Particle filters or Sequential Monte Carlo (SMC) methods are a set of on-line posterior density estimation algorithms that estimate the posterior density of the state-space by directly implementing the Bayesian recursion equations. SMC methods use a grid-based approach, and use a set of particles to represent the posterior density. These filtering methods make no restrictive assumption about the dynamics of the state-space or the density function. SMC methods provide a well-established methodology for generating samples from the required distribution without requiring assumptions about the state-space model or the state distributions. The state-space model can be non-linear and the initial state and noise distributions can take any form required. However, these methods do not perform well when applied to high-dimensional systems. SMC methods implement the Bayesian recursion equations directly by using an ensemble based approach. The samples from the distribution are represented by a set of particles; each particle has a weight assigned to it that represents the probability of that particle being sampled from the probability density function.
Weight disparity leading to weight collapse is a common issue encountered in these filtering algorithms; however it can be mitigated by including a resampling step before the weights become too uneven. In the resampling step, the particles with negligible weights are replaced by new particles in the proximity of the particles with higher weights.
The first traces of particle filters date back to the 50's; the 'Poor Man's Monte Carlo', that was proposed by Hammersley et al., in 1954, contained hints of the SMC methods used today. Later in the 70's, similar attempts were made in the control community. However it was in 1993, that Gordon et al., published their seminal work 'A novel Approach to nonlinear/non-Gaussian Bayesian State estimation', that provided the first true implementation of the SMC methods used today. The authors named their algorithm 'the bootstrap filter', and demonstrated that compared to other filtering methods, their algorithm does not require any assumption about that state-space or the noise of the system.
The objective of a particle filter is to estimate the posterior density of the state variables given the observation variables. The particle filter is designed for a hidden Markov Model, where the system consists of hidden and observable variables. The observable variables (observation process) are related to the hidden variables (state-process) by some functional form that is known. Similarly the dynamical system describing the evolution of the state variables is also known probabilistically.
A generic particle filter estimates the posterior distribution of the hidden states using the observation measurement process. Consider a state-space shown in the diagram (Figure 2).
The objective of the particle filter is to estimate the values of the hidden states x, given the values of the observation process y.
The particle filter aims to estimate the sequence of hidden parameters, xk for k = 0,1,2,3,…, based only on the observed data yk for k = 0,1,2,3,…. All Bayesian estimates of xk follow from the posterior distribution p(xk | y0,y1,…,yk). In contrast, the MCMC or importance sampling approach would model the full posterior p(x0,x1,…,xk | y0,y1,…,yk).
Particle methods assume and the observations can be modeled in this form:
- is a first order Markov process that evolves according to the distribution :
and with an initial distribution .
- The observations are conditionally independent provided that are known
- In other words, each only depends on . This conditional distribution for is written as
An example system with these properties is:
where both and are mutually independent and identically distributed sequences with known probability density functions and and are known functions. These two equations can be viewed as state space equations and look similar to the state space equations for the Kalman filter. If the functions and are linear, and if both and are Gaussian, the Kalman filter finds the exact Bayesian filtering distribution. If not, Kalman filter based methods are a first-order approximation (EKF) or a second-order approximation (UKF in general, but if probability distribution is Gaussian a third-order approximation is possible). Particle filters are also an approximation, but with enough particles they can be much more accurate.
Monte Carlo approximation
Particle methods, like all sampling-based approaches (e.g., MCMC), generate a set of samples that approximate the filtering distribution . For example, we may have samples from the approximate posterior distribution of , where the samples are labeled with superscripts as . Then, expectations with respect to the filtering distribution are approximated by
and , in the usual way for Monte Carlo, can give all the moments etc. of the distribution up to some degree of approximation.
Sequential importance resampling (SIR)
Sequential importance resampling (SIR), the original particle filtering algorithm (Gordon et al. 1993), is a very commonly used particle filtering algorithm, which approximates the filtering distribution by a weighted set of P particles
The importance weights are approximations to the relative posterior probabilities (or densities) of the particles such that .
SIR is a sequential (i.e., recursive) version of importance sampling. As in importance sampling, the expectation of a function can be approximated as a weighted average
For a finite set of particles, the algorithm performance is dependent on the choice of the proposal distribution
The optimal proposal distribution is given as the target distribution
However, the transition prior probability distribution is often used as importance function, since it is easier to draw particles (or samples) and perform subsequent importance weight calculations:
Resampling is used to avoid the problem of degeneracy of the algorithm, that is, avoiding the situation that all but one of the importance weights are close to zero. The performance of the algorithm can be also affected by proper choice of resampling method. The stratified sampling proposed by Kitagawa (1996) is optimal in terms of variance.
A single step of sequential importance resampling is as follows:
- 1) For draw samples from the proposal distribution
- 2) For update the importance weights up to a normalizing constant:
- Note that when we use the transition prior probability distribution as the importance function, , this simplifies to the following :
- Note that when we use the transition prior probability distribution as the importance function, , this simplifies to the following :
- 3) For compute the normalized importance weights:
- 4) Compute an estimate of the effective number of particles as
- 5) If the effective number of particles is less than a given threshold , then perform resampling:
- a) Draw particles from the current particle set with probabilities proportional to their weights. Replace the current particle set with this new one.
- b) For set
The term Sampling Importance Resampling is also sometimes used when referring to SIR filters.
Sequential importance sampling (SIS)
- Is the same as sequential importance resampling, but without the resampling stage.
"direct version" algorithm
|This section may be confusing or unclear to readers. (October 2011)|
The "direct version" algorithm is rather simple (compared to other particle filtering algorithms) and it uses composition and rejection. To generate a single sample at from :
- 1) Set n=0 (This will count the number of particles generated so far)
- 2) Uniformly choose an index L from the range
- 3) Generate a test from the distribution
- 4) Generate the probability of using from where is the measured value
- 5) Generate another uniform u from where
- 6) Compare u and
- 6a) If u is larger then repeat from step 2
- 6b) If u is smaller then save as and increment n
- 7) If n == P then quit
The goal is to generate P "particles" at using only the particles from . This requires that a Markov equation can be written (and computed) to generate a based only upon . This algorithm uses composition of the P particles from to generate a particle at and repeats (steps 2–6) until P particles are generated at .
This can be more easily visualized if is viewed as a two-dimensional array. One dimension is and the other dimensions is the particle number. For example, would be the Lth particle at and can also be written (as done above in the algorithm). Step 3 generates a potential based on a randomly chosen particle () at time and rejects or accepts it in step 6. In other words, the values are generated using the previously generated .
Other particle filters
- Auxiliary particle filter 
- Regularized auxiliary particle filter 
- Gaussian particle filter
- Unscented particle filter
- Gauss–Hermite particle filter
- Cost Reference particle filter
- Hierarchical/Scalable particle filter 
- Rao–Blackwellized particle filter 
- Rejection-sampling based optimal particle filter 
- Ensemble Kalman filter
- Generalized filtering
- Moving horizon estimation
- Recursive Bayesian estimation
- Monte Carlo localization
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- Feynman–Kac models and interacting particle algorithms (a.k.a. Particle Filtering) Theoretical aspects and a list of application domains of particle filters
- Sequential Monte Carlo Methods (Particle Filtering) homepage on University of Cambridge
- Dieter Fox's MCL Animations
- Rob Hess' free software
- SMCTC: A Template Class for Implementing SMC algorithms in C++
- Java applet on particle filtering
- vSMC : Vectorized Sequential Monte Carlo