Disclosure of Invention
The invention provides a narrow-band signal direction-of-arrival estimation method based on an expectation maximization algorithm, which mainly solves the problem that the estimation performance of subspace algorithms such as MUSIC (multiple signal to noise ratio) is reduced when the signal to noise ratio is low; the multi-dimensional search of the maximum likelihood estimation method is easy to have the problems of error, low estimation resolution and the like.
The technical scheme of the invention is as follows: a narrow-band signal direction-of-arrival estimation method based on an expectation-maximization algorithm comprises the following specific steps:
1) supposing that the receiving antenna array consists of M array element uniform linear arrays, the spacing of the array elements is d, P independent target signal sources from a space far-field narrow band are incident on the equal-spacing uniform linear arrays, and the incident direction of the signals is theta
1,θ
2,…θ
PWherein M is more than or equal to P,
λ is the wavelength of the incident signal;
2) sampling the space signal by an array antenna receiver to obtain a receiving signal Y (t);
3) constructing hidden variables
Which represents the output of the ith signal produced on the array, the output of the P signals on the array can be represented as
4) Construction of an implicit variable yi(t) a log-likelihood function for a parameter θ to be estimated;
5) deducing a narrow-band signal direction of arrival estimation objective function by utilizing an expectation maximization algorithm;
6) searching the target function respectively to obtain the direction of arrival of the information source;
further, an implicit variable y is constructedi(t) a log-likelihood function for a parameter θ to be estimated, said construction method being as follows:
the output produced on the array by the ith signal is:
in the formula, am(θi)=exp[(m-1)dsinθi]Representing the phase delay, theta, of the ith source at the mth element relative to the reference elementiIndicating the incident direction of the ith signal, si(t) denotes the envelope of the i-th signal, ni(t) represents the noise of the ith signal on the array, and the noise is assumed to be white gaussian noise with zero mean;
in the formula, Θ represents signal parameters including parameters such as amplitude and direction of arrival of a signal; p (y)i(t) | Θ) represents the probability density function of the array output under the condition of the signal parameter;
construction of an implicit variable yi(t) log-likelihood function for the parameter θ to be estimated:
∵InL(Θ|yi(t))=Inp(yi(t)|Θ)
wherein In represents logarithmic operation with e as the base, oc represents proportional proportion, and | | represents Frobenius norm of matrix;
further, the direction of arrival of each source is calculated by using an EM algorithm, and the calculation steps are as follows:
E-step:
Q(Θk+1|Θk)=E[InL(Θk+1|yi)|X,Θk]
wherein, E [. C]Expressing the mathematical expectation, and the superscript k expressing the estimated values of the variables, Q (theta), obtained during the k-th iteration
k+1|Θ
k) The condition expectation that is indicative of the signal parameter,
representing the estimated value.
M-step:
Obtaining the output generated by the signal i in k steps of iteration by using E-step solution
Combining the log-likelihood functions of the signal parameters to obtain an iterative formula of signal envelope and direction of arrival:
envelope of the signal:
direction of arrival:
where H represents the conjugate transpose of the array.
The invention has the beneficial effects that: the invention introduces the expectation maximization algorithm into the estimation of the direction of arrival of the narrow-band signal, avoids errors as much as possible, ensures the excellent performance of the estimation of the direction of arrival under the conditions of low signal-to-noise ratio and few fast beats, and has good stability and resolution.
Detailed Description
The invention is further described with reference to the following figures and specific examples.
Example 1: as shown in fig. 1, a method for estimating the direction of arrival of a narrowband signal based on expectation maximization specifically includes the following steps:
step 1: and forming an equidistant and uniform linear array by using the antenna receiver.
1 antenna receiver is placed at each interval distance d, M antenna receivers are placed together to form a uniform linear array, each antenna receiver becomes an array element, P far-field narrow-band signals are supposed to be incident on the uniform linear array, zero-mean Gaussian white noise is added in the signals in the transmission process, wherein M is more than or equal to P,
λ is the wavelength of the incident signal.
Step 2: the spatial signal is sampled to obtain a received signal y (t).
And step 3: constructing hidden variables
Which represents the output of the ith signal produced on the array, the output of the P signals on the array can be represented as
And 4, step 4: implicit variable y in the constructioni(t) a log-likelihood function with respect to the parameter θ to be estimated.
1) The output produced on the array by the ith signal is:
in the formula, am(θi)=exp[(m-1)d sinθi]Indicating the phase delay, theta, of the ith signal at the mth array element relative to the reference array elementiIndicating the incident direction of the ith signal, si(t) denotes the envelope of the i-th signal, ni(t) represents the noise of the ith signal on the array, and the noise is assumed to be white gaussian noise with zero mean;
2) the output of the P sources on the array can be represented as
In the formula, Θ represents a signal parameter and includes parameters p (y) such as the amplitude and the direction of arrival of a signali(t) | Θ) represents the probability density function of the array output under the condition of the signal parameter;
4) construction of an implicit variable yi(t) log-likelihood function for the parameter θ to be estimated:
∵InL(Θ|yi(t))=Inp(yi(t)|Θ)
wherein In represents logarithmic operation with e as the base, oc represents proportional proportion, and | | represents Frobenius norm of matrix;
and 5: and (3) deriving a narrow-band signal direction-of-arrival estimation objective function by using an expectation-maximization algorithm.
1)E-step:
Q(Θk+1|Θk)=E[InL(Θk+1|yi)|X,Θk]
Wherein, E [. C]Expressing the mathematical expectation, and the superscript k expressing the estimated values of the variables, Q (theta), obtained during the k-th iteration
k+1|Θ
k) The condition expectation that is indicative of the signal parameter,
representing the estimated value.
2)M-step:
Obtaining the output generated by the signal i in k steps of iteration by using E-step solution
Combining the log-likelihood functions of the signal parameters to obtain an iterative formula of signal envelope and direction of arrival:
envelope of the signal:
direction of arrival:
where H represents the conjugate transpose of the array.
Step 6: and searching the target function respectively to obtain the direction of arrival of the information source.
The method utilizes expectation-maximization algorithm (EM algorithm) to simplify calculation and construct hidden variable yi(t) and constructing an implicit variable yi(t) a log-likelihood function InL of a parameter theta to be estimated, wherein the InL is a nonlinear function of the parameter theta, and the optimal solution is solved only by one-dimensional search of the theta. The algorithm effectively avoids errors generated by multi-dimensional search of the ML algorithm on the space spectrum of the received signal, and simultaneously can ensure the performance and stability of estimation of the direction of arrival at low signal-to-noise ratio and few fast beats.
Example 2: the calculation was performed according to the method in example 1, wherein a uniform linear array consisting of 8 omnidirectional array elements was considered, the array element spacing was 0.5, the number of sampling points was 100, and the spatial angle search range was [ -90 ° 90 ° ].
The mean square error calculation equation is:
wherein I represents the number of Monte Carlo experiments,
denotes the angle of arrival, θ, of the ith test
pRepresenting the true direction of arrival angle of the signal.
Assuming that three independent information sources respectively enter an equidistant uniform linear array consisting of 8 omnidirectional array elements at an angle of-30 degrees to 60 degrees, the signal-to-noise ratio of the experiment is increased from-10 dB to 10dB, 50 independent direction-of-arrival estimation tests are carried out under each signal-to-noise ratio, the method disclosed by the invention is respectively compared with the direction-of-arrival estimation values obtained by the existing maximum likelihood estimation method and the MUSIC estimation method, the mean square errors of the three methods under different signal-to-noise ratios are respectively calculated, and the test result is shown in figure 2, wherein: in fig. 2, the abscissa represents the signal-to-noise ratio and the ordinate represents the mean square error.
It can be seen from fig. 2 that the estimation errors of the three methods are all reduced along with the increase of the signal-to-noise ratio, but under the conditions of low signal-to-noise ratio and few snapshot numbers, the mean square error of the method is obviously smaller than that of the other two algorithms, thereby illustrating the effectiveness of the method.
In conclusion, the method has excellent estimation performance, good stability and resolution ratio under the conditions of low signal-to-noise ratio and few fast beats.
While the present invention has been described in detail with reference to the embodiments shown in the drawings, the present invention is not limited to the embodiments, and various changes can be made without departing from the spirit of the present invention within the knowledge of those skilled in the art.