Wireless sensor networks (WSNs) have restricted use by both academic and industrial societies for a while. This is due to the limited electrical and computational power of a typical sensor node. By moving toward emergency and healthcare related applications such as the disaster response, mobility has become an important feature of WSNs. Having the mobile nodes in a WSNs causes new and even more challenging technical difficulties for its designers. The existing technologies should be revised in order to comply to the new form of applications. In this paper, an enhanced mobility-aware extension to T-MAC, as one of the popular medium access control protocols in WSNs is proposed. The simulation-based experiments show that the proposed method significantly increases the throughput and packet delivery ratio of T-MAC in exchange for a small increase in power consumption.

In this paper we present the design concept and experimental evaluation results of a newly developed DWDM Radio on Free Space Optics (RoFSO) system capable of transmitting simultaneously multiple RF service signals. The system performance is evaluated under effects of atmospheric turbulence over 1-km free-space transmission. Effectiveness of a proposed tracking scheme used to mitigate beam angle-of-arrival (AOA) effect and the viability of the communication system using a specially designed optical antenna are illustrated. An initial attempt to estimate an additional link margin to compensate for tracking error loss is also presented. Finally, we evaluate the performance of heterogeneous wireless signals after being transmitted over the system. We verify that the system can provide a stable connection for heterogeneous wireless services especially in the absence of strong atmospheric turbulence.

Position detection of wireless terminal tends to be important technology in wireless communication system. Demand for position detection is increasing. This paper proposes position detection method using bi-directional LCX (Leaky CoaXial cable). Due to bi-directional and the reflection at end of LCX on purpose, proposed method makes four propagation pathes. Proposed system can detect two dimensional position using the differences of four propagation times. This paper describes the equations for position detection and optimal arrangement about slots.

Recent rapid developments of high-speed wireless access technologies cause WLAN environments where QoS supported APs and not QoS supported APs are mixed. To provide optimal QoS guarantee in this mixed environment, this paper proposes AP selection agent system and its selection algorithm for mixed environment. In the system, agents equipped on some servers, APs and STAs in a network which cooperate with each other via SIP to provide optimal QoS and user oriented services. The proposed algorithm realizes an AP selection for a STA according to its required QoS and traffic load in an overall WLAN service area. Simulation results show improvement in the throughput and frequency utilization efficiency compared to conventional algorithms.

It is difficult to receive video download services in wireless environments. Some service management schemes to consume network resources effectively from the aspect of user's satisfaction (utility) are proposed. In video download services, user's satisfaction depends on both the latency time and the video quality. Some utility functions for the latency time or the video quality in video download service have been already measured. But these utility functions for video download services cannot be applied because users might become insensitive to the latency time when paying their attention to a video. In this paper, we firstly measure utility for the video quality and video download services by subjective assessments. Next, we newly propose a service management scheme to maximize utility for given throughput and show the effect of the proposed scheme applied to variable throughput with computer simulation.

Proposes a coded orthogonal frequency-division multiplexing (COFDM) based road-to-vehicle communication system. The proposed system makes efficient use of COFDM in order to get the implicit transmitter diversity effect. The paper then evaluates the received signal power and bit error rate performance in two diversity models. In one model, two antennas are vertically set up on top of a base station (BS) post. The other model uses two BS and the same signal is simultaneously transmitted from both BS. Computer simulation results show that the proposed scheme is capable of obtaining the transmitter diversity effect in both models.

This paper investigates the propagation characteristics of the inter-vehicle communication system operating in the millimeter band. First, this paper proposes a new propagation model, which takes into account the vehicle traffic, and theoretically derives the path loss and delay profile characteristics using the ray tracing method. Next, this paper investigates the high speed digital transmission performance of the inter-vehicle communication system. Namely this paper theoretically analyzes the bit error rate, required transmission power, and maximum available bit rate of the system. The results show that the vehicle traffic and the antenna size affect the transmission performance. Furthermore, this paper shows the improvement of propagation characteristics by diversity techniques.

This paper propose a new nonlinear distortion compensation scheme for OFDM systems in a multipath fading channel. The proposed scheme compensates for the performance degradation due to nonlinear distortion using the maximum likelihood (ML) detection criterion. Computer simulation results and theoretical analysis show that the proposed scheme can not only compensate for the degradation due to nonlinear distortion, but also give an implicit diversity effect in a multipath fading environment.

This paper discusses the transmission quality in the fiber optic microcellular communication system using a directly modulated laser diode and optical frequency discriminator. A new theoretical approach is used to describe the interaction between the nonlinearities of the directly frequency modulated laser diode and optical frequency discriminator, and the carrier to noise plus distortion ratio is theoretically analyzed. The results show that the receiver sensitivity can be improved by 3 dB using a Mach-Zehnder type frequency discriminator and balanced mixing photodetector. Furthermore, it is clarified that 3rd order intermodulation distortion is effectively suppressed due to the interaction between the laser diode and discriminator nonlinearities. This feature can be applied to adaptive post-distortion nonlinear compensation in the radio-over-fiber FM link.

This paper proposes radio modulation controlled level VPC in ATM networks. This technique combines the modulation level controlled radio (MLCR) and virtual path capacity control (VPC) method. The traffic fluctuation can be absorbed flexibly when applying the MLCR system to the radio channel of ATM networks which consist of wired and radio channels. We theoretically analyze the call blocking probability performance, and consider the simplicity of network design for different values of the ratio of radio links to all links.

This paper proposes a new non-linear distortion compensation scheme for orthogonal multi-carrier modulation systems. Multi-carrier modulation is considered as an effective technique for high speed digital transmission in a multipath fading environment. The proposed scheme, which is based on the maximum likelihood (ML) detection, can compensate the performance degradation due to non-linear distortion. However an ideal ML receiver requires a huge computational cost and is not feasible. To reduce the computational cost required for ML detection, the proposed scheme employs a sub-optimum estimation algorithm. Instead of evaluating all the possible sequences, the proposed scheme evaluates sequences which differs only by one bit from the sequence decoded by the conventional bit by bit receiver. Computer simulation results show that the proposed scheme can compensate the degradation due to non-linear distortion effectively.

This paper proposes a new joint symbol timing, symbol period and frequency offset estimation method for orthogonal multi-carrier modulation signal. The method is based on the maximum likelihood criterion making effective use of "cyclostationary property" of the orthogonal multi-carrier modulation signal, and requires no special signal such as pilot tones to assist the estimation. After a likelihood function for the estimation is derived, an optimum estimator which maximizes the likelihood function is proposed. In addition, in order to reduce the estimator complexity, a suboptimum estimator is also proposed. Computer simulation results show that the optimum estimator requires only about 10 observation symbols for the accurate estimation in a frequency selective slow Rayleigh fading channel as well as in an additive white Gaussian noise channel, and the suboptimum estimator is effective because of the low complexity, although it requires a longer observation period.

A new minimum-dispersion (MID) combiner, which reduces multipath degradation in a high capacity digital microwave radio, is proposed. A high capacity digital microwave radio is inherently very sensitive to waveform distortion caused by multipath in-band delay dispersion and in-band amplitude dispersion. To minimize the in-band dispersion, the combined-signal spectrum shape from the two antennas is monitored before and after a small change takes place in the combining phase. The phase shifter rotates in the direction of the flatter of the spectrum shapes, either the one before or the one after monitoring. Performance evaluations through simulation calculation and theoretical estimation using in-band amplitude dispersion probability density are given. More improvement can be obtained when the MID combiner is used instead of the maximum power (MAP) combiner currently in use. Laboratory and field experiments, using a 200 Mbit/ s 16-QAM signal, verify these analyses and show an additional outage reduction factor of more than 5.

A theoretical analysis is presented for a multi-hop relay system composed of QPSK-DPR's. A Direct Phase Regenerator (DPR) can regenerate a QPSK signal at carrier frequency. Special attention is given to jitter generation and accumulation. Circuit parameters characterizing jitter generation are clarified and obtained quantitatively. The required upper limit for jitter per DPR is also obtained from allotedC/Ndegradation in a multi-hop system. Simulation experiments were carried out using a novel 400 Mbit/s QPSK-DPR with less than 1 dB inherent equivalentC/Ndegradation. The results Show that a hybrid system, consisting of 4 to 9 DPR's and a base band regenerator, is feasible.

Multipath fading effects on a 200 Mbit/s 16 QAM digital radio system have been experimentally investigated. It was found that the outage probability can be accurately estimated using only the in-band frequency response without knowledge of the delay difference between two incident waves. New methods to evaluate the improvement factors for both space diversity reception and dynamic equalizing, which are required to combat multipath fading effects, were also proposed. In particular, the significance of noticing the delay dispersion sign was pointed out when attaining sufficient equalization effect. The validity of these evaluations was confirmed by simulation experiments and a field trial.

This paper describes a regenerator for quadriphase phase shift keying (QPSK) signals at the carrier frequency, which is called a "direct regenerator." The direct regenerator proposed here is realized using a tripler and a quadrupler. This regenerator is characterized by the symbolsm,nands, which denote the intermediate combiner amplitude ratio, the final combiner amplitude ratio and the limiter dynamic range, respectively. Complete regeneration is obtained atm = n = 1ands = \infty. Even if this condition is not satisfied, sufficient regenerative performance for practical use is preserved, when0.8 < m < 1.25, 0.7 < n < 1.4ands > 5dB. To verify the operation principle, an experiment was carried out for 1.7 GHz QPSK signals transmitted at 40 Mbits/s. It is shown that this regenerator has satisfactory decision and reshaping capability in dynamic operation. Using this direct regenerator, decision and reshaping of the QPSK signals can be made without detection. Thus, a repeater can be simplified, compared to a conventional repeater composed of a detector, a baseband regenerator and a modulator.