Multiplexing Communication Systems Research Articles

Impulse Noise Suppression based Iterative Hybrid Nonlinear in Power Line Communication

In power line orthogonal frequency division multiplexing (OFDM) communication systems, the presence of impulse noise can seriously affect system performance. One of the conventional solutions is to perform a mixed nonlinear processing on the time domain signal at the receiving end before performing subsequent OFDM demodulation. Therefore, this thesis studies an improved method based on iterative elimination of nonlinear distortion. Firstly, the received OFDM signal is subjected to hybrid nonlinear processing in the time domain. Secondly, the nonlinear distortion introduced by the time domain nonlinear processing is reconstructed by the symbols initially detected in the frequency domain, and the reconstruction accuracy is improved by iteration. Finally, the reconstructed nonlinear distortion is subtracted from the output of the hybrid nonlinear processing unit. The simulation results show that compared with the traditional hybrid nonlinear method, the performance of the proposed algorithm is much better than that of the traditional algorithm.

Multiple orbital angular momentum mode switching at multi-wavelength in few-mode fibers.

Mode division multiplexing has attracted great attention because it can potentially overcome the limitation of single-mode fiber traffic capacity. However, it has been challenging to realize multiple modes controlling and switching due to the intrinsic overlap of the modes in the transmission waveguide. As a solution, we propose a cascaded phase-shifted long-period fiber grating (PS-LPFG) based multiple mode switching scheme. Using the PS-LPFGs, the multiple guided orbital angular momentum (OAM) modes selective controlling and switching at multi-wavelength can be achieved in few-mode fibers by regulating the grating resonance condition. In principle, a N × N mode switch matrix can be realized by cascading CN2 gratings, where each grating acts as a mode switch element to achieve a couple selected OAM mode switching and meanwhile the other modes are under nonblocking status. As a proof of the concept, a 2 × 2 mode switching between two OAM modes at different wavelengths based on one PS-LPFG element is demonstrated in our experiments. The switching efficiency of the two modes at two wavelengths 1537nm and 1558nm are ∼98.4% and ∼98.7%, respectively. The proposed multiple OAM mode switch has potential applications in the future hybrid multi-dimensional multiplexing optical fiber communication systems.

Adaptive Transmission for Reconfigurable Intelligent Surface-Assisted OFDM Wireless Communications

Reconfigurable intelligent surfaces (RISs) have recently emerged as an innovative technology for improving the coverage, throughput, and energy/spectrum efficiency of future wireless communications. In this paper, we propose a new transmission protocol for wideband RIS-assisted single-input multiple-output (SIMO) orthogonal frequency division multiplexing (OFDM) communication systems, where each transmission frame is divided into multiple sub-frames to execute channel estimation simultaneously with passive beamforming. As the training symbols are discretely distributed over multiple sub-frames, the channel state information (CSI) associated with RIS cannot be estimated at once. As such, we propose a new channel estimation method to progressively estimate the associated CSI over consecutive sub-frames, based on which the passive beamforming at the RIS is fine-tuned to improve the achievable rate for data transmission. In particular, during the channel training, the RIS plays two roles of embedding training reflection states for progressive channel estimation and performing passive beamforming for data transmission on the data tones. Based on the estimated CSI in each sub-frame, we formulate an optimization problem to maximize the average achievable rate by designing the passive beamforming at the RIS, which needs to balance the received signal power over different sub-carriers and different receive antennas. As the formulated problem is non-convex and thus difficult to solve optimally, we propose two efficient algorithms to find high-quality solutions. Simulation results validate the effectiveness of the proposed channel estimation and beamforming optimization methods. It is shown that the proposed joint channel estimation and passive beamforming scheme is able to drastically improve the average achievable rate and reduce the delay for data transmission as compared to existing schemes.

A Quadrature Amplitude Modulation Receiver Model with Matched Filters

In this study, it is mathematically shown that the detector inputs obtained using the matched filters in the Quadrature Amplitude Modulation (QAM) receiver are the same expressions as those obtained using the integrators if a rectangular pulse is used in the transmitter and the channel is Additive White Gaussian Noise (AWGN). Under the assumption that there is a perfect synchronization between the transmitter and the receiver. QAM is intensively used at the present time for the data transmission on subchannels in orthogonal frequency-division multiplexing (OFDM) communication systems such as digital subscriber lines (DSLs) modems, wireless local area networks (WLANs) and cellular wireless communication systems (e.g. 4G LTE). Cyclic-prefix OFDM is deployed in 4G LTE systems and the rectangular pulse is used in the time domain to shape all subcarriers of OFDM.

Joint CFO and Channel Estimation in OFDM Systems Using Sparse Bayesian Learning

In this article, we study the problem of joint carrier frequency offset (CFO) and sparse channel estimation in orthogonal frequency-division multiplexing (OFDM) communication systems, from the perspective of sparse Bayesian learning (SBL) framework. We first consider the problem in the compressed sensing (CS) context and reformulate it as the problem of recovering a sparse vector from the received signal when CFO and noise variance are unknown parameters that should also be estimated. A novel SBL-based scheme has then been designed to iteratively estimate the CFO, channel impulse response (CIR), and variance of the noise jointly, using the expectation-maximization (EM) algorithm. Both theoretical analysis and simulation results confirm that the proposed scheme outperforms the existing methods while requiring a lower computational cost.

A Novel Ultra-Broadband Polarization Filter Based on a Microstructured Optical Fiber with a Gold-Coated Air Hole.

In this paper, a pentagonal microstructured optical fiber polarization filter by utilizing a surface plasmon resonance effect is proposed. The characteristics of the mode-coupling and filtering of the filter are studied by making use of the full-vector finite element method. The performance of the filter is greatly affected by the structure parameters. The losses of Y and X polarization of the fiber core are 665.97 and 0.17 dB/cm at 1.55 μm, respectively, and the loss ratio is 3917.47. This shows that the filter has a greater loss ratio. Moreover, both the extinction ratio and tolerance are also researched, which shows that the proposed filter has a wider filtering bandwidth and better fabrication tolerance. The designed filter has an important role in wavelength-division multiplexing (WDM) and coherent optical fiber communication systems.

A hybrid OFDM–CDMA-based robust image watermarking technique

Digital watermarking is a process of embedding hidden information called watermark into different kinds of media objects. It uses basic modulation, multiplexing and transform techniques of communication for hiding information. Traditional techniques used are least significant bit (LSB) modification, discrete cosine transform (DCT), discrete wavelet transform (DWT), discrete Fourier transform (DFT), code division multiple access (CDMA) or a combination of these. Among these, CDMA is the most robust against different attacks except geometric attacks. This paper proposes a blind and highly robust watermarking technique by utilizing the basis of orthogonal frequency division multiplexing (OFDM) and CDMA communication system. In this scheme, the insertion process starts by taking DFT of host images, permuting the watermark bits in randomized manner and recording them in a seed as a key. Then PSK modulation follows inverse DFT (IDFT) that gives watermark information as OFDM symbols. These symbols are spread using spreading codes and then arithmetically added to the host image. Finally, scheme applies inverse DCT (IDCT) to get watermarked host images. The simulation results of the proposed scheme are compared with CDMA-based scheme in DCT domain. The results show that the robustness of the proposed scheme is higher than the existing scheme for non-geometric attacks.

Experimental Demonstration of Nonlinear Frequency Division Multiplexing Transmission With Neural Network Receiver

Nonlinear frequency division multiplexing (NFDM) communication systems that are based on the nonlinear Fourier transform (NFT), have seen a rapid improvement in performance and transmission reach over just a few years. However, such an improvement is now being slowed down by fundamental challenges such as fiber loss and noise. As the NFT theory is defined over a lossless transmission fiber, a strong research focus has been dedicated to either improve the lossless assumption for practical fibers, by adapting the theory to approximately account for the fiber loss, or by devising encoding schemes that increase the robustness of the NFT to the fiber attenuation. However, the proposed solutions provide only minimal benefits to the system performance, especially for long fiber spans as in deployed links. Alternatively, a detection strategy based on replacing a conventional NFT receiver with a time-domain Neural network (NN)-based symbol decisor has been numerically proposed. Here, we extend such an idea by validating it experimentally. In order to apply the method in an experimental environment, the impact of phase noise, and receiver frequency offset needs to be addressed. We, therefore, propose a novel time-domain receiver architecture that combines a two-stage iterative carrier recovery with a NN-based symbol decisor. The carrier recovery, itself based on aNN for phase estimation, is numerically, and experimentally characterized. The proposed receiver has been evaluated for single-polarization two-eigenvalue transmission at 1 GBd. A two-fold increase in the transmission reach is enabled by the NN receiver (≈1600 km) compared to a conventional NFT receiver (≈560 km) for a practical link using 80-km spaced erbium-doped fiber amplifier (EDFAs).

Time-Varying Channel Equalization in Underwater Acoustic OFDM Communication System

In this paper, three time-varying channel equalization schemes are studied in the underwater acoustic (UWA) Orthogonal Frequency Division Multiplexing (OFDM) communication system. The equalization algorithms are the zero-forcing (ZF) equalization algorithm, and the minimum mean square error equalization (MMSE) algorithm and the serial interference cancellation (SIC) equalization algorithm. Among the schemes, there is a problem of needing a large amount of operation when obtaining the inversion of the channel matrix. Then, to reduce the computation complexity of channel matrix inversion, the band approximation of the channel matrix, the serial equalization and the LDLH decomposition are also studied. To evaluate the efficacy of the algorithms studied in this paper, numerical simulation and the field experiment are both conducted. The simulation results proof that each equalization algorithm can work appropriately under different time-varying conditions, and valid the reliability of each simplified algorithm under the same Doppler factor. The results of two sets of field experiment also prove that the simplified algorithm eliminates the influence of the residual narrow band Doppler to a certain extent, and a better effect is obtained while a channel estimation algorithm with higher accuracy is combined.

Sparse DFT Based Channel Estimation In OFDM Systems

Channel estimation is an essential part of Orthogonal Frequency Division Multiplexing (OFDM) communication systems. In this paper, two Discrete Fourier Transform (DFT) improvement algorithms are proposed and compared where the 1st one exploits channel sparsity concept while the other considers significant channel coefficients only. In the proposed algorithms; Enhanced and Sparse DFT (E-DFT and S-DFT), different number of significant channel components is selected either by a threshold determining procedure such as in E-DFT, or through determining channel sparsity level such as in S-DFT. In the presence of Doppler frequency shifts, the Inter Symbol Interference (ISI) effect on channel coefficients is successfully reduced using the proposed estimation algorithms. Vehicular A-ITU channel model is considered with a relatively high vehicle speed up to 68 Km/h in order to test the suitability of the proposed algorithms for mobile systems. E-DFT and S-DFT improves conventional as well as previous DFT improvement methods (I-DFT) suggested by [7], [8], [9], [15]. For 64 subcarriers, S-DFT outperforms E-DFT and I-DFT by about 3dB at a BER of 0.01 with a mobility reaches 45 Km/h, and by about 0.4dB and 2.5dB at a BER of 0.02 with a mobility reaches 68Km/h.

Density-matrix formalism for modal coupling and dispersion in mode-division multiplexing communications systems.

Borrowing methodology from quantum mechanics, we propose and develop a density-matrix formalism for modal coupling and dispersion in mode-division multiplexing communications systems. The central concept in our formalism is the density matrix, from which all observable information of an optical field can be handily accessed. In the formalism, we derive fundamental evolution equations and concatenation rules for the key elements that characterize essential modal properties, and construct a statistical model ready for the numerical analysis of stochastic light propagation in randomly perturbed fibers. Unlike the Stokes-vector formalism that requires J2 - 1 auxiliary Gell-Mann matrices, the density-matrix formalism can be directly formulated for arbitrary modal-space dimension J. Based on the density-matrix formalism, the statistical modal properties of a 4-mode fiber under random perturbation are numerically investigated, which raises an interesting possibility of optimizing the modal dispersion by manipulation of the random perturbation.

Joint low‐complexity nonlinear equalization and carrier frequency offset compensation for multiple‐input multiple‐output orthogonal frequency division multiplexing communication systems

AbstractThe conventional zero forcing (ZF) equalizer suffers from the noise enhancement problem and the increasing complexity with the increase of the number of subcarriers. On the other hand, the minimum mean square error (MMSE) equalizer mitigates the noise enhancement, but it needs estimation of the signal‐to‐noise ratio (SNR) to work properly. In addition, the Joint Low‐complexity Regularized ZF (JLRZF) equalization and carrier frequency offset compensation scheme mitigates the noise enhancement using a constant parameter called the regularization parameter without SNR estimation. In this paper, we present a JLRZF with successive iInterference cancelation (JLRZF‐SIC) scheme for multiple‐input multiple‐output (MIMO) discrete Fourier transform orthogonal frequency division multiplexing (DFT‐OFDM) system to cope with the above‐mentioned problems. The proposed JLRZF‐SIC scheme jointly performs the equalization and the carrier frequency offset (CFO) compensation processes in two steps. The coupling nature of the MIMO channel is canceled in the first step. A regularization term is added in the second step to avoid the inter‐symbol‐interference (ISI). Simulation results show that the proposed JLRZF‐SIC scheme improves the system performance with low complexity, especially in the presence of estimation errors.

Tuneable liquid crystal asymmetric dual-core photonic crystal fiber mode converter

In this work, a novel and compact mode selective coupler based on an asymmetric dual core photonic crystal fiber with a thermo-responsive core is proposed and numerically studied. Simulation results show that high conversion efficiencies are achieved between LP01 and LP11 modes in the O-band with a wide bandwidth of 24.55 nm and a mode conversion efficiency over 60% when it has a total length of 3.15 mm and the operating wavelength is 1310 nm. In addition, the thermally tuneability was evaluated when this device was submitted to thermal changes from 15°C to 35°C. The results evidence that thermal effect allows to control the operating wavelength and the mode coupling efficiency of the mode converter when it is used to couple energy from the LP01 mode to the LP11 mode. Therefore, the proposed device could be useful in high-bandwidth mode division multiplexed communication systems.

Performance Evaluation of Fiber Bragg Grating Strain Sensor Integrating in WDM Communication System

In this paper, a fiber Bragg grating (FBG) strain sensor is simulated utilizing OptiGrating software. Then the proposed sensor is integrated into the wavelength division multiplexing (WDM) communication system via OptiSystem software in order to evaluate its performance as a strain sensor. The proposed WDM system has 4–channels with NRZ modulation format and 100 GHz frequency spacing. According to the results, the degradation in the receiving signal reaching the limitation value (6 for Q–factor and 10E–9 for BER) at applied strain ranging from 170 µ –to– 180 µ.

Performance of chip-scale optical frequency comb generators in coherent WDM communications.

Optical frequency combs have the potential to become key building blocks of wavelength-division multiplexing (WDM) communication systems. The strictly equidistant narrow-band spectral lines of a frequency comb can serve either as carriers for parallel WDM transmission or as local-oscillator (LO) tones for parallel coherent reception. When it comes to highly scalable WDM transceivers with compact form factor, chip-sale comb sources are of particular interest, and recent experiments have demonstrated the viability of such devices for high-speed communications with line rates of tens of Tbit/s. However, the output power of chip-scale comb sources is generally lower than that of their conventional discrete-element counterparts, thus requiring additional amplifiers and impairing the optical signal-to-noise ratio (OSNR). In this paper, we investigate the influence of the power and optical carrier-to-noise ratio (OCNR) of the comb lines on the performance of the WDM link. We identify two distinctively different regimes, where the transmission performance is either limited by the comb source or by the link and the associated in-line amplifiers. We further investigate the impact of line-to-line power variations on the achievable OSNR and link capacity using a soliton Kerr frequency comb as a particularly interesting example. We believe that our findings will help to compare different comb generator types and to benchmark them with respect to the achievable transmission performance.

Compressed sensing channel estimation for STBC‐SM based hybrid MIMO‐OFDM system for visible light communication

SummaryThis paper presents the idea of sparse channel estimation using compressed sensing (CS) method for space–time block coding (STBC), and spatially multiplexing (SM) derived hybrid multiple‐input multiple‐output (MIMO) Asymmetrically clipped optical‐orthogonal frequency division multiplexing (ACO‐OFDM) optical wireless communication system. This hybrid system accounts multiplexing gain of SM and diversity gain of STBC technique. We present a new variant of sparsity adaptive matching pursuit (SaMP) algorithm called dynamic step‐size SaMP (DSS‐SaMP) algorithm. It makes use of the inherent and implicit structure of SaMP, along with dynamic adaptivity of step‐size feature which is compatible with the energy of the input signal, thus the name dynamic step size. Existing CS‐based recovery algorithms like orthogonal matching pursuit, SaMP, adaptive step‐size SaMP, and proposed DSS‐SaMP were compared for hybrid MIMO‐ACO‐OFDM visible light communication system. The performance analysis is demonstrated through simulation results with respect to bit error rate, symbol error rate, mean square error, computational complexity, and peak‐to‐average power ratio. Simulation results show that the proposed technique gives improved performance and lesser computational complexity in comparison with conventional estimation algorithms.

Stable generation of orbital angular momentum mode with an all-fiber laser

In this letter, an all-fiber laser that can generate stable OAM modes with topological charges of ±1 is demonstrated. A ring-core fiber designed in-house is used to reduce the crosstalk between OAM mode and other modes. A ring-core fiber-based long-period fiber grating (LPFG) and a fiber Bragg grating (FBG) are employed to select desired OAM mode. The coupling ratio of LPFG from HE11 mode to OAM mode exceeds 16 dB, and the mode purity of OAM mode is as high as 94.41%. This simple and compact fiber laser can be integrated into fiber-based OAM mode multiplexing optical communication system.

AoA-based channel estimation for massive MIMO OFDM communication systems on high speed rails

Channel estimation is a well-known challenge for wireless orthogonal frequency division multiplexing (OFDM) communication systems with massive antennas on high speed rails (HSRs). This paper investigates this problem and design two practicable uplink and downlink channel estimators for orthogonal frequency division multiplexing (OFDM) communication systems with massive antenna arrays at base station on HSRs. Specifically, we first use pilots to estimate the initial angle of arrival (AoA) and channel gain information of each uplink path through discrete Fourier transform (DFT), and then refine the estimates via the angle rotation technique and suggested pilot design. Based on the uplink angel estimation, we design a new downlink channel estimator for frequency division duplexing (FDD) systems. Additionally, we derive the Cramer-Rao lower bounds (CRLBs) of the AoA and channel gain estimates. Finally, numerical results are provided to corroborate our proposed studies.

Study on orthogonal superposition generation method of double-ring vortex beams

In the orbital angular momentum (OAM) multiplexing communication system, due to the orthogonality between OAM modes, two vortex beams with different topological charges are orthogonally superimposed to generate a double-ring vortex beam. The double-ring vortex beam generated by this method can ensure the independent propagation of information without interference. In this paper, the intensity and phase distributions of the double-ring vortex beam are theoretically analyzed and verified experimentally. The results show that the orthogonal superposition of any two vortex beams with different topological charges can generate the double-ring vortex beam. The distance between the double rings increases with the increase in the difference between the absolute values of the topological charges of two vortex beams. Using double-ring vortex beam to carry different information at the same time provides a new approach for improving channel capacity and transmission stability.

Tunable 50 GHz laser comb generation of multiwavelength Brillouin erbium fiber laser

We experimentally proposed widely tunable multiwavelength Brillouin erbium fiber laser with quintuple Brillouin frequency spacing (∼ 0.4 nm, ∼ 50 GHz). Only three dispersion compensation fiber spools with total length of 15 km as a Brillouin gain media are used. Two sections of an erbium-doped fiber amplifier with maximum pump power of 220 mW are utilized. In the developed structure, at port1, we achieved four quintuple Stokes lines of about 10 dB peak power and large optical signal to noise ratio of more than 50 dB. Wide tuning range of 40 nm (1540–1580 nm) in the absence of free running modes is recorded using high BP power, BP pre-amplification, and extracting the residual pump power outside the cavity. In addition, another 14 quintuple Stokes channels are produced at another four output ports (port 2 to port 5). The whole generated 50 GHz wavelength spacing signals can be used in dense wavelength division multiplexing communication systems as such large channel spacing can be easily recognized by the optical detector at the receiver terminal.