In recent years, the scheduling scheme of exploiting multiuser diversity gain has attracted much attention for the purpose of efficient spectrum utilisation. The authors consider the problem of analysing the quality of service in the scheduler extracting multiuser diversity gain. The authors use not only the channel state information but also the queue information in determining the optimal user to be scheduled. Through queuing analysis, the guaranteed rate, the average delay and the delay‐bound violation probability can be calculated.

In this study, the authors consider a delay control problem with proportional fair scheduling in time-varying wireless channels. For proportional fairness of transmission rates among users, a congestion control scheme is proposed for guaranteeing both throughput and delay of the wireless network. Specifically, a scheduling weight is introduced to control queue lengths without degrading the throughput of the network because the delay of users depends on their queue length. Using the asymptotic approximation technique, the throughput and delay performance of the proposed scheme are analysed and showed the multiuser diversity effect analytically in the Rayleigh-fading wireless channels. Finally, simulation results show that the proposed scheme improves delay without degrading throughput.

Over wireless multihop networks, the conventional transmission control protocol (TCP) cannot allocate the bandwidth fairly to flows. This is because flows with different locations competing for the same wireless channel may have very different perceptions on congestion in terms of packet delay and packet loss rate. In this study, the authors introduce a new window control algorithm and a new estimation method of congestion, which is called a delay information-based fair congestion control protocol (DFCCP). A new window control algorithm not only supports starved flows, but also allows starved flows to share the bandwidth fairly. Furthermore, since congestion occurs at multiple nodes over wireless multihop networks, the authors detect the level of congestion based on the summation of the average window sizes of multiple flows. The simulation results show that the proposed algorithm supports the starved flows while providing fairness among flows over wireless multihop networks.

Since the spaceborne spotlight synthetic aperture radar (SAR) has the long slant range and the high altitude, it generates the curved flight path due to the orbit curvature effect and the time delay due to the long round-trip time of the chirp pulse. Thus, to obtain the SAR image with high quality, we extend the omega-K algorithm. First, we convert the curved path into the straight-line path based on the resampling and the rotation of the transmitted-pulse path and the received pulse path. Then, using the modified matched filter and the modified Stolt interpolation based on the principle of the stationary phase, we compensate the phase error induced by the start-stop assumption of the omega-K algorithm.

We propose a method of obtaining the autocorrelation matrix to improve the performance of the subspace-based channel estimation method. By exploiting the circulant property of the received signal, the proposed method reduces the number of received blocks required to satisfy the persistency of excitation (POE) criterion for the input and decreases the residual error which affects the accuracy of the noise subspace. To verify the proposed method, we derive the analytical method for evaluating the POE criterion for the input. In simulation results, we demonstrate the efficiency of the proposed method.

The network capacity of wireless systems can be increased drastically by extracting multiuser diversity gain. The multiuser diversity gain is obtained by granting scheduling priority to the mobile station (MS) with the best current channel condition. There are a couple of drawbacks in multiuser diversity schedulers such as the linearly increasing feedback load with the number of MSs and the degrading fairness. To balance the capacity request against the fairness request, the authors need a scheduler achieving capacity and fairness tradeoff. In this study, the authors propose a new scheduling scheme of selecting a single MS, which can achieve the trade-off between capacity and fairness with reduced feedback. The simulation result shows that the proposed scheduling scheme attains better fairness compared with the conventional method for achieving capacity and fairness trade-off.

There is a trade-off problem between power consumption and streaming service quality over the wireless multihop networks. If the transmission power increases to overcome the bad link condition, the quality of streaming service can improve, but it can stress the other links and deplete the battery power of nodes. However, since the existing algorithms for streaming services do not consider this trade-off problem, streaming service quality cannot be guaranteed with the high power efficiency. In this study, the authors formulate the optimisation problem to solve the trade-off problem. Then, the authors propose a new cross-layer algorithm to achieve the derived optimal solution. The proposed cross-layer approach covers three layers: application layer, transport layer and physical layer. The newly proposed cross-layer algorithm not only avoids the underflow and the overflow of the decoder buffer, but also achieves the high playout rate and the high power efficiency. Through simulation results, it is shown that the proposed algorithm achieves the high and smooth playout rate with the high power efficiency.

Although the scheduling problem of the uplink transmission in the IEEE 802.16 broadband wireless access network is extensively discussed, most of the results are limited to the quality of service (QoS) upon throughput and delay requirement. But as in practice only limited wireless resources are made available, a fairness-based scheduling upon each connection's QoS provides better outcomes. In this study, the authors propose a new fair uplink scheduling for real-time polling service and non-real-time polling service with the proportional sharing of excess bandwidth of the network. To implement the proposed fair scheduling that satisfies the delay requirement and full bandwidth utilisation, the authors introduce the rate control algorithm. With the proposed scheduling, we guarantee the fairness, delay requirement and full bandwidth utilisation which are not fully achieved in the existing results.

The existing congestion control algorithms for streaming service experience the performance degradation over IEEE 802.11 multihop network because they mainly focus on the prevention of the buffer overflow at the specific node. However, congestion collapse happens not because of the buffer overflow but because of the serious contention at multiple nodes. As the MAC layer contention becomes serious, the end-to-end delay and the packet loss rate due to collision increase while throughput smoothness is not guaranteed due to fluctuation of the queueing delay at each node. Here, the authors propose a TCP-friendly congestion control algorithm based on the MAC layer contention state to improve streaming service. Specifically, each flow controls its transmission rate based on the estimated contention state which each node notifies to each flow by marking packets. The simulation results show that the proposed algorithm utilises the channel bandwidth more efficiently with providing throughput smoothness compared with the existing algorithms.

An approach to improve the performance of subspace-based channel estimation for multiple-input multiple-output (MIMO) systems is proposed. In a subspace-based channel estimation method, a small number of received blocks cause the residual error which affects the accuracy of the noise subspace because the covariance matrix is estimated from the received signal to find the noise subspace. To reduce the effect of residual error affecting the accuracy of the noise subspace, a measure called the Hermitian angle is introduced and the receive diversity is exploited based on the iterative algorithm.

For flexibility and cost reduction, the forward-looking bistatic spotlight synthetic aperture radar (FL-BSSAR) which uses the side-looking transmitter and the forward-looking receiver has been studied recently. For the FL-BSSAR imaging, we introduce a modified omega-k algorithm. Specifically, using rotation and shifting of the receiver path and resampling of the transmitter path, we convert the FL-BSSAR into the side-looking BSSAR. Then, we modify the matched filter and Stolt interpolation using the principle of the stationary phase to obtain the SAR image. Simulation results show that the modified omega-k algorithm enhances the image quality.

Bistatic spotlight synthetic aperture radar (SAR) which uses a separated transmitter and receiver has been studied intensively due to its flexibility. To reconstruct the image for the bistatic SAR in the parallel track configuration, where the transmitter and the receiver have equal velocity, we modify the omega-k algorithm. Specifically, using the extended Taylor approximation (ETA), we convert the parallel track configuration into the single track configuration. Then, using the principle of the stationary phase, we propose an analytical method to increase the image quality of bistatic SAR systems.

Congestion control for streaming real‐time applications, which need smoothness of the transmission rate, should be transmission control protocol (TCP)-friendly. Moreover, in wireless networks, TCP-friendly congestion control should be based on differentiation of packet losses due to congestion and wireless link error to improve network utilisation. The authors propose a TCP-friendly congestion control algorithm based on explicit congestion notification over the wireless networks. The simulation results show that the proposed algorithm utilises the link bandwidth efficiently, providing smoothness of the transmission rate.

The existing congestion control algorithms such as TCP Reno are not suitable for efficient utilisation of networks with a high bandwidth-delay product because it takes a very long time to achieve the full link utilisation. Furthermore, it is difficult to guarantee the fairness among the TCP connections with different round-trip times. To overcome these problems, a new window control algorithm using the buffer state and variable gains is proposed. The simulation results show that our algorithm improves the performance of TCP in the wireless networks.

Considers the nonblocking supervisory control problems of nondeterministic discrete event systems (DESs) which are modeled as nondeterministic automata with /spl epsiv/-moves. Based on language models, the paper presents the necessary and sufficient conditions for the existence of a nonblocking supervisor to achieve a given language specification for a nondeterministic DES. The developed nonblocking supervisor always guarantees the absence of blocked states in a controlled nondeterministic system. Moreover, when the language specification does not satisfy the existence conditions, the paper provides the computational algorithm for finding the supremal language of the specification which satisfies the conditions. Furthermore, the results developed are demonstrated through the example of an assembly work station.

This paper addresses the problem of fault-tolerant robust supervisory control of discrete event systems (DES) with model uncertainty. Necessary and sufficient conditions for the existence of a fault-tolerant robust supervisor are derived. If the conditions are satisfied, then tolerable fault event sequences embedded in the system can be achieved. The results obtained are applied to design, modeling, and control of a workcell in a production line consisting of two gas metal arc welding (GMAW) robots, a sensor, and a conveyor.

There has been much interest in studying the stability properties of discrete event dynamic systems (DEDS), and several definitions for stability with some kinds of stability analysis methods have been proposed, recently. However, they are too much theory-oriented and do not provide any sufficient understanding based on real applications. Therefore, in this paper, we explore the meaning of stability concepts in DEDS through the case study of a manufacturing system, the plasma etching system in semiconductor manufacturing processes. We apply the concepts of stability and stabilizability upon a general characterization of the stability properties of automata-theoretic DEDS models. The stability notion is expounded by considering the control-law synthesis of the plasma etching system in view of stabilizability.

A discrete event dynamic system (DEDS) approach is utilized to improve the reliability of a system from the fault-tolerance viewpoint. We propose a systematic way to classify faults and failures quantitatively and to find tolerable fault event sequences embedded in DEDSs. After this, the synthesis of a fault-tolerant supervisory control system is investigated. A case study of a photolithographic process in a semiconductor manufacturing system is provided to illustrate these techniques.

The theory developed by J.-T. Lim and S.M. Meerkov (see ibid., vol.COM-35, p.1278-88, Dec. 1987) is extended to channels with long propagation delays. The performance characteristics of Markovian access control protocols in satellite channels are evaluated. It is shown that the channel propagation delay may lead to improvements in systems behavior, i.e. to elimination of the saturation effects and to stabilization of the network in a steady state with relatively high performance characteristics.>

In this paper, a unified, analytical framework for analysis of Markovian access control protocols in collision channels with a finite number of buffered users is presented. The theory developed is based on: 1) an appropriate state-space modeling of a network, 2) the averaging theory for the slow-in-the-average Markov walks used for the local analysis, and 3) an asymptotic large deviations theory used for the global analysis. Specific new results obtained here are: 1) formulas for the steady-state buffer OCCupancy and the probability of packet rejection, 2) a relationship between the capacity of the users buffers and the steady-state throughput, time delay, and the probability of packet rejection, 3) an asymptotic formula for the saturation period, and 4) a new adaptive protocol, the adaptive TDMA, is constructed, and it is shown that it is superior to the adaptive ALOHA system.