Sadia Ahmed

The Roles of Ultra Wideband in Cognitive Networks

Cognitive radio is a recent concept that is expected to lead wireless communications to a new era. A proper candidate for the physical layer implementation of cognitive radio networks is ultra wideband (UWB), because UWB is highly competent in satisfying many basic requirements of cognitive radio. Beside this, UWB can also provide various kinds of supportive uses to cognitive radio systems that are realized by means of other wireless technologies. In this paper, a number of possible roles that UWB can take in cognitive networks are discussed in detail.

Evaluation of frequency offset and Doppler effect in terrestrial RF and in underwater acoustic OFDM systems

Frequency offset and Doppler effect cause loss of orthogonality in orthogonal frequency division multiplexing (OFDM), which in turn may severely degrade receiver performance. Traditionally, most of the literature dealt with non-varying Doppler effect over OFDM subcarriers. However, in terrestrial ultra-wideband (UWB) OFDM radio frequency (RF) and in high data rate underwater acoustic communication (UAC), the effect of Doppler varies considerably over OFDM band. This paper will investigate the frequency offset and the varying effect of Doppler through analysis and simulation in both domains in order to maintain orthogonality or to compensate loss of orthogonality. To the best knowledge of the authors, this type of detailed analysis is not present in these domains, and thus this paper provides a valuable contribution.

Inter-Frame Interference in Time Hopping Impulse Radio Based UWB Systems for Coherent Receivers

Ultra Wideband (UWB) Systems take advantage of collecting energy from a large number of multipaths. In high data rate systems for time hopping (TH) UWB transmission, this large number of multipaths may leak into neighboring frames causing inter-frame interference (IFI). Most studies so far avoid this IFI by placing one or more constraints on chip length, frame length, minimum pulse to pulse duration, etc. In this paper, IFI will be investigated in detail for BPSK and PPM modulations in coherent transceivers. It will be shown that IFI effect is insignificant for these modulation options and for these transceiver architectures. Therefore, the constraints to avoid IFI can be removed for those systems. To the best knowledge of the authors, this type of analysis has not been conducted before and the simulation and semi-analytical analysis both result in a novel conclusion.

Cognitive intelligence in UAC channel parameter identification, measurement, estimation, and environment mapping

The recent growing interest in underwater acoustic communication (UAC) requires an in depth understanding of a vast and diverse water medium. The UAC channel presents many difficulties such as high frequency, time, and space selectivity, frequency dependent noise, and significant range and band limitation on transmission. The traditional channel parameter measurement and estimation methods may not be sufficient and sometimes inapplicable to UAC, as many of the underwater situations are beyond human reach. This paper will first identify and classify the UAC channel environment, its parameters, parameter measurement and estimation methods. Then two novel cognitive intelligent algorithms will be presented to measure and estimate the parameters and map them to appropriate UAC channel environment models. To the best knowledge of the authors, the concept of cognitive intelligence (CI) and the proposed CI algorithms in UAC channel parameter measurement, estimation, and mapping ha ve not been addressed in literature to current date. Thus this research presents a pioneering work in this domain.

IFI and ISI in high data rate UWB coherent transceivers

Ultra Wideband (UWB) Systems involve gathering energy from a large number of multipaths. In high data rate UWB, the leakage of these paths among symbols can cause severe inter-symbol interference (ISI) and may degrade receiver performance. A few techniques were presented in the past to improve coherent receiver performance in the presence of ISI . In this paper, ISI is investigated for coherent receivers using PAM and PPM modulation schemes. It is shown with analysis and simulation that in high data rate systems, the overall ISI effect is negligible on the coherent receiver performance. The ISI is also observed in the presence of inter-frame interference (IFI) and similar result is obtained.

Estimation and compensation of Doppler scale in UAC OFDM systems

Doppler effect causes loss of orthogonality in Orthogonal Frequency Division Multiplexing (OFDM), which in turn may severely degrade receiver performance. In high data rate underwater acoustic communication (UAC), the effect of Doppler shift varies considerably over OFDM frequency band. In the past and current UAC OFDM literature, the frequency domain Doppler shift is presented in terms of path dependent Doppler scale in time domain. This paper proposes and presents two algorithms to estimate Doppler scale induced by time variability of the channel. The performance of these estimation methods is evaluated under varying sampling period, varying number of channel taps, and varying channels.

Estimation and compensation of Doppler effect using variable sub-carrier spacing in multiband UAC OFDM systems

Doppler shift causes loss of orthogonality in Orthogonal Frequency Division Multiplexing (OFDM), which in turn may severely degrade receiver performance. In high data rate underwater acoustic communication (UAC), the effect of Doppler varies considerably over OFDM subcarriers. This paper will propose an algorithm to estimate the varying effects of Doppler shift over subcarriers and to avoid/mitigate the ICI produced by such effects using variable sub-carrier spacing in multiband OFDM.

Inter-symbol Interference in High Data Rate Transmit Reference UWB Transceivers

Ultra Wideband (UWB) Systems take advantage of collecting energy from a large number of multipaths. In high data rate systems, the leakage of these paths among symbols may cause severe inter-symbol interference (ISI) and degrade performance. A few techniques were presented in the past to improve non-coherent receiver performance in the presence of ISI. In this paper, a new and simple transmit reference (TR) signaling scheme is proposed for high data rate systems. It is shown with analysis and simulation that the proposed TR scheme produces negligible ISI effect on the TR receiver performance.

Using varying channel impulse response to determine water pollution

Water occupies three forth of earths surface. Although human habitats reside on land, there is no denying of the vital connection between land and water. The future sustainability of human species on this planet depends on wise utilization of all available resources, including that provided by the vast water world. Therefore, it is imperative to explore, understand, define, and protect this massive, varying, and in many areas unexplored water domain. With the exponential growth in urbanization all over the world, water seems to be the common dumping ground of industrial waste. In addition, water is indirectly polluted in many ways in our everyday lives. Therefore, it is of vital importance to monitor water pollution levels effectively and regularly. It is a well known fact that sound wave propagation through water changes considerably in time, frequency, and in spatial domains. Any changes in the water channel and its parameters directly affect the propagation of the acoustic signal through it. Therefore, changes in acoustic channel impulse response (CIR) and acoustic signal level can be a valued indicator of water pollution. This paper presents preliminary laboratory results of the effects of two water contaminants, namely, motor oil and liquid detergent and shows how they change the water channel and in turn changes resulting acoustic CIR and signal level. The differences in CIR and signal level between clean water and that with pollutants can be utilized to monitor water pollution and its level effectively.

Analysis of Underwater Acoustic Communication Channels

The underwater acoustic communication (UAC) channel presents many difficulties such as high frequency, space, and time selectivity, frequency-dependent noise, and significant range and band limitation on transmission. Traditional UAC channel models that model such channels primarily include environmental models based on experimental data; models that are developed using mathematical equations such as wave equations, modal methods, and parabolic equations; and using statistical distributions. These methods/models are often limited in their coverage and accurate representations of every possible UAC channel environment. It is also physically impractical and cost ineffective to try to measure/estimate each channel to determine its model. In this paper, the authors will present the analysis of UAC channels according to the UAC channel environments classified and presented in a prior work by the authors, in which cognitive intelligence is used in the selection of the appropriate channel representations according to each sensed environment. To the best knowledge of the authors, this type of analysis and representation of UAC channels with respect to each UAC environment has not been addressed in the literature to date and therefore presents a significant contribution.