A New Definition of Mutual Impedance for Application in Dipole Receiving Antenna Arrays

A new definition of mutual impedance for two dipole antennas is introduced to characterize the mutual coupling effect between two dipole antennas in a more accurate manner. The calculation method and the measurement procedure for the new mutual impedance are given. Measurementand theoretical results on two monopole antennas were obtained as an example. The successful applications of the new mutual impedance in the compensation of mutual coupling effect in direction finding and adaptive interference suppression with significantly improved results showed the importance of the new mutual impedance

Multi-Cell Random Beamforming: Achievable Rate and Degrees of Freedom Region

Random beamforming (RBF) is a practically favourable transmission scheme for multiuser multi-antenna downlink systems since it requires only partial channel state information (CSI) at the transmitter. Under the conventional single-cell setup, RBF is known to achieve the optimal sum-capacity scaling law as the number of users goes to infinity, thanks to the multiuser diversity enabled transmission scheduling that virtually eliminates the intra-cell interference. In this paper, we extend the study of RBF to a more practical multi-cell downlink system with single-antenna receivers subject to the additional inter-cell interference (ICI). First, we consider the case of finite signal-to-noise ratio (SNR) at each receiver. We derive a closed-form expression of the achievable sum-rate with the multi-cell RBF, based upon which we show an asymptotic sum-rate scaling law as the number of users goes to infinity. Next, we consider the high-SNR regime and for tractable analysis assume that the number of users in each cell scales in a certain order with the per-cell SNR. Under this setup, we characterize the achievable degrees of freedom (DoF) for the single-cell case with RBF. Then we extend the analysis to the multi-cell RBF case by characterizing the DoF region. It is shown that the DoF region characterization provides useful guideline on how to design a cooperative multi-cell RBF system to achieve optimal throughput tradeoffs among different cells. Furthermore, our results reveal that the multi-cell RBF scheme achieves the "interference-free DoF" region upper bound for the multi-cell system, provided that the per-cell number of users has a sufficiently large scaling order with the SNR. Our result thus confirms the optimality of multi-cell RBF in this regime even without the complete CSI at the transmitter, as compared to other full-CSI requiring transmission schemes such as interference alignment.Comment: 28 pages, 6 figures, to appear in IEEE Transactions of Signal Processing. This work was presented in part at IEEE International Conference on Acoustics, Speech, and Signal Processing (ICASSP), Kyoto, Japan, March 25-30, 2012. The authors are with the Department of Electrical and Computer Engineering, National University of Singapore (emails: {hieudn, elezhang, elehht}@nus.edu.sg

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% and the isolations between two coils are more than 120 dB even at a low-field case of 0.5 T (f = 21.3 MHz). This method can be implemented by either software or hardware

A practical approach to compensate for the mutual coupling effect in an adaptive dipole array

A new method is introduced for the compensation of the mutual coupling effect of a linear adaptive dipole array employed in adaptive nulling of interference signals. The new method adopts a practical approach in that it needs only the measured voltages across the antenna terminal loads and an estimated current distribution for the calculation of the mutual impedances. The mutual impedance is defined and calculated differently from the conventional method and the results are more effective to remove the mutual coupling effect. The new method does not require the knowledge of the elevation angles of the signal of interest (SOI) and the interferences and still works if the elevation angles of the SOI and the interferences do not deviate, too much from the horizontal direction. This increases the capability of the array to work in three-dimensional signal environments. Computer simulations for a number of rather extreme signal environments have been carried out to testify the robustness and the capability of the new method

Antenna systems for diversity in the next-generation mobile handphones

In the cellular communications environment, short term or fast fading due to multipath has a significant impact on the overall system performance. This type of fading occurs when multiple replicas of the signal of interest arrive at the receiver over different paths, thus having different relative amplitude and phases. Antenna diversity can be easily employed to combat fast fading and enhance the performance of mobile communication systems. Utilization of spatial, polarization and pattern diversity is in widespread use at base stations. However, the commercial application of spatial antenna diversity in mobile phones is still very limited mainly due to the constraint of the small size of the modem handset In this project, the diversity performance of a dual helical antenna diversity system on a large ground plane and a mobile handset were investigated. Results for the S parameters, VSWR, frequency response and the radiation patterns of the antenna system were presented. The envelope correlation and diversity gain of the system were also calculated to provide justification for the employment of spatial diversity in mobile handsets. This report presents a description of the theory, experimental setup and procedures for electromagnetic simulation using IE3D software

Mutual coupling compensation for direction finding using receiving mutual impedance

The accuracy of direction finding using antenna arrays with different array apertures is studied. Numerical simulations are performed using full-wave moment method. The undesirable mutual coupling effect is properly modelled and compensated using receiving-mutual-impedance and the direction finding is obtained using the Matrix Pencil Method. The results show that increasing the number of elements with a small array aperture can significantly improve the accuracy in direction finding. The findings are crucial for the design of antenna arrays for direction finding

Direction-of-arrival estimation: measurement using compact antenna arrays under the influence of mutual coupling

This article investigates the accuracy of direction-of-arrival (DOA) estimations using compact antenna arrays under the influence of mutual coupling. We model the entire electromagnetic problem using the moment method such that the undesirable mutual coupling effect is taken into account accurately. We decouple the received signals using the receiving-mutual-impedance method (RMIM) and perform the DOA estimation using the matrix pencil method (MPM) through a Monte Carlo simulation approach. Although the signals are strongly distorted by mutual coupling, we show that increasing the number of elements within a small array aperture can significantly improve the estimation accuracy. The findings show that the RMIM can effectively compensate for mutual coupling for arrays with an aperture of a factor of a wavelength. Such findings are crucial to the development of compact arrays in various applications