Mutual coupling in multi-element array antennas and its influence on MIMO channel capacity

The effects of mutual coupling between receive elements in a multi-element antenna on multiple-input multiple-output (MIMO) system capacity are reported. The results are based upon analysis of mutual coupling calculated for a measured linear array of five printed dipole elements with 0.56 of a wavelength spacing. It is shown that the presence of mutual coupling at the receive array antenna leads to additional correlation between spatial channels and results in a loss of MIMO system capacity.     

Channel Capacity Between Antenna Arrays— Part I: Sky Noise Dominates

Space–time coding techniques can be used to achieve very high spectral efficiencies in highly scattering environments using multiple transmit and receive antennas. At the remote station, there is usually a more limited space allotted to the antenna array than at the base station. Since the spectral efficiency improves with the number of antennas, one is interested in how many antennas can be crammed into the limited space on the remote station. This paper (Part I of II) addresses some of the issues which affect the allowable density of antennas in the remote station. In particular, the mutual impedance between antenna elements in the remote array and the correlation between the signal and noise fields received by these elements are analyzed for their impact on the channel capacity achievable by such arrays. In particular, we assume the transmitter is radiating from$n_T$elements of uncoupled half-wave dipoles and knows nothing of the channel. A formula is given for the maximum channel capacity to a receiving array of$n_R$elements, coupled to each other in the presence of ambient noise or interference with a uniform angle of arrival distribution. This formula neglects amplifier noise in the receivers. It is shown that the channel capacity is already determined at the terminals of the receiving array, and can not be improved by internal coupling networks following the receiving array. When the propagation is by means of full three-dimensional scattering, the channel capacity is unaffected by mutual coupling in the receiving array.     

Channel-Optimized Quantization With Soft-Decision Demodulation for Space–Time Orthogonal Block-Coded Channels

We introduce three soft-decision demodulation channel-optimized vector quantizers (COVQs) to transmit analog sources over space–time orthogonal block (STOB)-coded flat Rayleigh fading channels with binary phase-shift keying (BPSK) modulation. One main objective is to judiciously utilize the soft information of the STOB-coded channel in the design of the vector quantizers while keeping a low system complexity. To meet this objective, we introduce a simple space–time decoding structure that consists of a space–time soft detector, followed by a linear combiner and a scalar uniform quantizer with resolution$q$. The concatenation of the space–time encoder/modulator, fading channel, and space–time receiver can be described by a binary-input,$2^q$-output discrete memoryless channel (DMC). The scalar uniform quantizer is chosen so that the capacity of the equivalent DMC is maximized to fully exploit and capture the system's soft information by the DMC. We next determine the statistics of the DMC in closed form and use them to design three COVQ schemes with various degrees of knowledge of the channel noise power and fading coefficients at the transmitter and/or receiver. The performance of each quantization scheme is evaluated for memoryless Gaussian and Gauss–Markov sources and various STOB codes, and the benefits of each scheme is illustrated as a function of the antenna-diversity and soft-decision resolution$q$. Comparisons to traditional coding schemes, which perform separate source and channel coding operations, are also provided.     

互耦效应对超大规模MIMO天线系统信道容量的影响

超大规模多输入多输出(Multiple-Input Multiple-Output,MIMO)天线系统是6G的关键技术,由于天线单元间距很小,多个天线单元的互耦效应是影响其性能的因素之一。建立了基于石墨烯基贴片天线阵列-子阵列架构的超大规模MIMO天线系统,推导出了互耦效应影响下的信道容量表达式。通过电磁场数值计算仿真了超大规模MIMO天线系统的信道容量,结果表明,在不考虑互耦效应时,超大规模MIMO天线系统的信道容量与子阵列天线单元数、子阵列数以及发射机功率正相关;在互耦效应的影响下,系统的信道容量降低,互耦效应的强弱与子阵列天线单元的间距有关,天线单元间间距越小,相邻天线间的互耦效应越明显,系统的信道容量越小。该仿真结果可以为6G中超大规模MIMO天线系统的设计提供参考。     

Adaptive Algorithm Using Hot-Electron Injection for Programming Analog Computational Memory Elements Within 0.2% of Accuracy Over 3.5 Decades

This paper describes a new predictive algorithm that can be used for programming large arrays of analog computational memory elements within 0.2% of accuracy for 3.5 decades of currents. The average number of pulses required are 7–8 (20$muhbox s$each). This algorithm uses hot–electron injection for accurate programming and Fowler–Nordheim tunneling for global erase. This algorithm has been tested for programming 1024$,times$16 and 96$,times$16 floating-gate arrays in 0.25$muhbox m$and 0.5$muhbox m$n-well CMOS processes, respectively.     

一种可用于卫星导航抗干扰天线的降耦方法

针对基于功率倒置算法的空时自适应抗干扰技术中天线阵元间互耦误差严重恶化算法性能这一问题,分析比较了目前常见的应对解决方案,并提出一种新的可用于抗干扰卫星导航终端的天线阵列降耦方法。该方法通过在卫星导航抗干扰天线阵列中加载电磁谐振吸波器,降低天线阵元间互耦。实验数据表明,利用该方法可以使阵元间互耦降低10 dB,使抗干扰接收机最大抑制干信比提升5 dB。     

The receive voltage transmission function of a phased array antenna element

A general method has been developed for determining the voltage response of a phased array element to an incoming electric field. The incident field may arrive from any scan angle, and mutual coupling effects are accounted for in terms of the array element active reflection coefficient. The method defines a receive voltage transmission functionbar{T}(theta, phi, f)as a term whose product with the incident electric fieldE_{i}(theta, phi, f) results in the array element voltage response. The derivation ofbar{T}(theta, phi, f)is obtained by treating each phased array antenna element as a receiving aperture, having an active impedance related to the mutual coupling in the array. The element effective length is formulated using an array element receive equivalent circuit, and the transmission functionbar{T}(theta, phi, f)is obtained using this effective length formulation. The usefulness of the transmission function for studying the response of a phased array element to particular radar signal waveforms is suggested, and an experimental evaluation ofbar{T}(theta phi, f)is described using mutual coupling measurements on a 37-element crossed dipole test array.     

Mutual coupling compensation of linear antenna array using evolutionary optimization technique

This paper presents an optimization-based approach to compensate for the mutual coupling effect and to reduce the sidelobe level (SLL) of the normalized radiation pattern by optimizing the current excitation amplitude of the antenna array elements. Due to the mutual coupling effect, the SLL of the radiation pattern is increased, and thereby, the field pattern of the antenna array is severely degraded. This causes interference with other communication systems working at the same frequency. Toward the compensation of the mutual coupling effect and reduction of SLL, the Cat swarm optimization (CSO) algorithm is employed, and the excitation amplitudes of the antenna elements are optimized. In this regard, optimizing the cost function is defined by introducing the impedance factor (IF), calculated by using the self-impedance and mutual impedance of the antenna elements. The proposed method for the synthesis of a mutually coupled linear antenna array is less expensive, simpler to use, and more effective. Array sets of 4, 6, 8, and 10 elements are considered for optimization.     

Mutual coupling, gain and directivity of an array of two identical antennas

It is shown that there is an optimum nonzero value of mutual coupling and passive reflection coefficient that is a function of only the embedded element pattern and element spacing. Optimum is defined as yielding maximum array gain. For closely spaced arrays, the optimum mutual coupling and passive reflection coefficient are quite large, but tend to cancel in the active mode to produce a low active reflection coefficient. A smaller value of mutual coupling would give lower gain than the optimum case. This again demonstrates that mutual coupling is a natural and desirable effect in array behavior. It is well known that it is possible to achieve array superdirectivity, in the sense that the directivity of an array of "N" elements is greater than "N" times the embedded element directivity; however, in this case the mutual coupling and passive reflection produce an active reflection coefficient that results in an array gain lower than "N" times the embedded element gain. In fact, higher superdirectivity always implies lower gain, relative to embedded element gain.     

Channel Capacity Between Antenna Arrays— Part II: Amplifier Noise Dominates

In this paper (Part II of two), we continue examining the use of space-time coding techniques to achieve very-high spectral efficiencies in highly scattering environments, using multiple transmit and receive antennas. The goal is to increase as much as possible the number of antenna elements, which is particularly difficult at the remote station, which usually has a more limited space allotted to the antenna array than at the base station. Under the assumption that sky noise was the dominant noise source, Part I addressed the channel-capacity effects of mutual impedance between antenna elements in the remote array, and the correlation between the signal and noise fields received by these elements. In Part II, we consider the same effects under the assumption that amplifier noise is the dominant noise source. The question of how closely the receiving array elements can be spaced depends on how precisely the channel can be estimated. This is related to the high-precision requirement experienced with supergain antenna arrays. The supergain connection is made explicit by showing that the optimum channel capacity for the case of a single transmitting element is achieved by using the supergain weights for the receiving array. To indicate the effect of noisy channel estimation, the loss in receiver antenna gain due to noise in weight estimates is computed with a simple simulation model of scattered propagation for the single-transmitting-antenna element case     

Impact of Mutual Coupling on Adaptive Switching Between MIMO Transmission Strategies and Antenna Configurations

Adaptive switching between multiple-input multiple-output (MIMO) transmission strategies like diversity and spatial multiplexing is a flexible approach to respond to channel variations. It is desirable to obtain accurate estimates of the switching points between these transmission schemes to realize the capacity gains made possible by adaptive switching. In this paper, it is shown that the accuracy of switching point estimates for switching between statistical beamforming and spatial multiplexing is improved by taking into account the effects of mutual coupling between antenna array elements. The impact of mutual coupling on the ergodic capacities of these two transmission strategies is analyzed, by deriving expressions for the same. Adaptive switching between combinations of transmission strategies and antenna array configurations (using reconfigurable antenna arrays) is shown to produce maximum capacity gains. Expressions for the switching points between transmission strategies and/or antenna configurations, including mutual coupling effects, are derived and used to explore the influence of mutual coupling on the estimates. Finally, measurements taken from reconfigurable rectangular patch antenna arrays are used to validate the analytical results.     

固定阵列长度两发多收MIMO系统功率偏移分析

高信噪比情况下,功率偏移是信噪比-容量曲线中信噪比轴的截距,优化功率偏移参数有助于提高系统容量。该文采用固定阵列长度的均匀直线阵,通过拟合三对角特普利茨矩阵行列式曲线,分析了接收天线间相关性以及收发天线数目的最大值对于两发多收单用户MIMO系统功率偏移的影响,得出了给定天线阵列长度情况下,功率偏移极值点的基本公式。仿真结果表明,可以利用极值点公式选择合理的接收天线数目,实现最优功率偏移。     

Dual-Band Microstrip Bandpass Filter Using Stepped-Impedance Resonators With New Coupling Schemes

A microstrip bandpass filter using stepped-impedance resonators is designed in low-temperature co-fired ceramic technology for dual-band applications at 2.4 and 5.2 GHz. New coupling schemes are proposed to replace the normal counterparts. It is found that the new coupling scheme for the interstages can enhance the layout compactness of the bandpass filter; while the new coupling scheme at the input and output can improve the performance of the bandpass filter. To validate the design and analysis, a prototype of the bandpass filter was fabricated and measured. It is shown that the measured and simulated performances are in good agreement. The prototype of the bandpass filter achieved insertion loss of 1.25 and 1.87 dB,$ S_11$of$-$29 and$-$40 dB, and bandwidth of 21% and 12.7% at 2.4 and 5.2 GHz, respectively. The bandpass filter is further studied for a single-package solution of dual-band radio transceivers. The bandpass filter is, therefore, integrated into a ceramic ball grid array package. The integration is analyzed with an emphasis on the connection of the bandpass filter to the antenna and to the transceiver die.     

Prediction of noise and interference cancellation with a moving compact receiver array in an indoor environment

An interference and noise cancellation technique based on a reference signal is studied for a compact receiver antenna array placed in an indoor environment. The exact active element pattern method is used to model the radiation characteristics of the array, including mutual coupling between the antenna elements. A two-dimensional ray-tracing technique describes the signal propagation in the indoor environment for a moving receiving antenna.     

Array Calibration for CDMA Smart Antenna Systems

In this paper, we investigate array calibration algorithms to derive a further improved version for correcting antenna array errors and RF transceiver errors in CDMA smart antenna systems. The structure of a multi‐channel RF transceiver with a digital calibration apparatus and its calibration techniques are presented, where we propose a new RF receiver calibration scheme to minimize interference of the calibration signal on the user signals. The calibration signal is injected into a multichannel receiver through a calibration signal injector whose array response vector is controlled in order to have a low correlation with the antenna response vector of the receive signals. We suggest a model‐based antenna array calibration to remove the antenna array errors including mutual coupling errors or to predict the element patterns from the array manifold measured at a small number of angles. Computer simulations and experiment results are shown to verify the calibration algorithms.     

Compensating for the mutual coupling effect in a normal-mode helical antenna array for adaptive ing

A practical normal-mode helical antenna array together with a new method for the compensation of the mutual coupling effect is proposed for the efficient adaptive ing of interferences in a mobile communication environment. The helical antenna array is shown to be able to generate deeper s than an equivalent monopole antenna array in the presence of the mutual coupling effect. The helical antenna elements are smaller in size and less susceptible to the mutual coupling effect than monopole antenna elements. The new method for compensation of the mutual coupling effect is shown to have a greater ability and flexibility than previous methods. A new definition of mutual impedance is introduced and used to find the coupling-free signal voltages across the antenna terminal loads. Numerical examples demonstrate the superior performance of the proposed adaptive array and the new compensation method.     

Adaptive processing using a single snapshot for a nonuniformlyspaced array in the presence of mutual coupling and near-fieldscatterers

This paper presents an adaptive technique to extract the signal of interest (SOI) arriving from a known direction in the presence of strong interferers using a single snapshot of data. The antenna elements in this method can be nonuniformly spaced and there can be mutual coupling between them. In addition, near-field scatterers can also be present. First, the voltages induced in the antenna elements of the array due to interferers, mutual coupling between the elements, and near-field scatterers is preprocessed by applying a transformation matrix to these voltages through a rigorous electromagnetic analysis tool. This electromagnetic preprocessing technique transforms the voltages that are induced in a nonuniformly spaced array containing real antenna elements to a set of voltages that will be produced in a uniform linear virtual array (ULVA) containing omnidirectional isotropic point radiators. In the transformation matrix we would like to include various electromagnetic effects like mutual coupling between the antenna elements, presence of near-field scatterers and the platform effects on which the antenna array is mounted. This transformation matrix when applied to the actual measured voltages yields an equivalent set of voltages that will be induced in the ULVA. A direct data domain least squares adaptive algorithm is then applied to the processed voltages to extract the SOI in the presence of interferers. Limited numerical examples are presented to illustrate the novelty of the proposed method     

Performance analysis of compact antenna arrays with MRC incorrelated Nakagami fading channels

This paper presents the average error probability performance of a compact space diversity receiver for the reception of binary coherent and noncoherent modulation signals through a correlated Nakagami (1960) fading channel. Analytical expressions of the average bit error rate (BER) are derived as a function of the covariance matrix of the multipath component signals at the antenna elements. Closed-form expressions for the spatial cross-correlation are obtained under a Gaussian angular power profile assumption, taking account of the mutual coupling between antenna elements. The effects of antenna array configuration (geometry and electromagnetic coupling) and the operating environment (fading, angular spread, mean angle-of-arrival) on the BER performance are illustrated     

Multiple access capacity of UWB m-ary impulse radio systems with antenna array

In this paper, the multiple access capacity of an Mary pulse position modulation (PPM) impulse radio (IR) system with antenna array is analyzed in dense multipath environments. An antenna array with Rake receivers is used to capture the signal energy from multipaths. Multiple access performance of the system is evaluated in terms of number of supported users for a given bit error rate and bit transmission rate with different number of antenna elements and selected paths. Numerical results show that the multiple access capacity of an M-ary IR system can be improved significantly by increasing the number of antenna elements and/or by adding more paths coherently at the receiver.     

考虑互耦影响下的智能天线数字波束赋形

本文分析了阵列单元之间的互耦对智能天线阵列数字波束赋形（DBF）的影响。通过对天线子系统建立等效接收模型，对由理想半波振子组成的8单元圆阵（用于TD-SCDMA系统）进行矩量法（MOM）分析，得到表征单元之间互耦的广义阻抗矩阵，由此得到智能天线的互耦信息及其对天线性能（SINR）的影响。通过数值分析结果与试验的结果与对比可以看出互耦对副瓣电平和智能天线抑制干扰的能力有一定的影响。本文的分析对智能天线的研究具有一定意义。