METHOD AND APPARATUS FOR ESTIMATING FREQUENCY OFFSET IN WIRELESS COMMUNICATION SYSTEM

An operating method of a receiver includes detecting a signal received from a transmitter at a plurality of sub-bands constituting the frequency band, determining first frequency offsets for the signal at reception sub-bands at which the signal has been detected among the plurality of sub-bands, and determining second frequency offsets by calibrating carrier frequency offsets based on a distance of each of the reception sub-bands from a center frequency of the frequency band.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application Nos. 10-2023-0087400, filed on Jul. 5, 2023, and 10-2023-0133696, filed on Oct. 6, 2023, in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference herein in their entireties.

BACKGROUND

Aspects of the inventive concept relate to a method and apparatus for determining a frequency offset so as to improve communication performance.

As an example of wireless communication, a wireless local area network (WLAN) is technology that connects two or more devices by using a wireless signal transmission method. A WLAN may be based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. The 802.11 standard has evolved into 802.11b, 802.11a, 802.11g, 802.11n, 802.11ac, 802.11ax, etc., and is capable of supporting a transmission rate of up to 1 Gbyte/s based on orthogonal frequency division multiplexing (OFDM).

In 802.11ac, data may be simultaneously transmitted to multiple users through multi-user multi-input multi-output (MU-MIMO). In 802.11ax, also called high efficiency (HE), orthogonal frequency division multiple access (OFDMA) as well as MU-MIMO is also applied to implement multiple access by splitting available subcarriers and providing the split subcarriers to users.

Accordingly, a WLAN system, to which 802.11ax is applied, may effectively support communication in dense areas and outdoors.

802.11be, also called extremely high throughput (EHT), seeks to support a 6-GHz unlicensed frequency band, to support various bandwidths per channel, to introduce hybrid automatic repeat and request (HARQ), and to support up to 16×16 MIMO. Accordingly, next-generation WLAN systems are expected to effectively support low latency and high-speed transmission such as new radio (NR), which is 5thgeneration (5G) technology. Recently, new technologies capable of supporting a bandwidth of up to 640 MHz per channel in 802.11be have been proposed to increase spectral efficiency and the transmission rate.

In wireless communication systems, a difference may occur between a carrier frequency of a transmission signal transmitted by a transmitter and a carrier frequency of a reception signal recognized by a receiver. Due to such a difference, the receiver may calculate a frequency offset and match the carrier frequency of the transmission signal to the carrier frequency of the reception signal.

In wideband wireless communication systems, due to various factors, a difference may occur between a carrier frequency of a transmission signal transmitted by a transmitter and a carrier frequency of a reception signal recognized by a receiver. Therefore, there is a need for a method of determining a frequency offset by taking into account various factors.

SUMMARY

Aspects of the inventive concept provide a method and apparatus for determining a frequency offset so as to improve communication performance.

According to an aspect of the inventive concept, there is provided an operating method of a receiver, the operating method including detecting a signal received from a transmitter at a plurality of sub-bands constituting the frequency band, determining first frequency offsets for the signal at reception sub-bands at which the signal has been detected among the plurality of sub-bands, and determining second frequency offsets by calibrating carrier frequency offsets based on a distance of each of the reception sub-bands from a center frequency of the frequency band.

According to another aspect of the inventive concept, there is provided a receiver for communicating with a transmitter through a frequency band, the receiver including a radio frequency integrated circuit (RFIC), and a processor configured to detect a signal received from the transmitter through the RFIC at a plurality of sub-bands constituting the frequency band, determine first frequency offsets for the signal at reception sub-bands at which the signal has been detected among the plurality of sub-bands, determine second frequency offsets by calibrating carrier frequency offsets based on the distance of each of the reception sub-bands from a center frequency of the frequency band, and determine a third frequency offset for the frequency band based on the second frequency offsets.

According to another aspect of the inventive concept, there is provided an operating method of a receiver, the operating method including splitting a frequency band of a reception signal into first sub-bands, detecting the reception signal at second sub-bands among the first sub-bands, determining first frequency offsets for the reception signal at the second sub-bands, determining second frequency offsets by calibrating the first frequency offset based on a distance of each of the second sub-bands from a center frequency of the frequency band, and determining a third frequency offset for the frequency band by using the second frequency offsets for each of the second sub-bands.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG.1is a diagram illustrating a wireless communication system10according to an embodiment.

Specifically,FIG.1illustrates a wireless local area network (WLAN) system as an example of the wireless communication system10. Detailed descriptions of embodiments focus on orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) based wireless communication systems, especially the IEEE 802.11 standard. However, aspects of the inventive concept may also be applied to other communication systems (e.g., cellular communication systems, such as long term evolution (LTE), LTE-advanced (LTE-A), new radio (NR), wireless broadband (WiBro), and global system for mobile communication (GSM), or short-range communication systems, such as Bluetooth and near field communication (NFC)) having similar technical background and channel forms through slight modifications without significantly departing from the scope of the inventive concept. This may be enabled by determination of those of ordinary skill in the art.

In addition, various functions described below may be implemented or supported by artificial intelligence technologies or one or more computer programs, each of which may be implemented as computer-readable program code and may be executed on a computer-readable recording medium. The terms “application” and “program” refer to one or more computer programs, software components, sets of instructions, procedures, functions, objects, classes, instances, associated data, or portions thereof suitable for implementation of appropriate computer-readable program code. The term “computer-readable program code” includes any type of computer code, including source code, object code, and executable code. The term “computer-readable recording medium” includes any type of memory that is accessible by a computer, such as read only memory (ROM), random access memory (RAM), a hard disk drive, a compact disk (CD), a digital video disk (DVD), or any other type of memory. The “non-transitory” computer-readable recording medium excludes wired, wireless, optical, or other communication links that transmit transient electrical or other signals. The non-transitory computer-readable recording medium includes a medium that allows data to be permanently stored and a medium that allows data to be stored and overwritten later, such as a rewritable optical disk or an erasable memory device.

In various embodiments described below, hardware approaches are described as an example. However, because various embodiments include technologies using both hardware and software, various embodiments do not exclude software-based approaches.

In addition, the terms referring to control information, the terms referring to entries, the terms referring to network entities, the terms referring to messages, the terms referring to elements of devices, and the like are exemplified for convenience of explanation. Therefore, the inventive concept is not limited to the terms to be described below, and other terms referring to an equivalent technical meaning may be used.

Referring toFIG.1, the wireless communication system10may include first and second access points AP1and AP2, a first station STA1, a second station STA2, a third station STA3, and a fourth station STA4. The first and second access points AP1and AP2may be connected to a network13including the Internet, an Internet protocol (IP) network, or any other networks. The first access point AP1may provide the first, second, third, and fourth stations STA1, STA2, STA3, and STA4with access to the network13within a first coverage area11, and the second access point AP2may provide the third and fourth stations STA3and STA4with access to the network13within a second coverage area12. In some embodiments, the first and second access points AP1and AP2may communicate with at least one of the first, second, third, and fourth stations STA1, STA2, STA3, and STA4based on wireless fidelity (Wi-Fi) or any other WLAN access technologies.

An access point may be referred to as a router, a gateway, etc., and a station may be referred to as a mobile station, a subscriber station, a terminal, a mobile terminal, a wireless terminal, user equipment, a user, etc. The station may be a mobile device, such as a mobile phone, a laptop computer, a wearable device, etc., or may be a stationary device, such as a desktop computer, a smart television (TV), etc.

The access point may determine a channel bandwidth used to communicate with the station to be one of a plurality of channel bandwidths. According to aspects of the inventive concept, the channel bandwidth may be referred to as a bandwidth. In an embodiment, the channel bandwidths may include 20 MHz, 40 MHz, 80 MHz, 160 MHz, 320 MHz, and 640 MHz.

The access point and the station, according to an embodiment, may detect signals received from other devices at a plurality of sub-bands constituting the frequency band. The access point and the station may determine first frequency offsets for the signal at reception sub-bands at which the signal has been detected among the sub-bands. The access point and the station may determine second frequency offsets by calibrating the first frequency offsets based on the distance of each of the reception sub-bands from the center frequency of the frequency band. The access point and the station may determine a third frequency offset for the frequency band based on the second frequency offsets.

The access point and the station, according to an embodiment, may determine the frequency offset by taking into account the difference between a frequency of a sampling clock of an analog-to-digital converter (ADC) and a frequency of a sampling clock of a digital-to-analog converter (DAC) as well as the difference between a frequency of a clock generated by a local oscillator (LO) of a transmitter and a frequency of a clock generated by an LO of a receiver.

The access point and the station, according to an embodiment, may calibrate a carrier frequency offset deviation of a reception signal according to a frequency position within a wideband channel.

The access point and the station, according to an embodiment, may determine a frequency offset that is not affected by a frequency band and a bandwidth of a reception signal.

The access point and the station, according to an embodiment, may determine a frequency offset, to which channel quality of each sub-band is reflected, by combining the calibrated carrier frequency offsets of the respective sub-bands in an in-phase and quadrature (I/Q) area.

The access point and the station, according to an embodiment, may have a diversity gain by combining the calibrated carrier frequency offsets of the respective sub-bands in an I/Q area.

For example, the access point and the station, according to an embodiment, may take into account the sampling clock offset (SCO) and carrier frequency offset (CFO) coupled effects, which appear differently according to the position away from the center frequency, at each sub-band constituting a wideband channel, and may determine an accurate frequency offset value for each sub-band, regardless of the frequency position within a channel. In addition, the access point and the station may perform appropriate coherent combining on the frequency offset values of all sub-bands at which the reception signal in the wideband channel is detected among the frequency offset values determined for each sub-band by reflecting the SCO and CFO coupled effects and may determine an accurate frequency offset value, regardless of the occupied frequency band and the bandwidth of the finally transmitted frame.

FIG.2illustrates a block diagram of a wireless communication device100according to an embodiment.

In a wireless communication system, it is difficult for clock frequencies of local oscillators (LO) respectively driven by a transmitter and a receiver to exactly match each other. Due to this, a difference may occur between a carrier frequency of a transmission signal transmitted by the transmitter and a carrier frequency of a reception signal recognized by the receiver. In accordance with aspects of the inventive concept described herein, a frequency offset that occurs for this reason is referred to as a CFO. When the CFO is present, the receiver recognizes a subcarrier of a transmission signal as being shifted by a certain frequency spacing.

On the other hand, a difference in frequencies of sampling clocks applied to analog-to-digital signal conversion may cause a frequency offset. In other words, a difference in frequencies of sampling clocks between the transmitter and the receiver may also cause a frequency offset. Specifically, a difference between a frequency of a sampling clock of an ADC and a frequency of a sampling clock of a DAC may cause a frequency offset between the transmitter and the receiver. In accordance with aspects of the inventive concept described herein, such a frequency offset is referred to as an SCO. The SCO may occur in conjunction with the CFO. Specifically, the presence of the SCO may influence the degree of the CFO. For example, the presence of the SCO may have different degrees of influence on the CFO according to the frequency band of the signal, and the CFO may have different deviations at sub-bands with different frequency positions.

The wireless communication device100according to aspects of the inventive concept may determine a frequency offset by taking into account both the CFO and the SCO. That is, the wireless communication device100may take into account that the CFO of the reception signal has different deviations for each sub-band due to the SCO effect.

Referring toFIG.2, the wireless communication device100may include a processor110, a radio frequency integrated circuit (RFIC)120, and antennas130. The wireless communication device100may further include a component for transmitting and receiving signals and data. For example, the wireless communication device100may further include an LO, etc.

The wireless communication device100may transmit or receive signals through the processor110, the RFIC120, and the antennas130. When the wireless communication device100transmits signals, the wireless communication device100may operate as a transmitter, and when the wireless communication device100receives signals, the wireless communication device100may operate as a receiver.

The processor110according to an embodiment may detect a signal received from the transmitter through the RFIC120at a plurality of sub-bands constituting a frequency band. The processor110may determine first frequency offsets for the signal at reception sub-bands at which the signal has been detected among the sub-bands. For example, the received signal may include a training sequence that is periodically repeated in a time domain. The processor110may determine the first frequency offset based on the training sequence. In addition, the processor110may determine the first frequency offset by extracting phase information for each of the sub-bands by using an auto-correlation function (ACF).

The processor110may determine second frequency offsets by calibrating CFOs based on the distance of each of the reception sub-bands from the center frequency of the frequency band. The presence of the SCO may influence the degree of the CFO of the frequency components within the channel. Specifically, the SCO may cause the CFO to appear differently according to how far the subcarrier is away from the center frequency of the frequency band. Accordingly, the second frequency offset that is generated by calibrating the first frequency offset based on the distance of each of the sub-bands from the center frequency of the frequency band may be a CFO from which the SCO effect is removed. On the other hand, the distance of each of the reception sub-bands from the center frequency of the frequency band may correspond to the magnitude of the index of each of the reception sub-bands.

The processor110may determine a third frequency offset for the frequency band based on the second frequency offsets.

For example, the processor110may determine the third frequency offset by performing weighted averaging on the second frequency offsets based on weights corresponding to the amplitude of the signal measured at each of the reception sub-bands.

For example, the processor110may re-construct I/Q vectors for each of the reception sub-bands by using the amplitude of the signal measured at each of the reception sub-bands and the second frequency offsets. The processor110may obtain a vector for the frequency band by combining the I/Q vectors. The processor110may determine the third frequency offset by calculating the phase of the vector for the frequency band.

The processor110may receive data from the transmitter through the RFIC120by using the third frequency offset.

FIG.3Aillustrates a block diagram of a wireless communication device200according to an embodiment.

Referring toFIG.3A, the wireless communication device200may include a band split circuit210, a CFO estimator220, a calibration circuit230, and a sub-band combiner240. The wireless communication device200may further include a component for transmitting and receiving signals and data.

The band split circuit210may split a channel of a reception signal. For example, when the wireless communication device200operates at a wideband of 320 MHz or more, the band split circuit210may split the channel into a plurality of sub-band units with a bandwidth of 20 MHz. For example, the band split circuit210may split a wideband channel signal into sub-band units based on band-split filtering.

The wireless communication device200ofFIG.3Ais an embodiment of the wireless communication device100ofFIG.2. The processor110may include a band division circuit210, a carrier frequency offset estimator220, a correction circuit230, and a subband combiner240.

The CFO estimator220may determine the CFO of each of the sub-bands. For example, the CFO estimator220may determine the CFO by using the periodicity of the reception signal. As a specific example, the CFO estimator220may calculate phase information for each of the sub-bands by using an ACF.

The calibration circuit230may calibrate the CFO when the SCO is present. The degree to which the CFO is affected by the SCO may change according to the position of the sub-band in the frequency domain. The calibration circuit230may remove the SCO effect from the CFO.

The sub-band combiner240may combine the calibrated CFOs corresponding to the respective sub-bands. The sub-band combiner240may obtain the frequency offset for the frequency band of the reception signal by combining the calibrated CFOs corresponding to the respective sub-bands.

FIG.3Bis a diagram for describing the SCO.FIG.3Cillustrates the CFO and the SCO in the frequency domain.FIG.3Dillustrates an embodiment of calibrating deviation due to an SCO effect from a CFO for each of 16 sub-bands with respect to a reception signal having a bandwidth of 320 MHz in a wideband channel.

The frequency offset may be present between the transmitter and the receiver due to LO clock mismatch. This may be expressed in units of parts per million (ppm). fCFO, which is the magnitude of the CFO of the reception signal due to the frequency offset FO(ppm), may depend on the center frequency fcof the wideband channel on which the receiver operates and may be expressed in units of Hz as shown in Equation 1 below.

The frequency offset FO(ppm)causes an offset in a sampling rate at which analog-to-digital signal conversion is performed between transmission and reception. Referring toFIG.3B, the period of sampling (Tx samples) of the transmitter is Ts, and the period of sampling (Rx samples) of the receiver is Ts′. The difference between Tsand Ts′ is ε.

This is referred to as an SCO or a sampling frequency offset (SFO). The magnitude of the SCO may depend on the bandwidth of the channel on which the receiver operates and may be expressed in frequency units of Hz as follows.

Tsand Ts′ represent the sampling period at the transmitting end and the sampling period at the receiving end, respectively, and fBWand fBWare the reciprocals of Tsand Ts′ and represent the sampling frequencies and bandwidths of the transmitting end and the receiving end, respectively. As such, when the SCO is present, the bandwidths recognized between the transmitter and the receiver in the frequency domain are different from each other, and thus, the difference may also occur in subcarrier spacing. When the wideband channel on which the receiver operates includes a total of N subcarriers, the deviation Δfsubcarrierof the subcarrier spacing due to the SCO is as follows.

The actual degree of the CFO experienced from the reception signal by the wireless communication device (see100ofFIG.2) deviates from fCFOas Δfsubcarrieris accumulated according to the distance from the center frequency for each subcarrier in the frequency domain.

Referring toFIG.3C, a difference between a first subcarrier TSC_1for the transmission signal and a first subcarrier RSC_1recognized by the receiver when there is no SCO effect is a CFO. Similarly, a difference between a second subcarrier TSC_2for the transmission signal and a second subcarrier RSC_2recognized by the receiver when there is no SCO effect is a CFO. As such, a difference between third to sixth subcarriers TSC_3to TSC_6for the transmission signal and third to sixth subcarriers RSC_3to RSC to 6 recognized by the receiver when there is no SCO effect is a CFO.

A difference occurs between a frequency position of a first subcarrier RSC_1′ recognized by the receiver when there is an SCO effect and a frequency position of a first subcarrier RSC_1recognized by the receiver when there is no SCO effect. This difference is based on a distance from a center frequency SCO_1. That is, the differences between frequency positions of second to sixth subcarriers RSC_2′ to RSC_6′ recognized by the receiver when there is the SCO effect and frequency positions of second to sixth subcarriers RSC_1to RSC_6recognized by the receiver when there is no SCO effect may change according to the distance from center frequencies SCO_2to SCO_6thereof, respectively. For example, as the distance from the center frequency increases, the difference between the position of the subcarrier recognized by the receiver when there is an SCO effect and the position of the subcarrier recognized by the receiver when there is no SCO effect may increase.

On the other hand, when the frequency index of the individual subcarrier is k, subcarriers are present in a range of −N/2≤k≤N/2 within the channel and the center frequency corresponds to k=0. The CFO deviation due to the SCO appears differently for each subcarrier depending on the position away from the center frequency of the entire channel in accordance with a certain rule. The mathematical relationship is derived as follows.

In Equation 4, fCFO, krepresents the magnitude of a CFO of a subcarrier located at an index k.

This is expressed in ppm as follows.

This may also be expressed as follows.

In Equation 6, α and δ are constants depending on a system environment. Because both CFO fCFOand SCO fSCOare associated with frequency offset FO(ppm)due to LO mismatch between the transmitter and the receiver, the degree fCFO, kof CFO experienced at any subcarrier position k is proportional to FO(ppm)and linearly increases or decreases as the linear function of the frequency index k. When the determined CFO value for each sub-band and the frequency index according to the sub-band position are substituted into Equation 6, the SCO effect may be inverted and calibrated and the fundamental frequency offset FO(ppm)may be obtained.

Referring toFIG.3D, the wideband channel operation with frequency offsets FO(ppm)=20 ppm and BW=320 MHz & N=1024 & fc=6 GHz between transmission and reception is assumed. The CFO value (CFO per subcarrier) experienced by each 20-MHz sub-band within a 320-MHz channel appears in as linear deviation according to a subcarrier index with respect to 20 ppm at the center band. When the SCO effect is calibrated so as not to be affected as shown in the direction of the arrow inFIG.3Dthrough the relational equation described above, a frequency offset corresponding to 20 ppm may always be obtained, regardless of the position of the sub-band.

Each point in the graph indicates the CFO of the subcarrier at the center frequency position of each of 16 20-MHz sub-bands in the 320-MHz wideband channel. The index kmof the subcarrier corresponding to the center frequency at any sub-band index m is defined as follows.

The value {tilde over (f)}CFO, m (ppm)obtained when the CFO is determined for each sub-band may be approximated as the CFO at the center frequency by the average of the subcarriers within the sub-band as follows.

Conversely, when the equation is modified to obtain the frequency offset FO(ppm)from the CFO determined for each sub-band, the frequency offset FO(ppm),mdetermined at the sub-band of the index m may be expressed as follows.

By substituting the index of the subcarrier into Equation 9, the frequency offset is expressed as follows.

FIG.4illustrates an operation procedure of a wireless communication device according to an embodiment.FIG.4may be described with reference toFIG.3A.

Referring toFIG.4, in operation S101, the wireless communication device200may detect a signal received from the transmitter at a plurality of sub-bands constituting the frequency band. For example, the bandwidth of the frequency band is 320 MHz, and the bandwidth of each of the sub-bands is 20 MHz. The bandwidths of the frequency band and the sub-bands may variously change and are not limited to the embodiment described above.

In operation S103, the wireless communication device200may determine first frequency offsets for the signal at reception sub-bands at which the signal has been detected among the sub-bands. The first frequency offset may be the CFO described above, and the reception sub-bands refer to sub-bands at which the signal has been detected among the sub-bands constituting the frequency band.

For example, the signal received by the wireless communication device200may include a training sequence that is periodically repeated in the time domain, and the wireless communication device200may determine the first frequency offset based on the periodicity of the training sequence. In addition, the wireless communication device200may determine the first frequency offset by extracting phase information for each of the sub-bands from the training sequence by using an ACF.

In addition, in operation S105, the wireless communication device200may determine second frequency offsets by calibrating the first frequency offsets based on the distance of each of the reception sub-bands from the center frequency of the frequency band. That is, the second frequency offset is a frequency offset calibrated in a direction to remove the SCO effect with respect to the first frequency offset.

As an example, the distance of each of the reception sub-bands from the center frequency of the frequency band may correspond to the magnitude of the index of each of the reception sub-bands.

In operation S107, the wireless communication device200may determine a third frequency offset for the frequency band based on the second frequency offsets.

For example, the wireless communication device200may determine the third frequency offset by performing weighted averaging on the second frequency offsets based on weights corresponding to the amplitude of the signal measured at each of the reception sub-bands.

For example, the wireless communication device200may re-construct I/Q vectors of each of the reception sub-bands by using the amplitude of the signal measured at each of the reception sub-bands and the second frequency offsets, to obtain a vector for the frequency band by combining the I/Q vectors, and may determine the third frequency offset by calculating the phase of the vector for the frequency band.

The wireless communication device200may receive data from the transmitter by using the third frequency offset. For example, the wireless communication device200may receive data by calibrating the frequency of the LO by using the third frequency offset. As another example, the wireless communication device200may receive data by calibrating the modulation of the reception signal by using the third frequency offset. The wireless communication device200may receive data from the transmitter in various methods by using the third frequency offset, and is not limited to the embodiments described above.

FIG.5Aillustrates an operation procedure of a wireless communication device according to an embodiment.FIG.5Amay be described with reference toFIG.2.

Referring toFIG.5A, in operation S201, the wireless communication device100may detect a reception signal for each sub-band based on an ACF and a cross-correlation function (CCF). For example, the wireless communication device100may split a wideband channel into sub-bands each having a bandwidth of 20 MHz and may calculate and update the ACF and the CCF for each sub-band. When the ACF and CCF values exceed a certain threshold, the wireless communication device100may consider that the reception signal has been detected at the corresponding sub-band, and a frame reception procedure through synchronization timing determination and frequency offset estimation may be started.

In operation S203, the wireless communication device100may calculate phase information (CFO) based on the ACF result of each sub-band. That is, the wireless communication device100does not combine the ACF of each of the sub-bands, instead the wireless communication device100first calculates the CFO value for each sub-band by extracting phase information from each ACF.

In operation S205, the wireless communication device100may calibrate phase information (CFO) based on the frequency position of the sub-band.

In operation S207, the wireless communication device100may re-construct an I/Q vector based on the absolute value of the AFC and the calibrated CFO.

In operation S209, the wireless communication device100may combine I/Q vectors of sub-bands at which the reception signal has been detected.

In operation S211, the wireless communication device100may determine a frequency offset by calculating phase information of one combined vector.

FIG.5Billustrates an operation procedure of a wireless communication device at an mthsub-band, according to an embodiment.FIG.5Bmay be described with reference toFIG.5A.

Operations S203′, S205′, and S207′ may correspond to operations S203, S205, and S207inFIG.5A, respectively.

The wireless communication device100may split a frequency band of a reception signal. For example, the wireless communication device100may split the frequency band into sub-bands each having a bandwidth of 20 MHz.

Referring toFIG.5B, in operation S203′, the wireless communication device100may calculate phase information (∠(ACF20M,m)) of the mthsub-band based on the ACF result (ACF20M,m) of the mthsub-band. The wireless communication device100may obtain the amplitude and phase of the reception signal as the ACF result. For example, the wireless communication device100may extract a synchronization timing from a reception signal including a training sequence and may calculate phase information (CFO) of an mthsub-band by using the synchronization timing.

In operation S205′, the wireless communication device100may calibrate the phase information (∠(ACF20M,m)) into phase information (∠(ACF20M,m)calib) by taking into account the SCO and CFO coupled effects based on the frequency position of the mthsub-band.

In operation S207′, the wireless communication device100may re-construct a phasor vector (|ACF20M, m|*ej2π{∠(ACF20M,m)calib}) of an I/Q area by using the amplitude of the reception signal, which is one of the ACF results, and the calibrated phase information (∠(ACF20M,m)calib).

FIG.5Cillustrates an operation of a wireless communication device at each sub-band and a method of combining a plurality of sub-bands, according to an embodiment.FIG.5Cmay be described with reference toFIGS.5A and5Band redundant descriptions may be omitted.

Referring toFIG.5C, operations S203a, S203b, and S203cmay correspond to operation S203′. Operations S205a, S205b, and S205cmay correspond to operation S205′. Operations S207a, S207b, and S207cmay correspond to operation S207′. Operations S201a, S201b, and S201cmay correspond to operation S201.

In operation S201a, the wireless communication device100may detect a reception signal at a zeroth sub-band. For example, the wireless communication device100may detect a reception signal at the zeroth sub-band based on an ACF and a CCF.

In operation S201b, the wireless communication device100may detect a reception signal at a first sub-band. For example, the wireless communication device100may detect a reception signal at the first sub-band based on an ACF and a CCF.

In operation S201a, the wireless communication device100may detect a reception signal at an mthsub-band. For example, the wireless communication device100may detect a reception signal at the mthsub-band based on an ACF and a CCF.

In operation S209a, the wireless communication device100may combine I/Q vectors of sub-bands at which the reception signal has been detected. For example, when the reception signal is detected at the zeroth sub-band, the wireless communication device100may use a first MUX MUX_0to select a phasor vector (|ACF20M, 0|*ej2π{∠(ACF20M,0)calib}) of an I/Q area of the zeroth sub-band. In this manner, the wireless communication device100may select phasor vectors of sub-bands at which the reception signal has been detected. The wireless communication device100may generate one phasor vector by combining all the selected phasor vectors.

In operation S211a, the wireless communication device100may determine the frequency offset of the frequency band by calculating phase information of one combined vector.

FIG.6illustrates a method of detecting a reception signal at a sub-band, obtaining a synchronization timing, and determining a frequency offset based on the synchronization timing and the reception signal, according to an embodiment.FIG.6may be described with reference toFIG.2.

Referring toFIG.6, in operation S301, the wireless communication device100may receive a signal.

In operation S303, the wireless communication device100may split a frequency band of the received signal into sub-bands each having a bandwidth of 20 MHz.

In operation S305, the wireless communication device100may detect a reception signal at a sub-band by using a CCF and may obtain a synchronization timing of the reception signal. For example, in operation S307, by detecting a peak of a periodic reception signal, the reception signal may be detected and the synchronization timing of the reception signal may be obtained.

In operation S309, the wireless communication device100may extract phase information for each of the sub-bands by using an ACF. For example, in operation S311, the phase information for each of the sub-bands may be extracted by using an ACF result and an ARCTAN function based on the synchronization timing. That is, the wireless communication device100may calculate an ACF value for each sub-band corresponding to a synchronization timing point.

In operation S313, the wireless communication device100may calibrate the extracted phase information by taking into account the SCO effect. When the wireless communication device100calibrates the deviation caused by the SCO from the measured CFO value according to the position of each sub-band from the center frequency within the channel, the wireless communication device100may determine a correct frequency offset, regardless of the position of the sub-band and the bandwidth of the reception signal.

In operation S315, the wireless communication device100may perform I/Q re-construction and sub-band combining. In operation S317, the wireless communication device100may extract phase information for the frequency offset through an I/Q area phasor vector and an ARCTAN function for the combined sub-bands.

FIG.7illustrates an operation procedure of a wireless communication device according to an embodiment.FIG.7may be described with reference toFIG.2.

Referring toFIG.7, in operation S401, the wireless communication device100may split a frequency band of a reception signal into first sub-bands.

In operation S403, the wireless communication device100may detect a reception signal at second sub-bands among the first sub-bands.

In operation S405, the wireless communication device100may determine first frequency offsets for the reception signal at the second sub-bands.

For example, the reception signal may include a training sequence that is periodically repeated in the time domain, and the wireless communication device200may determine the first frequency offset based on the periodicity of the training sequence. The wireless communication device100may determine the first frequency offset by extracting phase information for each of the second sub-bands from the training sequence based on an ACF.

For example, the wireless communication device100may obtain synchronization timings of the reception signal for each of the second sub-bands and may further determine the first frequency offset by extracting phase information for each of the second sub-bands from the training sequence based on the synchronization timings.

In operation S407, the wireless communication device100may determine the second frequency offset by calibrating the first frequency offset based on the distance of each of the second sub-bands from the center frequency of the frequency band.

In operation S409, the wireless communication device100may determine a third frequency offset for the frequency band by using the second frequency offset for each of the second sub-bands. For example, the wireless communication device100may determine the third frequency offset by performing weighted averaging on the second frequency offsets according to the amplitude of the reception signal for each of the second sub-bands.

For example, the wireless communication device100may re-construct a plurality of I/Q vectors corresponding to the second sub-bands by using the amplitude of the reception signal for each of the second sub-bands and the second frequency offset, to obtain one vector by combining the I/Q vectors, and may determine the third frequency offset by calculating the phase of the one vector.

FIG.8illustrates signals that are processed by a wireless communication device, according to an embodiment.FIG.8may be described with reference toFIG.2.

The wireless communication device100may perform synchronization by detecting a preamble of a frame signal for a signal reception operation and determining a frequency offset. The preamble of the frame signal may include a training sequence in which a sample array of a certain length is periodically repeated in the time domain. When s(n) represents the reception signal of the training sequence repeated at NLsample intervals defined by a WLAN standard, the relationship shown in Equation 1 below may be established in an ideal channel environment.

A CFO fCFOis present between transmission and reception, and an actual reception signal considering an additive white Gaussian noise (AWGN) channel may be expressed as Equation 2 below.

In Equation 12, s(n) represents an ideal reception signal, fCFOrepresents a CFO, η(n) represents noise, and Tsrepresents a sampling period.

Referring toFIG.8, in operation S501, the wireless communication device100may split a wideband channel into sub-bands. The wireless communication device100may split a channel of a reception signal r(n) into m sub-bands. For example, the wireless communication device100may split the reception signal r(n) into units of 20 MHz by filtering the reception signal r(n) in the time domain. A signal rm(n) corresponding to an mthsub-band may be expressed as Equation 13 below.

In Equation 13, fCFO,mrepresents a CFO experienced with respect to the signal of the mthsub-band by the wireless communication device100.

In operation S503, the wireless communication device100may process the signal rm(n) corresponding to the mthsub-band by using an ACF. For example, the wireless communication device100may calculate an ACF corresponding to a delay equal to a sample repetition period NLof the training sequence. The ACF may be expressed as Equation 14 below.

In Equation 14, Lautorepresents a buffer (window) length of accumulated samples for which the ACF is calculated.

In operation S505, the wireless communication device100may extract phase information (CFO) corresponding to the mthsub-band from the ACF. For example, the wireless communication device100may determine the CFO of the sub-band from the ACF value corresponding to the synchronization timing point n=nSYNCas follows.

In Equation 15, {circumflex over (f)}CFO, mrepresents the CFO of the mthsub-band. {circumflex over (f)}CFO,mincludes a certain proportion of deviation that occurs due to the SCO according to the position of the sub-band away from the center frequency within the entire wideband channel.

In operation S507, the wireless communication device100may calibrate the SCO effect. The wireless communication device100may obtain the determined CFO value in an environment without SCO by reversely calibrating the SCO effect, as shown in Equation 16 below.

In Equation 16, m may represent an index according to the position of each 20-MHz sub-band within the wideband channel. {circumflex over (f)}calib,mrepresents the calibrated CFO value. {circumflex over (f)}calib,mmay be the CFO of the channel center frequency generated by the fundamental frequency offset due to LO mismatch at the transmitting and receiving ends, regardless of the bandwidth of the reception signal and the position of the sub-band.

In operation S509, the wireless communication device100may re-construct the I/Q value for each sub-band by using the calibrated CFO value. The re-constructed phasor vector for each sub-band may be expressed as Equation 17 below.

In operation S511, the wireless communication device100may combine the calibrated CFO for each sub-band. For example, when the reception signal is a wideband signal occupying a plurality of 20 MHz sub-bands, a more accurate CFO value may be obtained by combining the calibrated CFO for each sub-band. In order to reflect the effect of different channel quality for each sub-band rather than simply performing arithmetic averaging on the CFO values for each sub-band, the wireless communication device100may re-construct and combine the phasor vectors in the I/Q area. That is, the wireless communication device100may obtain one phasor vector for all sub-bands within the channel by coherently combining vectors in a complex dimension.

In operation S513, the wireless communication device100may extract phase information from the phasor vector. That is, the wireless communication device100may determine the CFO obtained as an average for the entire channel band as follows.

{circumflex over (f)}CFO,calibrepresents an offset of a pure carrier in which the effect due to the SCO has been calibrated, and the wireless communication device100may determine the frequency offset as shown in Equation 19 below.

fcrepresents the center frequency of the frequency band.

FIGS.9A and9Billustrate frequency offsets in which an SCO effect is not calibrated in an AWGN channel.FIGS.9C and9Dillustrate frequency offsets in which an SCO effect is calibrated in an AWGN channel, according to an embodiment.

When determining the frequency offset of the reception signal in the wideband channel, the wireless communication device (see100ofFIG.2) according to the inventive concept may determine an accurate frequency offset value by calibrating the deviation of the CFO caused by the CFO and SCO coupled effects. The wireless communication device100may accurately predict and calibrate the deviation that occurs linearly due to the distance away from the center frequency of the wideband channel and may thus always determine a consistent frequency offset value for signals received over some or all of the sub-bands, regardless of the bandwidth and the frequency position. Accordingly, the wireless communication device100may ensure correct synchronization and reception performance.

InFIGS.9A to9D, it is assumed that a reception signal is present at only one specific 20-MHz sub-band in a 320-MHz wideband channel and a frequency offset of 20 ppm is present between transmission and reception.

FIGS.9A and9Billustrate the distribution of the CFO in the case of not taking into account the SCO effect in the AWGN channel as a cumulative distribution function (CDF). That is, this represents a case where the CFO is not calibrated. The position of the 20-MHz sub-band at which the reception signal is present has one of values within the range of −8≤CH_OFFSET≤−1 and +1≤CH_OFFSET≤+8 with respect to the center frequency of the 320-MHz channel according to the CH_OFFSET value.FIG.9Aillustrates the case of −8≤CH_OFFSET≤−1, andFIG.9Billustrates the case of +1≤CH_OFFSET≤+8.

FIGS.9C and9Dillustrate the distribution of the CFO in the case of taking into account the SCO effect in the AWGN channel as a CDF. That is, this represents a case where the CFO is calibrated.FIG.9Cillustrates the case of −8≤CH_OFFSET≤−1, andFIG.9Dillustrates the case of +1≤CH_OFFSET≤+8.

Referring toFIGS.9A to9D, when the CFO is not calibrated, the frequency offset is gradually shifted away from 20 ppm according to the frequency position. Specifically, in an environment where the frequency offset of 20 ppm is present, the CFO measured from the reception signal for each sub-band appears with a certain deviation according to the position CH_OFFSET of the frequency band due to the CFO and SCO coupled effects. The wireless communication device100may calibrate the deviation by using the mathematical relationship between the frequency offset and the CFO and SCO.

It may be confirmed that, when the CFO is calibrated by taking into account the SCO effect, the frequency offsets are all uniform at around 20 ppm. That is, the wireless communication device100may calibrate the CFO to accurately determine the original frequency offset of 20 ppm between transmission and reception, regardless of the frequency band position of the sub-band at which the signal is received.

FIGS.9E and9Fillustrate frequency offsets in which an SCO effect is not calibrated in a fading channel, according to an embodiment.FIGS.9G and9Hillustrate frequency offsets in which an SCO effect is calibrated in a fading channel, according to an embodiment.

InFIGS.9E to9H, it is assumed that a reception signal is present at only one specific 20-MHz sub-band in a 320-MHz wideband channel and a frequency offset of 20 ppm is present between transmission and reception.

FIGS.9E and9Fillustrate the distribution of the CFO in the case of not taking into account the SCO effect in the fading channel as a CDF. That is, this represents a case where the CFO is not calibrated. The position of the 20-MHz sub-band at which the reception signal is present has one of values within the range of −8≤CH_OFFSET≤−1 and +1≤CH_OFFSET≤+8 with respect to the center frequency of the 320-MHz channel according to the CH_OFFSET value.FIG.9Eillustrates the case of −8≤CH_OFFSET≤−1, andFIG.9Fillustrates the case of +1≤CH_OFFSET≤+8.

FIGS.9G and9Hillustrate the distribution of the CFO in the case of taking into account the SCO effect in the fading channel as a CDF. That is, this represents a case where the CFO is calibrated.FIG.9Gillustrates the case of −8≤CH_OFFSET≤−1, andFIG.9Hillustrates the case of +1≤CH_OFFSET≤+8.

Referring toFIGS.9E and9F, in an environment where the frequency offset of 20 ppm is present, the CFO measured from the reception signal for each sub-band appears with a certain deviation according to the position CH_OFFSET of the frequency band due to the CFO and SCO coupled effects. Referring toFIGS.9G and9H, it may be confirmed that, when the CFO is calibrated by taking into account the SCO effect, the CFOs are all uniform at around 20 ppm.

FIG.10illustrates a frequency offset of a frequency band, at which an SCO effect is not calibrated, and a frequency offset of a frequency band, at which an SCO effect is calibrated, in a fading channel.

When a reception signal is present over an entire 320-MHz wideband channel between transmission and reception with a frequency offset of 20 ppm, the CFO deviation due to the SCO effect appears differently for each sub-band. Even when the CFO of each sub-band is not calibrated (baseline), an average frequency offset may be approximated to 20 ppm. However, because the quality of the channel for each sub-band is different in the fading channel, the CFO deviation in a specific frequency band undergoes weighted averaging with a higher proportion, which may cause the determined value of the frequency offset to be biased. Because the wireless communication device (see100ofFIG.2) calibrates the CFO deviation due to the SCO for each sub-band and the calibrated CFO deviation is vector-combined in the I/Q area to determine the average frequency offset, a more accurate result may be obtained, regardless of the influence of different channel qualities for each frequency.

Referring toFIG.10, for a reception signal having a frequency offset of 20 ppm and a bandwidth of 320 MHz, a frequency offset value when an CFO is calibrated (proposed) has a smaller standard deviation than a baseline in the fading channel.

FIG.11is a diagram illustrating examples of devices for wireless communication, according to an embodiment.

Specifically,FIG.11illustrates an Internet-of-things (IoT) network system including a home gadget241, home appliances242, an entertainment device243, and an access point245.

In some embodiments, as described above, the devices for wireless communication illustrated inFIG.10may take into account the SCO and CFO coupled effects, which appear differently according to the position away from the center frequency, at each sub-band constituting the wideband channel, and may determine an accurate frequency offset value for each sub-band, regardless of the frequency position within the channel.

In addition, the devices for wireless communication illustrated inFIG.10may perform appropriate coherent combining on the frequency offset values of all sub-bands at which the reception signal in the wideband channel is detected among the frequency offset values determined for each sub-band by reflecting the SCO and CFO coupled effects and may determine an accurate frequency offset value, regardless of the occupied frequency band and the bandwidth of the finally transmitted frame.

While aspects of the inventive concept have been particularly shown and described with reference to embodiments thereof, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the following claims.