Diversity and channel estimation using time-varying signals and time-frequency techniques

The paper studies the capacities of time hopping ultra wideband (TH-UWB) systems by adopting orthogonal pulse-based modulation schemes in a multipath environment with the consideration of multiple access interference (MAI) and inter symbol interference (ISI). Recently, orthogonal pulse-based modulation schemes such as pulse shape modulation (PSM), biorthogonal PSM (BPSM) and the combination of orthogonal pulse position modulation (OPPM) and BPSM have been proposed for UWB system to reduce ISI and MAI. In this paper, we first investigate the influences of ISI and MAI on the capacities of TH-UWB systems in terms of signal-to-interference and noise ratio (SINR) for high-order modulation schemes based on various sets of orthogonal pulses. How mutual information varies with the number of multipath components (MPCs) is then analyzed. The complete mathematical analysis and simulation results are provided in detail. When maximizing the mutual information, the Gaussian distribution approximate is assumed to describe the characteristics of MAI and ISI. Simulation results are provided to verify the theoretical analysis. Some conclusions are drawn thereafter, such as that capacity depends on the pulse patterns adopted and mutual information varies with inversely proportional to the number of MPCs. We also observed that the capacities of UWB systems for OPPM-BPSM schemes are larger than those of PSM and BPSM schemes.

This chapter discusses techniques on multiple-input multiple-output (MIMO) visible light communications (VLC) system which improved the speed of data communications. It discusses the use of pilot signal in reducing the peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing (OFDM)-based intensity-modulated optical wireless communication system. The introduction of solid-state light-emitting diode (LED) lighting has attracted the attention of communications engineers worldwide, enabling the achievement of the dual functionality of room illumination while simultaneously transmitting wireless data via VLC. A Q factor analysis of the signal between single transmitter and single receiver as function of horizontal displacement beneath the center point of a transmitter was carried out, to investigate at what distance and how fast the quality of the signal degrades. The OFDM technique is attracting a lot of interest in optical wireless communications because of its resilience to multipath propagation-induced intersymbol interference (ISI), high spectral efficiency, and immunity to fluorescent light noise near digital converter (DC) region.

The primary objective of this paper is to develop output only modal identification and structural damage detection. Identification of multi-degree of freedom (MDOF) linear time invariant (LTI) and linear time variant (LTV—due to damage) systems based on Time-frequency (TF) techniques—such as short-time Fourier transform (STFT), empirical mode decomposition (EMD), and wavelets—is proposed. STFT, EMD, and wavelet methods developed to date are reviewed in detail. In addition a Hilbert transform (HT) approach to determine frequency and damping is also presented. In this paper, STFT, EMD, HT and wavelet techniques are developed for decomposition of free vibration response of MDOF systems into their modal components. Once the modal components are obtained, each one is processed using Hilbert transform to obtain the modal frequency and damping ratios. In addition, the ratio of modal components at different degrees of freedom facilitate determination of mode shape. In cases with output only modal identification using ambient/random response, the random decrement technique is used to obtain free vibration response. The advantage of TF techniques is that they are signal based; hence, can be used for output only modal identification. A three degree of freedom 1:10 scale model test structure is used to validate the proposed output only modal identification techniques based on STFT, EMD, HT, wavelets. Both measured free vibration and forced vibration (white noise) response are considered. The secondary objective of this paper is to show the relative ease with which the TF techniques can be used for modal identification and their potential for real world applications where output only identification is essential. Recorded ambient vibration data processed using techniques such as the random decrement technique can be used to obtain the free vibration response, so that further processing using TF based modal identification can be performed.

In this article, a band-limited three-level modulation scheme is proposed for a three-phase neutral-point-clamped (NPC) converter with an aim to balance the average capacitor voltages under all operating conditions. When conventional carrier-based pulsewidth modulation is applied, the amplitude of neutral point (NP)-voltage ripple tends to increase under linear imbalances, while the NP voltage can shift, leading to instability in the presence of nonlinear loads. A previously proposed three-level modulation scheme was able to eliminate all low-frequency capacitor voltage ripple by switching a phase between three levels in each switching period. This approach significantly increased the switching loss. The proposed modulation scheme limits the increase in switching loss by employing two operational bands, one fixed band applied to the capacitor voltages and another variable band applied to the phase currents. If the instantaneous capacitor voltages exceed the voltage band, then an appropriate phase-leg is switched in three levels to guarantee that the capacitor voltages cannot diverge further. If the instantaneous capacitor voltages reside within the voltage band, a variable current band is used to regulate switching in three voltage levels in order to obtain lower switching losses as compared to the conventional three-level modulation scheme. Therefore, the proposed modulation scheme presents a tradeoff between switching losses and capacitor voltage ripple. The experimental results are obtained under different loading conditions for validating the performance of the proposed modulation scheme.

Speckle reduction by energy time–frequency filtering

In this paper, an improved Space Vector Modulation (SVM) scheme is developed for matrix converter based rectifier. The conditions of using traditional SVM include small current ripple and continuous current operation mode (CCM). Thus the converter requires a very large inductor on DC side. To further reduce the size of converter, Discontinues current mode (DCM) is adopted for high efficiency compact design. The developed the modulation scheme works well with both CCM and DCM. The simulation results show that the line side current THD is further improved with proposed modulation scheme, especially for DCM operation.

Electric vehicles provide portable energy storage that can be used in Vehicle-to-Grid (V2G) applications. In order to source power to the grid, the power electronics on-board these vehicles must be bidirectional. A single stage bidirectional isolated AC-DC converter is discussed. A modulation scheme for this topology is proposed to provide control of real and reactive power. A control scheme is coupled with the modulation and allows the topology to act as an inductive or capacitive or load while sinking or supplying power. The proposed modulation scheme can charge the electric vehicle battery as well as provide ancillary services to the grid. The modulation and control scheme are analyzed and discussed. Simulation and hardware results are presented.

SummaryIn battery energy storage system (BESS) applications, the life of the battery depends on the quality of the charging/discharging current. However, the dual active bridge (DAB) converter, which interfaces BESS with the DC link, suffers from a backflow current in all modulation schemes, degrading the battery's performance. Hence, this paper proposes a particle swarm optimization (PSO) based universal phase shift (UPS) modulation scheme to improve the battery's life. In the proposed modulation scheme, the phase shift ratio of DAB to eliminate the backflow power is estimated using PSO. Also, it is observed that there is an improvement in the zero voltage switching (ZVS) range of the DAB. Hence, the overall efficiency of the DAB converter is increased over a wide operating region. The operating principle of the proposed modulation scheme, steady state analysis of power transmission, and backflow power are reported in this paper. The theoretical performance of the proposed modulation scheme is validated in a 2 kW experimental setup.

Optical CDMA systems have received much attention in fiber optic networks where the high-speed, the huge band width, the secure and immune communication, asynchronous and simultaneous access for multiple users on the same channel. Those features nominate strongly Optical CDMA for future optical multimedia networks. However, multiple access interference (MAI), which is originated from other simultaneous users, severely limits the capacity of the system. In this paper, an efficient transceiver architecture has been proposed using MAI cancellation to improve the performance of the existence simple transceiver without MAI cancellation. The strategy of the proposed transceiver is based on an estimation and cancellation of MAI in Optical CDMA system with employing a new spreading code named double padded modified prime code (DPMPC) and an efficient pulse-position modulation (PPM) scheme. In addition, for further improvement of the bit-error rate (BER) performance for the proposed transceiver Manchester coding (source coding) is used in signaling the transmitted data. The MAI estimation is achieved by pre-reserving one of optical spreading code sequences at the receiver based on the correlation property of the DPMPC. The proposed receiver estimates the interference and cancels it out from the received signal after the photo-detection process. The performance of the proposed transceiver architecture and its improvement will be compared with the performance of simple transceiver without MAI cancellation. The performance of various codes like modified prime code (MPC), new modified prime code (NMPC) are applied and compared with the proposed DPMPC to demonstrate its potential. An upper bound on the BER for the proposed system is derived and compared with a lower bound on the BER from the system without cancellation. The simulated computer results reveal that the performance, in terms of the BER, of the proposed system is significance improved compared with that of the system without cancellation.

Physical limitations due to fading and interference in wireless channels pose technical challenges in achieving reliable communication. To adapt to this adverse radio environment efficiently, we propose a fade-resistant transmission system based on code division multiple access (CDMA) technology. In this system, all the symbols of a user are transmitted simultaneously using parallel sequences over a time-varying wireless channel. We derive the relationship between the self-interference and the multiple-access interference in such a system. We prove that instantaneous signal-to-interference-plus-noise ratio (SINR) at the matched filter output of a system with parallel sequences is statistically higher than that of the conventional CDMA system. Numerical results demonstrate that the parallel sequence transmission scheme can provide capacity improvement over the conventional CDMA system depending on variations in the channel.

This paper concerns the estimation of a frequency offset of a known (pilot) signal propagated through a slowly fading multipath channel, such that channel parameters are considered to he constant over the observation interval. We derive a maximum-likelihood (ML) frequency estimation algorithm for additive Gaussian noise and path amplitudes having complex Gaussian distribution when covariance matrices of the fading and noise are known; we consider in detail the algorithm for the white noise and Rayleigh fading, in particular, for independent fading of path amplitudes and pilot signals with diagonal autocorrelation matrices. For the latter scenario, we also derive an ML frequency estimator when the power delay profile is unknown, but the noise variance and bounds for the path amplitude variances are specified; in particular, this algorithm can be used when path delays and amplitude variances are unknown. Finally, we consider frequency estimators which do not use a priori information about the noise variance; these algorithms are also operable without timing synchronization. All the frequency estimators exploit the multipath diversity by combining periodograms of multipath signal components and searching for the maximum of the combined statistic. For implementation of the algorithms, we use a fast Fourier transform-based coarse search and fine dichotomous search. We perform simulations to compare the algorithms. The simulation results demonstrate high accuracy performance of the proposed frequency estimators in wide signal-to-noise ratio and frequency acquisition range.

This paper proposes an input voltage disturbance suppression control strategy for the unidirectional matrix converter (UMC) with a new modulation scheme enhancing the stability. In the new scheme, the modulation index is directly, rather than reversely, proportional to the instantaneous amplitude of input filter capacitor voltages. Contrary to traditional schemes, the stability of the UMC with this new scheme is even better with the increase of the transferred active power, which is particularly suitable for applications with sinusoidal and balanced input conditions. As to the disturbed input conditions, the new scheme could introduce low-frequency harmonics into output currents. To address this issue, a feedback control strategy of output current amplitude is further proposed to eliminate the additional harmonics. Stability analysis of a UMC with the proposed modulation scheme and feedback control strategy is presented. Experimental results have verified the validity of the proposed control solution.

This paper proposes a novel modulation scheme for a multilevel Dual Active Bridge (DAB) DC–DC converter with direct power transfer capability in PV applications. The proposed modulation scheme has the ability to shape the high-frequency current waveforms, leading to lower peak/RMS values of current over a wide range of PV voltages. In addition, the multi-level structure allows for the use of low-voltage devices with significantly smaller channel resistance. The power circuit topology is based on a current-fed half-bridge (CF-HB) converter, with a coupled inductor, which facilitates the direct power transfer, resulting in significantly lower conduction losses. In summary, the proposed topology tackles both the conduction and switching losses through a multi-faceted approach by using the novel modulation scheme, the multi-level structure, the direct power transfer, and the inherent soft-switching. Detailed mathematical analysis and extensive experimental results demonstrate the superior performance of the proposed converter.

In body-centric communications, energy efficiency is a critical performance metric, while the achievable data rate is not of primary concern. In this paper we present a novel modulation scheme, which can be efficiently used in body-centric terahertz (THz) nanonetworks. The proposed scheme is a combination of the time-spread On-Off keying (TS–OOK) and the pulse position modulation (PPM) and presents lower energy consumption, compared to other existing methods as TS–OOK, at a minor cost to the data rate. Furthermore, another important aspect is that the proposed modulation scheme can be effectively used to mitigate the impact of the specific kind of noise in THz body-centric communications, thus leading to better error performance. Finally, we present analytical and simulation results in order to compare the new scheme with the existing TS–OOK.

Time-scale-chirp_rate operator for recovery of non-stationary signal components with crossover instantaneous frequency curves