KR102613817B1 - Method and apparatus for estimation and feedback of time offset for multiple access points in wireless communication system - Google Patents

Abstract

This disclosure relates to a time offset estimation and feedback method for a plurality of access points in a wireless communication system. A method of feeding back a time offset (TO) by a station (STA) in a wireless communication system according to an embodiment of the present disclosure includes receiving training information from each of a plurality of access points (AP); estimating a channel for each of the plurality of APs based on the training information, and estimating a first type TO for each of the plurality of APs based on the estimated channels; estimating a second type TO between the plurality of APs based on the estimated value for the first type TO; determining a beamformer for each of the plurality of APs based on one or more of the first type TO or the second type TO; and transmitting feedback information including one or more of a beamformer or the second type TO for each of the plurality of APs to each of the plurality of APs.

Description

Time offset estimation and feedback method and device for multiple access points in a wireless communication system {METHOD AND APPARATUS FOR ESTIMATION AND FEEDBACK OF TIME OFFSET FOR MULTIPLE ACCESS POINTS IN WIRELESS COMMUNICATION SYSTEM}

This disclosure relates to a wireless communication system, and specifically to a time offset estimation and feedback method and apparatus for a plurality of access points in a wireless communication system.

IEEE 802.11 wireless LAN (WLAN) is used in a variety of communication environments, and is required to support faster data rates to meet the demand for new and diverse services. Wireless devices that comply with the IEEE 802.11a/b/g/n/ac standard are still widely used today, and recently, wireless devices that comply with the IEEE 802.11ax standard are appearing. As the penetration rate of the WLAN system increases rapidly over the years, the current WLAN system has a higher density of access points (APs) in a given space compared to the past. In order to achieve a maximum throughput four times higher than that of IEEE 802.11ax while considering the high-density overlapping environment of these APs, the IEEE 802.11 Extreme high throughput (EHT) Study Group (SG) was formed in June 2018, and from May 2019, IEEE 802.11 After being promoted to the be Task Group (TGbe), development of the standard began. TGbe proposes AP coordination, in which multiple APs cooperate to transmit data, as one of the main techniques as an interference management and mitigation measure in a high-density overlapping WLAN environment. In other words, the conventional IEEE WLAN standard did not consider an environment in which multiple APs transmit data to one non-AP station (STA) (i.e., terminal) at the same time, but recently, cooperation techniques for multiple APs are being discussed. .

AP cooperation techniques currently being considered in TGbe can be classified into several types depending on the level of cooperation between APs. For example, in a situation where multiple APs share information with each other, distributed MIMO (distributed Multi-Input Multi-Output, D-MIMO), in which multiple APs simultaneously transmit data to be sent to one STA, is one of the cooperation techniques. It is being discussed. In D-MIMO, all APs participating in data transmission share the transmission data they want to send, which can be expected to improve not only network throughput but also the throughput of individual links. However, such AP cooperation operation may suffer serious performance degradation due to interference between APs if synchronization between APs is not supported. For example, the arrival time of data packets transmitted by each AP participating in AP cooperation at the STA may be different, which may cause reception performance deterioration.

In this way, accurate synchronization between APs is required for AP cooperation operation, but there is no effective method to improve synchronization performance between APs yet.

The technical problem of the present disclosure is to provide a method and device for estimating and feeding back by the STA the time offset between each of a plurality of APs and the STA in a wireless communication system.

An additional technical task of the present disclosure is to provide a method and device for estimating and feeding back time offsets between multiple APs by an STA in a wireless communication system.

An additional technical task of the present disclosure is to provide a method and device for performing synchronization between APs based on time offset between APs by utilizing a channel estimation procedure between APs and STAs in a wireless communication system.

The technical problems to be achieved by this disclosure are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by those skilled in the art from the description below. You will be able to.

A method of feeding back a time offset (TO) by a station (STA) in a wireless communication system according to an aspect of the present disclosure includes receiving training information from each of a plurality of access points (AP); estimating a channel for each of the plurality of APs based on the training information, and estimating a first type TO for each of the plurality of APs based on the estimated channels; estimating a second type TO between the plurality of APs based on the estimated value for the first type TO; determining a beamformer for each of the plurality of APs based on one or more of the first type TO or the second type TO; and transmitting feedback information including one or more of a beamformer or the second type TO for each of the plurality of APs to each of the plurality of APs.

A method of performing time synchronization by an access point (AP) in a wireless communication system according to an additional aspect of the present disclosure includes transmitting training information to a station (STA); Receiving feedback information including one or more of a beamformer for the AP or a time offset (TO) between the AP and another AP from the STA; and transmitting a data packet to the STA based on synchronized timing between the AP and the other AP based on the feedback information.

The features briefly summarized above with respect to the present disclosure are merely exemplary aspects of the detailed description of the present disclosure described below, and do not limit the scope of the present disclosure.

According to the present disclosure, a method and apparatus can be provided for accurately and efficiently estimating and feeding back by the STA the time offset between each of a plurality of APs and the STA in a wireless communication system.

According to the present disclosure, a method and device for accurately and efficiently estimating and feedbacking time offsets between multiple APs by an STA in a wireless communication system can be provided.

According to the present disclosure, by utilizing a channel estimation procedure between AP and STA in a wireless communication system, time offset between APs is alleviated based on simple feedback information of STA in an AP cooperation situation, and actual time offset is maintained even in a low signal-to-noise ratio section. A method and apparatus for effectively estimating and performing synchronization between APs may be provided.

The effects that can be obtained from the present disclosure are not limited to the effects mentioned above, and other effects not mentioned can be clearly understood by those skilled in the art from the description below. will be.

1 is a diagram illustrating an example of a single user joint D-MIMO system to which the present disclosure can be applied.
2 is a diagram illustrating an example of a time offset within an OFDM symbol to which the present disclosure can be applied.
Figure 3 is a diagram showing a legacy preamble structure to which the present disclosure can be applied.
4 is a diagram showing a first type TO and a second type TO according to an embodiment of the present disclosure.
Figure 5 is a diagram showing an example of training information to which the present disclosure can be applied.
FIG. 6 is a diagram illustrating an orthogonal mapping matrix according to an example of the present disclosure.
Figure 7 is a diagram showing an example of a feedback frame to which the present disclosure can be applied.
FIG. 8 is a diagram illustrating a frame including feedback information according to an embodiment of the present disclosure.
Figure 9 is a diagram showing a data packet transmission operation with AP time synchronization applied according to an embodiment of the present disclosure.
FIG. 10 is a flowchart for explaining the operation of an AP and an STA according to an embodiment of the present disclosure.
Figure 11 is a diagram showing the configuration of an AP device and an STA device according to the present disclosure.
12 to 16 are diagrams showing simulation results related to the present disclosure.

Hereinafter, with reference to the attached drawings, embodiments of the present disclosure will be described in detail so that those skilled in the art can easily practice them. However, the present disclosure may be implemented in many different forms and is not limited to the embodiments described herein.

In describing embodiments of the present disclosure, if it is determined that detailed descriptions of known configurations or functions may obscure the gist of the present disclosure, detailed descriptions thereof will be omitted. In addition, in the drawings, parts that are not related to the description of the present disclosure are omitted, and similar parts are given similar reference numerals.

In the present disclosure, when a component is said to be “connected,” “coupled,” or “connected” to another component, this is not only a direct connection relationship, but also an indirect connection relationship where another component exists in between. It may also be included. In addition, when a component is said to "include" or "have" another component, this does not mean excluding the other component, but may further include another component, unless specifically stated to the contrary. .

In the present disclosure, terms such as first and second are used only for the purpose of distinguishing one component from other components, and do not limit the order or importance of the components unless specifically mentioned. Accordingly, within the scope of the present disclosure, a first component in one embodiment may be referred to as a second component in another embodiment, and similarly, the second component in one embodiment may be referred to as a first component in another embodiment. It may also be called.

In the present disclosure, distinct components are intended to clearly explain each feature, and do not necessarily mean that the components are separated. That is, a plurality of components may be integrated to form one hardware or software unit, or one component may be distributed to form a plurality of hardware or software units. Accordingly, even if not specifically mentioned, such integrated or distributed embodiments are also included in the scope of the present disclosure.

In the present disclosure, components described in various embodiments do not necessarily mean essential components, and some may be optional components. Accordingly, embodiments consisting of a subset of the elements described in one embodiment are also included in the scope of the present disclosure. Additionally, embodiments that include other components in addition to the components described in the various embodiments are also included in the scope of the present disclosure.

This disclosure relates to communication between network nodes in a wireless communication system. A network node may include one or more of a base station, terminal, or relay. The term base station (BS) can be replaced by terms such as fixed station, Node B, eNodeB (eNB), ng-eNB, gNodeB (gNB), and access point (AP). . Terminal may be replaced by terms such as UE (User Equipment), MS (Mobile Station), MSS (Mobile Subscriber Station), SS (Subscriber Station), and non-AP station (non-AP STA).

A wireless communication system may support communication between a base station and a terminal, or may support communication between terminals. In communication between a base station and a terminal, downlink (DL) refers to communication from the base station to the terminal. Uplink (UL) refers to communication from the terminal to the base station. Communication between devices may include various communication methods or services such as Device-to-Device (D2D), Vehicle-to-everything (V2X), Proximity Service (ProSe), and sidelink communication. In device-to-device communication, the device may be implemented in the form of a sensor node, vehicle, disaster alarm, etc.

Additionally, examples of the present disclosure may be applied to a wireless communication system including a relay or relay node (RN). When a relay is applied to communication between a base station and a terminal, the relay may function as a base station for the terminal, and the relay may function as a terminal for the base station. Meanwhile, when a relay is applied to communication between terminals, the relay can function as a base station for each terminal.

The present disclosure can be applied to various multiple access methods of a wireless communication system. For example, multiple access methods include Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), and Single Carrier-FDMA (SC-FDMA). , OFDM-FDMA, OFDM-TDMA, OFDM-CDMA, NOMA (Non-Orthogonal Multiple Access), etc. In addition, a wireless communication system to which the present disclosure can be applied may support a Time Division Duplex (TDD) method using distinct time resources for uplink and downlink communications, and a Frequency Division Duplex (FDD) method using distinct frequency resources. Duplex) method can also be supported.

In the present disclosure, transmitting or receiving a channel includes transmitting or receiving information or signals through the channel. For example, transmitting a control channel means transmitting control information or signals through the control channel. Similarly, transmitting a data channel means transmitting data information or signals through a data channel.

Below, a time offset estimation and feedback method for multiple APs according to the present disclosure will be described.

This disclosure includes various examples for synchronization between APs when multiple APs cooperate in WLAN-based wireless communication. The examples of this disclosure are mainly explained assuming a WLAN system, but the examples of this disclosure are not limited to application to WLAN. That is, the scope of the present disclosure is the STA or terminal for synchronization between multiple APs or multiple base stations when multiple APs or base stations perform cooperative transmission for one STA or terminal in various wireless communication systems. Includes time offset estimation and feedback scheme.

In the examples of this disclosure, timing offset (TO) can be defined in two types. The first type TO may include the time difference between the AP and the STA (AP-STA TO). The second type TO may include the time difference between APs (AP-AP TO). Additionally, the TO for a plurality of APs may include a plurality of first type TOs for time differences between the plurality of APs and the STA, and a plurality of second type TOs for the time differences between the plurality of APs. For example, assume that the first, second, and third APs cooperate, and the first AP is the master AP or reference AP. In this case, the TO between the first AP and the STA, the TO between the second AP and the STA, and the TO between the third AP and the STA correspond to a plurality of first type TOs, and the TO between the first AP and the second AP and the first AP The TO between and the third AP (additionally the TO between the second AP and the third AP) may correspond to a plurality of second type TOs.

In examples of the present disclosure, the STA uses reception information (e.g., training information) from a plurality of APs to synchronize between APs based on the reception time error of packets or frames reaching the STA from each AP. It can be done. In addition, the STA estimates a channel based on training information received by the STA from a plurality of collaborating APs, estimates TO between the STA and each AP using the estimated channel information, and Channel information and TO can be fed back to (one or more of the APs). The AP can compensate for the channel and correct/adjust/align the transmission timing using the channel information and TO fed back from the STA.

For example, information received from multiple APs may include multiple training fields, and the multiple training fields may correspond to all antenna numbers of the multiple APs. That is, training information transmitted simultaneously by multiple APs may be common information, and differences may occur in the time it reaches the STA depending on the distance from each of the multiple APs to the STA, channel conditions, etc. For example, training information may be configured in the form of a long training field (EHT-LTF), and a frame containing such training information may be a null data packet (NDP) frame. However, the scope of the present disclosure is not limited thereto, and the STA can estimate and feed back TO through any frame containing training information corresponding to multiple APs (or antennas of multiple APs).

Information fed back by the STA (eg, channel information and/or TO) may be included in a beamforming report frame. However, the scope of the present disclosure is not limited thereto, and feedback information may be transmitted to the AP through any frame including the TO. For example, the training information and feedback information of the present disclosure can be transmitted and received between the AP and the STA using an NDP sounding procedure in which the AP and the STA transmit and receive NDP and beamforming reports (or compressed beamforming frames).

For example, in a single AP transmission situation in an existing WLAN environment, several techniques have been proposed for synchronization between STAs and APs, but in the case of a multi-AP transmission situation that TGbe has begun to consider, offset synchronization between APs that transmit data simultaneously is additionally performed. Should be. Currently, TGbe is considering simultaneous packet transmission based on trigger frames between APs in order to implement the simultaneous transmission procedure of NDP sounding and data packets for channel estimation in AP cooperation situations. Trigger frame-based simultaneous transmission assumes that a predetermined master AP transmits a trigger frame to one or more slave APs, and the slave AP and master AP that receive it simultaneously transmit packets. At this time, due to the different round-trip delays between the STA and each AP and differences in AP internal processing speeds, it is not guaranteed that the signals reaching the STA arrive at exactly the same time, and this time offset (timing offset) TO) can cause interference between STA received packets and act as a factor in degrading reception performance.

To solve this problem, the present disclosure provides a new method of estimating the time offset between the STA and each AP on the STA side using the transmission and reception channels estimated based on training information from a plurality of APs, and the estimated time offset value. We describe a new method that supports timing synchronization between APs participating in AP cooperation by providing feedback to the master AP or each AP.

Below, for clarity of explanation, examples of the present disclosure are described based on the NDP sounding procedure defined in the IEEE 802.11 standard. However, as described above, various examples of the present disclosure are applied to the IEEE 802.11 communication system or the NDP sounding procedure. is not limited to

1 is a diagram illustrating an example of a single user joint D-MIMO system to which the present disclosure can be applied.

In the example of Figure 1, it is assumed that N _APs cooperate (coordination). In the following example, for clarity of explanation, it is assumed that N _AP = 2, each AP is assumed to have N _t antennas (i.e., the number of antennas in AP1 and the number of antennas in AP2 are the same), and the STA is assumed to have one antenna. However, the scope of the present disclosure includes expanded application of the following examples to cases where the number of APs is 3 or more, each AP has a different number of antennas, and the STA has a plurality of antennas.

Based on the above assumption, the transmission baseband Orthogonal Frequency Division Multiplexing (OFDM) data symbol in the time domain can be expressed as Equation 1.

In Equation 1, i is the index of the AP, and a is the index of the antenna in each AP. Additionally, s _(i,a) (t) represents the time domain signal transmitted from the a-th antenna of the i-th AP. Additionally, X[k] represents the modulated symbol of the kth subcarrier. In addition, N represents the size of the FFT (fast fourier transform), T ^CP represents the length of the cyclic prefix (CP), and T ^S represents the length of the data symbol. Additionally, V(i,a) represents the beamformer (or beamforming matrix/vector, or precoding matrix/vector) applied to the a-th antenna of the i-th AP. Additionally, the time unit t may correspond to the time from the reference time point 0 to the time point corresponding to the sum of the lengths of the data symbol and CP.

When such OFDM data symbols are transmitted cooperatively from multiple APs, the symbol error rate (SER) may worsen when AP-AP TO and/or AP-STA TO occurs.

2 is a diagram illustrating an example of a time offset within an OFDM symbol to which the present disclosure can be applied.

Time offset (TO) can be divided into propagation offset and time synchronization offset depending on the cause of occurrence. First, the propagation offset refers to the offset that occurs when data packets transmitted from each AP reach the STA at different times due to propagation delay, etc. Time synchronization offset refers to the time synchronization error that occurs when the STA fails to estimate the exact location of the data symbol. The sum of these two offsets can be called total TO. In the examples below, TO may mean the entire TO, unless specifically stated otherwise.

Regarding the example in Figure 2, the sampling frequency (f _s ) is Define the sampling time (t _s ) as is defined as , and the TO between the STA and the ith AP is normalized by the sampling time t _s. It can be said that (i=1, 2). In this case, when TO exists between the STA and each AP in the discrete time domain, the received signal (y ^TO ) at the STA can be expressed as Equation 2.

In Equation 2, h _(i,a) (t) represents the channel impulse response at t for the i-th AP and a-th antenna, and s _(i,a) (t) represents the transmitted signal. Additionally, z(t) represents Gaussian noise.

Equation 2 can be expressed as Equation 3 as the product (or phase shift) of the exponential component on the frequency axis through FFT.

In Equation 3, H _(i,a) [k], S _(i,a) [k], and Z[k] are h _(i,a) [n], s _(i,a) [n], and It indicates that it has an FFT relationship with z[n].

Figure 3 is a diagram showing a legacy preamble structure to which the present disclosure can be applied.

The IEEE 802.11 physical layer (PHY) protocol data unit (Physical layer Protocol Data Unit (PPDU)) frame format may include a preamble and a Physical layer Service Data Unit (PSDU). The PSDU corresponds to a data field and includes data transmitted from the upper layer (eg, MAC) to the PHY or from the PHY to the upper layer and may have a variable length. The preamble may include a legacy preamble (L-preamble) and may include additional fields depending on the PPDU format. The legacy preamble may basically include a legacy-short training field (L-STF), a legacy-long training field (L-LTF), and a signal (SIG) field. The most basic non-HT (high throughput) PPDU frame format may consist of only a legacy preamble and PSDU (or data field). Additionally, depending on the type of PPDU frame format (e.g., HT-mixed format PPDU, HT-greenfield format PPDU, VHT (Very High Throughput) PPDU, etc.), additional (or different types of) elements may be added between the SIG field and the data field. STF, LTF, and SIG fields may also be included. In this case, the SIG field of the legacy preamble may be called an L-SIG (legacy-signal) field to distinguish it from the additional SIG field.

That is, the conventional time synchronization technique between a single AP and STA does not consider AP cooperation and therefore only considers TO between a single AP and STA. In the example of FIG. 3, the L-STF field and the L-LTF field are used for synchronization between a single AP and a single STA. It can be used for time synchronization between The legacy preamble includes an 8μs-long L-STF and an 8μs-long L-LTF, and the STA can accurately estimate the first starting point of the received frame using the correlation between the STA's time domain received signal and the legacy preamble.

However, the TO between APs cannot be estimated in an AP cooperation scenario using only the existing legacy preamble-based STA-AP time synchronization technique. Therefore, a new method for estimating TO between APs (i.e., first type TO) and TO between APs (i.e., second type TO) is required.

Below, the definitions of TO between STA and AP and TO between AP and AP in the AP cooperation situation according to this disclosure will first be described.

4 is a diagram showing a first type TO and a second type TO according to an embodiment of the present disclosure.

In the example of FIG. 4, TO is indicated when a slave AP (eg, AP2) transmits a downlink NDP frame based on a trigger of the master AP (eg, AP1) in an AP cooperation situation.

The NDP procedure (or NDP sounding procedure) is a procedure for the STA to feed back the beamformer calculation results and related information of the AP participating in data transmission. Since the conventional time synchronization technique does not consider the multi-AP transmission situation, the STA needs to find the starting point of the first received frame among the frames from multiple APs simultaneously received. Focus on making it 0. However, in reality, the TO between the subsequently received frame and the STA , and the TO information between AP1 and AP2 is difficult to obtain.

If the TO between APs increases, performance degradation may become severe in the STA's channel estimation procedure based on the field included in the additional preamble (e.g., EHT-preamble) following the legacy preamble.

Therefore, in the present disclosure, the STA uses the characteristics of a jointly stacked LTF structure to simultaneously acquire channels of multiple APs through NDP, and after conventional time synchronization in the legacy preamble, based on the EHT-LTF field. TO between the STA and each AP (e.g., and ), and TO between APs (e.g., ), and examples of estimating and compensating for this in the beamformer feedback step will be described.

Figure 5 is a diagram showing an example of training information to which the present disclosure can be applied.

In Figure 5, legacy preambles (e.g., L-STF, L-LTF, L-SIG) and EHT preambles (e.g., EHT-SIG/STF and EHT/LTF) that may be included in the preamble of the EHT PPDU format. represents. The scope of the present disclosure is not limited to the order and name of each field in the example of FIG. 5, and examples of the present disclosure may be applied to any frame containing training information corresponding to multiple antennas of multiple APs. . As such a frame, the EHT preamble of FIG. 5, particularly the frame including the EHT-LTF, will be described as an example.

The EHT-LTF structure can be formed by connecting LTF symbols greater than the sum of the number of all antennas of all APs. By expanding the structure of the existing single AP system in which an LTF symbol is assigned to each antenna, in this disclosure, for clarity of explanation, the number of LTF symbols included in the EHT-LTF is equal to the sum of the number of all antennas of all APs. Assume.

The LTF symbols corresponding to each antenna of each AP may have the same length in the time domain. As in the example of FIG. 5, an LTF symbol corresponding to each antenna of one AP may be sequentially assigned in the time domain, and an LTF symbol corresponding to each antenna of the next AP may be assigned. That is, the l-th LTF corresponding to the a -th antenna of the i-th AP in the EHT-LTF field may have the relationship l = (i-1)*N _t +a.

Accordingly, when S[k] is an EHT-LTF value assigned to the kth subcarrier in the frequency domain, the frequency domain signal Y _l [k] received from the STA during the lth LTF symbol duration is as shown in Equation 4. can be expressed.

In Equation 4, N _LTF represents the total number of LTF symbols included (or stacked) in the LTF field. For example, N _LTF =N _AP *N _t may be satisfied.

In addition, m is a spatial stream index, and is a replacement for the subscript (i,a), which indicates the a-th antenna of the i-th AP. Here, the relationship can be m=(i-1)*N _t +a.

T _CS ^m represents the cyclic shift value of the mth stream.

P is an orthogonal mapping matrix of size N _LTF *N _LTF , and P _{(m, l )} means the value of the mth column and lth row of P, and can have the value of -1 or 1. there is.

FIG. 6 is a diagram illustrating an orthogonal mapping matrix according to an example of the present disclosure.

As shown in Figure 6, P _{(m, l )} corresponds to the weight multiplied by S[k] of the l -th LTF duration for the m-th stream.

From the frequency domain received signal in Equation 4, based on the characteristic that the P matrix is orthogonal, the estimated channel of the channel corresponding to the kth subcarrier of the mth stream can be expressed as Equation 5.

TO (i.e., second type TO) and noise between APs can be mathematically expressed for the channel estimated through Equation 5.

First, in Equation 5 is the phase rotation by cyclic shift of the mth stream as in Equation 4 It includes, which can be offset with predefined/promised values for all APs and STAs.

Additionally, the stream index m in Equation 5 can be replaced again with m=(i-1)*N _t +a based on the AP and antenna index.

Additionally, based on STA, each AP Assuming that there are enough TOs (i.e., first type TO), the estimated channel corresponding to the kth subcarrier of the ath antenna of the ith AP can be expressed as Equation 6.

In Equation 6, W _(i,a) [k] corresponds to unwanted noise, and since it is the weighted sum of white Gaussian noise, it corresponds to white Gaussian noise.

The estimated channel calculated for each subcarrier is the TO between each AP and STA, as shown in Equation 6. (i.e., first type TO). In the case of a conventional single AP system, even if the exact TO between STA and AP cannot be estimated in the time synchronization procedure of the legacy preamble, even if the beamformer using the channel estimated based on the legacy preamble is applied if channel fading is not severe, Due to the presence of CP, performance degradation may not occur in the data decoding procedure. Meanwhile, in a multi-AP cooperation situation, unlike a single AP, the effective fading experienced by the STA increases due to TO between APs and APs, and accordingly, performance degradation may occur when fading longer than the CP length is experienced.

To solve this, it can be assumed that the actual channels for two adjacent subcarriers are similar ( ). This assumption is based on the characteristic that channels in the frequency domain become similar across all sections as fading decreases. Accordingly, the correlation of estimated channels between adjacent subcarriers can be derived as follows.

First, TO( ), the channel estimated from the two adjacent subcarriers of the a-th antenna of the i-th AP, and The correlation of Q _(i,a) [k] can be expressed as Equation 7.

In Equation 7, L _(i,a) means the number of nonzero channel taps of the channel corresponding to the a-th antenna of the i-th AP. It is assumed that different antennas of the same AP all have the same number of non-zero channel taps (i.e. )

Additionally, I _1(i,a) [k] represents the sum of different channel tap-related components in the time domain, and I _2(i,a) [k] represents the sum of the actual channel and non-channel noise in the frequency domain. It represents a component, which can be expressed as Equation 8.

In Equation 8, I _1(i,a) [k], h _(i,a) [ l ] and h _(i,a) [m] are independent of each other when l and m are not the same. Additionally, in the I _2(i,a) [k] equation, W _(i,a) [k] and H _(i,a) [k] are independent for different subcarrier indices k. Accordingly, the remaining subcarriers excluding the null subcarrier corresponding to each of the N _t antennas i For (where K is the set of subcarriers used for data transmission and reception), summing up all Q _(i,a) [k] gives I _{1 (i, a)} [k] and I ₂ for subcarrier index k. _(i,a) The sum of [k] all converges to 0, so the result of Equation 9 can be derived.

In equation 9: Group ingredients that are not related to It can be defined as: Here, A _i is a positive real number, Is It can be expressed as a real number. In this way, the bundle of components unrelated to TO can be affected by the number of channel taps and the instantaneous channel tap power. Specifically, has a value close to 0 in a slow fading situation, and can have a large value in a fast fading situation. As a result, an estimate for TO for the ith AP is obtained using the phase of the value derived from Equation 9. Can be expressed as Equation 10.

In Equation 10, ∠(x) represents the phase of x.

Hereinafter, the estimated value of TO (i.e., first type TO) with the STA for each AP, derived from the above-described process (i.e., ) Based on this, examples of estimating TO (i.e., second type TO) between APs will be described.

Assuming 2 APs, estimate of TO between AP1 and AP2 Can be expressed as Equation 11 based on the difference value of the estimated TO between AP1 and STA and the estimated TO between AP2 and STA.

In equation 10, Since it was confirmed that has a positive value and is affected by the number and power of instantaneous channel taps, in Equation 11 Is It has a range of , which can act as an error in the estimated TO between APs.

As described above, the STA may derive estimates of the first type TO between each AP and the STA and the second type TO between the APs based on the received training information (eg, EHT-LTF). Accordingly, the STA can transmit the estimated values of the first type TO and the second type TO to the AP as feedback information. For example, feedback information may be transmitted to each of a plurality of collaborating APs, or to one of the plurality of APs (eg, a master AP).

Accordingly, the AP may perform time synchronization between a plurality of APs based on feedback information from the STA (eg, estimated values of the first type TO and the second type TO).

The estimated value of the second type TO fed back in the above-described process (e.g., NDP sounding procedure) Corresponds to the relative TO of AP2 with respect to AP1. In order to obtain the beamformer fed back from the CBF (compressed beamforming) report transmitted from the STA to the AP, the STA applies the formula of Equation 11 to the channel estimated in Equation 6. can compensate.

Specifically, the TO-compensated estimated channel corresponding to the k-th subcarrier of the a-th antenna of the 2nd AP (i=2) can be expressed as Equation 12. Here, the estimated channel for calculating the beamformer of AP1 can use the estimated channel of Equation 6 as is.

If the number of APs is 3 or more, without loss of generality, the TO between APs is calculated based on the estimated channel of each AP in the same manner as above. After obtaining , Equation 12 can be used to derive an estimated channel in which the TO of the i=2, 3,...th AP has been compensated.

In this disclosure, the estimated channel-based beamformer is assumed to be a maximum ratio transmission (MRT) beamformer. In this case, V _(i,a) [k], which is the MRT beamformer corresponding to the k-th subcarrier of the a-th antenna of the i-th AP, can be expressed as Equation 13.

Figure 7 is a diagram showing an example of a feedback frame to which the present disclosure can be applied.

If the TO-compensated beamformer is calculated based on Equations 12 and 13, the beamformer transmission of each AP in the CBF reporting procedure can be performed in the same way as the existing CBF reporting procedure.

For example, the beamformer can be fed back using the format of the CBF action frame of FIG. 7. For example, the derived beamformer may be included and transmitted as quantized information in the CBF report frame.

FIG. 8 is a diagram illustrating a frame including feedback information according to an embodiment of the present disclosure.

As in the example of Figure 4, the TO that occurs between APs (i.e. ), in order to compensate for the data packet transmission situation performed later, the second type TO (TO) estimated in the STA ) may be provided to each AP.

For example, the estimated second type TO( ) can be added to the CBF reporting frame as shown in Figure 8 and transmitted to the AP. The added field may be named a TO report field, but the name does not limit the scope of the present disclosure. The TO report field can be implemented similarly to the MU exclusive beamforming report field, which is an extra field in the existing CBF action frame of FIG. 7, and therefore can be implemented within the scope of standard compliance.

Additionally, the feedback of the TO estimate value of the present disclosure may be transmitted to the AP through a frame of another format rather than a CBF reporting frame.

Figure 9 is a diagram showing a data packet transmission operation with AP time synchronization applied according to an embodiment of the present disclosure.

As mentioned above, the estimated TO fed back during the NDP process ( ) and each AP that receives the beamformer that compensates for this, determines the data transmission time in the subsequent data packet transmission procedure or before, as in the example of FIG. Multiple AP time synchronization can be performed by compensating/adjusting/aligning as much as possible. After AP time synchronization is applied, each AP can transmit data packets based on the beamformer fed back by the STA.

In this way, after performing time synchronization in which each AP corrects/adjusts/aligns the TO relative to the reference AP (e.g., master AP) in the time domain, when data packet transmission from multiple APs is performed, multiple Data packets simultaneously transmitted from APs are simultaneously received by the STA, and performance degradation due to TO between APs can be reduced in a situation where multiple APs cooperate and transmit at the same time.

FIG. 10 is a flowchart for explaining the operation of an AP and an STA according to an embodiment of the present disclosure.

In the example of FIG. 10, it is assumed that two APs perform cooperative transmission, but the same example can be extended and applied to cooperative transmission by three or more APs.

In step S1010, AP1 and AP2 may each transmit training information to the STA.

For example, training information may include a plurality of training symbols respectively corresponding to a plurality of antennas of a plurality of APs. For example, training information may correspond to the EHT-LTF field. Additionally, a frame containing training information may be an NDP frame.

Before step S1010, a specific AP (e.g., AP1, which is a master AP or a reference AP) transmits a trigger frame to another AP (e.g., a slave AP), and based on this, a plurality of APs simultaneously include training information. Frames may be transmitted.

If TO exists between APs, training information transmitted from each AP may not be simultaneously received by the STA.

In step S1020, the STA may perform channel estimation based on training information received from a plurality of APs.

For example, the signal transmitted from the a-th antenna of the i-th AP (see Equation 1) is received at the STA under the influence of channel and noise, and the received signal is divided into the time domain (see Equation 2) and the frequency domain (see Equation 2). (see Equation 3).

Based on this, when the STA receives a training field containing training symbols corresponding to all antennas of all APs, the signal received in the l -th training symbol can be expressed based on the P matrix for the training symbol (mathematics (see Equation 4), and based on this, the channel for the mth stream can be estimated (see Equation 5).

In step S1030, the STA may estimate the first type TO based on the estimated channel. The first type TO corresponds to the TO between each AP and STA.

For example, based on the estimated channel (see Equation 6) on each subcarrier assuming that there is a TO with the STA for each AP (i.e., first type TO), adjacent subcarriers Derive the correlation Q (see Equation 7) of the estimated channel for Noise-related components other than the channel (see Equation 8) converge to 0, and an estimated value of TO with the STA for each AP (see Equation 10) can be derived.

In step S1040, the STA may estimate the second type TO based on the first type TO for each AP. The second type TO corresponds to TO between APs.

For example, the STA may estimate the second type TO between AP1 and AP2 based on the first type TO for AP1 and the second type TO for AP2 (see Equation 11).

In step S1050, the STA may generate feedback information.

For example, the feedback information may include a beamformer (see Equation 13) based on the channel (see Equation 12) estimated for each AP based on the training information received in step S1010. Here, the estimated channel for each AP can be derived by compensating the estimated value of the first type TO, which is the TO for each AP, from the estimated value of the second type TO, which is the TO between APs.

In step S1060, the STA may transmit feedback information to each AP.

For example, feedback information 1 may include beamformer information derived based on the channel estimated for AP1, and feedback information 2 may include beamformer information derived based on the channel estimated for AP2. there is.

Additionally, each of feedback information 1 and feedback information 2 may further include an estimated value of a second type TO, which is an estimated TO between APs.

In step S1070, AP1 and AP2 may perform synchronization based on feedback information from the STA.

For example, time synchronization between APs may be performed based on an estimate of a second type TO, which is TO between APs.

AP1 and AP2, synchronized in step S1080, can cooperate to transmit data to the STA at the same time.

When the transmission timing is corrected/adjusted/aligned based on TO between APs, unlike step S1010, data transmitted from each AP in step S1080 may be simultaneously received by the STA.

Figure 11 is a diagram showing the configuration of an AP device and an STA device according to the present disclosure.

The AP device 1100 may include a processor 1110, an antenna unit 1120, a transceiver 1130, and a memory 1140.

The processor 1110 performs baseband-related signal processing and may include an upper layer processing unit 1111 and a physical layer processing unit 1115. The upper layer processing unit 1111 can process operations of the MAC layer or higher layers. The physical layer processing unit 1115 may process PHY layer operations (e.g., downlink transmission signal processing, uplink reception signal processing, etc.). In addition to performing baseband-related signal processing, the processor 1110 may also control the overall operation of the AP device 1100.

The antenna unit 1120 may include one or more physical antennas, and may support MIMO transmission and reception when it includes a plurality of antennas. Transceiver 1130 may include an RF transmitter and an RF receiver. The memory 1140 may store information processed by the processor 1110, software related to the operation of the AP device 1100, an operating system, applications, etc., and may also include components such as buffers.

The processor 1110 of the AP device 1100 may be configured to implement the operation of the i-th AP in the embodiments described in the present invention.

For example, the physical layer processing unit 1115 of the processor 1110 of the AP device 1100 may include a training information generating unit 1116 and a transmission timing adjusting unit 1117.

The training information generator 1116 may generate a frame containing training symbols corresponding to all antennas 1120 of the corresponding AP device 1100 and all antennas of other cooperative AP devices and transmit the frame to the transceiver 1130.

The transmission timing adjuster 1117 may correct/adjust/align the transmission timing based on the estimated value of the second type TO, which is the TO between APs fed back from the STA device 1150. For example, if the AP device 1100 is a master AP or a reference AP, transmission timing adjustment may not be performed, and if the AP device 1100 is a slave AP, the transmission timing may be adjusted based on the estimated value of the second type TO with the master/reference AP. Can be calibrated/adjusted/aligned.

The STA device 1150 may include a processor 1160, an antenna unit 1170, a transceiver 1180, and a memory 1190.

The processor 1160 performs baseband-related signal processing and may include an upper layer processing unit 1161 and a physical layer processing unit 1165. The upper layer processing unit 1161 can process the operations of the MAC layer or higher layers. The physical layer processing unit 1165 may process PHY layer operations (e.g., downlink transmission signal processing, uplink reception signal processing, etc.). In addition to performing baseband-related signal processing, the processor 1160 may also control the overall operation of the STA device 1150.

The antenna unit 1170 may include one or more physical antennas, and may support MIMO transmission and reception when it includes a plurality of antennas. Transceiver 1180 may include an RF transmitter and an RF receiver. The memory 1190 may store information processed by the processor 1160, software related to the operation of the STA device 1150, an operating system, applications, etc., and may also include components such as buffers.

The processor 1160 of the STA device 1150 may be configured to implement the operation of the STA in the embodiments described in the present invention.

For example, the physical layer processing unit 1165 of the processor 1160 of the STA device 1150 may include a channel estimation unit 1166, a TO estimation unit 1167, and a feedback information generation unit 1168.

The channel estimation unit 1166 may estimate the channel for the corresponding AP based on training information from each of the plurality of APs.

The TO estimation unit 1167 estimates a first type TO, which is a TO between each AP and an STA, based on the estimated channel, and estimates a second type TO, which is a TO between APs, based on the first type TO for a plurality of APs. can be estimated.

The feedback information generator 1168 may derive a beamformer for each AP based on the estimated channel for each AP derived based on the first type TO and the second type TO. The derived beamformer information and/or the second type TO estimate may be generated as feedback information and transmitted to the transceiver 1180.

In the operation of the AP device 1100 and the STA device 1150, the matters described for the AP and STA in the examples of the present invention can be applied in the same way, and overlapping descriptions will be omitted.

12 to 16 are diagrams showing simulation results related to the present disclosure.

The left and right graphs in FIG. 12 show the TO estimation probability between APs of the algorithm proposed by transmitting NDP in the TGn channel B and D models, respectively, according to the STA reception SNR performance for the estimated TO. In these results, the TO between AP 1 and AP 2 was assumed to be 350 ns (7 samples), and the actual TO between AP 1 and AP 2 was indicated on the graph using a red vertical line. Additionally, considering the estimated TO feedback overhead, the estimated TO was normalized to a sampling duration t _s = 50ns and then expressed as a rounded integer value.

From Figure 12, it can be seen that in the situation where the STA's reception SNR = 25dB, both channel models B and D estimate TO between APs within the error range, and it can also be seen that the error range decreases as the channel delay becomes shorter. Therefore, if the TO between APs is large, it can be expected that the delay in the effective channel experienced by the STA after the timing synchronization procedure can be reduced. In addition, as the received SNR at the STA level decreases, the probability of predicting an accurate TO decreases, but the estimation error range does not increase significantly, and it also shows a tendency to saturate with high SNR performance before the 15dB SNR situation, so the performance of the technique presented in this project is excellent. It can be confirmed that TO estimation is possible regardless of noise.

Figure 13 is a graph showing the frame error rate (FER) after estimating and compensating the TO between APs using the algorithm proposed in this project when the TO between APs is an integer multiple of the sampling period (multiple of 110 ns).

Figure 14 is a graph showing the throughput after estimating and compensating the TO between APs using the algorithm proposed in this project when the TO between APs is an integer multiple of the sampling period (multiple of 110 ns).

To derive the results shown in Figures 13 and 14, 1000 trials were repeated. Specifically, the [Proposed] method, indicated by a solid line on the graph, first performs the existing single AP-based coarse TO estimation in the L-preamble in the NDP procedure, and then performs the LTF of the EHT-preamble between each AP and STA presented in this study. The rounded value of the TO between the two APs estimated by applying the TO estimation technique and the compensated MRT beamformer to the beamformer of the AP that transmitted the estimated TO later among the two APs are simultaneously fed back, and then the data packet is sent. Sent again. At this time, it was assumed that the data transmitted by the two APs was the same, and the [w/o TO compensation] method indicated by the dotted line on the graph uses the estimated channel-based MRT beamformer obtained from the NDP procedure without using the TO estimation technique of this study. This is a case where two APs transmit the same data when used immediately. In the case where TO is not compensated overall, FER and throughput performance deteriorate as TO increases, while the technique in this study that compensates for TO shows relatively insignificant performance compared to the case where the compensation technique is not used when TO is small. Although there was damage, it was confirmed that performance did not occur regardless of the size of the TO.

Figures 15 and 16 show when the TO between APs is not an integer multiple of the sampling period (when TO is not a multiple of 50ns), the TO between APs is estimated using the algorithm proposed in this project and compensated in proportion to an integer multiple of the sampling period. This is a graph showing frame error rate (FER) and throughput. To derive the results, 1000 trials were repeated as shown in Figures 12 and 13. The [Proposed] and [w/o TO compensation] methods, indicated by solid and dotted lines on the graph, respectively, were tested in the same manner as in Figures 8 and 9.

Unlike the previous integer multiple feedback situation, in the case of FIGS. 15 and 16, there is a case where the STA-AP TO for the STA and one of the two APs is not an integer multiple, and therefore, when the STA samples the signal transmitted from the corresponding AP, the ISI It was confirmed that inter-symbol interference occurred and the FER and throughput performance deteriorated slightly compared to the integer TO situation. However, in this situation as well, similar to the integer multiple feedback situation, the proposed technique may have some performance loss at low TO, but shows robust performance as TO between APs increases. However, in the case where timing synchronization is not performed, TO increases and Accordingly, it was confirmed that the effective channel length experienced by STA exceeds the CP length and performance deteriorates rapidly. Therefore, if only the integer value of the TO between APs estimated through the technique of this study is fed back, not only can the STA's beamformer feedback procedure be fed back with less overhead, but it can also be confirmed that the resulting performance degradation is minimal.

Exemplary methods of the present disclosure are expressed as a series of operations for clarity of explanation, but this is not intended to limit the order in which the steps are performed, and each step may be performed simultaneously or in a different order, if necessary. In order to implement the method according to the present disclosure, other steps may be included in addition to the exemplified steps, some steps may be excluded and the remaining steps may be included, or some steps may be excluded and additional other steps may be included.

The various embodiments of the present disclosure do not list all possible combinations but are intended to explain representative aspects of the present disclosure, and matters described in the various embodiments may be applied independently or in combination of two or more.

Additionally, various embodiments of the present disclosure may be implemented by hardware, firmware, software, or a combination thereof. For hardware implementation, one or more ASICs (Application Specific Integrated Circuits), DSPs (Digital Signal Processors), DSPDs (Digital Signal Processing Devices), PLDs (Programmable Logic Devices), FPGAs (Field Programmable Gate Arrays), general purpose It can be implemented by a processor (general processor), controller, microcontroller, microprocessor, etc.

The scope of the present disclosure is software or machine-executable instructions (e.g., operating system, application, firmware, program, etc.) that cause operations according to the methods of various embodiments to be executed on a device or computer, and such software or It includes non-transitory computer-readable medium in which instructions, etc. are stored and can be executed on a device or computer.

Claims (19)

In a method of feeding back a time offset (TO) by a station (STA) in a wireless communication system, the method includes:
Receiving training information from each of a plurality of access points (APs), the plurality of APs including a first AP and a second AP;
Estimating a channel for each of the plurality of APs based on the training information and estimating a first type TO for each of the plurality of APs based on the estimated channel - the first type TO includes the STA and comprising a first type TO between the first AP, and a first type TO between the STA and the second AP;
Estimating a second type TO between the plurality of APs based on the estimate for the first type TO, wherein the second type TO includes a second type TO between the first AP and the second AP. ;
determining a beamformer for each of the plurality of APs based on the first type TO and the second type TO; and
A method comprising transmitting feedback information including a beamformer and the second type TO for each of the plurality of APs to each of the plurality of APs.
According to claim 1,
The method further comprising receiving a data packet from each of the plurality of APs to which time synchronization is applied based on a beamformer for each of the plurality of APs and the second type TO.
According to claim 1,
The training information includes a plurality of training symbols respectively corresponding to all antennas of the plurality of APs.
According to claim 3,
For a signal received in the l -th training symbol among the plurality of training symbols, a channel for the m-th stream is estimated based on an orthogonal mapping matrix P for the m-th stream and the l -th training symbol.
According to claim 4,
The frequency domain signal Y _l [k] received at the STA during the l th training symbol is expressed by the equation below:

Here, N _AP is the number of the plurality of APs,
N _t is the number of antennas for each of the plurality of APs,
m is the stream index, m=(i-1)*N _t +a, i is the index of the AP, a is the index of the antenna,
H _m [k] is the frequency domain channel for the mth stream on the kth subcarrier,
S _m [k] is the frequency domain received signal for the mth stream on the kth subcarrier,
P _{(m, l )} is the value of the mth column and lth row of the orthogonal mapping matrix P, and has a value of -1 or 1,
Z _l [k] is the frequency domain noise during the l th training symbol,
T _CS ^m is the cyclic shift value of the mth stream.
According to claim 5,
Estimated channel for the kth subcarrier of the mth stream is expressed by the equation below:

Here, N _LTF is the number of training symbols.
According to claim 6,
The step of estimating the first type TO is,
calculating an estimated channel on each subcarrier based on the assumption that the first type TO exists;
calculating correlation Q of estimated channels for adjacent subcarriers; and
and calculating an estimate of the first type TO based on the sum of the correlations Q for the entire subcarrier set K.
According to claim 7,
Estimated channel on the kth subcarrier for the ath antenna of the ith AP is expressed by the equation below:

here, is the first type TO for the ith AP,
W _(i,a) [k] is the unwanted noise component, method.
According to claim 8,
Estimated channel of kth subcarrier and the estimated channel at the k+1th subcarrier. The correlation between Q _(i,a) [k] is expressed by the equation below:

Here, L _(i,a) is the number of nonzero channel taps of the channel corresponding to the a-th antenna of the i-th AP,
I _1(i,a) [k] is the sum of different channel tap-related components in the time domain,
I _2(i,a) [k] is a component related to the actual channel and non-channel noise in the frequency domain.
According to clause 9,
The I _1(i,a) [k] and I _2(i,a) [k] are expressed by the equation below:

Here, h _(i,a) [ l ] and h _(i,a) [m] are independent of each other if l and m are not equal,
W _(i,a) [k] and H _(i,a) [k] are independent for different subcarrier indices k.
According to claim 10,
The sum of the correlation Q for the entire subcarrier set K is expressed by the equation below:

here, ego,
A _i has a positive real value,
Is A method with a real number value.
According to claim 11,
is an estimate of the first type TO for the i-th AP, is expressed by the equation below:

where ∠(x) is the phase of x, method.
According to claim 12,
The step of estimating the second type TO is,
Based on the estimated value of the first type TO for a reference AP among the plurality of APs and the estimated value of the first type TO for each of the remaining one or more APs among the plurality of APs, the estimated value of the second type TO is A method comprising the step of calculating.
According to claim 13,
The estimated value of the second type TO between the first AP and the second AP is is expressed by the equation below:

here, Is A method with a scope of .
According to claim 14,
The TO on the k-th subcarrier of the a-th antenna of the i-th AP is a compensated estimation channel. is expressed by the equation below:
, method.
According to claim 15,
V _(i,a) [k], the MRT (maximum ratio transmission) beamformer corresponding to the k-th subcarrier of the a-th antenna of the i-th AP, is expressed by the equation below,
, method.
In a method of performing time synchronization by an access point (AP) in a wireless communication system, the method includes:
Transmitting training information to a station (STA);
Receiving feedback information including a beamformer for the AP and a second type time offset (TO) between the AP and another AP from the STA; and
Transmitting a data packet to the STA based on synchronized timing between the AP and the other AP based on the feedback information,
The beamformer is estimated by the STA based on the first type TO and the second type TO between the AP and the STA.
In a station (STA) device that feeds back a time offset (TO) in a wireless communication system, the device:
transceiver;
Memory; and
Includes a processor,
The processor,
Receive training information from each of a plurality of access points (APs) through the transceiver, wherein the plurality of APs includes a first AP and a second AP;
Estimating a channel for each of the plurality of APs based on the training information, estimating a first type TO for each of the plurality of APs based on the estimated channel, and storing it in the memory - the first type TO is , including a first type TO between the STA and the first AP, and a first type TO between the STA and the second AP;
Based on the estimated value for the first type TO, a second type TO between the plurality of APs is estimated and stored in the memory, and the second type TO is a second type TO between the first AP and the second AP. Contains -;
determining a beamformer for each of the plurality of APs based on the first type TO and the second type TO and storing the beamformer in the memory; and
A device configured to transmit feedback information including a beamformer for each of the plurality of APs and the second type TO to each of the plurality of APs through the transceiver.
An access point (AP) device that performs time synchronization in a wireless communication system, the device comprising:
transceiver;
Memory; and
Includes a processor,
The processor,
transmitting training information to a station (STA) through the transceiver;
Receive feedback information including a beamformer for the AP and a second type time offset (TO) between the AP and another AP from the STA through the transceiver; and
is set to transmit a data packet to the STA through the transceiver based on the feedback information and synchronized timing between the AP and the other AP,
The beamformer is estimated by the STA based on the first type TO and the second type TO between the AP and the STA.

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