Mark S. Leeson

Recent developments in indoor optical wireless [Optical wireless communications]

An overview of the developments in optical wireless systems viewed from the traditional communications viewpoint of transmitter, channel and receiver is presented. The trends in modulation formats that match information to the optical wireless channel are considered. This is followed by the discussion of recent transmitter and receiver innovations, particularly the utilisation of diversity transceivers. As a preliminary to the following treatment, the nature and modelling of the optical wireless channel are introduced, with particular emphasis on its unique features in terms of transmitted power constraints and non-negativity. From the examination of modulation formats, on-off-keying remains the format of choice for basic binary transmission, whereas

Channel selection and classification of electroencephalogram signals

OBJECTIVE An electroencephalogram-based (EEG-based) brain-computer-interface (BCI) provides a new communication channel between the human brain and a computer. Amongst the various available techniques, artificial neural networks (ANNs) are well established in BCI research and have numerous successful applications. However, one of the drawbacks of conventional ANNs is the lack of an explicit input optimization mechanism. In addition, results of ANN learning are usually not easily interpretable. In this paper, we have applied an ANN-based method, the genetic neural mathematic method (GNMM), to two EEG channel selection and classification problems, aiming to address the issues above. METHODS AND MATERIALS Pre-processing steps include: least-square (LS) approximation to determine the overall signal increase/decrease rate; locally weighted polynomial regression (Loess) and fast Fourier transform (FFT) to smooth the signals to determine the signal strength and variations. The GNMM method consists of three successive steps: (1) a genetic algorithm-based (GA-based) input selection process; (2) multi-layer perceptron-based (MLP-based) modelling; and (3) rule extraction based upon successful training. The fitness function used in the GA is the training error when an MLP is trained for a limited number of epochs. By averaging the appearance of a particular channel in the winning chromosome over several runs, we were able to minimize the error due to randomness and to obtain an energy distribution around the scalp. In the second step, a threshold was used to select a subset of channels to be fed into an MLP, which performed modelling with a large number of iterations, thus fine-tuning the input/output relationship. Upon successful training, neurons in the input layer are divided into four sub-spaces to produce if-then rules (step 3). Two datasets were used as case studies to perform three classifications. The first data were electrocorticography (ECoG) recordings that have been used in the BCI competition III. The data belonged to two categories, imagined movements of either a finger or the tongue. The data were recorded using an 8 × 8 ECoG platinum electrode grid at a sampling rate of 1000 Hz for a total of 378 trials. The second dataset consisted of a 32-channel, 256 Hz EEG recording of 960 trials where participants had to execute a left- or right-hand button-press in response to left- or right-pointing arrow stimuli. The data were used to classify correct/incorrect responses and left/right hand movements. RESULTS For the first dataset, 100 samples were reserved for testing, and those remaining were for training and validation with a ratio of 90%:10% using K-fold cross-validation. Using the top 10 channels selected by GNMM, we achieved a classification accuracy of 0.80 ± 0.04 for the testing dataset, which compares favourably with results reported in the literature. For the second case, we performed multi-time-windows pre-processing over a single trial. By selecting 6 channels out of 32, we were able to achieve a classification accuracy of about 0.86 for the response correctness classification and 0.82 for the actual responding hand classification, respectively. Furthermore, 139 regression rules were identified after training was completed. CONCLUSIONS We demonstrate that GNMM is able to perform effective channel selections/reductions, which not only reduces the difficulty of data collection, but also greatly improves the generalization of the classifier. An important step that affects the effectiveness of GNMM is the pre-processing method. In this paper, we also highlight the importance of choosing an appropriate time window position.

Fault and attack management in all-optical networks

Network management for optical networks faces additional security challenges that arise by using transparent optical network components in communication systems. While some available management mechanisms are applicable to different types of network architectures, many of these are not adequate for all-optical networks. These have unique features and requirements in terms of security and quality of service, thus requiring a much more targeted approach in terms of network management. In this article we consider management issues with particular emphasis on complications that arise due to the unique characteristics and peculiar behaviors of transparent network components. In particular, signal quality monitoring is still a major complication in all-optical networks. Despite new methods for detection and localization of attacks having been proposed, no robust standards or techniques exist to date for guaranteeing the quality of service in these networks. Therefore, sophisticated mechanisms that assist in managing and assessing the proper function of transparent network components are highly desirable. Accordingly, we present an algorithm for multiple attack localization and identification that can participate in some tasks for fault management of all-optical networks

Developments in wavelength division multiple access networking

The last few years have seen a growing interest in WDM for optical networks in order to increase the capacity and overcome the electronic bottleneck. Alongside the improvement in the WDM components there has been continuing development of WDMA networks. A large number of different WDMA strategies have been investigated in terms of both network architecture and the associated protocol requirements. This article identifies major themes and provides examples of experimental and theoretical developments which are anticipated to facilitate WDMA networking.

Forward Error Correction for Molecular Communications

Communication between nanoscale devices is an area of considerable importance as it is essential that future devices be able to form nanonetworks and realise their full potential. Molecular communication is a method based on diffusion, inspired by biological systems and useful over transmission distances in the nm to m range. The propagation of messenger molecules via diffusion implies that there is thus a probability that they can either arrive outside of their required time slot or ultimately, not arrive at all. Therefore, in this paper, the use of a error correcting codes is considered as a method of enhancing the performance of future nanonetworks. Using a simple block code, it is shown that it is possible to deliver a coding gain of ∼ 1.7dB at transmission distances of 1 m. Nevertheless, energy is required for the coding and decoding and as such this paper also considers the code in this context. It is shown that these simple error correction codes can deliver a benefit in terms of energy usage for transmission distances of upwards of 25 m for receivers of a 5 m radius.

Underwater optical wireless communications: depth dependent variations in attenuation

Depth variations in the attenuation coefficient for light in the ocean were calculated using a one-parameter model based on the chlorophyll-a concentration C(c) and experimentally-determined Gaussian chlorophyll-depth profiles. The depth profiles were related to surface chlorophyll levels for the range 0-4  mg/m², representing clear, open ocean. The depth where C(c) became negligible was calculated to be shallower for places of high surface chlorophyll; 111.5 m for surface chlorophyll 0.8<C(c)<2.2  mg/m³ compared with 415.5 m for surface C(c)<0.04  mg/m³. Below this depth is the absolute minimum attenuation for underwater ocean communication links, calculated to be 0.0092  m⁻¹ at a wavelength of 430 nm. By combining this with satellite surface-chlorophyll data, it is possible to quantify the attenuation between any two locations in the ocean, with applications for low-noise or secure underwater communications and vertical links from the ocean surface.

Error correction coding for molecular communications

The emerging field of communications between nanoscale devices is one of considerable importance since it is essential that nanonetworks are formed to realize the potential of such devices. Molecular communication is a method based on diffusion, inspired by biological systems and useful over distances in the nm to μm range. Messenger molecules propagate via diffusion and there is thus a probability that they do not arrive at the receiver or are delayed so as to be delivered in the wrong communication time slot. In this paper, the use of error correction codes is considered to improve the transmission performance of molecular communications. Using a simple block code, it is possible to deliver a coding gain of ~1.6 dB. Nevertheless, energy is required for the coding and decoding when employing the code and this paper also considers this for the first time. It is shown that simple error correction delivers a benefit in terms of energy consumption for distances upwards of approximately 10 μm to 20 μm.

A Genetic Algorithm Method for Optical Wireless Channel Control

A genetic algorithm controlled multispot transmitter is proposed as an alternative approach to optimizing the power distribution for single element receivers in fully diffuse mobile indoor optical wireless communication systems. By specifically tailoring the algorithm, it is shown that by dynamically altering the intensity of individual diffusion spots, a consistent power distribution, with negligible impact on bandwidth and rms delay spread, can be created in multiple rooms independent of reflectivity characteristics and user movement patterns. This advantageous adaptability removes the need for bespoke system design, aiming instead for the use of a more cost effective, optimal transmitter and receiver capable of deployment in multiple scenarios and applications. From the simulations conducted it is deduced, that implementing a receiver with a FOV=55deg in conjunction with either of two notable algorithms, the dynamic range of the rooms, referenced against the peak received power, can be reduced by up to 26% when empty, and furthermore to within 12% of this optimized case when user movement perturbs the channel.

Multiple attack localization and identification in all-optical networks

The security characteristics of currently emerging all-optical networks display many unique features compared to traditional communication networks. In particular, network transparency raises many security vulnerabilities that differ substantially from conventional failures and should therefore be treated differently. One of the serious problems related to transparency lies in the fact that optical crosstalk is additive and can be exploited to perform service disruption attacks upon the network. Since these attacks can spread rapidly through the network, causing additional problems and triggering multiple alarms, they must be detected and identified at any point in the network where they may occur. However, to monitor all wavelength channels at several detection points into any node is likely to be very expensive. In this paper we provide formal specifications for optical crosstalk that can arise in optical cross-connect nodes. Based on these specifications, we propose an algorithm for localizing the sources of multiple attacks and identifying their nature in all-optical networks.

Tunable Pulse Amplitude and Position Modulation Technique for Reliable Optical Wireless Communication Channels

Modulation techniques have attracted increasing attention in optical wireless communications. Basic schemes such as on off keying (OOK), pulse amplitude modulation (PAM) and pulse position modulation (PPM) have been validated as suitable for the optical wireless channel. This paper starts from the analysis of these three modulation schemes in terms of their power and bandwidth requirements. As a result, a new tunable hybrid modulation technique is proposed. The proposed modulation scheme takes the real time channel conditions into account, which is different from other schemes. By employing amplitude and position modulation selectively, a guaranteed system performance can be secured, without compromising power and bandwidth efficiency. This is also a new approach to realize reliable optical wireless links.