KR100944754B1

KR100944754B1 - Apparatus of channel estimation for ofdm receiver, ofdm receiver having the same and method of channel estimation for ofdm receiver

Info

Publication number: KR100944754B1
Application number: KR1020080102440A
Authority: KR
Inventors: 조문호
Original assignee: (주)카이로넷
Priority date: 2008-10-20
Filing date: 2008-10-20
Publication date: 2010-03-03

Abstract

PURPOSE: An apparatus of channel estimation for an ofdm receiver, an ofdm receiver having the same and a method of channel estimation for the ofdm receiver are provided to selectively apply a channel estimation method according to the channel condition using output signal of an automatic gain control apparatus. CONSTITUTION: A timing synchronization part(110) implements the OFDM symbol synchronization of a received baseband signal. A time domain average unit(120) offers the average value of OFDM symbols as the average signal in the time domain. A Fourier transformer(130) performs Fourier transformation of the average signal. A first comparison unit(140) offers a first smoothing signal deciding the smoothing state of the channel estimation result. A guard band power calculation part(150) calculates the power of the guard band. A second comparison unit(160) offers a second smoothing signal deciding the smoothing state of the channel estimation result. A smoothing application part(170) offers the channel estimation result.

Description

A channel estimation apparatus for an orthogonal frequency division multiplexing receiver, an orthogonal frequency division multiplexing receiver including the same, and a channel estimation method for an orthogonal frequency division multiplexing receiver {APPARATUS OF CHANNEL ESTIMATION FOR OFDM RECEIVER, OFDM RECEIVER HAVING THE SAME AND METHOD OF CHANNEL ESTIMATION FOR OFDM RECEIVER}

The present invention relates to a reception apparatus based on Orthogonal Frequency Division Multiplexing (OFDM), and more particularly, to a channel estimation apparatus of an OFDM-based receiver selectively determining a channel state and selectively applying a channel estimation method. The present invention relates to an OFDM based receiver and a channel estimation method of an OFDM based receiver.

In general, when data is transmitted at high speed in a wireless channel, a high bit error rate is required due to the effects of multipath fading and Doppler spread, and thus a wireless access method suitable for a wireless channel is required. The OFDM scheme is suitable for high speed data transmission in a wireless channel.

OFDM is a type of multi-carrier transmission using multiple carriers.It converts a wideband signal serially input into N parallel data strings and divides them into several subcarriers having orthogonality to each other and transmits them in parallel so that each channel has the same number of carriers. The transmission period at will increase.

As a result, the interval of transmitted symbols is longer than the case of sequentially transmitting data using one carrier, and thus, the delay time of the channel is less affected. It has the effect of changing to the frequency non-selective channel characteristic. Therefore, the complexity of channel equalization can be reduced.

In addition, due to the orthogonality of subcarriers, each subcarrier is allowed to overlap in the spectrum, thereby increasing the efficiency of the spectrum compared to a general frequency division scheme, and the receiver can separate the subcarriers using simple signal processing techniques.

Although OFDM is robust to frequency selective attenuation, OFDM symbols suffer from magnitude and phase attenuation due to the passage of a radio channel. Therefore, the attenuation experienced in the channel must be estimated and compensated for. Therefore, accurate channel estimation plays an important role in increasing the overall performance of an OFDM system.

In general, OFDM channel estimation is performed by using a preamble signal. Channel estimation is performed by using algorithms such as LS (Least Square), LMMSE (Linear Minimum Mean Square Error), and Low Rank LMMSE in a training symbol of the preamble.

The LS algorithm is a technique for estimating the channel by dividing the training symbols of the received preamble into training symbols that are already known.The LS estimator is low in complexity and simple to implement, but it is sensitive to noise because it does not use the statistical characteristics of the channel. ) In terms of performance is poor.

The LMMSE algorithm is a channel estimate that minimizes the MSE between the actual channel value and the estimated channel value. The LMMSE algorithm performs channel estimation by applying an LS algorithm, and then smoothes the channel estimation result using the correlation value of the channel. The LMMSE algorithm uses the second order moments of channels such as autocorrelation and cross-correlation, so that the overall estimation error considering noise is minimized to achieve optimal performance in terms of MSE, but requires a large amount of computation to obtain statistical characteristics of the channel and noise. And the implementation is very complex.

In order to overcome the disadvantage that the LMMSE algorithm has to deal with a large amount of computation as described above, a low rank LMMSE algorithm has been proposed that reduces complexity by maintaining rank by using rank value decomposition (SVD). The channel estimation using the Low Rank LMMSE algorithm shows better MSE performance than the LS algorithm in the low signal-to-noise ratio (SNR) section, but degrades MSE performance in the high signal-to-noise ratio section, resulting in worse MSE performance than the LS algorithm.

Therefore, LS algorithm and Low Rank LMMSE algorithm divide performance superiority according to the status of wireless channel. In the conventional technique, one algorithm is common to all channels so that the channel estimation is not performed. Do not.

Accordingly, an object of the present invention is to provide an effective channel estimation apparatus by changing the channel estimation algorithm according to the channel state.

An object of the present invention is to provide an OFDM-based receiving apparatus including the channel estimation apparatus.

An object of the present invention is to provide an effective channel estimation method by changing the channel estimation algorithm according to the channel state.

In order to achieve the above object of the present invention, the channel estimation apparatus for an OFDM-based receiver according to an embodiment of the present invention includes a timing synchronizer, a time domain average unit, a Fourier transform unit, a first comparator, and a guardband power calculation. Part, a second comparator and a smoothing application part. The timing synchronizer uses an predefined preamble signal to match the OFDM symbol synchronization of the received baseband signal and calculates a maximum delay spread time value of the channel. The time domain average unit averages two consecutive OFDM symbols included in the received baseband signal in the time domain to provide an average signal. The Fourier transform unit performs a Fourier transform on the average signal to provide a frequency domain signal. The first comparator compares the calculated maximum delay spread time value with the maximum smoothing time value of the channel estimation result by the LS algorithm for applying the Low Rank LMMSE algorithm to determine whether to smooth the channel estimation result. Provides 1 smoothing signal. The guardband power calculator calculates a guardband power that is an average power of an unused subcarrier signal included in the frequency domain signal. The second comparator compares the calculated guardband power with a guardband power threshold and provides a second smoothing signal for determining whether the channel estimation result is smooth. The smoothing application unit smoothes or bypasses the channel estimation result based on the first smoothing signal and the second smoothing signal to provide a channel estimation result.

In an embodiment, the OFDM-based receiver may be a WLAN receiver of IEEE 802.11n protocol.

In example embodiments, the timing synchronizer calculates a correlation between the received baseband signal and the predefined preamble signal, synchronizes the OFDM symbol by using a point at which the calculated correlation is maximized, and performs the correlation. The interval between the point at which the value is maximum and the last point at which the correlation value becomes 1/4 or less of the maximum value after the point at which the correlation value is maximum may be set as the maximum delay spread time value.

The first comparer activates the first smoothing signal when the maximum delay spread time value is less than the maximum smoothable time value, and the second comparer activates the guardband power at the guardband power threshold. If greater than this value, the second smoothing signal may be activated.

In an embodiment, the guardband power threshold may be a guardband power value such that the signal-to-noise ratio is 20 dB.

In example embodiments, the smoothing applying unit first determines whether the first smoothing signal is activated, and if the first smoothing signal is deactivated, bypasses the channel estimation result without smoothing to provide a channel estimation result. When the first smoothing signal is activated, it is determined whether the second smoothing signal is activated. When the second smoothing signal is inactivated, the channel estimation result is bypassed without smoothing to provide a channel estimation result. When the second smoothing signal is activated, smoothing may be performed to provide a channel estimation result.

The smoothing application unit may determine whether the first smoothing signal is activated, and activate the second comparator when the first smoothing signal is activated, and when the first smoothing signal is inactive, the first smoothing signal. 2 You can disable the comparator.

In example embodiments, the smoothing applying unit may perform smoothing of the channel estimation result by adding subcarriers used in the frequency domain signal and first and second adjacent subcarriers in a ratio of 0.5: 0.25: 0.25.

In order to achieve the above object of the present invention, the channel estimation method of the OFDM-based receiver according to an embodiment of the present invention is to perform the OFDM symbol synchronization of the received baseband signal using a predefined preamble signal Calculating a maximum delay spread time value of the channel; averaging two consecutive OFDM symbols included in the received baseband signal in a time domain and providing the averaged signal; Fourier transforming the average signal to a frequency Providing a region signal, comparing the calculated maximum delay spread time value with a maximum smoothing time value of a channel estimation result by an LS algorithm for applying a low rank LMMSE algorithm, and determining whether the channel estimation result is smooth; Providing a first smoothing signal to determine, an unused subcarrier signal included in the frequency domain signal Calculating a guardband power, which is an average power of a call, comparing the calculated guardband power with a guardband power threshold to provide a second smoothing signal for determining whether to smooth the result of the channel estimation and the first Smoothing or bypassing the channel estimation result based on a smoothing signal and the second smoothing signal to provide a channel estimation result.

In order to achieve the above object of the present invention, an OFDM-based receiving apparatus according to an embodiment of the present invention receives a low noise amplifier (LNA) for amplifying the received data while reducing noise by receiving data passing through a wireless channel. Low Noise Amplifier, a mixer that extracts a baseband signal from the output signal of the low noise amplifier, an automatic gain control device that automatically adjusts gain and outputs the output signal of the mixer to have a constant gain. Automatic Gain Control), a channel estimation apparatus of an OFDM-based receiver that provides a channel estimation result by selectively applying a channel estimation method according to a channel state by using an output signal of the automatic gain control device, using the channel estimation result And a channel equalizer for compensating for the distorted channel characteristics for each subcarrier included in the output signal of the automatic gain control device. And a demodulator for demodulating the output signal of the channel equalizer into the original user signal.

The apparatus for estimating a channel may include: a timing synchronizer configured to synchronize OFDM symbols of the extracted baseband signal and calculate a maximum delay spread time value of a channel, wherein the channel estimator is included in the extracted baseband signal; A time domain average unit for averaging successive OFDM symbols in a time domain to provide an average signal, a Fourier transform unit for Fourier transforming the average signal to provide a frequency domain signal, the calculated maximum delay spread time value and the low rank A first comparator for applying a LMMSE algorithm to provide a first smoothing signal for determining whether the channel estimation result is smooth by comparing a maximum smoothing time value of the channel estimation result by an LS algorithm to the frequency domain signal; Guardband power calculation that calculates the power of the guardband, which is the average power of the unused subcarrier signals included A second comparator for comparing the calculated guardband power and the guardband power threshold to provide a second smoothing signal for determining whether the channel estimation result is to be smoothed and the first smoothing signal and the second smoothing signal. And a smoothing application unit for smoothing or bypassing the channel estimation result based on the channel estimation result.

Hereinafter, a channel estimating apparatus for an OFDM-based receiver, an OFDM-based receiving apparatus including the same, and a channel estimating method for an OFDM-based receiver according to embodiments of the present invention will be described in detail with reference to the accompanying drawings. It is not limited to the embodiments of the present invention, those skilled in the art will be able to implement the invention in various other forms without departing from the spirit of the invention.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprise" or "having" are intended to indicate that there is a feature, number, step, action, component, part, or combination thereof that is described, and that one or more other features or numbers are present. It should be understood that it does not exclude in advance the possibility of the presence or addition of steps, actions, components, parts or combinations thereof. Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art. Terms such as those defined in the commonly used dictionaries should be construed as having meanings consistent with the meanings in the context of the related art and shall not be construed in ideal or excessively formal meanings unless expressly defined in this application. Do not.

On the other hand, when an embodiment is otherwise implemented, a function or operation specified in a specific block may occur out of the order specified in the flowchart. For example, two consecutive blocks may actually be performed substantially simultaneously, and the blocks may be performed upside down depending on the function or operation involved. Like reference numerals in the drawings denote like elements.

1 is a block diagram illustrating an apparatus for estimating a channel of an OFDM based receiver according to an embodiment of the present invention.

Referring to FIG. 1, the channel estimating apparatus 100 includes a timing synchronizer 110, a time domain averager 120, a Fourier transform 130, a first comparator 140, and a guardband power calculator 150. ), A second comparator 160, and a smoothing application unit 170.

The timing synchronizer 110 synchronizes the OFDM symbol of the received baseband signal BS and calculates a maximum delay spread time value MDS of the channel by using a predefined preamble signal.

The time domain averaging unit 120 averages two consecutive OFDM symbols included in the received baseband signal BS in the time domain and provides the average signal AS. The time domain average unit 120 finds the starting point of the OFDM symbol by using the symbol synchronization set by the timing synchronizer 110.

The Fourier transform unit 130 performs a Fourier transform on the average signal AS and provides it as a frequency domain signal FS.

The first comparison unit 140 compares the maximum delay spread time value (MDS) and the maximum smoothing time threshold value of the channel estimation result by the LS algorithm to apply the Low Rank LMMSE algorithm, and thus whether the channel estimation result is smooth. It provides a first smoothing signal (S1) to determine the.

The guardband power calculator 150 calculates a guardband power (GBP), which is an average power of an unused subcarrier signal included in the frequency domain signal FS.

The second comparator 160 compares the guard band power (GBP) with the guard band power threshold and provides a second smoothing signal S2 for determining whether to smooth the channel estimation result by the LS algorithm.

The smoothing applying unit 170 smoothes or bypasses the channel estimation result by the LS algorithm based on the first smoothing signal S1 and the second smoothing signal S2 to provide the channel estimation result HCH.

FIG. 2 is a flowchart illustrating a smoothing determination procedure of the smoothing applying unit 170 of FIG. 1.

Referring to FIG. 2, it is determined whether the maximum delay spread time value (MDS) is greater than the maximum smoothing time threshold for applying the Low Rank LMMSE algorithm. It is determined whether the guard band power (GBP) is greater than the guard band power threshold, and if it is not greater than the threshold, smoothing is not applied. If the guard band power is greater than the threshold, the smoothing is applied to provide the channel estimation result.

Hereinafter, an operation of an apparatus for estimating a channel of an OFDM-based receiver according to an embodiment of the present invention will be described in detail with reference to FIGS. 1 and 2.

When the received baseband signal BS is input, the timing synchronizer 110 synchronizes OFDM symbols and calculates a maximum delay spread time value MDS of the channel.

3 is a graph illustrating a degree of correlation between a predefined preamble signal and a received baseband signal BS.

Referring to FIG. 3, since the point 310 at which the correlation value is maximum in the correlation value graph 300 between the predefined preamble signal and the received baseband signal BS becomes the start point of the OFDM symbol, the timing synchronizer ( 110 synchronizes OFDM symbols at this point. The time domain average unit 120 operates in accordance with the OFDM symbol synchronization. In addition, the maximum delay spread time value is the interval between the point 310 at which the correlation value is maximum and the last point 320 at which the correlation value becomes 1/4 or less of the maximum value after the point 310 at which the correlation value is maximum. Determined by (MDS).

The time domain average unit 120 averages two consecutive OFDM symbols included in the received baseband signal BS in the time domain using the OFDM symbol synchronization matched by the timing synchronizer 110, and then averages the average signal ( AS), and the Fourier transformer 130 performs Fourier transform on the average signal AS to provide the frequency domain signal FS.

The first comparator 140 calculates a maximum smooth time threshold value of the channel estimation result by the LS algorithm for applying the maximum delay spread time value MDS calculated by the timing synchronizer 110 and the Low Rank LMMSE algorithm. In comparison, a first smoothing signal S1 for determining whether to smooth the result of the channel estimation is provided.

The guardband power calculator 150 calculates a guardband power (GBP) which is an average power of an unused subcarrier signal among the subcarriers included in the frequency domain signal FS.

4 is a graph illustrating a comparison result of MSE performance of channel estimation by the LS algorithm without smoothing and channel estimation by the Low Rank LMMSE algorithm with smoothing when the maximum delay spread time value (MDS) is large.

Referring to FIG. 4, the low rank LMMSE algorithm using smoothing in the low signal-to-noise ratio 410 performs better in terms of MSE, but the performance of the LS algorithm without smoothing is better in the high signal-to-noise ratio 420. It can be seen that. Since the two graphs intersect at the point where the signal-to-noise ratio is 20 dB, the guardband power threshold in the second comparator is a value of the guard band power such that the signal-to-noise ratio is 20 dB. The method of obtaining guardband power so that the signal-to-noise ratio is 20dB is as follows.

When the power of one OFDM symbol included in the received signal is X, X is added to the power and noise power of the transmitted signal. If the noise is white noise, the average power of the unused subcarriers is the noise power Y, and the signal power is X minus the total subcarriers * Y. Therefore, the signal-to-noise ratio is expressed by the following equation.

[Equation]

SNR = X / (N * Y)-1

(Where N represents the total number of subcarriers.)

Therefore, if the average power (X) of one OFDM symbol included in the received baseband signal is obtained, the value of the guardband power (Y) to obtain a signal-to-noise ratio of 20 dB can be obtained by the above equation.

5 is a block diagram illustrating an OFDM based receiver including a channel estimation apparatus of an OFDM based receiver according to an embodiment of the present invention.

Referring to FIG. 5, the OFDM-based receiver 500 includes a low noise amplifier 510, a mixer 520, an automatic gain controller 530, a channel estimator 540, a channel equalizer 550, and a demodulator 560. ).

The low noise amplifier 510 amplifies the received data while receiving data passing through the wireless channel to reduce noise. The mixer 520 extracts the baseband signal from the output signal of the low noise amplifier 510. The automatic gain control device 530 automatically adjusts the gain, and outputs the output signal of the mixer 520 to have a constant gain. The channel estimator 540 provides a channel estimation result by selectively applying a channel estimation method according to the state of the channel using the output signal of the automatic gain control apparatus 530. The channel equalizer 550 compensates for the distorted channel characteristics for each subcarrier included in the output signal of the automatic gain control apparatus 530 by using the channel estimation result of the channel estimating apparatus 540. The demodulator 560 demodulates the output signal of the channel equalizer 550 into the original user signal.

The channel estimating apparatus 100 of FIG. 1 may be applied to the channel estimating apparatus 540 of the OFDM-based receiving apparatus 500 of FIG. 5.

The channel estimation apparatus of an OFDM-based receiver according to an embodiment of the present invention can effectively apply channel estimation in a wireless communication environment by changing a channel estimation algorithm according to a channel state, and thus can be efficiently applied to wireless communication.

As described above, the present invention has been described with reference to a preferred embodiment of the present invention, but those skilled in the art may vary the present invention without departing from the spirit and scope of the present invention as set forth in the claims below. It will be understood that modifications and changes can be made.

2 is a flowchart illustrating a procedure of determining whether a smoothing application unit of FIG. 1 is smoothed.

3 is a graph illustrating a degree of correlation between a predefined preamble signal and a received baseband signal.

FIG. 4 is a graph illustrating a comparison result of MSE performance of channel estimation by the LS algorithm without smoothing and channel estimation by the Low Rank LMMSE algorithm with smoothing when the maximum delay spread time value is large.

Claims

A timing synchronizer for adjusting OFDM symbol synchronization of a received baseband signal and calculating a maximum delay spread time value of the channel using a predefined preamble signal;

A time domain averaging unit for averaging two consecutive OFDM symbols included in the received baseband signal in a time domain to provide an average signal;

A Fourier transform unit for Fourier transforming the average signal to provide a frequency domain signal;

By comparing the calculated maximum delay spread time value and the maximum smoothing time value of the channel estimation result by the least square (LS) algorithm for applying the Low Rank Linear Minimum Mean Square Error (LMMSE) algorithm, A first comparator for providing a first smoothing signal for determining whether to smooth;

A guardband power calculator for calculating a guardband power which is an average power of an unused subcarrier signal included in the frequency domain signal;

A second comparison unit comparing the calculated guardband power with a guardband power threshold and providing a second smoothing signal for determining whether the channel estimation result is smoothed; And

And a smoothing application unit for smoothing or bypassing the channel estimation result based on the first smoothing signal and the second smoothing signal to provide a channel estimation result.

The method of claim 1, wherein the OFDM-based receiver,

Channel estimation apparatus of an OFDM-based receiver, characterized in that the wireless LAN receiver of the IEEE 802.11n protocol.

The method of claim 1, wherein the timing synchronization unit,

Calculate the degree of correlation between the received baseband signal and the predefined preamble signal, and synchronize the OFDM symbol using the point where the calculated correlation value is maximum, and the correlation with the point where the correlation value is maximum. And the maximum delay spread time value is an interval from a point at which a correlation value becomes one quarter or less of a maximum value after a point at which the value becomes maximum.

The method of claim 1, wherein the first comparison unit,

Activate the first smoothing signal when the maximum delay spread time value is smaller than the maximum smoothing time value;

The second comparison unit,

And if the guardband power is greater than the guardband power threshold, activating the second smoothing signal.

The method of claim 1, wherein the guardband power threshold,

A channel estimation apparatus for an OFDM-based receiver, characterized in that the value of the guardband power so that the signal to noise ratio is 20dB.

The method of claim 1, wherein the smoothing applying unit,

If the first smoothing signal is first determined to be activated, and if the first smoothing signal is deactivated, the channel estimation result is bypassed without smoothing to provide a channel estimation result. When the first smoothing signal is activated, the first smoothing signal is activated. If it is determined whether the second smoothing signal is activated, and if the second smoothing signal is inactivated, the channel estimation result is bypassed without smoothing to provide a channel estimation result, and if the second smoothing signal is activated, And a channel estimation result for smoothing the channel estimation result.

The method of claim 1, wherein the smoothing applying unit,

Determine whether to activate the first smoothing signal, activate the second comparator when the first smoothing signal is activated, and deactivate the second comparator when the first smoothing signal is deactivated. Channel estimation apparatus for OFDM-based receivers.

The method of claim 1, wherein the smoothing applying unit,

And a subcarrier used in the frequency domain signal and first and second adjacent subcarriers at a ratio of 0.5: 0.25: 0.25 to smooth the channel estimation result.

Calculating OFDM symbol synchronization of a received base band signal using a preamble signal and calculating a maximum delay spread time value of a channel;

Providing two average OFDM symbols included in the received baseband signal in an averaged time domain as an average signal;

Fourier transforming the average signal to provide a frequency domain signal;

A first smoothing signal for determining whether to smooth the channel estimation result is provided by comparing the calculated maximum delay spread time value with a maximum smoothing time value of the channel estimation result by the LS algorithm for applying the Low Rank LMMSE algorithm. Doing;

Calculating a power of a guard band, which is an average power of an unused subcarrier signal, included in the frequency domain signal;

Comparing the calculated guardband power with a guardband power threshold to provide a second smoothing signal for determining whether the channel estimation result is smoothed; And

And smoothing or bypassing the channel estimation result based on the first smoothing signal and the second smoothing signal to provide a channel estimation result.

A low noise amplifier (LNA) for amplifying the received data while reducing noise by receiving data passing through a wireless channel;

A mixer for extracting a baseband signal from an output signal of the low noise amplifier;

An automatic gain control device (AGC) for outputting the output signal of the mixer to have a constant gain;

A channel estimation apparatus of an OFDM-based receiver that provides a channel estimation result by selectively applying a channel estimation method according to a channel state by using an output signal of the automatic gain control apparatus;

A channel equalizer for compensating for a distorted channel characteristic for each subcarrier included in an output signal of the automatic gain control apparatus using the channel estimation result; And

And a demodulator for demodulating the output signal of the channel equalizer into an original user signal.

The apparatus of claim 10, wherein the channel estimating apparatus comprises:

A timing synchronizer configured to adjust OFDM symbol synchronization of the extracted baseband signal and calculate a maximum delay spread time value of the channel using a predefined preamble signal;

A time domain averaging unit for averaging two consecutive OFDM symbols included in the extracted baseband signal in a time domain to provide an average signal;

A first smoothing signal for determining whether to smooth the channel estimation result is provided by comparing the calculated maximum delay spread time value with a maximum smoothing time value of the channel estimation result by the LS algorithm for applying the Low Rank LMMSE algorithm. A first comparator;