Multi-input Multi-output Transmission Research Articles

Dual polarized optical multi‐input multi‐output transmission over 7‐core few‐mode fiber using adaptive recursive least squares constant modulus algorithm

AbstractWe describe a system with multi‐input multi‐output (MIMO) transmission over 7‐core few‐mode fiber (FMF) by incorporating polarization division multiplexing and mode division multiplexing. Three optical modes with x and y polarizations were used for 42 × 42 MIMO transmission over 7‐core FMF. We simulate the optical MIMO transmission at 20 Gbps with quadrature phase‐shift keying signals. By using the adaptive recursive least squares constant modulus algorithm (RLSCMA) at the receiver side, transmission data is successfully recovered by mitigating the effects of modal delay and mode coupling of received signals. The simulation results show that adaptive RLSCMA can improve the system performance of optical MIMO transmission systems.

Energy efficient secure MIMO transmission in the presence of smart attacker

The multi-input multi-output (MIMO) transmission against a smart attacker has recently been formulated as a non-collaborative game, in which the reinforcement learning-based transmitter is devised to suppress the attack motivation of the malicious attacker for more secure transmission. Basically, large enough transmission power is needed to obtain the Nash equilibrium in favour of the MIMO transmitter. When energy efficiency is required, a secure MIMO transmission design with reasonable energy efficiency will become highly desirable. This study focuses on how to make the MIMO transmitter not only resist malicious attacker but also realise reasonable energy efficiency. To this end, a new game between the MIMO transmitter and the malicious attacker is formulated to take into account the energy efficiency of the MIMO transmitter. Since the energy efficiency requirement tends to reduce the transmission power by the MIMO transmitter, which may degrade the realised security performance in an adverse environment, the probabilistic transmission strategy was employed to further enhance the secure transmission. The analysis results unveil that the proposed energy-efficient secure MIMO transmission design can not only ensure reasonable security performance but also noticeably improve energy efficiency performance.

Study of Optical MIMO Transmission Systems Using the MGDM Multiplexing Technique

In current local area networks, multimode fibers (MMFs), mainly graded index (GI) MMFs, are the main types of fibers used for data communications. Because of their high bandwidth, they are considered the main method of transmission that allows to offer multiservice broadband services using optical multiplexing techniques.
 
 The MGDM (ModeGroup Division Multiplexing) is a Multiplexing technique, which aims to improve the performance of the multimode optical fiber by spatially multiplexing the data streams to be transmitted. In this work, we study optical MIMO (multi-input multi-output) transmission systems on an MMF optical fiber, specifically the adaptation of the architecture of MIMO transmission systems. In this context, we have studied the mode group multiplexing technique (MDGM), to evaluate the transmission capacity. In fact, the latter depends on the injection conditions and the state of the optical fiber.

MIMO-OFDM NB-PLC Designs in Underground Medium-Voltage Networks

In this paper, we investigate the application of multi-input multi-output (MIMO) orthogonal frequency-division multiplexing (OFDM) to narrowband powerline communications (NB-PLC) over medium-voltage (MV) underground (UG) networks. We study different MIMO transmission scenarios with different injection configurations utilizing both the cable conductor and sheath phases. Multiconductor transmission line theory is used to accurately characterize the UG MV NB-PLC channel transfer function and evaluate the spatial correlation between the different conductor phases. The achievable data rates are evaluated for all configurations at different line lengths after optimizing the transmit energy allocation across the different spatial information streams subject to a transmit power spectral density constraint. The achievable data rates for MIMO configurations are shown to be significantly higher (by up to three times higher) compared to single-input single-output OFDM transmission. Moreover, we investigate the effects of practical design constraints, such as cyclic prefix length and bit caps, on the achievable data rates. Finally, we show how to reduce the complexity of both the transmitter and receiver implementations while minimizing the data rate loss.

Bridging Spatial Modulation With Spatial Multiplexing: Frequency-Domain ESM

Spatial modulation (SM) is a multi-input multi-output (MIMO) technique, which was originally introduced to reduce the number of radio frequency (RF) chains in a MIMO transmitter and thereby reduce cost and power consumption. Although it is very appealing in theory, this concept actually has two main problems: The first is that SM involves antenna switching, which degrades the spectral shaping and the Nyquist property of the transmitted signal. The second is the limited spectral efficiency due to the presence of silent antennas. In order to remedy the second problem, enhanced spatial modulation (ESM) was introduced in which multiple signal constellations are used, and information bits are conveyed not only by the transmitted symbols and the indices of the active antennas, but also by the constellation types activated. As for the problem of antenna switching, it actually disappears when SM is combined with OFDM, because the switching operation in frequency-domain SM (FD-SM) occurs at baseband. But the number of RF chains required becomes equal to the number of transmit antennas and the property of reduced number of RF chains, which was the original motivation for SM also disappears. FD-SM turns out to be a particular type of underloaded MIMO-OFDM system, which reduces the power requirements of the transmit power amplifiers rather than their number. In this paper, we introduce frequency-domain ESM (FD-ESM), which avoids both the spectral efficiency limitations and the antenna switching issues of the original SM. Exploiting the property that switching occurs at baseband and no savings in terms of the number of RF chains can be achieved in frequency-domain implementation, we design FD-ESM schemes in which the transmitted codewords do not include any zero-valued components. These designs bridge the concept of SM with spatial multiplexing, and they provide spectacular gains with respect to multi-stream SM (MSM) and also some significant gains compared to conventional spatial multiplexing.

A Study on Optimal Beam Patterns for Single User Massive MIMO Transmissions

This paper proposes optimal beam patterns of analog beamforming for SU (Single User) massive MIMO (Multi-Input Multi-Output) transmission systems. For hybrid beamforming in SU massive MIMO systems, there are several design parameters such as beam patterns, the number of beams (streams), the shape of array antennas, and so on. In conventional hybrid beamforming, rectangular patch array antennas implemented on a planar surface with linear phase shift beam patterns have been used widely. However, it remains unclear whether existing configurations are optimal or not. Therefore, we propose a method using OBPB (Optimal Beam Projection Beamforming) for designing configuration parameters of the hybrid beamforming. By using the method, the optimal beam patterns are derived first, and are projected on the assumed surface to calculate the achievable number of streams and the resulting channel capacity. The results indicate OBPB with a spherical surface yields at least 3.5 times higher channel capacity than conventional configurations.

Composite Neural Network Digital Predistortion Model for Joint Mitigation of Crosstalk, $I/Q$ Imbalance, Nonlinearity in MIMO Transmitters

Multi-input multi-output (MIMO) is anticipated to be a prominent technique proposed in the wireless communications to improve the system capacity and data rates of the wireless networks. However, the MIMO transmitter suffers from imperfections, such as crosstalk, power-amplifier (PA) nonlinearity, in-phase and quadrature (I/Q) imbalance, and dc offset. Investigating these effects, this paper proposes neural network (NN)-based digital predistortion (DPD) as an integral solution to compensate for crosstalk, PA nonlinearity, I/Q imbalance, and dc offset imperfections simultaneously in MIMO transmitters. The proposed NN DPD model provides a one-step single-model digital mitigation solution to multibranches of MIMO transmitters. With the increase in the dimensions of MIMO transmitter, the proposed NN-based DPD model provides a better compensation for transmitter imperfections and also reduces the complexity as compared to the state-of-the-art DPD methods. The proof-of-concept is provided with the 2×2 and 3×3 MIMO transmitters in the presence of strong PA nonlinearity, crosstalk, I/Q imbalance, and dc offset for homodyne as well as heterodyne transmitters' cases.

Power, Linearity, and Efficiency Prediction for MIMO Arrays With Antenna Coupling

Modern and future transmitter architectures are being driven toward multi-input multi-output (MIMO) transceivers, which make use of several radio frequency (RF) power amplifiers (PAs) to drive an antenna array. When moving to high integration scenarios, as is expected for fifth-generation communications, the isolation between the array and the PAs is often removed. Since the array elements are electromagnetically coupled, the waves fed to the antennas are also driving the output ports of the PAs. This effect creates an apparent variable load at the output of each PA, depending on the operation of the transmitter. In this system, the behavior of each PA cannot be fully described solely as a function of its input, as it will change according to the coupled signal. To predict and understand the behavior of these MIMO systems, simulation becomes a powerful tool. These simulators should be able to reproduce the main figures of merit (FoMs) of the transmitter, namely, output power, distortion, and efficiency. In this paper, we show how a recently proposed dc behavior model and an RF signal behavior model can be used in simulation setups to predict the MIMO transmitter FoMs of interest.

Exploitation of Dual-Polarization Diversity for 5G Millimeter-Wave MIMO Beamforming Systems

For the first time, to the best of our knowledge, this paper reports a complete millimeter wave (mmWave) wireless system, which coherently utilizes polarization diversity, multi-input multioutput (MIMO), and beamforming technologies applicable for upcoming fifth generation wireless mobile terminals. Using an interdisciplinary approach across the entire layers constituting of antennas, RF architecture, and digital modem, the polarization of the presented mmWave beamforming antenna system dynamically adapts to the channel environments and MIMO transmission modes. Empirical investigations, which are taken under real-life environments, ascertain that implementing switchable dual-polarized array antennas can alleviate the complexity of the beam searching procedure and MIMO channelization. In addition, experimental results acquired from intensive system level tests indicate more than 22 dB averaged cross-polarization discrimination under polarized MIMO channel conditions at the 60 GHz band. The beam selection algorithm that can be easily implemented based on power metric is confirmed to feature a 99.5% accuracy in comparison with the optimal selection algorithm derived from the theoretical maximal capacity.

On the Modeling and Compensation of Backward Crosstalk in MIMO Transmitters

In this letter, the impact of backward crosstalk at the output of power amplifiers (PAs) on the behavioral model of the multi-input multi-output (MIMO) transmitter is investigated. In this case, the interference signal from other branches leaks into the output of PA and changes the PA behavioral model. A novel generalized memory polynomial is proposed to model the nonlinear distortion caused by this type of crosstalk. Moreover, it is shown that a digital predistortion (DPD) algorithm based on a 2-D generalized memory polynomial can efficiently compensate for the nonlinearity and backward crosstalk impairment effects in MIMO transmitter. The experimental results validate the performance of the proposed model and the DPD method in four different scenarios. An adjusted channel power ratio of less than -50 dBc for the linearized output signal in the presence of -15/-10 dB crosstalk is achieved.

Frequency Synchronization for Multi-User MIMO Without Cyclic-Prefix Based on Space-Time Equalization

In this letter, we propose a new frequency synchronization scheme based on space-time equalization for multi-user multi-input multi-output transmissions. We specifically consider the case where no cyclic-prefix is employed such that the spectral efficiency can be maintained. The subspace analysis is first carried out for the received space-time snapshots from both the training and a few unknown data symbols. The proposed method then performs joint carrier frequency offset (CFO) estimation and space-time equalization design for each user individually, with constraining the beamforming vector to lie in the corresponding signal subspace. Numerical results are provided to corroborate the proposed studies. We show that the proposed method can achieve much better CFO estimation as well as data detection performance as compared to the other conventional approaches.

Soft Demodulation Algorithms for Generalized Spatial Modulation Using Deterministic Sequential Monte Carlo

Generalized spatial modulation (GSM) is a relatively new multi-input multi-output transmission technique that enables a flexible trade-off between the achievable transmission rate and the cost of radio frequency chains. However, due to the constraint of transmit antenna combination and the variation of interchannel interference, the efficient low-complexity demodulation of GSM signals is challenging, especially when soft demodulation is needed. In this paper, we propose two low-complexity algorithms based on the deterministic sequential Monte Carlo (SMC) technique for the demodulation of GSM. The type-I SMC demodulator, which uses the conventional successive interference cancellation as the kernel and draws antenna-wise samples from the extended constellation, is proposed for the overdetermined GSM system. The type-II SMC demodulator, which consists of two stages and uses the orthogonal matching pursuit as the kernel in the first stage, is proposed for the underdetermined GSM system. A key component in both algorithms is an efficient online scheme to eliminate the illegal samples during the sampling process. Both proposed algorithms achieve near-optimal performances with complexity linear in terms of the antenna size. Moreover, owing to their soft-input soft-output nature, they can be employed in a turbo receiver for a coded GSM system.

Near-Field Magnetic Induction MIMO Communication Using Heterogeneous Multipole Loop Antenna Array for Higher Data Rate Transmission

In this paper, we propose a novel method realizing multi-input–multi-output (MIMO) transmission in near-field magnetic induction (NFMI) communication for increasing channel capacity where limited capacity is a major bottleneck problem in magnetic communication systems. Since conventional magnetic communication systems using the same antenna patterns cannot easily implement MIMO transmission due to strong crosstalk between transmitters, we propose heterogeneous antenna arrays with multipole antennas, enabling crosstalk cancellation. We confirm the crosstalk cancellation of the proposed antenna array through numerical simulations and experiments, and verify the capacity enhancement of the proposed NFMI communication system over conventional NFMI communication schemes.

MIMO transmit scheme based on morphological perceptron with competitive learning

This paper proposes a new multi-input multi-output (MIMO) transmit scheme aided by artificial neural network (ANN). The morphological perceptron with competitive learning (MP/CL) concept is deployed as a decision rule in the MIMO detection stage. The proposed MIMO transmission scheme is able to achieve double spectral efficiency; hence, in each time-slot the receiver decodes two symbols at a time instead one as Alamouti scheme. Other advantage of the proposed transmit scheme with MP/CL-aided detector is its polynomial complexity according to modulation order, while it becomes linear when the data stream length is greater than modulation order. The performance of the proposed scheme is compared to the traditional MIMO schemes, namely Alamouti scheme and maximum-likelihood MIMO (ML-MIMO) detector. Also, the proposed scheme is evaluated in a scenario with variable channel information along the frame. Numerical results have shown that the diversity gain under space-time coding Alamouti scheme is partially lost, which slightly reduces the bit-error rate (BER) performance of the proposed MP/CL-NN MIMO scheme.

Multi-cell massive MIMO transmission with coordinated pilot reuse

In this paper, we propose a coordinated pilot reuse (CPR) approach to reduce the pilot overhead for multi-cell massive multi-input multi-output transmission. Unlike the conventional multi-cell pilot reuse approach in which pilots can only be reused among different cells, the proposed CPR approach allows pilots to be reused among both inter-cell and intra-cell user equipments, and thus, pilot overhead can be efficiently reduced. For spatially correlated Rayleigh fading channels, we first present a CPR-based channel estimation method and a low complexity pilot allocation algorithm. Because CPR might lead to additional pilot interference, we develop a statistically robust uplink receiver and downlink precoder that takes channel estimation errors into account. The proposed uplink receiver and downlink precoder are robust to channel state information inaccuracy, and thus, can guarantee a certain transmission performance. Monte-Carlo simulations illustrate the significant performance improvement in net spectral efficiency offered by the proposed CPR approach.

Capacity limits of spatially multiplexed free-space communication

Limits of orbital-angular-momentum multiplexing for free-space optical communication are revealed. Increasing the information capacity per unit bandwidth has been a perennial goal of scientists and engineers1. Multiplexing of independent degrees of freedom, such as wavelength, polarization and more recently space, has been a preferred method to increase capacity2,3 in both radiofrequency and optical communication. Orbital angular momentum, a physical property of electromagnetic waves discovered recently4, has been proposed as a new degree of freedom for multiplexing to achieve capacity beyond conventional multiplexing techniques5,6,7,8,9, and has generated widespread and significant interest in the scientific community10,11,12,13,14. However, the capacity of orbital angular momentum multiplexing has not been established or compared to other multiplexing techniques. Here, we show that orbital angular momentum multiplexing is not an optimal technique for realizing the capacity limits of a free-space communication channel15,16,17 and is outperformed by both conventional line-of-sight multi-input multi-output transmission and spatial-mode multiplexing.

Linear Propagation Effects in Mode-Division Multiplexing Systems

In this paper, we review linear propagation effects in a multimode fiber (MMF) and their impact on performance and complexity in long-haul mode-division multiplexing (MDM) systems. We highlight the many similarities to wireless multi-input multioutput (MIMO) systems. Mode-dependent loss and gain (MDL), analogous to multipath fading, can reduce average channel capacity and cause outage in narrowband systems. Modal dispersion (MD), analogous to multipath delay spread, affects the complexity of MIMO equalization, but has no fundamental effect on performance. Optimal MIMO transmission uses a basis of the Schmidt modes, which may be obtained by a singular value decomposition of the MIMO channel. In the special case of a unitary channel (no MDL), an optimal basis is the set of principal modes, which are eigenvectors of a group delay operator, and are free of signal distortion to first order. We present a concatenation rule for the accumulation of MD along a multisection link. We review mode coupling in MMF, including physical origins, models, and regimes of weak and strong coupling. Strong mode coupling is a key to overcoming challenges in MDM systems. Strong coupling reduces the group delay spread from MD, minimizing the complexity of MIMO signal processing. Likewise, it reduces the variations of loss and gain from MDL, maximizing channel capacity. In the strong-coupling regime, the statistics of MD and MDL depend only on the number of modes and the variance of accumulated group delay or loss/gain, and can be derived from the eigenvalue distributions of certain Gaussian random matrices.

Empirical MIMO beamforming and channel measurements at 57–64 GHz frequencies

AbstractIn this paper, we employ a 4‐port Agilent PNA‐X network analyzer with the expansion modules to measure a 2 × 2 multi‐input multi‐output (MIMO) propagation channel at 57–64 GHz frequency band. Moreover, we investigate on the beamforming capabilities of the millimetre‐wave (MMW) MIMO system. For both aforementioned architectures, that is, regular MMW MIMO and MMW MIMO with beamforming, the Shanon capacity criterion is used to evaluate the performance in terms of data transmission rate. The results reveal that we have a remarkable data rate gain due to MIMO transmission and even more when this MIMO is equipped with beamforming capabilities. Copyright © 2014 John Wiley & Sons, Ltd.

Averaging time required for measuring the specific absorption rate of a MIMO transmitter

This paper presents a new method to measure electric field (E-field) for human-body exposure to electromagnetic fields from Multi-Input Multi-Output (MIMO) wireless communication transmitters, and an estimation of averaging time required in the E-field measurement. The proposed method is to measure time-averaged E-field, and the averaging time is the duration required for a measurement system to make averaged E-field converged. The estimation is based on averaging the square of amplitude of received signals or measured E-field strength. Several simulation results with different measurement parameters for MIMO wireless communication transmitters are presented and discussed. It is pointed out that the averaging time in E-field measurements for multi-antenna transmitters is considerably longer than that for single antenna transmitters.

Joint Grassmann-Stiefel Quantization for MIMO Product Codebooks

We consider product codebook strategy where a single small codebook is implemented at the receiver to quantize larger multi-input multi-output (MIMO) channels, e.g. aggregate channels of cooperative MIMO base stations. The present work focuses on the codebook design, codebook construction, and codeword selection under this scenario, for single- or multistream MIMO transmission. Designing point-to-point unitary precoding codebook is related to a discretization problem on the Grassmann manifold, where a Grassmannian codeword is an equivalence class of rectangular-unitary/Stiefel matrices. For practical needs, one has to choose the rectangular unitary matrix to represent each Grassmann codeword. In this paper, we choose appropriate representatives so that product codebook quantization becomes competitive with global Grassmannian quantization. For this, we propose a novel joint Grassmann-Stiefel codebook design aiming at good quantization/discretization of Grassmann and Stiefel manifolds with a single codebook. To find low-distortion codebooks, we present a vector quantizer generating a Stiefel codebook conditioned on a fixed Grassmann codebook. For this purpose, we provide an exact solution for computing centroids in the Stiefel manifold with chordal distance. Furthermore, concrete examples of analytical joint Grassmann-Stiefel packings are given. Finally, we discuss low-complexity codeword selection methods.