Underwater optical wireless communication (UOWC) systems are attracting attention in the context of beyond 5G/6G networks for their capacity to ensure reliable and high-data-rate communications. The system under consideration consists of underwater nodes communicating with an onshore base station through relay nodes. In such scenarios, system performance can be affected by pointing errors resulting from the movements of both underwater and relay nodes. In this paper, we investigate the end-to-end performance of a UOWC-free space optical (FSO) system under the influence of pointing errors while considering the depth-dependent characteristics of the underwater link. We present a comprehensive mathematical model that accounts for relevant impairments, including attenuation, turbulence, and pointing errors. A closed-form expression for the outage probability is derived by applying the cascaded underwater turbulence model. The results obtained highlight the validity of using a cascaded turbulence model. We also show that it is possible to determine an optimal system configuration, specifically the beam radius at the receiver, to alleviate the induced fading caused by pointing errors, thereby enhancing the end-to-end system performance.

This study focuses on visible light communications using a high-speed display and rolling-shutter camera of a smartphone. We aim to achieve high-quality communication imperceptible to the human eye through the high-speed display. The capture rate of an image sensor is generally slow. Then, the image sensor cannot obtain some parts of a transmitted data image because of the high-speed display rate. We employ the rolling shutter camera that reads pixels row by row and propose a receiving scheme in which the camera obtains some transmitted data images in a captured image. The proposed scheme can acquire transmitted data, even if the display rate is higher than the capture rate. We experimentally evaluate the proposed scheme in terms of the quality of the visual information and the communication quality to clarify its superiority.

This study proposes a flight algorithm for delay-tolerant networks using unmanned aerial vehicles (UAVs) that cooperatively serve users on the ground. Information from a ground user is relayed across UAVs in a store-and-forward manner before transferring to another user. The network performance considerably depends on how UAVs move because of this feature. In this study, two types of intermittent communication between UAVs and between a UAV and a user are jointly considered to a varying degree in the UAV operation. The measure and optimization problem for UAVs to move are devised, and this measure is heuristically maximized. Simulation results demonstrated the effectiveness of the proposed algorithm in terms of information delivery with certain insights in the UAV movement based on UAV-to-user and UAV-to-UAV communication concepts.

In imperfect communication environments, such as those with high packet loss, systems using remote videos to perform robot teleoperations exhibit video quality degradation and reduced operability, which deteriorates work quality. In this study, we proposed using a video transmission method to suppress video quality degradation and associated reduction in work quality. Instead of using video compression and transmission, we combined motion JPEG, which compresses individual frames and transmits them sequentially, with erasure correction using parity, to reduce video freezing in remote video transmission and improve work quality. To validate this method, we measured the reception rates of video frames with and without erasure correction. We conducted subjective evaluations using an industrial robot that erasure correction was effective in teleoperation environments with sufficient bandwidth. However, in environments with limited bandwidth, the increase in data volume should be considered because of erasure probability and parity assignment. Furthermore the selection of an appropriate strength of erasure correction is necessary.

This paper proposes an industrial robot system that interpolates data loss due to imperfect communication. Most wireless communication standards adopt Carrier Sense Multiple Access protocol. This protocol may introduce intentional delay in packet transmissions to avoid interference, on top of the delay caused by transmission failure and retransmission. This delay can vary the reception timing of commands to the robot, and affect the behavior of the robot. To overcome this problem, we develop an algorithm that interpolates the data not received appropriately at the robot side. Our findings demonstrate that the proposed method reduces unnecessary behavior (vibration) compared to the conventional one, thus improving the operability robot.

Intelligent reflecting surfaces (IRSs) are an emerging technology that can be used to alleviate the requirements of a strict line of sight in free-space optical communication (FSO) systems. This paper investigates the performance of an IRS-assisted FSO system under the conditions of strong atmospheric turbulence fluctuations, taking into account the impact of the spatial jitter at the transmitter and the IRS. A closed-form for the bit-error rate (BER) has been derived. The impact of the strong atmospheric turbulence is characterized using the K-distribution. The obtained results demonstrate that the performance depends on the system configuration as well as the transmitter and IRS jitter. Furthermore, we show that optimal placement of the IRS can improve the BER.

This paper presents a high-speed uplink optical wireless communication system using multiuser multiple-input multiple-output (MIMO) technique. The proposed system is composed of multiple photodetectors installed on the ceiling and multiple transmitters located near the floor with multiple light-emitting diodes (LEDs). To improve the quality of the uplink channel, we apply angle diversity for each transmitter consisting of multiple LEDs with different orientations. The performance evaluation with one transmitter fixed and the other randomly moving is conducted. Based on the obtained results, we show that the proposed system achieves a BER of each transmitter better than 10−4over most locations while maintaining a data rate of 100 Mbps.

In free space optics (FSO) communication, one of the main challenges consists in ensuring a line of sight (LOS) link between the transmitter and the receiver. To overcome this stringent requirement, intelligent reflecting surfaces (IRSs) are considered as a promising solution to provide more controllable links and increase communication coverage. In this paper, we investigate the performance of an IRS-assisted FSO system considering the most degrading factors, including atmospheric turbulence and pointing error induced fading due to both the transmitter jitter and the IRS jitter angles. A closed-form expression of outage probability has been derived. Based on the obtained results, it is shown that the system performance depends on the placement of the IRS and the atmospheric turbulence. Furthermore, we show that it is possible to determine the optimal link distance depending on the placement of the IRS.

A wireless relay network using unmanned aerial vehicles (UAVs) to deliver messages, such as damage reports and safety confirmation, in wide areas affected by large-scale disasters is proposed. In this system, UAVs move autonomously following a predetermined flight method and deliver a message in a delay-tolerant manner by transmitting it hop by hop to other UAVs within the communication range. In this study, we propose a UAV flight method for non-uniform-user-distribution environments. The proposed method is based on coverage control and utilizes the Voronoi cell with a density function. We considered cases where the user distribution was available in advance and also where it was not. Simulation results showed that the proposed method could improve effectiveness in terms of average message delivery delay, especially when the user distribution was considerably non-uniform.

This study considers an autonomous inspection system of power transmission lines using multicopter-type un-manned aerial vehicles (UAVs). In this system, base stations (BSs) are placed on power transmission towers, and the control information for autonomous flight of UAVs is exchanged between the BSs and UAVs through wireless transmissions. In this study, we conduct transmission experiments between the transmitter of a UAV and a receiver placed on the power transmission tower. In the transmission experiments, we employ LoRa in 920 MHz band and measure the received signal strength indicator (RSSI) to investigate propagation characteristics. From the measured results, we calculate the mean and standard deviation of RSSI and discuss the feasibility of the system.

This paper discusses the uplink performance of an indoor data collection system. A base station (BS) collects information sent asynchronously from multiple transmitters using the optical code division multiple access (CDMA) technique. The communication performance may degrade because of the near-far problem caused by variations in the propagation path length between each transmitter and the BS receiver. We apply spatial diversity to mitigate the interference and improve the error rate performance. In our proposed method, the correlation values are normalized at each photodetector, and the largest value is selected for symbol estimation. The obtained results show that a performance improvement can be achieved using the proposed method for a small number of users.

This paper investigates underwater optical wireless communication (UOWC) systems. We introduce a system model including the main factors impairing the optical signal intensity, i.e., light absorption, scattering, together with water turbulence, which have not been thoroughly investigated in other UOWC studies. The optimal receiver design is based on maximum a posteriori (MAP) estimation while considering not only thermal noise but also shot noise. In addition to the MAP-based receiver, a simplified configuration of the receiver is also examined. The symbol error rate (SER) is analytically calculated for 3 different modulation schemes, i.e., pulse-position modulation (PPM), pulse-amplitude modulation (PAM), and pulse-amplitude-and-position modulation (PAPM). The obtained results show that a SER of $10^{-3}$ is achieved at a distance of 20 m for all the modulation schemes. The superiority of PPM over PAM and PAPM in terms of the SER is also shown. We demonstrate that the simplified receiver obtains almost the same SER performance as the MAP receiver.

A wireless LAN relay system is designed to extend the transmission range while maintaining stable communication. In this paper, we propose a wireless LAN relay system using multicopter unmanned aerial vehicles (UAVs) and evaluate the communication performance. For the demonstration experiment, we employ air-to-air and ground-to-air communications, where the former is a link between UAVs, and the latter is a link between a UAV and ground node. We measure the throughput and the received signal strength indicator, and clarify the communication performance of the wireless LAN relay system.

In a large-scale indoor environment such as an industrial one, a multiple-input-multiple-output optical wireless communication (MIMO-OWC) system represents a flexible and easy-to-deploy solution to achieve point-to-point long-distance and high-speed data transmission. The majority of imaging MIMO-OWC systems have mainly targeted high data rate communication over short distances, not exceeding 10 m. In this work, on the contrary, we focus on longer-distance links providing high-speed transmission data rate. For this purpose, we propose an imaging MIMO-OWC system that features an improved optical modulator array with a one-dimensional element arrangement, and a new imaging receiver using long focal length lenses placed in front of the receiving photo-detector array. The performance of the proposed system is then experimentally evaluated. The obtained results show that the measured MIMO channel matrices have sparse patterns, which means that the elements not corresponding to the emitted beams are nearly zero. We successfully demonstrate the transmission at a high data rate of 100 Mbps over a long distance of 50 m. This result surpasses the achieved distance corresponding to the conventional MIMO-OWC system using an independent optical system for each light-receiving element. Consequently, the experimental results confirm that our proposed system has the potential for higher speed with longer distances.

This study considers a power line communication (PLC) based multi-machine feedback control system. In PLC, channel characteristics have cyclostationary features synchronous to the mains voltage, and based on the multiple access schemes, the communication performance significantly differs. In this situation, previous work has evaluated the system which uses time division multiple access (TDMA) and found that the controlled quality of each machine becomes unequal. This manuscript thus proposes employing synchronous code division multiple access (SCDMA) and compares the control performance with the system using TDMA. As a result, using the proposed SCDMA, it is observed that all the machines achieve the same control quality, and the control quality itself is better than that with TDMA when the signals are much stronger than noise.

In this paper, we examine wireless relay networks using drones as emergency communication systems during large-scale disasters. Flying drones are inclined in the direction of their movement. This inclination causes a reduction in the received power when taking into account the effects of antenna directivity in the vertical plane and the polarization plane deviation. In this paper, we model the relationship between the inclination and flying speed of drones and derive the received power taking into account the inclination. We evaluate the influence of the drone's inclination on the delay time through simulation. The results indicate that the speed of the drone that achieves the shortest delay time is dependent on the number of drones.

For efficient collection of the leaf temperature data over the entire farm, we simultaneously use a drone in the sky and sensor nodes on the ground that are both equipped with infrared thermometers. In this study, we propose a communication protocol for wireless sensor network (WSN). The preliminary results of a prototype experiment show that the measurements are sufficiently synchronized for collecting useful leaf temperature data.

For large-scale disasters, it is important to ensure communications to confirm the information about safety and disaster situation. Then, this paper focuses on an emergency communication system using drones. In this system, an important issue is to shorten the delay time of transmitting information. Since delay time depends highly on a flight scheme of the drones, we propose two flight schemes: a rebounding-flight scheme and a common-flight-path scheme. Moreover, we consider breaks of drones due to their battery exhaustion because it is impossible for the drones to fly permanently. We evaluate the delay time with and without drones' breaks for the proposed flight schemes. Our results show that the proposed flight schemes are effective to reduce the delay time even if the drones' breaks are considered.

In this paper, a cross-layer design for communication and control layers for wireless feedback control systems is considered, and an optimized receiver design for state feedback is proposed. The proposed receiver is designed on the basis of a maximum à posteriori probability decision and utilizes the control layer's information, namely, the estimated state information of a Kalman filter-based state observer, as the à priori information of the transmitted state feedback. In addition, the proposed receiver provides the error covariance information of the received state feedback for the control layer. The recursive structure of the receiver and state observer is simple and suitable for the feedback control loop. The improvement in the channel error probability is supported by the control accuracy and vice versa. Numerical results show that the proposed receiver can effectively reduce the number of channel errors in the received state feedback and improve the control performance.

Packet losses degrade the performance of wireless feedback control systems. The use of channel coding may decrease the packet loss rate; however, its redundancy decreases the packet transmission rate under the same bit rate and thus degrades the control quality. In this paper, we propose a variable length channel coding scheme for state feedback without decreasing the packet transmission rate. The proposed scheme reduces the number of state variables to be fed back and instead adds redundant bits of channel coding on the basis of state estimation error. We show that the proposed scheme improves the control quality without decreasing the packet transmission rate.

In wireless feedback control systems with multiple plants, state information and control information are sent over a shared communication channel. The IEEE 802.15.4 beaconenabled mode has a contention access period (CAP) and a contention free period (CFP). In the CAP, carrier sense multiple access/collision avoidance (CSMA/CA) is used. In the CFP, time division multiple access (TDMA) is used. This paper considers wireless control over the IEEE 802.15.4 beacon-enabled mode and discusses how to use the CAP and the CFP. We propose two schemes that assign the plants to the CAP and the CFP; one is based on communication success/failure and the other is based on control error between state information and the target value. We show that the proposed assignment schemes improve the quality of an overall plant control compared with random assignment. In addition, the scheme based on the control layer achieves better control quality than that based on the communication layer.

Multiple access communication is required to control multiple plants via wireless communication channels. This study discusses time division multiple access (TDMA) schemes to improve control quality for feedback control of multiple plants. To improve the control quality, time slot allocation of the TDMA should be based on information of the control layer, i.e., control objective and plant's state. In wireless control of multiple plants, there are different target values for the plants and different packet losses on communication channels. This leads to different tracking performances among the controlled plants. Therefore, a flexible time slot allocation scheme that considers the tracking performances is required. In this paper, we propose time slot allocation schemes to minimize predicted tracking error on the controlled plants. One minimizes the predicted tracking error in all time slots, and the other sequentially minimizes that in each time slot. Numerical results show that the proposed schemes can improve an overall control quality under packet loss.

In this paper, we consider the reduction of the amount of transmission data in wireless feedback control systems. Because available frequency bandwidth is limited in wireless communication, the amount of control data that can be transmitted in a sampling period is limited. A source coding scheme that has variable length codes can efficiently reduce the amount of transmission data; however, a variation in packet loss rate due to the variable code length highly affects the control quality. If control data are assigned to long codes, packet loss rate of the data becomes large. Therefore, the control quality deteriorates if control data of high importance are assigned to long codes. From this view point, we propose a source coding scheme that considers the importance of control data. We evaluate the performance of the proposed coding scheme under a data-rate constraint and show that the proposed coding suppresses the deterioration of the control quality and reduces the amount of transmission control data.

This paper considers H∞control for wireless networked control systems with data packet loss. In wireless networked control systems, the control performance deteriorates due to packet loss. Therefore, packet loss is one of major weakness in the wireless networked control systems. To overcome this weakness, this paper proposes a new control design that considers the influence of packet loss as a disturbance. Our new control design belongs to the H∞control design and can reduce the influence of disturbances acting on the controlled object. Numerical examples show that the proposed H∞control design can more effectively reduce control performance deterioration due to packet loss. This paper provides not only the proposal of the new control design but also control performance comparisons of the proposed control design and the Linear Quadratic (LQ) control design. Numerical examples show that the control performance of the proposed H∞control design is better than that of the LQ control design.

Digital signage, which is advertising media using a liquid crystal display (LCD), is increasing in popularity. One possible application is wireless communications between digital signage and viewers. We focus on image-sensor-based visible light communication (VLC). Image-sensor-based VLC requires only an image sensor (IS), which is common equipment in smart phones. In the proposed method, an information signal is displayed on LCD. However, embedding the information signal into an advertisement may decrease the quality. We propose blue color difference modulation to make such signals imperceptible. We implemented a VLC system between digital signage and IS to demonstrate, the effectiveness of the proposed modulation in a real environment.

We focus on MIMO mesh networks, which consist of wireless nodes supporting MIMO transmission. Communication quality of a wireless link using MIMO spatial streams is sensitive to change in its channel, and performance will be unstable. For stable operation of MIMO mesh networks, a method analyzing the MIMO mesh networks is needed. In this paper, we aim to propose an analyzing method of MIMO mesh networks. We measure the time variation of throughput and Received Signal Strength Indicator (RSSI) as feature values of a link in our testbed of the MIMO mesh network, and analyze the measurement results by using Self-Organizing Map. As a result, we confirm that it is possible to find the links that have correlation with time variation of throughput and RSSI by applying SOM to the measured feature values.

Our goal is the improvement of the data collection in solar-powered wireless sensor networks. One of the methods to improve the data collection is cooperative transmission by which a node relays data from a source node to a base station. However, the cooperative transmission is not enough to prevent the data collection performance from the decline in low power generation, such as in a cloudy day and at night. We tackled this problem by introducing a repeater role to same nodes, which only relays data from the other nodes without transmitting its own data. Our proposal improves the data collection performance in the low power generation. Numerical examples compare and show the effectiveness of the conventional method and three assignment methods of the repeater role: one is to assign the repeater role to some nodes fixed in place around the farthest nodes, one is to assign the repeater role uniformly randomly to some nodes, and another is to assign the repeater role to some nodes that do not have sufficient battery levels.

An emergency node-combined wireless network that uses balloons, electric vehicles, and unmanned aerial vehicles (UAVs) was proposed. We discuss the construction and specific issues of this network. The communication area is evaluated by changing the node elevation. In this network, the elevations of balloon and UAV nodes vary, and the communication area changes with elevation. In addition, the network may be temporarily disconnected by changes of the communication area. To solve this problem, we employ a delay-tolerant network technique. We evaluate the number of nodes and node density according to the desired application and regional characteristics.

In this paper, we propose area transmission efficiency to evaluate the performance of single-user multiple-input multiple-output (SU-MIMO) and multi-user MIMO (MU-MIMO) in wireless mesh networks (WMNs). The area transmission efficiency is the number of packets that can be transmitted per unit time and per area, and is calculated by considering the multiplexing effect of MIMO and the density of transmitting nodes. The performance of SU-MIMO and MU-MIMO is evaluated and compared in term of the area transmission efficiency in WMNs. As a result, the area transmission efficiency of MU-MIMO can be superior to that of SU-MIMO when the number of queueing packets is small. Conversely, this relationship is reversed when the number of queueing packets is enough large. In addition, by selecting the transmission packets so as to reduce the number of receiving nodes, MU-MIMO is always superior to SU-MIMO.

In wireless feedback control systems, reducing packet loss is critical. The use of channel coding may decrease such packet loss; however, redundancy in channel coding also decreases the packet transmission rate although the bit rate remains the same. Thus, control systems are highly affected by disturbances. To improve control system quality, channel coding should be adaptively employed on the basis of packet loss and disturbances. Herein, an on-off error control coding scheme based on the communication and control layers is proposed. In this scheme, channel coding is adaptively used to minimize tracking errors in a controlled plant. Results demonstrate that the proposed on-off error control coding scheme improves the control system's reference tracking performance compared with that when using a fixed-rate coding scheme.

Packet loss is the major weakness of wireless feedback control systems. In this paper, we discuss the improvement of the control quality of wireless feedback control systems with Forward Error Correction (FEC) coding. If FEC coding is used, packet losses may decrease. But its redundancy decreases the packet transmission rate under the same bit rate, so that the control quality will be worse. Therefore, we propose an adaptive coding scheme to improve the control quality. In this scheme, FEC coding is used adaptively based on a predicted state of the controlled plant. Our proposal is the first approach in the field of adaptive channel coding for wireless feedback control systems because this scheme adapts channel coding to the quality of the control layer (application layer). In this paper, we show that the proposed adaptive coding scheme improves the control quality more than that of the fixed rate coding in Rayleigh fading channels.

We produced an ultra-compact bird-borne transceiver unit (weight of circuit board(5g) excluding solar battery and rechargeable battery system, harness of the crate) for S-band packet radio communications based on the CC2500 transceiver, a device manufactured by Texas Instruments (TI). We performed assessments to determine optimal parameters required to achieve successful bird-to-center communication over a distance of 18 km and bird-to-bird communication over a distance of 200 m, including optimal modulation method, transmission rates, and antenna. We equipped the transceiver unit with a modified dipole antenna (collinear antenna), which we tested in a 10-m anechoic chamber. Our experimental assessments and circuit design review pointed to the following parameters: 2FSK modulation method; 14.28 kHz frequency shift; 101.56 kHz IF reception bandwidth; and Manchester encoding(+). Our assessments showed bird-to-bird communications could be achieved over a distance of 200 m assuming MSK, FEC(+), and 500 kbps.

In wireless feedback control systems, it is important how to reduce packet losses. If channel coding is used, packet losses may decrease. But its redundancy decreases the packet transmission rate under the same bit rate, so that disturbances in the controlled plant may deteriorate the quality of the control system. To improve the quality of the control system, the channel coding should be used adaptively based on packet losses and disturbances. In this paper, we propose an adaptive coding scheme based on the quality of the communication layer and the control (application) layer. In this scheme, the channel coding is used adaptively based on a predicted state of the controlled plant under a packet loss rate. We show that the proposed adaptive coding improves the control quality compared with that of the fixed rate coding.

This paper proposes a joint coding scheme for information advertisement in wireless mesh networks. Random network coding is a technique that can mitigate the effect of a broadcast storm problem. However, it cannot correct bit errors, and it should forward more and more packets to recover bit errors. On the other hand, multi-route coding was proposed to reduce bit errors on wireless channels. This coding scheme can obtain a diversity gain by transmitting multiple coded packets on multiple routes. This paper proposes the joint coding scheme by using random network and multi-route coding to reduce the effect of the broadcast storm problem and bit errors. We evaluate the performance of the proposed coding scheme by simulation. The results show that the proposed scheme improves the transmission delay and packet delivery failure ratio compared with the conventional schemes.

This paper intends to point out some problem of telemetry for wild birds and future technical expectations to prevent Avian Influenza. For medium to large migratory birds, the data collection system aboard the NOAA meteorological satellites (the ARGOS system) has been able to map their migratory routes. However, recommended C/No for Doppler shift ill be 54dBHz, while calculated required C/No for 400 bps data uplink is 36dBHz. The gap between navigation function and data communication will be almost 18 dB. The bird has to pay this overload. We would like to propose the next generation digital transponder that is an advanced DCS with multibeam on Ku-band to perform the onboard processing (DCS and navigation system with Doppler shift). In case of 42dBi antenna of the spacecraft, the ground terminal with 13dBm RF-output and 2.25dBi antenna can transmit upto 50 bps with BPSK. This terminal attached with a leg of a bird will be suitable for a dove or a gull in size, if we could put new digital transponder on Ku-band.

This paper introduces a Visible Light Communication (VLC) system using an LED sign panel, which allows the provision of broadcast-type information in public spaces. This system provides an additional function to an LED advertisement panel; the panel not only displays an advertisement, but also transmits wirelessly to other devices. We suggest its use in benefit of the visually impaired. They can obtain local information by using this system, named i-LightHouse (i-LH). For this purpose, i-LH transmits a long-range navigation signal to direct users to the LED sign panel and a short-range signal to download content information to the users' devices. We propose its modulation scheme which can multiplex the navigation and content information at the same time. In this system, the interruption by a passerby and ambient lights may degrade the communication performance. On these issues, we propose an error correcting method and a filter to mitigate the interference of them and evaluate the effectiveness.

Our goal is the improvement of the node activity rate in solar-powered wireless sensor networks. We consider the application of a cooperative transmission scheme that uses a relay node when a node sends data to reduce the consumption of energy of the entire network. However, if we do not consider the variations in energy harvesting, the node activity rate declines during nighttime. In this paper, we smooth out the day-and-night node activity rate variations by introducing the new inactive node reactivation condition and achieve the required high node activity rate.

In this paper, we propose two dynamic bandwidth allocation (DBA) schemes, a game theory based DBA and a successive DBA, for satellite/terrestrial integrated mobile communication system (STICS). The game theory based DBA allocate the bandwidth according to a utility function based on a weighted demand bandwidth of both satellite and terrestrial system. The successive DBA recursively allocates an unit of bandwidth according to a weighted number of bandwidth unfilled cells of both satellite and terrestrial systems, until the total bandwidth is fulfilled. As results, both DBA outperform the conventional fixed bandwidth allocation scheme. Between two proposed DBA, the successive DBA shows better spectral improvement ratio than the game theory based DBA.

Pulse-shaping OFDM, or more generally BFDM and BFDM/OQAM, systems are originally proposed to combat intercarrier interference (ICI) caused by time-variance of the channel. The main property that makes these signals robust against ICI is that they allow to employ time-frequency well localized pulses, particularly Gaussian pulse which has the best time-frequency localization. However, this property also suppresses the outband or inband spectral leakage of the BFDM and BFDM/OQAM signal, making them attractive physical layer modulation scheme for cognitive radio. In this paper we analyze the power density spectra of the BFDM and BFDM/OQAM signals and numerically evaluate the inband and outband spectra for truncated Gaussian pulses.

In this paper, we consider visible light communication systems using LED array as a transmitter and high-speed camera as a receiver for Intelligent Transport System (ITS). Previously, we have proposed the hierarchical coding scheme which allocates data to spatial frequency components of the image depending on the priority. This scheme is possible to receive information of the high-priority even if communication distance is long. However, we need to distinguish multi-valued data from the received image by using a hierarchical coding. In this paper, we propose two improved decoding methods, and demonstrate to distinguish multi-valued data more correctly in the experiment.

Robust channel estimation scheme is essential for pulse-shaping OFDM systems in the multipath mobile environment. This paper proposes three types of channel estimation schemes for the general class of pulse-shaping OFDM systems. The first two types are suboptimal low-complexity maximum likelihood estimators. The last type is adaptive Kalman filter channel estimator. We numerically evaluate the performance of each estimator using computer simulation.

This manuscript proposes a cognitive radio system that shares a frequency with an existing primary system. In the proposed method, a secondary transmitter relays the primary signal and piggybacks its own data on it. Symbol error probabilities of the system are derived analytically. The result shows that the proposed system can communicate at a relatively high speed on the same frequency of the primary system without harm to it.

In this paper, we propose a new SC/MMSE Turbo equalizer for MC-CDMA to use in inter-vehicle communication (IVC). We modify the weight of the MMSE filter of SC/MMSE Turbo equalizer so that it can combat with frequency-selective fading in highly mobile IVC environment and also can mitigate the effect of multiple access interferences (MAI). As results, we show that better throughput is obtained by the proposed SC/MMSE equalization than the conventional one. The average reception range can be up to 70 [m] and the signals from 16 surrounding cars can be received. While for the conventional scheme, only three signals from the nearest cars are achieved.

We propose a contention-free medium access control (MAC) protocol for inter-vehicle communication systems that share information for the improvement of safety drive on the highway. In the proposed system, contention-free medium access is achieved by periodical one-way token transmissions. The token contains the frame information about the transmission/reception phase. While the token is relaid by one-way, the vehicle which has the token can send a packet to both up- and down-link. The token transmission interval is set not to be interfered from other transmit vehicles at a receive vehicle. In addition, multiple channels are used and allocated dynamically for every token transmission to reduce the transmission waiting time. We also evaluate the maximum end-to-end transmission delay of the proposed protocol, and clarify that the proposed protocol can guarantee the transmission within allowable delay time.

In this manuscript, a decode-and-forward cooperative diversity scheme for wireless packet transmission network with large and undetermined set of relay terminals is considered. Correctly received signals at some relay terminals are retransmitted to the receiver with the STBC-based cooperative relaying scheme. For large and undetermined set of relay terminals, performance of the conventional STBC-based cooperative diversity is known to be degraded due to the loss of the diversity gain and increasement of decoding complexity. In this manuscript, we propose a signaling scheme to reduce the performance loss. Our proposed scheme is based on the linear transformation of the STBC matrix by a signature vector, which has more degrees of freedom for the assignment of signals compared to the conventional STBC-based cooperative diversity.

For safety applications, automotive radar networks show an interesting potential. Apart from long range radars, networked short range radar systems are under development. Additional applications can be accomplished, such includes precrash detection, parking aid and blind spot surveillance. We research the automotive radar networks using short range radars. The network is built with multiple radars. In this paper, we consider data processing using measured ranges provided by the radars in order to estimate multiple target positions with high accuracy. Firstly, we derive a method based on MAP estimation. Secondly, we propose a method which reduces the calculation complexity compared to the above MAP method. Finally, we introduce a method which can detect targets with low undetected targets. The case often happens when the measured ranges are lost because of miss-detections. We evaluate the performance with computer simulations.

In the multihop cellular networks, multiple routes from a user station to a base station can be established because of the flexibility of network topology. In order to reduce packet errors of wireless transmissions, we proposed the multi-route coding scheme, in which packets are encoded and divided into subpackets, and they are transmitted on several routes from the user station to the base station. In order to improve the performance of the multi-route coding, a route selection scheme is very important issue. In this paper, we propose the route selection scheme for the multi-route coding in multihop cellular networks. The proposed scheme is based on the bit error rate of wireless links. We also apply the proposed route selection scheme to a hybrid routing protocol and evaluate its performance.

Pulse-shaping OFDM is well-known that it performs well in a mobile environment comparing with conventional OFDM. However, in highly mobile environment intersymbol and intercarrier interferences (ISI/ICIs) increase and can no longer be neglected. These ISI/ICIs deteriorate the performance of the systems. Proper channel equalization is needed for further improvement of the systems. In this paper, more general case, namely biorthogonal frequency division system based on offset QAM (BFDM/OQAM) is considered. We propose a multi-input multi-output (MIMO) transversal filter to equalize the time- frequency dispersive channel. Tap weights are calculated using the zero-forcing (ZF) algorithm. We also propose maximum- likelihood channel estimator and its low-complexity version. The proposed system can significantly improve the performance of BFDM/OQAM systems in the highly mobile environment.

This paper examines multihop cellular networks. There are some issues in these networks, such as route loss due to topological change and packet errors due to wireless nodes relay. To overcome these issues, we use a multiple route coding scheme in which packets are transmitted on multiple routes from a user node to multiple base stations. This scheme can achieve a diversity gain by combining packets that are transmitted along multiple routes. Some challenges must be met to establish multiple routes for this scheme. In this paper, we propose a routing protocol for multiple route coding. The key idea of this proposed scheme is using a hybrid routing protocol. This can mitigate an increase in control packets and multiple disjoint routes can be constructed. We evaluate the performance of this proposed scheme

For reduction in bit errors on wireless channels, we proposed the multi-route coding scheme on multiple routes for wireless multi-hop networks. In this paper, we introduce ARQ to the multi-route coding. In the multi-route coding, the destination node combines and decodes sub packets which are encoded and divided by the source node. The intermediate node relays a sub packet, that is, only a part of a packet. Therefore, the intermediate node cannot detect packet errors, and only the destination node can do after combining and decoding sub packets. We propose the ARQ scheme between the source node and the destination node. We analyze the proposed ARQ scheme and evaluate the throughput performance

This manuscript introduces a mathematically tractable and accurate model of narrowband power line noise based on experimental measurements. In this paper, the noise is expressed as a Gaussian process whose instantaneous variance is a periodic time function. With this assumption and representation, the cyclostationary features of power line noise can be described in close form. The periodic function that represents the variance is then approximated with a small number of parameters. The noise waveform generated with this model shows good agreement with that of actually measured noise. Noise waveforms generated by different models are also compared with that of the proposed model.

In this manuscript, wireless communication system for reliable wireless control of industrial machines in indoor environment is considered. In indoor environment, transmitted signal fluctuates due to fading and shadowing. To combat the fading and shadowing, macro/micro spatial diversity can be used as an effective countermeasure. In this manuscript, macro/micro diversity is achieved by multiple wireless relay terminals, which is convenient in setting and moving the macro diversity branches. In the proposed scheme, channel between the transmitter and the receiver through relay terminals is considered as a MIMO channel and separation of the signals re-transmitted from the relay terminals is not required. This feature makes it possible to have higher order diversity without the cost of bandwidth inefficiency. In numerical examples, the performance of the proposed scheme is evaluated by average frame-error rate and outage probability of frame-error rate. With two relay terminals, the diversity gain of the proposed scheme is about 5-6 dB, which is 1 dB higher than that of conventional macroscopic diversity

In real-time packet communications, packet retransmission is generally not used because it may produce undesired delays. In this case, however, packet errors caused by a channel may lead directly to a loss of transmitted information. Then it is necessary to employ a technique which can reduce the influence of packet errors. We propose to employ a multiple-route packet combining scheme in realtime communications on wireless multihop networks. The scheme, which we proposed previously (Nakagawa, N. et al., IEICE Commun. Soc. Conf., vol.B-5-119, p.496, 2003), can achieve diversity gain by combining multiple copies of the same packet that are transmitted along different routes. We analyze two characteristics of the proposed system. One is the average packet error probability, which is obtained as a function of the elapsed time after generating copied packet transmission requests. The other is the required time for achieving a required packet error probability. The results are evaluated from a quality of service (QoS) point of view. From numerical results, it is shown that for a given tolerable delay, there exists a number of multiple routes that minimizes the achievable average packet error probability. Moreover, if the number of multiple routes is selected appropriately, the required time for achieving a required packet error probability can be minimized.

In multimedia wireless communications, various types of traffic are handled. Generally, each type of this traffic has a different stochastic property. An access control protocol for multimedia wireless communications, therefore, are required to adopt a stochastic property of multimedia traffic. Conventional access control protocols, however, do not consider such characteristics of multimedia traffic. In this paper, we propose an access control protocol for multimedia CDMA packet communications. The proposed protocol controls the data packet transmissions based on the estimation of the instantaneous number of simultaneously transmitted voice and video packets. This estimation exploits the stochastic property of multimedia traffic. To carry out such estimation, we employ an adaptive algorithm as an estimation method. We evaluate the performance of the proposed protocol by comparing it with the performance in the case where the stochastic property of traffic is not considered.

A CDMA unslotted ALOHA system is a connectionless-type of CDMA packet communication system. In this system, a user station can transmit a packet asynchronously and randomly, and so the packet birth/death event is one of the most important problems for multiuser detection. We have proposed the CDMA unslotted ALOHA system using an adaptive filter receiver based on minimum mean square error criterion of Okada, Grant, Band and Ogawa (see IEEE International Symposium on Personal, Indoor and Mobile Radio Communication (PIMRC), Sep. 2000), and shown the improvement in the throughput performance even considering the effect of birth/death event. The ensemble-averaged squared error, however, increases at the point of packet birth. In this paper, we employ interleaving and forward error correction (FEC) coding techniques to mitigate the momentary increase in ensemble-averaged squared error. The use of FEC, however, causes the increase in MAI due to redundancy bits of FEC. Moreover, signal power is reduced under the condition that the energy of an information bit is the same. In such cases, whether an adaptive filter can operate effectively or not interests us. We evaluate the system performance and show that the improvement in throughput is achieved with interleaving and FEC techniques.

In this paper, we propose a new packet recognition and signal acquisition scheme with an adaptive linear filter based on a least mean square (LMS) algorithm for code-division multiple access (CDMA) packet systems. In CDMA systems, conventional receivers with matched filtering suffer from the near-far problem due to multiple access interference (MAI). In general, closed-loop power control is used in order to reduce the influence of the near-far problem. However, closed-loop power control is unsuitable for packet systems. Hence, in order to eliminate the influence of the near-far problem, we focus on an adaptive receiver based on an LMS algorithm for packet recognition and signal acquisition in CDMA packet systems.

Packet detection is one of the most important problems in packet communication systems. In a CDMA Unslotted ALOHA system, which is one of the wireless packet communication systems, multiple access interference (MAI) makes the performance of the packet detection worse. To reduce the effect of MAI, we propose a new packet detection scheme in this paper. In this scheme, Successive Interference Cancellation(SIC) is applied. The packet's signal is detected after the cancellation of MAI using SIC. This proposed scheme gives good performance of the packet detection.

We compare CDMA and FDMA (or TDMA) schemes in multibeam non-geostationary satellite systems, taking the effects of antenna patterns on inter-cell interference into consideration. The maximum acceptable number of users per cell in the uplink is employed as a measure of the system capacity. We have found that in FDMA (or TDMA) systems the maximum acceptable number of users varies according to the altitude of the satellites. While the performance is insensitive to the altitude in CDMA systems. For this reason, the altitude of the satellites is found to be an important factor for selecting multiple access schemes in non-geostationary satellite systems.

In wireless communication systems, as one of the effective techniques for combating fading, transmit diversity has attracted much attention, especially when receive diversity is expensive or impractical due to the constraint of terminal size. In this study we consider an adaptive carrier-by-carrier basis antenna selection scheme for transmit diversity in OFDM systems under the assumption that a transmitter has priori knowledge of the channel state information (CSI). In the proposed schemes transmit antennas are selected adaptively for each subcarrier according to CSI.

A modified version of the SAGE algorithm applied for joint estimation of delays, incidence azimuths and attenuation parameters characterizing the channel is presented. The modifications are based on using different observation intervals in optimizing the parameters and using time correlation in estimating delays, for the purpose of a further reduction in SAGE's computational weight, and improving its accuracy in delay estimation. We show analytically that short observation intervals are sufficient for optimizing the delays and the azimuths, and verify through simulation the reliability of using time correlation. Two other estimators are proposed combining the modified SAGE with a sequential estimation of the attenuation parameters suitable for slowly varying channels.

In an attempt to improve the performance under a frequency selective fading environment, we propose an orthogonal frequency division multiple access (OFDM) system in which MTCM (multiple trellis coded modulation) is applied to a set of multiple subcarriers. The code is designed to enlarge the summation of the Euclidean distance and the product distance. We also propose a scheme for assigning the symbols to subcarriers adaptively based on the channel state information (CSI) fed back from the reception receive end, and evaluate the performance applying these two codes and the proposed scheme under a multipath Rayleigh fading environment.

We propose the introduction of space diversity techniques to the code acquisition of a direct-sequence spread-spectrum signal. In this scheme, both a transmitter and a receiver have multiple antennas and the signals corresponding to all the combinations of the transmitter/receiver antennas are combined at the acquisition circuit of the receiver. The performance is evaluated for indoor packet radio communication systems from the viewpoints of average time for acquisition, probability of success of acquisition, and necessary preamble length. As the result, we show great performance improvements by the proposed scheme under slow and flat Rayleigh fading environment.

In this paper, we propose a CDMA slotted-ALOHA (CDMA S-ALOHA) system with low-bit rate and high-bit-rate classes of data users applying the multi code CDMA (MC-CDMA) scheme. Performance evaluation is obtained in the respect of the throughput. Contrary to one might expect, we show that the throughput of the class with higher bit rate can be improved by the MC-CDMA scheme. Moreover, we show that applying the modified channel load sensing protocol (MCLSP) to the system gives further improvement in the throughput of the higher bit rate class, as well as to the total throughput of the channel.

In this paper, we analyze the CDMA slotted ALOHA systems with finite buffer and finite population assumptions by expanding the linear approximate solution based on restricted occupancy urn models. We evaluate the system performance in terms of throughput, average delay, and rejection probability. Moreover, we clarify the effect of buffer capacity by changing the buffer size.

In this paper, we discuss the performance of a system in which multiprocessing gain (MPG) spread spectrum signals are transmitted to a base station within the same bandwidth. At the receiver of the base station, an adaptive array antenna is employed. We evaluate the system performance in terms of signal to interference-and-noise ratio (SINR) and also of outage probability of low-rate users. We find that the employment of the adaptive array antenna may result in serious degradation of the outage probability of low-rate users. We then show that the cause of this serious degradation is the inadequate guidelines of conventional adaptive array antennas.

We consider a receiver with satellite diversity by using DS/SS signals from different LEO satellites. Since the signals from the different LEO satellites may have different frequency offsets due to the Doppler effect, we propose an acquisition scheme which can estimate the code timings and carrier frequencies of these signals at once. We evaluate the characteristics of the proposed scheme by the mean acquisition time.

In multipath fading channels, the direct-sequence spread spectrum (DS/SS) technique is often employed because of the capability to resolve signals from different paths and realize path diversity at the receiver. This advantage of DS/SS, however, is only when the time difference of propagation delay between paths is greater than a chip duration. In indoor communications, the delay spread is often very small and the above condition on the delay and chip duration is no longer satisfied, and this performance improvement by the path diversity with conventional DS/SS systems cannot be expected. In this paper we propose a new diversity technique for indoor wireless communication systems where delay spread is smaller than a chip duration. In this system, the transmitter sends signals at the same time from multiple antennas with intentional time delays, which allow a receiver to distinguish and combine the signals from different antennas.

We evaluate the bit-error rate performance of direct-sequence spread spectrum (DS/SS) system using /spl pi//4-shift QPSK and /spl pi//2-shift BPSK and observe the effect of the nonlinearity of the high power amplifier (HPA) of transmitters which occurs when the HPA is operated near its saturation region. The hard limiter is chosen as the example of the nonlinear amplifier and the numerical results are reported by using coherent as well as differential detection at the receiving end. With the employment of either /spl pi//4-shift QPSK or /spl pi//2-shift BPSK, the DS/SS signal is not much influenced by the nonlinearity of the HPA as the two modulation schemes have the advantage of nearly constant envelope.

In this paper, we present the throughput performance of CDMA slotted ALOHA systems with modified channel load sensing protocol (MCLSP). In MCLSP, a hub station continuously senses channel load for a certain number of slots, and computes the probability with which each user station should transmit its packet, then broadcasts the computed value. As the access control is based on this probability, we can apply it to slotted system. We show that MCLSP can achieve significant improvement in throughput performance of CDMA slotted ALOHA systems even in large offered load. One of the issues in such a protocol is the access timing delay, which is the sum of process time and propagation time for the packet access control. We also show the effect of the access timing delay on the throughput performance.

A transmit permission control method for improving the throughput characteristics of a low Earth orbit (LEO) satellite communication system employing spread-slotted ALOHA multiple-access scheme is proposed. Both nonfading and fading satellite links are considered. The basic idea of the proposed scheme is to decrease the level of interference at each satellite and, hence, to increase the probability of packet success, by prohibiting the packet transmission from the users with relatively high propagation loss to their connecting satellites. It is shown that the method has the ability to improve the throughput performance in heavy traffic loads and the peak value of the throughput, significantly. It is also shown that the average delay performance of the system employing the proposed scheme is superior to that of the conventional system at heavy traffic loads.

Throughput analysis of direct-sequence spread spectrum multiple access (DS/SSMA) unslotted ALOHA with fixed packet length is presented. As the levels of multi-user interference fluctuate during the packet transmission, we calculate the packet error probability and the throughput by considering not only the number of overlapped packets but also the amount of time overlap. On the assumption that packet generation is Poisson, the system can be thought as the queueing system M/D//spl infin/. With Gaussian approximation of multi-user interference, we obtain the throughput as the function of the number of chips in a bit, the packet length, and the offered load of the system. We also analyze the channel load sensing protocol (CLSP), and obtain the optimum threshold of CLSP.

We analyze a system which simultaneously transmits spread-spectrum signals with different processing gains. The main purpose of this study is to give an analytical framework that describes the influence of the interfering signals with different processing gains. The results show that the power of the interference component after the despreading procedure, the power ratio of the desired to undesired component, and the bit error rate are not constant but vary symbol by symbol. We also show that the processing gain of the interfering signal affects the performance.

We propose a coordinate-by-coordinate interleaved trellis coded QPSK with the T-algorithm. A rate 1/2 convolutional code with constraint lengths of 7 and 15 are evaluated on Rayleigh fading channels in terms of the bit error rate (BER) by an analysis and a computer simulation. As the effective code length (ECL) is equal to the code constraint length for the case of a QPSK system, the proposed system achieves a satisfactory BER performance even in the situation where no channel state information (CSI) is available at the decoder. Further, in order to reduce the decoding complexity, we employ the T-algorithm. It is shown that as the T-algorithm can track the fluctuating gains introduced by the fading channel, we can achieve a satisfactory BER performance with 0.01% of the decoding complexity of the Viterbi algorithm.

We present a new direct-sequence spread-spectrum demodulator using the block signal processing in the presence of large carrier frequency offset such as Doppler shift in low Earth-orbital satellites systems. The proposed demodulator employs the initial acquisition scheme which can roughly estimate both the timing and the carrier frequency offset. This scheme realizes the maximum likelihood estimation with the help of the FFT. As the result of the analytical discussion, it is shown that the proposed scheme makes the initial acquisition possible even in the presence of a large frequency offset where the conventional demodulator cannot achieve the acquisition.

In some recent studies, the use of low Earth-orbit satellites in various applications is considered. In all of these studies, uniform distribution of traffic load is assumed. In this paper, the performance of a low Earth-orbit satellite communication system which is designed to service to two kinds of users; i.e., voice users and data users is estimated. The distribution of population of these users is assumed to be nonuniform. According to the simulation results, it is shown that the nonuniformity in traffic affects the performance of the system by decreasing the signal quality at the satellite which has to service to populated areas and increasing it superfluously at the satellite with not so populated service area, in a given period of time. By modeling the satellites during their movements, the change in signal quality while experiencing a peak of traffic load in their route is also determined. A modified power control method based on the amount of traffic load of each satellite is also examined and is shown that this method can make some performance improvements in signal quality, which is limited by special features of low Earth-orbit satellite systems.>

This paper investigates the bit error rate (BER) performance of hybrid direct-sequence/slow-frequency-hopping spread-spectrum multiple-access (DS/SFH-SSMA) operating in the environment with the near-far problem. Rate 1/2 convolutional code of constraint length 7 is applied with an appropriate interleaver to increase the diversity effect not only on time domain but also on frequency domain. As a result, we found that the BER performance of DS/SFH system was always satisfactory in the presence of near-far problem. On the other hand, poor BER performance of the direct-sequence system is found regardless of E/sub b//N/sub 0/ or number of users.

This paper presents a novel spread slotted Aloha system with channel load sensing protocol (CLSP). CLSP is an effective scheme to improve the throughput performance in spread unslotted Aloha systems. In spread slotted Aloha systems, however, it does not make sense to utilize CLSP because the slot size is usually the same as the packet size. The slot size of the proposed system is set less than the packet size, thereby enabling the authors to apply CLSP and improving the throughput performance. Another feature of the proposed system is that the system is not likely affected by the time difference between channel load sensing and timing of packet access, which they call the access timing delay. Throughput performance of the proposed system is evaluated in the presence of the access timing delay and a significant increase of the throughput is shown compared with that of spread unslotted Aloha with CLSP.>