Antenna diversity in mobile communications

TLDR

Mutual coupling plays a central role in mobile antennas, differing from conventional array antennas. The study investigates the conditions required for antenna diversity action. The authors derive that antenna diversity requires near‑orthogonality between incident and far fields, and use a mutual‑resistance condition as a design tool for mobile antennas. The study finds that low or zero mutual resistance between elements is sufficient for diversity in most practical mobile high‑gain antennas, while at base stations the condition is only necessary, and demonstrates that this mutual‑resistance criterion can guide the design of new mobile diversity antennas.

Abstract

The conditions for antenna diversity action are investigated. In terms of the fields, a condition is shown to be that the incident field and the far field of the diversity antenna should obey (or nearly obey) an orthogonality relationship. The role of mutual coupling is central, and it is different from that in a conventional array antenna. In terms of antenna parameters, a sufficient condition for diversity action for a certain class of high gain antennas at the mobile, which approximates most practical mobile antennas, is shown to be zero (or low) mutual resistance between elements. This is not the case at the base station, where the condition is necessary only. The mutual resistance condition offers a powerful design tool, and examples of new mobile diversity antennas are discussed along with some existing designs.