Stefan Mangold

LED-to-LED visible light communication networks

Visible Light Communication (VLC) with Light Emitting Diodes (LEDs) as transmitters and receivers enables low bitrate wireless adhoc networking. LED-to-LED VLC adhoc networks with VLC devices communicating with each other over free-space optical links typically achieve a throughput of less than a megabit per second at distances of no more than a few meters. LED-to-LED VLC adhoc networks are useful for combining a smart illumination with low-cost networking. We present and evaluate a software-based VLC physical layer and a VLC medium access control layer that retain the simplicity of the LED-to-LED approach. The design satisfies the requirement that LEDs should always be perceived as on with constant brightness. In each VLC device, in addition to an LED, only a low-cost microcontroller is required for handling the software-based communication protocol. The results of our performance measurements confirm recent claims about the potential of LED-to-LED VLC adhoc networks as a useful technology for sensor networks, smart and connected consumer devices, and the Internet-of-Things.

Connecting networks of toys and smartphones with visible light communication

Light emitting diodes are low-cost and energy- efficient. They are replacing incandescent bulbs as the primary source of illumination in residential and public environments. The brightness of LEDs can be modulated at a high rate, which enables the combination of illumination and wireless communication, imperceptible to humans. Such systems using LEDs as transceivers are called visible light communication systems. LEDs have also been extensively used in consumer electronics such as toys and smartphones, but primarily for reasons other than communication. We show various use cases of devices connected with VLC. Since LEDs can also be used as light receivers in VLC systems, adding microcontrollers to devices (if not already embedded) enables low-cost implementation of a wireless communication interface with VLC. This article reports on experience with several prototypes of practical VLC systems.

Low-complexity Visible Light Networking with LED-to-LED communication

Visible Light Communication (VLC) is an emerging technology in which Light Emitting Diodes (LEDs) transport information wirelessly, using the visible light spectrum. While most of the research on VLC has focused on wideband white LEDs used in ambient illumination, narrowband and colored LEDs have received little attention. Short-range free-space optical communication based on narrowband LEDs as visible light transmitters and receivers enable a variety of applications, a scenario we refer as LED-to-LED communication. In this paper, we introduce the communication and networking protocols of LED-to-LED communication. Our work addresses fundamental challenges such as efficient collision detection medium access protocol and elimination of light flicker. We build a prototype and demonstrate bi-directional data exchange in a network of up to four LEDs. We further study the trade-offs in the system design and measure the achievable bit-rate and transmission distances.

An LED-to-LED Visible Light Communication system with software-based synchronization

An LED can emit and receive light and provides therefore a simple building block for a Visible Light Communication (VLC) system. We describe a microcontroller-based system and report on its effectiveness in a testbed. The key idea is to use a microcontroller to provide synchronization so that the receiver can lock to the transmitted signal in a fast and efficient way. Further we propose a combined light emission and light measurement approach to receive with an LED while emitting light. The proposed system enables new entertaining applications by creating the illusion (for a human observer) that both transmitting and receiving LEDs are always switched on.

CAESAR: carrier sense-based ranging in off-the-shelf 802.11 wireless LAN

Wireless local area networks have been designed for wireless communication. Frames are acknowledged (ACKed) after a short and predefined MAC idle time. The MAC idle time varies with i) the physical distance between stations, caused by the delay of wireless signal propagation, and ii) the time to detect the ACK at the local station, which varies with the signal strength of the incoming ACK. We present CAESAR, CArriEr Sense-bAsed Ranging, that combines time of flight and signal-to-noise ratio measurements to calculate the distance between two stations. CAESAR measures the distance by estimating the MAC idle time in a data/ACK communication at a 44 MHz clock resolution and the ACK detection time on a per-frame basis. CAESAR is a software-based solution that is entirely implemented at the transmitter and it requires no protocol modifications and only a limited calibration in links with multi-path propagation. We implement CAESAR on commodity hardware and conduct extensive experiments both in controlled network conditions and dynamic radio environments. Our measurements confirm the accuracy of the solution and show the capability to track the distance to WLAN smartphones at pedestrian speeds.

Joint Scalable Coding and Routing for 60 GHz Real-Time Live HD Video Streaming Applications

Transmission of high-definition (HD) video is a promising application for 60 GHz wireless links, since very high transmission rates (up to several Gbit/s) are possible. In particular we consider a sports stadium broadcasting system where signals from multiple cameras are transmitted to a central location. Due to the high pathloss of 60 GHz radiation over the large distances encountered in this scenario, the use of relays might be required. The current paper analyzes the joint selection of the routes (relays) and the compression rates from the various sources for maximization of the overall video quality. We consider three different scenarios: (i) each source transmits only to one relay and the relay can receive only one data stream, and (ii) each source can transmit only to a single relay, but relays can aggregate streams from different sources and forward to the destination, and (iii) the source can split its data stream into parallel streams, which can be transmitted via different relays to the destination. For each scenario, we derive the mathematical formulations of the optimization problem and re-formulate them as convex mixed-integer programming, which can guarantee optimal solutions. Extensive simulations demonstrate that high-quality transmission is possible for at least ten cameras over distances of 300 m. Furthermore, optimization of the video quality gives results that can significantly outperform algorithms that maximize data rates.

Continuous synchronization for LED-to-LED visible light communication networks

Off-the-shelf Light Emitting Diodes (LEDs) and low-cost microcontrollers provide the foundation for networking using the visible light as communication medium. These networks require fast and stable synchronization and a distributed protocol to handle shared medium access. We present and evaluate a physical layer and a distributed, contention-based medium access control protocol that enables reliable communication over room-range distances; both are implemented in software using low-cost commercial off-the-shelf building blocks. Experiments with a testbed consisting of embedded devices equipped with only LEDs demonstrate the scalability of this approach. The performance evaluation indicates that Visible Light Communication is a reliable solution for more than ten devices to bring low-cost and non-complex connectivity to a large number of devices.

Supporting mobile devices with wireless LAN/MAN in large controlled environments

The functionality of mobile devices has grown exponentially in recent times. This has led to smart phones and the mobile Internet becoming a big success story. IEEE 802.11 wireless local area network, known as Wi-Fi, has become a standard feature on these devices and represents a viable alternative to using a mobile phone provider¿s network for connectivity. Users can surf the web, make VoIP calls, and more from their home WLAN networks or public hotspots. At present WLAN has too many outstanding issues to universally replace existing mobile phone networks. However, WLAN is ready to provide universal coverage for mobile devices in large controlled environments such as university and business campuses, sports stadia, and entertainment parks. In this article we outline the challenges in such a deployment and describe how the state of the art in WLAN can meet these challenges. Outstanding issues and areas requiring improvement are highlighted. With a view to overcoming these hurdles, some potential solutions and promising research directions are outlined.

EnLighting: An Indoor Visible Light Communication System Based on Networked Light Bulbs

The Internet of Things (IoT) envisions that many devices can connect to a network. Visible Light Communication (VLC) based on Light Emitting Diodes (LEDs) is an attractive communication fabric for the IoT, as LEDs are readily available and can serve as transmitters as well as receivers. LED light bulbs, enhanced with photodiodes, provide an attractive path to extend device-to-device communication to room area networking. This paper describes a VLC system, called EnLighting, based on light bulbs that support an embedded Linux version (and its complete networking stack). These programmable light bulbs can both send and receive; they can communicate with objects in a room as well as with other light bulbs nearby. Bidirectional communication allows a light bulb to actively participate in networking and simplifies deployment, maintenance, configuration, and controllability of indoor lighting installations and services. EnLighting supports low-bandwidth communication services in a room (and via a gateway, beyond the room), which provide the base for other applications, e.g., a location service. This paper includes an initial evaluation of a room area communication and localization network based on prototype Linux-enabled light bulbs, reporting the performance for different network traffic types, and shows the benefits obtained from bidirectional communication. This proof-of- concept system illustrates that simple devices can provide an attractive solution to future communication challenges in the saturated radio spectrum.

Quality-aware coding and relaying for 60 GHz real-time wireless video broadcasting

Wireless streaming of high-definition video is a promising application for 60GHz links, since multi-Gigabit/s data rates are possible. In particular we consider a sports stadium broadcasting system where video signals from multiple cameras are transmitted to a central location. Due to the high pathloss of 60GHz radiation over the large distances encountered in this setting, the use of relays is required. This paper designs a quality-aware coding and relaying algorithm for maximization of the overall video quality. We consider the setting that the source can split its data stream into parallel streams, which can be transmitted via different relays to the destination. For this, we derive the related formulation and re-formulate it as convex programming, which can guarantee optimal solutions.