As the communication mode expected to improve system throughput by omitting the specific means to acquire transmit rights, grant free methods, such as NOMA (Non-orthogonal Multiple Access), studied in various mobile networks as systems accommodate many terminals under the expansion of IoT. However, the system, which is premised on many terminals in core networks, has not been sufficiently investigated. Since the terminal location needs to be known and the location registration and its administrative function are particularly needed by mobile communication networks, the system is usually equipped in the core network where the whole network can be accessed easily. As the number of terminals housed in a network increases as the Internet of Things (IoT) expands, the throughput inevitably increases for location registration and the possibility of congestion developing increases. Thus, this article proposes a way to reduce the throughput for location registration where mMTC (massive Machine Type Communication) is prescribed by 5G premised on many simultaneous connection terminals.

High-resolution nowcasting flood prediction using streaming rainfall big data is desired for soft countermeasures against pluvial floods. Although an integrated flood analysis model consisting of a diffusion-wave equation, momentum equations and so on can simulate flood processes accurately, the model is unsuitable for nowcasting flood prediction due to its high computational cost. To realize accurate and computationally inexpensive nowcasting pluvial flood prediction, we have applied the knowledge distillation (KD) technique for training a lightweight neural network with the integrated flood analysis as a teacher. In the previous work, we showed its applicability in terms of prediction time and accuracy. However, this KD-based approach would not be sufficiently accurate in predicting floods caused by rainfall in minor rainfall patterns. To cope with it, we propose a prioritized sampling method that compensates for the imbalance between rainfall patterns. The rainfall patterns are mined from big rainfall data based on the spatiotemporal features of rainfall intensity over a rainfall duration. Compared to random sampling, we show that our prioritized sampling reduces the average prediction error of floods caused by rainfall in a minor rainfall pattern by 37 cm per mesh, which is equivalent to 96% reduction in error rate.

LED-to-camera communication for smartphones promises to enable low-cost, small-size, and intuitive data access to Internet-of-things (IoT) devices. However, for fast and reliable communication under the rolling shutter effect, the packet losses caused by inter-frame gaps and frame drops must be dealt with. In this paper, we introduce LED-to-camera communication between a commercial smartphone and an IoT device with LED flashlight, where the user can intuitively acquire the desired data transmitted from the IoT device to the smartphone. Specifically, we present IoTorch, a fast and reliable LED-to-camera communication that efficiently prevents the packet losses. IoTorch consists of two core mechanisms: 1) a minimum-repetition one-way reliable transmission focusing on the periodicity of inter-frame gaps and 2) an acknowledgement mechanism to overcome frame drops by using a smartphone's built-in flash focusing on its delay characteristics. Additionally, we propose an optimization method to increase the throughput up to 2.92 kbps (i.e., 2.43 times faster than that of the state-of-the-art method that overcomes the inter-frame gaps). Furthermore, we realize a remote wake-up function by using the smartphone's flash as the data transmission trigger. To demonstrate the benefit of IoTorch, we present a sensor data viewer using Arduino and a smartphone console system on TelosB.

As we are surrounded by countless sensing devices, “intuitive” communication, which allows a user to directly obtain information from a device of interest to the user, is needed. Towards this, we present long-range flicker-free LED(lightemitting diode)-to-camera communication in which flickering can be prevented by the rapid switching of LED colors and information can be acquired from remote LEDs that appear as only a few pixels on a camera screen. In the rolling shutter system (typically used in smartphones), the timing of an exposure varies depending on the in-screen position. Hence, the imaging time interval of the LED deviates from the constant frame cycle owing to hand micromotion during communication. Under this condition, when imaging an LED that switches color rapidly, the color captured in the frame changes depending on the degree of hand micromotion. To solve this problem, we present differential color shift keying (DCSK) as a mechanism for correcting the influence of hand micromotion. Experimental results using multiple smartphone models show that the mechanism using DCSK achieves a communication range exceeding 3 m with a throughput of 60 bps from an LED with a diameter of 10 mm compared to several centimeters achieved by state-of-the-art studies.

Magnetic resonant coupling wireless power transfer (MRC- WPT) using transmitter arrays is expected to be applied to power electric vehicles and electronic devices. Transmitter arrays can maintain power transfer efficiency despite lateral misalignment between the transmitter and receiver. However, methods of using transmitter arrays while reducing magnetic field leakage have not been studied. A new method of using transmitter arrays is necessary for electromagnetic compatibility and user safety. In this study, we optimized the input currents at transmitter arrays and the load at the receiver to reduce magnetic field leakage and maintain power transfer efficiency even with lateral misalignment between the transmitter arrays and the receiver. Simulations showed that the proposed method can reduce magnetic field leakage by more than 10 dB and 38 dB, with and without ferrite plates, respectively, compared to the existing method of maximizing power transfer efficiency.

LoRa networks achieve a remarkably wide coverage as compared to that of the conventional wireless systems by employing the chirp spread spectrum (CSS). However, single-hop LoRa networks have limited penetration in indoor environments and cannot efficiently utilize the multiple-access dimensions arising from different spreading factors (SF) because the star topology is typically used for LoRa. On the other hand, a multi-hop LoRa network has the potential to realize extensive network coverage by utilizing packet relaying and improving the network capacity by utilizing different SFs. We evaluated the LoRa receiver performance under co-technology interference via simulation and experiments to realize these potentials. The results show that LoRa can survive interference from time-synchronized packets transmitted by multiple transmitters with an identical SF. Furthermore, we examined the orthogonality between different SFs by evaluating the required signal-to-interference ratio (SIR). Finally, we demonstrated the possibility of employing different SFs to construct the pipeline in a multi-hop relay network to increase network efficiency.

This paper proposes to adopt concurrent trans- mission in a distributed antenna system to achieve the real- time capability and reliability for wireless automation systems. Our scheme is the simplest way to realize spatial diversity by deploying a large number, e.g. several tens, of transmit antennas and allow all of them to participate in simultaneous packet transmission without any cooperation. We examine the performance, including packet error rate and high- reliability area percentage, in simulation and over-the-air experiments under the IEEE 802.15.4 standard, where the experiments are conducted with up to 32 concurrent transmitters using Tmote Sky sensor nodes. We found that the presence of a timing offset limits the performance. By lowering the timing offset to be below half of a chip period, the reliable performance can be achieved without sacrificing the real-time capability.

Real-time and robust communication is essential for wireless control systems (WCS). Preambles are considered as significant communication overheads in WCS where payloads are typically small. To shorten the overhead in OFDM systems, previous works have considered fixed subcarrier sampling to obtain subcarrier-fixed short preambles (SFSP) and adopted frequency-direction interpolation (FDI) for channel estimation. Despite of advantage in fast fading channel, it results in poor performance in frequency-selective fading channel. In this work, we propose a subcarrier-selectable short preamble (SSSP) which provides full sampling coverage of all subcarriers with several preamble transmissions. By exploiting the periodicity of transmissions in WCS, the SSSP scheme obtains the channel estimation of non-selected subcarriers by not only FDI but also the time-direction extrapolation (TDE), which is robust in frequency-selective fading. In addition, in order to make the channel estimation robust in both fast fading and frequency-selective fading we adopt an adaptive algorithm altering between FDI or TDE mode. The feasibility of the proposed method is validated by evaluating symbol error rate (SER) in fast frequency-selective fading. Simulation results verify that unlike the SFSP the adaptive SSSP scheme shows no error floor in SER curve when the channel is frequency-selective fading, and performs similar to the SFSP scheme when the channel is fast fading.

Many works attributed the successful reception of concurrent transmission (CT) to the constructive interference. However, due to the inevitable carrier frequency offset (CFO) and the resulted beating effect, the claim of constructive interference is actually not valid. To clarify the reason behind the successful receptions under CT, we conduct extensive evaluations and identify the following findings. 1) We show that the IEEE 802.15.4 receivers survive the beating effect mainly because of the direct sequence spread spectrum (DSSS), while systems without the protection of DSSS are not applicable to CT. 2) We identify a counterintuitive phenomenon that the IEEE 802.15.4 receiver could survive only the beating results from large CFO, while performing poorly when CFO is small. 3) We demonstrate that, even if the receivers survive, CT links lead to little performance improvements compared to conventional signal transmission links from the SNR point of view.

Many researches have shown that concurrent transmission (CT) helps to improve the energy efficiency and latency of 2.4GHz IEEE-802.15.4-based multi-hop networks. However, it has been shown that the success of CT in IEEE 802.15.4 is mainly due to the DSSS. In view of the great potential of long range sub-GHz technologies for future smart meter applications, we assess the feasibility of applying CT to non-DSSS sub-GHz receivers. In this work, we evaluate the FSK-based receiver performance in the critical case - the scenario that concurrent transmitted signals are received with the same power such so the mutual interference become worst. Our evaluations are based on the 2-FSK mode of TI CC1120 chips. The results show that the basic 2-FSK mode is vulnerable to CT. On the other hand, if the FEC and the interleaver are adopted, the receiver can survive in the heavy interfering region as long as the timing offset is smaller than 1 μs and the CFO is larger than 5 KHz.

This paper presents an RSSI-based packet combining (PC) scheme to improve the packet reception reliability affected by a beat interference in wireless sensor networks (WSN) using concurrent transmission (CT). To overcome the beat problem, our PC scheme exploits multiple receptions to perform simple post-detection selection combining based on RSSI values. An implementation verifies its feasibility in resource-limited sensor nodes with time-sensitive CT. Evaluations using computer simulations and hardware experiments show significant reliability improvement.

In order to provide connectivity for various M2M devices in an easy and cost-effective manner, we propose the concept of "multi-service shared M2M area networks," which enables multiple M2M service providers to commonly use shared M2M gateways. The characteristics of M2M communications are analyzed in this paper based on various M2M use cases, and the technical challenges of shared M2M area networks are explained. Furthermore, we also propose a cell deployment analysis model suitable for shared M2M area networks, which can be applied for system design, especially from a cost-effectiveness viewpoint.

Spectrally efficient multiple access technologies, such as Multi-User Multiple-Input-Multiple-Output (MU-MIMO) system and Non-Orthogonal Multiple Access (NOMA), are the key enablers for the next generation communication systems to accommodate tremendous devices. Combining NOMA in MU-MIMO systems (NOMA-MIMO) is expected to boost the spectral efficiency even further. However, in order to solve the Inter-Antenna-Interference (IAI) and the Inter-Cluster-Interference (ICI) resulted from multiple-antenna and non-orthogonal transmissions, computation-intensive interference cancellation schemes are usually required in the receivers, which prohibit the employment of NOMA-MIMO system in systems with simple receivers, such as sensor networks. In this work, we propose a non-linear pre-coding method for two-cluster NOMA-MIMO systems that allows simple receiver implementation with only one antenna. Our main contribution is twofold: first, we adopt IAI-oriented pre-coding to spare the receiver from IAI cancellation; second, we propose to regulate the conventionally unsolvable IAI-corrupted ICI onto lattice points, so that the ICI can be removed by simple modulo operations instead of computational-intensive Successive Interference Cancellation (SIC) schemes. The ergodic sum capacity and whole system bit-error-rate (BER) performance are provided to show the validity of the proposed approach.

We present a communication protocol called Choco, which efficiently supports various traffic demands with end-to-end reliability in multi-hop Wireless Sensor Networks (WSNs). To achieve this, Choco uses fine-grained scheduling based on the insight that acknowledgements (ACKs) between ends can be delivered with little overhead using concurrent transmission. Specifically, the slot assignment is dynamically scheduled up to several times a second according to traffic loads and packet losses. We show that Choco achieves higher efficiency and reliability than ORPL+BF, which is a state-of-the-art collection protocol, for periodic traffic environments. Also, we show that in a networked imaging application, which generates burst traffic, Choco can successfully collect 5 VGA images with high efficiency from 5 nodes in approximately 168~s in an average 4.0~hop network.

Geofencing technology enables various location-based applications by detecting entrance to a predefined region and utilizes the event as a trigger to automatic communication or control of smart devices. In this paper, we implement the proposed geofencing system on Android based smartphone and evalua

This paper proposes a method for measuring the rapidly varying parameters of a power amplifier (PA) characterized by a memory polynomial model in less time than in the symbol period of the orthogonal frequency-division multiplexing (OFDM) signal of a long-term evolution-advanced (LTE-Advanced) system. The method fulfills the dual-updating scheme that we proposed in our previous papers, which described a method of determining slowly varying parameters. We used the dependence of bandwidth on the order of nonlinearity to distinguish the contributions from different orders of nonlinearities in the frequency domain. We assumed that fast Fourier transforms (FFTs) with shorter fundamental periods than the symbol period attain shorter measuring times. We conducted a computer simulation to confirm the applicable range of the described method for varying the speed of the model parameters. We assumed sinusoidal variations for the parameters of the PA, and used the proposed method to trace the varying parameters and used the traced results in a predistorter (PD). We estimated the improved spectrum for the PD to confirm the accuracy of the determined coefficients. The simulation shows that our method traces changes accurately up to a variation in amplitude of 20%. We also confirmed that the method can trace the sinusoidal variation frequency up to half of the symbol period.

TDMA is more suitable for high throughput data collection than CSMA. Yet, in wireless sensor network plain TDMA-based protocols require complex scheduling. In case of node failures, plain TDMA is hard to recover connection and achieve robust data collection due to the complex scheduling burden. Toward robust data collection, it is important to develop a collection protocol capable of topology adaptability. We have been developing a data gathering protocol named TKN-TWN. TKN-TWN exploits the advantages of TDMA and eases the scheduling burden by using two tokens to arbitrate transmission, providing high throughput via burst data transmission. Moreover, due to the simplified scheduling based on tokens, TKN-TWN is practical to provide adaptability for infrequent topology changes. We enhance the implementation of TKN-TWN over the previous work and carry out experiment on a deployed network in the office room to evaluate the throughput and topology adaptability as well. Experimental results in the network consisting of 32 TinyOS enabled sensor nodes show that TKN-TWN is robust to network changes caused by occasional node failures. Evaluation also shows that TKN-TWN is able to even provide throughput over 10 KByte/s.

To reduce the complexity of scheduling while exploiting the advantages of TDMA-based data collection, we propose TKN-TWN, a token-scheduled multi-channel data collection protocol. The TKN-TWN uses two tokens to arbitrate data-packet transmissions and associates the ownership of tokens with transmission slot assignment. For the reduction of scheduling burden, the TKN-TWN provides topology adaptability while maintaining high throughput in burst data transfer. In this paper, we present the system design of TKN-TWN with enhancement toward topology adaptability based on a previous work. We evaluate TKN-TWN on a local test bed using 31 TelosB sensor nodes. Evaluation results show that the TKN-TWN achieves throughput of 6.4 KByte/s with more than 99% delivery ratio even with occasional node failures.

The objective of this research is to realize a method that detects a user's entrance to predefined geographic area so that appropriate services are provided without explicit user action. Though such geofencing technology is gathering attention due to its wide range of applications, it is a challenge to overcome trade-off between power consumption and detection accuracy. Conventional variable interval positioning methods fall short on battery saving due to difficulties such as sparse and erroneous position measurement data, and the unpredictable speed and trajectory of the terminal. In this paper, we propose a method for position detection whose activation frequency is determined by speed toward the target spot. The method is robust against positioning error and fluctuation of the terminal's movement by leveraging the access angle to the target spot. Simulation results show that the proposed access speed correction method reduces power consumption from 42% to 52% against conventional method, while holding the false negative ratio to detect the target spot to less than 5%.

This paper presents a system design and experimental evaluations for ambient sound-based proximity detection with smartphones. Ambient sound is useful as spatiotemporal identifier. This is because ambient sound contains abundant information which depends on time and space. To detect proximity at high accuracy, we calculate cross correlation in frequency domain for the similarity measure of ambient sound. The results of experiments show that our system discriminates 5 rooms with the accuracy of true positive 94.9% and false positive 0.1%.

This paper presents how carrier frequency differences among transmitters affect concurrent transmission in wireless sensor networks. The carrier frequency differences cause two problems: reverse phase rotation and signal level fluctuations. The effects of these problems depend on modulation schemes. The demonstration shows these problems with moving images of superposed baseband signal and constellation transmitted from 1-6 nodes concurrently.

We present a high throughput data gathering protocol for wireless sensor network. Different from existing works we elaborate a token-scheduled multi-channel TDMA protocol named TKN-TWN that provides better scalability and topology adaptability. TDMA is more suitable for high throughput data collection than CSMA. However, TDMA-based protocols require complex scheduling of the transmission time slot. The scheduling burden is a primary stumbling block of topology adaptability. To ease the scheduling burden while exploiting the advantages of TDMA, we propose to use two tokens arbitrate transmission and associate the ownership of tokens with transmission slot assignment toward throughput optimization. Due to the simplified scheduling based on tokens, TKN-TWN is able to provide scalability and topology adaptability. In addition, TKN-TWN allows burst transmission by leveraging the contention-free nature of TDMA, and sustains high throughput with burst data transmission when sensor nodes have similar traffic pattern. An extensive simulation study indicates that TKN-TWN achieves robust data delivery even in densely deployed large scale sensor networks. Experimental results on Tmote Sky show that TKN-TWN is able to provide throughput over 10 KByte/s, an improvement of about 23% over a prior work.

We present a design and evaluation of Choco, a low-power, end- to-end reliable collection protocol for wireless sensor networks. We construct Choco on Glossy, which is a recently presented efficient flooding scheme exploiting constructive-interference phenomena. Although Glossy is originally developed for flooding, it can be regarded as a networking primitive which provides low-power, low- delay, any-to-any communication reachability. In Choco, all packet transmissions are based on Glossy. A sink node explicitly schedules each node operation in a centralized and fine-grained manner with Glossy, and controls retransmission and power saving mode of each node. Each sensor node transmits data packets to the sink node with Glossy according to the request from the sink node. Although Choco is very simple mechanism, we reveal that Choco has considerable merits over current data collection protocols through two different testbed evaluations. Testbed evaluations demonstrate that Choco provides 14.7 times more energy-efficient collection than the state-of-the-art, while achieving end-to-end reliability for every node.

Data-intensive wireless sensor network applications, such as structural health monitoring and earthquake monitoring, require high throughput bulk data collection. Based on the fact that each node stores the same amount of sensor data, we propose a Maximum-Subtree-First Collection Protocol (MSFCP), which adopts Maximum-Subtree-First scheduling on top of multi-channel block transfer to maximize overall throughput. We present the theoretical analysis that MSFCP can achieve optimal throughput in the ideal propagation environment, and we achieve overall throughput of 135 kbps on the IRIS Mote platform.

For the efficient use of spectrum resource, it is important to understand spectrum occupancy at a high spatial resolution, so a large number of measurement locations are required. In this work, in order to increase measurement locations, we design and implement a low-end spectrum sensor which is capable of switching to different channels in a spectrum range of the UHF band. The spectrum sensor can provide intuitive knowledge for spectrum usage.

This paper presents a preambleless communication system to realize industrial wireless control networks (WCN) which are characterized by short payloads and strict real-time requirements. The reduction of the preamble is greatly beneficial for the real-time operation because the transmission time of preambles is longer than that of the short payloads. Our system is implemented on a software defined radio (SDR) platform using GNU Radio toolkit. Experiments show the packet error rate (PER) performance is close to that of the theoretical value in an AWGN (additive white Gaussian noise) environment. Demonstrations show the real-time remote control of inverted pendulums on this system.

We study a network structure for efficient information dissemination that leverages location-based real-time applications operating in a distributed system. Gossip-based information dissemination is a promising method for a large-scale system to keep nodes in the network quickly updated with new information. The information dissemination efficiency depends strongly on the underlying network structure, so it is important to analyze how the network structure is optimized to achieve agile information dissemination, especially for time-sensitive applications, e.g., for dynamic pursuit of a moving target that requires frequent information updating. To this end, we construct a continuous-space network model extended from Kleinberg's small-world model so that it is applicable to location-based services. Analytical and simulation-based study shows that excellent information dissemination efficiency is achieved when the shortcuts overlaying a uniform cluster network are created nearly uniformly to nodes at all distance scales. We apply our model to the dynamic pursuit problem where a pursuer searches for a moving target. Monte-Carlo simulation shows that the network with well-structured shortcuts surpasses other types of networks in finding a moving target because the pursuer can steadily approach the target regardless of its location, and the network structure greatly influences the dynamic search ability in distributed systems.

Self-* properties have received considerable attention in the literature, and a wide variety of schemes have been proposed recently. In this survey, we introduce the fundamentals of self-stabilizing algorithm development in a highly dynamic environment. Roughly speaking, a self-stabilizing system recovers from recurrent transient faults that disturb its behavior after a finite convergence period. We put emphasis on the aspects that associate probability with self-stabilization. The introduced concepts and techniques might provide practical implications for designers of self-stabilizing systems.

In this demonstration we present an ubiquitous health monitoring system based on the NGN/IMS (next generation network/IP multimedia subsystem). It enables us to be taken care of by medical professionals anytime, anywhere using portable medical sensors and mobile networks. To provide seamless health monitoring services, an open, secure and functional platform is required. Such a platform should support multiple functions such as real-time transfer, event notification, and continuous data accumulation. The NGN/IMS is a potential platform to fulfill these requirements, because it is a standard- based open platform, which provides AAA (authentication, authorization and accounting), QoS (quality of service) support, event notification and a data management server. We present a system design and a prototype implementation on a NGN/IMS testbed.

Random access process of LTE (long term evolution) system is carried out exploiting the superior correlation property of random access code. Each user unit transmits randomly selected random access code, and the Node B (base station) detects each code by correlating the received signal with all possible random access codes. By comparing the correlation output power to the threshold, randomly selected codes are identified. This paper considers random access process of LTE system in relay network. A problem caused by the introduction of relay network is addressed, and a solution is provided. The proposed solution includes the power control strategy for both downlink broadcast signal and uplink random access signal. Simulation results show that the addressed problem is overcome by the proposed power control algorithm while the backward compatibility is maintained.

In distributed spectrum sensing, we deploy ten thousands of sensor nodes, and a low-cost sensor node is necessary. In this work, we design the sensor node specialized in power level measurement for distributed spectrum sensing, and we discuss 3 components of the node, which are frequency range, RBW and a detector.

Radio spectrum has turned into a precious natural resource from free goods due to the rapid development of wireless communication technology. In order to efficiently utilize radio spectrum, a spectrum policy should be enough appropriate to create new industries and new technologies. From this point of view, we have developed a distributed spectrum sensing system which collects dense information of spectrum usage over large area. In this work, we discuss the requirements and the design of the distributed spectrum sensing system.

Handoff procedure in IEEE 802.11 wireless networks must be accomplished with as little interruption as possible to maintain the required quality of service (QoS). We have developed a fast handoff scheme, called AuthScan, to provide a novel usage of the open system authentication phase to reduce channel scanning latency. AuthScan comprises two steps: firstly, a client caches its handoff history with beacon information. Secondly, when in need of handoff, a client transmits authentication request frames to the selected access points (APs) from the cache instead of broadcasting probe request frames as in active scan to discover the available APs. The next AP is selected by comparing quickly acquired handoff metrics during the handoff procedure. This fast handoff is possible by upgrading only software on the client side. This paper presents the theoretical handoff latency of AuthScan in comparison with other approaches, and shows the effectiveness of our system through implementation and experiments.

No abstract available.

Ranging process in IEEE 802.16 system provides a number of functionalities such as initial network entry, uplink synchronization, power adjustment, and system coordination. As the introduction of relay network in IEEE 802.16 systems, a new transmission power control algorithm is necessary. Therefore, this paper proposes a novel ranging transmission power control algorithm which reduces the interference between MS-BS ranging (i.e. ranging between MS and BS) and MS-RS ranging (i.e. ranging between MS and RS). Computer simulation results show the performance of ranging process in terms of ranging detection success probability and validate the efficiency of the proposed transmission power control algorithm. These results will be beneficial as the guideline for the design of IEEE 802.16 relay system.

In this paper, we propose a prototype of a guidance system for older people with dementia to support their activities of daily living (ADL), such as tea making, shaving, and self-hairdressing. We will demonstrate three ADL cases to show how our prototype works, and a developing case to show how to generalize our system to a new ADL in several minutes.

We have developed a fast handoff scheme, called AuthScan, to reduce the time-consuming channel scanning latency in IEEE 802.11 wireless networks. AuthScan comprises two steps: First, a client caches its handoff history with beacon information. Second, when in need of handoff, a client transmits Authentication Request frames to the selected Access Points (APs) from the cache instead of broadcasting Probe Request frames in active scan to discover the next AP. Our proposed method does not require any support from the infrastructure network and contributes to finish the handoff procedure with the lowest latency when compared to approaches that improve the efficiency of channel scanning. Furthermore, AuthScan requires neither hardware upgrades to the client, nor any modification to currently deployed APs. In this paper, we present the theoretical handoff latency of AuthScan and compare it to other approaches.

In this paper we present a high-density earthquake monitoring system using wireless sensor networks. For high-precision monitoring, we developed Pavenet OS, which is a hard-realtime operating system for sensor nodes, and acceleration sensor board. Sensor nodes of the system sample acceleration with less than 0.3 us jitter with Pavenet OS. The system provides earthquake engineering researchers the ability to measure vibrations of structures during earthquakes at less cost and higher node density than previous systems.

Handoff process in IEEE 802.11 wireless networks must be accomplished with as little interruption as possible to maintain the required quality of service (QoS). Previous research reported that channel scanning phase accounts for more than 90% of the overall link-layer handoff latency, ranging from 350 to 500 msec. In order to eliminate the time-consuming channel scanning latency we propose a stand-alone, shared beacon channel, where stations (STAs) can update information about neighboring access points (APs) via an extra receiver. Our proposed method contributes to completely eliminate the channel scanning phase, therefore, realizing fast handoff in IEEE 802.11 wireless networks. In this paper, we implement our proposed method on FreeBSD 6.1R kernel with Atheros AR5212-based 802.11 a/b/g chipsets. From experimental results, we find that it takes 6.063 msec delay on average during handoff from 802.11b AP to 802.11a AP while transmitting/receiving 480 byte ICMP frames with an interval of 10 msec.

The paper investigates and evaluates the application of log-likelihood ratio-based ordering in the decoding stage of amplify-and-forward cooperative spatial multiplexing system. The cooperative spatial multiplexing detects signals transmitted from the source with successive interference cancelation algorithm whose performance varies, depending on the ordering schemes applied. Log-likelihood ratio-based ordering uses and exploits the a posteriori information of the received signal at the destination to get a more accurate final estimation at the decoder output. Based on the evaluation, the authors demonstrate that with marginally extra complexity, log-likelihood ratio based-ordering offers better performance compared to signal-to-noise ratio-based ordering.

Abstract R&D on new generation mobile network has attracted a growing interest over the world on the background of rapid market growth for 2nd and 3rd generation cellular networks and wireless LANs/MANs. The National Institute of Information and Communications Technology (NICT) started the New Generation Mobile Network Project in April 2002, and has developed fundamental technologies to enable seamless and secure integration of various wireless access networks such as existing cellular networks, wireless LANs, home networks, intelligent transport systems (ITS), the Beyond‐3G (B3G) cellular and other wireless access systems. This paper overviews the achievements of the project focusing on network and access technologies. Copyright © 2007 John Wiley & Sons, Ltd.

This paper proposes a novel technique to enable flow-level multi-path forwarding on IPv6 networks. Generally on such networks, any packet from a node is forwarded toward a destination along the same path, even if some of the packets belong to a different flow with different characteristics. In accordance with packet characteristics and to fulfil demands such as traffic engineering and QoS routing, flow-level multi-path forwarding allows explicit selection of the paths on which to send packets. Multi protocol label switching (MPLS) technology has been used to perform this function so far. However, to apply MPLS on an existing IPv6 network, all non-MPLS-enabled routers in the network must be replaced with MPLS-enabled routers. Therefore, the migration cost is generally expensive. Our previous work has proposed a technique called flow-mapping explicit host routing (FMEHR,), that renders similar results to MPLS using unused address spaces such as IP private addresses. This paper describes the FMEHR concept and its realization on an IPv6 network and discusses its characteristics.

We design a scalable and expressive naming system called SENS, which can retrieve information of computing and content resources distributed widely on the Internet by exact queries and multi-attribute range queries over resource names. Our system utilizes a descriptive naming scheme to name resources and a multi-dimensional resource ID space for message routing through the overlay network of name servers (NSs). The resource ID space is constructed on the overlay network based on CAN routing algorithm. We propose a novel mapping scheme between resource names and resource IDs, which can preserve the locality of resource IDs while still achieving a good degree of load balancing regarding resource information distribution. We also propose a multicast routing algorithm to deliver resource information and a broadcast routing algorithm to route query messages to corresponding NSs at minimum cost. Our simulation results show that our system can achieve good routing performance and load balancing

The current channel scanning phase in IEEE 802.11 Wireless LAN does not fit fast handoff due to the following two reasons. First, IEEE 802.11 Wireless LAN operates based on CSMA/CA. Unlike cellular system, STAs can not scan other channels while exchanging data frames with currently associated AP, because it would directly cause frame loss. To attack this problem, related works have tried to utilize Power Saving Mode (PSM) or multiple NICs. Second, beacon interval is too far apart for fast handoff (100 msec by default). Since an STA lacks knowledge about which channel its nearby APs are operating on, it should redundantly scan every single channel regardless whether any AP exists on that channel. Though shorter beacon interval or active scan enable an STA to locate its nearby BSSs more quickly, these approaches increase extra traffic to the limited bandwidth. Previous works tried to provide an STA with nearby AP information approximated by APs collaboratively.

This paper proposes a MAC protocol for information discovery in ubiquitous networks. The proposed protocol is based on proactive discovery in which wireless devices periodically send advertisement broadcasts. The objective of the protocol is to provide the specified discovery time of the single-hop neighbors in a mobile ad hoc network. Since collisions in wireless network degrade performance on the discovery time, a node selects an adequate broadcast rate considering the network topology. In this paper, we show that using information about the number of neighbors in combination with framed Aloha scheme can provide the specified discovery time. We also consider a simple basic power saving mechanism which allows significantly reduce the power consumption

Application-level multicast (ALM) is a novel technology for multipoint applications, such as large scale file distribution, video and audio streaming, and video conference. Though several ALM systems have been proposed, all the multicast functions have been independently developed and integrated into individual applications. In such a situation, the development includes a lot of redundancy. Our goal is to develop RelayCast, the middleware which provides the common functions of ALM to eliminate the redundancy. RelayCast has some functional units which support fundamental and minimum functions respectively, and realizes one complete ALM system by cooperation of the units. The functional unit contains several components with different algorithms, and RelayCast meets the requirements of various applications by choosing the appropriate component. In this paper, we propose the RelayCast architecture, and present the implementation and experiment of a prototype.

This paper describes the design and implementation of a ubiquitous networking server that enables cross-device as well as cross-network mobility by using an out-of-band signaling method with a location sensing system for context-aware seamless network services. The context includes the user's presence, position of the user and neighboring devices, network interfaces attached to the mobile terminal, availability and available bandwidth of each network, priority in selecting networks, capabilities of the mobile terminal and devices, and installed applications. The server is designed to be placed not in operators' networks but in users' local networks such as home and enterprise networks to provide users with mobile IP-based handover capability between radio networks in a distributed manner. This would accelerate the introduction of Mobile IP to network services, offering flexibility in creating human-centric network services. The out-of-band signaling method we propose exchanges information between mobile terminals and the network about user and device authentication, session initiation protocol (SIP) signaling, mobile IP signaling, the position of users and devices, and signaling for cross-device mobility. These features provide us unprecedented flexibility in communication

When ad hoc networks are linked to wired networks by access points, the traffic tends to be non-uniformly distributed in the sense that it is more likely to be concentrated on the access points than on other nodes. However, most traditional schemes of ad hoc routing focus on the pure ad hoc network with a uniform traffic pattern. The purpose of this paper is to build multi-hop routes for non-uniform route connectivity with low routing overhead. To achieve efficient route discovery and route maintenance, we develop a new routing scheme of hopcount-based routing, which utilizes hopcount information to localize the route discovery within a limited area in order to reduce routing overhead. The simulation results show that the hopcount-based routing scheme highly reduces routing overhead while preserving high rate of successfully discovering route to the destination.

We have designed and implemented a compact and simple network oriented device, a NetSpeaker, which mainly consists of a DA converter, an amplifier and buffers, in order to enable setting many terminals and realize ubiquitous computing environment. It receives WAV formatted data via IP packets and simply plays it by DA converting. All the sound outputs of Windows XP/2000 applications can be played on remote NetSpeakers, since we implemented the sound forwarding function as a virtual sound device, which generates WAV data of sound outputs of applications and sends them to a selected NetSpeaker. In this paper, we show design of this NetSpeaker system and its application in a service mobility environment.

Tiny networked sensor devices will be disseminated over our physical life space, and take a significant role in realizing a ubiquitous computing environment. As sensor network technologies and its applications are still at its early stages, the architecture of the sensor network heavily relies on the applications. We believe it quite essential to come up with various kinds of applications, and obtain sufficient knowledge of sensor network architecture and its peripheral technologies through demonstrations. In this paper, we introduce our sensor network applications and present the way sensor network is used in them.

Wireless sensor network technologies are expected to be a key technology to support various innovative applications in the future ubiquitous computing environment. So far, the mainstream of sensor network research has been focused on simulation-based development of battery-aware communication protocols. However, when we apply sensor network technologies to practical applications such as environmental monitoring or factory automation, we find a lot of practical problems that are not considered in simulation-based approach. We believe that finding and solving such practical problems are quite important for deploying sensor network technologies beyond laboratory use. From this point of view, we have been trying to find such problems through various implementations of sensor network system for several years. This paper describes our opinions of designing sensor network system and introduces our approaches for developing practical systems.

An out-of-band basic access signaling (BAS) protocol is introduced for use in heterogeneous wireless networks. The BAS protocol enables a mobile terminal to communicate with an agent in a fixed network through any radio access network (RAN) chosen by the user as the basic access network (BAN). The agent uses the information on the location of mobile terminals and on the availability of RANs to decide which RAN to use, taking the user's preferences into account. This protocol was implemented in a demonstration system using existing radio systems and the standard mobile IP protocol. The separation of the signaling path from the data paths was found to save energy and to facilitate handover between RANs.

Ubiquitous computing as the integration of sensors, middleware, and networking technologies to form a "smart space" environment relies on the development of both software and hardware solutions. For over 3 years, our group has been developing a smart-space environment, involving the exploration of core technologies and attractive applications. We introduce the challenges faced and lessons learned in designing and developing a smart-space environment, including device control services, locating systems, wireless sensor nodes, and middleware services.

A context-based adaptive communication system is introduced for use in heterogeneous networks. The context includes the user's presence, location, available network interfaces, network availability, network priority, communication status, terminal features, and installed applications. An experimental system was developed to clarify the feasibility of using context information to flexibly control a network and applications. The system operates on a seamless networking platform we developed for heterogeneous networks. By using contexts, the system can show the caller and its callee the applications they can access that are available through the network before communication occurs. Changes of the contexts can switch the on-going application to another during communication. These features provide us with unprecedented styles of communications.

For IP-based always-on type multi-service user terminals, which may have multiple wireless interfaces, energy-efficiency is a key requirement. It is crucial to take into account the power consumption behavior and delay patterns associated with each state and transitions of the wireless devices to formulate efficient operation of multi-module terminals, especially in services where media/session handover between wireless networks are targeted. This paper investigates the power consumption pattern or behavior of some selected wireless interfaces that are good candidates for being part of the future of the multi-service user terminals. We propose a simple model for predicting energy consumption in a terminal attributed to the wireless network interfaces. We measured the actual consumption pattern to estimate the parameters of the model.

Application-level multicast (ALM) is being increasingly recognized as a solution to support multipoint applications without the need for a network layer multicast protocol. Though several ALM systems have been proposed, all the multicast functions are independently developed and integrated into individual applications. However, we can find out the common functions of ALM among the existing systems. Our goal is to abstract the common functions and incorporate them in the middleware called RelayCast. RelayCast meets the fundamental requirements of various applications by combining several components of ALM functions. In this paper, we firstly present the architecture and implementation of RelayCast. Then, we present the multipath routing mechanism as a component. The multi-path routing achieves fast tree rebuilding at the time of tree partition caused by end-host's leave or failure.

Decentralized and unstructured peer-to-peer (P2P) networks such as Gnutella are attractive for Internet-scale information retrieval and search systems because they require neither any centralized directory nor any centralized management of overlay network topology and data placement. However, due to this decentralized architecture, current P2P keyword search systems lack useful global knowledge such as popularity of data items and relationships between keywords and data items. As a result, current P2P keyword search systems supports only naive text-match search and can find only data items with a keyword (or meta-data) exactly indicated in a query. In this paper, we show an efficient P2P search system which increases possibility of discovering desired data items. The key mechanism is query expansion, where a received query is expanded based on keyword relationships managed in a distributed fashion by participating nodes. Keyword relationships are improved through search and retrieval processes and each relationships is shared among nodes holding similar data items. We also present implementation of our P2P search system.

High-speed wireless access networks have been studied for future mobile networking. In wireless links, the higher the transmission rate, the smaller its cell size becomes. As a result, a lot of discontinuous spotted areas will exist at many places in cities. In such an environment, a mobile host moving with high-speed like a car will go through cells during a very short time. Therefore, it will repeat discontinuous connection with the network, and will not be able to have enough time for communication. We present a prefetch architecture referred as to proactive data transfer for supporting these networks. In this architecture, a user utilizes a cellular network for file requests while she stays outside the area of the hot-spotted network. The requested files are prefetched at a spool server located in the hot-spotted network. Proactive data transfer enables a user to start high-speed data communication as soon as she enters into the area of high-speed wireless networks. Moreover, throughput deterioration caused by congestion of backbone networks can be alleviated. The experimental results show the effectiveness of proactive data transfer.

Layered multicasting with a layered source coding scheme enables efficient distribution of real-time streaming media over heterogeneous networks such as the Internet. In layered multicast, rate control is required to cope with congestion quickly because packet losses in a layer with higher priority lead greater degradation of the received quality. In addition, rate control for layered multicast is required to assure fair bandwidth allocation among competitive layered multicast sessions at a congested link independent of their start time. We present a novel rate control scheme for layered multicast, called network-supported layered multicast (NLM). In NLM, routers distributed within the network support rate control. NLM avoids packet losses on a layer with high priority and achieves fair bandwidth allocation by filtering and signaling. A router filters a layer with low priority when it detects congestion, or when it is informed that the layer is needless by signaling. Simulation results show NLM offers significant advantages compared with receiver-driven rate control.

For wireless channels, interference mitigation techniques are typically applied at the packet transmission level. In this paper, we present the Havana Framework for supporting integrated adaptive-QOS that also responds to impairments over multiple time scales that are present at the flow/session level. Our framework is based on three different mechanisms that operate over distinct adaptation time scales. At the packet transmission time scale, a channel predictor determines whether to transmit a packet or not depending on the state of the wireless channel. At the packet scheduling time scale, a compensator credits and compensates flows that experience bad link quality. Over even longer time scales an adaptor regulates flows taking into account the ability of wireless applications to adapt to changes in available bandwidth and channel conditions.

Detection of the existence of a desired user is considered in this paper. We assume that the signal to noise ratio is high enough to ignore the effects of noise compared with those of the interference by other users. The inter-user interference and user signals are modeled by the Nakagami model. The observation model for this situation is proposed, the locally optimum test statistic is derived under the model, and the asymptotic performance of the test statistic is compared with that of the envelope detector. We show that the locally optimum detector has performance better than the conventional envelope detector.

This paper presents a distributed power control scheme for next-generation multiservices CDMA systems. CDMA has inherent capability to control the quality-of-service (QoS) requirements by assigning different power levels to each traffic type. Toward this, optimum power control schemes have been investigated. The main drawback of the previously proposed algorithms is that they would require all users' transmission state necessitating a complicated control process or peak-rate bandwidth allocation. To overcome this, we exploit the Markovian property to obtain the statistics of the traffic. The statistical formulation is presented for allocating power distributedly so as to keep the "collision" probability below a predefined probability. Numerical examples show that the distributed power control scheme allows better utilization of wireless resources through statistical multiplexing than peak-rate bandwidth assignment, and it does not require a complicated control process while keeping total transmitted power at slightly greater than optimum power control.

Wireless ATM networks are motivated by an anticipated demand for wireless extensions of the integrated multimedia services. A TDMA-based wireless ATM is presented for the integration of wireless access with wired B-ISDN/ATM networks. The presented system is characterized by a reservation-based access protocol to handle diverse services and a dynamic duplexing scheme for TDD to support uplink/downlink asymmetric traffic. Resource allocation algorithms which aim at prorating the shared radio resources among ABR message traffic such as files and e-mails are also presented. Simulation results show that the equal sharing algorithm where the resources are equally shared among all mobile terminals can provide a higher average and lower variance of the throughput than the FIFO-based algorithm.

This paper presents a real-time VBR (variable bit rate) transport for next-generation multiservice personal communication networks. After discussing the characteristics of real-time VBR traffic, we propose a statistical multiplexing-based resource allocation scheme for real-time VBR traffic: "guaranteed" resources are assigned to high-priority packets, while "add-on" resources are assigned in a dynamic manner. The proposed scheme have been validated for an integrated voice/video environment, where video is encoded with a H.261-based two-layer video coder. The protocol was designed with the basic premise that there is a relatively large number of unused voice slots on average that can be used to carry "add-on" video packets. Numerical examples show that the prioritized transport under consideration can significantly improve the picture quality, and allow better utilization of wireless resources through statistical multiplexing.

Uplink/downlink independent radio resource management is one solution for emerging multimedia wireless networks. This paper presents six downlink channel assignment algorithms for packet-based TDMA/FDMA systems. The goal is to minimize the channel setup delay while attaining high transmission quality. Computer simulations are used to evaluate system performance in terms of dropped packet ratio and average channel setup delay. Only the bursty voice traffic is considered for real-time transmission. Priority-based two-stage algorithms are found to provide good performance, where the first stage is the selection of carrier to be scanned, and the second stage is the selection of time-slot to be assigned. The effect of priority determination algorithms is also discussed.

The performance of microcellular mobile communication systems such as the handover priority system, the overlaying macrocell system, and handover priority with overlaying macrocell system are evaluated and compared to that of the standard microcellular system by using spectrum efficiency, grade of service, and total efficiency as evaluation criteria. The total efficiency is composed of spectrum efficiency, blocking probability, and forced termination probability, and represents the comprehensive performance of the systems. Computational results suggest that high efficiency can be achieved by applying the overlaying macrocell system when the offered traffic is low and the handover priority system when it is high. Also the combined system, i.e., handover priority with overlaying macrocell system, is expected to have higher efficiency over the wide range of the offered traffic.>

Idle-slot broadcasting multiple access (IBMA) allows several information and control messages to share the same satellite channel efficiently. The IBMA protocol is particularly suitable for mobile satellite communications and vehicle positioning systems, in which some of the information messages, such as dispatch function, are classified as "periodic" and others, such as signalling, are classified as "random". When a new mobile terminal logins to the system. The Network Management Center reserves subsequent time slots for transmitting periodic messages without contention. Other random messages use the slotted ALOHA protocol to contend with other mobile terminals for an unreserved time slot. Computes simulations suggest that the use of the IBMA protocol can be regarded as a viable solution for advanced mobile satellite communications and positioning systems.>

An incremental approach to understanding 3-D structure and motion of rigid as well as nonrigid objects from image sequences is presented. To be applicable to natural imagery, the recovery process should meet two requirements: robustness to noise and the ability to cope with deformable objects. These requirements can be met by using a smoothness-of-motion constraint. The smoothness-of-motion constraint provides a framework for temporal integration of motion information over a longer sequence. The 3-D structure and motion information is smoothly and successively updated to agree with the observed transformation in the range. A model of rigid and nonrigid motion is introduced, and the smoothness-of-motion constraint is formulated as a stabilizing function. Some preliminary results are also given.>

An image sequence coding scheme based on the extracted information of 3-D structure and motion is presented. The goal is to achieve efficient and high-quality visual telecommunications through the use of 3-D scene characteristics inherent in image sequences. The structure and motion of rigid and nonrigid objects are robustly estimated from the displacement vectors between frames. As an image source model in the image coding process, the 3-D information provides more efficient representations of image sequences because they are derived directly from the scene characteristics. Experimental results are given and suggest that the scheme can be a viable solution in realizing high-quality visual communications.>