Spatial Domain Multiplexing Research Articles

Evolution of Short-Range Optical Wireless Communications

The optical wireless communication (OWC) technology explores the broad unregulated optical spectrum to provide high-speed wireless communications, and the visible, ultraviolet and near-infrared wavelength ranges all have been investigated. Compared with the conventional radio frequency (RF) band, the spectrum in OWC systems is much less congested and the interference is much lower. The OWC technology also provides enhanced physical layer security due to the natural confinement of optical beams that makes it difficult to intercept the transmitted signal. Hence, the OWC technology has been widely studied and considered as a promising candidate in future beyond-5G communications. The OWC technology has been considered for both long-range and short-range applications, and in this paper we will introduce the fundamentals and recent developments of short-range OWC systems. In particular, we will focus on the key short-range OWC applications in indoor personal or local area communications, underwater wireless communications, wireless data center networks, and vehicular communications. We will also introduce some recent widely studied advanced techniques to boost the performance of short-range OWC systems, including the spatial domain diversity, multiplexing and modulation principles to improve the system robustness and data rate, and the machine learning algorithms and hardware accelerators to suppress both linear and nonlinear effects to improve OWC signal quality and bit-error-rate performance.

Unified coupling and propagation model for spatially multiplexed optical communication systems using Bessel–Gaussian beams

Spatial domain multiplexing (SDM) offers a degree of photon freedom to optical fibers. One variant of SDM utilizes ring-like output profiles, created through specific oblique input angles, and shows high-performance gains. Conventional modeling of this system utilizes Laguerre–Gaussian based models; however, these models are more suitable for free-space environments as opposed to fiber propagation. Therefore, a diffraction-based approach to deriving a Bessel–Gaussian beam solution is presented that adheres well to experimental data while maintaining continuity requirements. The derived model shows an input angle to azimuthal index relationship proportional to the transverse wave number of the carrier optical fiber region of SDM architectures.

Two Novel Space-Time Coding Techniques Designed for UWB MISO Systems Based on Wavelet Transform.

In this paper two novel space-time coding multi-input single-output (STC MISO) schemes, designed especially for Ultra-Wideband (UWB) systems, are introduced. The proposed schemes are referred to as wavelet space-time coding (WSTC) schemes. The WSTC schemes are based on two types of multiplexing, spatial and wavelet domain multiplexing. In WSTC schemes, four symbols are transmitted on the same UWB transmission pulse with the same bandwidth, symbol duration, and number of transmitting antennas of the conventional STC MISO scheme. The used mother wavelet (MW) is selected to be highly correlated with transmitted pulse shape and such that the multiplexed signal has almost the same spectral characteristics as those of the original UWB pulse. The two WSTC techniques increase the data rate to four times that of the conventional STC. The first WSTC scheme increases the data rate with a simple combination process. The second scheme achieves the increase in the data rate with a less complex receiver and better performance than the first scheme due to the spatial diversity introduced by the structure of its transmitter and receiver. The two schemes use Rake receivers to collect the energy in the dense multipath channel components. The simulation results show that the proposed WSTC schemes have better performance than the conventional scheme in addition to increasing the data rate to four times that of the conventional STC scheme.

Extending the Effective Ranging Depth of Spectral Domain Optical Coherence Tomography by Spatial Frequency Domain Multiplexing

We present a spatial frequency domain multiplexing method for extending the imaging depth range of a spectral domain optical coherence tomography (SDOCT) system without any expensive device. This method uses two galvo scanners with different pivot-offset distances in two independent reference arms for spatial frequency modulation and multiplexing. The spatial frequency contents corresponding to different depth regions of the sample can be shifted to different frequency bands. The spatial frequency domain multiplexing SDOCT system provides an approximately 1.9-fold increase in the effective ranging depth compared with that of a conventional full-range SDOCT system. The reconstructed images of phantom and biological tissue demonstrate the expected increase in ranging depth. The parameters choice criterion for this method is discussed.

Wavelength independency of spatially multiplexed communication channels in standard multimode optical fibers

Spatial domain multiplexing (SDM), also known as space division multiplexing, adds a new degree of photon freedom to existing optical fiber multiplexing techniques by allocating separate radial locations to different channels of the same wavelength as a function of the input launch angle. These independent MIMO channels remain confined to their designated locations while traversing the length of the carrier fiber owing to helical propagation of light inside the fiber core. As a result, multiple channels of the same wavelength can be supported inside a single optical fiber core, thereby allowing spatial reuse of optical frequencies and multiplication of fiber bandwidth. It also shows that SDM channels of different operating wavelengths continue to follow an output pattern that is based on the input launch angle. As a result, the SDM technique can be used in tandem with wavelength division multiplexing (WDM), to achieve higher optical fiber bandwidth through increased photon efficiency and added degrees of photon freedom. This endeavor presents the feasibility of a hybrid optical fiber communication architecture in which the spectral efficiency of the combined system increases by a factor of “n” when each channel of an “ n ” channel SDM system carries the entire range of WDM spectra.

Combining spatial domain multiplexing and orbital angular momentum of photon-based multiplexing to increase the bandwidth of optical fiber communication systems

Spatial domain multiplexing/space division multiplexing (SDM) can increase the bandwidth of existing and futuristic optical fibers by an order of magnitude or more. In the SDM technique, we launch multiple single-mode pigtail laser sources of the same wavelength into a carrier multimode fiber at different angles. The launching angles decide the output of the carrier fiber by allocating separate spatial locations for each channel. Each channel follows a helical trajectory while traversing the length of the carrier fiber, thereby allowing spatial reuse of optical frequencies. We launch light from five different single-mode pigtail laser sources (of same wavelength) at different angles (with respect to the axis of the carrier fiber) into the carrier fiber. Owing to helical propagation, five distinct concentric donut-shaped rings with negligible crosstalk at the output end of the fiber were obtained. These SDM channels also exhibit orbital angular momentum (OAM), thereby adding an extradegree of photon freedom. We present the experimental data of five spatially multiplexed channels and compare them with simulated results to show that this technique can potentially improve the data capacity of optical fibers by an order of magnitude: A factor of five using SDM and another factor of two using OAM.

Design of Joint Spatial and Power Domain Multiplexing Scheme for Massive MIMO Systems

Massive Multiple-Input Multiple-Output (MIMO) is one of the key techniques in 5th generation wireless systems (5G) due to its potential ability to improve spectral efficiency. Most of the existing works on massive MIMO only consider Time Division Duplex (TDD) operation that relies on channel reciprocity between uplink and downlink channels. For Frequency Division Duplex (FDD) systems, with continued efforts, some downlink multiuser MIMO scheme was recently proposed in order to enable “massive MIMO” gains and simplified system operations with limited number of radio frequency (RF) chains in FDD system. However these schemes, such as Joint Spatial Division and Multiplexing (JSDM) scheme and hybrid precoding scheme, only focus on multiuser transmission in spatial domain. Different from most of the existing works, this paper proposes Joint Spatial and Power Multiplexing (JSPM) scheme in FDD systems. It extends existing FDD schemes from spatial division and multiplexing to joint spatial and power domain to achieve more multiplexing gain. The user grouping and scheduling scheme of JSPM is studied and the asymptotic expression for the sum capacity is derived as well. Finally, simulations are conducted to illustrate the effectiveness of the proposed scheme.

Attenuation and modal dispersion models for spatially multiplexed co-propagating helical optical channels in step index fibers

Spatial reuse of optical frequencies has been shown to be possible through a novel spatial domain multiplexing (SDM) technique that uses spatial multiplexer at the input end to launch multiple channels of the same wavelength inside a single strand of carrier fiber and then employs spatial filtering methods to de-multiplex the different optical channels at the output end. The individual SDM channels are confined to dedicated spatial locations inside the fiber while traversing through it owing to helical propagation of light. This presents attenuation and dispersion models of such a system. Experimentally obtained beam profile and resultant crosstalk of two such spatially multiplexed co-propagating SDM channels of the same wavelength over standard step index multimode optical fibers are also presented.

Experimental results from copropagating analog channels of same wavelength over <inline-formula><math display="inline" overflow="scroll"><mrow><mn>600</mn><mtext>-</mtext><mtext>meter</mtext></mrow></math></inline-formula>-long standard step index multimode fiber

Spatial domain multiplexing is a new dimension in fiber optic multiplexing with the potential to greatly increase the data-carrying capacity of optical fiber communication systems. Spatially multiplexed channels follow helical paths inside the fiber and do not interfere with each other as optical energy from individual channels is distributed in the radial direction at different distances from the origin of the fiber. The resultant output appears as concentric circles when projected on a screen. The experimental setup and results for two spatially modulated analog channels of the same wavelength over approximately 600 m of standard 62.5/125-µm multimode fiber, using 635-nm pigtail laser sources and 30-MHz sinusoidal modulation, are reported here.

Spatial domain multiplexing: A new dimension in fiber optic multiplexing

A novel multiplexing technique for fiber optic communications has been developed that supports multiple channels of optical energy inside an optical fiber by confining each individual channel to a unique spatial location. These channels can operate at exactly the same wavelength as well as differing wavelengths. The basic operating principle and experimental results for spatial domain multiplexed fiber optic communication systems is presented here. This technique adds a new dimension to currently available multiplexing schemes and has the potential to increase the bandwidth of existing and futuristic optical fiber systems by multiple folds.

Structural Testing of Navy Vessels Using Bragg Gratings and a Prototype Digital Spatial Wavelength Domain Multiplexing (DSWDM) System

ABSTRACTRepairing and maintaining the Fleet have been long‐standing problems for the Navy. In recent years, decreasing resources and Fleet assets have made repair and maintenance an even more difficult problem. In response to these pressures, Navy personnel have been seeking new methods to reduce overall maintenance costs. One such approach has been to instrument ships during operation with resistance strain gage technology in order to provide information for estimating the structural health of ship hulls. To date, a limited number of naval ships have been instrumented in this way, and have collected data to validate computer models that predict damage accumulation over the expected service life of the vessel (Adamchak 1984; Sikora 1983). The Navy is currently developing a new ship structural health monitoring system based on fiber optic technology. Application of this new health monitoring system to U.S. Navy vessels will improve maintenance programs by predicting need‐based maintenance schedules, thereby eliminating costly cyclic preventative maintenance schedules. This new system will also increase operational availability and ship survivability by providing ship operators real‐time information concerning the state of the ship structure. This paper describes the operational capabilities of this new system and recent test results on large‐scale naval structures including a U.S. Navy Landing Platform Dock 17 (LPD 17) ship propeller.