  This paper focuses on the design of low complexity hybrid analog/digital
precoding and diversity combining in millimeter wave (mmWave) Internet of
things (IoT) systems. Firstly, by exploiting the sparseness property of the
mmWave in the angular domain, we propose a spatial lobes division (SLD) to
group the total paths of the mmWave channel into several spatial lobes, where
the paths in each spatial lobe form a low-rank sub-channel. Secondly, based on
the SLD operation, we propose a low complexity hybrid precoding scheme, named
HYP-SLD. Specifically, for each low-rank sub-channel, we formulate the hybrid
precoding design as a sparse reconstruction problem and separately maximizes
the spectral efficiency. Finally, we further propose a maximum ratio combining
based diversity combining scheme, named HYP-SLD-MRC, to improve the bit error
rate (BER) performance of mmWave IoT systems. Simulation results demonstrate
that, the proposed HYP-SLD scheme significantly reduces the complexity of the
classic orthogonal matching pursuit (OMP) scheme. Moreover, the proposed
HYP-SLD-MRC scheme achieves great improvement in BER performance compared with
the fully digital precoding scheme.
