Real-time Multiple Input Multiple Output Research Articles

Efficient physics-based learned reconstruction methods for real-time 3D near-field MIMO radar imaging

Near-field multiple-input multiple-output (MIMO) radar imaging systems have recently gained significant attention. These systems generally reconstruct the three-dimensional (3D) complex-valued reflectivity distribution of the scene using sparse measurements. Consequently, imaging quality highly relies on the image reconstruction approach. Existing analytical reconstruction approaches suffer from either high computational cost or low image quality. In this paper, we develop novel non-iterative deep learning-based reconstruction methods for real-time near-field MIMO imaging. The goal is to achieve high image quality with low computational cost at compressive settings. The developed approaches have two stages. In the first approach, physics-based initial stage performs adjoint operation to back-project the measurements to the image-space, and deep neural network (DNN)-based second stage converts the 3D backprojected measurements to a magnitude-only reflectivity image. Since scene reflectivities often have random phase, DNN processes directly the magnitude of the adjoint result. As DNN, 3D U-Net is used to jointly exploit range and cross-range correlations. To comparatively evaluate the significance of exploiting physics in a learning-based approach, two additional approaches that replace the physics-based first stage with fully connected layers are also developed as purely learning-based methods. The performance is also analyzed by changing the DNN architecture for the second stage to include complex-valued processing (instead of magnitude-only processing), 2D convolution kernels (instead of 3D), and ResNet architecture (instead of U-Net). Moreover, we develop a synthesizer to generate large-scale dataset for training the neural networks with 3D extended targets. We illustrate the performance through experimental data and extensive simulations. The results show the effectiveness of the developed physics-based learned reconstruction approach compared to commonly used approaches in terms of both runtime and image quality at highly compressive settings. Our source codes and dataset are made available at https://github.com/METU-SPACE-Lab/Efficient-Learned-3D-Near-Field-MIMO-Imaging upon publication to advance research in this field.

Long-Haul Coupled 4-Core Fiber Transmission Over 7,200 Km With Real-Time MIMO DSP

In this work, we present an experimental demonstration of real-time transoceanic space-division multiplexed transmission with coupled-core multicore fibers. To compensate for modal coupling in the coupled-core multicore fibers, we implement real-time multiple-input multiple-output (MIMO) digital signal processing based on field programmable gate array circuits. Using optical receivers with a real-time MIMO, we demonstrate 16-channel wavelength-division multiplexed coupled four-core fiber transmission over 7200 km. The results show the feasibility of real-time coupled-core multicore fiber transmission.

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.

Inside view

Researchers at PoliCom, Politecnico di Milano have presented the first experimental proof of an interferometric orbital angular momentum mode multiplexer/demultiplexer, exploiting a very compact configuration based on cylindrical lenses. 20-Gbit/s error-free transmission demonstrates the effectiveness of the proposed energy-saving all-optical passive demultiplexing technique, which is very promising for increasing the capacity in short-range optical links for data communications, where the power consumption is a fundamental issue. The research group, PoliCom, Politecnico di Milano in Italy. Paolo Martelli, second from the right, told us about their research into multiplexing and beyond We spoke to Paolo Martelli, one of the authors of the research, to find out more about how light can carry angular momentum and how this can be used in communication. In 1992, the seminal paper of Allen et al. recognised for the first time that light can carry orbital angular momentum (OAM), which they demonstrated in the well-known Laguerre-Gaussian laser beams. More generally, a monochromatic light beam carries an OAM equal to lh/(2π) per photon when the spatial distribution of the field in the plane orthogonal to the propagation axis is characterised by an azimuthal phase term exp(ilθ), where θ is the azimuthal coordinate and l is the azimuthal index, which can assume any integer value. This phase term gives a complex phase structure to the beams carrying OAM, whose phase fronts consists in l intertwined helices. The integer l can be also seen as the topological charge of an “optical vortex” and gives the number of 2π phase variations that occur in one revolution of the azimuthal angle θ, as shown by the transverse phase patterns of the lower image. The possibility of generating and manipulating beams of light with different values of OAM opens new exciting possibilities in optics. The orbital angular momentum can be seen as a further physical observable held by the photon, which adds to the energy (related to the wavelength or frequency), the linear momentum (related to the propagation wave vector) and the spin angular momentum (related to the state of polarisation). One OAM application, of great relevance for optical communications, is that OAM represents a new dimension in multiplexing, in addition to the already deployed time, wavelength and polarisation. In fact, the orthogonality of OAM modes suggests the possibility of simultaneous transmission from different users, each on a separate channel. A possible approach for counteracting crosstalk in these systems is multiple-input multiple-output (MIMO) digital signal processing, which has already been exploited in polarisation-division multiplexing and in mode-division multiplexing (MDM) with linearly-polarised fibre-optic modes. However, real-time MIMO processing requires high-speed electronic circuits and should be avoided in applications where power consumption is a relevant issue, such as optical access networks, optical interconnects and data communication between servers. The interferometric scheme described in our work opens the immediate development of a passive, all-optical multiplexer/demultiplexer characterised by high compactness and modularity. In particular, our experiments represent a preliminary proof of principle in bulk optics, demonstrating the effectiveness of the proposed approach for OAM mode multiplexing/demultiplexing. The azimuthal phase terms of the first four orbital angular momentum modes – a unique signature We will follow this proof of concept with a micro-optic realisation of our interferometer with pairs of cylindrical lenses, where Fresnel lenses with short focal length, together with miniaturised prisms and beam splitters, will allow a very compact design. Such a component can be applied in very high capacity transmission systems, both for datacom and telecom applications. From this perspective, our research team will continue investigating the advantages and capabilities of the proposed optical solution, as alternative to different MDM approaches based on ultra-fast digital signal processing. We believe that a significant part of the technological development in the next-decade optical communications will be focused on the realisation of components for MDM, as mode multiplexer/demultiplexers, and for the management of network functionalities based on modes. In particular, the exploitation of OAM modes without MIMO could be an advantageous solution, in terms of cost and power consumption, for increasing the capacity and implementing network functionalities in data communications inter and intra data centres. On the other hand, optical transport would be ever more focused on SDM solutions able to cope with the cross-talk between multiplexed channels, as the exploitation of either multimode fibres with high-complexity MIMO or multicore fibres, where the limited overlap between fields propagating in different cores reduces the cross-talk with respect to multimode fibres.

Design Tradeoffs and Hardware Architecture for Real-Time Iterative MIMO Detection using Sphere Decoding and LDPC Coding

We explore the performance and hardware complexity tradeoffs associated with performing iterative multiple- input multiple-output (MIMO) detection using a sphere decoder and a low-density parity-check (LDPC) decoder. Iterations are performed both within the LDPC decoder as well as via an outer iteration loop through which refined soft information is fed back from the LDPC decoder to a MIMO detector. A hardware architecture and associated implementation results on Xilinx Virtex-5 field programmable gate array for a 4 x 4 QPSK MIMO system are presented. The system offers a performance improvement of approximately 1 dB over systems without the outer iteration loop, and provides an information bit throughput that ranges from 60 to 300 megabits per second when a length 1944 rate 1/2 LDPC code is used.

Prototype Implementation of Real-Time ML Detectors for Spatial Multiplexing Transmission

SUMMARY We developed two types of practical maximum-likelihooddetectors (MLD) for multiple-input multiple-output (MIMO) systems, us-ing a field programmable gate array (FPGA) device. For implementations,we introduced two simplified metrics called a Manhattan metric and a cor-relation metric. Using the Manhattan metric, the detector needs no multi-plication operations, at the cost of a slight performance degradation within1dB. Using the correlation metric, the MIMO-MLD can significantly re-duce the complexity in both multiplications and additions without any per-formance degradation. This paper demonstrates the bit-error-rate perfor-mance of these MLD prototypes at a 1Gbps-order real-time processingspeed, through the use of an all-digital baseband 4 ×4 MIMO testbed inte-grated on the same FPGA chip. key words: multiple-input multiple-output (MIMO), spatial multiplex-ing, maximum-likelihood detection (MLD), field programmable gate array(FPGA), real-time MIMO detector 1. Introduction Recently, wireless communications systems that exploitmultiple transmitting and receiving antennas have receiveda significant amount of attention. These multiple-inputmultiple-output (MIMO) systems are expected to achievea dramatic increase in spectral efficiency because of theantenna diversity in scattering-rich wireless environments.Therefore, the MIMO system has been one of currentlypromising techniques to realizeGbps-class high-speed wire-less transmission for future communications systems [1].Many researchers have extensively investigated severaltechniques for multiple-antenna systems, such as the BellLaboratories layered space-time architecture (BLAST) [2]and space-time coding (STC) schemes [3].In MIMO spatial multiplexing systems, maximum-likelihood detection (MLD) schemes offer excellent perfor-mance [4]. Since the receiver estimates the most-likely sig-nals out of all the possible transmitting signals, the compu-tational complexity of distance metric calculations becomesextremely high in general. Consequently, many researchershave focused on developing more computationally efficientschemes. As a kind of low-complexity detectors, we canuse spatial filters based on the minimum mean-square error

Measurement-Based Performance Evaluation of Advanced MIMO Transceiver Designs

This paper describes the methodology and the results of performance investigations on a multiple-input multiple-output (MIMO) transceiver scheme for frequency-selective radio channels. The method relies on offline simulations and employs real-time MIMO channel sounder measurement data to ensure a realistic channel modeling. Thus it can be classified in between the performance evaluation using some predefined channel models and the evaluation of a prototype hardware in field experiments. New aspects for the simulation setup are discussed, which are frequently ignored when using simpler model-based evaluations. Example simulations are provided for an iterative ("turbo") MIMO equalizer concept. The dependency of the achievable bit error rate performance on the propagation characteristics and on the variation in some system design parameters is shown, whereas the antenna constellation is of particular concern for MIMO systems. Although in many of the considered constellations turbo MIMO equalization appears feasible in real field scenarios, there exist cases with poor performance as well, indicating that in practical applications link adaptation of the transmitter and receiver processing to the environment is necessary.