Exploring Symmetry in Wireless Propagation Channels

TL;DR

This paper investigates the fundamental feature of wireless propagation asymmetry, highlighting its importance and potential causes, and presents preliminary measurements showing symmetry in ideal conditions, urging careful consideration in practical scenarios.

Contribution

It provides the first preliminary study on wireless propagation asymmetry, identifying potential causes and emphasizing the need for accurate modeling in various wireless applications.

Findings

Wireless channels are symmetric without impairments.

Propagation asymmetry can significantly affect modeling accuracy.

Further research is needed to understand asymmetry causes.

Abstract

Wireless communications literature is very rich with empirical studies and measurement campaigns that study the nature of the wireless propagation channel. However, despite their undoubted usefulness, many of these studies have omitted a fundamental yet key feature of the physical signal propagation, that is, wireless propagation asymmetry. This feature does not agree with the electromagnetic reciprocity theorem, and the many research papers that adopt wireless channel symmetry, and hence rendering their modeling, unexpectedly, inaccurate. Besides, asymmetry is unquestionably an important characteristic of wireless channels, which needs to be accurately characterized for vehicular/mobile communications, 5G networks, and associated applications such as indoor/outdoor localization. This paper presents a modest and a preliminary study that reports potential causes of propagation asymmetry. Measurements conducted on Khalifa University campus in UAE show that wireless channels are symmetric in the absence of symmetry impairments. Therefore, care should be taken when considering some practical wireless propagation scenarios. Key conclusions and recommendation are summarized. We believe that this study will be inspiring for the academic community and will trigger further investigations within wireless propagation assumptions.