WO2016125999A1 - Method for uplink multi-user transmission in wireless communication system and device for same - Google Patents

Abstract

Disclosed are a method for uplink multi-user transmission in a wireless communication system and a device for same. More particularly, a method for performing uplink multi-user (UL MU) transmission by a station (STA) device in a wireless communication system comprises the steps of: receiving pilot allocation information from an access point (AP); mapping a pilot to a pilot tone on the basis of the pilot allocation information; and transmitting to the AP a UL MU physical protocol data unit (PPDU) to which the pilot has been mapped, wherein one or more of a pilot tone, a pilot symbol and a pilot sequence can be particularly decided for each STA or STA group by means of the pilot allocation information.

Description

Method and apparatus for uplink multi-user transmission in wireless communication system

The present invention relates to a wireless communication system, and more particularly, to a method for performing or supporting uplink multi-user transmission and an apparatus for supporting the same.

Wi-Fi is a Wireless Local Area Network (WLAN) technology that allows devices to access the Internet in the 2.4 GHz, 5 GHz, or 60 GHz frequency bands.

WLANs are based on the Institute of Electrical and Electronic Engineers (IEEE) 802.11 standard. The Wireless Next Generation Standing Committee (WNG SC) of IEEE 802.11 is an ad hoc committee that considers the next generation wireless local area network (WLAN) in the medium to long term.

IEEE 802.11n aims to increase the speed and reliability of networks and to extend the operating range of wireless networks. More specifically, IEEE 802.11n supports High Throughput (HT), which provides up to 600 Mbps data rate, and also supports both transmitter and receiver to minimize transmission errors and optimize data rates. It is based on Multiple Inputs and Multiple Outputs (MIMO) technology using multiple antennas.

As the popularity of WLAN and the applications diversify using it, the next generation WLAN system supporting Very High Throughput (VHT) is the next version of the IEEE 802.11n WLAN system. IEEE 802.11ac supports data processing speeds of 1 Gbps and higher via 80 MHz bandwidth transmission and / or higher bandwidth transmission (eg 160 MHz) and operates primarily in the 5 GHz band.

Recently, there is a need for a new WLAN system to support higher throughput than the data throughput supported by IEEE 802.11ac.

The scope of IEEE 802.11ax, often discussed in the next-generation WLAN task group, also known as IEEE 802.11ax or High Efficiency (HEW) WLAN, includes: 1) 802.11 physical layer and MAC in the 2.4 GHz and 5 GHz bands; (medium access control) layer enhancement, 2) spectral efficiency and area throughput improvement, 3) environments with interference sources, dense heterogeneous network environments, and high user loads. Such as improving performance in real indoor environments and outdoor environments, such as the environment.

Scenarios considered mainly in IEEE 802.11ax are dense environments with many access points (APs) and stations (STAs), and IEEE 802.11ax discusses spectral efficiency and area throughput improvement in such a situation. . In particular, there is an interest in improving the performance of the indoor environment as well as the outdoor environment, which is not much considered in the existing WLAN.

In IEEE 802.11ax, we are interested in scenarios such as wireless office, smart home, stadium, hotspot, and building / apartment. There is a discussion about improving system performance in dense environments with many STAs.

In the future, IEEE 802.11ax improves system performance in outdoor basic service set (OBSS) environment, outdoor environment performance, and cellular offloading rather than single link performance in one basic service set (BSS). Discussion is expected to be active. The directionality of IEEE 802.11ax means that next-generation WLANs will increasingly have a technology range similar to that of mobile communication. Considering the situation where mobile communication and WLAN technology are recently discussed in the small cell and direct-to-direct communication area, the technical and business of next-generation WLAN and mobile communication based on IEEE 802.11ax Convergence is expected to become more active.

The next generation WLAN system considers support for uplink multi-user multiple input multiple output (MIMO) transmission. However, in this case, there is a limit in measuring a carrier frequency offset (CFO) for each user by applying an existing pilot method.

Accordingly, an object of the present invention is to propose a pilot frequency / time resource or pilot sequence allocation method for uplink multi-user transmission.

It is also an object of the present invention to propose a method for estimating a carrier frequency offset (CFO) for each user in uplink multi-user transmission.

The technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned above will be clearly understood by those skilled in the art from the following description. Could be.

An aspect of the present invention provides a method for performing uplink (UL) uplink (UL) multi-user (MU) multi-user (MU) transmission in a wireless communication system, wherein the pilot allocation information is obtained from an access point (AP). Receiving a pilot, mapping a pilot to a pilot tone based on the pilot allocation information, and transmitting a UL MU Physical Protocol Data Unit (PPDU) to which the pilot is mapped, to the AP; One or more of a pilot tone, a pilot symbol, and a pilot sequence may be specified for each STA or STA group.

According to another aspect of the present invention, in a STA (Station) device in which a device performs uplink (UL) multi-user (MU) transmission in a wireless communication system, RF (Radio) for transmitting and receiving a radio signal A processor for controlling a frequency unit and the RF unit, the processor receiving pilot allocation information from an access point (AP), mapping a pilot to a pilot tone based on the pilot allocation information, and mapping the pilot The UL MU Physical Protocol Data Unit (PPDU) is configured to be transmitted to the AP, and one or more of a pilot tone, a pilot symbol, and a pilot sequence may be specified for each of one or more STAs by the pilot assignment information.

According to another aspect of the present invention, a method for supporting an uplink (UL) multi-user (MU) transmission in an access point (AP) device in a wireless communication system includes: Transmitting pilot allocation information to an STA; and receiving a UL MU Physical Protocol Data Unit (PPUDU) in which a pilot is mapped to a pilot tone based on the pilot allocation information from the STA, and by the pilot allocation information One or more of a pilot tone, a pilot symbol, and a pilot sequence may be specified for one or more STAs.

According to another aspect of the present invention, in an access point (AP) device supporting uplink (UL) multi-user (MU) transmission in a wireless communication system, RF (Radio) for transmitting and receiving a radio signal Frequency) unit and a processor for controlling the RF unit, the processor transmits pilot allocation information to the STA participating in the UL MU transmission, the pilot is mapped to a pilot tone based on the pilot allocation information from the STA It is configured to receive the UL MU Physical Protocol Data Unit (PPDU), and one or more of a pilot tone, a pilot symbol, and a pilot sequence may be specified for each of one or more STAs by the pilot allocation information.

Preferably, the pilot tone may be randomly assigned to the STA or the STA group, or the same number may be allocated, or may be allocated to the differential number according to RSSI (Received Signal Strength Indication).

Preferably, a pilot tone may be re-assigned randomly or by a cyclic shift for each STA or STA group at a predetermined symbol period.

Preferably, the cyclic shift value may be determined randomly or according to a predetermined value or channel correlation between adjacent tones.

Preferably, when a specific pilot sequence is allocated for each STA or STA group, the pilot may be mapped to an adjacent pilot tone.

Preferably, a pilot tone may be re-assigned randomly or by a cyclic shift for each STA or STA group at a predetermined symbol period.

Preferably, the cyclic shift value may be determined randomly or according to a predetermined value or channel correlation between adjacent tones.

Preferably, when a specific pilot tone is allocated for each STA group, a specific pilot sequence may be allocated for each STA in the STA group.

Preferably, pilot tones allocated between the STA groups may be exchanged and used in a predetermined symbol period.

Preferably, the pilot symbols may be randomly assigned to the STA or the STA group, or may be allocated the same number or may be allocated to the differential number according to the received signal strength indication (RSSI).

According to an embodiment of the present invention, uplink multi-user transmission can be smoothly performed in a wireless communication system.

In addition, according to an embodiment of the present invention, the carrier frequency offset (CFO) estimation performance may be improved for each user by specifically assigning a pilot frequency / time resource or a pilot sequence to each user or user group.

The effects obtainable in the present invention are not limited to the above-mentioned effects, and other effects not mentioned will be clearly understood by those skilled in the art from the following description. .

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, included as part of the detailed description in order to provide a thorough understanding of the present invention, provide embodiments of the present invention and together with the description, describe the technical features of the present invention.

1 is a diagram illustrating an example of an IEEE 802.11 system to which the present invention can be applied.

2 is a diagram illustrating a structure of a layer architecture of an IEEE 802.11 system to which the present invention may be applied.

3 illustrates a non-HT format PPDU and a HT format PPDU of a wireless communication system to which the present invention can be applied.

4 illustrates a VHT format PPDU format of a wireless communication system to which the present invention can be applied.

5 is a diagram illustrating a constellation for distinguishing a format of a PPDU of a wireless communication system to which the present invention can be applied.

6 illustrates a MAC frame format of an IEEE 802.11 system to which the present invention can be applied.

7 illustrates the HT format of the HT Control field in a wireless communication system to which the present invention can be applied.

8 illustrates the VHT format of the HT Control field in a wireless communication system to which the present invention can be applied.

FIG. 9 is a diagram illustrating a general link setup procedure in a wireless communication system to which the present invention can be applied.

FIG. 10 is a diagram illustrating an arbitrary backoff period and a frame transmission procedure in a wireless communication system to which the present invention can be applied.

11 is a diagram illustrating a VHT-LTF in a wireless communication system to which the present invention can be applied.

12 through 15 are diagrams illustrating a High Efficiency (HE) format PPDU according to an embodiment of the present invention.

16 illustrates a method of generating a HE-LTF sequence according to an embodiment of the present invention.

17 illustrates a method of constructing a HE-LTF field according to an embodiment of the present invention.

18 is a diagram illustrating a CFO estimation method according to an embodiment of the present invention.

19 illustrates a method of allocating a pilot tone according to an embodiment of the present invention.

20 is a diagram illustrating an uplink multi-user transmission method according to an embodiment of the present invention.

21 is a block diagram illustrating a wireless device according to an embodiment of the present invention.

22 is a diagram illustrating in more detail the processor of FIG. 25 in accordance with an embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The detailed description, which will be given below with reference to the accompanying drawings, is intended to explain exemplary embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. The following detailed description includes specific details in order to provide a thorough understanding of the present invention. However, one of ordinary skill in the art appreciates that the present invention may be practiced without these specific details.

In some instances, well-known structures and devices may be omitted or shown in block diagram form centering on the core functions of the structures and devices in order to avoid obscuring the concepts of the present invention.

Specific terms used in the following description are provided to help the understanding of the present invention, and the use of such specific terms may be changed to other forms without departing from the technical spirit of the present invention.

The following techniques are code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), and NOMA It can be used in various radio access systems such as non-orthogonal multiple access. CDMA may be implemented by a radio technology such as universal terrestrial radio access (UTRA) or CDMA2000. TDMA may be implemented with wireless technologies such as global system for mobile communications (GSM) / general packet radio service (GPRS) / enhanced data rates for GSM evolution (EDGE). OFDMA may be implemented with wireless technologies such as Institute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), and the like. UTRA is part of a universal mobile telecommunications system (UMTS). 3rd generation partnership project (3GPP) long term evolution (LTE) is a part of evolved UMTS (E-UMTS) using E-UTRA, and employs OFDMA in downlink and SC-FDMA in uplink. LTE-A (advanced) is the evolution of 3GPP LTE.

Embodiments of the present invention may be supported by standard documents disclosed in at least one of the wireless access systems IEEE 802, 3GPP and 3GPP2. That is, steps or parts which are not described to clearly reveal the technical spirit of the present invention among the embodiments of the present invention may be supported by the above documents. In addition, all terms disclosed in the present document can be described by the above standard document.

For clarity, the following description focuses on IEEE 802.11 systems, but the technical features of the present invention are not limited thereto.

System general

1 is a diagram illustrating an example of an IEEE 802.11 system to which the present invention can be applied.

The IEEE 802.11 structure may be composed of a plurality of components, and a wireless communication system supporting a station (STA) station mobility that is transparent to a higher layer may be provided by their interaction. . A basic service set (BSS) may correspond to a basic building block in an IEEE 802.11 system.

In FIG. 1, there are three BSSs (BSS 1 to BSS 3) and two STAs are included as members of each BSS (STA 1 and STA 2 are included in BSS 1, and STA 3 and STA 4 are BSS 2. Included in, and STA 5 and STA 6 are included in BSS 3) by way of example.

In FIG. 1, an ellipse representing a BSS may be understood to represent a coverage area where STAs included in the BSS maintain communication. This area may be referred to as a basic service area (BSA). When the STA moves out of the BSA, the STA cannot directly communicate with other STAs in the BSA.

The most basic type of BSS in an IEEE 802.11 system is an independent BSS (IBSS). For example, the IBSS may have a minimal form consisting of only two STAs. In addition, BSS 3 of FIG. 1, which is the simplest form and other components are omitted, may correspond to a representative example of the IBSS. This configuration is possible when STAs can communicate directly. In addition, this type of LAN may not be configured in advance, but may be configured when a LAN is required, which may be referred to as an ad-hoc network.

The membership of the STA in the BSS may be dynamically changed by turning the STA on or off, the STA entering or exiting the BSS region, or the like. In order to become a member of the BSS, the STA may join the BSS using a synchronization process. In order to access all services of the BSS infrastructure, the STA must be associated with the BSS. This association may be set up dynamically and may include the use of a Distribution System Service (DSS).

The direct STA-to-STA distance in an 802.11 system may be limited by physical layer (PHY) performance. In some cases, this distance limit may be sufficient, but in some cases, communication between STAs over longer distances may be required. A distribution system (DS) may be configured to support extended coverage.

DS refers to a structure in which BSSs are interconnected. Specifically, instead of the BSS independently as shown in FIG. 1, the BSS may exist as an extended type component of a network composed of a plurality of BSSs.

DS is a logical concept and can be specified by the characteristics of the Distribution System Medium (DSM). In this regard, the IEEE 802.11 standard logically distinguishes between wireless medium (WM) and distribution system medium (DSM). Each logical medium is used for a different purpose and is used by different components. The definition of the IEEE 802.11 standard does not limit these media to the same or to different ones. In this way, the plurality of media are logically different, and thus the flexibility of the structure of the IEEE 802.11 system (DS structure or other network structure) can be described. That is, the IEEE 802.11 system structure can be implemented in various ways, the corresponding system structure can be specified independently by the physical characteristics of each implementation.

The DS may support mobile devices by providing seamless integration of multiple BSSs and providing logical services for handling addresses to destinations.

The AP means an entity that enables access to the DS through the WM to the associated STAs and has STA functionality. Data movement between the BSS and the DS may be performed through the AP. For example, STA 2 and STA 3 illustrated in FIG. 1 have a functionality of STA, and provide a function of allowing associated STAs STA 1 and STA 4 to access the DS. In addition, since all APs basically correspond to STAs, all APs are addressable entities. The address used by the AP for communication on the WM and the address used by the AP for communication on the DSM need not necessarily be the same.

Data transmitted from one of the STAs associated with an AP to the STA address of that AP may always be received at an uncontrolled port and processed by an IEEE 802.1X port access entity. In addition, when a controlled port is authenticated, transmission data (or frame) may be transmitted to the DS.

A wireless network of arbitrary size and complexity may be composed of DS and BSSs. In an IEEE 802.11 system, this type of network is referred to as an extended service set (ESS) network. The ESS may correspond to a set of BSSs connected to one DS. However, the ESS does not include a DS. The ESS network is characterized by what appears to be an IBSS network at the Logical Link Control (LLC) layer. STAs included in the ESS may communicate with each other, and mobile STAs may move from one BSS to another BSS (within the same ESS) transparently to the LLC.

In the IEEE 802.11 system, nothing is assumed about the relative physical location of the BSSs in FIG. 1, and all of the following forms are possible.

Specifically, BSSs can be partially overlapped, which is the form generally used to provide continuous coverage. Also, the BSSs may not be physically connected, and logically there is no limit to the distance between the BSSs. In addition, the BSSs can be located at the same physical location, which can be used to provide redundancy. In addition, one (or more) IBSS or ESS networks may be physically present in the same space as one or more ESS networks. This may be necessary if the ad-hoc network is operating at the location of the ESS network, if the IEEE 802.11 networks are physically overlapped by different organizations, or if two or more different access and security policies are required at the same location. It may correspond to an ESS network type in a case.

In a WLAN system, an STA is a device that operates according to Medium Access Control (MAC) / PHY regulations of IEEE 802.11. As long as the function of the STA is not distinguished from the AP individually, the STA may include an AP STA and a non-AP STA. However, when communication is performed between the STA and the AP, the STA may be understood as a non-AP STA. In the example of FIG. 1, STA 1, STA 4, STA 5, and STA 6 correspond to non-AP STAs, and STA 2 and STA 3 correspond to AP STAs.

Non-AP STAs generally correspond to devices that users directly handle, such as laptop computers and mobile phones. In the following description, a non-AP STA includes a wireless device, a terminal, a user equipment (UE), a mobile station (MS), a mobile terminal, and a wireless terminal. ), A wireless transmit / receive unit (WTRU), a network interface device (network interface device), a machine-type communication (MTC) device, a machine-to-machine (M2M) device, or the like.

In addition, the AP is a base station (BS), Node-B (Node-B), evolved Node-B (eNB), and Base Transceiver System (BTS) in other wireless communication fields. , A concept corresponding to a femto base station (Femto BS).

Hereinafter, in the present specification, downlink (DL) means communication from the AP to the non-AP STA, and uplink (UL) means communication from the non-AP STA to the AP. In downlink, the transmitter may be part of an AP and the receiver may be part of a non-AP STA. In uplink, a transmitter may be part of a non-AP STA and a receiver may be part of an AP.

2 is a diagram illustrating a structure of a layer architecture of an IEEE 802.11 system to which the present invention may be applied.

2, a layer architecture of an IEEE 802.11 system may include a MAC (Medium Access Control) sublayer / layer and a PHY sublayer / layer.

The PHY may be classified into a Physical Layer Convergence Procedure (PLCP) entity and a Physical Medium Dependent (PMD) entity. In this case, the PLCP entity plays a role of connecting a MAC and a data frame, and the PMD entity plays a role of wirelessly transmitting and receiving data with two or more STAs.

Both the MAC and the PHY may include a management entity, and may be referred to as a MAC sublayer management entity (MLME) and a PHY sublayer management entity (PLME), respectively. These management entities provide layer management service interfaces through the operation of layer management functions. The MLME may be connected to the PLME to perform management operations of the MAC, and likewise, the PLME may be connected to the MLME to perform management operations of the PHY.

In order to provide correct MAC operation, a Station Management Entity (SME) may be present in each STA. The SME is a management entity independent of each layer. The SME collects layer-based state information from MLME and PLME or sets values of specific parameters of each layer. The SME can perform these functions on behalf of general system management entities and implement standard management protocols.

MLME, PLME and SME can interact in a variety of ways based on primitives. Specifically, the XX-GET.request primitive is used to request the value of a Management Information Base attribute (MIB attribute), and the XX-GET.confirm primitive, if the status is 'SUCCESS', returns the value of that MIB attribute. Otherwise, it returns with an error indication in the status field. The XX-SET.request primitive is used to request that a specified MIB attribute be set to a given value. If the MIB attribute is meant for a particular action, this request requests the execution of that particular action. And, if the state is 'SUCCESS' XX-SET.confirm primitive, it means that the specified MIB attribute is set to the requested value. In other cases, the status field indicates an error condition. If this MIB attribute means a specific operation, this primitive can confirm that the operation was performed.

The PHY provides an interface to the MAC through TXVECTOR, RXVECTOR, and PHYCONFIG_VECTOR. TXVECTOR supports PPDU specific transmission parameters to PHY. Using RXVECTOR, the PHY informs the MAC of the PPDU parameter received. TXVECTOR is delivered from the MAC to the PHY via the PHY-TXSTART.request primitive, and RXVECTOR is delivered from the PHY to the MAC via the PHY-RXSTART.indication primitive.

Using PHYCONFIG_VECTOR, the MAC configures the PHY's behavior regardless of frame transmission or reception.

The operation in each sub-layer (or layer) will be briefly described as follows.

MAC is a MAC header and a frame check sequence (FCS) in a MAC Service Data Unit (MSDU) or a fragment of an MSDU received from an upper layer (eg, LLC). Attach to create one or more MAC Protocol Data Units (MPDUs). The generated MPDU is delivered to the PHY.

When an aggregated MSDU (A-MSDU) scheme is used, a plurality of MSDUs may be merged into a single A-MSDU (aggregated MSDU). The MSDU merging operation may be performed at the MAC upper layer. The A-MSDU is delivered to the PHY as a single MPDU (if it is not fragmented).

The PHY generates a physical protocol data unit (PPDU) by adding an additional field including information required by a physical layer transceiver to a physical service data unit (PSDU) received from the MAC. PPDUs are transmitted over wireless media.

The PSDU is substantially the same as the MPDU because the PSD is received by the PHY from the MAC and the MPDU is sent by the MAC to the PHY.

When an aggregated MPDU (A-MPDU) scheme is used, a plurality of MPDUs (where each MPDU may carry an A-MSDU) may be merged into a single A-MPDU. The MPDU merging operation may be performed at the MAC lower layer. A-MPDUs may be merged with various types of MPDUs (eg, QoS data, Acknowledge (ACK), Block ACK (BlockAck), etc.). The PHY receives the A-MPDU as a single PSDU to the MAC. That is, the PSDU is composed of a plurality of MPDUs. Thus, A-MPDUs are transmitted over the wireless medium in a single PPDU.

PPDU Physical Protocol Data Unit Format

Physical Protocol Data Unit (PPDU) refers to a block of data generated at the physical layer. Hereinafter, a PPDU format will be described based on an IEEE 802.11 WLAN system to which the present invention can be applied.

3 illustrates a non-HT format PPDU and a HT format PPDU of a wireless communication system to which the present invention can be applied.

3A illustrates a non-HT format PPDU for supporting an IEEE 802.11a / g system. Non-HT PPDUs may also be referred to as legacy PPDUs.

Referring to (a) of FIG. 3, the non-HT format PPDU includes an L-STF (Legacy (or Non-HT) Short Training field), L-LTF (Legacy (or, Non-HT) Long Training field) and It consists of a legacy format preamble and a data field composed of L-SIG (Legacy (or Non-HT) SIGNAL) field.

The L-STF may include a short training orthogonal frequency division multiplexing symbol. L-STF can be used for frame timing acquisition, automatic gain control (AGC), diversity detection, and coarse frequency / time synchronization. .

The L-LTF may include a long training orthogonal frequency division multiplexing symbol. L-LTF may be used for fine frequency / time synchronization and channel estimation.

The L-SIG field may be used to transmit control information for demodulation and decoding of the data field. The L-SIG field may include information about a data rate and a data length.

3B illustrates an HT-mixed format PPDU (HTDU) for supporting both an IEEE 802.11n system and an IEEE 802.11a / g system.

Referring to FIG. 3B, the HT mixed format PPDU includes a legacy format preamble including an L-STF, L-LTF, and L-SIG fields, an HT-SIG (HT-Signal) field, and an HT-STF (HT Short). Training field), HT-formatted preamble and data field including HT-LTF (HT Long Training field).

Since the L-STF, L-LTF, and L-SIG fields mean legacy fields for backward compatibility, they are the same as non-HT formats from L-STF to L-SIG fields. Even if the L-STA receives the HT mixed PPDU, the L-STA may interpret the data field through the L-LTF, L-LTF and L-SIG fields. However, the L-LTF may further include information for channel estimation that the HT-STA performs to receive the HT mixed PPDU and demodulate the L-SIG field and the HT-SIG field.

The HT-STA may know that it is an HT-mixed format PPDU using the HT-SIG field following the legacy field, and may decode the data field based on the HT-STA.

The HT-LTF field may be used for channel estimation for demodulation of the data field. Since IEEE 802.11n supports Single-User Multi-Input and Multi-Output (SU-MIMO), a plurality of HT-LTF fields may be configured for channel estimation for each data field transmitted in a plurality of spatial streams.

The HT-LTF field includes data HT-LTF used for channel estimation for spatial streams and extension HT-LTF (additional used for full channel sounding). It can be configured as. Accordingly, the plurality of HT-LTFs may be equal to or greater than the number of spatial streams transmitted.

In the HT-mixed format PPDU, the L-STF, L-LTF, and L-SIG fields are transmitted first in order to receive the L-STA and acquire data. Thereafter, the HT-SIG field is transmitted for demodulation and decoding of data transmitted for the HT-STA.

The HT-SIG field is transmitted without performing beamforming so that the L-STA and HT-STA can receive the corresponding PPDU to acquire data, and then the HT-STF, HT-LTF and data fields transmitted are precoded. Wireless signal transmission is performed through. In this case, the HT-STF field is transmitted to allow the STA to perform precoding to take into account the variable power due to precoding, and then the plurality of HT-LTF and data fields after that.

3 (c) illustrates an HT-GF format PPDU (HT-GF) for supporting only an IEEE 802.11n system.

Referring to FIG. 3C, the HT-GF format PPDU includes HT-GF-STF, HT-LTF1, HT-SIG field, a plurality of HT-LTF2 and data fields.

HT-GF-STF is used for frame timing acquisition and AGC.

HT-LTF1 is used for channel estimation.

The HT-SIG field is used for demodulation and decoding of the data field.

HT-LTF2 is used for channel estimation for demodulation of data fields. Similarly, since HT-STA uses SU-MIMO, channel estimation is required for each data field transmitted in a plurality of spatial streams, and thus HT-LTF2 may be configured in plural.

The plurality of HT-LTF2 may be configured of a plurality of Data HT-LTF and a plurality of extended HT-LTF similarly to the HT-LTF field of the HT mixed PPDU.

In (a) to (c) of FIG. 3, the data field is a payload, and includes a service field, a SERVICE field, a scrambled PSDU field, tail bits, and padding bits. It may include. All bits of the data field are scrambled.

3D illustrates a service field included in a data field. The service field has 16 bits. Each bit is assigned from 0 to 15, and transmitted sequentially from bit 0. Bits 0 to 6 are set to 0 and used to synchronize the descrambler in the receiver.

The IEEE 802.11ac WLAN system supports downlink multi-user multiple input multiple output (MU-MIMO) transmission in which a plurality of STAs simultaneously access a channel in order to efficiently use a wireless channel. According to the MU-MIMO transmission scheme, the AP may simultaneously transmit packets to one or more STAs that are paired with MIMO.

DL MU transmission (downlink multi-user transmission) refers to a technology in which an AP transmits a PPDU to a plurality of non-AP STAs through the same time resource through one or more antennas.

Hereinafter, the MU PPDU refers to a PPDU that delivers one or more PSDUs for one or more STAs using MU-MIMO technology or OFDMA technology. The SU PPDU means a PPDU having a format in which only one PSDU can be delivered or in which no PSDU exists.

The size of control information transmitted to the STA may be relatively large compared to the size of 802.11n control information for MU-MIMO transmission. An example of control information additionally required for MU-MIMO support includes information indicating the number of spatial streams received by each STA, information related to modulation and coding of data transmitted to each STA, and the like. Can be.

Therefore, when MU-MIMO transmission is performed to simultaneously provide data services to a plurality of STAs, the size of transmitted control information may be increased according to the number of receiving STAs.

In order to efficiently transmit the increased size of the control information, a plurality of control information required for MU-MIMO transmission is required separately for common control information common to all STAs and specific STAs. The data may be transmitted by being divided into two types of information of dedicated control information.

4 illustrates a VHT format PPDU format of a wireless communication system to which the present invention can be applied.

4 illustrates a VHT format PPDU (VHT format PPDU) for supporting an IEEE 802.11ac system.

Referring to FIG. 4, the VHT format PPDU includes a legacy format preamble consisting of L-STF, L-LTF, and L-SIG fields, a VHT-SIG-A (VHT-Signal-A) field, and VHT-STF (VHT Short Training). field), VHT Long Training field (VHT-LTF) and VHT-SIG-B (VHT-Signal-B) field.

Since L-STF, L-LTF, and L-SIG mean legacy fields for backward compatibility, they are the same as non-HT formats from L-STF to L-SIG fields. However, the L-LTF may further include information for channel estimation to be performed to demodulate the L-SIG field and the VHT-SIG-A field.

The L-STF, L-LTF, L-SIG field, and VHT-SIG-A field may be repeatedly transmitted in 20 MHz channel units. For example, when a PPDU is transmitted on four 20 MHz channels (i.e., 80 MHz bandwidth), the L-STF, L-LTF, L-SIG field, and VHT-SIG-A field are repeatedly transmitted on every 20 MHz channel. Can be.

The VHT-STA may know that it is a VHT format PPDU using the VHT-SIG-A field following the legacy field, and may decode the data field based on the VHT-STA.

In the VHT format PPDU, the L-STF, L-LTF and L-SIG fields are transmitted first in order to receive the L-STA and acquire data. Thereafter, the VHT-SIG-A field is transmitted for demodulation and decoding of data transmitted for the VHT-STA.

The VHT-SIG-A field is a field for transmitting control information common to the AP and the MIMO paired VHT STAs, and includes control information for interpreting the received VHT format PPDU.

The VHT-SIG-A field may include a VHT-SIG-A1 field and a VHT-SIG-A2 field.

The VHT-SIG-A1 field includes information on channel bandwidth (BW) used, whether space time block coding (STBC) is applied, and group identification information for indicating a group of STAs grouped in MU-MIMO. Group ID (Group Identifier), information about the number of space-time streams (NSTS) / Partial AID (Partial Association Identifier) and Transmit power save forbidden information. can do. Here, the Group ID means an identifier assigned to the STA group to be transmitted to support MU-MIMO transmission, and may indicate whether the currently used MIMO transmission method is MU-MIMO or SU-MIMO.

Table 1 is a table illustrating the VHT-SIG-A1 field.

The VHT-SIG-A2 field contains information on whether a short guard interval (GI) is used, forward error correction (FEC) information, information on modulation and coding scheme (MCS) for a single user, and multiple information. Information on the type of channel coding for the user, beamforming-related information, redundancy bits for cyclic redundancy checking (CRC), tail bits of convolutional decoder, and the like. Can be.

Table 2 is a table illustrating the VHT-SIG-A2 field.

VHT-STF is used to improve the performance of AGC estimation in MIMO transmission. The VHT-STF field duration is 4 ms.

VHT-LTF is used by the VHT-STA to estimate the MIMO channel. Since the VHT WLAN system supports MU-MIMO, the VHT-LTF may be set as many as the number of spatial streams in which a PPDU is transmitted. In addition, if full channel sounding is supported, the number of VHT-LTFs may be greater.

The VHT-SIG-B field includes dedicated control information required for a plurality of MU-MIMO paired VHT-STAs to receive a PPDU and acquire data. Therefore, the VHT-STA may be designed to decode the VHT-SIG-B field only when the common control information included in the VHT-SIG-A field indicates the MU-MIMO transmission currently received. . On the other hand, if the common control information indicates that the currently received PPDU is for a single VHT-STA (including SU-MIMO), the STA may be designed not to decode the VHT-SIG-B field.

The VHT-SIG-B field includes information on modulation, encoding, and rate-matching of each VHT-STA. The size of the VHT-SIG-B field may vary depending on the type of MIMO transmission (MU-MIMO or SU-MIMO) and the channel bandwidth used for PPDU transmission.

In order to transmit a PPDU of the same size to STAs paired to an AP in a system supporting MU-MIMO, information indicating a bit size of a data field constituting the PPDU and / or indicating a bit stream size constituting a specific field May be included in the VHT-SIG-A field.

However, the L-SIG field may be used to effectively use the PPDU format. In order to transmit the same size PPDU to all STAs, a length field and a rate field included in the L-SIG field and transmitted may be used to provide necessary information. In this case, since the MAC Protocol Data Unit (MPDU) and / or Aggregate MAC Protocol Data Unit (A-MPDU) are set based on the bytes (or octets) of the MAC layer, additional padding is applied at the physical layer. May be required.

In FIG. 4, the data field is a payload and may include a service field, a scrambled PSDU, tail bits, and padding bits.

Since the formats of various PPDUs are mixed and used as described above, the STA must be able to distinguish the formats of the received PPDUs.

Here, the meaning of distinguishing a PPDU (or meaning of distinguishing a PPDU format) may have various meanings. For example, the meaning of identifying the PPDU may include determining whether the received PPDU is a PPDU that can be decoded (or interpreted) by the STA. In addition, the meaning of distinguishing the PPDU may mean determining whether the received PPDU is a PPDU supported by the STA. In addition, the meaning of distinguishing the PPDU may also be interpreted to mean what information is transmitted through the received PPDU.

This will be described in more detail with reference to the drawings below.

5 is a diagram illustrating a constellation for distinguishing a format of a PPDU of a wireless communication system to which the present invention can be applied.

FIG. 5A illustrates a constellation of an L-SIG field included in a non-HT format PPDU, and FIG. 5B illustrates a phase rotation for HT mixed format PPDU detection. 5C illustrates phase rotation for VHT format PPDU detection.

Constellation of OFDM symbols transmitted after the L-SIG field and the L-SIG field in order for the STA to classify non-HT format PPDUs, HT-GF format PPDUs, HT mixed format PPDUs, and VHT format PPDUs. Phase is used. That is, the STA may distinguish the PPDU format based on the phase of the constellation of the OFDM symbol transmitted after the L-SIG field and / or the L-SIG field of the received PPDU.

Referring to FIG. 5A, binary phase shift keying (BPSK) is used for an OFDM symbol constituting an L-SIG field.

First, in order to distinguish the HT-GF format PPDU, when the first SIG field is detected in the received PPDU, the STA determines whether it is an L-SIG field. That is, the STA attempts to decode based on the constellation as illustrated in (a) of FIG. 5. If the STA fails to decode, it may be determined that the corresponding PPDU is an HT-GF format PPDU.

Next, in order to classify non-HT format PPDUs, HT mixed format PPDUs, and VHT format PPDUs, the phase of the constellation of OFDM symbols transmitted after the L-SIG field may be used. That is, the modulation method of OFDM symbols transmitted after the L-SIG field may be different, and the STA may distinguish the PPDU format based on the modulation method for the field after the L-SIG field of the received PPDU.

Referring to FIG. 5B, in order to distinguish the HT mixed format PPDU, the phase of two OFDM symbols transmitted after the L-SIG field in the HT mixed format PPDU may be used.

More specifically, the phases of OFDM symbol # 1 and OFDM symbol # 2 corresponding to the HT-SIG field transmitted after the L-SIG field in the HT mixed format PPDU are rotated by 90 degrees in the counterclockwise direction. That is, quadrature binary phase shift keying (QBPSK) is used as a modulation method for OFDM symbol # 1 and OFDM symbol # 2. The QBPSK constellation may be a constellation rotated by 90 degrees in a counterclockwise direction based on the BPSK constellation.

The STA attempts to decode the first OFDM symbol and the second OFDM symbol corresponding to the HT-SIG field transmitted after the L-SIG field of the received PPDU based on the properties as shown in FIG. If the STA succeeds in decoding, it is determined that the corresponding PPDU is an HT format PPDU.

Next, in order to distinguish between the non-HT format PPDU and the VHT format PPDU, the phase of the constellation of the OFDM symbol transmitted after the L-SIG field may be used.

Referring to FIG. 5C, in order to classify the VHT format PPDU, the phase of two OFDM symbols transmitted after the L-SIG field in the VHT format PPDU may be used.

More specifically, the phase of the OFDM symbol # 1 corresponding to the VHT-SIG-A field after the L-SIG field in the VHT format PPDU is not rotated, but the phase of the OFDM symbol # 2 is rotated by 90 degrees counterclockwise. . That is, BPSK is used for the modulation method for OFDM symbol # 1 and QBPSK is used for the modulation method for OFDM symbol # 2.

The STA attempts to decode the first OFDM symbol and the second OFDM symbol corresponding to the VHT-SIG field transmitted after the L-SIG field of the received PPDU based on the properties as shown in the example of FIG. If the STA succeeds in decoding, it may be determined that the corresponding PPDU is a VHT format PPDU.

On the other hand, if decoding fails, the STA may determine that the corresponding PPDU is a non-HT format PPDU.

MAC frame format

6 illustrates a MAC frame format of an IEEE 802.11 system to which the present invention can be applied.

Referring to FIG. 6, a MAC frame (ie, an MPDU) includes a MAC header, a frame body, and a frame check sequence (FCS).

MAC Header includes Frame Control field, Duration / ID field, Address 1 field, Address 2 field, Address 3 field, Sequence control It is defined as an area including a Control field, an Address 4 field, a QoS Control field, and an HT Control field.

The Frame Control field includes information on the MAC frame characteristic. A detailed description of the Frame Control field will be given later.

The Duration / ID field may be implemented to have different values depending on the type and subtype of the corresponding MAC frame.

If the type and subtype of the corresponding MAC frame is a PS-Poll frame for power save (PS) operation, the Duration / ID field is an AID (association identifier) of the STA that transmitted the frame. It may be set to include. Otherwise, the Duration / ID field may be set to have a specific duration value according to the type and subtype of the corresponding MAC frame. In addition, when the frame is an MPDU included in an A-MPDU format, the Duration / ID fields included in the MAC header may be set to have the same value.

The Address 1 to Address 4 fields include a BSSID, a source address (SA), a destination address (DA), a transmission address (TA) indicating a transmission STA address, and a reception address indicating a destination STA address (TA). RA: It is used to indicate Receiving Address.

Meanwhile, the address field implemented as a TA field may be set to a bandwidth signaling TA value, in which case, the TA field may indicate that the corresponding MAC frame contains additional information in the scrambling sequence. The bandwidth signaling TA may be represented by the MAC address of the STA transmitting the corresponding MAC frame, but the Individual / Group bit included in the MAC address may be set to a specific value (for example, '1'). Can be.

The Sequence Control field is set to include a sequence number and a fragment number. The sequence number may indicate a sequence number allocated to the corresponding MAC frame. The fragment number may indicate the number of each fragment of the corresponding MAC frame.

The QoS Control field contains information related to QoS. The QoS Control field may be included when indicating a QoS data frame in a subtype subfield.

The HT Control field includes control information related to the HT and / or VHT transmission / reception schemes. The HT Control field is included in the Control Wrapper frame. In addition, it exists in the QoS data frame and the management frame in which the order subfield value is 1.

The frame body is defined as a MAC payload, and data to be transmitted in a higher layer is located, and has a variable size. For example, the maximum MPDU size may be 11454 octets, and the maximum PPDU size may be 5.484 ms.

FCS is defined as a MAC footer and is used for error detection of MAC frames.

The first three fields (Frame Control field, Duration / ID field and Address 1 field) and the last field (FCS field) constitute the minimum frame format and are present in every frame. Other fields may exist only in a specific frame type.

7 illustrates the HT format of the HT Control field in a wireless communication system to which the present invention can be applied.

Referring to FIG. 7, the HT Control field includes a VHT subfield, an HT Control Middle subfield, an AC Constraint subfield, and a Reverse Direction Grant (RDG) / More PPDU (More PPDU). It may consist of subfields.

The VHT subfield indicates whether the HT Control field has the format of the HT Control field for the VHT (VHT = 1) or has the format of the HT Control field for the HT (VHT = 0). In FIG. 7, it is assumed that the HT Control field (that is, VHT = 0) for HT.

The HT Control Middle subfield may be implemented to have a different format according to the indication of the VHT subfield. A more detailed description of the HT Control Middle subfield will be given later.

The AC Constraint subfield indicates whether a mapped AC (Access Category) of a reverse direction (RD) data frame is limited to a single AC.

The RDG / More PPDU subfield may be interpreted differently depending on whether the corresponding field is transmitted by the RD initiator or the RD responder.

When transmitted by the RD initiator, the RDG / More PPDU field is set to '1' if the RDG exists, and set to '0' if the RDG does not exist. When transmitted by the RD responder, it is set to '1' if the PPDU including the corresponding subfield is the last frame transmitted by the RD responder, and set to '0' when another PPDU is transmitted.

The HT Control Middle subfield of the HT Control field for the HT includes a link adaptation subfield, a calibration position subfield, a calibration sequence subfield, a reserved subfield, and channel state information. And / or (CSI / Steering: Channel State Information / Steering) subfield, HT NDP Announcement (HT NDP Announcement) subfield, and Reserved subfield.

The Link Adaptation subfield is a training request (TRQ) subfield, an MCS request or antenna selection indication (MAI: MCS (Modulation and Coding Scheme) Request or ASEL (Antenna Selection) Indication) subfield, and an MCS feedback sequence indication (MFSI). MCS Feedback and Antenna Selection Command / data (MFB / ASELC) subfields.

The TRQ subfield is set to 1 when the responder requests sounding PPDU transmission and is set to 0 when the responder does not request sounding PPDU transmission.

If the MAI subfield is set to 14, this indicates an ASEL indication, and the MFB / ASELC subfield is interpreted as an antenna selection command / data. Otherwise, the MAI subfield indicates an MCS request and the MFB / ASELC subfield is interpreted as MCS feedback.

When the MAI subfield indicates an MCS Request (MRQ: MCS Request), it is interpreted that the MAI subfield is composed of an MRQ (MCS request) and an MSI (MRQ sequence identifier). The MRQ subfield is set to '1' if MCS feedback is requested and set to '0' if MCS feedback is not requested. When the MRQ subfield is '1', the MSI subfield includes a sequence number for specifying an MCS feedback request. When the MRQ subfield is '0', the MSI subfield is set to a reserved bit.

Each of the above-described subfields corresponds to an example of subfields that may be included in the HT control field, and may be replaced with another subfield or may further include additional subfields.

8 illustrates the VHT format of the HT Control field in a wireless communication system to which the present invention can be applied.

Referring to FIG. 8, the HT Control field includes a VHT subfield, an HT Control Middle subfield, an AC Constraint subfield, and a Reverse Direction Grant (RDG) / More PPDU (More PPDU). It may consist of subfields.

In FIG. 8, it is assumed that the HT Control field (that is, VHT = 1) for VHT. The HT Control field for the VHT may be referred to as a VHT Control field.

Since the description of the AC Constraint subfield and the RDG / More PPDU subfield is the same as the description of FIG. 7, the description thereof is omitted.

As described above, the HT Control Middle subfield may be implemented to have a different format according to the indication of the VHT subfield.

The HT Control Middle subfield of the HT Control field for VHT includes a reserved bit, a Modulation and Coding Scheme feedback request (MRQ) subfield, and an MRQ Sequence Identifier (MSI). Space-time block coding (STBC) subfield, MCS feedback sequence identifier (MFSI) / Least Significant Bit (LSB) of Group ID (LSB) subfield, MCS Feedback (MFB) Subfield, Group ID Most Significant Bit (MSB) of Group ID (MSB) Subfield, Coding Type Subfield, Feedback Transmission Type (FB Tx Type: Feedback transmission type) subfield and a voluntary MFB (Unsolicited MFB) subfield.

Table 3 shows a description of each subfield included in the HT Control Middle subfield of the VHT format.

In addition, the MFB subfield may include a VHT number of space time streams (NUM_STS) subfield, a VHT-MCS subfield, a bandwidth (BW) subfield, and a signal to noise ratio (SNR). It may include subfields.

The NUM_STS subfield indicates the number of recommended spatial streams. The VHT-MCS subfield indicates a recommended MCS. The BW subfield indicates bandwidth information related to the recommended MCS. The SNR subfield indicates the average SNR value on the data subcarrier and spatial stream.

Information included in each field described above may follow the definition of the IEEE 802.11 system. In addition, each field described above corresponds to an example of fields that may be included in the MAC frame, but is not limited thereto. That is, each field described above may be replaced with another field or additional fields may be further included, and all fields may not be necessarily included.

link set up  Procedure (Link Setup Procedure)

FIG. 9 is a diagram illustrating a general link setup procedure in a wireless communication system to which the present invention can be applied.

In order for an STA to set up a link and transmit / receive data with respect to a network, a STA must first undergo a scanning procedure, an authentication procedure, an association procedure, and the like for discovering the network. The link setup procedure may also be referred to as session initiation procedure and session setup procedure. In addition, the linking procedure may be collectively referred to as the scanning, authentication, and association procedure of the link setup procedure.

In the WLAN, a scanning procedure includes a passive scanning procedure and an active scanning procedure.

FIG. 9 (a) illustrates a link setup procedure according to passive scanning, and FIG. 9 (b) illustrates a link setup procedure according to active scanning.

As shown in FIG. 9A, the passive scanning procedure is performed through a beacon frame broadcasted periodically by the AP. A beacon frame is one of management frames in IEEE 802.11, which informs the existence of a wireless network and periodically (eg, allows a non-AP STA that performs scanning to find a wireless network and participate in the wireless network). , 100msec intervals). The beacon frame contains information about the current network (for example, information about the BSS).

In order to obtain information about the network, the non-AP STA passively moves channels and waits for reception of a beacon frame. The non-AP STA that receives the beacon frame may store information about a network included in the received beacon frame, move to the next channel, and perform scanning on the next channel in the same manner. The non-AP STA receives the beacon frame to obtain information about the network, thereby completing the scanning procedure on the corresponding channel.

As described above, the passive scanning procedure has the advantage that the overall overhead is small since the procedure is completed only when the non-AP STA receives the beacon frame without having to transmit another frame. However, there is a disadvantage in that the scanning execution time of the non-AP STA increases in proportion to the transmission period of the beacon frame.

On the other hand, the active scanning procedure as shown in Figure 9 (b) by the non-AP STA to broadcast the probe request frame (probe request frame) by actively moving the channel to discover what AP exists in the vicinity, Request network information from the AP.

In response to receiving the probe request frame, the responder waits for a random time in order to prevent frame collision, and transmits network information in a probe response frame to the corresponding non-AP STA. Upon receiving the probe response frame, the STA may store network related information included in the received probe response frame and move to the next channel to perform scanning in the same manner. The scanning procedure is completed by the non-AP STA receiving the probe response frame to obtain network information.

The active scanning procedure has an advantage that scanning can be completed in a relatively quick time compared to the passive scanning procedure. However, since an additional frame sequence is required, the overall network overhead is increased.

After completing the scanning procedure, the non-AP STA selects a network according to its own criteria and performs an authentication procedure with the corresponding AP.

The authentication procedure is a process in which a non-AP STA transmits an authentication request frame to the AP, and in response, the AP transmits an authentication response frame to the non-AP STA, that is, 2-way. This is done by handshaking.

An authentication frame used for authentication request / response corresponds to a management frame.

The authentication frame includes an authentication algorithm number, an authentication transaction sequence number, a status code, a challenge text, a Robust Security Network, and a finite cyclic group. Group) and the like. This corresponds to some examples of information that may be included in the authentication request / response frame, and may be replaced with other information or further include additional information.

The non-AP STA may transmit an authentication request frame to the AP. The AP may determine whether to allow authentication for the non-AP STA based on the information included in the received authentication request frame. The AP may provide a result of the authentication process to the non-AP STA through an authentication response frame.

Through the authentication procedure, the non-AP STA and the AP authenticate each other and then establish an association.

The association process is a process in which a non-AP STA transmits an association request frame to an AP, and in response, the AP transmits an association response frame to a non-AP STA, that is, 2-way. This is done by handshaking.

The association request frame includes information related to various capabilities of the non-AP STA, beacon listening interval, service set identifier (SSID), supported rates, supported channels, RSN, and mobility. It may include information on domain, supported operating classes, TIM Broadcast Indication Map Broadcast request, interworking service capability, and the like.

Based on this, the AP determines whether support for the corresponding non-AP STA is possible. After the determination, the AP transmits information on whether to accept the association request, the reason for the association request, and capability information that can be supported in the association response frame to the non-AP STA.

Association response frames include information related to various capabilities, status codes, association IDs (AIDs), support rates, Enhanced Distributed Channel Access (EDCA) parameter sets, Received Channel Power Indicators (RCPI), Received Signal to Noise Indicators (RSNI), mobility Information such as a domain, a timeout interval (association comeback time), an overlapping BSS scan parameter, a TIM broadcast response, and a Quality of Service (QoS) map may be included.

The information that may be included in the aforementioned association request / response frame corresponds to an example, may be replaced with other information, or may further include additional information.

If the non-AP STA successfully establishes an association with the AP, normal transmission / reception is performed. On the other hand, if the association is not successfully established with the AP, based on the reason, the non-AP STA may attempt to reassociate or attempt to associate with another AP.

Media access mechanism

In IEEE 802.11, communication is fundamentally different from the wired channel environment because the communication takes place over a shared wireless medium.

In a wired channel environment, communication is possible based on carrier sense multiple access / collision detection (CSMA / CD). For example, once a signal is transmitted from the transmitter, the channel environment does not change so much that the receiver does not experience significant signal attenuation. At this time, if two or more signals collided, detection was possible. This is because the power sensed by the receiver is instantaneously greater than the power transmitted by the transmitter. However, in a wireless channel environment, a variety of factors (e.g., large attenuation of the signal depending on distance, or instantaneous deep fading) can affect the channel so that the signal actually transmits properly at the receiving end. Whether this has occurred or a collision has occurred, the transmitter cannot accurately perform carrier sensing.

Accordingly, in a WLAN system according to IEEE 802.11, a carrier sense multiple access with collision avoidance (CSMA / CA) mechanism is introduced as a basic access mechanism of a MAC. The CAMA / CA mechanism is also called the Distributed Coordination Function (DCF) of the IEEE 802.11 MAC, and basically employs a "listen before talk" access mechanism. According to this type of access mechanism, the AP and / or STA may sense a radio channel or medium during a predetermined time interval (eg, DCF Inter-Frame Space (DIFS)) prior to starting transmission. Perform Clear Channel Assessment (CCA) sensing. As a result of sensing, if it is determined that the medium is in an idle state, the frame transmission is started through the medium. On the other hand, if the medium is detected to be occupied status, the AP and / or STA does not start its own transmission and assumes that several STAs are already waiting to use the medium. The frame transmission may be attempted after waiting longer for a delay time (eg, a random backoff period) for access.

By applying a random backoff period, assuming that there are several STAs for transmitting a frame, the STAs are expected to have different backoff period values, so that they will wait for different times before attempting frame transmission. This can minimize collisions.

In addition, the IEEE 802.11 MAC protocol provides a hybrid coordination function (HCF). HCF is based on the DCF and Point Coordination Function (PCF). The PCF refers to a polling-based synchronous access scheme in which polling is performed periodically so that all receiving APs and / or STAs can receive data frames. In addition, the HCF has an Enhanced Distributed Channel Access (EDCA) and an HCF Controlled Channel Access (HCCA). EDCA is a competition-based approach for providers to provide data frames to a large number of users, and HCCA is a non-competition-based channel access scheme using a polling mechanism. In addition, the HCF includes a media access mechanism for improving the quality of service (QoS) of the WLAN, and can transmit QoS data in both a contention period (CP) and a contention free period (CFP).

FIG. 10 is a diagram illustrating an arbitrary backoff period and a frame transmission procedure in a wireless communication system to which the present invention can be applied.

When a certain medium changes from occupy or busy to idle, several STAs may attempt to transmit data (or frames). At this time, as a method for minimizing the collision, STAs may select a random backoff count and wait for a slot time corresponding thereto to attempt transmission. The random backoff count has a pseudo-random integer value and may be determined as one of values uniformly distributed in the range of 0 to a contention window (CW). Where CW is a contention window parameter value. The CW parameter is given CW_min as an initial value, but may take a double value when transmission fails (eg, when an ACK for a transmitted frame is not received). If the CW parameter value is CW_max, data transmission can be attempted while maintaining the CW_max value until the data transmission is successful. If the CW parameter value is successful, the CW parameter value is reset to the CW_min value. The CW, CW_min and CW_max values are preferably set to (2 ^ n) -1 (n = 0, 1, 2, ...).

When the random backoff process begins, the STA counts down the backoff slot according to the determined backoff count value and continuously monitors the medium during the countdown. If the media is monitored as occupied, the countdown stops and waits, and when the media is idle the countdown resumes.

In the example of FIG. 10, when a packet to be transmitted to the MAC of the STA 3 arrives, the STA 3 may confirm that the medium is idle as much as DIFS and transmit the frame immediately.

Meanwhile, the remaining STAs monitor and wait for the medium to be busy. In the meantime, data may be transmitted in each of STA 1, STA 2, and STA 5, and each STA waits for DIFS when the medium is monitored in an idle state, and then backoff slots according to a random backoff count value selected by each STA. Counts down.

In the example of FIG. 10, STA 2 selects the smallest backoff count value and STA 1 selects the largest backoff count value. That is, at the time when STA 2 finishes the backoff count and starts frame transmission, the remaining backoff time of STA 5 is shorter than the remaining backoff time of STA 1.

STA 1 and STA 5 stop counting and wait while STA 2 occupies the medium. When the media occupancy of the STA 2 ends and the medium becomes idle again, the STA 1 and the STA 5 resume the stopped backoff count after waiting for DIFS. That is, the frame transmission can be started after counting down the remaining backoff slots by the remaining backoff time. Since the remaining backoff time of STA 5 is shorter than that of STA 1, frame transmission of STA 5 is started.

Meanwhile, while STA 2 occupies the medium, data to be transmitted may also occur in STA 4. At this time, when the medium is in the idle state, the STA 4 waits for DIFS and then counts down the backoff slot according to the random backoff count value selected by the STA.

In the example of FIG. 10, the remaining backoff time of STA 5 coincides with an arbitrary backoff count value of STA 4, and in this case, a collision may occur between STA 4 and STA 5. If a collision occurs, neither STA 4 nor STA 5 receive an ACK, and thus data transmission fails. In this case, STA4 and STA5 select a random backoff count value after doubling the CW value and perform countdown of the backoff slot.

Meanwhile, the STA 1 may wait while the medium is occupied due to the transmission of the STA 4 and the STA 5, wait for DIFS when the medium is idle, and then start frame transmission after the remaining backoff time passes.

The CSMA / CA mechanism also includes virtual carrier sensing in addition to physical carrier sensing in which the AP and / or STA directly sense the medium.

Virtual carrier sensing is intended to compensate for problems that may occur in media access, such as a hidden node problem. For virtual carrier sensing, the MAC of the WLAN system uses a Network Allocation Vector (NAV). The NAV is a value that indicates to the other AP and / or STA how long the AP and / or STA currently using or authorized to use the medium remain until the medium becomes available. Therefore, the value set to NAV corresponds to a period in which the medium is scheduled to be used by the AP and / or STA transmitting the frame, and the STA receiving the NAV value is prohibited from accessing the medium during the period. For example, the NAV may be set according to a value of a duration field of the MAC header of the frame.

The AP and / or STA may perform a procedure of exchanging a request to send (RTS) frame and a clear to send (CTS) frame to indicate that the AP and / or STA want to access the medium. The RTS frame and the CTS frame include information indicating a time interval in which a wireless medium required for transmission and reception of an ACK frame is reserved when substantial data frame transmission and acknowledgment (ACK) are supported. The other STA that receives the RTS frame transmitted from the AP and / or the STA to which the frame is to be transmitted or receives the CTS frame transmitted from the STA to which the frame is to be transmitted during the time period indicated by the information included in the RTS / CTS frame Can be set to not access the medium. This may be implemented by setting the NAV during the time interval.

residual carrier  Frequency carrier measurement and channel estimation

In an 802.11ac system, a pilot is used to measure a residual carrier frequency offset (CFO). Specifically, the remaining CFO is measured by allocating the same pilot to the same tone (or subcarrier) for each symbol and setting the same coefficient to measure the phase difference between pilot symbols.

In addition, channel estimation is performed in the VHT-LTF, and multiplies the P matrix to create orthogonality between symbols and streams of the same subcarrier and measure the same. The P matrix means a matrix having orthogonality for each row constituting the P matrix.

As described above, the VHT-LTF may be set as many as the space-time stream in which the PPDU is transmitted.

Table 4 illustrates the number of VHT-LTF fields.

Equations 1 to 4 below illustrate P matrix values.

Referring to Equation 1, the P_4 × 4 matrix (Equation 2) is used when the total number of space-time streams (N_STS, total) in the PPDU is 4 or less, and the P_6 × 6 matrix (Equation 3) when 5 or 6 is used. In the case of 7 or 8, P_8 × 8 (Equation 4) is used.

11 is a diagram illustrating a VHT-LTF in a wireless communication system to which the present invention can be applied.

11 illustrates the case of four space-time streams.

In the P matrix illustrated in Equations 2 to 4, each row corresponds to each space-time stream, and as shown in FIG. 11, matrix elements are multiplied by VHT-LTF as shown in FIG. 11.

Referring to FIG. 11, four VHT-LTFs of space-time stream 1 are multiplied by respective matrix elements belonging to the first row of Equation 2 above. The first VHT-LTF sequence element is multiplied by 1, the second VHT-LTF sequence element is multiplied by -1, the third VHT-LTF sequence element is multiplied by 1, and the fourth VHT-LTF sequence element by 1 Multiply.

For other space-time streams, the P matrix is multiplied as above.

Method for Uplink Multi-user Transmission

New frames for next-generation WLAN systems, 802.11ax systems, with increasing attention from vendors in various fields for next-generation WiFi and increased demand for high throughput and quality of experience (QoE) after 802.11ac. There is an active discussion of format and numerology.

IEEE 802.11ax is a next-generation WLAN system that supports higher data rates and handles higher user loads. One of the recently proposed WLAN systems is known as high efficiency WLAN (HEW: High). Called Efficiency WLAN).

The IEEE 802.11ax WLAN system may operate in the 2.4 GHz frequency band and the 5 GHz frequency band like the existing WLAN system. It can also operate at higher 60 GHz frequency bands.

In IEEE 802.11ax system, the existing IEEE 802.11 OFDM system (IEEE 802.11a, 802.11n) is used for outdoor throughput transmission for average throughput enhancement and inter-symbol interference in outdoor environment. , 4x larger FFT size for each bandwidth than 802.11ac. This will be described with reference to the drawings below.

Hereinafter, in the description of the HE format PPDU in the present invention, the description of the non-HT format PPDU, the HT-mixed format PPDU, the HT-greenfield format PPDU, and / or the VHT format PPDU described above will be described in HE format unless otherwise noted. May be incorporated into the description of the PPDU.

12 illustrates a High Efficiency (HE) format PPDU according to an embodiment of the present invention.

Referring to FIG. 12, a HE format PPDU for HEW may be largely composed of a legacy part (L-part) and an HE part (HE-part).

The L-part is composed of an L-STF field, an L-LTF field, and an L-SIG field in the same manner as the conventional WLAN system maintains. The L-STF field, L-LTF field, and L-SIG field may be referred to as a legacy preamble.

The HE-part is a part newly defined for the 802.11ax standard, and may include a HE-SIG field, a HE preamble, and a HE-data field. The HE-preamble may include a HE-STF field and a HE-LTF field. In addition, not only the HE-STF field and the HE-LTF field but also the HE-SIG field may be collectively referred to as HE-preamble.

12 illustrates the order of the HE-SIG field, the HE-STF field, and the HE-LTF field, but may be configured in a different order.

The L-part, the HE-SIG field, and the HE-preamble may be collectively referred to as a physical preamble (PHY).

The HE-SIG field may include information (eg, OFDMA, UL MU MIMO, enhanced MCS, etc.) for decoding the HE-data field.

L-part and HE-part (particularly, HE-preamble and HE-data) may have different fast fourier transform (FFT) sizes, and may use different cyclic prefixes (CP). That is, the L-part and the HE-part (particularly, the HE-preamble and the HE-data) may have different subcarrier frequency spacings.

The 802.11ax system can use four times larger (4x) FFT size than legacy WLAN systems. That is, the L-part may be configured with a 1x symbol structure, and the HE-part (particularly, HE-preamble and HE-data) may be configured with a 4x symbol structure. Here, 1x, 2x, 4x size FFT refers to the size relative to the legacy WLAN system (eg, IEEE 802.11a, 802.11n, 802.11ac, etc.).

For example, if the FFT size used for the L-part is 64, 128, 256, and 512 at 20 MHz, 40 MHz, 80 MHz, and 160 MHz, respectively, the FFT size used for the HE-part is 256 at 20 MHz, 40 MHz, 80 MHz, and 160 MHz, respectively. , 512, 1024, 2048.

As the FFT size is larger than that of the legacy WLAN system, the number of subcarriers per unit frequency is increased because the subcarrier frequency spacing is smaller, but the OFDM symbol length is longer.

That is, the use of a larger FFT size means that the subcarrier spacing becomes narrower, and similarly, an Inverse Discrete Fourier Transform (IDFT) / Discrete Fourier Transform (DFT) period is increased. Here, the IDFT / DFT period may mean a symbol length excluding the guard period (GI) in the OFDM symbol.

Therefore, if the HE-part (particularly HE-preamble and HE-data) is used with an FFT size four times larger than the L-part, the subcarrier spacing of the HE-part is 1/4 of the subcarrier spacing of the L-part. The ID-FT / DFT period of the HE-part is four times the IDFT / DFT period of the L-part. For example, if the subcarrier spacing of L-part is 312.5kHz (= 20MHz / 64, 40MHZ / 128, 80MHz / 256 and / or 160MHz / 512), the subcarrier spacing of HE-part is 78.125kHz (= 20MHz / 256 , 40MHZ / 512, 80MHz / 1024 and / or 160MHz / 2048). Also, if the IDFT / DFT period of the L-part is 3.2 ms (= 1 / 312.5 kHz), the IDFT / DFT period of the HE-part may be 12.8 ms (= 1 / 78.125 kHz).

Here, the GI can be one of 0.8 ㎲, 1.6 ㎲, 3.2 ㎲, so the OFDM symbol length (or symbol interval) of the HE-part including the GI is 13.6 ㎲, 14.4 ㎲, 16 according to the GI. It can be

Although FIG. 12 illustrates a case in which the HE-SIG field has a 1x symbol structure, the HE-SIG field may also have a 4x symbol structure like HE-preamble and HE-data.

Unlike the example of FIG. 12, the HE-SIG may be divided into an HE-SIG A field and an HE-SIG B field. In this case, the FFT size per unit frequency may be larger after HE-SIG B. That is, after the HE-SIG B, the OFDM symbol length may be longer than the L-part.

The HE format PPDU for the WLAN system to which the present invention can be applied may be transmitted through at least one 20 MHz channel. For example, the HE format PPDU may be transmitted in a 40 MHz, 80 MHz, or 160 MHz frequency band through a total of four 20 MHz channels. This will be described in more detail with reference to the drawings below.

13 is a diagram illustrating an HE format PPDU according to an embodiment of the present invention.

FIG. 13 illustrates a PPDU format when 80 MHz is allocated to one STA (or OFDMA resource units are allocated to a plurality of STAs within 80 MHz) or when different streams of 80 MHz are allocated to a plurality of STAs.

Referring to FIG. 13, L-STF, L-LTF, and L-SIG may be transmitted as OFDM symbols generated based on 64 FFT points (or 64 subcarriers) in each 20MHz channel.

The HE-SIG A field may include common control information transmitted in common to STAs receiving a PPDU. The HE-SIG A field may be transmitted in one to three OFDM symbols. The HE-SIG A field is copied in units of 20 MHz and includes the same information. In addition, the HE-SIG-A field informs the total bandwidth information of the system.

Table 5 is a table illustrating information included in the HE-SIG A field.

Information included in each field illustrated in Table 5 may follow the definition of the IEEE 802.11 system. In addition, each field described above corresponds to an example of fields that may be included in the PPDU, but is not limited thereto. That is, each field described above may be replaced with another field or additional fields may be further included, and all fields may not be necessarily included.

HE-STF is used to improve the performance of AGC estimation in MIMO transmission.

The HE-SIG B field may include user-specific information required for each STA to receive its own data (eg, PSDU). The HE-SIG B field may be transmitted in one or two OFDM symbols. For example, the HE-SIG B field may include information on the modulation and coding scheme (MCS) of the corresponding PSDU and the length of the corresponding PSDU.

The L-STF, L-LTF, L-SIG, and HE-SIG A fields may be repeatedly transmitted in units of 20 MHz channels. For example, when a PPDU is transmitted on four 20 MHz channels (ie, an 80 MHz band), the L-STF, L-LTF, L-SIG, and HE-SIG A fields may be repeatedly transmitted on every 20 MHz channel. .

As the FFT size increases, legacy STAs supporting legacy IEEE 802.11a / g / n / ac may not be able to decode the HE PPDU. In order for the legacy STA and the HE STA to coexist, the L-STF, L-LTF, and L-SIG fields are transmitted through a 64 FFT on a 20 MHz channel so that the legacy STA can receive them. For example, the L-SIG field may occupy one OFDM symbol, one OFDM symbol time is 4 ms, and a GI may be 0.8 ms.

The FFT size for each frequency unit can be made larger from the HE-STF. For example, 256 FFTs may be used in a 20 MHz channel, 512 FFTs may be used in a 40 MHz channel, and 1024 FFTs may be used in an 80 MHz channel. As the FFT size increases, the number of OFDM subcarriers per unit frequency increases because the interval between OFDM subcarriers becomes smaller, but the OFDM symbol time becomes longer. In order to improve the efficiency of the system, the length of the GI after the HE-STF may be set equal to the length of the GI of the HE-SIG A.

The HE-SIG A field may include information required for the HE STA to decode the HE PPDU. However, the HE-SIG A field may be transmitted through a 64 FFT in a 20 MHz channel so that both the legacy STA and the HE STA can receive it. This is because the HE STA can receive not only the HE format PPDU but also the existing HT / VHT format PPDU, and the legacy STA and the HE STA must distinguish between the HT / VHT format PPDU and the HE format PPDU.

14 is a diagram illustrating an HE format PPDU according to an embodiment of the present invention.

In FIG. 14, it is assumed that 20 MHz channels are allocated to different STAs (eg, STA 1, STA 2, STA 3, and STA 4).

Referring to FIG. 14, the FFT size per unit frequency may be larger from the HE-STF (or HE-SIG B). For example, from the HE-STF (or HE-SIG B), 256 FFTs may be used in a 20 MHz channel, 512 FFTs may be used in a 40 MHz channel, and 1024 FFTs may be used in an 80 MHz channel.

Since the information transmitted in each field included in the PPDU is the same as the example of FIG. 13, a description thereof will be omitted.

The HE-SIG B field may include information specific to each STA, but may be encoded over the entire band (ie, indicated by the HE-SIG-A field). That is, the HE-SIG B field includes information on all STAs and is received by all STAs.

The HE-SIG B field may inform frequency bandwidth information allocated to each STA and / or stream information in a corresponding frequency band. For example, in FIG. 14, the HE-SIG-B may be allocated 20 MHz for STA 1, 20 MHz for STA 2, 20 MHz for STA 3, and 20 MHz for STA 4. In addition, STA 1 and STA 2 may allocate 40 MHz, and STA 3 and STA 4 may then allocate 40 MHz. In this case, STA 1 and STA 2 may allocate different streams, and STA 3 and STA 4 may allocate different streams.

In addition, by defining the HE-SIG-C field, the HE-SIG C field may be added to the example of FIG. 14. In this case, in the HE-SIG-B field, information on all STAs may be transmitted over the entire band, and control information specific to each STA may be transmitted in units of 20 MHz through the HE-SIG-C field.

In addition, unlike the examples of FIGS. 13 and 14, the HE-SIG-B field may be transmitted in units of 20 MHz in the same manner as the HE-SIG-A field without transmitting over the entire band. This will be described with reference to the drawings below.

15 is a diagram illustrating a HE format PPDU according to an embodiment of the present invention.

In FIG. 15, it is assumed that 20 MHz channels are allocated to different STAs (eg, STA 1, STA 2, STA 3, and STA 4).

Referring to FIG. 15, the HE-SIG B field is not transmitted over the entire band, but is transmitted in 20 MHz units in the same manner as the HE-SIG A field. However, at this time, the HE-SIG-B is encoded and transmitted in 20 MHz units differently from the HE-SIG A field, but may not be copied and transmitted in 20 MHz units.

In this case, the FFT size per unit frequency may be larger from the HE-STF (or HE-SIG B). For example, from the HE-STF (or HE-SIG B), 256 FFTs may be used in a 20 MHz channel, 512 FFTs may be used in a 40 MHz channel, and 1024 FFTs may be used in an 80 MHz channel.

Since the information transmitted in each field included in the PPDU is the same as the example of FIG. 13, a description thereof will be omitted.

The HE-SIG A field is duplicated and transmitted in units of 20 MHz.

The HE-SIG B field may inform frequency bandwidth information allocated to each STA and / or stream information in a corresponding frequency band. Since the HE-SIG B field includes information about each STA, information about each STA may be included for each HE-SIG B field in units of 20 MHz. In this case, in the example of FIG. 15, 20 MHz is allocated to each STA. For example, when 40 MHz is allocated to the STA, the HE-SIG-B field may be copied and transmitted in units of 20 MHz.

It may be more preferable not to transmit the HE-SIG-B field over the entire band as in the case of allocating a part of the bandwidth having a low interference level from the adjacent BSS to the STA in a situation in which different bandwidths are supported for each BSS. .

12 to 15, the data field is a payload and may include a service field, a scrambled PSDU, tail bits, and padding bits.

Meanwhile, the HE format PPDU as shown in FIGS. 12 to 15 may be identified through a RL-SIG (Repeated L-SIG) field which is a repetition symbol of the L-SIG field. The RL-SIG field is inserted before the HE SIG-A field, and each STA may identify the format of the received PPDU as the HE format PPDU using the RL-SIG field.

The manner in which an AP operating in a WLAN system transmits data to a plurality of STAs on the same time resource may be referred to as DL MU transmission (downlink multi-user transmission). In contrast, a scheme in which a plurality of STAs operating in a WLAN system transmit data to an AP on the same time resource may be referred to as UL MU transmission (uplink multi-user transmission).

Such DL MU transmission or UL MU transmission may be multiplexed in a frequency domain or a spatial domain.

When multiplexing on the frequency domain, different frequency resources (eg, subcarriers or tones) may be allocated as downlink or uplink resources for each of a plurality of STAs based on orthogonal frequency division multiplexing (OFDMA). Can be. In this same time resource, a transmission scheme using different frequency resources may be referred to as 'DL / UL OFDMA transmission'.

When multiplexing on a spatial domain, different spatial streams may be allocated as downlink or uplink resources for each of the plurality of STAs. In this same time resource, a transmission expression through different spatial streams may be referred to as 'DL / UL MU MIMO' transmission.

The IEEE 802.11ax system is considering several technical introductions aimed at increasing average throughput more than four times compared to the existing 802.11ac system, one of which is 4x FFT. In this structure, the subcarrier spacing is reduced to one quarter compared to the conventional system, making the CFO sensitive, so the residual CFO must be compensated in an appropriate manner.

In addition, the support of UL MU MIMO and UL OFDMA is also considered in 802.11ax. In this situation, unlike the single user situation or the downlink MU MIMO situation, different CFOs are mixed at the receiving end. Existing CFO estimation method has difficulty in accurate user-specific CFO measurement.

To overcome this problem, the present invention proposes a new P matrix coded LTF (P matrix coded LTF) scheme capable of simultaneously performing channel estimation and CFO measurement in UL MU MIMO and / or UL OFDMA transmission. .

In addition, an additional method is proposed to overcome the limitations of using the existing P matrix and the limitations dependent on channel correlation. One is to cover all the numerologies that can be considered in 11ax using an existing P matrix without the definition of an additional P matrix, and the other is to reduce the effect of channel correlation. The number of users that can be supported while receiving is maintained.

Hereinafter, in the description of the present invention, the term user means an STA participating in MU MIMO and / or OFDMA transmission.

1. P matrix coded LTF method

1-A) HE-LTF Sequence Generation

By generating orthogonal LTF sequences for each user using the existing P matrix, we propose a new CFO measurement and channel estimation technique under the assumption that channel correlation is high in the frequency domain. .

Hereinafter, in describing a method of estimating a CFO and a channel according to a P matrix coded LTF method, for the convenience of description, the number of users participating in the UL MU MIMO and / or UL OFDMA transmission is two. Assume that the P matrix is assumed to reuse the P_4 × 4 matrix illustrated in Equation 2 above. In addition, it is assumed that each user is assigned one stream.

However, the present invention is not limited thereto and a different number of users may participate in UL MU MIMO and / or UL OFDMA transmission. In addition, a new P matrix orthogonal to each row may be defined and used instead of the previously defined P matrix. In addition, the present invention can be equally applied even when one or more streams are allocated to each user.

16 illustrates a method of generating a HE-LTF sequence according to an embodiment of the present invention.

In FIG. 16, L_i (that is, L_1, L_2, L_3, ...) represents a predefined common HE-LTF sequence. i represents the subcarrier index to which the HE-LTF is mapped.

s_i (k) (ie, s_1 (k), s_2 (k), s_3 (k) ...) represents the scrambled HE-LTF sequence for stream k. Here, k represents a stream index and i represents a subcarrier index to which the HE-LTF is mapped.

The predefined common HE-LTF sequence L_i is scrambled for each stream by using different rows (ie, P matrix codes) of the P matrix.

More specifically, matrix elements of different rows of the P matrix in units of N_f sub-sequences of a common HE-LTF sequence predefined for each stream (e.g., 4, etc.) Multiply by (matrix element) That is, sequence elements belonging to the sub-sequence and matrix elements constituting a specific row of the P matrix are multiplied in order.

As such, the scrambled HE-LTF sequence s_i is orthogonal for each stream in units of sub-sequences of N_f consecutive entries (eg, 4, etc.). That is, the scrambled HE-LTF sequence for each stream is piecewise orthogonal.

Referring to FIG. 16, in the case of stream 1, a matrix element of the first row of the P matrix is multiplied by a predefined common HE-LTF sequence, so that a (scrambled) HE-LTF sequence (that is, HE-LTF) Sequence 1) is generated. For stream 2, the HE-LTF sequence is multiplied by the elements of the second row of the P matrix to produce a (scrambled) HE-LTF sequence (ie HE-LTF sequence 2).

Specifically, in the case of stream 1, the first HE-LTF sequence element L_1 is multiplied by the first matrix element (ie, 1) of the first row of the P matrix, and the second HE-LTF sequence element L_2 is multiplied. The second matrix element (i.e. -1) of the first row of the P matrix is multiplied, and the third HE-LTF sequence element L_3 is multiplied by the third matrix element (i.e., the first row of the P matrix). 1) is multiplied, and the fourth HE-LTF sequence L_1 is multiplied by the first matrix element (ie, 1) of the fourth row of the P matrix. The HE-LTF sequence 1 (s_i) is scrambled by repeating the above process in four sequence units (ie, L_5 to L_8, L_9 to L_12, L_13 to L_16, ...) for the remaining HE-LTF sequences. (1)) is generated.

In the case of stream 2, HE-LTF sequence 2 (s_i (2)) is generated by performing the same process using the second row of the P matrix.

The HE-LTF sequence generated by the above method is orthogonal for each stream in units of 4 sub-sequences. That is, the HE-LTF sequence 1 and the HE-LTF sequence 2 are orthogonal in units of 4 sub-sequences.

Therefore, in case of UL MU MIMO and / or UL OFDMA transmission, different HE-LTF sequences are transmitted for each stream.

1-B) P matrix encoding in the time domain

Optionally, in order to maintain orthogonality in the time domain, P matrix encoding may be additionally applied to the HE-LTF sequence generated by the method 1-A). This will be described with reference to the drawings below.

17 illustrates a method of constructing a HE-LTF field according to an embodiment of the present invention.

In FIG. 17, c_j (k) (that is, c_1 (k), c_2 (k), c_3 (k), ...) denotes a row (ie, P matrix code) of the P matrix corresponding to the stream k. Indicates. Where k represents a stream index and j represents a symbol index.

The scrambled HE-LTF sequence may be mapped and transmitted on subcarriers of one or more symbols, where the number of symbols (hereinafter, referred to as 'HE-LTF symbols') to which the scrambled HE-LTF sequence is transmitted (or mapped) May be determined by the number of streams allocated for UL MU MIMO transmission.

Each matrix element belonging to one row of the P matrix (for example, a row corresponding to k in the stream) is multiplied by the HE-LTF sequence transmitted in each symbol in order. That is, for each stream, different rows of the P matrix are multiplied by data tones of the HE-LTF symbol (that is, tones to which the HE-LTF sequence is mapped).

That is, the scrambled HE-LTF sequence (s_i (k)) transmitted in the first HE-LTF symbol is multiplied by the first matrix element (ie, c_1 (k)) of a specific row of the P matrix, and 2 The scrambled HE-LTF sequence (s_i (k)) transmitted in the first HE-LTF symbol is multiplied by the second matrix element (ie c_2 (k)) of a specific row of the P matrix, and the third HE-LTF The scrambled HE-LTF sequence (s_i (k)) transmitted in the LTF symbol is multiplied by the third matrix element (i.e. c_3 (k)) of a specific row of the P matrix, and in the fourth HE-LTF symbol The scrambled HE-LTF sequence s_i (k) to be transmitted is multiplied by the fourth matrix element (ie c_4 (k)) of a specific row of the P matrix.

Referring to FIG. 17, the first matrix element (ie, 1) of the third row of the P matrix is multiplied by a scrambled HE-LTF sequence mapped to the first HE-LTF symbol and the third of the P matrix. The second matrix element of the row (ie 1) is multiplied by the scrambled HE-LTF sequence mapped to the second HE-LTF symbol and the third matrix element of the third row of the P matrix (ie , 1) is multiplied by a scrambled HE-LTF sequence that is mapped to a third HE-LTF symbol, and the fourth matrix element (ie, -1) of the third row of the P matrix is the fourth HE-LTF symbol. The scrambled HE-LTF sequence is mapped to.

As shown in the example of FIG. 16, orthogonality can be maintained in the frequency domain for each stream by multiplying different rows of the P matrix for each stream by the sub-sequence unit of the HE-LTF sequence. .

In addition, the HE-LTF symbol to which the scrambled HE-LTF sequence is transmitted is determined by the number of streams allocated to UL MU transmission. As shown in FIG. 17, different rows of the P matrix are generated for each stream. Orthogonality can be maintained in the time domain for each stream by multiplying the scrambled HE-LTF sequence mapped to each HE-LTF symbol.

1-C) CFO and Channel Estimation

Hereinafter, a method of estimating a CFO and a channel by using the HE-LTF received by a receiver (that is, an AP in case of UL MU MIMO and / or UL OFDMA transmission) will be described.

The P matrix code for a total of K streams (ie, stream index k (1 ≦ k ≦ K)) is expressed by Equation 5 below.

In Equation 5, since it is assumed that the matrix is P_4 × 4 as described above, when each P matrix code is a 1 × 4 matrix or the P matrix is a K × K matrix, the P matrix code for each stream is a 1 × K matrix. It may be configured as.

When the HE-LTF is transmitted as shown in the example of FIG. 17, the signal received in stream 1 in N_f (for example, 4) subcarriers and N_p symbols (ie, HE-LTF symbols) Y (1)) can be expressed as Equation 6 below.

In Equation 6, h_i (k) (that is, h_1 (k), h_2 (k), h_3 (k), ...) represents a channel passed from the transmitter to the receiver.

Jθ (k) (that is, θ (k), 2θ (k), 3θ (k), ...) represents the phase of a signal received in each HE-LTF symbol. Here, jθ (k) is expressed by excluding j representing a complex number in Equation 6, where j represents a symbol index. That is, the CFO of the first HE-LTF symbol is doubled in the second HE-LTF symbol in the time domain, and the CFO of the first HE-LTF symbol is tripled in the third HE-LTF symbol.

That is, since the signal transmitted from the transmitter is transmitted to the receiver through a channel, and a phase difference occurs for each symbol, the signal Y (1) received in stream 1 is channel h (1) and the scrambled HE-LTF sequence S (1). , P matrix code C (1), phase [e ^ jθ (1), e ^ j2θ (1), ..., e ^ jN_pθ (1)].

Here, the channel diagonal matrix dia (h (1)) is an N_f × N_f matrix, s (1) is an N_f × 1 matrix, c (1) is a 1 × N_p matrix, and a phase diagonal matrix ([e ^ jθ (1) , e ^ j2θ (1), ..., e ^ jN_pθ (1)]) may be represented by an N_p × N_p matrix, so that the signal Y (1) received at stream 1 at the receiver is N_f × It can be expressed as an N_P matrix, that is, in Equation 6, the row direction means the time axis and the column direction indicates the frequency axis.

Superimposed signals received from a total of K streams are represented by Equation 7 below.

18 is a diagram illustrating a CFO estimation method according to an embodiment of the present invention.

Referring to FIG. 18, as described above with reference to FIG. 16, the HE-LTF sequence is multiplied and scrambled by the HE-LTF sequence in units of N_f subcarriers, and thus, the HE-LTF sequence of each stream in units of N_f subcarriers. Are composed of orthogonal sequences.

Channels in all N_f consecutive subcarriers (e.g., 4, etc.) may be roughly unchanged (ie correlated). Therefore, the receiver may estimate a channel in each HE-LTF symbol in which the HE-LTF is transmitted under the assumption that the channel correlation is high in all N_f consecutive subcarriers (eg, 4, etc.) as described above. Can be.

On the other hand, due to the CFO, the channel may vary in time domain. Therefore, the receiver checks the phase difference of the signal transmitted in the HE-LTF symbol (that is, the HE-LTF sequence) to estimate the CFO.

In more detail, first, the receiver removes the scrambled HE-LTF sequence and the P matrix code from the signal received through N_f consecutive subcarriers in the HE-LTF symbol t for each stream.

Referring to stream 1, the receiver removes the HE-LTF sequence and the P matrix code from the signal Y (1) of stream 1 received through N_f consecutive subcarriers in the HE-LTF symbol t as shown in Equation 8 below. .

In equation (8), c * _t (k) represents the conjugate transpose matrix (k is the stream index) of the P matrix code for stream k, and s ^ H (k) is the scrambled HE for stream k. A transpose matrix (k is a stream index) of the -LTF sequence.

In Equation 8, assuming that a channel has a high correlation over an N_f subcarrier, it is calculated as in Equation 9 below.

In Equation 9, the scrambled HE-LTF sequence for each stream is orthogonal in units of N_f subcarriers, so the result of S ^ H (1) × s (k) (k ≠ 1) is '0' Signal components in all streams other than 1 are removed.

If the above method is performed in the same manner for the signals received in each stream, it is possible to remove signal components transmitted in all streams other than the corresponding stream in a particular HE-LTF symbol.

The receiver may derive a phase rotation from a specific HE-LTF symbol for each HE-LTF symbol as shown in Equation 10 below.

The CFO may be estimated from a phase difference between HE-LTF symbols based on the value derived from Equation 10 above as shown in Equation 11 below.

Based on the phase difference between the HE-LTF symbols derived from Equation 11, a CFO correction term is derived as shown in Equation 12 below.

That is, the receiver derives the CFO estimation value for each stream as shown in Equation (12).

In addition, if the value derived from Equation 12 is applied to Equation 9 above, the receiver estimates a channel for each HE-LTF symbol for each stream (that is, a channel estimation value).

Can be derived).

In the above-described method, there is a problem in that the residual CFO (CFO) measurement between the last symbol of the HE-LTF and the first symbol of the data field is impossible because there is no consideration of a pilot in the HE-LTF. In addition, if there is a pilot in the HE-LTF, it may not be necessary to calculate a CFO by applying a complex equation to all the tones (or subcarriers) of the HE-LTF as in the first method. In addition, since the first method uses only the previously defined P matrix illustrated in Equations 2 to 4, there is a limit to cover all the various numerologies of 802.11ax. As a result, it may be more effective to measure CFOs using pilots as in existing 802.11ac systems, rather than increasing the overhead of applying complex equations and defining new P matrices.

Therefore, the present invention proposes a method of measuring a residual CFO using a pilot in a UL MU MIMO situation as follows.

2. Use user specific pilot

Assign a specific pilot tone (or subcarrier) to each user. Each user sets and transmits a pilot coefficient to 0 for pilot tones not assigned to him.

In this case, power can be transmitted using power that should have been used for pilot tones not assigned to users.

Here, the pilot tones assigned to each user are different, and the method of allocating the pilot tones is as follows.

2-A) Pilot Assignment Method

Random assignment

You can randomly assign pilot tones to each user. This approach can reduce the overhead of additional algorithms for pilot tone allocation.

However, performance degradation of a user to which a pilot tone is relatively assigned may occur.

-Evenly assigned to each user

Each user is assigned the same number of pilot tones. By allocating pilot tones evenly to each user, similar performance can be guaranteed for all users.

For example, a pilot tone can be allocated to each user according to Equation 13 below.

In equation (13), u is the user index to which the pth pilot tone is assigned, U is the total number of users, and mod is the modulo operation.

For example, if the number of users participating in the UL MU transmission is 4, then 1 to each user in turn, such as User 1, User 2, User 3, User 4, User 1, ..., according to the index order of the pilot tones. Pilot tones may be allocated.

Alternatively, the pilot tones may be allocated from the front (ie, the lowest index of the pilot tones) by the number of pilot tones to be allocated to the first user, and the next pilot tones may be allocated to the next user.

Here, the number of pilot tones allocated to each user may be determined according to the UL MU transmission bandwidth (or the number of tones). For example, if 20 MHz (or 242 tones) is allocated for UL MU transmission and is set to a total of eight tones of pilot tones at 20 MHz (or 242 tones), then 4 users at that 20 MHz (or 242 tones) When participating in the MU transmission, the number of tones allocated to each user may be determined to be two.

Differential allocation according to RSSI

Performance gain can be achieved by allocating a pilot tone proportionally to a user with a high RSSI (Received Signal Strength Indication), or conversely, a higher quality of service (QoS) can be increased by assigning more pilot tones to a user with a lower RSSI.

For example, in the UL MU transmission process or before the UL MU transmission process, the AP may estimate an uplink channel by receiving a NDP (Null Data Packet) from STAs that will participate in the UL MU transmission. Can be measured.

In this case, one pilot tone may be allocated to each user in turn. Or, first, all pilot tones may be assigned to one user, and then the next pilot tones may be assigned to a user.

When using a user specific pilot as in the 2-A method above, the receiver receives only the signal of the user in the pilot tone assigned to the specific user. The system's method can be used to measure the user's CFO.

However, according to this method, if the channel state of the tone assigned to a particular user is not very good, reliable CFO measurement can be practically difficult even if a lot of power is used.

Therefore, in order to compensate for this, a new pilot tone is allocated to each user (ie, pilot tone reallocation) after a predetermined symbol or more passes.

Here, the period of the symbol can be determined arbitrarily or according to the size of the data. For example, it is desirable to set a minimum of two symbols or more as a period.

The period of the symbol may be determined by Equation 14 below according to the size of the data.

In Equation 14, Max (a, b) is an operation taking a larger value of a and b, and round (c) means a round operation.

The number k of the denominator is determined according to the degree of correlation, and as shown in Equation 15 below, a large value may be set when the correlation is strong and a small value when the correlation is weak.

Here, alpha is an average correlation value of adjacent tones and is a value between 0 and 1.

When Equation 15 is applied to Equation 14, it is equal to Max (2, round (1 / alpha)). At this time, when alpha = 0, k = 0 and do not reallocate a pilot tone. If 0 <alpha <0.5, Max (2, round (1 / alpha)) = round (1 / alpha), and if 0.5 <alpha <1, Max (2, round (1 / alpha)) = 2 same.

That is, when the channel correlation between the adjacent pilot tones is high, when the channel is in a bad situation, all the channels of the pilot assigned by a specific user may be bad, so that the reassignment period may be shortened to compensate for this.

2-B) How to Reallocate Pilot Tones After Each Symbol

Random assignment

Randomly reassign pilot tones to each user. This approach can reduce the overhead of additional algorithms for pilot tone allocation.

-Reassign by cyclic shift in current pilot tone

In the existing 802.11ac system, the pilot tone is cyclically shifted every symbol, and the pilot tone is cyclically shifted after the predetermined symbol instead of every symbol. do.

In this case, the cyclic shift direction may correspond to a left (low tone index direction) or a right (high tone index direction).

19 illustrates a method of allocating a pilot tone according to an embodiment of the present invention.

19 illustrates a situation in which a total number of users is 8 and one user is allocated to each user out of eight pilot tones.

As described above, when a total of eight pilot tones are allocated to each user, after a predetermined symbol, the pilot tones may be reassigned by a cyclic shift. 19 illustrates a case in which pilot tones are cyclically shifted and reallocated at one pilot tone interval.

In addition, the pilot tone may be reallocated again by cyclic shift even after a certain symbol passes. For example, in FIG. 19, user 1 is allocated pilot tone 1, and after a certain symbol, user 1 is reallocated pilot tone 2. And again, after a certain symbol, the user may be reassigned three pilot tones.

Here, the shift value can be arbitrarily determined. Or it may be fixed to a predetermined predetermined value (for example, 1). The shift value may also be determined according to the UL MU transmission bandwidth (or number of tones).

Alternatively, it may be determined according to the degree of channel correlation between adjacent tones. For example, as shown in Equation 16 below, the position of the pilot tone may be shifted in consideration of channel correlation.

In Equation 16, alpha is an average channel correlation value between adjacent tones, and is a value between 0 and 1.

If alpha is 1, the pilot tone is shifted by half the maximum number of available tones. This is to shift the pilot tone position to a larger value because the larger channel correlation may occur over several adjacent tones, so that the pilot may be located to avoid tones having bad channel conditions.

For example, if pilot 2's channel situation is not good, user 2's CFO measurement performance is not guaranteed until the pilot shift, but the pilot's position is shifted to take advantage of the relatively good channel condition of pilot 3 to degrade performance before the shift. To compensate.

In addition, the position of the pilot tone may be changed for each symbol period according to a constant shift value in a frequency hopping manner.

For example, if 20 MHz (or 242 tones) is allocated for UL MU transmission, and a total of eight tones are set to pilot tones, four users participate in the UL MU transmission at each 20 MHz (or 242 tones) and each user It is assumed that two pilot tones are allocated. Herein, when STA 1 is allocated pilot tones 1 and 2, the pilot tones 3 and 4 are allocated in the next symbol period, and the pilot tones 1 and 2 are allocated in the next symbol period. The location of the pilot tone allocated while hopping at may be changed.

Using the above-described method 2, more pilots are required than the number of users, so increasing the number of pilots to support a large number of users may reduce the tones to which data is mapped, thereby causing throughput degradation. In addition, to support a large number of users, the number of pilots assigned to each user is small, which can cause performance degradation of CFO measurements. However, the transmission power can be concentrated on some pilot tones (ie, assigned pilot tones) to compensate for the above performance deterioration.

3. Use orthogonal pilot for each user

The CFO can be measured by applying the aforementioned 1-C method to the pilot tone using an orthogonal pilot for each user such as an 802.11n multi-stream pilot.

That is, an orthogonal pilot sequence may be allocated to each user participating in the UL MU transmission. Accordingly, a user participating in the UL MU transmission may map and transmit an orthogonal pilot sequence allocated to each user on the same pilot tone.

In addition, since channel estimation of pilot tones can be performed using the orthogonal pilot using the aforementioned 1-C) method, additional performance gain can be obtained when channel estimation of peripheral data tones is performed.

In this case, however, if all the pilot tones are not located adjacent to each other, the channel conditions are not similar, so it is difficult to guarantee good performance when applying the aforementioned 1-C method. Accordingly, the present invention proposes the following pilot tone positions when using orthogonal pilot sequences in 802.11ax UL MU MIMO.

3-A) pilot tone position

Unlike existing systems in which pilot tones are located between data tones, we propose a structure in which pilot tones are located in a specific area. That is, the pilot tones are all located adjacent to each other.

Here, the position of the pilot tones can be anywhere in the usable tone, either in front (first low tone index), behind (first high tone index), or middle (middle tone index). For example, if the available tones are 1 to 26 and four pilots are used, the pilot tones can be placed on 1 to 4 or 2 to 5 or 3 to 6 or ..., 23 to 26 tones.

Here, the position of the pilot tone may be determined according to the UL MU transmission bandwidth (or the number of tones).

In this method, each user uses all pilots to measure the CFO, and the number of samples used when measuring the CFO is higher than that in the above method 2, so that reliable performance can be expected.

However, there is a possibility that a pilot may be allocated to a location of tones having poor channel condition. In particular, when channel correlation is large in the frequency domain, the channel state of all the tones allocated to the pilot may be bad.

In this case, it is possible to solve the problem by relocating the pilot tone after some symbols to compensate for performance degradation. The period of the symbol can be determined arbitrarily or according to the data size. For example, it is desirable to set at least two minimum two symbols or more as a period.

Depending on the size of the data, the period of the symbol may be determined by Equation 17 below.

In Equation 17, Max (a, b) is an operation taking a larger value of a and b, and round (c) means a round operation.

The number k of the denominator is a value determined by the degree of correlation, and as shown in Equation 18 below, a large value may be set when the correlation is strong and a small value when the correlation is weak.

Here, alpha is an average correlation value of adjacent tones between 0 and 1.

When Equation 18 is applied to Equation 17, it is equal to Max (2, round (1 / alpha)). At this time, when alpha = 0, k = 0 and do not reallocate a pilot tone. If 0 <alpha <0.5, Max (2, round (1 / alpha)) = round (1 / alpha), and if 0.5 <alpha <1, Max (2, round (1 / alpha)) = 2 same.

That is, when the channel correlation between the adjacent pilot tones is high, when the channel is in a bad situation, all the channels of the pilot assigned by a specific user may be bad, so that the reassignment period may be shortened to compensate for this.

The reassignment of pilot positions is proposed as follows.

3-B) How to reallocate pilot locations to each user after a certain symbol

Random assignment

Randomly reassign pilot locations to each user. This approach can reduce the overhead of additional algorithms for pilot location reallocation. However, tones with poor channel condition may be allocated to pilot tones again.

-Reassign pilot position by cyclic shift from current pilot tone

The pilot tone may be positioned by cyclically shifting left (low tone index direction) or right (high tone index direction) by the number of specific tones from the current pilot tone position.

In this case, the shift value may be arbitrarily determined, or may be fixed to a predetermined specific value (eg, 1). The shift value may also be determined according to the UL MU transmission bandwidth (or number of tones).

Alternatively, it may be determined according to the degree of channel correlation between adjacent tones. For example, as shown in Equation 19 below, the position of the pilot tone may be shifted in consideration of channel correlation.

In Equation 19, alpha is an average channel correlation value between adjacent tones, and is a value between 0 and 1.

If alpha is 1, the pilot tone is shifted by half the maximum number of available tones. This is to shift the pilot tone position to a larger value because the larger channel correlation may occur over several adjacent tones, so that the pilot may be located to avoid tones having bad channel conditions.

In addition, the position of the pilot tone may be changed for each symbol period according to a constant shift value in a frequency hopping manner.

For example, if 20 MHz (or 242 tones) is allocated for UL MU transmission, and a total of eight tones are set to pilot tones, four users participate in the UL MU transmission at each 20 MHz (or 242 tones) and each user It is assumed that two pilot tones are allocated. Herein, when STA 1 is allocated pilot tones 1 and 2, the pilot tones 3 and 4 are allocated in the next symbol period, and the pilot tones 1 and 2 are allocated in the next symbol period. The location of the pilot tone allocated while hopping at may be changed.

According to the aforementioned method 3, as in the method 2 described above, more pilots are required than the number of users, and a new orthogonal pilot other than the existing 11n multi-stream needs to be designed. However, it can be easily extended using the Hadamard method.

4. Use User group specific pilot

The CFO can be measured by dividing the user into groups, assigning different group specific pilot tones to each group, and applying the aforementioned method of 3, orthogonal pilot, within each group.

That is, the method of 2 and the method of 3 may be referred to as a hybrid method.

Here, the number of groups may be from at least one group to the maximum total number of users. That is, only one user in one group may belong.

The number of users in each group may be from 1 to the maximum total number of users.

The pilot tones assigned to each group are as follows.

4-A) How to Assign Pilot Tones

Random assignment

You can randomly assign pilot tones to each group. This approach can reduce the overhead of additional algorithms for pilot tone allocation.

However, performance degradation of a user in a group to which a pilot tone is relatively assigned may occur.

-Evenly assigned to each group

Each group is assigned the same number of pilot tones. By uniformly assigning pilot tones to each group, similar performance can be guaranteed for all users.

A pilot tone can be allocated to each group according to Equation 20 below.

In Equation 20, g is a group index to which a pth pilot tone is allocated, G is a total number of groups, and mod is a modulo operation.

For example, if the number of groups participating in the UL MU transmission is 4, 1 for each group in turn, such as group 1, group 2, group 3, group 4, group 1, ... according to the index order of pilot tones. Pilot tones may be allocated.

Alternatively, the pilot tones may be allocated from the front (ie, the lowest index of the pilot tones) by the number of pilot tones allocated to the first group, and the next pilot tones may be allocated to the next group.

Here, the number of pilot tones allocated to each group may be determined according to the UL MU transmission bandwidth (or the number of tones). For example, if 20 MHz (or 242 tones) is allocated for UL MU transmission and is set to a total of eight tones of pilot tones at 20 MHz (or 242 tones), then four groups at that 20 MHz (or 242 tones) are UL When participating in the MU transmission, the number of tones allocated to each group may be determined to be two.

Differential allocation according to RSSI

Performance gain can be achieved by allocating a pilot tone proportionally to a group having a high average RSSI of users in the group, or conversely, a higher pilot tone can be assigned to a group having a lower average RSSI to improve QoS.

For example, in the UL MU transmission process or before the UL MU transmission process, the AP may estimate an uplink channel by receiving a NDP (Null Data Packet) from STAs that will participate in the UL MU transmission. Can be measured. The AP may compare the RSSI for each group by averaging the RSSI values of the STAs belonging to the group.

In this case, the pilot tones may be allocated to each group one pilot tone in turn. Or, first, all pilot tones may be allocated to one group, and the next pilot tones may be allocated to the next group.

At this time, the pilot tones in the group have a form in one place. For example, suppose you have 26 usable tones, and you have two groups, and if there are two pilots in each group, each group's pilots can use one or two or three or four or five or six or more of the available tones. ..., 25 to 26 tones can be used as pilot tones. However, the two groups are assigned different pilot tones.

In this case, in order to compensate a group to which a pilot tone with a bad channel condition is allocated, the pilot tone may be changed after a certain number of symbols. The method is as follows.

Here, the period of the symbol can be determined arbitrarily or according to the size of the data. For example, it is desirable to set a minimum of two symbols or more as a period.

According to the size of the data, the period of the symbol may be determined by Equation 21 below.

In Equation 21, Max (a, b) is an operation taking a larger value of a and b, and round (c) means a round operation.

The number k of the denominator is determined according to the degree of correlation, and as shown in Equation 22 below, a large value may be set when the correlation is strong and a small value when the correlation is weak.

Here, alpha is an average correlation value of adjacent tones and is a value between 0 and 1.

When Equation 22 is applied to Equation 21, it is equal to Max (2, round (1 / alpha)). At this time, when alpha = 0, k = 0 and do not reallocate a pilot tone. If 0 <alpha <0.5, Max (2, round (1 / alpha)) = round (1 / alpha), and if 0.5 <alpha <1, Max (2, round (1 / alpha)) = 2 same.

That is, when the channel correlation between the adjacent pilot tones is high, when the channel is in a bad situation, all the channels of the pilot assigned by a specific user may be bad, so that the reassignment period may be shortened to compensate for this.

4-B) Reallocate pilot tones after each symbol

Random assignment

You can randomly reassign pilot tones to each group. This approach can reduce the overhead of additional algorithms for pilot tone allocation. However, tones with poor channel condition may be reassigned to pilot tones.

-Reassign by cyclic shift in current pilot tone

It is proposed to position the pilot tone by cyclic shifting to the left (low tone index direction) or right (high tone index direction) by a specific number of tones from the position of the current pilot tone.

In this case, the cyclic shift direction may correspond to a left (low tone index direction) or a right (high tone index direction).

In addition, the shift value may be arbitrarily determined, or may be fixed to a predetermined predetermined value (eg, 1). The shift value may also be determined according to the UL MU transmission bandwidth (or number of tones).

Alternatively, it may be determined according to the degree of channel correlation between adjacent tones. For example, as shown in Equation 23 below, the position of the pilot tone may be shifted in consideration of channel correlation.

In Equation 23, alpha is an average channel correlation value between adjacent tones, and is a value between 0 and 1.

If alpha is 1, the pilot tone is shifted by half the maximum number of available tones. This is to shift the pilot tone position to a larger value because the larger channel correlation may occur over several adjacent tones, so that the pilot may be located to avoid tones having bad channel conditions.

In addition, the position of the pilot tone may be changed for each symbol period according to a constant shift value in a frequency hopping manner.

For example, if 20 MHz (or 242 tones) are allocated for UL MU transmissions and a total of eight tones are set to pilot tones, then at that 20 MHz (or 242 tones), four groups participate in the UL MU transmission and each group It is assumed that two pilot tones are allocated. In this case, when group 1 is assigned pilot tones 1 and 2, the pilot tones 3 and 4 are allocated in the next symbol period, and the pilot tones 1 and 2 are allocated in the next symbol period. The location of the pilot tone allocated while hopping at may be changed.

-Exchange of pilot tone positions assigned by group

If the number of pilots allocated to each group is the same, the pilot tones may be exchanged between groups without having to reassign the positions of the pilot tones themselves.

This approach can reduce the additional overhead of pilot tone location reallocation. For example, suppose that Group 1 uses positions of 6 to 7 and Group 2 uses 20 to 21 pilot tones. After a certain symbol, 20 to 21 for Group 1 and 6 to 7 pilot tones for Group 2 You can assign the location of.

The advantage of this method is that pilots can be used in each group, so that a smaller pilot sequence can be used compared to the method described in 3, and thus, there is much room for reuse of the existing 802.11n multi-stream pilots. Is in. Of course, there is still room for designing a new orthogonal pilot sequence, depending on how many pilots will be used in each situation in 802.11ax.

Another advantage is that the pilots are grouped in one group, but the pilots in different groups can be separated, reducing the likelihood that the channel conditions of all pilot tones will worsen, as described above. In addition, compared to the second method, the number of samples used for CFO measurement can be taken to ensure the performance gain.

5. Use user specific symbols

In the method of 3 and 4 described above, pilot positions need to be gathered unlike DL or UL SU. In addition, in the method of 2, it is difficult to support more users than the number of pilots.

Therefore, unlike the above method, we propose a method that can support the number of pilots or more while using the same pilot tone location as in other situations as follows. That is, it is proposed to assign and use a symbol for each user, and a method of allocating a symbol is proposed as follows.

In this case, all pilot coefficients are set to 0 in the symbol to which each user is not assigned.

5-A) Symbol Assignment Method

Random assignment

Symbols can be randomly assigned to each user without certain rules. In this case, there is no additional overhead regarding the allocation scheme, but there may be a difference in CFO estimation performance for each user.

-Equal allocation

Symbols can be allocated evenly by user. By allocating symbols evenly per user, similar performance can be guaranteed for all users.

According to Equation 25 below, symbols can be allocated to each user evenly.

Where u is the user index to which the sth symbol is assigned, U is the total number of users, and mod is the modulo operation.

For example, if the number of users is 2 and the equal allocation method is used for each pilot symbol, the first symbol and the third symbol may be allocated to the first user. Then, the phase difference between the pilots of the two symbols can be divided by the symbol interval to estimate the residual CFO of the first user.

In this case, two or more pilot symbols may be allocated to each user. Here, the unit (ie, the number of symbols) to which the pilot symbols are allocated may be determined according to the UL MU transmission band.

However, according to this method, when the number of symbols is small, the number of symbols assigned by the user may decrease, which may cause performance degradation.

6. Use user group specific symbol

In order to overcome the disadvantages of the scheme 5 described above, it is proposed to use a user group specific symbol that can increase a symbol allocated to each user even when there are few symbols.

This may be referred to as a hybrid method of the 3 method and the 5 method. That is, according to the scheme 5 described above, a specific pilot symbol is allocated to each user group, and users belonging to the corresponding group may be assigned an orthogonal pilot. Accordingly, the users belonging to each group can transmit by mapping the orthogonal pilot sequences assigned to them to the same tone in the pilot symbols assigned to the group to which they belong.

Here, the number of groups may be from at least one group to the maximum total number of users. That is, only one user in one group may belong.

The number of users in each group may be from 1 to the maximum total number of users.

The symbols assigned to each group are as follows.

6-A) Symbol Assignment Method

Random assignment

You can assign symbols to each group at random. This approach can reduce the overhead of additional algorithms for symbol assignment. However, performance degradation of a user in a group in which fewer symbols are allocated may occur.

-Evenly assigned to each group

Assign the same number of symbols to each group. By equally assigning symbols to each group, similar performance can be guaranteed for all users.

A pilot tone can be allocated to each group according to Equation 26 below.

Where g is the group index to which the sth symbol is assigned, G is the total number of groups, and mod is the modulo operation.

For example, if the number of groups is 2 and the equal allocation method is used for each pilot symbol, the first symbol and the third symbol may be allocated to the first group. Then, the phase difference between the pilots of these two symbols is divided by the symbol interval to estimate the residual CFO of the first group.

In this case, two or more pilot symbols may be allocated to each group. Here, the unit (ie, the number of symbols) to which the pilot symbols are allocated may be determined according to the UL MU transmission band.

According to this method, pilots must be concentrated as in the method of 3. Therefore, a method of reallocating pilot positions, such as the 3-B) scheme, is further proposed.

Random assignment

Randomly reassign pilot locations to each user. This approach can reduce the overhead of additional algorithms for pilot location reallocation. However, tones with poor channel condition may be allocated to pilot tones again.

-Reassign pilot position by cyclic shift from current pilot tone

The pilot tone may be positioned by cyclically shifting left (low tone index direction) or right (high tone index direction) by the number of specific tones from the current pilot tone position.

In this case, the shift value may be arbitrarily determined, or may be fixed to a predetermined specific value (eg, 1). The shift value may also be determined according to the UL MU transmission bandwidth (or number of tones).

Alternatively, it may be determined according to the degree of channel correlation between adjacent tones. For example, as shown in Equation 27 below, the position of the pilot tone may be shifted in consideration of channel correlation.

In Equation 27, alpha is an average channel correlation value between adjacent tones, and is a value between 0 and 1.

If alpha is 1, the pilot tone is shifted by half the maximum number of available tones. This is to shift the pilot tone position to a larger value because the larger channel correlation may occur over several adjacent tones, so that the pilot may be located to avoid tones having bad channel conditions.

In addition, the position of the pilot tone may be changed for each symbol period according to a constant shift value in a frequency hopping manner.

For example, if 20 MHz (or 242 tones) is allocated for UL MU transmission, and a total of eight tones are set to pilot tones, four users participate in the UL MU transmission at each 20 MHz (or 242 tones) and each user It is assumed that two pilot tones are allocated. Herein, when STA 1 is allocated pilot tones 1 and 2, the pilot tones 3 and 4 are allocated in the next symbol period, and the pilot tones 1 and 2 are allocated in the next symbol period. The location of the pilot tone allocated while hopping at may be changed.

In addition, it is further proposed to divide the users in each group into sub-groups and apply the above-described method of 4.

For example, assume that users 1, 2, 3, and 4 belong to group 1, and users 5, 6, 7, and 8 belong to group 2. In addition, it is assumed that STAs 1 and 2 belong to subgroup 1 of group 1 and STAs 3 and 4 belong to subgroup 2. In addition, it is assumed that the first and third pilot symbols are allocated to group 1 and the second and fourth pilot symbols are allocated to group 2.

In this case, subgroup 1 of group 1 may transmit a pilot in the first symbol, and subgroup 2 may transmit a pilot in the third symbol. At this time, STAs 1 and 2 belonging to subgroup 1 of group 1 may map the orthogonal pilot sequences allocated to the pilot tones and transmit them in the first symbol.

20 is a diagram illustrating an uplink multi-user transmission method according to an embodiment of the present invention.

Referring to FIG. 20, an STA (or STA group) participating in UL MU transmission may receive pilot allocation information specifying a pilot tone, a pilot symbol, and / or a pilot sequence from the AP (S2001).

Here, the pilot allocation information means pilot tone, pilot symbol and / or pilot sequence information allocated according to the schemes 2 to 6 described above.

Here, the method of 2 to 6 described above is based on the STA index (or STA group index, stream index, and resource unit index in OFDMA) assigned to the STA (or STA group) participating in the UL MU transmission. Accordingly, pilot assignment information may not be received from the AP if a pilot tone, a pilot symbol, and / or a pilot sequence are specified. That is, step S2001 may be omitted.

For example, a STA (or STA group) participating in the UL MU transmission randomly pilots and / or pilots using a predetermined formula or the like based on the STA index (or STA group index) assigned to it. You can use symbols. In addition, one pilot tone may be used in sequence according to the STA (or STA group) index, and a predetermined number of pilot tones may be used from the front (that is, from the smallest tone index) according to the STA (or STA group) index. have.

In addition, the pilot allocation information may include reassignment period information of the pilot tones when the pilot tones are reassigned. In addition, if the reassignment period of the pilot tone can be specified based on the STA index (or the STA group index, the stream index, and the resource unit index in OFDMA) as described above, the reassignment period information of the pilot tone is not included in the pilot allocation information. You may not.

Hereinafter, for convenience of description, it is assumed that the STA participating in the UL MU transmission has received pilot allocation information from the AP in step S2001.

The STA participating in the UL MU transmission maps pilots based on the pilot allocation information received in step S2001 (S2002).

That is, when the STA receives the pilot tone and / or pilot symbol, the STA maps the pilot to the assigned pilot tone and / or pilot symbol. In addition, when the STA is assigned a pilot sequence, the STA maps the assigned pilot sequence to the pilot tone.

The STA participating in the UL MU transmission transmits a pilot mapped UL MU PPDU to the AP (S2003).

General apparatus to which the present invention can be applied

21 is a block diagram illustrating a wireless device according to an embodiment of the present invention.

Referring to FIG. 21, an apparatus 2110 according to the present invention may include a processor 2111, a memory 2112, and an RF unit 2113. The apparatus 2110 may be an AP or a non-AP STA for implementing an embodiment according to the present invention.

The RF unit 2113 may be connected to the processor 2111 to transmit / receive a radio signal. For example, the physical layer according to the IEEE 802.11 system may be implemented.

The processor 2111 may be connected to the RF unit 2113 to implement a physical layer and / or a MAC layer according to the IEEE 802.11 system. The processor 2111 may be configured to perform an operation according to various embodiments of the present disclosure as described above with reference to FIGS. 1 to 20. In addition, a module for implementing an operation of an AP and / or STA according to various embodiments of the present disclosure according to FIGS. 1 to 20 may be stored in the memory 2112 and executed by the processor 2111.

The memory 2112 is connected to the processor 2111 and stores various information for driving the processor 2111. The memory 2112 may be included in the processor 2111 or may be installed outside the processor 2111 and connected to the processor 2111 by known means.

In addition, the device 2110 may have a single antenna or multiple antennas.

As described above, the specific configuration of the device 2110 may be implemented so that the above-described matters described in various embodiments of the present invention may be independently applied or two or more embodiments may be simultaneously applied.

FIG. 22 is a diagram more specifically illustrating the processor of FIG. 21 according to an embodiment of the present invention.

FIG. 22 illustrates the control unit in more detail when the device 2110 of FIG. 21 operates as a receiver.

The control unit may include an LTF sequence unscrambling unit 2201, a CFO estimator 2202, and a legacy channel estimation unit 2203 using P matrix dispreading. have.

An LTF sequence unscrambling unit 2201 unscrambles the HE-LTF sequence from signals received from one or more wireless devices.

That is, as in the 1-C method, the LTF sequence unscrambling unit 2201 is a P matrix code (ie, applied to the HE-LTF sequence and the HE-LTF sequence from the signal received by Equation 8). One row of the P matrix) may be removed (ie, unscrambled).

CFO estimator 2202 estimates the CFO if necessary.

The CFO estimator 2202 can be estimated based on the phase difference between the symbols on which the received signal is transmitted, and in particular, the tones (or subs) of the length of the P matrix code (ie, one row of the P matrix). It can be estimated by assuming that channels are the same for each carrier).

That is, the CFO estimator 2202 can extract a signal for each stream by Equation 9 on the assumption that the channel has a high correlation over the N_f subcarriers as in the 1-C method. In addition, a phase rotation is derived for each HE-LTF symbol by Equation 10, and based on a phase difference between the HE-LTF symbols derived by Equation 11, CFO estimates can be derived.

The legacy channel estimation unit 2203 using P matrix dispreading performs channel estimation using P matrix dispreading.

That is, the legacy channel estimation unit 2203 using P matrix dispreading as in the 1-C) scheme applies the CFO estimation value derived by Equation 12 to Equation 9 for each stream. Channel estimation values can be derived for each HE-LTF symbol.

The embodiments described above are the components and features of the present invention are combined in a predetermined form. Each component or feature is to be considered optional unless stated otherwise. Each component or feature may be embodied in a form that is not combined with other components or features. It is also possible to combine some of the components and / or features to form an embodiment of the invention. The order of the operations described in the embodiments of the present invention may be changed. Some components or features of one embodiment may be included in another embodiment or may be replaced with corresponding components or features of another embodiment. It is obvious that the claims may be combined to form an embodiment by combining claims that do not have an explicit citation relationship in the claims or as new claims by post-application correction.

Embodiments according to the present invention may be implemented by various means, for example, hardware, firmware, software, or a combination thereof. In the case of a hardware implementation, an embodiment of the present invention may include one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), FPGAs ( field programmable gate arrays), processors, controllers, microcontrollers, microprocessors, and the like.

In the case of implementation by firmware or software, an embodiment of the present invention may be implemented in the form of a module, procedure, function, etc. that performs the functions or operations described above. The software code may be stored in memory and driven by the processor. The memory may be located inside or outside the processor, and may exchange data with the processor by various known means.

It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential features of the present invention. Accordingly, the above detailed description should not be construed as limiting in all aspects and should be considered as illustrative. The scope of the invention should be determined by reasonable interpretation of the appended claims, and all changes within the equivalent scope of the invention are included in the scope of the invention.

The scheme for uplink multi-user transmission in the wireless communication system of the present invention has been described with reference to the example applied to the IEEE 802.11 system, but can be applied to various wireless communication systems in addition to the IEEE 802.11 system.

Claims (13)

1. In a method of performing a uplink (UL) multi-user (MU) transmission by a STA (Station) device in a wireless communication system,

   Receiving pilot allocation information from an access point (AP);

   Mapping a pilot to a pilot tone based on the pilot assignment information; And

   Transmitting the pilot-mapped UL MU Physical Protocol Data Unit (PPDU) to the AP,

   The uplink multi-user transmission method in which at least one of a pilot tone, a pilot symbol, and a pilot sequence is specified for each STA or STA group by the pilot assignment information.
2. The method of claim 1,

   The uplink multi-user transmission method according to the STA or STA group is assigned a pilot tone randomly or the same number, or a differential number according to the received signal strength indication (RSSI).
3. The method of claim 2,

   An uplink multi-user transmission method in which pilot tones are randomly or cyclically shifted for each STA or STA group at a predetermined symbol period.
4. The method of claim 3,

   The cyclic shift value is determined by randomness or according to a predetermined value or channel correlation between adjacent tones.
5. The method of claim 1,

   If a specific pilot sequence is allocated for each STA or STA group, the pilot is mapped to an adjacent pilot tone.
6. The method of claim 5,

   An uplink multi-user transmission method in which pilot tones are randomly or cyclically shifted for each STA or STA group at a predetermined symbol period.
7. The method of claim 6,

   The cyclic shift value is determined by randomness or according to a predetermined value or channel correlation between adjacent tones.
8. The method of claim 1,

   If a specific pilot tone is assigned to each STA group, uplink multi-user transmission method is assigned a specific pilot sequence for each STA in the STA group.
9. The method of claim 8,

   The uplink multi-user transmission method in which allocated pilot tones are exchanged between the STA groups at predetermined symbol periods.
10. The method of claim 1,

    The uplink multi-user transmission method according to the STA or STA group is assigned a pilot symbol random (random), the same number or a differential number according to the received signal strength indication (RSSI).
11. In a STA (Station) device in which a device performs uplink (UL) multi-user (MU) transmission in a wireless communication system,

    An RF unit for transmitting and receiving radio signals; And

    A processor for controlling the RF unit,

    The processor receives pilot allocation information from an access point (AP),

    Map a pilot to a pilot tone based on the pilot allocation information,

    The pilot is configured to transmit the mapped UL MU Physical Protocol Data Unit (PPDU) to the AP,

    At least one of a pilot tone, a pilot symbol, and a pilot sequence is specified for each of one or more STAs by the pilot assignment information.
12. In a method for supporting an uplink (UL) multi-user (MU) transmission in an access point (AP) device in a wireless communication system,

    Transmitting pilot allocation information to an STA participating in the UL MU transmission; And

    Receiving a UL MU Physical Protocol Data Unit (PPDU) in which a pilot is mapped to a pilot tone based on the pilot allocation information from the STA,

    Uplink multi-user transmission method in which one or more of a pilot tone, a pilot symbol, and a pilot sequence are specified for each of one or more STAs by the pilot allocation information.
13. An access point (AP) device supporting uplink (UL) multi-user (MU) transmission in a wireless communication system,

    An RF unit for transmitting and receiving radio signals; And

    A processor for controlling the RF unit,

    The processor transmits pilot allocation information to the STA participating in the UL MU transmission,

    Receive a UL MU Physical Protocol Data Unit (PPDU) in which pilots are mapped to pilot tones based on the pilot allocation information from the STA,

    At least one of a pilot tone, a pilot symbol, and a pilot sequence is specified for each of one or more STAs by the pilot assignment information.

Images