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Massive multiple-input multiple-output (MIMO) promises significantly higherperformance relative to conventional multiuser systems. However, the promisedgains of massive MIMO systems rely heavily on the accuracy of the absolutefront-end calibration, as well as quality of channel estimates at the basestation (BS). In this paper, we analyze user equipment-aided calibrationmechanism to estimate the amplitude scaling and phase drift at eachradio-frequency chain interfacing with the BS array. Assuming a uniform lineararray at the BS and Ricean fading, we obtain the estimation parameters withmoment-based (amplitude, phase) and maximum-likelihood (phase-only) estimationtechniques. In stark contrast to previous works, we mathematically articulatethe equivalence of the two approaches for phase estimation. Furthermore, werigorously derive a Cramer-Rao lower bound to characterize the accuracy of thetwo estimators. Via numerical simulations, we evaluate the estimatorperformance with varying dominant line-of-sight powers, dominantangles-of-arrival, and signal-to-noise ratios.

Amplitude and Phase Estimation for Absolute Calibration of Massive MIMO Front-Ends