CN102664687B - CHIRP-OFDM system frequency domain diversity receiving method - Google Patents

Abstract

The invention discloses a CHIRP-OFDM (orthogonal frequency division multiplexing) system frequency domain diversity receiving method which comprises the following steps; at an emitting end, modulating data after serial-parallel conversion to a frequency spectrum, carrying out inverse discrete Fourier transform so as to obtain a group of time domain signals, carrying out spread spectrum modulation on the time domain signals by taking chirp signals as carrier waves, and determining a frequency modulation slope; at a receiving end, removing cyclic prefixes of received serial signals, carrying out despread modulation on the signals, and conjugating the frequency modulation slope and the emitting end in a same way of taking determined chirp signals as carrier waves; carrying out the inverse discrete Fourier transform to obtain a frequency domain frequency spectrum, carrying out phase compensation on each spectrum line of multi-path signals, and overlying with each direct signal spectrum line, so as to enable all spectrum lines to be added in a same-phase way, and realize diversity reception and further recover signal sending; and carrying out digital demodulation so as to obtain data output. The method can effectively overcome frequency selective fading in channels and improves the system performance.

Description

A kind of CHIRP-OFDM system frequency diversity method of reseptance

Technical field

What the present invention relates to is the communication technology in the fields such as a kind of underwater sound communication, input.

Background technology

The multi-path effect existing in underwater acoustic channel, make its channel transfer function there is the form of comb filter, exist between the decline and enhancement region of frequency, when signal frequency composition is between the fading zone of frequency, signal energy weakens even and disappears, and has had a strong impact on the transmission of information, particularly serious for this simple signal impact of sinusoidal signal, if employing broadband signal, even if some frequency content energy is affected, but integral body still can guarantee the normal transmission of signal.

Traditional ofdm system be take sinusoidal signal as carrier wave, effective information is modulated to the transmission of carrying out signal in mutually orthogonal sinusoidal signal with mapping modes such as QPSK.

But underwater acoustic channel is frequency selective fading channels mostly, when subchannel is in deep fade, the method for this channel equalization cannot play a role equally, and the performance of system is subject to severe inhibition.

Spread spectrum communication can resist channel fading, multipath to disturb, and have advantages of that long transmission distance, reliability are high, low by the property intercepted and captured, but transmission data rate is not high.OFDM is a kind of multicarrier parallel transmission method of bandwidth efficient.Both are carried out to organic combination, can obtain good stability.

Summary of the invention

The object of the present invention is to provide the frequency selective fading effectively overcoming in channel, a kind of CHIRP-OFDM system frequency diversity method of reseptance that improves systematic function.

The object of the present invention is achieved like this:

A kind of CHIRP-OFDM system of the present invention frequency diversity method of reseptance, is characterized in that:

(1), at transmitting terminal, by being modulated on frequency spectrum after serial data conversion, between adjacent spectral line, there is protection interval;

(2) carry out inverse discrete Fourier transformer inverse-discrete, obtain one group of time-domain signal, time-domain signal be take to linear FM signal and as carrier wave, do band spectrum modulation, determine chirp rate, before described band spectrum modulation, add Cyclic Prefix;

(3) at receiving terminal, the serial signal receiving is removed to Cyclic Prefix, to signal despreading modulation, adopt equally definite linear FM signal as carrier wave, chirp rate and transmitting terminal conjugation;

(4) carry out discrete Fourier transform (DFT) and obtain frequency domain frequency spectrum, each root spectral line of multi-path signals is done to phase compensation, and superimposed with direct signal spectral line, each spectral line homophase is added, reach the object of diversity reception, then recover transmitted signal;

(5) carry out digital demodulation, obtain data output.

The present invention can also comprise:

1, described protection is spaced apart:

x=[x ₁, x ₂... x _n-1] ^trepresent the data message that certain OFDM piece carries, vectorial S=[s ₁, s ₂... s _n-1] ^trepresent the time domain data after conversion.

2, described to time-domain signal take linear FM signal as carrier wave do band spectrum modulation as:

Copy tail data and add data front end to as Cyclic Prefix, and adopt certain definite Chirp signal as carrier wave, signal spread-spectrum to be modulated, obtain the mutually orthogonal Chirp of gang:

$\tilde{T}=T_{\mathrm{Chirp}}T$

Wherein, T=T _cPs, T _cPfor (N+N _cPthe Cyclic Prefix of) * N inserts matrix, Chirp signal T _chirp=[T _chirp(0), T _chirp(1) ... T _chirp(N+N _cP-1)],

3, at receiving terminal, signal is carried out to despreading modulation after removing Cyclic Prefix, adopt identical with transmitting terminal spread-spectrum signal size, the Chirp signal of the chirp rate that symbol is contrary:

$\tilde{V}=R_{\mathrm{Chirp}}{R}_{\mathrm{CP}}R$

Wherein, r _cPfor N * (N+N _cP) Cyclic Prefix remove matrix.

Advantage of the present invention is: the present invention is on the basis of conventional ofdm system, using linear FM signal as carrier wave, time-domain signal is carried out to despreading processing, multipath component and direct signal component are separated from each other, the impact of the frequency selectivity deep fade that has suppressed channel on signal.Subcarrier has certain protection interval on frequency domain between frequency, the time delay depending in channel of choosing at protection interval is expanded.The signal of receiving terminal is separated because of spectral line composition and direct sound wave spectral line that multipath produces on frequency domain, and each composition energy is separate, when rediscover signal, has effectively utilized the energy of multipath component.

Accompanying drawing explanation

Fig. 1 is the schematic diagram 1 that frequency diversity receives;

Fig. 2 is the schematic diagram 2 that frequency diversity receives;

Fig. 3 is the impulse Response Function of certain underwater acoustic channel;

Fig. 4 is-10dB white Gaussian noise background under, receive signal and carry out the signal spectrum of frequency diversity before and after receiving;

Fig. 5 is under 0dB white Gaussian noise background, receives signal and carries out the signal spectrum that frequency diversity receives front and back;

Fig. 6 adopts chirp-OFDM communication system and traditional ofdm system ber curve figure.

Embodiment

Below in conjunction with accompanying drawing, for example the present invention is described in more detail:

In conjunction with Fig. 1～6, at transmitting terminal, by being modulated on frequency spectrum by certain rule after serial data conversion, between adjacent spectral line, there is certain frequency protection interval, be greater than the maximum spectral line shift amount that in channel, main multipath component produces.Carry out inverse discrete Fourier transformer inverse-discrete, obtain one group of time-domain signal, time-domain signal be take to linear FM signal and do band spectrum modulation as carrier wave, chirp rate is determined.For intersymbol phase mutual interference before and after eliminating, before band spectrum modulation, add Cyclic Prefix.

At receiving terminal, first the serial signal receiving is removed to Cyclic Prefix, to signal despreading modulation, adopt equally definite linear FM signal as carrier wave, chirp rate and transmitting terminal conjugation.Then carry out discrete Fourier transform (DFT) and obtain frequency domain frequency spectrum, each root spectral line to multi-path signals is done phase compensation, and superimposed with direct signal spectral line, each spectral line homophase is added, reach the object of diversity reception, then recover transmitted signal, then carry out corresponding digital demodulation, obtain data output.

The present invention also comprises:

1. transmitting terminal is modulated to data message on the frequency of parallel frequency domain, carries out discrete Fourier inverse transformation, adds frequency protection interval between each carrier wave:

$S=F_N^{-1}X$

X=[x ₁, x ₂... x _n-1] ^trepresent the data message that certain OFDM piece carries.Vector S=[s ₁, s ₂... s _n-1] ^trepresent the time domain data after conversion.

2. copy tail data and add data front end to as Cyclic Prefix, and adopt certain definite Chirp signal as carrier wave, signal spread-spectrum to be modulated, obtain the mutually orthogonal Chirp of gang:

$\tilde{T}=T_{\mathrm{Chirp}}T$

Wherein, T=T _cPs, T _cPfor (N+N _cPthe Cyclic Prefix of) * N inserts matrix, Chirp signal T _chirp=[T _chirp(0), T _cirp(1) ... T _chirp(N+N _cP-1)],

3. after underwater acoustic channel, obtain receiving end signal:

$R=H\·\tilde{T}+n$

Wherein, H is transmission matrix, and n is white Gaussian noise vector.

4. at receiving terminal, signal is carried out to despreading modulation after removing Cyclic Prefix, adopt identical with transmitting terminal spread-spectrum signal size, the Chirp signal of the chirp rate that symbol is contrary.

$\tilde{V}=R_{\mathrm{Chirp}}{R}_{\mathrm{CP}}R$

Wherein,

r _cPfor N * (N+N _cP) Cyclic Prefix remove matrix.

5. the through and multipath component that receives signal forms the spectral line being separated from each other on frequency domain, and each multipath component spectral line is carried out to phase compensation by certain rule, makes all signals with superimposed, obtains classification receiving gain, and demodulation, recovers original information.

Conventional ofdm system, transmitting terminal is modulated to data message on parallel frequency after going here and there and changing, and after inverse discrete Fourier transformer inverse-discrete, obtains

$S=F_N^{-1}X---\left(1\right)$

X=[x ₁, x ₂... x _n-1] ^trepresent the data message that certain OFDM piece carries.Vector S=[s ₁, s ₂... s _n-1] ^trepresent the time domain data after conversion.

For eliminating intersymbol interference (ISI), at signal front end, add cyclic prefix CP, obtain:

T＝T _CPS (2)

T wherein _cPfor (N+N _cPthe Cyclic Prefix of) * N inserts matrix

$T_{\mathrm{CP}}=\left[\begin{array}{cc}{O}_{N_{\mathrm{CP}}\×(N-N_{\mathrm{CP}})}& I_{N_{\mathrm{CP}}\×C_{\mathrm{CP}}};I_{N\×N}\end{array}\right]---\left(3\right)$

If channel impulse response be sampled as h=[h (0) h (1) ... h (L-1)], we define transmission matrix

Through channel, reception signal is

R＝H·T+n (5)

In formula, n is white Gaussian noise vector.

At receiving terminal, remove Cyclic Prefix.Obtain

V＝R _CPR (6)

Wherein, R _cPfor N * (N+N _cP) Cyclic Prefix remove matrix

$R_{\mathrm{CP}}=\left[\begin{array}{cc}{O}_{N\×N_{\mathrm{CP}}}& I_{N\×N}\end{array}\right]---\left(7\right)$

Through discrete Fourier transform (DFT), obtain

Y＝F _NV (8)

Because we suppose the length of CP, be greater than the maximum delay of channel, so, at receiving terminal, remove after CP, the interference of adjacent OFDM interblock can be eliminated completely, by above-mentioned, variously obtains after comprehensive

$Y=F_N{R}_{\mathrm{CP}}{\mathrm{HT}}_{\mathrm{CP}}{F}_N^{-1}X+F_N{R}_{\mathrm{CP}}n---\left(9\right)$

Order

can find that it is N * N circular matrix, then make

above formula can abbreviation be

Y＝D _HX+N (10)

In formula, N=F _nr _cPn represents noise vector.Due to F _nbe the non-singular matrix of power normalization, the statistical property of the element in N is identical with n, separate and to be average be that 0 variance is the Gaussian Profile of σ.

Due to matrix

be a circular matrix, can be easy to the diagonalization of matrix by DFT.Make matrix D _hfor pair of horns battle array, the vector of samples h=[h that on its diagonal, each element is channel impulse response (0) h (1) ... h (L-1)] gained vector H=[H (0) H (1) after the conversion of N point Fourier ... H (N-1)],

Y＝diag(H)X+N (11)

Wherein, diag (H) represents by vectorial H=[H (0) H (1) ... H (N-1)] diagonal matrix that forms, wherein diagonal entry

from upper surface analysis, can find out, diagonal entry is to be formed by stacking by multipath signal, and phase place depends on carrier frequency and time delay, and this is the basic reason that produces frequency selective fading.When carrier wave k is in deep fade point time, H (k) levels off to zero, and transmitting information X (k) is destroyed, and cannot reduce by frequency domain equalization.

Therefore we make improvements traditional OFDM, at OFDM, signal are added after Cyclic Prefix, with certain definite Chirp signal spread-spectrum carrier wave,

$\tilde{T}=T_{\mathrm{Chirp}}T---\left(12\right)$

Wherein $T_{\mathrm{Chirp}}=[T_{\mathrm{Chirp}}\left(0\right),T_{\mathrm{Chirp}}\left(1\right)......T_{\mathrm{Chirp}}(N+N_{\mathrm{CP}}-1)],T_{\mathrm{Chirp}}\left(n\right)=\exp \left(j2\mathrm{\π}{\mathrm{\αn}}^2{T}_s^2\right),$ α is chirp rate, T _sfor the sampling period.After multiplying each other, make all carrier frequency of ofdm signal become that slope is identical, centre frequency is different, the Chirp signal of quadrature.

Through channel, receive signal and become

$R=H\·\tilde{T}+n---\left(13\right)$

At receiving terminal, signal is carried out to despreading modulation after removing Cyclic Prefix, adopt identical with transmitting terminal Chirp signal magnitude, the chirp rate that symbol is contrary, R _chirp=[R _chirp(N _cP), R _chirp(N _cP+ 1) ... R _chirp(N+N _cP-1)], obtain

$\tilde{V}=R_{\mathrm{Chirp}}{R}_{\mathrm{CP}}R---\left(14\right)$

Wherein, $R_{\mathrm{Chirp}}\left(n\right)=\exp \left(j2\mathrm{\π}{\mathrm{\αn}}^2{T}_s^2\right).$

Chirp carrier wave is different from single-frequency carrier wave, and simple signal time delay frequency is constant, only produces phase place and changes.And Chirp signal time delay not only phase place change, and centre frequency also changes.Remove after Cyclic Prefix, with same Chirp signal despreading modulation, make direct sound wave signal constant, multipath signal produces different frequency displacements and phase shift according to time delay.

By above-mentioned, variously obtain after comprehensive

$Y=F_N{R}_{\mathrm{Chirp}}{R}_{\mathrm{CP}}{\mathrm{HT}}_{\mathrm{Chirp}}{T}_{\mathrm{CP}}{F}_N^{-1}X+F_N{R}_{\mathrm{Chirp}}{R}_{\mathrm{CP}}n---\left(5\right)$

Order

matrix

still a circular matrix, can be by the diagonalization of Fourier transformation matrix, that is:

Y＝diag(H)X+N (16)

At this moment the multipath signal of a certain carrier wave is not to superpose on same frequency, but is extended on different frequent points.We do normalized, order

the length of channel impulse response function is L, and the maximum frequency shift that multipath signal produces is also L.Can obtain one group of spectral line bunch,

$H(k-l)=h\left(l\right)\exp (-j2\mathrm{\π}\frac{\mathrm{lk}}{N}+\mathrm{j\π}\frac{l^2}{N}),l=\mathrm{0,1}......L-1---\left(17\right)$

For fear of ICI, disturb, when transmitting, between each carrier wave, add frequency protection interval, protection interval is greater than L.According to formula 17, as shown in Figure 2, when the frequency resolution of FFT is enough high, as long as each root spectral line of multipath signal is done to phase compensation stack, make all signals with superimposed, can obtain classification receiving gain, fundamentally to have eliminated frequency selectivity interference.

On the one hand, this communication mode is separated signal because of spectral line composition and direct sound wave spectral line that multipath produces, the impact of the frequency selectivity deep fade that has suppressed channel on signal, on the other hand, when recovering actual signal, effectively utilized the energy of multipath component, improved the performance of system.As shown in schematic diagram 1, same delay volume has caused big or small identical frequency shift amount on frequency domain different frequent points, during recovery, has effectively utilized these frequency-shifted components, has strengthened the energy of primary signal.

Experiment condition is as follows, adopt the linear FM signal of bandwidth 1kHz as carrier wave, centre frequency interval 1.25kHz between adjacent signals, signal frequency range 2～6.75kHz, sample frequency should be greater than 2 times of signal highest frequency, here adopt 30kHz, the duration of a frame signal is 40ms, in having the underwater acoustic channel that multipath structure and noise background are white Gaussian noise, transmits.

Fig. 3 is the impulse Response Function of channel, except direct sound wave, because bottom and surface of sea reflection produces some multi-path signalses, Fig. 4, Fig. 5 is respectively the frequency domain spectral line contrast situation that receives part, signal diversifying reception front and back under signal to noise ratio-10dB and 0dB, the 131st and 201 places at frequency domain, it is the peak value of pulse that original signal produces, be subject to channel effect, the additional spectral line that both sides produce is serious on the detection impact of signal, when-10dB, the spectral line energy of multipath component has surpassed original spectral line, produce erroneous judgement, cannot correctly estimate primary signal, and after diversity reception is processed, the energy of original spectral line strengthens greatly, signal can well be judged and demodulation, guaranteed the performance of system, effectively suppress the frequency selective fading of channel.

Claims (4)

1. a CHIRP-OFDM system frequency diversity method of reseptance, is characterized in that:

(1), at transmitting terminal, by being modulated on frequency spectrum after serial data conversion, between adjacent spectral line, there is protection interval;

(2) carry out inverse discrete Fourier transformer inverse-discrete, obtain one group of time-domain signal, time-domain signal be take to linear FM signal and as carrier wave, do band spectrum modulation, determine chirp rate, before described band spectrum modulation, add Cyclic Prefix;

(3) at receiving terminal, the serial signal receiving is removed to Cyclic Prefix, to signal despreading modulation, adopt equally definite linear FM signal as carrier wave, chirp rate and transmitting terminal conjugation;

(4) carry out discrete Fourier transform (DFT) and obtain frequency domain frequency spectrum, each root spectral line of multi-path signals is done to phase compensation, and superimposed with direct signal spectral line, each spectral line homophase is added, reach the object of diversity reception, then recover transmitted signal;

(5) carry out digital demodulation, obtain data output.

2. a kind of CHIRP-OFDM system frequency diversity method of reseptance according to claim 1, is characterized in that: described protection is spaced apart:

represent the data message that certain OFDM piece carries, vector

represent the time domain data after conversion.

3. a kind of CHIRP-OFDM system frequency diversity method of reseptance according to claim 2, is characterized in that: described to time-domain signal take linear FM signal as carrier wave do band spectrum modulation as:

Copy tail data and add data front end to as Cyclic Prefix, and adopt definite Chirp signal as carrier wave, signal spread-spectrum to be modulated, obtain the mutually orthogonal Chirp of gang:

$\tilde{T}=T_{\mathrm{Chirp}}T$

Wherein, T=T _cPs, T _cPfor (N+N _cPthe Cyclic Prefix of) * N inserts matrix, Chirp signal T _chirp=[T _chirp(0), T _chirp(1) ... T _chirp(N+N _cP-1)],

4. a kind of CHIRP-OFDM system frequency diversity method of reseptance according to claim 3, is characterized in that:

At receiving terminal, signal is carried out to despreading modulation after removing Cyclic Prefix, adopt identical with transmitting terminal spread-spectrum signal size, the Chirp signal of the chirp rate that symbol is contrary:

$\tilde{V}=R_{\mathrm{Chirp}}{R}_{\mathrm{CP}}R$

Wherein,

r _cPfor N * (N+N _cP) Cyclic Prefix remove matrix.

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