Demonstration of field of view in a room with a number of downlights. When the hemispherical lens (right) is used, the images of many more lights are projected onto the surface than when the conventional lens (left) is used. This demonstrates the much wider field of view of the hemispherical lens. This is used in the MIMO system to provide diversity and in the positioning system to improve the accuracy of position estimation.
Orthogonal frequency division multiplexing (OFDM) is the modulation technique that underlies many new and emerging broadband communication systems. It is used in wireless LANs, digital television and radio, ADSL, power line communications, 4G mobile and many other wireless systems. Until recently it was thought to be incompatible with optical communication but following a number of research breakthroughs optical OFDM is now one of the hottest topics in optical communications. Asymmetrically clipped optical OFDM (ACO-OFDM) a technique which we invented specifically for optical systems is one of the most promising modulation techniques for optical wireless systems.
Our papers and patents on OFDM cover fundamental aspects of OFDM for wireless and optical communications. They range from the most fundamental: why OFDM is so sensitive to frequency offset? to the most practical: why digital television reception can be disrupted by the fridge turning on or off?
OFDM for Optical Wireless and other Intensity-Modulated Direct-Detection Optical Systems
Many optical communication systems and all indoor optical wireless systems use intensity modulation and direct detection (IM/DD). The standard form of OFDM developed for radio frequency wireless communications is incompatible with IM/DD. This is because in IM/DD the information is carried on the intensity of the light, which cannot be negative and OFDM signals are both positive and negative. We have written many papers about OFDM for IM/DD systems, particularly about a modulation scheme we invented called asymmetrically clipped optical OFDM (ACO-OFDM). ACO-OFDM has many attractive properties including being very energy efficient. ACO-OFDM is now a very hot research topic.
In OFDM data is transmitted in parallel on multiple sinusoids of slightly different frequencies. It is used in digital radio and digital television in Australia. OFDM was chosen to replace AM and FM as modulation techniques because it makes very efficient use of the spectrum and because distortions introduced in transmission can readily be corrected at the receiver.
Orthogonal Frequency Division Multiplexing (OFDM)
Optical wireless communication is an important and rapidly developing field. The RF spectrum 'crunch' means that RF based systems will not be able to meet the demand for very high data rate wireless communication. LED lighting and LED displays are becoming increasingly common and these can also be used as optical transmitters as part of high data rate optical communication systems.
Multiple input Multiple Output (MIMO) communication systems use multiple transmitters and multiple receivers to increase the overall capacity or reliability of the system and MIMO RF systems are now common. However MIMO transmission in optical wireless is very different from MIMO in RF wireless and this presents a wide range of theoretical and practical challenges.
Most optical wireless systems use intensity modulation and direct detection (IM/DD). As a result all of the elements of the MIMO channel matrix are real and positive. To ensure significant diversity either an imaging receiver or very directional receivers are required. Simply separating the receivers by a short distance does not provide diversity.
Our research on MIMO optical wireless has included developing novel wide field of view imaging receivers by using a hemispherical lens and analyzing the effect of various impairments on MIMO systems which use a pixelated transmitter (like an LED display) and a camera as a receiver.
Different data is transmitted from four different LEDs. At the receiver four different photodetectors receive the signals and digital signal processing is used to separate the data streams. A hemispherical lens in the receiver is used to ensure the receive diversity required for successful MIMO transmission
Optical signals transmitted by the lighting LEDs allow devices to accurately determine their position. Potential applications include device tracking, mobile robotics and position aware information systems.
MIMO Optical Wireless Communications
Optical Positioning and Localization
Positioning,also known as localization, is the process of determining the position of an object or person. Accurate positioning is critical for numerous applications.
The familiar Global Positioning System (GPS), originally a US military system,is now in everyday use around the world, often in new and unexpected ways. However, despite decades of research into indoor positioning using radio frequency signals, there is still no system which is cheap, accurate, and widely available.. The fundamental problem in radio based systems is that multipath transmission distorts the transmitted signals.
The widespread introduction of white light emitting diodes (LEDs) for illumination provides an unprecedented opportunity to fill this gap. Look up in almost any building and you will be able to see multiple light fittings, demonstrating that at most indoor locations, a receiver could be designed to receive signals from multiple light sources. LEDs can be modulated at much higher frequencies than conventional lighting, so the signals required for positioning can readily be transmitted at frequencies that do not cause visible flicker.
Visible light positioning VLP) using LEDs is a new and exciting research area with many multidisciplinary research questions and huge practical potential. Our recent papers on VLP cover topics as diverse as the fundamental Cramer-Rao bounds on positioning using time-of-arrival, to designing novel lens configurations to provide accurate angle-of-arrival information