Xingzhe Fan

Observer design for systems with multivariable monotone nonlinearities

Globally convergent observers are designed for a class of systems with multivariable nonlinearities. The approach is to represent the observer error system as the feedback interconnection of a linear system and a state-dependent multivariable nonlinearity. We first extend an earlier design (Automatica 37 (12) (2001) 1923) to multivariable nonlinearities, satisfying an analog of the scalar nondecreasing property. Next, we exploit the structure of the nonlinearity to relax the positive real restriction on the linear part of the observer error system. This relaxed design renders the feasibility conditions less restrictive, and widens the applicability of the observer, as illustrated with examples. Finally, output nonlinearities are studied and the design is extended to be adaptive in the presence of unknown parameters.

A passivity approach to game-theoretic CDMA power control

This paper follows a game-theoretical formulation of the CDMA power control problem and develops new decentralized control algorithms that globally stabilize the desired Nash equilibrium. The novel approach is to exploit the passivity properties of the feedback loop comprising the mobiles and the base station. We first reveal an inherent passivity property in an existing gradient-type algorithm, and prove stability from the Passivity Theorem. We then exploit this passivity property to develop two new designs. In the first design, we extend the base station algorithm with Zames-Falb multipliers which preserve its passivity properties. In the second design, we broaden the mobile power update laws with more general, dynamic, passive controllers. These new designs may be exploited to enhance robustness and performance, as illustrated with a realistic simulation study. We then proceed to show robustness of these algorithms against time-varying channel gains.

Robustness of network flow control against disturbances and time-delay ☆

This paper studies robustness of Kelly’s source and link control laws in (J. Oper. Res. Soc. 49 (1998) 237) with respect to disturbances and time-delays. This problem is of practical importance because of unmodelled

Generalized multicast congestion control

ows, and propagation and queueing delays, which are ubiquitous in networks. We ;rst show Lp-stability, for p ∈ [1; ∞], with respect to additive disturbances. We pursue L∞-stability within the input-to-state stability (ISS) framework of Sontag (IEEE Trans. Automat. Control 34 (1989) 435), which makes explicit the vanishing eAect of initial conditions. Next, using this ISS property and a loop transformation, we prove that global asymptotic stability is preserved for suBciently small time-delays in forward and return channels. For larger delays, we achieve global asymptotic stability by scaling down the control gains as in Paganini et al. (Proceedings of 2001 Conference on Decision and Control, Orlando, FL, December 2001, pp. 185–190) c

Delay robustness of a class of nonlinear systems and applications to communication networks

Efficient multicast congestion control (MCC) is one of the critical components required to enable the IP multicast deployment over the Internet. Previously proposed MCC schemes can be categorized in two: single-rate or multi-rate. Single-rate schemes make all recipients get data at a common rate allowed by the slowest receiver, but are relatively simple. Multi-rate schemes allow of heterogeneous receive rates and thus provide better scalability, but rely heavily on frequent updates to group membership state in the routers. A recent work by Kwon and Byers, combined these two methods and provided a multi-rate scheme by means of single-rate schemes with relatively low complexity. In this paper, we propose a new scheme called generalized multicast congestion control (GMCC). GMCC provides multi-rate features at low complexity by using a set of independent single-rate sub-sessions (a.k.a layers) as building blocks. The scheme is named GMCC because single-rate MCC is just one of its special cases. Unlike the earlier work by Kwon and Byers, GMCC does not have the drawback of static configuration of the source which may not match with the dynamic network situations. GMCC is fully adaptive in that (i) it does not statically set a particular range for the sending rates of layers, and (ii) it eliminates redundant layers when they are not needed. Receivers can subscribe to different subsets of the available layers and hence can always obtain different throughput. While no redundant layers are used, GMCC allows receivers to activate a new layer in case existing layers do not accommodate the needs of the actual receivers.

Normalized queueing delay: congestion control jointly utilizing delay and marking

In this paper we first study a class of nonlinear interconnected systems and derive sufficient conditions for their delay robustness. We then study a subclass with a specific interconnection structure that appears, among other applications, in several communication networks. To characterize their robustness to delays in backward and forward channels, we combine our main result with a passivity-based stability analysis, applicable to this subclass. We conclude with application examples from TCP/IP protocols in the Internet, and CDMA uplink power control in cellular networks.

L/sub p/ stability and delay robustness of network flow control

Depending upon the type of feedback that is primarily used as a congestion measure, congestion control methods can be generally classified into two categories: marking/loss-based or delay-based. While both marking and queueing delay provide information about the congestion state of a network, they have been largely treated with separate control strategies. In this paper, we propose the notion of the normalized queueing delay, which serves as a congestion measure by combining both delay and marking information. Utilizing normalized queueing delay (NQD), we propose an approach to congestion control that allows a source to scale its rate dynamically to prevailing network conditions through the use of a time-variant set-point. In ns-2 simulation studies, an NQD-enabled FAST TCP demonstrates a significant link utilization improvement over FAST TCP under certain conditions. In addition, we propose another NQD-based controller D + M TCP (Delay+Marking TCP) that achieves quick convergence to fair and stable rates with nearly full link utilization. Therefore, NQD is a suitable candidate as a congestion measure for practical congestion control.

A distributed congestion and power control algorithm to achieve bounded average queuing delay in wireless networks

This paper studies robustness of Kellys source and link control laws with respect to disturbances and time-delays. This problem is of practical importance because of unmodeled flows, and propagation and queueing delays, which are ubiquitous in networks. We first show L/sub p/-stability, for p /spl isin/ [l,/spl infin/], with respect to additive disturbances. We pursue L/sub /spl infin//-stability within the input-to-state stability (ISS) framework of Sontag which makes explicit the vanishing effect of initial conditions. Next, using this ISS property and a loop transformation, we prove that global asymptotic stability is preserved for sufficiently small time-delays in forward and return channels. For larger delays, we achieve global asymptotic stability by scaling down the control gains as in Paganini et al.

Transmission Rate Allocation in Multisensor Target Tracking Over a Shared Network

Allocating limited resources such as bandwidth and power in a multi-hop wireless network can be formulated as a Network Utility Maximization (NUM) problem. In this approach, both transmitting source nodes and relaying link nodes exchange information allowing for the NUM problem to be solved in an iterative distributed manner. Some previous NUM formulations of wireless network problems have considered the parameters of data rate, reliability, and transmitter powers either in the source utility function which measures an application’s performance or as constraints. However, delay is also an important factor in the performance of many applications. In this paper, we consider an additional constraint based on the average queueing delay requirements of the sources. In particular, we examine an augmented NUM formulation in which rate and power control in a wireless network are balanced to achieve bounded average queueing delays for sources. With the additional delay constraints, the augmented NUM problem is non-convex. Therefore, we present a change of variable to transform the problem to a convex problem and we develop a solution which results in a distributed rate and power control algorithm tailored to achieving bounded average queueing delays. Simulation results demonstrate the efficacy of the distributed algorithm.

A two-time-scale design for edge-based detection and rectification of uncooperative flows

In a multisensor target tracking application running on a shared network, at what bit rates should the sensors send their measurements to the tracking fusion center? Clearly, the sensors cannot use arbitrary rates in a shared network, and a standard network rate control algorithm may not provide rates amenable to effective target tracking. For Kalman filter-based multisensor target tracking, we derive a utility function that captures the tracking quality of service as a function of the sensor bit rates. We incorporate this utility function into a network rate resource allocation framework, deriving a distributed rate control algorithm for a shared network that is suitable for current best effort packet networks, such as the Internet. In simulation studies, the new rate control algorithm engenders significantly better tracking performance than a standard rate control method, while the ordinary data transfer flows continue to effectively operate while using their standard rate control methods.