MIMO Links Research Articles

Exploiting Successive Interference Cancellation for Spectrum Sharing Over Unlicensed Bands

Harmonious coexistence among different unlicensed wireless technologies has became increasingly important due to the spectrum shortage problem. As a representative case, we focus on addressing LTE-LAA and WiFi coexistence in unlicensed bands. Traditionally, collision avoidance-based medium access control (MAC) protocols adopted by both LAA and WiFi have led to low channel utilization and fairness. In this paper, we explore physical-layer interference suppression techniques (e.g., successive interference cancellation, SIC) to enhance the spectrum utilization of coexisting LAA and WiFi networks. We propose a SIC-aware MAC protocol that embraces concurrent transmissions and optimizes the channel access strategy at the MAC layer, so as to mitigate the interference (same-technology or cross-technology) due to excess channel contentions. We theoretically analyze the network throughput by extending Bianchi's Markov model, considering the impact of concurrent transmissions and SIC. We also extend the analysis to consider MIMO and MU-MIMO links with SIC. We validate our theoretical analysis and the effectiveness of the proposed MAC protocol via extensive simulations. We also implement a prototype LAA/WiFi SIC receiver on USRP devices to demonstrate the feasibility of cross-technology SIC and the proposed MAC protocol.

Space-time domain equalization algorithm based on complex-valued neural network in a long-haul photonic-aided MIMO THz system.

The urgent demand for high-bandwidth wireless services in enhanced mobile broadband networks needs innovative solutions for mobile front-haul systems. The terahertz (THz) band offers a promising candidate for ultrahigh-capacity data transmission. This study investigates the integration of photonics-aided THz signal generation with MIMO and PDM technologies. We proposed a novel, to the best of our knowledge, space-time domain equalization algorithm based on MIMO-complex-valued neural networks (CVNN), which can preserve the signal phase and the relation between the X- and Y-polarization. We experimentally demonstrate the transmission of 60-GBaud PDM-QPSK and 30-GBaud PDM-16QAM signals over a 100-m 2 × 2 wireless MIMO link at 320 GHz with BER below 3.8 × 10-3 and 1.56 × 10-2 for QPSK and 16QAM signals, respectively. Compared with the MIMO-Volterra, our MIMO-CVNN has an advantage in terms of calculation complexity and decision accuracy due to its effective handling of phase information and inter-polarization relationships simultaneously.

Analysis of full reference quality metrics for image transmission over a MIMO OWC channel under varying turbulent conditions

SummaryThe integrated OWC‐PON architecture has a potential to overcome the limitations of existent RF technology and eliminate the last mile bottleneck in geographically constrained access networks thus providing real sense broadband from core to user. However, phenomenon such as atmospheric turbulence causes performance degradation in such systems. In the dissemination of multimedia services, the performance is the ultimate quality as perceived by end user's visual senses. To ascertain this quality in quantitative terms, full reference quality metrics are employed for communication purposes. This paper deals with classification, evaluation and analysis of various image quality assessment metrics for image transmission over OWC channel under varying turbulent conditions. The performance enhancement in terms of these metrics is investigated by deploying MIMO diversity reception. Moreover, a prediction model for SSIM is developed and compared for both SISO and MIMO links. The results show an improvement in IQA metrics with diversity reception. SSIM can be predicted in terms of log of refractive index structure parameter by a fourth order polynomial model for both SISO and MIMO receptions under all regimes of turbulence. The goodness of the fit statistics is presented along with the models to provide for their accuracy and reliability.

Robust Power Allocation in Optical Satellite MIMO Links With Pointing Jitter

In this letter, an optical satellite system is considered comprised of a geostationary satellite with multiple transmitters on-board and an optical ground station with multiple receiving telescopes. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {\mathrm {A}}$ </tex-math></inline-formula> robust power allocation strategy for the downlink is investigated incorporating the effects of the atmospheric impairments as well as the optical beam pointing jitter. Firstly, a theoretical analysis for the optical satellite system under pointing jitter is given and a maximin optimization problem is formulated maximizing the worst-case network capacity. The power allocation is carried out under peak and total power constraints using standard convex techniques. The lower bound of the ergodic network capacity for independent log-normal optical channels over variable jitter variance is derived. The proposed methodologies are evaluated via simulations using experimental channel measurements from the ARTEMIS-optical satellite campaign and their performances are compared to baseline power allocation strategies. The presented results show the effects of jitter on the network capacity and confirm the superiority of the proposed allocation scheme.

Optimizing MIMO Link Scheduling

Optimizing MIMO Link Scheduling

Terahertz-Band MIMO-NOMA: Adaptive Superposition Coding and Subspace Detection

We consider the problem of efficient ultra-massive multiple-input multiple-output (UM-MIMO) data detection in terahertz (THz)-band non-orthogonal multiple access (NOMA) systems. We argue that the most common THz NOMA configuration is power-domain superposition coding over quasi-optical doubly-massive MIMO channels. We propose spatial tuning techniques that modify antenna subarray arrangements to enhance channel conditions. Towards recovering the superposed data at the receiver side, we propose a family of data detectors based on low-complexity channel matrix puncturing, in which higher-order detectors are dynamically formed from lower-order component detectors. We first detail the proposed solutions for the case of superposition coding of multiple streams in point-to-point THz MIMO links. We then extend the study to multi-user NOMA, in which randomly distributed users get grouped into narrow cell sectors and are allocated different power levels depending on their proximity to the base station. We show that successive interference cancellation is carried with minimal performance and complexity costs under spatial tuning. We derive approximate bit error rate (BER) equations, and we propose an architectural design to illustrate complexity reductions. Under typical THz conditions, channel puncturing introduces more than an order of magnitude reduction in BER at high signal-to-noise ratios while reducing complexity by approximately 90%.

COMBINED INFLUENCE OF DOPPLER EFFECT AND PILOT DE-ORTHOGONALIZATION ON 2×2 TO 4×4 MIMO SYSTEMS AND IMPROVEMENT OF ORTHOGONAL SEQUENCES

The Doppler effect in 2×2, 3×3, and 4×4 MIMO wireless communication systems with channel state estimation is studied. The orthogonal pilot signal approach is used for the channel estimation, where the Hadamard sequences are used for piloting along with the eight Romanuke orthogonal sets similar to the Walsh set. The Doppler effect is additionally aggravated by the pilot signal de-orthogonalization, where two negative-to-positive symbol errors are assumed to have occurred while signal is transmitted. MIMO transmissions are simulated for 10 cases of the frame length and pilot symbols per frame by no Doppler shift to 1100 Hz Doppler shift with a step of 100 Hz. By assuming that the carrier frequency is 5,9 GHz, the step corresponds to a motion speed of about 18.3 km/hr. Based on the simulations, it is ascertained that the Doppler effect negatively influences transmissions of long data packets. It is impracticable to apply MIMO transmissions of long packets at speeds exceeding 100 km/hr. To maintain an appropriate MIMO link data rate, the packet length should be shortened as the motion speed increases. On the other hand, the MIMO performance is substantially improved by increasing the number of antennas, except for the case of transmitting long packets. Besides, under the de-orthogonalization caused by two negative-to-positive symbol errors, the MIMO Walsh pilot sequences are outperformed by MIMO Romanuke pilot sequences, so the latter are considered as an improvement of MIMO orthogonal sequences. However, the performance difference between the Romanuke and Walsh pilot sequences decays as a greater number of transmit-receive antenna pairs is used and the motion speed increases.

INFLUENCE OF DOPPLER EFFECTS ON 2×2 TO 4×4 MIMO SYSTEMS WITH DIRECT LINE-OF-SIGHT PATH BY RICIAN FADING DISTRIBUTION

Background. For multiplying the capacity of a radio link by using multiple antennas at the transmitter and receiver ends, wireless communication networks use the MIMO technique. The quality of MIMO wireless communication may worsen when either the transmitter or receiver end is in motion, or they both are in motion. To sustain high quality of links, MIMO operates, in particular, on channel state information by using orthogonal pilot sequences.Objective. The goal is to estimate the bit-error-rate performance of 2×2, 3×3, and 4×4 MIMO systems with orthogonal pilot sequences for a range of Doppler shifts by the Rician fading distribution modeling the scenario of direct line-of-sight path. The estimation is believed to answer the question on MIMO efficiency by ascertaining whether increasing the number of antennas mitigates the Doppler effect in 2×2 to 4×4 MIMO systems with direct line-of-sight path.Methods. To achieve the goal, 2×2 to 4×4 MIMO systems with channel state estimation are simulated. The simulation is configured and carried out by using MATLAB® R2019a Communications System ToolboxTM functions. The 35 subcases of the frame length and number of pilot symbols per frame are considered.Results. The near-exponential-type decreasing of the bit-error-rate performance is almost not violated by no motion, whereas further speed increments destroy it for 2×2 MIMO systems. It is revealed that too long packet transmissions by short orthogonal pilot sequences (with respect to the packet length) are prone to errors. This especially concerns 2×2 MIMO systems, where channel state estimation by short orthogonal pilot sequences appears to be ineffective or weakly effective.Conclusions. In general, to maintain an appropriate MIMO link data rate, the packet length should be shortened as the motion speed increases. Increasing the number of antennas additionally mitigates Doppler effects. S o, 4 ×4 M IMO s ystem transmissions are indeed efficient. The shortest possible packet transmissions are not affected by Doppler effects if three or four antennas are used.

ParEst: joint estimation of the OFDM channel state information in MIMO systems

OFDM channel state information (CSI) is needed for determining key parameters in MIMO transmissions. In this paper, a novel CSI estimation method, ParEst, is proposed, which estimates the CSI from multiple transmitting antennas simultaneously. ParEst is based on a recent discovery that the CSI can be approximated very well by the linear combination of complex-based sinusoids on constant frequencies. ParEst finds the CSI of all antennas jointly by solving an optimization problem and achieves higher accuracy than existing heuristic methods. As the base sinusoids are on constant frequencies, ParEst pre-computes most key steps and reduces the run-time computation complexity to just a modest number of vector dot products equivalent to a few FFT calculations. ParEst can be applied to MIMO links in networks such as LTE, 5G, or Wi-Fi.

Networked multiple-input-multiple-output for optical wireless communication systems.

With the escalation of heterogeneous data traffic, the research on optical wireless communication (OWC) has attracted much attention, owing to its advantages such as wide spectrum, low power consumption and high security. Ubiquitous optical devices, e.g. light-emitting diodes (LEDs) and cameras, are employed to support optical wireless links. Since the distribution of these optical devices is usually dense, multiple-input-multiple-output (MIMO) can be naturally adopted to attain spatial diversity gain or spatial multiplexing gain. As the scale of OWC networks enlarges, optical MIMO can also collaborate with network-level operations, like user/AP grouping, to enhance the network throughput. Since OWC is preferred for short-range communications and is sensitive to the directions/rotations of transceivers, optical MIMO links vary frequently and sharply in outdoor scenarios when considering the mobility of optical devices, raising new challenges to network design. In this work, we present an overview of optical MIMO techniques, as well as the cooperation of MIMO and user/AP grouping in OWC networks. In consideration of the challenges for outdoor OWC, key technologies are then proposed to facilitate the adoption of optical MIMO in outdoor scenarios, especially in vehicular ad hoc networks. Lastly, future applications of MIMO in OWC networks are discussed. This article is part of the theme issue 'Optical wireless communication'.

Smart Diversity Through MIMO Satellite $Q/V$-Band Feeder Links

With sum throughputs exceeding 1 Tb/s, next-generation high throughput satellite will play a key role in 5G networks for applications including backhauling or broadband in-flight connectivity. Such satellites require the deployment of tens of $Q/V$ -band feeder links to support the data traffic in their uplink. Meanwhile, geographical restrictions and cost considerations significantly constrain the search for gateway deployment sites. Diversity strategies are, furthermore, necessary to cope with strong rain fades. In this paper, multiple-input-multiple-output (MIMO) feeder links are proposed to address these challenges. Using two active gateway antennas per feeder link, a MIMO-based solution enables to support twice the user link bandwidth per link compared to the state-of-the-art. On-ground and onboard processing strategies for such MIMO links are introduced, and the link outage probabilities are determined. An extension of smart diversity schemes to MIMO-based architectures is also presented to guarantee a sufficient robustness against rain fades. An analysis of the achieved uplink CINR demonstrates the superiority of the proposed concept and provides guidelines for an efficient design.

Neural Network-Based Joint Spatial and Temporal Equalization for MIMO-VLC System

The limited bandwidth of white light-emitting diode (LED) limits the achievable data rate in a visible light communication (VLC) system. A number of techniques, including multiple-input-multiple-output (MIMO) system, are investigated to increase the data rate. The high-speed optical MIMO system suffers from both spatial and temporal cross talks. The spatial cross-talk is often compensated by the MIMO decoding algorithm, while the temporal cross talk is mitigated using an equalizer. However, the LEDs have a non-linear transfer function and the performance of linear equalizers are limited. In this letter, we propose a joint spatial and temporal equalization using an artificial neural network (ANN) for an MIMO-VLC system. We demonstrate using a practical imaging/non-imaging optical MIMO link that the ANN-based joint equalization outperforms the joint equalization using a traditional decision feedback as ANN is able to compensate the non-linear transfer function as well as cross talk.

UAV-Aided MIMO Communications for 5G Internet of Things

The unmanned aerial vehicle (UAV) is a promising enabler of the Internet of Things (IoT) vision, due to its agile maneuverability. In this paper, we explore the potential gain of UAV-aided data collection in a generalized IoT scenario. Particularly, a composite channel model, including both large-scale and small-scale fading is used to depict typical propagation environments. Moreover, rigorous energy constraints are considered to characterize IoT devices as practically as possible. A multiantenna UAV is employed, which can communicate with a cluster of single-antenna IoT devices to form a virtual MIMO link. We formulate a whole-trajectory-oriented optimization problem, where the transmission duration and the transmit power of all devices are jointly designed to maximize the data collection efficiency for the whole flight. Different from previous studies, only the slowly varying large-scale channel state information is assumed available, to coincide with the fact that practically it is quite difficult to predictively acquire the random small-scale channel fading prior to the UAV flight. We propose an iterative scheme to overcome the nonconvexity of the formulated problem. The presented scheme can provide a significant performance gain over traditional schemes and converges quickly.

New insights into time synchronization of MIMO systems without and with interference

Abstract The time synchronization of (M × N) MIMO systems has been studied this last fifteen years, for both single-carrier (SC) and multi-carriers links. Without any interference, most of the available receivers assume orthogonal sequences. With interference, the current most powerful receiver is a generalized likelihood ratio test (GLRT) receiver, assuming unknown, stationary, circular, temporally white and spatially colored Gaussian noise. However, this receiver is more complex than its non-GLRT counterparts, which, unfortunately, do not perform as well in most cases. In this context, the purpose of this paper is to get new insights into the time synchronization of SC MIMO links, both without and with interference, in order to overcome the limitations of the available receivers. In the absence of interference, the MIMO GLRT receiver is computed and compared to the existing ones in a unified framework, enlightening its better performance. In the presence of interference, as the complexity is an important issue in practice, several ways to decrease the complexity of the available GLRT receiver while keeping its performance are proposed, enlightening the great practical interest of the proposed schemes. Finally, the optimization of the number of transmit antennas is investigated, enlightening the existence of an optimal value of M depending on the channel matrix.

On the Performance of Multiplexing FSO MIMO Links in Log-Normal Fading With Pointing Errors

In this paper, we employ a spatial multiplexing scheme with M transceivers to increase the data rate of a free-space optical (FSO) system. In our proposed scheme, each transmitter sends independent information toward the center of its related receiver aperture. We analyze the performance of the proposed scheme in terms of average achievable rate. To compare the performance of our proposed scheme with its spatial diversity FSO system counterpart, we define a case in which M transmitters send the same information. To facilitate performance analysis, we also derive the analytical expressions for spectral efficiency of both spatial multiplexing and diversity configurations, which reduces the (M+2)-dimensional integration of spatial multiplexing and (M2+2)-dimensional integration of spatial diversity FSO systems to a two-dimensional integration in both configurations. The adequacies of our analytical results are thoroughly confirmed by numerical simulations.

Low Complexity Capacity Maximizing Transmit Antenna Selection Schemes for Massive MIMO Wireless Communications

Recently massive MIMO systems have attracted a lot of attention. A serious limitation in implementing massive MIMO is the number of RF chains required, which is due to its high implementation cost and power consumption. Antenna selection is a wise remedy, that not only reduces the number of required RF chains, but also improves the system’s performance. In this work, two low complexity capacity-maximizing transmit antenna selection (TAS) schemes, one for low SNR region and one for medium to high SNR region, are proposed. It is shown that the proposed antenna selection schemes enhance the MIMO link’s capacity at low SNRs, and select only one antenna in more than $$90\%$$ of channel realizations, which in terms of numbers of required RF chains is very promising. The performance of the proposed schemes are compared to an existing high performance greedy TAS technique in terms of average capacity and complexity. Finally, a close analytical approximation for average capacity of the proposed capacity-based subset selection scheme is presented.

Balancing Interference and Delay in Heterogeneous Ad Hoc Networks With MIMO

Heterogeneous ad hoc networks with MIMO links can significantly improve transmission performance of the entire distributed wireless communication system. In this paper, we investigate how to increase total system throughput and decrease end-to-end delay with the help of heterogeneous characteristics of the ad hoc networks. Even if there are lots of references about distributed scheduling control considering multiple antennas, channel state, and so on, it still needs to be addressed how to guarantee destination to receive packets with a short delay. To resolve this issue, we first propose an interference-delay tradeoff method using convex optimization, which adjusts transmission rate and power to balance interference and delay. We then develop a speed power interference-based topology resource control algorithm with delay constraint to further adjust transmission power for reducing energy consumption. Simulation results show that the proposed algorithms can outperform the existing ones in terms of throughput, end-to-end delay, and power consumption.

Generalized Selection Multiuser Scheduling for the MIMO FSO Communication System and Its Performance Analysis

This paper analyzes the impact of generalized selection multiuser scheduling on the performance of the multiple-input-multiple-output (MIMO) free-space optical (FSO) communication system. We consider a general scenario that the point-to-multipoint system consists of a central transmitter equipped with K aperture groups containing M transmit apertures and K users equipped with N receive apertures communicating over gamma-gamma turbulence FSO channels. For the MIMO links between the central node and user, we derive the combining signal-to-noise ratio (SNR) distributions of MIMO repetition coding and MIMO transmit laser selection. Then, a statistics analysis is presented for the multiuser diversity scheme in which the user with the kth best combining SNR is selected. Based on the analysis, the closed-form expressions of the outage probability are derived, and the diversity benefit provided by the channel fluctuations is further discussed. Numerical results verify the accuracy of these analytical results.

Geodesical Refinement of MIMO Limited Feedback

In this paper, we investigate a codeword refinement technique called double codeword coding in the context of MIMO feedback. In the proposed method, the second codeword is fed back in addition to the best codeword as their interpolation at the transmitter improves the channel knowledge. The derivation of the optimal interpolation parameter is presented together with the derivation of an upper-bound to the interpolation parameter based on channel quality feedback available at the transmitter. We show that double codeword feedback refinement has slightly lower complexity than alternative techniques and still achieves similar performance. Furthermore, double codeword coding can be generalized to higher transmission ranks. Using extended multiuser MIMO link simulations, we verify that the double codeword feedback is able to provide significant gains in 3GPP LTE context.

CORRELATION EFFECTS ON THE MIMO CAPACITY FOR CONFORMAL ANTENNAS ON A PARABOLOID

The use of conformal antennas in a MIMO link scenario is investigated. Conformal slot antennas are considered both in the transmitter and the receiver. First, a new modified correlation coefficient is derived that goes beyond the Clarke coefficient and takes into account the element radiation pattern. Secondly, a hybrid formulation that accounts for the impact of the mutual coupling and the pattern dependent correlation on the capacity is presented. The mutual coupling for slots placed circumferentially on a paraboloid substrate is derived using a rigorous approach based on Uniform Theory of Diffraction (UTD). The capacity is evaluated for the case of Rayleigh fading channel considering the new pattern dependent correlation coefficient and the conformal antenna mutual coupling. The planar case is included as a limiting case. It is shown that for conformal antennas on a paraboloid the capacity degradation compared to the planar case is up to 0.5 bps/Hz due to coupling and correlation.