Optical Multiple-input Multiple-output Research Articles

Optical MIMO Communication Based on Joint Control of Base Station and OPA-Type OIRS

As a new type of optical communication device, the optical intelligent reflecting surface (OIRS) can reflect, shape and split optical signals in real time, and is expected to expand the application scenarios of optical wireless communication. However, in the existing research, a single OIRS cannot support optical multiple input multiple output (MIMO) communication since OIRS performs the same modulation on all incident light. In this paper, we propose an optical MIMO communication system based on joint control of base station and optical phased array (OPA)-type OIRS. The communication system utilizes the OPA -type OIRS's characteristic of superimposing the input optical phase and the OIRS's phase to implement a joint beam control scheme of the base station and the OIRS, thereby realizing optical MIMO communication with a single OIRS. Considering the OIRS's physical model, pointing error, atmospheric turbulence and attenuation, our work establishes the mathematical model of the optical MIMO communication system, and derives the closed-form expression of the probability density function (PDF) of the channel fading, asymptotic outage probability and bit error rate (BER) for the first time. Experiments and simulations are built to verify the feasibility and accuracy of our system model and theoretical derivation, which proves that our results are consistent with the actual system.

Using Standard 2×2 MIMO to Increase Capacity of Spatial Multiplexing with OAM Modes

Modal multiplexing can greatly increase fiber capacity, but often at the cost of high digital processing with multiple-input multiple-output (MIMO) algorithms. The use of orbital angular momentum (OAM) for multiplexing can support MIMO-free reception, but it always operates in an interferencelimited regime. Many previous MIMO-free demonstrations for OAM used a type of optical MIMO that is impractical for deployed systems. We compare the performance of MIMO-free and standard electronic 2 × 2 MIMO in a demonstration with no recourse to optical MIMO. We show that the receiver complexity of these two reception strategies is very similar in hardware and signal processing. We explore commercial coherent receivers with 2 × 2 MIMO, hence this solution for OAM demultiplexing can be deployed effectively with current technology. The 2 × 2 MIMO reception enhances the total bit rate and is more compatible with wavelength division multiplexing (WDM) as it equalizes performance across the C-band. At 1.3 km transmission, the net bit rate spans 426–450 Gb/s across wavelengths with 2 × 2 MIMO, while variability is high for MIMO-free reception, spanning 200–418 Gb/s.

OFDM-Based Generalized Optical MIMO

The combination of multiple-input multiple-output (MIMO) transmission and orthogonal frequency division multiplexing (OFDM) modulation has been shown to be an effective way to substantially enhance the capacity of bandlimited optical wireless communication (OWC) systems. In this paper, we propose four OFDM-based generalized optical MIMO (GO-MIMO) techniques for intensity-modulation and direct-detection OWC systems, including OFDM-based frequency-domain generalized spatial modulation (FD-GSM), frequency-domain generalized spatial multiplexing (FD-GSMP), time-domain generalized spatial modulation (TD-GSM) and time-domain generalized spatial multiplexing (TD-GSMP). For OFDM-based FD-GSM and FD-GSMP, spatial mapping is performed in the frequency domain, while it is carried out in the time domain for OFDM-based TD-GSM and TD-GSMP. To efficiently estimate both spatial and constellation symbols in each OFDM-based GO-MIMO technique, a corresponding maximum-likelihood (ML) detection algorithm is designed. Extensive simulations are conducted to evaluate and compare the performance of OFDM-based GO-MIMO techniques under various MIMO settings in a typical indoor environment. Simulation results demonstrate that there exists an optimal GO-MIMO technique to achieve a target spectral efficiency for a given receiver location, and the obtained optimal GO-MIMO technique is also related to the specific location of the receiver.

Optical MIMO communication with unequal power allocation to channels

Multiple input multiple output (MIMO) approach in fiber optical communication has emerged as an effective proposition to address the ever increasing demand for information exchange. In the ergodic case, the multiple channels, associated with multiple modes or cores or both in the optical fiber, is modeled by the Jacobi ensemble of random matrices. A key quantity for assessing the performance of MIMO systems is the mutual information (MI). We focus here on the case of an arbitrary transmission covariance matrix and derive exact determinant based results for the moment generating function (MGF) of mutual information (MI), and thereby address the scenario of unequal power per excited mode. The MGF is used to obtain Gaussian- and Weibull-distribution based approximations for the probability density function (PDF), cumulative distribution function (CDF) or, equivalently, the outage probability, and also the survival function (SF) or reliability function. Moreover, a numerical Fourier inversion approach is implemented to obtain the PDF, CDF, and SF directly from the MGF. The MGF is further used to investigate the ergodic capacity, which is the first moment (mean) of the mutual information. The analytical results are found to be in excellent agreement with Monte Carlo simulations. Our study goes beyond the earlier investigations where covariance matrix proportional to identity matrix has been considered which corresponds to equal power allocation per excited mode.

Multi-Gigabit Spatial-Division Multiplexing Transmission Over Multicore Plastic Optical Fiber

Today, we are evident to the revolution of the automotive industry and its demand for high bandwidth sensor fusion. Multiple high bandwidth sensors are connected, generating tremendous amount of data to be transferred in real-time in harsh environment, i.e., multi-gigabit intra-vehicle networks. Plastic optical fiber (POF) is an attractive medium for these multi-gigabit intra-vehicle networks due to its inherent EM compatibility, galvanic isolation, low weight, high tolerance to mechanical vibrations, low bending loss, and easy handling. However, commercial off-the-shelf PMMA based POFs are bandwidth-length product limited. Here, we suggest increasing the spectral efficiency using spatial-division multiplexing (SDM). In this article, we have experimentally demonstrated a low cost and eye-safe 3 × 3 multiple-input multiple-output (MIMO) SDM system over a 15 meters multicore (MC) POF using commercial optical components, e.g., RC-LEDs and Si-PIN photodiodes. A 3 × 1 spot-based cores coupler spatial multiplexer was developed to couple the optical components with the MC-POF. This system achieved raw data rates of 1.5 Gb/s and 2.4 Gb/s using offline-processed MIMO equalization. The proposed system enables a flexible and scalable optical MIMO system that can meet the requirements of the automotive industry.

Nonlinearities effects on optical MIMO systems

Multimode fibers (MMF) are enjoying a renewed attention boosted by recent advances in multimode complex nonlinear optics. Reason behind nonlinearity is occurred either due to intensity dependence of the medium refractive index or to inelastic-scattering phenomenon. The most commonly occurred non-linear effects in optical Multiple Input Multiple Output (O-MIMO) are the Kerr, Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS) effects. The impact of Kerr, SBS and SRS instigate distortions to the optical signal transmission over MIMO transmission system. In this work, we focus on the impact of the optical Kerr, SBS and SRS effects on the overall performance of a MIMO transmission system. It is observed that in O-MIMO system, nonlinear effects degrade the system performance since many channels propagate simultaneously. They can result in pulse distortion and crosstalk between channels. Here, a comparative study between MIMO communication system with and without nonlinearities allows an accurate performance evaluation. The comparison is performed using Q-factor and Bit Error Rate (BER). The outcome is analyzed using OptiSystem software tool.

Baseband linearity and interference effects in intensity modulated and direct detected optical MIMO systems

Abstract In short reach intensity modulated and direct detected (IM/DD) optical multiple-input multiple-output (MIMO) systems linear equalization strategies are commonly applied. However, the square-law detector inherently is a non-linear system element. This work investigates this discrepancy and shows for which laser types and under which mode coupling conditions baseband linearity applies for the first time. Based on a system model that integrates a multitude of aspects, e.g. the phase state of each fiber mode and mode coupling, it is shown that the received signal consists of a linear component and one that can be considered as interference. The simulation and measurement results highlight that only MIMO setups with weak and no mode cross-talk can be considered as linear. Systems with high mode cross-talk are impaired by significant interference, resulting in a highly degraded bit-error rate performance.

Weakly coupled 10-mode-division multiplexed transmission over 48-km few-mode fibers with real-time coherent MIMO receivers

For weakly coupled mode-division multiplexed (MDM) transmission systems, we design and implement optical coherent receiver prototypes with real-time multiple-input multiple-output (MIMO) digital signal processing to equalize two degenerate linearly polarized modes with dual polarization. Using field programmable gate array circuits, we implement real-value 8 × 2 MIMO adaptive equalization with externally separated phase compensators based on the least mean square algorithm, which enables not only training equalization but also fast carrier-phase tracking. With the optical coherent MIMO receiver prototype, we demonstrate real-time weakly coupled 10 × MDM wavelength-division multiplexed dual-polarization quadrature phase shift keying transmission over 48-km few-mode fibers. This report shows a record number of multiplexed spatial modes, namely, 10 modes with dual polarization, in real-time MDM transmission experiments.

Opto-Electron Eng, 2020, 47(3)] Research status and development of optical spatial modulation technology

Optical spatial modulation (OSM), a new optical multiple input multiple output (OMIMO) technique, effectively improves the transmission rate and energy efficiency by using the spatial domain laser index to carry additional information. Meantime, since only one laser is activated per symbol duration to transmit information, the problems of channel interference and synchronization are solved in traditional OMIMO system. This paper firstly introduces several OSM technologies and summarizes their research status both at home and abroad. Nextly, the OSM, optical space shift keying (OSSK), enhanced optical spatial modulation (EOSM) and differential optical spatial modulation (DOSM) schemes are compared and analyzed in terms of transmission rate, spectral efficiency, bit error rate (BER) and complexity. Finally, the key problems and future development direction are pointed out in OSM system.

Optical spatial modulation design.

Visible light communication (VLC) systems are inherently signal-to-noise ratio (SNR) limited due to link budget constraints. One favourable method to overcome this limitation is to focus on the pre-log factors of the channel capacity. Multiple-input multiple-output (MIMO) techniques are therefore a promising avenue of research. However, inter-channel interference in MIMO limits the achievable capacity. Spatial modulation (SM) avoids this limitation. Furthermore, the performance of MIMO systems in VLC is limited by the similarities among spatial channels. This limitation becomes particularly severe in intensity modulation/direct detection (IM/DD) systems because of the lack of phase information. The motivation of this paper is to propose a system that results in a multi-channel transmission system that enables reliable multi-user optical MIMO SM transmission without the need for a precoder, power allocation algorithm or additional optics at the receiver. A general bit error performance model for the SM system is developed for an arbitrary number of light-emitting diodes (LEDs) in conjunction with pulse amplitude modulation. Based on this model, an LED array structure is designed to result in spatially separated multiple channels by manipulating the transmitter geometry.This article is part of the theme issue ‘Optical wireless communication’.

Polarization-based 6 × 6 MIMO transmission over 75 km few-mode multicore fiber using recursive least squares constant modulus algorithm

In an optical multiple input multiple output (MIMO) transmission system, mode coupling and mode dispersion have a significant impact on the signal recovery processes at the receiver. We describe a polarization-based MIMO transmission by using mode division multiplexing and polarization division multiplexing over few-mode multicore fiber. We used three optical modes with x-y polarizations for 6 × 6 MIMO transmission over 75 km length of few-mode multicore fiber at 20 Gbps quadrature phase shift keying signals. Optical MIMO transmission is successfully recovered by using a recursive least squares constant modulus algorithm (RLSCMA). The simulation results show that RLSCMA enhanced the system performance of optical MIMO transmission system by employing sufficient adaptive equalization.

Iterative Unreplicated Parallel Interference Canceler for MDL-Tolerant Dense SDM (12-Core × 3-Mode) Transmission Over 3000 km

We propose a novel multiple-input multiple-output (MIMO) detection scheme based on iterative unreplicated parallel interference cancelling (UPIC) technique for mode dependent loss (MDL)-impaired few-mode fibre (FMF) transmission. An optical MIMO channel over an FMF exhibits a mode-dependent property where each mode division multiplexed signal is passed through a different lossy channel, known as an MDL phenomena. One challenging issue for realising future deployable space division multiplexing (SDM) systems is establishing reliable and robust signal transmission techniques in the presence of MDL. The main contributions of this paper include first, a proposal of newly developed iterative interference cancellation technique with parallel processing for MDL-impact mitigation, second, a comparative study on performance of MIMO detection schemes, and third, a demonstration of the longest dense SDM transmission over multicore (MC)-FMF. The proposed UPIC with iterative processing is experimentally shown to improve Q-factors by more than 3 dB, and record-long dense SDM transmission reach exceeding 3000 km over 12-core × 3-mode MC-FMF was achieved.

Optical Wireless MIMO Experiments in an Industrial Environment

This paper reports on a field trial in a robotic manufacturing cell. Channel measurements and transmission experiments were conducted, assisting the development of a light-emitting diode-based optical wireless communication (OWC) system tailored for the needs of industrial wireless applications. These are moderate data rates, reliability, and low latency. Since dedicated radio spectrum is hardly available, the usage of light is an interesting alternative. Due to its spatial confinement, OWC links provide enhanced security and are difficult to jam from outside a building—a particularly interesting property for industrial applications. We performed broadband distributed $8\times 6$ multiple-input multiple-output (MIMO) channel measurements in a manufacturing cell. Results confirm that the signal-to-noise ratio is sufficient if a line-of-sight (LOS) is available and bandwidth is limited to a few megahertz. However, small movements and rotations may lead to sudden fades of 10–20 dB, when an LOS is blocked. To improve reliability, antenna diversity concepts are applied. Results show that for reliable communication, MIMO diversity schemes are indispensable as it increases the probability of a free LOS significantly. Based on these insights, we suggest an OWC design tailored to industrial wireless applications.

Interference-Free LED Allocation for Visible Light Communications With Fisheye Lens

Due to the limited modulation bandwidth of commercial light emitting diodes (LEDs), imaging optical multiple-input multiple-output (MIMO) technology is applied in visible light communication (VLC) system to achieve high data rate. Since a receiver with a wide angle/field-of-view is preferred in the imaging optical MIMO VLC system, the fisheye lens can be utilized to concentrate the lights from the LEDs. To eliminate the inter-user interference and satisfy their target bit error rate (BER) requirements with the minimum number of LEDs, an interference-free LED allocation scheme is investigated in this paper, which is formulated as a combinatorial problem. The cost criterion of the combinatorial problem is defined as the number of the LEDs used to serve all users, and its discrete alternatives (i.e., feasible solutions) are the disjoint sets consisting of the LEDs that can be cooperatively utilized to satisfy the BER requirements for all users. For each LED in the disjoint set, its neighboring LEDs projected onto the same pixel are forbidden to serve different users. Moreover, due to the NP-hardness of the formulated problem, a location-based greedy algorithm is proposed, where the LEDs are allocated to the users sequentially based on their distances to the center of the LED array. Simulation results verify the effectiveness of our proposed algorithm and show that there exists no interference among all users while the target BER requirements for all users are satisfied with the proposed algorithm.

Decoupling of Broadband Optical MIMO Systems Using the Multiple Shift SBR2 Algorithm

Polynomial singular value decomposition (PSVD) plays a very important role in broadband multiple-input multiple-output (MIMO) systems. One of its applications lies in the decoupling of MIMO convolutive mixing channel matrixin order to recover the transmitted signals corrupted by the channel interference (CI) at the receiver. In this paper, a novel algorithm, known as multiple shift second order sequential best rotation (MS-SBR2), is proposed to compute the approximate PSVD of the broadband MIMO channel matrix. Experimental examples, including a measured (2 × 2) optical MIMO channel impulse response using the multi-mode fiber (MMF) testbed, are presented to examine the proposed algorithm. Bit error rate (BER) performances are evaluated among different transmission schemes. In addition, power allocation (PA) schemes are investigated to further optimize the BER performance.

Element-by-Element Full-Rank Optical Wireless MIMO Systems Using Narrow-Window Angular Filter Designed Based on One-Dimensional Photonic Crystal

In the field of radio-frequency (RF) communications, the multiple-input multiple-output (MIMO) strategy allows us to attain a higher rate of data transfer due to the multipath-rich environment. In contrast, it is challenging to achieve the same rate in a highly correlated channel that is specific to optical wireless communications. In this paper, we propose the novel concept of an element-by-element optical MIMO system that enables the interference-free reception of parallel symbol streams; it achieves this by using an angular filter (AF) with a narrow transparent window. The AF consists of a one-dimensional photonic crystal, and the angular selectivity is obtained by using the extremely small wave vector regime. We demonstrate that the capacity of the proposed system increases linearly with the minimum number of transmit source elements and receive detectors, which is similar to conventional RF MIMO systems.

Strongly-coupled multi-core fiber and its optical characteristics for MIMO transmission systems

We review recent progress on coupled multi-core fiber (MCF) technologies for optical multiple-input multiple-output (MIMO) transmission. First, we define types of MCF, namely non-coupled/coupled single-/few-mode MCF, and briefly report recent work on non-coupled/coupled MCF. We next describe the advantage of using coupled MCF in MIMO transmission systems, and present a coupled MCF design based on an analysis of coupling between super-modes in twisted bent MCF. We finally describe our experimental results for our strongly coupled MCF and its applicability for optical MIMO transmission systems.

Photodetector Selection Aided Multiuser MIMO Optical OFDM Imaging Visible Light Communication System

While playing an important role in radio frequency (RF)-based wireless systems, multiple-input multiple-output (MIMO) techniques have also found viable applications in visible light communication (VLC) systems. However, how to effectively reduce the inherent strong correlations between optical MIMO channels remains a typical issue for VLC. In this paper, we propose an MIMO-aided multiuser optical orthogonal frequency division multiplexing VLC system employing an imaging receiver (ImR). The proposed new scheme efficiently mitigates the optical MIMO channel correlations across the multiple photodetectors (PDs) of the same user equipment (UE) or those between different UEs, through invoking a new technique called photodetector selection (PDS). Two possible PDS designs, namely random PDS and modified maximum minimum singular value PDS are investigated. Through both analytical analysis and numerical simulations, we show that the proposed PDS-aided ImR system is capable of outperforming its conventional non-imaging receiver and non-PDS ImR counterparts, especially when UEs are close to each other. Results demonstrate that the proposed system offers a stably low bit error rate in most areas of the room.

On Capacity of Downlink Underwater Wireless Optical MIMO Systems With Random Sea Surface

In this letter, we focus on the relationship among channel capacity, signal-to-noise ratio (SNR), water types, wind speed, and characteristics of transmiter/receiver array such as inter-spacing and link range for downlink underwater wireless optical communications (UWOC) multiple-input multiple-output (MIMO) systems. Numerical results suggest that more turbid water, larger link range, and larger inter-spacing may reduce the channel capacity, and meanwhile more turbid water and larger link range can weaken the effects of random slopes and inter-spacing at the expense of larger SNR to balance the channel capacity.

Non-reciprocity compensation correction and antenna selection for optical large MIMO system

This paper exploits an optical large multiple input multiple output (MIMO) system. We first establish the non-reciprocity compensation correction factor to solve the channel non-reciprocity problem. Then we propose an antenna selection algorithm with the goal of realizing maximum energy efficiency (EE) when satisfying the outage EE. The simulation results prove that this non-reciprocity compensation correction factor can compensate beam energy attenuation gap and spatial correlation gap between uplink and downlink effectively, and this antenna selection algorithm can economize the number of transmit antennas and achieve high EE performance. Finally, we apply direct current- biased optical orthogonal frequency division multiplexing (DCO-OFDM) modulation in our system and prove that it can improve the bit error rate (BER) compared with on-off keying (OOK) modulation, so the DCO-OFDM modulation can resist atmospheric turbulence effectively.