Reflected Brownian motion


In probability theory, reflected Brownian motion is a Wiener process in a space with reflecting boundaries.
RBMs have been shown to describe queueing models experiencing heavy traffic as first proposed by Kingman and proven by Iglehart and Whitt.

Definition

A d–dimensional reflected Brownian motion Z is a stochastic process on uniquely defined by
where X is an unconstrained Brownian motion and
with Y a d–dimensional vector where
The reflection matrix describes boundary behaviour. In the interior of the process behaves like a Wiener process, on the boundary "roughly speaking, Z is pushed in direction Rj whenever the boundary surface is hit, where Rj is the jth column of the matrix R."

Stability conditions

Stability conditions are known for RBMs in 1, 2, and 3 dimensions. "The problem of recurrence classification for SRBMs in four and higher dimensions remains open." In the special case where R is an M-matrix then necessary and sufficient conditions for stability are
  1. R is a non-singular matrix and
  2. R−1μ < 0.

    Marginal and stationary distribution

One dimension

The marginal distribution of a one-dimensional Brownian motion starting at 0 restricted to positive values with drift μ and variance σ2 is
for all t ≥ 0, which yields when taking t → ∞ an exponential distribution
For fixed t, the distribution of Z coincides with the distribution of the running maximum M of the Brownian motion,
But be aware that the distributions of the processes as a whole are very different. In particular, M is increasing in t, which is not the case for Z.
The heat kernel for reflected Brownian motion at :
For the plane above

Multiple dimensions

The stationary distribution of a reflected Brownian motion in multiple dimensions is tractable analytically when there is a product form stationary distribution, which occurs when the process is stable and
where D = diag. In this case the probability density function is
where ηk = 2μkγk/Σkk and γ = R−1μ. Closed-form expressions for situations where the product form condition does not hold can be computed numerically as described below in the simulation section.

Simulation

One dimension

In one dimension the simulated process is the absolute value of a Wiener process. The following MATLAB program creates a sample path.

%rbm.m
n=10^4; h=10^; t=h.*; mu=-1;
X=zeros; M=X; B=X;
B=3; X=3;
for k=2:n+1
Y=sqrt*randn; U=rand;
B=B+mu*h-Y;
M=/2;
X=max;
end
subplot
plot;
subplot
plot;

The error involved in discrete simulations has been quantified.

Multiple dimensions

allows simulation of steady state RBMs.

Other boundary conditions

Feller described possible boundary condition for the process