In this paper, the capacity is analyzed for a CDMA system supporting heterogeneous traffic with on/off activity. The capacity is analyzed by modeling the on/off traffic activity as a binomial random variable and compared to the conventional analysis with the simplified traffic activity factor which is a constant. It has been shown that the capacity with the conventional analysis is more optimistic than the capacity analyzed with the binomial modeling of traffic activity. The effect of traffic activity on the capacity is further investigated for two different cases. One is the case under the same transmission rate where the average rate changes according to the traffic activity. The other is the case under the same average rate where the transmission rate changes according to the traffic activity. As the traffic activity factor becomes larger, the capacity increases for the case under the same average rate, whereas it decreases for the case under the same transmission rate.

An effective transmitter power control method is indispensable in combating the near-far problem and minimizing the effect of interference on the system capacity. This paper investigates the power control problem for the uplink direct sequence-code division multiple access (DS-CDMA) system. In recent works on power control problem, power levels are assumed to take on any positive real value. However, in practical systems, power levels are discrete. This paper therefore proposes a power control algorithm over a discrete set of power levels in which each base station locally optimizes the power set of all users in its own cell. Also, we investigate its convergence and power consumption properties. Simulation results indicate that the proposed power control algorithm converges faster and provides an improved performance.

This paper considers a wireless coherent system that enables high-speed-data transmission in the presence of carrier phase error over an additive white Gaussian noise (AWGN) channel. Carrier phase noise is caused by imperfect carrier tracking of the coherent demodulation. The channel characteristics of the system were modeled using phase noise whose stochastic process followed the Tikhonov distribution. For this model, we first propose an optimum detector that produces the most suitable decoding metric for a soft-input/soft-output (SISO) decoder, and then develop some simpler forms of the optimum detector to obtain efficient implementation at close to optimal performance. Those simple detectors that have a wide range of performance/complexity tradeoffs are promising in various applications. To evaluate the effectiveness of the proposed detectors, we have applied them to a bandwidth-efficient turbo-coded modulation scheme in which a component decoder based on SISO principles necessitates more exact channel measurement than is possible with a conventional decoder based on Viterbi decoding. Simulation results have demonstrated that the optimum detector enables excellent bit error rate (BER) performance that exceeds that with a normal detector designed for AWGN channels by more than 1 dB at a BER of 10^-6 under a severe phase noise environment.

We propose a new resource management scheme, Distributed Channel Allocation Protocol (DCAP), for multimedia Ad hoc Wireless LANs (AWLANs). This scheme implements a Quality-of-Service (QoS) providing distributed resource management on the Orthogonal Frequency Division Multiplexing-Code Division Multiple Access (OFDM-CDMA) channel architecture. According to the performance evaluation results for MPEG traffic sources, DCAP can be a good choice of resource management scheme for AWLANs supporting multimedia services on the Broadband Wireless Access (BWA)-type physical layer.

In this paper, a simple subcarrier selection combining technique is proposed for orthogonal frequency-division multiplexing (OFDM) systems with multiple receive antennas. The subcarrier-based selection algorithm is developed in the frequency domain to achieve an optimal selection combining gain for OFDM systems, instead of the antenna-based selection algorithms in the time domain or frequency domain. The proposed technique selects an optimal subcarrier with a maximum channel gain among all the receive antennas with the same subcarrier position, based on the estimated channel frequency response during the training period. Hardware complexity for the proposed technique is minimal since it requires single front-end with multiple receive antennas and single baseband demodulator. It is shown by computer simulation that a significant gain can be achieved by the proposed technique over the conventional selection combining technique for OFDM systems in practical situations.

This paper deals with the performance evaluation of different channel resource management techniques in LEO satellite systems based on an earth-fixed cell concept. Furthermore, in order to reduce the handover failure probability, we assumed that handover attempts can be queued. Both fixed and mobile users have been considered resulting in several classes of users. Each class requires a given Quality of Service (QoS) and thus a fixed part of the shared resource. Two channel allocation techniques are investigated: fixed channel allocation (FCA) and dynamic channel allocation (DCA). An analytical model is derived to analyze the performance of the FCA scheme supporting different kinds of users. A second analytical approch is proposed, in the FCA case, where a handover queuing strategy is taken into account. Implementation aspects for FCA and DCA strategies are discussed and compared in terms of blocking probabilities relative to each type of users.

This paper presents a two-tier coordinated checkpointing algorithm which can reduce the number of messages by being composed of two levels in mobile computing. Thus mobile devices have a high mobility and are lack of resources (e.g., storage, bandwidth, and battery power), traditional distributed algorithms like coordinated checkpointing algorithms could not be applied properly in mobile environment. In our proposed two-tier coordinated checkpointing algorithm, the messages to be transferred are requested by the mobile hosts and are handled by the appropriate MSS's (Mobile Support Stations). And the broadcast messages are handled by MSS instead of relaying the messages to all the mobile hosts directly as with the previous algorithms. This can reduce the communication cost and maintain the overall system consistency. In wireless cellular network, mobile computing based on a two-tier coordinated checkpointing algorithm reduces the number of synchronization messages. We perform performance comparisons by parametric analysis to show that a two-tier coordinated checkpointing algorithm can reduce communication cost compared to the previous algorithms in which the messages are directly sent to the mobile hosts.

This paper proposes a real-time scheduling mechanism for web server that finds a way to solve so-called priority inversion problem between tasks handling real-time web requests with higher priorities and tasks with lower priorities like HTML requests. The priority inversion problem of web server stems from operating two independent schedulers, web scheduler and kernel scheduler, without exchanging scheduling information or considering the requirement of each other. In the proposed mechanism, two schedulers are integrated in a kernel and the integrated scheduler schedules real-time web tasks and other application tasks together based on their priorities. Since the unified scheduler sees all tasks simultaneously in system, it can schedule the tasks with their absolute priorities and reduce the execution delay caused by the priority inversion. The proposed mechanism provides web server with a better chance to efficiently serve real-time web requests. The feasibility and performance of the proposed mechanism are empirically shown on an embedded system.

In this study, the frequency and spatial properties of undesired electromagnetic radiation distribution around a simple printed circuit board (PCB) model, which only has the mismatching printed line (PL) and ground, are estimated. Finite difference time domain (FDTD) modeling is developed for the analysis space, which is 500 400 51 mm³ in size, around the PCB. As the driving clock pulse has a very wide frequency bandwidth, ranging from kHz to GHz, basic and precise investigation of the noise emission mechanism from the basic model is performed. The results of the magnetic field H_x on the PCB as determined by FDTD simulation, and those of the experiment, driven by a clock pulse, agree well. The results show that although this approach is basic and simple, it becomes clear that the frequency and spatial characteristics of the electric and magnetic field near the PCB are influenced by the wavelength of the frequency and appling the driving clock pulse, and the low-frequency component of the electromagnetic distribution around the PCB is larger than the high-frequency components. It is suggested that the low-frequency noise problem should be carefully considered.

For common-mode noise current reduction, a CMC (Common Mode Choke) is widely used in signal transmission line circuits consisting of a ground and two conductors (a balanced transmission line). However, a common-mode noise current reduction characteristic is not clearly analyzed yet in the case that a CMC is inserted in a balanced transmission line. In this paper we propose the calculation method of CMC insertion losses and derive an equation to analyze the common-mode current characteristics of a balanced transmission line with a CMC inserted. The analyzed frequency range is from 100 kHz to 100 MHz. We conclude that in the frequency range up to 30 MHz: (1) the proposed insertion loss calculation method is useful for analyzing CMC insertion losses in differential-mode and in common-mode; (2) the derived circuit equation can be applied for analyzing the common-mode current characteristics of a balanced transmission line locally unbalanced with conditions of a CMC inserted; (3) the proposed calculation method may give the expected results that a CMC should be placed in a signal source side of an unbalanced point of a pair-cable for reduction of common-mode currents; and (4) if it is placed in a terminal (or load) side of an unbalanced point, there is no effect, or rather common-mode currents are increased by the insertion of a CMC.

This paper focuses on a global ultrasonic system for self-localization of a mobile robot. The global ultrasonic system consists of some ultrasonic generators fixed at some arbitrary position in the global coordinates and two receivers in the moving coordinates of the mobile robot. This system is used to obtain the state vector of the mobile robot in the global coordinates from the distance measurement between the ultrasonic generator and the receiver. In order to avoid the cross-talk and to synchronize the ultrasonic sensors, the sequential cuing technique using small-sized radio frequency module is adopted. An extended Kalman filter algorithm is used to process the noisy ultrasonic signal and to estimate the state vector. Computer simulations and experiments are conducted to verify the effectiveness of the proposed global ultrasonic system.

In 2002, Hwang and Yeh proposed some improved schemes to mend several security flaws in the Peyravian-Zunic password transmission scheme and password change scheme. However, this article will point out that there still exist some security flaws in the Hwang-Yeh schemes; at the same time, we shall also propose some improved versions of their schemes.

User authentication is necessary on the Internet and in mobile communications to protect the legal user's rights. One-time password authentication methods change the verifier every time by sending the present verifier along with the next verifier. However, such methods risk impersonation attacks because those protocols use two verifiers every session. The OSPA (Optimal Strong-Password Authentication) method is a one-time password method which prevents stolen-verifier problems, replay attacks, and denial of service attacks. In this letter, we devise an impersonation attack on the OSPA method and discuss how to break down the OSPA method.

A new clock and data recovery circuit (CDR) is realized for the application of data communication systems requiring GHz-range clock signals. The high frequency jitter is one of major performance-limiting factors in CDR, particularly when NRZ data patterns are used. A novel phase detector is able to suppress this noise, and stable clock generation is achieved. Furthermore, optical characteristics for fast locking are achieved with the adaptive delay cell in the phase detector. The circuit is designed based on CMOS 0.25 µm fabrication process and its performance is verified by measurement results.

A novel RF CMOS high Q-value active inductor is proposed in this work by using simple cascode RC feedback compensation technique. The performance of this active inductor has maximum Q-value about 1.2E6, inductance value from 3.5 nH to 4.5 nH and 3E-5Ω of minimum total equivalent loss, in the range of 1.2 GHz to 2 GHz.

The coarse frequency offset estimation algorithm has to provide an initial frequency error estimate, which is sufficiently accurate in order to operate reliably for the subsequent fine frequency synchronization algorithm. In this letter, we deal with a coarse frequency offset estimation for digital audio broadcasting. We propose an improved frequency synchronization scheme which uses the minimum energy detection scheme. We compare the performance of proposed scheme with that of conventional schemes under AWGN and Rayleigh channel. It has been shown that the proposed algorithm has high robustness against a large range of symbol timing offset with a low complexity.

This letter contains our proposal for a new iterative decoding algorithm for Turbo coded V-BLAST system. The proposed algorithm is based on maximum a posteriori (MAP) decision criterion. In a V-BLAST system concatenated with Turbo codes, the extrinsic information from the soft output channel decoder can be utilized as a priori probability, making it possible to apply MAP decision criteria to the V-BLAST decoding process. The MAP decision criterion is applied to the V-BLAST ordering and slicing procedure, resulting in a considerable gain in bit error performance. As the iteration rate increases, the proposed system exhibits performance similar to that of system with ideal sliced.

Without transmit power control (TPC) and Rake combining, the uplink capacity of a direct sequence code division multiple access (DS-CDMA) packet mobile communication system significantly degrades due to the near-far problem and multipath fading. In this letter, assuming a single cell system with an interference-limited channel, the impact of the joint use of Rake combining and TPC on the uplink capacity is evaluated by computer simulation. Slow TPC is found to give a link capacity larger than fast TPC. This is because, with slow TPC, the received signal power variations due to fading remain intact and this results in a larger capture effect.

A peak power reduction technique is proposed for subcarrier transmit power control applied orthogonal frequency division multiplexing (OFDM) system. In the proposed system, carrier-holes are created by applying a partial non-power allocation (PNPA) technique in which no transmit power is allocated to subcarriers with lower received E_b/N₀, and the amplitude and phase adjusted peak reduction subcarrier (PRS) is iteratively inserted in the non-power allocated subcarrier so as to suppress peak power. Computer simulation confirms that the proposed scheme can reduce peak power by 3.6 dB while keeping the same information bit-rate with conventional subcarrier transmit power control applied OFDM systems.

In this paper, we study a delay transmit diversity system combined with antenna diversity reception that transmits the time-delayed and weighted versions of the same signal from multiple antennas. At a receiver, multiple receive antennas are used and all delayed signals received on multiple antennas are coherently combined by a Rake receiver. The set of optimum antenna weights for maximizing the received signal-to-noise power ratio (SNR) after Rake combining is theoretically analyzed to show that the optimum solution is to transmit only from the best antenna that has the maximum equivalent channel gain seen after Rake combining. The bit error rate (BER) performance is theoretically analyzed and evaluated by computer simulation. The combined effect of transmit diversity and transmit power control (TPC) is also investigated.

We propose a new input arbitration method for an input buffered switch with a buffered crossbar. In the proposed method, each input module selects the first eligible queue from the starting point. The starting points of input modules are different from each other in any case. We show that the uniqueness of the starting points improves the switch performance. Finally, using computer simulations, we confirm the proposed method is better than the conventional method under the uniform and on-off traffic.

The conservative mode and the greedy mode scheduling algorithms for OBS switch with shared buffer are presented and discussed. Their performance is evaluated by computer simulations, as well as that of the greedy mode with void-filling algorithm. Simulation results show that the conservative mode and the greedy mode have different characteristics under different input load. The greedy mode and the conservative mode are more applicable in a real system than that with void-filling, owing to their lower computational complexity and FIFO characteristic. Finally, a composite algorithm integrated by the conservative mode and the greedy mode is proposed, which is adapted to the input load with the help of an input load monitor. The simulation results reveal that it has favorable performance under different load.

In this letter, we present a new IPv6 multihoming scheme, which complements in both load sharing and delay aspects. The advantages of the proposed scheme are as follows: 1) it provides efficient load sharing algorithm. 2) Since it distributes concentrated tunneling overhead to many other border routers, it can solve a point of failure problem. 3) Finally, it can drastically reduce end-to-end delay by means of establishing alternative optimized path. The simulation results show the distinct performance of our approach.

The technique of replica placement has been extensively employed to improve client perceived performance and disperse server workload. In this paper, we study some well-known algorithms of replica placement on the network and observe the logarithmic relationship between replica number and total access cost. Numerous simulations are done and it is found that some replica algorithms obey the logarithmic relationship with high correlation coefficients. A logrithmic function is proposed about replica number and total access cost. The logarithmic relationship is applied to the minimum facility problem and a function is deduced to get the optimal replica number.

The complexities of many previous scheduling algorithms for IRIS tasks are too high to apply them to practical systems handling many tasks. The high complexities stem from the fact that the algorithms perform scheduling on all tasks in a system, producing an optimal solution. As a way to reduce the complexity of scheduling, we propose a heuristic on-line scheduling algorithm that schedules the IRIS tasks only in a scheduling window, producing a sub-optimal solution. A scheduling window contains W important tasks determined by a selection policy. The performance of the proposed algorithm is verified by simulation.

In an attempt to enhance the impedance bandwidth and gain, a notched patch antenna is proposed that has a pair of L-strip feeders, and it has been experimentally studied. The enhanced features were confirmed and proved by comparing the proposed antenna with the antennae that have been reported in the relevant literatures. The experimental results showed that the impedance bandwidth (SWR2) of 35.74% and the peak gain of 8.69 dBi (at 2.17 GHz) were obtained by the frequency band under 2.5 GHz. Designed originally for PCS and IMT-2000 service band, the proposed antenna, with its simple structure, may easily be mass-produced and may have various commercial applications.

Wall shadow removal problems differ due to configuration of walls in different propagation scenarios. Applying Weiler-Atherton polygon clipping method helps calculate illuminated regions on the building walls, but unfortunately the technique has limitation when there are many walls and the walls configuration is complex. The modified Weiler-Atherton polygon clipping method proposed can solve the problem by regarding all vertices of the subject polygon or clipping polygon, that are also intersection points as simply intersection points. In the case that a certain vertex of the subject polygon is a vertex of the clipping polygon, this vertex of the subject polygon is still considered a vertex. It is found that wall shadow removal using the proposed modified Weiler-Atherton polygon clipping method is more efficient.

The authors propose a circular polarization generator using a square waveguide with L-type probe located on a side wall of waveguide. The sides of the L-probe forming the right angle are placed perpendicular to the direction of propagation. The position of the right angle is kept at a distance of a quarter-wave length from the side wall. The device provides 27% bandwidth for 3 dB axial ratio at 8.4 GHz on azimuth interval of 160.