STOR Colloquium: Abishek Sankararaman, UT-Austin
The University of Texas at Austin
Interference Queuing Networks and
Spatial Birth-Death Processes
Motivated by applications in wireless networks, we consider a class of spatial birth death process, both on the continuum and on the discrete space of grids. In the continuum, we consider an interacting particle birth death process on a compact torus and study questions of stochastic stability. In the discrete setting, we consider a countably infinite collection of coupled queues representing a large wireless network in the Euclidean space. There is one queue at each point of the d -dimensional integer grid. These queues have independent Poisson arrivals, but are coupled through their service rates. The service discipline is of the processor sharing type, with the service rate in each queue slowed down, when the neighboring queues have a larger workload. More precisely, the service rate is the signal to interference ratio of wireless network theory. This coupling is parameterized by a symmetric and translation invariant interference sequence. The dynamics is infinite dimensional Markov, with each queue having a non compact state space. It is neither reversible nor asymptotically product form, as in the mean-field setting. Coupling and percolation techniques are first used to show that this dynamics has well defined trajectories. Coupling from the past techniques of the Loynes’ type are then proposed to build its minimal stationary regime. This regime is the one obtained when starting from the all empty initial condition in the distant past. The rate conservation principle of Palm calculus is then used to identify the stability condition of this system, namely the condition on the interference sequence and arrival rates guaranteeing the finiteness of this minimal regime. We show that the identified condition is also necessary in certain special cases and conjecture to be true in all cases. Remarkably, the rate conservation principle also provides a closed form expression for its mean queue size. When the stability condition holds, this minimal solution is the unique stationary regime, provided it has finite second moments, and this is the case if the arrival rate is small enough. In addition, there exists a range of small initial conditions for which the dynamics is attracted to the minimal regime. Surprisingly however, there exists another range of larger though finite initial conditions for which the dynamics diverges, even though stability criterion holds.