  Due to the power consumption and high circuit cost in antenna arrays, the
practical application of massive multiple-input multiple-output (MIMO) in the
sixth generation (6G) and future wireless networks is still challenging.
Employing low-resolution analog-to-digital converters (ADCs) and hybrid analog
and digital (HAD) structure is two low-cost choice with acceptable performance
loss.In this paper, the combination of the mixed-ADC architecture and HAD
structure employed at receiver is proposed for direction of arrival (DOA)
estimation, which will be applied to the beamforming tracking and alignment in
6G. By adopting the additive quantization noise model, the exact closed-form
expression of the Cram\'{e}r-Rao lower bound (CRLB) for the HAD architecture
with mixed-ADCs is derived. Moreover, the closed-form expression of the
performance loss factor is derived as a benchmark. In addition, to take power
consumption into account, energy efficiency is also investigated in our paper.
The numerical results reveal that the HAD structure with mixed-ADCs can
significantly reduce the power consumption and hardware cost. Furthermore, that
architecture is able to achieve a better trade-off between the performance loss
and the power consumption. Finally, adopting 2-4 bits of resolution may be a
good choice in practical massive MIMO systems.
