  High hardware cost and high power consumption of massive multiple-input and
multiple output (MIMO) are still two challenges for the future wireless
communications including beyond 5G. 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 for massive MIMO receive array
with mixed-ADC structure is first investigated, where one part of radio
frequency (RF) chains are connected with high-resolution ADCs and the remaining
ones are connected with low-resolution ADCs. Moreover, the Cramer-Rao lower
bound (CRLB) for this architecture is derived based on the additive
quantization noise model approximation for the effect of low-resolution ADCs.
Then, the root-MUSIC method is designed for such a receive structure.
Eventually, a performance loss factor and the associated energy efficiency
factor is defined for analysis in detail. Simulation results find that a
mixed-ADC architecture can strike a good balance among RMSE performance,
circuit cost and energy efficiency. More importantly, just 1-4 bits of
low-resolution ADCs can achieve a satisfactory performance for DOA measurement.
