Effects of Mutual Coupling on Degree of Freedom and Antenna Efficiency in Holographic MIMO Communications

The holographic multiple-input-multiple-output (MIMO) communications refer to the MIMO systems built with ultra-dense antenna arrays, whose channel models and potential applications have attracted increasing attentions recently. When the spacing between adjacent array elements is larger than half wavelength, the effect of mutual coupling can generally be neglected in current antenna designs. However, in holographic MIMO communications, the influence of strong mutual coupling on antenna characteristics is inevitable, resulting in distorted radiation patterns and low radiation efficiencies. In this paper, starting from the analytical correlation and efficiency models, we investigate how the mutual coupling affects the capacity of a space-constrained MIMO system from the aspects of degree of freedom (DOF) and antenna efficiency. The involved fundamental concepts of correlation, DOF, efficiency and mutual coupling are crucial for both antenna and wireless-communication engineers when designing emerging MIMO communication systems

Capacity analysis for compact MIMO systems

We analyze the impact of mutual coupling on the capacity of MEMO systems with compact antenna arrays. Existing studies present conflicting views on the effect of mutual coupling. This is, in part, due to their different scopes and underlying assumptions of the system setups. In this paper, we aim to give a comprehensive picture by first examining the impact of mutual coupling on three capacity-related performance measures: antenna correlation, efficiency and bandwidth. While the first two aspects have received significant attention, antenna bandwidth with mutual coupling is a relatively uncharted territory. We show that while implementing a good matching network can drastically improve the system capacity for narrowband systems in the presence of strong mutual coupling, the same conclusions may not necessarily apply to wideband cases. To exemplify this, we carry out capacity simulations for an end-to-end MIMO system, where a recently proposed S-parameter approach is used in conjunction with the 3GPP-3GPP2 channel model to model realistic wideband channel and antenna effects at both transmit and receive ends

Mutual Coupling Reduction between Closely Spaced U-slot Patch Antennas by Optimizing Array Configuration and its Applications in MIMO

Multiple-input, multiple-output (MIMO) systems have received considerable attention over the last decade. There are some limitations when obtaining the most from MIMO,such as mutual coupling between antenna elements in an array. Mutual coupling and therefore inter-element spacing have important effects on the channel capacity of a MIMO communication system, its error rate, and ambiguity ofMIMO radar system. There is a huge amount of research that focuses on reducing the mutual coupling in an antenna array to improve MIMO performance. In this research, we focus on the antenna section of the system.Antenna design affects the performance of Multiple-Input-Multiple-output (MIMO) systems. Two aspects of an antenna‟s role in MIMO performance have been investigated in this thesis. Employing suitable an antenna or antenna array can have a significant impact on the performance of a MIMO system. In addition to antenna design, another antenna related issue that helps to optimize the system performance is to reduce mutual coupling between antenna elements in an array. Much research has focused on the reduction of mutual coupling. In this research, the effect of the antenna configuration in array on mutual coupling has been studied and the main purpose is to find the array configuration that providesthe minimum mutual coupling between elements. The U-slot patch antenna is versatile antennas that because of its features like wide bandwidth,multi-band resonance and the ease of achieving different polarizations. This research first investigated the u-slot patch antenna, its features and capabilities. Seconda CAD optimization to design a low profile, dual band U-slot patch antenna is provided. Designed antenna is a dual band antenna that is intended to work at 3.5 and 5 GHz and have sufficient gain of at least 3dB. The effect of mutual coupling on MIMO systems is studied and then different array configurations were considered for two closely spaced U-slot patch antennas. Different configurations show different mutual coupling behavior. After modeling and simulation, the array was designed, implemented and finally tested in an anechoic chamber. These results are compared to both simulation and theoretical results and the configuration with minimum amount of mutual coupling was found. Some radar experiments also have been done to prove the effect of mutual coupling on radar performanc

Miniaturized DGS and EBG structures for decoupling multiple antennas on compact wireless terminals

MIMO (Multiple Input Multiple Output) technology has been presented to significantly increase the wireless channel capacity and reliability without requiring additional radio spectrum or power. In MIMO systems, multiple antennas are mounted at both the transmitter and the receiver. When this technology is employed for a compact wireless terminal, one of the most challenging tasks is to reduce the high mutual coupling between closely placed antenna array elements. The high mutual coupling produces high correlation between antenna elements and affects the channel capacity of MIMO system. The objectives of this thesis are to design practical miniaturized structures to reduce high mutual coupling for small wireless terminals. The research is conducted in the following areas. Initially, a PIFA design and two-element PIFA array are proposed and optimized to operate at 1.9GHz. A pair of two coupled quarter-wavelength linear slits is inserted in a compact ground plane, resulting in significant reduction of the mutual coupling across antenna operating frequency band. In order to take up less space on the ground plane, instead of the linear slits, miniaturized convoluted slits are implemented between the two closely placed PIFAs. Although the convoluted slits have small area and are positioned close to the edges of the ground plane, the miniaturized convoluted slit structures achieve a reduction of mutual coupling between antenna elements and succeed in reducing the effect of the human body (head and hand) to the antennas. In order to further reduce the size of the slits etched on the compact ground plane, a novel double-layer slit-patch EBG structure is proposed. It consists of a two-layer structure including conducting patches and aperture slits placed on either side of a very thin dielectric layer. They are placed in very close proximity to each other (55μm). A two-element printed CPW-fed monopole array operating around 2.46GHz and a two-element UWB planar monopole array operating from 3GHz to 6GHz have been employed to investigate the proposed slit-patch EBG structures. The optimized double-layer slit-patch EBG structure yields a significant reduction of the mutual coupling and produces the maximum miniaturization of antenna array. Another novel convoluted slit-patch EBG structure has been presented to reduce the mutual coupling between two PIFAs operating at 1.9GHz. These results demonstrate that the slit-patch EBG structure is a feasible technology to reduce the mutual coupling between multiple antennas for compact wireless terminals

Spatial data stream multiplexing scheme for high-throughput WLANs

A novel scheme using spatial data stream multiplexing (SDSM) in the upcoming multiple-input multipleoutput (MIMO)-based IEEE 802.11n physical layer is proposed. It is shown that with SDSM, the same data rate can be achieved by using less number of transmit and receive antennas and therefore this scheme can reduce the number of antennas which results in reducing mutual coupling effects, hardware costs and implementation complexities. The maximum data rates that can be achieved using a 2 * 2 MIMO system is 270 Mbps and for a 4 * 4 MIMO system is 540 Mbps. The same data rates can be achieved using the SDSM technique which reduces the 2 * 2 MIMO system to 1 * 1 SISO system and the 4 * 4 MIMO system to a 2 * 2 MIMO system

Scaling up MIMO: Opportunities and Challenges with Very Large Arrays

This paper surveys recent advances in the area of very large MIMO systems. With very large MIMO, we think of systems that use antenna arrays with an order of magnitude more elements than in systems being built today, say a hundred antennas or more. Very large MIMO entails an unprecedented number of antennas simultaneously serving a much smaller number of terminals. The disparity in number emerges as a desirable operating condition and a practical one as well. The number of terminals that can be simultaneously served is limited, not by the number of antennas, but rather by our inability to acquire channel-state information for an unlimited number of terminals. Larger numbers of terminals can always be accommodated by combining very large MIMO technology with conventional time- and frequency-division multiplexing via OFDM. Very large MIMO arrays is a new research field both in communication theory, propagation, and electronics and represents a paradigm shift in the way of thinking both with regards to theory, systems and implementation. The ultimate vision of very large MIMO systems is that the antenna array would consist of small active antenna units, plugged into an (optical) fieldbus.Comment: Accepted for publication in the IEEE Signal Processing Magazine, October 201

Reciprocity Calibration for Massive MIMO: Proposal, Modeling and Validation

This paper presents a mutual coupling based calibration method for time-division-duplex massive MIMO systems, which enables downlink precoding based on uplink channel estimates. The entire calibration procedure is carried out solely at the base station (BS) side by sounding all BS antenna pairs. An Expectation-Maximization (EM) algorithm is derived, which processes the measured channels in order to estimate calibration coefficients. The EM algorithm outperforms current state-of-the-art narrow-band calibration schemes in a mean squared error (MSE) and sum-rate capacity sense. Like its predecessors, the EM algorithm is general in the sense that it is not only suitable to calibrate a co-located massive MIMO BS, but also very suitable for calibrating multiple BSs in distributed MIMO systems. The proposed method is validated with experimental evidence obtained from a massive MIMO testbed. In addition, we address the estimated narrow-band calibration coefficients as a stochastic process across frequency, and study the subspace of this process based on measurement data. With the insights of this study, we propose an estimator which exploits the structure of the process in order to reduce the calibration error across frequency. A model for the calibration error is also proposed based on the asymptotic properties of the estimator, and is validated with measurement results.Comment: Submitted to IEEE Transactions on Wireless Communications, 21/Feb/201