  This paper investigates the effect of various antenna array structures, i.e.,
uniform linear array (ULA), uniform rectangular array (URA), uniform circular
array (UCA), and concentric circular array (CCA), on cluster index modulation
(CIM) enabled massive multiple-input multiple-output (mMIMO) millimeter-wave
(mmWave) communications systems. As the CIM technique indexes spatial clusters
to convey additional information bits, the different radiation characteristics
caused by different array structures can significantly affect system
performance. By analyzing the effects of array characteristics such as
radiation pattern, array directivity, half-power beam width (HPBW), and
radiation side lobes on bit error rate (BER) performance, we reveal that URA
achieves better error performance than its counterparts in a CIM-enabled mmWave
system. We demonstrate that narrower beams alone cannot guarantee better BER
performance in a CIM-based system. Instead, other radiation characteristics,
especially radiation side lobes, can significantly influence system performance
by entailing extra interference in the non-intended directions. Illustrative
results show that URA owes its superiority to its lower side lobes. We also
propose an algorithm to implement fixed phase shifters (FPS) as a
hardware-efficient (HE) analog network structure (beamformer/combiner) to
reduce cost and energy consumption in mmWave systems and investigate the effect
of a non-ideal analog network on the BER performance for different array
structures. It is demonstrated that HE systems with a few FPSs can achieve
similar BER performance compared to the optimum (OP) analog network structure.
