Design and implementation of an uplink connection for a light-based IoT node

Abstract. In the wake of soaring demand for shrinking radio frequency (RF) spectrum, light-fidelity (LiFi) has been heralded as a solution to accommodate resources for future communication networks. Infrared (IR) and visible light communication (VLC) are meant to be used within LiFi because of numerous advantages. By combining the paradigm of internet of things (IoT) along with LiFi, light-based IoT (LIoT) emerges as a potential enabler of future 6G networks. With tremendous number of interconnected devices, LIoT nodes need to be able to receive and transmit data while being energy autonomous. One of the most promising clean energy sources comes from both natural and artificial light. In addition to providing illumination and energy, light can also be utilized as a robust information carrier. In order to provide bidirectional connectivity to LIoT node, both downlink and uplink have to be taken into consideration. Whereas downlink relies on visible light as a carrier, uplink approach can be engineered freely within specific requirements. With this in mind, this master’s thesis explores possible solutions for providing uplink connectivity. After analysis of possible solutions, the LIoT proof-of-concept was designed, implemented and validated. By incorporating printed solar cell, dedicated energy harvesting unit, power-optimised microcontroller unit (MCU) and light intensity sensor the LIoT node is able to autonomously transmit data using IR

Pointing-and-Acquisition for Optical Wireless in 6G: From Algorithms to Performance Evaluation

The increasing demand for wireless communication services has led to the development of non-terrestrial networks, which enables various air and space applications. Free-space optical (FSO) communication is considered one of the essential technologies capable of connecting terrestrial and non-terrestrial layers. In this article, we analyze considerations and challenges for FSO communications between gateways and aircraft from a pointing-and-acquisition perspective. Based on the analysis, we first develop a baseline method that utilizes conventional devices and mechanisms. Furthermore, we propose an algorithm that combines angle of arrival (AoA) estimation through supplementary radio frequency (RF) links and beam tracking using retroreflectors. Through extensive simulations, we demonstrate that the proposed method offers superior performance in terms of link acquisition and maintenance

Design, analysis and optimization of visible light communications based indoor access systems for mobile and internet of things applications

Demands for indoor broadband wireless access services are expected to outstrip the spectrum capacity in the near-term spectrum crunch . Deploying additional femtocells to address spectrum crunch is cost-inefficient due to the backhaul challenge and the exorbitant system maintenance. According to an Alcatel-Lucent report, most mobile Internet access traffic happens indoors. To alleviate the spectrum crunch and the backhaul challenge problems, visible light communication (VLC) emerges as an attractive candidate for indoor wireless access in the 5G architecture. In particular, VLC utilizes LED or fluorescent lamps to send out imperceptible flickering light that can be captured by a smart phone camera or photodetector. Leveraging power line communication and the available indoor infrastructure, VLC can be utilized with a small one-time cost. VLC also facilitates the great advantage of being able to jointly perform illumination and communications. Integration of VLC into the existing indoor wireless access networks embraces many challenges, such as lack of uplink infrastructure, excessive delay caused by blockage in heterogeneous networks, and overhead of power consumption. In addition, applying VLC to Internet-of-Things (IoT) applications, such as communication and localization, faces the challenges including ultra-low power requirement, limited modulation bandwidth, and heavy computation and sensing at the device end. In this dissertation, to overcome the challenges of VLC, a VLC enhanced WiFi system is designed by incorporating VLC downlink and WiFi uplink to connect mobile devices to the Internet. To further enhance robustness and throughput, WiFi and VLC are aggregated in parallel by leveraging the bonding technique in Linux operating system. Based on dynamic resource allocation, the delay performance of heterogeneous RF-VLC network is analyzed and evaluated for two different configurations - aggregation and non-aggregation. To mitigate the power consumption overhead of VLC, a problem of minimizing the total power consumption of a general multi-user VLC indoor network while satisfying users traffic demands and maintaining an acceptable level of illumination is formulated. The optimization problem is solved by the efficient column generation algorithm. With ultra-low power consumption, VLC backscatter harvests energy from indoor light sources and transmits optical signals by modulating the reflected light from a reflector. A novel pixelated VLC backscatter is proposed and prototyped to address the limited modulation bandwidth by enabling more advanced modulation scheme than the state-of-the-art on-off keying (OOK) scheme and allowing for the first time orthogonal multiple access. VLC-based indoor access system is also suitable for indoor localization due to its unique properties, such as utilization of existing ubiquitous lighting infrastructure, high location and orientation accuracy, and no interruption to RF-based devices. A novel retroreflector-based visible light localization system is proposed and prototyped to establish an almost zero-delay backward channel using a retroreflector to reflect light back to its source. This system can localize passive IoT devices without requiring computation and heavy sensing (e.g., camera) at the device end

Enhancement of Optics-Based Sensor Node Localization using Multiple Base Stations

학위논문 (석사)-- 서울대학교 대학원 : 기계항공공학부, 2013. 8. 박찬국.본 논문에서는 MEMS(Micro-Electro-Mechanical Systems) CCR(Corner Cube Retroreflector)을 이용한 광 신호 기반 무선 센서 네트워크의 센서 노드 위치 인식을 수행하기 위해 실제 환경에서 발생할 수 있는 문제점들을 분석하여 이를 극복하기 위한 다중 베이스 스테이션 시스템을 제안한다. 그 후 제안한 시스템을 이용한 위치 실험을 통하여 그 성능을 검증한다. 광 신호 기반 센서 노드 위치 인식 시스템은 회전하는 베이스 스테이션에서 발사된 광신호의 출발시간과 센서 노드에 부착된 MEMS CCR에 반사되어 되돌아오는 도착시간 차이를 이용하여 거리를 계산하고, 거리가 계산될 때의 베이스 스테이션의 회전각을 측정함으로써 이루어진다. MEMS CCR은 세 개의 반사면이 서로 수직을 이루고 있는 마이크로 미터 급의 크기의 반사체로써 입사한 광 신호를 입사 방향의 평행한 방향으로 반사시킬 수 있다. 실제 이 시스템을 가지고 MEMS CCR 을 위치 인식하는 센서 네트워크를 구성할 때 문제점이 발생한다. 첫 번째로 MEMS CCR에서 반사되어 베이스 스테이션으로 돌아오는 광 신호의 세기가 미약하여 약 30~40cm정도 밖에 측정 거리가 되지 않는 문제점과, 무작위적으로 분포된 센서 노드가 베이스 스테이션과 일직선으로 배열될 경우 광 신호의 직진성으로 인해 광 신호가 도달하지 못해 위치 인식이 되지 않는 문제점이다. 이를 해결하고자 본 논문에서는 다음과 같은 방안을 연구하였다. 먼저 MEMS CCR에 광신호가 반사되어 베이스 스테이션으로 돌아오는 과정을 분석하여 베이스 스테이션에서 MEMS CCR을 검출하기 위해 필요한 하드웨어의 사양에 대한 분석을 수행하였다. 그 다음으로 베이스 스테이션 시스템의 구조를 변경하여 다른 위치 인식 기법을 사용하거나 MEMS CCR의 변경을 제안하여 결과적으로 기존의 측정 가능 거리를 4배 이상 향상 시켰다. 무작위적으로 분포된 센서 노드의 문제는 기존의 단일 베이스 스테이션이 아닌 다중 베이스 스테이션 시스템을 제안하여 해결하였다. 그 후 위의 두 문제점을 해결하는데 사용된 다중 베이스 스테이션 시스템을 적용한 센서 노드 위치 인식 실험을 통하여 위치 인식 성능의 향상 면에서도 효용성이 있음을 확인하였다. 이를 통해 실제 센서 네트워크 구성에 다중 베이스 스테이션 시스템이 필수적이라는 결론을 내릴 수 있었다.목 차 초록 ⅰ 목차 ⅰⅴ 표 목차 ⅴⅰ 그림 목차 ⅴⅰⅰ 1. 서 론 1 1.1. 연구 배경 1 1.2. 연구의 목적 및 내용 6 2. 위치 인식 시스템 분석 9 2.1 ToF 스캐닝 시스템의 원리 9 2.2 실제 적용에 발생하는 문제점 분석 13 3. 실제 적용을 위한 시스템 개선 연구 15 3.1 거리에 따른 검출 조건 분석 17 3.1.1 입사 광 신호의 광 밀도 19 3.1.2 유효 반사 면적 21 3.1.3 반사율 24 3.1.4 광 분리기 25 3.1.5 최대 반사 광 신호 세기 26 3.1.6 이론 식과 측정치의 비교 27 3.1.7 이론 식을 통한 파라미터 식별 29 3.2 베이스 스테이션의 변경을 통한 성능 향상 31 3.2.1 AoA 위치 인식 기법 34 3.2.2 Lighthouse 위치 인식 기법 36 3.2.3 위치 인식 기법의 성능 비교 38 3.3 MEMS CCR의 변경을 통한 성능 향상 40 3.4 다중 베이스 스테이션 구성 43 4. 위치 인식 실험 및 결과 분석 45 4.1 실험 조건 46 4.2 실험 결과 분석 48 5. 결론 및 추후 연구 과제 52 참고문헌 55 Abstract 59Maste

Design and Analysis of Free Space Optical Sensor Networks for Short-Range Applications

Free space optical communication (FSOC) systems using direct detection and line of sight (LOS) laser links can provide spatially efficient and physically secure connectivity for wireless sensor networks. The FSOC system can be developed with low power microcontrollers so that the entire sensor system can be implemented on a single printed circuit board. Available data rates can range from kb/s to hundreds of Mb/s with the complete system consuming power only in the tens of mW. These features are advantageous for low-power communication networks over short distances in environments where LOS is available, and where radio frequency (RF) connectivity must be avoided because of interference or security issues. In particular, the faster data acquisition rates of FSOC systems are extremely attractive in applications where the sensor systems, or "motes", remain in sleep mode most of the time and need to transmit large amounts of data in extremely short bursts when they wake up. However, in order for directional FSO sensor networks to become viable short-range solutions, the networks must provide signal coverage over a wide field of view without strict optical alignment requirements, operate with efficient media access protocols that can handle network traffic in an efficient manner, and minimize random access times for the independent transmitting motes within the network. These challenges are the focus of this dissertation. In general, narrow optical beams used for FSOC require precise and complex pointing, acquisition, tracking and alignment methods. This dissertation addresses the challenge of alignment for FSO-based nodes by designing optical transceiver architectures with multiple narrow field of view (FOV) transmitters and a single, wide angle receiver. The architecture consists of rings of multiple transmitters surrounding a photodiode for light collection. Each ring is tilted at a different angle so that a wide transmission FOV can be obtained, thereby allowing point-multipoint communication. Depending on the number of transmitters and the transmitter's divergence angle, different FOVs can be tailored to fit the requirements of the target application. The developed transmitter design requires only a few milliwatts of transmission power from each transmitter to cover its respective FOV, which is sustainable with drive currents up to 10 milliamps using vertical cavity surface emitting lasers (VCSELs), making it a more practical strategy for a compact battery driven device. The other major challenge is designing the proper media access control (MAC) protocol, which provides nodes with addresses and channel access capability so that directional links between multiple nodes can be formed. The challenge lies in the fact that most nodes are blind to other nodes' transmissions because of their relatively narrow directional links. Because of this blindness, packet collisions are inevitable. Therefore, an efficient multiple access protocol needs to be designed for the FSOC system to ensure successful directional communication between the motes and cluster heads for data collection and relaying. While there are many protocols that allow multiple access and provide collision avoidance for traditional RF systems, these protocols are not optimized for FSOC systems consisting of multiple narrow FOV transmitters. Instead, a directional MAC (DMAC) protocol is developed from existing RF protocols, but modified for FSOC technology. It overcomes the limitations in FSOC communication resulting from directionality by setting up a master-slave network architecture where communication takes place between a sensing system, "mote", and a central control station, or "cluster head", which is designed with a multiple VCSEL transmitters. In this way, the physical transmitter sources of the cluster head become an integral part of the FSOC DMAC protocol. In this type of architecture, the master node, or cluster head, has the dual functionality of coordinating network traffic and aggregating data from all the slave nodes, or motes, that are within its field of view (FOV). Multiple cluster heads can form a directional network backbone, and can relay signals collected from a mote through other cluster heads, until the signal is delivered to its destination. In summary, this dissertation provides: 1) the design and implementation of small and inexpensive short-range FSOC systems that can be implemented using standard "off the shelf" components including a microcontroller and sensor device to form a complete standalone package; 2) development of a DMAC protocol that is optimized for the implemented FSOC system and target network applications; 3) network performance evaluation and optimization for the combined FSOC hardware, network architecture, and DMAC protocol. This is done through a series of hardware tests on an experimental prototype FSOC sensor network consisting of 10 motes and 1 cluster head and simulations of larger network sizes