  The problem of designing downlink transmission schemes for supporting
heterogeneous ultra-reliable low-latency communications (URLLC) and/or with
other types of services is investigated. We consider the broadcast channel,
where the base station sends superimposed signals to multiple users. Under
heterogeneous blocklength constraints, strong users who are URLLC users cannot
wait to receive the entire transmission frame and perform successive
interference cancellation (SIC) due to stringent latency requirements, in
contrast to the conventional infinite blocklength cases. Even if SIC is
feasible, SIC may be imperfect under finite blocklength constraints. To cope
with the heterogeneity in latency and reliability requirements, we propose a
practical downlink transmission scheme with discrete signaling and single-user
decoding (SUD), i.e., without SIC. We carefully design the discrete input
distributions to enable efficient SUD by exploiting the structural
interference. Furthermore, we derive the second-order achievable rate under
heterogenous blocklength and error probability constraints and use it to guide
the design of channel coding and modulations. It is shown that in terms of
achievable rate under short blocklength, the proposed scheme with regular
quadrature amplitude modulations and SUD can operate extremely close to the
benchmark schemes that assume perfect SIC with Gaussian signaling.
