Spectral density estimation
In statistical signal processing, the goal of spectral density estimation (SDE) is to estimate the spectral density (also known as the power spectral density) of a random signal from a sequence of time samples of the signal. Intuitively speaking, the spectral density characterizes the frequency content of the signal. One purpose of estimating the spectral density is to detect any periodicities in the data, by observing peaks at the frequencies corresponding to these periodicities.
SDE should be distinguished from the field of frequency estimation, which assumes that a signal is composed of a limited (usually small) number of generating frequencies plus noise and seeks to find the location and intensity of the generated frequencies. SDE makes no assumption on the number of components and seeks to estimate the whole generating spectrum.
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Spectrum analysis, also referred to as frequency domain analysis or spectral density estimation, is the technical process of decomposing a complex signal into simpler parts. As described above, many physical processes are best described as a sum of many individual frequency components. Any process that quantifies the various amounts (e.g. amplitudes, powers, intensities, or phases), versus frequency can be called spectrum analysis.
Spectrum analysis can be performed on the entire signal. Alternatively, a signal can be broken into short segments (sometimes called frames), and spectrum analysis may be applied to these individual segments. Periodic functions (such as ) are particularly well-suited for this sub-division. General mathematical techniques for analyzing non-periodic functions fall into the category of Fourier analysis.
The Fourier transform of a function produces a frequency spectrum which contains all of the information about the original signal, but in a different form. This means that the original function can be completely reconstructed (synthesized) by an inverse Fourier transform. For perfect reconstruction, the spectrum analyzer must preserve both the amplitude and phase of each frequency component. These two pieces of information can be represented as a 2-dimensional vector, as a complex number, or as magnitude (amplitude) and phase in polar coordinates (i.e., as a phasor). A common technique in signal processing is to consider the squared amplitude, or power; in this case the resulting plot is referred to as a power spectrum.
In practice, nearly all software and electronic devices that generate frequency spectra apply a fast Fourier transform (FFT), which is a specific mathematical approximation to the full integral solution. Formally stated, the FFT is a method for computing the discrete Fourier transform of a sampled signal.
Because of reversibility, the Fourier transform is called a representation of the function, in terms of frequency instead of time; thus, it is a frequency domain representation. Linear operations that could be performed in the time domain have counterparts that can often be performed more easily in the frequency domain. Frequency analysis also simplifies the understanding and interpretation of the effects of various time-domain operations, both linear and non-linear. For instance, only non-linear or time-variant operations can create new frequencies in the frequency spectrum.
The Fourier transform of a stochastic (random) waveform (noise) is also random. Some kind of averaging is required in order to create a clear picture of the underlying frequency content (frequency distribution). Typically, the data is divided into time-segments of a chosen duration, and transforms are performed on each one. Then the magnitude or (usually) squared-magnitude components of the transforms are summed into an average transform. This is a very common operation performed on digitally sampled time-domain data, using the discrete Fourier transform. This type of processing is called Welch's method. When the result is flat, it is commonly referred to as white noise. However, such processing techniques often reveal spectral content even among data which appears noisy in the time domain.
Suppose that a signal is sampled at different times, with the samples uniformly spaced by , giving values . Since the power spectral density of a continuous function defined on the entire real line is the modulus squared of its Fourier transform, the simplest technique is to estimate the spectrum is the periodogram, given by the modulus squared of the discrete Fourier transform,
Despite the simplicity of the periodogram, the method suffers from severe deficiencies. It is an inconsistent estimator because it does not converge to the true spectral density as . It exhibits very high spectral leakage although this can be reduced by multiplying by a window function. In the presence of additive noise, the estimate has a positive bias.
Many different techniques for spectral estimation have been developed to overcome the problems of the naive periodogram. These techniques can generally be divided into non-parametric and parametric methods. The non-parametric approaches explicitly estimate the covariance or the spectrum of the process without assuming that the process has any particular structure. The periodogram itself is a non-parametric approach, and is essentially equivalent to the Fourier transform of the biased autocovariance convolved with a Fejér kernel. Some of the most common estimators in use for basic applications (e.g. Welch's method) are non-parametric estimators closely related to the periodogram. By contrast, the parametric approaches assume that the underlying stationary stochastic process has a certain structure which can be described using a small number of parameters (for example, using an auto-regressive or moving average model). In these approaches, the task is to estimate the parameters of the model that describes the stochastic process.
Following is a partial list of non-parametric spectral density estimation techniques:
- Periodogram, the basic modulus-squared of the discrete Fourier transform
- Bartlett's method is the average of the periodograms taken of multiple segments of the signal to reduce variance of the spectral density estimate
- Welch's method a windowed version of Bartlett's method that uses overlapping segments
- Multitaper is a periodogram-based method that uses multiple tapers, or windows, to form independent estimates of the spectral density to reduce variance of the spectral density estimate
- Least-squares spectral analysis, based on least squares fitting to known frequencies
- Non-uniform discrete Fourier transform is used when the signal samples are unevenly spaced in time
- Singular spectrum analysis is a nonparametric method that uses a singular value decomposition of the covariance matrix to estimate the spectral density
- Short-time Fourier transform
Below is a partial list of parametric techniques:
- Autoregressive model (AR) estimation, which assumes that the nth sample is correlated with the previous p samples.
- Moving-average model (MA) estimation, which assumes that the nth sample is correlated with noise terms in the previous p samples.
- Autoregressive moving average (ARMA) estimation, which generalizes the AR and MA models.
- Maximum entropy spectral estimation is an all-poles method useful for SDE when singular spectral features, such as sharp peaks, are expected.
In parametric spectral estimation, one assumes that the signal is modeled by a stationary process which has a spectral density function (SDF) that is a function of the frequency and parameters . The estimation problem then becomes one of estimating these parameters.
where the are fixed coefficients and is a white noise process with zero mean and innovation variance . The SDF for this process is
with the sampling time interval and the Nyquist frequency.
There are a number of approaches to estimating the parameters of the process and thus the spectral density::452-453
- The Yule-Walker estimators are found by recursively solving the Yule-Walker equations for an process
- The Burg estimators are found by treating the Yule-Walker equations as a form of ordinary least squares problem. The Burg estimators are generally considered superior to the Yule-Walker estimators.:452 Burg associated these with maximum entropy spectral estimation.
- The forward-backward least-squares estimators treat the process as a regression problem and solves that problem using forward-backward method. They are competitive with the Burg estimators.
- The maximum likelihood estimators assume the white noise is a Gaussian process and estimates the parameters using a maximum likelihood approach. This involves a nonlinear optimization and is more complex than the first three.
Frequency estimation is the process of estimating the complex frequency components of a signal in the presence of noise given assumptions about the number of the components. This contrasts with the general methods above, which do not make prior assumptions about the components.
Finite number of tones
A typical model for a signal consists of a sum of complex exponentials in the presence of white noise,
The power spectral density of is composed of impulse functions in addition to the spectral density function due to noise.
The most common methods for frequency estimation involve identifying the noise subspace to extract these components. These methods are based on eigen decomposition of the autocorrelation matrix into a signal subspace and a noise subspace. After these subspaces are identified, a frequency estimation function is used to find the component frequencies from the noise subspace. The most popular methods of noise subspace based frequency estimation are Pisarenko's method, the multiple signal classification (MUSIC) method, the eigenvector method, and the minimum norm method.
Minimum norm method
If one only wants to estimate the single loudest frequency, one can use a pitch detection algorithm. If the dominant frequency changes over time, then the problem becomes the estimation of the instantaneous frequency as defined in the time–frequency representation. Methods for instantaneous frequency estimation include those based on the Wigner-Ville distribution and higher order ambiguity functions.
If one wants to know all the (possibly complex) frequency components of a received signal (including transmitted signal and noise), one uses a discrete Fourier transform or some other Fourier-related transform.
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