Due to the power consumption and high circuit cost in antenna arrays, the
practical application of massive multipleinput multiple-output (MIMO) in the
sixth generation (6G) and future wireless networks is still challenging.
Employing lowresolution 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 mixedADC 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 closedform
expression of the Cramer-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 mixedADCs 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.Comment: 11 pages, 7 figure
