Space-division Multiplexing Systems Research Articles

Mode Coupling Effects in Ring-Core Fibers for Space-Division Multiplexing Systems

An optical fiber with weak mode coupling is desirable for future ultrahigh capacity space-division multiplexing (SDM) systems because mode coupling in an optical fiber results in extrinsic loss of the fiber and crosstalk between guided optical modes. To study the feasibility of a ring-core fiber (RCF) for SDM systems, in this paper, we investigate the mode coupling in the RCF supporting five or seven guided mode groups (MGs) at a wavelength of 1550 nm. For this purpose, the coupled mode/power theory with identified spatial power spectrum of random perturbations of fiber axis is used to estimate the bend loss/crosstalk of the RCF due to microbending. It is shown that based on the identified parameters for the spatial power spectrum in the 5/7-MG RCF, the estimated bend loss/crosstalk of the RCF agrees well with experimental results. In addition, the impact of the gradient parameter α and refractive index contrast Δ of the fiber refractive index profile on bend loss and crosstalk of the RCF is explored. Simulation results indicate that the Δ instead of the α significantly affects bend loss and crosstalk of the RCF. The magnitude improvement in bend loss by increasing the Δ is dependent on the spatial power spectrum.

Low complexity MIMO method based on matrix transformation for few-mode multi-core optical transmission system

This paper proposes and demonstrates a low complexity multiple-input multiple-output (MIMO) equalization digital signal processing (DSP) method for the few mode multi-core (FMMC) fiber optical transmission system. The MIMO equalization algorithm offers adaptive equalization taps according to the degree of crosstalk in cores or modes, which eliminates the interference among different modes and cores in space division multiplexing (SDM) transmission system. Compared with traditional MIMO method, the proposed scheme has increased the convergence rate by 4 times and reduced the number of finite impulse response (FIR) filters by 55% when the numbers of mode and core are three.

A multi-orbital-angular-momentum multi-ring micro-structured fiber with ultra-high-density and low-level crosstalk

In this paper, a novel multi-orbital-angular-momentum multi-ring micro-structured fiber (MOMRMF) is proposed for the first time. This compact MOMRMF is presented with 19 rings, each ring supporting 36 modes (34 of them are OAM ones i.e., 684 channels in total) over the whole C wavelength band (1520–1580nm). Due to the large refractive index difference between silica and air, the effective refractive index (neff) differences between the adjacent eigenmodes are all larger than 10−3, which is practically impossible to gain in other traditional high-density OAM fibers fabricated by modified chemical vapor deposition (MCVD) method. Compared with the trench-assisted multi-OAM multi-ring fiber (TA-MOMRF), the lower-level inter-ring crosstalk in the designed MOMRMF also can be achieved by the air hole-assisted. It is believed that both the high-density and low-level crosstalk make such a fiber of great potential in the space-division multiplexing (SDM) system.

Fisheye-lens-based space division multiplexing system for visible light communications

This paper presents the high-diversity space division multiplexing (SDM) visible light communications (VLCs) utilizing a fisheye-lens-based imaging receiver. The receiver features ultra-wide field-of-view, good illumination uniformity, ultra-high imaging quality, and compact size; therefore, it can realize omnidirectional receiving and provide high spatial diversity for the SDM VLC system. In the presented system, four data streams are parallelly transmitted by commercial phosphorescent white light-emitting diodes, and discrete multitone signals based on quadrature amplitude modulation and bit loading are employed to achieve high spectral efficiency. The experiment verifies that the interchannel interference is effectively alleviated due to the high spatial diversity provided by the fisheye-lens-based receiver, and an aggregate data rate of 1.3 Gbit/s is achieved with the limited 3-dB bandwidth of 20 MHz. Besides, it is feasible to extend the system to more channels to achieve higher capacity. The experimental results indicate that the fisheye-lens-based imaging receiver is a potential candidate for high-speed VLC applications.

Investigation of nonlinear effects in few-mode fibers

In this work, nonlinear optical effects are exploited for future implementations of space-division multiplexing fiber systems. The paper first presents the fundamentals of intermodal nonlinear phenomena over few-mode fibers, such as cross-phase modulation, four-wave mixing (FWM), stimulated Brillouin scattering and stimulated Raman scattering. Second, the potential applications of few-mode nonlinear effects are discussed for sensing and optical signal processing. We demonstrated how fiber mode symmetries and linear mode coupling affect intermodal power transfer and spectral broadening. Lastly, the paper proposes a ultrafast all-optical simultaneous wavelength and mode conversion scheme based on intermodal FWM, with the capability of switching state of polarization and mode degeneracy orientation. Under this scheme, cross-polarization modulation and cross-mode modulation can be achieved.

BER Performance of MDL-Impaired MIMO-SDM Systems With Finite Constellation Inputs

We evaluate the effect of mode-dependent loss (MDL) on the bit-error-ratio (BER) performance of optical multiple-input-multiple-output space-division multiplexing (MIMO-SDM) systems using finite signal constellations and maximum likelihood detection. Through analyses and numerical simulations, we study the signal-to-noise-ratio penalty incurred by MDL as a function of various system parameters.

Breakthroughs in Photonics 2012: Space-Division Multiplexing in Multimode and Multicore Fibers for High-Capacity Optical Communication

We summarize the latest advances in space-division multiplexing (SDM) for increasing the capacity per fiber strand. We discuss SDM fibers, recent SDM transmission experiments, subsystems suitable for SDM systems, as well as integrated components able to manipulate multiple spatial modes within a single optical device.

Geometric requirements for photonic lanterns in space division multiplexing

We investigate the use of "photonic lanterns" as adiabatic mode converters for space-division multiplexing (SDM) systems to interface multiple single-mode fibers to a multi-mode fiber. In a SDM system, minimizing the coupling loss and mode-dependent loss best utilizes all spatial modes of the fiber which increases the capacity, the transmission distance, and minimizes the outage probability. We use modal analysis, the beam propagation method, and a transfer matrix technique to analyze the lanterns throughput along with its mode dependent loss and show that unitary coupling between single-mode fibers and a multi-mode fiber is only possible by optimizing the arrangements of the cores. Results include simulations for three, 12, 15, and 51 core lanterns to couple to six, 24, 30, and 102 spatial and polarization modes, respectively.

Fiber LPG Mode Converters and Mode Selection Technique for Multimode SDM

The use of multimode fibers in mode division multiplexed space-division multiplexing systems offers one solution to the capacity limitations of single-mode fiber transmission. Passive components to control the individual modes in few-mode fiber (FMF) are key elements to build more complex modules and components necessary for a high performance system. Fiber-based mode converters are important elements in the FMF mux/demux, and long-period gratings have been investigated to provide mode conversion in two- and four-mode fibers. A method to separate and monitor the modes in real time is described as the basis of a method to measure the individual modal performance of a component during fabrication.

MIMO capacities and outage probabilities in spatially multiplexed optical transport systems

With wavelength-division multiplexing (WDM) rapidly nearing its scalability limits, space-division multiplexing (SDM) seems the only option to further scale the capacity of optical transport networks. In order for SDM systems to continue the WDM trend of reducing energy and cost per bit with system capacity, integration will be key to SDM. Since integration is likely to introduce non-negligible crosstalk between multiple parallel transmission paths, multiple-input multiple output (MIMO) signal processing techniques will have to be used. In this paper, we discuss MIMO capacities in optical SDM systems, including related outage considerations which are an important part in the design of such systems. In order to achieve the low-outage standards required for optical transport networks, SDM transponders should be capable of individually addressing, and preferably MIMO processing all modes supported by the optical SDM waveguide. We then discuss the effect of distributed optical noise in MIMO SDM systems and focus on the impact of mode-dependent loss (MDL) on system capacity and system outage. Through extensive numerical simulations, we extract scaling rules for mode-average and mode-dependent loss and show that MIMO SDM systems composed of up to 128 segments and supporting up to 128 modes can tolerate up to 1 dB of per-segment MDL at 90% of the system's full capacity at an outage probability of 10(-4).

MIMO MC-CDMA系統中基於選擇性映射(SLM)降低峰均功率比

本研究提出了將選擇影射法應用於空域複用MIMOMC-CDMA系統降低峰均功率比。同時針對傳統的選擇影射法需傳輸的邊帶資訊大的缺陷，提出將一種改進的SLM技術應用於系統中的方法。並對應用SLM前後系統發射端和接收端的性能進行了理論上的分析和仿真比較。理論分析和仿真結果表明：在發射端，應用SLM後降低了系統的峰均功率比，並且隨著向量個數的增加峰均功率比性能會進一步改善；在接收端，SLM對系統的BER有一定的影響，而改進的SLM對系統的BER影響極小。A method of peak-to-average power ratio (PAPR) reduction in the space division multiplexing MIMO MCCDMA system using Selected Mapping (SLM) technique is presented in this paper. Moreover, in order to reduce number of side information, a novel improvement SLM approach on the system is also presented in this paper. A comparison of the difference of the transmitter and receiver performance between with SLM and without SLM in terms of theory and simulation indicates that the average PAPR can be lowered with the novel improvement SLM approach besides with the increase in the number of vector PAR the performance will be further improved in the transmitter; and the influence to a certain degree of BER is less than without the novel improvement SLM approach in the receiver.

A Lattice-Reduction-Aided Soft Demapper for High-Rate Coded MIMO-OFDM Systems

We extend the lattice-reduction (LR) method to MIMO detection for coded space division multiplexing OFDM systems with soft-input decoding. The scheme employs a novel combination of LR, K-best candidate generation, and simple candidate pruning. With a handful of candidates, the receiver is able to approach packet error rate performance of full maximum likelihood detector to within 1 dB, over varying data rates and channel correlations. It is appealing for high-rate coded-MIMO systems operating over quasi-static wireless channels, where the infrequent LR computation amortized over many symbols becomes negligible

Iterative Power Allocation Scheme for MIMO Systems

In closed-loop multiple-input and multiple-output space-division multiplexing (MIMO-SDM) systems, allocating power among multiple transmit data streams improves the channel capacity. However, the optimum power allocation values are not always available in closed-form. For instance, when we use transmission and reception schemes that do not transfer the MIMO channel into parallel orthogonal channels (e.g., eigen-mode SDM), the signal to interference plus noise ratio (SINR) of each data stream at the output of the receiver is not proportional to its corresponding transmit power. This feature makes it difficult to obtain the optimal closed-form power allocation value for each data stream. Thus, in this paper, we propose an iterative power.allocation scheme for MIMO-SDM systems where the SINK is not proportional to the transmit power. Furthermore, we incorporate a transmit antenna selection scheme into the proposed power allocation scheme in order to attain further capacity enhancement. Computer simulation results are provided to show the effectiveness of the proposed power allocation schemes.

A Simplified Maximum Likelihood Detector for OFDM-SDM Systems in Wireless LAN

This paper presents a reduced-complexity maximum likelihood detection (MLD) scheme for orthogonal frequency division multiplexing with space division multiplexing (OFDM-SDM) systems. Original MLD is known to be an optimal scheme for detecting the spatially multiplexed signals. However, MLD suffers from an exponentially computational complexity because it involves an exhaustive search for the optimal result. In this paper, we propose a novel detection scheme, which drastically reduce the complexity of MLD while keeping performance losses small. The proposed scheme decouples the spatially multiplexed signals in two stages. In stage one, the estimated symbols obtained from zero-forcing (ZF) are used to limit the candidate symbol vectors. In stage two, to form a final estimate of the transmitted symbol vector, the Euclidean or original defined likelihood metric is examined over all symbol vectors obtained from stage 1. Both the bit error rate (BER) and packet error rate (PER) performances are evaluated over a temporally and spatially uncorrelated frequency selective channel through the computer simulations. For a four-transmit and four-receive OFDM-SDM system transmitting data at 144 Mbit/s and 216 Mbit/ss i.e., employing 16 Quadrature Amplitude Modulation (16QAM) and 64QAM subcarrier modulation over 16.6 MHz bandwidth channel, the degradation in required SNR from MLD for PER = 1% are about 0.6 dB and 1.5 dB, respectively. However, the complexity of MLD is reduced to 0.51000% and 0.01562%.

A Reduced-Complexity Signal Detection Scheme Employing ZF and K-Best Algorithms for OFDM/SDM

This paper proposes a reduced-complexity signal detection scheme for Orthogonal Frequency Division Multiplexing with Space Division Multiplexing (OFDM/SDM) systems that utilize Zero-Forcing (ZF) and K-best algorithms. It is known that Maximum Likelihood Detection (MLD) with exhaustive search achieves mathematically optimal performance for SDM signal detection. However, it also suffers from exponential computational complexity against the number of transmit antennas and modulation order. In order to reduce the computational complexity of MLD, we apply the K-best algorithm for signal detection. It is known that the K-best algorithm itself inherently reduces the computational complexity of MLD because it avoids exhaustive search. In this paper, we propose the modified K-best algorithm, which exploits the ZF algorithm for initial symbol estimation. This initial symbol estimation improves the decoding accuracy of the original K-best algorithm. We evaluate the performance of the proposed scheme through computer simulations. The computer simulation results show that the performance degradation from the MLD algorithm is suppressed to just 1 dB or so in terms of the required E b /N 0 for packet error rate (PER) = 10 -2 , When either 16 Quadrature Amplitude Modulation (16QAM) or 64QAM is applied with three transmit and three receive antennas. In these cases, 87% and 99% fewer metric computations are required than the MLD algorithm. It is confirmed that the proposed MLD algorithm offers a significant reduction in the computational complexity from the MLD algorithm while suppressing the performance degradation.

Peak-to-average power reduction in space division multiplexing based OFDM systems through spatial shifting

A new method to reduce the peak-to-average power ratio (PAPR) in space division multiplexing systems applying orthogonal frequency division multiplexing is proposed. The method applies spatial shifting to partial transmit sequences to achieve a decreased PAPR on all transmit branches.

A joint-channel diagonalization for multiuser MIMO antenna systems

In this paper, we address the problem of improving the performance of multiuser space-division multiplexing (SDM) systems where multiple independent signal streams can be transmitted in the same frequency and time slot. The problem is important in multiuser multiple-input multiple-output systems where communication from one base station to many mobile stations can occur simultaneously. Our objective is to devise a multiuser linear space-time precoder for simultaneous channel diagonalization of the multiuser channels enabling SDM. Our new approach is based on diagonalizing the multiuser channel matrices and we use a variation of successive Jacobi rotations. In addition to the diagonalization, our approach attempts to optimize the resultant channel gains for performance enhancement. Our method is valid for both frequency-flat and frequency-selective fading channels but we assume that the base station knows all the channels and that they are quasi-stationary.

Performance enhancement of multiuser MIMO wireless communication systems

This paper describes a new approach to the problem of enhancing the performance of a multiuser multiple-input-multiple-output (MIMO) system for communication from one base station to many mobile stations in both frequency-flat and frequency-selective fading channels. This problem arises in space-division multiplexing systems with multiple users where many independent signal streams can be transmitted in the same frequency and time slot through the exploitation of multiple antennas at both the base and mobile stations. Our new approach is based on maximizing a lower bound for the product of signal-to-interference plus noise ratio (SINR) of a multiuser MIMO system. This provides a closed-form (noniterative) solution for the antenna weights for all the users, under the constraint of fixed transmit power. Our solution is shown by simulation to have better performance than previously proposed iterative or noniterative solutions. In addition, our solution requires significantly reduced complexity over a gradient search-based method that directly optimizes the product SINRs while still maintaining similar performance. Our solution assumes channel state information is present at the base station or transmitter.