Division Multiplexing Communication Systems Research Articles

Equalization and Carrier Frequency Offset Compensation for Underwater Acoustic OFDM Systems

Due to noise enhancement, conventional Zero Forcing (ZF) equalizers are not suitable for wireless Underwater Acoustic (UWA) Orthogonal Frequency Division Multiplexing (OFDM) communication systems. Furthermore, these systems suffer from increasing complexity due to the large number of subcarriers, especially in Multiple-Input Multiple-Output (MIMO) systems. On the other hand, the Minimum Mean Square Error equalizer suffers from high complexity. This type of equalizers needs an estimation of the operating Signal-to-Noise Ratio to work properly. In this paper, we propose a Joint Low-Complexity Regularized ZF equalizer for MIMO UWA-OFDM systems to cope with these problems. The main objective of the proposed equalizer is to enhance the system performance with a lower complexity by performing equalization in two steps. The co-channel interference can be mitigated in the first step. A regularization term is added in the second step to avoid the noise enhancement. Simulation results show that the proposed equalization scheme has the ability to enhance the UWA system performance with low complexity.

MMSE-based-filter and artificial noise design for MIMO–OFDM systems

Physical layer security is a crucial issue in wireless networks to prevent legitimate communication from eavesdropping. This paper investigates the physical layer security of a wireless multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) communication system in the presence of a multiple-antenna eavesdropper. We first propose a minimum mean square error (MMSE)-based-filter secure MIMO–OFDM system which can destroy the orthogonality of eavesdropper’s signals. The MMSE-based-filter is used at the receiver to suppress the inter-subcarrier interference and disturb the reception of eavesdropper while maintaining the quality of legitimate transmission. Next, we propose another artificial noise (AN)-assisted secure MIMO–OFDM system to further improve the security of the legitimate transmission. The time-domain AN signal is designed to be canceled out at the legitimate receiver. Therefore, AN will disturb the reception of eavesdropper while the legitimate transmission will not be affected. Finally, power allocation for AN signals and subcarriers will be discussed since AN is generated utilizing the residual power of the MIMO–OFDM system. Simulation results are presented to demonstrate the security performance of the proposed transmit filter design and AN-assisted scheme in the MIMO–OFDM system.

A multiple-input multiple-output orthogonal frequency division multiplexing underwater communication system using vector transducers

Vector sensors and transducers are compact multichannel devices that can be used for underwater communication via acoustic particle velocity channels (A. Abdi and H. Guo, “A new compact multichannel receiver for underwater wireless communication networks,” IEEE Transactions on Wireless Communications, vol. 8, pp. 3326-3329, 2009). In this paper, a multiple-input multiple-output (MIMO) underwater acoustic communication system is presented using orthogonal frequency division multiplexing (OFDM) modulation. Upon transmitting multiple independent data streams simultaneously over several channels, this MIMO system can increase the transmission rate, whereas the OFDM modulation mitigates the highly frequency selective underwater channels. Various components of the system including vector transducers and algorithms for synchronization, channel estimation, MIMO detection, channel coding, etc., are designed and implemented. Using this system, experiments are conducted to measure and study acoustic particle velocity channels in the MIMO setup. Additionally, system performance parameters such as bit error rate and spectral efficiency are measured and discussed for various conditions and configurations, to understand the performance of the developed vector MIMO-OFDM system. [The work was supported in part by the National Science Foundation (NSF), Grant IIP-1500123.]

An Overview on the Applications of Matrix Theory in Wireless Communications and Signal Processing

This paper overviews the key applications enabled by matrix theory in two major fields of interest in electrical engineering, namely wireless communications and signal processing. The paper focuses on the fundamental role played by matrices in modeling and optimization of wireless communication systems, and in detection, extraction and processing of the information embedded in signals. Among the major applications in wireless communications, the role of matrix representations and decompositions in characterizing multiple-input multiple-output (MIMO) and orthogonal frequency division multiplexing (OFDM) communication systems is described. In addition, this paper points out the important contribution made by matrices in solving signal estimation and detection problems. Special attention is given to the implementation of matrices in sensor array signal processing and the design of adaptive filters. Furthermore, the crucial role played by matrices in representing and processing digital images is depicted by several illustrative applications. This paper concludes with some applications of matrix theory in the area of compressive sensing of signals and by outlining a few open research problems for future study.

SPM Supression in Long Haul CO-OFDM System using SPWR-OPC Modules

Objective: The main aim of this research paper is to enhance the reach of Long-haul Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) communication system. Methods/Statistical Analysis: The purpose of SPWR and OPC in CO-OFDM system is to palliate dispersion and (Non-linearity) SPM respectively. In this proposed system Symmetrical Pulse Width Reduction (SPWR) and Optical Phase Conjugate modules (OPC) are used. The CO-OFDM system is investigated to compensate SPM by using up to 6 OPCs. Findings: While analyzing results it reveals that with the increase in the number of OPCs up to 6 enhances the transmission distance of CO-OFDM system and improves the Bit Error Rate (BER) performance but if increased beyond 6 OPCs deteriorates its performance. When using six OPCs in comparison to a single OPC, the system reach extends six times. Application/Improvement: The proposed system is simulation based and in future can be implemented commercially in Digital Terrestrial Television (DTT) and real-time audio and video communication.Keywords: Coherent Optical Orthogonal Frequency Division Multiplexing, Optical Phase Conjugate, Self Phase Modulation, Symmetrical Pulse Width Reduction

Simulation of 32-Point Split-Radix Multipath Delay Commutator (SRMDC) based FFT Architecture

Background: Fast Fourier Transform (FFT) processor is the important method in Orthogonal Frequency Division Multiplexing (OFDM) communication systems. Split-Radix Fast Fourier Transform (SRFFT) estimates the least number of multiplications compared to other FFT algorithm; hence SRFFT is the best for the implementation of low power FFT processor. Methods: In this paper, the proposed “pipelined 32-point Split-Radix Multi-path Delay Commutator (SFMDC) FFT” architecture is designed. The MDC architecture has advantages in reducing the low chip area and lower power consumption. The SR-FFT algorithm has changing the numbers of butterfly elements in successive stages. The 32-point SR-MDC FFT architecture is to determine the frequency transformation techniques. The Split-Radix FFT algorithm is based on radix-2 and radix-4 algorithm and it is related to mixed radix FFT. Findings: The performance evaluation of proposed architecture is determined through VLSI System. Less area, low power consumption and high speed are the main parameters in this design. The proposed structure provides high throughput rate and low hardware complexity. Improvements: The goal of this proposed work is to reduce the slices and LUTs and power consumption and also to enhance the performances of the FFT processor than the existing method. The proposed method has been evaluated by using ModelSim 6.3C and synthesis results has been validated by using Xilinx Plan ahead 12.4.

Combined peak-to-average power ratio reduction and physical layer security enhancement in optical orthogonal frequency division multiplexing visible-light communication systems

A physical encryption scheme for discrete Hartley transform (DHT) precoded orthogonal frequency division multiplexing (OFDM) visible-light communication (VLC) systems using frequency domain chaos scrambling is proposed. In the scheme, the chaos scrambling, which is generated by a modified logistic mapping, is utilized to enhance the physical layer of security, and the DHT precoding is employed to reduce of OFDM signal for OFDM-based VLC. The influence of chaos scrambling on peak-to-average power ratio (PAPR) and bit error rate (BER) of systems is studied. The experimental simulation results prove the efficiency of the proposed encryption method for DHT-precoded, OFDM-based VLC systems. Furthermore, the influence of the proposed encryption to the PAPR and BER of systems is evaluated. The experimental results show that the proposed security scheme can protect the DHT-precoded, OFDM-based VLC from eavesdroppers, while keeping the good BER performance of DHT-precoded systems. The BER performance of the encrypted and DHT-precoded system is almost the same as that of the conventional DHT-precoded system without encryption.

An Optical Power Divider Based on Two-dimensional Photonic Crystal Structure

Abstract In this paper, an optical power divider with one input and four outputs has been proposed in a two-dimensional photonic crystal with triangular lattice and simulated using dielectric holes in an air substrate. The dividing properties of the power divider have been numerically simulated and analyzed using the plane wave expansion and finite difference time domain methods. The results show that the transmittance of this divider can be as high as 94.22 % for λ=1.55 µm; thus, the proposed structure is suitable for wavelength division multiplexing communication systems. Also, due to the small footprint of the proposed structure, this optical power divider is applicable for optical-integrated circuit design.

An Improved Compressed Sensing based Sparse Channel Estimation for MIMO-OFDM Systems with an Efficient Pilot Insertion Scheme

In this paper, we proposed an improved recovery algorithm, with efficient pilot placement, for Compressed Sensing (CS)-based sparse channel estimation based on Sparsity Adaptive Matching Pursuit (SAMP) in Orthogonal Frequency Division Multiplexing (OFDM) communication systems. The proposed algorithm does not require a priori knowledge of the sparsity, and to approach the true sparsity, adjusts the step size adaptively. Furthermore, estimation accuracy can be affected by different measurement matrices in CS, due to different pilot arrangements. It is known that, the Cyclic Difference Set (CDS) is the optimal setoff pilot locations when the signal is sparse on the unitary Discrete Fourier Transform (DFT) matrix by minimizing the mutual coherence of the measurement matrix. Based on this, an efficient pilot insertion scheme is introduced in cases where Cyclic Different Set does not exist. Simulation results in the paper show that the channel estimation algorithm, with the new pilot placement scheme, which offers a better trade-off between the performances in terms of Bit Error Rate (BER), Mean Squared Error (MSE) and the complexity, when compared to previous estimation algorithms.

Selective mapping and restorable clipping joint scheme for light-emitting diode nonlinearity alleviation in visible light communication system

A joint algorithm, integrating selective mapping (SLM) and restorable clipping (RC), is proposed for the direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) and visible light communication (VLC) system to reduce the nonlinearity impacts of light-emitting diode (LED) aggravated by high peak-to-average power ratio (PAPR) and DC-bias. The performance of DCO-OFDM VLC system is analyzed and discussed with different techniques of LED nonlinearity alleviation. The simulation results show that compared to the original DCO-OFDM VLC system, the system with the proposed scheme can achieve about 4.8 dB improvement of PAPR reduction and 7 dB improvement of bit error rate (BER) performance. The reason is that the signals acquiring the desired shape in LED linear region can be recovered correctly without distortion induced by LED nonlinearity. It is demonstrated that the proposed SLM-RC technique effectively reduces not only PAPR but also the impacts of LED nonlinearity without BER deterioration.

Novel Compressed Sensing-Based Channel Estimation Algorithm and Near-Optimal Pilot Placement Scheme

This paper presents a novel recovery algorithm based on sparsity adaptive matching pursuit (SaMP) and a new near-optimal pilot placement scheme, for compressed sensing (CS)-based sparse channel estimation in orthogonal frequency division multiplexing (OFDM) communication systems. Compared with other state-of-the-art recovery algorithms, the proposed algorithm possesses the feature of SaMP of not requiring a priori knowledge of the sparsity level, and moreover, adjusts the step size adaptively to approach the true sparsity level. Furthermore, we focus on the pilot pattern design in sparse channel estimation. Although a brute-force search guarantees the optimal pilot pattern, it is prohibitive to examine all possibilities due to high computational complexity. It is known that by minimizing the mutual coherence of the measurement matrix when the signal is sparse on the unitary discrete Fourier transform (DFT) matrix, the optimal set of pilot locations is a cyclic difference set (CDS). Based on this, we propose an efficient near-optimal pilot placement scheme in cases where CDS does not exist. Simulation results show that the proposed channel estimation algorithm, with the new pilot placement scheme, offers a better tradeoff between the performance-in terms of mean-squared-error (MSE) and bit-error-rate (BER)-and complexity, when compared to other estimation algorithms.

Estimating the Performance of DD OFDM Optical Systems Impaired by In-Band Crosstalk Using the Moment Generating Function

A theoretical method to assess the impact of in-band crosstalk on the performance of orthogonal frequency division multiplexing (OFDM) communication systems with direct-detection (DD) is presented. The proposed method relies on the moment generating function (MGF) to estimate the bit error rate (BER) of the DD OFDM communication system and takes into account in-band crosstalk, detected noise, and arbitrary electrical filtering at the receiver. By comparison with Monte Carlo simulation, we show that the MGF-based method predicts accurately the BER for different crosstalk levels, subcarriers numbers, 4-ary, 16-ary, and 64-ary quadrature amplitude modulation mappings, carrier-to-signal power ratios, ideal and nonideal electrical filtering, and different types of linearly mapped OFDM signals used in DD. The accuracy of the proposed method may be severely diminished in presence of significant signal distortion due to the nonlinearity of the optical modulator as, in this case, the assumptions of the proposed method do not hold. We also show that the DD OFDM optical systems tolerance to in-band crosstalk is lower than in single-carrier coherent detection optical systems, and it is significantly lessened when the modulation order in the OFDM subcarriers increases.

Photonic crystal-based optical filters for operating in second and third optical fiber windows

In this paper, the filtering properties of photonic crystals (PCs) to perform narrow-channel transmission-type filters in second and third optical fiber telecommunication windows have been studied. Filtration of these zero dispersion and low-loss windows have simultaneously been established by utilizing of a triple-cavity transmission-type one-dimensional PC that provides perfect transmittances and narrow-channels at corresponding wavelengths. Such PC-based optical filter can be used in wavelength division multiplexing (WDM) optical communications systems.

Robust QoS-Aware Cross-layer Design of Adaptive Modulation Transmission on OFDM Systems in High-Speed Railway

In this paper, we consider an orthogonal frequency division multiplexing communication system that adopts frame-by-frame transmission in high-speed railway (HSR) scenario. Due to the increase in demand for the QoS sensitive services, an efficient QoS-aware transmission strategy that improves the system performance is required urgently. Many efforts have been devoted to addressing this problem with the assumption of block fading channel in a frame duration. However, due to the frequent channel quality variation in a frame duration and serious inter channel interference in HSR scenario, the throughput of the QoS sensitive services degrades severely. Hence, a robust cross-layer transmission strategy that combines adaptive modulation (AM) scheme with truncated automatic repeat request protocol is proposed. In this cross-layer formulation, the normalized average throughput is optimized subject to the average power and the packet loss rate (PLR) requirements. First, we derive the closed form average bit error rate that represents the PLR requirement at the physical layer. Second, we obtain the solution of robust AM scheme and power allocation policy in the case of continuous rate. Third, we present the adaptive bits and power allocation scheme in the case of discrete rate, which can be implemented in practice. Finally, the performance of the proposed transmission strategy is evaluated by extensive simulations. Comparing with the constant transmit power AM scheme, the throughput increases by 20%, which demonstrates that the proposed robust cross-layer design is suitable for the HSR communication systems.

Optimum Resource Allocation in Orthogonal Frequency Division Multiplexing Communication System using Fuzzy Rule Base System and Particle Swarm Optimization and Comparison with Conventional other Techniques

OFDM is a technique that is chosen for high data rate communication and is important for 4th generation communication systems. To provide best possible quality of service, intelligent allocation of resources such as power, bandwidths are necessary. Various algorithms have been proposed for allocation of resources. Fuzzy Rule Based System (FRBS) and Particle Swarm Optimization (PSO) algorithm are used for optimization of code rate, modulation and power. FRBS is used to choose the best possible modulation and code rate and PSO is used for better power allocation.

Improved superimposed training sequence-based timing synchronization for space optical orthogonal frequency division multiplexing system

This paper investigates the timing synchronization problem of a space optical orthogonal frequency division multiplexing (OOFDM) communication system. First, based on the good autocorrelation property of gen- eralized chirp-like sequence, a training sequence is constructed to fit the non-negative light intensity signal requirement of the OOFDM system, of which the front and rear portions are cyclical and the whole is mirror- symmetric. No longer a periodic-repetition structure, the mirror-symmetric structure can effectively avoid the side lobe of objective function and reduce the complexity of correlation calculation, and thereby can improve the synchronization performance. Then, the constructed training sequence is superimposed on a complete data symbol for transmission to efficiently utilize transmitting power and spectrum resources of the communication system. At the receiver, the position of timing synchronization is estimated using maximum-likelihood algorithm and the correlation between the local sequence and the received signal. Simulation results show that, compared with several existing methods, the proposed timing synchronization method achieves better synchronization performances under both strong and weak atmospheric turbulence channels. © 2015 Society of Photo-Optical

BER analysis of hybrid carrier system based on WFRFT with carrier frequency offset

Carrier frequency offset (CFO) due to Doppler shift or misaligned frequency between the transmitter and the receiver has a serious impact on both orthogonal frequency division multiplexing (OFDM) and single carrier (SC) communication systems. Inter-carrier interference coefficients of OFDM system produced by CFO have been proposed and applied in analysing exact bit error rate (BER) upon diverse channel models. In this Letter, interference of hybrid carrier communication system based on weighted-type fractional Fourier transform impaired by CFO is investigated, which is a weighted form of multi-carrier and SC schemes, and then exact BER expression of BPSK modulation HC system is derived and verified by simulations.

Design and Analysis of 25 and 35 Gbps Optical Communication System based on APDCDM Technique

and 35 Gbps Absolute Pole Duty cycle division multiplexing (APDCDM) based communication system is designed successfully. A technique for estimation of BER is developed for four users APDCDM based optical communication system, referring to the data recovery rules. The performance parameters like SNR, Q-factor and BER are calculated for the transmission rate

Frequency-Domain Iterative SDFE for MIMO-OFDM Systems

This paper proposes a frequency-domain iterative soft decision feedback equalization (SDFE) scheme for multiple-input---multiple-output orthogonal frequency division multiplexing communication systems in multi-path fading channels. Error propagation can seriously affect the performance of the adaptive decision feedback equalization. We propose an iterative SDFE to not only combat error propagation but also be suitable for multilevel modulation systems. Under a Gaussian approximation of the a priori information and the SDFE output, the output of an equivalent AWGN channel and expressions for the a posteriori log likelihood ratio (LLR) and the extrinsic LLR are derived for multilevel modulation. In addition, we also investigate the iterative receivers by employing irregular low-density parity check codes (LDPC) and turbo codes. The performance of the LDPC-coded iterative receiver is better than that of the turbo-coded iterative receiver. Some simulation examples are given to show the effectiveness and comparisons of the proposed receiver.

On the Performance of Coherent OFDM Systems in Free-Space Optical Communications

This paper presents a new model of a coherent orthogonal frequency division multiplexing free space optical communication (OFDM-FSO) system that can be tuned with the virtual local oscillator, under the Gamma-Gamma atmospheric turbulence channel, not only considering the intensity scintillation but considering the phase noise caused by atmospheric turbulence effect on the performance of the system as well. The mapping mode of the OFDM signal uses 16-quadrature amplitude modulation (QAM) with better performance. On this basis, we derived the closed form of the symbol error rate and interrupt probability, respectively. In this paper, we analyzed under the premise of the different number of subcarriers, the influence of each parameter of atmospheric channel on the bit error, and the communication interrupt performance of the OFDM-FSO system, respectively. The results show that the coherent OFDM-FSO system can obtain higher sensitivity and can overcome the bad atmosphere influence to obtain good performance of the system.