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A reliable vehicle-to-vehicle communication (V2VC) is essential to enable safe and highly automated driving system. Addition of a relay station (RS) can improve packet delivery rate (PDR) of CSMA/CA-based V2VC. However, the improvement is limited by packet collision at RS due to a hidden terminal (HT). This HT problem can be alleviated by employing sectorised receive antenna at RS, i.e. sectorised relay-assisted (SR-) V2VC. However, SR-V2VC still faces another problem, i.e. packet congestion at RS under high traffic conditions due to the improved packet reception rate. This leads to packet drop at RS and hence limits the improvement brought by SR-V2VC. Consequently, it is crucial to introduce a kind of traffic congestion avoidance methods to RS. In this article, the authors propose SR-V2VC with a payload combining forwarding (PCF) method (SR-V2VC/PCF) and theoretically evaluate the system performance using a CSMA/CA collision model. Large-scale computer simulations are also conducted to confirm the performance improvement brought by SR-V2VC/PCF. It is shown that the lowest broadcast PDR of 62% for the non-relay system is improved to higher than 81% by SR-V2VC/PCF and the directivity of the sector antenna has impact on the benefit of the proposed scheme.

A novel Infrastructure to Vehicle (I2V) multi-hop broadcast communication technique is proposed. The proposed Time-Controlled Broadcast Forwarding (TC-BF) method enables more efficient information delivery on the road compared to the existing broadcast packet forwarding schemes such as flooding and Irresponsible Forwarding (IF) methods. Packet delivery rate (PDR) and forwarding efficiency of the proposed scheme are evaluated by computer simulation. It is concluded that the proposed TC-BF can cut unnecessary forwarding and keep high PDR with extended communication range.

This paper theoretically analyzes the throughput of the dynamic multi-hop networks under high traffic load condition. In order to reduce the number of hops and improve the throughput performance, the shortcut path between the non-neighboring nodes is established. However, the preceding packet hinders the establishment of the shortcut path when the traffic load is high. To eliminate the interference from the preceding packet, we introduce intra-flow interference canceller (IFIC). We derive the theoretical packet delivery rate (PDR) expression by taking into account the existence of preceding packets. Theoretical analysis and numerical evaluation elucidate the effectiveness of IFIC to improve the throughput performance in dynamic multi-hop networks even under high traffic load condition.

Conventional Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) multi-hop transmission suffers severely from intra-flow interference (IFI) in dense traffic, due to the hidden terminal (HT) problem. We propose a non-CSMA/CA multi-hop transmission scheme with IFI-canceler (IFIC) that can efficiently relay packets under high traffic loads in fading environments. This interference cancellation technique employs adaptive equalization with a normalized least mean square (NLMS) algorithm for channel estimation and has good BER/PER performance under a wide range of SNR and SIR conditions. It is also capable of canceling multiple interference sources. A multi-hop packet transmission frame format dedicated to the IFIC proposal is designed. An analysis model is created that considers possible outage events in multi-hop traffic flows. Simulations of multi-hop transmission indicate that IFIC throughput under slow Rayleigh fading linearly increases with traffic load and is 3.5 times higher than that of current CSMA/CA systems at the highest normalized traffic load. When 2-branch maximal-ratio combining (MRC) reception is added after IC, the throughput is further improved to a level very close to the theoretical maximum for additive white Gaussian noise (AWGN) channels. We also assess the impact of maximum Doppler frequency, and show that the best performance is maintained if the frequency is less than 10 Hz and the performance under higher Doppler frequencies can be effectively improved by retransmission.

Most of reactive ad hoc routing schemes establish a route by using route request (RREQ) and route reply (RREP) messages. In actual wireless environments, however, route construction with this method is not always successful, nor is the established route stable. This paper analyses two issues that prevent stable route establishment. They are the packet collision of RREQ/RREP and the decrease of channel correlation between RREQ/RREP. In order to overcome these problems, an improved reactive routing scheme is proposed that employs received threshold and reply timing control. The route construction performance for the proposed scheme is evaluated by network simulation. The simulated results show that the proposed scheme can improve route construction probability under fading environment with a small increase of route construction delay.

In this study, we integrate overlay cognitive radio technology into 4G cellular networks for the sharing of TV spectrum. On one hand, OFDM is a promising technique for high-speed data transmission over multipath fading channels and has been considered to be the best candidate for 4G mobile networks. On another hand, the overlay cognitive radio model makes it possible to have two concurrent transmissions in a given interference region, where conventionally only one communication takes place at a given time. We investigate different service provision scenarios and propose both time domain and frequency domain overlay cognitive radio OFDM systems for next generation cellular networks. Numerical results show our proposed schemes can achieve satisfying performance in different use cases.

Importance of wireless M2M (machine-to-machine) applications is rapidly increasing, and more effective interference management of large-scale ad hoc networks becomes essential. Study on the properties of interference in multi-hop ad hoc network and effective use of interference canceller is promising to achieve better performance. In this paper, an intra-flow interference canceller with a slotted CSMA/CA mechanism is proposed for improving packet delivery performance under the environment where both intra-flow and inter-flow interferences exist. The results from computer simulation with 100 nodes show that the interference canceller can improve global packet delivery rate up to 47%.

Highly reliable vehicular communication is a key to the future automated driving. CSMA/CA packet relay-assisted schemes can improve reliability of ITS vehicle-to-vehicle (V2V) communications by compensating shadowing loss and fading. However, their performance is affected by hidden terminal (HT) problem. An improved relay-assisted V2V scheme employing a sectorized relay station (RS) was proposed to mitigate the problem and its effectiveness has been studied for intersection environment. However, effectiveness of the scheme with multiple RSs along a street has not been studied yet. This paper first analyzes the effect of HT problem on the relay-assisted V2V schemes with multiple RSs along a street, and shows that the sectorized RSs cooperate with each other to mitigate the problem and obtain diversity gain. Large-scale system simulations of 240 vehicles confirm that the relay-assisted scheme with sectorized RSs can achieve the highest reliability of V2V communications even in straight street scenario.

Multi-hop wireless ad hoc networks suffer from temporary link error due to fading. In order to improve packet transmission reliability and achieve efficient transmission in fading environment, a new cognitive temporary bypassing scheme is proposed based on a cross-layer approach and cognitive behavior of local nodes. The proposed scheme enables neighboring nodes to prepare and create a temporary bypass for lost-packets. This is done by monitoring message packets that include information of the multi-hop route and link-acknowledgement. The scheme also includes an anti-collision function that is necessary to prevent contention among multiple bypassing nodes. Packet success probability with the proposed scheme is studied both by theoretical analysis and time-domain computer simulation for Rayleigh faded single- and multi-hop links. Network simulation using a modified QualNet simulator validate that packet success probability is remarkably improved with the scheme for maximum Doppler frequencies up to 30Hz.

This paper proposes a low power Vehicular-to-Machine (V2M) communication system consisting of crowd servers, moving vehicles and low-power fixed roadside devices. In order to save energy of roadside devices, it employs an enhanced ZigBee protocol with fast network association and switching capability. The vehicles can collect information from various types of roadside devices and process them for their own, as well as uploading some of them to corresponding servers for public use. In the system, a roadside device, playing role of the ZigBee end device, detects a vehicular station (VS) by hearing its beacon signal that requests necessary types of information. The device then associates the personal area network (PAN) that is formed by the VS and repeats the association and disconnection for different PANs so that it can send information to all requesting VSs. With current ZigBee protocol, however, it is difficult for the devices to handover between PANs passing by at high speed. In order to enable smooth and fast network association, we propose a fast PAN association / handover algorithm that refers to PAN subscription history and receiving power of beacon. It is confirmed via computer simulations that the proposed method enables faster network association and remarkably improves information delivery rate compared with the original ZigBee protocol.

CSMA/CA packet relay schemes can improve reliability of ITS V2V communication by compensating propagation loss and fading. However, their performance is affected by hidden terminal (HT) problem, especially when the density of vehicle stations is high. An improved scheme employing sectorized relay station has been proposed to mitigate the problem, and its effectiveness has been shown by some simulations. However, potential of the scheme has not been analyzed in detail nor optimized yet. In this paper, a theoretical model is proposed and performance of the packet relay assisted V2V communication with sectorized RS is analyzed. The accuracy of the proposed model is validated by simulations. Some suggestions to enhance the reliability of V2V communication are provided based on the results.

Sensor networks always face bottleneck problem at sink nodes where communication traffic concentrates. In order to achieve highly efficient communication in such networks, a multi-code reception spread-spectrum (MCR-SS-) CSMA/CA protocol has been proposed and shown to be effective to solve the uplink bottleneck when gathering information from sensor nodes. However, actual throughput for the network is limited not only by the airtime usage efficiency of uplink but also by that of downlink acknowledgement (ACK) transmission. This issue is remarkable when the data packet size is not much larger than that of ACK packet. This paper proposes a new practical group ACK method that can improve the network performance as a result of downlink throughput enhancement. Instead of sending individual ACK packets, a multiplexed group ACK packet is broadcasted to the data sender nodes that transmit data packets in the preceding communication frame. Network throughput and delay performance of the proposed method are evaluated by calculations and simulations based on the IEEE 802.15.4 PHY specification and the enhanced MCR-SS-CSMA/CA MAC. The results show that the proposed group ACK method exploits full potential of the MCR-SS-CSMA/CA and improves total network throughput significantly.

The next generation cellular networks target significant capacity improvements and require dense frequency reuse of the scarce radio spectrum, resulting in a danger of severe interference. Fractional frequency reuse (FFR) techniques have been proposed for macro-cell networks to mitigate interference. However, these static schemes are not adaptive to meet the traffic demand changes from mobile users, and thus inefficient with respect to power consumption in many scenarios. This paper starts with the basic soft fractional frequency reuse (SFFR) framework and proposes a traffic demand orientated green power control scheme that can make power allocation by considering different traffic demands. Afterwards, this green concept is extended to heterogeneous network (HetNet) for the power control strategies in low-power nodes (LPNs). Numerical results show that our proposed schemes can achieve considerable improvement in terms of throughput and power consumption, as well as extra rewards of high flexibility and low complexity.

Vehicle-roadside-vehicle relaying scheme has been studied as an effective means for improving the reliability of ITS V2V communication at intersection. However, hidden terminal (HT) problem that is caused by non-line-of-sight (NLOS) propagation at the intersection severely deteriorates packet reception at relay station (RS), which reduces the gain of relaying scheme. On the other hand, relaying increases air traffic, which results in interruption of packet transmission when the number of vehicles is large. In order to mitigate these two issues and achieve higher communication reliability, an improved packet payload combining relay scheme with sectorized RS (S-PCRL) is proposed. Simulated results show that the proposed scheme can effectively mitigate the issues and remarkably improve the reliability of ITS V2V communication at intersection environment with up to three hundreds of vehicles.

The use of multihop connection, in which a station in between the mobile station and the base station performs hopping relay of the transmission, in CDMA cellular systems should expand the area coverage and reduce the transmitted power per hop. There is a possibility that interference power may be reduced and that system capacity may be enlarged by this approach. Depending on the method of application, however, the system capacity may be decreased due to the interference during hopping. In this study, a basic investigation of interference reduction by multihop connection is first presented. Then, a procedure for selection of the hop stations with a strong capability for reducing the interference power is proposed. Considering a multicellular environment with a CDMA (code division multiple access) cellular scheme, the characteristics of multihop connections are evaluated by computer simulation, assuming packet access. It is shown that multihop connection has the effect of improving system capacity. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 86(10): 17–24, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.10185

Vehicle-to-vehicle (V2V) communication is regarded as an essential means to avoid traffic accidents and enhance vehicle safety. However, reliability of the communication severely suffers from wireless propagation on the road such as fading and shadowing. Vehicle-roadside- vehicle relay communication schemes have been proposed to improve the reliability of the V2V communication. Although the scheme can ease fading and compensate shadowing loss for intersection environments, it increases airtime occupancy ratio which results in more chances of packet collisions. This paper proposes a network coding broadcast relay scheme considering the location of transmitting vehicle stations. It can reduce the air traffic caused by relaying and improve the performance of V2V communication. Simulated results show that the proposed scheme can effectively reduce airtime occupancy ratio under intersection environment and improve the performance of relaying for high traffic conditions.

CSMA/CA protocol shows poor performance for multi-hop transmission due to intra-flow interference (IFI) caused by hidden terminal (HT) problem. We propose an IFI-cancelling (IFIC) multi-hop transmission scheme which can efficiently relay packets under high traffic load and fading environment by using a Known interference cancellation technique. It employs the least mean square (LMS) equalization process of signal reception. A packet frame format dedicated to the proposed IFIC is designed. Theoretical model and a physical layer-included simulator are created for validation and performance evaluation. The results show that the throughput of the IFIC transmission under Rayleigh fading environment linearly increases with traffic load. When traffic load is high, it has about 3 times higher throughput than the multi-hop network with normal CSMA/CA. When 2-branch maximal-ratio combining (MRC) reception is added after IC, the throughput is further enhanced very close to the theoretical maximum.

The forthcoming mobile communication systems are expected to provide much variety of services from high quality voice to high definition videos through high data rate wireless channels at anywhere in the world. High data rate requires broad frequency bands, and sufficient broadband can be achieved in higher frequency bands such as microwave, Ka-band and millimeter-wave. Broadband wireless channels have to be connected to broadband fixed networks such as the Internet and local area networks. The future generation systems will include not only cellular phones, but also many new types of communication systems such as broadband wireless access systems, millimeter-wave LAN, intelligent transport systems (ITS) and high altitude stratospheric platform station (HAPS) systems. The key words in the future generations of mobile communications are ``multimedia communications'', ``wireless access to broadband fixed networks'' and ``seamless roaming among different systems''. This article discusses the future generations mobile communication systems.

In this paper, we propose a bandwidth efficient distributed network code, termed superimposed adaptive network coded cooperation (SANCC), for a network consisting of M users sending different information packets to a common destination through independent block fading channels. Different from conventional cooperative communications with half-duplex constraint, our proposed approach does not reduce the bandwidth efficiency with the aid of high order modulation while its encoding process is still performed by simple XOR operation similar to the conventional adaptive network coded cooperation (ANCC). In order to exploit the structure of the proposed approach, effective mapping and corresponding decoding are investigated. We further propose a modified encoding for SANCC to improve the inherent error floor behavior and analyze its performance based on extrinsic information transfer (EXIT) chart. Finally, the effectiveness of our proposed approach is confirmed via computer simulations.

Performance of CSMA/CA multi-hop networks is severely affected by intra-flow interference caused by Hidden Terminal (HT) problem. HT problem leads to failure of carrier sense and causes packet error due to collision. However, there is no precise analysis method that can estimate the performance of multi- hop CSMA/CA networks under fading environment. In this paper, a CINR-based analysis method is proposed that well predicts the packet delivery probability for CSMA/CA multi-hop network under such environment. With the proposed method, the impact of HT-caused intra-flow interference on the network performance and the effect of carrier sense level are analyzed for offered load traffic rate of 1/2 and 1/3. Higher carrier sensitivity can compromise HT-caused intra-flow interference at the first hop but cannot improve the network's packet delivery ratio at the end node significantly. It is concluded that the proposed method is useful for the design and improvement of CSMA/CA multi-hop networks.

The reliability of CSMA/CA vehicle-to-vehicle (V2V) broadcast communication is seriously affected by propagation issues such as propagation loss, fast fading by multipath propagation, and hidden terminal problem. In order to improve the V2V communication reliability, a vehicle-roadside-vehicle relaying communication scheme has been proposed and its effectiveness has been shown. However, relaying increases air time occupancy ratio in a wireless channel which results in more chances of collision and limits the improvement by relaying. This paper proposes an improved V2V packet relay scheme with payload combining at relay station to save air time and mitigate the collision. Simulated results assuming an intersection show that the average packet delivery ratio is significantly improved with the proposed scheme.

The reliability of CSMA/CA vehicle-to-vehicle (V2V) communication under actual road environment suffers from fading, shadowing and hidden terminal (HT) problem. In order to improve the communication reliability, a V2V broadcast scheme with relay stations has been proposed and its effectiveness was shown by some simulations. However, the potential of the CSMA/CA broadcast relay network has not been well analyzed and optimized. In this paper, a theoretical model is proposed to analyze the performance of such network in detail. In order to fit the realistic vehicular environment, the model assumes a typical crossroad and takes into account fading, shadowing, hidden terminal problem and capture effect, as well as distances between nodes. The influence of system parameters including RF frequency band and carrier sense threshold on the reliability of the network is also studied through the analysis. The accuracy of the proposed analytical model was validated by simulations. Some suggestions to enhance the reliability of V2V communication are provided based on the results.

Performance of CSMA/CA wireless networks is severely affected by hidden terminal (HT) problem that leads to failure of carrier sense and causes packet error due to collision. However, no mathematical analysis method has been available that can estimate the performance of two-dimensional CSMA/CA network under fading environment with hidden terminal. The analysis method proposed in this paper enables to precisely predict the probability of successful communication for CSMA/CA network under such environment. The probability maps obtained by the method visualize the effect of HT problem under fading environment. They show the dependence of the packet transmission performance on interferer's position and carrier sense level. The results as well as the proposed analysis method are quite useful for CSMA/CA network design.

Vehicle to vehicle (V2V) communication is regarded as an important means to avoid traffic accidents and improve vehicle safety. The reliability of the V2V communication strongly depends on propagation issues such as propagation loss determined by node distance, fast fading by multipath propagation, and hidden terminal problem when multiple vehicles exist. However, analysis of the packet delivery rate considering hidden terminals in fading environment has not been sufficiently investigated. This paper presents the method for analyzing the broadcast packet delivery rate of V2V communication by setting a mathematical model considering CSMA/CA mechanism. The obtained results show that the number and the position of interfering vehicles, as well as carrier sense level, significantly affect the packet delivery rate.

Vehicle to vehicle (V2V) communication is regarded as an essential means to avoid traffic accidents and increase vehicle safety. However, reliability of the communications severely suffers from wireless propagation on the road. A vehicle-roadside-vehicle relay communication scheme has been proposed to improve the reliability of the V2V communications. The packet broadcast transmission performance of the proposed scheme with large number of transmitting vehicle stations around intersection is evaluated by computer simulations that assume fading, corner loss attenuation and carrier sense range. The combinations of two frequency bands, 700 MHz and 5.9 GHz, are also compared for the direct and relay transmissions. Simulated results show that the average packet reception ratio is remarkably improved with the proposed scheme regardless of number of the vehicle stations. The frequency combination of 700 MHz as vehicular station transmission and 5.9 GHz as relay station retransmission shows better performance.

Multi-hop packet transmission error due to packet collision is a serious issue for realizing large-scale wireless ad hoc networks. In order to solve the issues, a multicode-reception spread-spectrum (MCR-SS) -CSMA/CA was proposed as a multiple access scheme for layered-tree networks. This paper proposes a new SS-CSMA scheme that combines a time group access (TGA) technique with the above multiple access scheme to gain higher interference reduction between nodes. The proposed scheme divides the layered-tree network into upper subnets group and lower subnets group, thus reducing collision probability among subnets. Network throughput and data delivery ratio for the proposed scheme are analyzed by simulations based on the IEEE 802.15.4/ZigBee specification. Simulation results show that the proposed scheme can improve network throughput remarkably.

Multi-hop wireless ad hoc networks suffer from temporary link error due to fading. In order to improve packet transmission reliability and achieve efficient transmission in fading environment, this paper proposes an integrated dynamic multi-hopping scheme based on a cross-layer approach and cognitive behavior of local nodes. The scheme provides two important functions, temporary bypassing and dynamic shortcut, to achieve the goals. The proposed MAC protocol avoids their conflict by giving priority to the shortcut path over the bypass and original passes. Moreover, the transmission reliability can be further improved by triple-path diversity. Packet transmission success probability and average transmission delay were analyzed and evaluated for a basic two-hop model. The results show that the proposed scheme can achieve better transmission reliability with reduced transmission delay, compared with previous multi-hop schemes.

In the CSMA wireless networks, the carrier sense mechanism is severely affected by hidden terminal problem and frequency of packet collision increases. However, the detailed analysis of the packet transmission success ratio suffered from hidden terminals in fading environment has not been studied. This paper presents the method for analyzing the ratio by setting a mathematical model and considering three parameters: distances between nodes, propagation characteristic of radio wave and carrier sense level. The network simulations for the same model were conducted to validate the analysis. The obtained results show that the carrier sense level has significant impact on the packet transmission performance. The presented method can be widely used to analyze the CSMA/CA networks.

Vehicle to vehicle (V2V) communication is regarded as an essential means for enabling vehicle safety communications (VSC) to avoid traffic accidents. However, reliability of the communications severely suffers from wireless propagation on the road, such as deep fading due to multi-path and shadowing by surrounding buildings and vehicles. In this paper, a new vehicle-roadside-vehicle relay communication network employing multiple frequency bands is proposed to achieve more reliable V2V communications. By assigning lower frequencies such as 700 MHz for relay transmission, and higher frequencies of 5.8 GHz for direct communication, remarkable improvement can be obtained. In order to confirm the performance of the proposed relay network, computer simulations using QualNet network simulator assuming 200 vehicles at an intersection were conducted. The simulation results show that the packet delivery rate is remarkably improved by the proposed relay network.

Multi-hop delay and packet error due to packet collision are serious issues for realizing large-scale wireless ad hoc networks. A cluster-tree network can reduce multi-hop delay by limiting hop counts. However, the network suffers from bottleneck at a network sink node and it limits data throughput. Also, hidden terminals bring packet collision issues when using CSMA/CA. In this paper, spread spectrum multiple access (SSMA) employing multicode reception and CSMA/CA are combined as a new MAC protocol for solving the issues. A carrier sense method after despreading and a double-spreading code assignment method for the network are also proposed for improving the performance. Throughput and data delivery ratio for the proposed method are analyzed via simulations based on the IEEE 802.15.4/ZigBee. Simulation results show that the throughput is remarkably improved and the bottleneck at a network coordinator is mitigated with the proposed method.

In wireless ad hoc networks, the link SNR varies due to fading and this causes a temporary link error and blocks the multi-hop packet transmission. The cognitive temporary bypassing method was proposed to improve the transmission quality. In this paper, the proposed protocol is improved by adding acknowledgement and anti-collision functions necessary to achieve reliable multi-hop transmission. Packet success probability with the improved method has been analyzed for Rayleigh faded wireless links as functions of hop count. Simulation results using a modified QualNet simulator validate that packet success probability is remarkably improved with the method for maximum Doppler frequencies up to 30Hz.

With the rapidly increasing demand for providing highspeed mobile multimedia and supporting diversified requirements of users, wireless quality of service (QoS) resource management becomes more and more indispensable, especially for the upcoming IMT-2000 system and beyond. In this paper, a wireless QoS architecture and two QoS resource allocation algorithms for CDMA cellular systems are proposed to meet these requirements. Based on the proposed QoS architecture, an adaptive QoS resource management is proposed to assign radio resource corresponding to its availability and the users' maximum and minimum QoS requirements. A service-rank-based QoS resource management is further proposed to satisfy more high-rank users as much as possible while maintaining the minimum requirement of low-rank users. Computer simulations indicate that the proposed QoS resource management algorithms exhibit superior performance with respect to dropping probability, rate of satisfied users and necessary transmission time, which indicate higher satisfaction rate of users and better performance.

The growth in mobile multimedia services will accelerate over the next decade. This paper discusses service aspects and system requirements for the fourth generation mobile communication (4G) system that will allow us to provide higher-speed communications and deal with user demand in the next decade. This paper also proposes a new radio access network (RAN) structure suited to the 4G system. The principles of the proposed cluster-cellular RAN are localized handover processing and horizontal structure. The advantages of the proposed RAN are also discussed.

Multi-hop packet transmission error due to packet collision is a serious issue for realizing large-scale wireless ad hoc networks. In order to solve the issues, a multicode-reception spread-spectrum (MCR-SS)-CSMA/CA scheme is proposed as multiple access scheme for layered-tree networks. This scheme is further improved by adding a time group access (TGA) technique to gain higher interference reduction between nodes. The proposed TGA scheme divides the layered-tree network into upper subnets group and lower subnets group, thus reducing collision probability among subnets. Network throughput and data delivery ratio for the proposed scheme are analyzed by simulations based on the IEEE 802.15.4/ZigBee specification. Simulation results show that the proposed schemes can improve packet delivery ratio and network throughput remarkably.