We consider uplink transmission for a massive multi-user multiple-input multiple-output (MU-MIMO) system in which the base station (BS) is equipped with a mixed analogto-digital converter (ADC) architecture. In this architecture, a portion of the antennas at the BS is connected to lowpower low-resolution ADCs while the other is fed to powerhungry high-resolution ADCs. By taking into account the mixed-ADC architecture, we derive a closed-form expression for the sum spectral efficiency (SE) when maximum ratio combining (MRC) detection is employed at the BS. Then, we formulate an optimization problem to determine, for a given power budget at the BS, what is the optimal distribution of low and highresolution ADCs that maximizes the sum SE. Interestingly, it is shown that in most realistic scenarios, using a larger number of antennas with only one-bit ADCs is the best approach.
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