Robust Vehicular Communications for Traffic Safety---Channel Estimation and Multiantenna Schemes

Vehicular communications, where vehicles exchange information with other vehicles or entities in the road traffic environment, is expected to be a part of the future transportation system and promises to support a plethora of applications for traffic safety and efficiency. In particular, vehicle-to-vehicle (V2V) communication promises to support numerous traffic safety applications by enabling a vehicle to broadcast its current status to all the other vehicles in its surrounding.\ua0 \ua0 Vehicular wireless channels can be highly time- and/or frequency-selective due to high mobility of the vehicles and/or large delay spreads. IEEE 802.11p has been specified as the physical layer standard for vehicular communications, where the pilots are densely concentrated at the beginning of a frame. As a consequence, accurate channel estimation in later parts of the frame becomes a challenging task. In this thesis, a solution to overcome the ill-suited pilot pattern is studied; a cross-layered scheme to insert complementary pilots into an 802.11p frame is proposed. The scheme does not require modifications to the 802.11p standard and a modified receiver can utilize the complementary pilots for accurate channel estimation in vehicular channels.\ua0 \ua0 The metallic components of present-day vehicles pose a challenge in designing antenna systems that satisfy a minimum required directive gain in the entire horizontal plane. Multiple antennas with contrasting directive gain patterns can be used to alleviate the problems due to low directive gains. A scheme that combines the output of L antennas to the input of a single-port receiver is proposed in the thesis. The combining scheme is designed to minimize the probability of a burst error, i.e., an unsuccessful decoding of K consecutive packets from a transmitter arriving in the direction of low directive gains of the individual antennas. To minimize complexity, the scheme does not estimate or use any channel state information. It is shown using measured and simulated directive gain patterns that the probability of burst errors for packets arriving in the direction of low directive gains of the individual antenna elements can be minimized.\ua0 \ua0 The enhanced distributed channel access (EDCA) scheme is used in V2V communications to facilitate the sharing of allocated time-frequency resources. The packet success ratio (PSR) of the broadcast messages in the EDCA scheme depends on the number of vehicles and the packet transmission rate. The interference at a receiving vehicle increases due to multiple simultaneous transmissions when the number of vehicles grows beyond a limit, resulting in the decrease of the PSR. A receiver setup with sector antennas, where the output of each antenna can be processed separately to decode a packet, is described in the thesis with a detailed performance analysis. A significant increase in the PSR is shown in a dense vehicular scenario by using four partially overlapping sector antennas compared with a single omnidirectional antenna setup

Wi-Fi의 신뢰성 및 에너지 효율성 향상을 위한 MAC/PHY 기법

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2015. 8. 최성현.Over the last quarter century, wireless local area network (WLAN) technology has become an essential and indispensable part of our daily lives. Recently, a tremendously growing number of portable devices, such as smartphones, tablets and laptops, are being equipped with Wi-Fi, the hallmark of the IEEE 802.11 WLAN, in order to meet ever-increasing traffic demands at extremely low cost. Encouraged by this remarkable success, Wi-Fi is facing two trends. First, the state-of-the-art IEEE 802.11 specifications, e.g., IEEE 802.11n and 802.11ac, have focused on improving physical layer (PHY) rate by enabling multiple antennas, called multiple-input multiple-output (MIMO), and bandwidth widening, known as chan- nel bonding [1, 2]. Second, to achieve high throughput and transmission efficiency at medium access control (MAC) layer, IEEE 802.11n/ac standards have defined two types of frame aggregation techniques: MAC service data unit (MSDU) aggregation and MAC protocol data unit (MPDU) aggregation which amortize PHY/MAC proto- col overhead e.g., binary random backoff, physical layer convergence protocol (PLCP) preamble, and acknowledgement (ACK), over multiple frames by packing several MS- DUs and MPDUs into a single aggregate MSDU (A-MSDU) and aggregate MPDU (A-MPDU), respectively. Apparently, there is no doubt that these state-of-the-art features meet and fulfill i the requirements of Wi-Fi equipped device users by offering several hundred-fold in- creases in PHY rate and ubiquitous access. However, with respect to the demands of the battery-powered portable device users, the belief is easily broken from two per- spectives. First, since using higher PHY rate and longer A-MPDU are more vulnerable to channel errors especially for mobile users, the significant throughput performance degradation can be observed. Second, the emerging Wi-Fi chipsets, based on IEEE 802.11n/ac, consume much more energy than its legacy IEEE 802.11a/b/g counterparts due to the usage of MIMO and channel bonding. Nowadays, as the battery-powered portable device users place increasingly complex demands on the functionality of their devices which have strict power limitation, satisfaction with robust communication and battery life time is becoming increasingly important. In this dissertation, to address these challenges, we propose robust and energy efficient MAC/PHY layer strategies of Wi-Fi. First of all, to confirm those changes, we have conducted extensive experiments using state-of-the-art commercial IEEE 802.11n/ac-equipped devices and Microsofts Software Radio (Sora) platform. Our experiment results have revealed strong evidence that the use of long A-MPDU frames seriously deteriorates the Wi-Fi performance, i.e., throughput, especially for the pedestrian mobile users. Besides, we have found that the use of channel bonding remarkably consumes more energy, thus making Wi- Fi a primary energy consumer in the battery-powered portable devices. Especially, the energy cost is dominated by excessive and unnecessary listening and receiving operations. We begin an intra-frame rate control algorithm (Intra-RCA) design, called SNR- aware Intra-frame Rate Adaption (SIRA), which enhances the system performance of Wi-Fi in fast time-varying environments [3]. Widely used inter-frame rate control algorithms (Inter-RCAs), which select the PHY rate of each frame based on the time- ii averaged frame loss rate and the signal strength statistics, perform poorly for a long A-MPDU due to the channel variation in mobile environments. Unlike the previous ap- proaches, SIRA adapts the PHY rate on intra-frame basis, i.e., the PHY rate is updated in the middle of a frame according to user mobility. The performance of the proposed scheme is also evaluated by a trace-driven link level simulator employing the collected channel traces from real measurements. The simulation results show that SIRA outper- forms a standalone Inter-RCA in all tested traces. Despite its enhanced performance and considerable frame error reduction, the performance degradation caused by the impact of user mobility still remains due to the inherent limitation of IEEE 802.11 PHY design. Therefore, we conclude that this challenge should be solved with the assistance of PHY modification, and propose Channel-Aware Symbol Error Reduction (ChASER), a new practical channel estimation and tracking scheme for Wi-Fi receivers [4]. ChASER utilizes re-encoding and re-modulation of the received data symbol to keep up with the wireless channel dynamics at the granularity of orthogonal frequency division multi- plexing (OFDM) symbols. In addition, its low-complexity and feasibility of standard compliance is demonstrated by Microsofts Sora prototype implementation and experi- mentation. To our knowledge, ChASER is the first IEEE 802.11n-compatible channel tracking algorithm since other approaches addressing the time-varying channel con- ditions over a single (aggregated) frame duration require costly modifications of the IEEE 802.11 standard. Even though the above proposed approaches enhance the Wi- Fis throughput performance and robustness over conventional technique, the rest re- quirement of the portable device user, i.e., energy efficient Wi-Fi system design, should be addressed as ever. Accordingly, we propose a new power save operation as well as the corresponding protocol, called WiFi in Zizz (WiZizz), which judiciously exploits the characteristic iii of the channel bonding defined in IEEE 802.11ac and efficiently handles the channel bandwidth in an on-demand manner to minimize the traumatic energy spent by IEEE 802.11ac devices [5]. Our extensive measurement and simulation show significant per- formance improvement (as high as 73% energy saving) over a wide range of commu- nication scenarios. In addition, the feasibility of easy implementation is demonstrated by a prototype with a commercial 802.11ac device. To the best of our knowledge, WiZizz is the first IEEE 802.11ac-congenial energy efficient bandwidth management while other existing approaches require costly modifications of the IEEE 802.11ac specification. In summary, we propose a number of compelling algorithms and protocols to im- prove the robustness and energy efficiency in accordance with the paradigm shift of Wi-Fi. Moreover, our evaluation results show that the proposed schemes in this disser- tation are effective and yield considerable performance gain based on both the trace- driven link level simulation and the network level simulation which well reflects the wireless channel characteristics of the real world and the operation of IEEE 802.11 WLAN, respectively. We demonstrate the feasibility of our approaches by implement- ing prototypes in off-the-self IEEE 802.11n/ac devices and software-defined radio.Abstract Contents List of Tables List of Figures 1 Introduction 1.1 Paradigm Shift of Wi-Fi 1.2 DevelopmentTrend of IEEE802.11 1.2.1 MAC Features 1.2.2 PHY Features 1.3 Overview of Existing Approaches 1.3.1 Rate Adaptation 1.3.2 MPDU Length Optimization 1.3.3 Channel Estimation 1.3.4 Demystifying Wi-Fi Power Consumption 1.3.5 Minimizing Energy Consumption of Wi-Fi 1.4 Main Contributions 1.4.1 Impact of Mobility Analysis 1.4.2 Intra-frame Rate Adaptation 1.4.3 Enhanced Channel Estimation and Tracking 1.4.4 802.11ac Power Consumption Analysis 1.4.5 Energy Efficient Bandwidth Management 1.5 Organization of the Dissertation 2 Impact of Mobility 2.1 Introduction 2.2 Background 2.2.1 Channel Estimation and Compensation 2.2.2 Role of Pilot Subcarriers 2.2.3 Frame Aggregation 2.3 Measurement Study 2.3.1 Experimental Settings 2.3.2 Temporal Selectivity 2.3.3 Unreliability of A-MPDU in Mobile Environments 2.3.4 Relation between Symbol Dispersion and Mobility 2.4 Summary 3 SIRA: SNR-aware Intra-frame Rate Adaptation 3.1 Introduction 3.1.1 Revisit of Rate Adaptation Algorithms 3.1.2 Channel and Mobility Condition 3.2 ProposedAlgorithm 3.2.1 Pilot-based SNR Estimation 3.2.2 Mobility Detection 3.2.3 Unequal Modulation and Coding Scheme 3.2.4 Zero-overhead Feedback 3.2.5 SIRA Structure 3.3 Simulation 3.3.1 Trace-drivenLinkLevelSimulation 3.3.2 Mobility Detection Accuracy 3.3.3 Performance Comparison 3.4 Summary 4 ChASER: Channel-Aware Symbol Error Reduction 4.1 Introduction 4.2 Revisit of Channel Estimation Algorithms 4.3 ChASER Design 4.3.1 Channel Estimation using Unknown Data Symbols 4.3.2 Adaptive Filter 4.3.3 Adaptive Filter for MIMO 4.3.4 CRC-assisted Channel Correction 4.3.5 Summary of ChASER Operation 4.3.6 Impact of Step Size μ 4.4 Testbed Experiments 4.4.1 Prototype Implementation on SDR Platform 4.4.2 Testbed Settings 4.4.3 Performance Comparison 4.5 Simulation 4.5.1 Simulation Methodology 4.5.2 Estimation Accuracy 4.5.3 Impact of the A-MPDU Duration 4.5.4 Throughput Performance 4.6 Summary 5 Power Consumption of Wi-Fi: Modeling and Testbed Validation 5.1 Introduction 5.2 Revisit of IEEE 802.11ac Features 5.2.1 Wider Bandwidth Channel 5.2.2 MIMO and Higher Order Modulation 5.2.3 Relation between 802.11ac Features and Power 5.3 Modeling 802.11ac Power Consumption 5.3.1 Power Model of IEEE 802.11ac Receiver 5.3.2 Power Model of IEEE 802.11ac Transmitter 5.4 Power Consumption Measurement 5.4.1 Experimental Setting 5.4.2 Idle State Power Consumption 5.4.3 Receive State Power Consumption 5.4.4 Transmit State Power Consumption 5.4.5 PowerModelVerification 5.4.6 Summary 6 WiZizz: Energy Efficient Bandwidth Management 6.1 Introduction 6.2 WiFi Need to Zizz 6.3 WiZizz Design 6.3.1 Dynamic Mode 6.3.2 Pseudo-dynamic Mode 6.3.3 PHY-level Filtering 6.4 Testbed Experiments 6.4.1 Prototype Implementation and Testbed Setting 6.4.2 Bandwidth Switching Delay 6.4.3 Performance Evaluation 6.5 SimulationResults 6.5.1 Simulation Methodology 6.5.2 Constant Traffic Source with FixedMCS 6.5.3 Comprehensive Traffic Patterns 6.5.4 Collaboration with SMPS 6.6 Summary 7 Conclusion and Future Work 7.1 Research Contributions 7.2 Further Research Plans Abstract (In Korean) 감사의 글Docto

Simulated Assessment of Interference Effects in Direct Sequence SpreadSpectrum (DSSS) QPSK Receiver

This research developed and validated a generic simulation for a direct sequence spread spectrum (DSSS), using differential phase shift keying (DPSK) and phase shift keying (PSK) modulations, providing the flexibility for assessing intentional interference effect using DSSS quadrature phase shift keying receiver (QPSK) with matched filtering as a reference. The evaluation compares a comprehensive pool of jamming waveforms at pass-band that include continuous wave (CW) interference, broad-band jamming, partial-band interference and pulsed interference. The methodology for jamming assessment included comparing the bit error rate (BER) versus required jamming to signal ratio (JSR) for different interferers using the Monte Carlo approach. This thesis also analyzes the effect of varying the jammer bandwidth for broad-band jammers including broad-band noise (BBN), frequency hopping interference (FHI), comb- spectrum interference (CSI), multi-tone jamming (MTJ), random frequency modulated interference (RFMI) and linear frequency modulated interference (LFMI). Also, the effect of changing the duty cycle for pulsed CW waveforms is compared with the worst case pulsed jamming equation. After the evaluation of different interferers, the research concludes that pulsed binary phase shift keying (BPSK) jamming is the most effective technique, whereas the CW tone jamming and CW BPSK interference result are least effective. It is also concluded that by finding an optimum bandwidth, FHI and BBN improves the required JSR by approximately 2.1 dB, RFMI and LFMI interference by 0.9 and 1.5 dB respectively. Alternately, MTJ and CSI improves their effectiveness in 4.1 dB and 3.6 dB respectively, matching the performance of the pulsed BPSK jammer

Cooperative Radio Communications for Green Smart Environments

The demand for mobile connectivity is continuously increasing, and by 2020 Mobile and Wireless Communications will serve not only very dense populations of mobile phones and nomadic computers, but also the expected multiplicity of devices and sensors located in machines, vehicles, health systems and city infrastructures. Future Mobile Networks are then faced with many new scenarios and use cases, which will load the networks with different data traffic patterns, in new or shared spectrum bands, creating new specific requirements. This book addresses both the techniques to model, analyse and optimise the radio links and transmission systems in such scenarios, together with the most advanced radio access, resource management and mobile networking technologies. This text summarises the work performed by more than 500 researchers from more than 120 institutions in Europe, America and Asia, from both academia and industries, within the framework of the COST IC1004 Action on "Cooperative Radio Communications for Green and Smart Environments". The book will have appeal to graduates and researchers in the Radio Communications area, and also to engineers working in the Wireless industry. Topics discussed in this book include: • Radio waves propagation phenomena in diverse urban, indoor, vehicular and body environments• Measurements, characterization, and modelling of radio channels beyond 4G networks• Key issues in Vehicle (V2X) communication• Wireless Body Area Networks, including specific Radio Channel Models for WBANs• Energy efficiency and resource management enhancements in Radio Access Networks• Definitions and models for the virtualised and cloud RAN architectures• Advances on feasible indoor localization and tracking techniques• Recent findings and innovations in antenna systems for communications• Physical Layer Network Coding for next generation wireless systems• Methods and techniques for MIMO Over the Air (OTA) testin

Advanced Applications of Rapid Prototyping Technology in Modern Engineering

Rapid prototyping (RP) technology has been widely known and appreciated due to its flexible and customized manufacturing capabilities. The widely studied RP techniques include stereolithography apparatus (SLA), selective laser sintering (SLS), three-dimensional printing (3DP), fused deposition modeling (FDM), 3D plotting, solid ground curing (SGC), multiphase jet solidification (MJS), laminated object manufacturing (LOM). Different techniques are associated with different materials and/or processing principles and thus are devoted to specific applications. RP technology has no longer been only for prototype building rather has been extended for real industrial manufacturing solutions. Today, the RP technology has contributed to almost all engineering areas that include mechanical, materials, industrial, aerospace, electrical and most recently biomedical engineering. This book aims to present the advanced development of RP technologies in various engineering areas as the solutions to the real world engineering problems

Cooperative Radio Communications for Green Smart Environments

The demand for mobile connectivity is continuously increasing, and by 2020 Mobile and Wireless Communications will serve not only very dense populations of mobile phones and nomadic computers, but also the expected multiplicity of devices and sensors located in machines, vehicles, health systems and city infrastructures. Future Mobile Networks are then faced with many new scenarios and use cases, which will load the networks with different data traffic patterns, in new or shared spectrum bands, creating new specific requirements. This book addresses both the techniques to model, analyse and optimise the radio links and transmission systems in such scenarios, together with the most advanced radio access, resource management and mobile networking technologies. This text summarises the work performed by more than 500 researchers from more than 120 institutions in Europe, America and Asia, from both academia and industries, within the framework of the COST IC1004 Action on "Cooperative Radio Communications for Green and Smart Environments". The book will have appeal to graduates and researchers in the Radio Communications area, and also to engineers working in the Wireless industry. Topics discussed in this book include: • Radio waves propagation phenomena in diverse urban, indoor, vehicular and body environments• Measurements, characterization, and modelling of radio channels beyond 4G networks• Key issues in Vehicle (V2X) communication• Wireless Body Area Networks, including specific Radio Channel Models for WBANs• Energy efficiency and resource management enhancements in Radio Access Networks• Definitions and models for the virtualised and cloud RAN architectures• Advances on feasible indoor localization and tracking techniques• Recent findings and innovations in antenna systems for communications• Physical Layer Network Coding for next generation wireless systems• Methods and techniques for MIMO Over the Air (OTA) testin

Applications of Non-Orthogonal Waveforms and Artificial Neural Networks in Wireless Vehicular Communications

Ph. D. ThesisWe live in an ever increasing world of connectivity. The need for highly robust, highly efficient wireless communication has never been greater. As we seek to squeeze better and better performance from our systems, we must remember; even though our computing devices are increasing in power and efficiency, our wireless spectrum remains limited. Recently there has been an increasing trend towards the implementation of machine learning based systems in wireless communications. By taking advantage of a neural networks powerful non-linear computational capability, communication systems have been shown to achieve reliable error free transmission over even the most dispersive of channels. Furthermore, in an attempt to make better use of the available spectrum, more spectrally efficient physical layer waveforms are gathering attention that trade increased interference for lower bandwidth requirements. In this thesis, the performance of neural networks that utilise spectrally efficient waveforms within harsh transmission environments are assessed. Firstly, we investigate and generate a novel neural network for use within a standards compliant vehicular network for vehicle-to-vehicle communication, and assess its performance practically in several of the harshest recorded empirical channel models using a hardware-in-the-loop testing methodology. The results demonstrate the strength of the proposed receiver, achieving a bit-error rate below 10−3 at a signal-to-noise ratio (SNR) of 6dB. Secondly, this is then further extended to utilise spectrally efficient frequency division multiplexing (SEFDM), where we note a break away from the 802.11p vehicular communication standard in exchange for a more efficient use of the available spectrum that can then be utilised to service more users or achieve a higher data throughput. It is demonstrated that the proposed neural network system is able to act as a joint channel equaliser and symbol receiver with bandwidth compression of up to 60% when compared to orthogonal frequency division multiplexing (OFDM). The effect of overfitting to the training environment is also tested, and the proposed system is shown to generalise well to unseen vehicular environments with no notable impact on the bit-error rate performance. Thirdly, methods for generating inputs and outputs of neural networks from complex constellation points are investigated, and it is reasoned that creating ‘split complex’ neural networks should not be preferred over ‘contatenated complex’ neural networks in most settings. A new and novel loss function, namely error vector magnitude (EVM) loss, is then created for the purposes of training neural networks in a communications setting that tightly couples the objective function of a neural network during training to the performance metrics of transmission when deployed practically. This loss function is used to train neural networks in complex environments and is then compared to popular methods from the literature where it is demonstrated that EVM loss translates better into practical applications. It achieved the lowest EVM error, thus bit-error rate, across all experiments by a margin of 3dB when compared to its closest achieving alternative. The results continue and show how in the experiment EVM loss was able to improve spectral efficiency by 67% over the baseline without affecting performance. Finally, neural networks combined with the new EVM loss function are further tested in wider communication settings such as visible light communication (VLC) to validate the efficacy and flexibility of the proposed system. The results show that neural networks are capable of overcoming significant challenges in wireless environments, and when paired with efficient physical layer waveforms like SEFDM and an appropriate loss function such as EVM loss are able to make good use of a congested spectrum. The authors demonstrated for the first time in practical experimentation with SEFDM that spectral efficiency gains of up to 50% are achievable, and that previous SEFDM limitations from the literature with regards to number of subcarriers and size of the transmit constellation are alleviated via the use of neural networksEPSRC, Newcastle Universit