CN106100692A - MIMO OFDM underwater sound communication system doppler spread method of estimation - Google Patents

Abstract

The invention discloses a kind of doppler spread method of estimation being applicable to MIMO OFDM mobile underwater sound communication system, send two sections of training sequences repeated at transmitting terminal, receiving terminal uses the docking collection of letters number of multiple parallel correlators to carry out time delay auto-correlation computation.The length of window of the correlator according to maximum output result carries out doppler spread factor estimation.A kind of training sequence structure being applicable to the program that the present invention provides, the diversity gain that MIMO technology brings can be made full use of, compared with traditional method based on linear FM signal estimating Doppler spreading factor, it is remarkably improved doppler spread factor estimated accuracy.

Description

MIMO-OFDM underwater sound communication system doppler spread method of estimation

Technical field

The present invention relates to field of underwater acoustic communication, particularly relate to Doppler in a kind of MIMO-OFDM mobile underwater sound communication system Spread estimation method.

Background technology

Bandwidth and the availability of frequency spectrum are two key factors affecting communication system information transfer rate, and MIMO technology can Launched by many antennas simultaneously and receive signal and significantly improve the availability of frequency spectrum, thus in the underwater sound communication system of Bandwidth-Constrained System obtains pay close attention to widely and study, in single carrier transmission and OFDM multi-carrier transmission, all introduced MIMO technology.

In digital communication, the most in an ofdm system, the Frequency Synchronization of sending and receiving end is the problem that a comparison is crucial, The accuracy that frequency deviation is estimated influences whether the receiving terminal process to the demodulation coding etc. of signal.Due to ofdm signal be modulated at many The form of the signal sum in individual orthogonal sub-carriers, it is indefinite that the superposition of sub-carrier signal can make ofdm signal envelope rise and fall, and Many higher peak values inevitably occur, thus brings bigger peak power and average power ratio (Peak-to- Average Power Ratio,PAPR).In the Doppler frequency offset estimation method of MIMO-OFDM mobile underwater sound communication system, For reducing the PAPR impact on ofdm signal, the present invention proposes a kind of training sequence being applicable to this system, and this sequence has low PAPR and high autocorrelation performance, it is adaptable to the doppler spread factor in underwater sound communication system is estimated.

Underwater acoustic channel be multi-path jamming serious time, frequently, space-variant channel, its complexity and polytropy limit underwater sound communication Performance, particularly mobile underwater sound communication system, owing to sound wave spread speed in the seawater is only 1500m/s, sending and receiving end Mobile cause signal of communication extension in time or compression much larger than radio communication.Therefore, for mobile underwater sound communication system, Must first estimate the doppler spread factor, then receive signal resampling according to the factor pair estimated, eliminate Doppler The effect impact on signal.Existing doppler spread algorithm for estimating is used mostly linear frequency modulation (Linearly Frequency Modulated, LFM) signal is as front and back's synchronizing signal, and receiving terminal utilizes known sequence to enter with receiving signal Row computing cross-correlation, according to peak intervals and the mathematic interpolation doppler spread at actual signal interval of front and back's synchronizing signal output The factor.There are two main shortcomings in this method of estimation: receiver needs to cache before and after all of reception data could calculate The peak value of synchronizing signal, is unfavorable for real time signal processing；Computing cross-correlation is made with locally known signal owing to receiving signal, it is impossible to The impact of the signal frequency shift that elimination causes due to sending and receiving end crystal oscillator frequency difference etc..

The present invention proposes to use the HT-LTF part of 40MHz mixed model lead code in IEEE802.11n agreement as instruction Practicing sequence, this sequence comprises two sections of identical OFDM symbol.Receiving terminal uses the docking collection of letters number of multiple parallel correlators to prolong Time auto-correlation processing, carry out doppler spread factor estimation according to the length of window of correlator of maximum output result.The program In terms of real time signal processing and elimination sending and receiving end fix frequency deviation, have the advantage that the sequential structure of repetition is positioned at Frame front end, It is not required to cache whole frame data can carry out synchronizing and Doppler's estimation etc.；Two sections of repetitive sequences are by the fixing frequency in identical sending and receiving end Impact, takes conjugation and can eliminate fixing frequency deviation during auto-correlation computation partially.

Summary of the invention

Technical problem: for the problem that Doppler's estimation scheme estimated accuracy existing in underwater sound communication system is relatively low, this Invention provides the doppler spread method of estimation of a kind of MIMO-OFDM mobile underwater sound communication system, makes full use of training sequence good The diversity gain that good autocorrelation performance and MIMO technology bring, can obtain higher doppler spread factor estimated accuracy.

Technical scheme: for achieving the above object, a kind of MIMO-OFDM mobile underwater sound communication system that the present invention uses is many Doppler spread method of estimation, introduces MIMO-OFDM skill in the underwater sound communication system that, multi-path jamming limited in channel width is serious Art, and a kind of training sequence structure being applicable to this system is proposed.Described scheme comprises the steps:

1) inserting two sections of training sequences repeated before sending Frame, different launching uses unequal length on antenna Cyclic shift, to prevent beam shaping effect；

2) signal is through time-varying Multipath Time Delay Channels；

3) receiving terminal uses the docking collection of letters number of multiple parallel correlators to carry out time delay auto-correlation computation；

4) doppler spread factor estimation is carried out according to the length of window of the correlator of maximum output result.

Described step 1) in, the training sequence inserted before Frame uses 40MHz mixing in IEEE802.11n agreement Two sections of identical OFDM symbol that the HT-LTF part of pattern lead code obtains through inverse Fourier transform.

Described step 1) in, the different methods launching the cyclic shift using unequal length on antenna is: to a code OFDM symbol in unit's cycleAs 0≤t≤T+T_csTime, useReplaceAnd work as T+T_csDuring≤t≤T, useReplaceWherein, T is an OFDM symbol persistent period, T_csIt is the length of cyclic shift.

Described step 2) in, time-varying Multipath Time Delay Channels is: impulse response function can be expressed as:

$h(\mathrm{\τ},t)=\underset{p}{\mathrm{\Σ}}{A}_p\left(t\right)\mathrm{\δ}(\mathrm{\τ}-{\mathrm{\τ}}_p(t\left)\right)$

Wherein, subscript p represents multipath number, A_pT () is path gain, τ_pT () is the path delay of time, it is assumed that all paths are many Doppler spread factor a is identical, the path delay of time τ_p(t), path gain A_pT () keeps constant in a frame symbol duration, i.e. remember For τ_p,A_p。

Described step 3) in, receiving terminal uses the docking collection of letters number of multiple parallel correlators to carry out a side for time delay auto-correlation computation Method is:

$C_j\left(n\right)=\underset{k=0}{\overset{K_l-1}{\mathrm{\Σ}}}{y}^j(n+k)\·{\left(y^j\right)}^{*}(n+k+K_l)$

Wherein subscript j represents jth root reception antenna, K_lRepresenting the length of window of correlator, y represents reception signal, and n represents N-th sampled point.Energy function P is:

$P_j\left(n\right)=\frac{1}{2}\underset{k=0}{\overset{K_l-1}{\mathrm{\Σ}}}\[y^j(n+k)\·{\left(y^j\right)}^{*}(n+k)+y^j(n+k+K_l)\·{\left(y^j\right)}^{*}(n+k+K_l)\]$

Described step 4) in, the decision method of maximum output result is:

$M_n=\underset{j=1}{\overset{Nr}{\mathrm{\Σ}}}\left|C_j\right(n){|}^2/\underset{j=1}{\overset{Nr}{\mathrm{\Σ}}}{P}_j^2\left(n\right)$

N_rFor reception antenna number, M_nExport result for correlator, choose maximum therein, be designated as M_max, P_jN () is energy Function.

Described step 4) in, the method carrying out doppler spread factor estimation is: take the window of the maximum judgment variables of output Length valueCan calculate the doppler spread factor is:

$\hat{K}=\frac{K_F}{(1+\hat{a})}\⇒\hat{a}=\frac{K_F-\hat{K}}{\hat{K}}$

Wherein, K_FFor sending the physical length of training sequence,For the estimated value of the doppler spread factor, make in sea water The velocity of sound is c, and accordingly, the speed of related movement that can obtain transceiver is:

$\hat{v}=c\hat{a}$

Beneficial effect: the doppler spread factor of a kind of MIMO-OFDM mobile underwater sound communication system that the present invention provides is estimated Meter method, uses time delay auto-correlation algorithm to carry out doppler spread factor estimation at receiving terminal, it is not necessary to cache whole frame data i.e. Doppler spread factor estimation can be carried out, it is simple to real-time signal processing；And owing to two sections of repetitive sequences are by identical receipts Fixing frequency deviation of making a start affects, and takes conjugation and can eliminate sending and receiving end and fix the impact that frequency deviation is brought during auto-correlation computation.The present invention is carried The scheme gone out remains to more accurate carry out doppler spread factor estimation under time-varying Multipath Time Delay Channels, it is possible to fully profit Obtain diversity gain with MIMO technology, improve estimated accuracy.

Accompanying drawing explanation

Fig. 1 is the training sequence domain symbol structure chart that the present invention proposes to use；

Fig. 2 is the data frame structure figure sending signal in the present invention；

Fig. 3 is the multiple parallel correlator schematic diagram of the receiving terminal in the present invention；

Fig. 4 is doppler spread factor a=0.005, and in the case of multipath number path=3, estimation difference is with the change of signal to noise ratio The simulation curve changed and change, wherein, estimation difference refers to the deviation between estimated value and the actual value of the doppler spread factor.

Fig. 5 is doppler spread factor a=0.005, and in the case of multipath number path=7, estimation difference is with the change of signal to noise ratio The simulation curve changed and change.

Fig. 6 is doppler spread factor a=0.009, and in the case of multipath number path=3, estimation difference is with the change of signal to noise ratio The simulation curve changed and change.

Fig. 7 is doppler spread factor a=0.009, and in the case of multipath number path=7, estimation difference is with the change of signal to noise ratio The simulation curve changed and change.

Detailed description of the invention

Below in conjunction with the accompanying drawings the present invention is further described.

If it is N that MIMO-OFDM system launches antenna number_t, reception antenna number is N_r, signal uses based on Cyclic Prefix The OFDM modulation system of (Cyclic Prefix, CP), to prevent intersymbol interference, making B is channel width, and K is subcarrier number, Then subcarrier spacing is Δ f=B/K, and an OFDM symbol persistent period is T=1/ Δ f=K/B, the circulation of each OFDM symbol The a length of T of prefix time_g。

As shown in Figure 1, 2,40MHz during the frequency-domain structure of training sequence of the present invention is IEEE802.11n agreement The HT-LTF part of mixed model lead code.Training sequence comprises two sections of identical OFDM symbol, make s=[s [0], s [1] ..., s[K-1]]^TRepresent the frequency domain sequence of an OFDM symbol, then the base band training sequence of m frame data can be write as:

$x\left(t\right)=\underset{k=0}{\overset{K-1}{\Σ}}s\[k\]e^{j2\mathrm{\π}\frac{k}{T}t},t\∈\[t_m,t_m+2T\]$

Wherein, K is subcarrier number, t_mBeing m frame signal initial time, T is the persistent period of an OFDM symbol.Phase Answer, be f through frequency_cCarrier wave up-conversion, the bandpass signal obtained is:

$\tilde{x}\left(t\right)=\mathrm{Re}\{\underset{k=0}{\overset{K-1}{\mathrm{\Σ}}}s\[k\]e^{j2\mathrm{\π}(\frac{k}{T}+f_c)t}\},t\∈\[t_m,t_m+2T\]$

In mimo systems, in order to eliminate undesirable beam shaping effect, single spatial flow is released from different antennae Dependency between the signal propagated, launches the transmission sequence on antenna to difference and carries out the cyclic shift (T of unequal length_CS), right OFDM symbol in one code-element period $\tilde{x}\left(t\right),$ As 0≤t≤T+T_csTime, useReplaceAnd work as T+T_cs≤t≤T Time, useReplaceT is an OFDM symbol persistent period, T_csIt is the length of cyclic shift.Then send out for i-th The m frame signal penetrating antenna transmitting is:

${{\tilde{x}}_{cs}}^i(t;{T_{cs}}^i)=\left\{\begin{array}{c}\mathrm{Re}\left\{\underset{k=0}{\overset{K-1}{\mathrm{\Σ}}}s\[k\]e^{j2\mathrm{\π}\left(\frac{k}{T}+f_c\right)\left(t-T_{cs}^i\right)}\right\},t_m\≤t\≤t_m+2T+T_{cs}^i\\ \mathrm{Re}\left\{\underset{k=0}{\overset{K-1}{\mathrm{\Σ}}}s\[k\]e^{j2\mathrm{\π}\left(\frac{k}{T}+f_c\right)\left(t-T_{cs}^i-T\right)}\right\},t_m+2T+T_{cs}^i\≤t\≤t_m+2T\end{array}\right.$

The impulse response function of time-varying multidiameter delay underwater acoustic channel can be expressed as:

$h(\mathrm{\τ},t)=\underset{p}{\mathrm{\Σ}}{A}_p\left(t\right)\mathrm{\δ}(\mathrm{\τ}-{\mathrm{\τ}}_p(t\left)\right)$

Wherein, A_pT () is path gain, τ_pT () is the path delay of time, it is assumed that:

1) doppler spread factor a in all paths is identical, it may be assumed that

τ_p(t)≈τ_p-at

2) τ in the path delay of time_p(t), path gain A_pT () keeps constant in a frame symbol duration, be i.e. designated as τ_p,A_p。 Signal reception signal on above-mentioned channel, receiving terminal jth root reception antenna can be expressed as:

${\tilde{y}}^j\left(t\right)=\underset{i=1}{\overset{N_t}{\mathrm{\Σ}}}\left(\mathrm{Re}\left\{\underset{k=0}{\overset{K-1}{\mathrm{\Σ}}}s\[k\]H_k{e}^{j2\mathrm{\π}\left(\frac{k}{T}+f_c\right)\left(1+a\right)t}\right\}+\tilde{w}\left(t\right)\right),t\∈\[t_m,t_m+\frac{2T}{\left(1+a\right)}\]$

Wherein, $\tilde{w}\left(t\right)$ It is additive white Gaussian noise, channel transfer function H_kDefinition be:

$H_k=\left\{\begin{array}{c}\underset{p}{\Σ}{A}_p{e}^{-j2\mathrm{\π}(\frac{k}{T}+f_c)({\mathrm{\τ}}_p+T_{cs}^i)},t_m\≤t\≤t_m+\frac{2T+T_{cs}^i}{(1+a)}\\ \underset{p}{\Σ}{A}_p{e}^{-j2\mathrm{\π}(\frac{k}{T}+f_c)({\mathrm{\τ}}_p+T_{cs}^i+T)},t_m+\frac{2T+T_{cs}^i}{(1+a)}\≤t\≤t_m+\frac{2T}{(1+a)}\end{array}\right.$

Reception signal is down-converted to baseband signal is:

$y^j\left(t\right)=e^{j\mathrm{\Ω}t}\underset{i=1}{\overset{N_t}{\mathrm{\Σ}}}\left(\underset{k=0}{\overset{K-1}{\mathrm{\Σ}}}s\[k\]H_k{e}^{j2\mathrm{\π}\frac{k}{T}\left(1+a\right)t}+w\left(t\right)\right),t\∈\[t_m,t_m+\frac{2T}{\left(1+a\right)}\]$

W (t) is baseband Gaussian white noise.Ω=2 π af_cBe carrier wave frequency deviation (carrier frequency offset, CFO).It can be seen that Doppler effect causes 2 impacts to transmission signal from formula:

1) cause signal extension in time or compression, i.e. persistent period become 2T/ (1+a) from 2T.

2) Doppler effect causes each subcarrier there occursFrequency shift (FS), and due to underwater acoustic channel Bandwidth is more or less the same with carrier frequency, and the frequency shift (FS) of different subcarriers is different, so at can not be as narrow band signal Reason.Therefore before carrying out OFDM demodulation, signal have to be carried out Doppler frequency deviation compensation, to prevent inter-sub-carrier interference (Inter-Carrier Interference, ICI).

Sample at receiving terminal, then can obtain baseband digital signal is:

$y^j\left(n\right)=y^j\left(t\right){|}_{t=t_m+n{T}_s}$

Wherein K_FFor sending the physical length of training sequence,It is the sampling interval,It it is over-sampling rate.

As it is shown on figure 3, the present invention uses the method using multiple parallel correlators to process signal at receiving terminal, real The estimation of the existing doppler spread factor.Specifically, the correlation window of each correlator takes different length, and the docking collection of letters number is entered Line delay auto-correlation computation, if the length of window of correlator is K_l, utilize two sections of repetitive sequences in reception signal to make auto-correlation, Arrive:

$C_j\left(n\right)=\underset{k=0}{\overset{K_l-1}{\mathrm{\Σ}}}{y}^j(n+k)\·{\left(y^j\right)}^{*}(n+k+K_l)$

Wherein subscript j represents jth root reception antenna, and y represents reception signal, and n represents the n-th sampled point.Energy function P counts In calculation correlation window, whole energy of sequence, can suppress effect of noise, reduce and fluctuate:

$P_j\left(n\right)=\frac{1}{2}\underset{k=0}{\overset{K_l-1}{\mathrm{\Σ}}}\[y^j(n+k)\·{\left(y^j\right)}^{*}(n+k)+y^j(n+k+K_l)\·{\left(y^j\right)}^{*}(n+k+K_l)\]$

Judgment variables is:

$M_n=\underset{j=1}{\overset{Nr}{\mathrm{\Σ}}}\left|C_j\right(n){|}^2/\underset{j=1}{\overset{Nr}{\mathrm{\Σ}}}{P}_j^2\left(n\right)$

Then, all correlators output M is chosen_nIn maximum, be designated as M_max, work as M_maxExceed the decision threshold of setting Γ_thTime, i.e. think and have signal frame to arrive:

H₁if:M_max＞ Γ_th

When the output of all correlators is all higher than Γ_thTime, threshold value need to be again chosen according to channel conditions.

Take the length of window value of the maximum judgment variables of outputThen think that this value with the signal length by Doppler contribution is Immediate, so, can calculate the doppler spread factor is:

$\hat{K}=\frac{K_F}{(1+\hat{a})}\⇒\hat{a}=\frac{K_F-\hat{K}}{\hat{K}}$

K_FFor sending the physical length of training sequence,For the estimated value of the doppler spread factor, accordingly, can receive and dispatch The speed of related movement of machine is:

$\hat{v}=c\hat{a}$

Wherein, c is sound wave spread speed in the seawater.

Receiver utilization estimatesThe docking collection of letters number carries out resampling, can eliminate Doppler effect to transmission signal Impact.

In the case of Fig. 4 Fig. 7 gives the different doppler spread factor and different channels multipath number, estimation difference with The change of signal to noise ratio and the simulation curve that changes.Visible, the performance of the present invention is all substantially better than tradition in all of the embodiments illustrated Linear frequency modulation method, along with increase and the increase of the doppler spread factor of multipath number, the estimation of linear FM signal is by mistake Difference all dramatically increases, and the training sequence structure that the present invention is suggested plans is due to the existence of Cyclic Prefix, can be effective against many Footpath is disturbed, and estimates that the stability of performance is relatively strong, and the increase of multipath number and the increase of the doppler spread factor are to estimation difference shadow Ring little；And utilize MIMO technology can promote the estimation performance of the doppler spread factor further.

Claims (7)

1. the doppler spread method of estimation being applicable to MIMO-OFDM mobile underwater sound communication system, it is characterised in that: at letter Introducing MIMO-OFDM technology in the underwater sound communication system that road Bandwidth-Constrained, multi-path jamming are serious, employing OFDM symbol as this is The training sequence of system, described method comprises the steps:

1) inserting two sections of training sequences repeated before sending Frame, different launches the circulation using unequal length on antenna Displacement, to prevent beam shaping effect；

2) signal is through time-varying Multipath Time Delay Channels；

3) receiving terminal uses the docking collection of letters number of multiple parallel correlators to carry out time delay auto-correlation computation；

4) doppler spread factor estimation is carried out according to the length of window of the correlator of maximum output result.

The doppler spread method of estimation being applicable to MIMO-OFDM mobile underwater sound communication system the most according to claim 1, It is characterized in that: described step 1) in, the training sequence inserted before Frame uses 40MHz in IEEE802.11n agreement Two sections of identical OFDM symbol that the HT-LTF part of mixed model lead code obtains through inverse Fourier transform.

The doppler spread method of estimation being applicable to MIMO-OFDM mobile underwater sound communication system the most according to claim 1, It is characterized in that: described step 1) in, the different methods launching the cyclic shift using unequal length on antenna is: to one OFDM symbol in code-element periodAs 0≤t≤T+T_csTime, useReplaceAnd work as T+T_csDuring≤t≤T, useReplaceWherein, T is an OFDM symbol persistent period, T_csIt is the length of cyclic shift.

It is applicable to the doppler spread method of estimation of MIMO-OFDM mobile underwater sound communication system the most according to claim 1, its It is characterised by: described step 2) in, time-varying Multipath Time Delay Channels is: impulse response function is expressed as:

$h(\mathrm{\τ},t)=\underset{p}{\Σ}{A}_p\left(t\right)\mathrm{\δ}(\mathrm{\τ}-{\mathrm{\τ}}_p(t\left)\right)$

Wherein, subscript p represents multipath number, A_pT () is path gain, τ_pT () is the path delay of time, it is assumed that the Doppler in all paths Spreading factor is identical, the path delay of time τ_p(t), path gain A_pT () keeps constant in a frame symbol duration, be designated as τ_p, A_p。

It is applicable to the doppler spread method of estimation of MIMO-OFDM mobile underwater sound communication system the most according to claim 1, its It is characterised by: described step 3) in, receiving terminal uses the docking collection of letters number of multiple parallel correlators to carry out time delay auto-correlation computation Method is:

$C_j\left(n\right)=\underset{k=0}{\overset{K_l-1}{\Σ}}{y}^j(n+k)\·{\left(y^j\right)}^{*}(n+k+K_l)$

Wherein j represents jth root reception antenna, K_lRepresenting the length of window of correlator, y represents reception signal, and n represents the n-th sampling Point；Energy function P is:

$P_j\left(n\right)=\frac{1}{2}\underset{k=0}{\overset{K_l-1}{\Σ}}\[y^j(n+k)\·{\left(y^j\right)}^{*}(n+k)+y^j(n+k+K_l)\·{\left(y^j\right)}^{*}(n+k+K_l)\].$

It is applicable to the doppler spread method of estimation of MIMO-OFDM mobile underwater sound communication system the most according to claim 1, its It is characterised by: described step 4) in, the decision method of maximum output result is:

$M_n=\underset{j=1}{\overset{Nr}{\Σ}}|C_j\left(n\right){|}^2/\underset{j=1}{\overset{Nr}{\Σ}}{P}_j^2\left(n\right)$

N_rFor reception antenna number, M_nExport result for correlator, choose maximum therein, be designated as M_max, P_jN () is energy letter Number.

It is applicable to the doppler spread method of estimation of MIMO-OFDM mobile underwater sound communication system the most according to claim 1, its It is characterised by: described step 4) in, the method carrying out doppler spread factor estimation is: take the window of the maximum judgment variables of output Length valueCalculating the doppler spread factor is:

$\hat{K}=\frac{K_F}{(1+\hat{a})}\⇒\hat{a}=\frac{K_F-\hat{K}}{\hat{K}}$

Wherein, K_FFor sending the physical length of training sequence,Estimated value for the doppler spread factor.