  High hardware cost and high power consumption of massive multiple-input and
multiple output (MIMO) are two challenges for the future wireless
communications including beyond fifth generation (B5G) and sixth generation
(6G). Adopting the low-resolution analog-to-digital converter (ADC) is viewed
as a promising solution. Additionally, the direction of arrival (DOA)
estimation is an indispensable technology for beam alignment and tracking in
massive MIMO systems. Thus, in this paper, the performance of DOA estimation
with mixed-ADC structure is firstly investigated. The Cramer-Rao lower bound
(CRLB) for this architecture is derived based on the additive quantization
noise model. Eventually, a performance loss factor and the associated energy
efficiency factor is defined for analysis in detail. Simulation results show
that the mixed-ADC architecture can strike a good balance among performance
loss, circuit cost and energy efficiency. More importantly, just a few bits (up
to 4 bits) of low-resolution ADCs can achieve a satisfactory performance for
DOA measurement.
