KR101040465B1 - Method for channel estimation in a cazac code based mobile system - Google Patents

Abstract

Disclosed is a channel estimation method in a mobile communication system using a constant amplitude zero auto-correlation (CAZAC) code as a pilot. According to one embodiment, the method comprises receiving a pilot sequence from a transmitter, code compensating for the received pilot sequence, discrete Fourier transform (DFT) or inverse discrete Fourier transform (IDFT) for the repeated signal. And obtaining a channel estimate of the signal on which the DFT or the IDFT is performed. According to the above embodiment, effective channel estimation is possible in a system based on a CAZAC code including 3GPP LTE.

Long Term Evolution (LTE), Constant Amplitude Zero Auto-Correlation (CAZAC) code, Orthogonal Frequency Division Multiplexing (OFDM), Channel Estimation

Description

Channel Estimation Method in CAAC Code-Based Mobile Communication System {METHOD FOR CHANNEL ESTIMATION IN A CAZAC CODE BASED MOBILE SYSTEM}

1 illustrates a 3GPP LTE uplink frame structure.

2 is a block diagram illustrating one embodiment of a 3GPP LTE uplink channel estimator structure.

3 illustrates one embodiment of a method of repeating a received pilot sequence.

4 illustrates one embodiment of a muting window.

5A and 5B show channel response results of signals according to FIG. 4 for 3GPP LTE uplink.

<Explanation of symbols for main parts of the drawings>

N: FFT size of the system

N _k : Size of subcarrier allocated to k th user

L: magnitude of pilot or reference signal

R: Total number of iterations of pilot signal (R = R _L + R _R )

M: size of the muting window

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mobile system, and in particular, channel estimation in a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system based on a Constant Amplitude Zero Auto-Correlation (CAZAC) code. ).

3GPP LTE is a wireless transmission technology that 3GPP, the third generation asynchronous mobile communication technology standardization organization, aims to complete standardization in September 2007, and is considered as the fourth generation in consideration of technological completion and commercialization time. It is expected to be the most influential among them. 3GPP LTE is defined based on Orthogonal Frequency Division Multiplexing (OFDM), and the sequence used for the pilot is based on the CAZAC code.

High-speed data transmission over a wireless channel results in high bit error rates due to effects such as multipath fading, Doppler spread, and additive white Gaussian noise (AWGN). Connection method is required. As the wireless access method, a spread spectrum method having advantages such as low output and low detection probability is widely used. The spread spectrum method can be largely classified into a direct sequence spread spectrum (DSSS) method and a frequency hopping spread spectrum (FHSS) method. The DSSS scheme has an advantage of actively coping with a multi-path phenomenon occurring in a wireless channel by using a Rake receiver using path diversity of the channel. However, the DSSS method can be efficiently used up to a transmission rate of 10Mbps, but in the case of more high-speed data transmission, hardware complexity increases rapidly as inter-chip interference increases, and it can be accommodated by multi-user interference. It is known that there is a limit to the capacity of a user. On the other hand, the FHSS scheme has an advantage of reducing the effects of multi-channel interference and narrowband impulsive noise because data is transmitted while moving frequencies by random sequences. In FHSS, accurate synchronization between transmitters and receivers is very important, but it is difficult to extract synchronization during high-speed data transmission.

Recently, many OFDM schemes have been adopted as a standard considering the complexity of a transceiver and suitable for high-speed data transmission. In the OFDM method, since a plurality of carriers having mutual orthogonality are used, the frequency utilization efficiency is increased, and the process of transforming / demodulating the plurality of carriers in the transmitter / receiver has an inverse discrete Fourier transform (IDFT) and a discrete Fourier (DFT), respectively. This results in the same result as performing a transform, and can be implemented at high speed using inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). For this reason, the OFDM scheme is suitable for high-speed data transmission, so high-speed wireless LAN of IEEE 802.11a and HIPELAN / 2, Broadband Wireless Access (BWA) of IEEE 802.16, and Digital Audio Broadcasting (DAB) And Digital Terrestrial Television Broadcasting (DTTB), the standard of ADSL and VDSL. Although the 3GPP standard has used wireless communication based on the frequency spreading method, LTE (Long Term Evolution), which is recently discussed, uses OFDM for downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) for uplink. do. SC-FDMA is used to reduce the Peak to Average Power Ratio (PAPR). SC-FDMA is also an OFDM-based technology, and solves the PAPR problem by performing DFT at the OFDM front end to show a single carrier characteristic. .

Coherent demodulation in the receiver requires a pilot or reference signal. That is, channel estimation may be performed through a pilot or reference signal, and data distortion may be compensated for through the channel estimation value. In the conventional code division multiple access (CDMA), a sliding window method is used for channel estimation. In a high spread spectrum of 1 to 5 MHz, a pseudo random code, There was no problem in channel estimation due to orthogonal variable spreading factor (OVSF). In OFDM / SC-FDMA using the coherent method, since the frequency band used by the system is allocated to multiple users, the frequency allocated to one user is only several hundred kHz, so the length of the pilot / reference signal is frequency spread communication. Compared to the fall significantly. Even if the frequency allocation is small, the existing technology can be used. The 3GPP LTE uplink is as follows.

As shown in FIG. 1, in the frame structure of the 3GPP LTE uplink, 14 OFDM symbols constitute one subframe, and the 4th and 11th OFDM symbols are used for pilot. The remaining OFDM symbols are used for user data. The 3GPP LTE standard recommends that data and pilot use the same frequency band. The scheduler of the base station determines which user is allocated to each frequency band, and informs the frequency offset and the number of available subchannels (N _k ). The user can perform modulation, discrete fourier transform (DFT), and subcarrier mapping. data is transmitted through a process such as subcarrier mapping or inverse fast Fourier transform (IFFT). However, in the OFDM symbol that the pilot enters, a constant amplitude zero auto-correlation (CAZAC) code having a complex value is input to the DFT without performing data modulation. The receiver performs N-point FFT for demodulation to obtain the user's data and performs channel estimation to know the channel response due to the fading channel.

According to the conventional channel estimation method, after subcarrier demapping, the pilot uses a certain portion of the frequency domain, and the required portion of the pilot performs code compensation for channel estimation. The code-compensated signal moves from left to right by defining a sliding window of a certain size to remove white noise and interference, and averages the entire pilot period. If N _k is large, a sufficient sliding window size can be determined, so that a relatively accurate value can be obtained. However, since the minimum value of N _k is set to 12 in the 3GPP LTE uplink standard, it is not only difficult to determine the size of the sliding window, but also a short pilot length to remove white noise and interference. That is, according to OFDM, which allocates a small frequency band to a user, channel estimation may be inaccurate when the sliding window method is used because the pilot length is short.

An object of the present invention is to provide a method for channel estimation using characteristics of a constant amplitude zero auto-correlation (CAZAC) code used as a pilot. In addition, an object of the present invention is to enable efficient communication by knowing channel response and channel quality and transmission timing of a subcarrier.

According to one aspect of the present invention, a channel estimation method in a system using a CAZAC code as a pilot is provided. The method includes receiving a pilot sequence from a transmitter, code compensating the received pilot sequence, performing a DFT or IDFT on the compensated signal, and performing a signal on which the DFT or IDFT is performed. Obtaining a channel estimate.

According to an embodiment of the present invention, the method may further include repeating a predetermined amount of signals with respect to the code compensated sequence before performing the DFT or IDFT.

In order to achieve the above object, the present invention provides a method for channel estimation in the 3GPP LTE system using the characteristics of the Constant Amplitude Zero Auto-Correlation (CAZAC) code. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2 illustrates a structure of a 3GPP LTE channel estimator according to an embodiment. As shown in FIG. 2, the channel estimator 200 according to an embodiment includes a pilot sequence receiver 210, a code compensator 220, an iterator 230, and a discrete fourier transform (DFT) performer 240. , A muting unit 250 and an inverse discrete Fourier transform (IDFT) execution unit 260.

When the terminal has data to transmit and is allocated a radio resource that can be used by the base station, as shown in FIG. 1, the 14 OFDM symbols constitute one subframe, and the 4th and 11th OFDM symbols are used for pilot. It has a frame structure used, and transmits a signal using N _k subcarriers (subcarriers). However, the terminal may transmit a pilot regardless of data, and the operation according to the present embodiment is not limited to the uplink.

The base station receives the pilot transmitted from the terminal in the pilot sequence receiving unit 210, performs fast Fourier transform (FFT) on the OFDM symbol corresponding to the pilot to estimate the user's channel to obtain the desired subcarrier information. The received pilot sequence thus obtained becomes code compensation in the code compensation unit 220. Then, in the repeating unit 230, as shown in FIG. 3, some codes on the left edge are repeated R _L times and some codes on the right edge are repeated R _R times so that a total (R _L + R _R ) signal is obtained. Is repeated. The DFT is performed by the discrete fourier transform (DFT) performing unit 240 on the repeated signal. Here, the sequence length of the signal after the DFT is the sum of the magnitude (L) of the pilot signal and the number of repetitions (R = R _L + R _R ) of the edge code of the pilot signal, that is, (L + R), respectively. The signal includes interference and white noise (AWGN) of adjacent cells. In particular, when a pilot structure is used as CDM (code division multiplexing) in MU-MIMO, multi-user pilots exist simultaneously.

In this situation, in order to obtain a desired channel response, the muting unit 250 performs muting through a window having a size of M. In consideration of multipath fading and the magnitude of a pilot signal, The size (M) of the muting window and the position of the window must be determined. 4 illustrates one embodiment of a muting window for 24 subcarriers. In this case, since the timing of the transmission signal is correct, the point of the desired signal is seen at a point on the x-axis. In addition, another peak is seen at points 12-13, caused by another user's pilot at MU-MIMO. To estimate the pilot of another user, adjust the position of the muting window.

After removing white noise, interference, and other user's signals through muting, an inverse discrete fourier transform (IDFT) performer 260 may obtain a desired user's channel response 270 through the IDFT. In this case, the length of the pilot may be different from N _k . In this case, the channel response is obtained through specific signal cancellation and interpolation.

5A and 5B show a result of obtaining a channel response of the signal of FIG. 4 with respect to 3GPP LTE uplink. Since there is no difference in the edge part because it is not repeated, it can be seen that the channel estimated envelope and phase are well matched.

Although the above embodiments have been described with reference to the embodiments shown in the drawings for clarity, this is merely exemplary, and various modifications and equivalent other embodiments are possible to those skilled in the art. Will understand. In addition, although the above embodiments have been described with respect to 3GPP LTE system, those skilled in the art will understand that the present invention is applicable to all systems based on CAZAC code.

According to the above embodiments, channel estimation may be effectively applied to a system based on not only 3GPP LTE but also CAZAC code. Through efficient channel estimation, a receiver can provide accurate channel information to an equalizer, and multiple users can obtain a channel response of each user in a system in which a pilot is configured in a CDM manner. The information obtained through channel estimation helps determine channel quality and transmission timing of subchannels as well as equalizers.

Claims (12)

A channel estimation method in a system using a constant amplitude zero auto-correlation (CAZAC) code as a pilot, Receiving a pilot sequence from a transmitter, Code compensation on the received pilot sequence, Performing a discrete Fourier transform (DFT) or an inverse discrete Fourier transform (IDFT) on the compensated signal, and Obtaining a channel estimate for the signal on which the DFT or IDFT is performed How to include. The method of claim 1, Repeating a predetermined amount of signal for the code compensated sequence before performing the DFT or IDFT. The method of claim 2, The repeated amount of signal is a code at the edge of the code compensated sequence. The method of claim 1, Obtaining a channel estimate for the signal on which the DFT or IDFT has been performed, comprising using a peak or threshold of the signal. 5. The method of claim 4, Obtaining a channel estimate for the signal on which the DFT or IDFT has been performed includes identifying a position of the vertex or a position above the threshold and calculating a transmission timing of the transmitter. The method of claim 5, Informing the transmitter of the calculated transmission timing to adjust transmission timing. The method of claim 1, Finding a signal transmitted by the transmitter and removing a signal other than the desired signal to remove interference of another user and to minimize white noise. The method of claim 7, wherein An IDFT when the first DFT is performed after obtaining the desired signal, and a DFT when the first IDFT is performed. The method of claim 8, And adjusting the size and length of the signal in accordance with a channel response with respect to the signal on which the DFT or IDFT is performed. The method according to claim 1 or 2, The receiver further using the channel estimate for data modulation. The method according to claim 1 or 2, The receiver further using the channel estimate in scheduling for channel environment determination of the transmitter. The method of claim 1, The transmitter simultaneously transmits data and pilots, or only pilots.

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