C. Waldschmidt

Transient responses of a Vivaldi antenna and a logarithmic periodic dipole array for ultra wideband communication

Antennas are mandatory system components for UWB communication systems. The paper presents a comprehensive approach for the characterization of UWB antenna concepts. Measurements of the transient responses of a LPDA and a Vivaldi antenna prove the effectivity of the presented model.

Influence and modelling of mutual coupling in MIMO and diversity systems

MIMO (multiple input multiple output) and diversity systems can increase distinctly the system performance of future wireless communication systems. The correlation of signals, depending on the power azimuth spectrum, between single transmit and receive antennas is a criterion to asses smart antenna systems. The assumption of independent antennas often made is not vindicated any more for small antenna spacing, since mutual coupling between single antenna elements occurs. This paper discusses the influence of mutual coupling on the correlation and shows the consequences for MIMO systems.

Compact MIMO-arrays based on polarisation-diversity

The application of MIMO (multiple input multiple output) systems in small handheld devices is difficult, since usually large antenna spacings are required to obtain uncorrelated channel coefficients. This paper shows, using dipoles, that a combination of pattern- and polarisation diversity yields uncorrelated channel coefficients that can be exploited by MIMO system. A way of modeling the impact of antennas on MIMO systems is presented here. The complex radiation pattern, polarisation, mutual coupling of the antennas and cross polarisation coupling within the transmission channel are taken into account. It is shown that a combination of polarisation and pattern diversity leads to very compact antenna arrays, that still yield uncorrelated channels.

A small planar inverted F antenna with capacitive and inductive loading

The development of very small antennas significantly influences the miniaturization of handheld devices. A planar and symmetric antenna is built up. A capacitive loading for this planar inverted F antenna (IFA) is presented. For a further size reduction, an inductive loading is investigated. The method of moments (MoM) is used to model the antenna and measurements are carried out for verification.

Measurements and simulations of compact MIMO-systems based on polarization diversity

A comparison of different very compact antenna arrays for MIMO systems is presented. It is shown that a combination of different diversity techniques, like spatial, pattern and polarization diversity, leads to compact capable MIMO systems. First, simulations showing the fundamentals of polarization diversity in MIMO systems are given. Second, channel measurements with different antenna array configurations are shown.

On the integration of MIMO systems into handheld devices

This work shows that it is possible to integrate several antennas into small hand-held devices to setup MIMO systems for mobile communications. The integration of three and four Inverted-F antennas into a small housing is presented. These integrated antenna arrays are analyzed by simulations of complete MIMO systems. The simulations are verified by measurements. Different quality measures for antenna arrays like the mean effective gain and for MIMO systems like the correlation properties of the channel matrix are investigated. It is shown, that a low correlation among the elements of the channel matrix can easily be obtained, even for very small antenna spacings. But realizing antenna arrays with a high efficiency, which is essential for mobile devices, is difficult The influence of a user, who holds the small device nearby the head, is also investigated. The user mainly influences the efficiency of the arrays, whereas the correlation properties remain unchanged.

Analysis of compact arrays for MIMO based on a complete RF system model

A model for an extended MIMO transmission channel including transmitter, receiver, antennas and the physical channel is presented. For the analysis of compact MIMO systems for small handheld devices, it is inevitable to consider this extended model, since the antennas couple and interact, which influences both SNR and the channel matrix H. Capacity calculations for diversity and beamforming based MIMO systems are given.

Electromagnetic Characterization of Ultra Wideband Antennas

Ultra Wideband applications in communication and Radar have stimulated research for several 10 years. The possibilities for high resolution and high data rates have been the driving visions. Especially the military research added the feature of low probability of detection. Since the United States Federal Communication Commission (FCC) has opened the spectrum from 3.1 to 10.6 GHz for unlicensed, low power applications a new dimension is added to ultra wideband research. This papers focuses on state of the art research in ultra wideband antennas. Especially the Vivaldi antenna and the logarithmic periodic dipole array (LPDA) of small size for applications in communications are treated. It is shown that both can handle very well the more than 100 % relative bandwidth. While the Vivaldi antenna shows low distortion and only small ringing, the logarithmic periodic dipole array exhibits high distortion, ringing and higher order modes of radiation. This behavior has to be optimized together with the modulation schemes for ultra wideband communications.

Broadband multimode antennas for MIMO applications

This paper presents compact broadband multimode antennas for MIMO and diversity applications. The antenna system is riot based on spatial diversity. as usual MIMO system, but on a combination of pattern and polarisation diversity. Different modes of self-complementary spiral antennas we used to decorrelate the signals. It is shown that only one antenna is necessary to receive three uncorrelated signals, thus the space required to place the MIMO antenna is very small.

Quality measures and examples of arrays for MIMO in hand-held devices

The paper presents different quality measures for the assessment of compact antenna arrays for MIMO systems. Usually, only the capacity and the correlation properties for a MIMO system are considered, but, for the development of compact arrays, the behavior in terms of the power of an array has to be taken into account. The power transmission gain of a MIMO system and the effective gain of the antennas in an array are used as quality measures. These measures are statistical measures that describe the antenna array performance for specific propagation channels. To illustrate the assessment of different arrays for MIMO, arrays in small handheld devices are used as an example.