  High power consumption and expensive hardware are two bottlenecks for
practical massive multiple-input multiple-output (mMIMO) systems. One promising
solution is to employ low-resolution analog-to-digital converters (ADCs) and
digital-to-analog converters (DACs). In this paper, we consider a general
multipair mMIMO relaying system with a mixed-ADC/DAC architecture, in which
some antennas are connected to low-resolution ADCs/DACs, while the rest of the
antennas are connected to high-resolution ADCs/DACs. Leveraging on the additive
quantization noise model, both exact and approximate closed-form expressions
for the achievable rate are derived. It is shown that the achievable rate can
approach the unquantized one by using only 2-3 bits of resolutions. Moreover, a
power scaling law is presented to reveal that the transmit power can be scaled
down inversely proportional to the number of antennas at the relay. We further
propose an efficient power allocation scheme by solving a complementary
geometric programming problem. In addition, a trade-off between the achievable
rate and power consumption for different numbers of low-resolution ADCs/DACs is
investigated by deriving the energy efficiency. Our results reveal that the
large antenna array can be exploited to enable the mixed-ADC/DAC architecture,
which significantly reduces the power consumption and hardware cost for
practical mMIMO systems.
