Wireless Orthogonal Frequency Division Multiplexing Systems Research Articles

An efficient deep neural network channel state estimator for OFDM wireless systems

AbstractChannel state estimation (CSE) is essential for orthogonal frequency division multiplexing (OFDM) wireless systems to deal with multipath channel fading. To attain a high data rate with the use of OFDM technology, an efficient CSE and accurate signal detection are required. The use of machine learning (ML) to improve channel estimates has attracted a lot of attention lately. This is because ML techniques are more adaptable than traditional model-based estimation techniques. The present study proposes a receiver for low-spectrum usage in OFDM wireless systems on Rayleigh fading channels using deep learning (DL) long short-term memory (LSTM). Before online deployment and data retrieval, the proposed DL LSTM estimator gathers channel state information from transmit/receive pairs using offline training. Based on the simulation results of a comparative study, the proposed estimator outperforms conventional channel estimation approaches like minimum mean square error and least squares in noisy and interfering wireless channels. Furthermore, the proposed estimator outperforms the DL bidirectional LSTM (BiLSTM)-based CSE model. In particular, the proposed CSE performs better than other examined estimators with a reduced number of pilots, no cycle prefixes, and no prior knowledge of channel statistics. Because the proposed estimator relies on a DL neural network approach, it holds promise for OFDM wireless communication systems.

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>

On the performance of IRS‐assisted OFDM system with non‐ideal oscillator and amplifier

Intelligent reflective surface (IRS), a software‐controlled metasurface, is now a proven and promising candidate technology to achieve superior and reliable data transmission for the next‐generation mobile communication systems. This letter investigates the performance of an IRS‐assisted orthogonal frequency division multiplexing (OFDM) wireless system in the presence of a non‐ideal oscillator and amplifier. The analytical framework comprises of deriving the closed‐form expressions for the outage probability, spectral efficiency, energy efficiency, and diversity order. The result shows that the phase noise and distortion significantly limit the performance gain of an IRS‐assisted OFDM wireless communication system.

Proposed Different Signal Processing Tools for Efficient Optical Wireless Communications

Optical Wireless Communication (OWC) is a new trend in communication systems to achieve large bandwidth, high bit rate, high security, fast deployment, and low cost. The basic idea of the OWC is to transmit data on unguided media with light. This system requires multi-carrier modulation such as Orthogonal Frequency Division Multiplexing (OFDM). This paper studies optical OFDM performance based on Intensity Modulation with Direct Detection (IM/DD) system. This system requires a non-negativity constraint. The paper presents a framework for wireless optical OFDM system that comprises (IM/DD) with different forms, Direct Current biased Optical OFDM (DCO-OFDM), Asymmetrically Clipped Optical OFDM (ACO-OFDM), Asymmetrically DC-biased Optical OFDM (ADO-OFDM), and Flip-OFDM. It also considers channel coding as a tool for error control, channel equalization for reducing deterioration due to channel effects, and investigation of the turbulence effects. The evaluation results of the proposed framework reveal enhancement of performance. The performance of the IM/DD-OFDM system is investigated over different channel models such as AWGN, log-normal turbulence channel model, and ceiling bounce channel model. The simulation results show that the BER performance of the IM/DD-OFDM communication system is enhanced while the fading strength is decreased. The results reveal also that Hamming codes, BCH codes, and convolutional codes achieve better BER performance. Also, two algorithms of channel estimation and equalization are considered and compared. These algorithms include the Least Squares (LS) and the Minimum Mean Square Error (MMSE). The simulation results show that the MMSE algorithm gives better BER performance than the LS algorithm.

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 of Orthogonal Frequency Division Multiplexing Based Advanced Encryption Standard

Currently, there are a lot of secure communication schemes have been proposed to hide secret contents. In this work, one of the methods deploying encryption to cipher data is represented. The primary object of this project is applying Advanced Encryption Standard (AES) in communications based Orthogonal Frequency Division Multiplexing (OFDM). This article discusses the security of the method encrypting directly QAM symbols instead of input bit-stream. This leads to improving the security of transmitting data by utilization of authentication key between the mobile and base station. The archived results demonstrate that the performance of the AES-OFDM system is completely acceptable to compare with the criteria for 4G.
 Keywords:
 Orthogonal Frequency Division Multiplexing (OFDM), Advanced Encryption Standard (AES), Quadrature Amplitude Modulation (QAM), Authentication Key, Cellular Network, Encryption, Physical Layer, 4G, LTE.
 References
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A Carrier Frequency Offset Estimation for OFDM System via Local Searching Capon Algorithm

Orthogonal frequency division multiplexing (OFDM) technology is an important modulation technology in wireless communication. One of the main disadvantages of OFDM system is that it is very sensitive to carrier frequency deviation. Therefore, estimation of carrier frequency deviation is one of the key technologies of OFDM system. In this paper, the global search Capon algorithm is improved, and a local search Capon algorithm is proposed. Simulation results show that the algorithm has low complexity and high performance. In the local search, P spectral peaks are superimposed, which can effectively overcome the problem that the estimation performance of traditional Capon algorithm is seriously reduced due to the unapparent spectral peaks or the missing spectral peaks in the case of low SNR., and it has a good application prospect in power wireless OFDM communication system. Since contemporary information-retrieval systems rely heavily on the content of titles and abstracts to identify relevant articles in literature searches, great care should be taken in constructing both.

Secure Image Transmission in Wireless OFDM Systems Using Secure Block Compression-Encryption and Symbol Scrambling

In this paper, a joint compression-encryption and symbol scrambling scheme is proposed for orthogonal frequency-division multiplexing (OFDM) systems for image date. First, in the compression-encryption phase, the original image data is fist divided into sub-blocks and then an secure image block compressive sensing (BCS) is applied to resulting image data, in which a chaotic sparse basis matrix and a chaotic measurement matrix based on the discrete cosine transform (DCT) are used. Secondly, the resulting secure CS data is transform into binary phase shift keying (BPSK) symbols and then a chaotic scrambling operation is applied to the obtained BPSK symbols. The simulation results show that the proposed secure BCS scheme can achieve better peak signal-to-nose ratio (PSNR) performance than that of the conventional chaotic measurement method. The simulation results also show that a good trade-off performance of signals between PSNR and computational complexity can be achieved when the proper sub block size of image is adopted. In addition, the performances of the BPSK OFDM signal, which is generated from the secure BCS of the image, are evaluated over AWGN and multipath fading channels. The simulation results show that the proposed scheme not only achieve effective secure block compressive sensing and achieve secure transmission in physical layer. The security of the transmitted image data is enhanced by the joint compression-encryption and symbol scrambling.

Suppressing the OFDM CFO‐Caused Constellation Symbol Phase Deviation by PAPR Reduction

The well‐known major drawbacks of the Orthogonal Frequency‐Division Multiplexing (OFDM), namely, the transmitter versus receiver Carrier Frequency Offset (CFO), and the Peak‐to‐Average Power Ratio (PAPR) of the transmitted OFDM signal, may degrade the error performance, by causing Intercarrier Interference (ICI), as well as in‐band distortion and adjacent channel interference, respectively. Moreover, in spite of the utmost care given to CFO estimation and compensation in OFDM wireless systems, such as wireless local networks or the mobile radio systems of the fourth generation, e.g., the Long‐Term Evolution (LTE), still some residual CFO remains. With this regard, though so far the CFO and the PAPR have been treated independently, in this paper, we develop an Error Vector Magnitude (EVM) based analytical model for the CFO‐induced constellation symbol phase distortion, which essentially reveals that the maximal CFO‐caused squared phase deviation is linear with the instantaneous (per‐OFDM‐symbol) PAPR. This implies that any PAPR reduction technique, such as simple clipping or coding, indirectly suppresses the CFO‐induced phase deviation, too. The analytically achieved results and conclusions are tested and successfully verified by conducted Monte Carlo simulations.

Structure-Based Low Complexity MMSE Channel Estimator for OFDM Wireless Systems

Wireless communication systems utilizing orthogonal frequency division multiplexing (OFDM) transmissions are capable of delivering high data rates over multipath frequency selective channels. This paper deals with joint estimation/interpolation of wireless channel using pilot symbols transmitted concurrently with the data. We propose a low complexity, spectrally efficient minimum mean square error channel estimator which exploits the correlation structure of channel frequency response for reducing the complexity. Specifically, it is shown that if pilots are inserted appropriately across OFDM subcarriers, the proposed algorithm requires no matrix inversion, thereby significantly relieving the computational burden without deteriorating the performance. Moreover, the knowledge of channel correlation is also not required for the proposed estimator. Simulation results validate that the proposed technique outperforms existing low-complexity variants in terms of mean square error and computational complexity.

A novel reduced complexity optimized PTS technique for PAPR reduction in wireless OFDM systems

In this paper, we propose a novel low complexity Partial Transmit Sequence (PTS) technique employing Random phase sequence matrix (RPSM) for peak to average power ratio (PAPR) reduction in orthogonal frequency division multiplexing (OFDM) systems. The main goal of our suggested scheme is to achieve the optimum phase sequence matrix to minimize PAPR and simultaneously reduce the computational complexity by decreasing the number of Inverse Fast Fourier Transform (IFFT) operations required. Lower PAPR reduces the complexity of Digital to Analog converters (DAC) and increases the efficiency of power amplifiers. Analytical expressions for Complementary Cumulative Distribution Function (CCDF), Number of subcarriers, subblocks and Total computational complexity are derived. Simulation results match closely with the analytical results. It is demonstrated that a favorable tradeoff can be achieved between the reduction of PAPR and computational complexity. It is observed that the suggested modified PTS technique outperforms the traditional PTS (T-PTS) technique.

Shift Estimation with Delayed Correlation Cyclic-Selective Mapping (Cyclic-SLM) PAPR Reduction in OFDM System

Cyclic-selective mapping (SLM) for wireless orthogonal frequency division multiplexing (OFDM) systems combines an original signal and its cyclic shifted signal to reduce a peak-to-average power ratio (PAPR) in time domain. However, side information (SI) regarding to the amount of the cyclic shift is required at the receiver, and how to transmit it is one of the most challenging issues of SLM schemes because it decreases the throughput of wireless OFDM systems. The delayed correlation based estimation scheme is proposed in this paper to avoid the transmission of the SI. This scheme exploits the correlation between a guard interval sequence and a received signal with delay to estimate the amount of the cyclic shift at the receiver side. Numerical results obtained through computer simulation show that the proposed scheme can estimate the delay with an accuracy of more than 0.99 at an $$E_b/N_0$$Eb/N0 of 8 dB and achieve the PAPR that is equivalent to the conventional SLM while the bit error rate degradation is slightly different from that with perfect shift estimation.

Performance Evaluation of Coded Cooperative Relay Based OFDM Wireless Communication System

This paper investigates the performance of a coded cooperative relay based Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system using native rate 1/2 convolutional code and rate 1/3 turbo code. These investigations are focused on evaluating the performance of the proposed coded cooperative wireless communication system over both Additive white Gaussian Noise (AWGN) channel and different Stanford University Interim (SUI) channel models. The performance of the proposed coded cooperative relay based system is evaluated using MATLAB computer simulations and compared with direct transmission and other cooperative protocols i.e., Amplify-and-Forward (AAF) and Decode-andForward (DAF). The performance indicators used for comparison are Bit-Error Rate (BER) and spectral efficiency. The results show that the proposed coded cooperative system achieves lower Signal-to-Noise Ratio (SNR) values for the desired BER and achieves high spectral efficiency as compared to direct transmission, AAF and DAF based cooperative protocols. Finally, the results of computer simulations are included to demonstrate the efficiency of the proposed scheme.

Spectral Efficiency of OFDM Systems With Random Residual CFO

Orthogonal frequency division multiplexing (OFDM) over wireless channels is sensitive to carrier frequency offset (CFO), which destroys orthogonality amongst sub-carriers, giving rise to inter-carrier interference (ICI). Different techniques are available for estimating and compensating for the CFO at the receiver. However, in practice, a residual CFO remains at the receiver after CFO estimation, where the estimation accuracy depends primarily on the fractions of time and power used by the estimator. In this paper, we propose to measure the efficiency of OFDM systems with CFO estimation errors in terms of the spectral efficiency, which accounts for both, the degradation in signal-to-interference plus noise ratio (SINR) due to the residual CFO, and the penalty of the extra power and spectral resources allocated to achieve the desired CFO estimation accuracy. New accurate expressions are derived for the spectral efficiency of wireless OFDM systems in the presence of residual CFO and frequency-selective multipath fading channel. These are used to compare between two common CFO estimation methods in wireless OFDM systems, namely, the cyclic prefix based and the training symbols based CFO estimation techniques for fixed and variable pilot power. These results are further extended to include OFDM systems with transmit diversity techniques. In addition, the impact of imperfect channel estimation on the overall spectral efficiency is also included. Numerical results reveal that the cyclic prefix based CFO technique is more efficient than the training symbols based CFO technique when perfect channel state information (CSI) is known blindly at the receiver. Furthermore, fixed pilot power results in a spectral efficiency ceiling as SNR increases, whereas spectral efficiency increases with SNR without bound in the equal pilot and signal powers case.

Hybrid MIMO Cooperative Scheme for Improved Throughput in Wireless OFDM Systems

In this paper, a new hybrid multiple-input multiple-output (MIMO) cooperative scheme is proposed in wireless orthogonal frequency division multiplexing (OFDM) systems. MIMO---OFDM systems have allowed the improvement of the system performance as reliability and data rate. However, MIMO---OFDM systems have limitations of supporting reliable transmission and high data rate due to size, cost and complexity limitation of device. So, the hybrid MIMO cooperative communication has been studied to compensate the defects. The hybrid MIMO cooperative scheme has advantages of MIMO and cooperative communication. But the conventional hybrid MIMO cooperative scheme has a few defects under particular environments. So, the proposed hybrid MIMO cooperative scheme supplements the defects of the conventional schemes.

An Efficient Lattice Sphere Decoding Technique for Multi-Carrier Systems

A low-complexity detection scheme, which consists of a lattice sphere decoder (LSD), is proposed for Orthogonal Frequency Division Multiplexing (OFDM) systems. The inter-carrier interference resulting from selective fading channel destroys the orthogonality of the OFDM signal and poses a significant limitation on wireless OFDM systems. The aim of this paper is to propose an efficient detection technique in order to provide reliable communications using OFDM in selective fading channels. The proposed detection technique combines the new LSD with two regularization methods. Theoretical analysis and simulation results show that the proposed detection scheme substantially improves the symbol error rate performance of OFDM systems. Moreover, its complexity is 22 times lower than that reported in a previous work, and 51 times lower than the complexity of the original sphere decoding technique.

Blind and semi-blind ML detection for space-time block-coded OFDM wireless systems

This paper investigates the joint maximum likelihood (ML) data detection and channel estimation problem for Alamouti space-time block-coded (STBC) orthogonal frequency-division multiplexing (OFDM) wireless systems. The joint ML estimation and data detection is generally considered a hard combinatorial optimization problem. We propose an efficient low-complexity algorithm based on branch-estimate-bound strategy that renders exact joint ML solution. However, the computational complexity of blind algorithm becomes critical at low signal-to-noise ratio (SNR) as the number of OFDM carriers and constellation size are increased especially in multiple-antenna systems. To overcome this problem, a semi-blind algorithm based on a new framework for reducing the complexity is proposed by relying on subcarrier reordering and decoding the carriers with different levels of confidence using a suitable reliability criterion. In addition, it is shown that by utilizing the inherent structure of Alamouti coding, the estimation performance improvement or the complexity reduction can be achieved. The proposed algorithms can reliably track the wireless Rayleigh fading channel without requiring any channel statistics. Simulation results presented against the perfect coherent detection demonstrate the effectiveness of blind and semi-blind algorithms over frequency-selective channels with different fading characteristics.

Energy-Efficient Resource Allocation in Downlink OFDM Wireless Systems With Proportional Rate Constraints

Orthogonal frequency-division multiplexing (OFDM), with its own advantages of spectral efficiency (SE) enhancement and flexible resource allocation, is a promising basic technique for fulfilling the ever increasing demand for high-data-rate transmission and various service type support for mobile multimedia communications. In the meantime, energy efficiency (EE) has now become a critical metric for green system design. In this paper, energy-efficient and fair resource allocation is investigated in a downlink OFDM-based mobile communication system. Given a subcarrier assignment, the bisection-based optimal power allocation (BOPA) is proposed at first, which achieves the maximum EE and guarantees proportional data rates for users. Then, a two-step subcarrier assignment is designed to avoid unaffordable computational complexity of exhaustive search. In the first step, the estimated energy-efficient transmit power is found via the assumptions on flat fading and subcarrier sharing. In the second step, the traditional spectral-efficient subcarrier assignment (SESA) is introduced to complete the bandwidth resource allocation among users. Although the two-step subcarrier assignment is suboptimal due to the fact that the optimization is done independently in two separate steps, numerical results demonstrate that its performance is very close to the optimum. This paper also studies the difference between the energy-efficient solution and the traditional spectral-efficient policy and observes that they are similar with each other in the low channel-gain-to-noise ratio (CNR) regime. This observation is helpful and could enable that the energy-efficient design can be turned into the relatively simpler spectral-efficient policy when the CNR is low.

On Statistical Properties of Co-channel Interference in OFDM Systems

Co-channel interference is a key problem in orthogonal frequency division multiplexing (OFDM) wireless communication systems. Channel coding is an effective means of combating the interference. In order to achieve a good decoding performance, the accurate interference distribution should be known at receiver. In most of the previous studies, the co-channel interference was usually modeled as a Gaussian random variable, however, was found to be inaccurate for OFDM cellular systems recently in few works. In this paper, we study the statistic properties of co-channel interference in wireless OFDM systems. By using Monte Carlo simulation and curve-fitting, non-Gaussian distributions are found to be more accurate than the Gaussian distribution. Based on the derived interference distributions, the log-likelihood ratio (LLR) in a maximum a posteriori (MAP) decoder is calculated to enhance the error correction ability. Simulation results show that a dramatic performance gain can be achieved by using the derived interference distributions compared to the conventional decoder using the Gaussian model.

A New Modified SLM Scheme for Wireless OFDM Systems Without Side Information

To avoid the transmission of side information and decrease the computational complexity in the conventional selected mapping (SLM) scheme, a novel set of conversion matrices (CMs) is proposed for the modified SLM scheme, and the signal processing scheme to remove the weighted factors on each subcarrier is also derived for wireless orthogonal frequency-division multiplexing (OFDM) systems. Computer simulations show that, without side information, the new scheme can achieve similar peak-to-average power ratio (PAPR) reduction and bit error rate (BER) performance with even lower computational complexity as compared with the available modified SLM schemes with the CMs.