Orthogonal Frequency Division Multiplexing Frame Research Articles

Nonlinear distortion mitigation of DML-based OFDM optical systems with non-orthogonal DFT-precoding.

We propose a non-orthogonal discrete Fourier transform (DFT) matrix precoding scheme for the mitigation of nonlinear distortion induced by the interaction between laser chirp and fiber dispersion in a directly modulated laser (DML)-based orthogonal frequency division multiplexing (OFDM) transmission system. Compared with conventional OFDM, the proposed method can decrease the peak-to-average power ratio (PAPR) and significantly reduce the nonlinear distortion without sacrificing spectral efficiency (SE). The cascaded binary-phase-shift-keying iterative detection (CBID) algorithm is used to eliminate the inter-carrier interference (ICI) that is purposely induced by the non-orthogonal precoding. The performance of the proposed scheme is experimentally evaluated, achieving ∼0.4-dB sensitivity improvement at the KP4-forward error correction (FEC) threshold over the T/2-spaced third-order Volterra nonlinear equalizer (VNLE). Meanwhile, compared to the VNLE, the reduction in computational complexity of one OFDM frame is 90% for multiplication and 88.32% for addition in this work.

A Novel Method of Removing the Influence of Continuous Electromagnetic Wave Disturbances in OFDM Systems

This article describes a novel technique for removing the influence of a continuous wave (or narrowband) electromagnetic disturbance in orthogonal frequency division multiplexing (OFDM) systems with quadrature amplitude modulation or phase-shift keying modulation schemes. The technique relies on a mathematical derivation of how a continuous wave electromagnetic disturbance induces spectral leakage in the OFDM system. Using this derivation, an algorithm is obtained that aims to cancel the continuous wave electromagnetic disturbance by estimating its frequency, retrieving its amplitude and phase from the corrupted OFDM frame. Note that the algorithm does not require any prior information about the disturbance. The proposed algorithm is validated through thorough simulations, covering different modulations, noise variations, and spectral leakage cases, and compared with standard OFDM performance without the algorithm. Through our experimentation, it has been demonstrated that for a disturbance frequency not equal to one of the OFDM subcarriers, the algorithm can estimate the disturbance frequency with high precision, resulting in a gain of more than 80 dB when compared to the case without the algorithm. For a disturbance frequency equal to one of the OFDM subcarriers, a simple coding technique such as Hamming & interleaving enables the user to remove the disturbance.

Optimized Waveforms for 5G–6G Communication With Sensing: Theory, Simulations and Experiments

Joint communication and sensing (JCAS) is an emerging technology for managing efficiently the scarce radio frequency (RF) spectrum, and is expected to be a key ingredient in beyond fifth-generation (5G) networks. We consider a JCAS system, where the full-duplex radar transceiver and the communication transmitter are the same device, and pursue orthogonal frequency-division multiplexing (OFDM) waveform optimization by jointly minimizing the lower bounds of delay and Doppler estimation. This is attained by filling the empty subcarriers within the OFDM frame with optimized samples while reallocating a proportion of the communication subcarriers’ power, which essentially controls the fairness between the two functionalities. Both communication and filled <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">radar subcarriers</i> are used for radar processing. The optimized sample values are found analytically, and a computationally feasible algorithm is presented for this task. We also address how the peak-to-average power ratio of the waveform can be controlled and minimized along the optimization process. The results are then numerically evaluated in 5G New Radio (NR) network context, which indicate a trade-off between the minimization of the lower bounds. The main-lobe width and the peak side-lobe level (PSL) of the range and velocity profiles of the radar image are also analyzed. An inverse relation between the lower bounds and the PSLs is observed, while the main-lobe width can be minimized simultaneously. The trade-off between communication and sensing is investigated, which indicates that the lower bounds can be improved at the cost of the communication capacity. Moreover, over-the-air RF measurements are carried out with unoptimized and optimized 5G NR waveforms at the 28 GHz mm-wave band, to validate the range profile’s PSL improvement in an outdoor mapping scenario, depicting considerable performance gain.

Adaptive subcarrier nulling for error vector magnitude and peak to average power ratio reduction in block coded OFDM-VLC

A subcarrier nulling technique based on Reed–Solomon (RS) encoding and nonlinear distortion measurement in orthogonal frequency-division multiplexing (OFDM)-based visible-light communication (VLC) is proposed. The effect of nonlinear clipping imposed by the low dynamic range in an OFMD-VLC leads to a non-uniform distortion on the data-bearing OFDM subcarriers. This distortion combined with the low bandwidth modulation of the light-emitting diode could limit the throughput of the VLC system. The proposed technique employs the clipping distortion to adaptively select and null the group of subcarriers that are responsible for the peak values in the OFDM symbol. Using the frequency domain RS block code, the OFDM transmitter is provided with the flexibility to find and remove the accumulated in-band distortion anywhere within the OFDM frame without the need for prior subcarrier reservation. With the help of log-likelihood ratio detection, nulled subcarriers are estimated at the receiver, and the RS codewords with erasure are recovered and decoded without using side information. This technique mitigates the error vector magnitude and reduces the peak to average power ratio in an OFDM-VLC under severe clipping conditions with reduced complexity. The proposed technique bit error rate performance is studied and compared with traditional techniques in a VLC system with clipping constraint.

Low Complexity Transmitter Architecture for Optical OFDM Systems

A novel low-complexity optical orthogonal frequency division multiplexing (LCO-OFDM) transmission architecture is proposed in this letter. The LCO-OFDM transmitter consists of two branches, wherein the first and second branches use odd and even subcarriers, respectively. Specifically, only complex multiplications are required in modulated symbols of each branch without Hermitian symmetry and inverse fast Fourier transform operations. Transmission signals of LCO-OFDM can be obtained by transmitting real and imaginary parts of generated complex signals in each branch over two successive orthogonal frequency division multiplexing frames and adding a direct current bias to avoid zero clipping noise. Simulation results showed the optimum performance of LCO-OFDM in terms of both peak-to-average power ratio and bit error rate.

Subspace-Based Estimation of Rapidly Varying Mobile Channels for OFDM Systems

It is well-known that time-varying channels can provide time diversity and improve error rate performance compared to time-invariant fading channels. However, exploiting time diversity requires very accurate channel estimates at the receiver. In order to reduce the number of unknown channel coefficients while estimating the time-varying channel, basis expansion models can be used along with long transmission frames that contain multiple orthogonal frequency division multiplexing (OFDM) symbols that experience the channel variation. The design of these OFDM frames need to judiciously incorporate training and data insertions in the transmitted signal while maintaining orthogonality. In this work, we propose an inter channel interference (ICI)-free training model depending on pilot symbols only and provide a corresponding time-varying channel estimation method. This scheme relies on an algorithm to determine the number of OFDM symbols per frame and the number of basis functions per path with minimal information about the Doppler bandwidth. As a performance benchmark, Bayesian Cramer Rao lower bound (CRLB) and the corresponding MSE bound are derived analytically for the proposed training model. Theoretical MSE expressions of the proposed estimation scheme are also derived as well as the MSE expressions in the presence of Doppler frequency mismatch. Simulations exhibit substantial MSE improvement and the corresponding Symbol Error Rate (SER) performances of the low complexity estimation scheme. They also corroborate that, unlike the common results in the literature, an OFDM system can perform better as the Doppler frequency increases with judicious design of training and channel estimation schemes.

OFDM synchronization system using wavelet transform for symbol rate detection

In radio communications, using wavelet signal analysis to recover the symbol rate timing clock of orthogonal frequency-division multiplexing (OFDM) is a new approach that can tolerate signal distortion from intersymbol interference (ISI) and intercarrier interference of encoding digital data on multiple carrier frequencies. Typically, the reception synchronization with wavelet signal analysis in OFDM can improve the performance over the fourier transform-based OFDM. However, a synchronization procedure that is stable against distortion and noise is essential to diminish the symbol synchronization establishment and operation sampling period. In this paper, we propose an OFDM synchronization system and analyze the impact of the wavelet denoise procedure on the OFDM system, which extracts the symbol rate of the OFDM frame. The evaluation results show that the proposed system can optimize the frequency window size to enable an efficient timing and frequency offset estimation with high and stable performance in terms of bit error rate (BER) and Frame Error Rate (FER) especially when the value of EbN0 (a normalized signal-to-noise ratio SNR measure) is greater than 8 dB, thanks to the wavelet transform.

Performance Prediction of OFDM Based Cognitive Radio for Next Generation Networking Capabilities

As a pioneering technology, next generation of mobile networks would overcome many problems of daily life activities. This new technology has many challenges that are tried to be destroyed. In this regard, to optimize the utilization of the allowed spectrum, an essential tool such as cognitive radio (CR) must be employed. It provides opportunity to use spectrum in strategic manner to both licensed and unlicensed users in such a way that the available spectrum is exploited in an efficient strategy. Meanwhile, the orthogonal frequency division multiplexing (OFDM) multicarrier transmission mechanism represents one of the more familiar techniques that are widely applied in free space systems of communication. Since this algorithm has the capability of satisfying the prime aspect of CR, which is associated with locally exploiting the unused spectrum autonomously, OFDM procedure is investigated as a CR system’s candidate. In other words, it is of interest to study its behavior when it is combined with CR technology. Here, our goal is to treat the problem of spectrum sensing techniques in conjunction with OFDM signal. Simulation results show that OFDM spectral correlation can be enhanced via varying the number of samples. Also, OFDM pilot is mandatory and acts as a flag for OFDM frames. So, it is of importance to boost it for better detection. Additionally, the probability of detection (Pd) is estimated for different values of signal strength when the false alarm probability (Pf) is fixed. Moreover, receiver operating characteristic, which is the variation of Pd as a function of Pf for a fixed SNR, is drawn for two cases; theoretical and simulated. It is found that the two cases are of high degree of coincidence.

Comparison of interpolation-based sampling frequency offset compensation schemes for practical OFDM-VLC systems.

Sampling frequency offset (SFO) is an important issue in the orthogonal frequency-division multiplexing (OFDM)-based visible light communication (VLC) systems with low-cost analog-to-digital or digital-to-analog converters (ADCs/DACs). A digital interpolation or resampling filter can be used to effectively compensate the SFO. In such case, oversampling at the receiver ADC is required to mitigate the aliasing effect due to imperfect DACs and nonlinearity of visible light sources that cause extra frequency components inside/outside the OFDM signal spectrum. The oversampling factor (rate) is mainly determined by the order of the digital interpolation filter and nonlinear VLC links. The design of the OFDM-VLC receiver incorporating the digital interpolation filter is vital as it affects not only the transmission performance but also the complexity of digital signal processing (DSP). To evaluate the feasibility of the digital interpolation-based SFO compensation schemes for cost-sensitive VLC applications, in this paper, a real-time OFDM-VLC receiver incorporating the 2nd/3rd/4th order interpolation filters is experimentally demonstrated. An OFDM frame structure is designed for the synchronization including SFO estimation and compensation, in which the precision and latency of DSP are considered. On the basis of the real-time OFDM-VLC receiver, the comparison in the VLC transmission performance and DSP complexity between different interpolation-based SFO compensation schemes is discussed.

On practical cooperative multi point transmission for 5G networks

5G networks are expected to drastically enhance the existing broadband wireless connectivity in many ways including higher bandwidth, lower delay, and wider coverage. To provide the required spectral efficiency, a key technology that could support the 5G vision is a transmission scheme based on Cooperative Multi Point Transmission (CoMP) combined with Orthogonal Frequency-Division Multiplexing (OFDM). This, however, requires an optimum resource allocation in a CoMP-OFDM system, which is shown to be a highly complex problem. Our proposal, referred to as scalable CoMP (SCoMP), tackles this complexity by dividing the optimal resource allocation problem into four subproblems such that CoMP becomes scalable and applicable in a practical 5G network without a huge overhead in terms of protocol signaling and backhaul requirements. Namely, these four subproblems are as follows: base station (BS) group formation (clustering), assignment of mobile stations (MS) to BS groups, MS group formation and OFDM frame construction, which are solved independently and therefore at lower complexity. Our approach improves its scalability even further by applying a static but overlapping BS grouping scheme which is MS-unaware. We decrease the potential performance loss due to MS-unawareness that is needed for improved scalability by two mechanisms. First, we propose to use overlapping BS clusters supporting variable cluster sizes which mitigates inter-cluster interference. Second, we apply dynamic grouping of MSs to make sure, only spatially-compatible MSs are being served simultaneously in the same group. Finally, our approach is able to deal with bursty network traffic. The realisation of SCoMP is enabled through the deployment of the software defined networking approaches in the backhaul network.

ACO–OFDM with Improved Bandwidth Efficiency over Long Haul and MIMO Optical Fiber Communication Systems

Authors propose an improved orthogonal frequency division multiplexing (OFDM) scheme in which the information samples are transmitted in otherwise discarded negative frame of asymmetrically clipped OFDM (ACO–OFDM). ACO–OFDM has two similar positive and negative OFDM envelopes so that clipping does not result in loss of signal information. The flipped polarity negative samples are inserted at the zeros of positive frame or back to back with positive frame, in time-compressed frame such that composite signal has the same frame duration as ACO–OFDM but improved transmission power, bandwidth efficiency. Simulation results for bit error rate with respect to number of subcarriers and optical launch power show that the proposed time-compressed OFDM frames perform better in high-dispersion condition over long distance, high data rate, and high transmission power over single-mode fiber. It has been demonstrated that the proposed time-compressed frame structure also performs well with the 2*2 MIMO setup for multimode fiber link.

An Efficient OFDM-Based Encryption Scheme Using a Dynamic Key Approach

Physical layer (PHY) security has emerged as a promising methodology for securing current and future networks that employ orthogonal frequency-division multiplexing (OFDM) technology. OFDM is the basic building block for multicarrier modulation in most contemporary networks such as vehicular ad hoc networks, Internet of Things (IoT), as well as 4G/5G systems. Most existing OFDM-based security solutions lack the notion of secrecy and dynamicity when combining a secret key with random information extracted from the physical channel. Yet, some solutions perform encryption preinverse fast Fourier transform and some postinverse fast Fourier transform, without clear guidelines concerning the impact on performance and security. In this paper, OFDM-based encryption schemes at the PHY are investigated, analyzed, and weaknesses are identified. It is shown that encryption in the frequency domain slightly mitigates the effects of channel fading and improves the bit error-rate performance. On the other hand, time-domain encryption is shown to be more secure. Furthermore, a dynamic secret key approach that enhances the security level of OFDM-based encryption schemes, in addition to a new technique for updating cipher primitives for input OFDM symbols or frames, are proposed. These schemes are shown to strike a good balance between performance and security robustness as demonstrated through experimental simulations.

An erasure-based scheme for reduction of PAPR in spatial multiplexing MIMO-OFDM using Reed-Solomon codes over GF(216 + 1)

ABSTRACTIn this paper, a novel erasure-based scheme which uses long Reed-Solomon (RS) codes over GF(65537) is proposed for the reduction of the peak-to-average power ratio (PAPR) in coded orthogonal frequency division multiplexing (OFDM). The motivation for using the field GF(65537) is to generate long code words (up to 65536 symbols in just one code word). Using long codes results in greater flexibility to search for low PAPR OFDM frames within the subsets of symbols of a code word because RS codes are maximum distance separable and any subset of a high enough number of symbols is sufficient for the recovery of data. Over this field, the lengths of code words are exponents of 2. Hence, low-complexity radix-2 fast Fourier transform can be exploited. RS codes are deployed for both PAPR reduction and error correction. Simulation results show that in similar PAPR reduction performances, the proposed scheme outperforms the previously reported work with RS codes in both error correction and computation complexity. The proposed scheme can be applied to both single-input single-output and multi-input multi-output systems.

Performance of a frequency-domain OFDM-frame detector

A frequency-domain algorithm for orthogonal frequency-division multiplexing (OFDM)-frame detection is considered useful when the OFDM system is to operate in a known narrowband interference (NBI) channel, e.g., in a cognitive radio OFDM-based overlay system. While frame detection algorithm based on state-of-the-art time-domain correlation perform poorly in such a channel, we propose a frame detection algorithm based on energy detection in frequency domain. By exploiting the NBI band information obtained during sensing period, the proposed algorithm exhibits strong performance and outperforms other existing frequency-domain OFDM frame detection alternatives even at the signal-to-interference ratio (SIR) below 0 dB. Moreover, simulation results are confirmed by the analytical performance presented as a linear combination of the incomplete gamma function. The weight of each component is a function of the eigenvalues of the matrix associated with the detection metric and the number of summation is proportional to the number of non-zero eigenvalues of the matrix associated with the detection metric.

Pilot-Based Time Domain SNR Estimation for Broadcasting OFDM Systems

The estimation of signal-to-noise ratio (SNR) is a major issue in wireless orthogonal frequency-division multiplexing (OFDM) system. In OFDM system, each frame starts with a preamble symbol that facilitates the SNR estimation. However, the performance of preamble-based SNR estimation schemes worsens in the fast-changing environment where channel changes symbol to symbol. Accordingly, in this paper, we propose a novel pilot-based SNR estimation scheme that optimally exploits the pilot subcarriers that are inserted in each data symbol of the OFDM frame. The proposed scheme computes the circular correlation between the received signal and the comb-type pilot sequence to obtain the SNR. The simulation results are compared with the conventional preamble-based Zadoff-Chu sequence SNR estimator. The results indicate that the proposed scheme generates near-ideal accuracy; especially in low SNR regimes, in terms of the normalized mean square error (NMSE). Moreover, this scheme offers a significant saving of computation over a conventional time domain SNR estimator.

Channel Estimation for Code Division Multiple Access Combined with Space Time Block Coding

In this paper, we present a combination of code division multiple access, space time block coding and orthogonal frequency division multiplexing technology. Also, the channel estimator related to this combination is proposed. In code division multiple access system, as the number of users increases the performance decreases. To overcome this problem, we apply space time block coding to improve the performance of the system. This approach is valid for non time dispersive channels. In the case of time dispersive channels, the problem of inter-symbol interference occurs. To cancel the inter-symbol interference, we use orthogonal frequency division multiplexing technology to transform the time dispersive channel into a number of non time dispersive channels. In this case, the received signal is simply decoded. To further improve the performance, channel estimator has to be very efficient. Therefore, we propose channel estimator based on the pilot symbols inserted periodically into the orthogonal frequency division multiplexing transmitted frame. At the receiver, interpolation filtering with specific gain and cutoff frequency is used to estimate the channel at all locations corresponding to orthogonal frequency division multiplexing frame. Several examples of real and estimated channels with different Doppler shifts are addressed. Simulation results of these examples show high performance and robustness of our approach.

Performance of Differential-Phase OFDM Systems over Selective Multipath Fading Channels

This paper investigates the performance of differential-phase Orthogonal Frequency Division Multiplexing (OFDM) over frequency-selective multipath Nakagami-m radio fading channels. A closed form for the average signal to noise/interference ratio in the presence of the selective multipath fading channel and additive white Gaussian noise (AWGN) is derived. The results reveal that the system performance is impacted by the interference between the adjacent OFDM frames in two successive signaling intervals which is called Inter-Symbol-Interference (ISI). In addition, the system will possibly be distorted when the orthogonality between the adjacent subcarriers is ceased, creating Inter-Channel-Interference (ICI). This paper also studies the bit-error-rate (BER) performance of the differential-phase OFDM system over the Nakagami-m channel in the presence of AWGN, ISI, and ICI under different conditions and parameters. Moreover, the effect of adding the guard period and the number of subchannels on the probability of error is analyzed. The IEEE 802.11a standard parameters with 64 subcarriers and a decaying exponential power delay profile with root-mean-square value of 129 ns are used in this study. The system performance is also simulated at different guard intervals, number of OFDM subcarriers, Nakagami severity parameter values, and different numbers of possible differential phases.

Performance-Improved Secure OFDM Transmission Using Chaotic Active Constellation Extension

We propose and demonstrate a physical-layer security scheme using chaotic active constellation extension (ACE) in orthogonal frequency division multiplexing (OFDM). A super OFDM frame is generated by chaotic frame interleaving in ACE, where hyper digital chaos is applied to generate the chaotic interleaving vectors. A huge key space of size 2048! is provided via the multi-fold encryption to guarantee the secure OFDM data transmission, while the transmission performance is effectively improved via the chaotic ACE algorithm. Secure transmission of 8.9-Gb/s 16-QAM encrypted OFDM signals is demonstrated over a 20-km standard single-mode fiber, which shows reliable robustness against any exhaustive attacks. Moreover, the proposed chaotic ACE scheme improves the spectral efficiency of OFDM transmission, since it does not require any sideband to transmit the frame-interleaving information.

Computational Efficient SLM–OFDM Receiver for Time-Invariant Indoor Fading Channel

This paper addresses receiver related side information (SI) estimation issues when selected mapping is used to reduce peak-to-average power ratio in orthogonal frequency division multiplexing (OFDM) systems. The SI contains critical information and its accurate estimation is required to enable successful recovery of payload data regardless of the channel condition. However, the need for SI estimation poses some practical issues in the form of high computational complexity and implementation challenges. Through simulations, this paper investigates the performance of an alternative data decoding approach called Embedded Coded Modulation (ECM), which requires no SI estimation. Using a form of block-type OFDM frame structure, results show that the ECM technique produces identical data decoding performance as other methods even in the presence of some non-linear amplifier distortions. In addition, it is shown that the ECM method eliminates SI related computational complexity and implementation problems.

Experimental Demonstration of Software-Configurable Asynchronous Real-Time OFDM Signal Transmission in a Hybrid Fiber-VLLC System

In this paper, a real-time hybrid fiber and visible laser light communication orthogonal frequency division multiplexing (OFDM) transport system is experimentally demonstrated. Based on virtual input/output technique, the system can be configured dynamically by using software. Although there is an approximate 12-ppm sampling frequency offset (SFO) between the transmitter and receiver, by adjusting the symbols number of one OFDM frame and the mapping scheme, the experiment results show that without SFO compensation, the system can achieve an up to 2.65 Gb/s data rate with a bit error rate at below 1e-3 after 100-km fiber and 6-m free-space transmission. As far as we know, this is the first time a real-time OFDM transmission has been reported in such a hybrid system.