Arbitrary Precoding with Single-Fed Parasitic Arrays: Closed-Form Expressions and Design Guidelines

Single-fed compact arrays known as electronically steerable parasitic antenna radiators (ESPARs) have recently emerged as a new paradigm for spatial multiplexing that requires only a single radio-frequency (RF) chain and a few easy-to-implement analog tunable loads. Besides the remarkable hardware savings, the ESPAR capabilities are still limited as it is not possible to find appropriate loadings that multiplex any signaling format over the air. Indeed, commonly the loading values are obtained through time-consuming iterative algorithms, e.g. exhaustive search, and can only emulate MIMO transmission of signals emerging from low-order constellations. This paper constitutes a clear step forward in solving this problem, as it defines the necessary guidelines for the design of single-fed ESPAR systems that are able to support an arbitrary precoding scheme. To alleviate the need for iterative processes, the proposed design methodology uses complex tunable loads and introduces new closed-form expressions for the exact computation of the loads and the feeding voltage.