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DOI： 10.1109/tgcn.2020.3033844

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Language：English   Publisher：Information Processing Society of Japan  

Autonomous mobile devices, such as robots and unmanned aerial vehicles, as alternatives to humans, are expected to be applied to searching for and manipulating a variety of emergent events of which the location and number of occurrences are unknown. When an autonomous mobile device searches for an event, it needs to sense a physical signal emitted by an event, such as radio waves, smell or temperature. After a device finds an event, it must manipulate the event. We previously proposed Mobile Sensing Cluster (MSC), which applies swarm intelligence to multiple autonomous mobile devices to quickly search for and manipulate multiple events using dynamically formed multiple swarms of mobile devices. However, in an environment that the physical signal emitted by an event and sensed by a device includes some random noises, the behavior of swarms in MSC becomes unstable. As a result, MSC requires a long time to search and manipulate. In this paper, we propose a dynamic swarm spatial scaling MSC for improving the tolerance of MSC against such random noises, and show its effectiveness.

DOI： 10.2197/ipsjjip.29.140

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

Stable wireless connections are required for realtime applications employing high data rate video streaming. Each of Multi-AP system and Multi-Link aggregation is effective to make WLAN connections stable, and we propose a combined Multi-AP Multi-Link system for realizing more stability in the practical use. In our proposed system, APs, which have different types of transmission queues, transmit a stream collaboratively with the support of a coordinator. In the demonstration, we will show that the proposed system realizes high quality video streaming even when WLAN links suffer from interference caused by surrounding devices.

DOI： 10.1109/CCNC46108.2020.9045205

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Reliable and real-time vital information monitoring is required to promote health and prevent disease/injury for a group of exercisers spread in a sports ground [1], such as schoolchildren and professional athletes during physical training.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

"AccuWiSe: Accurate Wireless vital Sensing" is a multi-hop wireless network system which can collect and monitor vital information such as heart rate (HR), energy expenditure (EE) and core body temperature (BT) from tens of exercisers spread in a sports ground periodically, reliably and in real-time.In this paper, we explain the AccuWiSe hardware/software configuration and show its vital sensing performance.

DOI： 10.1145/3278576.3278580

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this paper, we investigate how to improve the reliability of heart-rate monitoring of a large number of people playing sports over a limited area of sports field. Densely-located nodes attached to exercisers transmit heart-rate data to a data collection node simultaneously, which negatively affects the reliability of data collection due to severe congestions caused by excessive traffic load. In order to solve this problem, we introduce a heart-rate data transmission control scheme where the sensed heart-rate is transmitted only when each node detects a larger variation than a fixed threshold. In order to analyze trade-off between the traffic suppression and increased sensing error through the introduced threshold, we first conduct experiments to obtain heart-rate variations during typical scenarios of exercises in a sports-day at schools in Japan. The obtained experimental data are then used to construct a model of heart-rate variations, which is then employed for computer simulations to evaluate the effectiveness of the introduced heart-rate transmission control. Our numerical results show that the heart-rate transmission control is effective, especially in a scenario where exercisers with different patterns of heart-rate variations coexist.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

Wireless sensor and actuator networks (WSANs) are required to achieve both energy-efficiency and low-latency in order to prolong the network lifetime while being able to quickly respond to actuation commands transmitted based on the real-time sensing data. These two requirements are in general in a relationship of trade-off when each node operates with well-known duty-cycling modes: nodes need to make their radio interfaces (IFs) frequently active in order to promptly detect the communication requests from the other nodes. One approach to break this inherent trade-off, which has been actively studied in recent literature of wireless sensor networks (WSNs), is the introduction of wake-up receiver that is installed into each node and used only for detecting the communication requests. The main radio IF in each node is woken up only when needed, i.e., in an on-demand manner, through a wake-up message received by the wake-up receiver. In this paper, we introduce radio-on-demand sensor and actuator networks (ROD-SAN) where the concept of wake-up receiver is applied to realize on-demand WSANs. We first evaluate data collection rate, packet delivery latency, and energy-efficiency of ROD-SAN and duty-cycling modes defined in IEEE 802.15.4e by computer simulations. Then, we present our test-bed implementation of RODSAN including all protocols from the lowest layer of wake-up signaling to the application layer offering the functionalities of information monitoring and networked control. Finally, we show experimental results obtained through our field trial in which 20 nodes are deployed in an outdoor area with the scale of 450m x 200 m. The numerical results obtained by computer simulations and experiments confirm the effectiveness of ROD-SAN to realize energy-efficient and high-response WSANs.

DOI： 10.1587/transcom.2016SNP0013

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Real-time vital sensing for persons during exercises is important for effective physical training, healthcare and injury/disease prevention. We have developed a vital sensor node which can sense heart rate (HR), energy expenditure (EE) and body temperature (BT), and furthermore can transmit the sensed data by wireless in real-time, just by hanging itself with the clip at the rim of undershorts. The vital sensor node utilizes a motion artifact (MA) cancellation technique to accurately sense HR during vigorous exercises and an EE estimation technique by means of a motion sensor to accurately sense EE. In this paper, we outline the essence of the two techniques.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

Energy harvesting wireless sensor networks (EH-WSNs) are being actively studied in order to solve the problems faced by battery-operated WSNs, namely the cost for battery replacement and the negative impact on the environment. In EH-WSNs, each node harvests ambient energy, such as light, heat, vibration, and uses it for sensing, computations, and wireless communications, where the amount of harvested energy of each node varies depending on their environments. MAC protocols for EH-WSNs need to be designed to achieve high throughput and fairness, however, the conventional MAC protocols proposed for EH-WSNs do not adapt to the harvesting rate of each node, resulting in poor fairness. In this paper, we propose a fair MAC protocol based on polling scheme for EH-WSNs. The proposed scheme adjusts contention probability of each node according to its harvesting rate, thereby increasing the throughput of nodes with low harvesting rate. We evaluate throughput and fairness of the proposed fair polling scheme by theoretical analysis and computer simulations, and show that the proposed scheme can improve fairness with little degradation of the overall network throughput.

DOI： 10.1587/transcom.2015EBP3479

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this paper, we focus on oxygen consumption (VO2) estimation using 6-axis motion sensor (3-axis accelerometer and 3-axis gyroscope) for people playing sports with diverse intensities. The VO2 estimated with a small motion sensor can be used to calculate the energy expenditure, however, its accuracy depends on the intensities of various types of activities. In order to achieve high accuracy over a wide range of intensities, we employ an estimation framework that first classifies activities with a simple machine-learning based classification algorithm. We prepare different coefficients of linear regression model for different types of activities, which are determined with training data obtained by experiments. The best-suited model is used for each type of activity when VO2 is estimated. The accuracy of the employed framework depends on the trade-off between the degradation due to classification errors and improvement brought by applying separate, optimum model to VO2 estimation. Taking this tradeoff into account, we evaluate the accuracy of the employed estimation framework by using a set of experimental data consisting of VO2 and motion data of people with a wide range of intensities of exercises, which were measured by a VO2 meter and motion sensor, respectively. Our numerical results show that the employed framework can improve the estimation accuracy in comparison to a reference method that uses a common regression model for all types of activities.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/EMBC.2016.7591786

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Other Link： https://dblp.uni-trier.de/rec/conf/embc/2016

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

This paper considers on demand WiFi wake-up where a wake up receiver is installed into each WiFi device. The wake-up receiver detects a wake-up call by finding the predefined length of WiFi frames, which corresponds to a wake up ID, through envelope detection with limited signal processing. Since each wake up receiver continuously observes the WiFi channel, an adverse event of False Positive (FP), where a WiFi device is falsely turned on without actual wake-up calls, can occur when the length of non-wake-up, background data frames match with predefined length. In this paper, we suggest using the received signal strength (RSS) of WiFi frames to differentiate the real and false wake-up calls. The proposed scheme exploits the correlation among RSSs of WiFi frames received from a single station located in a fixed position. Using measured RSS data obtained under various settings and different degrees of mobility, we investigate not only the FP reduction rate but also its impact on the probability of detecting real wake-up calls. We also present experimental results obtained with our prototype in which the proposed scheme is implemented.

DOI： 10.1587/transcom.2015EBP3437

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

This paper investigates the impact of hidden nodes (HNs) on on-demand access point (AP) wake-up that is employed to realize energy-efficient wireless LANs (WLANs). The considered wake-up signaling exploits IEEE 802.11 signals transmitted by a WLAN station (STA) to remotely activate a sleeping AP: a STA with communication demands transmits a series of WLAN frames with their length corresponding to the wake-up ID. A wake-up receiver attached to each AP detects the length of WLAN frames with the low-power operations of envelope detection and on-off-keying (OOK) demodulation. Since WLAN frames constituting a wake-up signal are transmitted by a STA following carrier sense multiple access (CSMA) protocol, they are vulnerable to the well-known hidden node (HN) problem. The impact of HNs on wake-up signaling is different from that on data communications since the wake-up receiver employs unconventional frame length detection to extract the information on the wake-up ID from the received signal. In this paper, we first investigate the impact of HNs on wake-up failure probability with theoretical and experimental evaluations. If the degradation of wake-up signalling due to HNs is observed for a STA, the corresponding STA may suffer from collisions due to the same HNs for its data communications even if it manages to succeed in the wake-up process. In this case, the wake-up operation itself may not be necessary. Therefore, we also compare the impact of HNs on wakeup signaling and that on data communications after the wake-up process. These results and discussions provide us with an insight on the impact of HNs on on-demand AP wake-up exploiting WLAN signals.

DOI： 10.1587/transcom.2015EBP3311

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

In the field of education such as elementary and middle schools, teachers want to take care of schoolchildren during physical trainings and after-school club activities. On the other hand, in the field of sports such as professional and national-level sports, physical or technical trainers want to manage the health, physical and physiological conditions of athletes during exercise trainings in the grounds. In this way, it is required to monitor vital signs for persons during exercises, however, there are several technical problems to be solved in its realization. In this paper, we present the importance and necessity of vital monitoring for persons during exercises, and to make it possible periodically, reliably and in real-time, we present the solutions which we have so far worked out and point out remaining technical challenges in terms of vital/physical sensing, wireless transmission and human interface.

DOI： 10.1587/transcom.2015MII0001

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Real-time vital sensing/monitoring for persons during exercises is essential for not only avoiding injuries and diseases but also effective training. For the vital sensing, we have developed a heart rate sensing technique canceling motion artifact induced during exercise and an energy expenditure sensing technique using tri-axial accelerometer. On the other hand, for the wireless transmission, mainly assuming several tens of persons during exercises, we have developed a broadcast/unicast-based networking technique placing data forwarding nodes around an athletic field.
In this paper, paying attention to the wireless networking, we show the challenges we have so far made to select the broadcast/unicast-based technique. Furthermore, we discuss the necessity of real-time vital sensing/monitoring from one thousand persons during exercises and present future challenges in wireless networking towards its realization.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In frame-length based wake-up control, a wake-up receiver smoothens the signal envelope by low-pass-filtering (LPF) before frame length detection. However, the LPF also has a side effect of distorting the rising and falling edges of the envelope. This letter analyzes the effect of LPF on frame length detection, and suggests measuring potential frame lengths with multiple thresholds and from them selecting the correct one. The analysis shows that using thresholds with exponential intervals helps to achieve a good performance with only a small number of thresholds. The effectiveness of the proposed scheme is also confirmed by experimental results.

DOI： 10.1109/LWC.2015.2475268

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Energy harvesting wireless sensor networks (EH-WSNs) are being actively studied in order to solve the problems faced by battery-operated WSNs, namely the cost for battery replacement and impacts on the environment. In EH-WSNs, each node harvests ambient energy, such as light, heat, vibration, and uses it for sensing, computations, and wireless communications, where the amount of harvested energy of each node varies depending on their surrounding environments. MAC protocols for EH-WSNs need to be designed to achieve high throughput and fairness, however, the conventional MAC protocols proposed for EH-WSNs do not adapt to the harvesting rate of each node, resulting in poor fairness. In this paper, we propose a fair MAC protocol based on polling scheme for EH-WSNs. The proposed scheme adjusts contention probability of each node according to the individual harvesting rate, thereby increasing throughput achieved by nodes with low harvesting rate. We evaluate throughput and fairness of the proposed fair polling scheme by computer simulations, and show that the proposed scheme can improve fairness with little degradation of the overall network throughput.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Vital signs obtained from people playing various types of sports can be exploited not only for diagnosing their physical conditions but also for learning the best practice to be followed to enhance their sporting abilities and skills. In this paper, focusing on a framework to collect vital data from a human group consisting of a massive number of people densely located in a limited area, we develop two new scenarios in a school environment: vital sensing in a class of physical training and large-scale sports events. The feasibility of the developed scenarios largely depends on the ability to collect data from a large number of densely-located sensor nodes, therefore, we investigate its limitation from a viewpoint of communication performance. Assuming IEEE 802.15.4g as an enabling technology, we evaluate the packet delivery rate under the two developed scenarios by computer simulations and discuss its potential to support a large volume of vital data traffic generated by densely-located nodes.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

In this paper, green wireless local area networks (WLANs), where idle access points (APs) are put into sleep and activated upon the request of mobile nodes, are realized by exploiting WLAN signals to convey wake-up messages. Specifically, wake-up messages, sent by nodes, are modulated onto frame lengths (physical transmission time) of successive WLAN signals and detected by non-WLAN low-cost receivers equipped at APs. This method, however, is susceptible to serious 1) false negative events due to low signal quality or collisions with background WLAN frames and 2) false positive events where background WLAN frames happen to have the same frame lengths as those of wake-up messages. In the proposed scheme, WLAN frames forming a wake-up message are transmitted in a burst and interpreted as an equivalent message. On this basis, false probability is reduced from two aspects: 1) Modulation constellations of frame lengths are optimized to maximize the Hamming distance between equivalent messages, and 2) preamble frame and envelope smoothing are used to mitigate false events. In addition to theoretical analysis and simulation, a prototype test bed is built and experimented on. Extensive evaluations confirm that the proposed scheme helps to greatly improve the reliability of wake-up control compared with state-of-the-art methods.

DOI： 10.1109/TVT.2014.2325643

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Radio-On-Demand Networks (ROD) has been proposed in order to improve energy-efficiency of WiFi device, which offers wireless access in an on-demand manner. Each WiFi device in ROD is equipped with a low-power wake-up receiver, which detects the length of wireless LAN frames through envelope detection and on-off-keying (OOK) processing. A STA, upon communications demands, transmits a wake-up signal consisting of WiFi frames with their length corresponding to a target wake-up ID. The receiver detects a wake-up request by finding the predefined length of WiFi frames. However, when WiFi devices transmitting non-wake-up data frames are present around the wake-up receiver, it is possible that the length of these frames match with the predefined length. This causes an adverse event of False Positive ( FP), where WiFi device is turned on without actual wake-up requests. In this paper, in order to reduce the number of FPs, we propose a wake-up frame detection exploiting the correlation among the received signal strength of WiFi frames received from a single STA in a fixed position. By computer simulations using the experimentally measured values of received signal strength, we show that the proposed scheme can significantly reduce the number of FPs without degrading the wake-up success probability.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Physical layer network coding (PLNC) has been proposed to improve throughput of the two-way relay channel, where two nodes communicate with each other, being assisted by a relay node. Most of the works related to PLNC are focused on a simple three-node model and they do not take into account the impact of interference from other transmissions. Unlike these conventional studies, in this paper, we apply PLNC to a large-scale cellular network in the presence of intercell interference (ICI). In cellular networks, a terminal and a Base Station (BS) have different transmission power, which causes different impact of ICI on downlink (DL) and uplink (UL) phase. We theoretically derive outage probability with a tractable approach based on stochastic geometry which accurately models ICI. Moreover, we compare the performance of PLNC with Direct and conventional Relay scheme. With the obtained numerical results, we discuss how the interference and the difference of transmission power affect outage probability achieved by PLNC.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In order to reduce energy wastefully consumed by WiFi device, Radio-On-Demand Networks (ROD) has been proposed, which offers wireless access in an on-demand manner. Each WiFi device in ROD is equipped with a low-power wake-up receiver which is capable of detecting the length of frames observed over WiFi channels through envelope detection and on-off-keying (OOK) processing. Once the wake-up receiver detects the predefined set of length of WiFi frames included in the wake-up signal transmitted by the other WiFi device, it turns its main WiFi module on. The wake-up receiver extracts WiFi signals over all WiFi channels at 2.4 GHz band. On the other hand, a WiFi device, which tries to wake up the other device, selects a wake-up channel, a channel to transmit its wake-up signal. In this case, the receiver observes an envelope which is the combination of the wake-up signal and the interfering signals existing over all WiFi channels. Here, how to select the wake-up channel is a key issue when the degree of interference is different among multiple WiFi channels. In this paper, with different degree of interference in each channel, we evaluate the impact of wake-up channel selection on the wake-up success probability of WiFi device. Besides simulations results, we also present experimental results which are obtained with our developed prototype of wake-up receiver.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Wireless local area networks (WLANs) are widely deployed recently. But many basic service sets (BSSs) nearby have to share a common channel due to the limitation in the spectrum resource. To get higher throughput with newly deployed access points (APs), it is necessary to improve spatial reuse of the channels by transmit power control (TPC). The achievable throughput, however, heavily depends on other factors such as rate adaptation (RA). Moreover, TPC without careful design may lead to asymmetric links and degrade fairness. In this paper, we jointly design TPC and RA to further improve total throughput of WLANs, and suggest (i) choosing power for each BSS by maximizing throughput which takes tradeoff between transmit rate and spatial reuse of channels, and, (ii) avoiding potential asymmetric links by explicit coordination among APs: each BSS uses almost the same power as its co-channel neighbors while BSSs far from each other may use different power levels as required. Extensive simulation evaluations confirm that the proposed scheme can greatly improve total throughput of dense WLANs, meanwhile fairness is retained.

DOI： 10.1007/s11277-013-1300-z

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Physical layer network coding (PLNC) has the potential to improve throughput of multi-hop networks. However, most of the works are focused on the simple, three-node model with two-way relaying, not taking into account the fact that there can be other neighboring nodes that can cause/receive interference. The way to deal with this problem in distributed wireless networks is usage of MAC-layer mechanisms that make a spatial reservation of the shared wireless medium, similar to the well-known RTS/CTS in IEEE 802.11 wireless networks. In this paper, we investigate two-way relaying in presence of interfering nodes and usage of spatial reservation mechanisms. Specifically, we introduce a reserved area in order to protect the nodes involved in two-way relaying from the interference caused by neighboring nodes. We analytically derive the end-to-end rate achieved by PLNC considering the impact of interference and reserved area. A relevant performance measure is data rate per unit area, in order to reflect the fact that any spatial reservation blocks another data exchange in the reserved area. The numerical results carry a cautionary message that the gains brought by PLNC over one-way relaying may be vanishing when the two-way relaying is considered in a broader context of a larger wireless network. © 2013 IEEE.

DOI： 10.1109/ICCW.2013.6649241

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this paper, we design a simple, low-cost, and low-power wake-up receiver which can be used for an IEEE 802.11-compliant device to remotely wake up the other devices by utilizing its own wireless LAN (WLAN) signals. A typical usage scenario of such a wake-up receiver is energy management of WiFi device: a device equipped with the wake-up receiver turns WiFi interface off when there is no communication demand, which is powered-on only when the wake-up receiver detects a wake-up signal transmitted by the other WiFi device. The employed wake-up mechanism utilizes the length of 802.11 data frame generated by a WiFi transmitter to differentiate the information conveyed to the wake-up receiver. The wake-up receiver is designed to reliably detect the length of transmitted data frame only with simple envelope detection and limited signal processing. We develop a prototype of the wake-up receiver and investigate the detection performance of the envelope of 802.11 signals. Based on the obtained experimental results, we select appropriate parameters employed by the wake-up receiver to improve the detection performance. Our numerical results show that the proposed wake-up receiver achieves much larger detection range than the off-the-shelf, commercial receiver having the similar functionality.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

This paper investigates wake-up ID and protocol design for radio-on-demand (ROD) wireless LAN (WLAN) where wake-up radio is applied to access point (AP) in order to save energy consumed by WLAN. Each AP in ROD WLAN is transited to a sleep state when there is no associated user. A wake-up receiver installed into each AP is used to detect a wake-up signal transmitted by a station (STA) upon communications demands. Each STA specifies an AP to wake up by embedding ID of the target AP into the wake-up signal. In this paper, we propose a wake-up ID assignment and ID matching which can reduce the probability of false wake-up caused by bit errors over wireless channels. The proposed scheme generates wake-up ID of each AP based on ESSID in such a way that certain number of hamming distances is maintained among different wake-up IDs. The AP wakes up when the hamming distance between the received ID, possibly containing bit errors, and its assigned ID is less than a predetermined value. The numerical results obtained by theoretical analysis and computer simulation show that the proposed scheme can effectively reduce the false wake-up probabilities with short ID length and very simple operations at wake-up receivers. We also propose a wake-up protocol for ROD WLAN to reduce energy wastefully consumed by APs which are redundantly woken up with the proposed ID assignment/matching. Our simulation results show that ROD WLAN with the proposed protocol achieves much better energy-efficiency than ROD WLAN without the proposed protocol and WLAN without applying ROD technologies.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

This paper considers a radio-on-demand (ROD) wireless LAN (WLAN) in which access points (APs) are put into a sleep mode during idle periods and woken up by stations (STAs) upon communications demands. The on-demand wake-up is realized by a wake-up receiver which is equipped with each AP and is used to detect a wake-up signal transmitted by STA. In order to reduce the hardware installation cost at STA, we advocate to utilizing wireless LAN frames transmitted by each STA as a wake-up signal. We generate a wake-up signal based on frame length modulation (FLM) where each STA creates a series of WLAN frames with different length to which the information on wake-up ID is embedded. The simple and low-power wake-up receiver extracts the wake-up ID from the received frames. In this paper, we design and develop a prototype of the wake-up receiver and propose a wake-up protocol which defines a procedure to realize the on-demand AP wake-up in ROD WLAN. We evaluate system-level performance of ROD WLAN based on our prototype and our proposed wake-up protocol, and investigate appropriate settings of parameters for our proposed FLM to achieve the required system-level performance. Our numerical results confirm that the proposed wake-up protocol with FLM achieves smaller delay than a conventional AP employing passive scanning while maintaining small probability to be falsely woken up by continuous interference. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.comcom.2012.04.022

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Recent investigations show that (i) access points (APs) of wireless local area networks (WLANs) are idle during much of the time, and, (ii) an AP in its idle state without forwarding any packets still consumes a large percentage of power. Therefore, it is necessary to put idle APs into sleep so as to realize green WLANs. The problem, however, is how to quickly and reliably activate APs from sleep when nodes initiate new data flows. Aiming at realizing Radio-On-Demand WLANs, in this paper, we suggest exploiting burst transmission of WLAN frames to convey wake-up IDs from nodes to APs. Our contribution is two-fold: (i) The burst transmission prevents interfering WLAN signals from breaking in, and, (ii) We re-interpret the sequence of WLAN frames for wake-up signaling as an equivalent ID. Based on the analysis of hamming distance among equivalent IDs, we further suggest using partial correlation to reduce the error rate of wake-up signals. The effectiveness of the proposed scheme is confirmed by both theoretical analysis and simulation evaluations.

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Recent investigations show that access points (APs) of wireless local area networks (WLANs) are idle during much of the time and that an AP in its idle state still consumes a large percentage of power. Wake-up receivers can be used to realize radio-on-demand WLANs, activating APs from the sleep mode only in times of active data communications. A wake-up receiver, sharing the antenna (and the same ISM band) with its co-located WLAN module and exploiting RF energy detection, can be implemented at low cost and run with low power consumption. In this article, we evaluate the effect of an imperfect RF band pass filter (BPF), and suggest a new soft decision method to (i) resist adjacent channel interference leaked by BPF, and, (ii) distinguish wake-up signals from WLAN signals. Extensive simulation and testbed experimental results confirm that the proposed scheme, at a moderate cost, has good performance in delivering wake-up signals and controlling false wake-up events caused by WLAN signals.

DOI： 10.1186/1687-1499-2012-42

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Optimized resource allocation of the Downlink (DL) in wireless systems utilizing Multi-Carrier (MC) transmission requires Channel State Information (CSI) feedback for each user/subchannel to the Base Station (BS), consuming a high amount of Uplink (UL) radio resources. To alleviate this problem, several works have considered contention-based CSI feedback in the UL control channel. We propose such a feedback scheme for a generic MC system, based on the idea of variable collision protection, where the probability that a feedback information experiences a collision depends on its importance. By partitioning the CSI into orthogonal layers of priority, and allocating different numbers of feedback slots to each layer, this scheme ensures that the feedback success probability is higher for the CSI with better quality, which is more likely to be used by the scheduler. Furthermore, we present a theoretical performance analysis of the proposed scheme, assuming Maximum CSI (Max CSI) and normalized Proportional Fair Scheduler (PFS), where a tight approximation of the achievable throughput is obtained assuming discrete Adaptive Modulation (AM) and CSI feedback which are relevant for the practical systems. Analytical and simulation results show that our proposed scheme provides an excellent trade-off between system performance and feedback overhead.

DOI： 10.1109/TWC.2011.101211.110062

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Access points (APs) of wireless LANs (WLANs), always powered on and ready to serve mobile nodes, consume a large amount of power in total, but are idle during much of the time. Previous protocol-based sleep-wakeup scheduling schemes partially solve this problem, but the large wakeup delay remains a problem. Aiming at realizing Radio-On-Demand WLANs, in this paper, we suggest using an additional wakeup transceiver to convey wakeup signals from nodes to APs. APs in the sleep mode are activated by nodes when new data flows are initiated, where the wakeup delay is very low. The proposed wakeup transceiver works on the 2.4GHz ISM band and shares antenna with a co-located WLAN module to reduce hardware cost. The wakeup receiver is designed to be simple but very reliable, and consume a very low power. The contribution of this paper is two-fold: (i) Wakeup signals are designed to co-exist with WLAN signals by exploiting the carrier sense mechanism of WLAN devices, and, (ii), explicit signal recognition is used to achieve an extremely low false wakeup probability in an environment where the number of WLAN signals is overwhelming. Testbed experiments confirm that the proposed scheme, besides its simplicity, has good performance in both frame error rate and false wakeup probability.

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This paper considers secondary usage of spectrum resources that are allocated to frequency division duplexing (FDD)-based cellular systems. We propose an interference cancellation technique using the concept of distributed antenna array. We show that the interference from the primary cellular system can be cancelled at a secondary node equipped with single antenna by introducing a helper that decodes the interference and transfers it to the secondary node. Interference is forwarded during the time periods in which training signals are transmitted within the primary cellular system. Since the duration of those intervals is short relative to periods for data transmission in the primary system, we introduce a scheme by which the helper compresses the interference before forwarding it. The performance advantage of our scheme is verified by computer simulation, where both analytical and measured channel models are used. The measurement data used in evaluations have been obtained through outdoor experiments while assuming a short-range secondary system. The results indicate that the proposed method for distributed interference cancellation leads to performance benefit, in particular under heterogeneous path loss conditions between the helper and the primary/secondary nodes.

DOI： 10.1109/TWC.2011.062911.101524

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In this paper, we introduce Radio-On-Demand (ROD) wireless LAN (WLAN) in which access points (APs) are put into a sleep mode during idle periods and woken up by stations (STAs) upon communications demands. The on-demand wake-up is realized by a wake-up receiver which is equipped with each AP and is used to detect a wake-up signal transmitted by STA. In this paper, in order to reduce the hardware installation cost at STA, we advocate to utilize wireless LAN frames transmitted by each STA as a wake-up signal to awake the target AP. The STA generates a wake-up signal by devising WLAN signal: each STA creates a series of WLAN frames with different length to which the information on wake-up ID is embedded. The wake-up receiver extracts the wake-up ID from the received frames with a simple detector which ensures its low-power operation. We evaluate false negative (STA fails to wake up the target AP) and false positive (AP falsely wakes up without an intended wake-up signal) probabilities of our proposed on-demand wake-up scheme with computer simulations. The numerical results show that the proposed scheme achieves the false negative probability of about 10(-2) when the detection error ratio of '1' is less than 10(-3). We also show that the false positive probability can be largely reduced by employing long WLAN frames to generate each wake-up signal. These results confirm that the proposed wake-up scheme is a promising approach to reducing wasteful energy consumed by idle APs in WLAN.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

This paper discusses the possibility of deploying a short-range cognitive radio (secondary communication system) within the service area of a primary system. Although the secondary system interferes with the primary system, there are certain locations in the service area of the primary system where the cognitive radio can reuse the frequency of the primary system without causing harmful interference to it and being disturbed by the primary system. These locations are referred to as having a spatial opportunity for communications in the secondary system, since it can reuse the frequency of the primary system. Simulation results indicate that the antenna gain, beamwidth, and propagation path loss greatly affect the spatial opportunity of frequency reuse for the secondary users. The results show that spatial spectrum reuse can be significantly increased when the primary system users are equipped with directional antennas. An important component in this study is the heterogeneous path loss model, i.e., the path loss model within the primary system is different from the model used to calculate the interference between the primary and the secondary systems. Our results show that the propagation models corresponding to the actual antenna heights in the primary/secondary system can largely impact the possibilities for spectrum reuse by the cognitive radios.

DOI： 10.1587/transcom.E94.B.1243

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Network coding is an effective method to improving relay efficiency, by reducing the number of transmissions required to deliver data from source(s) to destination(s). However, its performance may be greatly degraded by rate mismatch, which is seldom touched in previous works and remains a challenge. In this paper, we reinterpret network coding as a mapping of modulation constellation. On this basis, we extend the mapping to support full rate network coding (FRNC), enabling simultaneous use of different modulations by nesting the low level constellation as a subset of the high level constellation. When relay links have different qualities, the messages of different flows are combined via network coding in such a way that for each relay link, its desired message is transmitted at its own highest rate. The limit in constellation size is also addressed. Compared with the state-of-the-art solutions to rate mismatch, the proposed scheme achieves the full rate of all relay links on the broadcast channel.

DOI： 10.1155/2011/780632

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In this paper, a primary (licensed) user leases part of its resources to independent secondary (unlicensed) terminals in exchange for a tariff in dollars per bit, under the constraint that secondary transmissions do not cause excessive interference at the primary receiver (PRX). The PRX selects a power allocation (PA) for the secondary user that maximizes the secondary rate (and thus its revenue) and enforces it by the following mechanism: Upon violation of a predefined interference level, PRX keeps silencing randomly selected secondary users, until the aggregate secondary interference is below the required threshold. This mechanism ensures that secondary users may not be willing to deviate from the allocated PA. Specifically, the scenario gives rise to a Stackelberg game, in which the primary determines the PA and a Nash equilibrium (NE) constraint is imposed on the PA to ensure that secondary users do not have incentives to deviate, given their knowledge of the silencing mechanism run at the PRX. In principle, the primary should find the set of all PAs that are NE and among them choose the one that maximizes the aggregate secondary utility, and thereby the revenue of the primary. For the most general setting of channel gains, we investigate the conditions for NE for a subset of PAs. When the scenario is symmetric in the sense that all secondary users have the same channel gains in the direct/interfering links, we prove that only two optimal power allocations exist. Finally, for the case of general channel gains with strong interference, we show that there is a unique NE of the game.

DOI： 10.1109/TWC.2010.092810.091799

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Mobile nodes in wireless LANs connect themselves to the Internet via their associated access points (AP). Although more and more APs are being deployed, nodes tend to gather around some common hotspots, contending for few APs and leaving other APs idle. The traffic unbalance affects both per-node throughput and network throughput. In this paper, we aim to solve this problem by AP selection. We jointly consider the two key factors--channel availability and link quality--that determine the achievable throughput of a node, and suggest the potential throughput (PT) metric for AP selection. The PT metric is defined as the maximal throughput that can be achieved by a node if it exclusively occupies the remaining idle channel. In this way, a node can achieve higher throughput by associating with a farther but less used AP and the congestion of the network can be alleviated. The simulation results show that the PT metric can greatly improve the total throughput when nodes are unevenly distributed around APs. The testbed experiments with the offthe- shelf WLAN cards also confirm that the per-node throughput can be effectively improved with the proposed method.

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This paper addresses a transmission scheduling for wireless multi-hop network employing physical layer wireless network coding (PL-WNC). With PL-WNC, two distant nodes first transmit packets to an intermediate relay simultaneously, which then amplifies and forwards the received signal. The destination nodes decode the desired signals by using the a priori information which are their own packets or packets overheard from the other nodes. The achievable performance of PL-WNC depends not only on traffic pattern within a network but also on the probability for each destination node to have the a priori information for decoding the desired information. In this paper, we introduce a scheduling strategy for PL-WNC, which decides source-destination pairs of PL-WNC, taking account of traffic pattern as well as overhearing probability among nodes. The selected sources transmit packets only when the destinations are expected to have the a priori information with high probability. We define a parameter for each source node to decide whether it should transmit a packet when scheduled, and investigate its impact on the achievable throughput. Our simulation results show that our proposed scheduling significantly improves throughput of PL-WNC.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

This paper proposes multipoint-to-multipoint (MPtoMP) real-time broadcast transmission using network coding for ad-hoc networks like video game networks. We aim to achieve highly reliable MPtoMP broadcasting using IEEE 802.11 media access control (MAC) that does not include a retransmission mechanism. When each node detects packets from the other nodes in a sequence, the correctly detected packets are network-encoded, and the encoded packet is broadcasted in the next sequence as a piggy-back for its native packet. To prevent increase of overhead in each packet due to piggy-back packet transmission, network coding vector for each node is exchanged between all nodes in the negotiation phase. Each user keeps using the same coding vector generated in the negotiation phase, and only coding information that represents which user signal is included in the network coding process is transmitted along with the piggy-back packet. Our simulation results show that the proposed method can provide higher reliability than other schemes using multi point relay (MPR) or redundant transmissions such as forward error correction (FEC). We also implement the proposed method in a wireless testbed, and show that the proposed method achieves high reliability in a real-world environment with a practical degree of complexity when installed on current wireless devices.

DOI： 10.1587/transcom.E93.B.2316

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electronics, Information and Communication Engineers  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electronics, Information and Communication Engineers  

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Intelligent Transportation System (ITS) sets its target at a quite high level such as zero fatalities, to which safety driving support with inter-vehicle communications can largely contribute. Because code division multiple access (CDMA) technology provides delay-free channel access, we developed a CDMA-based inter-vehicle communications system, called a multi-carrier multi-code spread aloha (MM-SA) system, to meet challenging requirements on supporting safety driving. This paper introduces a transmission scheduling method that is a key function of MM-SA to provide highly reliable communications between vehicles. The performances of MM-SA are evaluated under realistic traffic accident scenarios. Our evaluation results show that MM-SA system is a promising solution to offering safety driving support.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Information Processing Society of Japan  

Although the near-far effect has been considered to be the major issue preventing CDMA from being used in ad-hoc networks, in this paper, we show that the near-far effect is not a severe issue in inter-vehicle networks for safety driving support, where packet transmissions are generally performed in the broadcast manner. Indeed, the near-far effect provides extremely reliable transmissions between near nodes, regardless of node density, which cannot be achieved by CSMA/CA. However, CDMA cannot be directly applied in realistic traffic accident scenarios, where highly reliable transmissions are required between far nodes as well. This paper proposes to apply packet forwarding and transmission scheduling methods that try to expand the area, where reliable transmissions are achievable. Simulation results show that the proposed scheme significantly excels a CSMA/CA-based scheme in terms of delivery ratio and delay under realistic traffic accident scenarios. Specifically, the proposed scheme achieves approximately 90% of delivery ratio and 4 milliseconds of end-to-end delay in a scenario, where the CSMA/CA scheme achieves 60% of delivery ratio and 80 milliseconds of delay.

DOI： 10.2197/ipsjjip.18.1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Driving safety support is one of the most attractive applications of an intervehicle communications system. Real-time and reliable packet exchange among vehicles is key to offering timely warnings to drivers in order to avoid fatal accidents. In this article we introduce multicarrier multicode spread Aloha (MM-SA) system, an intervehicle communications system developed to satisfy challenging requirements for supporting driving safety. MM-SA is based on code-division multiple access technology, and has inherent robustness to the increase of node density as well as the hidden terminal problem. We present an overview of MM-SA system including protocol design and prototyping as well as the GUI tool, which are all developed for avoiding intersection collisions. We also present performance comparison between the MM-SA system and a CSMA-based system under a realistic accident scenario. Our evaluation results show that the MM-SA system has superior performance to the CSMA-based system, and achieves the communication performance required for driving safety support.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Safety driving support is one of the most attractive applications of an inter-vehicle communications system. Real-time and reliable packet exchange among vehicles is a key to offering timely warnings to drivers in order to avoid fatal accidents. In this paper, we introduce the multi-carrier multi-code spread aloha (MM-SA) system, an inter-vehicle communications system developed for satisfying challenging requirements on supporting safety driving. MM-SA is based on code division multiple access (CDMA) technology, and has inherent robustness to the increase of node density as well as the hidden terminal problem. This paper presents an overview of MM-SA system including protocol design and prototyping as well as graphical user interface (GUI) tool, which are all developed for avoiding intersection collisions.

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This paper proposes bidirectional packet aggregation and coding (BiPAC), a packet mixing technique which jointly applies packet aggregation and network coding in order to increase the number of supportable VoIP sessions in wireless multi-hop mesh networks. BiPAC applies network coding for aggregated VoIP packets by exploiting bidirectional nature of VoIP sessions, and largely reduces the required protocol overhead for transmitting short VoIP packets. We design BiPAC and related protocols so that the operations of aggregation and coding are well-integrated while satisfying the required quality of service by VOW transmission, such as delay and packet loss rate. Our computer simulation results show that BiPAC can increase the number Of Supportable VoIP sessions maximum by around 87% as compared with the case when the packet aggregation alone is used, and 600% in comparison to the transmission without aggregation/coding. We also implement BiPAC in a wireless testbed, and run experiments in an actual indoor environment. Our experimental results show that BiPAC is a practical and efficient forwarding method, which can be implemented into the current mesh hardware and network stack.

DOI： 10.1587/transcom.E92.B.3060

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Network coding is an effective method to improve forwarding efficiency in multi-hop wireless networks. Previous network coding schemes proposed for unicast transmissions focus on scheduling where paths are already established, and assume a fixed rate for all links, neglecting link rate heterogeneity. In this paper we study a network coding based pseudo bidirectional relay scheme for wireless LAN, taking link quality into account. We first show when network coding based relay improves channel efficiency and how to select such a relay in a distributed way. Then we enhance network coding based transmissions from three aspects: (i) Coordinated rate adaptation is adopted to reduce the probability with which a priori packet is missing in times of network decoding. (ii) The retransmission scheduling completely solves the lack-of-a-priori problem. (iii) Piggybacking receiving status in ACK/NAK further enables opportunistic reception and avoids unnecessary transmissions. Simulation results confirm that compared with direct transmissions, the proposed relay scheme can reduce per-packet transmission time by up to 39% in Rayleigh fading environment.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This letter addresses a scheduling problem for wireless network coding (WNC). In our previous work, we have theoretically shown that the optimum number of nodes to be included into a network-coded packet as well as its transmission rate depends on time-varying link condition between a transmitting node and receiving nodes [1]. Based on this observation, this letter designs practical scheme which opportunistically selects scheduled nodes, packets to be coded and an employed modulation level according to time-varying channel conditions and packet length. The numerical results show that the proposed opportunistic scheduling can improve the overall throughput as compared with non-opportunistic approach.

DOI： 10.1109/TWC.2009.080266

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this paper, we investigate inter vehicle networks offering an application of safety driving support. For appropriately evaluating the efficiency of inter vehicle communication systems to support such an application, it is of paramount importance to make extensive evaluations under realistic accident model. In this paper, assuming realistic intersection collisions scenarios, we provide simulation results of an inter vehicle network based on an experimental guide line defined for inter vehicle communications system in Japan. Besides the guide line system, we also present our newly developed inter vehicle communication system based on code division multiple access (CDMA) scheme. Our numerical results show that the proposed CDMA based system can effectively combat hidden terminal problem as well as high level of congestion around an intersection, and is the only system to satisfy the requirements set by the application for safety driving support.

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This paper proposes bidirectional packet aggregation and coding (BiPAC), a packet mixing technique which jointly applies packet aggregation and network coding in order to increase the number of supportable VoIP sessions in wireless multi-hop networks. BiPAC applies network coding for aggregated VoIP packets by exploiting bidirectional nature of VoIP sessions, and largely reduces the required protocol overhead for transmitting short VoIP packets. We design BiPAC and related protocols so that the operations of aggregation and coding are well-integrated while satisfying the required quality of service by VoIP transmission, such as delay and packet loss rate. Our computer simulation results show that BiPAC can increase the number of supportable VoIP sessions maximum by around 70% as compared with the case when the packet aggregation alone is used, and 450% in comparison to the transmission without aggregation/coding. We also implement BiPAC in a wireless testbed, and run experiments in an actual indoor environment. Our experimental results show that BiPAC is a practical and efficient forwarding method, which can be implemented into the current mesh hardware and network stack. ©2009 IEEE.

DOI： 10.1109/ICC.2009.5198784

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Network coding is an effective method to improve channel efficiency by reducing the number of transmissions. However, most existing network coding schemes are based on network layer broadcast. As a result packets become vulnerable to channel errors due to lack of retransmissions. In this paper we present a cross-layer signal processing technique and show that network coded signal mixed from different flows can be separated in the physical layer by exploiting a priori knowledge. Then we propose the soft network coding framework for multi-hop forwarding. This framework enables layer-2 retransmission and further reduces packet errors by combining retransmitted copies. It supports both conventional unicast and the proposed network coding based bi-cast. Simulation results show that compared with plain Decode-and-Forward (DF), the proposed scheme can reduce air-time by about 30% for Poisson arrival traffic while retaining almost the same packet error rate as DF with frame combining.

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Technological advances coupled with regulatory initiative for more efficient utilization of radio spectrum resulted in the introduction of dynamic spectrum access enabled radio called Cognitive Radio (CR). A CR network is allowed to access a frequency band owned by primary user, which can provide higher throughput and better serviceability in wireless networks. The availability of multiple primary bands requires a CR network to select the best operating band which can maximize the total system performance. The selection should be made according to heterogenous properties of primary bands which offer different maximum data rate for secondary use and generate diverse traffic pattern. In this paper, assuming such heterogenous primary bands for secondary use, simple and distributed dynamic channel selection strategies are proposed and evaluated. In addition, we introduce a measurement metric for interference experienced by primary users due to secondary network's access in primary band. We investigate the impact of different channel selection strategies and parameters on the primary and secondary performance.

DOI： 10.1007/s11277-008-9464-7

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This paper proposes a new interference cancellation/avoidance scheme for secondary spectrum usage, which takes advantage of the features of Time Division Duplex (TDD) based primary systems. We focus on the co-existence between the heterogeneous systems, where the primary and the secondary are using the same spectrum simultaneously. The secondary systems have to accurately find the possible interference from/to the primary systems for discreet and efficient secondary transmission. In order to judge the arrival of interference from/to the primary system, the secondary access point (AP) compares the difference in power between the up and downlink signals in TDD-based primary systems. This scheme enables the secondary AP to know the level of interference from the primary systems that arrives at the AP without employing complex signal detection. The proposed scheme also determines the level of interference induced to the primary system by measuring the difference in power. To cancel and avoid interference in the proposed scheme, we introduce an adaptive array at only the AP that creates a null toward the interference with a higher power level between the up and downlink signals. Moreover, new algorithms by using scheduling information of the multiple primary nodes are introduced in the proposed method, in order to enhance the transmission quality of a secondary mobile station (MS) while maintaining a simple control scheme for the MS. Finally, we verify the effectiveness of the proposed method based on computer simulations.

DOI： 10.1007/s11277-008-9472-7

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In a commonly accepted usage scenario, a cognitive radio appears as a secondary user of certain spectrum which is licensed to another, primary system. A prominent example of cognitive system is a mesh network operating under the interference from primary system. For such a scenario, we propose techniques for efficient secondary usage of spectrum, which rely on the adaptive array antenna in order to reduce the interference between the primary and the cognitive system. In order to keep the hardware complexity as small as possible, the number of antennas at each cognitive node should be small. However, with the simplest 2-element linear adaptive array, the created antenna pattern can result in non-optimized pattern between cognitive nodes in the mesh network. In order to solve such a problem, this paper introduces a simple antenna pattern switching where each cognitive node is equipped with three antennas, and tries to select the antenna configuration constituting 2-element linear array with the best antenna pattern for each link. The proposed configuration requires three antennas but only two transceiver chains, which can reduce the hardware complexity. We also introduce 3-element linear array and design a simple procedure to heuristically select the pattern. Our numerical results show that the proposed techniques can significantly increase the available bandwidth and networking connectivity with small complexity when a cognitive mesh network is located inside the communication area of the primary system.

DOI： 10.1007/s11277-008-9477-2

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This paper proposes a novel interference cancellation technique using the concept of a distributed array, for the secondary use of the frequency resources in frequency division duplexing (FDD)-based cellular systems. In order to achieve efficient distributed interference cancellation, the secondary system exploit intervals for periodic training signals transmitted by FDD-time division multiple access (TDMA)-based cellular systems. We show that an efficient interference cancellation can be achieved by introducing helpers that decode the interference and transfer it to the secondary nodes. We take advantage of the high-speed links within the secondary system and devise a data compressing scheme that reduces time for transferring the decoded interference. We clarify the effectiveness of the proposed scheme by using computer simulation. As a parameter, the transmission quality between the helper and secondary nodes is varied. The results indicate that the proposed distributed interference cancellation is effective compared to a conventional adaptive array using a zero forcing algorithm, when the signal to noise ratio between the helper and the target secondary node is greater than 20 dB. ©2008 IEEE.

DOI： 10.1109/ICC.2008.573

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This paper studies various cooperative diversity schemes for Orthogonal Frequency Division Multiple Access (OFDMA)- Time Division Duplex (TDD) based two-hop cellular networks in low mobility scenarios, where the instantaneous channel state information is available at the base station. A user scheduling and radio resource allocation technique is developed in order to efficiently integrate various cooperative diversity schemes for the emerging IEEE 802.16j based systems. The analysis of the system with this scheduler shows that a simple cooperative diversity scheme which dynamically selects the best scheme between conventional relaying and direct transmission is promising in terms of throughput and implementation complexity. The conventional relaying refers to the scheme where the destination relies solely on the signals received through the relay.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This letter proposes a method for channel state information (CSI) estimation in a wireless relay network, which consists of a base station (BS), a relay station (RS), and a mobile station (MS). The proposed method exploits the fact that the link condition between fixed BS and fixed RS tends to be stable, and the frequent CSI update is not necessary for this link. In order for the BS to obtain the CSI of the distant link (i.e. RS-MS link), RS amplifies a pilot signal received from MS with a pre-defined amplification factor, and forwards it to BS. This enables the BS to obtain the CSI of the RS-MS link based on the received pilot signal and pre-knowledge on the CSI of BS-RS link, which can reduce the required overhead to explicitly exchange CSI.

DOI： 10.1109/LCOMM.2007.070160

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KOREAN INST COMMUNICATIONS SCIENCES (K I C S)  

We propose a link adaptation and selection method for the links constituting an orthogonal frequency division multiplexing (OFDM) based wireless relay network. The proposed link adaptation and selection method selects the forwarding, modulation, and channel coding schemes providing: the highest end-to-end throughput and decides whether to use the relay or not. The link adaptation and. selection is done for each sub-channel based on instantaneous signal to interference plus noise ratio (SINR) conditions in the source-to-destination, source-to-relay and relay-to-destination links. The considered forwarding schemes are amplify and forward (AF) and simple adaptive decode and forward (DF). Efficient adaptive modulation and coding decision rules are provided for various relaying schemes. The proposed end-to-end link adaptation and selection method ensures that the end-to-end throughput is always larger than or equal to that of transmissions without relay and non-adaptive relayed transmissions. Our evaluations show that over the region where relaying improves the end-to-end throughput, the DF scheme provides significant throughput gain over the AF scheme provided that the error propagation is avoided via error detection techniques. We provide a frame structure to enable the proposed link adaptation and selection method for orthogonal frequency division multiple access (OFDMA)-time division duplex relay networks based on the IEEE 802.16e standard.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

This paper addresses a scheduling problem for wireless network coding which has been recently proposed as a novel method to enhance the throughput in wireless networks. The wireless network coding involves broadcast transmission of a network-coded packet which contains unicast data to several receiving nodes. These receiving nodes have time-varying fading links to the transmitting node, which can generate different instantaneous conditions for different links. In this paper, we introduce an opportunistic scheduling for such a wireless network coding, which selects a set of nodes whose packets are network-coded as well as the data rate for the broadcast transmission according to the instantaneous link conditions. We analyze the average capacity of such a scheduling, and discuss the impact of different parameters on the average capacity. We show that the opportunistic scheduling can maximize the average capacity by choosing the appropriate set of network-coded nodes according to the instantaneous link conditions. We also discuss the practical factors which can additionally affect the best scheduling strategy for wireless network coding.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

It has recently been recognized that the wireless networks represent a fertile ground for devising communication modes based on network coding. A particularly suitable application of the network coding arises for the two-way relay channels, where two nodes communicate with each other assisted by using a third, relay node. Such a scenario enables application of physical network coding, where the network coding is either done (a) jointly with the channel coding or (b) through physical combining of the communication flows over the multiple access channel. In this paper we first group the existing schemes for physical network coding into two generic schemes, termed 3-step and 2-step scheme, respectively. We investigate the conditions for maximization of the two-way rate for each individual scheme: (1) the Decode-and-Forward (DF) 3-step schemes (2) three different schemes with two steps: Amplify-and-Forward (AF), JDF and Denoise-and-Forward (DNF). While the DNF scheme has a potential to offer the best two-way rate, the most interesting result of the paper is that, for some SNR configurations of the source-relay links, JDF yields identical maximal two-way rate as the upper bound on the rate for DNF.

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this paper, the possibility for a short-range cognitive radio (secondary communication system) to be located within the service area of the primary system is discussed. Although the secondary system interferes with the primary system, there can be certain locations in the service area of the primary system where the cognitive radio can reuse the frequency of the primary system without disturbing it or being disturbed by the primary system. We say that in those locations there is a spatial opportunity for communication in the secondary system. The primary and secondary systems have different features and usage scenarios, which result in different deployment conditions. This gives rise to differences between the path loss for a link in the primary system and the path loss for the interference from the primary to secondary systems. This paper investigates the impact of the heterogeneous path loss conditions on the spatial opportunity for the secondary system. Our approach can be applied as a general way to investigate the coexistence among multiple systems. We clarify that the change of the path loss coefficient due to the different antenna heights of the primary and secondary systems can largely affect the spatial opportunity for frequency reuse by the cognitive radios.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this paper we investigate the problem of spectrally efficient operation of a cognitive radio (CR), also called Secondary System (SS), under an interference from the primary system (PS). A cognitive receiver (CRX) observes a multiple (MA) access channel of two users, the secondary and the primary transmitter, respectively. We advocate that the SS should apply Opportunistic Interference Cancellation (OIC) and decode the PS signal when such an opportunity is created by the rate selected in the PS and the power received from the PS. We derive the achievable data rate in the SS when OIC is applied. When the PS is decodable, we devise a method applied by the SS to achieve the maximal possible secondary rate. This method has a practical significance, since it enables rate adaptation without requiring any action from the PS. We investigate the power allocation in the SS when OIC is applied over multiple channels. We show that the optimal power allocation can be achieved with intercepted water-filling instead of the conventional water-filling. The results show a significant gain for the rate achieved by OIC.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

This paper proposes a simple antenna switching and a relay technique for cognitive radios using adaptive array antenna to effectively reduce the interference from the primary system. While the adaptive array antenna can create null towards the direction of primary system, the created antenna pattern can result in non-optimized pattern between cognitive nodes. In order to solve such a problem, this paper first introduces a simple antenna pattern switching. With this technique, each cognitive radio node is equipped with three antennas and each node tries to select the antenna configuration resulting in the best antenna pattern for each link. Furthermore, we propose a combination between the above simple antenna switching and the interference-induced multi-hop transmission where the multi-hop transmission is utilized when a link in the network suffers from the low transmission quality due to the created antenna pattern. Our numerical results show that the proposed techniques can significantly improve the achievable data rate in cognitive networks.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Frequency Hopping (FH) technology has been widely used for short-range networks operating in unlicensed band. As the short-range FH networks gain momentum in ubiquitous usage, the interference that collocated FH networks cause to each other, termed self-interference, becomes one of the major sources that degrade the communication performance. This paper proposes the Adaptive Frequency Rolling (AFR), a particular adaptive instance of FH that enables FH networks to cooperate and effectively avoid the self-interference. The AFR uses as input solely the observed packet error rate (PER) and it does not require any exchange of information among the collocated networks. The effect of the AFR over a longer time interval is that the networks use the complete set of disposable channels in an implicit time-division and cooperative manner. The parameter choice is such that a network which uses AFR never occupies the channels in the unlicensed spectrum more than what is permitted by the current regulation. AFR is designed to be robust towards the noise-induced channel errors. We also design AFR with probing (AFR-P), a modified version of AFR, which can also overcome frequency-static interference from collocated non-FH network by introducing channel removal strategy with probing. Our simulation results show that AFR and AFR-P have superior goodput performance to pseudorandom frequency hopping (PFH), and also standard adaptive frequency hopping (AFH), where the latter is designed to exclusively combat frequency-static interference. All these features promote the great potential of AFR as a coexistence mechanism for unlicensed operation.

DOI： 10.1109/TWC.2007.05315

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This paper introduces and analyzes relaying techniques that increase the achievable throughput in multi-hop wireless networks by applying network coding over bi-directional traffic flows. We term each such technique as Bi-directional Amplification of Throughput (BAT)-relaying. While network coding is normally performed by combining decoded packets, here we introduce a relaying method based on Amplify-and-Forward (AF), where the relay node utilizes the inherent combining of packets provided by simultaneous transmissions over a multiple access channel. Under low noise levels, AF BAT-relaying offers a superior throughput performance. The unconventionality of AF BAT relaying opens many possibilities for further research.

DOI： 10.1109/LCOMM.2007.061436

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Mechanisms based on frequency hopping have been widely used to enable short-range wireless networks to use resources from the unlicensed spectrum without frequency planning. Bluetooth piconet is a prime example of an FH-based network with unlicensed operation. As a price for open access, the piconet may experience adverse interference from other collocated FH piconets or other wireless devices that are transmitting in the same unlicensed band. A basic approach to mitigate this interference is that the piconet applies adaptive FH (AFH) and attempts to hop over a set (hopset) of less interfered channels. On the other hand, the regulation of unlicensed operation sets constraints on possible hopset adaptations. In this article we present two novel AFH strategies: adaptive frequency rolling (AFR) and dynamic AFH (DAFH). AFR avoids self-interference while preserving the dynamics of spectrum usage as required by the current regulation. DAFH is a distributed mechanism by which collocated piconets select nonconflicting hopsets while trying to keep the hopset size as large as possible. DAFH is not completely compliant with current regulations, but the rationale given for its design contains new rules of behavior for the unlicensed spectrum. Both approaches significantly outperform the conventional AFH strategy.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Bluetooth provides wireless networking among proximate nodes. The Bluetooth nodes are usually battery-powered, which puts a strong demand on the design of energy-efficient network structures. In the conventional piconets and scatternets, the roles of master, slave, and bridge are assigned to the nodes in a semi-static manner. This produces unfairness, since the master and bridge roles have higher activity and thus higher energy consumption. The logical piconet/scatternet structure decreases the energy efficiency by requiring packet forwarding even when a direct wireless link is possible. In this paper, we propose Bhie-Park, a flexible networking structure that achieves energy efficiency and fairness, without requiring change to the Bluetooth Specification. Blue-Park utilizes the Bluetooth park mode and can be described as a self-referencing structure, where each node parks all the other nodes as a master. The packets are exchanged between nodes with dynamic park/unpark procedures, which results in dynamic role assignment. We evaluate the Blue-Park in two scenarios - comparison with a piconet and comparison with a scatternet among nodes that are physically within a single hop, The simulation results show that, when the incoming traffic at each device is bursty, in both scenarios Blue-Park can achieve higher energy efficiency and lifetime. Furthermore, the delay is not increased significantly due to the dynamic link establishment. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.comcom.2006.01.018

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Language：English   Publishing type：Research paper (international conference proceedings)  

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DOI： 10.1109/PIMRC.2006.254358

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE COMPUTER SOC  

As the Wireless Personal Area Network (WPAN) gets utilized by more individuals, the interference that collocated WPANs cause to each other, termed self-interference, will be one of the major sources that degrade WPAN's communication performance. The conventional Adaptive Frequency Hopping ( AFH) strategies avoid frequency-static interference by reducing the hopset, but this deteriorates the performance if there is also self-interference. In this paper, we propose Dynamic AFH ( DAFH) mechanisms that are concurrently employed by collocated WPANs in order to avoid the self-interference. With DAFH, WPAN adaptively self-allocates a subset of frequency channels to be hopped, such as to minimize the experienced interference. The packet error rate is the only input to the proposed mechanisms, which enables DAFH to also avoid interference from frequency-static interferer. The optimization of the throughput should not be the sole target of the DAFH because WPAN operates in unlicensed spectrum and arbitrary adaptation of the FH pattern may be harmful to proximate non-WPAN devices. Therefore, we define and adopt an etiquette rule to characterize the behavior of the collocated WPANs with DAFH as a single collective entity that produces interference. The operation of DAFH is robust and adaptive to the dynamic changes in the environment and to the noise errors in the channel. Simulation results show that DAFH significantly increases the throughput of the WPANs in presence of both self-interference and frequency-static interference, while the WPANs employ best effort to minimize changes in the overall interference pattern.

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

This paper considers relaying techniques that increase the achievable throughput in multi-hop wireless networks by taking advantage of the bi-directional traffic flow. Such a relaying technique is termed relaying with Bi-directional Amplification of Throughput (BAT-relaying). The BAT-relaying is utilizing the concept of anti-packets, defined for bi-directional traffic flows. The relay node combines the anti-packets that are destined for different nodes and broadcasts the combined packet. Two BAT-relaying techniques have been proposed previously, Decode-and-Forward (DF) BAT-relaying and Amplify-and- Forward (AF) BAT-relaying. While in DF the relay node combines the packets by an XOR operation, AF BAT-relaying utilizes the inherent packet combining provided by the multiple access channel. In an errorless channel, AF has always higher achievable throughput than DF, but in noisy channels the noise amplification can severely degrade the performance of AF. In this paper we introduce a new scheme for BAT-relaying, termed Denoise-And-Forward (DNF) BAT-Relaying. The DNF BAT-relaying also makes use of the combining provided by the multiple access channel, but it removes the noise from the combined anti-packets before broadcasting to the destinations. While in the noiseless channel DNF and AF offer the same throughput performance which is superior to DF BAT-relaying, in large regions of the lower SNR values DNF BAT-relaying has the best throughput performance of all three schemes. Due to the unconventional nature of the BAT-relaying schemes, there are many open issues for further investigation. The design of a practical DNF scheme concerns several protocol layers, including modulation and coding.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

This paper discusses what a new paradigm can be in wireless communication systems of the twenty-first century. First, it suggests two directions for the new paradigm; one is "micro- and nano-device communication system" which is the projected scenario considering that the entities in source and destination have been shrinking throughout the history of wireless communication systems. The second direction is "networked robot system", which emerges as a natural extension of mobile ad hoc networking where the networking is closely related to motion control of robots. Secondly, it shows two interesting research topics, "the new communication protocol design" and "signal processing", respectively, that arise in the wake of the fusion between the two directions in the novel communication paradigm. Finally, it considers a new science of wireless communications in the twenty-first century.

DOI： 10.1007/s11277-006-9036-7

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In wireless networks, the shared broadcast medium enables interactions among nodes and thus introduction of novel communication modes. This paper introduces and analyzes relaying techniques that increase the achievable throughput in multi-hop wireless networks by taking advantage of the bi-directional traffic How. Such a relaying technique is termed relaying with Bi-directional Amplification of Throughput (BAT-relaying). The BAT-relaying is utilizing the concept of anti-packets, defined for bi-directional traffic flows. The relay node combines the packets (anti-packets) that are destined for different nodes and broadcasts the combined packet. The first variant, termed Decode-and-Forward (DF) BAT relaying, has been proposed before in the literature. It combines the packets by using the XOR operation, which makes such proposal closely related to the network coding approaches. We proposed another type of BAT-relaying based on Amplify-and-Forward (AF), which utilizes the inherent packet combining that emerges from simultaneous utilization of a multiple access channel. We analyze the achievable throughput of the DF and AF BAT-relaying, regarding the impact of the traffic asymmetry and the channel errors. The unconventionality of this relaying, in particular AF BAT-relaying, opens many possibilities for further research.

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

A Wireless Personal Area Network (WPAN) provides wireless networking among proximate devices, usually carried by an individual. Bluetooth is a first instance of the WPAN technology. The basic networking entity in Bluetooth is a piconet. Several piconets (WPANs) can be interconnected into a scatternet, which can be considered as an extendable multi-hop ad hoc networking structure. Since Bluetooth operates in the unlicensed ISM band, each piconet uses pseudorandom frequency hopping. If collocated piconets use the same channel simultaneously, the piconets interfere with each other and the transmitted packets are lost in collisions. This interference is termed self-interference. The piconets that are networked into scatternet exhibit spatial overlapping and naturally produce multi-piconet self-interference. The collisions cause retransmissions and increase the energy spent per data portion, which results in energy-inefficient operation. To tackle this problem, in our previous work we have proposed a self-interference avoidance (SIA) mechanism. However, this basic SIA mechanism is oblivious with respect to the physical topology and does not account for the mitigation of self-interference due to the propagation effects. Furthermore, the basic SIA mechanism relies on the assumption that all piconets are using packets of identical and fixed length. In this paper we will generalize the SIA mechanism to overcome the stated restrictions. We propose the adaptive SIA (A-SIA) algorithm, which adapts the SIA algorithm to the actual interference. The simulation results show that A-SIA largely retains the energy gain offered by the SIA algorithm, while significantly improving the goodput. We also design an instance of the SIA mechanism that operates with variable-length packets, referred as generalized SIA (G-SIA) algorithm. Our simulation results show that the G-SIA algorithm offers good performance in terms of goodput and energy efficiency, but the goodput is degraded if inappropriate segmentation/reassembly policy is used.

DOI： 10.1007/s11277-005-8732-z

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INST ELECTRONICS TELECOMMUNICATION ENGINEERS  

This paper presents an overview of physical (PHY) and medium access control (MAC) layers for IEEE 802.11a, which is a standard of a broadband high-speed wireless local area network (WLAN). IEEE 802.11a uses 5 GHz frequency band and its PHY is based on orthogonal frequency division multiplexing (OFDM). In this paper, we evaluate the OFDM-based PHY and MAC performance of IEEE 802.11a under various practical wideband channel models, which are abstracted from actual propagation measurements. The effects of different transmission modes defined in PHY on the IEEE 802.11 system performance are investigated. We also discuss the impact of link adaptation, which attempts to choose the best-suited transmission mode according to channel conditions, on the overall system performance.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this paper, we propose two time slot allocation algorithms for code division multiple access scheme with time division duplex (CDMA/TDD) systems to efficiently accommodate multimedia traffic. Assuming a practical multiple cell environment and a multimedia service model which consists of several kinds of circuit-switched and packet-switched services with different quality of services (QoSs), we evaluate the average delay of the CDMA/TDD system with the proposed algorithms, and compare the performance with that of CDMA with frequency division duplex (CDMA/FDD) and time division multiple access with TDD (TDMA/TDD) systems. Our computer simulation results show that the CDMA/TDD system with one of the proposed algorithms, which can effectively avoid interference among users with different QoSs, can improve the average delay performance as compared with the other systems.

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This correspondence proposes a flexible wireless multimedia communications system where time-division multiple-access (TDMA) and direct sequence code-division multiple-access (DS-CDMA) schemes are flexibly selectable according to user's quality of services (QoSs) and channel conditions. We evaluate the traffic performance of the proposed system and discuss the impact of the access scheme selectability on the traffic performance. The key element of the proposed system is how to determine an appropriate access scheme for a user. Therefore, we theoretically derive a simple selection algorithm to maximize the system capacity. Our computer simulation results show that the proposed system with the derived access scheme selection algorithm can significantly improve the traffic performance as compared with conventional TI)NUL and CDMA systems.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

In software-based wireless multimedia communications systems, each mobile terminal will be able to select its best-suited transmission format according to its quality of service (QoS) and channel condition. In this paper, we focus attention on " access scheme selectability" in such a software-based system, and discuss the traffic performance improvement due to adaptive access scheme selection. Assuming a software-based TDMA/CDMA system where time division multiple access (TDMA) and direct sequence code division multiple access (DS-CDMA) schemes are flexibly selectable, we evaluate the traffic performance in terms of average delay with a typical multimedia service model to be supported in future wireless communications systems. In the TDMA/CDMA system, how to determine an appropriate access scheme for a user is a key issue. Therefore, we discuss the selection algorithm for efficiently supporting heterogeneous multimedia services. Our computer simulation results show that the software-based system with a simple access scheme selection algorithm can significantly improve the traffic performance as compared with conventional hardware-based systems.

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Venue：Tempe, Arizona, USA  

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Venue：Shanghai, China  

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