CN113612709B - Channel estimation method based on joint placement of orthogonal time-frequency space OTFS pilots - Google Patents

Abstract

The invention discloses a channel estimation method based on joint placement of orthogonal time-frequency space OTFS pilots. The implementation steps are: 1. Place orthogonal time-frequency space OTFS data block pilots and guard intervals; Perform the inverse symplectic Fourier transform, and then perform the Heisenberg transform on the signal block to send the obtained time domain signal; 3. The orthogonal time-frequency space OTFS data block signal received by the receiving end is subjected to threshold detection; 4. Extraction exceeds the threshold The data block signal of the receiving end is calculated, and the antenna channel coefficient is calculated. The present invention places a guard interval at a position within half of the maximum delay spread and the maximum Doppler spread around the pilot symbol, reduces the interference received by the pilot symbol at the receiving end, improves the accuracy of the estimated channel information, and reduces the Pilot energy requirements.

Description

Channel estimation method based on joint placement of orthogonal time-frequency space OTFS pilots

technical field

The present invention belongs to the field of communication technologies, and further relates to a channel estimation method based on joint placement of Orthogonal Time Frequency Space (OTFS) pilots in the field of wireless communication technologies. The present invention can be used to estimate corresponding channel information from pilot signals received in an OTFS system.

Background technique

Currently, the Orthogonal Frequency Division Multiplexing (OFDM) modulation technology widely used in 5G and WIFI wireless networks is easily affected by the Doppler effect. OTFS has better performance than OFDM in high mobility wireless communication scenarios. Orthogonal Time-Frequency-Space OTFS is a two-dimensional modulation scheme that modulates in the delay-Doppler domain. Through a series of two-dimensional transformations, the dual-dispersion channel is converted into an approximately non-fading channel in the delay-Doppler domain. One of the main challenges faced by the OTFS system is how to accurately estimate the delay-Doppler channel state information (CSI). For the channel estimation of the OTFS system, the main challenge is how to place the pilots. If the pilot placement scheme is not good, the accuracy of the channel estimation of the entire system will be affected, and the hardware equipment requirements of the transmitting end will be too high, which is not conducive to the actual system. practicality.

Weijie Yuan, Shuangyang Li et al. mentioned a superimposed pilot placement method in their paper "Data-Aided Channel Estimation for OTFS Systems with A Superimposed Pilot and Data Transmission Scheme" (IEEE Wireless Communications Letters, 2021). In the method, the data symbols and pilot symbols are superimposed and placed at the transmitting end, and then the channel noise is passed through, and then at the receiving end of the OTFS system, the corresponding channel information is found through threshold judgment, and the channel information is estimated by using the received signal. This technical solution can effectively improve the utilization rate of frame data. However, this method still has the disadvantage that since the data symbols and noise interfere with the placement of the superimposed pilot, the pilot requires a large amount of signal-to-noise. The channel estimation can be more accurate.

In its patent document "OTFS method for data channel characterization and its use" (Patent Application No. 2015800492013, Authorized Announcement No. CN 106716824 B), Cohill Technology Co., Ltd. discloses a method of placing multiple data in one frame of data. Pilots, the way the data symbols are placed around the signal placement of the pilots. In this manner, for each wireless antenna and each stream identifiable multiple OTFS pilot symbol waveforms, the receiver is used to determine the 2D channel state from the antenna. The method can place multiple orthogonal pilot signals at the same time, the pilot signals do not interfere with each other, and the channel information can be estimated independently. However, this method still has the disadvantage that since the data symbols are placed around the pilot symbols in this pilot placement method, the interference of the data symbols is very large, and the channel information is estimated inaccurately.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a channel estimation method based on joint placement of orthogonal time-frequency space OTFS pilots, aiming at solving the problem of accurately estimating the delay Doppler channel state in the OTFS communication system. Information (CSI) problems and excessive pilot energy requirements.

The idea of realizing the purpose of the present invention is to place a pilot symbol at the center of each frame of data at the transmitting end, and according to the maximum delay spread and maximum Doppler spread of the OTFS system, the maximum delay spread around the pilot symbol The guard interval is placed within half of the maximum Doppler spread, and the data symbols are placed outside the guard interval. The pilot symbols in the data block at the receiving end will be interfered by the symbols placed around the pilot in the center of the OTFS data block at the transmitting end, and the guard interval placed around the pilot in the center of the OTFS data block at the transmitting end will not affect the pilot symbols. The guard interval can effectively reduce the interference from the data symbols on the pilot symbols received by the receiving end, so that the channel coefficients of the antennas can be estimated more accurately. The guard interval between the pilot signal and the data signal placed in the OTFS data block is transmitted to the receiving end through the antenna. The interference to the pilot symbol only exists in the antenna channel noise, so that the pilot symbol received by the receiving end is more accurate. , better channel estimation performance can be obtained, thereby reducing the energy of pilot symbols at the transmitting end, so that the OTFS system can obtain similar channel estimation performance under the condition of lower pilot signal-to-noise ratio.

The scheme that realizes the object of the present invention comprises the following steps:

Step 1, according to the following formula, place the pilot in the orthogonal time-frequency space OTFS data block:

where x[k,l] represents the kth data symbol on the lth subcarrier in the orthogonal time-frequency space OTFS data block; k=1,...,M-1 and l=1,..., N-1, M and N represent the total number of OTFS system sub-carriers and the total number of symbols determined by the number of transmitter antennas, respectively, x _p represents the k- _p -th row and l- _p column pilots in the orthogonal time-frequency space OTFS data block matrix symbol, 0 indicates that in an orthogonal time-frequency space OTFS data block matrix, the k _p -k _max row to k _p +k _max row, the l _p -l _max /2 column to the l _p +l _max /2 column are set The guard interval is to place 0 around x _p ; k _max represents the maximum Doppler shift determined by the moving speed of the orthogonal time-frequency space OTFS communication system, and l _max represents the sending end of the orthogonal time-frequency space OTFS communication system The maximum delay determined by the distance from the receiving end, x _d represents the data symbols at other positions in the orthogonal time-frequency space OTFS data frame;

Step 2, send the time domain signal:

Perform the inverse symplectic Fourier transform ISFFT on each orthogonal time-frequency space OTFS data block to obtain the signal block in the time-frequency domain, and then perform the Heisenberg transform on the signal block to obtain the time-domain signal of the data block. Send time domain signals;

Step 3, extract the received quadrature time-frequency space OTFS data block signal:

(3a) The receiving end performs an operation opposite to step 2 on the received time domain signal to obtain a data block in the time delay-Doppler domain;

(3b) The data symbols of each coordinate position in the data block that exceed the threshold are retained in the data block matrix, and the remaining data symbols are discarded.

Step 4, according to the following formula, estimate the antenna channel coefficient:

表示多普勒偏移为k-k_p，时延为l-l_p的天线信道系数，y[k,l]表示接受端的正交时频空OTFS数据块矩阵中第k行第l列数据符号。in,

Represents the antenna channel coefficient with Doppler offset kk _p and time delay ll _p , y[k,l] represents the data symbol in the kth row and the lth column of the orthogonal time-frequency space OTFS data block matrix at the receiving end.

Compared with the prior art, the present invention has the following advantages:

First, because the present invention is based on the maximum delay spread and the maximum Doppler spread of the OTFS system, the guard interval is placed at a position within half of the maximum delay spread and the maximum Doppler spread around the pilot symbol, which overcomes the prior art. The shortcoming of excessive interference to the pilot symbols in the middle makes the interference of the pilot symbols of the receiving end greatly reduced in the present invention, and the accuracy of the estimated channel information is also improved accordingly.

Second, since the present invention places a guard interval at a position within half of the maximum delay spread and the maximum Doppler spread around the pilot symbol according to the maximum delay spread and the maximum Doppler spread of the OTFS system, it overcomes the problems of the prior art. The problem that the demand for the signal-to-noise ratio of the pilot frequency is too high enables the present invention to effectively reduce the demand for the energy of the pilot frequency.

Description of drawings

Fig. 1 is the flow chart of OTFS system modulation of the present invention;

Fig. 2 is a pilot placement scheme diagram in the OTFS transmission frame of the present invention;

FIG. 3 is a simulation result diagram of channel estimation of the OTFS system of the present invention.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and embodiments.

Referring to FIG. 1 , the specific steps for realizing the present invention are described as follows.

Each frame of data sent at the sender is shown in step 1:

Step 1, according to the following formula, place the pilot in the orthogonal time-frequency space OTFS data block:

where x[k,l] represents the kth data symbol on the lth subcarrier in the orthogonal time-frequency space OTFS data block; k=1,...,M-1 and l=1,..., N-1, M and N represent the total number of OTFS system sub-carriers and the total number of symbols determined by the number of transmitter antennas, respectively, x _p represents the k- _p -th row and l- _p column pilots in the orthogonal time-frequency space OTFS data block matrix symbol, 0 indicates that in an orthogonal time-frequency space OTFS data block matrix, the k _p -k _max row to k _p +k _max row, the l _p -l _max /2 column to the l _p +l _max /2 column are set The guard interval is to place 0 around x _p ; k _max represents the maximum Doppler shift determined by the moving speed of the orthogonal time-frequency space OTFS communication system, and l _max represents the sending end of the orthogonal time-frequency space OTFS communication system The maximum delay determined by the distance from the receiving end, x _d represents the data symbols at other positions in the orthogonal time-frequency space OTFS data frame.

Step 2, sending a time domain signal.

Perform inverse symplectic Fourier transform ISFFT on each orthogonal time-frequency space OTFS data block to obtain the signal block in the time-frequency domain, and then perform Heisenberg transform on the signal block to obtain the time-domain signal of the data block. The channel transmits time-domain signals.

Step 3, extract the received orthogonal time-frequency space OTFS data block signal.

The receiving end performs the opposite operation of step 2 on the received time domain signal to obtain a data block in the delay-Doppler domain.

The data symbols of each coordinate position in the data block exceeding the threshold are kept in the data block matrix, and the rest of the data symbols are discarded.

The thresholds described are as follows:

Among them, γ represents the threshold value, * represents multiplication, coef represents the coefficient factor determined by the maximum time domain and maximum Doppler, N ₀ represents the noise energy of the transmitting channel, and Es represents an orthogonal time-frequency space _OTFS data at the transmitting end. Average of all data symbol energies in the block.

Referring to FIG. 2 , the corresponding relationship between the symbols in the OTFS transmission frame of the present invention after being transmitted through the wireless channel will be further described.

Fig. 2(a) is a schematic diagram of pilot placement in an OTFS data frame in step 1. The position marked "p" in the center of Fig. 2(a) represents the pilot symbol after placement, and "X" in Fig. 2(a) The part marked with "" represents the data symbol, and the part marked with "O" in Fig. 2(a) represents the guard interval.

Fig. 2(b) is a schematic diagram of pilot placement in an OTFS data frame of the receiving end in step 3. The position marked with a "⊕" shape in Fig. 2(b) represents the four pilot symbols after transmission through the wireless channel. Each pilot symbol corresponds to one pilot symbol in FIG. 2(a), and this correspondence is related to the number of wireless channels. In the embodiment of the present invention, the number of wireless channels is four, so the pilot symbol is in the One-to-four relationship before and after transmission. Similarly, the part marked with a "◇" shape in Figure 2(b) represents the data symbol after wireless channel transmission, and the data symbol is that each data symbol in Figure 2(a) corresponds to four data after wireless transmission. symbol, the corresponding data symbols at each position are superimposed on each other to obtain the data symbols on the entire data frame number.

Step 4, according to the following formula, estimate the antenna channel coefficient:

表示多普勒偏移为k-k_p，时延为l-l_p的天线信道系数，y[k,l]表示接受端的正交时频空OTFS数据块矩阵中第k行第l列数据符号。in,

Represents the antenna channel coefficient with Doppler offset kk _p and time delay ll _p , y[k,l] represents the data symbol in the kth row and the lth column of the orthogonal time-frequency space OTFS data block matrix at the receiving end.

The effect of the present invention can be further explained by the following simulation.

1. Simulation conditions:

The hardware platform of the simulation experiment of the present invention is: the processor is Intel i5 7300CPU, the main frequency is 2.5GHz, and the memory is 8GB.

The software platform of the simulation experiment of the present invention is: Windows 10 operating system and MATLAB R2021a.

The OTFS system used in the simulation experiment of the present invention adopts the system in which the total number of sub-carriers M is equal to 64 and the total number of carrier symbols N is equal to 64, the modulation mode of the data vector is QPSK, the channel type is complex Gaussian channel, and the number of channel paths is 4 respectively. In this case, the receiving end passes the threshold detection, estimates the channel information, and counts the channel estimation errors for 10,000 cycles.

2. Simulation content and result analysis:

The simulation experiment of the present invention is that the transmitter adopts the pilot placement scheme of the present invention (joint placement scheme) and a prior art placement scheme (superimposed pilot placement scheme), and sends two pilot placement schemes at the transmitter. The receiving end performs threshold detection, estimates the antenna channel coefficient, and calculates the error rate of the estimated value of the channel. The transmission data frame of the OTFS system is 10000 frames, and the number of symbols is 64*64. The corresponding channel estimation NMSE _h results are shown in Figure 3.

In the simulation experiment of the present invention, the superimposed pilot placement scheme adopted refers to,

Weijie Yuan, Shuangyang Li et al. mentioned a superimposed pilot placement method in "Data-Aided Channel Estimation for OTFS Systems with A Superimposed Pilot and Data Transmission Scheme" (IEEE Wireless Communications Letters, 2021).

The NMSE _h index referred to in the simulation experiment of the present invention is as follows:

表示估计的信道系数，h_w表示实际的信道系数，||·||²表示求绝对值的平方。where NMSE _h is the channel estimation error rate,

represents the estimated channel coefficient, h _w represents the actual channel coefficient, and ||·|| ² represents the square of the absolute value.

The effect of the present invention will be further described below in conjunction with the simulation FIG. 3 .

The abscissa in Fig. 3 represents the signal-to-noise ratio of the transmitted symbol, in dB; the ordinate represents the error rate of channel estimation.

The curve marked with a triangle in Fig. 3 shows that the transmitter of the OTFS system uses the superimposed pilot placement method, and the channel is estimated at the receiver. When the signal-to-noise ratio of the pilot is 40dB, the error rate of the channel estimation increases with Variation curve of signal-to-noise ratio of transmitted symbols. This curve is a curve drawn with the signal-to-noise ratio of the transmitted symbol as the abscissa and the error rate of the channel estimation as the ordinate by estimating the channel of the OTFS communication system when there are four paths in the physical channel.

The curve marked by the dots in Fig. 3 shows that the joint pilot placement scheme of the present invention is used for the OTFS system, and then the received symbol block is subjected to threshold detection. The curve of the signal-to-noise ratio with the transmitted symbol. This curve is a curve drawn by estimating the channel of the OTFS system when there are 4 paths in the physical channel.

The above simulation experiments show that: the method of the present invention places a pilot symbol at the center of each frame of data at the transmitting end, and according to the maximum delay spread and maximum Doppler spread of the OTFS system, the maximum delay spread around the pilot symbol The guard interval is placed at the position within half of the maximum Doppler extension, and the data symbol is placed outside the guard interval. The joint placement of the pilot frequency reduces the interference received by the pilot frequency; in the method of the present invention, the pilot frequency is in the 38dB signal. In the case of noise ratio, the channel estimation error rate of the pilot in the superimposed pilot scheme is obtained under the condition of 40dB signal-to-noise ratio. The joint pilot placement method has a 2dB signal-to-noise ratio compared with the superimposed pilot placement method The gain overcomes the shortcoming of high pilot energy requirement, so that the present invention reduces the energy required by the transmitting end pilot, improves the accuracy of channel estimation, and is a very practical method for placing pilots in the OTFS system.

Claims (2)

1. a channel estimation method based on joint placement of orthogonal time-frequency space OTFS pilots, is characterized in that, pilots are placed in the center of orthogonal time-frequency space OTFS data blocks, and the maximum time delay spread around pilot symbols The guard interval is placed within half of the maximum Doppler spread; the steps of the channel estimation method include the following: Step 1, according to the following formula, place the pilot in the orthogonal time-frequency space OTFS data block:

where x[k,l] represents the kth data symbol on the lth subcarrier in the orthogonal time-frequency space OTFS data block; k=1,...,M-1 and l=1,..., N-1, M and N represent the total number of OTFS system sub-carriers and the total number of symbols determined by the number of transmitter antennas, respectively, x _p represents the k- _p -th row and l- _p column pilots in the orthogonal time-frequency space OTFS data block matrix symbol, 0 indicates that in an orthogonal time-frequency space OTFS data block matrix, the k _p -k _max row to k _p +k _max row, the l _p -l _max /2 column to the l _p +l _max /2 column are set The guard interval is to place 0 around x _p ; k _max represents the maximum Doppler shift determined by the moving speed of the orthogonal time-frequency space OTFS communication system, and l _max represents the sending end of the orthogonal time-frequency space OTFS communication system The maximum delay determined by the distance from the receiving end, x _d represents the data symbols at other positions in the orthogonal time-frequency space OTFS data frame; Step 2, send the time domain signal: Perform inverse symplectic Fourier transform ISFFT on each orthogonal time-frequency space OTFS data block to obtain a signal block in the time-frequency domain, and then perform Heisenberg transform on the signal block to obtain the time-domain signal of the data block. domain signal; Step 3, extract the received quadrature time-frequency space OTFS data block signal: (3a) The receiving end performs an operation opposite to step 2 on the received time domain signal to obtain a data block in the time delay-Doppler domain; (3b) the data symbols of each coordinate position exceeding the threshold in the data block are retained in the data block matrix, and the remaining data symbols are discarded; Step 4, according to the following formula, estimate the antenna channel coefficient:

in,

Represents the antenna channel coefficient with Doppler offset kk _p and time delay ll _p , y[k,l] represents the data symbol in the kth row and the lth column of the orthogonal time-frequency space OTFS data block matrix at the receiving end. 2. the channel estimation method based on joint placement of orthogonal time-frequency space OTFS pilots according to claim 1, is characterized in that, the threshold value described in step (3b) is as follows:

Among them, γ represents the threshold value, * represents multiplication, coef represents the coefficient factor determined by the maximum time domain and maximum Doppler, N ₀ represents the noise energy of the transmitting channel, and Es represents an orthogonal time-frequency space _OTFS data at the transmitting end. Average of all data symbol energies in the block.

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