KR20090119582A - A method and apparatus for channel estimation - Google Patents

Abstract

PURPOSE: A method and an apparatus for estimating a channel are provided to reduce a noise and a channel distortion phenomenon by adaptively estimating a channel according to a signal-to-noise ratio. CONSTITUTION: A transmitter transmits a signal to a receiver(200). The receiver receives a signal from the transmitter through a receiving antenna(210-1~210-Nr). The receiver receives a signal changed by a channel and a noise. A receiving unit(220) outputs an OFDM symbol from the received signal. An OFDM demodulating part(230) removes a CP(Cyclic Prefix) added to the OFDM symbol. The OFDM demodulating part obtains a signal of a frequency domain through a FFT(Fast Fourier Transform) process. A channel estimating device(300) includes a first channel estimator(310), a second channel estimator(320), and an effective section setting part(330). The first channel estimator calculates a base channel value based on a pilot extracted from the signal of the frequency domain. The second channel estimator and the effective section setting part calculate a channel estimation value by adaptively processing a noise of the base channel value according to a signal-to-noise ratio.

Description

A method and apparatus for channel estimation

The present invention relates to wireless communication, and more particularly, to a channel estimation method and apparatus for estimating a channel in consideration of a modulation and coding scheme.

Digital wired / wireless communication systems are widely used to provide various types of communication such as voice and packet data.

In digital communication, information is converted into digital data in bits. The transmitter modulates the input bit stream into a signal for transmission, and the receiver demodulates the received signal into bits to recover information. The phase and amplitude of the signal change in the transmission of communication data from the transmitter to the receiver. This is because channels are frequency-selective and time-varying. The transmitter must take these changes into account to recover the original data.

The change in phase and amplitude that occurs as a signal propagates along a channel is called the channel response. Channel response generally depends on frequency and time. If the receiver can determine the channel response, the received signal can be corrected to compensate for channel degradation. Determining the channel response at the receiver is called channel estimation.

The accuracy of the channel estimation performed by the receiver in a changing channel environment greatly affects system performance. Therefore, as a multi-carrier communication system using a plurality of subcarriers has recently been developed, there is a need for increasing the accuracy of channel estimation. In the multi-carrier communication system, the channel characteristics of each subcarrier are different, and the subcarriers undergoing large fading have a relatively high error rate. In order to demodulate a transmission signal in a fading channel environment, the receiver may estimate channel information using pilots promised to each other during a transmission and reception period.

In order to improve the accuracy of channel estimation, the noise included in the received signal must be efficiently removed, and the distortion of the channel that may occur when the noise is removed should be reduced. However, in the conventional channel estimation method of mapping the channel information obtained in the frequency domain to a signal in the time domain to remove noise and converting it back to the frequency domain, the channel estimation method is performed regardless of the signal to noise ratio (SNR) of the received signal. Since only samples included in a certain time period in the region are used for channel estimation, there is a problem in that it is impossible to satisfy both factors, such as noise processing and reduction of channel information distortion, which are factors of accurate channel estimation. This is because samples that are not included in a certain time period also have components of the original signal. This distortion of channel information eventually leads to deterioration of data restoration and leads to a decrease in data throughput.

Accordingly, there is a need for a method and apparatus capable of performing accurate channel estimation in consideration of the signal-to-noise ratio of a received signal.

An object of the present invention is to provide a channel estimation method and apparatus for estimating a channel in consideration of a modulation and coding scheme.

According to one aspect of the present invention, a channel estimation method is provided. The method includes obtaining a channel estimate in a frequency domain based on the pilot extracted from a received signal including a plurality of subcarriers to which data or pilot is mapped, and mapping a time domain signal obtained by mapping the channel estimate to the time domain. And obtaining a channel estimate corrected in the frequency domain by taking an effective period set according to the modulation and coding scheme of the received signal.

According to another aspect of the present invention, a channel estimation method is provided. The method includes receiving a signal including a plurality of subcarriers to which data or pilots are mapped, obtaining a channel estimate in a frequency domain based on the pilot extracted from the received signal, and converting the channel estimate into a time domain. Obtaining the corrected channel estimate in the frequency domain by taking the time-domain signal obtained by mapping according to a valid period set according to the modulation and coding scheme of the received signal, and restoring the data using the corrected channel estimate. do.

According to another aspect of the present invention, there is provided a channel estimating apparatus. The apparatus includes a first channel estimator obtaining a channel estimate in a frequency domain based on the pilot extracted from a received signal including a plurality of subcarriers to which data or pilots are mapped, and a time obtained by mapping the channel estimate to a time domain. An effective section setter for setting an effective section according to a modulation and coding scheme of the received signal, and a second channel estimator for obtaining a corrected channel estimate by mapping the time domain signal belonging to the valid section to a frequency domain Include.

By taking into account the signal-to-noise ratio, adaptive channel estimation can provide high-quality channel estimation with reduced channel distortion and noise.

1 is a block diagram illustrating a wireless communication system. Wireless communication systems are widely deployed to provide various communication services such as voice and packet data.

Referring to FIG. 1, a wireless communication system includes a base station 20 (BS) and a user equipment (UE) 10. The base station 20 generally refers to a fixed station that communicates with the terminal 10, and in other terms, such as a Node-B, a Base Transceiver System, or an Access Point. Can be called. One or more cells may exist in one base station 20. The terminal 10 may be fixed or mobile and may be called by other terms such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), and a wireless device.

Hereinafter, downlink means communication from the base station 20 to the terminal 10, and uplink means communication from the terminal 10 to the base station 20. In the downlink, the transmitter may be part of the base station 20 and the receiver may be part of the terminal 10. In uplink, the transmitter may be part of the terminal 10 and the receiver may be part of the base station 20.

Multiple access schemes for downlink and uplink transmission may be different. For example, downlink may use Orthogonal Frequency Division Multiple Access (OFDMA) and uplink may use Single Carrier-Frequency Division Multiple Access (SC-FDMA).

There is no limitation on the multiple access scheme applied to the wireless communication system. Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Single-Carrier FDMA (SC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), or other known modulation techniques. It can be based on the same multiple access techniques. These modulation techniques demodulate signals received from multiple users of a communication system to increase the capacity of the communication system.

The wireless communication system may be an Orthogonal Frequency Division Multiplexing (OFDM) / Orthogonal Frequency Division Multiple Access (OFDMA) based system. The OFDM technique divides a high-rate data stream into a large number of low-rate data streams and transmits them simultaneously using multiple carriers. Each of the plurality of carriers is called a subcarrier.

Since orthogonality exists between a plurality of carriers, the receiver can detect a signal even if frequency components of subcarriers overlap each other. The data stream having the high data rate is converted into a plurality of low data rate data streams through a serial to parallel converter, and each subcarrier is multiplied by the data streams converted in parallel. Each subcarrier may be converted into an OFDM symbol in a time domain by an inverse discrete fourier transform (IDFT) and transmitted. The IDFT can be efficiently implemented using an Inverse Fast Fourier Transform (IFFT).

Since the symbol duration of the low carrier subcarrier is increased, relative signal dispersion in time caused by multipath delay spread is reduced. Inter-symbol interference can be reduced by inserting a guard interval longer than the delay spread of the channel between OFDM symbols. In addition, if a part of the OFDM signal is copied to the guard period and placed at the beginning of the symbol, the OFDM symbol may be protected by being cyclically extended.

2 is a block diagram illustrating an example of a transmitter. The transmitter is based on a Single Carrier-FDMA (SC-FDMA) system. SC-FDMA is a technique that is mainly applied to uplink transmission, and prior to generating an OFDM signal, spreading is first applied to a DFT matrix in a frequency domain and then modulated and transmitted using the OFDM scheme.

Referring to FIG. 2, the transmitter 100 includes a transmit processor 110, a serial to parallel converter 120, a DFT spreader 130, and a subcarrier mapper 140. , IDFT unit 150, parallel to serial converter 160, cyclic prefix inserting unit 170, transmitting unit 180 and transmitting antenna 190-1, ..., 190-N _Nt ).

The transmission processor 110 receives data such as voice or packet data. The transmission processor 110 processes (source coding, channel coding, mapping) the input data to generate a plurality of serial data symbols having a high data rate.

The serial / parallel converter 120 converts the plurality of serial data symbols into a parallel data stream having a low data rate.

The DFT spreader 130 distributes the parallel data stream as shown in Equation 1 using a DFT matrix.

는 데이터 심벌 벡터 s를 분산시키기 위해서 사용된 N_b 크기의 DFT 행렬이다. Here, x represents a vector in which the data stream is distributed in the frequency domain, N _b represents the number of subcarriers for any user, and s represents a data symbol vector.

Is the N _b used to spread the data symbol vector s DFT matrix of magnitude.

The subcarrier mapper 140 maps the distributed vector x to the subcarrier.

The IDFT unit 150 generates an OFDM symbol as shown in Equation 2 by converting the subcarrier to which the vector x is mapped to the time domain.

는 주파수 영역의 신호를 시간 영역의 신호로 변환하기 위해 사용되는 크기 N의 DFT 행렬이다. DFT 행렬의 크기는 특정한 목적을 위해 다양하게 제어될 수 있다.Here, y represents an OFDM symbol vector transmitted in the time domain, and N represents the number of subcarriers included in the OFDM symbol.

Is a DFT matrix of size N that is used to convert a signal in the frequency domain into a signal in the time domain. The size of the DFT matrix can be variously controlled for a specific purpose.

The parallel / serial converter 160 converts an OFDM symbol including a parallel subcarrier string in series. The CP inserter 170 inserts a CP into the serial-converted OFDM symbol. CP is added at the transmitter to remove inter-symbol interference (ISI) to turn the frequency-selective channel into a flat-fading channel, also known as a guard interval do.

The transmitting unit 180 converts the sample signal output from the CP inserting unit 170 into an analog signal and transmits it through the transmitting antennas 190-1,..., 190 -N _t .

3 is a block diagram illustrating a receiver according to an example of the present invention.

Referring to FIG. 3, the receiver 200 includes reception antennas 210-1,..., 210 -N _r , a reception unit 220, an OFDM demodulator 230, a reception processor 240, and a channel estimation device. (300).

Receive antennas 210-1,..., 210 -N _r receive signals from the transmitter. When the transmitter 100 transmits the signal X to the receiver 200, the receiver 200 receives the signal Y changed by the channel H and the noise N. Transmitted signal X = [X ₁ , X ₂ , ..., X _Nt ], Received signal Y = [Y ₁ , Y ₂ , ..., Y _Nr ], Noise N = [N ₁ , N ₂ , .. , N _Nr ], the relationship shown in the following equation 3 is established between X and Y.

Where N _t is the number of transmit antennas 110-1, ..., 110-N _Nt , N _r is the number of receive antennas 210-1, ..., 210-N _r , and X _Nt is the transmit The signal transmitted from the antenna N _t , Y _Nr is a signal received from the receiving antenna N _r , and H _NrNt is a channel through which the signal transmitted from the transmitting antenna N _t reaches the receiving antenna N _r . The receiver 200 needs to know the channel H correctly to obtain the actual transmitted signal X from the received signal Y. The channel H can be estimated by using a pilot that the transmitter 100 and the receiver 200 know each other. The estimated value of the channel H is called a channel estimation value. The pilot is also referred to as a reference signal (RS).

The receiving unit 220 outputs an OFDM symbol from the received signal. The OFDM demodulator 230 removes a cyclic prefix (CP) added to an OFDM symbol and performs a fast fourier transform (FFT) to obtain a signal in a frequency domain. The signal in the frequency domain includes a plurality of subcarriers. Data or pilots are mapped to each subcarrier. However, data or pilot may not be mapped to a subcarrier corresponding to a guard period.

The channel estimating apparatus 300 includes a first channel estimator 310, a second channel estimator 320, and an effective section setting unit 330. The first channel estimator 310 extracts a pilot from the signal in the frequency domain and obtains a base channel value based on the extracted pilot. If the signal-to-noise ratio (SNR) is relatively large, the influence of the white noise is small, so that the base channel value can be relied on. However, if the signal-to-noise ratio is relatively small, the base channel value may be dominated by white noise. To solve this problem, the second channel estimator 320 and the effective section setting unit 330 adaptively process the noise of the base channel value according to the signal-to-noise ratio to obtain a channel estimate.

The second channel estimator 320 maps the base channel value to the time domain by IFFT and converts the base channel value into a time domain signal. The second channel estimator 320 zero-replaces all remaining portions of the time domain signal except the valid section set by the valid section setting unit 330 by zero, takes a signal of the valid section, and FFTs the frequency. Obtain the noise-treated channel estimate in the domain.

The valid period setting unit 330 sets a valid section of the time domain signal. The valid section is also known as the time window. The time domain signal generally includes noise, and the effective section is a section in which the second channel estimator 320 determines only which time delay point signal is validly allowed in obtaining a channel estimate. As described above, signals other than the valid interval are all replaced by zeros by the second channel estimator 320. Because, after a certain time delay point, the original signal almost disappears and only noise remains. This noise does not need to be taken into account to obtain the channel estimate.

However, even if the signal outside the effective period, the noise is relatively large and the original signal may be small, but may exist. When such a small amount of the original signal is replaced with zero by the second channel estimator 320, a negative effect of distorting channel information may be generated in addition to the positive effect of noise reduction. In order to reduce such negative effects, the effective period setting unit 330 adaptively adjusts the size of the effective period according to a modulation and coding scheme (MCS). This is because the signal-to-noise ratio changes according to the channel situation, so that this change is reflected in the calculation of the channel estimate to reduce the distortion.

The reception processor 240 extracts and restores data symbols from the signal in the frequency domain by using the channel estimation value input from the channel estimating apparatus 300, and demodulates and decodes the restored data symbols. Output the data stream.

4 is an explanatory diagram illustrating a method of setting an effective section by the valid section setting unit according to an embodiment of the present invention.

Referring to FIG. 4, the time-domain signal has an impulse peak value in an ideal case where there is no noise. That is, the peak value measured every time delay from t1 to t5 should be discontinuous as in the solid line. However, since the actual time-domain signal is interpolated between each peak value and includes noise, it has a continuous form in which noise is applied after interpolation between each peak value as shown by a dotted line.

As the effective section setting unit 330 shortens the valid section, since a large portion of the original signal will be replaced with zero, the distortion of the channel estimation value may become more severe, and the influence of noise may be reduced. That is, the minimization of distortion and the minimization of noise are in a trade-off relationship. Therefore, the effective section setting unit 330 must adaptively adjust the size of the valid section in consideration of the quality of the received signal such as the signal-to-noise ratio. If the noise is uniformly processed according to the size of the fixed effective period despite the change in the channel state, the reliability of the channel estimation is lowered, which may cause deterioration of data recovery.

Assuming a low signal-to-noise ratio, the time-domain signal contains more noise. Therefore, the effective section setting unit 330 sets the valid section 1 (V ₁ ) relatively short to reduce the effect of noise as much as possible rather than the effect of distortion.

5 is an explanatory diagram for explaining a method for setting an effective section by an effective section setting unit according to another example of the present invention.

Referring to FIG. 5, assuming that the signal-to-noise ratio is high, relatively little noise is included in the time-domain signal. Therefore, the effective section setting unit 330 sets the valid section 2 (VS ₂ ) relatively long in order to reduce distortion of the channel estimate value rather than the effect of noise. That is, when compared with the effective period in Fig. 4, VS ₁ <VS ₂ .

In this way, if the effective section setting unit 330 variably sets the size of the valid section according to the channel state, the reliability of channel estimation may be increased. The valid section setting unit 330 may set the size of the valid section in consideration of a modulation and coding scheme. In a system using Adaptive Modulation and Coding (AMC), the modulation and coding scheme is changed according to the Channel Quality Information (CQI) fed back. For example, the transmitter increases the transmission rate by applying a high modulation and coding rate in a channel environment having a high quality of the received signal, and lowers the transmission rate by applying a low modulation and coding rate in a channel environment having a low quality of the receiving signal. Lowers. The transmitter informs the receiver of the modulation and coding scheme determined according to the quality of the received signal through the control channel.

6 is a block diagram illustrating a first channel estimator according to an embodiment of the present invention.

Referring to FIG. 6, the first channel estimator 310 includes a pilot extractor 311 and a least square estimator 312. The pilot extractor 311 extracts pilots R [0], R [1], ..., R [N-1] from the signals in the frequency domain.

The LS estimator 312 performs the pilots P [0], P [1], ... promised in the transmission and reception period with the extracted pilots R [0], R [1], ..., R [N-1]. The base channel values H '[0], H' [1], ..., H '[N-1] are found in the frequency domain by using Least Square Estimation with P [N-1]. . The least-squares estimation technique has a lower channel estimation performance than the minimum mean sqaure (MMSE) technique, but has an advantage of easy implementation and low complexity. The extracted pilot has a form of a product of a pilot promised in a transmission period and a channel in a frequency domain as shown in Equation 4.

Where k (0≤k≤N-1) is the subcarrier index in the frequency domain, R [k] is the pilot mapped to the subcarrier at index k actually received by the receiver 200, and H [k] is the receiver 200 The base channel value, P [k], of the subcarrier of index k to be estimated by P denotes a pilot signal promised between the transmitter 100 and the receiver 200, and N [k] denotes a noise signal.

The LS estimator 212 obtains the base channel value H 'by dividing the pilot actually received by the pilot 100 and the pilot 200 already known as Equation 5.

The base channel value obtained by Equation 5 is input to the second channel estimator 320.

7 is a block diagram illustrating a second channel estimator according to an embodiment of the present invention.

Referring to FIG. 7, the second channel estimator 320 includes an IDFT unit 321, a zero-replace unit 322, and a DFT unit 323.

The IDFT unit 321 maps the elementary channel values H '[0], H' [1], ..., H '[N-1] to signals in the time domain, thereby channel values h [0], h [1], ..., h [N-1] are obtained. The zero-replacement unit 322 obtains the channel values h_zero [0], h_zero [1], ... h_zero [N-1] of each sample in which the signal part after the effective period is replaced with all zeros.

The DFT unit 323 performs a DFT on the channel values h_zero [0], h_zero [1], ... h_zero [N-1], and again channel estimate values H [0], H [1], ... in the frequency domain. , H [N-1].

As described above, the second channel estimator 320 inversely transforms the channel value obtained through the Fourier transform during the pilot reception period into the impulse response of the channel, removes the noise and the interference signal, and then Fourier transforms it. Obtaining the frequency response removes the components of the noise and interference signals to obtain better channel estimates.

8 is a simulation graph showing performance when channel estimation is performed using the channel estimation apparatus and method according to the present invention. The simulation environment of FIG. 8 is shown in Table 1.

It is assumed that the uplink MCS uses 16QAM at a signal-to-noise ratio of 16 dB or less, and 64 QAM at 16 dB or more, and the effective period of the prior art and the present invention is equal to 16 dB or less.

Referring to FIG. 8, it can be seen that the prior art in which the effective period is fixed and the present invention use the same valid period in the case of 16QAM, so that the two channel estimation techniques have the same performance. However, in the case of 64QAM applied at 16dB or more, the present invention can confirm that the performance improvement effect can be obtained by reducing the channel distortion phenomenon by setting the effective period longer. As such, by using an effective period that can be adaptively adjusted according to the channel state, a high quality channel estimation value can be obtained by supplementing a channel distortion phenomenon, which has been a problem of the prior art.

The channel estimation technique described above may be implemented by various means. For example, the channel estimator may be implemented in hardware, software or a combination thereof. In hardware implementation, an application specific integrated circuit (ASIC), a digital signal processing (DSP), a programmable logic device (PLD), a field programmable gate array (FPGA), a processor, a controller, and a microprocessor are designed to perform the above functions. , Other electronic units, or a combination thereof. In a software implementation, the channel estimator may be implemented as a module that performs the functions described above. The software may be stored in a memory unit and executed by a processor. The memory unit or processor may employ various means well known to those skilled in the art.

Although the present invention has been described above with reference to the embodiments, it will be apparent to those skilled in the art that the present invention may be modified and changed in various ways without departing from the spirit and scope of the present invention. I can understand.

1 is a block diagram illustrating a wireless communication system.

2 is a block diagram illustrating an example of a transmitter.

3 is a block diagram illustrating a receiver according to an example of the present invention.

4 is an explanatory diagram illustrating a method of setting an effective section by the valid section setting unit according to an embodiment of the present invention.

5 is an explanatory diagram for explaining a method for setting an effective section by an effective section setting unit according to another example of the present invention.

6 is a block diagram illustrating a first channel estimator according to an embodiment of the present invention.

7 is a block diagram illustrating a second channel estimator according to an embodiment of the present invention.

8 is a simulation graph showing performance when channel estimation is performed using the channel estimation apparatus and method according to the present invention.

Claims (8)

Obtaining a channel estimate in a frequency domain based on the pilot extracted from a received signal including a plurality of subcarriers to which data or pilots are mapped; And And taking the time domain signal obtained by mapping the channel estimate to the time domain by an effective period set according to the modulation and coding scheme of the received signal, to obtain a channel estimate corrected in the frequency domain. The method of claim 1, The higher the order of modulation and the higher the coding rate (coding rate), the larger the size of the effective period, the channel estimation method. The method of claim 1, Wherein the channel estimate is obtained by a Least Square (LS) estimation technique. Receiving a signal comprising a plurality of subcarriers to which data or pilots are mapped; Obtaining a channel estimate in a frequency domain based on the pilot extracted from the received signal; Obtaining a corrected channel estimate in the frequency domain by taking the time domain signal obtained by mapping the channel estimate to the time domain by an effective period set according to a modulation and coding scheme of the received signal; And Reconstructing the data using the corrected channel estimate. The method of claim 1, And transmitting Channel Quality Information (CQI) used to determine the modulation and coding scheme. A first channel estimator obtaining a channel estimate in a frequency domain based on the pilot extracted from a received signal including a plurality of subcarriers to which data or pilots are mapped; An effective section setter configured to set an effective section of the time domain signal obtained by mapping the channel estimate to the time domain according to a modulation and coding scheme of the received signal; And And a second channel estimator for mapping the time domain signal belonging to the valid interval to the frequency domain to obtain a corrected channel estimate. The method of claim 6, And the first channel estimator obtains the channel estimate using a least squares estimating technique. The method of claim 6, And the valid period setter replaces the time domain signal other than the valid period with zero.

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