The performance of GNSS based positioning, not only the accuracy but also the outage (where there is no sufficient number of satellites for position computation), is greatly degraded in urban canyons. To solve this problem, previous works have exploited vehicles and road side units as anchors, and suggested estimating the distance using phase information of V2X (vehicle-to-everything) OFDM signals. To further reduce the outage and improve the positioning accuracy, it is reasonable to use as positioning anchors the large number of LEO (Low Earth Orbit) satellites, which currently are mainly deployed for ubiquitous communications. Experiments in some literature revealed that LEO satellites use an OFDM-like structure to transmit data, and it is possible to exploit the OFDM phase information of LEO satellite signals to estimate the distance. It is well known that the impact of Doppler shift is serious at high speeds. To verify the feasibility of using the OFDM-based ranging with LEO satellites, this paper analyzes the impact of Doppler shift on the phase measurement of OFDM signals, and reports the ranging performance under 3D ray tracing simulations.

Over-the-air computation has attracted much attention in wireless data aggregation, which collects and processes data simultaneously with high efficiency. Different propagation distances from nodes to the sink or channel fading may lead to misalignment in signal magnitude and result in computation error. In our previous work, amplify and forward (AF) based relay has been investigated. With multiantenna available at the sink, it is possible to achieve better performance by the misalignment allowed optimization (Miso), compared with zero forcing (ZF) which requires all signals aligned in signal magnitude. But Miso does cause many signals misaligned in signal magnitude, leading to a biased result. Aiming to further improve system performance, in this work, we study how to combine the AF based relay with Miso by a joint optimization, and solve it with an iterative algorithm. Extensive simulations verify the effectiveness of the proposed method.

Over-the-air computation (AirComp), as an effective method to wireless data aggregation, has attracted much attention recently. It helps to improve network efficiency and scalability by integrating communication and computation in the air. In AirComp, both signal magnitude misalignment and noise lead to computation error. In the single antenna case, by allowing misalignment in signal magnitude, a good tradeoff can be achieved between signal distortion and noise power, which leads to a minimal error. In the multi-antenna case, usually the zero-forcing policy is used to enforce signal magnitude alignment (no distortion), which, however, increases noise and affects the overall computation error. To better exploit multiple antennas at the sink, in this paper, we propose a misalignment allowed optimization (Miso) method for AirComp. Specifically, a group of nodes whose signals may be misaligned are dynamically selected, and other signals are aligned to a higher level with higher quality. On this basis, the optimization of multi-antenna AirComp is converted to a difference of convex problem and is solved iteratively. Simulations confirm that the proposed method greatly reduces computation error and scales better with the number of nodes, compared with previous methods.

Vehicular communication plays an important role in cooperative perception. 3GPP has successively standardized LTE-V2X and NR-V2X to support different kinds of vehicular communications, especially local information dissemination among vehicles by broadcast. There is no feedback for broadcast transmission. Therefore, blind retransmission usually is used to improve system reliability. But its effect is limited in the non-line-of-sight (NLOS) scenario. In this paper, with NR-V2X mode 2 as an example, we study how to improve the reliability of local broadcast transmission by using relay, from two aspects. (i) A vehicle allocates resources for the retransmissions at a relay vehicle in advance to reduce the delay. (ii) A relay vehicle is selected to help multiple vehicles simultaneously, considering the link quality between vehicles. Simulation evaluations confirm that using relay helps to improve information dissemination at the NLOS intersections, even when there is no vehicle at the intersection center.

IoT systems typically involve separate data collection and processing, and face the scalability issue when the number of nodes increases. For some tasks, only the result of data fusion is needed. Then, the whole process can be realized in an efficient way, integrating the data collection and fusion in one step by over-the-air computation (AirComp). Its shortcoming, however, is signal distortion when channel gains of nodes are different, which cannot be well solved by transmission power control alone in times of deep fading. To address this issue, in this paper, we propose a multi-slot over-the-air computation (MS-AirComp) framework for the sum estimation in fading channels. Compared with conventional data collection (one slot for each node) and AirComp (one slot for all nodes), MS-AirComp is an alternative policy that lies between them, exploiting multiple slots to improve channel gains so as to facilitate power control. Avoiding to obtain instantaneous channel gains of all nodes at the sink is a key point. Specifically, the transmissions are distributed over multiple slots and a threshold of channel gain is set for distributed transmission scheduling. Each node transmits its signal only once, in the slot when its channel gain first gets above the threshold, or in the last slot when its channel gain remains below the threshold. Theoretical analysis gives the closed-form of the computation error in fading channels, based on which the optimal parameters are found. Noticing that computation error tends to be reduced at the cost of more transmission power, a method is suggested to control the increase of transmission power. Simulations confirm that the proposed method can effectively reduce computation error, compared with state-of-the-art methods.

Prevention of pedestrians traffic accidents has become an important issue in intelligent transportation system. Pedestrian to vehicle communication, in which pedestrians send their position information to surrounding vehicles, is effective in reducing accidents when pedestrians are out of sight of vehicles. However, in urban canyons, the precision of pedestrian positioning via GPS may be significantly degraded due to obstructions and reflections of roadside buildings. To solve this problem, our previous work has suggested using vehicles as anchors for pedestrian positioning, and estimating pedestrian-vehicle distance from instantaneous channel state information (CSI), by using Support Vector Regression (SVR). In this paper, based on the fact that pedestrian-vehicle distance changes continuously, we propose to estimate the distance from a CSI sequence by using an LSTM (Long short-term memory) model, and solve the problem of packet loss that may affect the CSI collection. Simulations using 3D ray tracing show that the proposed method reduces the distance error and the horizontal positioning error by 24.8 % and 41.7%, respectively, compared to the previous method using SVR.

Position information plays an important role in preventing pedestrian accidents by pedestrian-to-vehicle communication. But the computation of pedestrian position, based on GPS, may fail in urban canyons due to the obstruction of roadside building. This problem can be solved by using vehicles and roadside units as anchors for pedestrian positioning, where trilateration is used to compute pedestrian position based on distances to anchors. But the performance is degraded by multipath propagation and limited by the time resolution. To address this problem, in this paper, we investigate how phase information of OFDM signals in V2X communications varies with the propagation distance, and exploit the phase difference of arrival to estimate the distance difference. We study how to deal with the inter-symbol interference in OFDM symbols and combine multiple estimations of distance difference to improve the accuracy. Simulation evaluations by 3D ray-tracing confirm the effectiveness of the proposed method.

Pedestrian-to-vehicle (P2V) communication, which sends information from pedestrians to nearby vehicles by wireless signals, has attracted much attention recently. In P2V communication, when many pedestrians share the same channel and transmit frequently, congestion occurs, which not only increases the delay but also degrades packet delivery rate. In our previous work, a method was proposed to estimate the degree of risk of pedestrians according to the context information of pedestrians (e.g., pedestrian property, distance to vehicles, etc.), and on this basis differentiate pedestrians by assigning different transmission priorities to pedestrians, with a lower priority leading to a longer transmission interval. However, this method does not consider actual channel utilization and cannot well adapt to different scenarios. In this paper, we extend our previous method, and use channel utilization rate as the feedback of the transmission control. This feedback is essential in making the algorithm track the changes in the number of pedestrians and their degrees of risk. Evaluations on network simulator confirm that the proposed method effectively improves packet delivery rate by 12.3% on average meanwhile retaining the same delay compared with the previous method.

Inter-vehicle communication (IVC) helps to prevent vehicle accidents by disseminating vehicle position and speed information, and road/traffic information among vehicles. Relay vehicle selection control (for disseminating road/traffic information in a wide range) and prioritized transmission control (for disseminating fast and reliably urgent information such as sudden brake information) play important roles in IVC. Our previous work proposed to integrate the two control functions explicitly in the MAC (Media Access Control) layer. Relay vehicle selection control is realized by sorting vehicles in the descending order of their distances from the transmitting vehicle and on this basis a different waiting time is set to each vehicle via using CW (Contention Window) in the EDCA (Enhanced Distributed Channel Access) mechanism of wireless LANs. In this paper, we further consider the coexistence of ordinary and large vehicles. A large vehicle, with a high antenna and a long transmission distance, is more preferentially selected as relay by introducing a new metric-potential relay distance. Simulation results show that by preferentially selecting large vehicles as relay, the proposed method can improve the reachability by up to 10% and reduce the delay by 35% in the highway scenario, and can improve the reachability by up to 4% and reduce the delay by up to 13% in the urban scenario.

This paper studies the potential problem and performance when tightly integrating a low power wake-up radio (WuR) and a power-hungry wireless LAN (WLAN) module for energy efficient channel access. In this model, a WuR monitors the channel, performs carrier sense, and activates its co-located WLAN module when the channel becomes ready for transmission. Different from previous methods, the node that will be activated is not decided in advance, but decided by distributed contention. Because of the wake-up latency of WLAN modules, multiple nodes may be falsely activated, except the node that will actually transmit. This is called a false wake-up problem and it is solved from three aspects in this work: (i) resetting backoff counter of each node in a way as if it is frozen in a wake-up period, (ii) reducing false wake-up time by immediately putting a WLAN module into sleep once a false wake-up is inferred, and (iii) reducing false wake-up probability by adjusting contention window. Analysis shows that false wake-ups, instead of collisions, become the dominant energy overhead. Extensive simulations confirm that the proposed method (WuR-ESOC) effectively reduces energy overhead, by up to 60% compared with state-of-the-arts, achieving a better tradeoff between throughput and energy consumption.

Inter-vehicle communication (IVC), which will play an important role in applications such as driving safety support system via periodical exchange of position and speed information among vehicles and autonomous driving which relies on local distribution of road and traffic information, has attracted much attention recently. But IVC does not scale well with the number of requests in the request/response-based communication. In this paper, we propose to apply the technique of Content Centric Network (CCN) to solve this problem, by a cross-layer design, which combines relay vehicle selection, packet forwarding, and cache search, in the same framework. First, we study 1) unique content naming and 2) robust routing (relay vehicle selection, packet forwarding and cache search) to realize the basic function of CCN. Next, we extend the routing method so as to overcome the cache miss problem in the mobile environment. Extensive simulation evaluations confirm the effectiveness of the proposed methods. Compared with the method without caching, the basic method effectively improves system performance. By mitigating the cache miss problem, the extended method (ECCN-IVC) further reduces the average hop count of data packets by up to 46%, and improves the content acquisition success rate by up to 204%.

In large-scale wireless sensor networks (WSNs) for wild environments, nodes once deployed run only on battery. Moreover, nodes close to a sink consume energy more quickly than other nodes due to packet forwarding. Mobile sink is a good solution to this issue, although it causes two new problems to nodes: (i) overhead of updating routing information and (ii) increased operating time due to aperiodic query. To solve these problems, this paper proposes an energy efficient data collection protocol for mobile sink, where sink-based centralized transmission scheduling (SC-Sched) is realized by using asymmetric communication and wake-up radio (WuR). Specifically, nodes do not update routing information. Instead, the sink determines the transmission order. In addition, each node is equipped with a WuR. At the time that a packet transmission between two nodes is scheduled, the sink transmits a wake-up message using a large transmission power, directly activating both nodes simultaneously to communicate. Extensive simulation evaluations confirm that the proposed method is more energy efficient than conventional methods, reducing energy consumption to 1/9 in a WSN with 600 nodes. This method is further enhanced to deal with frame loss caused by multipath fading, improving frame delivery rate meanwhile suppressing energy consumption and data collection time.

In the downlink of a wireless LAN, power-save mode is a typical method to reduce power consumption. However, it usually causes large delay. Recently, remote wake-up control via a low-power wake-up radio (WuR) has been introduced to activate a node to instantly receive packets from an access point (AP). But link quality is not taken into account and protocol overhead of wake-up per node is relatively large. To solve these problems, in this paper, a broadcast-based wake-up control framework is proposed, and a low-power WuR is used to receive traffic indication map from an AP, monitor link quality, and perform carrier sense. Among the nodes which have packets buffered at the AP, only those whose SNR is above a threshold will be activated, contending via a proper contention window to receive packets from the AP. Optimal SNR threshold, deduced by theoretical analysis, helps to reduce transmission collisions and false wake-ups (caused by wake-up latency) and improve transmission rate. Extensive simulations confirm that the proposed method (i) effectively reduces power consumption of nodes compared with other methods, (ii) has less delay than power-save mode in times of light traffic, and (iii) achieves higher throughput than other methods in the saturation state.

In wireless LANs (WLANs), a power-hungry transceiver is used for both the control (carrier sense) and data exchanges. As a result, when many nodes contend to access a same channel, the WLAN transceiver performs carrier sense during most time, which wastes much energy. To solve this problem, we study separating the control and data operations in WLANs, by tightly integrating a lower power wake-up radio (WuR) with the WLAN module. Specifically, the WuR is used to monitor the channel and conduct carrier sense. It activates the WLAN module for actual transmissions when the channel gets ready. This is a contention-based self wake-up, which is different from previous schemes where a specific receiver is remotely activated. Due to the hardware constraint, a WLAN module is susceptible to a non-negligible wake-up latency. Our analysis shows that this wake-up latency may break the carrier sense mechanism and lead to false wake-up events. Then, we propose to recover the carrier sense mechanism by resetting the backoff counter of a falsely activated node in a way as if it is frozen in the wake-up period. Simulation evaluations confirm that the proposed scheme effectively mitigates the impact of wake-up latency by reducing the duty time of WLAN modules.

CSMA is the de facto standard in wireless LANs (WLANs), where one transceiver is used for both the control and data planes. When many nodes contend to access a same channel, the WLAN transceiver performs carrier sense during most time, which wastes much energy. Dynamic adjustment of parameters (contention window) is suggested in previous works for improving network throughput, but this is also realized at the cost of continuously monitoring the channel with large power consumption. To solve these problems, in this paper, we suggest tightly integrating a lower power wake-up radio (WuR) with the power-hungry WLAN module. Specifically, (i) The WuR is used to monitor the channel and conduct carrier sense. It activates the WLAN module for actual transmissions when the channel gets ready. (ii) The WuR is used to measure the inter-frame space, based on which contention window is adjusted accordingly. Extensive simulation evaluations confirm that the proposed scheme (WuR-CSMA) effectively reduces the duty ratio of WLAN modules and improves system throughput, achieving both energy and spectral efficiency compared with the conventional schemes.

Sensor networks are playing more and more important roles, e.g., accurately monitoring the generation and consumption of electric power for the purpose of smart grid. In these applications, sensor nodes, working with battery, should be put into sleep in the idle state to prolong network lifetime and be activated on demand to ensure real-time response. State-of-the-art schemes merely exploiting duty-cycling cannot simultaneously satisfy these two requirements. In this paper, we exploit frame length of 802.15.4g signals for the wake-up control of sensor nodes. A wake-up ID is modulated onto frame lengths of consecutive signals defined by the mode switch mechanism, transmitted by a sensor node using the standard 802.15.4g protocol, and detected by a non-802.15.4g, low-power wake-up receiver. A prototype wake-up receiver is implemented by using FPGA. It consists of two-stage wake-up control in the sense that duty-cycling is also exploited in the detection of wake-up signals. The proposed scheme achieves a better tradeoff between power-consumption and wake-up latency, compared with the conventional duty-cycling schemes. The experimental evaluation confirms that the wake-up control meets the sensitivity requirement of data communications.

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.

Abstract We investigate the challenge of splitting a traffic flow over WiMAX and WiFi links. For traffic load distribution over heterogeneous RATs (Radio Access Technologies), heterogeneous link resources need to be commonly measured and fairly compared. To this end, an airtime cost model is considered as a common resource measure. The model can be used to estimate channel time consumed for a successful packet transmission. As a traffic split mechanism, an airtime‐balance method is proposed. Using the airtime cost model, the offered traffic load (in Mbps) is converted into airtime cost required for its transmission, and IP packets are distributed to multiple RATs so that airtime is equally balanced between RATs. We have implemented a practical test‐bed system, including both WiMAX and WiFi systems, and used it to conduct extensive experiments indoor and outdoor. Experimental results confirm that airtime‐balance can achieve an improved flow split to reduce the waiting packets at the reorder buffer of the receiver. Moreover, it could realize a more rapid adaptation to link variations with local measurements, when compared to the RTT‐based method, which also requires extra system overhead due to the use of probe packets. Copyright © 2010 John Wiley & Sons, Ltd.

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.

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 10-2when 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.

Throughput of wireless LANs is interference-limited. Newly deployed access points (APs) can hardly bring throughput gain without careful design. Although transmit power control (TPC) can improve spatial reuse of the channels, the achievable throughput also heavily depends on other factors, such as rate adaptation, which are seldom touched in previous works. In addition, TPC protocols, designed with the assumption that all nodes have the same capability, do not work well when legacy nodes not supporting TPC coexist. In this paper, (i) we exploit joint rate adaptation and TPC to improve system performance and suggest choosing power to maximize throughput after taking tradeoff between transmit rate and spatial reuse of channels, and, (ii) we suggest using the NAV (network allocation vector) mechanism to create virtual slots in the scenario where legacy nodes coexist with new nodes running TPC. Simulation results show that (i) the proposed TPC scheme can effectively improve both total throughput and fairness, and, (ii) the slotted channel access scheme ensures that throughput gain can be achieved by nodes running TPC while the performance of legacy nodes not supporting TPC is not degraded.

Network coding is an effective method to improving relay efficiency by reducing the number of transmissions. However, its performance is limited by several factors such as packet length mismatch and rate mismatch. Although the former may be solved by re-framing, the latter remains a challenge and is likely to greatly degrade the efficiency of network coding. In this paper, we re-interpret network coding as a mapping of modulation constellation. On this basis, we extend such mapping to enable 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 together in such a way that for each relay link its desired message is transmitted at its own highest rate. Compared with previous solutions to rate mismatch, the proposed scheme achieves the full rate of all relay links on the broadcast channel.

As advanced integrated network architecture, "cognitive radio technologies," which aim to improve the spectrum efficiency, have been studied. In the cognitive radio networks, each node recognizes radio conditions, and according to them, optimizes their wireless communication routes with the integration of the heterogeneous wireless media not only by switching over them but also aggregating and utilizing them simultaneously. The adaptive control of switchover use and concurrent use of various wireless media will offer a stable and flexible wireless communication. In this paper, we introduce our cognitive radio testbed that has IEEE802.16 and IEEE802.11 interfaces, and perform the field trial in order to experimentally examine the effectiveness of the cognitive radio technologies. The experimental results show that the concurrent use of IEEE802.16 and IEEE802.11 wireless media offers high IP throughput performance and furthermore, the adaptive route control according to the radio conditions improves the IP throughput by more than 20% and reduce the one-way delay to less than 1/6. It is found that the cognitive radio technologies can provide the appropriate wireless communication routes to meet various demands for application QoS.

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.

Carrier sense multi-access (CSMA) is a typical method to share the common channel in a wireless LAN (WLAN). It works fairly well in times of light traffic. However as the number of nodes in a WLAN increases quickly, severe collision greatly degrades network performance. In this paper we propose a distributed multi-user scheduling (DMUS) scheme to solve this problem, taking time-variant link quality and rate adaptation into account. Instead of all nodes, only nodes with high instantaneous link quality are allowed to contend for the channel. By setting a suitable SNR threshold, at any instance only a small percentage of nodes join the contention. As a result, collision is mitigated, fairness is retained by independent fading, and the total throughput is increased since transmissions are finished at higher rates. Simulation results show when there are 40 nodes in a WLAN, the DMUS scheme improves total throughput by up to 49.6% compared with the contention-free scheme, and by up to 194.6% compared with the CSMA/CA scheme.

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.

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.

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 performed in the broadcast manner. Indeed, the near-far effect provides extremely reliable transmissions between near nodes, regardless of node density, which can not be achieved by CSMA/CA. However, CDMA can not 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 achieves approximately 100% of delivery ratio and 4 milliseconds of delay in a realistic traffic accident scenario, where CSMA/CA achieves approximately 60% of delivery ratio and 80 milliseconds of delay.

A multi-player video game via wireless connections using portable devices is one of the most popular applications of ad-hoc networks. Broadcast transmissions can be used for many- to-many communications in multi-player games. However, reliable broadcast communications are hard to realize because of the lack of retransmissions in the medium access control (MAC) layer. In this paper, we propose a broadcast method using random linear network coding in order to enhance the reliability of many-to-many and real-time communications. Exploiting the periodic nature of game traffic as well as inherent robustness offered by random network coding, the proposed method can provide reliable packet deliveries between 2 nodes which have a link under constant fade or even under temporary loss. Our simulation results show that the proposed method can provide higher reliability than the 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 practical degree of complexity when installed on current wireless devices.

Multipath fading greatly degrades system performance of wireless networks. Conventionally diversity is introduced to mitigate fading. In this paper we study space diversity in multi-hop wireless networks under power constraints. Point-to-point links are extended to group-to-group virtual MIMO links. Then we propose a cross-layer design of MAC and routing that exploits opportunistic cooperation and selective forwarding (OCSF). Packets are forwarded around the pre-calculated anchor route and the proposed OCSF scheme enables local post selection of both transmitter and forwarder according to channel state information. Simulation results confirm that OCSF outperforms existing schemes that utilize distributed space-time coding or selection diversity.

In wireless communication networks, there is growing interest in using mobile terminals with multiple wireless interfaces in order to achieve increased throughput and reliability. However, the performance of communications using multiple wireless interfaces can be severely impaired by fluctuations in quality of the wireless links. We designed and implemented a novel interface status monitoring scheme which supports the detection of changes in wireless link characteristics for multiple wireless interfaces and proactive decision making about the status of interfaces which can be used for communication. The monitoring scheme acquires information about link quality parameters, such as signal strength, retries and rate, directly from the wireless device driver, which allows it to acquire a large amount of information about wireless transmissions available in the driver, and to reduce latency in deciding the status of interfaces. In this paper we describe the design of the scheme and demonstrate its application to sustaining throughput on aggregated Wi-Fi links.

In this paper we study the channel efficiency of mobile ad hoc networks (MANET) and try to trade off between the progress and the instantaneous rate when forwarding packets in the fading environment. We define a new metric-bit transfer speed (BTS)-as the ratio of the progress made towards the destination to the equivalent time taken to transfer a payload bit. This metric takes the overhead, rate and progress into account. Then we propose an opportunistic progressive routing (OPR) protocol, jointly optimizing routing and MAC by the cross-layer design. In OPR, a node selects the forwarder with the biggest BTS for a packet and the forwarder changes as the packet size or channel status varies. The extensive simulation shows that OPR greatly reduces channel occupation time and packet loss compared with the normalized advance (NADV) scheme and contention-based forwarding (CBF) scheme.

Recent studies on directional media access control (MAC) protocols using smart antennas for wireless ad hoc networks have shown that directional MAC protocols outperform traditional omni-directional MAC protocols. Those studies evaluate performance primarily based on a simulations, where antenna beam is assumed to be ideal, i.e., with neither side-lobes nor back-lobes. Propagation conditions are also assumed to be based on a mathematical model without realistic fading. However, for the real application of ad hoc networks those optimistic assumptions do not hold anytime. In this paper, we develop at first a testbed for directional MAC protocols which enables investigation of the performance of MAC protocols in a real environment. It incorporates ESPAR as a practical smart antenna, IEEE802.15.4/ZigBee, GPS and gyro modules to allow easy installation of different MAC protocols. To our knowledge, it is the first compact testbed with a practical smart antenna for directional MAC protocols. We implement a directional MAC protocol called SWAMP to evaluate it in a real environment. The empirical discussion based on the experimental results shows the degradation of the protocol with ideal antennas, and it shows that the protocol still achieves the SDMA effect of spatial reuse and the effect of communication range extension.

In next generation wireless networks, a variety of heterogeneous radio access technologies are expected to be available simultaneously within a single wireless terminal. This paper addresses the challenge of link aggregation in multi-access networks, where a key issue is how to optimally aggregate bandwidth offered by the individual radio link. Effective link throughput model is employed as a common resource measure for heterogeneous links. Based on the model, we present a packet allocation technique to optimize link aggregation. The mapping of packets to radio access links is performed to maximize the overall expected system throughput. Airtime constraint is added for an equal load balance. We have conducted extensive simulations with two scenarios. One is at the co-existence of WiMAX and WiFi, and the other includes only WiFi links. The proposed technique adaptively operates to time-varying link resource, and achieves better the aggregated link performance than WRR with fixed distribution ratio.

We demonstrate a method for improving quality of ad-hoc multihop wireless communications between moving vehicles. This method is designed to overcome problems of high packet loss and route instability common in inter-vehicular communications. We implemented the proposed method as middleware supporting real-time packet transmissions such as VoIP, and evaluated the performance in experimental tests, including a 3-vehicle test on a public road. We obtained measurement results showing maximum packet loss lower than 5 %, compared to more than 80% with existing implementations of the ad hoc routing protocols OLSR and AODV. We also showed that it is possible to support good quality VoIP broadcasts at normal traffic speeds of from 40km/h to 60 km/h on open roads.

For multi-access networks with heterogeneous radio access techniques, the challenges include the problem of how to optimally distribute network traffics into each radio link in order to enhance the aggregated link capacity. As an effective load balance mechanism, traffics may be equivalently assigned to each RA, in proportion to its available capacity. To this end, airtime cost is identified as a common resource measure for WiMAX and WiFi links. Due to their different data transmission mechanisms, the calculation of airtime cost is separately derived. Based on the common metric, the offered traffic for each link is converted into airtime cost required for its transmission. Traffics are distributed so that cumulative airtime cost between RAs is fair. Evaluation results show that airtime cost model can commonly measure the link resources of heterogeneous wireless links, and this measurement enables traffics to be equally distributed, in proportion to their transmission capacity.

In emerging wireless communication environments, a variety of wireless systems will coexist using a variety of applications. On the other hand, concern is increasing that the growing use of wireless systems will exhaust finite radio resources. Cognitive radio, which aims to optimize the utilization efficiency of radio resources by combining multiple wireless systems, has been proposed as a solution to this problem. In a cognitive wireless network composed of nodes equipped with multiple wireless systems, we have proposed a traffic control method that decreases delay and increases throughput by using wireless link aggregation. In this paper, our proposed method is presented, and its performance is shown through evaluation with IEEE802.11 and IEEE802.16.

Performance of mobile ad hoc networks (MANET) routing protocols may be greatly degraded due to involvement of weak links in the routes. One solution is to use directional antenna to improve link quality. The other is to less prefer weak links. In the experiments we found that even with the two methods the system performance is still limited in the presence of mobility and multipath fading due to the following facts: (1) beam scanning overhead and frequent beam variations, (2) route instability due to metric variation and false link breaks. Then we improve it by three schemes: (i) Calculate the antenna beam by position information, (ii) Calculate a stable link metric from the expectation value of received signal strength indication (RSSI). (iii) Avoid false link breaks. These schemes are suitable for the scenarios where both line of sight path and multipath fading exist. The experiment and simulation results indicate that the schemes can effectively reduces PER and improve throughput of AODV in the Rician fading situations.

A cognitive wireless network is mostly deployed as wireless access network to optimize the utilization efficiency of radio resource by using multiple wireless systems. In such deployment, the network topology based on a tree structure can be efficiently and quickly constructed among the cognitive terminals (CT) by using the cognitive base station (CBS) as a root, to support the multihop communication. However, the original tree-based routing (TBR) protocol is designed to handle a single wireless system such as IEEE802.11a or lib, and thus can not be applied to the cognitive networks configured with multiple wireless systems, which may have the different bandwidths and transmission ranges. To solve this problem, we propose in this paper an efficient and practical protocol, called cognitive tree-based routing (CTBR) protocol, which extends and significantly enhances the ability of the known TBR protocol to enable it to support multiple wireless systems such as IEEE802.11g and IEEE802.11j. Simulation results reveal that our proposed CTBR protocol that utilizes the cognitive-aware link metric to select a route with the best end-to-end metric and an interface with the least local load for any source-destination pair, achieves much higher performance than the utilization of normal metric associated with the hop count.

Congestion bottlenecks at nodes where flows cross can severely limit the number of simultaneous flows in ad hoc wireless networks using 802.11 media access. We show how routing protocols and packet generation parameters can be modified to reduce congestion at such bottlenecks. We also present results of implementation tests which show dramatic reduction of packet error rate and delay, and increase in the number of VoIP sessions due to reduction in congestion

We demonstrate the use of ad hoc multi-hop flooding for sharing information among mobile terminals. PDA devices with 802.11 WLAN cards are used as mobile terminals which periodically broadcast position and other presence information. Each terminal relays packets broadcast by neighbors using ayooding algorithm which has been optimized for fast propagation of information over multiple hops. We also report results of systematic experiments using 50 terminals showing characteristic propagation time distributions for up to 5 hops.

Ad hoc wireless networks are expected to have many useful applications, such as emergency communications after accidents and natural disasters, distributed sensing in buildings and natural environments, versatile wireless monitoring in health care and support system, and to make our daily lives easier, more productive and more enjoyable. In order to fully realize the potential of wireless ad hoc networks, we need to be able to realize large-scale networks, which connect large numbers of ad hoc terminals over wide areas. Previous research on ad hoc networks has been based mainly on computer simulations and small-scale implementations, and there have been few successful implementations of large-scale ad hoc networks. We have made significant breakthroughs toward realizing real-world operation of large-scale ad hoc networks, by the testbed based approach. In this paper, we discuss the breakthroughs toward realizing large-scale ad hoc networks and inter-vehicle communications for vehicle safety, which is one of the promising applications of ad hoc networks.

Several positioning systems to obtain precise location information have been researched. However, they have scalability and flexibility problems because they need completely configured space with a large number of sensors. To avoid the problems, we have proposed a self-organizing location estimation method that uses ad hoc networks and Self- Organizing Maps and needs no prepared space with a large number of sensors. In this paper and our demonstration, the proposed method and its characteristic are presented.

Our major contribution in this paper is to devise a MAC protocol that exploits the advantages of directional antenna in ad hoc networks for improved system performance. In order to implement effective MAC protocol in this context, a node should know how to set its transmission direction to transmit a packet to its neighbors and to avoid transmission in other directions where data communications are already in progress. In this paper, we are proposing a receiver-centric approach for direction and communication-aware MAC (DCA-MAC) protocol, so that, nodes become aware of its neighborhood and also the direction of the nodes for communicating directionally. Our proposed directional MAC protocol can be effective in both ITS (intelligent transportation system), which we simulate in string and parallel topology, and in any community network, which we simulate in random topology. The performance evaluation on QualNet network simulator clearly indicates the efficiency of our protocol. In addition, under the fading channel conditions, it's difficult for each node to perceive the status of the neighbors. In AWGN, Ricean and fading environments, we compare and analyze the performance of omnidirectional MAC, conventional directional MAC and our proposed directional MAC

Despite many routing protocols proposed for mobile ad hoc networks few have considered link quality and route stability. Weak links contained in the routes often lead to route breaks. Multipath fading and mobility make link quality timevariant. When the link metric is associated with link quality the routes may change frequently. In times of transmission failure due to collision or fading, the routing packets are susceptible to loss owing to the lack of retransmission mechanism for the broadcast packets. For these reasons, the routes become unstable. In this paper, we investigate the long-term link quality and route stability in the on-demand routing protocols. We compare the link metric policies, optimize the route discovery and use the soft detection scheme to avoid false link breaks. The simulation results show that both Packet Error Ratio (PER) and throughput in the enhanced on-demand routing protocol can be greatly improved under the Rayleigh fading environment.

In mobile ad hoc networks on-demand routing protocols are very attractive due to low overhead. In these protocols, however, (1) the routes tend to contain weak links that have a low rate and high packet error ratio; (2) the routes are susceptible to breaks in the presence of mobility. Our major concern in this paper is to locally optimize the routes in the on-demand routing protocols in terms of link quality and mobility. The local optimization is realized by two main techniques: relating link metric with received signal strength and preferring the route with a shorter metric through trigger update (TU). In this manner the initial routes converge to the local optimum in the static case, and adapt to topology variations and link quality changes caused by mobility. Also, the routing overhead is controlled. Simulation results show that AODV with the application of local optimization (AODV-TU) achieves a much higher performance than the original AODV

Network monitoring is one of the most significant functions in network management to understand the state of a network in real-time. In network management, such as SNMP (simple network management protocol), polling is used for this purpose. If the time interval for two consecutive polling requests is too long, then we cannot understand the state of the network in real-time. Conversely, if it is too short, then the polling message traffic increases and imposes a heavy load on the network although we can understand the state of the network in real-time. Many schemes have been proposed for controlling the overheads of dynamic polling by throttling polling rates. Unfortunately, by only considering overheads, these schemes fail to take into account the rate at which information must be obtained in order to achieve management tasks. Examples of these tasks include checking for threshold violations and determining if additional capacity should be allocated dynamically. This paper proposes a new scheme for dynamic polling that considers both the overhead of polling traffic and the message rates required for management tasks. Our scheme examines polling traffic, applying the discrete Fourier transformation to extract the desired polling rate. We demonstrate the availability of this approach through simulations in which polling requests are varied dynamically. Our scheme adjusts polling rates to make them more consistent and to reduce resource overheads.

For efficient and integrated network management of SDH/PDH mixed large-scale transport networks, KDD has developed a TMN-based integrated management system. This paper describes an architecture of the system and its implementation. The features of the architecture are: (1) to employ a distributed system configuration based on TMN multi-layered functional architecture, (2) to newly define an integrated management information model which unifies different management information models for many kinds of NEs (Network Elements) and which enables us to manage a network with mixed SDH and PDH hierarchies of, and (3) to introduce a new alarm correlation mechanism for high system performance.

OSI Management model defines only peer to peer communication process between a managing application (AP) and a managed AP. Thus, when more than one managed AP stores portions of Management Information Base (MIB) distributively, the managing AP needs a sophisticated distributed processing function. This paper proposes an agent, called Management System Agent (MSA) which provides location transparency and schema transparency of MIB for managing APs, to ease the realization of managing APs. Location transparency hides the distribution of MIB from managing APs. Schema transparency also enables managing APs to invoke management operation on unified schema. This paper demonstrates the effectiveness of the MSA model through applying it to implementation of Customer Network Management (CNM) system focusing X.25 public data networks.

The Worldwide Intelligent Network (WIN) group is studying and developing robust network operation and traffic control techniques to realize international dynamic routing. So far, the WIN group has developed the TMN based interface specification for an exchange of routing information among WIN systems at each telecommunication administration (TA). This paper describes the TMN based traffic management data exchange architecture, WIN system implementations and evaluation of the data exchange performance.

Results are presented of a study of the feasibility realizing a real-time information retrieval service operating between different worldwide videotex systems by means of gateway facilities that use conversion of data syntax and processing of the videotex information structure. Two separate evaluations were carried out, both using response time as the performance criterion. One was for communication processes at both of two gateways, and the other was for data syntax conversion at the local host. Both approaches are considered as kernel functions in the gateway system. The results of each evaluation were favorable in the sense that response time for the total system using gateways compared favorably to the response time in direct videotex access over the packet-switched public data network.>