Orthogonal Frequency Division Multiplexing Modulation Research Articles

Joint signal-to-signal beat interference mitigation for the field recovery of symmetric carrier-assisted differential detection with low carrier-to-signal power ratio

As a combination of direct detection and coherent detection technologies, self-coherent detection has the advantages of low cost and optical field recovery ability. However, most of the self-coherent detection techniques are limited to single sideband (SSB) signals. Recently, carrier-assisted differential detection (CADD) has been proposed to realize complex-valued double sideband (DSB) signals, but it requires a high carrier-to-signal power ratio (CSPR) to mitigate the signal-to-signal beat interference (SSBI). Later, a more cost-effective symmetric CADD (S-CADD) has been proposed while the required CSPR is still high. In order to alleviate the high requirements of CSPR, we propose a scheme based on the joint of digital pre-distortion (DPD) at transmitter and clipping at receiver to further improve the S-CADD system performance. This joint processing can not only solve the problem of non-uniform distribution of subcarrier signal-to-noise ratio (SNR) caused by non-ideal transfer function, but also the error propagation problem caused by enhanced SSBI under low CSPR. After the validation of the 64 Gbaud 16-ary quadrature amplitude modulation (16-QAM) orthogonal frequency division multiplexing (OFDM) signal transmitted over 80 km standard single mode fiber (SSMF), the CSPR required by the proposed scheme to reach the 20% soft decision-forward error correction (SD-FEC) and 7% hard decision-forward error correction (HD-FEC) can be reduced by 1.3 dB and 2.8 dB, respectively, with a comparison of the conventional S-CADD. The results show the potential of the proposed scheme in the short-reach optical transmissions.

“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.

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.

“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.

Equalizer Parameters’ Adjustment Based on an Oversampled Channel Model for OFDM Modulation Systems

A physical model of a wireless transmission channel in the time domain usually consists of the main propagation path and only a few reflections. The reasonable assumptions made about the channel model can improve its parameters’ estimation by a greedy OFDM (Orthogonal Frequency Division Multiplexing) equalizer. The equalizer works flawlessly if delays between propagation paths are in the sampling grid. Otherwise, the channel impulse response loses its compressible characteristic and the number of coefficients to find increases. It is possible to get back to the simple channel model by data oversampling. The paper describes how the above idea helps the OMP (Orthogonal Matching Pursuit) algorithm estimate channel coefficients. The authors analyze the oversampling algorithm on the one hand to assess the influence of filtering function and signal resolution on the quality of the channel impulse response reconstruction. On the other hand, the abilities of the OMP algorithm are analyzed to distinguish components of the oversampled signal. Based on these analyses, we proposed modifications to the compressible channel’s impulse response reconstruction algorithm to minimize the number of transmission errors. A distinction was made between the filters used in the OMP search and channel reconstruction stages before calculating equalizer coefficients. Additionally, the results of the search stage were considered as elements within the groups.

Suitability analysis of optical filters for enhancing performance parameters in a QPSK CO-OFDM modulation system

Abstract Optical filtering affects the fiber impairments in a fiber optic transmission system and its overall efficiency. The fiber impairments like attenuation, dispersion, and nonlinearities are challenges to overcome. The optical orthogonal frequency division multiplexing (O-OFDM) modulation is among the techniques to overcome these challenges. This paper uses O-OFDM with coherent transmission (denoted as CO-OFDM) and quadrature phase shift keying (QPSK) for modulating subcarriers. It is denoted as QPSK CO-OFDM modulation. This QPSK CO-OFDM (denoted here as QC) modulation system is further modified by introducing different optical filters in its fiber optic link. Different variants of QC are created by using optical filters in its fiber link. For example, the QC_BL is the variant having an optical Bessel filter in its fiber link. Performances of each variant have been evaluated and compared in terms of BER. This comparison allows us to select a suitable filter for a particular parameter, such as dispersion, fiber length, input power, etc. The QC variant having no filter is denoted as QC_WF and is taken as a reference for comparison.

Performance Analysis of Subcarrier Index Modulation OFDM in a DWDM-FSO link

Free Space Optics (FSO) is a wireless optical communication system using light to establish high-capacity links between points without the need for physical cables. However, its susceptibility to atmospheric conditions such as fog, rain, and turbulence presents challenges to signal quality maintenance. To address this, Dense Wavelength Division Multiplexing (DWDM), traditionally employed in fiber optics, is leveraged to optimize bandwidth utilization and scalability in high-capacity communications. The integration of DWDM into FSO systems offers redundancy and resilience against signal degradation induced by atmospheric conditions. To the best of our knowledge, this is the first time a paper explores the performance of the Subcarrier Index Modulation Orthogonal Frequency Division Multiplexing (SIM-OFDM) technique within a DWDM-FSO system. The objective is to reinforce FSO link robustness and improve transmission rates. Through a comparative analysis with conventional OFDM in an Additive White Gaussian Noise (AWGN) channel, the study reveals a notable Signal-to-Noise Ratio (SNR) gain of at least 3dB with enhanced SIM-OFDM (ESIM-OFDM). Simulations, conducted with realistic parameters within the DWDM-FSO architecture, demonstrate that ESIM-OFDM modulation enables a transmission range of 2 km with an aggregated rate of 10.1 Gb/s. These results underscore the potential of SIM-OFDM in enhancing FSO system performance through the integration of DWDM technology. The proposed ESIM-OFDM DWDM-FSO scenario could represent a highly interesting solution for establishing 5G Backhaul links.

Communication Channel Influence on Self Interference Cancellation for In-Band Full-Duplex Underwater Acoustic Systems

This paper employs a self-interference cancellation (IBFD-UWA) system for shallow-water acoustic channels by depending on the model of communication channel for SI cancellation, which is based on geometry to accommodate any possible changes in the underwater environment. UWA channels have propagation characteristics based on the configurations of straight and multi-direction mirrors between a transmitter and a receiver. This propagation at the node itself and between the two nodes to transfer information between the nodes operating in the IBFD mode, orthogonal frequency division multiplex (OFDM) modulation with quadrature phase-shift keying (QPSK) mapping. By using numerical simulations, it can be shown that the developed model is able to accurately describe the underwater acoustic environment. The results indicate that local multipath propagation delays can exceed 1.4s with transmission losses of up to 80dB, and the SIC requires about 4 seconds to reduce SI signals to background noise levels successfully.

Evaluating Performance: A Study of Encrypted vs. Unencoded Signals in SISO-OFDM with LS and MMSE Estimations

The continuous progress in digital communication has played a crucial role in meeting the increasing need for faster data rates. Orthogonal Frequency Division Multiplexing (OFDM), a pivotal methodology in this progression, attains improved data rates through the efficient utilisation of densely packed carriers within a specified channel bandwidth. This article focuses on the investigation of channel estimation in OFDM systems. the study of search lies in its examination of the performance consequences associated with the incorporation or lack thereof of a convolutional encoder in OFDM systems and looks at how well two well-known channel estimation algorithms, Least Square (LS) and Minimum Mean Square Error (MMSE), work in 4-Quadrature amplitude modulation (4-QAM) OFDM systems with and without a convolutional encoder with a comprehensive evaluation of the efficacy of the OFDM system across various channel conditions. It uses MATLAB implementations as its main tool. The findings of the study indicate that the MMSE algorithm, despite its higher complexity, exhibits superior performance in comparison to the LS algorithm when combined with a convolutional encoder. The gain in terms of bit error rate (BER) improvement approximately 12 dB. This represents the logarithmic scale improvement in BER from BER1(uncoded) to BER2(coded) at the same the energy per bit to noise power spectral density ratio (Eb/N0) of 40 dB.

A Survey on Spatial Modulation OFDM: Optimal Detection and Performance Analysis

Spatial modulation is a novel modulation scheme that aims to acquire a trade-off between the two adverse purposes in wireless communications, namely the improvements of data rate and data reliability, by adjusting the operational transmit antenna index in the emitted bits. To fight against inter-symbol interference (ISI) present in frequency selective channels, orthogonal frequency division multiplexing (OFDM) as become a extensively accepted and used mechanism. We investigate in this work the performance of SM-OFDM modulation over frequency selective MIMO channel using quadrature amplitude modulation (QAM) scheme. At the receiver side, a group of detectors is proposed to recover the active transmit antenna index (Tx-antenna) and the emitted data sequence. In this paper, a new SM-OFDM-based scheme is proposed, which adds another element of information in the transmitted SM-data block consisting of a chosen constellation among the available rotated set. To attain this, an additional index is used to select the specific constellation. At the receiver, a set of detectors is proposed to jointly estimate the transmitted symbol, as well as the used constellation and the active transmit antenna indices. The comparison of the performance of this new scheme against the conventional SM-OFDM shows an increased spectral efficiency at the price of a slight degradation in bit error rate. The proposed scheme permits a trade-off between the quality of the communication and the achieved spectral efficiency. Some concluding remarks are drawn over simulation results.

A Novel OFDM-Based Time Domain Quadrature GSM for Visible Light Communication System

In order to improve the spectral efficiency (SE) as well as the receiver performance of band-limited visible light communications (VLCs), two orthogonal frequency division multiplexing (OFDM)-based quadrature generalized multiple-input multiple-output (QG-MIMO) transmission schemes, including time domain (TD) quadrature generalized spatial modulation (TD-QGSM) and TD quadrature generalized spatial multiplexing (TD-QGSMP), are proposed in this paper. Firstly, the constellation symbols in the frequency domain are split into in-phase and quadrature components to perform the OFDM modulation separately. Then, the corresponding time domain signal is spatially mapped on different light emitting diodes (LEDs) for achieving the diversity or multiplexing. In addition, we also propose an illegal vector correction (IVC)-based orthogonal matching pursuit (OMP) detection algorithm to deal with the error propagation and noise amplification effect, where a novel correction criterion is involved for assisting the index vectors estimation and thus for improving the demodulation performance. The simulation results demonstrate that the SE can be significantly improved by the proposed schemes as compared with the existing OFDM-based generalized MIMO schemes, with the TD-QGSM increasing by at least 56.5% and the TD-QGSMP increasing by at least 72.3%. Moreover, the bit error rate (BER) performance can be further improved when applying the proposed IVC-OMP detection method, which outperforms the traditional maximum-likelihood and maximum ratio combining (ML-MRC) detection by at least 62.5%.

Super-Regenerative Receiver Wake-Up Radio Solution for 5G New Radio Communications

Wake-up radio is a promising solution to reduce the energy wasted by mobile devices during an idle state. In this paper, we propose a new wake-up radio solution for 5G mobile devices based on a super-regenerative receiver characterized by its low cost and low power consumption and investigate how to build on the orthogonal frequency-division multiplexing (OFDM) modulation capability at the base station to generate optimal wake-up signals. After presenting the relevant features and limitations of super-regenerative receivers operating in different 5G New Radio (NR) frequency bands, we evaluate how the numerology, the number of resource blocks, and the quadrature amplitude modulation (QAM) scheme used affect the sensitivity of the super-regenerative wake-up receiver. The results show that a 256-QAM modulation scheme, together with the highest numerology values, achieves optimal receiver sensitivity with a minimal number of resource blocks, yielding higher duty cycle pulses that also facilitate symbol synchronization tasks.

Channel-Independent Quantum Mapping-Substitution OFDM-Based Spatial Modulation for Physical-Layer Encryption in VLC Networks

Physical-layer encryption has lately been highlighted as a promising security approach for visible light communications (VLC). Orthogonal frequency-division multiplexing (OFDM) is the main structural entity of multiple-carrier modulation in most modern communication systems, including vehicle ad hoc networks, 5G systems, and the Internet of Things. The integration of OFDM and spatial modulation (SM) can improve the capacity of low-complication VLC systems. Two common methods have been proposed for OFDM-based SM schemes: frequency-domain SM (FD-SM) and time-domain SM (TD-SM). TD-SM supports higher spectral efficiency than FD-SM with low complication. In this paper, we propose a physical-layer encryption method, referred to as channel-independent quantum mapping-substitution (CIQMS) TD-SM, for enhancing VLC's confidentiality. The random properties of the input data are used for generating dynamic cypher keys that are updated for each frame using hashing and stream cypher methods. The proposed approach performs four encryption operations: quantum noise stream cypher, substitution of radiated data symbols mapping, substitution of light-emitting-diode information mapping, and polarity encryption of time-domain OFDM subcarriers. The findings show that our CIQMS method provides a considerably higher secrecy rate than channel-dependent methods proposed in the literature. Also, it provides high robustness against both authorized and non-authorized eavesdroppers. In addition to that, the results show that the CIQMS method provides high confidentiality against several attacks, e.g., statistical attacks, chosen-/known-plaintexts, and brute-force attacks.

Combined intra symbol frequency domain averaging channel estimation for polarization division multiplexed coherent optical OFDM/OQAM systems

Intrinsic imaginary interference (IMI) induced by chromatic dispersion (CD) and polarization mode dispersion (PMD) seriously degrades the performance of optical orthogonal frequency division multiplexing offset-quadrature amplitude modulation (O-OFDM/OQAM) systems. Therefore, effective channel estimations are required for such systems to maintain good transmission performance. In the previous publications, both the full loaded (FL) and the half loaded (HL) frequency-domain channel estimation methods for polarization-division-multiplexed (PDM) coherent optical OFDM/OQAM (CO-OFDM/OQAM) systems have been proposed. However, for these frequency-domain methods, the channel frequency-domain responses caused by the correlation of the amplified spontaneous emission (ASE) noise are not considered. In this paper, we systematically discuss the frequency-domain channel transmission model for PDM CO-OFDM/OQAM systems. With the analysis of the correlation of the ASE noise, we propose a combined intra symbol frequency-domain averaging (CISFDA) channel estimation method for PDM OFDM/OQAM systems. Compared with the full-loaded frequency-domain channel estimation (FL-FD) method, the CISFDA method promotes the system robustness against the ASE noise and fiber nonlinear effect significantly. The proposed method is validated by numerical Monte Carlo simulations of the PDM CO-OFDM/OQAM system. Our new algorithm outperforms the FL-FD method in the 36 Gb/s PDM OFDM CO-OFDM/OQAM system, and the transmission distance is improved by 500 km with the bit-error-rate (BER) target at 3.8×10-3.

Iterative Filtering PAPR Reduction Method for OFDM Modulation in Fifth-generation Cellular Networks

Background & Objective: Orthogonal Frequency Division Multiplexing (OFDM) is an ordinarily used waveform in the fifth generation (5G) cellular networks for uplink links. However, there is a prominent disadvantage in the form of a high peak-to-average power ratio (PAPR) which yields distortion in the timing signal generated at the output of the high-power amplifier (HPA). Methods: A new method called Iterative Filtering PAPR Reduction (IFP) has been suggested in this paper and maintains backward compatibility. The basic concept behind this algorithm is to obtain a filter based on a constant-envelope signal that is intimate to the original signal as far as power is concerned. The constant-envelope signal is then compared to the output between the product of the convolution of the original signal with the filter in question, allowing for the calculation of the impulse response of the filter. Such a process can be repeated several times with different filters to realize the best reduction in PAPR. Results: The simulated results of the IFP method proved better performance in terms of PAPR reduction, Bit Error Rate (BER), and computational complexity requiring two iterations only. We can see a gain of 3.1dB in terms of PAPR reduction, 17dB in terms of BER, and a factor of 33 times in terms of computational complexity compared to the TR method. The Complementary Cumulative Complementary Density Function (CCDF) has assisted in measuring and improving the PAPR performance of the system. Conclusion: The theoretical analysis shows that a single iteration (NF= 1) is sufficient, and the simulation results exposed in this paper show a gain of 3.1 dB in PAPR reduction.

Discrete Hartley transform-based dual-mode index modulation OFDM for visible light communications

Using discrete Hartley transform (DHT) instead of discrete Fourier transform (DFT) in the optical index modulation (IM) aided orthogonal frequency division multiplexing (OFDM) systems for visible light communications (VLC) results in improvement in terms of the spectral efficiency, energy efficiency, peak-to-average power ratio, and bit error rate (BER) performance with a lower computational complexity, benefiting from the removal of the Hermitian symmetry. In this paper, by employing dual-mode (DM) index modulation in the DHT-based OFDM systems, more spectral efficient schemes for VLC are proposed. A reduced-complexity maximum likelihood (ML) detector and a simple log-likelihood ratio (LLR) detector for the DM-OFDM-IM systems are presented, achieving a better performance with a lower complexity over the traditional detectors. The spectral efficiency and the average BER are analyzed. The minimum distance between different transmitted subblock vectors is calculated for characterizing the performance. The simulation results demonstrate that the DHT-based DM-OFDM-IM systems achieve significant performance gains over the DHT-based OFDM-IM counterparts and perform better than the DFT-based counterparts in terms of the spectral efficiency and BER performance.

Slepian Transform Based Detectors for HF Skywave Massive MIMO-OFDM Systems

In this paper, we investigate signal detection for high frequency (HF) skywave massive multiple-input multiple-output (MIMO) systems with orthogonal frequency division multiplexing (OFDM) modulation. We first introduce the beam based channel model for HF skywave massive MIMO-OFDM channels and show the relationship of sparse supports between the beam domain channel and the Fourier spectrum of the space domain channel. Based on the sparse beam domain channel, we propose a separate Slepian transform (SST) based detector, where a set of modulated Slepian sequences are designed independently for user terminals (UTs). Before the minimum mean-squared error (MMSE) detection, the Slepian transform is performed for each UT to reduce the dimension of the observation vector and the channel matrix, thus avoiding high dimensional matrices multiplications and inversions. Due to the sparsity of the beam domain channel, the Slepian transform of channel vectors can be efficiently implemented by low-dimensional matrix-vector multiplications. To further reduce the computational complexity, we propose a joint Slepian transform (JST) based detector, where a fixed set of modulated Slepian sequences are designed. As a result, Slepian transforms of the observation vector can be efficiently implemented using low-dimensional fast Fourier transform (FFT). Simulation results demonstrate the advantages of high performance and low complexity of proposed detectors.

Multifunctional difluoroboron β-diketonate-based luminescent receiver for a high-speed underwater wireless optical communication system.

The last decade has witnessed considerable progress in underwater wireless optical communication in complex environments, particularly in exploring the deep sea. However, it is difficult to maintain a precise point-to-point reception at all times due to severe turbulence in actual situations. To facilitate efficient data transmission, the color-conversion technique offers a paradigm shift in large-area and omnidirectional light detection, which can effectively alleviate the étendue limit by decoupling the field of view and optical gain. In this work, we investigated a series of difluoroboron β-diketonate fluorophores by measuring their photophysical properties and optical wireless communication performances. The emission colors were tuned from blue to green, and >0.5 Gb/s data transmission was achieved with individual color channel in free space by implementing an orthogonal frequency-division multiplexing (OFDM) modulation scheme. In the underwater experiment, the fluorophore with the highest transmission speed was fabricated into a 4×4 cm2 luminescent concentrator, with the concentrated emission from the edges coupled with an optical fiber array, for large-area photodetection and optical beam tracking. The net data rates of 130 Mb/s and 217 Mb/s were achieved based on nonreturn- to-zero on-off keying and OFDM modulation schemes, respectively. Further, the same device was used to demonstrate the linear light beam tracking function with high accuracy, which is beneficial for sustaining a reliable and stable connection in a dynamic, turbulent underwater environment.

Least Square Estimation-Based Different Fast Fading Channel Models in MIMO-OFDM Systems

In cellular wireless communication systems, channel estimation (CE) plays a pivotal role as a crucial technique applied in orthogonal frequency division multiplexing (OFDM) modulation. CE utilizes a variety of methods, including decision-directed channel estimation, pilot-assisted channel estimation (PACE), and blind channel estimation. Among these options, PACE is widely favored for its remarkable stability and consistent superior performance. The idea of massive multiple-input multiple-output (MIMO) shows tremendous potential for the future of wireless communications. However, existing massive MIMO systems face challenges with their high computational complexity and intricate spatial structures, preventing efficient utilization of channel and sparsity features in these multiantenna systems. In communication channels, the signal received is often influenced by the characteristics of the channel and noise present at the receiver. To address this issue, an efficient dataset is utilized, employing the least square (LS) algorithm for minimization. OFDM is a commonly and widely used modulation method in communication systems utilized to specifically combat resonance fading in wireless channels. In wireless communication systems employing OFDM-MIMO, frequency selectivity and time-varying attributes due to multipath channels cause Intercarrier Interference (ICI) among symbols. Channel estimation is a vital aspect for mitigating the effects of fading channels. This investigation focuses on the application of a method examined in the study, which involves a block-type pilot symbol-assisted estimation technique for Rayleigh and Rician fading channel models. The research assesses the performance of the least square (LS) channel estimators in fast-fading channel models while employing various symbol mapping techniques focusing on bit error rate, throughput, and mean square error. The results indicate that the LS estimator exhibits excellent performance in Rayleigh and AWGN channels within the pedestrian A (PedA) model for both uplink and downlink scenarios. It outperforms the PedA model without channel estimation.

Enhancing security and capacity in FSO transmission for next-generation networks using OFDM/OCDMA-based ICSM codes

In order to address the growing demands for both enhanced security levels and increased transmission capacity, this research proposes a novel approach for free space optical (FSO) transmission. The proposed design incorporates an identity column shift matrix (ICSM) code to ensure robust security. Additionally, capacity enhancement is achieved through the utilization of a 4-level quadrature amplitude modulation (4-QAM) scheme in conjunction with an orthogonal frequency division multiplexing (OFDM) modulator. The performance of the system is evaluated across three channels, each transmitting data at a rate of 20 Gb/s, while operating in an FSO channel that is subjected to varying atmospheric attenuation effects and atmospheric turbulence. Real meteorological data from three different cities [Alexandria, Egypt; Jeddah, Kingdom of Saudi Arabia (KSA); and Hyderabad, India], situated across two continents, are incorporated to demonstrate the practicality of implementing the proposed model in real-world environments. The experimental results reveal that an increase in atmospheric turbulence leads to a higher bit error rate (BER) and lower received optical power (ROP), resulting in degraded data reception. Furthermore, the study examines the impact of weather conditions, indicating that the longest and shortest propagation ranges of 12.5 and 0.286 km, respectively, are achieved under clear weather and heavy dust storms. These conditions yield an ROP of −9.5 dBm and a log (BER) of approximately −2.5. Conversely, in the presence of strong turbulence, the performance further deteriorates. The proposed model demonstrates its ability to transmit a message signal over a distance of 0.8375 km, with a log (BER) of −2.6 under weak atmospheric turbulence. However, under strong atmospheric turbulence at the same distance, the log (BER) increases to −0.5. Regarding specific cities, the FSO range for transmitting information signals is found to be 9.58 km in Jeddah, which decreases to 6.58 km in Alexandria and 5.17 km in Hyderabad due to the increased atmospheric attenuation in these cities.