Wing Shing Wong

Systems with finite communication bandwidth constraints. II. Stabilization with limited information feedback

For part I, see ibid., vol.42, p.1294-8, 1997. In this paper a new class of feedback control problems is introduced. Unlike classical models, the systems considered here have communication channel constraints. As a result, the issue of coding and communication protocol becomes an integral part of the analysis. Since these systems cannot be asymptotically stabilized if the underlying dynamics are unstable, a weaker stability concept called containability is introduced. A key result connects containability with an inequality equation involving the communication data rate and the rate of change of the state.

Systems with finite communication bandwidth constraints. I. State estimation problems

In this paper, we investigate a state estimation problem involving finite communication capacity constraints. Unlike classical estimation problems where the observation is a continuous process corrupted by additive noises, there is a constraint that the observations must be coded and transmitted over a digital communication channel with finite capacity. This problem is formulated mathematically, and some convergence properties are defined. Moreover, the concept of a finitely recursive coder-estimator sequence is introduced. A new upper bound for the average estimation error is derived for a large class of random variables. Convergence properties of some coder-estimator algorithms are analyzed. Various conditions connecting the communication data rate with the rate of change of the underlying dynamics are established for the existence of stable and asymptotically convergent coder-estimator schemes.

Formal Aspects of Serializability in Database Concurrency Control

An arbitrary interleaved execution of transactions in a database system can lead to an inconsistent database state. A number of synchronization mechanisms have been proposed to prevent such spurious behavior. To gain insight into these mechanisms, we analyze them in a simple centralized system that permits one read operation and one write operation per transaction. We show why locking mechanisms lead to correct operation, we show that two proposed mechanisms for distributed environments are special cases of locking, and we present a new version of lockdng that alows more concurrency than past methods. We also examine conflict graph analysis, the method used in the SDD-1 distributed database system, we prove its correctness, and we show that it can be used to substantially improve the performance of almost any synchronization mechanisn.

User speed estimation and dynamic channel allocation in hierarchical cellular system

The huge amount of handoffs generated by microcells creates a problem for the future PCN. To alleviate the problem, we propose a hierarchical cellular system which comprises cells of different sizes. Ideally, one would like to use large cells to serve high-mobility users. A challenging issue is to obtain a good estimate of the user speed. A simple speed estimation is proposed and based on this estimate one can implement a number of dynamic channel allocation algorithms on such a hierarchical network. A comparative study of these algorithms will be presented based on a detailed simulation model.<<ETX>>

A distributed fixed-step power control algorithm with quantization and active link quality protection

A distributed fixed-step power control algorithm is presented. It is a simple feedback adjustment algorithm using only local information. In the ideal case where there is no power constraint, it is guaranteed that existing users will not be dropped due to admission of new users. If it is infeasible to accommodate all of them, the new user will be blocked. When the constraint on the maximum power is imposed, it is shown by simulation that blocking a new call is more probable than dropping any existing calls, if the capacity is exceeded. Besides, its convergence property is demonstrated. The convergence rate, which depends on the step size, is studied through simulation. In addition, the issue of power quantization is addressed.

Power control and rate management for wireless multimedia CDMA systems

We consider a wireless multimedia code-division multiple-access system, in which the terminals transmit at different rates. We formulate the problem as a constrained optimization problem, with the objective of maximizing the total effective rate. An optimal power control strategy is derived. When the scale of the system is large, the optimal solution takes a simple form, which is easy to be applied practically. Furthermore, our basic model can be extended to include delay-sensitive traffic.

On a necessary and sufficient condition for finite dimensionality of estimation

Ever since the technique of the Kalman–Bucy filter was popularized, there has been an intense interest in finding new classes of finite dimensional recursive filters. In the late seventies, the concept of the estimation algebra of a filtering system was introduced. It has proven to be an invaluable tool in the study of nonlinear filtering problems. In this paper, a simple algebraic necessary and sufficient condition is established for an estimation algebra of a special class of filtering systems to be finite-dimensional. Also presented is a rigorous proof of the Wei–Norman program which allows one to construct finite-dimensional recursive filters from finite dimensional estimation algebras.

Hot-spot traffic relief in cellular systems

By analyzing mathematical models, it is shown that combining channel borrowing with a coordinated sectoring or overlying scheme provides effective ways to handle hot-spots in the system. Blocking probabilities with these arrangements are derived, and the dynamic sharing with bias (DSB) rule is suggested for increasing the trunking efficiency. A simple handoff model is formulated and analyzed for comparing the probabilities of additional handoffs due to sectoring and overlaying of cells. With the nominal allocation of 60 channels per cell and a donor cell having a load of 30 Erlangs, numerical results show that at a blocking requirement of 1%, the traffic load in the hot-spot cell can be increased from 47 to 63 Erlangs with the use of the channel borrowing with the cell sectoring scheme: while with the use of the DSB rule, the load can be increased further to 71 Erlangs. A slightly higher load can be carried in the hot-spot cell with the use of cell overlaying arrangement. >

Sequential packing algorithm for channel assignment under cochannel and adjacent-channel interference constraint

Generally, the channel-assignment problem (CAP) for mobile cellular systems is solved by graph-coloring algorithms. These algorithms, though sometimes can yield an optimal solution, do not supply any information on whether an optimal solution has been found or bow far away it is from the optimum. In view of these undesirable features, two relevant results are presented. First, a lower bound for the minimum number of channels required to satisfy a given call-traffic demand is derived. This lower bound is tighter than the existing ones under certain conditions and can be used as a supplement for other approximate algorithms. Second, we propose an efficient heuristic algorithm to solve this problem. Although the CAP is nondeterministic polynomial (NP) complete in general, our algorithm provides an optimal solution for a special class of network topologies. For the general case, promising results are obtained, and numerical examples show that our algorithm has a better performance than many existing algorithms.

Performance analysis of locking and optimistic concurrency control algorithms

Abstract New analytic models are presented which predict the maximum throughput of locking and optimistic concurrency control algorithms for a centralized database system. By making several simplifying assumptions, these models can be easily solved. The analytic results are tested against simulation and are shown to have an accuracy considerably better than some previously reported methods. The models are used to carry out a comparison between locking and optimistic control under stated assumptions. It is found that locking schemes consistently have higher maximum throughput than optimistic schemes.