Niek Bouman

Delay performance of backlog based random access

Backlog-based CSMA strategies provide a popular mechanism for distributed medium access control in wireless networks. When suitably designed, such strategies offer the striking capability to match the optimal throughput performance of centralized scheduling algorithms in a wide range of scenarios. Unfortunately, however, the activation rules used in these schemes tend to yield excessive backlogs and delays. More aggressive activation rates can potentially improve the delay performance, but may not allow provable maximum-stability guarantees. In order to gain a fundamental understanding how the shape of the activation function affects the queueing behavior, we focus on a single- node scenario, thus separating the impact of the network topology. We demonstrate that three qualitatively different regimes can arise, depending on how rapidly the activation function increases with the backlog. Simulation experiments are conducted to validate the analytical findings.

Delays and mixing times in random-access networks

We explore the achievable delay performance in wireless random-access networks. While relatively simple and inherently distributed in nature, suitably designed backlog-based random-access schemes provide the striking capability to match the optimal throughput performance of centralized scheduling mechanisms. The specific type of activation rules for which throughput optimality has been established, may however yield excessive backlogs and delays. Motivated by that issue, we examine whether the poor delay performance is inherent to the basic operation of these schemes, or caused by the specific kind of activation rules. We derive delay lower bounds for backlog-based activation rules, which offer fundamental insight in the cause of the excessive delays. For fixed activation rates we obtain lower bounds indicating that delays and mixing times can grow dramatically with the load in certain topologies as well.

Achievable delay performance in CSMA networks

We explore the achievable delay performance in wireless CSMA networks. While relatively simple and inherently distributed in nature, suitably designed backlog-based CSMA schemes provide the striking capability to match the optimal throughput performance of centralized scheduling mechanisms in a wide range of scenarios. The specific type of activation rules for which throughput optimality has been established, may however yield excessive backlogs and delays. Motivated by that issue, we examine whether the poor delay performance is inherent to the basic CSMA operation of these schemes, or caused by the specific kind of activation rules. We first establish lower bounds for the delay in the case of fixed activation rates. The bounds indicate that the delay can dramatically grow with the load in certain topologies. We also discuss to what extent the bounds apply to backlog-based activation rules. Simulation experiments are conducted to illustrate and validate the analytical results.

Experimental Validation of an Explicit Power-Flow Primary Control in Microgrids

The existing approaches to control electrical grids combine frequency and voltage controls at different time-scales. When applied in microgrids with stochastic distributed generation, grid quality of service problems may occur, such as under- or overvoltages as well as congestion of lines and transformers. The COMMELEC framework proposes to solve this compelling issue by performing explicit control of power flows with two novel strategies: 1) a common abstract model is used by resources to advertise their state in real time to a grid agent; and 2) subsystems can be aggregated into virtual devices that hide their internal complexity in order to ensure scalability. While the framework has already been published in the literature, in this paper, we present the first experimental validation of a practicable explicit power-flow primary control applied in a real-scale test-bed microgrid. We demonstrate how an explicit power-flow control solves the active and reactive power sharing problem in real time, easily allowing the microgrid to be dispatchable in real time (i.e., it is able to participate in energy markets) and capable of providing frequency support, while always maintaining quality of service.

Queues with random back-offs

We consider a broad class of queueing models with random state-dependent vacation periods, which arise in the analysis of queue-based back-off algorithms in wireless random-access networks. In contrast to conventional models, the vacation periods may be initiated after each service completion, and can be randomly terminated with certain probabilities that depend on the queue length. We first present exact queue-length and delay results for some specific cases and we derive stochastic bounds for a much richer set of scenarios. Using these, together with stochastic relations between systems with different vacation disciplines, we examine the scaled queue length and delay in a heavy-traffic regime, and demonstrate a sharp trichotomy, depending on how the activation rate and vacation probability behave as function of the queue length. In particular, the effect of the vacation periods may either (i) completely vanish in heavy-traffic conditions, (ii) contribute an additional term to the queue lengths and delays of similar magnitude, or even (iii) give rise to an order-of-magnitude increase. The heavy-traffic trichotomy provides valuable insight into the impact of the back-off algorithms on the delay performance in wireless random-access networks.

Delay performance in random-access networks

We explore the achievable delay performance in wireless random-access networks. While relatively simple and inherently distributed in nature, suitably designed queue-based random-access schemes provide the striking capability to match the optimal throughput performance of centralized scheduling mechanisms in a wide range of scenarios. The specific type of activation rules for which throughput optimality has been established, may however yield excessive queues and delays. Motivated by that issue, we examine whether the poor delay performance is inherent to the basic operation of these schemes, or caused by the specific kind of activation rules. We derive delay lower bounds for queue-based activation rules, which offer fundamental insight in the cause of the excessive delays. For fixed activation rates, we obtain lower bounds indicating that delays can grow dramatically with the load in certain topologies as well.