  Massive multiple-input multiple-output (MIMO) promises significantly higher
performance relative to conventional multiuser systems. However, the promised
gains of massive MIMO systems rely heavily on the accuracy of the absolute
front-end calibration, as well as quality of channel estimates at the base
station (BS). In this paper, we analyze user equipment-aided calibration
mechanism to estimate the amplitude scaling and phase drift at each
radio-frequency chain interfacing with the BS array. Assuming a uniform linear
array at the BS and Ricean fading, we obtain the estimation parameters with
moment-based (amplitude, phase) and maximum-likelihood (phase-only) estimation
techniques. In stark contrast to previous works, we mathematically articulate
the equivalence of the two approaches for phase estimation. Furthermore, we
rigorously derive a Cramer-Rao lower bound to characterize the accuracy of the
two estimators. Via numerical simulations, we evaluate the estimator
performance with varying dominant line-of-sight powers, dominant
angles-of-arrival, and signal-to-noise ratios.
