  The non-orthogonal coexistence between the enhanced mobile broadband (eMBB)
and the ultra-reliable low-latency communication (URLLC) in the downlink of a
multi-cell massive MIMO system is rigorously analyzed in this work. We provide
a unified information-theoretic framework blending an infinite-blocklength
analysis of the eMBB spectral efficiency (SE) in the ergodic regime with a
finite-blocklength analysis of the URLLC error probability relying on the use
of mismatched decoding, and of the so-called saddlepoint approximation.
Puncturing (PUNC) and superposition coding (SPC) are considered as alternative
downlink coexistence strategies to deal with the inter-service interference,
under the assumption of only statistical channel state information (CSI)
knowledge at the users. eMBB and URLLC performances are then evaluated over
different precoding techniques and power control schemes, by accounting for
imperfect CSI knowledge at the base stations, pilot-based estimation overhead,
pilot contamination, spatially correlated channels, the structure of the radio
frame, and the characteristics of the URLLC activation pattern. Simulation
results reveal that SPC is, in many operating regimes, superior to PUNC in
providing higher SE for the eMBB yet achieving the target reliability for the
URLLC with high probability. Moreover, PUNC might cause eMBB service outage in
presence of high URLLC traffic loads. However, PUNC turns to be necessary to
preserve the URLLC performance in scenarios where the multi-user interference
cannot be satisfactorily alleviated.
