Orthogonal Frequency Division Multiplexing Applications Research Articles

Investigations on OFDM UAV-based free-space optical transmission system with scintillation mitigation for optical wireless communication-to-ground links in atmospheric turbulence

The potential integration of unmanned aerial vehicles (UAVs) with free space optical (FSO) communication systems stands as a promising innovation in the realm of wireless network infrastructures. This study presents a comprehensive investigation into the application of orthogonal frequency division multiplexing (OFDM) in conjunction with UAV-based FSO technology, with a specific focus on establishing robust wireless communication links to ground sites within the evolving landscape of 5G networks. The research introduces a pioneering 4-level quadrature amplitude modulation (4-QAM)-OFDM-FSO framework tailored for UAV-to-ground communication, revolutionizing the prospects for seamless and high-throughput data transmission within dynamic network environments. Through comprehensive simulations and theoretical analyses, we unveil the system's efficacy in mitigating atmospheric turbulence, achieving heightened signal integrity, and ensuring performance adaptability over varying link distances, thus significantly addressing present limitations in traditional wireless communication models. Anchored within the context of modern wireless network infrastructures, this work serves as a crucial stepping stone for the practical application of OFDM-UAV-FSO communication systems, representing a paradigm shift in fostering resilient and agile wireless connectivity in the era of 5G networks. The inception of cutting-edge wireless networks expected to outperform the capabilities of 5G necessitates an infrastructure that can handle vast amounts of data. This infrastructure must be not only cost-effective and simple to deploy but also readily scalable to accommodate the diverse demands of front-haul and backhaul applications. Motivated by the growing interest in harnessing UAVs to extend the reach and enhance the operational efficacy of conventional cellular networks, this work introduces a novel application of UAV-ground station connections. The concept employs FSO to facilitate network traffic within both the segments. To optimize throughput, resilience, and spectral efficiency, the application of OFDM is proposed. The research considers the transmission of a 20 Gbps 4-QAM data signal across various channel conditions. It thoroughly assesses the performance implications of factors such as transmission distance and beam divergence. The study explores the correlation between pointing error, scintillation, beam divergence angle, and average spectral efficiency. A slight increase in pointing error results in a rapid rise in the scintillation index, while a larger beam divergence angle can help minimize the impact of scintillation. Adapting the beam's divergence angle based on the pointing error between the optical transceivers can reduce the effects of scintillation and improve the average spectral efficiency and channel capacity. Additionally, the relationship between pointing error, scintillation, and the determination of the optical beam divergence angle in terms of beam divergence and average spectral efficiency and channel capacity is examined, and theoretical evaluations further confirm the method's effectiveness in reducing scintillation in the presence of pointing errors. Furthermore, the simultaneous use of OFDM adaptive beam divergence control and modulation could significantly enhance the data rate. This approach aims to reduce the impact of scintillation in UAV FSO links, which often experience significant losses due to unpredictable fluctuations in the atmosphere's refractive index. The results of the simulations indicate that the integrated 4-QAM-OFDM-FSO framework can realize high data transmission rates, efficiently serving front-haul and backhaul needs, thereby signifying a significant evolutionary leap for the next generation of wireless technology. The numerical findings demonstrate the significant impact of the coherent FSO OFDM optical wireless communication (OWC) setup in UAV wireless communications to ground links, particularly in mitigating the effects of strong turbulence and pointing errors (PEs). Through the integration of spatial coherence diversity and adaptive modulation OFDM in the coherent OWC, there has been a noticeable enhancement in the average spectral efficiency (ASE). Notably, our results indicate an ASE of 53 bits/s/Hz and 37 bits/s/Hz achieved at an average transmitted optical power of 10 dBm for an aperture diameter of 10 cm, without and with PEs for the coherent OWC-FSO OFDM UAV technique, respectively. The proposed method was validated through simulations, demonstrating both improved average spectral efficiency and effective reduction of the scintillation effect. This approach holds promise for mitigating scintillation effects in UAV-FSO links.

Effect of DNN Approximation for Channel Estimation and Signal Detection on OFDM Applications

This paper offers a deep learning approximation to realize channel estimation and signal detection that creates the main communication structure skeleton for the orthogonal frequency-division multiplexing (OFDM) system known as an efficient modulation type on 5G. This letter offers an application of deep learning to handle the wireless OFDM channels' end-to-end conduct. First, channel state information (CSI) is predicted explicitly that differs from existing OFDM receivers, then detected the transmitted symbols utilizing the predicted CSI. In the end, CSI is predicted by the suggested deep learning approximation indirectly and transmitted symbols are directly recovered. The structure of the designed receiver occurs of a layer of DNN and soft decisions, which resolves the issues channel estimation error, time delay, and limitation of decoding between users in classic detection techniques. In the simulation results, it is observed that the receiver has powerful stability on the power distribution of user, not only convenient for the linear channel, but also for nonlinear channel when enhancement the number of users, also detection can be well on the receiver. Generally, the efficiency of the modulation system decreases with the features of the multipath channel utilized for transmission. Channel estimation and detection of symbols utilize to reduce the impacts of the channel, which needs high computation and bandwidth conventionally. This paper is used deep neural networks (DNN) for detecting the signal, in this way much effort in detecting the channel is prevented. The proposed method saves priceless bandwidth via used CP in OFDM with a big increase in SNR.

G-OFDM Variants Evaluation for Transmitter and Receiver Implementation

Orthogonal Frequency Division Multiplexing (OFDM) is chosen as a multiplexing technique and broadly used in today’s radiocommunication environments to overcome spectrum insufficiency. In current OFDM applications, the IDFT/DFT algorithms are used for modulation and demodulation, efficiently implemented using the IFFT/FTT. The IFFT and FFT are some of the main components of OFDM systems, requiring intensive computation, especially for a high number of sub-channels. Reducing the computational burden of the IFFT/FFT would offer an advantage in reducing the total OFDM system complexity. In this proposed system, the idea of implementing the very fast Fourier transform (VFFT) in OFDM (later, it is called G-OFDM) is based on a trade-off between performance and complexity. The implementation complexity of G-OFDM is lower than OFDM. However, there is a performance cost. G-OFDM has been studied both analytically and in simulation over the AWGN channel. In particular, performance is marred by the non-uniformity of SNR among sub-carriers. In this study, it was proposed two G-OFDM variants called G1-OFDM and G2-OFDM. G1-OFM is the least complex among all G-OFDM scenarios but gives the worst performance, while G2-OFDM gives the best performance but is the most complex. The results show that the performance of G-OFDM and its variants can be improved through the application of different values of n-quantization levels. In other words, using n-quantization levels, we can decrease the processing loss of the G system.

Experimental Evaluation of RF Waveform Designs for Wireless Power Transfer Using Software Defined Radio

The possibility to harvest energy from ambient radio-frequency (RF) sources has intrigued humankind for the past several decades. In this context, there has been a tremendously growing research interest in the field of wireless power transfer (WPT) using the RF range of the electromagnetic (EM) spectrum. In this paper, we experimentally investigate the aspect of real-time energy harvesting (EH) via different types of waveform designs such as orthogonal frequency division multiplexing (OFDM), square, triangular, sinusoidal, and sawtooth. We make use of a Software Defined Radio (SDR) and a Powercast P21XXCSR-EVB EH module to carry out the experiments on a practical device to assess performance. Specifically, we are interested in obtaining some insights based on the comparison between the aforementioned waveform designs from the perspectives of the separation distance between the USRP and P21XXCSR-EVB EH module, and power emission via USRP. In this vein, we perform additional subsequent experiments after reporting the practical effectiveness of the OFDM waveform, which also follows our intuitive analysis. Correspondingly, we study the effect on WPT with variable USRP transmit power, the separation distance between the USRP and EH antennas, number of OFDM sub-carriers, and multipath setting. As an application of OFDM, the effectiveness of fifth generation-new radio (5G-NR) and long-term evolution (LTE) waveforms are also tested for the WPT mechanism. The demonstration of the EH is provided in terms of the above-mentioned investigation metrics while seeking the best waveform to support WPT.

Sub-Sampled Time Domain Sparse Information in Wired OFDM and STBC-OFDM Systems

A sub-sampling method developed for wired or wireless Orthogonal Frequency Division Multiplexing (OFDM) systems is presented. The proposed technique can be activated during the exchange of sparse data. The sub-sampling method is implemented with low cost and complexity hardware since it requires the customization of a few specific OFDM stages like channel interleaving, sampling control, etc, and does not require any iterative optimization procedure. Employing special Inverse-Fast Fourier Transform (I-FFT) input structures, the recovery of the original data from fewer samples than those required by the receiver FFT is possible without any other cost for general OFDM applications. A special detector initiates the sub-sampling mode when sparse information is exchanged like sensor data or uncompressed real-time video with stationary views like night surveillance cameras. Up to ¾ of the time, the system can operate in sub-sampling mode reducing the power consumed by the Analogue/Digital Converter (ADC), the FFT and the IFFT. Memory needed for sample storage, intermediate results and FFT/IFFT coefficients can be released in real time if the proposed architecture is adopted. A Bit Error Rate (BER) below 10−3 or full information recovery is achieved in most of the cases. An image reconstruction example is also tested in a wireless environment with Normalised Mean Square Error (NMSE) ranging from 10−5 to less than 0.1.

Evolution of PAPR Reduction Techniques: A Wavelet Based OFDM Approach

Orthogonal frequency division multiplexing (OFDM) is considered among the most suitable multiplexing technique for realizing high speed wireless communication in the present era. Substantially immune to frequency selective fading, high spectral efficiency and superior data rate made OFDM a preferred and popular modulation technique in 3rd generation and 4th generation mobile communication systems. The high value of peak to average power ratio (PAPR) and inter carrier interference (ICI) are the most severe bottleneck for OFDM applications to deliver high performance in communication system. The present work portraits most of conventional and current PAPR reduction techniques through comparative study assisted by simulative analysis which foretells future research potential in wavelet based OFDM to generate more efficient system. PAPR optimization is achieved through hybridization of wavelet with clipping and companding techniques. Different modulation techniques have been analyzed (through Matlab simulation) over proposed hybrid model to observe the PAPR reduction in OFDM system.

Adaptive SOICAF Algorithm for PAPR Mitigation in OFDM Systems

Orthogonal frequency division multiplexing (OFDM) technology widely used in wireless communications applications for its high data rate. The major limitation of OFDM is the peak-to-average power (PAPR) problem. Several techniques were applied to mitigate PAPR in OFDM application systems. One of the schemes used for PAPR mitigation is Iterative clipping and filtering (ICAF). It uses the same or identical signals for recursive clipping with a constant threshold level for distinct levels of iterations. This scheme drives signal processing blocks like Fast-Fourier transform (FFT)/inverse FFT (IFFT) blocks the number of times to achieve the required level of PAPR, thereby consumes the average system power and takes more computational time. For further improvement, a new adaptive algorithm to mitigate PAPR in OFDM systems proposed. The proposed adaptive algorithm Lagrange multiplier optimization. Results after simulation justify that depending on the threshold value, PAPR reduction achieved with a slight degradation in BER compared with other existing techniques. BER remains same for low threshold level value, but for high threshold level values it is little bit degraded. Over all PAPR reduction, it performs better by 4 dB when compared to the other schemes. In the case of BER tradeoffs, the complexity of the design and computational speed of resources usage minimized. By carefully selecting the clipping threshold, we can achieve same BER with good reduction in PAPR.

OFDM Based RADAR-Communication System Development

Abstract This paper highlights the hardware implementation of orthogonal frequency division multiplexing (OFDM) based joint radar and communication (RADCOM) system for the vehicular application. Improvement of the communication system due to the application of OFDM is well established and the same advantages can be exploited to enhance the radar performance. The frequency diversity of the OFDM signal improves the radar performance significantly. This motivates the authors to develop a RADCOM system based on OFDM waveform. This paper includes the performance analysis of the OFDM based RADCOM system under the dynamic vehicular channel conditions. Communication system performance is analyzed based on the bit error rate (BER) and the performance of the radar system is evaluated based on target detection.

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.

The application of OFDM in optical fiber communication systems

Orthogonal frequency division multiplexing (OFDM) is a special multi-carrier transmission technology. It utilizes digital signal processing (DSP) to perform inverse Fast Fourier transform (IFFT) and generates a set of orthogonal subcarriers for parallel transmission of low-rate digital signals, achieving the transmission of high-speed digital signals. It has the advantages of high spectral efficiency and anti-dispersion, and has broad applications in large-capacity and long-distance optical fiber communication systems and optical access networks. In this paper, the working principle, structure and application of OFDM in optical fiber communication systems are investigated. Then the problems which should be solved in the application of OFDM in optical communication systems are discussed. Finally, the development of OFDM system is prospected.

Bit-error rate based Doppler estimation for shallow water acoustic OFDM communication

Orthogonal frequency division multiplexing (OFDM) has drawn extensive attention in the field of underwater acoustic (UWA) communication due to its advantages of high transmission rate and high spectrum efficiency. Unfortunately, the practical application of OFDM in UWA communication is subject to severe multi-path as well as Doppler sensitivity. To overcome the impact of Doppler, generally the value of Doppler is estimated by searching the best correlation with the standard copy in time or frequency domain, to enable Doppler compensation via the resampling. However, it has been recognized that, at the presence of diffuse or time varying multi-path, the Doppler estimated by searching signal-level correlation may cause significant mismatch and thus deteriorate the demodulation. The reason is that, while the Doppler is searched in terms of signal-level correlation function, the purpose of the demodulation is to achieve the minimum bit error rate (MBER) at bit-level. In this paper, a Doppler estimation method based on BER search is proposed to transform the signal-level Doppler estimation into a bit-level one. Finally, numerical simulation and sea trial experiments under different Doppler scenarios are performed to verify the effectiveness of the proposed Doppler estimation method.

Impulsive Noise Recovery and Elimination: A Sparse Machine Learning Based Approach

The performance of orthogonal frequency division multiplexing (OFDM) based wireless vehicular communication systems is faced with the great challenge of impulsive noise (IN), which could limit the application of OFDM in ultra-reliable low-latency communication scenarios. In this paper, the challenge of IN elimination for OFDM-based wireless systems is efficiently overcome by the proposed sparse learning algorithms and probabilistic framework inspired by the emerging machine learning theories. For the first time, the sparse machine learning theory is introduced to IN recovery and elimination. Exploiting the measurement vector of IN observed from the reserved null sub-carriers as the input, a novel sparse machine learning based algorithm of sparse cross-entropy minimization is proposed, in which the probability distribution of the IN support is iteratively updated by minimizing the loss function, i.e. the cross-entropy. The proposed algorithm is able to effectively and efficiently learn the sparse pattern and converge to the accurate distribution of IN support. To facilitate an accelerated and even more efficient learning process, regularization is imposed on the loss function by adding a weighting parameter in favor of the accurate distribution. The computer simulation results confirm that the proposed scheme outperforms conventional methods while utilizing fewer spectrum resources over wireless vehicular channels.

A Survey on FFT/IFFT Processors for Next Generation Telecommunication Systems

The Fast Fourier Transform and Inverse Fast Fourier Transform (FFT/IFFT) are the most significant digital signal processing (DSP) techniques used in Orthogonal Frequency Division Multiplexing (OFDM)-based applications which include day-to-day wired/wireless communications, broadband access, and information sharing. The advancements in telecommunication technologies require an efficient FFT/IFFT processing device to meet the necessary specifications which depend on the particular application. A real-time implementation of high-speed FFT/IFFT processor with less area that operates in minimal power consumption is essential in designing an OFDM integrated chip. A comparative study of efficient algorithms and architectures for FFT chip design is presented in this paper. It is also recommended that mixed-radix/higher-radix algorithm combined with Single-path Delay Commutator (SDC) architecture is appropriate for massive MIMO in 5G, optical OFDM, cooperative MIMO and multi-user MIMO-based applications.

SQNR Improvement Enabled by Nonuniform DAC Output Levels for IM-DD OFDM Systems

We propose and demonstrate via both simulation and experiment a scheme for a low-bit resolution digital-to-analog convertor (DAC) to improve the signal-to-quantification noise ratio (SQNR) of an intensity-modulated direct-detection (IM-DD) orthogonal frequency division multiplexing (OFDM) signal. In this scheme, the discrete output levels of a DAC are nonuniform and properly assigned based on the distribution of the OFDM signal. Such nonuniformity allows a better exploitation of output levels for the OFDM signal so that low-level waveforms that appear much more frequently experience mitigated quantization noise. To calculate these optimized output levels with the distribution of an OFDM signal, a numerical algorithm is proposed and verified. Our proposed scheme is proven effective in the simulations for Gaussian-distributed signals and brings over 3.6-dB and ∼2.5-dB improvements on SQNR and receiver sensitivity, respectively. The feasibility is also verified by a proof-of-concept experiment with ∼7.4-Gbps quadrature phase shift keying (QPSK)-modulated OFDM signals output by a 3-bit DAC. Our scheme enables transmission of such signals with no further optimization with ∼−25-dBm receiver sensitivity, while it is impossible using a conventional way. Both simulation and experimental results validate our scheme as a promising way to be implemented in low-bit resolution DACs for IM-DD OFDM applications.

Complementary M ‐ary orthogonal spreading OFDM architecture for HF communication link

A complementary M-ary orthogonal spreading (CMOS) architecture with orthogonal frequency division multiplexing (OFDM) is proposed to improve performance of bit-error rate (BER) and spectrum efficiency for high frequency (HF) communications. The CMOS-OFDM architecture is based on orthogonal complementary pairs with their central elements being zero. It is well known that complementary pairs theoretically reduce peak-to-mean envelope power ratio (PMEPR) of OFDM due to their ideal aperiodic auto-correlation properties. However, many practical OFDM applications, such as HF OFDM and IEEE 802.11, require that no data should be modulated to direct current (DC) subcarrier, which will destroy ideal properties of traditional complementary pairs and hence increase their PMEPRs. Compared with traditional complementary pairs, the proposed zero-centre complementary pairs (ZCCPs) in CMOS-OFDM architecture may maintain ideal properties and hence be more suitable for OFDM systems as long as zero element in the centre is modulated to DC subcarrier. The corresponding simulation results and numerical analyses over the HF fading channel modelled by ITU-R F.1487 are provided. Different from the previous researches on HF OFDM systems, the proposed HF CMOS-OFDM architecture using the constructed ZCCPs not only possesses excellent BER performance, but also may firmly resist PMEPR to 3 dB or near to 3 dB.

An area-efficient and low-power 64-point pipeline Fast Fourier Transform for OFDM applications

In an orthogonal frequency division multiplexing (OFDM) based wireless systems, Fast Fourier Transform (FFT) is a critical block as it occupies large area and consumes more power. In this paper, we present an area-efficient and low power 16-bit word-width 64-point radix-22 and radix-23 pipelined FFT architectures for an OFDM-based IEEE 802.11a wireless LAN baseband. The designs are derived from radix-2k algorithm and adopt a Single-Path Delay Feedback (SDF) architecture for hardware implementation. To eliminate the complex multipliers and read-only memory (ROM) which is used for internal storage of twiddle factor coefficients, the proposed 64-point FFT employs a Canonical Signed Digit (CSD) complex constant multiplier using adders, multiplexers and shifters. The complex constant multiplier (CCM) is modified using common sub-expression sharing block that reduces the area of the design. The proposed radix-22 and radix-23 pipelined FFT architectures are modeled and implemented using TSMC 180nm CMOS technology with a supply voltage of 1.8V. The implementation results show that the proposed architectures significantly reduces the hardware cost and power consumption in comparison to existing 64-point FFT architectures.

Low Power 64 Point FFT Processor

Objectives: This paper proposes a design of Low power FFT (Fast Fourier Transform) processor used in OFDM (Orthogonal Frequency Division Multiplexing) application as there is demand for low power design of portable communication device. Methods: This FFT processor is based on SDF (Single Path Delay Feedback) pipelined Architecture. Digit slicing multiplier less architecture aids in realizing the complex Multiplication. To reduce power dynamic power dissipation, the proposed architecture applies clock gating buffer. Control circuit is implemented using Gray code sequence instead of binary code sequence. The design proposed here is implemented in Verilog HDL. Cadence tool is used for synthesizing the proposed design Findings: The number of complex multiplication is also reduced by using radix -25 algorithms. The result shows reduced power consumption up to 25%. Improvements: This paper is presented for 64 Point FFT design; this can also be extended for Higher N point FFT design.

Efficient VLSI implementation of FFT for orthogonal frequency division multiplexing applications

In an orthogonal frequency division multiplexing (OFDM)-based digital transmitter, the inverse fast Fourier transform (IFFT) processing unit consumes the most hardware area and power, especially because of the twiddle multipliers in the Cooley–Tukey-based decimation-infrequency (DIF) IFFT architecture. This work concentrates on the trivial multiplications in the input stage of the IFFT unit and replaces them by the proposed ‘pass-logic’. These replacements can be possible because the inputs are bitwise with binary-phase shift keying (PSK) or qudrature-PSK digital modulation. The input stage of DIF-FFT for 8 to 128 points (N) were implemented with multipliers and ‘pass-logics’. The performance improvements (PIs) of the proposed FFT/IFFT implementation have been analysed. For a 64-point FFT in FPGA, the number of slices was reduced by 9% and the total power by 6.5%. The same implementation on an ASIC, consumed 28% less power and 27% lesser gates. In 128-point implementation, these PIs are more than those of the 64-point, thus PI is in upward trend as N increases. A chip for FFT processing as per IEEE 802.11a specifications (64-point, 16-bit data) is designed with pass-logics, which uses 24 947 gates and consumes 6.45 mW at 1.8 V, 20 MHz in 0.18 µm 1P6M CMOS process.

Application of OFDM Technology in 4G Mobile Network

OFDM (Orthogonal Frequency Division Multiplexing) is a special kind of multi-carrier digital modulation technique. Each OFDM subcarrier exists orthogonality and great use of spectrum resources. To a certain extent, the problem of shortage of spectrum resources will be eased in the future. OFDM has a good ability to resist inter symbol interference and fight against frequency selective fading, and be able to adapt to the future wireless high-speed data transmission without distortion. Each channel modulation and demodulation with FFT and IFFT is easy to implement and meet the need of the business in the wireless communication system. The paper introduces the basic principle of OFDM technology, describes the advantages of OFDM and the defects needed to be overcome. Moreover, we analyzes the characteristics of 4G technology and discusses the application of OFDM in 4G mobile network.

Design of A Novel 8-point Modified R2MDC with Pipelined Technique for High Speed OFDM Applications

This study presents, design of modified R2MDC with pipelined Technique. Radix-2 Multipath Delay commutators is the one type of Radix-2 FFT. Wireless networks have found a significant place in the growing technologies. Orthogonal Frequency Division Multiplexing (OFDM) is a key technique used in wireless networks for high speed operation, high data capacity and spectral efficiency. The Fast Fourier Transform (FFT) are essential in the field of Digital Signal Processing (DSP), widely used in communication systems, especially in Orthogonal Frequency Division Multiplexing (OFDM) systems. Not a lot of research has been done on modified R2MDC FFT in OFDM. In this study, we propose a modified R2MDC 8-Point FFT in which pipelining technique is used in each Butterfly unit architecture. Using this technique through VLSI simulations it is found to reduce the delay and increase the throughput and hence is suitable for OFDM. This method can be extended to OFDM implementation that is apt for Wireless Networks.