In this paper, we develop a low-complexity channel estimation for hybrid
millimeter wave (mmWave) systems, where the number of radio frequency (RF)
chains is much less than the number of antennas equipped at each transceiver.
The proposed mmWave channel estimation algorithm first exploits multiple
frequency tones to estimate the strongest angle-of-arrivals (AoAs) at both base
station (BS) and user sides for the design of analog beamforming matrices. Then
all the users transmit orthogonal pilot symbols to the BS along the directions
of the estimated strongest AoAs in order to estimate the channel. The estimated
channel will be adopted to design the digital zero-forcing (ZF) precoder at the
BS for the multi-user downlink transmission. The proposed channel estimation
algorithm is applicable to both nonsparse and sparse mmWave channel
environments. Furthermore, we derive a tight achievable rate upper bound of the
digital ZF precoding with the proposed channel estimation algorithm scheme. Our
analytical and simulation results show that the proposed scheme obtains a
considerable achievable rate of fully digital systems, where the number of RF
chains equipped at each transceiver is equal to the number of antennas.
Besides, by taking into account the effect of various types of errors, i.e.,
random phase errors, transceiver analog beamforming errors, and equivalent
channel estimation errors, we derive a closed-form approximation for the
achievable rate of the considered scheme. We illustrate the robustness of the
proposed channel estimation and multi-user downlink precoding scheme against
the system imperfection.

本文提出了一种适用于混合毫米波系统的低复杂度信道估计算法，其中 RF 链路数远小于每个收发器上配备的天线数，该算法利用多个频率进行强角度到达估计，并针对该估计结果设计模拟波束成形矩阵，以此估算信道，进而进行多用户下行传输。该算法适用于非稀疏和稀疏毫米波信道环境，并且分析和仿真结果表明，该算法对于系统缺陷具有较强的容错性能。