forward discrete probability propagation rasit onur topaloglu ph.d. candidate [email protected]
Post on 21-Dec-2015
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SPICE Hierarchy and the Problem
ex: gm=2*k*ID
Level1
Level4
Level0 TLeffWefftox
Level2
Level3
0 NSUBms
CoxF
k Qdep
Vth
ID
gm rout Level5
•SPICE formulas are hierarchical; hence can progressively relate physical parameters to device parameters in connectivity graphs•Recent models attribute process variations to physical parameters
physical
electrical/mathematical
•Probability density functions can be input at Level0 independently
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Probability Propagation
•Estimation of device parameters at highest level needed to examine effects of process variations•An analytic solution not possible since functions highly non-linear and Gaussian approximations not accurate in deep sub-micron
GOALS
Algebraic tractability : enable manual applicability by designers
Speed : be comparable or outperform Monte Carlo in quick estimation
•A method to propagate pdf’s to highest level necessary
Flexibility : be able to use non-standard densities to outperform parametric belief propagation
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Shortcomings of Monte Carlo
•No algebraically tractability : No manual estimation by designers possible due to large number of iterations and random sampling
•Limited to standard distributions : Random number generators in CAD tools only provide certain distributions, hence a new module usually needs to be programmed
•Speed : No quick convergence to an estimate distribution due to random sampling unless a large number of costly
iterations employed
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A Reminder on Applying Monte Carlo for Probability Propagation
gm
Level1
Level4
Level0 nVFBNSUBLW
Vth Cox
tox
ID
k Level2
Level3
•Pick independent samples from distributions of Level0 parameters
•Compute functions using these samples until highest level reached •Construct a histogram to approximate the distribution
•Repeat while desired accuracy is not yet reached:
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Parametric Belief Propagation
•Each node receives and sends messages to parents and children until equilibrium•Parent to child () : causal information
•Parent to parent () : diagnostic information
Calculationshandledat each node:
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Parametric Belief Propagation
•When arrows in the hierarchy tree indicate linear addition operations on Gaussians, analytic formulations possible•Not straightforward for other distributions or non-standard distributions
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Implementing FDPP
•F (Forward) : Given a function, estimates the distribution of next node in the formula hierarchy using samples
•Q (Quantize) : Discretizes a pdf to operate on its samples
Analytic operation on continuous distributions difficult; instead work in discrete domain and convert back to continuous
domain at the end:
•Q-1 (De-Quantize) : Converts a discrete pdf back to continuous domain : implemented as an interpolation function
•B (Band-pass) : Decrements number of samples using a threshold on sample probabilities
•R (Re-bin) : Decreases number of samples by combining close samples together
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T NSUB
PHIf
Necessary Operators (Q, F, B, R, Q-1) on a Connectivity Graph
Q Q
F
B
•F, B and R repeated until we acquire the distribution of a high level parameter; Q and Q-1 used just once
R
Q-1
TLeffWefftox 0 NSUBms
CoxF
k
Vth
ID
gmrout
Qdep
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pdf(X)Q Operator
•QN band-pass filters pdf(X) and divides into bins
))(()( XpdfQX N N in QN indicates number of bins
spdf(X)=(X)
X
pdf(
X)
spdf
(X)
X
•Use N>(2/m), where m is maximum derivative of pdf(X), thereby obeying a bound similar to Nyquist •If quantizer uniform and small, quantization error
random variable Q is uniformly distributed, then2/ 22 2 2
12/ 2[ ] ( )Q E Q q pdf Q dq
Variance of quantization error:
•Increase number of bins to reduce quantization error
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F Operator •F operator implements a function over spdf’s using
deterministic sampling
•Corresponding function in connectivity graph applied to deterministic pair-wise combination of impulse values to get the value of the new sample
•Heights of impulses (probabilities) multiplied to get probability of new sample
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Effect of Non-linear Functions
•Application of functions cause accumulation in certain ranges
Band-pass and re-bin operations needed after F operation
Impulses after F, before B and R
•De-quantization would not result in a pdf•Increased number of samples would induce a computational burden
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Band-pass, Be, Operator•Eliminate samples having values out of range (6): might
cut off tails of bi-modal or long-tailed distributions
Margin-based Definition:
Error-based Definition:•Eliminate samples having probabilities least likely to occur :
can also eliminate samples in useful range hence offers more computational efficiency
•Implementation : eliminate samples with probabilities less than 1/e times the sample with the largest probability
•e should be chosen such that it is smaller than the ratio of products of maximum and minimum probability
samples for nodes to which F is applied
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Re-bin, RN, OperatorImpulses after F Resulting spdf(X)Unite into one bin
•Samples falling into the same bin congregated in one
jbjiji
ji pwmdi
)(:,
),(•Total distortion given by
2)(),( jiji wmwmd mi : center of i’th bin
can be used to select bin locations, where
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Experimental Results
•Impulse representation for threshold voltage and transconductance are obtained through FDPP on the graph
(X) for gm(X) for Vth
•Matlab R12 used to evaluate FDPP method
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•A close match is observed after interpolation
Monte Carlo – FDPP Comparison
solid : FDPP dotted : Monte Carlo
Pdf* of VthPdf* of ID
•Correlation error introduced by the independence assumption of F operator results in negligible error as R operator helps distribute this error over the pdf state space
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Monte Carlo – FDPP Comparison with a Low Sample Number
•Monte Carlo inaccurate for moderate number of samples•Indicates FDPP can converge to an acceptable estimate with far less number of samples
solid : FDPP with 100 samples
Pdf* of FPdf* of F
noisy : Monte Carlo with 1000 samples
solid : FDPP with 100 samples
noisy : Monte Carlo with 100000 samples
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•Edges define a linear sum, ex: n5=n2+n3
Monte Carlo – FDPP ComparisonPdf of n7Benchmark example
solid : FDPP dotted : Monte Carlo triangles:belief propagation
•Monte Carlo result is separated as FDPP and belief propagation neglect correlation
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•When distributions at internal nodes n4, n5, n6 re-sampled using Monte Carlo, all methods converge
Faulty Application of Monte CarloPdf of n7Benchmark example
solid : FDPP dotted : Monte Carlo triangles:belief propagation
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Conclusions•Forward Discrete Probability Propagation is introduced as
an alternative to Monte Carlo and parametric belief propagation methods for quick estimation :
•FDPP should be preferred to MC when a faster convergence to real distribution is necessary with limited number of samples
•FDPP provides an algebraic intuition due to deterministic sampling and manual applicability due to using less number of samples
•FDPP can account for non-standard pdf’s where parametric methods are limited to certain ones