  Channel estimation is of critical importance in millimeter-wave (mmWave)
multiple-input multiple-output (MIMO) systems. Due to the use of large antenna
arrays, low-complexity mmWave specific channel estimation algorithms are
required. In this paper, an auxiliary beam pair design is proposed to provide
high-resolution estimates of the channel's angle-of-departure (AoD) and
angle-of-arrival (AoA) for mmWave MIMO systems. By performing an amplitude
comparison with respect to each auxiliary beam pair, a set of ratio measures
that characterize the channel's AoD and AoA are obtained by the receiver.
Either the best ratio measure or the estimated AoD is quantized and fed back to
the transmitter via a feedback channel. The proposed technique can be
incorporated into control channel design to minimize initial access delay.
Though the design principles are derived assuming a high-power regime,
evaluation under more realistic assumption shows that by employing the proposed
method, good angle estimation performance is achieved under various
signal-to-noise ratio levels and channel conditions.
