Conventional OFDM Research Articles

Non-coherent OFDM-Subcarrier Power Modulation for Low Complexity and High Throughput IoT Applications

Aiming to reduce the transceiver complexity and power consumption of communication systems dedicated to serving future Internet of Things (IoT) applications, researchers have taken many approaches with varying degrees of success. A promising candidate solution that can reduce complexity significantly and thus also enhance power-saving is the use of non-coherent modulation-based schemes. Utilizing a non-coherent structure rids the system of any dependency on the knowledge of the phase of the transmitted signal during demodulation. However, although most of the available non-coherent techniques in the literature reduce the complexity of the system, they, unfortunately, suffer from a drawback of some sort, especially in reducing the overall spectral efficiency of the system. To address this problem, we propose a new non-coherent modulation scheme called orthogonal frequency division multiplexing with subcarrier power modulation and differential phase-shift keying (OFDM-SPM-DPSK), as an effective modulation technique for future 6G and beyond systems. The proposed technique has the potential to reduce complexity, enhance power-saving while improving spectral efficiency significantly, resulting in a spectral efficiency performance twice that of a conventional non-coherent OFDM system. Furthermore, receive diversity is implemented through maximal ratio combining to overcome the bit error degradation brought by the adoption of a non-coherent structure. Additionally, the computational complexity is discussed and shown to be as efficient as OFDM, where the proposed scheme does not add further complexity over conventional OFDM.

Dual frame OFDM with optical phase conjugation for MIMO system in multimode fiber

For nonlinearity and dispersion mitigation in optical fiber, the authors propose a dual frame OFDM transmission, based on optical phase conjugation. The proposed dual frame OFDM is modification of asymmetrically clipped OFDM (ACO-OFDM) frame. The proposed OFDM frame consist of two positive frames, first one is time compressed ACO-OFDM frame and second one is optical phase conjugated version of first frame. This increases the bandwidth efficiency and total transmitted power for the system. For electronic dispersion compensation (EDC) in conventional OFDM, signal pre-processing is applied at transmitter side where it requires channel information. In case of OFDM and multiple input multiple output (MIMO) system setup for multi mode fiber, channel becomes almost unpredictable. The proposed dual frame OFDM does not require EDC at transmitter side. For received optical power $$-12$$ dbm conventional ACO-OFDM records bit error rate (BER) $$4.48*10^{-1}$$ and proposed dual frame ACO-OFDM with OPC records BER $$2.58*10^{-4}$$ . Simulation results for variable fiber length show BER $$2.20*10^{-1}$$ for phase conjugated subcarrier coding and $$2.24*10^{-3}$$ for proposed ACO-OFDM with OPC for multi-mode fiber length 500 m. It also allows different multiframe transmission based on time and wavelength diversity.

Q-Ary Multi-Mode OFDM With Index Modulation

In this letter, we propose a novel orthogonal frequency division multiplexing with index modulation (OFDM-IM) scheme, which we call Q -ary multi-mode OFDM-IM (Q -MM-OFDM-IM). In the proposed scheme, Q disjoint M-ary constellations are used repeatedly on each subcarrier, and a maximum-distance separable code is applied to the indices of these constellations to achieve the highest number of index symbols. A low-complexity subcarrier-wise detection is shown possible for the proposed scheme. Spectral efficiency (SE) and the error rate performance of the proposed scheme are further analyzed. It is shown that the proposed scheme exhibits a very flexible structure that is capable of encompassing conventional OFDM as a special case. It is also shown that the proposed scheme is capable of considerably outperforming the other OFDM-IM schemes and conventional OFDM in terms of error and SE performance while preserving a low-complexity structure.

Experimental Demonstration of an SFO-Robustness Scheme With Fast OFDM for IMDD Passive Optical Network Systems

A sampling frequency offset (SFO)-robustness scheme with zero padding (ZP)-fast orthogonal frequency-division multiplexing (FOFDM) is proposed and experimentally demonstrated in intensity-modulated and direct-detection (IMDD) passive optical network (PON) systems. It can be used to mitigate the impact of SFO-induced inter-carrier interference (ICI). The performance comparison between the proposed FOFDM scheme and conventional OFDM scheme in the presence of SFO is presented. Experimental results indicate that, as SFO-induced subcarrier phase offset (SPO) is compensated within the estimated error of ±0.1 ppm, at the same spectral efficiency (SE) and bandwidth granularity, the error vector magnitude (EVM) of ZP-FOFDM is superior to the OFDM at different received optical power (ROP). In addition, when the ROP is −1 dBm, the average error vector magnitude (EVM) of all the data subcarriers for ZP-FOFDM is 3.3-dB superior to that of OFDM. It means that the ZP-FOFDM scheme can effectively improve the SFO-robustness compared with OFDM.

Transformasi Wavelet dengan Teknik Clipping Filtering untuk Mereduksi PAPR pada OFDM

Orthogonal Frequency Division Multiplexing (OFDM) is chosen as multiplexing techniques and broadly used in today’s radiocommunication environments to overcome spectrum insufficiency. With several superior advantages, however, OFDM is terribly affected by high peak to average power ratio (PAPR) due to offset frequency errors and local oscillator (LO) frequency synchronization errors. The high PAPR can cause nonlinear distortion, which outcomes in intermodulation and spectral leakage. This study aims to model the use of wavelet transform (discrete wavelet transform (DWT)) to replace Fourier transform (discrete Fourier transform (DFT)) that used in conventional OFDM, later in this paper is termed as DFT-OFDM. Clipping filtering techniques then applied to DWT-OFDM. The model was proposed to reduce PAPR in DFT-OFDM. The model was compared to DFT-OFDM using Matlab simulation method. The performance was evaluated using the Complementary Cumulative Distributive Function (CCDF) vs. PAPR. The results show that at PAPR 10-3for DFT-OFDM, it was produced PAPR of 10.6 dB whereas in DWT-OFDM, using Daubechies orde 7 (Daubechies7), Symlet orde 7 (Symlet7), Coiflet orde 2 (Coiflet2), were reached PAPR 4.8 dB, PAPR 3.3 dB, PAPR 3 dB, respectively. It means Coiflet2 providing the best PAPR reduction among other orthogonal wavelets. By applied clipping filtering to wavelet Coiflet2, it was produced PAPR of 2.9 dB for classical clipping and 2.8 dB for deep clipping. It show that wavelet Coiflet2 with deep clipping provided the best PAPR.

Performance Investigations and PAPR Reduction Analysis Using Very Efficient and Optimized Amended SLM Algorithm for Wireless Communication OFDM System

The orthogonal frequency division multiplexing (OFDM) is the most encouraging multi-carrier modulation system chosen for the high data rates but the objective is to resolve intrinsic common issue of peak to average power ratio (PAPR). The projected algorithm is illustrated in this research study which is established upon selected mapping (SLM) with pseudo-random sequence under time domain μ law companding of signal. Besides the significant competitive characteristics of conventional SLM algorithm (ConvenSLM-Algo) (Mestdagh et al. in Electron Lett 32(22):2056–2057, 1996. https://doi.org/10.1049/el:19961384 ), it undergoes the tedious complexity in generation of phase sequence and for the recovery of sequence, side information is mandatory. The key concern, thus, to eliminate the tedious problem of designing of phase sequence along with the aim of reducing the fluctuation of signal with high PAPR. Henceforth, in the projected algorithm (Projected-Algo) a noteworthy strategy has been investigated and followed for the designing of phase sequence which is very easy and for the recovery of information at receiver, side information of index of column of phase sequences can be used because of straightforwardness in its establishment. Hence, we demonstrate the effective overall excellent performance of Projected-Algo along with analysis and also comparative study of Projected-Algo outperforms the conventional OFDM system (unchanged), ConvenSLM-Algo [16], SLM with new pseudo random phase sequences (ModSLM-Algo) given in literature (Ali et al. in Canadian conference on electrical and computer engineering, 2018. https://doi.org/10.1109/CCECE.2018.8447835 ).

Performance Comparison of Proposed Scheme Coded DWT-OFDM System with that of Conventional OFDM

In this paper, we have investigated the Proposed scheme coded DWT-OFDM system in comparison to conventional OFDM system over AWGN channel. 16, 32, 64 – QAM and PSK modulation schemes have been considered to evaluate the system. Moreover, two different wavelets have been used for examining the system. Simulations are performed using MATLAB software. It is found that proposed scheme based DWT-OFDM system performs better than conventional OFDM system under all the considered modulation schemes

HYBRIDIZATION OF SUBCARRIER INDEX MODULATION AND OFDM WITH MIMO SYSTEM APPLIED TO FSO

Presently free space optical communication (FSO) is a promising technique on which various researches are carried on. The integration of orthogonal frequency division multiplexing (OFDM) and Subcarrier index modulation, conventional OFDM, Multi input multi output OFDM (MIMO-OFDM) concepts are analyzed in this paper. In free space optical communication these techniques are utilized as well as the performance analysis is simulated using MATLAB. This work investigates the significant of SIM modulated MIMO-OFDM in FSO. Also it analyses the superiority of SIM modulated MIMO-OFDM than SIM modulated classical OFDM and simple Subcarrier index modulation over free space optical communication. Here different turbulence conditions are taken to analyze the concept of SIM along with MIMO-OFDM, classical OFDM in free space. Ultimately SIM modulated MIMO-OFDM exhibits better BER performance in the order of 10-6 in comparison to simple SIM and SIM associated OFDM in free space.

Low Complexity Equalization of Orthogonal Chirp Division Multiplexing in Doubly-Selective Channels.

Orthogonal Chirp Division Multiplexing (OCDM) is a modulation scheme which outperforms the conventional Orthogonal Frequency Division Multiplexing (OFDM) under frequency selective channels by using chirp subcarriers. However, low complexity equalization algorithms for OCDM based systems under doubly selective channels have not been investigated yet. Moreover, in OCDM, the usage of different phase matrices in modulation will lead to extra storage overhead. In this paper, we investigate an OCDM based modulation scheme termed uniform phase-Orthogonal Chirp Division Multiplexing (UP-OCDM) for high-speed communication over doubly selective channels. With uniform phase matrices equipped, UP-OCDM can reduce the storage requirement of modulation. We also prove that like OCDM, the transform matrix of UP-OCDM is circulant. Based on the circulant transform matrix, we show that the channel matrices in UP-OCDM system over doubly selective channels have special structures that (1) the equivalent frequency-domain channel matrix can be approximated as a band matrix, and (2) the transform domain channel matrix in the framework of the basis expansion model (BEM) is a sum of the product of diagonal and circulant matrices. Based on these special channel structures, two low-complexity equalization algorithms are proposed for UP-OCDM in this paper. The equalization algorithms are based on block LDL factorization and iterative matrix inversion, respectively. Numerical simulations are finally proposed to show the performance of UP-OCDM and the validity of the proposed low complexity equalization algorithms. It is shown that when the channel is doubly selective, UP-OCDM and OCDM have similar BER performance, and both of them outperform OFDM. Moreover, the proposed low complexity equalizers for UP-OCDM both show better BER performance than their OFDM counterparts.

Visible light communications: increasing data rates with polarization division multiplexing.

In this Letter, we propose a new configuration for visible light communication systems, which results in doubling of the data rate due to the use of polarization division multiplexing. As light-emitting diodes are unpolarized incoherent light sources, we isolate both the perpendicular s and parallel p modes for independent modulation. For the first time, to the best of our knowledge, we show that it is possible to transmit and successfully recover two separate orthogonal frequency division multiplexing (OFDM) signals on each polarization (pol-OFDM). Furthermore, we compare the performance of the pol-OFDM system with the transmission of a single conventional OFDM signal without a polarizer over the same physical link. We show that similar bit error rates can be achieved while obtaining ∼45% improvement in both the data rate and spectral efficiency due to polarization multiplexing.

Modified OFDM configurations with equalization and CFO compensation for performance enhancement of OFDM communication systems using symmetry of the Fourier transform

The Orthogonal Frequency Division Multiplexing (OFDM) systems allow efficient spectrum usage and simple Frequency Domain Equalization (FDE). However, a large number of sub-carriers increases the computational complexity of the equalization process. In this paper, the conventional OFDM system is modified to enhance the Bit-Error-Rate (BER) performance or reduce the computational complexity. Also, the banded-matrix approximation is taken into account to reduce the equalization complexity. Simulation results show that the 1st proposed OFDM system based on Binary Phase Shift Keying (BPSK) can reduce the computational complexity to O(N/2) instead of O(N) compared to the conventional OFDM configuration based on the banded-matrix approximation concept. The 2nd proposed OFDM system based on BPSK requires approximately the same computational complexity of the conventional OFDM configuration and gives a Signal-to-Noise Ratio (SNR) gain of about 7.05 dB at BER = 10−3 compared to the corresponding conventional OFDM configuration. The 3rd proposed OFDM system based on Quadrature Phase Shift Keying (QPSK) gives an SNR gain of about 4.51 dB at a BER = 10−3 compared to the corresponding conventional OFDM configuration. The 2nd proposed OFDM system based on BPSK outperforms the different schemes in the presence of estimation errors.

Performance of Filtered OFDM Over Different Fading Channels

Orthogonal frequency division multiplexing (OFDM) schemes are used extensively nowadays in large number of practical systems, but OFDM may not be the ideal solution for 5G scenarios because OFDM has high Out of band emission (OOBE) due to high spectral leakage and high peak to average power ratio (PAPR). Therefore Filtered OFDM (F-OFDM), which uses sub band filtering, is proposed as the most flexible waveform for 5G. However there is no work listed in the literature so far that compares the bit error rate (BER) performance of F-OFDM and conventional OFDM in AWGN, Rayleigh and Rician channel. This paper presents the simulation results on the BER performance of F-OFDM and OFDM in presence of AWGN, Rayleigh and Rician channel. The comparison is also done in terms of OOBE and PAPR factor. Results indicate that F-OFDM shows significantly less spectral leakage with same performance in terms of BER compared to conventional OFDM. However for the cases considered, OFDM performance is slightly better than F-OFDM in terms of PAPR factor.

MDW and EDW/DDW codes with AND subtraction/single photodiode detection for high performance hybrid SAC-OCDMA/OFDM system

In this paper, a hybrid system is introduced using orthogonal frequency division multiplexing (OFDM) technique with spectral amplitude coding optical code division multiple access (SAC-OCDMA). In this system, the AND subtraction detection is used with the modified double weight (MDW) and enhanced double weight (EDW) codes, in addition to the single photodiode detection with the diagonal double weight (DDW) code. The proposed system achieves performance enhancement in the performance enhancement concerning the maximum allowable number of users, signal to noise ratio and (minimum) bit error rate (BER) $${\Re}$$. The proposed codes that used with AND subtraction detection techniques are: MDW and EDW, while the DDW codes are used with single photodiode detection (SPD). Detection techniques are investigated to reduce the effect of both the multiple access interference and phase induced intensity noise. A comparison is performed between the conventional SAC-OCDMA systems and OFDM with SAC-OCDMA. The obtained results reveal that OFDM with SAC-OCDMA gives better performance than conventional SAC-OCDMA. OFDM with SAC-OCDMA using DDW code with SPD detection can support a high bit rate up to 10 Gbps for 70 users with a BER of 10–9.

RETRACTED ARTICLE: Peak to average power ratio reduction of ZT DFT-s-OFDM signals using improved monarch butterfly optimization-PTS scheme

For traditional orthogonal multiplexing frequency division (OFDM) systems, a novel Zero Tail Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing Division (ZT DFT-s-OFDM) waveform architecture has recently been proposed as a problem solution of low peak to average power ratio (PAPR) efficiency. The ZT DFT-s-OFDM system allows the delay in the transmission of the channel with multipath to be dynamically copied, thus the limitations to be rectified with the hard-coded Cyclic Prefix (CP). However it is affected by few roadblocks during the transmission of data, and the primary one involves the high peak-to-average power ratio (PAPR), which results in saturation observed in the power amplifier, production of more amount of interference and decreased resolution in elements such as digital/analog converters, which were considered as nonlinear. Partial transmit sequences (PTSs) is a promising plan and direct technique, ready to accomplish a viable PAPR decrease execution, yet it requires a thorough hunt to locate the ideal stage factors, which causes high computational multifaceted nature expanded with the quantity of sub-blocks. Right now, the author proposed a reduced computational complexity PTS scheme, in view of a new swarm knowledge algorithm, which is termed as Improved Monarch Butterfly Optimization (IMBO) for PAPR decrease with the ZT DFT-s-OFDM framework. The IMBO is a new swarm intelligence algorithm, with the capability of achieving an efficient optimization search that implements phase weighting process with less complexity for selecting optimum phase factors. Also, the proposed technique is quite efficient in looking for a fusion with optimal character of phase rotation factors for minimizng the computational difficulty involved. The outcomes of simulation indicate that IMBO-based PTS algorithm can substantially reduce PAPR employing an easy network structure in comparison with classical algorithms such as PTS scheme for conventional OFDM and PTS with ZT-DFT-s-OFDM.

PWM‐based dimmable hybrid optical OFDM for visible‐light communications

In visible-light communication (VLC) system, hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) is an attractive multi-carrier modulation to achieve high power and spectrum efficiency. Dimming control is deemed to be an essential technique in VLC since it can meet diverse luminance demands. However, severe non-linear distortion occurs when dimming the HACO-OFDM VLC system, especially with the pulse-width modulation (PWM) dimming technique. In this work, a novel PWM-based dimmable hybrid optical OFDM (DHO-OFDM) scheme is proposed to simultaneously support different illumination requirements and realise a spectral-efficient VLC link. The transmitted symbols in the authors' proposed DHO-OFDM can be readily estimated by the standard HACO-OFDM receiver without requiring the additional detecting the PWM signal, thus reducing the receiver complexity compared to the other PWM-based dimmable schemes. Moreover, the proposed DHO-OFDM scheme can support a broad dimming range with a relatively stable communication performance, which makes it more suitable for broadband data transmission. Numerical results have verified the superiorities of DHO-OFDM over the conventional OFDM schemes in terms of achievable spectral efficiency and bit-error rate.

An improved backtracking search optimization algorithm for cubic metric reduction of OFDM signals

Abstract The large amplitude variations of OFDM signals generate in-band distortion and out-of-band radiation. In recent years, cubic metric (CM) has been verified as a more accurate metric to measure the amplitude variations. In this paper, the PTS technique is used to decrease the CM of OFDM signals. To overcome the search complexity of an exhaustive search based PTS technique, we introduce an improved backtracking search (IBS) optimization algorithm. Simulations are conducted to show the advantages of the proposed IBS based PTS approach compared with the conventional OFDM, and several state-of-the-art methods in terms of search complexity and CM reduction performance.

Enhanced sampling frequency offset resistance method based on precoding technique in W-band OFDM RoF systems

Precoding technique is proposed and experimental demonstrated to resist sampling frequency offset (SFO) in W-band OFDM RoF systems. Walsh–Hadamard transform (WHT)-precoding, discrete Hartley transform (DHT)-precoding, Zadoff–Chu transform (ZCT)-precoding, and orthogonal circulant matrix transform (OCT)-precoding are investigated and compared. SFO-induced inter-carrier interference (ICI) on each subcarrier of conventional OFDM and OFDM with precoding (precoded-OFDM) are theoretical analyzed. Experimental results indicate OFDM with OCT-precoding (OCT-OFDM) can obtain a remarkable resistance to the SFO-induced ICI in W-band OFDM RoF systems.

OFDM Chirp Waveform Design Based on Imitating the Time–Frequency Structure of NLFM for Low Correlation Interference in MIMO Radar

The orthogonal frequency division multiplexing (OFDM) chirp signal has its special merits of high-range resolution by exploiting a full bandwidth as multiple input multiple output (MIMO) radar signal. After simulation, there are some high peaks in the autocorrelation function and cross correlation function of conventional OFDM chirp signals, which may increase the correlation interference and reduce the detection performance of MIMO radar signal. This letter proposes to reduce the autocorrelation sidelobe peak (ASP) of OFDM chirp signal by subsection imitating the nonlinear frequency modulation (NLFM) signal's time-frequency structure. Since a suitable difference should be made between the subchirp rates and subchirp carrier frequencies of each of the two designed OFDM chirp signals for reducing the cross correlation peaks (CP), the imitated NLFM waveforms' design problem is solved by the proposed optimized method. At last, subchirps of the designed signals are coded by a group of optimized codes. The simulation shows the designed OFDM chirp signals have lower ASP and CP than the conventional ones and referenced previous OFDM signals, and their ASP decreases as the number of subchirps increases.

Performance of OFDM‐based massive MIMO OTFS systems for underwater acoustic communication

In this study, multi-user (MU) underwater acoustic communication is investigated using massive multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM)-based orthogonal time frequency space modulation (OTFS) systems. The performance of a 4-user scenario is evaluated over a simulated 1 km vertically-configured time-varying underwater acoustic channel (UAC) in terms of bit error rate and maximum achievable bit rate. Considering 64-QAM and frequency-domain pilot-based channel estimation, it is shown that the underwater vehicles employing the OFDM-based OTFS modulation outperform those using the conventional OFDM modulation in a dynamic UAC. The application of massive MIMO allows the four underwater vehicles to use the same time and frequency resources to transmit their information reliably to a surface station. Furthermore, it is shown that each underwater vehicle in the MU-massive MIMO transmission scenario can achieve an effective bit rate as high as 198.7 kbps over the 1 km UAC using four transmitting transducers.

Multipath Diversity for OFDM Based Visible Light Communication Systems Through Fractional Sampling

In this paper, we propose fractional sampling for optical orthogonal frequency division multiplexing (O-OFDM) in order to achieve diversity gain for indoor visible light communication systems. Fractional sampling (FS) is a diversity technique that generates different replicas of the received OFDM symbol by sampling the received signal higher than the baud rate. Indoor optical wireless channels exhibit frequency selectivity and FS increases the reliability of the transmission by exploiting the multipath diversity. Our numerical results show that for a realistic indoor channel model, proposed FS receiver provides significant performance improvement over conventional OFDM receiver.