Hon Tat Hui

A Note on the Mutual-Coupling Problems in Transmitting and Receiving Antenna Arrays

In antenna arrays, mutual coupling between antenna elements is well known as an undesired effect, which degrades the performance of array signal-processing algorithms. The compensation of such an undesired effect has been a popular research topic throughout the years. Various approaches for mutual-coupling compensation have been developed, and they can easily be found in the open literature. In general, the mutual-coupling problems for a transmitting and receiving array are different, even if the physical geometry of the array remains unchanged. However, it seems that antenna engineers are not well aware of such differences in the analysis of receiving antenna arrays. In this note, the mutual-coupling problems in transmitting and receiving antenna arrays are revisited. The differences between the mutual coupling and mutual impedances for transmitting and receiving antenna arrays are explained. It is concluded that the mutual-coupling problems of transmitting and receiving arrays are in general different, and hence different mutual impedances should be used for mutual-coupling analysis and compensation.

Decoupled 2D Direction of Arrival Estimation Using Compact Uniform Circular Arrays in the Presence of Elevation-Dependent Mutual Coupling

Based on the rank reduction theory (RARE), a decoupled method for 2D direction of arrival (DOA) estimation in the presence of elevation-dependent mutual coupling is proposed for compact uniform circular arrays (UCAs). Using a new formulation of the beamspace array manifold in the presence of mutual coupling, the azimuth estimates are decoupled from the elevation estimates and obtained with no need for the exact knowledge of mutual coupling. For the elevation estimation, a 1D parameter search in the elevation space for every azimuth estimate is performed with the elevation-dependent mutual coupling effect compensated efficiently. Though the computational load for the elevation estimation is increased compared to that of the original UCA-RARE algorithm, the 1D parameter search in our method overcomes most of the inherent shortcomings of the UCA-RARE algorithm. This enables unambiguous and paired 2D DOA estimation with the elevation-dependent mutual coupling effect being compensated for effectively. Numerical examples are presented to demonstrate the effectiveness of the proposed method.

Optimizing MIMO Channel Capacities Under the Influence of Antenna Mutual Coupling

This letter investigates the effect of antenna mutual coupling involving the unconventional concepts of transmitting and receiving mutual impedances on the multiple-input multiple-output (MIMO) channel capacity. Suitable expressions for the optimum power allocation strategy under the influence of antenna mutual coupling are derived. The undertaken numerical analysis shows a significant difference in the calculated results for the optimized average channel capacity for the cases when antenna mutual coupling is taken into account or neglected. The obtained results indicate that to obtain the correct optimum power allocation strategy antenna mutual coupling effects need to be properly taken into consideration. This claim is supported by computer simulation examples.

Compensation for the mutual coupling effect in the ESPRIT direction finding algorithm by using a more effective method

An effective method is introduced to compensate for the mutual coupling effect in the estimation of signal parameter via rotational invariance techniques (ESPRIT) direction finding algorithm. The compensation method seeks to quantify the mutual coupling more accurately by using a new mutual impedance. Study of two closely spaced arriving signals shows that the high-resolution capability of ESPRIT can be achieved only when the mutual coupling effect is compensated for by using our new compensation method. Critical situations with a large signal level difference, with an increased mutual coupling effect resulting from a more compact-size antenna array, and with signals coming from a nonhorizontal elevation angle are also studied using the ESPRIT algorithm. Results show that the new compensation method, when applied to ESPRIT, is more accurate, more robust, and more flexible than the previous open-circuit voltage method.

Antenna Designer's Notebook - Receiving mutual impedance between two normal-mode helical antennas (NMHAs)

A new mutual impedance - the receiving mutual impedance - between two normal-mode helical antennas is defined, measured, and theoretically calculated. The variations of the receiving mutual impedance with antenna separation, with frequency, and with excitation source direction are critically investigated. An application of the receiving mutual impedance in direction finding demonstrates its more accurate description of the mutual coupling effect than that using the conventional mutual impedance

Compensation for the mutual coupling effect in uniform circular arrays for 2D DOA estimations employing the maximum likelihood technique

An effective compensation method for the mutual coupling effect in uniform circular arrays (UCAs) employed for two-dimensional (2D) direction-of-arrival (DOA) estimations is introduced. A new 2D DOA searching algorithm using the maximum likelihood technique optimized by the emperor selective genetic algorithm (ML-EMSGA) is introduced for use with UCAs. This method circumvents the difficulty of dealing with coherent signals in 2D DOA estimations. ML-EMSGA is less computationally demanding than the maximum likelihood method (MLM) and statistically more efficient. Our study shows that ML-EMSGA can be effectively combined with the proposed compensation method, which is based on the introduction of a new mutual impedance, to give very accurate and robust 2D DOA estimation results. The structure of mutual impedance matrix for UCAs under the compensation method is fully explained. The theory of the ML-EMSGA for the UCAs is formulated. Computer simulation examples on several synthetic scenarios are presented to demonstrate the effectiveness of the mutual coupling compensation method and the superior performance of the ML-EMSGA for UCAs.

Improved DOA Estimations Using the Receiving Mutual Impedances for Mutual Coupling Compensation: An Experimental Study

An experimental study is performed to investigate the improvement of direction-of-arrival (DOA) estimation using the recently proposed receiving mutual impedances for mutual coupling compensation. A seven-monopole antenna array is constructed and used in DOA estimation employing the MUSIC algorithm. Comparisons are made with the case of using the conventional mutual impedances for mutual coupling compensation. All experiments are carried out inside an anechoic chamber. Results are obtained for one- and two-source experiments which indicate that the performance of DOA estimation in the presence of mutual coupling can be significantly improved when the mutual coupling effect is compensated by using the receiving mutual impedances.

Mutual Coupling Compensation for Direction-of-Arrival Estimations Using the Receiving-Mutual-Impedance Method

A short review of the receiving-mutual-impedance method (RMIM) for mutual coupling compensation in direction finding applications using linear array is conducted. The differences between the conventional-mutual-impedance method (CMIM) and RMIM, as well as the three different determination methods for receiving mutual impedance (RMI), will be discussed in details. As an example, direction finding with better accuracies is used for demonstrating the superiority of mutual coupling compensation using RMIM.

An effective compensation method for the mutual coupling effect in phased arrays for magnetic resonance imaging

An effective compensation method to compensate for the mutual coupling effect in magnetic resonance imaging (MRI) phased arrays is introduced. This method uses the knowledge of the position of the signal source in MRI, i.e., the active slice, to define a new mutual impedance that accurately quantifies the coupled voltages and enables them to be removed from the terminal voltages almost completely. Numerical results using the method of moments show that the percentage errors in the compensated voltage are at least on the order of 10/sup -5/% and the isolations between two coils are more than 120 dB even at a low-field case of 0.5 T (f/sub 0/=21.3 MHz). This method can be implemented by either software or hardware.

Effective mutual coupling compensation for direction-of-arrival estimations using a new, accurate determination method for the receiving mutual impedance

Performance degradation due to the mutual coupling effect of antenna arrays in direction-of-arrival (DOA) estimations has been well known, and many mutual coupling compensation methods have been proposed throughout the years. In particular, the mutual coupling effect becomes significant when the antenna elements are closely spaced, which is difficult to compensate. In this paper, a new, accurate method is introduced to determine the receiving mutual impedances of antenna arrays with closely spaced omni-directional antenna elements. This new method uses a number of plane wave sources coming from different directions to determine the receiving mutual impedances of a particular antenna element with all other antenna elements in the array in a single step. This step is repeated for all the antenna elements in the array, and the full mutual impedance matrix of the array is thus obtained. Using the Matrix Pencil Method (MPM), DOA estimations using eight-element arrays were used to demonstrate the validity and accuracy of this new method.