CN102868659A - Symbol synchronization and Doppler compensation method for mobile orthogonal frequency division multiplexing (OFDM) underwater sound communication signal - Google Patents

Abstract

The invention relates to a symbol synchronization and Doppler compensation method for a mobile orthogonal frequency division multiplexing (OFDM) underwater sound communication signal. The method comprises the following steps of: obtaining rough estimation of a first OFMD symbol start position according to a frame header signal, extracting a symbol, then obtaining Doppler expansion rough estimation of the symbol according to a frequency measurement simple-frequency signal in the symbol, converting the symbol to a frequency domain according to Chirp-z conversion, searching a precise start moment and a Doppler expansion amount of the symbol by taking a pilot of the frequency domain and a relevant value of a local pilot as cost functions, presuming the start position of the next symbol according to the start position of the current symbol, extracting the next symbol, and repeating the operations till the end of a frame of signal. According to the Doppler measurement and compensation method, based on the Chirp-z conversion, the measurement and compensation precision is high; the Chirp-z conversion is implemented according to fast Fourier transformation (FFT); the practicability is high; according to the Doppler expansion search method which is in a manner that large step length is superior to small step length, so that the search efficiency is high; a Doppler measurement range is not limited; and the method can be used for constant-speed motion and variable-speed motion.

Description

A kind of sign synchronization of mobile OFDM water sound communication signal and Doppler Compensation Method

Technical field

The present invention relates to the underwater sound communication field, more definite in saying, relate to the OFDM underwater sound communication method, mainly be a kind of sign synchronization and Doppler Compensation Method of mobile OFDM water sound communication signal.

Background technology

Orthogonal frequency division multiplexi (OFDM) has good anti-frequency selective fading performance and high bandwidth availability ratio, can be used for the serious underwater high-speed sound communication in many ways.Yet OFDM has the shortcoming that himself determines, namely receptivity is responsive to sign synchronization deviation and frequency departure.

In underwater sound communication, owing to being usually operated at low frequency, and the velocity of sound is far below radio, therefore when having Doppler, compare the performance that radio has seriously influenced ofdm system more, Doppler on underwater sound ofdm communication affect essence be large Doppler on the impact of broadband connections, therefore, the Doppler's modification method in the narrowband radio ofdm communication is not suitable for underwater sound ofdm communication.

Doppler Compensation Method in the underwater sound communication, relatively typical method is piece Doppler estimation, the time domain linear difference approach of Sharif B S employing in 1998.Baosheng Li in 2007 use for reference the piece processing method of Sharif, for the compensating for doppler problem in the underwater sound ofdm communication, propose at first to carry out the broadband Doppler effect correction, namely record first doppler spread amount (spreading factor), in time domain, signal is resampled, then carry out the arrowband Doppler effect correction, be about to approximate carrier frequency shift (the Carrier Frequency Offset that regards as of residual Doppler, CFO), carry out CFO and estimate, then compensate.Said method need to be preserved the complete signal of a frame, and EMS memory occupation is large, is unfavorable for real-time implementation, and adopts approximate evaluation method was, and the Doppler effect correction precision is not high, and decoding effect is not good.

Summary of the invention

The object of the present invention is to provide a kind of sign synchronization and Doppler Compensation Method of mobile OFDM water sound communication signal, compensation is carried out in frequency domain, and precision is high, practicality good.

The object of the present invention is achieved like this: the method comprises the steps:

(1) at transmitting terminal, at header signal of initial use (such as linear FM signal LFM) of every frame, be used for the synchronously thick of first symbol, add a frequency measurement spectral line in the outside of each OFDM symbol frequency spectrum, be used for Doppler's rough estimate, in the OFDM frequency domain, insert pseudo random sequence [d ₀, d ₁..., d _M-1] as pilot frequency sequence, pilot frequency sequence not only can be used as channel estimating, can also be as the precision of each symbol synchronously and Doppler's estimation;

(2) at receiving terminal, adopt the copy correlation method to catch header signal, calculate the constantly τ that reaches of header signal ₀, by τ ₀Pusher obtains the start bit of first OFDM symbol

Rough estimate;

(3) with

For initial time takes out an OFDM symbol, according to the frequency offset of frequency measurement signal, calculate the rough estimate of doppler spread in this symbol (or compression) amount

(4) with

With

Be search center, when arranging-region of search frequently, with Chirp-Z conversion (chirp Z transform) ofdm signal transformed to frequency domain, extract the pseudorandom pilot frequency sequence, calculation cost function, and size relatively make cost function reach maximum With

Be exactly accurate original position and the doppler spread amount of this symbol, in order to improve doppler searching efficient, doppler searching can be divided into for two steps, first with large step search, search for half step distance again;

(5) according to the symbol initial time that obtains in the step (4) Take out a symbol, carry out the Chirp-Z conversion, obtain the OFDM frequency-region signal behind the Doppler effect correction, then carry out channel estimating and equilibrium, recover raw information;

(6) again with the initial time of current sign

Be benchmark, obtain the original position of next OFDM symbol Repeating step (3) ~ (6) are until a frame signal finishes.

In the technique scheme, the rough estimate of doppler spread amount in the described step (3)

Obtain like this: the frequency of supposing the frequency measurement signal is f _z, affected by Doppler, the frequency shift (FS) of the frequency measurement signal that receives is to f ' _z, then

Computing formula be:

${\widehat{\mathrm{\β}}}_0=\frac{f_z^{\′}-f_z}{f_z}$

When

The time, the expression signal is compressed, when

The time, the expression signal is expanded.

In the technique scheme, in the described step (4) with

With

Be search center, when arranging-region of search refers to frequently: setup times is interval The size of T is relevant with the maximum movement speed of mobile platform, and movement velocity is larger, and expansion or the decrement of signal are larger, and T is just larger, otherwise T is just less; Arrange between Doppler region

The size of δ is by the received signal to noise ratio decision, and signal to noise ratio is larger, Doppler's rough estimate Just more near actual value, δ just can be less, otherwise δ is just larger.

In the technique scheme, with the Chirp-Z conversion ofdm signal is transformed to frequency domain in the described step (4), the to received signal Chirp-Z conversion of y (n), can be write as with formula:

$Y\left(z_k\right)=\underset{n=0}{\overset{N-1}{\mathrm{\Σ}}}y\left(n\right)A^{-n}{W}^{\mathrm{nk}}$

Wherein,

f ₀Be initial frequency, φ ₀Be frequency resolution, f _sSample rate, f ₀And φ ₀Determined by doppler spread amount β, specifically can be write as:

f ₀=f _low*(1+β)

${\mathrm{\φ}}_0=\frac{B}{N}*(1+\mathrm{\β})$

Wherein, f _LowBe the frequency of the minimum subcarrier of OFDM, B is signal bandwidth.

In the technique scheme, extract the pseudorandom pilot frequency sequence in the described step (4), the calculation cost function, the computing formula of cost function is:

Wherein, || the expression modulo operation, conj () represents conjugate operation, [d ₀, d ₁..., d _M-1] be local pseudo random sequence,

It is the pseudo random sequence that receiving terminal extracts.

In the technique scheme, described step was divided into for two steps with doppler searching in (4), first with large step search, referred to the half step distance search: in the region of search again

In, first with large step search, search makes cost function Xcr (t _i, β _i) reach maximum β _c, again with β _cCentered by, carry out fine searching with half step distance, estimated accurately at last

In the technique scheme, the symbol initial time that obtains in the described step (5) Get symbol, the method for carrying out the Chirp-Z conversion is: according to initial time

Getting length is Signal (T _OfdmThe emission symbol lengths) carry out the Chirp-Z conversion, initial frequency and frequency resolution are respectively:

$f_0=f_{\mathrm{low}}*(1+\widehat{\mathrm{\β}})$

${\mathrm{\φ}}_0=\frac{B}{N}*(1+\widehat{\mathrm{\β}})$

The invention has the beneficial effects as follows: compare with Doppler Compensation Method in the past, compensation method of the present invention is carried out in frequency domain, adopts the Chirp-Z conversion that signal is transformed into frequency domain, and compensation precision is high, can utilize FFT to realize the Chirp-Z conversion, and practicality is good; Utilize frequency domain pseudorandom pilot frequency sequence, can realize the estimation of uniting to sign synchronization and doppler spread; Searching method to doppler spread adopts first large step pitch, rear half step distance, and search efficiency is high; The present invention can be used for uniform motion, also can be used for variable motion.

Description of drawings

Fig. 1 sign synchronization and Doppler effect correction process flow block diagram;

In Fig. 2 computer simulation experiment, in translational speed 6 joint (the doppler spread amount is 0.002) situations, without the decoding planisphere of sign synchronization and Doppler effect correction;

In Fig. 3 computer simulation experiment, in translational speed 6 joint (the doppler spread amount is 0.002) situations, through the decoding planisphere of sign synchronization and Doppler effect correction;

Transceiver relative position figure in the experiment on Fig. 4 lake;

Underwater acoustic channel impulse response figure when experiment is apart from 3km on Fig. 5 lake;

On Fig. 6 lake in the experiment translational speed 6 joint situations, the time-cost function Xcr (t during the two-dimentional precise search of frequency _i, β _i);

Test in the translational speed 6 joint situations, through the decoding constellation of sign synchronization and Doppler effect correction on Fig. 7 lake.

Embodiment

Below in conjunction with accompanying drawing and implementation example to further detailed description of the present invention.

The sign synchronization of mobile OFDM water sound communication signal of the present invention and Doppler Compensation Method, the method mainly comprises the steps:

1. the generation method transmits

The first of the every frame of underwater sound ofdm communication signal is one section header signal, is used for the synchronously thick of first symbol, and header signal can be selected linear frequency modulation LFM or pseudorandom phase-modulated signal, is thereafter one section idle guard time, then is the ofdm communication signal.

An OFDM symbol can be expressed as:

$x\left(t\right)=\mathrm{Re}\left\{\underset{k=0}{\overset{K-1}{\mathrm{\&Sigma;}}}{X}_{\mathrm{<figure-callout\; id="2"\; label="k\; e\; j"\; filenames="766380DEST\_PATH\_IMAGE002.png,DEST\_PATH\_HDA00002257564100031.png"\; state="\{\{state\}\}">k</figure-callout>}}{e}^j{2\mathrm{\&pi;}(f_{\mathrm{low}}+\mathrm{k\&Delta;f})t}^{}\right\}---\left(1\right)$

K is number of subcarriers in the formula (1), Δ f be subcarrier spacing (

T _SymAn OFDM symbol lengths), f _LowMinimum sub-carrier frequencies, { X _k, k=1 ..., K is modulated to K the frequency domain data on the subcarrier, and K subcarrier is divided into M pilot sub-carrier and K-M data subcarrier, and pilot sub-carrier is spacedly distributed, pilot interval is Q* Δ f(Q*M=K), pilot frequency information [d ₀, d ₁..., d _M-1] be the pseudo random sequence of M for length.Pilot frequency information not only can be used as channel estimating, can also synchronous and Doppler's estimation as the precision of each symbol.

Outer survey at the OFDM carrier wave increases a frequency measurement spectral line; be used for Doppler's rough estimate; the frequency of this spectral line can be higher or lower than the frequency of ofdm signal; and and the frequency domain that leaves proper width between the ofdm signal is protected the interval; when having Doppler; the ofdm signal spectral line can be to low or to height skew, and the width at protection interval should be unlikely to cause the erroneous judgement of spectral line and the frequency measurement spectral line of ofdm signal.For the ease of the detection of receiving terminal, the energy intensity of this spectral line can suitably be higher than the energy of ofdm signal.

2. Doppler is to the impact analysis of ofdm signal

When having Doppler, on time domain, because causing the expansion of symbol or compression, Doppler can not ignore, and it affects the location of symbol; On frequency domain, the subcarrier pair of different frequency is not identical in the frequency shift (FS) that identical doppler spread amount β produces in the ofdm signal, but subcarrier spacing remains identical after the skew, is analyzed as follows:

The frequency of supposing k subcarrier is f _k, Δ f is subcarrier spacing, is formulated as:

f _k=f _low+k*Δf （2）

When the doppler spread amount was β, the frequency that receives was f ' _k, be formulated as:

f′ _k=f _k*(1+β) （3）

Subcarrier spacing is Δ f ', is formulated as:

Δf′=f′ _k+1-f′ _k=Δf*(1+β) （4）

Therefore, the zero-time of correct sprocket bit on time domain, on frequency domain with the minimum subcarrier f of OFDM _Low* (1+ β) is starting point, extracts frequency domain information take Δ f ' as the interval, just can realize sign synchronization and the Doppler of OFDM are revised.Because β is not integer (between 0.01 ~ 0.0001) usually, the high resolution that the present invention utilizes the Chirp-Z conversion to provide is realized the Accurate Analysis to the OFDM frequency spectrum.

3. the sign synchronization of receiving terminal and Doppler revise

The receiving terminal handling process as shown in Figure 1.Adopt the copy correlation method to catch header signal, calculate the constantly τ that reaches of header signal ₀, because the free time after the frame head is known, by τ ₀Can obtain the original position of first OFDM symbol

Rough estimate;

With

For initial time takes out an OFDM symbol, affected by Doppler, suppose that the frequency of frequency measurement signal is by f _zBe offset to f ' _z, then can obtain the rough estimate of doppler spread amount in this symbol

Computing formula is:

${\widehat{\mathrm{\&beta;}}}_0=\frac{f_z^{\&prime;}-f_z}{f_z}---\left(5\right)$

When

The time, the expression signal is compressed, when

The time, the expression signal is expanded.

With

With

Centered by, at time interval

And between Doppler region

Upward symbol original position and doppler spread amount are carried out two-dimentional fine searching.

Get OFDM and receive signal y (t), t ∈ [t _i: t _i+ T _Sym* (1+ β _i)],

If y (n) is the discrete form of y (t), ask the Chirp-Z conversion of y (n), can be write as:

$Y\left(z_k\right)=\underset{n=0}{\overset{N-1}{\mathrm{\&Sigma;}}}y\left(n\right)A^{-n}{W}^{\mathrm{nk}}---\left(6\right)$

Wherein,

f ₀Be initial frequency, φ ₀Be frequency resolution, by changing doppler spread amount β _iAffect f ₀And φ ₀, specifically can be write as:

f ₀=f _low*(1+β _i) （7）

${\mathrm{\&phi;}}_0=\frac{B}{N}*(1+{\mathrm{\&beta;}}_i)---\left(8\right)$

Wherein, f _LowBe the frequency of the minimum subcarrier of OFDM, B is signal bandwidth, f _sIt is sample rate.

The frequency of supposing the 1st pilot sub-carrier is f _Low, then with f _Low* (1+ β _i) be starting point, with Q* Δ f* (1+ β _i) be the interval, from Y (z _k) M pilot frequency pseudo-random sequence of middle extraction

With local pseudo random sequence [d ₀, d ₁..., d _M-1] make related operation, make cost function with correlation:

$\mathrm{Xcr}(t_i,{\mathrm{\&beta;}}_i)={|\underset{i=0}{\overset{M-1}{\mathrm{\&Sigma;}}}{\tilde{d}}_i*\mathrm{conj}\left(d_j\right)|}^2---\left(9\right)$

Wherein, || the expression modulo operation, conj () represents conjugate operation.Suppose that real symbol original position and doppler spread amount are respectively t and β, in the ideal case, work as t _i=t, β _iDuring=β, cost function Xcr (t _i, β _i) reach maximum, that is to say:

The size of region of search T is relevant with the maximum movement speed of mobile platform, and movement velocity is larger, and expansion or the decrement of signal are larger, and T is just larger, otherwise T is just less; The size of δ is by the received signal to noise ratio decision, and signal to noise ratio is larger, and Doppler's rough estimate is just more near actual value, and δ just can be less, otherwise δ is just larger.

In order to improve doppler searching speed, in the region of search

In, first with large step search, search makes cost function Xcr (t _i, β _i) reach maximum β _c, again with β _cCentered by, carry out fine searching with half step distance, estimated accurately at last

Obtain the symbol initial time by said method

With the doppler spread amount After, according to initial time

Getting length is

Signal do the Chirp-Z conversion, initial frequency and frequency resolution are respectively:

${\mathrm{<figure-callout\; id="0"\; label="f"\; filenames="766380DEST\_PATH\_IMAGE002.png,DEST\_PATH\_HDA00002257564100031.png"\; state="\{\{state\}\}">f</figure-callout>}}_0=f_{\mathrm{low}}*(1+\widehat{\mathrm{\&beta;}})---\left(11\right)$

${\mathrm{\&phi;}}_0=\frac{B}{N}*(1+\widehat{\mathrm{\&beta;}})---\left(12\right)$

Then carry out channel estimating and equilibrium, recover raw information.

Again with the initial time of current sign Be benchmark, obtain the original position of next OFDM symbol

Repeat said process, until a frame signal finishes.

4. concrete underwater sound ofdm communication embodiment

Effect of the present invention is verified in computer and lake experiment, and the below provides a specific embodiment effect of the present invention is described.

System parameters and operational environment are as follows: system bandwidth 4kHz, carrier frequency 8kHz, sub-carrier number 1024, minimum sub-carrier frequencies f _Low=6kHz, subcarrier spacing 3.9063Hz, pilot sub-carrier is several 256, symbol lengths 256ms, intersymbol guard time 64ms, information modulation system QPSK; Frame head linear FM signal bandwidth 6kHz-10kHz, length 64ms; Frequency measurement signal frequency f _z=5906.25kHz.

Carried out respectively testing on Computer Simulation and the lake.In Computer Simulation, in simulation translational speed 6 joint (the doppler spread amount is 0.002) situations, Fig. 2 is the decoding planisphere without sign synchronization and Doppler effect correction, and Fig. 3 is the decoding planisphere through sign synchronization and Doppler effect correction.

In lake experiment, the transceiver relative position as shown in Figure 4, receiver is static, transmitter is made circular motion, the relative motion of sending and receiving end is that non-uniform movement exists acceleration.Fig. 5 is the underwater acoustic channel impulse response figure when experiment is apart from 3km on the lake.Fig. 6 is on the lake in the experiment translational speed 6 joint situations, the time-cost function Xcr (t during the frequency precise search _i, β _i).Fig. 7 tests on the lake in the translational speed 6 joint situations, receives the decoding constellation of signal.

Claims (6)

1. the sign synchronization of a mobile OFDM water sound communication signal and Doppler Compensation Method, it is characterized in that: the method comprises the steps:

(1) at transmitting terminal, at header signal of initial use of every frame, be used for the synchronously thick of first symbol, add a frequency measurement spectral line in the outside of each OFDM symbol frequency spectrum, be used for Doppler's rough estimate, in the OFDM frequency domain, insert pseudo random sequence [d ₀, d ₁..., d _M-1] as pilot frequency sequence;

(2) at receiving terminal, adopt the copy correlation method to catch header signal, calculate the constantly τ that reaches of header signal ₀, by τ ₀Pusher obtains the start bit of first OFDM symbol

Rough estimate;

(3) with For initial time takes out an OFDM symbol, according to the frequency offset of frequency measurement signal, calculate the rough estimate of the interior doppler spread of this symbol or decrement

(4) with

With

Be search center, when arranging-region of search frequently, with the Chirp-Z conversion ofdm signal transformed to frequency domain, extract the pseudorandom pilot frequency sequence, calculation cost function, and size relatively make cost function reach maximum

With

Be exactly accurate original position and the doppler spread amount of this symbol, doppler searching was divided into for two steps, first with large step search, searched for half step distance again;

(5) according to the symbol initial time that obtains in the step (4)

Take out a symbol, carry out the Chirp-Z conversion, obtain the OFDM frequency-region signal behind the Doppler effect correction, then carry out channel estimating and equilibrium, recover raw information;

(6) again with the initial time of current sign Be benchmark, obtain the original position of next OFDM symbol

Repeating step (3) ~ (6) are until a frame signal finishes.

2. the sign synchronization of mobile OFDM water sound communication signal according to claim 1 and Doppler Compensation Method is characterized in that: with the Chirp-Z conversion ofdm signal is transformed to frequency domain in the described step (4), its formula is:

$Y\left(z_k\right)=\underset{n=0}{\overset{N-1}{\mathrm{\&Sigma;}}}y\left(n\right)A^{-n}{W}^{\mathrm{nk}}$

Wherein, y (n) receives signal,

f ₀Be initial frequency, φ ₀Be frequency resolution, f _sSample rate, f ₀And φ ₀Determined by doppler spread amount β, its formula is:

f ₀=f _low*(1+β)

${\mathrm{\&phi;}}_0=\frac{B}{N}*(1+\mathrm{\&beta;})$

Wherein, f _LowBe the frequency of the minimum subcarrier of ofdm signal, B is the ofdm signal bandwidth.

3. the sign synchronization of mobile OFDM water sound communication signal according to claim 1 and Doppler Compensation Method is characterized in that: in the described step (4) with

With

Be search center, when arranging-region of search refers to frequently: setup times is interval

T is search radius, arranges between Doppler region

δ is search radius.

4. the sign synchronization of mobile OFDM water sound communication signal according to claim 1 and Doppler Compensation Method is characterized in that: extract the pseudorandom pilot frequency sequence in the described step (4), and the calculation cost function, the computing formula of cost function is:

Wherein, || the expression modulo operation, conj () represents conjugate operation, [d ₀, d ₁..., d _M-1] be local pseudo random sequence,

It is the pseudo random sequence that receiving terminal extracts.

5. the sign synchronization of mobile OFDM water sound communication signal according to claim 1 and Doppler Compensation Method, it is characterized in that: doppler searching was divided into for two steps in the described step (4), with large step search, refer to the half step distance search again: interval at doppler searching first

In, first with large step search, search makes cost function Xcr (t _i, β _i) reach maximum β _c, again with β _cCentered by, carry out fine searching with half step distance, estimated accurately at last

6. the sign synchronization of mobile OFDM water sound communication signal according to claim 1 and Doppler Compensation Method is characterized in that: in the described step (5) according to the symbol initial time that obtains in the step (4)

Take out a symbol, the method for carrying out the Chirp-Z conversion is: according to initial time

Getting length is

Signal, T _OfdmBe the emission symbol lengths, carry out the Chirp-Z conversion, initial frequency and frequency resolution are respectively:

$f_0=f_{\mathrm{low}}*(1+\widehat{\mathrm{\&beta;}})$

${\mathrm{\&phi;}}_0=\frac{B}{N}*(1+\widehat{\mathrm{\&beta;}})$

Wherein, f _LowBe the frequency of the minimum subcarrier of ofdm signal, B is the ofdm signal bandwidth.

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