Linear Precoding with Low-Resolution DACs for Massive MU-MIMO-OFDM Downlink

We consider the downlink of a massive multiuser (MU) multiple-input multiple-output (MIMO) system in which the base station (BS) is equipped with low-resolution digitalto-analog converters (DACs). As opposed to recent results for such BS architectures, we assume that the system operates over a frequency-selective wideband channel and uses orthogonal frequency division multiplexing (OFDM) to simplify equalization at the user equipments (UEs). Furthermore, we consider the practically relevant case of oversampling DACs. We analyze the uncoded bit error rate (BER) performance with linear precoders (e.g., zero forcing) and quadrature phase-shift keying using Bussgang’s theorem. We further develop a lower bound on the information-theoretic sum-rate throughput achievable with Gaussian inputs, which can be evaluated exactly for the case of 1-bit DACs. For the case of multi-bit DACs, we derive approximate yet accurate expressions for the distortion caused by the low-resolution DACs, which can be used to evaluate the aforementioned sum-rate throughput lower bound. Our results suggest that, for a massive MU-MIMO-OFDM system with a 128-antenna BS serving 16 UEs simultaneously, only 3– 4 DAC bits are required to achieve an uncoded BER of 10−4 with a negligible performance penalty compared to the infiniteresolution case at the cost of additional out-of-band emissions.