Björn Lindmark

On the available diversity gain from different dual-polarized antennas

Dual-polarized antennas are traditionally characterized in terms of port-to-port isolation and co- and cross-polar radiation patterns. For base station antennas in a mobile communications system, the critical parameter is instead the far-field coupling between the two channels. In a mobile communication system, base station antennas with a nominal /spl plusmn/45/spl deg/ to vertical linear polarization are commonly used. Such antennas are difficult to design with constant polarization characteristics in azimuth. We calculate the antenna output power correlation and diversity gain under Rayleigh fading conditions and different values of the environment cross-polar discrimination. Two different antennas are compared: a dual-polarized aperture coupled patch and a slanted dipole configuration, both over an infinite groundplane. We show that the aperture coupled patch provides lower output correlation and higher diversity gain than the slanted dipoles in all investigated cases.

Dual-polarized array for signal-processing applications in wireless communications

A novel dual-polarized antenna array designed for a spatial division multiple access (SDMA) system working in the 1850-1990-MHz band is designed and built. The antenna is designed to have similar element patterns, and measurements of S-parameters and radiation patterns are presented. The array signal processing performance of the array using all elements and with no compensation for mutual coupling or differences in element patterns is studied through direction-of-arrival (DOA) estimation using total least squares estimation of signal parameters via rotational invariance techniques (TLS-ESPRIT). The results show that the accuracy of the DOA estimates is quite acceptable for wireless communication applications.

A novel dual polarized aperture coupled patch element with a single layer feed network and high isolation

The aperture coupled patch geometry has been extended to dual polarization by several authors. In Tsao et al. (1988) a cross-shaped slot is fed by a balanced feed network which allows for a high degree of isolation. However, the balanced feed calls for an air-bridge which complicates both the design process and the manufacture. An alleviation to this problem is to separate the two channels onto two different substrate layers separated by the ground plane. In this case the disadvantage is increased cost. Another solution with a single layer feed is presented in Brachat and Baracco (1995) where one channel feeds a single slot centered under the patch whereas the other channel feeds two separate slots placed near the edges of the patch. Our experience is that with this geometry it is hard to achieve a well-matched broadband design since the slots near the edge of the patch present very low coupling. All the above geometries maintain symmetry with respect to the two principal planes if we ignore the small spurious coupling from feed lines in the vicinity of the aperture. We propose to reduce the symmetry to only one principal plane which turns out to be sufficient for high isolation and low cross-polarization. The advantage is that only one layer of feed network is needed, with no air-bridges required. In addition the aperture position is centered under the patch. An important application for dual polarized antennas is base station antennas. We have therefore designed and measured an element for the PCS band (1.85-1.99 GHz).

The Evolution of Base Station Antennas for Mobile Communications

This paper gives a general overview of the design of base station antennas for mobile communications. It explains underlying theoretical and practical implementation aspects in mobile communication networks of today and the future. In the first part the fundamental parameters of a base station antenna are discussed in the context of radio network design. In particular we discuss parameters such as gain, radiation patterns, frequency bands and power handling and put them in the context of cell planning, propagation and capacity. In the final parts of the paper we give an overview of the underlying theory of diversity and MIMO systems.

Joint estimation of mutual coupling, element factor, and phase center in antenna arrays

A novel method is proposed for estimation of the mutual coupling matrix of an antenna array. The method extends previous work by incorporating an unknown phase center and the element factor (antenna radiation pattern) in the model, and treating them as nuisance parameters during the estimation of coupling. To facilitate this, a parametrization of the element factor based on a truncated Fourier series is proposed. The performance of the proposed estimator is illustrated and compared to other methods using data from simulations and measurements, respectively. The Cramér-Rao bound (CRB) for the estimation problem is derived and used to analyze how the required amount of measurement data increases when introducing additional degrees of freedom in the element factor model. We find that the penalty in SNR is 2.5 dB when introducing a model with two degrees of freedom relative to having zero degrees of freedom. Finally, the tradeoff between the number of degrees of freedom and the accuracy of the estimate is studied. A linear array is treated in more detail and the analysis provides a specific design tradeoff.

Measurements of the spatio-temporal polarization characteristics of a radio channel at 1800 MHz

We present a combined measurement and analysis of the spatial, temporal, and polarisation characteristics of a 1800 MHz radio channel. The measurements were performed using a correlative channel sounder. A complex chirp signal with a bandwidth of 150 MHz was transmitted by a vertically polarized omnidirectional antenna and received by a 8+8 dual polarized planar antenna array. Measurements were taken at two different base station locations, one being suburban and the other urban. Using this data we characterize the radio channel in terms of azimuth and delay spread of the vertical and horizontal polarization.

Measurements of MIMO indoor channels at 1800MHz with multiple indoor and outdoor base stations

This paper proposes several configurations for multiple base stations in indoor MIMO systems and compares their performance. The results are based on channel measurements realized with a MIMO testbed. The receiver was moved along several routes and floors on an office building. Both outdoor and indoor locations are considered for the transmitters or base stations, which allow the analysis of not only indoor but also outdoor-to-indoor environment. The use of 2 base stations with different system level combinations of the two is analyzed. We show that the configuration with base station selection provides almost as good performance as a full water-filling scheme when the 2 base stations are placed at different locations. Also the spatial correlation properties for the different configurations are analyzed and the importance of considering path loss when evaluating capacity is highlighted.

Comparison of mutual coupling compensation to dummy columns in adaptive antenna systems

Compensation for mutual coupling in antenna arrays by matrix multiplication is compared to the use of dummy elements. A least squares estimation of the coupling matrix is made, including co- and cross-polar coupling. We show that compensation for measurements off the phase center is important as well as a proper assumption of the ideal element pattern. We study the performance of a dual polarized patch array with respect to far-field phase error, signal-to-interference reduction, and cross-polarization level. In all aspects, the performance of the compensation method exceeds or equals the use of dummy elements.

Capacity of a 2/spl times/2 MIMO antenna system with mutual coupling losses

The capacity of multiple-input multiple-output (MIMO) systems is strongly dependent on the channel matrix which in turn can be expressed in integrals over the antenna far-field patterns. The effect of mutual coupling on the capacity of a 2/spl times/2 MIMO system is studied. The results show that, even though the correlation is reduced, the losses caused by mutual coupling decrease the capacity as we reduce the antenna spacing.

RF characterization of low-voltage high-isolation MEMS series switch based on a S-shaped film actuator

This paper presents on an electrostatically actuated microelectromechanical series switch for DC to RF signals, based on a flexible S-shaped film actuator moving between two electrodes in touch mode actuation. This novel concept, in contrast to most other MEMS switches, allows a low actuation voltage design independent on the off-state gap height, which makes larger switching contact areas for low insertion loss and higher current handling capability possible, by obtaining high isolation. Furthermore, the double anchor principle avoids self-biasing and is favorable of switching higher power signals. The actuation voltages of the first prototype switches are 12 V to open and 15.8 V to close the metal contact. The RF isolation with a gap distance of 14.2 /spl mu/m is better than -45 dB up to 2 GHz and -30 dB at 15 GHz despite a large switching contact area of 3500 /spl mu/m/sup 2/.