CN113612709B - Channel estimation method based on joint placement of orthogonal time-frequency-space OTFS pilot frequency - Google Patents
Channel estimation method based on joint placement of orthogonal time-frequency-space OTFS pilot frequency Download PDFInfo
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Abstract
The invention discloses a channel estimation method for placing orthogonal time-frequency-space OTFS pilot frequency based on a combination mode, which comprises the following implementation steps: 1. placing orthogonal time-frequency space OTFS data block pilot frequency and a guard interval; 2. carrying out inverse Fourier transform on the data block, carrying out Heisebauer transform on the signal block, and sending the obtained time domain signal; 3. carrying out threshold detection on orthogonal time-frequency space OTFS data block signals received by a receiving terminal; 4. and extracting the data block signals of the receiving end exceeding the threshold value, and calculating the antenna channel coefficient. The invention places the guard interval at the position within half of the maximum delay spread and the maximum Doppler spread around the pilot frequency symbol, reduces the interference on the pilot frequency symbol at the receiving end, improves the accuracy of the estimated channel information and reduces the requirement of pilot frequency energy.
Description
Technical Field
The invention belongs to the technical field of communication, and further relates to a channel estimation method based on joint placement of Orthogonal Time Frequency Space (OTFS) pilot Frequency in the technical field of wireless communication. The invention can be used for estimating corresponding channel information from pilot signals received by an OTFS system.
Background
Currently, orthogonal Frequency Division ofdm (orthogonal Frequency Division multiplexing) modulation techniques widely used in 5G and WIFI wireless networks are susceptible to the doppler effect. OTFS has better performance than OFDM in high mobility wireless communication scenarios. The orthogonal time-frequency-space OTFS is a two-dimensional modulation scheme for modulating in a delay-Doppler domain, and a double-dispersion channel is converted into a channel which is approximately non-fading in the delay-Doppler domain through a series of two-dimensional transformation. One of the main challenges facing OTFS systems 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 pilot, and if the pilot placement scheme is not good, the accuracy of the channel estimation of the whole system is caused, and the requirement of the hardware equipment of the transmitting end is also too high, which is not favorable for the practicability of the actual system.
Weijie Yuan, Shuangyang Li et al, in its published article "Data-aid Channel Estimation for OTFS Systems with A superior Pilot and Data Transmission Scheme" (IEEE Wireless Communications Letters, 2021), mention a Superimposed Pilot placement. The method includes the steps that data symbols and pilot symbols are placed on a sending end in an overlapping mode, channel noise is conducted, corresponding channel information is found at a receiving end of an OTFS system through threshold judgment, and the channel information is estimated through receiving signals. The technical scheme can effectively improve the utilization rate of frame data, but the method still has the defects that data symbols and noise are interference when the superposed pilot frequency is placed, so the pilot frequency needs a large signal-to-noise ratio, and the channel estimation can be more accurate.
The kohl technology corporation in its patent document "OTFS method for data channel characterization and its use" (patent application No. 2015800492013, publication No. CN 106716824B) discloses a signal placement method in which a plurality of pilots are placed in one frame of data, and data symbols are placed around the pilots. This approach determines the 2D channel conditions from each wireless antenna and from each stream using a receiver for a number of OTFS pilot symbol waveforms that are identifiable by the antenna. The method can simultaneously place a plurality of orthogonal pilot signals, the pilot signals do not interfere with each other, and channel information can be independently and separately estimated. However, the method still has the disadvantages that the pilot frequency placement mode is that the data symbols are placed around the pilot frequency symbols, the data symbols interfere greatly, and the channel information is estimated inaccurately.
Disclosure of Invention
The present invention aims to provide a channel estimation method based on joint placement of orthogonal time-frequency-space-OTFS pilot, aiming at solving the problem of accurate estimation of delay-doppler Channel State Information (CSI) and the problem of high pilot energy requirement in an OTFS communication system.
The idea of realizing the purpose of the invention is that a pilot frequency symbol is placed at the central position in each frame data of a sending end, a protection interval is placed at the position within half of the maximum delay spread and the maximum Doppler spread around the pilot frequency symbol according to the maximum delay spread and the maximum Doppler spread of the OTFS system, and a data symbol is placed outside the protection interval. The pilot symbols in the data block of the receiving end can be interfered by symbols placed around the pilot at the center of the OTFS data block of the sending end, and the guard interval placed around the pilot at the center of the OTFS data block of the sending end can not cause interference to the pilot symbols, so that the guard interval effectively reduces the interference from the data symbols received by the receiving end, and the channel coefficient of the antenna is estimated more accurately. The guard interval between the pilot signal and the data signal which are placed in the OTFS data block is transmitted to the receiving end through the antenna, and the interference to the pilot symbol only has antenna channel noise, so that the pilot symbol received by the receiving end is more accurate, and better channel estimation performance can be obtained, therefore, the energy of the pilot symbol of the transmitting end can be reduced, and an OTFS system can obtain similar channel estimation performance under the condition of lower pilot signal-to-noise ratio.
The scheme for realizing the aim of the invention comprises the following steps:
wherein, x [ k, l]Representing the kth data symbol on the l subcarrier in the orthogonal time-frequency space OTFS data block; k 1, M-1 and l 1, N-1, M and N denote the total number of OTFS system subcarriers and the total number of symbols, respectively, determined by the number of transmitter antennas, xpRepresenting the kth in the orthogonal time-frequency space-OTFS data block matrixpLine lpColumn pilot symbol, 0 represents the kth in an orthogonal time-frequency space-OTFS data block matrixp-kmaxLine to kp+kmaxLine, lp-lmaxColumn/2 to lp+lmaxGuard interval arranged in column/2, i.e. at xpIs placed around 0; k is a radical ofmaxRepresents the maximum Doppler shift, l, determined by the velocity of movement of an orthogonal time-frequency-space OTFS communications systemmaxRepresents the maximum time delay, x, determined by the distance between the sending end and the receiving end of the orthogonal time-frequency-space OTFS communication systemdData symbols representing other positions in the orthogonal time-frequency space OTFS data frame;
step 2, sending a time domain signal:
performing inverse Fourier transform ISFFT on each orthogonal time-frequency space OTFS data block to obtain a signal block of the time-frequency domain, performing Heisenberg transform on the signal block to obtain a time-domain signal of the data block, and transmitting the time-domain signal through an antenna;
step 3, extracting the received orthogonal time-frequency-space OTFS data block signals:
(3a) the receiving end carries out the operation opposite to the step 2 on the received time domain signal to obtain a data block in a delay-Doppler domain;
(3b) and keeping the data symbols of each coordinate position in the data block exceeding the threshold value in the data block matrix, and discarding the rest data symbols.
wherein,indicating a Doppler shift of k-kpWith a time delay of l-lpOf the antenna channel coefficient y k, l]And the ith row and ith column data symbols in the orthogonal time-frequency space OTFS data block matrix of the receiving end are represented.
Compared with the prior art, the invention has the following advantages:
first, because the invention places the guard interval at the 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, the defect of the prior art that the pilot symbol is interfered too much is overcome, so that the interference on the pilot symbol at the receiving end in the invention is greatly reduced, and the accuracy of the estimated channel information is correspondingly improved.
Secondly, because the invention places the guard interval at the position within half of the maximum delay spread and the maximum Doppler spread around the pilot frequency symbol according to the maximum delay spread and the maximum Doppler spread of the OTFS system, the problem of over-high requirement of the signal-to-noise ratio of the pilot frequency in the prior art is overcome, and the invention can effectively reduce the requirement of the energy of the pilot frequency.
Drawings
FIG. 1 is a block flow diagram of the OTFS system modulation of the present invention;
FIG. 2 is a diagram of a pilot placement scheme in an OTFS send frame according to the present invention;
fig. 3 is a diagram of simulation results of channel estimation of the OTFS system of the present invention.
Detailed Description
The invention is further described below with reference to the figures and examples.
With reference to fig. 1, the specific steps for implementing the present invention are described as follows.
Each frame data sent at the sending end is as shown in step 1:
wherein, x [ k, l]Representing the kth data symbol on the l subcarrier in the orthogonal time-frequency space OTFS data block; k 1, M-1 and l 1, N-1, M and N denote the total number of OTFS system subcarriers and the total number of symbols, respectively, determined by the number of transmitter antennas, xpRepresenting the kth in the orthogonal time-frequency space-OTFS data block matrixpLine lpColumn pilot symbol, 0 represents the kth in an orthogonal time-frequency space-OTFS data block matrixp-kmaxLine to kp+kmaxLine, lp-lmaxColumn/2 to lp+lmaxGuard interval arranged in column/2, i.e. at xpIs placed around 0; k is a radical ofmaxRepresents the maximum Doppler shift, l, determined by the velocity of movement of an orthogonal time-frequency-space OTFS communications systemmaxRepresents the maximum time delay, x, determined by the distance between the sending end and the receiving end of the orthogonal time-frequency-space OTFS communication systemdAnd data symbols representing other positions in the orthogonal time-frequency space OTFS data frame.
And step 2, sending the time domain signal.
And performing inverse Fourier transform ISFFT on each orthogonal time-frequency space OTFS data block to obtain a signal block of the time-frequency domain, performing Heisenberg transform on the signal block to obtain a time-domain signal of the data block, and sending the time-domain signal through a wireless channel.
And 3, extracting the received orthogonal time-frequency space OTFS data block signals.
And (3) the receiving end carries out the operation opposite to the step (2) on the received time domain signal to obtain a data block in the delay-Doppler domain.
And keeping the data symbols of each coordinate position in the data block exceeding the threshold value in the data block matrix, and discarding the rest data symbols.
The threshold values are as follows:
where γ denotes a threshold value, x denotes a multiplication, coef denotes a coefficient factor determined by the maximum time domain and the maximum doppler, N0Representing the noise energy of the transmission channel, EsAnd the average value of the energy of all data symbols in one orthogonal time-frequency space OTFS data block at the sending end is represented.
With reference to fig. 2, the correspondence relationship of the symbols in the OTFS transmission frame after being transmitted through the wireless channel is further described.
Fig. 2(a) is a schematic diagram of pilot placement in an OTFS data frame in step 1, where the position of the central part marked with "p" in fig. 2(a) indicates a pilot symbol after placement, the part marked with "X" in fig. 2(a) indicates a data symbol, and the part marked with "O" in fig. 2(a) indicates a guard interval.
Fig. 2(b) is a schematic diagram of pilot placement in an OTFS data frame at the receiving end in step 3, where the position indicated by [ ] in fig. 2(b) indicates four pilot symbols after transmission through a wireless channel, and the four pilot symbols all correspond to one pilot symbol in fig. 2(a), and this correspondence is related to the number of wireless channels. Similarly, the portion denoted by the shape "in fig. 2(b) represents the data symbols after being transmitted through the wireless channel, where each data symbol in fig. 2(a) is wirelessly transmitted and corresponds to four data symbols, and the corresponding data symbols at each position are superimposed to obtain the data symbols over the whole data frame number.
wherein,indicating a Doppler shift of k-kpWith a time delay of l-lpOf the antenna channel coefficient y k, l]And the ith row and ith column data symbols in the orthogonal time-frequency space OTFS data block matrix of the receiving end are represented.
The effects of the present invention can be further illustrated by the following simulations.
1. Simulation conditions are as follows:
the hardware platform of the simulation experiment of the invention is as follows: the processor is an Intel i 57300 CPU, the main frequency is 2.5GHz, and the memory is 8 GB.
The software platform of the simulation experiment of the invention is as follows: the Windows 10 operating system and MATLAB R2021 a.
The OTFS system used in the simulation experiment of the invention adopts a system that the total number M of subcarriers is equal to 64 and the total number N of carrier symbols is equal to 64, the modulation mode of data vectors is QPSK, the channel type is complex Gaussian channel, the number of channel paths is respectively selected under the condition of 4, the receiving end estimates channel information by threshold detection, and the cycle number of statistical channel estimation error is 10000.
2. And (3) analyzing the contents of the simulation and the results thereof:
the simulation experiment of the invention is that the sending end respectively adopts the pilot frequency placing scheme (combined placing scheme) of the invention and the placing scheme (superposed pilot frequency placing scheme) of the prior art, the sending end sends the data blocks of the two pilot frequency placing schemes, and the receiving end carries out thresholdValue detection, estimating antenna channel coefficients, calculating error rates of estimated values of the channels. The data frame sent by OTFS system is 10000 frames, the symbol number is 64 x 64, and the corresponding channel estimation NMSE is obtainedhThe results are shown in FIG. 3.
In the simulation experiment of the invention, the adopted superimposed pilot frequency placement scheme means that,
weijie Yuan, Shuangyang Li et al, in Data-aid Channel Estimation for OTFS Systems with A superior Pilot and Data Transmission Scheme (IEEE Wireless Communications Letters, 2021) mention a Superimposed Pilot placement.
NMSE as referred to in the simulation experiments of the inventionhThe indexes are as follows:
wherein NMSEhWhich is indicative of the error rate of the channel estimate,representing the estimated channel coefficient, hwRepresenting the actual channel coefficients, | · | > non-woven phosphor2Indicating squaring the absolute value.
The effect of the invention is further described below in conjunction with the simulation of 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 the channel estimate.
The curve marked by triangles in fig. 3 represents a curve of the error rate of channel estimation along with the change of the signal-to-noise ratio of the transmitted symbol, which is obtained by performing channel estimation at the receiving end in a manner of using an overlay type pilot frequency placement at the transmitting end of the OTFS system, under the condition that the signal-to-noise ratio of the pilot frequency is 40 dB. The curve is a curve which is obtained by estimating a channel of the OTFS communication system when 4 paths exist in a physical channel, and is drawn by taking the signal-to-noise ratio of a transmitted symbol as an abscissa and taking the error rate of channel estimation as an ordinate.
The curve marked with dots in fig. 3 represents the variation of the channel estimation error rate with the signal-to-noise ratio of the transmitted symbol, which is obtained when the combined pilot placement scheme of the present invention is used for an OTFS system and then the received symbol block is subjected to threshold detection, under the condition that the pilot signal-to-noise ratio is 38 dB. The curve is a curve which is obtained by estimating a channel of the OTFS system when 4 paths exist in a physical channel, and is drawn by taking the signal-to-noise ratio of a transmitted symbol as an abscissa and taking the error rate of channel estimation as an ordinate.
The above simulation experiments show that: the method of the invention places a pilot frequency symbol at the center position in each frame data of a sending end, according to the maximum time delay expansion and the maximum Doppler expansion of an OTFS system, places a protection interval at the position within half of the maximum time delay expansion and the maximum Doppler expansion around the pilot frequency symbol, places a data symbol outside the protection interval, and the pilot frequency is placed in a combined mode, so that the interference on the pilot frequency is reduced; in the method, the pilot frequency obtains the channel estimation error rate under the condition of the signal-to-noise ratio of 40dB in the superimposed pilot frequency scheme under the condition of the signal-to-noise ratio of 38dB, and the combined pilot frequency placing mode has the signal-to-noise ratio gain of 2dB compared with the superimposed pilot frequency placing mode, thereby overcoming the defect of high pilot frequency energy requirement, reducing the energy required by the pilot frequency of a sending end, improving the accuracy of channel estimation and being a practical OTFS system pilot frequency placing method.
Claims (2)
1. A channel estimation method based on joint placement of orthogonal time-frequency space OTFS pilot frequency is characterized in that the pilot frequency is placed in the center of an orthogonal time-frequency space OTFS data block, and a protection interval is placed at a position within a half of the maximum delay spread and the maximum Doppler spread around a pilot frequency symbol; the channel estimation method comprises the following steps:
step 1, placing pilot frequency in the orthogonal time-frequency space OTFS data block according to the following formula:
wherein, x [ k, l]When representing orthogonalityA kth data symbol on the l subcarrier in the frequency-space OTFS data block; k 1, M-1 and l 1, N-1, M and N denote the total number of OTFS system subcarriers and the total number of symbols, respectively, determined by the number of transmitter antennas, xpRepresenting the kth in the orthogonal time-frequency space-OTFS data block matrixpLine lpColumn pilot symbol, 0 represents the kth in an orthogonal time-frequency space-OTFS data block matrixp-kmaxLine to kp+kmaxLine, lp-lmaxColumn/2 to lp+lmaxGuard interval arranged in column/2, i.e. at xpIs placed around 0; k is a radical ofmaxRepresents the maximum Doppler shift, l, determined by the velocity of movement of an orthogonal time-frequency-space OTFS communications systemmaxRepresents the maximum time delay, x, determined by the distance between the sending end and the receiving end of the orthogonal time-frequency-space OTFS communication systemdData symbols representing other positions in the orthogonal time-frequency space OTFS data frame;
step 2, sending a time domain signal:
performing inverse Fourier transform ISFFT on each orthogonal time-frequency space OTFS data block to obtain a signal block of a time-frequency domain, performing Heisenberg transform on the signal block to obtain a time-domain signal of the data block, and transmitting the time-domain signal through an antenna;
step 3, extracting the received orthogonal time-frequency-space OTFS data block signals:
(3a) the receiving end carries out the operation opposite to the step 2 on the received time domain signal to obtain a data block in a delay-Doppler domain;
(3b) keeping the data symbols of each coordinate position exceeding the threshold value in the data block matrix, and discarding the rest data symbols;
step 4, estimating the antenna channel coefficient according to the following formula:
2. The method for channel estimation based on joint placement of orthogonal time-frequency-space-OTFS pilots as claimed in claim 1, wherein the threshold values in step (3b) are as follows:
where γ denotes a threshold value, x denotes a multiplication, coef denotes a coefficient factor determined by the maximum time domain and the maximum doppler, N0Representing the noise energy of the transmission channel, EsAnd the average value of the energy of all data symbols in one orthogonal time-frequency space-OTFS data block at the transmitting end is represented.
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CN114584239B (en) * | 2022-03-07 | 2022-11-18 | 大连理工大学 | OTFS underwater acoustic communication sparse channel estimation method based on learning denoising |
CN114884777B (en) * | 2022-04-28 | 2024-07-19 | 中国科学院计算技术研究所 | Channel estimation method based on transform domain |
CN115086114B (en) * | 2022-06-10 | 2023-08-15 | 西安电子科技大学 | Channel estimation method based on distributed placement of orthogonal time-frequency space OTFS pilot frequency |
CN115051894B (en) * | 2022-06-10 | 2024-02-06 | 西安电子科技大学 | OTFS system symbol detection method based on serial interference elimination |
CN115442189B (en) * | 2022-08-23 | 2024-05-14 | 成都中科微信息技术研究院有限公司 | Multi-antenna channel estimation method, medium and device suitable for time-frequency space domain |
CN115426224B (en) * | 2022-11-07 | 2023-02-03 | 成都航天通信设备有限责任公司 | Channel estimation method and system based on OTFS (optical transport plane) signal |
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