Intensity Modulation Direct Detection Research Articles

A GMM-based non-uniform quantization scheme for improving low-resolution IMDD-UFMC system performance

In this paper, a Gaussian mixture model (GMM) based nonuniform quantization scheme with low-bit resolution is firstly presented in optical intensity-modulated direct detection universal filter multicarrier (IMDD-UFMC) system for increasing system performance. By utilizing the Expectation-Maximum (EM) iterative algorithm of GMM model, the optimal quantized level can be directly obtained, and then is used to realize the nonuniform quantization for reducing the digital-to-analog converter (DAC) bit resolution of UFMC signals. In this processing, the extra estimation operation of signals’ probability density distribution (PDF) is not involved, which can reduce the complexity of the common non-uniform quantization scheme compared to the method based on the PDF estimation. And, the proposed scheme is further validated in a 10 Gb/s IMDD-UFMC transmission system through a 20 km fiber based the commercially OptiSystem15.0 software. And, all the results are obtained by this numerical simulation. Results verified that, in the cases of 3 ∼ 5 bit, 2.7%∼7.8% the error vector magnitude (EVM) and at least 1-dB received sensitivity improvements can be achieved compared to the uniform quantization schemes.

Mixed FSO/RF Based Multiple HAPs Assisted Multiuser Multiantenna Terrestrial Communication

In this work, a mixed free-space optics (FSO)/radio-frequency (RF) based multiple serial high altitude platforms (HAPs) assisted multiuser multiantenna terrestrial communication system is considered. For the considered multi-hop system, earth station to HAP and HAP to HAP links are assumed as FSO links, and HAP to terrestrial mobile users (MUs) link is assumed as RF link. At the FSO detector both the heterodyne detection and intensity modulation direct detection techniques are considered. Atmospheric turbulence of the ES to HAP FSO link is modeled with Gamma-Gamma fading along with the pointing error impairments. As HAP situates in the stratosphere, negligible atmospheric turbulence exists at HAP altitude (20 km), hence, only pointing error is considered for the inter HAP links. Further, the HAP consists of multiantenna array to provide high data-rates to the terrestrial users via RF links, considered to be Nakagami-m distributed. The intermediate HAPs perform selective decode-and-forward relaying and opportunistic user scheduling is performed for the best user selection. For the performance analysis, analytical expression of overall outage probability is obtained and the impact of various selection parameters like pointing error, FSO detection type, MU selection, number of antennas, and RF fading severity are observed on the outage performance. Asymptotic outage probability is also derived to obtain the diversity order of the considered communication system. Further, considering various modulation schemes, a generalized average bit-error-rate expression is derived and the impact of pointing error, FSO detection type, MU selection, number of antennas, and RF fading severity are observed on its performance. Finally, derived results are validated through Monte-Carlo simulations.

Higher-Order Modulation vs Faster-Than-Nyquist PAM-4 for Datacenter IM-DD Optics: An AIR Comparison Under Practical Bandwidth Limits

The 4-ary pulse amplitude modulation (PAM-4) has been widely deployed to both electrical and optical interfaces in datacenter applications. Based on 100-GBd PAM-4, the 200G per lane optical interface is being actively developed in both industrial interoperability groups and standard bodies. Going beyond 200G, it is questionable if PAM-4 could support an even faster interface with higher baud-rate, considering the bandwidth growth for both electronics and optical components has started lagging the ever-increasing capacity demand. Under limited bandwidth, one option is to improve the spectral efficiency by higher order formats like PAM-6 and PAM-8, but this induces backward incompatibility as well as potential modulation mismatch between electrical and optical interfaces that requires additional energy for gearboxing. This revives the study of faster than Nyquist (FTN) signaling to pursue higher PAM-4 baud-rate breaking the bandwidth limit. In ECOC 2021, we demonstrated a reduced-complexity maximum a posteriori probability (MAP) decoder in intensity-modulation (IM) direct-detection (DD) systems with superior FTN performance. In this paper, we extend our work by adding more algorithm details and experimental results. Moreover, we perform the achievable information rate (AIR) comparison between the FTN PAM-4 and the higher-order modulation within the Nyquist limit in different channel conditions. Remarkably, in a practical IM-DD channel with gradually decayed frequency response, FTN shows AIR gain over the Nyquist approach even at a low FTN decoding complexity. The finding suggests FTN PAM could be a promising candidate at post 200G era to offer both AIR and implementation advantages.

An SNR-improved Transmitter of Delta-sigma Modulation Supported Ultra-High-Order QAM Signal for Fronthaul/WiFi Applications

In this paper, we proposed an improved mobile fronthaul architecture employing the SNR-improved delta-sigma digitization scheme and 4-level pulse amplitude modulation (PAM-4) format. Different from traditional 2-bit quantization, this scheme deploys two-fold delta-sigma quantization, which uses 1-bit delta-sigma modulator to quantize the signal, and another 1-bit delta-sigma modulator to quantize the in-band noise. A significant reduction of the in-band noise can be achieved only applying a differentiator, bringing about a much better noise shaping performance. Meanwhile, the two 1-bit streams can be combined and transmitted in a PAM-4 manner through an intensity modulation direct detection (IM-DD) channel. We have successfully experimentally demonstrated the digitization and transmission of 65536 quadrature amplitude modulation (QAM) baseband orthogonal frequency division multiplexing (OFDM) signal with sampling rate of 1.25 GSa/s over 20-km fiber with standard 10-Gbaud PAM-4 signal, with a signal to noise ratio (SNR) of 57.7 dB. Compared to the conventional 2-bit quantized OFDM scheme, experimental results show that a significant SNR improvement of 17.7 dB has been achieved by the proposed scheme. In addition, we also realized a transmission of 16384-QAM intermediate frequency (IF) signal at the center frequency of 3.5 GHz (with an SNR of 49.6 dB). An improvement of 15.9 dB can also be maintained.

C-band 200 Gbit/s/λ PAM-4 transmission over 2-km SSMF using look-up-table pre-distortion combined with nonlinear Tomlinson-Harashima pre-coding.

The performance of high baud-rate intensity modulation direct detection (IM-DD) transmissions is severely degraded by both the linear and nonlinear inter-symbol interference (ISI). Here, we propose and experimentally demonstrate a transmitter-side look-up-table pre-distortion combined with nonlinear Tomlinson-Harashima pre-coding (LUT_PD-NTHP) scheme with the capability of mitigating the linear and nonlinear ISI simultaneously, enabling a C-band 200 Gbit/s/λ PAM-4 transmission over 2-km standard single mode fiber (SSMF), under an end-to-end 10-dB bandwidth of about 20 GHz. The proposed LUT_PD-NTHP scheme is experimentally verified to be superior to the LUT pre-distortion combined with linear THP (LUT_PD-LTHP) scheme, in terms of both the receiver sensitivity and the LUT storage requirement, when only the feed-forward equalization (FFE) is used at the receiver-side. In particular, after the 200 Gbit/s PAM-4 signal transmission over the 2-km SSMF without the chromatic dispersion (CD) compensation, the proposed LUT_PD-NTHP scheme with a LUT pattern length of 3 possesses not only 0.25 dB improvement of the receiver sensitivity but also about 99% LUT pattern reduction, in comparison with the LUT_PD-LTHP scheme with a LUT pattern length of 5.

Unified Performance Analysis of Reconfigurable Intelligent Surface Empowered Free-Space Optical Communications

Reconfigurable intelligent surface (RIS) is an excellent use case for line-of-sight (LOS) based technologies such as free-space optical (FSO) communications. In this paper, we analyze the performance of RIS-empowered FSO (RISE-FSO) systems by unifying Fisher-Snedecor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathcal{F}}$ </tex-math></inline-formula> ), Gamma-Gamma ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cal {GG}$ </tex-math></inline-formula> ), and Malága ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cal {M}$ </tex-math></inline-formula> ) distributions for atmospheric turbulence with zero-boresight pointing errors over deterministic as well as random path-loss in foggy conditions with heterodyne detection (HD) and intensity modulation/direct detection (IM/DD) methods. By deriving the probability density function (PDF) and cumulative distribution function (CDF) of the direct-link (DL) with the statistical effect of atmospheric turbulence, pointing errors and random fog, we develop exact expressions of PDF and CDF of the resultant channel for the RISE-FSO system. Using the derived statistical results, we present exact expressions of outage probability, average bit-error-rate (BER), ergodic capacity, and moments of signal-to-noise ratio (SNR) for both DL-FSO and RISE-FSO systems. We also develop an asymptotic analysis of the outage probability and average BER and derive the diversity order of the considered systems. We validate the analytical expressions using Monte-Carlo simulations and demonstrate the performance scaling of the FSO system with the number of RIS elements for various turbulence channels, detection techniques, and weather conditions.

Performance of UAV-Assisted Multiuser Terrestrial-Satellite Communication System Over Mixed FSO/RF Channels

In this letter, performance of a multiantenna multiuser unmanned aerial vehicle (UAV)-assisted terrestrial-satellite communication system over mixed free space optics (FSO)/radiofrequency (RF) channels is analyzed. Downlink transmission from the satellite to the UAV is completed through FSO link which follows gamma&#x2013;gamma distribution with pointing error impairments. Both the heterodyne detection and intensity modulation direct detection techniques are considered at the FSO receiver. To avail the antenna diversity, multiple transmit antennas are considered at the UAV. Selective decode-and-forward scheme is assumed at the UAV and opportunistic user scheduling is performed while considering the practical constraints of outdated channel state information (CSI) during the user selection and transmission phase. The RF links are assumed to follow Nakagami-m distribution due to its versatile nature. In this context, for the performance analysis, analytical expressions of outage probability, asymptotic outage probability, ergodic capacity, effective capacity, and generalized average symbol-error-rate expressions of various quadrature amplitude modulation (QAM) schemes such as hexagonal-QAM, cross-QAM, and rectangular QAM are derived. A comparison of various modulation schemes is presented. Further, the impact of pointing error, number of antennas, delay constraint, fading severity, and imperfect CSI are highlighted on the system performance. Finally, all the analytical results are verified through the Monte&#x2013;Carlo simulations.

56-Gb/s/λ C-band DSB IM/DD PAM-4 40-km SSMF transmission employing a multiplier-free MLSE equalizer.

Chromatic dispersion, which introduces pattern-dependent inter-symbol interference (ISI), appears to be a long-standing performance limiting problem in optical intensity modulation direct detection (IM/DD) transmission systems. In this paper, we propose a multiplier-free maximum likelihood sequence estimation (MLSE) equalizer for C-band double-sideband IM/DD transmission. It models the IM/DD channel with dispersion-induced ISI and Gaussian noise. A look-up table is applied to record ISI for transition probability calculation and the Viterbi algorithm for decision sequence acquisition. Specifically, to reduce the number of multipliers, a refined construction of Viterbi algorithm based on tentative path decisions is adopted, which compresses the complexity of branch metric calculation to less than 1/4 for PAM-4 format. Moreover, approximation calculation is employed to realize multiplier-free hardware implementation, which greatly reduces the hardware consumption. The proposed MLSE equalizer offers superior performance and lower complexity over conventional equalizers. In the experimental verification, we experimentally demonstrate a C-band 56-Gb/s double-sideband 4-level pulse amplitude modulation (PAM-4) IM/DD transmission over 40-km standard single mode fiber exploiting the proposed refined MLSE without any optical amplifier, filter or dispersion managed modules at the receiver end, achieving a bit-error-ratio of 2.65×10-4, which is 2.28 orders of magnitude lower than the scheme using Volterra nonlinear equalizer.

Frequency domain anti-flickering in visible light communication system

Light emitting diodes (LED) is a promising device in visible light communication (VLC), enabling simultaneous illumination and data transmission via visible light. However, the generation of modulated visible light signals causes variation to the LED intensity in intensity modulation/direct detection (IM/DD) transmission, which may cause flickering — the fast blinking of the LED (when signals are below 200 Hz). Flicker may cause migraines, headaches, and even repetitive behavior among persons with autism. Therefore, flicking mitigation approaches must be applied for VLC with simultaneous illumination scenarios. While the optimization of the LED driving circuit and encoding can be employed to reduce flickering, driving circuit dramatically increase the system cost and communication requirements are usually excluded. As to encoding methods, they will result in reduced data rate and hardware overhead, leading to performance bottlenecks. In this paper, we proposed a signal processing method in the frequency domain to mitigate LED flickering. Low-frequency components (3–70 Hz) of modulated signals are extracted by using Fourier transform and inversely restored for transmission. Besides flickering, it is shown that interference caused by other artificial light sources can also be mitigated with the proposed method. Simulation results show that by applying the proposed frequency domain signal processing technique, communication performance of the system is enhanced with the cancellation of interference while flicker is reduced significantly. Under the condition of a 13 dB signal-to-noise (SNR) ratio, the bit error rate can reach the order of 10−3.

Direct detection transmission of a PAM signal with power fading mitigation based on Alamouti coding and dual-drive MZM.

Power-fading impairment induced by fiber dispersion and photodiode detection imposes a fundamental limitation on the intensity-modulation direct-detection (IM-DD) transmission systems. In this work, we propose a cost-effective pulse-amplitude modulation (PAM) signal transmission scheme with power-fading mitigation enabled by Alamouti coding and dual-drive Mach-Zehnder modulator (DDMZM). By interleaving the symbol blocks in the time domain for upper- and lower-arm of the DDMZM, flat end-to-end frequency response can be obtained without spectral nulls after combining the photocurrents at odd and even time slots. For single channel demonstration, we experimentally transmit up to 160Gb/s PAM-4, 140Gb/s PAM-6, and 108Gb/s PAM-8 signals over 80 km standard single-mode fiber (SSMF) with bit-error rates (BERs) below the 20% soft-decision forward error correction (SD-FEC) threshold of 2.0×10-2. For wavelength division multiplexing (WDM) transmission, 8λ×150Gb/s PAM-4 signals spacing at 100 GHz can also achieve 80 km reach. Moreover, we compare the optical signal-to-noise ratio (OSNR) sensitivity with single sideband (SSB) scheme and evaluate the tolerance of bias deviation in numerical simulation. The proposed Alamouti coding-based scheme provides a hardware-efficient and dispersion-tolerant candidate for high-speed inter-data center interconnect (DCI) applications.

Prototype of DSP-Free IM/DD MDM Transceiver for Datacenter Interconnection

New transmission techniques for short-reach optical transmission applications especially datacenter interconnections are highly expected to break current bandwidth bottleneck, among which mode division multiplexing (MDM) based on few-mode fiber (FMF) is promising by exploring different linearly-polarized (LP) modes or mode groups as spatial transmission channels. However, it has been a great challenge to deal with degenerate modes in FMFs. In this paper, a FMF design utilizing multiple ring perturbations in fiber core to suppress modal crosstalk during fiber transmission is firstly proposed and a 6-LP-mode double-ring-core (DRC) FMF is fabricated with a high minimum refractive index difference (min|neff|) of 1.4910-3 among all the modes. Secondly, a novel degenerate-mode-selective coupler (DMSC) converting each degenerate LPmn mode into the LP11 mode in a two-mode fiber is proposed. Then intensity-modulation/direct-detection (IM/DD) MDM transmission scheme without any digital signal processing (DSP) can be realized utilizing mode multiplexer/demultiplexer (MUX/DEMUX) consisting of cascaded mode-selective couplers (MSC) and DMSCs, in which each degenerate or non-degenerate LP mode is considered as an independent transmission channel. Then based on the proposed DRC-FMF and mode MUX/DEMUX, we demonstrate a 4-LP-mode prototype system using commercial single-mode 10 Gbps SFP + modules and 4K video transceivers without any hardware or software modifications. Finally, the intra-LP-mode dispersion (ILMD) effect in a weakly-coupled FMF is analyzed and a modified fixed-analyzer method is proposed for its measurement. Experimental results show that it may be one of the major impairments for IM/DD MDM transmission.

Dynamic polarization-insensitive BOTDA in direct-detection OFDM with CNN-based BFS extraction.

We propose a dynamic polarization-insensitive Brillouin optical time domain analyzer (D/PI-BOTDA) with orthogonal frequency division multiplexing (OFDM) based on intensity-modulated direct-detection (IM-DD). A polarization-division-multiplexed (PDM) pump signal enables polarization diversity of the stimulated Brillouin scattering while a multi-frequency OFDM probe signal realizes dynamic sensing with single-shot transmission. We experimentally demonstrated distributed temperature sensing along a total 940-meter fiber with a temperature sensing coefficient of 1.2°C/MHz. The experimental results indicated a remarkable suppression of Brillouin gain fluctuation up to 4.38 times compared to the case without polarization diversity. To facilitate the Brillouin frequency shift (BFS) extraction process, we also implement a CNN-based BFS extraction method with SE-Res2Net block. The adopted algorithm achieves a higher accuracy than conventional curve fitting method, with a 10-time enhancement in the time efficiency.

Advanced DSP Enabled C-Band 112 Gbit/s/λ PAM-4 Transmissions With Severe Bandwidth-Constraint

The performance of high-speed intensity modulation direct detection (IM-DD) transmissions is severely degraded by the linear inter-symbol interference (ISI), due to both bandwidth-constrained optoelectrical components and the chromatic dispersion (CD). Meanwhile, nonlinear ISI due to intensity modulation and square-law detection can further deteriorate the IM-DD system performance significantly. Here, we first propose three metrics to quantify those impairments in IM-DD transmissions. Then, we compare both the BER performance and computational complexity of different DSP schemes to enable short-reach high-speed IM-DD transmissions with severe bandwidth constraint, using a C-band PAM-4 experimental test-bed with an end-to-end 10-dB bandwidth of 17 GHz. Those DSP schemes include receiver-side feedforward equalization (FFE), receiver-side Volterra filter equalization (VFE), transmitter-side Tomlinson-Harashima precoding (THP) together with receiver-side FFE, transmitter-side THP together with receiver-side VFE, channel-shortening filter (CSF) together with maximum likelihood sequence estimation implemented with linear branch metrics (MLSE-LI-BM), and CSF together with MLSE implemented with nonlinear branch metrics (MLSE-NL-BM). Our experimental results indicate that, only the combination of CSF and MLSE-NL-BM enables a 112 Gbit&#x002F;s transmission over 5 km fiber at the KP4-FEC threshold, but with an increased computational complexity.

Channel Estimation Based on Complex-Valued Neural Networks in IM/DD FBMC/OQAM Transmission System

Filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) is a promising candidate for 5G mobile fronthaul system. Due to the inherent imaginary interference of FBMC/OQAM signals, an accurate channel estimation is particularly indispensable. In this paper, an efficient method is proposed based on complex-valued neural networks (CVNN) for an accurate channel estimation of FBMC/OQAM signals with low computational complexity and pilot overhead. Here, the channel frequency response (CFR) is first calculated for training the CVNN. Next, the CFR estimated from the extracted pilots is exploited as the input of the trained CVNN to yield the CFR of data symbols. We experimentally demonstrate a 12.5-GBd intensity modulation direct detection (IM/DD) FBMC/OQAM transmission system over 30-km and 50-km standard single mode fibers (SSMF). The experimental results show that the proposed method achieves 3-dB and 1-dB receiver sensitivity improvement at the bit error ratio (BER) of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> with only 5% pilot overhead respectively, compared to the conventional least square (LS) and linear minimum mean error (LMMSE) methods. When the pilot overhead decreases from 10% to 1%, its BER performance is always better than LS and LMMSE. For the computational complexity, its complexity is the same order of magnitude as LS and lower than LMMSE. On the other hand, the CVNN requires less hidden neurons to keep the similar BER performance as real-valued neural networks (RVNN). Meanwhile, it has excellent resilience to fiber chromatic dispersion over 30-km and 50-km SSMF.

On the Capacity Performance of Hybrid FSO/RF System With Adaptive Combining Over Generalized Distributions

This paper investigates the ergodic capacity performance of hybrid free space optics (FSO)/ radio frequency (RF) system by employing an adaptive-combining-based switching scheme. In the adaptive combining scheme, the FSO and RF links are combined using the maximal-ratio combining (MRC) at the output, when the instantaneous signal-to-noise ratio (SNR) of the FSO link drops below a predefined threshold SNR value. The FSO link experiences the atmospheric turbulence, which is modeled using the generalized Malaga distribution, non-zero boresight misalignment or pointing errors, and atmospheric attenuation, under two types of detection techniques, i.e. heterodyne detection (HD) and intensity modulation/direct detection (IM/DD). Similarly, the fading of the RF link is characterized using the generalized <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\kappa$</tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> distribution. In particular, we derive the probability density function (PDF) of the instantaneous SNR of the adaptive combining scheme. Capitalizing on the SNR statistics, we obtain the unified closed-form ergodic capacity expressions for the adaptive-combining-based hybrid FSO/RF system. In addition, the asymptotic expressions for the ergodic capacity of the system at the high-SNR region are presented. All the derived analytical expressions are validated by using Monte-Carlo simulations. Numerical results and discussions are provided to compare the ergodic capacity performance of the adaptive-combining-based hybrid FSO/RF system with the existing system models such as single-link FSO, MRC-based hybrid FSO/RF, and hard-switching-based hybrid FSO/RF systems.

A Mixed FSO/RF Integrated Satellite-High Altitude Platform Relaying Networks for Multiple Terrestrial Users with Presence of Eavesdropper: A Secrecy Performance

In this paper, the secrecy performance of a mixed free space optical (FSO)/radio frequency (RF) integrated satellite-high altitude platform (HAP) relaying networks for terrestrial multiusers with the existence of an eavesdropper is investigated. In this network, FSO is adopted to establish the link between the satellite and HAP for which it experiences Gamma-Gamma distributions under different detection schemes (i.e., heterodyne and intensity modulation direct detection). The transmission between the amplify-and-forward (AF) relaying HAP and terrestrial multiusers is through the RF and is modeled as shadowed-Rician fading distribution. Owning to broadcasting nature of RF link, it is assumed that an eavesdropper attempts to intercept the users’ confidential message, and the eavesdropper link is subjected to Rician distributions. Specifically, the closed-form expression for the system equivalent end-to-end cumulative distribution function is derived by exploiting the Meijer’s G and Fox’s H functions. Based on this expression, the exact closed-form expressions of the system connection outage probability, secrecy outage probability, and strictly positive secrecy capacity are obtained under the different detection schemes at HAP. Moreover, the asymptotic analyze of the system secrecy outage probability is provided to obtain more physical insights. Furthermore, the accuracy of all the derived analytical closed-form expressions is verified through the Monte-Carlo simulations. In addition, the impact of atmospheric turbulence, pointing errors, shadowing severity parameters, and Rician factor are thoroughly evaluated. Under the same system conditions, the results depict that heterodyne detection outperforms the intensity modulation direct detection.

Coded DCO-OFDM Techniques in Intensity Modulation/Direct Detection (IM/DD) Systems

Optical wireless communications (OWC) are among the best alternative techniques for transmitting information-laden optical radiation across a free-space channel from one place to another. DC-biased optical OFDM (DCO-OFDM) is a technique that sacrifices the power efficiency to transmit unipolar OFDM signals. The primary drawback with DCO-OFDM is its clipping noise, which causes distortion and lowers the bit error rate (BER). Thus, in this paper, we show the performance of DCO with different coded techniques to improve the BER in additive white Gaussian noise (AWGN) for IMDD systems. The experimental results show that the coded DCO-OFDM has the best performance. Furthermore, turbo coding has the best coding technique added to the DCO-OFDM system.

A Performance Limit Estimation Framework for Multihop Repeated/Regenerated Optical Links

We propose and demonstrate a generalized framework for performance limit evaluation and comparison of multihop optical repeated and regenerated links. The model developed is implementation agnostic and applies to multihop optical links of varied forms, including fiber, free space, and underwater links. The framework estimates the best-case performance gains of deploying an all-regenerative link over an all-repeater link for any given implementation. The implementation-independent technique is then illustrated using guided and free-space optical links. An abstract model is developed first with the evolution of signal, noise power, and bit error rate down the link compared and contrasted for both cases. The model is then evaluated using physical parameters for a typical fiber optic intensity-modulated direct detection link, and the obtained all-regenerator link performance advantage is translated to extra reach and lower transmission power requirements. Further, certain approximations are provided to reduce computational complexity and improve the analytical tractability of the procedure, which could be particularly helpful when employed in specialized hardware or for dynamic reconfiguration networks. Finally, the framework&#x2019;s versatility is established by employing it in analyzing an ideal free-space link and comparing amplify and forward links against decode and forward counterparts. Similar results are also reproduced on a commercial optical link simulation suite. Detailed literature on link analysis is provided for fiber, free space, and underwater links, bringing out their similarities. We conclude by elaborating on various current and emerging application domains and certain limitations of the proposed technique.

Real-time reception of NHS-OFDM signal with SPA-enhanced channel estimation for intensity-modulated direct-detection systems

In this Letter, we have experimentally verified a low-complexity subcarrier pairwise-averaging (SPA)-enhanced channel estimation (CE) method for small-size fast Fourier transform (FFT) non-Hermitian symmetric orthogonal frequency-division multiplexing (NHS-OFDM) transceivers. Compared with intra-symbol frequency averaging (ISFA), more than 20% look-up tables and 10% logic power consumption can be saved. The least-square (LS), ISFA, and SPA CE methods are compared by offline and real-time digital signal processing approaches. The results show that the receiver sensitivity of the SPA NHS-OFDM transmission system with 64/128-point FFT can be improved by more than 1 dB at the bit error rate of 3.8 × 10-3 compared to the LS.

Performance analysis of MIMO FSO systems with EGC diversity receiver over lognormal-Rician turbulence

Two approximate closed-form probability density function expressions for the sum of lognormal-Rician random variates (RVs) and the sum of square root of lognormal-Rician RVs are obtained. The proposed expressions are shown to provide high accuracy over a wide range of channel conditions according to Kolmogorov–Smirnov goodness-of-fit statistical tests results. Also, the analysis of the approximation error is presented to indicate that a higher approximation accuracy can be achieved for larger r or smaller variance σz2. To reveal the importance of the proposed approximation, new approximate closed-form expressions of the ergodic capacity, average bit error rate for the intensity-modulation direct-detection (IMDD), and coherent multiple-input multiple-output (MIMO) free-space optical (FSO) systems with equal gain combining diversity technique over lognormal-Rician turbulence channels are developed. It is observed that MIMO technology can offer a significant improvement in system performance and the performance of coherent FSO systems outperforms that of IMDD systems. The Monte Carlo simulation results are further utilized to illustrate the accuracy of the proposed approach.