In this paper, we provide various perspectives related to wireless sensor and mobile ad hoc networking research. We reveal their commonalities and differences and suggest the need for participation from the computer science community, in addition to communications and protocols. In particular, we reveal the various issues that demand deeper investigation, collaborative research, and standardization. We introduced the aim, structure, objectives, and goals of the World Wireless Research Forum (WWRF) and highlight the activities conducted by the WWRF Working Group 3. Finally, we discuss the status of various standardization efforts and present new research issues and challenges.

The development of ad hoc networking is now being watched with keen interest, because it is a key technology for realizing ubiquitous networks and ubiquitous society. In Japan, on ad hoc network, much R&D and related works have been done and on going in academia, industries and governments. Test-bed based research is an important aspect of research on ad hoc networks. The performance of ad hoc networks in real-world situations typically depends on many complex phenomena. This paper introduces a few topics of test-bed based ad hoc network research activities in Japan.

In contention-based wireless ad hoc networks, power control is an efficient way to improve the spatial reuse by allowing multiple pairs to communicate simultaneously. In this paper, we propose a game-theoretical approach for joint power and rate control in ad hoc networks, where the transmit rate of each link is maximized. Meanwhile we consider the transmit power as the cost, since higher power leads to higher interference and more energy consumption. In particular, we introduce a novel auction-like pricing algorithm in which the cost per unit power steps up until the network settles down at a Nash equilibrium, which is a feasible power and rate allocation, even when the Signal to Interference and Noise Ratio (SINR) requirements are initially infeasible. Numerical results show significant throughput improvement and energy consumption savings compared with the previously proposed algorithm that defers the link with minimum SINR.

Wireless networks have been rapidly integrated with the wired Internet and considered as the most popular MAC technology for Ad Hoc mobile communications. In WLAN technologies, IEEE 802.11b WLAN is the most widespread wireless network today. The IEEE 802.11b WLAN supports multiple transmission rates and the rate is chosen in an adaptive manner by an auto rate control algorithm. This auto rate control algorithm deeply affects the total system performance of the IEEE 802.11b WLAN. In this paper, we examine the WLAN performance with regard to the auto rate control algorithm, especially the ARF scheme, which is the most popular auto rate control algorithm in 802.11b based WLAN products. The experimental results indicate that the ARF scheme works well in the face of signal noise due to node location. However, the ARF scheme severely degrades system performance when multiple nodes contend to obtain the wireless channel and the packet is lost due to signal collision.

We investigate the impact of symbol rate control, modulation level control, and the number of hops on the area spectral efficiency of interference-limited multihop radio networks. By controlling symbol rate and modulation level, data rate can be adapted according to received power. In addition, varying the number of hops can control received power. First, we evaluate the achievable end-to-end throughput of multihop transmission assuming symbol rate and modulation level control. Numerical results reveal that by controlling symbol rate or using multihop transmission, the end-to-end communication range can be extended at the cost of end-to-end throughput, and this may result in lower area spectral efficiency. Next, an expression for the area spectral efficiency of multihop radio networks is derived as a function of the number of hops and the end-to-end throughput. Numerical results also reveal that the resulting area spectral efficiency depends on the specific circumstances, which, however, can be increased only by using multihop transmission.

A multihop connection scheme, where one or more mobile terminals relay transmission signals using the same access scheme between an end user terminal and its destination base station, is a promising approach to overcome reduction in cell size caused by high bit-rate data transmission. In a general radio communication system, the coverage area and system throughput are closely interrelated. In this paper, the performance of a multihop cellular network employing a CDMA access scheme, which is a promising candidate for beyond the third generation, is studied in terms of the coverage area and system throughput by conducting a link level simulation. The results show that a multihop connection expands the coverage area, especially in the case of light traffic, and also has an advantage in system throughput.

In this paper, we evaluate the effect of space-time coded cooperative relaying technique in multihop inter-vehicle communication (IVC) networks. The IVC systems have an issue that communication links are often blocked by obstacles such as heavy vehicles. The breakage of a radio link in multihop connections may significantly decrease the system throughput in multihop IVC networks. It is demonstrated through system-level evaluations that the cooperative relaying can offer remarkable capacity enhancement by exploiting multi-route diversity and overcoming accidental link breakage resulting from frequent topological changes.

The issue of importance of multiuser detection for CDMA-based mobile ad hoc networks is addressed in this paper. For conventional scheme, each terminal in the network uses matched filter to receive packets, so the performance (e.g., throughput) of the network suffers from multi-access interference (MAI). Different from above scheme, in this paper, each terminal of the ad hoc network is equipped with an adaptive blind linear multiuser detector, so the ability of MAI-resistance is gained. Employing slotted-ALOHA protocol in MAC layer and using fully-connected network model, the throughput of ad hoc network is studied. Theoretic analysis and simulation results show that multiuser detection can remarkably improve throughput performance of ad hoc networks.

Flooding is usually utilized to find a multi-hop route toward a node which is not within transmission range. However, existing flooding schemes deteriorate the performance of network because of periodic message exchanges, frequent occurrence of collisions, and redundant packet transmission. To resolve the problem, a lightweight and novel flooding scheme is proposed in this paper. The scheme employs ongoing packets for constructing a cluster architecture as the existing on-demand clustering scheme. Unlike to the existing schemes, it makes use of unicast packet transmission to reduce the number of collisions and to find the flooding candidates easily. As a result, the proposed scheme yields fewer flooding nodes than other schemes. Simulation results prove that it causes fewer packet transmissions and fewer collisions than those of two other schemes.

How to safely or reliably flood a packet via broadcast scheme to all nodes in computer networks is an important issue. However, it is a big challenge and critical problem to broadcast data packets over mobile ad hoc networks reliably due to the unsettled wireless links, high mobility, and the lack of the acknowledgment (ACK) scheme. Many solutions deal with this problem by adopting multiple unicast transmissions to achieve reliable broadcast transmission in network layer. Unfortunately, it will cause severe duplicate transmissions and thus rapidly consume the limited network bandwidth. One simplest way to solve this drawback is to broadcast data packets in data link layer. But a serious problem will be arisen that replied ACK frames will collide at the sending node if we enforce each mobile node to reply an ACK after receiving the broadcast data frame. Therefore, in order to overcome the thorny problem, we proposed a broadcast engagement ACK mechanism (BEAM), which is completely compatible with the IEEE 802.11 protocol, for reliable broadcast transmission in the data link layer. We also show that the overhead of raising the reliability of broadcast transmission in network layer would be significantly reduced in data link layer. Simulation results show that the proposed BEAM can reach approximate 100% reliability even in heavy traffic load. We also indicate that the BEAM could be combined with other network layer broadcast schemes to approach higher flooding ratio as well as reduce bandwidth consumption effectively.

Overlay multicast schemes for Mobile Ad hoc Networks (MANET) [1] have been proposed to remove non group members' dependency for multicast forwarding. However, since all packets are relayed from one group member to other group member in a form of unicast packet, they sometimes cause a lot of packets' collision and low resource utilization particularly where group members densely locate. In this paper, we propose a new forwarding scheme, DESIRE (Density awarE heterogeneouS overlay multIcast foRwarding schemE in MANET), for overlay multicast in MANET. DESIRE is significantly different from previous overlay multicast schemes in that it creates separate data forwarding path over overlay DDT (Data Delivery Tree) depending on density of group members. In dense environment, a zone is constructed and a scoped flooding is applied within it. On the other hand, in sparse environment, data dissemination is accomplished in a form of unicast packet similar to previous overlay multicast. By adapting scoped flooding in dense environment, it can reduce a lot of packets' collision and balance battery consumption by making use of mobile node's broadcast capability entirely. Also, in sparse environment, DESIRE can cope with network mobility very well since overlay DDT can remain static regardless of changes in physical topology. Such distinct advantages are evaluated by simulation. Generally, better performance is shown with large group members than small group members.

Many routing protocols have been proposed for mobile ad hoc networks. Among these protocols, the on-demand routing protocols are very attractive because they have low routing overhead. However, few of the existing on-demand routing protocols have considered the link heterogeneity, such as the different communication rate, different Packet Error Ratio (PER). As a result, the routes tend to have the shortest hop count and contain weak links, which usually provide low performance and are susceptible to breaks in the presence of mobility. In this paper, we analyze the existing on-demand routing protocols and propose a Link Heterogeneity Aware On-demand Routing (LHAOR) protocol, where the link quality and mobility are considered. Specifically, the Local Update (LU) is proposed and the link metric is inversely related with the Received Signal Strength Indicator (RSSI). By using the LU method and RSSI information, the routes adapt to the topology variation and link quality changes, and reach the local optimum quickly, which contains strong links and has a small metric. Simulation and experiment results show that our LHAOR protocol achieves much higher performance than the classical on-demand routing protocols.

Unlike traditional networks, the characteristics of mobile wireless devices that can dynamically form a network without any infrastructure and wired line mean that mobile ad hoc networks frequently display partition owing to node mobility or link failures. Consequently, an ad hoc network is difficult to provide on-line access to trusted authorities or centralized servers. Despite the existence of well-known security mechanisms, the absence of a stationary central authorization facility in an open and distributed communication environment is a major challenge. Consequently, applying traditional Public Key Infrastructure (PKI) security architecture to mobile ad hoc networks will create secure blind sides. Based on this perspective, this study proposes a novel scalable and robust cluster-organized key management scheme. Distribution of trust to an aggregation of cluster heads using a threshold scheme faculty provides mobile ad hoc networks with robust key management. Furthermore, the proposed approach provides Certificate Authority (CA) with a fault tolerance mechanism to prevent a single point of compromise or failure, and saves CA large repositories from maintaining member certificates, making the proposed approach more suitable for numerous mobile devices. Additionally, this study proposes a Cluster Secure Based Routing Protocol (CSBRP) to integrate into the key management to enhance non-repudiation of routing information and routing performances. Finally, this study introduces a mathematical model to demonstrate that the proposed cluster-based communication outperforms the node-based approach.

In this paper we present a network layer based admission control and simple class based service differentiation model to support QoS in mobile ad hoc network. Our distributed admission control procedure works along with the route finding phase of reactive routing protocols for mobile ad hoc network (AODV, DSR etc). We also propose a simple class based distributed service differentiation system to support QoS once a traffic is admitted by our admission control mechanism. The proposed service differentiation is based on DiffServ model and includes modifications like configuration of each node with edge and core functionality, dynamic selection of edge/core functionality, use of minimal and simple classes. Simulation results show that our system allows seven times more real time traffic in the network than the proposed QoS for AODV model while satisfying the demanded end-to-end delay and providing low jitter.

This paper proposes the MultiPath streaming scheme with Media Synchronization control (MPMS) for audio-video transmission in wireless ad hoc networks. In many audio-video streaming applications, media compensate each other from a perceptual point of view. On the basis of this property, we treat the two streams as separate transport streams, and then the source transmits them into two different routes if multiple routes to the destination are available. The multipath transmission disturbs the temporal structure of the streams; in MPMS, the disturbance is remedied by media synchronization control. In order to implement MPMS in this paper, we enhance the existing Dynamic Source Routing (DSR) protocol. We compare the application-level QoS of MPMS and three other schemes for audio-video transmission by simulation with ns-2. In the simulation, we also assess the influence of the multipath transmission on other traffic. The simulation result shows that MPMS is effective in achieving high QoS at the application-level.

This paper proposes a user revocation scheme for decentralized networks. User revocation is a method to distribute a group decryption key that is shared by n users in a group so that all but d revoked users can obtain the key. In decentralized networks such as ad-hoc networks, mesh networks, and Peer to Peer (P2P) networks, a sender should revoke the access of a dishonest user or an unauthorized user as soon as possible to protect the security of group communication. However, if the sender distributes the group key to all users aside from the revoked user, it would take a long time to revoke a user in a large group. In addition, users must set shared group keys for each user without a privileged center. We propose a scheme in which the amount of transmission and the key storage of each user are small.

In virtual cellular network (VCN), proposed for high-speed packet mobile communications, the signal transmitted from a mobile terminal is received by wireless ports distributed in each virtual cell and relayed to the central port that acts as a gateway to the core network. In this letter, we apply the multi-hop maximal ratio combining (MHMRC) diversity and propose the route modification algorithm in order to improve transmit power efficiency degradation caused by the carrier frequency difference between the control and the data communication channels for VCN. The transmit power efficiency and the distribution of the number of hops are evaluated by computer simulation for a VCN.

A method of evaluating the wavelength filter spectrum response is introduced. The increase of the crosstalk level due to the filtering and the relation between the total crosstalk and the spectral efficiency are derived in detail using the Gaussian filter. Since this method can be applied to various kinds of filter spectrum responses, the ultimate spectral efficiencies of filters are compared. In this comparison, the problem of the box-like filter, which has been considered to be desirable, is revealed, and this is improved by cascading the filter spectrum. The requirement on the rejection floor that inheres in the filter is also made clear.

Deflection routing is one of the promising approaches to resolve contention in the optical burst switching networks. In the conventional deflection routing scheme, optical bursts may be unable to traverse the route evaluated to select an outgoing link because of the contention at succeeding downstream transit nodes. As a result, the optical bursts may traverse a different route resulting in a long distance or decreased performance. This paper proposes a deflection routing scheme that considers the possibility of the contention at downstream nodes. This scheme utilizes the "expected route distance" instead of the static route distance toward a destination node. The expected route distance considers the possibility of contention at each downstream transit node and is calculated using measured link blocking probabilities at each downstream transit node. The selection priority of each outgoing link is given dynamically based on its expected route distance toward a destination node. By considering the possibility of contention at downstream nodes, a routing scheme with high performance can be realized. The loss rate of optical bursts is improved when an imbalanced load is applied to the network, and the loss rate of optical bursts is also improved when the network includes links with extremely different distances.

A new expression for cell/packet loss probability in an ATM and packet switched queue system with a finite buffer is presented. Cell and packet loss analysis is based on the new concept of a "buffer overflow cluster" and the overflow probability for a queue with an infinite buffer. This approach holds for a wide variety of long-range dependent traffic sources typical of wide-area networks, as well as Internet and other communication traffics. The method is verified by simulations of two long-range dependent traffic scenarios: fractional Gaussian noise and multifractal wavelet model traffic with a beta marginal distribution.

The recent Internet needs a network structure and traffic engineering that can support various applications requiring differentiated traffic processing and a high quality of service. The extension of the Internet from wired to wireless systems that generate location-dependent and burst errors has made the support of good services more difficult with existing packet scheduling algorithms. Accordingly, this paper proposes a wireless differentiated service packet scheduling (WDSPS) algorithm that can provide reliable and fair services in differentiated wireless internet service networks. As such, the proposed scheduling algorithm solves the HOL blocking problem within a class packet queue that occurs in a wireless network, supports differentiated services for each class defined in a differentiated service network, and facilitates gradual and efficient service compensation not only among classes but also among flows within a class, thereby preventing a monopoly by one class or one flow. Simulations confirmed that the proposed WDSPS scheduling algorithm could provide the required QoS differentiation between classes and enhanced the service throughput under various wireless network conditions.

We propose a novel routing algorithm for reverse proxy servers, called load balancing content address hashing (LB-CAH), and evaluate the performance of the proposed routing algorithm compared with that of the content address hashing (CAH) and the hash and slide (HAS) routing algorithms. The proposed LB-CAH routing algorithm calculates the popularity of pages in the load balancer using an LFU caching technique and periodically makes a popularity list. Using this popularity list, the proposed routing algorithm selects a reverse proxy server as follows. When the requested page appears in the popularity list, the request is routed according to the round robin method; otherwise, it is routed according to the content address hashing method. We evaluate and compare the LB-CAH, CAH and HAS routing algorithms by simulation experiments from the viewpoints of load balancing, consumed cache space and cache hit rate. Simulation experiments show that the proposed LB-CAH routing algorithm achieves almost the same degree of load balancing as the HAS algorithm and the same cache hit rate as the CAH algorithm for reverse proxy servers in various web site environments.

For a CDMA system with a single carrier, we consider a call control policy at each cell, which gives priority to handoff calls over new calls while meeting the overall call quality. New calls are first under the call control of the threshold type, and then receive services together with the handoff calls but under the outage restriction guaranteeing a pre-specified call quality. An optimization model with such quality-guaranteeing constraints is formulated, which is to determine the threshold value for each cell, minimizing the new call blocking probability. We propose a solution heuristic, with which a number of simulations are conducted under a variety of traffic environments. The computational experiments evaluate the usefulness of our call control scheme in that handoff calls are given an appropriate level of priority while the system capacity is effectively utilized.

This paper presents a new transmission scheme of M-ary biorthogonal pulse position modulation (BPPM) in ultra wideband systems. The proposed scheme incorporates position-wise parity information to improve the probability of symbol detection over multipath channels. A linear filter-based channel modification is also introduced to mitigate multipath degradation and maximize the probability of symbol detection by using parity information. The analytical and numerical results show that the proposed scheme achieves a significant improvement of symbol error rate (SER) with very low computational complexity and no symbol delay.

Estimation of unknown signal parameters with sensor array measurements has been investigated quite extensively. Also, there has been in recent years an explosive increase in the number of mobile users in wireless cellular systems, thus contributing to growing levels of multi-user interference. To overcome this problem, application of adaptive antenna array techniques to further increase the channel capacity has been discussed. In this paper, a new model of locally scattered signals in the vicinity of mobiles is proposed by defining the mean steering vector and manipulate it mathematically for several distributions. Under this model an estimation method of the direction of arrival is investigated based on a weighted subspace fitting technique. Statistical analysis and simulations are also considered.

The power line communication (PLC) system should be investigated with respect to the influence on electromagnetic environments. Longitudinal conversion loss (LCL) and input impedance are important parameters for evaluating the influence because they are closely related to the radiated, conducted, and inducted emission. An indoor AC mains system consisting of electrical equipment and an AC mains line was modeled by four-port networks, and the LCL and the input impedance were calculated. The parameters of the four-port networks were determined from theory and measurement. The analytical model was examined using a simple network and the results show that the calculated values agreed with the measured ones. The LCL and the input impedance were investigated at the AC mains port in some existing buildings, and the measured results almost agreed with the calculated results derived from the indoor AC mains system model.

In this paper, the electrical constants of a biological tissue-equivalent agar-based solid phantom from 3.0 to 6.0 GHz are described. The developed phantom can reproduce the electrical constants of biological tissues from 3.0 to 6.0 GHz, and it is not necessary to change the phantom for each frequency band in the range of 3.0 to 6.0 GHz during the measurements. Moreover, adjustments to the dielectric constants of the phantom at 3.0, 3.8, 5.2, and 5.8 GHz are presented. The constants of this phantom can be adjusted mainly by using polyethylene powder and sodium chloride. The phantom can be used to evaluate the Specific Absorption Rate (SAR) as well as the antenna characteristics in the range of 3.0 to 6.0 GHz. Furthermore, the effect of the electrical constants of the phantom on the SAR is investigated. The investigation of SAR measurements is performed on the phantom at 5.2 GHz using the thermographic method. Calculations using the FD-TD method and the finite difference method based on the heat conduction equation are carried out in order to evaluate the thermal diffusion in the measurements using the thermographic method. The measured and calculated results are in good agreement. There is evidence that the thermal diffusion influences the SAR estimation at 5.2 GHz more than in a lower frequency range even though this method basically does not depend on the frequency.

ERS-1/2 wind scatterometers (WSC) data sets may contain cells in which two, instead of three, measurements are available. These data sets are called incomplete. Operational inversion procedures discard such data sets and therefore no wind field is estimated. This is very limiting in semi-closed seas such as the Mediterranean Sea. In this paper we propose a new inversion procedure capable to retrieving near surface wind fields from incomplete data sets. This procedure is an enhancement of an algorithm already proposed and tested by the authors. A set of comprehensive experiments are presented and discussed. It is shown that the inversion procedure gets to remarkable results even in presence of a large number of doublets.

A fast encoding technique is described for vector quantization (VQ) of line spectral frequency parameters. A reduction in VQ encoding complexity is achieved by using a preliminary test that reduces the necessary codebook search range. The test is performed based on two criteria. One criterion uses the distance between a specific single element of the input vector and the corresponding element of the codevectors in the codebook. The other criterion makes use of the ordering property of LSF parameters. The fast encoding technique is implemented in the enhanced variable rate codec (EVRC) encoding algorithm. Simulation results show that the average searching range of the codebook can be reduced by 44.50% for the EVRC without degradation of spectral distortion (SD).

Data broadcast has become a promising approach to achieving information dissemination in wireless environments due to the limited channel bandwidth and the power constraints of portable devices. In this paper, a restricted dynamic programming approach which generates broadcast programs is proposed to partition data items over multiple channels near optimally. In our approach, a function to predict the optimal average expected delay, in terms of the number of channels, the summation of the access frequencies of data items, and the ratio of the data items is developed by employing curve fitting. Applying this function, we can find a cut point that may be very close to the optimal cut. Thus, the search space in dynamic programming can be restricted to the interval around a determined cut point. Therefore, our approach only takes O(N log K) time, where N is number of data items and K is the number of broadcast channels. Simulation results show that the solution obtained by our proposed algorithm is near-optimal.

In 2002, Yeh, Shen, and Hwang proposed a one-time password authentication scheme using smart cards. However, Tsuji et al. and Ku et al. showed that it is vulnerable to the stolen verifier attack. Therefore, this paper proposes an improved one-time password authentication scheme, which not only keeps the security of the scheme of Yeh-Shen-Hwang but also can withstand the stolen verifier attack.

Adaptive Virtual Queue (AVQ) introduces a novel implementation algorithm for Active Queue Management (AQM). The stability criterion for AVQ was deduced in literature [1], but it lacks practicability due to the difficulty of solving the transcendental equation. In this letter, the AVQ stability is further investigated based on the characteristic roots of delay-differential equation. Another stability criterion explicitly associated with parameters of network configuration is deduced and the upper bound of delay time for stable AVQ algorithm is determined. Finally, the conclusion is validated through simulation experiments.

In this letter, we propose a novel approach for use in the analytical modeling of the overall performance of a Hybrid ARQ (type I and II) together with arbitrary channel model, based on Hidden Markov Model (HMM). Using the combined HMM model developed for involved ARQ protocols with the finite state channel model, such critical performance measure as throughput and delay can be derived in closed form. Analytical results are derived for Stop-and-Wait as well as Go-back-N type together with the type I and type II Hybrid ARQ scheme adopted. We compare the analytical results along with the simulation results in order to check the correctness our model, and show the efficiency of our approach by applying it to realistic environments such as the CDMA IS-95 system with its derived equations.

A novel phase error correction scheme is proposed for the high rate OFDM-based wireless local area networks (WLANs). The proposed scheme makes the system capable of efficiently compensating the whole phase error due to the residual sampling clock offset and frequency offset estimation error after timing and frequency offset compensation, as well as the phase noise.

In this letter, we propose an efficient transmission slot selection scheme for Band Division Multi-Carrier-CDMA (BD-MC-CDMA) systems under the constraints of packet loss and delay bound for each individual session. By utilizing channel dynamics together with the delay deadline and loss history, one can determine whether to transmit or not during each time slot, based on the prediction of future channel variations. To validate the efficiency of the proposed algorithm, we model each sub-band as a discrete time Markov Chain using a finite state Markov channel (FSMC) and derive the criteria required for transmission decision. Simulation results show that our proposed scheme can satisfy quality of service (QoS) requirements for real-time traffic with minimum use of resources, while increasing throughput of non-real-time traffic with the resources saved from real-time traffic.

In this paper, a refined analytic model is presented for the IEEE 802.11 distributed coordination function (DCF) in a time-varying channel environment. In the proposed model, the channel is modelled using a finite-state Markov (FSM) chain. The saturation throughput and average packet delay are analyzed from the proposed model. It is shown using OPNET^TM and UltraSAN^TM simulations that the proposed model accurately predicts the performance of the IEEE 802.11 DCF.

Space-time array manifold model is usually used in a fast fading channel to estimate delay for the radio location. The existing additive white Gaussian noise (AWGN) estimation error model significantly overestimates the delay estimation. In this paper, we model the estimation error of the space-time array manifold channel impulse response (CIR) matrix as a correlated AWGN matrix and its performance is shown to be closer to the estimation error of practical systems than the existing model.

Using several high rate recursive convolutional codes as the basic element and the trace criteria as the designing principle, a new kind of recursive space-time trellis code with more flexible and higher data rate is presented for the serially concatenated space-time code. When 2^b-ary modulation and N transmit antennas are used, the data rate of the new code can be arranged from b bps/Hz to Nb-1bps/Hz by modifying the number of recursive convolutional codes and the data rate of each code.

Diagonally weighted space-time trellis codes using one-bit feedback of channel information are considered. We derive performance criteria for designing STTC and weighting matrix in perfect feedback channel. A simple modification of conventional STTC using unitary rotation is proposed and some modified STTCs are found through computer search. Simulation results show that the modified STTCs achieve the improved performance over a wide range of feedback channel condition.

We investigate the optimal chip rate of power or rate adapted direct-sequence code division multiple access (DS/CDMA) communication systems in Nakagami fading channels. We find that the optimal chip rate that maximizes the spectral efficiency depends upon both the channel parameters, such as multipath intensity profile (MIP) and line-of-sight (LOS) component, and the adaptation scheme itself. With the rate adaptation, the optimal chip rate is less than 1/T_m irrespective of the channel parameters, where T_m is multipath delay spread. This indicates that with the rate adaptation, correlation receiver achieves higher spectral efficiency than RAKE receiver. With the power adaptation, however, the optimal chip rate and the corresponding number of tabs in RAKE receiver are sensitive to MIP and LOS component.

This letter proposes two very-low-complexity maximum-likelihood (ML) detection algorithms based on QR decomposition for the quasi-orthogonal space-time code (QSTBC) with four transmit antennas [3]-[5], called VLCMLDec1 and VLCMLDec2 decoders. The first decoder, VLCMLDec1, can be used to detect transmitted symbols being extracted from finite-size constellations such as phase-shift keying (PSK) or quadrature amplitude modulation (QAM). The second decoder, VLCMLDec2, is an enhanced version of the VLCMLDec1, developed mainly for QAM constellations. Simulation results show that both of the proposed decoders enable the QSTBC to achieve ML performance with significant reduction in computational load.

This letter proposes to employ the Discrete Cosine Transform (DCT) interpolation-based channel estimation (DCTI-CE) method for practical MIMO-OFDM (Multiple Input Multiple Output--Orthogonal Frequency Division Multiplexing) system so as to achieve its potential ability for enabling high transmission data rate. The various computer simulations are conducted to evaluate the DCTI-CE method in MIMO-OFDM system under the multipath fading channel for wireless LANs.

This paper proposes the design to reduce the number of fed elements by replacing with parasitic elements in an array antenna. The study depends on the analysis of electromagnetic wave fields in consideration of the coupling between the half-wavelength dipoles. The case of 2 fed elements and 2 parasitic elements is considered as a unit cell to form the total array. After optimizing the element arrangement, the antenna gain can match that of the equivalent 4-fed element case. Feeding networks in a high power radiating system are analyzed in terms of the length and matching of feed lines, and the arrangement of amplifiers.

The transient scattering characteristics of millimeter waves from a cylindrical object near a flat boundary were measured by the 50 GHz scatterometer to evaluate the multiple interactions of scattered waves with the objects and the boundary. Both perfectly conducting and dielectric cylinders are considered as a scattering object. The pulse intensities including waves scattered first from the object and then from the flat boundary or vice versa are shown to be significantly influenced by the distance from the object to the boundary, depending on the refractive index of the object. The observed higher order responses including the multiple scattering between the object and the boundary are also discussed. A preliminary comparison of the measured and calculated pulse responses for the perfectly conducting object is presented at slightly oblique incidence on a flat boundary.

Unwanted electromagnetic emission occurs due to the common-mode current on the cables entering a PC's metal enclosure and can be treated as wire antennas passing through the apertures of the enclosure. To reduce the emission, a stack of metal rings is suggested to be placed around the cable and external to the aperture, adopting the concept of a Coaxial Band-Stop Filter, for the first time. The influence of this novel structure on the common-mode current is examined in the FDTD-method frame work.