CN109743106B - FTN rate transmission method suitable for atmospheric laser communication - Google Patents
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Abstract
An FTN speed transmission method suitable for atmospheric laser communication, the originating of the method introduces FTN technology, convert QPSK signal into FTN signal, make the symbol rate greater than Nyquist rate; the receiving end utilizes a Digital Signal Processing (DSP) technology, namely a CMA linear equalizer technology, to effectively compensate the intersymbol interference brought by introducing the FTN technology. Compared with the traditional QPSK transmission system, the introduction of the FTN technology effectively improves the transmission rate and the spectrum efficiency of the atmospheric laser communication system, improves the error code performance of the atmospheric laser communication system, and has a certain reference value for the specific design of the mobile communication system in the actual engineering.
Description
Field of the art
The invention relates to an FTN rate transmission method suitable for atmospheric laser communication, which further improves the transmission rate and the spectrum efficiency of an atmospheric laser communication system by combining an FTN technology and a QPSK modulation mode, and belongs to the technical field of wireless optical communication.
Background
With the increasing demand of users for transmission rate of communication systems, research on high optical spectrum efficiency in optical communication has attracted much attention. In traditional atmospheric laser communication, a high-order modulation mode is often adopted to improve the spectrum efficiency. However, in this method, as the modulation order is increased, the further improvement of the spectrum efficiency is limited by the complexity of the system and the tolerance to noise. In recent years, Mazo et al have proposed a Faster Than Nyquist (FTN) transmission system that improves the spectral efficiency of the system by reducing the signal bandwidth.
The FTN technique is a non-orthogonal transmission method, which breaks through the limitation of the Nyquist criterion, and can increase the transmission rate of the communication system when the symbol rate is greater than the Nyquist rate. Although FTN techniques introduce ISI while increasing transmission rates, these effects can be well compensated for using Digital Signal Processing (DSP) techniques. Therefore, the FTN technology provides an effective means for solving the high requirement of the atmospheric laser communication system on the transmission rate.
Although the system performance can be effectively improved by introducing the FTN technology into the atmospheric laser communication, when a laser signal is transmitted in an atmospheric channel, the signal is easily affected by atmospheric turbulence, and the atmospheric turbulence effect may cause light spot drift, light intensity fluctuation (flicker), and the like of a transmission light beam, so that the transmission quality of the optical signal is seriously degraded, and further the system performance degradation such as increase of the bit error rate, reduction of channel capacity, increase of interruption probability, and the like of the atmospheric laser communication system is further caused, thereby affecting the improvement degree of the FTN technology on the atmospheric laser communication system performance.
Therefore, the invention introduces FTN technology into atmospheric laser communication, provides an FTN speed transmission method suitable for atmospheric laser communication, and analyzes the system error code performance under the method aiming at atmospheric turbulence.
Disclosure of Invention
The invention aims to provide an FTN rate transmission method suitable for atmospheric laser communication, which realizes the improvement of the transmission rate of an atmospheric laser communication system by improving the spectral efficiency of the system.
The invention relates to an FTN speed transmission method suitable for atmospheric laser communication, which introduces FTN technology into an atmospheric laser communication system, and a receiving end effectively compensates intersymbol interference caused by the FTN technology by utilizing digital signal processing technology, namely CMA linear equalizer technology, thereby realizing the improvement of the frequency spectrum efficiency of the system.
The invention has the advantages that: the FTN technology is combined with a QPSK modulation mode, so that the limitation of a Nyquist criterion is broken through, the transmission rate and the spectral efficiency of the atmospheric laser communication system are effectively improved, and an effective measure is provided for constructing a large-capacity and high-rate atmospheric laser communication system.
Drawings
Fig. 1 is a block diagram of an FTN atmospheric laser communication system under a turbulent flow channel, fig. 2 is a comparison of spectral efficiencies before and after adopting an FTN transmission technique, fig. 3 is a relationship between a system BER and a baud rate before and after introducing an FTN, fig. 4 to fig. 9 are a comparison of constellation diagrams of QPSK and FTN-QPSK systems under different flicker indexes, and fig. 10 is a relationship between OSNR and BER under different flicker indexes.
Detailed Description
The invention relates to an FTN rate transmission method suitable for atmospheric laser communication, which introduces FTN technology into an atmospheric laser communication system, and a receiving end adopts a CMA linear equalization algorithm to process digital signals so as to compensate intersymbol interference caused by the FTN technology and improve the spectrum efficiency
The method comprises the following specific steps:
step 1: at the transmitting end, X, Y two binary information sequences are first mapped into QPSK signals x respectivelynAnd ynThen the signal is processed by an FTN shaping filter to generate an FTN signal xFTNAnd yFTN
Wherein x isn、ynAre respectively independent complex symbols, g (t) is signal pulse waveform, and
k is 0, ± 1, ± 2, ·, τ (0 < τ < 1) is an acceleration factor, and T is a symbol interval satisfying Nyquist transmission;
the two FTN signals are respectively modulated on laser through an IQ modulator to form x (t), y (t) two polarization signals, and s (t) is transmitted by an optical antenna after being coupled by a polarization coupler; suppose that two optical signals received by the optical receiving antenna are r respectivelyx(t)、ry(t); two optical signals and local oscillator optical signal rLO(t) is divided intoRespectively passing through a 2 x 490-degree mixer and a photoelectric balance detector to form four paths of electric signals; combining the four paths of electric signals into r1(t)、r2(t) two paths of complex signals are respectively subjected to ADC sampling after passing through a matched filter to obtain
Wherein E issIs the pulse energy, PtIs the average emitted optical power, PLOThe local oscillation light power, h is the light intensity fading coefficient, eta is the photoelectric conversion coefficient,
x, Y the correlation of the noise of the two-path signal can be expressed as
For noise signals
The variance is
Wherein G (f) is a spectral function of g (t), and N0/2 is a noise power spectral density, which is a constant; assuming perfect matching of the matched filter in the ideal case, then
Step 2: sending the sampled signals to a DSP module for processing and recovering user information; wherein, the equalizer adopts a CMA linear equalizer;
then, the decision variable output by the CMA equalization algorithm is represented as
Wherein,
respectively are output results of noise components of the X branch and the Y branch after being sampled by the ADC;
judging the judgment variable by setting a judgment threshold v, and demodulating two paths of signals; the decision rule is as follows:
calculating the average bit error rate of the FTN transmission system under the turbulent flow channel according to the obtained decision variable to obtain
Wherein f isI(h) Is the probability density function of the light intensity fading coefficient h, and I is the light intensity of the receiving end when the turbulent flow exists.
The present invention will be described in detail below with reference to specific embodiments thereof.
The invention is achieved by the following technical measures:
1. the basic assumption is that:
the present invention assumes an average emitted optical power PtQPSK modulation is adopted, and the optical power P of local oscillator is adoptedLOThe channel is a flat slow fading channel. Assuming that the background light has been filtered out by the filter, only polarization noise is considered. This assumption is typical of such systems and is not a particular requirement of the present invention.
2. Detailed description of the preferred embodiment
At the transmitting end, X, Y two paths of binary sequences form FTN signals through QPSK mapping and FTN shaping filters. Setting symbol period as tau (tau is more than 0 and less than 1) as acceleration factor), since X, Y two branches are no longer orthogonal, FTN complex signal is expressed as
Wherein x isn、ynAre respectively independent complex symbols, g (t) is signal pulse waveform, and
k is 0, ± 1, ± 2, ·, T is the symbol interval that satisfies Nyquist transmission.
The two FTN signals are respectively modulated by an IQ modulator to form two polarized light signals, namely
Wherein, PtIn order to average the emitted optical power,
for initial phase of signal light, omegasIs the frequency of the signal light.
And x (t), y (t) and the two paths of polarized light signals pass through the polarization coupler. Assuming that the polarization coupling ratio is 100%, a polarization multiplexing signal s (t) is obtained
s(t)＝x(t)+j·y(t) (4)
Laser signals are subject to atmospheric turbulence when transmitted in an atmospheric channel. Let r (t) ═ hs (t) + z (t) be the optical antenna received signal. Where z (t) represents a noise signal. h is the light intensity fading coefficient caused by the atmospheric turbulence, and the probability density function follows the distribution of
Wherein h is I/I0Exp (2 x), I is the intensity of the signal received by the receiver in the presence of turbulence, I0The signal light intensity received by the receiving end when no turbulent flow exists. Mu.sχIs the mean value of the x values,
is the variance of χ. Normalizing the light intensity signal, i.e. making E (I) equal to 1, then muχAnd
satisfy
At the receiving end, the optical signal is received by an optical receiving antenna after transmission through the air channel. The optical signal received by the receiving end can be represented as the channel transmission model
Wherein E issIs pulse energy, zx(t)，zyAnd (t) are respectively the noise components of the X, Y two paths of signals.
Let the local oscillator optical (LO) signal be
Wherein, PLOIs the optical power of the local oscillator light,
is the initial phase, omega, of the local oscillator lightLOIs the local oscillator frequency.
Two received signals rx(t)、ry(t) are respectively passed through 2X 490 deg. frequency mixers and local oscillator light rLO(t) mixing to produce an output field
After mixing (E)11,E21)、(E31,E41)、(E12,E22)、(E32,E42) Respectively connected into four balance detectors, the outputs of the balance detectors are respectively
The X, Y paths of in-phase components and orthogonal components are respectively combined to form two paths of complex signals
Two paths of complex signals enter an ADC for sampling after passing through a matched filter, and the output signals of an X branch and an Y branch can be respectively expressed as
Wherein,
x, Y the correlation of the noise of the two-path signal can be expressed as
For noise signals
The variance is
Wherein G (f) is a spectral function of g (t), N0The/2 is the noise power spectral density, which is a constant. Assuming perfect matching of the matched filter in the ideal case, then
And sending the sampled signals to a DSP module for processing, and compensating intersymbol interference by using a CMA linear equalizer. The equalizer adopts a CMA linear equalizer, and the main purpose is to compensate intersymbol interference between combined complex signals.
When the CMA equalization algorithm is adopted, under the criterion of reducing the mean square error, the iterative equation of the tap coefficient of the filter is
Where μ is the iteration step size, ε is the feedback error, h11、h12、h21、h22Respectively four filter tap coefficients, are provided,
and
is the output of the equalizer.
The algorithm for the feedback error ε is
The decision variable output via the CMA equalizer is then expressed as
Wherein,
the noise components of the X branch and the Y branch are output results after ADC sampling.
And (4) judging the judgment variable through a set judgment threshold v, and demodulating two paths of signals. The decision rule is as follows:
in order to obtain the reliability of the invention, the error rate is deduced by taking the X branch as an example, and the expression of the error rate of the X branch is
Pex(h)＝P(0)P(e/0)+P(1)P(e/1) (21)
In the formula, P (0) and P (1) are probabilities of transmitting data 0 and 1, respectively; p (e/0) is the probability of erroneous judgment when data 0 is transmitted, and P (e/1) is the probability of 0 code when data 1 is transmitted.
According to equation (21), when the transmission data is 0 and 1, respectively, the decision variable is changed
Obey distributions are respectively
At this time correspond toP (e/0) and P (e/1) of (A) may be represented as
Where erfc (·) is the complementary error function. The error rate of the X branch can be obtained according to the formula (21), and the error rate of the Y branch can be obtained similarly, and the expressions are respectively
Then, the average error rate of the FTN transmission system is
Therefore, the average bit error rate of the FTN transmission system under the turbulent flow channel is expressed as
In order to obtain an optimal decision threshold vtThe invention selects the average bit error rate<Pe>The minimum decision threshold value is the optimal decision threshold vt. To pair<Pe>Partial derivatives are calculated for v and made
The optimal decision threshold value v obtained by solving the equation (27)tIs 0. Then, the formula (26) can be converted into
To further illustrate the correctness of the present invention and the influence of the atmospheric turbulence on the system error rate, a Monte Carlo (Monte Carlo) method is adopted to perform simulation verification on the system error rate. The simulation conditions are as follows, (1) the transmitted signal is modulated by QPSK; (2) average emitted light power P t1, photoelectric conversion efficiency η 0.5, wavelength λ 1550nm, transmission rate 21G baud, acceleration factor τ 0.67, pulse energy
Local oscillator optical power P LO1 is ═ 1; (3) turbulence intensity: no turbulence, s.i. -, 0.4, s.i. -, 0.8.
Fig. 2 is a graph comparing spectral efficiency before and after FTN transmission techniques. It can be seen from the figure that, by adopting the FTN transmission technique, the spectral efficiency of the atmospheric laser communication system is greatly improved. When OSNR is 18dB, the spectral efficiency with FTN systems is as high as 1.9 Baud/Hz. Compared with a system without FTN, the frequency spectrum efficiency is improved by 15%.
Fig. 3 is a diagram of system BER versus baud rate before and after introduction of FTN. As can be seen from the figure, at the threshold of the bit error rate (i.e., BER ═ 3.8 × 10)-3) When no turbulence exists, the transmission rate of the system adopting the FTN technology is as high as 30G baud. Compared with the system before adopting FTN technology, the transmission rate is improved by 5G baud. When the flicker indexes are 0.4 and 0.8 respectively, the transmission rates of the system adopting the FTN technology are respectively as high as 21.5G baud and 13G baud. Compared with the system before adopting the FTN technology, the transmission rate is respectively improved by 4.5G baud and 4G baud. This shows that the introduction of FTN technology greatly increases the transmission rate of the original system, but the degree of rate increase gradually decreases with the increase of turbulence intensity.
Fig. 4 to 9 are constellation diagrams on simulation curves when the Baud rate is 18G Baud in fig. 3. Wherein fig. 4 and 7 show constellations without turbulence; FIGS. 5 and 6 are
Is 0.4A constellation of time; FIGS. 8 and 9 are
The constellation at 0.8. As can be seen from fig. 4 to 9, the introduction of the FTN technique effectively improves the error performance of the communication system. At the same time, an increase in turbulence intensity leads to an increase in noise, thereby increasing the error rate of the system, and this effect is more pronounced for systems using the FTN method.
FIG. 10 is a graph of OSNR versus BER for different flicker indexes. It can be seen that the system BER gradually decreases as the OSNR increases, and the magnitude of the decrease in the system BER gradually decreases as the flicker index increases. Meanwhile, it can be seen that when the s.i. is 0.6 and 0.8, the influence of the atmospheric turbulence on the FTN-QPSK optical communication system is similar. When BER is 3.8X 10-3The required osnr increases by 2dB, 2.6dB, 2.7dB and 5.9dB for s.i. values of 0.4, 0.6, 0.8 and 1, respectively, compared to the case without turbulence. This shows that the stronger the turbulence intensity, the greater the influence on the error code performance of the FTN system.
From the above description of the embodiments, it is clear for a person skilled in the art that the present invention can be implemented in software or by hardware. Based on the above understanding, the contribution of the technical solution of the present invention to the prior art can be realized by software or hardware to execute the method of the embodiment of the present invention.
Claims (1)
1. An FTN speed transmission method suitable for atmospheric laser communication is characterized in that FTN technology is introduced into an atmospheric laser communication system, a receiving end adopts a CMA linear equalization algorithm to process digital signals to compensate intersymbol interference brought by the FTN technology and improve spectral efficiency, and the method comprises the following steps:
step 1: at the transmitting end, X, Y two binary information sequences are first mapped into QPSK signals x respectivelynAnd ynThen the signal is processed by an FTN shaping filter to generate an FTN signal xFTNAnd yFTN
Wherein x isn、ynAre respectively independent complex symbols, g (t) is signal pulse waveform, and
k is 0, ± 1, ± 2, …, τ (0 < τ < 1) is the acceleration factor, T is the symbol interval that satisfies the Nyquist transmission;
the two FTN signals are respectively modulated on laser through an IQ modulator to form x (t), y (t) two polarization signals, and s (t) is transmitted by an optical antenna after being coupled by a polarization coupler; suppose that two optical signals received by the optical receiving antenna are r respectivelyx(t)、ry(t); two optical signals and local oscillator optical signal rLO(t) forming four paths of electric signals after passing through a 2 x 490-degree mixer and a photoelectric balance detector respectively; combining the four paths of electric signals into r1(t)、r2(t) two paths of complex signals are respectively subjected to ADC sampling after passing through a matched filter to obtain
Wherein E issIs the pulse energy, PtIs the average emitted optical power, PLOThe local oscillation light power, h is the light intensity fading coefficient, eta is the photoelectric conversion coefficient,
x, Y the correlation of the noise of the two-path signal can be expressed as
For noise signals
The variance is
Wherein G (f) is a spectral function of g (t), and N0/2 is a noise power spectral density, which is a constant; assuming perfect matching of the matched filter in the ideal case, then
Step 2: sending the sampled signals to a DSP module for processing and recovering user information; wherein, the equalizer adopts a CMA linear equalizer;
then, the decision variable output by the CMA equalization algorithm is represented as
Wherein,
respectively are output results of noise components of the X branch and the Y branch after being sampled by the ADC;
judging the judgment variable by setting a judgment threshold v, and demodulating two paths of signals; the decision rule is as follows:
calculating the average bit error rate of the FTN transmission system under the turbulent flow channel according to the obtained decision variable to obtain
Wherein f isI(h) Is the probability density function of the light intensity fading coefficient h, and I is the light intensity of the receiving end when the turbulent flow exists.
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