Browsing by Author "Rao, Raghunandan M."

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    "Adaptive Pilot Patterns for CA-OFDM Systems in Non-stationary Wireless Channels"
    Rao, Raghunandan M.; Marojevic, Vuk; Reed, Jeffrey H. (IEEE, 2017-09-12)
    In this paper, we investigate the performance gains of adapting pilot spacing and power for Carrier Aggregation (CA)-OFDM systems in nonstationary wireless channels. In current multi-band CAOFDM wireless networks, all component carriers use the same pilot density, which is designed for poor channel environments. This leads to unnecessary pilot overhead in good channel conditions and performance degradation in the worst channel conditions. We propose adaptation of pilot spacing and power using a codebook-based approach, where the transmitter and receiver exchange information about the fading characteristics of the channel over a short period of time, which are stored as entries in a channel profile codebook. We present a heuristic algorithm that maximizes the achievable rate by finding the optimal pilot spacing and power, from a set of candidate pilot configurations. We also analyze the computational complexity of our proposed algorithm and the feedback overhead. We describe methods to minimize the computation and feedback requirements for our algorithm in multi-band CA scenarios and present simulation results in typical terrestrial and air-to ground/ air-to-air nonstationary channels. Our results show that significant performance gains can be achieved when adopting adaptive pilot spacing and power allocation in nonstationary channels. We also discuss important practical considerations and provide guidelines to implement adaptive pilot spacing in CAOFDM systems.
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    Enhancing Performance of Next-Generation Vehicular and Spectrum Sharing Wireless Networks: Practical Algorithms and Fundamental Limits
    Rao, Raghunandan M. (Virginia Tech, 2020-08-20)
    Over the last few decades, wireless networks have morphed from traditional cellular/wireless local area networks (WLAN), into a wide range of applications, such as the Internet-of-Things (IoT), vehicular-to-everything (V2X), and smart grid communication networks. This transition has been facilitated by research and development efforts in academia and industry, which has resulted in the standardization of fifth-generation (5G) wireless networks. To meet the performance requirements of these diverse use-cases, 5G networks demand higher performance in terms of data rate, latency, security, and reliability, etc. At the physical layer, these performance enhancements are achieved by (a) optimizing spectrum utilization shared amongst multiple technologies (termed as spectrum sharing), and (b) leveraging advanced spatial signal processing techniques using large antenna arrays (termed as massive MIMO). In this dissertation, we focus on enhancing the performance of next-generation vehicular communication and spectrum sharing systems. In the first contribution, we present a novel pilot configuration design and adaptation mechanism for cellular vehicular-to-everything (C-V2X) networks. Drawing inspiration from 4G and 5G standards, the proposed approach is based on limited feedback of indices from a codebook comprised of quantized channel statistics information. We demonstrate significant rate improvements using our proposed approach in terrestrial and air-to-ground (A2G) vehicular channels. In the second contribution, we demonstrate the occurrence of cellular link adaptation failure due to channel state information (CSI) contamination, because of coexisting pulsed radar signals that act as non-pilot interference. To mitigate this problem, we propose a low-complexity semi-blind SINR estimation scheme that is robust and accurate in a wide range of interference and noise conditions. We also propose a novel dual CSI feedback mechanism for cellular systems and demonstrate significant improvements in throughput, block error rate, and latency, when sharing spectrum with a pulsed radar. In the third contribution, we develop fundamental insights on underlay radar-massive MIMO spectrum sharing, using mathematical tools from stochastic geometry. We consider a multi-antenna radar system, sharing spectrum with a network of massive MIMO base stations distributed as a homogeneous Poisson Point Process (PPP) outside a circular exclusion zone centered around the radar. We propose a tractable analytical framework, and characterize the impact of worst-case downlink cellular interference on radar performance, as a function of key system parameters. The analytical formulation enables network designers to systematically isolate and evaluate the impact of each parameter on the worst-case radar performance and complements industry-standard simulation methodologies by establishing a baseline performance for each set of system parameters, for current and future radar-cellular spectrum sharing deployments. Finally, we highlight directions for future work to advance the research presented in this dissertation and discuss its broader impacts across the wireless industry, and policy-making.
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    LTE PHY Layer Vulnerability Analysis and Testing Using Open-Source SDR Tools
    Rao, Raghunandan M.; Ha, Sean; Marojevic, Vuk; Reed, Jeffrey H. (2017-09-10)
    This paper provides a methodology to study the PHY layer vulnerability of wireless protocols in hostile radio environments. Our approach is based on testing the vulnerabilities of a system by analyzing the individual subsystems. By targeting an individual subsystem or a combination of subsystems at a time, we can infer the weakest part and revise it to improve the overall system performance. We apply our methodology to 4G LTE downlink by considering each control channel as a subsystem. We also develop open-source software enabling research and education using software-defined radios. We present experimental results with open-source LTE systems and shows how the different subsystems behave under targeted interference. The analysis for the LTE downlink shows that the synchronization signals (PSS/SSS) are very resilient to interference, whereas the downlink pilots or Cell-Specific Reference signals (CRS) are the most susceptible to a synchronized protocol-aware interferer. We also analyze the severity of control channel attacks for different LTE configurations. Our methodology and tools allow rapid evaluation of the PHY layer reliability in harsh signaling environments, which is an asset to improve current standards and develop new robust wireless protocols.
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    Performance Analysis of a Mission-Critical Portable LTE System in Targeted RF Interference
    Marojevic, Vuk; Rao, Raghunandan M.; Ha, Sean; Reed, Jeffrey H. (2017-09)
    Mission-critical wireless networks are being up-graded to 4G long-term evolution (LTE). As opposed to capacity, these networks require very high reliability and security as well as easy deployment and operation in the field. Wireless communication systems have been vulnerable to jamming, spoofing and other radio frequency attacks since the early days of analog systems. Although wireless systems have evolved, important security and reliability concerns still exist. This paper presents our methodology and results for testing 4G LTE operating in harsh signaling environments. We use software-defined radio technology and open-source software to develop a fully configurable protocol-aware interference waveform. We define several test cases that target the entire LTE signal or part of it to evaluate the performance of a mission-critical production LTE system. Our experimental results show that synchronization signal interference in LTE causes significant throughput degradation at low interference power. By dynamically evaluating the performance measurement counters, the k- nearest neighbor classification method can detect the specific RF signaling attack to aid in effective mitigation.
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    Perspectives of Jamming, Mitigation and Pattern Adaptation of OFDM Pilot Signals for the Evolution of Wireless Networks
    Rao, Raghunandan M. (Virginia Tech, 2016-09-15)
    Wireless communication networks have evolved continuously over the last four decades in order to meet the traffic and security requirements due to the ever-increasing amount of traffic. However this increase is projected to be massive for the fifth generation of wireless networks (5G), with a targeted capacity enhancement of 1000× w.r.t. 4G networks. This enhanced capacity is possible by a combination of major approaches (a) overhaul of some parts and (b) elimination of overhead and redundancies of the current 4G. In this work we focus on OFDM reference signal or pilot tones, which are used for channel estimation, link adaptation and other crucial functions in Long-Term Evolution (LTE). We investigate two aspects of pilot signals pertaining to its evolution - (a) impact of targeted interference on pilots and its mitigation and (b) adaptation of pilot patterns to match the channel conditions of the user. We develop theoretical models that accurately quantify the performance degradation at the user’s receiver in the presence of a multi-tone pilot jammer. We develop and evaluate mitigation algorithms to mitigate power constrained multi-tone pilot jammers in SISO- and full rank spatial multiplexing MIMO-OFDM systems. Our results show that the channel estimation performance can be restored even in the presence of a strong pilot jammer. We also show that full rank spatial multiplexing in the presence of a synchronized pilot jammer (transmitting on pilot locations only) is possible when the channel is flat between two pilot locations in either time or frequency. We also present experimental results of multi-tone broadcast pilot jamming (Jamming of Cell Specific Reference Signal) in the LTE downlink. Our results show that full-band jamming of pilots needs 5 dB less power than jamming the entire downlink signal, in order to cause Denial of Service (DoS) to the users. In addition to this, we have identified and demonstrated a previously unreported issue with LTE termed ‘Channel Quality Indicator (CQI) Spoofing’. In this scenario, the attacker tricks the user terminal into thinking that the channel quality is good, by transmitting interference transmission only on the data locations, while deliberately avoiding the pilots. This jamming strategy leverages the dependence of the adaptive modulation and coding (AMC) schemes on the CQI estimate in LTE. Lastly, we investigate the idea of pilot pattern adaptation for SISO- and spatial multiplexing MIMO-OFDM systems. We present a generic heuristic algorithm to predict the optimal pilot spacing and power in a nonstationary doubly selective channel (channel fading in both time and frequency). The algorithm fits estimated channel statistics to stored codebook channel profiles and uses it to maximize the upper bound on the constrained capacity. We demonstrate up to a 30% improvement in ergodic capacity using our algorithm and describe ways to minimize feedback requirements while adapting pilot patterns in multi-band carrier aggregation systems. We conclude this work by identifying scenarios where pilot adaptation can be implemented in current wireless networks and provide some guidelines to adapt pilots for 5G.