Cornelis Hoek

Channel measurements for large antenna arrays

Equipping base stations (BSs) with very large antenna arrays is a promising way to increase the spectral and energy efficiency of mobile communication systems without the need for new cell sites. However, the prominently theoretical works on this topic are based on several crucial assumptions about the wireless channel which have not been sufficiently validated by measurements. In this paper, we report on an outdoor measurement campaign with a scalable virtual antenna array consisting of up to 112 elements. The large amount of acquired data allows us to study several important aspects of large-scale MIMO systems. For example, we partially confirm the theoretical results based on uncorrelated channels which predict that the channels at different positions become more and more orthogonal as the number of antennas grows. However, for the measured channels, the marginal gain of an additional antenna quickly diminishes. Nevertheless, our results indicate that most of the theoretical benefits of large-scale MIMO could be realized also over the measured channels.

3D beamforming trials with an active antenna array

Beamforming techniques for mobile wireless communication systems like LTE using multiple antenna arrays for transmission and reception to increase the signal-to-noise-and-interference ratio (SINR) are state of the art. The increased SINR is not only due to a larger gain in the direction of the desired user, but also due to a better control of the spatial distribution of interference in the cell. To further enhance the system performance not only the horizontal, but also the vertical dimension can be exploited for beam pattern adaptation, thus giving an additional degree of freedom for interference avoidance among adjacent cells. This horizontal and vertical beam pattern adaptation is also referred to as 3D beamforming in the following. This paper describes investigations of the potential of 3D beamforming with lab and field trial setups and provides initial performance results.

3D beamforming: Performance improvement for cellular networks

The beam pattern of a mobile communication base station has significant impact on the performance of a cellular network. Three-dimensional (3D) beamforming combines the horizontal beam pattern adaptation, as applied for beamforming and multiple input multiple output (MIMO) schemes, with a vertical antenna pattern adaptation. The recent availability of new flexible antenna techniques enables a fully dynamic antenna pattern adaptation which can be specified per resource block and user equipment (UE), and makes 3D beamforming practically feasible. This paper describes the basic principles of 3D beamforming, including the impact of downtilt adaptation on the physical layer as well as the potential of its combination with beam coordination involving the media access control (MAC) layer. Our investigations assumed the vertical main lobe of the beam pattern was geometrically pointed towards the UE. We discuss a number of different realization options and simulation results including a Bell Labs field trial with vertical beam steering. Using wireless systems with state of the art Long Term Evolution (LTE) signal format and including the lightRadio™ antenna array, our trials in the Stuttgart testbed verified the basic predicted properties and potential advantages of 3D beamforming.

Interference avoidance with dynamic vertical beamsteering in real deployments

This paper gives an overview on the possibilities and potential of dynamic vertical beamsteering in a cellular mobile radio system with focus on the interference limited macro-cell scenario. Different realization options for dynamic terminal specific downtilt adaptation at the base station (eNB) are introduced and simulated performance figures are given. Beam coordination methods for interference avoidance without and with the requirement for inter-eNB communications are considered. The impact of the most important system parameters like downtilt angle variation and coordination algorithm parameter setting is investigated. Basic measurements in real environment for proof of concept are introduced and their relation to simulation results are discussed. Finally the major conclusions and an outlook for future investigations are given.

Investigation of diversity and beamforming techniques for UMTS high speed downlink packet access

This paper deals with the high speed downlink packet access (HSDPA) in UMTS/FDD system. The transmit diversity and beamforming techniques are focused on, namely space time transmit diversity (STTD), closed loop mode 1 transmit diversity, conventional beamforming and closed loop beamforming with 2 transmit antennas. The link level system performance of the various methods is investigated and compared for the fading environment. At low user speeds, closed loop transmit diversity turns out to be superior. STTD has the lowest system complexity and achieves an acceptable performance at high SNR. Both beamforming schemes are the good solutions for high speed mobile terminal at low SNR

Performance assessment of multi user inter cell interference alignment with measured channels

It has been shown that in a multi-user multi-cell network, the Multi User Inter Cell Interference Alignment (MUI-CIA) based transmit precoding outperforms the non-alignment based state of the art precoding schemes. The theory of MUICIA is based on the assumption that the partial and stale inter-cell interference (ICI) information is still useful. This ICI is used by the transmitter to design he precoding such that the ICI and the multi-user interference (MUI) are aligned at the receiver. In this paper, we provide the proof of this concept with the help of measured channels. By using a measurement test-bed, we demonstrate that in slow time variant channels, the partial ICI contains useful information. Our outcomes are based on two different indoor scenarios for a multi user multi cell system with multiple antenna users. We also take into account the impact of training overhead on the performance in order to realize the feasibility of MUICIA with nominal overhead. Moreover, we compare the performance of MUICIA with other non-alignment based multi user precoding schemes. The results show that the concept of MUICIA is valid and it provides higher gains in a multi cell setup than other non-alignment based precodings with high as well as nominal training overhead.