Chin Tau Lea

Multi-log/sub 2/N networks and their applications in high-speed electronic and photonic switching systems

A class of switching networks is proposed to remove the time and space bottlenecks of conventional RAM-controlled switching architectures. The advantages of these networks, such as tolerance of faults O(log/sub 2/N) stages between each inlet-outlet pair, self-routing capability, easy path hunt, and easy fault diagnosis, and their implementation are discussed. Graph theory is used to construct strictly nonblocking or rearrangeable nonblocking networks that are based on the new architecture. >

Received Signal Strength-Based Wireless Localization via Semidefinite Programming: Noncooperative and Cooperative Schemes

The received signal strength (RSS)-based approach to wireless localization offers the advantage of low cost and easy implementability. To circumvent the nonconvexity of the conventional maximum likelihood (ML) estimator, in this paper, we propose convex estimators specifically for the RSS-based localization problems. Both noncooperative and cooperative schemes are considered. We start with the noncooperative RSS-based localization problem and derive a nonconvex estimator that approximates the ML estimator but has no logarithm in the residual. Next, we apply the semidefinite relaxation technique to the derived nonconvex estimator and develop a convex estimator. To further improve the estimation performance, we append the ML estimator to the convex estimator with the result by the convex estimator as the initial point. We then extend these techniques to the cooperative localization problem. The corresponding Cramer-Rao lower bounds (CRLB) are derived as performance benchmarks. Our proposed convex estimators comply well with the RSS measurement model, and simulation results clearly demonstrate their superior performance for RSS-based wireless localization.

Strictly nonblocking directional-coupler-based switching networks under crosstalk constraint

Directional-coupler (DC)-based switching systems can switch signals at the rate of several terabits per second. Such switches can also transmit signals with multiple wavelengths simultaneously. Despite these advantages, DCs suffer from an intrinsic crosstalk problem that must be overcome in building a robust switching system. In this paper, the principles of constructing strictly nonblocking DC-based photonic switching systems under various crosstalk constraints are explored. We demonstrate how crosstalk adds a new dimension to the theory of switching systems. We find the sufficient nonblocking condition for photonic networks under crosstalk constraints and demonstrate that some well-known nonblocking networks can tolerate a stricter crosstalk constraint while retaining their hardware complexity. The theory developed in the paper can guide us in making the design tradeoff between the level of crosstalk and the amount of hardware.

Log/sub 2/ (N, m, p) strictly nonblocking networks

A class of self-routing strictly nonblocking switching networks, called the Log/sub 2/ (N, m, p) network, is introduced. The proposed networks are self-routing and have O (Log/sub 2/ N) stages between each inlet-outlet pair. The self-routing capability makes it attractive for high-speed switching; the small number of crosspoints between each inlet-outlet pair also makes the network well suited for many nonlogic photonic switching technologies. The complexity and the fault-tolerant capability of the strictly nonblocking Log/sub 2/ (N, m, p) network are analyzed. To show the merits of the proposed network, the authors compare it to the Cantor network, which is also strictly nonblocking and has similar characteristics. >

Stability and Optimization of the CSMA and CSMA/CD Channels

A comprehensive study of the stability and optimization of the infinite population, slotted, nonpersistent CSMA and CSMA/ CD channels is presented. The approach to both stability and performance optimization differs significantly from previous work, and provides a number of new results including robustness in stability and performance in the presence of channel and control parameter variations. It is first shown that both channels are unstable under the usual assumption of random retransmission delay. Pakes lemma is then applied to study the properties of a type of distributed retransmission control which provides stable channels. Basic results are in the form of inequalities which define stability regions in the space of channel and control parameters, and further permit one to specify controls which maximize channel throughput as a function of packet length and CD time with stability guaranteed. The delay versus throughput characteristic for the stabilized channels is derived and used to demonstrate the performance achievable with these channels.

Architecture design, frequency planning, and performance analysis for a microcell/macrocell overlaying system

An innovative hierarchical microcell/macrocell architecture is presented. By applying the concept of cluster planning, the proposed sectoring arrangement can provide good shielding between microcells and macrocells. As a result, underlaid microcells can reuse the same frequencies as overlaying macrocells without decreasing the macrocell system capacity. With the proposed method, microcells not only can be gradually deployed, but they can be extensively installed to provide complete coverage and increase capacity throughout the service area. With these flexibilities, the proposed method allows existing macrocellular systems to evolve smoothly into a hierarchical microcell/macrocell architecture.

Improving call admission policies in wireless networks

It is well known that the call admission policy can have a big impact on the performance of a wireless network. However, the nonlinear dependence of new calls and handoff calls makes the search for a better call admission policy – in terms of effective utilization – a difficult task. Many studies on optimal policies have not taken the correct dependence into consideration. As a result, the reported gains in those studies cannot be confirmed in a real network. In this paper we develop a solution to the problem of finding better call admission policies. The technique consists of three components. First, we search for the policy in an approximate reduced‐complexity model. Second, we modify the Linear Programming technique for the inherently nonlinear policy‐search problem. Third, we verify the performance of the found policy in the exact, high‐complexity, analytical model. The results shown in the paper clearly demonstrate the effectiveness of the proposed technique.

Wide-sense nonblocking Banyan-type switching systems based on directional couplers

Directional couplers are capable of switching signals with multiple wavelengths. They are the ideal technology for building light-path switches to configure wavelength division multiplexing (WDM) transmission facilities. This technology, however, has a major constraint: crosstalk. Design of a strictly nonblocking directional coupler-based photonic switching system under a specified crosstalk constraint has previously been studied. The results indicate that a lower level of crosstalk can be achieved at a cost of some extra amount of hardware. In this paper, we show that if a simple rule is followed in setting up the connections, both the amount of hardware for nonblocking networks and the connection setup time can be greatly reduced. The resulting networks thus belong to the class of wide-sense nonblocking networks.

Bipartite graph design principle for photonic switching systems

Several attributes of a bipartite graph are exploited in designing switching systems. First, in a network based on bipartite graphs, no two paths are allowed to intersect at a vertex. This attribute is used to design a directional coupler-based photonic switching network with very low crosstalk. Since crosstalk is the most limiting factor in constructing a large directional-coupler-based photonic switching network, crosstalk reduction on the device and architecture levels is an important design issue. Secondly, broadcast, which is an intrinsic property of a bipartite graph (the same is always true with a crossbar representation), is used in designing multiconnection switching networks. Thirdly, it is shown that the nonplanar nature of bipartite graphs makes them well suited for designing three-dimensional free-space-transmission photonic switching systems. >

On call admission control in DS/CDMA cellular networks

Analytical models are proposed for various direct sequence code-division multiple-access (DS/CDMA) call admission control schemes. Many mathematical call admission models for DS/CDMA cellular networks have been proposed. However, they have shortcomings. First, by ignoring the stochastic traffic load variation or call blocking effect, they failed to sufficiently characterize the second moment of other-cell interference. This leads to inaccurate analysis of a real network. Second, the optimal control parameters were often obtained through an exhaustive search which was very time consuming. Finally, the estimation of system capacity in previous models was obtained by using a simple one-slope path-loss propagation model. However, it is well known that a two-slope path loss propagation model is needed in a line-of-sight (LOS) microcell propagation environment. We propose an analytical model for call admission to overcome these drawbacks. In addition, we combine a modified linear programming technique with the built analytical model to find better call admission control schemes for a DS/CDMA cellular network.