Analysis and measurement of multiple antenna systems for fading channel prediction in moving relays

The performance of wireless data transmission to mobile vehicles is improved if channel state information is available at the transmitter but movement of vehicles causes outdating of channel estimates. The concept of a predictor antenna has recently been proposed, where an antenna is placed in front of other antennas on the roof of the vehicle to sense the radio environment in advance. This can comparatively provide an order-of-magnitude improvement in channel prediction performance. A potential problem with this idea is that closely placed antennas will experience mutual electromagnetic couplings. These may reduce the efficiency of the predictor antenna concept if they are not taken into account. In this paper, we discuss about how to treat the forgoing issue and eventually evaluate a promising candidate on measured channels. We argue that only open-circuit voltage method would be realistic for the present application. The usefulness of the proposed decoupling method is demonstrated on field measurements obtained in downtown Dresden, Germany. We also partly address the sensitivity of the open-circuit decoupling method to the accuracy of the utilized network parameters.

I. Introduction

In modern wireless communication systems, the presence of channel state information (CSI) at the transmitter enables link adaptation, multi-user scheduling, beamforming, spatial multiplexing, space division multiple access, and coordinated multipoint transmission. Delays in the feedback control loop cause outdating of the CSI when transmitting to a moving vehicle. This problem becomes increasingly severe with larger delays, higher vehicle velocities, and shorter carrier wave-lengths. Prediction based on past received channel estimates by Kalman or Wiener prediction can to some extent alleviate the problem [1]. However, these methods become inadequate in multipath fading environments when the required prediction horizon in time corresponds to a movement of more than 0.3 carrier wavelengths in space. For high mobility users, this limitation prevents accurate CSI to be obtained in the current 3GPP Long Term Evolution (LTE) systems with carrier frequencies exceeding 1 GHz [2].