Rui Valadas

Optical interference produced by artificial light

Wireless infrared transmission systems for indoor use are affected by noise and interference induced by natural and artificial ambient light. This paper presents a characterisation (through extensive measurements) of the interference produced by artificial light and proposes a simple model to describe it. These measurements show that artificial light can introduce significant in‐band components for systems operating at bit rates up to several Mbit/s. Therefore it is essential to include it as part of the optical wireless indoor channel. The measurements show that fluorescent lamps driven by solid state ballasts produce the wider band interfering signals, and are then expected to be the more important source of degradation in optical wireless systems.

Multiscale Fitting Procedure Using Markov Modulated Poisson Processes

This paper proposes a parameter fitting procedure using Markov Modulated Poisson Processes (MMPPs) that leads to accurate estimates of queuing behavior for network traffic exhibiting long-range dependence behavior. The procedure matches both the autocovariance and marginal distribution of the counting process. A major feature is that the number of states is not fixed a priori, and can be adapted to the particular trace being modeled. The MMPP is constructed as a superposition of L 2-MMPPs and one M-MMPP. The 2-MMPPs are designed to match the autocovariance and the M-MMPP to match the marginal distribution. Each 2-MMPP models a specific time-scale of the data. The procedure starts by approximating the autocovariance by a weighted sum of exponential functions that model the autocovariance of the 2-MMPPs. The autocovariance tail can be adjusted to capture the long-range dependence characteristics of the traffic, up to the time-scales of interest to the system under study. The procedure then fits the M-MMPP parameters in order to match the marginal distribution, within the constraints imposed by the autocovariance matching. The number of states is also determined as part of this step. The final MMPP with M2L states is obtained by superposing the L 2-MMPPs and the M-MMPP. We apply the inference procedure to traffic traces exhibiting long-range dependence and evaluate its queuing behavior through simulation. Very good results are obtained, both in terms of queuing behavior and number of states, for the traces used, which include the well-known Bellcore traces.

Modeling IP traffic: joint characterization of packet arrivals and packet sizes using BMAPs

This paper proposes a traffic model and a parameter fitting procedure that are capable of achieving accurate prediction of the queuing behavior for IP traffic exhibiting long-range dependence. The modeling process is a discrete-time batch Markovian arrival process (dBMAP) that jointly characterizes the packet arrival process and the packet size distribution. In the proposed dBMAP, packet arrivals occur according to a discrete-time Markov modulated Poisson process (dMMPP) and each arrival is characterized by a packet size with a general distribution that may depend on the phase of the dMMPP. The fitting procedure is designed to provide a close match of both the autocovariance and the marginal distribution of the packet arrival process, using a dMMPP; a packet size distribution is fitted individually to each state of the dMMPP. A major feature of the procedure is that the number of states of the fitted dBMAP is not fixed a priori; it is determined as part of the procedure itself. In this way, the procedure allows establishing a compromise between the accuracy of the fitting and the number of parameters, while maintaining a low computational complexity.We apply the inference procedure to several traffic traces exhibiting long-range dependence. Very good results were obtained since the fitted dBMAPs match closely the autocovariance, the marginal distribution and the queuing behavior of the measured traces. Our results also show that ignoring the packet size distribution and its correlation with the packet arrival process can lead to large errors in terms of queuing behavior.

MPLS over WDM network design with packet level QoS constraints based on ILP models

MPLS (multiprotocol label switching) over WDM (wavelength division multiplexing) networks are gaining significant attention due to the efficiency in resource utilization that can be achieved by jointly considering the two network layers. This paper addresses the design of MPLS over WDM networks, where some of the WDM nodes may not have packet switching capabilities. Given the WDM network topology and the offered traffic matrix, which includes the location of the edge LSRs (label switched routers), we jointly determine the location of the core LSRs (i.e. the core WDM nodes that also need to include packet switching capabilities) and the lightpath routes (which are terminated on the LSRs) that minimize the total network cost. We consider constraints both at the optical and packet layers: an MPLS hop constraint on the maximum number of LSRs traversed by each LSP (label switched path), which guarantees a given packet level QoS, and a WDM path constraint on the maximum length of lightpaths, which accommodates the optical transmission impairments. A novel integer linear programming (ILP) formulation based on an hop-indexed approach, which we call the HOP model, is proposed. A two-phase heuristic, derived from a decomposition of the HOP model in two simpler ILP models that are solved sequentially, is also developed. The computational results show that the heuristic is efficient and produces good quality solutions, as assessed by the lower bounds computed from the HOP model. In some cases, the optimal solution is obtained with the branch-and-bound method.

The infrared physical layer of the IEEE 802.11 standard for wireless local area networks

The new IEEE 802.11 standard for wireless local area networks defines a specification for an infrared physical layer. This article gives an overview of infrared technology and describes the IEEE 802.11 specification in detail, presenting a historical perspective of its development. The infrared physical layer was designed for diffuse systems supporting two data rates (1 and 2 Mb/s) and includes provisions for a smooth migration to higher data rates. The specification is suitable for low-cost transceivers but allows interoperability with higher-performance systems. The main application envisaged for IEEE 802.11 infrared wireless local area networks is ad hoc networks.

Characterisation and modelling of artificial light interference in optical wireless communication systems

Wireless indoor infrared transmission systems are affected by noise and interference induced by natural and artificial ambient light. While the shot noise induced on the receiver photodiode by steady ambient light has been extensively described and included in system models, the interference produced by artificial light has only been mentioned as a source of degradation and quite simple descriptions have been presented. This paper presents a characterisation (through extensive measurements) of the interference produced by artificial light and proposes a simple model to describe it. These measurements show that artificial light can introduce significant in-band components for systems operating at bit rates up to several Mbit/s. Therefore it is essential to include it as part of the optical wireless indoor channel. The measurements show that fluorescent lamps driven by solid state ballasts produce the wider band interfering signals, and are then expected to be the more important source of degradation in optical wireless systems.

Sectored receivers for indoor wireless optical communication systems

This document proposes and studies a novel receiver structure for indoor optical wireless communication systems that exploits the directional nature in both signal and noise propagation through the use diversity techniques. In this document we concentrate on the ability of the proposed sectored receiver in combating the ambient noise. Significant optical power gains are demonstrated and are seen to increase with the relative weight of the directional noise within the cell, with the sharpness of the directional noise source beam width and in environments where there are noise sources positioned outside the cell. Also the SNR of a sectored receiver is seen to be much less sensitive to the position and beamwidth of the noise sources than the SNR of a non-sectored receiver allowing for more universal transceiver designs.

Robust feature selection and robust PCA for internet traffic anomaly detection

Robust statistics is a branch of statistics which includes statistical methods capable of dealing adequately with the presence of outliers. In this paper, we propose an anomaly detection method that combines a feature selection algorithm and an outlier detection method, which makes extensive use of robust statistics. Feature selection is based on a mutual information metric for which we have developed a robust estimator; it also includes a novel and automatic procedure for determining the number of relevant features. Outlier detection is based on robust Principal Component Analysis (PCA) which, opposite to classical PCA, is not sensitive to outliers and precludes the necessity of training using a reliably labeled dataset, a strong advantage from the operational point of view. To evaluate our method we designed a network scenario capable of producing a perfect ground-truth under real (but controlled) traffic conditions. Results show the significant improvements of our method over the corresponding classical ones. Moreover, despite being a largely overlooked issue in the context of anomaly detection, feature selection is found to be an important preprocessing step, allowing adaption to different network conditions and inducing significant performance gains.

Sectored receivers to combat the multipath dispersion of the indoor optical channel

This paper presents the first results of a study developed to evaluate the performance of sectored receivers in combating the multipath dispersion of the indoor optical channel. We intend to: (1) evaluate the performance of sectored receivers in minimizing multipath dispersion and, (2) conclude if sectored receivers may be used successfully to reduce both ambient noise and multipath dispersion, by estimating only the ambient noise at each sector of the receiver. The results show that sectored receivers can reduce significantly the multipath dispersion, but selection of the sector with the best impulse response is required. Also, to achieve significant gains the field-of-view of the sectors have to be smaller than 50/spl deg/.

Modulation methods for wireless infrared transmission systems: performance under ambient light noise and interference

The major aspects that impair the performance of optical wireless transmission systems are the shot noise induced by the steady ambient light level, transmitted optical power limitations (high path losses), channel bandwidth limitations due to multipath dispersion and the interference produced by artificial light sources. Several modulation and encoding schemes have been proposed for this channel and their performance has been studied and presented by several authors. The work reported in this paper extends the previous analysis by taking into account the optical power penalty induced by artificial light interference. An analytical approach is used to estimate the optical power penalty induced by artificial light interference. In practical systems, the effect of the interference is usually mitigated using electrical high- pass filters. In this paper the combined effect of interference and high-pass filter is evaluated by resorting to simulation. The presented results show that interference produced by fluorescent lamps driven by electronic ballasts induce high power penalties in OOK and PPM systems, even when high-pass filtering is used. For the interference produced by incandescent lamps and fluorescent lamps driven by conventional ballasts, the power penalty induced in OOK systems can be effectively reduced using high-pass filtering, while PPM is very tolerant to that interference even without any high-pass filtering.