Conventional Single-carrier Research Articles

Attention-assisted autoencoder neural network for end-to-end optimization of multi-access fiber-terahertz communication systems

We propose an end-to-end (E2E) fiber-terahertz (THz) integrated communication system based on an attention-assisted multi-access autoencoder (AMAE) neural network. The AMAE neural network comprises artificial neural networks (ANNs) that function as transmitters (T-ANNs), channel models, and receivers (R-ANNs) for multiple users. By connecting the computational graph of multiple T-ANNs and R-ANNs, we jointly optimize the AMAE to facilitate E2E multi-access communication. Attention mechanisms guide the optimization process to achieve fair and efficient power allocation and orthogonality among different users. We experimentally evaluated the performance of our proposed E2E framework in a 60 Gbit/s multi-channel (1, 5, and 10 km) fiber-THz hybrid system. The results indicate that our AMAE approach outperforms the conventional single-carrier quadrature amplitude modulation scheme by over 3 dB in receiver sensitivity and 11 Gbit/s in capacity under the 20% soft-decision forward error correction threshold in the same-channel back-to-back condition. Additionally, under the performance balance constraint, our approach achieves a transmission speed of 60 Gbit/s within a 10 GHz bandwidth in the multi-channel setting.

Socioeconomic Analysis of Integrated Energy System of China for 2020

Fossil fuel source depletion, oil price fluctuations, less-efficient conventional single energy carrier network, and renewable energy intermittency are the most significant issues of the power system. An integrated energy system is a promising measure to mitigate the dependence on fossil fuel sources and decarbonize the global economy by omitting the challenges posed by integrating renewable energy sources. However, the fluctuations in fuel and energy prices make it a rigorous task to find out the most cost-effective approach of the integrated energy system. Therefore, this article aims to determine the most economically beneficial alternative to attain a sustainable energy supply. In this article, a feasibility analysis is conducted for three different fuel prices and socioeconomic analysis is performed by investigating market exchange feasibility using EnergyPLAN market simulation strategy. The proposed energy analysis software computes the yearly energy system simulation within a few seconds, which makes it more efficient than existing energy analysis tools. Socioeconomic costs are evaluated to find out the least cost solution. Results substantiate that when compared with feasibility analysis and scenario 5 (previous study most optimal scenario), the market exchange case proves to be socioeconomically optimal due to maximum net earnings on energy exports .

Enhanced Data Transmission Rate of XCTD Profiler Based on OFDM

An expendable conductivity-temperature-depth profiler (XCTD) is one of the most important instruments used to obtain hydrological data, such as temperature and conductivity, and detect ocean depth in a large area. However, the XCTD channel provides poor time-varying performance, narrowband, and low signal-to-noise ratio (SNR), which severely restricts the data transmission rate. In contrast to conventional single-carrier modulation techniques, such as amplitude-shift keying and differential phase-shift keying, this article provides a new method, based on orthogonal frequency division multiplexing (OFDM) to enhance the data transmission rate of deep-sea abandoned profilers. We apply the OFDM to enhance the SNR of the XCTD, which is achieved by reducing the data transmission rate of each sub-channel. Moreover, the bandwidth utilization may be improved by increasing the number of subcarriers in a given bandwidth, which enhances the data transmission rate. Based on analysis of the XCTD channel model, OFDM with different parameters such as constellation mapping, number of subcarriers, subcarrier spacing, signal period and cyclic prefix are achieved. To verify the effectiveness of the OFDM, this study investigates the influence of different parameters on the data transmission rate at different noise levels, i.e., -20 dB and -40 dB.

Optimal operation strategy for multi-carrier energy systems including various energy converters by multi-objective information gap decision theory and enhanced directed search domain method

Multi-carrier energy systems can increase energy efficiency due to the ability of these systems to consider and optimize the interactions of various energy carriers. However, the operation of these systems is somehow different from the operation of conventional single-carrier energy systems. In this paper, a new operation strategy for multi-carrier energy systems including natural gas and electricity is proposed. Various energy converters, including conventional and renewable electricity generators, gas furnace, and combined heat and power generator, are modeled in the proposed strategy to supply different electric, heat, and gas loads in the output. The proposed operation strategy employs a multi-objective information gap decision theory approach to model the uncertainty sources of multi-carrier energy systems, such as the uncertainties of demand forecasts, wind power forecast, and photovoltaic power forecast. For solving the multi-objective optimization problem, based on information gap decision theory, for multi-carrier energy system operation, an enhanced directed search domain method is proposed as a new multi-objective optimization approach. The performance of the proposed multi-carrier energy system operation optimization model and the proposed enhanced directed search domain solution method is investigated on the IEEE 118-bus test system by comparing with the other alternatives.

Probabilistic Amplitude Shaping for a 64-QAM OFDM W-Band RoF System

In this letter, probabilistic amplitude shaping (PAS) is performed in the case of orthogonal frequency division multiplexing (OFDM) by combining with orthogonal circulant matrix transform (OCT) precoding. Thanks to the precoding method, one optimized probabilistic distribution is globally implemented across 240 subcarriers by merely one distribution matcher (DM). The proposed probabilistic shaping scheme for OFDM is experimentally demonstrated in a 64-ary quadrature amplitude modulation (QAM) W-band radio-over-fiber (RoF) system using direct-detection (DD), with 20-km single-mode fiber (SMF), and 0.5-m free-space transmission. The experimental results show that, compared to the uniform 64-QAM, there is 1.07-dB shaping gain for system sensitivity at BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> . Meanwhile, it inherits the advantage of flexible capacity granularity from the conventional single-carrier PAS.

A Dual-Mode Index Modulation Scheme With Gray-Coded Pairwise Index Mapping

In this letter, we propose a single-carrier (SC) dual-mode (DM) index modulation scheme with Gray-coded pairwise index mapping. To improve the index error rate performance of DM schemes, we apply Gray-coding to the mapping of modulation patterns. The error propagation within each symbol group in DM schemes can also be avoided due to the pairwise pattern design. Moreover, the mapping ideas in the proposed scheme can be easily extended to other index modulation schemes to obtain better bit error rate performance. Simulation results demonstrate the performance advantage of the proposed scheme over conventional SC and DM schemes in frequency selective fading channels.

Software-Defined Optoelectronics: Space and Frequency Diversity in Heterodyne Interferometry

In this paper, a general architecture for the implementation of software-defined optoelectronic systems is described. This concept harnesses the flexibility of software-defined hardware to implement optoelectronic systems which can be configured to adapt to multiple high speed optical engineering applications. As an application example, a software-defined optical interferometer using a commercial software-defined radio platform is built. The system is tested by performing high speed optical detection of laser-induced photoacoustic signals in a concentrated dye solution. Using software modifications only, conventional single carrier and also multicarrier heterodyne techniques with space and frequency diversity are performed. The latter technique shows a noticeable performance improvement in single-shot interferometric measurements.

FPGA-Based Reconfigurable PWM Generator for Power Electronic Converter Applications

Application of digital hardware based on FPGA in power electronic converters control systems has grew up in recent years. One of the most important issues in hardware implementation of power converters controller systems is the PWM signal generation. This issue is more important in case of multi-level converters with several carries. Implementation of these types of PWM strategies with conventional PWM peripherals in existing DSPs and microcontroller is very complicated and sometimes impossible, because existing PWM generation peripherals are based on single carrier and optimized for two-level inverter applications. This paper presents a multi-carrier-based generalized reconfigurable PWM generator hardware that can be used in power electronic converters control systems. The proposed hardware can be implemented on both FPGA and ASIC as a peripheral for a main processor. The proposed architecture can be easily configured to support several PWM strategies including conventional single-carrier and novel multi-carrier PWM techniques. The proposed hardware includes output control and programmable dead-band generator module as well. The system function selection and other configurations can be carried out using dedicated control and configuration registers. Whole architecture and internal modules are designed and implemented in Verilog HDL and simulated then synthesized and tested successfully on Altera Cyclone II FPGA using FPGA development board and laboratory prototypes. Simulation and experimental results for various types of modulation strategies are presented verifying generality and reconfigurability of the proposed hardware.

Single-Carrier Frequency-Domain Equalization With Index Modulation

Motivated by the recent index modulation (IM) concept in the spatial, frequency, and space-time domains, we propose a novel broadband single-carrier (SC)-based IM (SC-IM) scheme as well as its low-complexity frequency-domain equalization (FDE) algorithm. The proposed FDE-aided SC-IM scheme outperforms the conventional FDE-aided SC scheme, while achieving a comparable low level of detection complexity. In addition, in order to reduce the inter-channel correlation arising from the activated symbol indices, we propose a symbol mapping algorithm, which further improves the achievable error-rate performance of the SC-IM scheme. Our simulation results demonstrate the performance advantage of the proposed FDE-aided SC-IM scheme over the conventional FDE-aided SC scheme.

Dual-Mode Time-Domain Index Modulation for Nyquist-Criterion and Faster-Than-Nyquist Single-Carrier Transmissions

In this paper, we propose a novel dual-mode time-domain single-carrier (SC) index modulation (DM-SCIM) scheme, where the combination of two constellation modes carries information bits further to modulated symbols, hence increasing the transmission rate. Accordingly, the proposed DM-SCIM scheme is capable of attaining a higher transmission rate than the conventional SC and SCIM schemes, without imposing any additional performance penalty. Furthermore, in order to maintain the detection complexity low enough to be tractable at the handset receiver, a successive detection algorithm, comprising minimum mean-square error (MMSE)-based frequency-domain equalization and log-likelihood ratio detection, is developed for both the conventional SCIM and the proposed DM-SCIM schemes. We also derive the error-rate bound of the proposed DM-SCIM scheme under the assumption of a channel-uncoded scenario. Moreover, in order to further increase bandwidth efficiency, the proposed DM-SCIM scheme is extended to the scenario supporting faster-than-Nyquist signaling, where the symbol interval in the time domain is set to be smaller than that defined by the Nyquist criterion. Our simulation results demonstrate the explicit performance advantage of the proposed schemes over the existing SC schemes. It is also revealed that our DM-SCIM scheme is capable of achieving a significantly lower peak-to-average power ratio than orthogonal frequency division multiplexing (OFDM) and OFDM with index modulation, while benefiting from a high reliability, which is achievable by multipath diversity.

Logarithmic Companding Technique to Improve Performance of SC-FDMA Systems

Objectives: This paper proposes logarithmic companding technique to improve PAPR and BER performance of conventional Single Carrier Frequency Division Multiple Access (SC-FDMA) system. Methods/Analysis: The lower signal envelope fluctuations in SC-FDMA signal results in low peak to average power ratio, in contrast to that of OFDMA. But still, there is a need to minimize PAPR in SC-FDMA systems. Although there are many techniques to minimize PAPR in SC-FDMA systems they are highly complex or need side information to be transmitted. This paper proposes a companding technique based on a logarithmic function which provides better PAPR reduction when compared to μ law companding. Findings: Simulation results demonstrate that proposed method provides better PAPR and BER performance when compared to conventional SC- FDMA system and μ law companding.

Chromatic dispersion mitigation in long-haul fiber-optic communication networks by sub-band partitioning.

We propose and experimentally demonstrate a novel sub-band multiplexed data architecture for chromatic dispersion (CD) mitigation. We have demonstrated 32 GBaud multi-sub-band (MSB) dual-polarization (DP) 16QAM transmission over 2400 km. Using this approach, the transmitted signal bandwidth is divided into multiple narrow-bandwidth sub-bands, each operating at a lower baud rate. Within each sub-band bandwidth, the CD frequency response can be approximated as a linear-phase band-pass filter, which can be considered as an analog delay that does not require compensation. Therefore, the resulting receiver digital signal processing (DSP) is simplified due to the removal of the CD compensation equalizer. In addition, this leads to efficient parallelization of DSP tasks by deploying multiple independent sub-band processors running at a lower clock rate. The proposed system reduces receiver computational complexity and offers 1 dB higher Kerr-nonlinearity tolerance and 2% extended transmission reach in comparison to the conventional single carrier systems.

Multicarrier Offset-QAM for Long-Haul Coherent Optical Communications

The availability of high-speed digital-to-analog converters for coherent optical communication systems makes it possible to digitally divide the available electrical bandwidth into subchannels on each optical carrier which enables a parallel implementation of the digital signal processing. In this paper, we use electrical multicarrier offset-QAM modulations to achieve crosstalk-free modulation for subcarriers spaced at the symbol rate. We show that in coherent optical communication systems, several impairments are bound to break the subcarrier orthogonality and cause failures of payload symbol recovering. We propose a dedicated digital signal processing architecture which implements channel estimation and a new algorithm to track the phase in multicarrier offset-QAM modulation. We experimentally compare the latter with conventional multicarrier and single carrier QAM modulation so as to assess the benefits of crosstalk mitigation via offset-QAM modulation. We carry out experiments to test the transmission performance of multicarrier offset-QAM modulation in a dispersion-unmanaged long-haul transmission link.

Extending Reading Range of Commercial RFID Readers

In this paper, multisine excitation signals are used to extend the reading range of commercial RF identification readers. To do so, a commercial reader is equipped with an external multisine front-end that implements previous mathematical proposals. A mathematical description is presented in order to show the ability of multisine signals to communicate data, with minimal changes in the downlink path, while no changes are required in the conventional tag architecture. Moreover, and most important, if a proper multisine design is performed, a conventional reader receiver is still able to demodulate and decode the backscattered multisine signal from the tag, without any hardware change. Thus, guidelines are presented for multisine design, including multisine nature, central tone positioning, tone separation, and bandwidth requirements. In order to evaluate the reading range improvement, when compared with the conventional single carrier approach with the same average power, two experiments are conducted: in the first one, an oscilloscope is used to measure the tag response and to decide whether the tag does or does not respond. In the second measurement scenario, the downlink path is implemented by the reader combined with the front-end and the uplink is implemented solely by the reader. In this case, the decision on successful tag response is taken when the reader reads the tag identification. The first measurement scenario has pointed out for a maximum reading range improvement of near 43% for an eight-tone multisine signal with 2-MHz tone separation. In the second scenario, a more realistic one, a reading range improvement of almost 25% has been obtained for a 8 + 1 tones multisine.

An analytical study of the improved nonlinear tolerance of DFT-spread OFDM and its unitary-spread OFDM generalization

DFT-spread (DFT-S) coherent optical OFDM was numerically and experimentally shown to provide improved nonlinear tolerance over an optically amplified dispersion uncompensated fiber link, relative to both conventional coherent OFDM and single-carrier transmission. Here we provide an analytic model rigorously accounting for this numerical result and precisely predicting the optimal bandwidth per DFT-S sub-band (or equivalently the optimal number of sub-bands per optical channel) required in order to maximize the link non-linear tolerance (NLT). The NLT advantage of DFT-S OFDM is traced to the particular statistical dependency introduced among the OFDM sub-carriers by means of the DFT spreading operation. We further extend DFT-S to a unitary-spread generalized modulation format which includes as special cases the DFT-S scheme as well as a new format which we refer to as wavelet-spread (WAV-S) OFDM, replacing the spreading DFTs by Hadamard matrices which have elements +/-1 hence are multiplier-free. The extra complexity incurred in the spreading operation is almost negligible, however the performance improvement with WAV-S relative to plain OFDM is more modest than that achieved by DFT-S, which remains the preferred format for nonlinear tolerance improvement, outperforming both plain OFDM and single-carrier schemes.

Mitigation of Intercarrier Interference for OFDM Over Time-Varying Underwater Acoustic Channels

Orthogonal frequency division multiplexing (OFDM) has emerged as a promising modulation scheme for underwater acoustic (UWA) communications, thanks to its robustness to channels with severe time dispersion. Compared to conventional single-carrier systems, for which complicated equalization schemes are usually required, OFDM systems are in general much simpler to implement as detection can be carried out symbol-by-symbol over time-dispersive channels. In this paper, we focus on cyclic-prefixed OFDM over time-varying UWA channels. To cope with the intercarrier interference (ICI) that arises at the receiver side because of the time variations in the channel, we consider two ICI-mitigation techniques. In the first scheme, the ICI coefficients are explicitly estimated, and minimum mean square error linear equalization based on such estimates is performed. In the second approach, no explicit ICI estimation is performed, and detection is based on an adaptive decision-feedback equalizer applied in the frequency domain across adjacent subcarriers. To cope with the phase variations of the ICI coefficients, phase-tracking loops are introduced in both ICI-mitigation schemes. The effectiveness of the presented schemes is demonstrated through simulation results, as well as real data collected in a recent experiment conducted in shallow water off the western coast of Kauai, HI, in June 2008.

MMSE Beamforming for SC-FDMA Transmission over MIMO ISI Channels

We consider transmit beamforming for single-carrier frequency-division multiple access (SC-FDMA) transmission over frequency-selective multiple-input multiple-output (MIMO) channels. The beamforming filters are optimized for minimization of the sum of the mean-squared errors (MSEs) of the transmitted data streams after MIMO minimum mean-squared error linear equalization (MMSE-LE), and for minimization of the product of the MSEs after MIMO MMSE decision-feedback equalization (MMSE-DFE), respectively. We prove that for SC-FDMA transmission in both cases eigenbeamforming, diagonalizing the overall channel, together with a nonuniform power distribution is the optimum beamforming strategy. The optimum power allocation derived for MMSE-LE is similar in spirit to classical results for the optimum continuous-time transmit filter for linear modulation formats obtained by Berger/Tufts and Yang/Roy, whereas for MMSE-DFE the capacity achieving waterfilling strategy well known from conventional single-carrier transmission schemes is obtained. Moreover, we present a modification of the beamformer design to mitigate an increase of the peak-to-average power ratio (PAPR) which is in general associated with beamforming. Simulation results demonstrate the high performance of the proposed beamforming algorithms.

Time-Frequency Cyclic Shift Keying Transceiver for Low PAPR MC-CDMA Uplink System over Multipath Fading Channels

A low peak-to-average-power-ratio (PAPR) transceiver with a time-frequency cyclic shift orthogonal keying (TF-CSOK) technique is proposed for the uplink multi-carrier CDMA (MC-CDMA) system over multiple access interference (MAI) and multipath interference (MPI) channels. The low complexity structure of the TF-CSOK MC-CDMA system is designed to involve the FCSOK and TCSOK techniques to combat MPI and MAI effects, respectively. In particular, at the besestation, the multiuser detector employs the maximum likelihood (ML) rule and the TFSOK despreading and demapping techniques to acquire the M-ary modulation gain and diversity gain. Simulation results show that the multuser receiver has the robustness against strong MAI. Moreover, it outperforms the conventional single-carrier frequency division multiple access (SC-FDMA) system and the conventional MC-CDMA system under MAI and MPI environments.

A flexible multicarrier CDMA system with time‐varying data rates and services

AbstractWe consider an enhanced multicarrier code‐division multiple access (CDMA) system that performs the spreading in the time and frequency domains simultaneously, where time‐domain spreading codes are used as a main code and frequency‐domain spreading codes (FDSCs) are used as a subcode. By allocating the FDSCs to users, rates and services can be dynamically matched to users' needs. The resulting system can support different time‐varying source rates and, therefore, enable multimedia applications. In addition, since the proposed scheme uses multiple carriers, it has some advantages such as the mitigated multipath fading, frequency/path diversity and narrowband interference suppression capability, in contrast to a single carrier (SC) system such as the multicode CDMA scheme. Finally, the closed‐form expression for probability of bit error is derived for a frequency‐selective Rayleigh fading channel. It has been also shown that the proposed system has outperformed the conventional SC RAKE system if the available bandwidth is fixed. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Maximum-Likelihood Estimator for Coarse Carrier Frequency Offset Estimation in OFDM Systems

Orthogonal frequency division multiplexing (OFDM) systems are more sensitive to carrier frequency offset (CFO) compared to the conventional single carrier systems. CFO destroys the orthogonality among subcarriers, resulting in inter-carrier interference (ICI) and degrading system performance. To mitigate the effect of the CFO, it has to be estimated and compensated before the demodulation. The CFO can be divided into an integer part and a fractional part. In this paper, we investigate a maximum-likelihood estimator (MLE) for estimating the integer part of the CFO in OFDM systems, which requires only one OFDM block as the pilot symbols. To reduce the computational complexity of the MLE and improve the bandwidth efficiency, a suboptimum estimator (Sub MLE) is studied. Based on the hypothesis testing method, a threshold Sub MLE (T-Sub MLE) is proposed to further reduce the computational complexity. The performance analysis of the proposed T-Sub MLE is obtained and the analytical results match the simulation results well. Numerical results show that the proposed estimators are effective and reliable in both additive white Gaussian noise (AWGN) and frequency-selective fading channels in OFDM systems.