WO2024070446A1

WO2024070446A1 - Access point device and processing device that communicates with wireless device via plurality of access point devices

Info

Publication number: WO2024070446A1
Application number: PCT/JP2023/031469
Authority: WO
Inventors: 一生菅野; 武雄大関; 良晃天野
Original assignee: Kddi株式会社
Priority date: 2022-09-26
Filing date: 2023-08-30
Publication date: 2024-04-04
Also published as: JP2024047102A

Abstract

This processing device comprises: an acquisition means configured to acquire, from a plurality of AP devices, first characteristic information indicating uplink channel characteristics estimated by each of the plurality of AP devices on the basis of reference signals received from a wireless device; a control means configured to schedule the transmission and reception of a test signal by each of the plurality of AP devices, and perform a correction process for acquiring, from the plurality of AP devices, second characteristic information indicating the channel characteristics between the AP devices estimated on the basis of the test signal received by each of the plurality of AP devices from the other AP devices and generating correction information based on a reference AP device; an estimating means configured to estimate third characteristic information indicating downlink channel characteristics on the basis of the first characteristic information and the correction information; and a transmitting means configured to transmit information for precoding transmission signals transmitted to the wireless device by the plurality of AP devices on the basis of the third characteristic information to each of the plurality of AP devices.

Description

Access point device and processing device for communicating with wireless devices via a plurality of access point devices - Patents.com

The present disclosure relates to a mobile communication system that communicates with a wireless device (WD) via multiple access point (AP) devices.

For example, in cell-free massive MIMO (CFmMIMO), a central processing unit (CPU) located in an aggregate station communicates with wireless devices (WDs) via multiple access points (APs) located at different geographical locations. The AP device located in each AP is simply referred to as "AP" below. The AP has an antenna and a wireless device. The wireless device has a wireless transmitter (TX) that transmits wireless signals to the WD via the antenna, and a wireless receiver (RX) that receives wireless signals from the WD via the antenna. The communication link connecting the CPU of the aggregate station and the AP is also called a fronthaul.

The CPU generates a transmission signal based on data to be sent to one or more WDs, and sends the generated transmission signal to the TX of each of the multiple APs. The TX of each AP transmits a wireless signal via an antenna based on the transmission signal from the CPU. The RX of each AP receives wireless signals from one or more WDs via an antenna. The RX of each AP transmits a received signal based on the received wireless signal to the CPU. The CPU demodulates the data from each WD based on the received signal from each AP.

Here, the CPU precodes a transmission signal generated based on data to be transmitted to one or more WDs based on the downlink (DL) channel matrix between the one or more WDs and transmits the signal to each of the multiple APs. For example, in the case of time division duplex (TDD), when there is a contradiction between the DL channel characteristics and the uplink (UL) channel characteristics, the DL channel matrix can be estimated based on a reference signal or standard signal that the CPU receives from one or more WDs via each AP.

However, even if reciprocity is recognized in the channel characteristics of the wireless section, the TX and RX characteristics of each AP are generally different from each other. For this reason,

Patent Documents

1 and 2 disclose a configuration in which test signals are transmitted and received between multiple APs to obtain the ratio of the TX and RX transfer functions of a reference AP among multiple APs to the TX and RX transfer functions of other APs, and the DL channel matrix is obtained by correcting the UL channel matrix based on the ratio.

As described in

Patent Documents

1 and 2, CFm MIMO in which channel estimation and precoding are performed in the CPU is referred to as "centralized." In contrast, Non-Patent Document 1 discloses "decentralized" or "distributed" CFm MIMO. In distributed CFm MIMO, channel estimation and precoding are performed by the AP.

JP 2019-091974 A JP 2019-004345 A

This disclosure provides a technology for estimating downlink channel characteristics from uplink channel characteristics in distributed cell-free massive MIMO.

According to one aspect of the present disclosure, a processing device that communicates with one or more wireless devices via a plurality of access point (AP) devices that transmit and receive wireless signals includes: an acquisition means configured to acquire from the plurality of AP devices first characteristic information indicating uplink channel characteristics estimated by each of the plurality of AP devices based on a reference signal received from the one or more wireless devices; a control means configured to schedule transmission and reception of test signals by each of the plurality of AP devices, acquire from the plurality of AP devices second characteristic information indicating channel characteristics between the AP devices estimated by each of the plurality of AP devices based on the test signal received from the other AP devices, and perform a correction process to generate correction information based on the second characteristic information acquired from the plurality of AP devices, using a reference AP device among the plurality of AP devices as a reference; an estimation means configured to estimate third characteristic information indicating downlink channel characteristics between the one or more wireless devices based on the first characteristic information and the correction information; and a transmission means configured to transmit information for precoding a transmission signal to be transmitted by the plurality of AP devices to the one or more wireless devices based on the third characteristic information to each of the plurality of AP devices.

According to this disclosure, it is possible to estimate downlink channel characteristics from uplink channel characteristics in distributed cell-free massive MIMO.

Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which the same or similar components are designated by the same reference numerals.

1 is a configuration diagram of a mobile communication system according to an embodiment. FIG. 2 is a block diagram of a processing device according to an embodiment. FIG. 2 is a configuration diagram of an AP device according to an embodiment. FIG. 2 illustrates an example of scheduling information according to one embodiment.

The embodiments are described in detail below with reference to the attached drawings. Note that the following embodiments do not limit the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be combined in any desired manner. In addition, the same reference numbers are used for the same or similar configurations, and duplicate descriptions will be omitted.

FIG. 1 is a diagram showing the configuration of a mobile communication system according to this embodiment. The mobile communication system includes a CPU 1, N (N is an integer of 2 or more) access point devices (APs) 2, and two WDs 3. In the following description, when each AP 2 is to be distinguished, they are written as AP#1 to AP#N as shown in FIG. 1. Similarly, when two WDs 3 are to be distinguished, they are written as WD#1 and WD#2 as shown in FIG. 1. Although the number of WDs 3 is 2 in FIG. 1, the number of WDs 3 can be any number of 1 or more. The mobile communication system according to this embodiment uses time division duplex (TDD). In other words, the downlink (DL) from AP 2 to WD 3 and the uplink (UL) from WD 3 to AP 2 are separated by time, and the frequency band of the radio signal used in the DL direction is the same as that of the radio signal used in the UL direction.

CPU1 is a processing device (also referred to as a central processing device) located in the aggregation station, and as described above, is connected to N APs 2 via the fronthaul. For example, when CPU1 transmits data to WD#1 and WD#2, CPU1 generates a transmission signal including data addressed to WD#1 and WD#2, and transmits the generated transmission signal to each of APs#1 to AP#N. APs#1 to AP#N each have a TX and RX, and one or more antennas. APs#1 to AP#N transmit wireless signals based on the transmission signal from CPU1. The wireless signals transmitted by each of APs#1 to AP#N are received by WD#1 and WD#2.

In addition, each of AP#1 to AP#N receives wireless signals from WD#1 and WD#2, estimates channel characteristics between each WD3 based on the wireless signals, performs signal processing to separate the WD3 signal from other WD signals, and transmits the corresponding channel estimation results and separated signals to CPU1. Note that the wireless signals from WD#1 and WD#2 include data that WD#1 and WD#2 transmit to the communication partner. CPU1 estimates the data from WD#1 and WD#2 based on the received signals from AP#1 to AP#N.

FIG. 2 is a block diagram of the CPU 1 according to this embodiment. The transmitter 11 generates a transmission signal to be sent to each AP 2 based on the transmission data addressed to WD#1 and WD#2, and transmits it to each AP 2. The receiver 12 demodulates and outputs the received data from WD#1 and WD#2 based on the signal received from each AP 2. The controller 13 controls the entire CPU 1, transmits control signals to each AP 2 via the transmitter 11, and receives control signals from each AP 2 via the receiver 12.

FIG. 3 is a configuration diagram of AP2 according to this embodiment. The control unit 24 receives a control signal from the CPU 1 via the receiving unit 25, and transmits a control signal to the CPU 1 via the transmitting unit 26. The receiving unit 25 also outputs a transmission signal from the CPU 1 to the TX 22. The TX 22 precodes the transmission signal based on the DL precoding matrix W' notified by the control unit 24, and outputs the precoded transmission signal to the antenna 21. The antenna 21 converts the transmission signal from the TX 22 into a radio signal and transmits it.

The antenna 21 also converts the radio signal received from WD3 into a received signal and outputs it to RX 23. RX 23 transmits the received signal to CPU 1 via the transmitter 26. The receiver 12 also outputs a sounding reference signal (SRS) received from WD3 to the controller 24. The SRS is an UL reference signal defined by the Third Generation Partnership Project (3GPP). The controller 24 estimates the UL channel characteristic value between WD3 and WD3 based on the SRS, and notifies the CPU 1 of UL characteristic information indicating the estimated UL channel characteristic value by a control signal.

The control unit 13 of the CPU 1 estimates the UL channel matrix H _UL between the WD 3 based on the UL characteristic information received from each AP 2 by a control signal. Then, the control unit 13 estimates the channel matrix H _DL by correcting the channel matrix H _UL with the correction matrix C. The control unit 13 generates a precoding matrix W by a method such as ZF, MMSE, or PMMSE based on the DL channel matrix H _DL . For example, each column of the precoding matrix W corresponds to one AP 2. The control unit 13 extracts a column corresponding to AP #k (k is an integer from 1 to N) from each column of the precoding matrix W to generate a precoding matrix W' for each AP. The control unit 13 transmits the corresponding precoding matrix W' to each AP 2 using a control signal. Note that a configuration may be adopted in which some or all elements of the estimated channel matrix are transmitted to the AP 2, and the corresponding precoding matrix is calculated in the AP 2. In this case, the corresponding precoding matrix can be generated based on a method such as LPMMSE.

The control unit 13 of the CPU 1 controls the execution of a calibration process (correction process) to generate the above-mentioned correction matrix C. The calibration process will be described below. The calibration process can be executed when a new AP 2 is added, when the TX or RX of the AP 2 is replaced, or when it is estimated that the characteristics of the TX or RX of the AP 2 have changed significantly due to aging or temperature changes in the equipment.

In the calibration process, the control unit 13 schedules AP#1 to AP#N in order as transmitting APs, so that one transmitting AP transmits a test signal and the other AP2 (receiving AP) receives this test signal. The control unit 13 notifies the control unit 24 of each AP2 of scheduling information indicating the scheduling result by a control signal. FIG. 4 shows an example of the scheduling information. FIG. 4 shows that AP#k transmits a test signal at timing T#k, and therefore, that an AP2 different from AP#k receives the test signal from AP#k at timing T#k. More specifically, the scheduling information indicates the radio resources from which each AP2 transmits the test signal. In other words, FIG. 4 shows only the timing at which the test signal is transmitted, but in reality, the frequency band for transmitting the test signal may also be scheduled.

The control unit 24 of each AP 2 controls TX 22 and RX 23 to transmit a test signal according to the scheduling information and to receive test signals from other APs 2. If the timing to transmit a test signal is the timing to transmit a DL radio signal, the receiving AP receiving the test signal controls TX 22 and RX 23 to perform a receiving operation but not a transmitting operation at this timing. If the timing to transmit a test signal is the timing to receive a UL radio signal, the transmitting AP transmitting the test signal controls TX 22 and RX 23 to perform a transmitting operation but not a receiving operation at this timing. RX 23 outputs the test signal received from other APs 2 to the control unit 24.

The control unit 24 of AP#k estimates channel characteristics in the direction from the other AP2 to AP#k based on the received test signal. For example, based on the test signals transmitted by AP#2 to AP#N and received by AP#1, the control unit 24 of AP#1 can estimate channel characteristic values _h2,1 to hN _,1 from AP#2 to AP#N to AP#1, respectively. The control unit 24 of AP#k notifies the CPU1 of each of the (N-1) inter-AP channel characteristic values in the direction from the other AP2 to AP#k by a control signal.

Therefore, the control unit 13 of the CPU 1 receives (N-1) channel characteristic values from each of the N APs 2, that is, bidirectional channel characteristic values for each pair of two APs 2 among the N APs 2. Here, if the transfer functions of TX22 and RX23 of AP#k are TX _k and RX _k , the channel characteristic value h _1,2 from AP#1 to AP#2 is TX ₁ ×h _p1-2 ×RX ₂ , and the channel characteristic value h _2,1 from AP#2 to AP#1 is TX ₂ ×h _p2-1 ×RX _1. Note that h _p1-2 is the transfer function of the wireless section from AP#1 to AP#2, and h _p2-1 is the transfer function of the wireless section from AP#2 to AP#1.

When reciprocity of the wireless section is recognized, h _p1-2 =h _p2-1 . Therefore, in this case, the correction coefficient C _1,2 of AP#2 based on AP#1 is calculated as C _1,2 =h _1,2 /h _2,1 =(TX ₁ ×RX ₂ )/(TX ₂ ×RX ₁ ). The correction coefficient C _1,2 is the ratio of the transfer functions of TX22 and RX23 of AP#2 based on the transfer functions of TX22 and RX23 of AP#1.

The control unit 13 first selects one of the N APs 2 as the reference AP. The criteria for selecting the reference AP are arbitrary, but for example, the signal-to-noise ratio (SNR) can be estimated based on the test signal received by the AP 2, and the AP 2 with the best average SNR with other APs 2 can be selected as the reference AP. For this reason, the control unit 24 of each AP 2 can be configured to notify the CPU 1 of the SNR of the test signal as well. In the following description, it is assumed that AP #1 is selected as the reference AP. In this case, the control unit 13 obtains correction coefficients C _1,2 to C _1,N . The correction coefficient C _1,1 is 1=(TX ₁ ×RX ₁ )/(TX ₁ ×RX ₁ ). The control unit 13 obtains a diagonal matrix with the correction coefficients C _1,1 to C _1,N as elements as the correction matrix C. That is, C = diag[ _C1,1 , _C1,2 , ..., C1 _,N ] = ( _TX1 / _RX1 ) diag[ _RX1 / _TX1 , _RX2 / _TX2 , ..., RXN _/ _TXN ].

The control unit 13 estimates the DL channel matrix H _DL by multiplying the UL channel matrix H _UL by the inverse matrix of the correction matrix C, and generates the precoding matrix W as described above. Then, as described above, the control unit 13 transmits the precoding matrix W′ corresponding to each AP 2 to each AP 2 by a control signal.

The above configuration makes it possible to estimate downlink channel characteristics in distributed cell-free massive MIMO.

The test signal may be transmitted in any format and using any radio resource. As an example, a channel state information reference signal (CSI-RS) defined in 3GPP, which is transmitted by AP2 to WD3 to estimate DL channel characteristics, may be used as the test signal. In this case, the CSI-RS transmitted by the transmitting AP may be used both to estimate DL channel characteristics by WD3 and to estimate inter-AP channel characteristics by the receiving AP. Furthermore, the CSI-RS as a test signal may be transmitted using radio resources defined in 3GPP as being capable of transmitting CSI-RS.

As another example, the SRS transmitted by WD3 to estimate UL channel characteristics can be used as a test signal. In this case, the SRS as a test signal can be transmitted using radio resources specified by 3GPP as being capable of transmitting the SRS.

In this embodiment, AP#1 to AP#N are sequentially set as transmitting APs, and only one transmitting AP transmits a test signal at a certain timing, and the remaining AP2 (receiving AP) is scheduled to receive the test signal from the transmitting AP. This is because, by scheduling in this manner, a test signal is transmitted and received in a pair of any two AP2s among AP#1 to AP#N. In addition, this is also to select a reference AP based on the transmission and reception results of the test signal of each pair. However, for example, when the reference AP is AP#1, what is required is to transmit and receive a test signal in (N-1) pairs including AP#1. Therefore, for example, AP#1 transmits a test signal at a certain timing, and AP#2 to AP#N receive the test signal, so that the channel characteristic values h _1,2 to h _1,N can be estimated. In addition, when multiple APs 2 can transmit test signals using different frequency bands at a certain timing, for example, when (N-1) APs 2 can transmit test signals at a certain timing, APs #2 to #N transmit test signals, and AP #1 receives the test signals, so that channel characteristic values h _2,1 to h _N,1 can be estimated, and thus correction matrix C can be estimated. In addition, for example, h _2,1 can be obtained by multiplying h _2,m and h _m,1 . Note that m is any number from 3 to N. Therefore, for example, when a reference AP or a plurality of candidate APs to be the reference AP can be determined or selected in advance based on the placement positions of APs #1 to AP #N, it is not necessary to schedule transmission and reception of test signals between any two pairs of APs 2.

In the above embodiment, the mobile communication system uses TDD. However, the above embodiment can be applied to a mobile communication system that uses frequency division duplex (FDD) as long as reciprocity of the wireless channel is recognized even in FDD.

In addition, in the above embodiment, the terms channel matrix, precoding matrix, and correction matrix are used. A channel matrix is information indicating the characteristics of multiple channels, a precoding matrix is information indicating the processing to be applied to wireless signals transmitted from multiple APs 2, and a correction matrix is information indicating the amount of correction for the values of each element of the channel matrix. Therefore, the term "matrix" can be replaced with the term "information."

The CPU1 or AP2 according to the present disclosure may be realized by a computer program that, when executed by one or more processors of a device having the processor, causes the device to function as the CPU1 or AP2. The computer program may include program instructions that are executable by one or more processors. The computer program may be stored in a non-transitory computer-readable storage medium or may be distributed via a network.

The invention is not limited to the above-described embodiments, and various modifications and variations are possible within the scope of the invention.

This application claims priority based on Japanese Patent Application No. 2022-152527, filed on September 26, 2022, the entire contents of which are incorporated herein by reference.

Claims

A processing device for communicating with one or more wireless devices via a plurality of access point (AP) devices for transmitting and receiving wireless signals, comprising:
an acquisition means configured to acquire, from the plurality of AP devices, first characteristic information indicating uplink channel characteristics estimated by each of the plurality of AP devices based on a reference signal received from the one or more wireless devices;
a control means configured to schedule transmission and reception of a test signal by each of the plurality of AP devices, acquire second characteristic information from the plurality of AP devices indicating channel characteristics between the AP devices estimated based on the test signal received by each of the plurality of AP devices from other AP devices, and perform correction processing to generate correction information based on the second characteristic information acquired from the plurality of AP devices and using a reference AP device among the plurality of AP devices as a reference;
an estimation means configured to estimate third characteristic information indicating channel characteristics of a downlink between the one or more wireless devices and the one or more wireless devices based on the first characteristic information and the correction information;
A transmitting means configured to transmit information for performing precoding of a transmission signal to be transmitted by the plurality of AP devices to the one or more wireless devices based on the third characteristic information, to each of the plurality of AP devices;
The processing device comprises:
The processing device according to claim 1, wherein the control means is configured to schedule transmission and reception of the test signal so that each of the multiple AP devices transmits the test signal at a different timing during the correction process.
The processing device according to claim 1 or 2, wherein the control means is configured to notify each of the plurality of AP devices of scheduling information indicating the result of the scheduling.
A computer-readable storage medium storing a computer program that, when executed by one or more processors of a device having one or more processors, causes the device to function as a processing device according to any one of claims 1 to 3.
An access point (AP) device that transmits a transmission signal received from a processing device as a wireless signal and transmits a reception signal corresponding to a wireless signal received from one or more wireless devices to the processing device,
The processing device communicates with the AP device and with the one or more wireless devices via one or more other AP devices;
The AP device includes:
a first transmitting means configured to transmit, to the processing device, first characteristic information indicating uplink channel characteristics estimated based on reference signals received from the one or more wireless devices;
a second transmitting means configured to transmit a wireless signal including a test signal according to scheduling information received from the processing device, and to transmit to the processing device second characteristic information indicating a channel characteristic between the AP devices estimated based on the test signal included in the wireless signal received from the one or more other AP devices according to the scheduling information;
A precoding means configured to perform precoding of the transmission signal based on precoding information received from the processing device;
The AP device includes:
The AP device according to claim 5, wherein the precoding information is generated by the processing device based on the first characteristic information and the second characteristic information received from the AP device and the one or more other AP devices, respectively.
A period for transmitting a radio signal to the one or more wireless devices is different from a period for receiving a radio signal from the one or more wireless devices,
The AP device according to claim 5 or 6, wherein a timing for transmitting a wireless signal including the test signal is within a period for receiving a wireless signal from the one or more wireless devices.
The AP device of claim 7, wherein the reference signal and the test signal are both sounding reference signals.
A period for transmitting a radio signal to the one or more wireless devices is different from a period for receiving a radio signal from the one or more wireless devices,
The AP device according to claim 5 or 6, wherein the timing for receiving the wireless signal including the test signal from the one or more other AP devices is within a period for transmitting a wireless signal to the one or more wireless devices.
the reference signal is a sounding reference signal;
The AP device of claim 9 , wherein the test signal is a channel state information reference signal.
A computer-readable storage medium storing a computer program that, when executed by one or more processors of an apparatus having one or more processors, causes the apparatus to function as an AP apparatus according to any one of claims 5 to 10.