CN107465639B - Multi-path delay synchronous decision demodulation method based on short-time discrete Fourier transform - Google Patents
Multi-path delay synchronous decision demodulation method based on short-time discrete Fourier transform Download PDFInfo
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Abstract
The invention provides a multi-path delay synchronous decision demodulation method based on short-time discrete Fourier transform, which comprises the following steps: the receiving end firstly receives a carrier signal on a power line, and then the carrier signal is filtered out of band noise through a band-pass filter; then sampling and quantizing the carrier signal subjected to band-pass filtering, and discretizing the signal; performing short-time discrete Fourier transform on a discrete sequence obtained by row discretization; obtaining amplitudes at two carrier frequencies through approximate processing; selecting an optimal branch; and performing demodulation judgment by using the optimal branch information to complete signal demodulation. The invention has the technical effects that: the method can quickly realize symbol synchronization and accurately realize signal demodulation, and the system is not influenced by carrier initial phase and carrier frequency estimation error, has strong robustness and is easy to realize. Simulation results show that the performance of the method is close to coherent demodulation performance, and field operation results show that the method has extremely high communication achievement rate and practical application value.
Description
Technical Field
The invention relates to an incoherent 2FSK demodulation method, in particular to a multi-path delay synchronous decision demodulation method based on Short-time Discrete Fourier Transform (DSTFT), belonging to the field of communication.
Background
In recent years, a power line carrier communication technology is gradually and widely applied to a power consumption information acquisition system, and as a power line is a non-uniformly distributed transmission line, the channel characteristics of a power network are very bad, and the transmission of power line carrier communication is greatly influenced, so that the accuracy of signal demodulation is improved, and the method is very important for improving the user experience of the power line carrier communication system.
The 2FSK modulation mode is mostly adopted in the current narrowband power line carrier communication system, and the demodulation of 2FSK signals can be divided into two categories, namely coherent demodulation and noncoherent demodulation. Coherent demodulation utilizes frequency and phase information of carriers to demodulate, and has good anti-noise performance, but the phenomenon of inverted pi exists, two synchronous carriers need to be extracted simultaneously, the complexity is extremely high, and the realization is difficult in a short time, so the error rate is high in practical application; while the incoherent demodulation scheme is widely applied because carrier extraction is not needed, common incoherent demodulation methods include a frequency discrimination method, a zero-crossing detection method, a Hilbert method, a differential detection method and the like. However, some methods have unsatisfactory performance, and some methods are complicated to describe, and are not easy to implement under the condition of assuming that various kinds of information are known.
Disclosure of Invention
In order to overcome the problems, the invention provides a multi-path delay synchronous decision demodulation method based on short-time discrete Fourier transform, which can realize the quick timing synchronization of code elements by utilizing a multi-path delay joint detection method, then can accurately judge the information based on the optimal branch frequency domain information, and the simulation result shows that the performance of the method is close to 2FSK coherent demodulation performance, and the method has excellent performance in the field operation of a power line carrier system and has good practical application value.
The technical scheme of the invention is as follows: a multi-path delay synchronous decision demodulation method based on short-time discrete Fourier transform comprises the following steps:
the receiving end firstly receives a carrier signal on a power line, and then the carrier signal is filtered out of band noise through a band-pass filter;
then sampling and quantizing the carrier signal subjected to band-pass filtering, and discretizing the signal;
delaying the discrete sequence obtained by row discretization by taking N/8 as a unit so as to obtain 8 groups of discrete sequences, and dividing the 8 groups of sequences into 8 paths to perform short-time discrete Fourier transform;
each branch circuit carries out short-time discrete Fourier transform according to the window length N, and the amplitude values at two carrier frequencies are obtained through approximate processing;
calculating the difference value of the amplitude values of the carrier frequency 1 and the carrier frequency 2, and selecting an optimal branch;
and performing demodulation judgment by using the optimal branch information, judging as 1 if the element in the difference sequence is greater than 0, otherwise judging as 0, and completing signal demodulation.
The method for selecting the optimal branch comprises the steps of obtaining a sequence of two carrier frequency amplitude difference values after each branch is subjected to multiple short-time discrete Fourier transform, calculating the variance of the difference value sequence of each branch, and taking the branch with the largest variance as the optimal branch.
The window length N is the ratio of the sampling rate of the receiving end to the transmission rate of the signal.
The invention has the technical effects that: the method can quickly realize symbol synchronization and accurately realize signal demodulation, and the system is not influenced by carrier initial phase and carrier frequency estimation error, has strong robustness and is easy to realize. Simulation results show that the performance of the method is close to coherent demodulation performance, and field operation results show that the method has extremely high communication achievement rate and practical application value.
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FIG. 1 is a schematic flow diagram of the process of the present invention;
fig. 2 is a baseband-to-frequency band signal waveform comparison chart of the modulation method of the present invention;
FIG. 3 is a diagram illustrating simulation results of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more clearly and completely apparent, the technical solutions in the embodiments of the present invention are further clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention, and it is obvious that the described embodiments are one, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention provides an incoherent multi-path delay joint demodulation method based on short-time discrete Fourier transform aiming at 2FSK modulation, and the method steps are shown in figure 1. Suppose that the two carrier frequencies of the 2FSK signal are respectively f1And f2The transmission rate R of the signalsThe receiving end has a sampling rate of fs. The receiving end firstly receives a carrier signal on a power line, then out-of-band noise is filtered through a band-pass filter, then sampling quantization is carried out, and discretization processing is carried out on the signal. The method carries out short-time discrete Fourier transform based on the obtained discretization data, thereby completing the time synchronization and the demodulation judgment of the symbol. The basic idea of the DSTFT to perform 2FSK decision is to take N discrete sampling points each time, and perform short-time discrete Fourier transform on the N sampling points, thereby obtaining the amplitude-frequency characteristics of different discrete frequency components (where N represents the number of sampling points in one symbol period, i.e., N represents the number of sampling points in one symbol period
TsRepresenting one symbol period) and then frequency identification and signal decision are achieved by comparing the magnitude of the amplitude-frequency values at the two carrier frequencies.
Because the discrete Fourier transform is carried out on the N discrete sampling points each time, and the symbol synchronization is not realized in the received sampling signal, f contained in the N discrete sampling points is extracted1And f2Both frequency components are also unknown. If a window of length N covers exactly one complete symbol period, the spectral characteristics obtained at this time reach a maximum at one carrier frequency and a maximum at another carrier frequencyA small value; with the shift of the window position, two pieces of frequency component information may exist in the window function at the same time, for example, the first L sampling points are f1Then the last N-L sampling points are f2After the short-time discrete fourier transform is performed, the amplitudes of the two carrier frequencies are closer to each other, but the frequency domain amplitude of the carrier frequency signal with a smaller number of points in the window is smaller than the frequency domain amplitude of the carrier frequency signal with a larger number of points.
Based on the analysis, the invention provides a multi-path time delay short-time combined synchronous judgment method based on short-time discrete Fourier transform. As shown in fig. 1, the discrete sequence obtained by sampling is delayed by N/8, so as to obtain 8 groups of discrete samples. And if the original sampling sequence is x (N), the discrete sequence of all branches subjected to DSTFT conversion is x (N-i × N/8), i is more than or equal to 0 and less than or equal to 7, and i represents a branch index. The 8 branch sequences simultaneously carry out short-time discrete Fourier transform, the window length is N, and the short-time discrete Fourier transform formula is as follows:
in the formula (I), the compound is shown in the specification,
after discrete Fourier transform, each branch obtains a group of discrete amplitude sequences at every N sampling points, and the frequency corresponding to the kth amplitude is fk＝k*fsN, the frequency point obtained after DSTFT conversion does not necessarily correspond to two carrier frequencies f1And f2Therefore, in order to compare the amplitude and frequency values of the two carrier frequencies, the frequency value indexes corresponding to the two carrier frequencies need to be obtained through approximation processing, and the calculation is as follows:
K1＝[Nf1/fs]
K2＝[Nf2/fs]
wherein [. ]]Representing a rounding operation, the Kth of X (K) is taken1、K2Value, i.e. two carrier frequencies f1And f2For the corresponding amplitudes, both values are retained and calculated for each discrete Fourier transformThe difference value.
Taking branch i as an example, after the mth discrete short-time Fourier transform, the difference value delta of the two carrier frequency amplitudes is takenim＝Xim(K1)-Xim(K2)，Xim(k) And k is more than or equal to 0 and less than or equal to N-1, which represents a frequency domain sequence obtained after the mth DSTFT transformation of the branch i. After the branch i is subjected to multiple short-time discrete fourier transforms, a set of difference sequences can be obtained, and the corresponding difference sequence can be expressed as { Δ }imM is more than or equal to 0 and less than or equal to M-1, M represents the total times of calculating short-time discrete Fourier transform of each branch, and the difference value sequence of all branches is shown in figure 2; and then calculating the variance of the difference value sequence of each branch, and taking the branch with the largest variance as the optimal branch, namely the branch has the best time synchronization characteristic. The best branch index is calculated as follows:
where Var represents variance, symbol synchronization of signals is accomplished by selecting the optimal branch, and synchronization error is at TsWithin/8
And then, carrying out demodulation judgment by using the optimal branch information, judging as 1 if the element in the difference sequence is more than 0, otherwise, judging as 0, namely
Thereby completing the signal demodulation, and the simulation result shows that the performance of the proposed demodulation scheme is close to that of the coherent demodulation scheme (about 1.2dB performance difference), as shown in fig. 3. The field operation result shows that the provided scheme has high communication success rate, low implementation complexity and high practical application and popularization value.
Although embodiments of the present application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present application, and that variations, modifications, substitutions and alterations may be made to the above embodiments by those of ordinary skill in the art within the scope of the present application.
Claims (2)
1. A multi-path delay synchronous decision demodulation method based on short-time discrete Fourier transform is characterized by comprising the following steps:
the receiving end firstly receives a carrier signal on a power line, and then the carrier signal is filtered out of band noise through a band-pass filter;
then sampling and quantizing the carrier signal subjected to band-pass filtering, and discretizing the signal;
delaying the discrete sequence obtained by discretization by taking N/8 as a unit so as to obtain 8 groups of discrete sequences, and dividing the 8 groups of sequences into 8 paths to perform short-time discrete Fourier transform;
each branch circuit carries out short-time discrete Fourier transform according to the window length N, and the amplitude values at two carrier frequencies are obtained through approximate processing;
calculating the difference value of the amplitude values of the carrier frequency 1 and the carrier frequency 2, and selecting an optimal branch;
carrying out demodulation judgment by using the optimal branch information, judging as 1 if elements in the difference sequence are more than 0, otherwise judging as 0, and finishing signal demodulation;
the method for selecting the optimal branch comprises the steps of obtaining a sequence of two carrier frequency amplitude difference values after each branch is subjected to multiple short-time discrete Fourier transform, calculating the variance of the difference value sequence of each branch, and taking the branch with the largest variance as the optimal branch.
2. The method as claimed in claim 1, wherein the window length N is a ratio of a sampling rate of a receiving end to a transmission rate of a signal.
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| CN1819572A (en) * | 2006-03-23 | 2006-08-16 | 上海交通大学 | Demodulating method of binary carrier shift key-controlling system based on Hibert transform |
| CN102510292A (en) * | 2011-10-09 | 2012-06-20 | 浙江大学 | NAVTEX demodulator based on DSTFT (discrete short time Fourier transform) and demodulation method thereof |
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| CN1819572A (en) * | 2006-03-23 | 2006-08-16 | 上海交通大学 | Demodulating method of binary carrier shift key-controlling system based on Hibert transform |
| CN102510292A (en) * | 2011-10-09 | 2012-06-20 | 浙江大学 | NAVTEX demodulator based on DSTFT (discrete short time Fourier transform) and demodulation method thereof |
| CN103138807A (en) * | 2011-11-28 | 2013-06-05 | 深圳市财付通科技有限公司 | Implement method and system for near field communication (NFC) |
Non-Patent Citations (1)
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| 基于时频分析的BFSK信号解调的研究和实现;陈大平;《中国优秀硕士学位论文全文数据库 信息科技辑》;20150615;第7，10，43-47页 * |
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