  Advances in analog-to-digital converter (ADC) technology have opened up the
possibility to directly digitize wideband radio frequency (RF) signals,
avoiding the need for analog down-conversion. In this work, we consider an
orthogonal frequency-division multiplexing (OFDM)-based massive multi-user (MU)
multiple-input multiple-output (MIMO) uplink system that relies on direct
RF-sampling at the base station and digitizes the received RF signals with
1-bit ADCs. Using Bussgang's theorem, we provide an analytical expression for
the error-vector magnitude (EVM) achieved by digital down-conversion and
zero-forcing combining. Our results demonstrate that direct RF-sampling 1-bit
ADCs enables low EVM and supports high-order constellations in the massive
MU-MIMO-OFDM uplink.
