Orthogonal Frequency Division Multiplexing Demodulation Research Articles

Performance analysis of 4QAM-OFDM-FSO link under rain weather conditions

Abstract Free space optical communication (FSO) is a technique based on transmission of data by propagating the light in free space. FSO is inexpensive, providing high data rates, high capacity, high security, and low power and uses the license free frequency spectrum. Nevertheless, FSO is still facing many problems such as bad weather conditions, atmospheric turbulence, and multipath fading. In this research work, orthogonal frequency division multiplexing (OFDM) FSO system is analyzed for low to heavy rain fall intensities. Different rain rates have been considered for low, moderate, heavy, very heavy, and torrential rains. For all the rain rates, a direct detection OFDM–FSO system and a coherent detection system have been simulated and results are compared. Analysis has been done on the basis of SNR, received power, and constellation diagrams. According to these parameters, link length of both the systems for different rain rates was found. Coherent detection is applied to improve the system sensitivity compared to conventional direct detection. In tropical regions where heavy rainfall is the main reason for the degradation of the system, the OFDM system with coherent detection can perform better with high data rates. Second, over a 11 km FSO, a 4-QAM wireless signal is successfully transmitted under very heavy rainfall conditions. The distance achieved by moderate and heavy rains is 25 km and 15.5 km with an acceptable SNR of 25 dB and a system sensitivity (received power) of −65.1 dBm.

“Hybrid MAC Methodology for Improving the QOS in Fiber Wireless Network”

The Fi-Wi (Fiber-Wireless) approach stands out as a crucial element in the realm of networks, demonstrating superiority over various technologies. With the exponential growth in Internet users, significant strides have been made in the evolution of Fi-Wi networking systems in recent years. This mechanism offers broader bandwidth and network stability, ensuring high-speed connectivity with "Anytime Anywhere" availability for end users. However, the escalating energy demand in networking systems poses a constraint on the network's lifespan, impacting transmission.
 Over the years, researchers have proposed and tested various Media Access Control (MAC) protocols to address transmission and energy consumption issues. Despite these efforts, existing protocols have encountered challenges such as overheating, delays, throughput issues, and collisions. This research paper introduces a combination of Time Division Multiple Access (TDMA) and Carrier Sense Multiple Access (CSMA) to tackle associated challenges. The primary objective is to enhance throughput and reduce delays in Fi-Wi networks.
 To achieve this goal, the study employs techniques that involve and Utilizing an Orthogonal Frequency Division Multiplexing (OFDM) modulator, a free-space optical (FSO) communication channel, an OFDM demodulator, and Opti-system for the analysis and enhancement of received signals, our study demonstrates that the proposed MAC protocol surpasses conventional MAC protocols in terms of delay, data throughput, and transmission efficiency.

“Hybrid MAC Methodology for Improving the Qos in Fiber Wireless Network”

The Fi-Wi (Fiber-Wireless) approach stands out as a crucial element in the realm of networks, demonstrating superiority over various technologies. With the exponential growth in Internet users, significant strides have been made in the evolution of Fi-Wi networking systems in recent years. This mechanism offers broader bandwidth and network stability, ensuring high-speed connectivity with "Anytime Anywhere" availability for end users. However, the escalating energy demand in networking systems poses a constraint on the network's lifespan, impacting transmission.
 Over the years, researchers have proposed and tested various Media Access Control (MAC) protocols to address transmission and energy consumption issues. Despite these efforts, existing protocols have encountered challenges such as overheating, delays, throughput issues, and collisions. This research paper introduces a combination of Time Division Multiple Access (TDMA) and Carrier Sense Multiple Access (CSMA) to tackle associated challenges. The primary objective is to enhance throughput and reduce delays in Fi-Wi networks.
 To achieve this goal, the study employs techniques that involve and Utilizing an Orthogonal Frequency Division Multiplexing (OFDM) modulator, a free-space optical (FSO) communication channel, an OFDM demodulator, and Opti-system for the analysis and enhancement of received signals, our study demonstrates that the proposed MAC protocol surpasses conventional MAC protocols in terms of delay, data throughput, and transmission efficiency.

Performance of 5G NR-polar QAM-OFDM in nonlinear distortion plus Non-Gaussian noise over Rayleigh fading channel

The polar code readily provides rate adaptation, reaching Shannon’s capacity at low encoding and decoding complexity. It has entered the 5G New Radio (5G-NR) as a result of these alluring features to improve the bit error rate (BER) performance in Gaussian noise. In challenging environments for modern broadband communication systems, the amplitude and phase distortion caused by nonlinear distortion (NLD) of high-power amplifiers (HPAs), non-Gaussian noise or impulsive noise (IN), and multipath fading channels (H) play a significant role in BER degradation in an orthogonal frequency-division multiplexing (OFDM) system. Therefore, to improve the BER degradation of the conventional NR-polar QAM-OFDM system over such environments, different novel closed-form expressions of combined NLD-HPA and IN distributions and the log-likelihood ratios (Ls) at the OFDM demodulator output with and without Rayleigh fading channel have been derived. In addition, the novel derived closed-form BER expressions validate the derived distributions using Monte-Carlo simulation in the downlink (DL) and uplink (UL) scenarios. This system demonstrates a significant performance improvement and achieves a coding gain of 28 dB and 25.5 dB compared to the traditional coded OFDM system, which achieved only 10 dB in the case of IBO = 4 dB over the Rayleigh fading channel.

Efficient Index Modulation-Based MIMO OFDM Data Transmission and Detection for V2V Highly Dispersive Channels

Vehicle-to-vehicle (V2V) communication networks are based on vehicles that wirelessly exchange data, traffic congestion, and safety warnings between them. The design of new V2V systems requires increasingly energetically and spectrally efficient systems. Conventional multiple-input–multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems have been used successfully for the last decade. However, MIMO-OFDM systems need to be improved to face future communication networks in high-mobility environments. This article proposes an efficient index modulation (IM)-based MIMO-OFDM system for V2V channels. The proposed transmission system is evaluated in high Doppler-spread channels. The results demonstrate that the proposed scheme reduces the required computational complexity in data detection and exhibits gains of up to 3 dB in bit error rate (BER) performance when compared to the conventional MIMO-OFDM system under the same conditions and parameters, along with achieving superior spectral efficiency. The results show the viability of implementing the proposed system in practical applications for high-transmission-rate V2V channels.

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.

Multiuser Adversarial Attack on Deep Learning for OFDM Detection

Adversarial attack has been widely used to degrade the performance of deep learning (DL), especially in the field of communications. In this letter, we evaluate different white-box and black-box adversarial attack algorithms for a DL-based multiuser orthogonal frequency division multiplexing (OFDM) detector subject to multiuser adversarial attack. The bit error rates under different adversarial attacks are compared. The results show that, the perturbation efficiency of adversarial attack is higher than conventional multiuser interference. Virtual adversarial methods (VAM) and zeroth-order-optimization (ZOO) attacks perform the best among white-box and black-box methods, respectively. They are also effective when the attack changes the starting time. Additionally, adding the number of attackers is found useful to improve the VAM attack but not for ZOO. This letter shows that adversarial attack is powerful to generate adversarial against multiuser OFDM communications.

An optimised hardware architecture of the angular-domain cyclostationary detector for cognitive radio communications

This paper proposes a novel method to improve the performance and reduce the implementation design complexity of the angular domain cyclostationary feature detection (CFD). This method implements Orthogonal Frequency Division Multiplexing (OFDM) signal detection for cognitive radio applications. The originality of this work is based on using a selective sampling process for the OFDM signal detection by using only samples related to the cyclic prefix (CP). Using only the first NCP (CP length) angles of the angular domain cyclic autocorrelation function and eliminating the remaining angles has significantly improved the detector sensitivity. Results from Matlab simulations revealed that the proposed method reaches a detection probability of 100% at an SNR (signal-to-noise ratio) equal to −16 dB in the case of an additive white Gaussian noise (AWGN) channel. Moreover, the proposed method outperforms the other CFD methods with a performance range up to 5 dB. The proposed algorithm is coded using the VHDL language and implemented with the minimum of counters and comparators on Field-programmable gate array (FPGA) platform without using multipliers and DSP blocks. The proposed design requires a lesser power, logic and memory bits by 5.95, 60.96 and 87.5%, respectively, compared to the conventional angular domain CFD design. The proposed hardware architecture can be more efficient for spectrum sensing in many communication systems, such as LTE-Advanced and the L-band Digital Aeronautical Communication System (LDACS).

OFDM Radar and Communication Joint System Using Opto-Electronic Oscillator With Phase Noise Degradation Analysis and Mitigation

Orthogonal frequency division multiplexing (OFDM) signal is a superior dual-functional waveform for the integration of radar sensing and communication in intelligent transportation. But the sensitivity to phase noise is a serious issue introducing interference and causing performance degradation during demodulation. In this paper, we explore the essential mechanism of the action and generation of phase noise through theoretical analysis, where the OFDM demodulation process and power spectrum density (PSD) of phase noise is discussed in the frequency domain, and draw the conclusion that high-speed phase jitter will cause unrecoverable deterioration of OFDM demodulation. Therefore, a photonics-aided radar and communication integrated system based on Optoelectronic oscillator (OEO) is proposed. The positive feedback oscillation with long energy storage time make the phase noise pattern of OEO just suitable to against the phase noise sensitivity of OFDM. A proof-of-concept experiment is demonstrated at 24 GHz with 2 GHz bandwidth to verify the radar sensing and communication function. A two-dimensional radar imaging with a range resolution of 0.075 m and velocity resolution of 4.4 km/h, a communication capacity of 6.4 Gbps is obtained. A quantitative performance comparison is also carried out. By using an ordinary microwave source and OEO separately, the demodulation constellation and error vector magnitude (EVM) under different subcarrier spacing is measured and compared. The result is corresponding to our analysis with the EVM decreasing from 12.5% to 4.7% under subcarrier spacing of 125 kHz.

Hardware Chip Performance of CORDIC Based OFDM Transceiver for Wireless Communication

The fourth-generation (4G) and fifth-generation (5G) wireless communication systems use the orthogonal frequency division multiplexing (OFDM) modulation techniques and subcarrier allocations. The OFDM modulator and demodulator have inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) respectively. The biggest challenge in IFFT/FFT processor is the computation of imaginary and real values. CORDIC has been proved one of the best rotation algorithms for logarithmic, trigonometric, and complex calculations. The proposed work focuses on the OFDM transceiver hardware chip implementation, in which 8-point to 1024-point IFFT and FFT are used to compute the operations in transmitter and receiver respectively. The coordinate rotation digital computer (CORDIC) algorithm has read-only memory (ROM)-based architecture to store FFT twiddle factors and their angle generators. The address generation unit is required to fetch the data and write the results into the memory in the appropriate sequence. CORDIC provides low memory, delay, and optimized hardware on the field-programmable gate array (FPGA) in comparison to normal FFT architecture for the OFDM system. The comparative performance of the FFT and CORDIC-FFT based OFDM transceiver chip is estimated using FPGA parameters: slices, flip-flops, lookup table (LUTs), frequency, power, and delay. The design is developed using integrated synthesis environment (ISE) Xilinx version 14.7 software, synthesized using very-high-speed integrated circuit hardware description language (VHDL), and tested on Virtex-5 FPGA.

Modeling and spectral analysis of high speed optical fiber communication with orthogonal frequency division multiplexing

Abstract Optical communication has emerged as the best solution to high-speed transmission systems due to its higher bandwidth and higher data rates. The higher data rates with simple transmitter and receiver modeling are one of the best solutions to design the spectrally efficient high-speed optical transmission system using different modulation techniques. In the recent past, optical communication networks have been specifically designed and optimized to support different communication standards increasing the capabilities of optic fiber, integrated waveguides, and related optical fiber hardware, and signal conditioning circuits. So, this research paper presents a study, modeling, simulation, and analysis of optical performance under different transmission systems which are considered in a practical high-speed design using orthogonal frequency division multiplexing (OFDM). This can be done by using both coherent detection OFDM and direct detection OFDM to minimize dispersion effects in optical communication because OFDM is basic for modern technology to increase the data rate and increase the requirement of bandwidth in broadband services. Many optical fiber problems, such as chromatic dispersion (CD) and polarization mode dispersion (PMD) are solved using OFDM. The performance analyses made in this work by using different parameters such as bit error rate (BER), optical signal-to-noise ratio (OSNR), quality (Q) factor at different distance transmission by using different quadrature modulation such as 4-QAM, 16-QAM, 32-QAM, and 64-QAM to maintain the OSNR value, spectral efficiency, and the required data rates. The simulated results have shown that the coherent detection OFDM and direct detection OFDM with considered QAM provides the best value of BER and quality of the signal at a specific distance.

PERFORMANCE OF QAM-OFDM SYSTEM OVER RAYLEIGH FADING CHANNEL IN WEI BULL NOISE DISTRIBUTION

The phenomenal increase in the usage of mobile devices and wireless networking tools in recent years has resulted in the communication industry needing higher data speeds for connections and bandwidth. As a result, multi-carrier modulation has been suggested as a reliable and effective method of transmitting data over difficult communication networks such as selective fading channels. Orthogonal Frequency Division Multiplexing (OFDM) is a highly effective multi-carrier technique that can meet users' high demands. Many studies have looked into this technique, mostly as a way to counteract fading and Additive White Gaussian Noise (AWGN). As a result, the performance evaluation of the QAM-OFDM system in the presence of multi-path Rayleigh fading in Weibull noise is examined in this article. Furthermore, bit error rate performance (BER) is computed using the optimal derivation of the real system contaminated by compound Gaussian and non-Gaussian (Weibull) noise distributions at the OFDM demodulator output. The derived result is an exact match to the simulated result over various scenarios introduced by the MATLAB software package.

Phase Retrieval of Complex OFDM Signal by Solving Temporal Transport-of-Intensity Equation

We simulate the complex signal reconstruction by solving the temporal transport-of-intensity equation (TIE) in a direct detection orthogonal frequency division multiplexing (OFDM) system. At the receiver, by measuring two intensities before and after a dispersive element, we construct a TIE and retrieve the phase information of the complex OFDM signal by solving it. We investigate the influence of the carrier-signal power ratio (CSPR), the value of the dispersion, signal baud rate, sampling rate, laser linewidth, and frequency offset on the performance of phase retrieval (PR). In a 20 Gbaud 16 quadrature-amplitude-modulation (16QAM) OFDM system, the TIE-based direct-detection requires a 12 dB CSPR, which is comparable to Kramers–Kronig (KK)-based receiver and the single carrier TIE scheme. A proof-of-concept adaptive modulation scheme has also been applied to the TIE-based OFDM system.

A Survey on the Energy Detection of OFDM Signals with Dynamic Threshold Adaptation: Open Issues and Future Challenges.

Cognitive radio (CR), as a concept based on the ability to detect and share the unutilised spectrum, has been envisioned as a promising candidate to improve the efficiency of frequency spectrum assignments. For the realisation of the CR concept, energy detection (ED), as one of the available spectrum sensing methods, is broadly considered because of its low computational complexity and implementation costs. Due to the vast usage of the orthogonal frequency division multiplexing (OFDM) technique in contemporary communication systems, the ED of OFDM signals in the CR networks has become important for practical realisation. Since the ED accuracy of the OFDM signals can be improved by the sensing threshold adaptation, this paper surveys the impact of noise variations and dynamic threshold (DT) adaptation on the ED performance of OFDM signals. Analyses were performed by the simulation of the ED related to OFDM signals transmitted in the margin or rate adaptive and combined margin and rate adaptive OFDM systems. The results obtained through extensive simulations provide fundamental insights into how different factors, including the transmission power, the signal to noise ratio, the false alarm probability and the sample quantity, affect the ED efficiency. Comprehensive analyses of the obtained results indicate the main ED weaknesses and how the appropriate selection of analysed factors can enhance the ED processes for different OFDM systems. The observed ED weaknesses were further thoroughly surveyed, and the open issues and challenges related to the enhancement of the main ED limitations have been elaborated. The presented survey results can serve as a basis for the improvement of a broadly accepted ED method in CR networks.

Multiple blanking preprocessors for impulsive noise mitigation in OFDM-based power-line communication systems

Power-line communication (PLC) technology has been a critical enabler for smart grid systems. It provides two-way communication links without new infrastructure deployment. However, as the power-line was not designed to carry data, there are many drawbacks when it is used as a communication channel. One of the major drawbacks is impulsive noise occurrence. Blanking nonlinear preprocessor has been commonly used for mitigating impulsive noise in orthogonal frequency-division multiplexing (OFDM)-based PLC. It uses a threshold to detect whether a signal sample is contaminated by impulsive noise. Optimum performance is obtained when the optimal blanking threshold is used. The optimal blanking threshold depends on noise parameters, which are usually assumed to be available to the receiver. However, they may not be known in practice. In this paper, we propose a blanking preprocessor bank consisting of multiple blanking preprocessors with different thresholds which blindly handles the impulsive noise. Each blanking preprocessor is considered to provide an independent channel. The output of all preprocessors is combined using the maximal ratio combining (MRC) technique at the OFDM demodulator. Simulation results show that the proposed blanking preprocessor bank method results in better performance compared to the one using the optimal threshold method.

A Frequency-Diversity System With Diversity Encoder and OFDM Modulation

In this work, a diversity encoder maps vectors of N modulated symbols to vectors of N diversity symbols using an orthogonal matrix. Each modulated symbol appears in N diversity symbols. An Orthogonal-Frequency-Division-Multiplexing (OFDM) modulator converts the diversity vectors to OFDM symbols. The number of subcarriers in the OFDM symbol equals N. Each diversity symbol modulates one subcarrier; therefore, each modulated symbol is transmitted by N orthogonal subcarriers. The diversity bandwidth of the modulated symbols is N times the subchannel bandwidth. The modulated symbols experience independent fading gains on each subchannel since the diversity bandwidth (OFDM bandwidth) is chosen to be multiple of the coherence bandwidth of the transmission channel. In the receiver, an OFDM demodulator converts back the OFDM symbols to vectors of diversity symbols. The diversity detector maps the diversity vectors to vectors of modulated symbols. The noise samples at the output of the diversity detector are uncorrelated. Maximum likelihood and linear decorrelator detectors are used as diversity detectors. The transmitter and the receiver of this work are implemented using Field-Programable-Gate-Array (FPGA) technology. The performance of the implemented system is the same as the performance of the N channels diversity system with the Maximal-Ratio-Combiner (MRC) receiver.

Impulse Noise Suppression based Iterative Hybrid Nonlinear in Power Line Communication

In power line orthogonal frequency division multiplexing (OFDM) communication systems, the presence of impulse noise can seriously affect system performance. One of the conventional solutions is to perform a mixed nonlinear processing on the time domain signal at the receiving end before performing subsequent OFDM demodulation. Therefore, this thesis studies an improved method based on iterative elimination of nonlinear distortion. Firstly, the received OFDM signal is subjected to hybrid nonlinear processing in the time domain. Secondly, the nonlinear distortion introduced by the time domain nonlinear processing is reconstructed by the symbols initially detected in the frequency domain, and the reconstruction accuracy is improved by iteration. Finally, the reconstructed nonlinear distortion is subtracted from the output of the hybrid nonlinear processing unit. The simulation results show that compared with the traditional hybrid nonlinear method, the performance of the proposed algorithm is much better than that of the traditional algorithm.

A Simple Blind Sampling Frequency Offset Estimation Scheme for Short-Reach DD-OFDM Systems

In this paper, a simple and overhead-free sampling frequency offset (SFO) blind estimation and compensation scheme is proposed and experimentally demonstrated in a short-reach direct detection orthogonal frequency division multiplexing (DD-OFDM) transmission system. After extracting subcarriers of OFDM symbols from multiple OFDM frames, the phase shift is obtained by the fourth-power algorithm, and the obtained results are averaged to perform phase estimation. It can achieve high-precision SFO estimation while ensuring low computational complexity as well as high spectral efficiency. The experimental results show that the SFO estimation accuracy using the proposed scheme can be significantly reduced to as low as ±3 ppm when SFO ranges from -1000 ppm to +1000 ppm after 20-km standard single-mode fiber (SSMF) transmission. Moreover, for the QPSK/16QAM encoded DD-OFDM systems aided by the proposed scheme, up to ±200 ppm SFO can be compensated with slight error vector magnitude (EVM) penalties after 20-km SSMF transmission. Furthermore, the results demonstrate that despite the existence of severe SFO, our proposed SFO compensation scheme can offer nearly negligible power penalties in the DD-OFDM system with optical back-to-back (OBTB) and 20-km SSMF transmission as compared to the ideal case without SFO.

Joint MAP channel estimation and data detection for OFDM in presence of phase noise from free running and phase locked loop oscillator

This paper addresses a computationally compact and statistically optimal joint Maximum a Posteriori (MAP) algorithm for channel estimation and data detection in the presence of Phase Noise (PHN) in iterative Orthogonal Frequency Division Multiplexing (OFDM) receivers used for high speed and high spectral efficient wireless communication systems. The MAP cost function for joint estimation and detection is derived and optimized further with the proposed cyclic gradient descent optimization algorithm. The proposed joint estimation and detection algorithm relaxes the restriction of small PHN assumptions and utilizes the prior statistical knowledge of PHN spectral components to produce a statistically optimal solution. The frequency-domain estimation of Channel Transfer Function (CTF) in frequency selective fading makes the method simpler, compared with the estimation of Channel Impulse Response (CIR) in the time domain. Two different time-varying PHN models, produced by Free Running Oscillator (FRO) and Phase-Locked Loop (PLL) oscillator, are presented and compared for performance difference with proposed OFDM receiver. Simulation results for joint MAP channel estimation are compared with Cramer-Rao Lower Bound (CRLB), and the simulation results for joint MAP data detection are compared with “NO PHN” performance to demonstrate that the proposed joint MAP estimation and detection algorithm achieve near-optimum performance even under multipath channel fading.

Direct Data Detection of OFDM Signals Over Wireless Channels

This paper presents a novel efficient receiver design for wireless communication systems that incorporate orthogonal frequency division multiplexing (OFDM) transmission. The proposed receiver does not require channel estimation or equalization to perform coherent data detection. Instead, channel estimation, equalization, and data detection are combined into a single operation, and hence, the detector performs a direct data detector (D <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ). The D <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> is applied to key practical wireless systems such as the Long Term Evolution (LTE) and the New Radio (NR) of the fifthgeneration (5G) system. The performance of the proposed D <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> is thoroughly analyzed theoretically in terms of bit error rate (BER), where closed-form accurate approximations are derived for several cases of interest, and validated by Monte Carlo simulations. Moreover, extensive complexity analysis are performed to evaluate the system suitability for implementation. The obtained theoretical and simulation results demonstrate that the BER of the proposed D3 is only 3 dB away from coherent detectors with perfect knowledge of the channel state information (CSI) in flat and frequency-selective fading channels for a wide range of signal-to-noise ratios (SNRs). If CSI is not known perfectly, then D <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> outperforms the coherent detector substantially, particularly at high SNRs with linear interpolation. The computational complexity of D <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> depends on the length of the sequence to be detected, nevertheless, a significant complexity reduction can be achieved using the Viterbi algorithm.