CN109450489B - Pilot sequence interference cancellation method for spread spectrum underwater acoustic communication - Google Patents

Pilot sequence interference cancellation method for spread spectrum underwater acoustic communication Download PDF

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CN109450489B
CN109450489B CN201811004032.3A CN201811004032A CN109450489B CN 109450489 B CN109450489 B CN 109450489B CN 201811004032 A CN201811004032 A CN 201811004032A CN 109450489 B CN109450489 B CN 109450489B
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pilot
interference cancellation
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pilot sequence
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熊省军
杜鹏宇
朱小辉
谢哲
王超
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715th Research Institute of CSIC
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7097—Interference-related aspects
    • H04B1/711—Interference-related aspects the interference being multi-path interference
    • H04B1/7115—Constructive combining of multi-path signals, i.e. RAKE receivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B11/00—Transmission systems employing sonic, ultrasonic or infrasonic waves
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B13/00—Transmission systems characterised by the medium used for transmission, not provided for in groups H04B3/00 - H04B11/00
    • H04B13/02—Transmission systems in which the medium consists of the earth or a large mass of water thereon, e.g. earth telegraphy

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Abstract

The invention discloses a pilot sequence interference cancellation method for spread spectrum underwater acoustic communication, which aims at the problem that the cross-correlation value of a pilot sequence and an information sequence in the spread spectrum underwater acoustic communication is not 0, and the interference of the pilot sequence is introduced by the related despreading processing of the information sequence, and provides the pilot sequence interference cancellation method based on sequential Least Squares (LS). And time synchronization and carrier synchronization of a pilot sequence are adopted, then pilot sequence interference cancellation of the LS is carried out sequentially, and finally output after pilot sequence interference cancellation processing is carried out. The invention has the beneficial effects that: the method does not need matrix inversion operation, has low complexity, can adapt to underwater acoustic channels with serious multipath, and has good practicability.

Description

Pilot sequence interference cancellation method for spread spectrum underwater acoustic communication
Technical Field
The invention relates to the technical field of underwater acoustic communication, in particular to a pilot frequency sequence interference cancellation method for spread spectrum underwater acoustic communication.
Background
The underwater acoustic channel has serious multipath interference, the multipath expansion in shallow sea environment is dozens of milliseconds to hundreds of milliseconds, the multipath expansion in deep sea environment is up to several seconds, the serious multipath interference brings great challenges to the design of an equalizer, and the communication technology relying on channel equalization is difficult to ensure the communication robustness under the complex multipath condition. The spread spectrum underwater acoustic communication utilizes the good orthogonality of the pseudo-random sequence to effectively reduce the multipath interference, the spread spectrum communication usually adopts a RAKE receiver to realize multipath diversity combination, a complex equalizer is avoided, the multipath interference is changed into valuable, the multipath diversity gain is obtained, the self-processing gain of the pseudo-random spread spectrum sequence is combined, the anti-interference capability of the spread spectrum underwater acoustic communication is effectively improved, the spread spectrum technology has good confidentiality, the spread spectrum technology is widely applied to the underwater acoustic communication, and the communication technology becomes the preferred communication technology of naval underwater acoustic communication equipment.
The RAKE receiver is one of the core technologies in spread spectrum underwater acoustic communication, and realizes the measurement of parameters such as multipath delay, strength and the like of a channel by matching and filtering a pilot frequency sequence (also called a measurement sequence), designs a linear combined weighting network by using the measurement result of the multipath parameters of the channel, and separates and combines the related output of information sequences to realize multipath diversity combination. However, in practice, it is difficult to ensure strict orthogonality between the pilot sequence and the information sequence, and even in the case of a pseudo-random sequence of the same family (such as Gold sequence or Kasami sequence), the cross correlation value between the two is not 0, and especially when the sequence length is short, the interference of the pilot sequence on the information sequence is not negligible. In a spread spectrum underwater acoustic communication system of code modulation, the lowest communicable signal-to-noise ratio of spread spectrum communication is further reduced by channel coding gain, and in order to improve the channel multipath parameter measurement accuracy of a pilot sequence under the condition of low signal-to-noise ratio, the length of the pilot sequence is often designed to be several times of the length of an information sequence, so that different families of pseudo-random sequences are caused, and the interference of the pilot sequence on the information sequence is further increased.
Disclosure of Invention
The invention aims to overcome the defects of the prior art, and provides a pilot sequence interference cancellation method for spread spectrum underwater acoustic communication, which is applied to development of underwater acoustic positioning communication nodes and low-frequency remote underwater acoustic communication sonar.
The object of the present invention is achieved by the following technical means.
The invention provides a pilot sequence interference cancellation method based on sequential Least Square (LS) aiming at the cross-correlation value of a pilot sequence and an information sequence in spread spectrum underwater acoustic communication being not 0 and the interference of the pilot sequence introduced by the related despreading processing of the information sequence.
The invention mainly comprises the following steps:
(1) time synchronization and carrier synchronization of pilot sequences
Before pilot sequence interference cancellation, time synchronization and carrier synchronization are required to be performed on spread spectrum symbols, so that a received sequence and a locally copied pilot sequence have the same frequency and phase. After coarse synchronization of a receiving sequence at a receiving end of spread spectrum underwater acoustic communication, due to Doppler stretching effect caused by clock deviation and relative motion of a communication receiving end and a communication transmitting end, it is necessary to further accurately synchronize spread spectrum symbol time and carrier along with time accumulation so as to finish time synchronization deviation and Doppler correction. The pilot frequency sequence generally selects a pseudo-random sequence, has a pin-shaped time-frequency fuzzy characteristic, adopts a receiving sequence and a plurality of pilot frequency sequence copies of different Doppler spread factors for matched filtering processing, the position of the peak value of a time-frequency fuzzy function on a time axis is a synchronous moment, and the position of the peak value on a frequency axis is Doppler estimation; and intercepting the spread spectrum symbol corresponding to the synchronization time, and completing the carrier synchronization of the spread spectrum symbol by interpolation resampling processing by utilizing the estimated value of the Doppler spread factor.
(2) Pilot sequence interference cancellation for sequential LS
The cross correlation value of the pilot sequence and the information sequence is not 0, interference is introduced to the information sequence despreading, when the length of the pilot sequence and the information sequence is short or the pilot sequence and the information sequence are in different families, the interference of the pilot sequence to the information sequence despreading is not negligible, and pilot sequence interference cancellation and suppression processing is required. Through the step 1, a receiving sequence with the same frequency and phase as the pilot sequence of the local copy is obtained, but because the receiving sequence is the result of the coherent superposition of the multipath signals, the interference of the pilot sequence cannot be directly eliminated by subtraction, and therefore the pilot sequence of the local copy needs to be matched with the receiving sequence through filtering. The invention designs a filter H based on LS criterionn(z) tap coefficients such that each time instant
Figure GDA0002610076960000021
Formula (1), wherein u [ n ]]For locally copied pilot sequences, L is the length of the pilot sequence, hn[l]For the filter's l-th tap coefficient at time n, equation (1) adds a "forgetting factor" λ for the adaptive channel impulse response change. By minimizing J (n), one can obtain
Figure GDA0002610076960000022
The solution of (1).
Figure GDA0002610076960000023
The invention adopts sequential LS to estimate in formula (1)
Figure GDA0002610076960000024
Order to
Figure GDA0002610076960000025
u[n]＝[u[n]u[n-1]…u[n-p+1]]H，[·]HFor the conjugate transpose operation, the phase shift is performed by the quadrature principle,
Figure GDA0002610076960000026
in the formula (2)
Figure GDA0002610076960000027
Figure GDA0002610076960000028
Σ[n]＝(1-K[n]hH[n])Σ[n-1] (5)
V [ n ] in formula (3) is the received sequence after precise time and carrier synchronization, e [ n ] is the output of subtracting the filtering result of the locally copied pilot sequence from v [ n ], i.e. the output after the interference cancellation processing of the pilot sequence. K [ n ] in the formula (4) is a gain factor, and Σ [ n ] in the formula (5) is a p × p dimensional covariance matrix, and the matrix inversion operation is avoided by adopting sequential recursion calculation, so that the method has strong practicability.
The invention has the beneficial effects that: aiming at the problem that the pilot frequency sequence and the information sequence in the spread spectrum underwater acoustic communication are difficult to ensure strict orthogonality, so that the information sequence de-spreads interference containing the pilot frequency sequence, the invention provides a pilot frequency sequence interference offset method based on sequential LS, matrix inversion operation is not needed, the filter coefficient is updated in a self-adaptive manner, and the method can adapt to an underwater acoustic channel with serious multipath interference and has better practicability.
Drawings
Fig. 1 is a spreading symbol structure.
Fig. 2 is a diagram of time synchronization and carrier synchronization of a pilot sequence.
Fig. 3 is a schematic block diagram of adaptive pilot sequence interference cancellation.
Fig. 4 is a block diagram of filter coefficient estimation for sequential LS.
Detailed Description
The invention will be described in detail below with reference to the following drawings:
FIG. 1 shows a common spreading symbol structure, where N ≧ 1, and the spread signal generated by FIG. 1 can be represented as
Figure GDA0002610076960000031
In formula (1), u [ n ]]Is the nth chip value of the pilot sequence, L is the length of the pilot sequence, qm[n]The nth chip value of the mth information sequence, L' being the length of the information sequence,
Figure GDA0002610076960000032
t is the time length of the chip, g (T) is the pulse shaping function of the chip, and if it is a rectangular window, it can be expressed as
Figure GDA0002610076960000033
FIG. 2 is a diagram of time synchronization and carrier synchronization of a pilot sequence, where a received signal r (n) is mixed with a carrier, as shown in equation (3), fcIs a carrier frequency, fsAnd (3) performing low-pass filtering on x (n) for the sampling frequency, as shown in formula (4), to obtain a complex envelope signal y (n), wherein b (n) is a low-pass filter coefficient, and then performing Q-time extraction on y (n) to obtain a baseband receiving sequence z (n), as shown in formula (5).
x(n)＝r(n)exp(-j2πfcn/fs) (3)
Figure GDA0002610076960000034
z(n)＝y(m)(m-Qn)n,m＝0,1,2,… (5)
The pilot sequence is generally selected from pseudo-random sequence with spike-shaped time-frequency fuzzy characteristic, and the receiving end prestores a plurality of pilot sequence frequency-domain copies S of different Doppler spread factorsiPerforming multi-channel matched filtering processing with the base band pilot frequency sequence, wherein the formula (6) is the ith channel matched filtering output, wherein (·)*For conjugate operation, the arrival time of pilot sequence is estimated by finding the position and channel number corresponding to the maximum peak output by matched filtering
Figure GDA0002610076960000041
And doppler spread factor
Figure GDA0002610076960000042
Intercepting synchronization moments
Figure GDA0002610076960000043
Corresponding baseband pilot sequence, using estimated values of Doppler spread factor
Figure GDA0002610076960000044
And (4) carrying out interpolation resampling treatment to obtain a spread spectrum sequence v (n) after Doppler correction.
Ri＝ifft(Si·(fft(v))*) (6)
After the time synchronization and the carrier synchronization as shown in fig. 2, a receiving sequence v (n) with the same frequency and phase as the pilot sequence copied locally is obtained, but because of the multipath effect of the underwater acoustic channel, v (n) is the result of multipath coherent superposition of a plurality of spread spectrum sequences, the receiving sequence v (n) cannot be directly subtracted from the pilot sequence u (n) copied locally to eliminate the interference of u (n), and considering that the impact response of the underwater acoustic channel changes along with time, the invention adopts the self-adaptive interference cancellation technology, as shown in fig. 3, by designing a self-adaptive filter, u (n) is matched with v (n) after being subjected to the self-adaptive filtering processing, and then is subtracted, so that the interference of u (n) can be cancelled.
The invention adopts a sequential LS estimation method to estimate the filter H by minimizing the cost function of the formula (7)nTap coefficient of (z)
Figure GDA0002610076960000045
So that each moment of time
Figure GDA0002610076960000046
In order to adapt to the change of channel impulse response, the formula (1) introduces a forgetting factor lambda, 0<λ<1 to adjust down the weight of the previous error in j (n).
Figure GDA0002610076960000047
FIG. 4 is a block diagram of filter coefficient estimation for sequential LS, with sequential LSE of the filter coefficients represented as
Figure GDA0002610076960000048
By the principle of orthogonality,
Figure GDA0002610076960000049
in the formula (8), e [ n ] is the innovation data of the estimator, and is also the output of subtracting the filtering result of the pilot frequency sequence of the local copy from v [ n ], namely the output after the interference cancellation processing of the pilot frequency sequence. K [ n ] is a gain factor, is a vector of p × 1, as formula (10), u [ n ] is a locally copied pilot frequency sequence of p × 1 dimension, as formula (11), and Σ [ n ] is a covariance matrix of p × p dimension, as formula (12), because of the calculation in a sequential recursion mode, matrix inversion operation is avoided, and the method has strong practicability. After the interference is offset, the number of error packets and the error rate are both improved, and the effectiveness of the method is proved.
Figure GDA00026100769600000410
Figure GDA00026100769600000411
u[n]＝[u[n]u[n-1]…u[n-p+1]]H (11)
Σ[n]＝(1-K[n]hH[n])Σ[n-1] (12)
It should be understood that equivalent substitutions and changes to the technical solution and the inventive concept of the present invention should be made by those skilled in the art to the protection scope of the appended claims.

Claims (2)

1. A pilot frequency sequence interference cancellation method for spread spectrum underwater acoustic communication is characterized in that: the method comprises the following steps:
(1) before pilot frequency sequence interference cancellation, time synchronization and carrier synchronization are carried out on spread spectrum symbols, so that a receiving sequence and a locally copied pilot frequency sequence have the same frequency and phase;
(2) pilot sequence interference cancellation for sequential LS: obtaining a receiving sequence with the same frequency and phase as the pilot frequency sequence copied locally through the step (1); designing filters
Figure DEST_PATH_IMAGE001
Such that each time instant
Figure DEST_PATH_IMAGE002
Of formula (1), wherein
Figure DEST_PATH_IMAGE003
Is a locally copied pilot sequence, L is the length of the pilot sequence,
Figure DEST_PATH_IMAGE004
for the filter at time n
Figure DEST_PATH_IMAGE005
A tap coefficient, and a forgetting factor is added "
Figure DEST_PATH_IMAGE006
By minimization
Figure DEST_PATH_IMAGE007
，
Figure 712094DEST_PATH_IMAGE007
Is referred to as a cost function, is available
Figure DEST_PATH_IMAGE008
The solution of (1);
Figure DEST_PATH_IMAGE009
estimation of in equation (1) using sequential Least Squares (LS) method
Figure DEST_PATH_IMAGE010
Let us order
Figure DEST_PATH_IMAGE011
，
Figure DEST_PATH_IMAGE012
，
Figure DEST_PATH_IMAGE013
For the conjugate transpose operation, the phase shift is performed by the quadrature principle,
Figure DEST_PATH_IMAGE014
(2)
in the formula (2)
Figure DEST_PATH_IMAGE015
(3)
Figure DEST_PATH_IMAGE016
(4)
Figure DEST_PATH_IMAGE017
(5)
Of formula (3)
Figure DEST_PATH_IMAGE018
For the received sequence after precise time and carrier synchronization, of the formula (4)
Figure DEST_PATH_IMAGE019
As a gain factor, of formula (5)
Figure DEST_PATH_IMAGE020
Is composed of
Figure DEST_PATH_IMAGE021
A dimensional covariance matrix is then created,
Figure DEST_PATH_IMAGE022
is composed of
Figure DEST_PATH_IMAGE023
And subtracting the output of the filtering result of the locally copied pilot frequency sequence, namely the output after the interference cancellation processing of the pilot frequency sequence.
2. The method of claim 1, wherein the pilot sequence interference cancellation method comprises: the pilot frequency sequence selects a pseudo-random sequence, has a pin-shaped time-frequency fuzzy characteristic, adopts a receiving sequence and a plurality of pilot frequency sequence copies of different Doppler spread factors for matched filtering processing, the position of the peak value of a time-frequency fuzzy function on a time axis is a synchronous moment, and the position of the peak value on a frequency axis is Doppler estimation; and intercepting the spread spectrum symbol corresponding to the synchronization time, and completing the carrier synchronization of the spread spectrum symbol by interpolation resampling processing by utilizing the estimated value of the Doppler spread factor.
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