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CN113098815A - Frequency domain suppression method and system for interference between underwater sound OFDM carriers - Google Patents

Frequency domain suppression method and system for interference between underwater sound OFDM carriers Download PDF

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CN113098815A
CN113098815A CN202110247293.3A CN202110247293A CN113098815A CN 113098815 A CN113098815 A CN 113098815A CN 202110247293 A CN202110247293 A CN 202110247293A CN 113098815 A CN113098815 A CN 113098815A
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CN113098815B (en
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李渝舟
邱吉慧
王志战
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Huazhong University of Science and Technology
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2689—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation
    • H04L27/2691—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation involving interference determination or cancellation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B11/00—Transmission systems employing sonic, ultrasonic or infrasonic waves
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B13/00—Transmission systems characterised by the medium used for transmission, not provided for in groups H04B3/00 - H04B11/00
    • H04B13/02—Transmission systems in which the medium consists of the earth or a large mass of water thereon, e.g. earth telegraphy
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L25/03159—Arrangements for removing intersymbol interference operating in the frequency domain
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2668—Details of algorithms
    • H04L27/2669—Details of algorithms characterised by the domain of operation
    • H04L27/2672—Frequency domain

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Abstract

本发明公开了一种水声OFDM载波间干扰的频域抑制方法与系统，包括：S1、基于最小均方误差原则构建ICI频域抑制目标优化模型，根据前导码中的导频信号，通过最小化ICI频域抑制目标优化模型的目标函数，估计当前数据帧的最佳基准频偏；S2、将接收信号以当前数据帧的最佳基准频偏的分数倍的频率间隔进行若干次频谱搬移后，对频谱搬移后的信号分别进行相同点数的FFT运算，得到各子载波处的多路FFT信号；S3、将产生的多路FFT信号逐子载波地进行加权组合，得到载波间干扰频域抑制后的信号；本发明将最佳基准频偏的选取转化为多变量联合优化问题，实现了多变量联合优化，从而最大程度地提升ICI频域抑制精度，在中高多普勒因子的情况下，系统检测性能有显著的提升。

Figure 202110247293

The invention discloses a frequency domain suppression method and system for underwater acoustic OFDM inter-carrier interference, including: S1, constructing an ICI frequency domain suppression target optimization model based on the principle of minimum mean square error, Calculate the objective function of the ICI frequency-domain suppression target optimization model, and estimate the best reference frequency offset of the current data frame; S2. Perform spectrum shifting for the received signal several times at frequency intervals that are fractional times the best reference frequency offset of the current data frame After that, perform FFT operations with the same number of points on the signals after spectrum shifting, to obtain multi-channel FFT signals at each sub-carrier; S3, weight and combine the generated multi-channel FFT signals sub-carrier by sub-carrier to obtain the inter-carrier interference frequency domain The suppressed signal; the invention transforms the selection of the optimal reference frequency offset into a multivariable joint optimization problem, and realizes the multivariable joint optimization, thereby maximizing the suppression accuracy of the ICI frequency domain. In the case of medium and high Doppler factors , the system detection performance has been significantly improved.

Figure 202110247293

Description

Frequency domain suppression method and system for interference between underwater sound OFDM carriers
Technical Field
The invention belongs to the technical field of underwater sound wireless communication, and particularly relates to a frequency domain suppression method and a frequency domain suppression system for interference between underwater sound OFDM (orthogonal frequency division multiplexing) carriers.
Background
The ocean occupies more than two thirds of the surface area of the earth, contains extremely rich resources such as organisms, oil gas, mineral products and the like, and has important strategic value. With the continuous deep ocean exploration and the rapid development of information technology, the rapid accumulation of ocean data makes the highly reliable underwater acoustic wireless communication theory and technology to be urgently developed. However, the lack of standardized channel models, severe time-frequency double dispersion effects in underwater acoustic environments, and extremely limited communication bandwidth all make it difficult to achieve reliable underwater acoustic communication in complex and variable marine environments.
Due to the great advantages shown in the aspects of effectively resisting multipath interference, reducing the design complexity of a receiver under a high-dispersion channel and the like, an Orthogonal Frequency Division Multiplexing (OFDM) technology becomes a standard technology for realizing underwater medium-long distance transmission. A frequency selective channel is divided into a plurality of flat fading sub-channels which are not overlapped with each other, so that single-tap equalization and symbol-by-symbol detection with low complexity can be carried out at a receiving end, intersymbol interference caused by time dispersion of a wireless channel is effectively reduced, and the complexity of equalization in a receiver is reduced. However, in a marine environment with high doppler distortion, due to severe doppler frequency offset, orthogonality among subcarriers of OFDM is destroyed, and challenges of interference suppression and signal recovery increase dramatically, whereas conventional processing methods such as a block equalizer based on minimum mean square error are difficult to be practically applied due to high complexity (about the third power of block length); on the other hand, the conventional coherent detection algorithms all rely heavily on channel state information, but the lack of an accurate underwater acoustic channel model makes the algorithms difficult to show better performance, so that the accuracy of data detection is limited to generate performance bottleneck; in addition, the synchronization overhead generated by conventional channel estimation methods based on pilot assistance and the like further increases the cost and thus the loss of very limited spectrum resources.
Therefore, in order to ensure the detection performance of the underwater acoustic OFDM system, accurate suppression of Inter-Carrier Interference (ICI) is one of the essential signal processing flows. The existing ICI suppression method mainly focuses on two processing angles of a time domain and a frequency domain, and takes Partial Fast Fourier Transform (P-FFT) and Fractional Fourier Transform (F-FFT) as representatives, and practice shows that the performance of the frequency domain ICI suppression method is obviously superior to that of the time domain suppression method. However, the classical F-FFT processing method uses subcarrier spacing as reference frequency offset, so that the system fails to suppress ICI to the maximum extent, and the detection performance of the system is limited under medium and high doppler factors. Therefore, a high-precision ICI (inter-carrier interference) suppression method of the underwater acoustic OFDM (orthogonal frequency division multiplexing) system needs to be researched.
Disclosure of Invention
Aiming at the defects or improvement requirements of the prior art, the invention provides a frequency domain suppression method and a frequency domain suppression system for inter-carrier interference of underwater acoustic OFDM (orthogonal frequency division multiplexing), which are used for solving the technical problem that the data detection performance of the underwater acoustic OFDM system with a medium-high Doppler factor is limited due to the fact that ICI (inter-carrier interference) cannot be suppressed to the greatest extent by taking a subcarrier interval as a reference frequency offset in the prior art.
In order to achieve the above object, in a first aspect, the present invention provides a frequency domain suppression method for inter-carrier interference in an underwater acoustic OFDM system, including the following steps:
s1, constructing an ICI frequency domain suppression target optimization model based on the minimum mean square error principle, and estimating the optimal reference frequency offset of the current data frame by minimizing the objective function of the ICI frequency domain suppression target optimization model according to the pilot signals in the lead codes;
s2, after carrying out frequency spectrum shifting on the received signal for a plurality of times at preset frequency intervals, carrying out FFT operation on the signals subjected to frequency spectrum shifting by the same point number respectively to obtain a plurality of paths of FFT signals at each subcarrier; the preset frequency interval is a fraction times of the optimal reference frequency offset of the current data frame;
and S3, carrying out weighted combination on the generated multipath FFT signals subcarrier by subcarrier to obtain signals after the frequency domain suppression of the inter-carrier interference.
Further preferably, the step S1 includes:
s11, constructing an ICI frequency domain suppression target optimization model based on a minimum mean square error principle;
s12, dividing the pilot signals in the lead code into two parallel signals A, B; the channel A signal is a pilot signal in the lead code, and the channel B signal is a product of the pilot signal and the sampling moment;
s13, initializing the current reference frequency offset and the weight value of the sub-carrier corresponding to the pilot signal;
s14, carrying out frequency spectrum shifting on the A, B two paths of signals for a plurality of times at a frequency interval preset by the fractional multiple of the current reference frequency offset, and carrying out FFT operation on the signals subjected to frequency spectrum shifting with the same number of points to obtain a plurality of paths of FFT signals;
s15, weighting and combining the multiple paths of FFT signals corresponding to the path A signals to obtain demodulated signals on the path A pilot frequency; forming vectors by using multiple paths of FFT signals corresponding to the B path of signals, and performing weighted combination after a group of Hadamard product operation to obtain demodulated signals of the B path of pilot frequency;
s16, calculating a target value of the ICI frequency domain suppression target optimization model based on the demodulated signal on the pilot frequency of the path A, judging whether the ratio of the target values of the two adjacent times is smaller than a preset threshold value, and if so, turning to the step S17; otherwise, the operation is finished, and the current reference frequency offset is the optimal reference frequency offset of the current data frame;
s17, updating the current reference frequency offset and weight value based on the demodulated signal of the pilot frequency of the path A and the demodulated signal of the pilot frequency of the path B, and turning to the step S14;
the target function of the ICI frequency domain suppression target optimization model is:
Figure BDA0002964545290000041
wherein f iseIs the current reference frequency offset;
Figure BDA0002964545290000042
the weight value at the p pilot signal subcarrier; bkIs the original data symbol; kP＝{k1，...，kP-1，kP}；kpA subcarrier subscript corresponding to the p-th pilot signal in the preamble; p1, 2,. said, P; p is the number of pilot signals;
Figure BDA0002964545290000043
Figure BDA0002964545290000044
the demodulated signal on the p pilot frequency of the A path;
Figure BDA0002964545290000045
and a column vector formed by FFT signals corresponding to the p-th pilot signal of the A path.
Further preferably, in step S13, the A, B two signals are respectively spaced at intervals of frequency in frequency interval
Figure BDA0002964545290000046
Performing 2I-time frequency spectrum shifting to obtain (2I +1) paths of signals respectively;
wherein, the path A signal is v (t), and the path B signal is tv (t); the frequency spectrum shifted signal corresponding to the A path signal is
Figure BDA0002964545290000047
The frequency spectrum shifted signal corresponding to the B path signal is
Figure BDA0002964545290000048
I is a positive integer.
Further preferably, the ith FFT signal corresponding to the pth pilot signal of the a channel is:
Figure BDA0002964545290000051
vi(t) is the signal after the frequency spectrum shift corresponding to the A path signal, and Δ f is the subcarrier interval.
Further preferably, the demodulated signal on the p-th pilot in the B-path is:
Figure BDA0002964545290000052
wherein,
Figure BDA0002964545290000053
column vectors formed by FFT signals corresponding to the p-th pilot signal of the B path; β [ -I, · 1, 0, 1,. 1, I]T；
Figure BDA0002964545290000054
Representing a Hadamard product operation;
the ith FFT signal corresponding to the p-th pilot signal of the B path is:
Figure BDA0002964545290000055
tvi(t) is a signal after the frequency spectrum corresponding to the B-path signal is shifted, and Δ f is a subcarrier interval.
Further preferably, in step S2, the multipath FFT signals at the k-th subcarrier are:
zk＝[zk，-I，...，zk，-1，zk，0，zk，1，...，zk，I]T
Figure BDA0002964545290000056
wherein,
Figure BDA0002964545290000057
i is a positive integer, r (t) is a received signal, fbaseThe best reference frequency offset for the current data frame.
Further preferably, the multiple FFT signals z at the k-th sub-carrier are combinedkWeighting and combining to obtain a signal x after the inter-carrier interference frequency domain suppression on the kth path of sub-carrierkWherein
Figure BDA0002964545290000058
wkis the weight value at the k-th subcarrier.
Further preferably, the frequency domain suppression method for inter-carrier interference in the underwater acoustic OFDM system is used for symbol detection; the method specifically comprises the following steps: after steps S1-S2 are performed, the generated multiple FFT signals are weighted and combined subcarrier by subcarrier, and symbol detection is performed; and when the symbol detection is finished once, updating a weight value corresponding to the next subcarrier based on the current symbol detection error, and carrying out symbol detection after carrying out weighted combination on the multipath FFT signals of the next subcarrier by adopting the updated weight value.
Further preferably, the update formula for updating the weight value corresponding to the k +1 th subcarrier based on the detection error detected by the symbol at the k +1 th subcarrier is as follows:
wk+1＝wk+μgk
wherein, wk+1The weight value of the k +1 th path of subcarrier; mu is an iteration step length; gkThe iterative gradient of the weight specifically includes:
Figure BDA0002964545290000061
"+" indicates a conjugate operation.
In a second aspect, the present invention provides a frequency domain suppression system for inter-carrier interference in an underwater acoustic OFDM system, including:
the optimal reference frequency offset estimation module is used for constructing an ICI frequency domain suppression target optimization model based on the minimum mean square error principle, estimating the optimal reference frequency offset of the current data frame by minimizing the objective function of the ICI frequency domain suppression target optimization model according to the pilot signals in the lead codes, and outputting the optimal reference frequency offset to the multi-path FFT signal acquisition module;
the multi-path FFT signal acquisition module is used for carrying out frequency spectrum shifting on the received signal for a plurality of times at preset frequency intervals, respectively carrying out FFT operation on the signals subjected to frequency spectrum shifting with the same number of points to obtain multi-path FFT signals at each subcarrier, and outputting the multi-path FFT signals to the symbol detection module; the preset frequency interval is a fraction times of the optimal reference frequency offset of the current data frame;
and the weighting combination module is used for carrying out weighting combination on the generated multipath FFT signals subcarrier by subcarrier to obtain signals after the inter-carrier interference frequency domain suppression.
In a third aspect, the present invention also provides a machine-readable storage medium storing machine-executable instructions which, when invoked and executed by a processor, cause the processor to implement any one of the above-mentioned methods for frequency-domain suppression of inter-carrier interference in an underwater acoustic OFDM system.
Generally, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects:
1. the invention provides a frequency domain suppression method for inter-carrier interference in an underwater acoustic OFDM system, which is characterized in that a target optimization model of an ICI frequency domain suppression system is constructed based on a minimum mean square error criterion, the selection of the optimal reference frequency offset is converted into a multivariable joint optimization problem through a two-way parallel processing structure, and the multivariable joint optimization is realized under the condition of ensuring more ideal performance, namely, the optimal reference frequency offset of the current data frame is estimated by minimizing a target function of the ICI frequency domain suppression target optimization model, so that the ICI frequency domain suppression precision is improved to the maximum extent, and the system detection performance can be obviously improved under medium-high Doppler factors.
2. Compared with the existing F-FFT, the frequency domain suppression method for the inter-carrier interference in the underwater sound OFDM system carries out frequency spectrum shifting on the received signal for a plurality of times by taking the fractional multiple of the optimal reference frequency offset as the frequency interval without considering only [ F [ -F ] isk-Δf，fk+Δf]Sub-carrier interference in a frequency range, where fkAnd f is the center frequency of the kth carrier, and is the subcarrier interval. Under the condition of medium and high Doppler factors, the detection performance of the frequency ICI suppression method can be remarkably improved, and the application range of the frequency ICI suppression in different Doppler expansion scenes is widened.
Drawings
Fig. 1 is a flowchart of a frequency domain suppression method for inter-carrier interference in an underwater acoustic OFDM system according to the present invention;
fig. 2 is a block diagram of an ICI suppressing method according to embodiment 1 of the present invention;
fig. 3 is a path gain diagram of a simulated underwater acoustic channel under test according to embodiment 1 of the present invention;
FIG. 4 is a graph of the MSE performance of the method of the present invention versus the Doppler factor of a prior art method;
FIG. 5 is a graph of the MSE performance as a function of signal-to-noise ratio for the method of the present invention versus prior art methods.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
To achieve the above object, as shown in fig. 1, the present invention provides a frequency domain suppression method for inter-carrier interference in an underwater acoustic OFDM system, including the following steps:
s1, constructing an ICI frequency domain suppression target optimization model based on the minimum mean square error principle, and estimating the optimal reference frequency offset of the current data frame by minimizing the objective function of the ICI frequency domain suppression target optimization model according to the pilot signals in the lead codes;
s2, after carrying out frequency spectrum shifting on the received signal for a plurality of times at preset frequency intervals, carrying out FFT operation on the signals subjected to frequency spectrum shifting by the same point number respectively to obtain a plurality of paths of FFT signals at each subcarrier; the preset frequency interval is a fraction times of the optimal reference frequency offset of the current data frame;
and S3, carrying out weighted combination on the generated multipath FFT signals subcarrier by subcarrier to obtain signals after the frequency domain suppression of the inter-carrier interference.
To further illustrate the methods provided by the present invention, the following examples are given:
examples 1,
In this embodiment, differential coherent detection is taken as an example to describe the method flow of the present invention in detail, and the frequency domain suppression method for inter-carrier interference in an underwater acoustic OFDM system provided by the present invention is mainly applied to a receiving end.
Specifically, after applying the frequency domain suppression method for inter-carrier interference provided by the present invention to a receiving end, the complete communication process of the underwater acoustic OFDM system is as follows:
a sending end:
PSK symbol b to be transmittedkAfter serial-to-parallel conversion, differential coding in frequency domain is carried out in sequence to obtain symbol d to be modulatedkI.e. by
Figure BDA0002964545290000091
Wherein, bk∈{a0，a1，...，aQ-1}，aq＝ej2πq/QQ is 0, 1,., Q-1, Q is the modulation order, and K is the total number of subcarriers.
Will encode the symbol dkThe signal is transmitted to a channel through operations of IFFT modulation, cyclic prefix addition, parallel-to-serial conversion, and frequency up-conversion, and the transmission signal can be represented as
Figure BDA0002964545290000092
Wherein f isk＝f0+ k Δ f is the kth subcarrier frequency, f0For the lowest frequency of the system, Δ f — B/K is the subcarrier spacing, T is the OFDM block duration, and B is the signal bandwidth.
Receiving end:
the received signal is processed by down-conversion, rough sampling, cyclic prefix removal and other operations to obtain a signal to be demodulated;
specifically, as shown in fig. 2, the frequency domain suppression method for inter-carrier interference in an underwater acoustic OFDM system provided by the present invention includes the following steps:
s1, constructing an ICI frequency domain suppression target optimization model based on the minimum mean square error principle, and estimating the optimal reference frequency offset of the current data frame by minimizing the objective function of the ICI frequency domain suppression target optimization model according to the pilot signals in the lead codes;
specifically, a two-way parallel processing structure is adopted, and the optimal reference frequency offset in the data frame is estimated according to the pilot frequency in the preamble signal v (t). The double-path parallel processing structure comprises a signal preprocessing module, a multi-path FFT operation module, a Hadamard product operation module, a combiner and a gradient updating module; the signal preprocessing module is used for generating A, B two-path signals which are processed in parallel; the multi-path FFT operation module is used for carrying out frequency spectrum shifting on the preprocessed signals at preset frequency intervals and then respectively carrying out FFT operation on the same points; the Hadamard product operation module consists of a group of Hadamard product devices and is used for Hadamard product operation of one path of signals; and the combiner is used for carrying out weighted combination on the multipath FFT-processed signals to obtain modulated signals on each subcarrier. And the gradient updating module is used for updating the reference frequency offset value according to the signal processing information at the pilot frequency until a certain cutoff condition is met.
The specific process is as follows:
s11, constructing an ICI frequency domain suppression target optimization model based on a minimum mean square error principle;
s12, dividing the pilot signals in the lead code into two parallel signals A, B; the path A signal is a pilot signal v (t) in the lead code, and the path B signal is a product tv (t) of the pilot signal and the sampling time;
s13, initializing the current reference frequency offset and the weight value of the sub-carrier corresponding to the pilot signal; specifically, the initial value of the current reference frequency offset may be set to Δ f or an integer multiple of the doppler frequency offset; where Δ f is the inter-subcarrier spacing.
S14, carrying out frequency spectrum shifting on the A, B two paths of signals for a plurality of times at a frequency interval preset by the fractional multiple of the current reference frequency offset, and carrying out FFT operation on the signals subjected to frequency spectrum shifting with the same number of points to obtain a plurality of paths of FFT signals;
specifically, the A path signal v (t) is separated by a preset frequency interval
Figure BDA0002964545290000111
In 2I frequency spectrum shifts, a (2I +1) channel signal (including the original preamble signal v (t)) can be generated, and the mathematical expression is as follows:
Figure BDA0002964545290000112
wherein f ise(j) The reference frequency offset is the reference frequency offset under the jth iteration number;
for generated multipath signal vi(t) respectively performing the same Fourier operation to obtain the ith path of FFT signal corresponding to the pth pilot signal:
Figure BDA0002964545290000113
wherein k ispA subcarrier subscript corresponding to the p-th pilot signal in the preamble; k is a radical ofp∈{k1，...，kP-1，kP}; p is the number of pilot signals.
The path B signal is the product of the pilot signal and the sampling time, i.e. the signal tv (t), and the same processing is performed as above, so that the ith path FFT signal corresponding to the p-th pilot signal of the path B is obtained as follows:
Figure BDA0002964545290000114
s15, weighting and combining the multiple paths of FFT signals corresponding to the path A signals to obtain demodulated signals on the path A pilot frequency; forming vectors by using multiple paths of FFT signals corresponding to the B path of signals, and performing weighted combination after a group of Hadamard product operation to obtain demodulated signals of the B path of pilot frequency;
specifically, for the p-th pilot, the (2I +1) -way output on the A-way
Figure BDA0002964545290000121
Forming a column vector
Figure BDA0002964545290000122
The demodulated signal on the p pilot frequency can be obtained after weighted combination by the combiner
Figure BDA0002964545290000123
Wherein,
Figure BDA0002964545290000124
the weight value of the p pilot signal subcarrier under the jth iteration number is obtained; similarly, the p-th pilot signal subcarrier is constituted by (2I +1) channels, and the weight value here is a vector constituted by weight values of the p-th pilot signal subcarrier.
(2I +1) path output on B path
Figure BDA0002964545290000125
Forming a column vector
Figure BDA0002964545290000126
Figure BDA0002964545290000127
Then obtaining the demodulation signal on the p pilot frequency after Hadamard product operation and weighted combination:
Figure BDA0002964545290000128
wherein β [ -I, · 1, 0, 1,. 1, I]T，
Figure BDA0002964545290000129
Representing a hadamard product operation.
S16, calculating a target value of the ICI frequency domain suppression target optimization model based on the demodulated signal on the pilot frequency of the path A, judging whether the ratio of the target values of the two adjacent times is smaller than a preset threshold value, and if so, turning to the step S17; otherwise, the operation is finished, and the current reference frequency offset is the optimal reference frequency offset of the current data frame;
specifically, the objective function of the ICI frequency domain suppression objective optimization model is:
Figure BDA00029645452900001210
wherein f iseIs the current reference frequency offset;
Figure BDA00029645452900001211
the weight value at the p pilot signal subcarrier; bkThe method comprises the steps of obtaining original data symbols, namely PSK symbols to be transmitted; kP＝{k1，...，kP-1，kP}；kpA subcarrier subscript corresponding to the p-th pilot signal in the preamble; p1, 2,. said, P; p is the number of pilot signals;
Figure BDA0002964545290000131
Figure BDA0002964545290000132
the demodulated signal on the p pilot frequency of the A path;
Figure BDA0002964545290000133
and a column vector formed by FFT signals corresponding to the p-th pilot signal of the A path.
The invention judges whether the cut-off condition is met by judging whether the ratio of the target values of two adjacent times is smaller than a preset threshold value, wherein the cut-off condition in the embodiment is as follows: judging whether the requirements are met
Figure BDA0002964545290000134
Wherein E (j) is a target value under the j iteration number; e (j-1) is the target value for the j-1 th iteration number.
S17, based on the demodulated signal of the pilot frequency in the A path and the demodulated signal of the pilot frequency in the B path, the current reference frequency offset and the weight value are updated, and the step goes to S14.
Specifically, the embodiment is based on the minimum mean square error principle, and adopts a coordinate descent method to perform iterative update to solve the optimal reference frequency offset of the current data frame; the specific updating process is as follows:
Figure BDA0002964545290000135
Figure BDA0002964545290000136
……
Figure BDA0002964545290000137
for each sub-variable, the reference frequency offset iterative formula is:
fe(j+1)＝fe(j)+μγ(j)
Figure BDA0002964545290000138
wherein,
Figure BDA0002964545290000139
is the k-thpSymbol error at each pilot, "+" indicates conjugate operation.
The weight iterative formula is
Figure BDA0002964545290000141
Figure BDA0002964545290000142
S2, after carrying out frequency spectrum shifting on the received signal for a plurality of times at preset frequency intervals, carrying out FFT operation on the signals subjected to frequency spectrum shifting by the same point number respectively to obtain a plurality of paths of FFT signals at each subcarrier; the preset frequency interval is a fraction times of the optimal reference frequency offset of the current data frame;
specifically, the multipath FFT signals at the kth subcarrier in the received signal are:
zk＝[zk，-I，...，zk，-1，zk，0，zk，1，...，zk，I]T
Figure BDA0002964545290000143
wherein,
Figure BDA0002964545290000144
i is a positive integer, r (t) is a received signal, fbaseThe best reference frequency offset for the current data frame.
And S3, carrying out weighted combination on the generated multipath FFT signals subcarrier by subcarrier to obtain signals after the frequency domain suppression of the inter-carrier interference.
Preferably, the frequency domain suppression method for inter-carrier interference in the underwater acoustic OFDM system can be used for symbol detection; at this time, the above-mentioned two-way parallel processing structure further includes a data detection module for recovering the original data symbols from the demodulated signal, as shown in fig. 1.
The method specifically comprises the following steps: after steps S1-S2 are performed, the generated multiple FFT signals are weighted and combined subcarrier by subcarrier, and symbol detection is performed (i.e., symbol detection is performed after the multiple FFT signals at the subcarriers are weighted and combined in sequence for each subcarrier in the received signal); and when the symbol detection is finished once, updating a weight value corresponding to the next subcarrier based on the current symbol detection error, and carrying out symbol detection after carrying out weighted combination on the multipath FFT signals of the next subcarrier by adopting the updated weight value. The specific process is as follows:
s31, making k equal to 0;
s32, receiving the multipath FFT signal z at the k path subcarrier in the signalkWeighted combination to obtain the demodulated signal x on the k path sub-carrierkWherein
Figure BDA0002964545290000151
wkthe weight value at the k-th path of subcarrier, i.e. each path of weight value of the combiner.
S33, demodulating signal x on k path sub carrierkCarrying out data detection to obtain detection symbols
Figure BDA0002964545290000152
And for the detected symbol
Figure BDA0002964545290000153
Performing symbol decision to obtain decision symbol
Figure BDA0002964545290000154
Specifically, the data detection in the present embodiment is differential coherent detection; data symbols on the kth subcarrier
Figure BDA0002964545290000155
The expression of (a) is as follows:
Figure BDA0002964545290000156
after symbol decision, the decision symbol is obtained as:
Figure BDA0002964545290000157
wherein,
Figure BDA0002964545290000158
indicating symbol decision, i.e. judgement
Figure BDA0002964545290000159
The Hamming distance from each point in the Q-order PSK symbol set is the final one when the Hamming distance is the shortest
Figure BDA00029645452900001510
And (4) taking values.
S34, calculating decision symbol
Figure BDA00029645452900001511
And detecting the symbol
Figure BDA00029645452900001512
The difference is obtained to obtain the detection error e of symbol detection at the k path subcarrierkAnd calculating an iterative gradient g of the weightsk(ii) a Judging whether e is satisfiedkLess than a predetermined error threshold ethAnd is
Figure BDA00029645452900001513
Less than a predetermined gradient threshold gth(ii) a If yes, go to step S35; otherwise, updating the weight value corresponding to the k +1 th path of subcarriers based on the detection error of the symbol detection at the k path of subcarriers according to the minimum mean square error principle;
specifically, the detection error e of symbol detection at the k-th subcarrierkComprises the following steps:
Figure BDA00029645452900001514
according to the minimum mean square error principle, the update formula of the iterative gradient of the weight is as follows:
Figure BDA0002964545290000161
if | ek|＜ethAnd is
Figure BDA0002964545290000162
The weights at the next carrier are updated
wk+1＝wk+μgk
Wherein, wk+1The weight value of the k +1 th path of subcarrier; mu is an iteration step length and is a constant close to 0; gkThe iterative gradient of the weight specifically includes:
Figure BDA0002964545290000163
Figure BDA0002964545290000164
"+" indicates a conjugate operation.
Otherwise, no update is performed.
In this embodiment, the iterative gradient g of the weightthA value of 2, a predetermined error threshold ethThe value is 1.
S35, let k equal to k + 1;
and S36, repeating the steps S32-S35 to iterate until the symbol detection at each subcarrier is completed.
In order to further explain the intercarrier interference suppression effect of the method provided by the invention, the intercarrier interference suppression precision of the method (marked as A-FFT) provided by the invention and the classical P-FFT and F-FFT in the underwater sound OFDM is respectively tested. Fig. 3 is a time delay-path gain diagram of an underwater acoustic simulation channel for testing, and based on the simulation environment setting, the inter-carrier interference suppression precision of the classic P-FFT, F-FFT and the method a-FFT of the present invention for underwater acoustic OFDM is analyzed and compared. The specific experimental results are as follows:
(1) detection results under different Doppler factors
As shown in fig. 4, which is a graph of the MSE performance of the method provided by the present invention and the existing method according to the test result of Doppler factor (Doppler a) variation, it can be seen from fig. 3 that, under the condition of SNR 30dB, the a-FFT provided by the present invention has the performance equivalent to the classical P-FFT and F-FFT at low Doppler factor, while in the differential underwater acoustic OFDM system with medium and high Doppler factor, the a-FFT provided by the present invention exhibits the performance advantage of Mean Square Error (MSE) which is more significant.
(2) Detection results at different SNR of medium Doppler factor
FIG. 5 is a graph showing the MSE performance of the method of the present invention and the prior art method as a function of the signal-to-noise ratio, and FIG. 4 shows that the MSE performance is 2.5 × 10-4Compared with P-FFT and F-FFT, the MSE performance of the A-FFT method provided by the invention is reduced by 16.61-52.91%, and the performance is better.
Examples 2,
A frequency domain suppression system for inter-carrier interference in an underwater acoustic OFDM system, comprising:
the optimal reference frequency offset estimation module is used for constructing an ICI frequency domain suppression target optimization model based on the minimum mean square error principle, estimating the optimal reference frequency offset of the current data frame by minimizing the objective function of the ICI frequency domain suppression target optimization model according to the pilot signals in the lead codes, and outputting the optimal reference frequency offset to the multi-path FFT signal acquisition module;
the multi-path FFT signal acquisition module is used for carrying out frequency spectrum shifting on the received signal for a plurality of times at preset frequency intervals, respectively carrying out FFT operation on the signals subjected to frequency spectrum shifting with the same number of points to obtain multi-path FFT signals at each subcarrier, and outputting the multi-path FFT signals to the symbol detection module; the preset frequency interval is a fraction times of the optimal reference frequency offset of the current data frame;
and the weighting combination module is used for carrying out weighting combination on the generated multipath FFT signals subcarrier by subcarrier to obtain signals after the inter-carrier interference frequency domain suppression.
The related technical scheme is the same as embodiment 1, and is not described herein.
In a third aspect, the present invention also provides a machine-readable storage medium storing machine-executable instructions, which when invoked and executed by a processor, cause the processor to implement the method for frequency-domain suppression of inter-carrier interference in an underwater acoustic OFDM system according to embodiment 1.
The related technical scheme is the same as embodiment 1, and is not described herein.
It will be appreciated by those skilled in the art that the foregoing is only a preferred embodiment of the invention, and is not intended to limit the invention, such that various modifications, equivalents and improvements may be made without departing from the spirit and scope of the invention.

Claims (10)

1. A frequency domain suppression method for inter-carrier interference in an underwater acoustic OFDM system is characterized by comprising the following steps:
s1, constructing an ICI frequency domain suppression target optimization model based on a minimum mean square error principle, and estimating the optimal reference frequency offset of the current data frame by minimizing the objective function of the ICI frequency domain suppression target optimization model according to the pilot signals in the lead codes;
s2, after carrying out frequency spectrum shifting on the received signal for a plurality of times at preset frequency intervals, carrying out FFT operation on the signals subjected to frequency spectrum shifting by the same point number respectively to obtain a plurality of paths of FFT signals at each subcarrier; the preset frequency interval is a fraction times of the optimal reference frequency offset of the current data frame;
s3, carrying out weighted combination on the multiple paths of FFT signals subcarrier by subcarrier to obtain signals after inter-carrier interference frequency domain suppression.
2. The method for suppressing the inter-carrier interference in the underwater acoustic OFDM system as claimed in claim 1, wherein said step S1 comprises:
s11, constructing an ICI frequency domain suppression target optimization model based on a minimum mean square error principle;
s12, dividing the pilot signals in the lead code into two parallel signals A, B; the path A signal is a pilot signal in the lead code, and the path B signal is a product of the pilot signal and the sampling time;
s13, initializing the current reference frequency offset and the weight value of the sub-carrier corresponding to the pilot signal;
s14, carrying out frequency spectrum shifting on the A, B two paths of signals for a plurality of times at a frequency interval preset by the fractional multiple of the current reference frequency offset, and carrying out FFT operation on the signals subjected to frequency spectrum shifting with the same number of points to obtain a plurality of paths of FFT signals;
s15, weighting and combining the multiple paths of FFT signals corresponding to the path A signals to obtain demodulated signals on the path A pilot frequency; forming vectors by using multiple paths of FFT signals corresponding to the B path of signals, and performing weighted combination after a group of Hadamard product operation to obtain demodulated signals of the B path of pilot frequency;
s16, calculating a target value of the ICI frequency domain suppression target optimization model based on the demodulated signal on the pilot frequency of the path A, judging whether the ratio of the target values of the two adjacent times is smaller than a preset threshold value, and if so, turning to the step S17; otherwise, the operation is finished, and the current reference frequency offset is the optimal reference frequency offset of the current data frame;
s17, updating the current reference frequency offset and weight value based on the demodulated signal of the pilot frequency of the path A and the demodulated signal of the pilot frequency of the path B, and turning to the step S14;
wherein, the objective function of the ICI frequency domain suppression objective optimization model is as follows:
Figure FDA0002964545280000021
wherein f iseIs the current reference frequency offset;
Figure FDA0002964545280000029
the weight value at the p pilot signal subcarrier; bkIs the original data symbol; kP＝{k1，...，kP-1，kP}；kpA subcarrier subscript corresponding to the p-th pilot signal in the preamble; p1, 2,. said, P; p is the number of pilot signals;
Figure FDA0002964545280000022
Figure FDA0002964545280000023
the demodulated signal on the p pilot frequency of the A path;
Figure FDA0002964545280000024
and a column vector formed by FFT signals corresponding to the p-th pilot signal of the A path.
3. The method for suppressing the intercarrier interference in the underwater acoustic OFDM system as claimed in claim 2, wherein in said step S13, A, B two signals are respectively spaced at intervals of frequencies
Figure FDA0002964545280000025
Performing 2I-time spectrum shiftingRespectively obtaining (2I +1) paths of signals;
wherein, the path A signal is v (t), and the path B signal is tv (t); the frequency spectrum shifted signal corresponding to the A path signal is
Figure FDA0002964545280000026
The frequency spectrum shifted signal corresponding to the B path signal is
Figure FDA0002964545280000027
I is a positive integer.
4. The method according to claim 3, wherein the i-th path FFT signal corresponding to the p-th pilot signal in the A-path is:
Figure FDA0002964545280000028
vi(t) is the signal after the frequency spectrum shift corresponding to the A path signal, and Δ f is the subcarrier interval.
5. The method as claimed in claim 3, wherein the demodulated signal on the p-th pilot in the B-channel is:
Figure FDA0002964545280000031
wherein,
Figure FDA0002964545280000032
column vectors formed by FFT signals corresponding to the p-th pilot signal of the B path; β [ -I, · 1, 0, 1,. 1, I]T；
Figure FDA0002964545280000033
Representing a Hadamard product operation;
the ith FFT signal corresponding to the p-th pilot signal of the B path is:
Figure FDA0002964545280000034
tvi(t) is a signal after the frequency spectrum corresponding to the B-path signal is shifted, and Δ f is a subcarrier interval.
6. The method for suppressing the inter-carrier interference in the underwater acoustic OFDM system according to claim 1, wherein in step S2, the multiple FFT signals at the k-th sub-carrier are:
zk＝[zk，-I，...，zk，-1，zk，0，zk，1，...，zk，I]T
Figure FDA0002964545280000035
wherein,
Figure FDA0002964545280000036
i is a positive integer, r (t) is a received signal, fbaseThe best reference frequency offset for the current data frame.
7. The method as claimed in claim 6, wherein the multiple FFT z signals at k-th sub-carrier are processed by the frequency domain suppression methodkWeighting and combining to obtain a signal x after the inter-carrier interference frequency domain suppression on the kth path of sub-carrierkWherein
Figure FDA0002964545280000037
wkis the weight value at the k-th subcarrier.
8. The method for suppressing the inter-carrier interference in the underwater acoustic OFDM system according to any one of claims 1-7, wherein the method is used for symbol detection; the method specifically comprises the following steps: after steps S1-S2 are performed, the generated multiple FFT signals are weighted and combined subcarrier by subcarrier, and symbol detection is performed; and when the symbol detection is finished once, updating a weight value corresponding to the next subcarrier based on the current symbol detection error, and carrying out symbol detection after carrying out weighted combination on the multipath FFT signals of the next subcarrier by adopting the updated weight value.
9. A frequency domain suppression system for inter-carrier interference in an underwater acoustic OFDM system, comprising:
the optimal reference frequency offset estimation module is used for constructing an ICI frequency domain suppression target optimization model based on the minimum mean square error principle, estimating the optimal reference frequency offset of the current data frame by minimizing the objective function of the ICI frequency domain suppression target optimization model according to the pilot signals in the lead codes, and outputting the optimal reference frequency offset to the multi-path FFT signal acquisition module;
the multi-path FFT signal acquisition module is used for carrying out frequency spectrum shifting on the received signal for a plurality of times at preset frequency intervals, respectively carrying out FFT operation on the signals subjected to frequency spectrum shifting with the same number of points to obtain multi-path FFT signals at each subcarrier, and outputting the multi-path FFT signals to the symbol detection module; the preset frequency interval is a fraction times of the optimal reference frequency offset of the current data frame;
and the weighting combination module is used for carrying out weighting combination on the generated multipath FFT signals subcarrier by subcarrier to obtain signals after the inter-carrier interference frequency domain suppression.
10. A machine-readable storage medium having stored thereon machine-executable instructions which, when invoked and executed by a processor, cause the processor to implement the method for frequency-domain suppression of inter-carrier interference in an underwater acoustic OFDM system as claimed in any one of claims 1 to 8.
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