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State Space Reconstruction
NG SOOK KIEN
Prepared by: Ng Sook Kien
NG SOOK KIEN
Table of Contents: Time Series Phase Space Phase Space Reconstruction
Delay Reconstructiona. Minimal Embedding Dimension
➢ Method: 1. False Nearest Neighbours 2. Saturation of Invariant of Attractors 3. True Vector Fields
b. Optimal Time Delay (lag)
➢ Method: 1. Mutual Information 2. Autocorrelation
➢ What Is It ?➢ Why Minimum?
➢ What Is It ?➢ Why ?
A
B
NG SOOK KIEN
What Is Time Series?
Time series: a scalar sequence of measurements taken at fixed sampling time
Time series analysis accounts for the fact that data points taken over time may have an internal structure (such as autocorrelation, trend or seasonal variation) that should be accounted for
Time series analysis accounts for the fact that data points taken over time may have an internal structure (such as autocorrelation, trend or seasonal variation) that should be accounted for
What is time series?
Main goal TS analysis
Time Series VS Phase Space
What is Phase Space
Phase Space (Math)
(Attractor, Trajectory)
Why PS Reconstruction
What Info?
eg. Fig
Summary
NG SOOK KIEN
Time Series Analysis
➢ Identifying the nature of the phenomenon ➢ predicting future values
2 main goals of time series analysis:
We can extrapolate the identified pattern to predict future events.
Requirement: the pattern of observed time series data is identified Requirement: the pattern of observed time series data is identified
We can extrapolate the identified pattern to predict future events.
What is time series?
Main goal TS analysis
Time Series VS Phase Space
What is Phase Space
Phase Space (Math)
(Attractor, Trajectory)
Why PS Reconstruction
What Info?
eg. Fig
Summary
NG SOOK KIEN
Time Series VS Phase Space
What we observe?
Time Series Phase Space
Therefore, Convert observation into state vector (phase space)
So, how ?
PHASE SPACE RECONSTRUCTION
What is time series?
Main goal TS analysis
Time Series VS Phase Space
What is Phase Space
Phase Space (Math)
(Attractor, Trajectory)
Why PS Reconstruction
What Info?
eg. Fig
Summary
NG SOOK KIEN
Phase Space I➢ Demonstrate & visualise the changes in the dynamical variables
➢ May contain many dimensions
➢ Phase diagram represents all that the system can be, (its shape describes qualities of the system)
➢ Succession of plotted points => system's state evolving over time.
➢ Every possible state of the system => a point plotted in multidimensional space.
What is time series?
Main goal TS analysis
Time Series VS Phase Space
What is Phase Space
Phase Space (Math)
(Attractor, Trajectory)
Why PS Reconstruction
What Info?
eg. Fig
Summary
NG SOOK KIEN
Phase Space II
A state is specified by a vector
Is a finitedimensional vector space
Dynamics can be describe by:
➢ An explicit system of mfirstorder ordinary differential eq. continuous flow:
➢ mdimensional map – time is a discrete variable:➢ mdimensional map – time is a discrete variable:
What is time series?
Main goal TS analysis
Time Series VS Phase Space
What is Phase Space
Phase Space (Math)
(Attractor, Trajectory)
Why PS Reconstruction
What Info?
eg. Fig
Summary
NG SOOK KIEN
Phase Space III
A sequence of points that fulfill the above eq. => trajectory of the dynamical system
Trajectory run away to infinity
stay in a bounded area forever
Basin of attraction => a set of initial conditions attracted to same asymptotic behaviour.
Attractors: ➢ subset of the phase space to which the system evolves after a long enough time➢ Is invariant under the dynamical evolution
What is time series?
Main goal TS analysis
Time Series VS Phase Space
What is Phase Space
Phase Space (Math)
(Attractor, Trajectory)
Why PS Reconstruction
What Info?
eg. Fig
Summary
NG SOOK KIEN
Why Phase Space Reconstruction?
Deterministic system, once present state fixed, states at all future times are determined as well.
Establish a phase space for the system such that specifying a point in this space specifies the state of the system, then.... We can study the dynamics of the system by studying the dynamics of phase space.
Example.......
Why Phase Space Reconstruction?
Establish a phase space for the system such that specifying a point in this space specifies the state of the system, then....
What is time series?
Main goal TS analysis
Time Series VS Phase Space
What is Phase Space
Phase Space (Math)
(Attractor, Trajectory)
Why PS Reconstruction
What Info?
eg. Fig
Summary
NG SOOK KIEN
What Information From Phase Space Reconstruction ?A = Too many predators.B = Too few prey.C = Few predator and prey; prey can grow.D= Few predators, ample prey.
Drawn from point A is an arrow (vector) showing how the system would change from that point.
Phase Space Reconstruction
Original phase space of Henon Map
Reconstructed phase space of Henon Map, delay = 1
NG SOOK KIEN
NG SOOK KIEN
Short Summary:
Time series measure only 1 / limited variable
Phase space reconstruction enable us to study unobserved variables Attractors contain geometrical & dynamical properties of the original phase space Study properties of attractors = study properties of the system
NG SOOK KIEN
Derivative coordinates Principal value decomposition
Delay coordinates:
Methods of Phase Space Reconstruction
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
Combination
NG SOOK KIEN
Delay Reconstruction We have:
➢ S – measurement function➢ measurement noise
Delay reconstruction in m dimensions is formed by the vector :
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
NG SOOK KIEN
In practice, 2 problems exist:
➢ Optimal delay coordinates ?
➢ Necessary dimension of the reconstructed phase space (minimum embedding dimension) is unknown
Problems:
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
NG SOOK KIEN
Embedding Theorems:
Guarantee 1: for ideal noisefree data there exists a embedding dimension m such that vectors are equivalent to original phase space vectors.
Guarantee 2: Attractor formed by => equivalent to the attractor in the original phase space if m > 2D
F
Most important embedding parameter: product of (m. )
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
NG SOOK KIEN
Minimum Embedding Dimension Embedding => Condition when the attractor in the original phase space is completely unfolded in the reconstructed phase space.
Dattractor
<< Doriginal phase space
Unnecessary to reconstruct the original phase space when...
Precise knowledge – IMPOSSIBLE to reconstruct the original phase space.
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
NG SOOK KIEN
Why Minimum Embedding Dimension?
No. of points on the reconstructed attractor is too few to obtain reliable estimates
Computational "cost" rises exponentially with the m
In the presence of noise, the unnecessary dimensions of the phase space are not populated by new information (already captured in smaller m)
m too large, chaotic data add redundancy & degrade the performance of many algorithms
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
NG SOOK KIEN
Problems With Embedding of Scalar Data
2 aspects:➢ To characterise the state of a system uniquely
Manifold => CURVED
Construct D independent variables for every time t the signal is sampled.
➢ Find an embedding of a curved manifold in Cartesian space – Whitneys' Theorem
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
NG SOOK KIEN
Whitney Embedding Theorem
Any smooth Ddimensional manifold can be embedded in Euclidean 2D+1 space
➢ Proved only for integer D➢ Doesn't say how likely it is that a given map really forms an embedding
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
If the original attractor has dimension D, then an embedding dimension of m = 2D + 1 will be adequate for reconstructing the attractor.
NG SOOK KIEN
Takens' Embedding Theorem
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
If a time series comes from a dynamical system that is on an attractor, the trajectories constructed from the time series by embedding will have the same topological properties as the original one.
Requirement
NG SOOK KIEN
X(ti ) observable state variable at discrete time (t
i ),
is the time delay,τm is the embedding dimension. For example,
Construct from the original time series x(ti ) vectors
of dimension m,
These points are then assumed to approximate the reconstructed attractor.
For time series X1, X
2, X
3, . . . , X
10, the reconstructed
attractor with = 3 and m = 2 has points (Xτ1, X
4), (X
2, X
5),
(X3, X
6), . . . , (X
7, X
10) in a 2dimensional phase space.
Takens' Embedding Theorem
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
DD
NG SOOK KIEN
False Nearest Neighbours
Property: m too low, distant points in the original phase space are close points in the reconstructed phase space
SO
Increasing m, No more false nearest neighbour
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
NG SOOK KIEN
Saturation of Invariant on The Attractor
Property: attractor unfolded, any invariant on the attractor is independent of m
Attractor not completely unfolded, invariants depend on the embedding dimension.
SO
Increasing m, Value of invariant on the attractor stops changing
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
NG SOOK KIEN
True Vector Fields: Property: vector field associated with the vector function is unambiguous (attractor unfolded).
Tangents to the evolution of the vector function are smooth & unique throughout the phase space.
m too low, the vector field in some neighborhoods is not unique (tangents point in different directions).
SO
Directional vectors in each neighborhood point in the same direction.
Increasing m,
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
Time Lag
How?
NG SOOK KIEN
Optimum Delay (Time Lag)
too small: attractor restricted to the diagonal of the reconstructed phase space too large: components uncorrelated. Reconstructed attractor no longer represents the true dynamics.
Not be close to an integer multiple of a periodicity of the system
Almost every value of time lag should work.
Not the subject of the embedding theorem – data with infinite precision.
Reconstruction method
Delay Reconstruction (math)
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
Time Lag
How?
NG SOOK KIEN
How To Choose The Time Lag?suggestions :
➢ Let the lag be a quarter of the period of the characteristic frequency
➢ Let the lag be the autocorrelation time when the autocorrelation approaches 1/e
➢ Let the lag be the time when the mutual information function reaches its first minimum
Problems ( m, lag)
Embedding Theorem (Guarantee)
Minimum Embedding
Y Minimum Embedding
Problems with Embedding
Whitneys' Theorem
Takens' Theorem
False Nearest Neighbours
Saturation of Invariant
True Vector Fields
Time Lag
How?
Examples
Very strong periodic signals
NG SOOK KIEN
Examples:Henon original
Reconstructed, delay = 1
NG SOOK KIEN
Reconstructed, delay = 3
Reconstructed, delay = 2
Examples:
NG SOOK KIEN
Examples:
Reconstructed Phase Space of Lorenz Attractor, delay = 1
Original Phase space of Lorenz Attractor
Examples:
NG SOOK KIEN
Reconstructed Phase Space of Lorenz Attractor, delay = 3
Reconstructed Phase Space of Lorenz Attractor, delay = 2
Examples:
NG SOOK KIEN
Reconstructed Phase Space of Lorenz Attractor, delay = 7
Reconstructed Phase Space of Lorenz Attractor, delay = 15
NG SOOK KIEN
Examples:
Reconstructed Phase Space of Lorenz Attractor, delay = 30
Reconstructed Phase Space of Lorenz Attractor, delay = 35
NG SOOK KIEN
Examples:
Reconstructed Phase Space of Lorenz Attractor, delay = 50
Reconstructed Phase Space of Lorenz Attractor, delay = 60
Examples:
NG SOOK KIEN
Reconstructed Phase Space of Lorenz Attractor, delay = 80
Reconstructed Phase Space of Lorenz Attractor, delay = 100
NG SOOK KIEN
Examples:
Reconstructed Phase Space of Lorenz Attractor, delay = 150
Reconstructed Phase Space of Lorenz Attractor, delay = 200
Examples:
NG SOOK KIEN
Reconstructed Phase Space of Lorenz Attractor, delay = 300
Reconstructed Phase Space of Lorenz Attractor, delay = 500
NG SOOK KIEN
Examples:
NG SOOK KIEN
Examples:
Examples:
NG SOOK KIEN
NG SOOK KIEN
Examples:
NG SOOK KIEN
Examples:
NG SOOK KIEN
Examples:
NG SOOK KIEN
Summary: Phase diagram represents all that the system can be, (its shape describes qualities of the system) Is invariant under the dynamical evolution Really possible to reconstruct the phase space from only one time series. Methods: Delay embedding Theorem
NG SOOK KIEN