Thomas Svantesson

Mutual coupling effects on the capacity of multielement antenna systems

A study of the capacity of multiple element antenna systems is presented, with particular emphasis on the effect that mutual coupling between the antenna elements has on the capacity. The results presented here show, contrary to some earlier claims, that correlation between different channel coefficients as a function of antenna spacing, can in fact decrease when the mutual coupling effect is accounted for. As a consequence, capacity also improves. A realistic channel model is used to perform simulations to support these claims.

Correlation and channel capacity of MIMO systems employing multimode antennas

A novel way of exploiting higher modes of antennas as diversity branches in multiple-input-multiple-output (MIMO) systems is introduced. Essentially, antennas employing multiple modes offer characteristics similar to an antenna array, through multiple modes and using only a single element. The physical mechanism that yields different received signals is the fact that each mode has a different radiation pattern. Analytical expressions for the correlation between signals received by different modes are presented for a biconical and a circular microstrip antenna that employs higher order modes. It is found that the correlation is low enough to yield a significant diversity gain. Furthermore, the channel capacity of a MIMO system using a multimode antenna, i.e., an antenna employing multiple modes, is found to be comparable to the capacity of an array. Since only one element is needed, the multimode antenna offers several advantages over traditional arrays, and is an interesting antenna solution for future high capacity MIMO systems.

Modeling and estimation of mutual coupling in a uniform linear array of dipoles

The mutual coupling in a uniform linear array (ULA) of dipoles is calculated using basic electromagnetic concepts. Since the coupling often is unknown and needs to be estimated, a simpler model is proposed based on the electromagnetic analysis. The parameterization of this model is shown to be locally unambiguous. A necessary condition for the joint solution of directions and coupling parameters to be unique is also derived. Finally, the directions and coupling parameters are estimated using a maximum likelihood method. It is found that the simpler coupling model with just a few parameters well describes the full electromagnetic model.

An antenna solution for MIMO channels: the switched parasitic antenna

We investigate the switched parasitic antenna (SPA), which is a novel technique for electronically directing the radiation pattern in a MIMO system. The correlation between the received signal modes is shown to be sufficiently low to yield a diversity gain. The capacity limit using the SPA is investigated for different SPA configurations and it is found that the capacity is comparable with an array antenna configuration in certain situations. Finally, a space time block coding scheme is used to evaluate the bit error rate of a MIMO-SPA system. It is found that the SPA requires a 5 dB higher SNR than an antenna array solution to achieve a BER=10/sup -2/. However, the array antenna requires a radio transceiver for every antenna, as opposed to the SPA which uses only one transceiver.

A physical MIMO radio channel model for multi-element multi-polarized antenna systems

A physical multi-input multi-output (MIMO) spatio-temporal channel model for simulation of multi-element multi-polarized antenna systems is presented. The model is based on electromagnetic (EM) scattering, hence including many of the channel characteristics encountered in practice. A compact channel expression is obtained by introducing a dyad formulation of the scattering from arbitrary objects. The polarization properties of the channel and the antennas are included in the model, thereby allowing studies of the impact of different antenna arrangements and polarizations. Simulation results are presented where channel properties such as spatial correlation, channel capacity, and time evolution are calculated for microcell and picocell scenarios.

The effects of mutual coupling using a linear array of thin dipoles of finite length

The effects of mutual coupling on the direction finding accuracy of a linear array of dipole elements are studied. An approximative expression of the measured voltages when a plane wave is incident upon the array is derived using mutual impedances. The direction finding accuracy is then investigated by calculating the Cramer-Rao lower bound. It is found that a known coupling does not affect the estimation performance much. In the case of an unknown coupling, the estimation procedure can be simplified by using only a few off-diagonal elements of the coupling matrix. Depending on how many parameters are included, the RMS error can actually be smaller than the CRB for the case when all off-diagonals are estimated.

Mutual coupling compensation using subspace fitting

The effects of an unknown mutual coupling is compensated for by estimating the coupling along with the direction of arrival (DOA) using a modified version of the noise subspace fitting (NSF) algorithm. It is possible to concentrate the NSF criterion regarding time coupling parameters, and the DOAs are obtained by a numerical search just as in the coupling free case. To evaluate the proposed method, the mutual coupling in a uniform linear array (ULA) of dipoles is calculated using fundamental electromagnetics. It is found that the effects of an unknown coupling can be reduced by estimating the coupling as well as the DOAs.

On capacity and correlation of multi-antenna systems employing multiple polarizations

Wireless communications have experienced an explosive growth the last decades. So far, the main applications have been voice service and low rate data services such as short messaging. However, the next generation of communication systems aim at providing wireless Intemet and multimedia services that require high or very high data rates. A promising way of achieving these data rates is to use multiple antennas at both the transmitter and the receiver, i.e. MultiInput Multi-Output (MIMO) systems [2, 41. These systems typically employ arrays of spatially separated elements at both ends. However, more compact implementations are possible if the polarization properties of the antenna also are exploited. An interesting concept that exploits polarization was introduced in [l], where a sixfold increase in capacity over a single polarized antenna was reported. This dramatic capacity increase was obtained by exploiting the full ElectroMagnetic (EM) field instead of just a single field component. In [l] neither the impact of the radiation pattem of the antenna nor the angular spread of the multipath components were analyzed. This paper will study the performance of a system that employs a multi-element multi-polarized antenna consisting of three electric dipoles and three magnetic dipoles, that measures all six components of the EM field. The correlation and channel capacity of such a system will be analyzed for various angular spreads and the combined effects of the radiation pattems and the polarizations of the antennas will be discussed.

High-resolution direction finding using a switched parasitic antenna

Direction finding by exploiting the directional radiation patterns of a switched parasitic antenna (SPA) is considered. By employing passive elements (parasites), which can be shorted to ground using pin diodes, directional radiation patterns can be obtained. The direction finding performance of the SPA is examined by calculating a lower bound on the direction finding accuracy, the Cramer-Rao lower bound (CRB). It is found that the SPA offers a compact implementation with high-resolution direction finding performance using only a single radio receiver. Thus, exploiting SPAs for direction finding is an interesting alternative to traditional antenna arrays offering compact and low-cost antenna implementations.

An antenna solution for MIMO channels: the multimode antenna

A novel way of exploiting diversity by employing the higher order modes of a biconical antenna is introduced. Essentially, the biconical antenna offers characteristics similar to an antenna array through multiple modes using just a single antenna element. An analytical expression for the correlation between different modes in a realistic environment is presented. It is found that the correlation is low enough to yield a diversity gain. Furthermore, the channel capacity of a multi-input multi-output (MIMO) system using a multimode antenna, i.e. an antenna employing multiple modes, is found to be comparable to that of an array. Since only one antenna element is needed, the multimode antenna offers several advantages over traditional arrays and is an interesting antenna solution for future high capacity MIMO systems.