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.

翻译：由于天线阵列中的功耗和高电路成本，大规模多输入多输出（MIMO）在第六代（6G）及未来无线网络中的实际应用仍面临挑战。采用低分辨率模数转换器（ADC）及混合模数（HAD）结构是两种性能损失可接受的低成本方案。本文提出在接收端采用混合ADC架构与HAD结构相结合的方法，用于波达方向（DOA）估计，该技术将应用于6G中的波束赋形跟踪与对准。基于加性量化噪声模型，推导了混合ADC下HAD架构的Cramér-Rao下界（CRLB）精确闭式表达式。此外，作为基准，本文还推导了性能损失因子的闭式表达式。同时，为考虑功耗因素，本文进一步研究了能效。数值结果表明，采用混合ADC的HAD结构可显著降低功耗与硬件成本，并且该架构能够在性能损失与功耗之间实现更好的权衡。最后，在实际大规模MIMO系统中，采用2-4比特分辨率可能是一个良好选择。