Multi-in Multi-out Research Articles

A mm-Wave Frequency Modulated Transmitter Array for Superior Resolution in Angular Localization Supporting Low-Latency Joint Communication and Sensing

Joint sensing-and-communication is envisioned to play a major role in emerging mm-Wave wireless systems to perform rapid tracking of wireless nodes for reliable wireless communication and ubiquitous distributed sensing. Phased arrays are essential to realize high array gains and enhance the communication distance. However, their pencil sharp beams require precise localization and tracking of wireless nodes to maintain beam alignment and reliable communication, especially in mobile wireless links. In parallel, array-based mm-Wave radars require large arrays for precise angular resolution that is fundamentally limited by the form factor and power consumption of the radar systems. We propose a frequency modulated array (FMA) transmitter (Tx) to convert locations/positions to timing information, which supports dual-modes for joint sensing-and-communication: rapid and precise full field-of-view (FoV) receiver (Rx) localization in the Tx/Rx localization mode for wireless communication and target angular detection in its radar mode. Exploiting its unique joint space-time-frequency dependent signals, the FMA distinguishes spatial signatures far below its array 3-dB beamwidth, i.e., achieving super-resolution and can simultaneously localize multiple Rx nodes or detect multiple targets in “one-shot.” Moreover, FMA only adds minimal circuit/computation overhead on traditional multi-in multi-out (MIMO) arrays. A proof-of-concept 28-GHz four-element multi-mode FMA Tx array is implemented in a 45 nm CMOS silicon on insulator (SOI) process. Over-the-air (OTA) measurements with Rx nodes in the communication-mode or reflectors in radar-mode demonstrate the four-element FMA Tx array operations and achieve a full-FoV coverage with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&lt;$</tex-math> </inline-formula> 2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> –4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> accuracy in angular localization with a 10 ns timing resolution (i.e., 100 MHz digital clock in Rx). Sensing aided communication is demonstrated to create sharp and secure constellation decomposition array (CDA) 64 quadratic-amplitude modulation (QAM) beams toward target Rx at low rms error vector magnitude (EVM) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&lt;$</tex-math> </inline-formula> 5.33% using the sensed data from FMA based Rx localization scheme.

Time-Delay Estimation Based on Graph Global Smoothness

Time-delay is an essential factor affecting the performance of time series-related tasks such as time series forecasting, system modeling, etc. Therefore, the time-delay estimation (TDE) issue has attracted more and more attention. In this paper, a TDE method for the open-loop multi-in multi-out (MIMO) delay system with an unknown structure is proposed. In our approach, the theory of graph Laplacian is introduced to the TDE task for the first time. The correspondence between a delay system and a graph is established by constructing an N-linked graph from input-output samples of the delay system, and the global smoothness of the N-linked graph is considered as a time-delay metric. The TDE process of a delay system is then implemented by tracking the minimum of the global smoothness. The sparsity of the N-linked graph is further leveraged to effectively reduce the computational load of the TDE process. Simulation experiments and wind speed forecasting based on real data validate the effectiveness of the proposed method.

Demonstration of 100-Gbit/s 32-QAM signal transmission in a radio-over-fiber system with 2-bit DAC.

In this Letter, a low-cost radio-over-fiber (RoF) system at the Ka band based on a low-resolution digital-to-analog converter (DAC) is proposed and investigated. The noise shaping (NS) technique is adopted to suppress the in-band quantization noise induced by the low-resolution DAC. To evaluate the performance of the proposed RoF system, the transmission of a 80/100-Gbit/s dual-polarization 16/32-QAM signal over 20-km single-mode fiber (SMF) and 1-m 2 × 2 multi-in multi-out (MIMO) wireless link coupled with a 2/3/4-bit DAC is experimentally demonstrated. The results show that the bit error rate (BER) of the signal generated by the 2-bit DAC can be effectively reduced by more than one order of magnitude when noise shaping is applied.

Tracking of Evasive Objects Using Bistatic Doppler Radar Operating in the Millimeter Wave Regime

In this study, we propose a range detection (RD) ability by a continuous wave (CW) bistatic Doppler radar (RDCWB) of small and fast targets with very high range resolution. The target’s range and velocity are detected simultaneously. The scheme is based on the transmission of a continuous wave (CW) at millimeter wavelength (MMW) and the measurement of the respective Doppler shifts associated with target movements in different directions. The range resolution in this method is determined by the Doppler resolution only, without the necessity to transmit the modulated waveforms as in frequency modulation continuous wave (FMCW) or pulse radars. As the Doppler resolution in CW depends only on the time window required for processing, a very highrange resolution can be obtained. Most other systems that perform target localization use the transmission of wide-band waveforms while measuring the delay of the received signal scattered from the target. In the proposed scheme, the range resolution depends on the processed integration time of the detected signal and the velocity of the target. The transmission is performed from separated antennas and received by a single antenna. The received signal is heterodyned with a sample of the transmitted signal in order to obtain the Doppler shifts associated with the target’s movement. As in a multi-in multi-out (MIMO) configuration, the presented scheme allows for the accumulation of additional information for target classification. Data on the target’s velocity, distance, direction, and instantaneous velocity can be extracted. Using digital processing, with the additional information obtained by analyzing the difference between the resulting intermediate frequencies caused by the Doppler effect, it is possible to calculate the distance between the radar and the target at high resolution in real-time. The presented method, which was tested experimentally, proved to be highly effective, as only one receiver is required for the detection, while the transmission is carried out using a fixed, single-frequency transmission.

Frequency-Domain-Multiplexing Single-Wire Interface and Harmonic-Rejection-Based IF Data De-Multiplexing in Millimeter-Wave MIMO Arrays

Recently, mm-wave multi-in multi-out (MIMO) arrays are garnering significant attention because of their ability to form multiple spatial beams, achieving significantly higher data rates compared to traditional phased array-based approaches. Furthermore, scalable MIMO arrays can provide many other functionalities, such as full digital beamforming and per-power amplifier (PA) digital pre-distortion. In this work, an MIMO four-element TX array architecture is presented with a frequency-domain multiplexing-based single-wire interface (SWI), breaking the tradeoff between channel-to-channel isolation and single wire (SW) bandwidth (BW). The concept of harmonic-rejection mixing (HRM) is used to de-multiplex the four modulated signals simultaneously from the SW using a single passive mixer driven by a multi-phase local oscillator (LO). The two-stage harmonic recombination circuits are implemented in baseband and, hence, achieve high channel-to-channel isolation with a low power overhead. A 60-GHz four-element TX prototype demonstrates the proposed architecture in 45-nm RF silicon-on-insulator (RFSOI) CMOS. The frequency-division-multiplexed (FDM)-based SWI can support 8-GHz total intermediate frequency (IF) BW across the four channels with 30-40-dB channel-to-channel isolation. Each TX element in the array achieves 20-35-dB conversion gain and +8.8-10.9-dBm OP1dB while consuming 225 mW/element.

Cognitive Antenna Selection in MIMO Imaging Radar

A cognitive antenna selection strategy for multi-in multi-out (MIMO) imaging radar is proposed in this article. The basic idea of our strategy relies on the dynamic selection of the antenna location according to the feedback information to enhance the image quality and reduce the computational burden simultaneously. The aim of our strategy is to indirectly minimize the mean squared error associated with the amplitudes and positions of the strong scattering centers of the target through the frame potential. Specifically, it is assumed that the imaging process is initially performed via a conventional uniform linear array/random sparse array of collocated MIMO radar; hence, based on the collected data, an initial image of the target is derived (perception). Then, the accuracy of low-resolution images is enhanced progressively according to the cognitive paradigm via a specific antenna location selection at the next transmission (action). Benefit from the enhanced accuracy, the support area of targets can be estimated to reduce the dimension of the undersampling matrix and, finally, the computational burden in the subsequent high-resolution image reconstruction process is reduced. The simulation results highlight the capabilities of our cognitive approach to provide more interesting benefits in imaging than the random selection strategy and demonstrate the enhanced imaging performance of cognitive sparse MIMO array under the condition of limited antennas and noise.

Structured Clutter Covariance Matrix Estimation for Airborne MIMO Radar With Limited Training Data

This letter studies the estimation of structured clutter covariance matrix (CCM) for space-time adaptive processing (STAP)-based airborne multiin multiout (MIMO) radar with limited training data. The Kronecker-product-expansion structure of the CCM is considered, where each term involves two Kronecker product operators. By exploiting the low-rankness of two permutations of the CCM, we propose a novel estimator based on least squares penalized by two nuclear norms. The estimator is also extended by considering the linear structures of the CCM. We examine the structure of the proposed solution and demonstrate its superior performance through simulation studies.

Coherent Measurements With MIMO Radar Networks of Incoherent FMCW Sensor Nodes

This work introduces a method for coherent multi-in multi-out (MIMO) processing over a network of frequency-modulated continuous wave (FMCW) radar sensor nodes with independent signal synthesis. The coherency between the bistatic channels is established algorithmically based on the frequency and phase-offset estimation from pairs of sensor nodes and subsequent least-squares estimation on the overdetermined equation system. For the chirp-sequence radar, the proposed processing allows range and speed measurements, as offsets are estimated and corrected for each individual ramp. The applicability of the method is demonstrated in a prototypical setup of a radar network with three sensor nodes comprising integrated 122-GHz transceivers. The obtained bistatic phase precisions of <; 0.1 rad approximately match those of the monostatic measurements of the same radar nodes. For a sequence of 30 ramps of 1-ms duration, this translates to ambiguous range precisions <; 10 μm and speed precision <; 1 mm/s. The absence of wired high-frequency links between the nodes makes the approach well suited for shortrange radar networks in cost-sensitive applications.

Pattern-Reconfigurable Planar Array Antenna Characterized by Digital Coding Method

We propose a low-profile and high-gain pattern-reconfigurable planar array antenna (PRPAA) based on digital coding characterization. The radiating elements in the PRPAA are designed ingeniously to realize digital “0” and “1” elements. By dynamically encoding the digital radiating elements with different coding patterns, various radiation beams can be achieved and switched in real time based on one antenna aperture. More importantly, for different coding patterns, the number and direction of the radiation beams of the digital PRPAA can be well predicted. We fabricate such a PRPAA and validate its digital-controllable radiation patterns experimentally. This communication provides many opportunities to realize low-cost, lightweight, high-gain, and multi-unit PRPAA, which offers promising applications in massive multi-in multi-out (MIMO) antenna array system, smart antenna, radar system, and dynamic microwave imaging.

Massive MIMO With Massive Connectivity for Industrial Internet of Things

In this paper, we investigate the feasibility of using massive multi-in multi-out (MIMO) to provide connectivity to Industrial Internet of Things (IIoT) wirelessly. Single-cell massive MIMO is used to support a large number of low-power IIoT devices in the uplink simultaneously. To support a large number of devices simultaneously, orthogonal uplink reference signals are heavily reused, which severely compromises the quality of the channel estimation for all devices. We show that the attempt by zero-forcing (ZF) decoding to zero out the interference from a large number of devices results in very poor performance, whereas maximum ratio (MR) decoding outperforms ZF decoding by a large margin. We present theoretical closed-form equations of spectral efficiency for both MR and ZF processings with massive connectivity. Simulation results validate our theoretical analysis. Furthermore, we provide analyses that are useful for designing low-power massive IIoT networks using massive MIMO.

A 28-nm 75-fsrms Analog Fractional-$N$ Sampling PLL With a Highly Linear DTC Incorporating Background DTC Gain Calibration and Reference Clock Duty Cycle Correction

An analog fractional- $N$ sampling phase-locked loop (PLL) is presented. It achieves 75-fs rms jitter, integrated from 10 kHz to 10 MHz, and a −249.7-dB figure of merit (FoM) at the fractional- $N$ mode with a 52-MHz reference clock. The measured fractional spur is less than −64 dBc across the 5.5–7.3-GHz output frequency band. The PLL employs digital-to-time converter (DTC)-based sampling PLL architecture, high linearity DTC design techniques, background DTC gain calibration, and reference clock duty cycle correction (DCC) to improve the integrated phase noise (IPN) and fractional spur. This design meets the performance requirement of the 5G cellular 64-quadratic-amplitude modulation (QAM) standard in the 28-/39-GHz band, supporting $2 \times 2$ multi-in multi-out (MIMO). This paper, implemented in a 28-nm CMOS process, is integrated in a 5G millimeter-wave cellular transceiver. This PLL consumes 18.9 mW and occupies 0.45 mm2.

Modeling Multi-User WLANs Under Closed-Loop Traffic

In this paper, we present the first cross-layer analysis of wireless LANs operating under downlink multi-user multi-in multi-out (MU-MIMO), considering the fundamental role played by the closed-loop (TCP) traffic. In particular, we consider a scenario in which the access point transmits on the downlink via MU-MIMO, whereas stations must employ single-user transmissions on the uplink, as is the case in IEEE 802.11ac. With the help of analytical models built for different regimes that can occur in the considered system, we identify and explain crucial performance anomalies that can result in very low throughput in some scenarios, completely offsetting the theoretical gains achievable by MU-MIMO. We discuss solutions to mitigate the risk of this performance degradation and alternative uplink strategies allowing WLANs to approach their maximum theoretical capacity under MU-MIMO.

Transceivers and Spectrum Usage Minimization in Few-Mode Optical Networks

Metro-area networks are likely to create the right conditions for the deployment of few-mode transmission (FMT) due to limited metro distances and rapidly increasing metro traffic. To address the new network design problems arising with the adoption of FMT, integer linear programming (ILP) formulations have already been developed to optimally assign modulation formats, baud rates, and transmission modes to lightpaths, but these formulations lack scalability, especially when they incorporate accurate constraints to capture inter-modal coupling. In this paper, we propose a heuristic approach for the routing, modulation format, baud rate and spectrum allocation in FMT networks with arbitrary topology, accounting for inter-modal coupling and for distance-adaptive reaches of few-mode (specifically, up to five modes) signals generated by either full multi-in multi-out (MIMO) or low-complexity MIMO transceivers and for two different switching scenarios (i.e., spatial full-joint and fractional-joint switching). In our illustrative numerical analysis, we first confirm the quasi-optimality of our heuristic by comparing it to the optimal ILP solutions, and then we use our heuristic to identify which switching scenario and FMT transceiver technology minimize spectrum occupation and transceiver costs, depending on the relative costs of transceiver equipment and dark fiber leasing.

Dynamics of Antenna Reactive Energy Using Time-Domain IDM Method

A general computational paradigm, the time-domain infinitesimal dipole model (IDM) method, is developed and applied to compute reactive energy and radiated power for arbitrary shaped antennas with different time-domain excitations. The proposed technique first utilizes finite-difference time-domain method to compute the time-domain radiating antenna currents from near-zone electric/magnetic fields. Further, the far-zone time-domain radiated power is analytically estimated using a suitable IDM of the antenna spacetime current distributions. The time-domain reactive energy is calculated by adopting a rigorous energy subtraction approach. The proposed time-domain IDM technique is first validated by reproducing the standard antenna-Q-factor results of half-wavelength thin-wire dipoles. Second, time-domain reactive energy signatures are obtained for pulse-excited single and multiband dipole antennas, both in isolated condition and two-element multi-in multi-out (MIMO) topology. Third, the effects of parasitic reflector and director elements on the spatio-temporal energy dynamics of thin-wire Yagi-Uda antennas are demonstrated. The proposed algorithm is very general, applicable to arbitrary antenna shape and spacetime excitations, and hence is expected to enable engineers to look beyond traditional single frequency Q-factors of electrically small antennas to probe into the more fundamental spatio-temporal energy dynamics of general radiating structures, especially future generation 5G MIMO antennas.

Optimum Wideband High Gain Analog Beamforming Network for 5G Applications

A broadband high-gain millimeter-wave (mmWave) array beamforming network (BFN) design, analysis, and implementation based on the Rotman lens antenna array feeding are presented. The BFN is intended for operation in the (26-40) GHz frequency band for a wide range of potential applications in the fifth generation (5G). The system is made on Rogers substrate, RO6010, to provide compatibility with standard planar low-cost processing techniques for millimeter-wave monolithic integrated circuit (MMIC). The measured results show the system capability of 80° beam scanning for different angles at -39.7°, -26.5°, -13.3°, 0°, +13.3°, +26.5°, and +39.5° at 28 GHz. With these features in addition to being compact size, low profile, and lightweight, this BFN is suitable for various millimeter-wave and 5G applications such as the advanced multi-in multi-out (MIMO) systems, remote sensing, and automotive radar.

Fog Massive MIMO: A User-Centric Seamless Hot-Spot Architecture

The decoupling of data and control planes in the forthcoming 5G wireless networks enables the efficient implementation of multitier architectures, where coverage and connectivity are guaranteed by top-tier macro-cells, and at the same time, high throughput and low latency are achieved locally by lower tiers in the hierarchy. This paper considers a new architecture for such lower tiers, dubbed fog massive multi-in multi-out (MIMO), where the user equipment (UE) nodes can connect to a “fog” of the densely deployed remote radio heads (RRHs) in a seamless, user-centric, and opportunistic manner. In the case of dense small cells, traditional cellular architectures inherently give rise to frequent handovers and pilot sequence re-assignments, which typically incur excessive protocol overhead and latency. In the proposed fog massive MIMO architecture, the UEs implicitly associate themselves with the most convenient RRHs in a completely autonomous manner. Each UE makes use of the uplink pilot sequences that contain equal-weight codewords and enable a novel “on-the-fly” pilot contamination control mechanism. We analyze the spectral efficiency and the outage probability of the proposed architecture via stochastic geometry, using some recent results on unique coverage in Boolean models, and provide a detailed comparison with the benchmark represented by massive MIMO cellular system with a genie-aided minimum-distance user-cell association. Our analysis, corroborated by extensive system simulation, reveals that there exists a “sweet spot” of the per pilot user load (number of users per pilot), such that the proposed system achieves a spectral efficiency close to that of the genie-aided cellular system. In these conditions, it is possible to achieve the low protocol overhead and low user RRH association latency promised by the fog massive MIMO at virtually no significant performance cost in terms of system spectral efficiency with respect to the cellular benchmark.

Closed-Form Expressions for Channel Shortening Receivers Using $A\ Priori$ Information

Channel shortening has been studied in the context of intersymbol interference and multi-in multi-out channels as a means to compute a posteriori probabilities with a BCJR algorithm at a reduced computational complexity. This is done by considering an approximate channel response of reduced length. In a turbo receiver, soft a priori information can be linearly combined with the received sequence to form a new input to the BCJR trellis-based processing. In this letter, we provide closed-form expressions for the channel shortening filters using a generalized mutual information objective function. The proposed receiver allows a complexity reduction with respect to numerical optimization approaches which may also present stability, precision, and convergence issues.

Downlink Beamforming for Energy-Efficient Heterogeneous Networks With Massive MIMO and Small Cells

A heterogeneous network (HetNet) of a macrocell base station equipped with a large-scale massive multi-in multi-out (MIMO) antenna array overlaying a number of small cell base stations (small cells) can provide high quality of service (QoS) to multiple users under low transmit power budget. However, the circuit power for operating such a network, which is proportional to the number of transmit antennas, poses a problem in terms of its energy efficiency (EE). This paper addresses the beamforming design at the base stations to optimize the network EE under the QoS constraints and a transmit power budget. Beamforming tailored for weak, strong, and medium cross-tier interference HetNets is proposed. In contrast to the conventional transmit strategy for power efficiency in meeting the users’ QoS requirements, which suggest the use of a few hundred antennas, it is found out that the overall network EE quickly drops if this number exceeds 50. It is found that, for a given number of antennas, HetNet is more energy efficient than massive MIMO when considering the overall energy consumption.

Low-Cost Multimode Patch Antenna for Dual MIMO and Enhanced Localization Use

This communication proposes a simple, low-cost multimode patch antenna combining good multi-in multi-out (MIMO) performance with precise angle of arrival (AoA) estimation. The AoA is based on the monopulse antenna concept; however, unlike in radar applications, the necessity for complex circuitry is replaced by the intrinsic properties of even and odd resonant patch modes. This capability is advantageous for future “Internet of Things” antennas, embedded into low-cost and size-constrained devices. The envelope correlation coefficient, measured in an anechoic chamber, is below 1.5%, ensuring good MIMO performance. An exemplary addition to localization algorithm exploiting antenna properties is demonstrated.

High-Frequency Acoustic Estimation of Time-Varying Underwater Sparse Channels Using Multiple Sources and Receivers Operated Simultaneously

In this paper, the authors propose a robust underwater acoustic field estimation of the time-varying channel impulse response for simultaneous transmissions using multiple sources and multiple receivers [multi-in multi-out (MIMO)] that closely follows the rapidly time-varying nature of the underwater acoustic channel. The presented algorithm outlines a time-varying underwater channel estimation method based on the channel sparsity characteristic. The method uses the MIMO P-iterative greedy orthogonal matching pursuit algorithm and takes advantage of the sparsity of the underwater acoustic channel. This technique is compared to a trended least square estimation method presented by the authors in a previous publication. Both techniques are demonstrated in a fully controlled environment, using simulated and experimental data, and the results in each case show a significant improvement in the estimation of the channel impulse response.