to the flying object;
a process of receiving, in the flying object, the calibration signal transmitted from the second antenna element unit as a second received signal;
calculating the calibration value of the transmitter between the first antenna element section and the antenna element section based on a comparison of the first received signal and the second received signal. Method. A calibration method according to claim 1,
The plurality of antenna element units includes a first antenna element unit and a second antenna element unit,
The calibration value calculation process includes:
a process of transmitting the calibration signal from the flying object to the array antenna;
a process of receiving, in the first antenna element unit, the calibration signal transmitted from the flying object as a first received signal;
a process of receiving, in the second antenna element unit, the calibration signal transmitted from the flying object as a second received signal;
and calculating the calibration value of the receiver between the first antenna element section and the antenna element section based on a comparison of the first received signal and the second received signal. Method. The calibration method according to any one of claims 1 to 3,
The K factor calculation process includes:
a process of measuring the received power of the radio wave and the fluctuation amount of the received power on the receiving side;
and a process of calculating the K factor based on the received power and the amount of variation. A calibration method. The calibration method according to any one of claims 1 to 4,
The calibration method, wherein the radio wave is transmitted from the array antenna to the flying object in the K factor calculation process. A calibration method applied to an array antenna having a plurality of antenna element units,
a process of transmitting radio waves from one of the array antenna and the flying object to the other;
altitude adjustment processing for adjusting the altitude of the flying object until the radio waves received by the other of the array antenna and the flying object can be regarded as direct waves only;
After the altitude adjustment process, a calibration signal is transmitted from one of each of the plurality of antenna element units and the flying object to the other, and transmission between the plurality of antenna element units is performed based on a reception result of the calibration signal. a calibration value calculation process for calculating a calibration value of a device or a receiver; and a calibration method. an array antenna having a plurality of antenna element parts;
including a projectile and
one of the flying object and the array antenna receives radio waves transmitted from the other of the flying object and the array antenna, calculates a K factor based on the reception result of the radio waves,
the flying object performs altitude adjustment processing for adjusting the altitude of the flying object so that the K factor is equal to or greater than a threshold;
After the altitude adjustment process, each of the plurality of antenna element units and one of the flying objects on the transmission side transmits a calibration signal to each of the plurality of antenna element units and the other of the flying object on the receiving side. send and
The calibration system, wherein the receiving side calculates a calibration value of a transmitter or a receiver between the plurality of antenna element units based on reception results of the calibration signal.

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