  This paper investigated the uplink of multi-user massive multi-input
multi-output (MIMO) systems with a mixed analog-to-digital converter (ADC)
receiver architecture, in which some antennas are equipped with costly
full-resolution ADCs and others with less expensive low-resolution ADCs. A
closed-form approximation of the achievable spectral efficiency (SE) with the
maximum-ratio combining (MRC) detector is derived. Based on this approximated
result, the effects of the number of base station antennas, the transmit power,
the proportion of full-resolution ADCs in the mixed-ADC structure, and the
number of quantization bits of the low-resolution ADCs are revealed. Results
showcase that the achievable SE increases with the number of BS antennas and
quantization bits, and it converges to a saturated value in the high user power
regime or the full ADC resolution case. Most important, this work efficiency
verifies that for massive MIMO, the mixed-ADC receiver with a small fraction of
full-resolution ADCs can have comparable SE performance with the receiver with
all full-resolution ADCs but at a considerably lower hardware cost.
