Spatial Division Multiplexing Research Articles

Discrete Multitone Modulation for Short-Reach Mode Division Multiplexing Transmission

Spatial division multiplexing (SDM) can support the capacity demand increase in short-range optical link, but the communication is affected by the intermodal crosstalk. In order to target a low-cost energy-efficient solution, we propose to exploit direct modulation and direct detection without MIMO post-processing in a mode division multiplexed (MDM) system. We theoretically analyze the frequency distribution of the crosstalk contributions taking into account that directly modulated sources at the same nominal wavelength are actually characterized by a certain frequency shift. Our model demonstrates that this shift originates a non-uniform beating noise in the frequency domain due to the intermodal crosstalk. We propose to exploit discrete multitone modulation and a guard band in the multicarrier spectrum to cope with the non-uniform crosstalk issue for typical crosstalk values due to present commercial multi-plane light conversion MUX/DEMUX devices and propagation in few few-mode fiber (FMF) kilometers. The model is validated by comparing simulations and experimental results in the case of a 2-mode MDM system. We demonstrate in a pure SDM scenario that, with respect to single mode transmission, the capacity can be increased of 2.3 times in a 10-km FMF link with 4-modes exploiting direct detection, limited-bandwidth directly modulated lasers and commercial mode MUX/DEMUXs.

High-capacity spatial-division multiplexing with orbital angular momentum based on multi-ring fiber

We present a simple method of spatial-division multiplexing/demultiplexing with orbital angular momentum (OAM) based on a multi-ring optical fiber. Multi-ring fibers are ideal spatial division multiplexing carriers, providing more communication channels including orthogonal OAM multiplexing channels and spatial channels. We propose an OAM encoding scheme using multiplexing perfect vortex array (MPVA) to make full use of these channels and achieve high-capacity data communication links. For improving the coupling efficiency, perfect vortices whose radii independent of topological charge are employed to generate the intensity rings in each ring core region. Furthermore, a practical encoding scheme of multiplexing OAM is proposed to optimize the use of available OAM states. The encoded information in one core could reach L bits as there are L available OAM states in the fiber. To decode the multiplexing perfect vortex, an additional correction phase is employed to transform the perfect vortex into conventional optical vortex. By detecting the states of multiplexing OAM of each spot in this array with a perfect vortex detection phase plate, information encoded by perfect vortices is decoded. The efficient data encoding and decoding method with MPVA could be expanded to almost all kinds of multi-ring fibers to achieve high coupling efficiency and high-capacity data transmission.

Modeling optical fiber space division multiplexed quantum key distribution systems.

We report a model to use to evaluate the performance of multiple quantum key distribution (QKD) channel transmission using spatial division multiplexing (SDM) in multicore (MCF) and few-mode fibers (FMF). This model is then used to analyze the feasibility of QKD transmission in 7-core MCFs in two practical scenarios involving the (1) transmission of only QKD channels and (2) simultaneous transmission of QKD and classical channels. In the first case, standard homogeneous MCFs enable transmission distances per core compatible with transmission parameters (distance and net key rate) very close to those of single-core single-mode fibers. For the second case, heterogeneous MCFs must be employed to make this option feasible.

Spatial Channel Network (SCN): Opportunities and Challenges of Introducing Spatial Bypass Toward the Massive SDM Era [Invited

Considering middle-term predictions of the need for commercial 10-Tb/s optical interfaces working in 1-P b/s optical transport systems by 2024 and recalling the introduction of the optical bypass when entering the wavelength-abundant era in the early 2000s, we re-evaluate the values of hierarchical optical network architecture in light of the forthcoming massive spatial division multiplexing (SDM) era. We introduce a spatial channel (SCh) network (SCN) architecture, where the SDM layer is explicitly defined as a new networking layer that supports the new multiplexing technology of SDM. In an SCN, optical channels (OChs) accommodated in an express SCh bypass the overlying wavelength cross-connects (WXCs) using spatial cross-connects (SXCs) on the route. As one challenge that SCNs will present, we point out that an excessively large SXC insertion loss reduces the optical reach for spectrally groomed OChs. We show that the optical reach of spectrally groomed OChs can be maintained at almost the same level as that for an OCh transported through a conventional single-layer WDM network thanks to the low-loss features of commercially available spatial switches and foreseeable low-loss SDM multiplexers and demultiplexers. As another challenge, we discuss how to achieve growable, reliable, and cost-effective SXCs. We propose two SXC architectures based on sub-matrix switches and core-selective switches. Simple cost assessment shows that the proposed architectures are more cost-effective than the full-size matrix switch-based architecture with 1 + 1 equipment protection and conventional stacked WXC architecture.

Unified Derivation of Optimal Feedback Rate in Downlink Cellular Systems With Multi-Antenna Base Stations

A downlink cellular network in which base stations (BSs) are distributed according to a homogeneous Poisson point process is considered. Each BS simultaneously serves multiple single-antenna users for downlink transmission; zero-forcing beamforming (ZFBF) is used for spatial division multiplexing. Each user quantizes and feeds back the channel state information (CSI) to the transmitter to ensure that at least partial CSI is available at the transmitter. The net spectral efficiency is introduced as a main performance metric; it measures the net profit achieved by using limited-feedback-based ZFBF. This paper mainly analyzes the optimal number of feedback bits that maximizes the net spectral efficiency. The optimal number is analyzed with respect to various important system parameters by extending and generalizing previous studies. Specifically, this paper analyzes the asymptotic behaviors of the optimal number with respect to the signal-to-noise ratio (SNR) and the channel coherence time; it does so by providing close approximations to the optimal number that is generally satisfied regardless of the values of the SNR, number of users, path loss exponent, and channel coherence time. It is demonstrated by simulation that the proposed approximations are extremely close to the optimal number on the region of interest of the system parameters.

Crosstalk-aware routing, spectrum, mode and time assignment using FMF with partial MIMO equalization in flexible grid datacenter networks

Abstract Recently, lots of datacenters (DCs) have been built to enhance reliability and expand processing ability for computing service in cloud computing. Furthermore, spatial division multiplexing (SDM) technology and elastic optical network (EON) are also considered as the candidate solutions to solve the requirements of huge bandwidth and flexible connections. However, it is much more difficult to use few mode fiber (FMF) in DCs-based EON due to the existing modal crosstalk among different modes. Faced with the challenges, we propose a routing, spectrum, mode and time assignment algorithm to decrease the blocking probability and increase the spectrum utilization. To deal with the problem more efficiently, we propose a method called flexible sliding three-dimensional block (3D block) which represents the dimensions of spectrum, space and time in our algorithm. 3D block slides in the resource space to find out proper resources. The simulations are conducted on datacenter networks (DCNs) and the results show that lower blocking probability and higher spectrum utilization are achieved by efficiently utilizing the spectrum, space and time domains.

SDN Control of Sliceable Multidimensional (Spectral and Spatial) Transceivers with YANG/NETCONF

The combination of spatial division multiplexing (SDM) with flexi-grid wavelength division multiplexing (WDM) enables optical transceivers to provide super-channels that exploit the spectral and spatial dimensions. For the first time to the best of our knowledge, in this paper we extend SDM-WDM transceivers to support sliceability and make effective use of spatial and spectral resources. The proposed architecture provides multiple independent spectral-spatial super-channels (i.e., slices) sharing the same transceiver and exploiting different spectrum/core/mode resources. The proposed architecture also enables us to monitor the bit error rate of the provisioned slices in order to detect any signal-quality degradation due to the provisioning of additional slices. On the other hand, there is a general trend toward the disaggregation of optical networks. In this paper, we also propose the first open application program interface based on a YANG data model and NETCONF protocol for software-defined-networking-enabled sliceable SDM-WDM transceivers. The proposed transceiver is experimentally validated in a joint test bed between CTTC and KDDI research.

Modeling Parametric Processes in Few-Mode Semiconductor Optical Amplifiers

Few-mode semiconductor optical amplifiers (FM SOAs) allow more than one transverse modes to propagate inside of the waveguide region. They are useful in optical signal processing of distinct spatial mode signals when applied to spatial division multiplexing in an optical fiber communication. A model for FM SOAs including carrier rate and coupled-mode equations, which govern linear and nonlinear effects, is derived for the first time. The model reveals two distinctions of parametric process in FM SOAs through carrier gratings. First, the carrier gratings with transverse variation patterns provide unique mode selectivity, which only allows efficient interaction between certain modes. This mode selectivity is summarized as a parallelogram rule, a simple method to find the interacting modes and categorize them. Second, degenerate four wave mixing (FWM) via static carrier grating diffraction distinguishes itself from other types of FWMs for the highest wave interaction efficiency. Theoretical analysis and numerical simulation show that optical signal processing based on the degenerate FWM features high power efficiency, broadband operation, phase sensitivity, and mode selectivity. The processing can be done either in the spatial mode domain or in the frequency domain, or by combing them together. These features bestowed by the introduction of spatial degree of freedom, make FM SOAs promising in optical signal processing.

Long distance crosstalk-supported transmission using homogeneous multicore fibers and SDM-MIMO demultiplexing.

We propose and evaluate the use of spatial-division multiplexing (SDM) multiple input multiple output (MIMO) systems to support long distance transmission using single-mode homogeneous multicore fibers. We show that on a uniform link with per-span inter-core skew compensation, the required SDM-MIMO memory length corresponds to the largest inter-core skew per span on the link. Furthermore, we show that with inter-core skew compensation, the required memory length of the SDM-MIMO is nearly constant with the transmission distance for accumulated crosstalk below -11 dB. We experimentally demonstrate the use of SDM-MIMO with a memory length of 20 ns on a long distance transmission link using 20 GBaud PDM-QPSK signals. We achieve a reach of 9780 km, which corresponds to a 9% improvement over the case without SDM-MIMO. We also show that the use of SDM-MIMO is applicable to the transmission of signals with higher modulation order, achieving transmission reach improvements of 14% for 20 GBaud PDM-16QAM and PDM-64QAM signals.

Network Performance Trade-Off in Modular Data Centers With Optical Spatial Division Multiplexing

Modular design based on spatial division multiplexing switches is a promising way to improve the capacity and reduce the cabling complexity of data center networks. However, due to the coexistence of mice and elephant flows, in modular data center networks a trade-off between the blocking probability and total throughput arises. In fact, blocking elephant flows would lead to a relevant penalty on the throughput but a small penalty on the blocking probability. In this paper, we investigate the relation between the blocking and throughput in modular data center networks based on optical spatial division multiplexing. We combine the two metrics linearly by a weight factor that prioritizes them relatively. To solve the resource allocation problem, we propose both mixed integer linear programming formulations and close-to-optimal heuristics for three different spatial division multiplexing switching schemes. Simulation results demonstrate that a carefully chosen weight factor is necessary to achieve a proper balance between the blocking probability and throughput for all the schemes.

Spatial Division Multiplexing-Based Reflective Intensity-Modulated Fiber Optics Displacement Sensor

Due to the benefit of multiple parallel channels within single cladding and easy implementation of multi-input multioutput technique, we present the spatial division multiplexing (SDM)-based reflective intensity-modulated fiber optics displacement sensor (RIMFODS). Under the condition of single-input single-output, we experimentally compare the transfer function of RIMFODS by using a pair of standard single-mode fibers (SSMFs), single seven-core-single-mode fiber, and single seven-core-few-mode fiber. In comparison with the conventional SSMF-based RIMFODS, the use of seven-core-few-mode fiber can reduce the dead zone of measurement range from 200 to 100 μ m. Meanwhile, the system sensitivity can be enhanced from 0.14 to 12.2 nW/ μ m. In particular, with the help of two-input-five-output technique, the dead zone can be further reduced to 70 μ m, with sensitivity reaching as high as 53.87 nW/ μ m. Finally, we theoretically investigate the effect of both the core size and the core spacing of multicore fiber on the transfer function of RIMFODS. Such an SDM-based RIMFODS with compact footprint has lots of potential to be exploited.

Demonstration of an SDM Network Testbed for Joint Spatial Circuit and Packet Switching †

We demonstrate a spatial division multiplexing (SDM) network testbed composed of three nodes connected via 19-core multi-core fibers. Each node is capable of joint spatial circuit switching and joint packet switching to support 10 Tb/s spatial circuit super channels and 1 Tb/s line rate spatial packet super channels. The performance of the proposed hybrid network is evaluated, showing successful co-existence of both systems in the same network to provide high capacity and high granularity services. Finally, we demonstrate an optical channel selection associated with the quality of service requirements on the SDM network testbed.

Nested Precoding in MU-MIMO Channels: The Superposition Transmission of Space-Time Coding and Spatial-Division Multiplexing

Nonorthogonal multiple access (NOMA) can improve the spectrum efficiency through reusing the power-level space by different users. Although NOMA in single-input single-output channels has been thoroughly explored, it is still underexploited in multiple-input multiple-output (MIMO) channels. Precoding design in multiuser MIMO channels with nonorthogonal transmission is a multidimensional nesting problem, it is hard to compare two matrix channels if only their power levels are considered. In this paper, we first propose a method to compare the covariance matrices of two channel matrices, and then derive a channel condition under which the linear precoding scheme can achieve the sum capacity. Considering arbitrary realizations of multiuser channels, we provide a nested precoding algorithm that can work well in highly correlated multiuser channels. Based on this idea, a specialized superposition transmission scheme between signaling and data information is proposed, where plenty of resources beneath the signaling signal are utilized to transmit data information for dedicated users, and a large amount of extra rates are achieved. In addition, space-time coding is used for signaling information and spatial-division multiplexing is used for multiuser data information. Under the constraint that signaling information should be correctly decoded at any position in the cell, we seek to maximize the sum rate of the underneath multiuser data streams. To this end, the optimal and a low-complexity simplified precoding algorithms are developed. Finally, simulation results verify the throughput improvement and the correct decoding probability of the signaling information.

Time-domain characteristics of ultrafast transverse mode switching based on Si nanowires.

We investigate the potential of an ultrafast transverse mode switch based on silicon on insulator (SOI) platform for the switching applications in the spatial division multiplexing (SDM) systems. The impact of waveguide geometry, operation wavelength, pulse width of the pump and probe waves, as well as the temporal delay between the pulses are discussed with a special focus on the temporal switching windows of the switch. The optimized switching performances are further applied to a switching node in a hybrid optical time division-multiplexing (OTDM) and SDM system. The switching energy of the Si-based switch is shown to be potentially smaller than that of the Si3N4 based integration platform.

Node Architectures and Cost Evaluation for Optical Networks that Adopt Spatial Division Multiplexing

Node Architectures and Cost Evaluation for Optical Networks that Adopt Spatial Division Multiplexing

Low Complexity Wireless Powered Information Transfer Strategy for Multiuser MIMO Systems

In this paper, the minimum achievable rate maximization problem for wireless-powered multiple users is investigated. Zero-forcing (ZF)-based precoding is considered for information transmission (IT) in the uplink from multi-antenna users to a multi-antenna access point that charges the users through wireless power transmission (PT) in the downlink. A max–min user rate optimization problem is formulated when a ZF-based transmission is used for spatial division multiplexing in the IT phase. We propose a low-complexity method to design the energy beamformer for the PT phase, the transmit power allocation for multi-stream transmission for the IT phase, and the time ratio between PT and IT phases in terms of maximizing the minimum achievable rate. The validity of the proposed scheme is shown through complexity analysis and numerical simulations.

Allocation of spectral and spatial modes in multidimensional metro-access optical networks

Introduction of spatial division multiplexing (SDM) has added a new dimension in an effort to increase optical fiber channel capacity. At the same time, it can also be explored as an advanced optical networking tool. In this paper, we have investigated the resource allocation to end-users in multidimensional networking structure with plurality of spectral and spatial modes actively deployed in different networking segments. This presents a more comprehensive method as compared to the common practice where the segments of optical network are analyzed independently since the interaction between network hierarchies is included into consideration. We explored the possible transparency from the metro/core network to the optical access network, analyzed the potential bottlenecks from the network architecture perspective, and identified an optimized network structure. In our considerations, the viability of optical grooming through the entire hierarchical all-optical network is investigated by evaluating the effective utilization and spectral efficiency of the network architecture.

Design and characterization of 16-mode PANDA polarization-maintaining few-mode ring-core fiber for spatial division multiplexing

A PANDA polarization-maintaining few-mode ring-core fiber (PM-FM-RCF) structure with two air holes around the ring core is proposed. The relative mode multiplicity factor (RMMF) is defined to evaluate the spatial efficiency of the designed PM-FM-RCF. The performance analysis and comparison of the proposed PANDA PM-FM-RCFs considering three different types of step-index profiles are detailed. Through modal characteristic analysis and numerical simulation, the PM-FM-RCF with a lower refractive index difference (Δnoi=1.5%) between the ring core and the inner central circle can support up to 16 polarization modes with large RMMF at C-band, which shows the optimum modal properties compared with the PM-FM-RCF with higher Δnoi. All the supported polarization modes are effectively separated from their adjacent polarization modes with effective refractive index differences (Δneff) larger than 10−4, which also show relatively small chromatic dispersion (−20 to 25 ps/nm/km), low attenuation (<1.4 dB/km), and small bending radius (∼8 mm) over the C-band. The designed PM-FM-RCF can be compatible with standard single-mode fibers and applied in multiple-input multiple-output-free spatial division multiplexing optical networks for short-reach optical interconnection.

Vertical-Cavity Surface-Emitting Lasers With Two Emission-Controllable Transverse Modes

We propose a vertical-cavity surface-emitting laser (VCSEL) with two transverse modes independently controlled by two contacts. By directly etching a trench in the p-doped distributed Bragg reflector, we form an air gap and two sub-mesas as waveguides above the oxide layer. Contacts are deposited on each sub-mesa. Emission control on two different spatial modes in a single VCSEL is achieved by changing the driving condition of one-contact or two-contact injection. Owing to this feature, the proposed VCSEL has a potential as a cost-effective source for optical communications using spatial-division multiplexing.

Spatial-division-multiplexing in time-variant and multi-path channel based on vector adaptive time reversal technique

In the field of multistatic remote detection for underwater target, as a time-variant and multi-path channel interfered by environmental noise, complex underwater acoustic environment limits the spatial share of channel and poses a great challenge against spatial-division-multiplexing (SDM) in multistatic detection. In this paper, a kind of method of spatial-division-multiplexing(SDM) based on vector adaptive time-reversal technique is proposed. By use of time-reversal technique, multi-path structure of channel is efficiently restrained; Meanwhile, with adaptive filtering applied, time-variant characteristic of channel is effectively suppressed. Moreover, with single vector hydrophone, spatial focus of target echoes and noise interference suppression is accomplished by spatial filtering. Finally, excellent results are performed in SDM of multistatic detection, supported by the experimental simulation.