Seongah Jeong

Beamforming Design for Joint Localization and Data Transmission in Distributed Antenna System

A distributed antenna system whose goal is to provide data communication and positioning functionalities to mobile stations (MSs) is studied. Each MS receives data from a number of base stations (BSs) and uses the received signal not only to extract the information but to determine its location as well. This is done based on time-of-arrival or time-difference-of-arrival measurements, depending on the assumed synchronization conditions. The problem of minimizing the overall power expenditure of the BSs under data throughput and localization accuracy requirements is formulated with respect to the beamforming vectors used at the BSs. The analysis covers both frequency-flat and frequency-selective channels and accounts for robustness constraints in the presence of parameter uncertainty as well. The proposed algorithmic solutions are based on rank-relaxation and difference-of-convex programming.

Mobile Edge Computing via a UAV-Mounted Cloudlet: Optimization of Bit Allocation and Path Planning

Unmanned aerial vehicles (UAVs) have been recently considered as means to provide enhanced coverage or relaying services to mobile users (MUs) in wireless systems with limited or no infrastructure. In this paper, a UAV-based mobile cloud computing system is studied in which a moving UAV is endowed with computing capabilities to offer computation offloading opportunities to MUs with limited local processing capabilities. The system aims at minimizing the total mobile energy consumption while satisfying quality of service requirements of the offloaded mobile application. Offloading is enabled by uplink and downlink communications between the mobile devices and the UAV, which take place by means of frequency division duplex via orthogonal or nonorthogonal multiple access schemes. The problem of jointly optimizing the bit allocation for uplink and downlink communications as well as for computing at the UAV, along with the cloudlets trajectory under latency and UAVs energy budget constraints is formulated and addressed by leveraging successive convex approximation strategies. Numerical results demonstrate the significant energy savings that can be accrued by means of the proposed joint optimization of bit allocation and cloudlets trajectory as compared to local mobile execution as well as to partial optimization approaches that design only the bit allocation or the cloudlets trajectory.

Cooperative jammer design in cellular network with internal eavesdroppers

In this paper, we consider a cooperative jammer to improve secrecy of the wireless transmission in a cellular downlink network. The private message intended for a single user should be kept from the remainder of the users who are regarded as internal eavesdroppers. To improve the secrecy of the intended user, we propose an employment of a helper with multiple antennas and design its optimal transmit beamforming vector. Specifically, the helper node generates the artificial interference to the internal eavesdroppers and so enhances the security by increasing the ambiguity at the eavesdroppers. Based on a framework of power gain region in [1], we optimize the transmission strategy for the helper which maximizes the secrecy capacity of the intended user. The analytical and simulation results show that the proposed scheme enhances the secrecy capacity. In addition, all users whose secrecy rates are zero with no helpers cooperation can achieve the positive secrecy rate by the proposed scheme.

Optimal Fronthaul Quantization for Cloud Radio Positioning

Wireless positioning systems that are implemented by means of a cloud radio access network (C-RAN) may provide cost-effective solutions, particularly for indoor localization. In a C-RAN, baseband processing, including localization, is carried out at a centralized control unit (CU) based on quantized baseband signals received from the radio units (RUs) over finite-capacity fronthaul links. In this paper, the problem of maximizing the localization accuracy over fronthaul quantization/compression is formulated by adopting the Cramér-Rao bound (CRB) on the localization accuracy as the performance metric of interest and information-theoretic bounds on the compression rate. The analysis explicitly accounts for the uncertainty of parameters at the CU via a robust, or worst-case, optimization formulation. The proposed algorithm leverages the Charnes-Cooper transformation and difference-of-convex (DC) programming and is validated via numerical results.

Secure beamforming and self-energy recycling with full-duplex wireless-powered relay

In this paper, we investigate a two-hop full-duplex wireless-powered relaying system consists of a source, a relay, and a destination in the presence of a passive eavesdropper. The relay assists the transmission of confidential information from the source to the destination, while simultaneously harvesting the energy with time switching scheme by the radio-frequency (RF) signals. Our goal is to maximize the physical-layer security under harvested energy constraints by designing the full-duplex wireless-powered relay, whose two relaying strategies are considered, namely amplify-and-forward (AF) and decode-and-forward (DF). The relay beamforming vector design is proposed for AF protocol, and is jointly optimized with the time ratio parameter in the case of DF protocol to maximize the physical-layer security under harvested energy constraints. Moreover, for the simultaneous energy and secure message transfer at the relay, a two-phase method is provided, which enables the relay to avoid the self-interference caused by full-duplex operation, and also to harvest the energy from the self-interference. The proposed algorithmic solutions leverage the rank relaxation, Majorization-Minimization (MM) programming, and line search method. Numerical results show that the proposed full-duplex relaying system outperforms the half-duplex relaying system in energy harvesting. Moreover, the trade-off between AF and DF protocols according to the occurrence probability of non-zero secrecy rate can be observed in terms of physical-layer security.

Secure Transmission in Downlink Cellular Network with a Cooperative Jammer

In this paper, a novel transmission scheme is proposed to improve the security of downlink cellular network. The confidential message intended to one of K mobile users (MUs) should be securely kept from the undesired recipients. In this work, the K-1 remaining users are regarded as potential eavesdroppers and called as internal eavesdroppers. For the security enhancement, we propose an adaptation of a single cooperative jammer (CJ) to increase the ambiguity at all malicious users by distracting them with artificial interference. With the help of CJ, we derive the optimal joint transmission scheme by beamforming solution to maximize the secrecy rate of the intended user. Numerical results show the performance improvement of the proposed scheme.

Beamforming design for joint localization and data transmission in distributed antenna systems

A distributed antenna system whose goal is to provide data communication and positioning functionalities to mobile stations (MSs) is studied. Each MS receives data from a number of base stations (BSs) and uses the received signal not only to extract the information but to determine its location as well. This is done based on time-of-arrival or time-difference-of-arrival measurements, depending on the assumed synchronization conditions. The problem of minimizing the overall power expenditure of the BSs under data throughput and localization accuracy requirements is formulated with respect to the beamforming vectors used at the BSs. The analysis covers both frequency-flat and frequency-selective channels and accounts for robustness constraints in the presence of parameter uncertainty as well. The proposed algorithmic solutions are based on rank-relaxation and difference-of-convex programming.

Joint TOA/AOA-based localization in wireless sensor networks

Wireless sensor network (WSN) localization has drawn attention in a broad spectrum of applications for security, disaster response, monitoring, surveillance and tactical system. The deployment of a large number of low-cost, low-power and multi-functional sensors is preferred, which provides an exceptional localization performance via cooperative communication. In this paper, we consider a cooperative localization in WSNs consisting of multiple anchor nodes (ANs) equipped with a linear antenna array of M elements, a single-antenna target and a central unit (CU). We focus on a synchronous network and all the channels are supposed to be frequency-flat. Our objective is to evaluate the performance of localization technique for the targets position that jointly exploits both Time-of-Arrival (TOA) and Angle-of-Arrival (AOA) measurements. We derive the Cramér-Rao bound (CRB) for a joint TOA/AOA-based localization. Additionally, for the performance comparison, the CRBs of solely TOA- and AOA-based localizations are also derived. By numerical results, our analysis is verified and it is observed that the joint TOA/AOA-based localization has a superior performance than either TOA- or AOA-based localization.

Transmit beamforming with imperfect CSIT in spectrum leasing for physical-layer security

In spectrum leasing, primary users (PUs) lease the part of their spectral resources to secondary users (SUs) in exchange for appropriate remuneration. In this paper, we consider spectrum leasing via cooperation for physical-layer security that the secondary cooperation exists for improving the primary secrecy rate while maintaining its quality of service (QoS). Unlike the previous researches with the perfect channel state information (CSI) at transmitter (CSIT) assumption, we study when no information regarding the eavesdropper is available at the transmitter. This imperfect CSIT makes the SUs transmission strategy limited. To find the optimal transmission technique for the SU, we formulate a problem appropriate for the imperfect CSIT case. By using the proposed problem, we design the optimal transmit beamforming for the SU. Also, we analyze the ergodic rate of the secondary link at high signal-to-noise ratio (SNR) when the secondary cooperation focuses only on maximizing the primary secrecy rate for the viable choice of the secondary QoS level. The numerical result shows that the primary secrecy rate by the proposed transmit beamforming is comparable to that based on the perfect CSIT.

Positioning via direct localisation in C-RAN systems

Cloud Radio Access Network (C-RAN) is a prominent architecture for 5G wireless cellular system that is based on the centralization of baseband processing for multiple distributed radio units (RUs) at a control unit (CU). In this work, it is proposed to leverage the C-RAN architecture to enable the implementation of direct localization of the position of mobile devices from the received signals at distributed RUs. With ideal connections between the CU and the RUs, direct localization is known to outperform traditional indirect localization, whereby the location of a source is estimated from intermediary parameters estimated at the RUs. However, in a C-RAN system with capacity limited fronthaul links, the advantage of direct localization may be offset by the distortion caused by the quantization of the received signal at the RUs. In this paper, the performance of direct localization is studied by accounting for the effect of fronthaul quantization with or without dithering. An approximate Maximum Likelihood (ML) localization is developed. Then, the Cramer-Rao Bound (CRB) on the squared position error (SPE) of direct localization with quantized observations is derived. Finally, the performance of indirect localization and direct localization with or without dithering is compared via numerical results.