Massive MIMO Systems with Non-Ideal Hardware: Energy Efficiency, Estimation, and Capacity Limits

The use of large-scale antenna arrays can bring substantial improvements in energy and/or spectral efficiency to wireless systems due to the greatly improved spatial resolution and array gain. Recent works in the field of massive multiple-input multiple-output (MIMO) show that by increasing the number of antennas at base stations (BSs) the user channels decorrelate, thus strong signal gains are achievable with little inter-user interference. Since these results rely on asymptotics, it is important to investigate whether the conventional system models are reasonable in the asymptotic regimes. This paper considers transceiver hardware impairments at both the BSs (equipped with large antenna arrays) and the single-antenna user equipments (UEs). As opposed to ideal hardware cases, we show that practical hardware impairments create finite ceilings on the estimation accuracy and capacity of each user link. Surprisingly, the performance is mainly limited by the hardware at the UEs, while the impact of impairments at the large-scale arrays vanishes asymptotically and pilot contamination becomes negligible. Furthermore, we prove that an arbitrarily high energy efficiency can be achieved by reducing the transmit power while increasing the number of antennas. Alternatively, a non-zero capacity can be retained while the hardware quality is degraded as the array grows, thus enabling use of inexpensive antenna elements.