Correlated Multiple-input Multiple-output Channels Research Articles

Synchronous time multiplexing for non-line-of-sight multiple-input multiple-output optical camera communications.

In typical multiple-input multiple-output (MIMO) optical camera communication (OCC) systems, the spatial correlation of MIMO channels is large. Optical signals between light sources can easily interfere with each other, negatively impacting the overall transmission performance. In this work, we propose a time-multiplexing integral modulation scheme for a non-line-of-sight MIMO OCC system, where each LED transmits different signals to improve both the data rate and security of the system. A genetic algorithm (GA)-based adaptive multi-threshold scheme is designed to demodulate the blurred fringes in multi-level pulse amplitude modulation. The experimental results show that at a distance of 2.5 m, a data rate of 16.4 kb/s can reach with the BER performance of 3.01 × 10-3, which validates the superiority and reliability of our proposed schemes.

IMRecoNet: Learn to Detect in Index Modulation Aided MIMO Systems With Complex Valued Neural Networks

Index modulation (IM) reduces the power consumption and hardware cost of the multiple-input multiple-output (MIMO) system by activating part of the antennas for data transmission. However, IM significantly increases the complexity of the receiver and needs accurate channel estimation to guarantee its performance. To tackle these challenges, in this paper, we design a deep learning (DL) based detector for the IM aided MIMO (IM-MIMO) systems. We first formulate the detection process as a sparse reconstruction problem by utilizing the inherent attributes of IM. Then, based on greedy strategy, we design a DL based detector, called IMRecoNet, to realize this sparse reconstruction process. Different from the general neural networks, we introduce complex value operations to adapt the complex signals in communication systems. To the best of our knowledge, this is the first attempt that introduce complex valued neural network to the design of detector for the IM-MIMO systems. Finally, to verify the adaptability and robustness of the proposed detector, simulations are carried out with consideration of inaccurate channel state information (CSI) and correlated MIMO channels. The simulation results demonstrate that the proposed detector outperforms existing algorithms in terms of antenna recognition accuracy and bit error rate under various scenarios.

Flipped superposed constellation design for MIMO visible-light communication systems

Multiple-input multiple-output (MIMO) visible-light communication (VLC) systems based on the concept of superposed constellation have attracted increasing attention because multiplexing gains can be achieved even in a highly correlated MIMO channel. In this paper, a novel superposed constellation scheme is proposed for 2×2 MIMO VLC systems, where a 2n-order quadrature amplitude modulation (QAM) signal is obtained by superposing 4QAM and processed 2n-2-order QAM signals. Based on the original 2n-2-order QAM signal, the processed signal is generated by adding an optimal offset, power normalization, and flipping according to the value of the 4QAM signal. Thus, the required power ratio between the two superposed signals is equal to 1, which not only avoids the power competition in the receiver, but also reduces the risk of the nonlinear distortion of the light-emitting diode at the transmitter. Moreover, benefitting from the flipped superposition method, two additional performance gains are provided. First, thorough Gray coding can be achieved. Second, the received power can be improved because the two transmitted signals are correlated. An algorithm for solving the optimal offset is proposed based on the target of the equal power ratio, and the expression of the received power is derived as well. Then, the performance of the proposed scheme is investigated using detailed simulations in an additive white Gaussian noise channel, where the superposed 64QAM constellation is assumed as an example. Further, we perform an experimental demonstration and examine the performance of the proposed system when a practical optical channel is considered, where nonideal impacts, such as nonlinearity and power competition, may occur. The experimental results confirm that the proposed scheme achieves a lower bit error rate (BER) and a larger dynamic range of driving peak-to-peak voltage compared with the existing superposed constellation schemes. Considering the 7% pre-forward error correction BER threshold of 3.8 × 10-3, the proposed superposed 64QAM constellation system can achieve a maximum transmission rate of 3 Gb/s.

Ultra-High Mobility Analysis of MIMO Wireless System in Tunnel Scenarios

This letter studies the effect of ultra-high mobility on the performance of multiple-input multiple-output (MIMO) wireless communication system in tunnel scenarios. It is found that the Doppler frequency shift generated by the ultra-high mobility reduces the correlation coefficient. Importantly, it is observed that there exists a key inflection point distance in the ultra-high speed tunnel scene. When this distance is exceeded, the channel correlation of MIMO can be significantly reduced. Moreover, it is perceived that the inflection point distance is relatively stable and is less affected by the communication frequency. On this basis, we propose a high-quality communication area with low MIMO channel correlation. For the ultra-high speed of 720km/h, the inflection distance is about 95m, and the reduction of MIMO channel correlation at 3.5GHz and 28GHz can reach up to 52.7% and 41.2%, respectively. The findings are extremely useful for the applications of the mobile MIMO communication system in tunnel and railway scenarios.

Compressive Subspace Learning With Antenna Cross-Correlations for Wideband Spectrum Sensing

Compressive subspace learning (CSL) with the exploitation of space diversity has found a potential performance improvement for wideband spectrum sensing (WBSS). However, previous works mainly focus on either exploiting antenna auto-correlations or adopting a multiple-input multiple-output (MIMO) channel without considering the spatial correlations, which will degrade their performances. In this paper, we consider a spatially correlated MIMO channel and propose two CSL algorithms (i.e., mCSLSACC and vCSLACC) which exploit antenna cross-correlations, where the mCSLSACC utilizes an antenna averaging temporal decomposition, and the vCSLACC uses a spatial-temporal joint decomposition. For both algorithms, the conditions of statistical covariance matrices (SCMs) without noise corruption are derived. Through establishing the singular value relation of SCMs in statistical sense between the proposed and traditional CSL algorithms, we show the superiority of the proposed CSL algorithms. By further depicting the receiving correlation matrix of MIMO channel with the exponential correlation model, we give important closed-form expressions for the proposed CSL algorithms in terms of the amplification of singular values over traditional CSL algorithms. Such expressions provide a possibility to determine optimal algorithm parameters for high system performances in an analytical way. Simulations validate the correctness of this work and its performance improvement over existing works in terms of WBSS performance.

Pairwise Coding for MIMO-OFDM Visible Light Communication

This work proposes a dual pairwise coding (DPWC) technique for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) systems. DPWC is a technique designed to mitigate the signal-to-noise ratio (SNR) imbalance created in the received information symbols due to the light emitting diodes’ bandwidth limitations and the strong correlation of typical indoor VLC MIMO channels. With singular value decomposition precoding, the MIMO VLC channel is decomposed into independent parallel sub-channels. The DPWC technique is then applied to further improve the system performance by pairing and interleaving OFDM subcarriers and MIMO sub-channels with imbalanced SNR. The error performance of the proposed DPWC technique is investigated in detail via simulation and experimental demonstration. Its effectiveness with different degrees of bandwidth-limitation (data rate) and channel correlation is presented, and the results are compared with other variants of the pairwise coding technique. DPWC provides performance improvement with a significant reduction in the SNR required to achieve a specific bit error rate (BER).

Chromaticity-Adaptive Generalized Spatial Modulation for MIMO VLC With Multi-Color LEDs

In this paper, we propose a chromaticity-adaptive generalized spatial modulation (CA-GSM) method for multiple-input multiple-output (MIMO) visible light communication (VLC) systems with quadrichromatic LEDs (QLEDs). After the MIMO-VLC system model is constructed, the CA-GSM method based on chromaticity-adaptive constellation optimization is brought forward accounting for the correlated MIMO channel with multi-color crosstalk and quadrangle chromaticity region. This work mainly investigates the optimal active QLED selection pattern in CA-GSM aiming at maximizing the minimum Euclidean distance of received multi-color constellation. Considering power, amplitude and chromaticity constraints, the optimization problem of chromaticity-adaptive constellation is transformed and solved by the linear approximation of Taylor series. Therefore, the design algorithm of CA-GSM is derived with the optimal QLED selection pattern. Simulation results verify that our proposed CA-GSM can achieve performance improvement compared to the multi-stream spatial modulation and the conventional design method without considering channel.

On Optimizing Feedback Interval for Temporally Correlated MIMO Channels With Transmit Beamforming and Finite-Rate Feedback

A receiver with perfect channel state information (CSI) in a point-to-point multiple-input multiple-output (MIMO) channel can compute the transmit beamforming vector that maximizes the transmission rate. For frequency-division duplex, a transmitter is not able to estimate CSI directly and has to obtain a quantized transmit beamforming vector from the receiver via a rate-limited feedback channel. We assume that time evolution of MIMO channels is modeled as a Gauss-Markov process parameterized by a temporal-correlation coefficient. Since feedback rate is usually low, we assume rank-one transmit beamforming or transmission with single data stream. For given feedback rate, we analyze the optimal feedback interval that maximizes the average received power of the systems with two transmit or two receive antennas. For other system sizes, the optimal feedback interval is approximated by maximizing the rate difference in a large system limit. Numerical results show that the large system approximation can predict the optimal interval for finite-size system quite accurately. Numerical results also show that quantizing transmit beamforming with the optimal feedback interval gives larger rate than the existing Kalman-filter scheme does by as much as 10% and than feeding back for every block does by 44% when the number of feedback bits is small.

Performance Analysis of OSTBC Transmission in Lossy Forward MIMO Relay Networks

We analyze the outage probability for the orthogonal space-time block code based multiple-input multiple-output (MIMO) relay networks, composed of one source, one relay, and a single destination. The relay forwards the decoded and interleaved information sequence even though the information part may contain error(s), according to the lossy forward strategy. In spatially independent MIMO channels, we find that the diversity order of the relay network can be interpreted and formulated by the well-known max-flow min-cut theorem . Moreover, we extend the analysis to the case of spatially correlated MIMO channels. Approximated explicit expressions for the outage probabilities are obtained in high signal-to-noise ratio regime.

MIMO Channel Reconstruction from Lower Dimensional Multiple Antenna Measurements

A method for reconstructing multiple-input multiple-output (MIMO) channel correlation matrices from lower dimensional channel measurements is presented. Exploiting the symmetry of correlation matrix structure enables reproducing higher dimensional MIMO channel matrices from available lower order measurements. This leads to practically important applications allowing prediction of higher dimensional MIMO system capacity. In particular, we study Kronecker-type MIMO channels suitable for reconstructing full channel matrices from partial information about transmit-receive fading in spatial and polarimetric domains and analyze validity conditions for such models. One of the important channel conditions is Doppler frequency related to non-stationarity in the environment. We present simulations of cluster-type scattering model using $$2\times 2$$ MIMO channel correlation matrices to predict performance of $$2\times 4$$ MIMO system including recovery of angular power spectrum. An example of dual circular polarized $$2\times 4$$ MIMO land mobile satellite measurements in 2.5 GHz frequency band illustrates applicability of the method to reconstruct spatial and polarimetric channel correlation matrices for estimating ergodic channel capacity from single-antenna or uni-polarized measurements.

On the Performance of SR and FR Protocols for OSTBC-Based AF-MIMO Relay System With Channel and Noise Correlations

This paper proposes a selection relaying (SR) protocol for a cooperative multiple-input–multiple-output (MIMO) relay system that consists of a direct link between a source and a destination. The system has only receive-side channel state information (CSI) and spatially correlated MIMO channels, and the receiver nodes observe spatially correlated noise. The transmit nodes employ orthogonal space–time block codes (OSTBCs), whereas the receiver nodes employ optimum minimum mean square error (MMSE) detection. The SR protocol, which transmits via the relay only when the direct link between the source and destination is in outage, is compared with the fixed relaying (FR) protocol, which always uses the relay. By deriving novel asymptotic expressions of the outage probabilities, it is analytically shown that both protocols provide the same diversity gain. However, the coding gain (CG) of the SR protocol can be much better than that of the FR protocol. In particular, when all MIMO links have the same effective rank, irrespective of its value, the SR protocol provides better CG than the FR scheme if the target information rate is greater than $\ln_2(\mbox{3})$ bits per channel use. Simulation results support theoretical analysis and show that the SR scheme can significantly outperform the FR method, which may justify the increased complexity due to the one-bit feedback requirement in the SR protocol.

Open-Loop Precoder Design for Spatial Multiplexing Systems in Transmit Correlated MIMO Channels

The performance of spatial multiplexing (SM) systems is significantly degraded in transmit correlated multiple-input multiple-output (MIMO) channels. In communication systems that are available for feedback, precoding can enhance performance by exploiting channel state information (CSI) at the transmitter. However, broadcasting systems have no reverse channel nor any CSI. Furthermore, channel mismatches in communication systems severely affect performance. For these systems, open-loop precoders that require no feedback CSI were proposed. However, an analytical precoder design method was not presented in the previous works. In this paper, we present an open-loop precoder design method for SM systems with a maximum-likelihood receiver in transmit correlated MIMO channels. We first describe how to design an open-loop precoder using a rotational matrix over the amplitude of the correlation. Our precoder design method is derived by observing the effect of correlation mismatch between the feedback CSI and the actual channel condition on a minimum Euclidean distance-based precoder. Next, we present a method to design an open-loop precoder that takes into account the impact of the phase of the correlation. For this precoder, a phase rotation matrix is added on the rotational matrix, and optimum parameters are obtained by maximizing the worst-case minimum distance for the phase of the correlation. The simulation results illustrate the validity of proposed open-loop precoder in transmit correlated MIMO channels.

Investigation of MIMO Channel Correlation and Capacity in Curved Subway Tunnel

This letter explores the influence of curved subway tunnel on multiple-input–multiple-output (MIMO) channel. In order to study in detail and comprehensively, extensive simulations have been carried out with different curvatures, receiver positions, and antenna configurations. Ray-tracing was utilized to obtain the MIMO channel matrices at 1.8 GHz. The influence of curved tunnel has been highlighted by comparing with that of straight tunnel in terms of a statistical analysis of correlation between antennas of receiver, singular values of channel transfer matrix, and channel capacity. The calculation of channel capacity is based on both a constant signal-to-noise ratio and a fixed transmitted power. The results provide reference for the other relevant research and engineering community in the future.

Three-Dimension Kronecker Channel Modeling and Correlation Analysis

In this paper a three-dimension (3D) multiple-input multiple-output (MIMO) channel model is derived by considering the elevation dimension and the azimuth dimension together. To get a more accurate performance analysis for 3D MIMO channel, both Tx and Rx correlation matrices are derived, respectively, in closed form, which consist of 3D Kronecker channel model. This novel 3D Kronecker channel model is developed for arbitrary antenna arrays with non-isotropic antenna patterns and also for any propagation environment of 3D MIMO systems. In order to quantify the performance of 3D MIMO systems, the capacity in multi-user cases is analyzed. Simulation results validate the proposed 3D Kronecker channel model and study the impact of elevation and azimuth angular spread and that of Rx antenna element spacing on the correlation. The proposed capacity analysis in multi-user cases for 3D MIMO systems is also verified by simulation.

채널 상관 및 직접파가 존재하는 해상 통신 환경에서 개루프 프리코딩 기반의 공간다중화 전송 기법

Recently, high data rate transmission is required in maritime communication. In this paper, we consider multiple input multiple output (MIMO) spatial multiplexing (SM). However, the performance of SM is severly degraded due to spatial channel correlation and line-of-sight (LOS) component. In the maritime communication, the MIMO channel correlation and LOS are critical due to the lack of scatteres around the transmitter and/or the receiver. When the feedback of channel information is available, precoding can enhance the error performance by exploiting the channel information. However, it is difficult to derive closed-form solution considering both the correlation and LOS. In this paper, we present open-loop precoding-based spatial multiplexing transmission method by showing that the effect of performance for the correlation and LOS. It is shown that the open-loop precoding can mitigate the performance degradation due to the LOS as well as the correlation. Consequently, we expect that the proposed open-loop precoding can be adopted to the maritime communication system.

Eigenvalue Dynamics of a Central Wishart Matrix With Application to MIMO Systems

We investigate the dynamic behavior of the stationary random process defined by a central complex Wishart (CW) matrix ${\bf{W}}(t)$ as it varies along a certain dimension $t$. We characterize the second-order joint cdf of the largest eigenvalue, and the second-order joint cdf of the smallest eigenvalue of this matrix. We show that both cdfs can be expressed in exact closed-form in terms of a finite number of well-known special functions in the context of communication theory. As a direct application, we investigate the dynamic behavior of the parallel channels associated with multiple-input multiple-output (MIMO) systems in the presence of Rayleigh fading. Studying the complex random matrix that defines the MIMO channel, we characterize the second-order joint cdf of the signal-to-noise ratio (SNR) for the best and worst channels. We use these results to study the rate of change of MIMO parallel channels, using different performance metrics. For a given value of the MIMO channel correlation coefficient, we observe how the SNR associated with the best parallel channel changes slower than the SNR of the worst channel. This different dynamic behavior is much more appreciable when the number of transmit ($N_T$) and receive ($N_R$) antennas is similar. However, as $N_T$ is increased while keeping $N_R$ fixed, we see how the best and worst channels tend to have a similar rate of change.

Optimal SP training for spatially correlated MIMO channels under coloured noises

Based on convex programming for optimisation, the optimal superimposed (SP) training signal design is proposed for spatially correlated multiple-input–multiple-output (MIMO) channels in the presence of correlated symbols and coloured Gaussian noises. Simulation results show that the proposed training design can effectively estimate the channel and outperforms the existing designs.

A novel cuckoo search approach to detection in spatially correlated MIMO channels

In this paper, we propose a cuckoo search approach to detect the transmitted symbols in spatially correlated multiple input multiple output (MIMO) channels. Detection is considered to be a challenging task in correlated channels due to the fact that the channel matrix tends to be ill conditioned or sometimes rank deficient. Currently, lattice reduction aided detection is understood to be the best approach for detection in spatially correlated channels. We propose cuckoo search as an effective approach to solve this problem. We provide the performance comparison of the proposed detector with various other detectors including lattice reduction aided detector. Through the results, we demonstrate the efficiency of cuckoo search detector over other detectors in uncorrelated and correlated channels as well. We corroborate that cuckoo search detector is a better choice over other existing detectors especially in spatially correlated MIMO channels.

Transmit Beamforming Analysis for MIMO Systems in Indoor Residential Environment Based on 3D Ray Tracing

In this paper, an evaluation of transmit beamforming (TxBF) contribution is analyzed in the context of an indoor residential environment. Using an optimized 3D ray tracing tool, the multiple-input multiple-output (MIMO) propagation channel is simulated. The algorithm of singular value decomposition is applied and a new transmission scheme is considered. Comparisons of cases with/without TxBF are realized by analyzing the received power, the signal to noise ratio of each spatial stream and the channel capacity. It is shown that the TxBF capacity gain increases when the MIMO channel correlation at the transmit side reaches high values, especially when only the dominant eigen mode is selected. The main result concerns the antenna spacing: a judicious choice of the transmitting antenna spacing can improve the effective transmission range by more than 45 % in non-line of sight conditions. Several MIMO configurations (number of antennas and spatial streams) are also compared to determine the relevance of a possible activation of TxBF. The interference influence is studied and it is shown that under some conditions, TxBF improves the signal to interference ratio.

Performance of opportunistic fixed-beam spatial multiplexing in OFDMA uplink

Adaptive spatial multiplexing (SM) Multiple Input Multiple Output (MIMO) techniques enhance the spectral efficiency of wideband wireless communications in favorable radio channel conditions. In this study, we show the benefits of combining a traditional fixed-beam scheme and multiuser opportunistic radio resource allocation in space-time-frequency domains. This combining is feasible in wideband Orthogonal Frequency Domain Multiple Access (OFDMA) systems such as the Universal Mobile Telecommunications System (UMTS) Long Term Evolution (LTE). This study brings novel knowledge which indicates that an orthogonal fixed beam approach benefits from opportunistic radio resource allocation clearly more than a conventional antenna domain approach in frequency-selective radio channels. It was found out that the fixed beam radio link performance is enhanced in wideband adaptive radio links due to the fact that orthogonal beams reduce correlation between the MIMO channels in the allocated sub-bands. Moreover, beamforming gains bring higher Eigenvalues of the MIMO channel matrix in the opportunistically selected sub-bands. It is shown that the fixed beam deployment changes the distribution of the MIMO channel correlation values and Eigenvalues in a manner which enhances opportunistic multiuser gains in wideband time-frequency-selective MIMO radio channels. Simulation results with 2 × 2 spatial multiplexing in an OFDMA uplink indicate that the proposed beam domain scheme gives up to 80% data throughput gain over the corresponding antenna domain scheme in a pedestrian microcell environment.