CN102497351A - Constant-envelop light OFDM-MSK (orthogonal frequency division multiplexing-minimum shift keying) modulation method - Google Patents

Abstract

The invention discloses a constant-envelop (CE) light OFDM-MSK (orthogonal frequency division multiplexing-minimum shift keying) modulation method, which is capable of minimizing PAPR (peak to average power ratio) and dropping the border of the frequency spectrum quickly so as to reduce interference among carrier waves, enhances resistance capability of the light OFDM system to interference of nonlinearity and subcarriers and accordingly improves the performances of long-distance large-capacity light OFDM transmission system. The modulation method includes the following steps: (1) a transmitting end generates OFDM-MSK signals; (2) the signals generated in the step (1) are subjected to digital-to-analogue conversion and optical phase modulation so as to generate CE-OFDM-MSK light signals to be sent to optical fibers; (3) a receiving end receives the CE-OFDM-MSK light signals by means of relative detection and the CE-OFDM-MSK light signals are subjected to phase modulation to be OFDM-MSK signals; and (4) source end data are acquired by means of modulation reverse to the modulation process of generating the OFDM-MSK signals.

Description

Permanent envelope optical OFDM-MSK modulator approach

Technical field

The present invention relates to a kind of new light OFDM modulation format method, in particular, relate to the new type light OFDM modulator approach of a kind of permanent envelope OFDM of combination (CE-OFDM) modulation and MSK OFDM (OFDM-MSK) modulation advantage.

Background technology

Light positive hand over frequency multiplexing technique (O-OFDM) because have anti-chromatic dispersion and polarization mode dispersion, the availability of frequency spectrum is high and balanced advantage such as simple, the hot research that has become optical communication field is technological.In addition, the develop rapidly of Digital Signal Processing (DSP) technology also further promotes the application of OFDM technology at optical communication field in recent years.The big volume transport information of long distance is pursuing one's goal of optical communication field always, but OFDM is prone to cause severe nonlinear owing to have higher peak-to-average power ratio (PAPR) in the transmission of optical fiber middle and long distance; In addition, the ofdm system sub-carriers is little at interval, and is slow if the frequency spectrum edge descends, and is easy to generate interchannel interference (ICI).To the bigger problem of ofdm system PAPR, can carry out conversion to reduce PAPR to signal at transmitting terminal, come restituted signal in the signal transformation mode of receiving terminal employing and transmitting terminal contrary.Wherein, permanent envelope OFDM (CE-OFDM) modulator approach based on the phase modulated converter technique can make PAPR near minimum 0dB.The CE-OFDM modulator approach is through being loaded into phase-modulator to original ofdm signal, thereby is transferred to information on the phase place, reaching the constant purpose of signal envelope, thereby makes that PAPR is 0dB.But because the fluctuation of light wave itself, PAPR can not be entirely 0dB, but this modulation format has made PAPR be reduced to bottom line, thereby has reduced effect of nonlinear in the optical fiber.In addition, in order to make the frequency spectrum edge of signal descend fast, people have proposed the method for Continuous Phase Modulation OFDM (OFDM-CPM), the continuous modulator approach of phase place between data on promptly a kind of each number of sub-carrier.But existing system is not owing to realize that the phase place of data on each number of sub-carrier realizes real Continuous Phase Modulation continuously and not; And MSK OFDM (OFDM-MSK) modulator approach can make the data phase on the subcarrier continuous; Thereby realized the OFDM-CPM modulation fully; Reach the frequency spectrum edge and descended fast, significantly reduced the effect of carrier frequency shift (CFO), ICI and intersymbol interference (ISI).

Therefore the present invention proposes a kind of new CE-OFDM-MSK modulator approach; In conjunction with the advantage that CE-OFDM modulates and OFDM-MSK modulates; At utmost reduced the PAPR of ofdm signal on the one hand; The nonlinear effect of introducing in reducing to transmit makes the frequency spectrum edge descend fast on the other hand, thereby improves the performance of the big Capacity Optical OFDM transmission system of long distance.

Summary of the invention

The object of the invention: propose a kind of new CE-OFDM-MSK modulator approach, strengthen optical OFDM system and resist ability non-linear and subcarrier interference, thereby improve the long distance performance of Capacity Optical OFDM transmission system greatly.

Main contents of the present invention have: combine the advantage of existing OFDM-MSK and CE-OFDM system to propose a kind of new modulator approach; This method not only makes PAPR reduce to bottom line; And the frequency spectrum edge is descended with the reduction inter-carrier interference fast, thereby improve the performance of system.

Concrete implementation method of the present invention is: utilize the OFDM-MSK signal to go the phase place of modulated laser to form horizontal envelope ofdm signal at transmitting terminal and send.

The concrete performing step of the present invention is:

1) transmitting terminal produces the OFDM-MSK signal;

2) with 1) signal that produces is through digital-to-analogue conversion, and carry out light phase modulation and the CE-OFDM-MSK light signal that obtains is sent into optical fiber;

3) receiving terminal adopts the mode of coherent detection to receive the line phase demodulation of going forward side by side of CE-OFDM-MSK signal and obtains the OFDM-MSK signal;

4) adopt and the demodulation mode acquisition source end data that produces OFDM-MSK signal modulated process contrary.

Description of drawings

Through the description of carrying out below in conjunction with accompanying drawing to embodiment, above-mentioned or other advantages of the present invention will become apparent, wherein:

Fig. 1 shows the complete block diagram of CE-OFDM-MSK system

Fig. 2 illustrates OFDM-MSK transmitter block diagram

Fig. 3 illustrates the OFDM-MSK transmitter block diagram based on FFT

Fig. 4 illustrates the CE-OFDM system block diagram

Fig. 5 shows the preceding planisphere of CE-OFDM-QAM phase modulation

Fig. 6 shows planisphere behind the CE-OFDM-QAM phase modulation

Fig. 7 shows the planisphere of CE-OFDM-QAM signal behind optical fiber

Fig. 8 show after the CE-OFDM-QAM equilibrium through separating the phase signals planisphere

Fig. 9 shows the qam constellation figure behind the CE-OFDM-QAM signal equalization

Figure 10 shows the preceding planisphere of CE-OFDM-MSK phase modulation

Figure 11 shows planisphere behind the CE-OFDM-MSK phase modulation

Figure 12 shows CE-OFDM-MSK signal planisphere behind optical fiber

Figure 13 show after the CE-OFDM-MSK equilibrium through separating the phase signals planisphere

Figure 14 shows the msk signal planisphere after the CE-OFDM-MSK equilibrium

Figure 15 shows the error rate of CE-OFDM-MSK and CE-OFDM-QAM system with the signal to noise ratio change curve

Embodiment

Through with reference to down in the face of the embodiment and the detailed description of accompanying drawing of exemplary indefiniteness, advantage of the present invention and characteristic and realize that method of the present invention can be easier to understanding.Yet the present invention can implement with multiple different form, and should not be construed as limited to the instance in this explaination.In addition, thus provide these embodiment this openly will be completely and completely, and will intactly design of the present invention be conveyed to those skilled in the art, the present invention will only be defined by accompanying claims.

To combine accompanying drawing that execution mode of the present invention is described in detail below.

Fig. 1 has provided the complete block diagram of CE-OFDM-MSK system.At first the source end data is carried out MSK mapping, add training sequence, pilot tone then, go here and there again and change, afterwards data are carried out the IFFT computing, obtain the OFDM-MSK signal after adding Cyclic Prefix (CP) again.Afterwards the continuous OFDM-MSK signal that produces through digital-to-analogue conversion is carried out light phase modulation and sends into optical fiber.

Receiving course and process of transmitting contrary.At first adopt the mode detection signal of coherent detection, and obtain the Digital Discrete signal, after string and conversion, removing CP, frequency domain equalization and separate phase modulated, separate mapping through FFT computing and MSK and obtain to send data sequence through analog-to-digital conversion.

Below will provide the concrete realization details of modulated terminal.

Fig. 2 has schematically provided OFDM-MSK transmitter block diagram.At first send data and go here and there and change, form N parallel data α ₀..., α _NAfter the MSK mapping, the signal that only contains phase information can be expressed as

$X\left(t\right)=e^{j({\mathrm{\θ}}_n+{\mathrm{\α}}_n\·\frac{\mathrm{\π\; t}}{2T})},$ (formula one a) for 0≤t≤T

Wherein, θ _nThe initial phase of expression sampling instant.For making phase place continuous, then have in sampling instant

${\mathrm{\θ}}_n=\frac{\mathrm{\π}}{2}\underset{q=0}{\overset{n-1}{\mathrm{\Σ}}}{\mathrm{\α}}_q+\mathrm{\φ}$ (getting initial phase φ=0) (formula one b)

Make T=NT _s, and get

$\tilde{S}\left(t\right)=\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{X}_k\left(t\right)*{\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="HSA00000636299700012.png,HSA00000636299700072.png"\; state="\{\{state\}\}">e</figure-callout>}}^j,$ 0≤t≤T (formula two)

Wherein,

0≤t≤T, k represent the k number of sub-carrier of ofdm signal, θ _{K, n}The initial phase of representing n data on the k number of sub-carrier, α _{K, n}Represent n data of importing constantly on the k number of sub-carrier, get any two sub-carrier signals A (t) wherein, B (t) has

$\mathrm{Re}=\{\underset{0}{\overset{{\mathrm{NT}}_s}{\&Integral;}}A\left(t\right)*B^{*}\left(t\right)\mathrm{dt}\}=0,$

Hence one can see that any two sub-carrier signals quadratures.

Because msk signal is not a constant in a mark space; Therefore can not directly carry out IFFT, need do following processing it: to (formula two) multiply by the factor

signal after the conversion

$S{\left(t\right)}^{\&prime;}=\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{X}_k\left(t\right)*e^{j\frac{\mathrm{\&pi;}(2k+\frac{1}{2})t}{{\mathrm{NT}}_s}}$ (formula three)

Get (formula one) substitution (formula three)

$S{\left(t\right)}^{\&prime;}=\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{e}^{j({\mathrm{\&theta;}}_{k,n}+{\mathrm{\&alpha;}}_{k,n}*\frac{\mathrm{\&pi;\; <figure-callout\; id="2"\; label="t"\; filenames="HSA00000636299700012.png,HSA00000636299700072.png"\; state="\{\{state\}\}">t</figure-callout>}}{2{\mathrm{NT}}_s})}*e^{j\frac{\mathrm{\&pi;}(2k+\frac{1}{2})t}{{\mathrm{NT}}_s}}$ (formula four)

$=\underset{k=0}{\overset{N-1}{\mathrm{\&Sigma;}}}{e}^{j{\mathrm{\&theta;}}_{k,n}^{\&prime;}}*e^{j\frac{k^{\&prime;}2\mathrm{\&pi;<figure-callout\; id="2"\; label="t"\; filenames="HSA00000636299700012.png,HSA00000636299700072.png"\; state="\{\{state\}\}">t</figure-callout>}}{2{\mathrm{NT}}_s}}$

Wherein,

θ ' _{K, n}The initial phase of n data on the k number of sub-carrier after the expression conversion, the subcarrier after the k ' expression conversion.In order to keep the continuity of phase place, we make again

$U_{v,n}=\left\{\begin{array}{cc}{e}^{j{\mathrm{\&theta;}}_{k,n}^{\&prime;}}& v=k^{\&prime;}\\ 0& v\&NotEqual;k^{\&prime;}\end{array}\right.$ (formula five)

Obtain

$S{\left(t\right)}^{\&prime;}=\underset{v=0}{\overset{2N-1}{\mathrm{\&Sigma;}}}{U}_{v,n}*{\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="HSA00000636299700012.png,HSA00000636299700072.png"\; state="\{\{state\}\}">e</figure-callout>}}^j$ (formula six)

So far obtain one can be directly with the msk signal of IFFT.OFDM-MSK transmitter block diagram based on IFFT is as shown in Figure 3.

Fig. 4 has provided the CE-OFDM system block diagram, sets forth how the OFDM-MSK signal is carried out permanent envelope modulation.Consider general situation, go to handle the sampled signal of each OFDM symbol with NIFFT point IFFT, be real signal for making output x [n], requiring the data of input IFFT is to satisfy the data vector of conjugation symmetry, zero padding, promptly

$[0,X[1],X[2],...,X[N],0_{1\&times;N_{\mathrm{ZP}}},0.X^{*}[N],...,X^{*}[2],X^{*}[1\left]\right],$

Wherein, Be the data symbol after the mapping,

Be to comprise N _ZPThe row vector of individual 0 element.Therefore, counting of IFFT is N _IFFT=2N+N _ZP+ 2.The 1st 0 and N+N _ZP+ 10 is in order to satisfy the conjugation symmetry, remaining N _ZPIndividual 0 is in order to realize the over-sampling of time domain sequences.Oversample factor is defined as C _OS=N _IFFT/ (N _IFFT-N _ZP).Adopt this method, the output of IFFT can be expressed as

(formula seven)

N=0 wherein, 1 ...., N _IFFT-1,

The OFDM sequence of the high PAPR of above-mentioned generation { x [n] } at first is transformed to { x (t) } through a digital to analog converter (D/A), removes to obtain the signal of 0dB PAPR again through a phase-modulator, s (t)=exp (jCx (t)), and wherein C is a proportionality constant.Signal after the conversion is injected in the optical fiber and transmits.The equivalent low-pass signal of CE-OFDM signal can be expressed as

s(t)＝Aexp{j[2πhm(t)+θ]}，-T _CP≤t≤T，

Wherein A is a signal amplitude, and 2 π h represent modulation index, and θ is a phase place of an extra interpolation in order to reach Continuous Phase Modulation.The signal m (t) that comprises transmitted information is real-valued ofdm signal, and form is following:

(formula eight)

We contrast CE-OFDM-QAM and two kinds of systems of CE-OFDM-MSK in emulation; Followed the tracks of the planisphere of signal, this emulation initial parameter is a modulation index 0.1, data rate 10Gb/s; Sub-carrier number 8; Fill Cyclic Prefix ratio 1/16, laser power 20mw, transmission range is 100km.

Fig. 5 shows the preceding planisphere of CE-OFDM-QAM phase modulation, because system produces the QAM signal and passed through real number IFFT computing, so constellation point all drops on the real axis, the data that produce like this are convenient to carry out phase modulated.

Fig. 6 shows planisphere behind the CE-OFDM-QAM phase modulation; On phase place, remove the data-moving of real number IFFT output; So planisphere is an amplitude is 1 circular arc, the length of circular arc is relevant with modulation index, and planisphere can obviously be found out thus; The amplitude perseverance of signal is 1, thereby makes that the PAPR that sends signal is 0dB.

Fig. 7 shows the planisphere of CE-OFDM-QAM signal behind optical fiber, because CHROMATIC DISPERSION IN FIBER OPTICS, nonlinear effect and inevitable random noise, thereby causes the disperse of planisphere.

Fig. 8 shows and separates phase signals after the CE-OFDM-QAM equilibrium, and with the modulated terminal contrary, signal constellation point all drops on the real axis.

Fig. 9 shows the qam constellation figure behind the CE-OFDM-QAM signal equalization, and figure can find out thus, signal degradation through balanced, has still obtained reasonable qam constellation figure though the noise of introducing in the channel makes.

Figure 10 shows the preceding planisphere of CE-OFDM-MSK phase modulation, and is consistent with QAM, because system produces the QAM signal and passed through real number IFFT computing, so constellation point all drops on the real axis, the data that produce like this are convenient to carry out phase modulated.

Figure 11 shows planisphere behind the CE-OFDM-MSK phase modulation; On phase place, remove the data-moving of real number IFFT output; So planisphere is an amplitude is 1 circular arc, the length of circular arc is relevant with modulation index, and planisphere can obviously be found out thus; The amplitude perseverance of signal is 1, thereby makes that the PAPR that sends signal is 0dB.

Figure 12 shows CE-OFDM-MSK signal planisphere behind optical fiber, because CHROMATIC DISPERSION IN FIBER OPTICS, nonlinear effect and inevitable random noise, thereby causes the disperse of planisphere.

Figure 13 shows and separates the phase signals planisphere after the CE-OFDM-MSK equilibrium, and with the modulated terminal contrary, signal constellation point all drops on the real axis.

Figure 14 shows the msk signal planisphere after the CE-OFDM-MSK equilibrium, and figure can find out thus, signal degradation through balanced, has still obtained reasonable MSK planisphere though the noise of introducing in the channel makes.

We have provided the ber curve under this simulation parameter; Shown in figure 15, can know by simulation curve, be under the situation of 10-3 in the error rate; The desired signal to noise ratio of CE-OFDM-MSK is than the low 2dB of CE-OFDM-QAM; And along with the reduction of the error rate, CE-OFDM-MSK has embodied the trend more excellent than CE-OFDM-QAM, and the desired signal to noise ratio of CE-OFDM-MSK can be hanged down more than the 2dB than CE-OFDM-QAM.

This theoretical derivation with us is consistent; Because the introducing of OFDM-MSK has realized that the phase place of data on each number of sub-carrier is continuous, has really realized Continuous Phase Modulation; Reach the frequency spectrum edge and descended fast, significantly reduced the effect of carrier frequency shift (CFO), ICI and ISI.Therefore OFDM-MSK is better than OFDM-QAM in the effect that reduces aspect carrier frequency shift (CFO), ICI and the ISI.On the other hand,, reduced the PAPR of signal, helped high-power, the long Distance Transmission of signal in optical fiber owing to the introducing of CE-OFDM.

Therefore the present invention is reducing signal(-) carrier frequency skew (CFO), ICI, ISI, and realizes having embodied advantage aspect high-power, the long Distance Transmission of signal in optical fiber.

The invention is not restricted to the foregoing description, without departing from the present invention, can carry out various distortion and modification.

Claims (2)

1. a new light OFDM modulation format method in particular, relates to the new type light OFDM modulator approach of a kind of permanent envelope OFDM of combination (CE-OFDM) modulation and MSK OFDM (OFDM-MSK) modulation advantage.

This method comprises the steps:

(1) transmitting terminal produces the OFDM-MSK signal;

(2) the signal process digital-to-analogue conversion that (1) is produced is carried out light phase modulation and the CE-OFDM-MSK light signal that obtains is sent into optical fiber;

(3) receiving terminal adopts the mode of coherent detection to receive the line phase demodulation of going forward side by side of CE-OFDM-MSK signal and obtains the OFDM-MSK signal;

(4) adopt and the demodulation mode acquisition source end data that produces OFDM-MSK signal modulated process contrary.

2. the method for claim 1 resists ability non-linear and subcarrier interference in order to strengthen optical OFDM system, thereby improves the long distance performance of Capacity Optical OFDM transmission system greatly, and we have proposed new CE-OFDM-MSK modulator approach.

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