  Although the hybrid of cell-free (CF) massive multiple-input multiple-output
(MIMO) and non-orthogonal multiple access (NOMA) promises massive spectral
efficiency gains, the type of precoders employed at the access points (APs)
impacts the gains. In this paper, we thus comprehensively evaluate the system
performance with maximum ratio transmission (MRT), full-pilot zero-forcing
(fpZF) and modified regularized ZF (mRZF) precoders. We derive their
closed-form sum rate expressions by considering Rayleigh fading channels, the
effects of intra-cluster pilot contamination, inter-cluster interference, and
imperfect successive interference cancellation (SIC). Our results reveal that
this system supports significantly more users simultaneously at the same
coherence interval compared to its OMA equivalent. However, intra-cluster pilot
contamination and imperfect SIC degrade the system performance when the number
of users is low. Moreover, with perfect SIC, mRZF and fpZF significantly
outperform MRT. Also, we show that this system with either mRZF or fpZF
precoding outperforms OMA systems with MRT. The analytical findings are
verified by numerical results.
