WO2016122156A1 - Cfo estimation method using reference signal in wireless lan system - Google Patents

Abstract

Disclosed is a CFO estimation method comprising: generating a first function which is defined by an RS received from two consecutive OFDM symbols with respect to a certain subcarrier; repeatedly performing a process of generating the first function with respect to all sets of subcarriers to which reference signals are transmitted; and determining, as a first residual CFO, the phase of a second function, which is the sum of repeatedly performed results, by using information regarding a pre-known reference signal.

Description

CFO Estimation Method Using Reference Signal in Wireless LAN System

The following description relates to a wireless communication system, and more particularly, to a method and apparatus for estimating a CFO using a reference signal in a WLAN system.

Recently, with the development of information and communication technology, various wireless communication technologies have been developed. Among these, WLAN is based on radio frequency technology, and can be used in homes, businesses, or businesses by using portable terminals such as personal digital assistants (PDAs), laptop computers, and portable multimedia players (PMPs). It is a technology that allows wireless access to the Internet in a specific service area.

In order to overcome the limitation of communication speed, which has been pointed out as a weak point in WLAN, recent technical standards have introduced a system that increases the speed and reliability of the network and extends the operating distance of the wireless network. For example, IEEE 802.11n supports High Throughput (HT) with data throughput up to 540 Mbps or more, and also uses multiple antennas at both the transmitter and receiver to minimize transmission errors and optimize data rates. Application of Multiple Inputs and Multiple Outputs (MIMO) technology has been introduced.

As next-generation communication technology, machine-to-machine communication technology has been discussed. In IEEE 802.11 WLAN system, a technical standard for supporting M2M communication is being developed as IEEE 802.11ah. In M2M communications, you may want to consider a scenario where you occasionally communicate a small amount of data at low speeds in an environment with many devices.

Communication in a WLAN system is performed in a medium shared between all devices. When the number of devices increases, such as M2M communication, it is necessary to improve the channel access mechanism more efficiently in order to reduce unnecessary power consumption and interference.

The present invention has been made to solve the above-mentioned problems of the general technique, and an object of the present invention is to accurately measure a CFO at a receiver.

Another object of the present invention is to improve the efficiency of the CFO estimation method using the blind method by utilizing the reference signal.

It is still another object of the present invention to ensure the performance of CFO estimation even in an environment where residual CFOs are represented by large values.

Technical objects to be achieved in the present invention are not limited to the above-mentioned matters, and other technical problems not mentioned above are provided to those skilled in the art from the embodiments of the present invention to be described below. May be considered.

CFO estimation method for solving the above technical problem, generating a first function defined by the reference signal (RS) received in two consecutive OFDM symbols for a particular subcarrier, the process of generating the first function Repeatedly performing the entire subcarrier set to which the reference signal is transmitted; determining the phase of the second function summating the repeated results as the first residual CFO by using information about a known reference signal; Compensating the received data using the one remaining CFO, and estimating a second remaining CFO from the compensated data using a blind scheme, for the subcarrier set for which the reference signal is not transmitted. .

The CFO estimation method may further include removing the effect of the CFO from the received data using the third remaining CFO defined as the sum of the first remaining CFO and the second remaining CFO.

The RS may be a Cell-specific RS (CRS), a Channel State Information Reference Signal (CSI-RS), or a DeModulation RS (DMRS).

The first function is defined according to the following equation, [Equation]

Where n is an OFDM symbol index, k is a subcarrier index,

Is the first function,

May represent a signal received from the k th subcarrier of the n th OFDM symbol.

The second function is defined according to the following equation, [Equation]

In the equation

Is the first residual CFO, n is the OFDM symbol index, L is the number of total OFDM symbols-1, k is the subcarrier index,

Is a set of subcarriers for which an RS is received in an nth OFDM symbol,

Is the first function, N is the length of OFDM symbol,

May indicate the length of a cyclic prefix (CP).

Data transmitted from the transmitter may be received using Binary Phase Shift Keying (BPSK), Quadrature BPSK (Quadrature BPSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (16-QAM), or 64-QAM.

The receiver for solving the above technical problem includes a transmitter, a receiver, and a processor operating in connection with the transmitter and the receiver. The first function defined by the method is generated, and the process of generating the first function is repeatedly performed for the entire subcarrier set to which the reference signal is transmitted, and the repeated results are performed by using information about a known reference signal. The phase of the summed second function is determined as the first residual CFO, the data received using the first residual CFO is compensated, and for the set of subcarriers for which the reference signal is not transmitted, the blind method is used from the compensated data. Estimate the second remaining CFO.

According to embodiments of the present invention, the following effects can be expected.

First, by measuring the residual CFO, the accuracy of the CFO estimation result of the preamble portion can be compensated.

Second, by performing the process of estimating the CFO using the signal known in advance in the receiver, it is possible to improve the performance of the CFO estimation using the blind method.

Third, CFO estimation performance can be optimized with low complexity even in environments where residual CFO values are high.

Effects obtained in the embodiments of the present invention are not limited to the above-mentioned effects, and other effects not mentioned above are commonly known in the art to which the present invention pertains from the description of the embodiments of the present invention. Can be clearly derived and understood by those who have In other words, unintended effects of practicing the present invention may also be derived by those skilled in the art from the embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are provided to facilitate understanding of the present invention, and provide embodiments of the present invention together with the detailed description. However, the technical features of the present invention are not limited to the specific drawings, and the features disclosed in the drawings may be combined with each other to constitute a new embodiment. Reference numerals in each drawing refer to structural elements.

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

2 is a diagram illustrating another exemplary structure of an IEEE 802.11 system to which the present invention can be applied.

3 is a diagram illustrating another exemplary structure of an IEEE 802.11 system to which the present invention can be applied.

4 is a diagram illustrating an exemplary structure of a WLAN system.

5 is a diagram illustrating a link setup process in a WLAN system.

6 is a diagram for describing a backoff process.

7 is a diagram for explaining hidden nodes and exposed nodes.

8 is a diagram for explaining an RTS and a CTS.

9 is a diagram for describing a power management operation.

10 to 12 are diagrams for explaining in detail the operation of the STA receiving the TIM.

13 is a diagram for explaining a group-based AID.

14 through 16 are diagrams illustrating examples of an operation of an STA when a group channel access interval is set.

17 to 19 are diagrams showing a frame structure and constellation related to the present invention.

20 is a diagram showing a pilot signal on a frequency axis according to the present invention.

21 and 22 are diagrams for explaining a BPSK-based CFO estimation method.

23 is a flowchart illustrating a BPSK based CFO estimation method.

24 to 26 are diagrams for explaining a CFO estimation method according to the present invention.

27 is a flowchart illustrating a CFO estimating method according to the present invention.

28 is a diagram illustrating a resource block for explaining a proposed embodiment.

29 is a flowchart illustrating a CFO estimation method according to an exemplary embodiment.

30 is a diagram illustrating a configuration of a terminal and a base station according to an embodiment.

The terms used in the present invention have been selected as widely used general terms as possible in consideration of the functions in the present invention, but this may vary according to the intention or precedent of the person skilled in the art, the emergence of new technologies and the like. In addition, in certain cases, there is also a term arbitrarily selected by the applicant, in which case the meaning will be described in detail in the description of the invention. Therefore, the terms used in the present invention should be defined based on the meanings of the terms and the contents throughout the present invention, rather than the names of the simple terms.

The following embodiments combine the components and features of the present invention in a predetermined form. Each component or feature may be considered to be optional unless otherwise stated. Each component or feature may be embodied in a form that is not combined with other components or features. In addition, some of the components and / or features may be combined to form an embodiment of the present 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.

In the description of the drawings, procedures or steps which may obscure the gist of the present invention are not described, and procedures or steps that can be understood by those skilled in the art are not described.

Throughout the specification, when a part is said to "comprising" (or including) a component, this means that it may further include other components, except to exclude other components unless specifically stated otherwise. do. In addition, the terms "... unit", "... group", "module", etc. described in the specification mean a unit for processing at least one function or operation, which is hardware or software or a combination of hardware and software. It can be implemented as. Also, "a or an", "one", "the", and the like are used differently in the context of describing the present invention (particularly in the context of the following claims). Unless otherwise indicated or clearly contradicted by context, it may be used in the sense including both the singular and the plural.

In the present specification, embodiments of the present invention have been described based on data transmission / reception relations between a base station and a mobile station. Here, the base station is meant as a terminal node of a network that directly communicates with a mobile station. The specific operation described as performed by the base station in this document may be performed by an upper node of the base station in some cases.

That is, various operations performed for communication with a mobile station in a network consisting of a plurality of network nodes including a base station may be performed by the base station or network nodes other than the base station. In this case, the 'base station' may be replaced by terms such as a fixed station, a Node B, an eNode B (eNB), an advanced base station (ABS), or an access point.

In addition, a 'mobile station (MS)' may be a user equipment (UE), a subscriber station (SS), a mobile subscriber station (MSS), a mobile terminal, an advanced mobile station (AMS), a terminal. (Terminal) or a station (STAtion, STA) and the like can be replaced.

Also, the transmitting end refers to a fixed and / or mobile node that provides a data service or a voice service, and the receiving end refers to a fixed and / or mobile node that receives a data service or a voice service. Therefore, in uplink, a mobile station may be a transmitting end and a base station may be a receiving end. Similarly, in downlink, a mobile station may be a receiving end and a base station may be a transmitting end.

In addition, the description that the device communicates with the 'cell' may mean that the device transmits and receives a signal with the base station of the cell. That is, a substantial target for the device to transmit and receive a signal may be a specific base station, but for convenience of description, it may be described as transmitting and receiving a signal with a cell formed by a specific base station. Similarly, the description of 'macro cell' and / or 'small cell' may not only mean specific coverage, but also 'macro base station supporting macro cell' and / or 'small cell supporting small cell', respectively. It may mean 'base station'.

Embodiments of the present invention may be supported by standard documents disclosed in at least one of the wireless access systems IEEE 802.xx system, 3GPP system, 3GPP LTE system and 3GPP2 system. That is, obvious steps or parts which are not described among the embodiments of the present invention may be described with reference to the above documents.

In addition, all terms disclosed in the present document can be described by the above standard document. In particular, embodiments of the present invention may be supported by one or more of the standard documents P802.16e-2004, P802.16e-2005, P802.16.1, P802.16p, and P802.16.1b standard documents of the IEEE 802.16 system. have.

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.

In addition, specific terms used in the embodiments of the present invention are provided to help the understanding of the present invention, and the use of the specific terms may be changed to other forms without departing from the technical spirit of the present invention.

1.IEEE 802.11 system general

1.1 Structure of WLAN System

1 is a diagram showing an exemplary structure 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 WLAN supporting station mobility (STA) that is transparent to a higher layer may be provided by their interaction. The Basic Service Set (BSS) may correspond to a basic building block in an IEEE 802.11 LAN. FIG. 1 exemplarily shows that two BSSs (BSS1 and BSS2) exist and include two STAs as members of each BSS (STA1 and STA2 are included in BSS1 and STA3 and STA4 are included in BSS2). do. 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 LAN is an independent BSS (IBSS). For example, the IBSS may have a minimal form consisting of only two STAs. In addition, the BSS (BSS1 or BSS2) 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, and 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).

2 is a diagram illustrating another exemplary structure of an IEEE 802.11 system to which the present invention can be applied. In FIG. 2, components such as a distribution system (DS), a distribution system medium (DSM), and an access point (AP) are added in the structure of FIG. 1.

The station-to-station distance directly in the LAN can be limited by physical performance. In some cases, this distance limit may be sufficient, but in some cases, communication between more distant stations may be necessary. The 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 nature of the distribution system media (DSM). In this regard, the IEEE 802.11 standard logically separates Wireless Medium (WM) and Distribution System Media (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 logically different, the flexibility of the IEEE 802.11 LAN structure (DS structure or other network structure) can be described. That is, the IEEE 802.11 LAN structure can be implemented in various ways, the corresponding LAN structure can be specified independently by the physical characteristics of each implementation.

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

An AP means an entity that enables access to a DS through WM for associated STAs and has STA functionality. Data movement between the BSS and the DS may be performed through the AP. For example, STA2 and STA3 shown in FIG. 2 have the functionality of a STA, and provide a function to allow associated STAs STA1 and STA4 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.

3 is a diagram illustrating another exemplary structure of an IEEE 802.11 system to which the present invention can be applied. 3 conceptually illustrates an extended service set (ESS) for providing wide coverage in addition to the structure of FIG. 2.

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 called an 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 LLC (Logical Link Control) layer. STAs included in the ESS may communicate with each other, and mobile STAs may move from within one BSS to another BSS (within the same ESS) transparently to the LLC.

In IEEE 802.11, nothing is assumed about the relative physical location of the BSSs in FIG. 3, and all of the following forms are possible. BSSs can be partially overlapped, which is a form commonly 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 than one) ESS network. This may be necessary if the ad-hoc network is operating at the location of the ESS network, if 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.

4 is a diagram illustrating an exemplary structure of a WLAN system. In FIG. 4, an example of an infrastructure BSS including a DS is shown.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In a WLAN system, an STA is a device that operates according to MAC / PHY regulations of IEEE 802.11. The STA includes an AP STA and a non-AP STA. Non-AP STAs are devices that users typically handle, such as laptop computers and mobile phones. In the example of FIG. 4, STA1, STA3, and STA4 correspond to non-AP STAs, and STA2 and STA5 correspond to AP STAs.

In the following description, a non-AP STA includes a terminal, a wireless transmit / receive unit (WTRU), a user equipment (UE), a mobile station (MS), and a mobile terminal. May be referred to as a Mobile Subscriber Station (MSS). In addition, the AP may include a base station (BS), a node-B, an evolved Node-B (eNB), and a base transceiver system (BTS) in other wireless communication fields. , A concept corresponding to a femto base station (Femto BS).

1.2 Link setup process

FIG. 5 is a diagram illustrating a general link setup process.

In order for an STA to set up a link and transmit / receive data with respect to a network, an STA first discovers the network, performs authentication, establishes an association, and authenticates for security. It must go through the back. The link setup process may also be referred to as session initiation process and session setup process. In addition, a process of discovery, authentication, association, and security establishment of a link setup process may be collectively referred to as association process.

An exemplary link setup procedure will be described with reference to FIG. 5.

In step S510, the STA may perform a network discovery operation. The network discovery operation may include a scanning operation of the STA. That is, in order for the STA to access the network, the STA must find a network that can participate. The STA must identify a compatible network before joining the wireless network. A network identification process existing in a specific area is called scanning. There are two types of scanning methods, active scanning and passive scanning.

5 exemplarily illustrates a network discovery operation including an active scanning process. In active scanning, the STA performing scanning transmits a probe request frame and waits for a response to discover which AP exists in the vicinity while moving channels. The responder transmits a probe response frame to the STA that transmits the probe request frame in response to the probe request frame. Here, the responder may be an STA that last transmitted a beacon frame in the BSS of the channel being scanned. In the BSS, the AP transmits a beacon frame, so the AP becomes a responder. In the IBSS, since the STAs in the IBSS rotate and transmit a beacon frame, the responder is not constant. For example, an STA that transmits a probe request frame on channel 1 and receives a probe response frame on channel 1 stores the BSS-related information included in the received probe response frame and stores the next channel (eg, number 2). Channel) to perform scanning (i.e., probe request / response transmission and reception on channel 2) in the same manner.

Although not shown in FIG. 5, the scanning operation may be performed by a passive scanning method. In passive scanning, the STA performing scanning waits for a beacon frame while moving channels. The beacon frame is one of management frames in IEEE 802.11. The beacon frame is notified of the existence of a wireless network and is periodically transmitted to allow the STA performing scanning to find the wireless network and participate in the wireless network. In the BSS, the AP periodically transmits a beacon frame, and in the IBSS, STAs in the IBSS rotate and transmit a beacon frame. When the STA that performs the scanning receives the beacon frame, the STA stores the information on the BSS included in the beacon frame and records beacon frame information in each channel while moving to another channel. Upon receiving the beacon frame, the STA may store BSS related information included in the received beacon frame, move to the next channel, and perform scanning on the next channel in the same manner.

Comparing active and passive scanning, active scanning has the advantage of less delay and power consumption than passive scanning.

After the STA discovers the network, an authentication process may be performed in step S520. This authentication process may be referred to as a first authentication process in order to clearly distinguish from the security setup operation of step S540 described later.

The authentication process includes a process in which the STA transmits an authentication request frame to the AP, and in response thereto, the AP transmits an authentication response frame to the STA. 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 STA may send an authentication request frame to the AP. The AP may determine whether to allow authentication for the corresponding STA based on the information included in the received authentication request frame. The AP may provide a result of the authentication process to the STA through an authentication response frame.

After the STA is successfully authenticated, the association process may be performed in step S530. The association process includes a process in which the STA transmits an association request frame to the AP, and in response thereto, the AP transmits an association response frame to the STA.

For example, the association request frame may include information related to various capabilities, beacon listening interval, service set identifier (SSID), supported rates, supported channels, RSN, mobility domain. Information about supported operating classes, TIM Broadcast Indication Map Broadcast request, interworking service capability, and the like.

For example, an association response frame may 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 Information, such as an indicator, a mobility domain, a timeout interval (association comeback time), an overlapping BSS scan parameter, a TIM broadcast response, and a QoS map.

This corresponds to some examples of information that may be included in the association request / response frame, and may be replaced with other information or further include additional information.

After the STA is successfully associated with the network, a security setup process may be performed at step S540. The security setup process of step S540 may be referred to as an authentication process through a Robust Security Network Association (RSNA) request / response. The authentication process of step S520 is called a first authentication process, and the security setup process of step S540 is performed. It may also be referred to simply as the authentication process.

The security setup process of step S540 includes, for example, performing a private key setup through 4-way handshaking through an Extensible Authentication Protocol over LAN (EAPOL) frame. can do. In addition, the security setup process may be performed according to a security scheme not defined in the IEEE 802.11 standard.

2.1 Evolution of WLAN

In order to overcome the limitation of communication speed in WLAN, IEEE 802.11n exists as a relatively recently established technical standard. 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) with data throughput of up to 540 Mbps and also uses multiple antennas at both the transmitter and receiver to minimize transmission errors and optimize data rates. It is based on Multiple Inputs and Multiple Outputs (MIMO) technology.

As the spread of the WLAN is activated and the applications using the same are diversified, a need for a new WLAN system for supporting a higher throughput than the data processing speed supported by IEEE 802.11n has recently emerged. Next-generation wireless LAN systems that support Very High Throughput (VHT) are the next version of the IEEE 802.11n wireless LAN system (e.g., IEEE 802.11ac, etc.), which are used in MAC Service Access Points (SAPs). It is one of the recently proposed IEEE 802.11 WLAN system to support data processing speed of 1Gbps or more.

The next generation WLAN system supports MU-MIMO (Multi User Multiple Input Multiple Output) transmission in which a plurality of STAs simultaneously access a channel in order to use the wireless channel efficiently. According to the MU-MIMO transmission scheme, the AP may simultaneously transmit packets to one or more STAs that are paired with MIMO.

In addition, supporting the WLAN system operation in whitespace has been discussed. For example, the introduction of WLAN systems in TV whitespace (TV WS), such as the idle frequency band (eg, 54-698 MHz band) due to the digitization of analog TV, is being discussed as an IEEE 802.11af standard. . However, this is merely an example, and whitespace may be referred to as a licensed band that can be preferentially used by a licensed user. An authorized user refers to a user who is authorized to use an authorized band and may also be referred to as a licensed device, a primary user, an incumbent user, or the like.

For example, an AP and / or STA operating in a WS should provide protection for an authorized user. For example, if an authorized user such as a microphone is already using a specific WS channel, which is a frequency band divided in a regulation to have a specific bandwidth in the WS band, the AP may be protected. And / or the STA cannot use a frequency band corresponding to the corresponding WS channel. In addition, the AP and / or STA should stop using the frequency band when the authorized user uses the frequency band currently used for frame transmission and / or reception.

Therefore, the AP and / or STA must be preceded by a procedure for determining whether a specific frequency band in the WS band is available, that is, whether there is an authorized user in the frequency band. Knowing whether there is an authorized user in a specific frequency band is called spectrum sensing. As the spectrum sensing mechanism, energy detection, signal detection, and the like are used. If the strength of the received signal is greater than or equal to a predetermined value, it may be determined that the authorized user is in use, or if the DTV preamble is detected, the authorized user may be determined to be in use.

In addition, M2M (Machine-to-Machine) communication technology is being discussed. In IEEE 802.11 WLAN system, a technical standard for supporting M2M communication is being developed as IEEE 802.11ah. M2M communication refers to a communication method that includes one or more machines (Machine), may also be referred to as MTC (Machine Type Communication) or thing communication. Here, a machine refers to an entity that does not require human direct manipulation or intervention. For example, a device such as a meter or a vending machine equipped with a wireless communication module, as well as a user device such as a smartphone that can automatically connect to a network and perform communication without a user's operation / intervention, can be used. This may correspond to an example. The M2M communication may include communication between devices (eg, device-to-device (D2D) communication), communication between a device, and an application server. Examples of device and server communication include communication between vending machines and servers, point of sale devices and servers, and electricity, gas or water meter readers and servers. In addition, applications based on M2M communication may include security, transportation, health care, and the like. Considering the nature of these applications, M2M communication should generally be able to support the transmission and reception of small amounts of data at low speeds in the presence of very many devices.

Specifically, M2M communication should be able to support a large number of STAs. In the currently defined WLAN system, it is assumed that a maximum of 2007 STAs are associated with one AP, but in M2M communication, there are methods for supporting a case where a larger number (approximately 6000 STAs) are associated with one AP. Is being discussed. In addition, many applications are expected to support / require low data rates in M2M communication. In order to support this smoothly, for example, in a WLAN system, an STA may recognize whether data to be transmitted to it is based on a TIM (Traffic Indication Map) element, and methods for reducing the bitmap size of the TIM are discussed. It is becoming. In addition, M2M communication is expected to be a lot of traffic with a very long transmission / reception interval. For example, very small amounts of data are required to be sent and received every long period (eg, one month), such as electricity / gas / water use. Accordingly, in the WLAN system, even if the number of STAs that can be associated with one AP becomes very large, it is possible to efficiently support the case where the number of STAs having data frames to be received from the AP is very small during one beacon period. The ways to do this are discussed.

As such, WLAN technology is rapidly evolving and, in addition to the above examples, technologies for direct link setup, media streaming performance improvement, support for high speed and / or large initial session setup, support for extended bandwidth and operating frequency, etc. Is being developed.

2.2 Media Access Mechanism

In a WLAN system according to IEEE 802.11, a basic access mechanism of MAC (Medium Access Control) is a carrier sense multiple access with collision avoidance (CSMA / CA) mechanism. The CSMA / CA mechanism is also called the Distributed Coordination Function (DCF) of the IEEE 802.11 MAC. It 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 period (e.g., during a DCF Inter-Frame Space (DIFS), before starting transmission. When the medium determines that the medium is in an idle state, the frame transmission is started through the medium, while the medium is occupied. If it is detected that the AP and / or STA does not start its own transmission, it waits by setting a delay period (for example, a random backoff period) for medium access and attempting frame transmission. By applying a random backoff period, since several STAs are expected to attempt frame transmission after waiting for different times, collisions can be minimized.

In addition, the IEEE 802.11 MAC protocol provides a hybrid coordination function (HCF). HCF is based on the DCF and the 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 multiple users, and HCCA uses 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).

6 is a diagram for describing a backoff process.

An operation based on an arbitrary backoff period will be described with reference to FIG. 6. When a medium that is occupy or busy is changed to an idle state, 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 the corresponding slot time, respectively, before attempting transmission. The random backoff count has a pseudo-random integer value and may be determined to be one of values in the range of 0 to CW. Here, CW is a contention window parameter value. The CW parameter is given CW_min as an initial value, but may take a double value in case of transmission failure (for example, when 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 2n-1 (n = 0, 1, 2, ...).

Once the random backoff process begins, the STA continues to monitor the medium while counting down the backoff slots according to the determined backoff count value. If the medium is monitored as occupied, the countdown stops and waits; if the medium is idle, it resumes the remaining countdown.

In the example of FIG. 6, when a packet to be transmitted to the MAC of the STA3 arrives, the STA3 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 also be transmitted in each of STA1, STA2, and STA5, and each STA waits for DIFS when the medium is monitored idle, and then counts down the backoff slot according to a random backoff count value selected by the STA. Can be performed.

In the example of FIG. 6, STA2 selects the smallest backoff count value and STA1 selects the largest backoff count value. In other words, the remaining backoff time of the STA5 is shorter than the remaining backoff time of the STA1 at the time when the STA2 finishes the backoff count and starts the frame transmission. STA1 and STA5 stop counting for a while and wait for STA2 to occupy the medium. When the occupation of the STA2 ends and the medium becomes idle again, the STA1 and the STA5 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 the STA5 is shorter than that of the STA1, the STA5 starts frame transmission.

 Meanwhile, while STA2 occupies the medium, data to be transmitted may also occur in STA4. At this time, when the medium is in an idle state, the STA4 waits for DIFS, performs a countdown according to a random backoff count value selected by the STA4, and starts frame transmission. In the example of FIG. 6, the remaining backoff time of STA5 coincides with an arbitrary backoff count value of STA4. In this case, a collision may occur between STA4 and STA5. If a collision occurs, neither STA4 nor STA5 receive an ACK, and thus data transmission fails. In this case, STA4 and STA5 may double the CW value, select a random backoff count value, and perform a countdown. Meanwhile, the STA1 waits while the medium is occupied due to transmission of the STA4 and STA5, waits for DIFS when the medium is idle, and starts frame transmission after the remaining backoff time passes.

2.3 Sensing operation of STA

As described above, the CSMA / CA mechanism 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 may use a network allocation vector (NAV). The NAV is a value in which an AP and / or STA currently using or authorized to use a medium instructs another AP and / or STA how long to 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. The NAV may be set, for example, according to the value of the "duration" field of the MAC header of the frame.

In addition, a robust collision detect mechanism has been introduced to reduce the possibility of collision. This will be described with reference to FIGS. 7 and 8. Although the actual carrier sensing range and transmission range may not be the same, it is assumed to be the same for convenience of description.

7 is a diagram for explaining hidden nodes and exposed nodes.

7A illustrates an example of a hidden node, in which STA A and STA B are in communication and STA C has information to transmit. In more detail, although STA A is transmitting information to STA B, it may be determined that the medium is idle when STA C performs carrier sensing before sending data to STA B. This is because transmission of STA A (ie, media occupation) may not be sensed at the location of STA C. In this case, since STA B receives the information of STA A and STA C at the same time, a collision occurs. At this time, STA A may be referred to as a hidden node of STA C.

FIG. 7B is an example of an exposed node, and STA B is a case in which STA C has information to be transmitted from STA D while transmitting data to STA A. FIG. In this case, when STA C performs carrier sensing, it may be determined that the medium is occupied by the transmission of STA B. Accordingly, since STA C is sensed as a medium occupancy state even if there is information to be transmitted to STA D, it must wait until the medium becomes idle. However, since STA A is actually outside the transmission range of STA C, transmission from STA C and transmission from STA B may not collide with STA A's point of view, so STA C is unnecessary until STA B stops transmitting. To wait. At this time, STA C may be referred to as an exposed node of STA B.

8 is a diagram for explaining an RTS and a CTS.

In order to efficiently use a collision avoidance mechanism in an exemplary situation as shown in FIG. 7, a short signaling packet such as a request to send (RTS) and a clear to send (CTS) may be used. The RTS / CTS between the two STAs may allow the surrounding STA (s) to overhear, allowing the surrounding STA (s) to consider whether to transmit information between the two STAs. For example, when an STA to transmit data transmits an RTS frame to an STA receiving the data, the STA receiving the data may inform the neighboring terminals that it will receive the data by transmitting the CTS frame to the surrounding terminals.

8A illustrates an example of a method for solving a hidden node problem, and assumes that both STA A and STA C try to transmit data to STA B. FIG. When STA A sends the RTS to STA B, STA B transmits the CTS to both STA A and STA C around it. As a result, STA C waits until data transmission between STA A and STA B is completed, thereby avoiding collision.

FIG. 8 (b) is an example of a method for solving an exposed node problem, and STA C overhears RTS / CTS transmission between STA A and STA B, so that STA C is another STA (eg, STA). It may be determined that no collision will occur even if data is transmitted to D). That is, STA B transmits the RTS to all the surrounding terminals, and only STA A having the data to actually transmit the CTS. Since STA C receives only RTS and not STA A's CTS, it can be seen that STA A is out of STC C's carrier sensing.

2.4 Power Management

As described above, in the WLAN system, channel sensing must be performed before the STA performs transmission and reception, and always sensing the channel causes continuous power consumption of the STA. The power consumption in the receive state is not significantly different from the power consumption in the transmit state, and maintaining the receive state is also a great burden for the power limited STA (ie, operated by a battery). Therefore, if the STA maintains a reception standby state in order to continuously sense the channel, it inefficiently consumes power without any particular advantage in terms of WLAN throughput. In order to solve this problem, the WLAN system supports a power management (PM) mode of the STA.

The power management mode of the STA is divided into an active mode and a power save (PS) mode. The STA basically operates in the active mode. The STA operating in the active mode maintains an awake state. The awake state is a state in which normal operation such as frame transmission and reception or channel scanning is possible. On the other hand, the STA operating in the PS mode operates by switching between a sleep state and an awake state. The STA operating in the sleep state operates at the minimum power, and does not perform frame scanning as well as channel scanning.

As the STA operates in the sleep state for as long as possible, power consumption is reduced, so the STA has an increased operation period. However, it is impossible to operate unconditionally long because frame transmission and reception are impossible in the sleep state. If there is a frame to be transmitted to the AP, the STA operating in the sleep state may transmit the frame by switching to the awake state. On the other hand, when the AP has a frame to transmit to the STA, the STA in the sleep state may not receive it and may not know that there is a frame to receive. Accordingly, the STA may need to switch to the awake state according to a specific period in order to know whether or not the frame to be transmitted to (or, if there is, receive it) exists.

9 is a diagram for describing a power management operation.

Referring to FIG. 9, the AP 210 transmits a beacon frame to STAs in a BSS at regular intervals (S211, S212, S213, S214, S215, and S216). The beacon frame includes a traffic indication map (TIM) information element. The TIM information element includes information indicating that the AP 210 is present with buffered traffic for STAs associated with it and will transmit a frame. The TIM element includes a TIM used to inform unicast frames and a delivery traffic indication map (DTIM) used to inform multicast or broadcast frames.

The AP 210 may transmit the DTIM once every three beacon frames. STA1 220 and STA2 222 are STAs operating in a PS mode. The STA1 220 and the STA2 222 may be configured to receive a TIM element transmitted by the AP 210 by switching from a sleep state to an awake state at every wakeup interval of a predetermined period. . Each STA may calculate a time to switch to the awake state based on its local clock. In the example of FIG. 9, it is assumed that the clock of the STA coincides with the clock of the AP.

For example, the predetermined wakeup interval may be set such that the STA1 220 may switch to the awake state for each beacon interval to receive the TIM element. Accordingly, the STA1 220 may be switched to an awake state when the AP 210 first transmits a beacon frame (S211) (S221). STA1 220 may receive a beacon frame and obtain a TIM element. When the obtained TIM element indicates that there is a frame to be transmitted to the STA1 220, the STA1 220 sends a PS-Poll (Power Save-Poll) frame requesting the AP 210 to transmit the frame, and the AP 210. It may be transmitted to (S221a). The AP 210 may transmit the frame to the STA1 220 in response to the PS-Poll frame (S231). After completing the frame reception, the STA1 220 switches to the sleep state again.

When the AP 210 transmits the beacon frame for the second time, the AP 210 does not transmit the beacon frame at the correct beacon interval because the medium is busy, such as another device accessing the medium. It can be transmitted at a delayed time (S212). In this case, the STA1 220 switches the operation mode to the awake state according to the beacon interval, but fails to receive the delayed beacon frame, and switches back to the sleep state (S222).

When the AP 210 transmits a beacon frame for the third time, the beacon frame may include a TIM element set to DTIM. However, since the medium is occupied (busy medium) state, the AP 210 delays transmission of the beacon frame (S213). The STA1 220 may operate by switching to an awake state according to the beacon interval, and may obtain a DTIM through a beacon frame transmitted by the AP 210. It is assumed that the DTIM acquired by the STA1 220 indicates that there is no frame to be transmitted to the STA1 220 and that a frame for another STA exists. In this case, the STA1 220 may determine that there is no frame to receive, and then switch to the sleep state again. The AP 210 transmits the frame to the STA after transmitting the beacon frame (S232).

The AP 210 transmits a beacon frame fourthly (S214). However, the STA1 220 cannot adjust the wakeup interval for receiving the TIM element because the STA1 220 cannot obtain information indicating that there is buffered traffic for itself through the previous two times of receiving the TIM element. Alternatively, when signaling information for adjusting the wakeup interval value of the STA1 220 is included in the beacon frame transmitted by the AP 210, the wakeup interval value of the STA1 220 may be adjusted. In this example, the STA1 220 may be configured to switch the operating state by waking up once every three beacon intervals from switching the operating state for TIM element reception every beacon interval. Accordingly, the STA1 220 cannot acquire the corresponding TIM element because the AP 210 maintains a sleep state at the time when the AP 210 transmits the fourth beacon frame (S214) and transmits the fifth beacon frame (S215).

When the AP 210 transmits a beacon frame for the sixth time (S216), the STA1 220 may operate by switching to an awake state and may acquire a TIM element included in the beacon frame (S224). Since the TIM element is a DTIM indicating that a broadcast frame exists, the STA1 220 may receive a broadcast frame transmitted by the AP 210 without transmitting the PS-Poll frame to the AP 210. (S234). Meanwhile, the wakeup interval set in the STA2 230 may be set in a longer period than the STA1 220. Accordingly, the STA2 230 may switch to the awake state at the time S215 at which the AP 210 transmits the beacon frame for the fifth time (S215) and receive the TIM element (S241). The STA2 230 may know that there is a frame to be transmitted to itself through the TIM element, and transmit a PS-Poll frame to the AP 210 to request frame transmission (S241a). The AP 210 may transmit the frame to the STA2 230 in response to the PS-Poll frame (S233).

For power saving mode operation as shown in FIG. 9, the TIM element includes a TIM indicating whether a frame to be transmitted to the STA exists or a DTIM indicating whether a broadcast / multicast frame exists. DTIM may be implemented through field setting of a TIM element.

10 to 12 are diagrams for explaining the operation of the STA receiving the TIM in detail.

Referring to FIG. 10, the STA may switch from a sleep state to an awake state to receive a beacon frame including a TIM from an AP, interpret the received TIM element, and know that there is buffered traffic to be transmitted to the AP. . After contending with other STAs for medium access for PS-Poll frame transmission, the STA may transmit a PS-Poll frame to request an AP to transmit a data frame. After receiving the PS-Poll frame transmitted by the STA, the AP may transmit the frame to the STA. The STA may receive a data frame and transmit an acknowledgment (ACK) frame thereto to the AP. The STA may then go back to sleep.

As shown in FIG. 10, the AP may operate according to an immediate response method after transmitting a data frame after a predetermined time (for example, short inter-frame space (SIFS)) after receiving a PS-Poll frame from the STA. Can be. Meanwhile, when the AP fails to prepare a data frame to be transmitted to the STA during the SIFS time after receiving the PS-Poll frame, the AP may operate according to a deferred response method, which will be described with reference to FIG. 11.

In the example of FIG. 11, the STA transitions from the sleep state to the awake state to receive the TIM from the AP and transmits the PS-Poll frame to the AP through contention as in the example of FIG. 10. If the AP does not prepare a data frame during SIFS even after receiving the PS-Poll frame, the AP may transmit an ACK frame to the STA instead of transmitting the data frame. When the data frame is prepared after transmitting the ACK frame, the AP may transmit the data frame to the STA after performing contention. The STA may transmit an ACK frame indicating that the data frame was successfully received to the AP and go to sleep.

12 illustrates an example in which the AP transmits a DTIM. STAs may transition from a sleep state to an awake state to receive a beacon frame containing a DTIM element from the AP. STAs may know that a multicast / broadcast frame will be transmitted through the received DTIM. The AP may transmit data (ie, multicast / broadcast frame) immediately after the beacon frame including the DTIM without transmitting and receiving the PS-Poll frame. The STAs may receive data while continuously awake after receiving the beacon frame including the DTIM, and may switch back to the sleep state after the data reception is completed.

2.5 TIM Structure

In the method of operating a power saving mode based on the TIM (or DTIM) protocol described with reference to FIGS. 9 to 12, the STAs have a data frame to be transmitted for themselves through STA identification information included in the TIM element. You can check. The STA identification information may be information related to an association identifier (AID), which is an identifier assigned by the STA at the time of association with the AP.

AID is used as a unique identifier for each STA in one BSS. For example, in the current WLAN system, the AID may be assigned to one of values from 1 to 2007. In the currently defined WLAN system, 14 bits may be allocated for an AID in a frame transmitted by an AP and / or STA, and an AID value may be allocated up to 16383, but in 2008, 16383 is set as a reserved value. It is.

The TIM element according to the existing definition is not suitable for the application of M2M application, where a large number of (eg, more than 2007) STAs may be associated with one AP. Extending the existing TIM structure as it is, the TIM bitmap size is so large that it cannot be supported by the existing frame format, and is not suitable for M2M communication considering low transmission rate applications. In addition, in M2M communication, it is expected that the number of STAs in which a received data frame exists during one beacon period is very small. Therefore, considering the application example of the M2M communication as described above, since the size of the TIM bitmap is expected to be large, but most bits have a value of 0, a technique for efficiently compressing the bitmap is required.

As a conventional bitmap compression technique, there is a method of defining an offset (or starting point) value by omitting consecutive zeros in front of a bitmap. However, if the number of STAs in which a buffered frame exists is small but the AID value of each STA is large, the compression efficiency is not high. For example, when only frames to be transmitted to only two STAs having AIDs of 10 and 2000 are buffered, the compressed bitmap has a length of 1990 but all have a value of 0 except at both ends. If the number of STAs that can be associated with one AP is small, the inefficiency of bitmap compression is not a big problem, but if the number of STAs increases, such inefficiency may be a factor that hinders overall system performance. .

As a solution to this problem, the AID may be divided into groups to perform more efficient data transmission. Each group is assigned a designated group ID (GID). AIDs allocated on a group basis will be described with reference to FIG. 13.

FIG. 13A illustrates an example of an AID allocated on a group basis. In the example of FIG. 13A, the first few bits of the AID bitmap may be used to indicate a GID. For example, the first two bits of the AID bitmap may be used to represent four GIDs. In the case where the total length of the AID bitmap is N bits, the first two bits (B1 and B2) indicate the GID of the corresponding AID.

FIG. 13A illustrates another example of an AID allocated on a group basis. In the example of FIG. 13B, the GID may be allocated according to the location of the AID. In this case, AIDs using the same GID may be represented by an offset and a length value. For example, if GID 1 is represented by an offset A and a length B, it means that AIDs A through A + B-1 on the bitmap have GID 1. For example, in the example of FIG. 13 (b), it is assumed that AIDs of all 1 to N4 are divided into four groups. In this case, AIDs belonging to GID 1 are 1 to N1, and AIDs belonging to this group may be represented by offset 1 and length N1. Next, AIDs belonging to GID 2 may be represented by offset N1 + 1 and length N2-N1 + 1, AIDs belonging to GID 3 may be represented by offset N2 + 1 and length N3-N2 + 1, and GID AIDs belonging to 4 may be represented by an offset N3 + 1 and a length N4-N3 + 1.

When the AID allocated based on such a group is introduced, by allowing the channel access to different time intervals according to the GID, the TIM element shortage problem for a large number of STAs can be solved, and an efficient data transmission and reception can be performed. have. For example, channel access may be allowed only to STA (s) corresponding to a specific group during a specific time interval, and channel access may be restricted to other STA (s). As such, a predetermined time interval in which only specific STA (s) are allowed to access may be referred to as a restricted access window (RAW).

Channel access based on the GID will be described with reference to FIG. 13 (c). FIG. 13C illustrates a channel access mechanism according to the beacon interval when the AID is divided into three groups. The first beacon interval (or the first RAW) is a period in which channel access of an STA corresponding to an AID belonging to GID 1 is allowed, and channel access of STAs belonging to another GID is not allowed. To implement this, the first beacon includes a TIM element only for AIDs corresponding to GID 1. The second beacon frame includes a TIM element only for AIDs having GID 2, so that only the channel access of the STA corresponding to the AID belonging to GID 2 is allowed during the second beacon interval (or second RAW). The third beacon frame includes a TIM element only for AIDs having GID 3, and thus only channel access of the STA corresponding to the AID belonging to GID 3 is allowed during the third beacon interval (or third RAW). The fourth beacon frame again includes a TIM element for only AIDs having GID 1, and thus only channel access of the STA corresponding to the AID belonging to GID 1 is allowed during the fourth beacon interval (or fourth RAW). Then, even in each of the fifth and subsequent beacon intervals (or fifth and subsequent RAWs), only channel access of the STA belonging to the specific group indicated in the TIM included in the beacon frame may be allowed.

In FIG. 13C, the order of GIDs allowed according to beacon intervals is cyclic or periodic, but is not limited thereto. That is, by including only the AID (s) belonging to a particular GID (s) in the TIM element, allowing channel access only to the STA (s) corresponding to the particular AID (s) during a particular time period (eg, a particular RAW). And operate in a manner that does not allow channel access of the remaining STA (s).

The group-based AID allocation scheme as described above may also be referred to as a hierarchical structure of the TIM. That is, the entire AID space may be divided into a plurality of blocks, and only channel access of STA (s) (that is, STAs of a specific group) corresponding to a specific block having a non-zero value may be allowed. Accordingly, the TIM can be divided into small blocks / groups so that the STAs can easily maintain the TIM information and manage the blocks / groups according to the class, quality of service (QoS), or purpose of the STA. 13 illustrates a two-level hierarchy, but a hierarchical TIM may be configured in the form of two or more levels. For example, the entire AID space may be divided into a plurality of page groups, each page group may be divided into a plurality of blocks, and each block may be divided into a plurality of sub-blocks. In this case, as an extension of the example of Fig. 13 (a), in the AID bitmap, the first N1 bits represent a page ID (i.e., PID), the next N2 bits represent a block ID, and the next N3 bits. Indicates a sub-block ID and may be configured in such a way that the remaining bits indicate the STA bit position within the sub-block.

In the examples of the present invention described below, various methods of dividing and managing STAs (or AIDs assigned to each STA) into predetermined hierarchical group units may be applied. It is not limited to the above examples.

2.6 Improved Channel Access Scheme

When AID is assigned / managed on a group basis, STAs belonging to a specific group may use the channel only in the "group channel access interval" (or RAW) assigned to the group. If the STA supports the M2M application, the traffic for the STA may have a characteristic that occurs according to a long period (eg, tens of minutes or hours). Since the STA does not need to maintain the awake state frequently, it is preferable to operate in the sleep mode for a long time and to occasionally switch to the awake state (that is, the wakeup interval of the corresponding STA is set long). As described above, an STA having a long period of wake-up interval may be referred to as an STA operating in a "long-sleeper" or "long-sleep" mode. However, the reason why the wakeup period is set to be long is not necessarily limited to M2M communication, and the wakeup interval may be set to be long according to the state of the STA or the surrounding situation in general WLAN operation.

If the wakeup interval is set, the STA may determine whether the wakeup interval has been elapsed based on its local clock. However, since the local clock of the STA generally uses a low-cost oscillator, there is a high probability of error, and furthermore, if the STA operates in the long-sleep mode, the error may become larger over time. Therefore, the time synchronization of the STA that wakes up from time to time may not match the time synchronization of the AP. For example, the STA calculates that it is a timing at which it can receive a beacon and switches to an awake state, but may not receive a beacon actually transmitted by the AP at that timing. That is, the STA may miss the beacon frame due to a clock drift, and this problem may occur a lot when the long-sleep mode is operated.

14 through 16 are diagrams illustrating examples of an operation of an STA when a group channel access interval is set.

In the example of FIG. 14, STA3 is an STA belonging to group 3 (ie, GID = 3), and wakes up from a channel access interval assigned to group 1 (ie, GID = 1) to request a PS-Poll requesting transmission of a frame to the AP. Can be done. The AP, which receives the PS-Poll from the STA, transmits an ACK frame to the STA3. If the AP has buffered data to transmit to the STA3, the AP provides information indicating the ACK frame (that is, the information to be transmitted). can do. For example, the information may be indicated by setting the value of the 1-bit “More Data” field (or MD field) included in the ACK frame to 1 (that is, MD = 1).

Here, since the STA3 transmits the PS-Poll belongs to the channel access interval for group 1, the AP does not transmit data immediately after transmitting the ACK frame even if there is data to be transmitted to STA3, but is allocated to group 3 to which STA3 belongs. Data to STA3 is transmitted in the received channel access interval (GID 3 channel access of FIG. 14).

Since STA3 has received an ACK frame set to MD = 1 from the AP, it continues to wait for data to be transmitted from the AP. That is, in the example of FIG. 14, since STA3 does not receive a beacon immediately after waking up, the time point at which the user wakes up may be a channel access interval assigned to the group to which he or she belongs according to a calculation based on his local clock. The PS-Poll is sent to the AP, assuming there may be data to be sent to the AP. Or, suppose that time synchronization may not be correct because it operates in the long-sleep mode, and STA3 may send a PS-Poll to the AP to receive if there is data to be transmitted to it. Since the ACK frame received from the AP indicates that there is data to be transmitted to the STA3, the STA3 continues to wait for data reception under the assumption that its channel access is allowed. The STA3 consumes power unnecessarily even though data reception is not allowed until the time synchronization is correctly performed from the information included in the next beacon.

In particular, when the STA3 operates in the long-sleep mode, since the beacon is not frequently received, unnecessary power consumption may occur, such as performing CCA even when the STA3 does not belong to the channel access interval.

Next, the example of FIG. 15 illustrates a case in which an STA having GID 1 (ie, belonging to group 1) misses a beacon frame at a wake up timing. That is, an STA that does not receive a beacon including a GID (or PID) assigned to itself continues to wait in an awake state until it receives a beacon including its GID (or PID). That is, even though the time when the STA wakes up is a channel access interval assigned to the STA, the STA has not checked whether the TIM transmitted through the beacon includes its GID (or PID), and thus the timing is It is not known whether the channel access interval is assigned to the group.

As such, in the example of FIG. 15, the STA transitioned from the sleep state to the awake state continues after the first beacon until it receives the fourth beacon including its GID (ie, GID 1). This consumes unnecessary power. After all, after consuming unnecessary power, the STA can receive the beacon including the GID 1 and thus perform the RTS transmission, the CTS reception, the data frame transmission, and the ACK reception.

FIG. 16 illustrates a case where the STA wakes up at a channel access interval for another group. For example, an STA with GID 3 may wake up at a channel access interval for GID 1. That is, the STA having the GID 3 consumes power unnecessarily until the beacon corresponding to its GID is received after waking up. When receiving the TIM indicating the GID 3 in the third beacon, it can recognize the channel access interval for its group and perform operations such as data transmission and ACK reception after CCA through RTS and CTS.

3. Proposed CFO Estimation Method 1

In an environment where diverse interests in next-generation WiFi and demands for improving 802.11a and higher quality of experience (QoE) performance are increasing, it is necessary to define a new frame format for the next-generation WLAN system. In particular, the most important part of the new frame format is the preamble part, and the design of the preamble that plays the role of synchronization, channel tracking, channel estimation, adaptive gain control (AGC), etc. This is even more important because it can have a direct and big impact.

In a next generation WiFi system in which a large number of APs and STAs simultaneously access data and attempt to transmit and receive data, system performance may be limited by a conventional preamble design scheme. That is, each preamble block (eg, a short training field (STF) that plays a role of AGC, CFO estimation / compensation, timing adjustment, etc.), or a channel estimation / compensation, residual CFO compensation, etc. If the responsible Long Training Field (LTE) performs only the respective roles defined in the conventional preamble structure, the frame length may be increased and a burden on overhead may occur. Therefore, if a specific preamble block can support various functionalities in addition to a predetermined role, an efficient frame structure can be designed.

In addition, since the next generation WiFi system considers data transmission not only indoors but also in outdoor environments, the structure of the preamble may need to be designed differently according to the environment. Of course, designing a unified preamble format that is independent of environmental changes may help system implementation and operation, but it is desirable to be adaptively designed for the system environment.

Hereinafter, a preamble design for efficiently supporting various functions as described above is proposed. For convenience, the new WLAN system is called a High Efficiency (HE) system or a High Efficiency WLAN (HEW) system, and the frame of the HE system and the Physical Layer Convergence Procedure (PLCP) Protocol Data Unit (PPDU) are named HE frame and HE PPDU, respectively. do. However, it is apparent to those skilled in the art that the proposed preamble can be applied not only to the HE system but also to other WLAN systems and cellular systems.

Table 1 below shows Orthogonal Frequency Division Multiplexing (OFDM) numerology, which is a premise of the CFO estimation method proposed below. Table 1 is an example of a new OFDM numerology proposed by the HE system, the figures and items described are merely examples and other values may be applied. Table 1 is based on an example of applying a FFT four times larger than the conventional BW, and it is assumed that three DCs are used for each BW.

Table 1

Parameter	CBW20	CBW40	CBW80	CBW80 + 80	CBW160	Description
N FFT	256	512	1024	1024	2048	FFT size
N SD	238	492	1002	1002	2004	Number of complex data numbers per frequency segment
N
SP	4	6	8	8	16	Number of pilot values per frequency segment
N ST	242	498	1010	1010	2020	Total number of subcarriers per frequency segment. See NOTE.
N SR	122	250	506	506	1018	Highest data subcarrier index per frequency segment
N Seg	One	One	One	2	One	Number of frequency segments
Δ F	312.5 kHz					Subcarrier frequency spacing for non-he portion
Δ F_H E	78.125 kHz					Subcarrier frequency spacing for HE portion
T DFT	3.2 μs					IDFT / DFT period for non-HE portion
T DFT_HE	12.8 μs					IDFT / DFT period for HE portion
T GI	0.8 μs = T DFT / 4					Guard interval duration for non-HE portion
T GI_HE	3.2 μs = T DFT_HE / 4					Guard interval duration for HE portion
T GI2	1.6 μs					Double guard interval for non-HE portion
T GIS_HE	0.8 μs = T DFT_HE / 16 [Alternative: 0.4 μs (1/32 CP)]					Short guard interval Duration (used only for HE data)
T SYML	4 μs = T DFT + T GI					Long GI symbol interval for non-HE portion
T SYML_HE	16 μs = T DFT_HE + T GI_HE					Long GI symbol interval for HE portion
T SYMS_HE	13.6 μs = T DFT_HE + T GIS_HE [Alternative: 13.2 μs (with 1/32 CP)]					Short GI symbol interval (used only for HE data)
T SYM	T SYML or T SYMS depending on the GI used					Symbol interval for non-HE portion
T SYM_HE	T SYML_HE or T SYMS_HE depending on the GI used					Symbol interval for HE portion
T L-STF	8 μs = 10 * T DFT / 4					Non-HE Short Training field duration
T L-LTF	8 μs = 2 x T DFT + T GI2					Non-HE Long Training field duration
T L-SIG	4 μs = T SYML					Non-HE SIGNAL field duration
T HE-SIGA	12.8 μs = 2 (T SYML + 3T GI ) in HE- PPDU format-1 or TSYML_HE in HE- PPDU format-2 and HE-PPDU format-3					HE Signal A field duration
T HE-STF	T SYML_HE					HE Short Training field duration
T HE-LTF	T SYML_HE					Duration of each HE LTF symbol
T HE-SIGB	T SYML_HE					HE Signal B field duration
N service	16					Number of bits in the SERVICE field
N tail	6					Number of tail bits per BCC encoder
NOTE: N ST = N SD + N SP

17 is a diagram illustrating a frame structure according to an embodiment of the present invention. As shown in the examples of FIGS. 17A, 17B, and 17C, the frame structure may be implemented in various forms. The proposed CFO estimation method is one of preambles among the frame structures. It is mainly related to HE-STF (High Efficiency Short Training Field).

18 and 19 illustrate a frame structure and a constellation according to another embodiment. 18 (a) shows a frame structure on a time axis of a high throughput (HT) system according to 802.11n. In FIG. 18 (a), L-SIG and HT-SIG represent a Legacy Signal Field and a High Throughput Signal Field, respectively. When defining one OFDM symbol length as 4us, L-SIG corresponds to one OFDM symbol, while HT-SIG corresponds to two OFDM symbols.

Meanwhile, when data transmission is performed according to the frame structure shown in FIG. 18 (a), system information transmitted to the terminal is mapped and transmitted to the constellation diagram as shown in FIG.

19 (a) shows a frame structure of a very high throughput (VHT) according to 802.11ac. Similar to the case of FIG. 18, in the VHT system, system information is transmitted to the terminal using the L-SIG and VHT-SIG-A fields of FIG. 19 (a), and is mapped to the constellation diagram of FIG. 19 (b) and transmitted. do.

20 illustrates a pilot signal on a frequency axis in accordance with a proposed embodiment.

As described in FIGS. 18 and 19, the terminal (or receiver) receives a fast fourier transform (FFT) operation according to a field of L-SIG, HT-SIG, VHT-SIG-A, etc. from the base station (or transmitter). The operation result can be expressed as shown in FIG. 20.

20 shows a pilot signal converted on a frequency axis for each OFDM symbol. In FIG. 20, a signal received at each subcarrier may be expressed as Equation 1 below.

[Equation 1]

In Equation 1, k denotes an index of a subcarrier, and n denotes an index of an OFDM symbol.

Denotes a channel in the nth OFDM symbol and the kth subcarrier,

Data signal transmitted through

Suppose, the received signal is in Equation 1

It can be expressed as.

Meanwhile, as shown in FIG. 20, some subcarriers are configured with a guard interval or direct current (DC) component, and these subcarriers are set to null without carrying a data signal. On the other hand, the index set of subcarriers carrying a data signal is defined as C below.

Before explaining the proposed CFO estimation method, the concept of CFO is explained first. Carrier Frequency Offset (CFO) is caused by the Doppler effect or the performance of the oscillator included in the transmitter and receiver. CFOs can be divided into integer and fractional parts (for example, if CFO is 2.5, integer is 2, fraction is 0.5), and integer CFO will result in circular shifting of subcarriers by that value, Fractional CFO causes interference between subcarriers.

Meanwhile, in the HT and VHT systems, the receiver estimates the CFO value using the L-STF field and the L-LTF field, and applies the estimation result to the received OFDM symbol after the estimation of the CFO, thereby influencing the CFO as shown in Equation 2. Remove it.

[Equation 2]

In equation (2)

Is the actual CFO value,

Denotes an estimated CFO value. y denotes a received signal vector when there is a CFO, x denotes a received signal vector when there is no CFO, and n denotes a noise vector. Meanwhile, in Equation 2, the diagonal matrix D is defined as Equation 3 below.

[Equation 3]

If the receiver completely estimates the CFO value using L-STF and L-LTF (

The receiver can completely remove the CFO from the received signal using Equations 2 and 3

). However, because CFOs change little by little over time, CFOs are difficult to estimate perfectly (

Accordingly, the residual CFO is defined as in Equation 4.

[Equation 4]

In equation (4)

Means the CFO value changed over time. The receiver uses the pilot signals included in the L-SIG and HT-SIG to reestimate the remaining CFO. In the HT system, four pilot signals are used to estimate the residual CFO. However, in the HT system, since the number of pilot signals is relatively small, the performance of the CFO estimation is drastically reduced when the SNR is low. To compensate for this problem, the number of pilot signals must be increased, but a decrease in yield occurs as a trade-off. Therefore, there is a need for a CFO estimation method for minimizing performance degradation at low SNR while still using the structure of the HT system.

Hereinafter, the CFO estimation method will be described with reference to FIGS. 21 to 23. According to the proposed embodiment, the receiver can estimate the CFO through a blind method using a data signal rather than a pilot signal.

21 and 22 are diagrams for explaining a proposed CFO estimation method. 21 and 22 illustrate a case in which data transmission is performed according to a binary phase shift keying (BPSK) scheme or a quadrature phase shift keying (QBPSK) scheme.

According to the proposed CFO estimation method, the received signals of two consecutive OFDM symbols are first considered based on Equation (1).

Is defined as in Equation 5.

[Equation 5]

In Equation 5, L is defined as (number of total OFDM symbols-1 to which the proposed CFO estimation method is applied). For example, L = 1 when two OFDM symbols are used as shown in FIG. 20, and L = 3 when three OFDM symbols such as L-SIG and HT-SIG shown in FIG. 18A are used.

Equation 5 will be described in detail. In two consecutive n, n + 1 th OFDM symbols, the channel does not change rapidly. Accordingly, in equation (5)

Is defined on the assumption that the channels of two consecutive OFDM symbols are the same.

In the proposed CFO estimation method,

When is calculated, the calculation process of Equation 6 below is performed.

[Equation 6]

That is, in equation (6)

According to the real part of

Is determined. Then, Equation 7 below is a process of determining the final residual CFO.

[Equation 7]

In equation (7)

Is the final calculated residual CFO value, where N and

Denotes the length of the OFDM symbol and the cyclic prefix (CP) length, respectively. It can be seen from Equation 7 that the processes according to Equations 5 and 6 are performed for the entire subcarrier set C containing data.

Hereinafter, the process of Equations 5 to 7 will be described in detail. First, assuming n = 1 in Equation 5,

Is expressed by Equation 8 below.

[Equation 8]

In Equation 8,

Data signal

Represents the power component of,

The function has a value of 1 if the sign of variable a is positive and -1 if negative. On the other hand, the approximation of the second line in Equation 8 is made by ignoring the interference between adjacent subcarriers due to the remaining CFO. Also, the approximation of the fourth line is a channel of two OFDM symbols.

Wow

Is made through the assumption that is equal. Considering that the BPSK method is applied together with the above assumptions,

Relationships are always established.

In equation (8) If is

Is expressed as in Equation (9).

[Equation 9]

Referring to FIG. 21, Equation 9

The component represents the radius of the circle shown,

It can be seen that the component represents the phase value of the illustrated point. At this time,

The phase value of is the residual CFO (

) Is proportional to the value of the remaining CFO. For example, if the remaining CFO value is zero,

The phase of is also zero. if

If the phase of is less than 2π, the residual CFO and

The phase of 1 corresponds to 1: 1. therefore,

The residual CFO value can be estimated from the phase of.

Contrary,

In Equation 9, Equation 9 is expressed as shown in FIG. If the receiver is

If you know that this is a simple calculation

The residual CFO can be estimated from the phase of. However, this calculation has a problem that the receiver must know in advance about the received data. Since the proposed CFO estimation method is a blind method that does not use a pilot signal,

You need to know whether the value is positive or negative so that you can accurately estimate the residual CFO.

To solve this process, consider the case where the residual CFO value is a relatively small portion of the total CFO. That is, the receiver first estimates the CFO value using preambles such as L-STF and L-LTF, and secondly estimates the residual CFO value through L-SIG and HT-SIG. You can consider. If the approximate CFO value is estimated through the primary CFO estimation process, the phase of the remaining CFO becomes a relatively small value. therefore,

The range of phases of do not have a large value, and Equation 10 may be derived.

[Equation 10]

In other words,

If the phase of corresponds to quadrant or quadrant,

Can be seen as. Also,

If the phase of corresponds to quadrant 2 or quadrant 3,

Can be seen as. Equation 6 described above is described by the relationship between the residual CFO and Equation 10. That is, according to equation (6)

In relation to

It can be seen that the phase of is changed by π.

On the other hand, in the noise-free environment, all calculated through the equation (6)

Phase of

to be. That is, the calculation result for all subcarriers is in-phase or co-phase. Thus, Equation 7 enables accurate estimation of the residual CFO in a noisy environment. In Equation 7, since both the phase and the power of the signal are summed, the result is robust to the dominant noise.

According to the method described above, the total CFO can be accurately measured by estimating the remaining CFO using the blind method. In addition, even when the SNR / SINR is low, the overhead of additionally transmitting pilot signals is not required, so that efficient communication is also possible.

Meanwhile, according to an embodiment of the present disclosure, the subcarrier set to which the CFO estimation method is applied may be replaced with C ', which is a subset of C, rather than the entire subcarrier set C containing data. That is, the process of estimating the residual CFO through the phase of the sum of all the samples in Equation 7 is performed. In this case, the residual CFO can be estimated even if only some subcarriers are used instead of all subcarrier samples. Therefore, a subset of C on which the data signal is loaded is defined as C ', and C' may be applied instead of C in Equation 7.

This means that if a fading phenomenon occurs seriously in a particular subcarrier, the size of data received in that subcarrier is very small. In this case, that data makes little contribution to estimating the residual CFO compared to other data samples. In other words, even if only relatively large data samples are used, the residual CFO can be estimated, and even if some small samples are excluded, there is almost no degradation in performance.

According to another embodiment using this embodiment, when the receiver knows the magnitude of the received data signal for each subcarrier, the subcarriers having the magnitude greater than or equal to the threshold value are aligned to C 'in order of increasing magnitude. Can be defined According to this embodiment, as the complexity of the receiver performing the processes of Equations 5 to 7 is reduced, it is possible to minimize the decrease in the estimated performance of the remaining CFO.

On the other hand, only the case where the BPSK method or the QBPSK method is applied has been considered. However, the case where the BPSK scheme and the QBPSK scheme are used alternately for each OFDM symbol may also be considered. That is, when two consecutive OFDM symbols are mapped to different constellations, even if they are multiplied by two received signals, 1 or -1 cannot be obtained, and thus the method proposed above cannot be applied as it is.

In this case, the residual CFO is estimated through Equation 11 below.

[Equation 11]

Equation 11 replaces Equation 5 described above. In Equation 11, a received signal in an OFDM symbol using QBPSK

Change phase by On the other hand, a received signal in an OFDM symbol using BPSK does not phase change. According to Equation 11, even if a series of OFDM symbols alternately use BPSK and QBPSK, the results according to Equations 6 and 7 can be obtained in the same manner.

23 is a flowchart illustrating a proposed CFO estimation method. 23 illustrates a time series flow of a CFO estimating method according to an embodiment described above. Therefore, although not specifically illustrated or described in FIG. 23, it can be easily understood that the contents described above with reference to FIGS. 18 through 22 may also be applied to FIG. 23.

First, the transmitter transmits data to the receiver (S2310). Data is transmitted in units of frames defined by OFDM symbols and subcarriers, and is mapped to specific constellations and transmitted to a receiver. In this constellation, either BPSK or QBPSK may be used, and BPSK and QBPSK may be alternately used for a series of OFDM symbols.

On the other hand, the receiver first estimates the CFO value from the received signal (S2320). This process is performed using a preamble portion such as L-STF, L-LTF in the frame. However, since the CFO value changes with time, the CFO value estimated in S2320 may not be an accurate CFO value.

Therefore, the receiver first estimates the residual CFO value to compensate for the estimated CFO value (S2330). As described above, the receiver estimates the residual CFO under the assumption that the channels of the received signals of two consecutive OFDM symbols are the same. Specifically, the receiver checks the sign of the real part of the result of multiplying two received signals, assuming that the residual CFO is smaller than the first estimated CFO value. When the entire subcarrier is in-phase, the receiver can obtain the residual CFO from the phase value calculated by summing up the results for all subcarriers.

Finally, the receiver can accurately decode the data transmitted by the transmitter by removing the influence of the CFO value estimated at S2320 and the residual CFO value estimated at S2330 in the received signal.

4. Proposed CFO Estimation Method 2

24 to 26 illustrate a CFO estimating method according to an exemplary embodiment. In the above, the CFO estimation method when the BPSK and / or QBPSK method is used has been described, and the CFO estimation method when the data transmission using the Quadrature Phase Shift Keying (QPSK) method is described below.

Similar to the process described in Equation 5, first, considering the received signals of two consecutive OFDM symbols based on Equation 1

This is defined as in Equation 12.

[Equation 12]

In Equation 12, L is defined as (number of total OFDM symbols-1 to which the proposed CFO estimation method is applied). For example, L = 1 when two OFDM symbols are used as shown in FIG. 20, and L = 3 when three OFDM symbols such as L-SIG and HT-SIG shown in FIG. 18A are used.

Explaining Equation 12 in detail, for two consecutive n, n + 1 th OFDM symbols, the channel does not change rapidly. Accordingly, in Equation 12

Is defined on the assumption that the channels of two consecutive OFDM symbols are the same.

Then, at the receiver

When is calculated, the calculation process of Equation 12 below is performed.

[Equation 13]

In Equation 13,

According to the sign and magnitude of the real and imaginary parts of

Is determined. As will be described later in detail, four different cases in Equation 13 correspond to four different quadrants on the constellation. therefore,

According to each quadrant to which

The function is generated in a different way as defined in equation (13).

Equation 14 is then a process of determining the final residual CFO based on Equation 13.

[Equation 14]

In equation (14)

Is the residual CFO value finally calculated by the receiver, where N and

Denotes the length of the OFDM symbol and the cyclic prefix (CP) length, respectively. It can be seen from Equation 14 that the processes according to Equations 12 and 13 are performed for the entire subcarrier set C on which data is loaded.

Hereinafter, the process of Equations 12 to 14 will be described in detail. first,

May be expressed as in Equation 15 below through the approximation process described in Equation 8.

[Equation 15]

In case of data transmission using QPSK method,

The distribution of values is one of four points shown in FIG. That is, the phase of the product of data transmitted through two consecutive OFDM symbols has a value of any one of four points on the constellation shown in FIG. Meanwhile, in FIG. 24

The value is

Is defined as

[Equation 16]

As shown in FIG. 24,

It can be seen that the phase of is one of {0, π / 2, π, π / 3}.

On the other hand, if the size of the residual CFO to be measured at the receiver is relatively small compared to the total CFO value,

The value of may be represented by the examples shown in FIG. 25. That is, the residual CFO is relatively small compared to the CFO value primarily estimated through the preamble portion. Thus, a function used to estimate residual CFO

It can be seen that the phase of does not have a large value, as shown in FIG.

The phase of does not deviate significantly from the phase values of the four points on the QPSK constellation.

If there is no assumption that the size of the residual CFO is relatively small, a phase ambiguity problem occurs in the process of measuring the residual CFO. However, according to the assumptions described above,

The magnitude of the phase change of {0, π / 2} (first embodiment of FIG. 25), {π / 2, π} (second embodiment of FIG. 25), {π, 3π / 2} (FIG. 25) 3), or {3π / 2, 2π} (fourth embodiment of FIG. 25).

From this result, Equation 17 below is derived.

[Equation 17]

That is, according to Equation 17

By comparing and analyzing the sign and magnitude of the real and imaginary parts of

You can see which of the four quadrants in this constellation. E.g,

this

If you satisfy

Corresponds to the first embodiment in Equation 17 and FIG. 25 (

You can see that. In other words,

Even though the receiver does not correctly decode

Through the phase value of

It is possible to estimate the phase of. In other words, even if the CFO is estimated according to the blind decoding method, the phase ambiguity problem does not occur.

Meanwhile,

If is confirmed which of the four cases, according to equation (13)

This function

Is generated. All generated according to equation (13)

Is in-phase (or co-phase) state, so

Finally, the residual CFO is estimated based on Eq.

On the other hand, according to the embodiment proposed above,

The larger the size and the smaller the residual CFO, the better the accuracy of the estimated residual CFO. This is because the effect of noise on the final residual CFO can be reduced.

Meanwhile, according to another proposed embodiment, a case in which a series of OFDM symbols alternately use BPSK and QPSK may be considered. That is, similarly to the case where the BPSK scheme and the QBPSK scheme are alternately applied, the BPSK and the QPSK may be alternately used in two consecutive OFDM symbols. In this case, the phase of the result of multiplying the two received signals is not located at the four points previously shown in FIG. 24, and thus the embodiment described above cannot be applied as it is.

In this case, the residual CFO is estimated through Equation 18.

Equation 18

Equation 18 replaces Equation 12, and in Equation 18, a phase of a received signal in an OFDM symbol using BPSK is uniformly changed by π / 4. In this case, the constellation of the BPSK whose phase is changed as shown in FIG. 26 becomes the same as part of the constellation of the QPSK. Therefore, after Equation 18, the above-described embodiments may be applied as it is.

Meanwhile, in this embodiment, the angle of changing the phase is

Can be (

Is a set of integers). That is, it is meaningful to change the BPSK constellation to be a part of the QPSK constellation, and the exact value of the phase value may vary. According to the embodiment described above, in all OFDM symbols using BPSK

Their phase value uniformly

As long as it is changed.

However, each

Even if these phases are rotated using different values, the same results as described above are obtained. E.g,

Phase value of is rotated by π / 4,

Even when the phase value of is rotated by 3π / 4, the same result as in the case where the same phase value is rotated for two received signals according to equation (18). This is because Equation 18 is simply for moving the points on the BPSK constellation to the points of the QPSK.

The embodiment described above may be equally applied to the case where QBPSK and QPSK are alternately used for each OFDM symbol. That is, when QBPSK and QPSK are used in two consecutive OFDM symbols, the above-described CFO estimation process may be applied as it is by changing the phase value of a symbol in which QBPSK is used instead of BPSK in Equation 18.

According to another embodiment, when a series of OFDM symbols alternately use BPSK and QPSK, Equation 19 below may be used instead of Equation 12.

[Equation 19]

Unlike Equation 18, Equation 19 does not change the phase of the received signal for each OFDM.

Change the phase of the whole by π / 4. Equation 19 holds regardless of whether the n th OFDM symbol is BPSK or QPSK. This is because the BPSK and the QPSK are alternately used for each consecutive OFDM symbol, so as to match the phase difference between the BPSK and the QPSK.

In equation (19),

Rotating the phases to different values produces the same result. E.g,

Rotate the phase of π / 4

Even if the phase of is rotated by 3π / 4, the same result. In addition, the embodiment described in Equation 19 may be equally applied not only to the case where BPSK and QPSK are used for each OFDM symbol but also to the case where QBPSK and QPSK are used.

Hereinafter, the embodiments described above will be described with reference to the operation of the terminal. First, in the process of transmitting data transmitted from a transmitter to a receiver, the CFO experienced by the nth OFDM symbol is

Defined as The receiver estimates the CFO using the preamble portion of the received frame first to remove the CFO on the time axis in the n + n th OFDM symbols. Estimated CFO is

Defined as However, since the first estimated CFO is not perfect (

) Remaining CFO (

) Will exist. Accordingly, the receiver estimates the residual CFO by using the above-described embodiments alone or in combination.

Once the residual CFO is estimated, the receiver receives the received signal at subcarrier k in order to remove the influence of the estimated residual CFO from the received signal.

) Phase is modified as in Equation 20 below.

[Equation 20]

Through this process, the receiver can compensate for the phase distortion due to the residual CFO in the received signal, and finally improve the received SINR.

On the other hand, unlike the method of removing the CFO on the time axis, the method of removing the CFO on the frequency axis as described above can not remove the effect of the leakage signal caused by the CFO. Therefore, from the n + 2 and n + 3 th OFDM symbols, a CFO estimation value as shown in Equation 21 below is used to remove the influence of the CFO on the time axis.

[Equation 21]

Is

see

It has a value closer to, resulting in smaller residual CFO. Therefore, since the received signal on the frequency axis after the FFT operation generates less signal leakage, the efficiency is increased in view of the received SNR. Next, using the proposed embodiments

After estimating the value, the phase of the subcarrier received signal can be corrected as described in Equation 20.

27 is a flowchart illustrating a CFO estimation method according to an embodiment of the present disclosure. FIG. 27 illustrates a CFO estimating method according to the embodiments described with reference to FIGS. 24 through 26 according to a time series flow. Accordingly, the contents described with reference to FIGS. 24 to 26 may be applied to the same or similarly, although not specifically described with reference to FIG. 27.

First, the transmitter transmits data to the receiver (S2710). Data is transmitted in units of frames defined by OFDM symbols and subcarriers, and is mapped to specific constellations and transmitted to a receiver. With this constellation, QPSK may be used, and BPSK (or QBPSK) and QPSK may be alternately used for a series of consecutive OFDM symbols.

On the other hand, the receiver first estimates the CFO value from the received signal (S2720). This process is performed using a preamble portion such as L-STF, L-LTF in the frame. However, since the CFO value changes with time, the CFO value estimated in S2720 may not be an accurate CFO value.

Accordingly, the receiver estimates the residual CFO value in order to compensate the estimated CFO value primarily (S2730). As described above, the receiver estimates the residual CFO under the assumption that the channels of the received signals of two consecutive OFDM symbols are the same. Specifically, the receiver compares the sign and magnitude of the real part and the imaginary part of the result of multiplying two received signals, assuming that the residual CFO is smaller than the first estimated CFO value. When the received signal of the entire subcarriers is in-phase through the comparison result, the receiver can obtain the residual CFO from the phase value calculated by summing the results for all subcarriers.

Finally, the receiver can accurately decode the data transmitted by the transmitter by removing the influence of the CFO value estimated at S2720 and the residual CFO value estimated at S2730 in the received signal.

5. Proposed CFO Estimation Method 3

In the above, the CFO estimation method of the receiver when the data transmission using BPSK (QBPSK) or QPSK is made. In this embodiment, the receiver operates efficiently when the residual CFO has a relatively small size compared to the CFO estimated primarily. However, when estimating the residual CFO in this blind manner, the receiver may not be sure that the size of the residual CFO is small enough. Therefore, the following describes the CFO estimation method of the receiver when the size of the residual CFO is large.

28 and 29, the CFO estimation method of the receiver will be described by dividing into four steps. FIG. 28 is a diagram illustrating a resource block for explaining a proposed embodiment and includes a plurality of resource elements (REs) defined in Long Term Evolution (LTE) / LTE-A (LTE-A). A resource block (RB) is shown. In FIG. 28, one RB is defined by 14 OFDM symbols arranged on a horizontal axis and 12 subcarriers arranged on a vertical axis, and includes a total of 168 REs.

In FIG. 28, the REs indicated by hatching indicate RS (Reference Signal), the REs indicated by hatching from the upper left to the lower right are CRS (Cell-specific RS), and the REs indicated by hatching from the upper right to the lower left are DMRS (DeModulation RS). ). These RSs are known to the receiver in advance, and the receiver uses the RS to determine channel state information (CSI) or perform channel estimation for demodulation.

According to the proposed embodiment, the receiver primarily estimates a residual CFO using RS (hereinafter, referred to as a first residual CFO). The receiver compensates the received data using the estimated first residual CFO, thereby adjusting the size of the residual CFO within a small range when the magnitude of the total residual CFO is unknown. Subsequently, the receiver estimates the residual CFO according to the CFO estimation method 1 or 2 described above (hereinafter, the second residual CFO). In other words, in order to efficiently apply the blind CFO estimation method proposed above, the receiver needs to adjust the residual CFO to a small size, and thus estimate the first residual CFO through the embodiments described below. The total remaining CFO (hereinafter, the third remaining CFO) is the sum of the first remaining CFO and the second remaining CFO. Hereinafter, the proposed embodiment will be described in detail.

First, the receiver estimates the first remaining CFO using the RS. Similar to the CFO estimation methods 1 and 2 described above, the receiver uses the received signal in two consecutive OFDM symbols as a function of Equation 22

Create

[Equation 22]

Equation 22 is distinguished from Equation 5 and 12, k is

Not an element of

Is an element of.

Is a set representing the index of the subcarrier where the RS is located in the nth OFDM symbol. That is, the receiver uses Equation 22 by using a relationship with consecutive OFDM symbols for the RE where the RS is located.

Create Meanwhile,

Is a set representing indices of subcarriers in which data other than RS is located. Since only one of RS and data is transmitted in one RE,

To satisfy.

Following Equation 22, Equation 23 below describes a process in which the receiver measures the first residual CFO.

[Equation 23]

In equation (23)

Denotes a first residual CFO measured using RS for the n th OFDM symbol. That is, the first remaining CFO is the subcarrier where the RS is located

Represents a CFO measured only for.

When the first residual CFO is determined by Equation 22 and Equation 23, the receiver compensates the received data using the first residual CFO. This process may be understood as a process of changing the phase of the received data by the calculated first residual CFO. Since RS is a value previously known to the receiver, the first residual CFO calculated by Equation 22 and Equation 23 can compensate the entire CFO having a large value to a sufficiently small value. Even if the first remaining CFO is not exactly the same as the entire CFO, the remaining residual CFO is reduced to a sufficiently small value through the process of compensating the data using the first remaining CFO.

Meanwhile, when Equation 23 is described in detail,

May be expressed as in Equation 24 below.

[Equation 24]

The first equation of equation (24) is derived from equations (8) and (9). Meanwhile, as described above, since RS is a value previously known to the receiver, the receiver

Know in advance about the phase of Thus, in equation (24)

When the component of is removed, the second equation of Equation 24 is derived.

Since information on the phase value of the component is known to the receiver in advance, a process for solving the phase ambiguity problem is unnecessary in Equations 22 and 23.

On the other hand, as described above, the process of compensating data as the first remaining CFO is determined is represented by Equation 25 below.

[Equation 25]

Equation 25 is the first remaining CFO measured in Equation 23 (

The process of changing the phase of the received signal by (). As such, when the data compensation process using the first residual CFO is completed, the receiver uses the CFO estimation method 1 or 2 described above to determine the total remaining CFO (

, The second remaining CFO (

Estimate). When data transmission is performed using QPSK, a process of estimating the second remaining CFO is expressed by Equation 26.

[Equation 26]

Equation 26 is the same as Equation 13, and represents a process of converting the phase of the entire data into the in-phase state to estimate the second residual CFO reduced to a small size. If BPSK or QBPSK is used instead of QPSK, Equation 6 is used for the same process. Meanwhile, when BPSK and QBPSK are alternately used for two consecutive OFDM symbols, Equation 11 may be used instead of Equation 25. Alternatively, when BPSK (or QBPSK) and QPSK are alternately used for two consecutive OFDM symbols, Equation 18 may be used instead of Equation 25.

Following Equation 26, a second residual CFO is estimated according to Equation 27.

[Equation 27]

Equation 27 is different from Equation 23,

The residual CFO is calculated for. That is, the residual CFO is calculated only for subcarriers in which data exists except for subcarriers in which RS exists. When the second remaining CFO is finally calculated by Equation 27, the total CFO is represented as the sum of the first remaining CFO and the second remaining CFO (Equation 28).

[Equation 28]

Hereinafter, various embodiments that can be applied to the process of estimating the CFO according to the series of processes described above will be described.

According to an embodiment of the present disclosure, in the process of estimating the second residual CFO, the receiver may calculate the residual CFO together with the subcarriers with RS as well as the subcarriers with data. That is, unlike Equation 27, the second residual CFO may be calculated by Equation 29.

[Equation 29]

According to this embodiment, the number of samples used to estimate the second residual CFO is increased by the number of subcarriers in which RS is present, thereby improving the estimation performance of the second residual CFO. For example, if the number of data subcarriers and the number of RS subcarriers are the same (

), The estimated performance of the second residual CFO according to Equation 29 is 3dB better than that of Equation 27. Furthermore, the second residual CFO is defined in such a way as to improve the accuracy of the first residual CFO, so that the performance of the total residual CFO (or third residual CFO) is also improved by 3 dB.

In the above, when the first residual CFO is calculated, the receiver compensates the data using the first residual CFO, and then calculates the second residual CFO. On the other hand, according to an embodiment, the process of compensating the data and the process of calculating the second residual CFO may be integrated and performed as one process according to Equation 30.

Equation 30

Specifically, when the first remaining CFO is calculated, the third remaining CFO is calculated immediately without compensating the data and calculating the second remaining CFO. To this end, in Equation 30, the received signal (

) Phase is changed by the first remaining CFO. In addition, in the present embodiment, similarly to Equation 29, the third residual CFO is calculated by using both the data subcarrier and the RS subcarrier. Through mathematical calculation, Equation 30 may be expressed as Equation 31, through which the total residual CFO calculated according to Equation 30 is the same as the total residual CFO according to Equations 22 to 28 described above. Can be.

Equation 31

In the above, the condition that the subcarriers in which the RS is located in two consecutive OFDM symbols is the same (

CFO estimation method is described. However, depending on how data and RS are arranged in the resource zone, this condition may not be established.

or

Consider the case of. Under these conditions, the proposed embodiment

silver

Is defined as

That is, the process of estimating the first residual CFO by the receiver through the RS may be performed only for the subcarrier in which the RS exists in both the n th OFDM symbol and the n + 1 th OFDM symbol. This means that the receiver

Wow

This is because a phase ambiguity problem occurs when only one of the values is known.

Furthermore, the first remaining CFO

Is estimated according to the condition that is a set of subcarriers where data is located in the nth OFDM symbol.

Also

It can be defined as. That is, subcarriers other than the subcarriers in which RS exists in both consecutive OFDM symbols are excluded in the process of estimating the first residual CFO and used as samples in the process of estimating the first residual CFO.

In the above, the CFO estimation method has been described based on a subcarrier in which RSs are located in two adjacent OFDM symbols. However, there is no subcarrier in which all RSs are located in consecutive OFDM symbols.

May also occur. In this case, the above-described process of estimating the first residual CFO can be omitted. That is, the first remaining CFO (

) Is set to 0 so that subsequent processes can be performed.

On the other hand, the first residual CFO estimation process proposed above may be applied to OFDM symbols separated by G. That is, as shown in Equation 32, the first residual CFO may be calculated for the subcarriers in which both RSs exist in the n-th OFDM symbol and the n + G-th OFDM symbol.

Equation 32

In Equation 32,

Is defined as In other words, the CFO estimation method has been described under the condition that the channel and the CFO do not change in two consecutive OFDM symbols. On the other hand, according to Equation 32, the receiver estimates the CFO under the condition that the channel and the CFO do not change during G + 1 OFDM symbols. Error due to this process

Is defined as in Equation 33.

[Equation 33]

error

Is larger as the distance G between two OFDM symbols becomes larger. Therefore, in such an embodiment

And the efficiency of CFO estimation is increased when the channel and CFO change is not large during G + 1 OFDM symbols.

In the above, the CFO is estimated on the same subcarrier for two OFDM symbols (contiguous OFDM symbols or OFDM symbols separated by G). However, according to another embodiment, the CFO may be estimated through adjacent subcarriers instead of the same subcarrier. For example, the process of estimating the first remaining CFO may be changed as shown in Equation 34.

[Equation 34]

In (34)

Denotes the distance between two subcarriers used for CFO estimation. When the first remaining CFO is calculated based on Equation 34, the process of calculating the second remaining CFO may be changed as shown in Equation 35.

[Equation 35]

Of equation (35)

It also shows the distance between adjacent subcarriers. In Equations 34 and 35, it is assumed that channels between adjacent subcarriers are identical to each other (

). Errors resulting from these assumptions

Is defined similarly to (33) and is represented by (36).

[Equation 36]

error

Is

Wow

The larger the difference is, the larger. Therefore, in this embodiment, the efficiency is increased as the selectivity of the frequency axis is small. For example, when the delay profile of the channel appears small, such as in an indoor environment, the above embodiment operates efficiently.

Meanwhile, in Equation 34 and Equation 35

Wow

May be set to different values, and if both values are 0, Equations 34 and 35 are the same as Equations 22 and 25.

The proposed CFO estimating method 3 estimates the first remaining CFO by using the RS previously known by the receiver. The first residual CFO is used in the process of compensating the data, and from the compensated data, the second remaining CFO is calculated according to CFO estimation method 1 or 2.

On the other hand, the first residual CFO estimation process using the RS has been described under the condition that the channels of the two OFDM symbols are the same. However, in case A of FIG. 28, CRSs are transmitted through different antenna ports in the first OFDM symbol and the second OFDM symbol. Therefore, the channel described in the two OFDM symbols are different, the above-described technique cannot be applied as it is. However, in case of an Evolved Physical Downlink Control CHannel (E-PDCCH) in LTE / LTE-A, since two OFDM symbols are transmitted through the same antenna port, the proposed CFO estimation method can be applied as it is.

Subsequently, RSs are not disposed in the third OFDM symbol and the fourth OFDM symbol in FIG. 28. Therefore, the total remaining CFO is estimated without estimating the first remaining CFO. In the fourth and fifth OFDM symbols, RS exists only in the fifth OFDM symbol.

Therefore, the process of estimating the first remaining CFO is similarly omitted.

Meanwhile, in case B of FIG. 28 (6th and 7th OFDM symbols), twice as many RSs as in case A are arranged. Therefore, in case B, the number of samples for estimating the first residual CFO is twice as large, and the estimation performance of the first residual CFO is improved by 3 dB compared to case A.

In case C (the 9th and 11th OFDM symbols), the first residual CFO may be estimated based on Equation 33 on the basis of OFDM symbols separated by G = 1. However, compared with cases A and B, the estimation performance is likely to deteriorate.

According to the embodiments described above, by estimating the first residual CFO using the data known to the receiver in advance, a part of the CFO of the data is compensated (the phase ambiguity problem does not occur in this process), and the remaining second residual CFO. Estimates the CFO according to the blind method described above. Through this process, the blind scheme can be applied to the case where the size of the residual CFO is small, thereby increasing the residual CFO estimation performance and efficiency.

29 is a flowchart illustrating a CFO estimation method according to an exemplary embodiment.

FIG. 29 illustrates a CFO estimating method according to the embodiments described with reference to FIG. 28 according to a time series flow. Thus, the embodiments described in FIG. 28 may be applied to the same or similarly, although not specifically described in FIG. 29.

First, the transmitter transmits data to the receiver (S2910). Data may be transmitted in units of frames defined by OFDM symbols and subcarriers, and may be mapped to specific constellations and transmitted to a receiver. As the constellation, BPSK, QBPSK, QPSK, etc. may be used, and BPSK and QBPSK may be used alternately for a series of OFDM symbols, or BPSK (or QBPSK) and QPSK may be used alternately.

On the other hand, the receiver first estimates the CFO value from the received signal (S2920). This process is performed using a preamble portion such as L-STF, L-LTF in the frame. However, since the CFO value changes with time, the CFO value estimated in S2920 may not be an accurate CFO value. Thus, the receiver subsequently performs a process for estimating the residual CFO.

The receiver estimates the first residual CFO value using the RS (S2930). The process of estimating the first residual CFO measures the first residual CFO by using RS received in both adjacent OFDM symbols in a specific subcarrier. The receiver compensates the phase value of the data by using the first residual CFO, and estimates the second residual CFO based on the compensated data (S2940). Since data is compensated through the first residual CFO estimated in S2930, the second residual CFO of S2940 becomes a relatively small value. Accordingly, the receiver estimates the second residual CFO according to S2330 of FIG. 23 and S2730 of FIG. 27, and the sum of the two residual CFOs estimated in S2930 and S2940 becomes the total residual CFO.

Finally, the receiver can accurately decode the data transmitted by the transmitter by removing the effects of the CFO estimated at S2920 and the overall residual CFO estimated at S2930 and S2940.

6. Device Configuration

30 is a diagram illustrating a configuration of a receiver and a transmitter according to an embodiment of the present invention. In FIG. 30, the receiver 100 and the transmitter 200 may include radio frequency (RF) units 110 and 210, processors 120 and 220, and memories 130 and 230, respectively. Although FIG. 30 illustrates only a 1: 1 communication environment between the receiver 100 and the transmitter 200, a communication environment may also be established between a plurality of receivers and a plurality of transmitters. In addition, the transmitter 200 illustrated in FIG. 30 may be applied to both the macro cell transmitter and the small cell transmitter.

Each RF unit 110, 210 may include a transmitter 112, 212 and a receiver 114, 214, respectively. The transmitter 112 and receiver 114 of the receiver 100 are configured to transmit and receive signals with the transmitter 200 and other receivers, and the processor 120 is functionally functional with the transmitter 112 and the receiver 114. In connection, the transmitter 112 and the receiver 114 may be configured to control a process of transmitting and receiving signals with other devices. In addition, the processor 120 performs various processes on the signal to be transmitted and transmits the signal to the transmitter 112, and performs the process on the signal received by the receiver 114.

If necessary, the processor 120 may store information included in the exchanged message in the memory 130. With such a structure, the receiver 100 may perform the method of various embodiments of the present invention described above.

The transmitter 212 and the receiver 214 of the transmitter 200 are configured to transmit and receive signals with other transmitters and receivers, and the processor 220 is functionally connected to the transmitter 212 and the receiver 214 to transmit the signal. 212 and the receiver 214 may be configured to control the process of transmitting and receiving signals with other devices. In addition, the processor 220 may perform various processing on the signal to be transmitted, transmit the signal to the transmitter 212, and may perform processing on the signal received by the receiver 214. If necessary, the processor 220 may store information included in the exchanged message in the memory 230. With such a structure, the transmitter 200 may perform the method of the various embodiments described above.

Processors 120 and 220 of the receiver 100 and the transmitter 200 respectively instruct (eg, control, adjust, manage, etc.) operation at the receiver 100 and the transmitter 200, respectively. Respective processors 120 and 220 may be connected to memories 130 and 230 that store program codes and data. The memories 130 and 230 are coupled to the processors 120 and 220 to store operating systems, applications, and general files.

The processor 120 or 220 of the present invention may also be referred to as a controller, a microcontroller, a microprocessor, a microcomputer, or the like. The processors 120 and 220 may be implemented by hardware or firmware, software, or a combination thereof.

When implementing an embodiment of the present invention using hardware, application specific integrated circuits (ASICs) or digital signal processors (DSPs), digital signal processing devices (DSPDs), and programmable logic devices (PLDs) configured to perform the present invention. Field programmable gate arrays (FPGAs) may be provided in the processors 120 and 220.

Meanwhile, the above-described method may be written as a program executable on a computer, and may be implemented in a general-purpose digital computer which operates the program using a computer readable medium. In addition, the structure of the data used in the above-described method can be recorded on the computer-readable medium through various means. Program storage devices that may be used to describe storage devices that include executable computer code for performing the various methods of the present invention should not be understood to include transient objects, such as carrier waves or signals. do. The computer readable medium includes a storage medium such as a magnetic storage medium (eg, a ROM, a floppy disk, a hard disk, etc.), an optical reading medium (eg, a CD-ROM, a DVD, etc.).

It will be understood by those skilled in the art that embodiments of the present invention can be implemented in a modified form without departing from the essential characteristics of the above description. Therefore, the disclosed methods should be considered in descriptive sense only and not for purposes of limitation. The scope of the present invention is shown in the claims rather than the detailed description of the invention, and all differences within the equivalent scope should be construed as being included in the scope of the present invention.

The CFO estimation method as described above has been described with reference to the example applied to the IEEE 802.11 system and the HEW system, but it is possible to apply to various wireless communication systems in addition to the IEEE 802.11 system and the HEW system.

Claims (12)

1. A method for estimating a carrier frequency offset (CFO) in a wireless communication system,

   Generating a first function defined by a reference signal (RS) received in two consecutive Orthogonal Frequency Division Multiplexing (OFDM) symbols for a particular subcarrier;

   Repeatedly performing the process of generating the first function for the entire subcarrier set to which the reference signal is transmitted;

   Determining, as a first residual CFO, a phase of a second function sum of the repeated results by using information about the reference signal known in advance;

   Compensating for data received using the first remaining CFO; And

   Estimating a second residual CFO from the compensated data using a blind scheme for the subcarrier set for which the reference signal is not transmitted.
2. The method of claim 1,

   The CFO estimation method,

   Using the third remaining CFO defined by the sum of the first remaining CFO and the second remaining CFO, removing the effect of the CFO from the received data.
3. The method of claim 1,

   The RS is a Cell-specific RS (CRS), Channel State Information-RS (CSI-RS) or DeModulation RS (DMRS), CFO estimation method.
4. The method of claim 1,

   The first function is defined according to the following equation,

   [Equation]

   

   N is an OFDM symbol index, k is a subcarrier index,

   

   Is the first function,

   

   Is a signal received from the kth subcarrier of the nth OFDM symbol.
5. The method of claim 1,

   The second function is defined according to the following equation,

   [Equation]

   

   In the above equation

   

   Is the first residual CFO, n is the OFDM symbol index, L is the number of total OFDM symbols-1, k is the subcarrier index,

   

   Is a set of subcarriers for which an RS is received in an nth OFDM time.

   

   Is the first function, N is the length of an OFDM symbol,

   

   Is a length of a cyclic prefix (CP).
6. The method of claim 1,

   Data transmitted from a transmitter is received using Binary Phase Shift Keying (BPSK), Quadrature BPSK (Quadrature BPSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (16-QAM), or 64-QAM. Estimation method.
7. A receiver for estimating a CFO in a wireless communication system,

   A transmitter;

   Receiving unit; And

   A processor operating in connection with the transmitter and the receiver,

   The processor,

   Generate a first function defined by a reference signal (RS) received in two consecutive OFDM symbols for a particular subcarrier,

   Generating the first function by repeating the entire subcarrier set to which the reference signal is transmitted;

   Using the information on the reference signal known in advance, the phase of the second function summating the repeated results is determined as the first residual CFO,

   Compensating for the received data using the first remaining CFO;

   And for a subcarrier set for which the reference signal is not transmitted, estimating a second residual CFO from the compensated data using a blind scheme.
8. The method of claim 7, wherein

   The processor,

   And using a third residual CFO defined as the sum of the first residual CFO and the second residual CFO to remove the effect of the CFO from the received data.
9. The method of claim 7, wherein

   Wherein the RS is CRS, CSI-RS or DMRS.
10. The method of claim 7, wherein

    The first function is defined according to the following equation,

    [Equation]

    

    N is an OFDM symbol index, k is a subcarrier index,

    

    Is the first function,

    

    Is a signal received from a kth subcarrier of an nth OFDM symbol.
11. The method of claim 7, wherein

    The second function is defined according to the following equation,

    [Equation]

    

    In the above equation

    

    Is the first residual CFO, n is the OFDM symbol index, L is the number of total OFDM symbols-1, k is the subcarrier index,

    

    Is a set of subcarriers for which an RS is received in an nth OFDM symbol,

    

    Is the first function, N is the length of an OFDM symbol,

    

    Is the length of the CP.
12. The method of claim 7, wherein

    The data transmitted from the transmitter is received using BPSK, QBPSK, QPSK, 16-QAM, or 64-QAM.

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