Ahsan Ali

Identification of Box–Jenkins models for spatially interconnected systems in closed-loop

This paper presents an identification method for spatially interconnected distributed systems operating in closed-loop. The proposed approach makes use of refined instrumental variable method to identify spatially interconnected systems in Box–Jenkins form, where the controller is assumed to be known. The method presented here can yields statistically optimal estimates, and compared with other approaches to identify such systems under similar scenarios, takes far less time. The approach is applicable to both separable and non-separable systems and takes into account the boundary conditions. Though described only for two-dimensional systems, it is readily extendible to systems having more spatial dimensions. The effectiveness of the method is shown by a simulation example.

Fixed structure Hα control of couple tank system and anti-integral windup PID control strategy for actuator saturation

This paper considered two main issues. First is about the complete mathematical modelling and Fixed-Structure α robust controller design for couple tank system in frequency domain. This system is nonlinear and classical Proportional Integral Derivative (PID) controller may not give appropriate performance. Although classical PID controllers are more practical due their simplicity and linearity but they have some limitations. Especially in case of nonlinear system or small variations in parameters, they may not give desired transient specification. The proposed controller is a robust controller which contains a fixed structure and fixed order, and its parameters are optimized using non smooth Hα optimization in frequency domain. Also Fixed-Structure Hα controller also gives satisfied performance in case of parameters variation. Couple tank system is widely used in industry. For example its most important applications are water purification, effluent treatment, dairy filtration, chemical processing and also this system is used in pharmaceutical industry. Actuator saturation is a major nonlinearity that may exist in couple tank systems. And this nonlinearity may lead to performance degradation or instability of controller and whole system. This problem is known as integral windup and to deal this problem, an Anti-Integral Windup PID control strategy is introduced in the second part of this paper.

Guard zone-based scheduling in ad hoc networks

Abstract Scheduling algorithms in ad hoc networks allow nodes to share the wireless channel so that concurrent transmissions can be decoded successfully. On one hand, scheduling needs to be efficient to maximize spatial reuse and minimize retransmissions due to collisions. But on the other hand, the scheduling algorithm needs to be easily implementable in a distributed fashion with little, if any, coordination with other nodes in the network. In this paper we propose and evaluate a simple scheduling technique that suppresses transmissions by nodes around the desired receiver. Using stochastic geometry, we derive a near-optimal guard zone which can be easily realized in a distributed manner, and exhibits about a 2–40 fold increase in capacity compared to ALOHA; the capacity increase depending primarily on the required outage probability and node density. The capacity loss is about 15–25% compared to a well-known near-optimal centralized scheme. In contrast to centralized scheduling (which is highly impractical), our scheme lends itself to distributed implementation and also protects active links. Our derivations cleanly capture how the optimal guard zone size varies with different network parameters like path loss, outage, spreading gain, and node density, and we show how these results can be used to provide protocol design guidelines.

Stochastic geometry of thinned nodes in ad hoc networks

In the absence of multi-user detection techniques in ad hoc networks, interference management is done through MAC scheduling by creating suitable exclusion zones around active receivers. Applying MAC mechanisms alters the spatial distribution Φi of parent contending nodes to thinned daughter distribution Φs. The knowledge of post-MAC geometrical distribution of nodes is important for efficient MAC implementation. The paper provides a thinning model to capture the MAC implementation in ad hoc networks and investigates the stochastic geometry of thinned nodes. It is shown that the thinning process results in different spatial distributions that is sensitive to the robustness of the physical layer.