  A large-scale fully-digital receive antenna array can provide very
high-resolution direction of arrival (DOA) estimation, but resulting in a
significantly high RF-chain circuit cost. Thus, a hybrid analog and digital
(HAD) structure is preferred. Two phase alignment (PA) methods, HAD PA (HADPA)
and hybrid digital and analog PA (HDAPA), are proposed to estimate DOA based on
the parametric method. Compared to analog phase alignment (APA), they can
significantly reduce the complexity in the PA phases. Subsequently, a fast root
multiple signal classification HDAPA (Root-MUSIC-HDAPA) method is proposed
specially for this hybrid structure to implement an approximately analytical
solution. Due to the HAD structure, there exists the effect of
direction-finding ambiguity. A smart strategy of maximizing the average receive
power is adopted to delete those spurious solutions and preserve the true
optimal solution by linear searching over a set of limited finite candidate
directions. This results in a significant reduction in computational
complexity. Eventually, the Cramer-Rao lower bound (CRLB) of finding emitter
direction using the HAD structure is derived. Simulation results show that our
proposed methods, Root-MUSIC-HDAPA and HDAPA, can achieve the hybrid CRLB with
their complexities being significantly lower than those of pure linear
searching-based methods, such as APA.
