WO2018147520A1

WO2018147520A1 - Interference cancellation apparatus and method robust to frequency offset in wireless communication system

Info

Publication number: WO2018147520A1
Application number: PCT/KR2017/008959
Authority: WO
Inventors: 고영채; 임병주
Original assignee: 고려대학교 산학협력단
Priority date: 2017-02-07
Filing date: 2017-08-17
Publication date: 2018-08-16
Also published as: KR101962273B1; KR20180091550A

Abstract

Provided are an interference cancellation apparatus and an interference cancellation method robust to a frequency offset in a wireless communication system. At this time, the interference cancellation apparatus receives a signal modulated according to a predetermined modulation scheme, estimates a carrier frequency offset on the basis of a preamble and a pilot of the received signal, detects an optimal filter coefficient matched with the estimated carrier frequency offset among optimum filter coefficients calculated for each of a plurality of carrier frequency offsets, and demodulates the received signal with a demodulation scheme corresponding to the modulation scheme by using the detected optimal filter coefficient in a reception filter and thus detects data.

Description

Apparatus and method for interference cancellation robust to frequency offset in wireless communication system

The present invention relates to an apparatus and method for canceling interference according to a frequency offset at a receiving end of a wireless communication system.

Orthogonal Frequency Division Multiplexing (OFDM) technology is widely used as a core technology for 4G mobile communication and wireless LAN systems. However, OFDM systems are very sensitive to synchronization and have out-of-band channels (OBBs). Due to this problem, OFDM technology is not suitable for the fifth generation network scenarios, such as machine-to-machine (M2M) and vehicle-to-vehicle (V2V).

Accordingly, a generalized frequency division multiplexing (GFDM) communication technique has been proposed that overcomes the shortcomings of OFDM and uses a new waveform suitable for the fifth generation network scenario. The GFDM method is a block-based filtering multicarrier technique, which is suitable for low latency communication and has a low peak-to-average power ratio (PAPR).

The GFDM scheme has various advantages over OFDM, but there is a problem that interference occurs because the orthogonality between subcarriers is lost by using a non-orthogonal signal.

In this regard, Korean Patent Laid-Open Publication No. 10-2009-0075730 (name of the invention: a method and apparatus for interference cancellation in a wireless communication system) includes a window applied to a signal before the signal is transmitted in a wireless communication system such as an OFDM system. A method for removing interference caused by a function and an apparatus for performing the method are disclosed. Such an interference cancellation apparatus includes a receiver device for receiving a signal to which a window function is applied, and a processing device coupled to the receiver device, the processing device comprising a plurality of coefficients derived from the window function (generally generated in real time or the By retrieving from a storage device coupled to the processing device) and applying the coefficients to the received signal to remove at least some of the interference from the received signal.

One embodiment of the present invention relates to an interference canceling apparatus and a method for canceling interference using a reception filter robust to a frequency offset in a wireless communication system.

However, the technical problem to be achieved by the present embodiment is not limited to the technical problem as described above, and other technical problems may exist.

In order to achieve the above technical problem, an interference cancellation apparatus based on a frequency offset of a wireless communication system according to an aspect of the present invention, when receiving a transmission signal modulated according to a predetermined modulation scheme, preamble and pilot of the transmission signal A frequency offset estimator for estimating a carrier frequency offset based on the frequency offset; A filter coefficient detector configured to store an optimum filter coefficient calculated in advance for each of a plurality of carrier frequency offsets and to detect an optimum filter coefficient matched to the estimated carrier frequency offset among the previously stored optimal filter coefficients; And a reception filter which detects data by demodulating the transmission signal by a demodulation scheme corresponding to the modulation scheme using the detected optimal filter coefficients.

In another aspect of the present invention, a method of canceling interference based on a frequency offset includes: receiving a transmission signal modulated according to a preset modulation scheme; Estimating a carrier frequency offset based on the preamble and pilot of the transmitted signal; Detecting an optimum filter coefficient matched to the estimated carrier frequency offset among the optimal filter coefficients calculated for each of a plurality of carrier frequency offsets stored in advance; And demodulating the transmission signal by a demodulation scheme corresponding to the modulation scheme by using the detected optimum filter coefficient as a reception filter to detect data.

According to the aforementioned problem solving means of the present invention, it is possible to effectively remove the interference of the received signal generated the frequency offset due to the oscillator and the Doppler effect in a wireless communication system using a variety of communication schemes, including GFDM as well as OFDM.

Further, according to the problem solving means of the present invention, it is possible to design a filter that is robust to the frequency offset without changing the basic structure of the receiving end of the wireless communication system.

1 is a block diagram of a wireless communication system according to an embodiment of the present invention.

2 is a block diagram of a receiving apparatus according to an embodiment of the present invention.

3 is a structural diagram of GFDM data applied to an embodiment of the present invention.

4 is a graph comparing performance when a reception filter robust to a frequency offset according to an embodiment of the present invention and another conventional reception filter are used.

5 is a flowchart illustrating a method for canceling interference based on a frequency offset according to an embodiment of the present invention.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. In the drawings, parts irrelevant to the description are omitted in the drawings, and like reference numerals designate like parts throughout the specification. In addition, while describing with reference to the drawings, even if the configuration shown by the same name may be different according to the drawing number, the drawing number is just described for convenience of description and the concept, features, functions of each configuration by the corresponding reference number Or the effects are not to be construed as limiting.

Throughout the specification, when a part is "connected" to another part, this includes not only "directly connected" but also "electrically connected" with another element in between. . In addition, when a part is said to "include" a certain component, which means that it may further include other components, except to exclude other components unless specifically stated otherwise, one or more other It is to be understood that the present invention does not exclude the possibility of the presence or the addition of features, numbers, steps, operations, components, parts, or combinations thereof.

1 is a block diagram of a wireless communication system according to an embodiment of the present invention. 2 is a block diagram of a receiving apparatus according to an embodiment of the present invention.

Hereinafter, to describe that the wireless communication system 10 according to an embodiment of the present invention is a GFDM communication system. However, the wireless communication system 10 is not limited to the GFDM communication system, and may be a wireless communication system according to various communication methods such as OFDM communication.

The GFDM communication system 10 includes a transmitting device 100 and a receiving device 200.

The transmitter 100 transmits the GFDM signal by performing GFDM modulation on the data "d". The GFDM signal is transmitted to the receiving device 200 through a noise channel. The receiving device 200 performs GFDM demodulation on the received GFDM signal to detect detection data ("

Outputs ").

Before describing the configuration of the GFDM communication system 10 in detail, the GFDM signal will be described with reference to FIG. 3.

As shown in FIG. 3, the GFDM data consists of blocks of K subcarriers and M subsymbols. In this case, the GFDM communication system 10 is characterized by using a filter for each subcarrier in the transmitting device, and when passing through the GFDM modulation filter, an out-of-band channel (OOB) can be reduced and a cyclic prefix (CP) is used. Because of this, the equalization process at the receiving device is simple.

1 again, the transmitting apparatus 100 converts the data “d” to be transmitted into parallel data through an S / P encoder. The transmitter 100 modulates the parallel data through the GFDM modulator 110 including the preset filter. At this time, the transmission device 100 allows each subcarrier to pass through a predetermined filter. Then, the transmitting device 100 adds a CP to the GFDM modulated GFDM signal, upconverts through digital-analog converting (“D / A”), and transmits the same.

The GDFM signal (that is, the transmission signal) transmitted by the transmission apparatus 100 may be represented by Equation 1 below.

In Equation 1

Is a filter passed for each subcarrier,

to be. In other words,

Is a filter coefficient that is multiplied by the data transmitted to the kth subcarrier and the mth subsymbol. And

Denotes data transmitted to the kth subcarrier and the mth subsymbol.

Equation 1 as described above may be expressed in a matrix form as shown in Equation 2 below.

At this time,

Is

as

A matrix of magnitudes and N denotes an upsampling coefficient.

For reference, the GFDM communication system 10 according to an embodiment of the present invention may be applied to an OFDM communication system. In general, an OFDM communication system includes an OFDM transmitter for performing data inverse fast fourier transform (IFFT) and a receiver for restoring data by performing a fast fourier transform (FFT) signal from the OFDM transmitter. The transmission apparatus 100 according to an embodiment of the present invention may use a raised cosine filter and a square root raised cosine filter.

And M = 1 may be referred to as an OFDM communication system. That is, in Equation 2

Since the matrix becomes an Inverse Fast Fourier Transform (IFFT) matrix, it is the same as the structure of OFDM, so the difference between GFDM and OFDM

Matrix

And how much M is.

The receiving device 200 receives the GFDM signal modulated by the GFDM method and down-converts it through analog-digital converting (A / D). The receiver 200 removes the CP from the digitally converted GFDM signal, and removes the CP from the GFDM demodulator 210 through the GFDM demodulator 210 including the preset filter. Next, the reception apparatus 200 converts the GFDM demodulated signal into serial data through a P / S encoder to detect data ("

In this case, the filter removes the interference by applying a predetermined optimal coefficient to the GFDM signal.

Referring to FIG. 2, the reception apparatus 200 converts a modulation matrix (") from the transmission apparatus 100 to data" d ".

Receives a GFDM signal multiplied by "), wherein the received GFDM signal includes a carrier frequency offset (CFO) (").

Is generated and noise is added. Thus, if a frequency offset occurs in the transmission signal from the transmission apparatus 100, performance will fall.

The GFDM demodulator 210 of the receiving device 200 estimates the frequency offset of the received GFDM signal and removes interference. First, the frequency offset estimator 211 estimates a frequency offset generated in a transmission signal based on a preamble and a pilot present at the front of the received data. Since the frequency offset estimation method can use various proposed algorithms, a detailed description thereof will be omitted. And filter coefficient detector 212 determines the estimated frequency offset " in a filter codebook in which filter coefficients are calculated and stored.

A filter coefficient corresponding to "" is detected. Next, interference is removed from the received GFDM signal through the reception filter 213 to which the detected filter coefficient is applied.

Hereinafter, a process in which the filter coefficient detector 212 calculates and stores filter coefficients for a plurality of frequency offsets will be described in detail.

The reception device 200 may use a matched filter as a reception filter as shown in FIG. 2, or may detect data through a minimum mean-squared error filter (MMSE). have.

When a matched filter is used, the detected data ("

") Can be expressed as Equation 3 below.

At this time,

silver

Means noise with power of.

For reference, the data detected when the MMSE filter is used ("

") Can be expressed as Equation 4 below.

Meanwhile, the GFDM signal in which the frequency offset is generated due to the oscillator and the Doppler effects of the transmitting apparatus 100 and the receiving apparatus 200 may be represented as in Equation 5 below.

here

Is the frequency offset matrix

Indicates. At this time,

Means the diagonal matrix,

Denotes a normalized frequency offset as an interval of the GFDM carrier.

In the case of using a matched filter, conventionally

The Hermition matrix of the matrix

The data could be detected by multiplying by.

On the other hand, the receiver 200 according to an embodiment of the present invention is a filter robust to the frequency offset

Is used as a matching filter, and the detected data can be represented by Equation 6 below.

At this time,

Is

as,

It means a matrix of size. Also

as,

Is

Means a filter shifted with respect to time and frequency.

Data detected using a filter strong against such a frequency offset (that is, data shown in Equation 6) can be developed as shown in Equation 7 below.

In Equation 7, the diagonal component is the signal to be obtained (that is, the actual data) and the remaining part is the interference signal.

If the frequency offset is removed, the matrix of Equation 7 is closer to the identity matrix, so that the interference is less, so that the desired signal can be detected. Therefore, since the desired signal (that is, the actual data) can be detected by minimizing the interference, the signal-to-interference ratio (SIR) can be maximized for Equation (7).

Obtain

That is, in Equation 7, since the diagonal component is the desired signal and the remaining portion is the interference signal, the signal-to-interference ratio is calculated by obtaining the power of the diagonal component and the power of the remaining component.

In this case, the signal-to-interference ratio may be expressed by Equation 8 below.

To simplify the Equation 8

Among the properties of, use the property shown in Equation 9 below.

As in Equation 9 above,

Did not shift time and frequency

Is a matrix that shifts time

And a matrix for shifting frequencies

Can be multiplied by

Therefore, the expression (8)

Substituting Equation 9 into Equation 9 can be arranged as in Equation 10 below.

At this time,

Is a value related to the received power of the signal transmitted on the k th subcarrier and the m th subsymbol,

Is a value related to the sum of the interference power generated due to the reception power and the frequency offset of the data when transmitting the data.

In equation (10)

And

Can be represented by Equations 11 and 12, respectively.

Thus, a filter capable of maximizing the value of Equation 10

By using, it is possible to minimize interference in the environment where frequency offset occurs and improve the performance of the system.

To this end, an optimal solution of Equation 13 is calculated.

The optimal solution of equation (13) can be obtained by the following equation (14) by Rayleigh-Ritz theorem. For reference, Rayleigh-Litz theory is a method of expressing an unknown function as a linear combination of a finite number of basis functions that satisfy a displacement boundary condition, and using the basis function as a weighted function to solve the solution. Weighted residual method

In Equations 11 and 12 above

And

So you can get

After calculating, the eigenvalue can be calculated. Find the maximum of the eigenvalues and find the corresponding eigenvector

Used as. At this time,

Hence, the filter coefficient obtained from Equation 14 is used for the reception filter 213.

The filter coefficients obtained through Equation 14 are not calculated and used in real time every time the GFDM signal is received, but are calculated and calculated by the filter coefficient detector 212 for each frequency offset, and are generated and stored in the codebook. The filter coefficient detector 212 uses matched filter coefficients among the filter coefficients stored in the codebook according to the frequency offset of the received GFDM signal.

In FIG. 4, in the OFDM communication method (indicated by "OFDM" in FIG. 4), when an existing matching filter is used in the GFDM communication system (indicated by "GFDM (MF method)" in FIG. 4), zero in the GFDM communication system. In the case of using forcing (indicated by "GFDM (ZF method)" in FIG. 4), in the case of using a reception filter robust to a frequency offset in the GFDM communication system according to an embodiment of the present invention (" Signal-to-interference ratio). In FIG. 4, a reception filter optimized for a frequency offset value of 0.2 is illustrated, and it can be seen that a reception filter according to an embodiment of the present invention exhibits superior performance compared to other filters in a frequency offset 0.2 section.

Hereinafter, an interference cancellation method in a wireless communication system through an interference cancellation apparatus robust to a frequency offset according to an embodiment of the present invention will be described with reference to FIG. 5. In the GFDM communication system according to an embodiment of the present invention, the receiving device may operate as an interference cancellation device for processing a GFDM signal.

When receiving a signal modulated by the GFDM scheme transmitted by the transmitter (S510), the frequency offset for the received signal is estimated (S520).

For reference, before estimating the frequency offset for the received signal, a procedure of analog-digital converting (A / D) the received signal and removing the CP may be performed.

Next, a filter coefficient matched to the estimated frequency offset is detected among the previously calculated filter coefficients for each frequency offset (S530).

In this case, previously calculated filter coefficients may be generated and stored in a codebook. In addition, since the method for calculating the filter coefficient has been described above through Equations 3 to 14, detailed description thereof will be omitted.

Thereafter, the detected filter coefficients are applied to the reception filter to process GFDM demodulation to detect and output data from which the interference is removed from the received signal (S540).

In this case, the reception filter may be a matching filter or an MMSE filter. In addition, the GFDM demodulated signal may be converted into serial data through a P / S encoder and output as detection data.

The interference cancellation method robust to the frequency offset of the wireless communication system according to the above-described embodiments of the present invention may be implemented in the form of a recording medium including instructions executable by a computer, such as a program module executed by a computer. have. Computer readable media can be any available media that can be accessed by a computer and includes both volatile and nonvolatile media, removable and non-removable media. Computer-readable media can also include both computer storage media and communication media. Computer storage media includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Communication media typically includes a mechanism for computer-readable instructions, data structures, program modules, or other data or other transmission of a modulated data signal such as a carrier wave, and includes any information delivery medium.

The foregoing description of the present invention is intended for illustration, and it will be understood by those skilled in the art that the present invention may be easily modified in other specific forms without changing the technical spirit or essential features of the present invention. will be. Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive. For example, each component described as a single type may be implemented in a distributed manner, and similarly, components described as distributed may be implemented in a combined form.

The scope of the present invention is shown by the following claims rather than the above description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present invention. do.

Claims

An interference cancellation apparatus robust to a frequency offset of a wireless communication system,

A frequency offset estimator for estimating a carrier frequency offset based on a preamble and a pilot of the received signal when receiving a signal modulated according to a preset modulation scheme;

A filter coefficient detector for storing an optimum filter coefficient calculated in advance for each of a plurality of carrier frequency offsets and detecting an optimum filter coefficient matched to the estimated carrier frequency offset among the stored optimal filter coefficients; And

And a reception filter which demodulates the received signal by a demodulation scheme corresponding to the modulation scheme using the detected optimum filter coefficients to detect data.
The method of claim 1,

And the filter coefficient detector generates and stores an optimum filter coefficient for each carrier frequency offset in a codebook, wherein the filter coefficient detector is robust to a frequency offset of a wireless communication system.
The method of claim 1,

The filter coefficient detector,

Calculating a diagonal matrix for a normalized frequency offset which is a carrier spacing applied during the modulation,

Calculating the signal-to-interference ratio based on the power of the diagonal component and the power of the remaining components of the diagonal matrix;

And a filter coefficient for maximizing the signal-to-interference ratio is calculated and stored as the optimum filter coefficient.
The method of claim 3, wherein

The filter coefficient detector,

Signal with Frequency Offset
For the signal-to-interference ratio (SIR) is calculated according to Equation 1 below,

Based on the calculated signal-to-interference ratio, the optimal solution is calculated according to Rayleigh-Ritz theorem according to Equation 2 below.

And the optimum solution is stored as the optimum filter coefficient.

<Equation 1>

<Equation 2>

Where d is the transmission data,
Is
as
A modulation matrix of magnitude, where g k, m are filter coefficients multiplied by the transmitted data, K is the number of subcarriers, M is the number of subsymbols, N is the upsampling coefficient,

C is the frequency offset matrix
This,
Is a diagonal matrix,
Is the normalized frequency offset, which is the carrier spacing,
Is the receiving filter
As a filter shifted in time and frequency
This,
Is the filter coefficient not shifted by time and frequency,
Is a time shift matrix,
Is a matrix for shifting frequencies,

Where U is a value based on the received power of the signal transmitted to the k th subcarrier and the m th subsymbol, V is a value based on the sum of the interference power generated due to the received power and the frequency offset for the data,

But the optimal one
The eigenvector corresponding to the maximum value of the eigenvalues of the matrix.)
The method of claim 1,

The reception filter,

An interference canceling apparatus robust to frequency offsets in a wireless communication system, which is either a matched filter or a minimum mean-square error filter (MMSE).
The method of claim 1,

The modulation scheme is Generalized Frequency Division Multiplexing (GFDM) modulation, interference cancellation apparatus robust to the frequency offset of the wireless communication system.
In the interference cancellation method of the interference cancellation apparatus robust to the frequency offset of the wireless communication system,

Receiving a signal modulated according to a preset modulation scheme;

Estimating a carrier frequency offset based on the preamble and pilot of the received signal;

Detecting an optimum filter coefficient matched to the estimated carrier frequency offset among the optimal filter coefficients calculated for each of a plurality of carrier frequency offsets stored in advance; And

And demodulating the received signal by a demodulation scheme corresponding to the modulation scheme by using the detected optimum filter coefficient as a reception filter to detect data.
The method of claim 7, wherein

The optimal filter coefficients calculated for each of the plurality of carrier frequency offsets are generated and stored in a codebook.
The method of claim 7, wherein

Before the step of detecting the optimum filter coefficients,

Calculating a diagonal matrix for a normalized frequency offset which is a carrier spacing applied during the modulation;

Calculating a signal-to-interference ratio based on the power of the diagonal component and the power of the remaining components of the diagonal matrix, and

Calculating a filter coefficient for maximizing the signal-to-interference ratio and storing the filter coefficient as the optimum filter coefficient.
The method of claim 7, wherein

The reception filter,

A method of interference cancellation, which is either a matched filter or a Minimum Mean-Squared Error filter (MMSE).
The method of claim 7, wherein

The modulation scheme is Generalized Frequency Division Multiplexing (GFDM) modulation.