Martin J.N. Sibley

Integrated transceivers for optical wireless communications

Line-of-sight free-space optical links can provide extremely high bandwidth communications, but this usually requires that transmitter and receiver are precisely aligned. In order to allow terminals to be mobile, links must be able to track users within their field of view so that the link is maintained. There are various means to do this, but all require complex subsystems with a number of different optical, optoelectronic, and electrical components. A solid-state tracking architecture is introduced and a seven-channel tracking system demonstration described. The system is designed to operate at 155 Mb/s and is, to the best of our knowledge, the first that uses an integrated approach. Arrays of novel resonant cavity LED (RCLED) emitters that operate at 980 nm are used as sources. These are flip-chip bonded to arrays of CMOS driver circuits and integrated with the necessary transmitter optics. The receiver uses a back-illuminated detector array flip-chip bonded to arrays of custom CMOS receivers. All these components are custom and have performance substantially better than nonoptimized commercially available components. In the paper, the design and fabrication of the optics, optoelectronics, and electronics required for this is described. Successful operation of all the subsystems is detailed, together with results from an initial link demonstration.

High-speed integrated transceivers for optical wireless

Optical wireless LANs have the potential to provide bandwidths far in excess of those available with current or planned RF networks. There are several approaches to implementing optical wireless systems, but these usually involve the integration of optical, optoelectronic, and electrical components in order to create transceivers. Such systems are necessarily complex, and the widespread use of optical wireless is likely to be dependent on the ability to fabricate the required transceiver components at low cost. A number of UK universities are currently involved in a project to demonstrate integrated optical wireless subsystems that can provide line-of-sight in-building communications at 155 Mb/s and above. The system uses two-dimensional arrays of novel microcavity LED emitters and arrays of detectors integrated with custom CMOS integrated circuits to implement tracking transceiver components. In this article we set out the basic approaches that can be used for in-building optical wireless communication and argue the need for an integrated and scalable approach to the fabrication of transceivers. Our work aimed at implementing these components, including experimental results and potential future directions, is then discussed.

A comparison of coherent digital PPM with PCM

Digital pulse position modulation (PPM) is a preferred modulation format for the ideal photon counting channel and optical intersateilite links. Here we examine its potential for the coherent optical fibre communications channel. We present a thorough performance and optimisation analysis. Comparisons, at a wavelength of 1.5 m, are made with shot-noise limited coherent PCM (homodyne and heterodyne ASK, FSK and PSK) over a range of fibre band-widths and varying PPM word size. We conclude that for moderate to high fibre bandwidths homodyne digital PPM should achieve an improvement in sensitivity of typically 5 dB over homo-dyne PSK PCM

High speed integrated optical wireless transceivers for in-building optical LANs

12 Maintaining high bandwidth indoor optical wireless channels under a wide range of operating conditions usually requires relatively complex transceiver components. Integrating optical, optoelectronic and optical components using techniques that are suitable for mass manufacture is an important step in the development of these systems. This paper describes work to develop low cost integrated tracking transmitter and receiver components for use in a cellular indoor optical wireless network. A seven channel demonstrator operating at 155 Mb/s is under construction, using arrays of Resonant Cavity LEDs, PIN detectors, Silicon CMOS driver circuits and associated optics. Development of components, design methodology and initial results are detailed.

A monolithic common-collector front-end optical preamplifier

A monolithic transimpedance preamplifier has been developed having a common-collector cascode configuration with shunt feedback, using an advanced bipolar IC process. The measured sensitivity was -35.0 dBm at 140 Mbit/s for an error rate of 10-9and a p-in photodiode responsivity of 0.5 A/W.

Investigation of Doppler Effects on high mobility OFDM-MIMO systems with the support of High Altitude Platforms (HAPs)

The merging of Orthogonal Frequency Division Multiplexing (OFDM) with Multiple-Input Multiple- Output (MIMO) systems is a promising mobile air interface solution for the next generation wireless local area networks (WLANs) and 4G mobile cellular wireless systems. The main aim of this research is to design a highly robust and efficient OFDM-MIMO system to support permanent accessibility and high data rates to users moving at high mobility and speeds up to 300km/h. The paper discusses a comprehensive literature review focused in both technologies and the contributions that have been anticipated during the last years, followed by a research project planning with discussions and what experiments will be carried out using the simulation programs. Furthermore, building the hybrid architecture involves merging the novel model with High Altitude Platform system (HAPs) technology, to analyze the overall performance of the network in delivering IP-broadband services and high load application to users.

Transit-time limitations in p-i-n photodiodes

A novel derivation for predicting transit-time effects in p-i-n photodiodes is described in which the diode current is found by simple integration. Experimental results obtained from two wide-area photodiodes compare well with theoretical predictions. It is also shown that previously published theory does not agree with these experimental observations.

Detection of digital pulse position modulation over highly/slightly dispersive optical channels

In this paper we present experimental results for a digital pulse position modulation (PPM) system that uses a low-bandwidth receiver. It is shown that the performance of such a system is relatively independent of channel dispersion. Thus a simple low-bandwidth receiver can be used on low- or high-dispersion digital PPM links.

Design implications of high-speed digital PPM

Work in the area of digital pulse position modulation (digital PPM) has shown that this type of modulation can yield sensitivities that are typically 4 - 5 dB better than an equivalent PCM system. Recent experimental work has shown that the receiver in a digital PPM system does not need to have a wide bandwidth. Instead, the bandwidth can be very low so that the receiver is effectively impulsed by the digital PPM signal. The advent of very high speed Si digital ICs, and fast lasers, means that digital PPM can now be used to code gigabit PCM signals. This paper presents original theoretical results for a digital PPM system coding 1 Gbit/s PCM signals into 8 Gbit/s digital PPM signals. The paper also addresses the difficulties that the system designer is likely to encounter, and discusses some possible solutions.

Fault detection algorithm for multiple GCPV array configurations

In this paper, a fault detection algorithm for multiple grid-connected photovoltaic (GCPV) array configurations is introduced. For a given set of conditions such as solar irradiance and photovoltaic module temperature, a number of attributes such as power, voltage and current are calculated using a mathematical simulation model. Virtual instrumentation (VI) LabVIEW software is used to monitor the performance of the GCPV system and to simulate the theoretical I-V and P-V curves of the examined system. The fault detection algorithm is evaluated on multiple GCPV array configurations such as series, parallel and series-parallel array configuration. The fault detection algorithm has been validated using 1.98 kWp GCPV system installed at the University of Huddersfield. The results indicates that the algorithm is capable to detect multiple faults in the examined GCPV plant and can therefore be used in large GCPV installations.