This paper presents simple throughput models for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems with hybrid automatic repeat request (HARQ). In ...particular, we consider the long term evolution (LTE) systems. Antenna correlations and power imbalances are implicitly included in the models. In order to explicitly characterize the antenna effects on throughput, we resort to the spatial multiplexing (SM) efficiency. The SM efficiencies of a MIMO antenna with zero forcing (ZF), minimum mean square error (MMSE), and maximum likelihood (ML) receivers are studied via numerical simulations using the throughput models. To verify the throughput models, the throughputs of two LTE terminals (including antenna correlations and power imbalances) are measured in a reverberation chamber (RC). Good agreements between the measured and modeled throughputs are observed.
The turn-table platform stirring technique has been used to improve the measurement accuracy of a reverberation chamber (RC), where half-wavelength spatial sampling on the platform is usually used as ...a rule of thumb. A smaller sampling distance (scaling factor) was suggested experimentally by curve fitting in a previous work. However, an analytical study of this scaling factor is still missing in the literature. By expanding the electromagnetic plane wave using the Jacobi-Anger expansion, the scaling factor is determined when the truncation error falls below certain threshold. The result is verified by measurements in a RC.
This paper provides a systematic review of the mutual coupling in multiple-input multiple-output (MIMO) systems, including the effects on performances of MIMO systems and various decoupling ...techniques. The mutual coupling changes the antenna characteristics in an array, and therefore, degrades the system performance of the MIMO system and causes the spectral regrowth. Although the system performance can be partially improved by calibrating out the mutual coupling in the digital domain, it is more effective to use decoupling techniques (from the antenna point) to overcome the mutual coupling effects. Some popular decoupling techniques for MIMO systems (especially for massive MIMO base station antennas) are also presented.
Grant-free random access is a promising protocol to support massive access in beyond fifth-generation (B5G) cellular Internet-of-Things (IoT) with sporadic traffic. Specifically, in each coherence ...interval, the base station (BS) performs joint activity detection and channel estimation (JADCE) before data transmission. Due to the deployment of a large-scale antennas array and the existence of a huge number of IoT devices, JADCE usually has high computational complexity and needs long pilot sequences. To solve these challenges, this paper proposes a dimension reduction method, which projects the original device state matrix to a low-dimensional space by exploiting its sparse and low-rank structure. Then, we develop an optimized design framework with a coupled full column rank constraint for JADCE to reduce the size of the search space. However, the resulting problem is non-convex and highly intractable, for which the conventional convex relaxation approaches are inapplicable. To this end, we propose a logarithmic smoothing method for the non-smoothed objective function and transform the interested matrix to a positive semidefinite matrix, followed by giving a Riemannian trust-region algorithm to solve the problem in complex field. Simulation results show that the proposed algorithm is efficient to a large-scale JADCE problem and requires shorter pilot sequences than the state-of-art algorithms which only exploit the sparsity of device state matrix.
In this paper, the distribution of the measured antenna efficiency in a reverberation chamber (RC) is derived, based on which the statistics (e.g., mean and variance) of the measured antenna ...efficiency are studied. It is shown that the standard method (proposed in IEC 61000-4-21) for measuring antenna efficiency results in a consistent estimate. However, the standard estimator is only asymptotically unbiased (as the independent sample number N goes to infinity) and it is biased for a finite N. The derived analytical expressions for the statistics of the antenna efficiency estimator are verified by simulations as well as RC measurements. In order to use the analytical expressions of the estimation statistics in practice, one needs to estimate the independent sample number N from the measurement. This can be done based on a single measurement. Thus, using the derived estimation statistics, one can characterize the measurement performance of the antenna efficiency based on a single measurement, instead of directly estimating the statistics by performing many independent measurements.
Light field (LF) photography is an emerging paradigm for capturing more immersive representations of the real world. However, arising from the inherent tradeoff between the angular and spatial ...dimensions, the spatial resolution of LF images captured by commercial micro-lens-based LF cameras is significantly constrained. In this paper, we propose effective and efficient end-to-end convolutional neural network models for spatially super-resolving LF images. Specifically, the proposed models have an hourglass shape, which allows feature extraction to be performed at the low-resolution level to save both the computational and memory costs. To fully make use of the 4D structure information of LF data in both the spatial and angular domains, we propose to use 4D convolution to characterize the relationship among pixels. Moreover, as an approximation of 4D convolution, we also propose to use spatial-angular separable (SAS) convolutions for more computationally and memory-efficient extraction of spatial-angular joint features. Extensive experimental results on 57 test LF images with various challenging natural scenes show significant advantages from the proposed models over the state-of-the-art methods. That is, an average PSNR gain of more than 3.0 dB and better visual quality are achieved, and our methods preserve the LF structure of the super-resolved LF images better, which is highly desirable for subsequent applications. In addition, the SAS convolution-based model can achieve three times speed up with only negligible reconstruction quality decrease when compared with the 4D convolution-based one. The source code of our method is available online.
In this paper, we consider a large scale multiple input multiple output (LS-MIMO) relaying system, where an information source sends the message to its intended destination aided by an LS-MIMO relay, ...while a passive eavesdropper tries to intercept the information forwarded by the relay. The advantage of a large scale antenna array is exploited to improve spectral efficiency and enhance wireless security. In particular, the challenging issue incurred by short-distance interception is well addressed. Under very practical assumptions, i.e., no eavesdropper channel state information (CSI) and imperfect legitimate CSI at the relay, this paper gives a thorough secrecy performance analysis and comparison of two classic relaying techniques, i.e., amplify-and-forward (AF) and decode-and-forward (DF). Furthermore, asymptotical analysis is carried out to provide clear insights on the secrecy performance for such an LS-MIMO relaying system. We show that under large transmit powers, AF is a better choice than DF from the perspectives of both secrecy performance and implementation complexity, and prove that there exits an optimal transmit power at medium regime that maximizes the secrecy outage capacity.
As a complement to high-layer encryption techniques, physical layer security has been widely recognized as a promising way to enhance wireless security by exploiting the characteristics of wireless ...channels, including fading, noise, and interference. In order to enhance the received signal power at legitimate receivers and impair the received signal quality at eavesdroppers simultaneously, multiple-antenna techniques have been proposed for physical layer security to improve secrecy performance via exploiting spatial degrees of freedom. This paper provides a comprehensive survey on various multiple-antenna techniques in physical layer security, with an emphasis on transmit beamforming designs for multiple-antenna nodes. Specifically, we provide a detailed investigation on multiple-antenna techniques for guaranteeing secure communications in point-to-point systems, dual-hop relaying systems, multiuser systems, and heterogeneous networks. Finally, future research directions and challenges are identified.
Due to the complicated and time-varying boundary conditions, reverberation chamber (RC) measurements are usually studied using statistical analysis. Hence, it is of importance to know the number of ...independent samples of the measurements. Different methods for estimating the number of independent samples has been proposed in the literature. This paper uses the spatial degrees of freedom method to estimate the independent sample number. Comparing with the commonly used autocorrelation function method, the proposed method requires less computation time and provides a smaller estimation bias for correlated samples and a smaller estimation variance once above a certain sample number. The estimated independent sample number can be used to explain the dependences of the measurement uncertainty on frequency, mode stirrers, and RC loading. Finally, the estimated independent sample number is used to predict the standard deviation (STD) of the average power level for different loading configurations, where good agreements are observed.
Throughputs of 2 Formula Omitted 2 long term evolution (LTE) multiple-input multiple-output (MIMO) systems in both open- and closed-loop configurations have been studied previously. However, in the ...previous work, the study of the closed-loop MIMO system is limited to receive semi-correlation (i.e., correlations exist only at the receive side only), resulting in inaccurate modeling of the closed-loop throughput with large transmit antenna correlations and incomplete conclusion about the effectiveness of codebook based precoding based on 2-bit feedback. In this work, the effect of the transmit antenna correlation on the closed-loop throughput is examined. Unlike in the previous work, an inversely weighted mean square error (MSE) matrix is proposed as the precoding selection criterion, resulting in good agreements between the simulations and the RC measurements of a closed-loop LTE system with various correlations. It is found that the closed-loop MIMO system is superior to its open-loop counterpart in the presence of large transmit antenna correlation in the low and medium signal-to-noise (SNR) regimes.
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