Performance Of Orthogonal Frequency Division Multiplexing System Research Articles

Comparative study of DCT-and DHT-based OFDM systems over doubly dispersive fading channels

Abstract In high-mobility operating scenarios orthogonal frequency division multiplexing (OFDM) system lacks its optimality, intercarrier interference (ICI) occurs and the Doppler shifts deteriorate the orthogonality of the subcarriers. However, this problem can be overcome by utilizing complicated equalizers at the receiver. Discrete cosine transform (DCT) and discrete Hartley transform (DHT) have been used instead of discrete Fourier transform (DFT) in the standard OFDM system. The performance results achieved enhancement over dispersive selective channels that cannot be accomplished with a standard OFDM system, even with utilizing complex equalizers. In this paper, the performance of the DCT-based OFDM system and DHT-based OFDM system is compared and analyzed with respect to DFT-based OFDM system performance over doubly dispersive fading channel at various Doppler shifts, the block diagrams of the proposed systems are provided to simplify the theoretical analysis by making it easier to follow. Simulation results emphasized that the performance of DCT and DHT-based OFDM systems under doubly dispersive fading conditions scenario outperforms DFT-based OFDM system of order 3 dB energy per bit to noise power spectral density ratio (Eb/N0).

Recurrent neural networks for enhanced joint channel estimation and interference cancellation in FBMC and OFDM systems: unveiling the potential for 5G networks

FBMC is a pivotal system in 5G, serving as a cornerstone for efficient use of available bandwidth while simultaneously meeting stringent requirements for high spectral efficiency. Notably, FBMC harnesses the power of multicarrier modulation (MC), a good alternative to orthogonal frequency division multiplexing (OFDM) technology that supports fourth-generation (4G) systems. The wireless communications field is full of challenges, the most important of which are channel estimation and interference cancellation, both of which deserve comprehensive study to increase the efficiency of data transmission. In this paper, our investigation takes a deliberate step towards the convergence of two prominent modulation models: OFDM and FBMC. We specifically contrast these modulation techniques with the intricate field of joint channel estimation and interference cancellation (JCEIC). In this research study, we take advantage of recurrent neural networks' (RNNs') efficiency as a vehicular channel to perform precise channel estimation and recovery of uncorrupted transmitted signals, thereby lowering the bit error rate (BER). Our channel estimation for a dual selective channel is based on the thoughtful placement of pilots scattered over the temporal and frequency dimensions, and is further improved by the interference cancellation method of low complexity that was selected. Our JCEIC proposal aims to integrate RNNs carefully, using the output sequences of JCEIC algorithms as useful inputs to this neural architecture. By clearly demonstrating a decrease in BER as compared to traditional approaches, it is evident that the performance of the novel approach is near to that of a perfect channel. Additionally, a comparison of the performance of FBMC and OFDM systems at various signal-to-noise ratios reveals a clear performance divide that favors the former in terms of system efficiency. The BER is restricted by FBMC to a commendable threshold of less than 0.1 at a modest 5 dB, continuing the higher trend started by its improved RNN-based channel estimate. The accuracy of channel estimation is clearly improved by this paradigm shift, and the computing complexity typical of 5G networks is also clearly reduced.

Federated Learning With Over-the-Air Aggregation Over Time-Varying Channels

We study federated learning (FL) with over-the-air aggregation over time-varying wireless channels. Independent workers compute local gradients based on their local datasets and send them to a parameter server (PS) through a time-varying multipath fading multiple access channel via orthogonal frequency-division multiplexing (OFDM). We assume that the workers do not have channel state information, hence the PS employs multiple antennas to alleviate the fading effects. Wireless channel variations result in inter-carrier interference, which has a detrimental effect on the performance of OFDM systems, especially when the channel is rapidly varying. We examine the effects of the channel time variations on the convergence of the FL with over-the-air aggregation, and show that the resulting undesired interference terms have only limited destructive effects, which do not prevent the convergence of the learning algorithm. We also validate our results via extensive simulations, which corroborate the theoretical expectations.

Deep Learning-Assisted OFDM Channel Estimation and Signal Detection Technology

The orthogonal frequency division multiplexing (OFDM) technique has received wide attention because of its high spectrum utilization. However, the drawback of inter-subcarrier interference in OFDM systems makes the channel estimation and signal detection performance of OFDM systems with few pilots and short cyclic prefixes (CP) poor. In this paper, we use deep learning to assist OFDM in recovering nonlinearly distorted transmission data. Specifically, we use a self-normalizing network (SNN) for channel estimation, combined with a convolutional neural network (CNN) and a bidirectional gated recurrent unit (BiGRU) for signal detection, thus proposing a novel SNN-CNN-BiGRU network structure (SCBiGNet). The simulation results show that the SCBiGNet model outperforms the existing techniques for the different numbers of pilots and lengths of CPs. The BER performance is improved by 0.2~9 dB.

Post-Processing of Iterative Estimation and Cancellation Scheme for Clipping Noise in OFDM Systems

Clipping is an efficient and simple method that can reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. However, clipping causes in-band distortion referred to as clipping noise. To resolve this problem, a novel iterative estimation and cancellation (IEC) scheme for clipping noise is one of the most popular schemes because it can significantly improve the performance of clipped OFDM systems. However, IEC exploits detected symbols at the receiver to estimate the clipping noise in principle and the detected symbols are not the sufficient statistic in terms of estimation theory. In this paper, we propose the post-processing technique of IEC, which fully exploits given sufficient statistic at the receiver and thus further enhances the performance of a clipped OFDM system as verified by simulations.

Enhanced diversity scheme for orthogonal frequency division multiplexing systems over doubly selective fading channels

AbstractFrequency‐time selective fading degrades the performance of communication systems, but it also provides an opportunity to collect multipath diversity and Doppler diversity. In this paper, an oversampled grouped‐linear‐constellation‐precoding (GLCP) scheme is proposed for orthogonal frequency division multiplexing (OFDM) systems to extract the diversity inherent in doubly selective fading channels. In the proposed scheme, GLCP is employed to precode the information symbols of OFDM at the transmitter, and the precoded OFDM symbols are oversampled in the time domain at the receiver. A full‐groups maximum likelihood (ML) decoder and a successive group decoder to decode the oversampled OFDM symbols with diversity gains are developed. The successive group decoder combines the group ML decoding algorithm with a block‐decision‐feedback‐equalization (BDFE) algorithm. The proposed scheme can take advantage of both the GLCP scheme and the oversampling scheme to achieve multipath diversity gains and Doppler diversity gains in doubly selective fading channels. Simulation results show that the proposed scheme can significantly improve the performance of OFDM systems in the frequency selective fading channel and the doubly selective fading channel in terms of bit error ratio, indicating that the diversity gains can be greatly enhanced by the proposed scheme in doubly selective fading channels.

PAPR Analysis of OFDM using Selective Mapping Method

A common standard modulation for several high data rate wireless communication systems is Orthogonal Frequency-Division Multiplexing (OFDM). It operates by splitting up large streams of data into several parallel streams with minimal data rates. Compared to single carrier modulation systems, OFDM offers various benefits. Some of these benefits are high transmission bit rates, great spectral efficiency, and sensitivity to frequency fading channels. However, there are drawbacks with OFDM. Among the major issues is the high Peak to Average Power Ratio (PAPR), due to which the system complexity increases. The Selective Mapping approach is used in this study to lower the OFDM PAPR. In this article, simulation has been done using MATLAB, evaluated PSD, spectral efficiency, BER performance and PAPR analysis of OFDM system.

Impact of carrier frequency offset and in-phase and quadrature imbalance on the performance of wireless precoded orthogonal frequency division multiplexing

<p>Precoding in orthogonal frequency division multiplexing (OFDM) system proved to reduce the peak-to-average power ratio (PAPR), so that it improves BER. However, from the existing literature, the effect of carrier frequency offset (CFO), in-phase and quadrature (IQ) imbalance on precoded wireless OFDM systems has not been carried out. Therefore, this study evaluated the precoded OFDM (P-OFDM) system performance by considering the impact of CFO and IQ imbalance. P-OFDM performance evaluation is expressed in signal-to-interference noise ratio (SINR) and bit error rates (BER). The communication channels used are the additive white Gaussian noise (AWGN) channel and the frequency-selective Rayleigh fading (FSRF) channel, while the channel equalization process is considered perfect. The results of the analysis and simulation show that P-OFDM is greater affected by the presence of CFO and IQ imbalance than conventional OFDM system. In AWGN channel, P-OFDM experiences different SINR for each subcarrier. This is different from conventional OFDM system, where all SINRs are the same for all subcarriers. In the FSRF channel, both the POFDM system and the OFDM system experience different SINR for each subcarrier, where the SINRs fluctuation in the P-OFDM system is much larger than in the OFDM system.</p>

BER Performance of Pre-Coded Space-Time Conjugate Two-Path OFDM Systems

It’s critical to design a robust wireless communication system that achieves an improved bit error rate (BER) in mobile channels. In this paper, a 2x2 space-time (ST) orthogonal frequency division multiplexing (OFDM) system is proposed with conjugate cancellation (CC) two-path transmission scheme. The 2x2 STCC-OFDM system features in the excellent inter-carrier interference (ICI) mitigation in the mobile channels with a better BER than that of a conventional 2x2 ST-OFDM system. Five orthogonal transforms are employed as the precoder to enhance the BER further, such as Zadoff-Chu transform (ZCT), slant transform (ST), discrete Hartley transform (DHT), Haar transform (HT), and Walsh–Hadamard transform (WHT). ZCT row wise precoder provide the lowest peak-to-average power ratio and good BER performance. Simulations demonstrate that the proposed STCC-OFDM systems are robust to various channel environments Although the STCC-OFDM systems improve BER with the cost of reducing bandwidth efficiency when compared to a ST-OFDM, they can be compensated by a higher QAM constellation size.

An Efficient ICI Cancellation Scheme to Mitigate the Effect of ICI on OFDM Systems.

Inter-carrier interference (ICI) emerges in orthogonal frequency division multiplexing (OFDM) systems used for mobile communication as a consequence of the Doppler Effect's loss of orthogonality among subcarriers. Inter-Carrier Interference (ICI), which affects every subcarrier, drastically lowers performance. The performance of OFDM systems may be enhanced using a variety of ICI mitigation strategies. Comparable subcarrier frequency offsets are guaranteed by the premise that the OFDM transmission bandwidth is suitably modest in the majority of ICI mitigation strategies, on the other hand. The frequency offsets between each subcarrier might change, hence a wideband OFDM system in a situation with high mobility is investigated. Furthermore, the suggested ICI cancellation approach, Total ICI Cancellation, does not reduce bandwidth efficiency or transmission rate. As an example, the Total ICI Cancellation approach uses the ICI matrix's orthogonality to provide perfect ICI cancellation and a significant boost in BER at a linearly increasing cost. The suggested technique, which matches the BER performance of a wideband OFDM system without ICI, offers the best BER performance possible in the presence of frequency offset and time shifts in the channel, according to simulation findings in the AWGN and multipath fading channels.

An efficient and improved PTS algorithm for PAPR reduction in OFDM system

ABSTRACT In this research, an improved PTS algorithm is projected for diminishing peak to average power ratio (PAPR) of well accepted and effective orthogonal frequency division multiplexing (OFDM) system. OFDM is also considered as trustworthy method for several wireless communication applications and obtained a lot of research interests but unfortunately, one of the most important concern is very high PAPR in OFDM signals, which deteriorates system performance thus this is the most important drawback. To address this problem, a novel partial transmit sequence (PTS) algorithm is projected based upon centering phase sequences matrix (PTS-CPSM-Algo). These phase sequences are used for optimization of data so that minimum PAPR can be obtained. This algorithm obtains significant reduction in PAPR which leads to the enhancement in performance of OFDM system. From the developed analysis and simulation results, supremacy can be clearly seen for proposed PTS-CPSM-Algo over already existing OFDM system, selected mapping (SLM) and partial transmit sequence (PTS).

When high PAPR reduction meets CNN: A PRD framework.

One of the most important factors limiting the performance of OFDM (Orthogonal Frequency Division Multiplexing) system is high PAPR (Peak to Average Power Ratio). Great efforts have been made in suppressing PAPR, but their implementation often requires pre-processing all input signals, leading to excessive calculation overhead. When the transmission speed is high, much more time will be taken to process the input signal with the traditional methods, which will reduce the performance of the system. In this background, this paper firstly presents an algorithm, called PRD, to identify the high PAPR sequence without IFFT (Inverse Fast Fourier Transform) operations, in which a CNN (Convolutional Neural Network) for identifying PAPR sequences is trained first before applying further PAPR reduction schemes. Experimental results show that the proposed algorithm can identify the high PAPR sequences with 92.3% accuracy and reduce PAPR with extremely low calculations.

OFDM System Performance, Variability and Optimality With Design Imperfections and Channel Impediments

An important indicator commonly used to evaluate the quality of a communication link is signal-to-interference ratio (SIR). However, studies in the literature dealing with SIR for orthogonal frequency division multiplexing (OFDM) systems are relatively few. This paper analytically investigates OFDM system performance by including major channel impairments such as excess delay spread, synchronization error, sample timing error, frequency offset, and time-varying fading. We derive a closed-form expression for SIR, accounting for both variations of inter-carrier interference (ICI) and inter-symbol interference (ISI) with the number of subcarriers, and the susceptibility to channel impairments. A condition relating peak ISI to peak ICI is derived. An expression quantifying the impact of synchronization error in ISI is obtained. SIR behavior with respect to the tradeoff among numbers of subcarriers and other system parameters are investigated. A numerical method, as well as a polynomial equation for an analytical solution, is formulated to find the optimal number of subcarriers for a given set of constraints. We demonstrate the variations of SIR with various parameters and analyze the leakage of power (unwanted power distribution across subcarriers).

DFT-Spread OTFS Communication System with the Reductions of PAPR and Nonlinear Degradation

The orthogonal time frequency space (OTFS) system with two-dimensional (2-D) modulation technique is an attractive technology to show the better performance than the conventional orthogonal frequency division multiplexing (OFDM) system, particularly in the high-speed vehicle communications and high-level dimensions of multiple-input multiple-output systems. However, the OTFS system has high peak-to-average power ratio (PAPR), like OFDM system. In this paper, we analyze the PAPR characteristics of the OTFS signal and propose a concept of discrete Fourier transform (DFT)-Spread OTFS system to reduce the PAPR efficiently. The key idea of the DFT precoding technique is to spread the energy of a subcarrier throughout the subcarriers. Therefore, the PAPR is decreased. In the practical wireless communication systems, the power efficiency could become worse, or the bit error rate (BER) is seriously degraded by the high peak signal power in a high power amplifier (HPA). So, by reducing the PAPR, the efficiency of the HPA can be significantly improved and the nonlinear distortion caused by HPA can be decreased, too. We evaluate the spectrum characteristics of conventional OFDM and OTFS schemes and the proposed DFT-Spread OTFS scheme according to the different nonlinear conditions of HPA. Simulation results demonstrate that the power spectrums of the proposed system are less sensitive than the conventional schemes in HPA nonlinear environments. In addition, we compare the uncoded BER performance among the different schemes over the AWGN and delay-Doppler channels according to the several HPA nonlinear conditions for the different mobility speeds of user equipment. The BER performance of the conventional OFDM system breaks down completely in high-mobility scenarios for both HPA linear and nonlinear conditions. The proposed scheme improves the BER performance over AWGN and delay-Doppler channels in HPA nonlinear environments, as compared to the conventional OTFS system. In high-mobility scenarios and at BER of $$1 \times 10^{ - 3}$$ , simulation results verify that the OTFS and our proposed scheme can achieve a gain of 12.8 dB compared to OFDM scheme. Moreover, the proposed DFT-Spread OTFS scheme can be reduced PAPR by 2.2 dB and 1.8 dB compared to OFDM and OTFS schemes, respectively.

A new nonlinear companding algorithm based on tangent linearization processing for PAPR reduction in OFDM systems

In this paper, a novel nonlinear companding transform (NCT) is proposed to reduce the Peak-to-Average Power Ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. The companding function is designed based on continuously differentiable reshaping of the probability density function (PDF) of signal amplitudes. The original PDF is cut off for PAPR reduction, and lower and medium segments of original PDF are scaled and linearized respectively, for maintaining power and cumulative distribution constraints. The linearized segment is set to be the tangent of the scaled version at the inflexion point, so as to reduce the out-of-band (OOB) radiation as much as possible. Parameters of the proposed scheme are solved under joint constraints of constant power and unity cumulative distribution. A new receiving method is also proposed to improve the bit error rate (BER) performance of OFDM systems. Simulation results indicate the proposed scheme can achieve better OOB radiation and BER performance at same PAPR levels, compared with existing similar companding algorithms.

Performance evaluation of high mobility OFDM channel estimation techniques

In wireless communication, Orthogonal Frequency Division Multiplexing (OFDM) has been adopted due to its robustness to multipath fading and high data rate transmissions. At the other hand, the performance of OFDM systems severely degraded due to multi-path fading and Doppler frequency shifts in mobile systems, which causes inter-carrier-interference (ICI). Thus, Estimation of channel parameters is required at the receiver using a pre designed estimator where pilot tones are inserted in each OFDM symbol. In this paper, a random pilot data are generated and inserted in each OFDM symbol at equally spaced locations. The performance test of Least Square (LS) and Linear Minimum Mean Square (LMMSE) estimation methods are proposed with Discrete Fourier Transform (DFT) based on both LS and LMMSE, where different ITU channel models are considered in order to compare their performance for data transmission in high mobile systems with different Doppler frequencies exceeds 200 Hz and minimal number of pilots.

Effect of Controlling Parameters of Tone Reservation Based on Null Subcarriers (TRNS) on the Performance of OFDM Systems

High data rate communication systems usually implement Orthogonal Frequency Division Multiplexing (OFDM) to face frequency selectivity. High Peak to Average Power Ratio (PAPR) is an OFDM disadvantage that causes Bit Error Rate (BER) degradation and out-of-band (OOB) radiation when OFDM signal pass through nonlinear Power Amplifier (PA). In order to overcome this problem larger Input Back-Off (IBO) is required. However, large IBO decreases the PA efficiency. PAPR reduction techniques are used to reduce the required IBO, so that PA efficiency is saved. Several PAPR reduction methods are introduced in literature, among them Tone Reservation based on Null Subcarriers (TRNS) is downward compatible version of Tone Reservation (TR) with small excess in the average power and low computational complexity compared to others. As will be shown, TRNS is the best practical one of the four downward compatible techniques. Performance of TRNS is controlled by three parameters; number of peak reduction tones (PRTs), predefined threshold (Amax), and number of iterations (Itr). In order to increase PAPR reduction gain, enhance BER performance, and reduce the required IBO to follow the given power spectral density (PSD), we have to choose the values of these parameters adequately. Results showed that, we have to reduce the threshold value to the average (i.e. Amax =0 dB). Also, we have to increase number of PRTs. However, we have to maintain the spectrum shape. Finally, we have to choose moderate number of iterations (e.g. Itr ≈50), as excessive increase in number of iterations is not useful, especially at high PAPR values.

Collaborative Blind Equalization for Time-Varying OFDM Applications Enabled by Normalized Least Mean and Recursive Square Methodologies

In this work, a collaborative blind equalization method for the orthogonal frequency division multiplexing (OFDM) signals in time-varying channels is presented. Equalizers are eliminated the inter-symbol interference (ISI) in the received signals caused by channel distortions. The conventional adaptive equalization requires sending the training-sequences periodically to synthesize the channel model, which can only provide redundant information, and consequently decrease the channel utilization and complicate the system. To overcome this drawback, the blind equalization methods, which need not send the training-sequences periodically, is developed. However, the conventional blind equalization methods still suffer from various disadvantages. The normalized least mean square (NLMS) method is able to converge rapidly, whereas its equalization error is relatively large. The recursive least square (RLS) method has smaller steady-state error but low convergence rate, demanding massive training sequences. To further enhance the equalization performance of time-varying OFDM systems, which is typically with massive calculation, a collaborative blind equalization is proposed in this work, which is able to effectively combine the characteristics of the conventional NLMS and RLS methods together. The numerical simulations demonstrate the proposed LM-RS method can exhibit quite good performance. Particularly, as compared with the conventional NLMS and RLS methods, the proposed LM-RS method achieves smaller steady-state error and lower complexity, as well as similar convergence rate. All these results indicate that the proposed collaborative LM-RS blind equalization method is suitable for the OFDM transmission under the time-varying wireless application environments.

Blind Algorithm Development for Peak to Average Power Ratio Reduction in OFDM Systems under Frequency Selective Channels

One major drawback of orthogonal frequency division multiplexing (OFDM) system is peak to average power ratio (PAPR). This effect causes high power amplifier (HPA) to introduce intermodulation and out of band radiation as the signal goes through, thus degrades the performance of OFDM systems. This paper proposes blind algorithms which takes advantage of signal transformation technique and signal distortion technique. Simulation results show that at complementary cumulative distribution function (CCDF) level of 10-3 , the proposed algorithm achieved 3.2 dB PAPR improvement compared to discrete Fourier transform with interleaved frequency division multiple access (DFT-IFDMA) based algorithm. The bit error rate (BER) performance has degraded by 2 dB compared to the original OFDM signal with no distortion under frequency selective channel (FCS) at BER of 10-4 . These presented results, mark this algorithm as a better candidate for PAPR reduction algorithm in long term evolution (LTE) network. Under AWGN channels, the proposed algorithm performs better both in low and high signal power values. Under frequency selective channels, the existing and proposed algorithm converges after 10 dB of signal to noise power values. The low BER transmissions at low signal power values signify energy efficiency, ideal for portable wireless devices with limited battery power.

An Underwater Acoustic OFDM System Based on NI CompactDAQ and LabVIEW

Recently the orthogonal frequency division multiplexing (OFDM) technique has attracted increasing interests in underwater acoustic (UA) communications. In this paper, we present an OFDM-based UA communication system, which is designed based on the National Instruments LabVIEW software and the CompactDAQ device. Details on both the transmitter and receiver system design are discussed. The performance of this UA OFDM system is verified through recent UA communication experiments performed in a tank, in the Canning River, and in the estuary of the Swan River, Western Australia. Experimental results show that the system achieves a reliable bit error rate performance even with high-order modulation schemes. Compared with conventional field-programmable gate array and digital signal processor based designs, the proposed implementation simplifies the prototype design process and reduces the software development time. The proposed system provides a flexible and reconfigurable prototype for researchers to test and validate the performance of UA communication algorithms in real UA channels.