  In this paper, we focus on one of the key technologies for the
fifth-generation wireless communication networks, massive
multiple-input-multiple-output (mMIMO), by investigating two of its most
relevant architectures: 1) to provide in-band backhaul for the ultra-dense
network (UDN) of self-backhauled small cells (SCs), and 2) to provide direct
access (DA) to user equipments (UEs). Through comprehensive 3GPP-based
system-level simulations and analytical formulations, we show the end-to-end UE
rates achievable with these two architectures. Differently from the existing
works, we provide results for two strategies of self-backhauled SC deployments,
namely random and ad-hoc, where in the latter SCs are purposely positioned
close to UEs to achieve line-of-sight (LoS) access links. We also evaluate the
optimal backhaul and access time resource partition due to the in-band
self-backhauling (s-BH) operations. Our results show that the ad-hoc deployment
of self-backhauled SCs closer to the UEs with optimal resource partition and
with directive antenna patterns, provides rate improvements for cell-edge UEs
that amount to 30% and tenfold gain, as compared to mMIMO DA architecture with
pilot reuse 3 and reuse 1, respectively. On the other hand, mMIMO s-BH
underperforms mMIMO DA above the median value of the UE rates when the effect
of pilot contamination is less severe, and the LoS probability of the DA links
improves.
