Christiane Kuhnert

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.

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.

A modelling approach for multiuser MIMO systems including spatially-colored interference [cellular example]

This paper deals with the investigation of MIMO (multiple input multiple output) systems on a system level, including interference. The interference affects the capacity of each user and depends on the spatial characteristics of the propagation channel. It is spatially-colored. A necessary step towards a realistic performance evaluation is to include interference into the channel modelling process. A new multiuser double-directional channel model (MDDCM), that enables a correct interference description, is presented in this paper. It is based on a distributed scatterer approach and includes line-of-sight areas, far clusters and new models for street guided and over-rooftop propagation. Simulation results of a cellular multiuser MIMO system for the downlink case, based on the MDDCM, are presented. With this tool, it becomes possible to evaluate the performance of multiuser MIMO systems under realistic propagation and interference conditions.

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.

Three Dual-Band Miniaturized Inverted F Antennas Integrated in a PDA for MIMO Applications

For multimedia applications three antennas are placed inside the housing of a personal digital assistant (PDA) with respect to MIMO. One planar and two newly designed buckled inverted F antennas are used. MIMO capacity is calculated with a MIMO system model including accurate antenna modeling and a double-directional multipath indoor channel model. It is shown, that for new, efficient MIMO systems, antenna design and placement must take into account the complete system including the user

Frequency dependence of correlation coefficients in ultra wideband antenna arrays

In order to optimize the performance of multi-antenna systems for communications applications it is desirable to have little correlation between the signals at the different receiving antennas. This can be achieved by choosing appropriate antenna characteristics and positions. For ultra wideband (UWB) applications this task is even more challenging, since correlation coefficients are dependent on frequency. This paper presents an investigation of the frequency dependence of the correlation coefficients of a linear antenna array in an UWB indoor propagation scenario. The investigated array itself consists of omni-directional elements. Thus only the correlation behaviour that is introduced by the propagation channel is regarded. It turns out from the conducted simulations that the correlation coefficients are highly altering with frequency and distance of the antennas. Furthermore, they severely depend on the position of the array inside the room. The simulation results show that, even if omni-directional antenna elements are used, with a dedicated placement of the antenna array pretty low correlation coefficients can be obtained over the entire frequency range regulated by the FCC for UWB applications.

Capacity of the MIMO broadcast channel under realistic propagation conditions

In this paper simulation results of the multiuser MIMO broadcast channel are presented. The MIMO broadcast channel corresponds to the downlink case where the base station sends simultaneously different information to several users in the cell. In contrast to many information theoretical studies, the presented results are based on a realistic MIMO channel model and are therefore a first step towards realistic performance evaluations. The channel model is called multiuser double-directional channel model (MDDCM) and was introduced recently in T. Fugen et al., (2004). It is based on a geometry approach with fixed scatterers. It enables a realistic description of the desired link as well as the spatially-colored interference in a macro-cellular environment. Based on the MDDCM the sum capacity of the MIMO broadcast channel for a varying number of users and varying antenna configurations in an isolated communication cell is presented. To achieve the sum capacity the duality to the MIMO multiple access channels is used

A Hybrid Framework for Incorporating Analog Front-End Effects in MIMO OFDM Design

This paper presents a framework including both digital signal processing and the analog RF part of the system. A MIMO OFDM system with various numbers of antennas at transmit and receive is investigated in a fast-fading urban environment. The hybrid framework is applied to assess performance degradation due to impairments in the RF part, e.g. fading, nonlinear high power amplifiers, phase noise and frequency offset of local oscillators. It is shown that bit error performance is affected most by distortion due to frequency offset of the local oscillator in the receive front-end. A simple frequency estimator designed for single antenna OFDM systems can be adopted to the MIMO case as it stands to combat for frequency offsets.

Impact of receiver inter-chain coupling on BER performance of space-time coded MIMO systems

In multiple input multiple output (MIMO) communication systems, multiple radio frequency (RF) chains are required at both transmit and receive side of the link, each corresponding to one antenna. Mutual coupling effects may occur not only at the antennas, but also among the RF chains integrated on a single chip. At the receive side, inter-chain coupling leads to a degradation of the receive signal power and a generation of correlated noise. The impact of these effects on bit error rate (BER) performance in space-time block coded MIMO systems is calculated analytically. Further on, design considerations are discussed