  Ultra Reliable and Low Latency Communications (URLLC) is deemed to be an
essential service in 5G systems and beyond to accommodate a wide range of
emerging applications with stringent latency and reliability requirements.
Coexistence of URLLC alongside other service categories calls for developing
spectrally efficient multiplexing techniques. Specifically, coupling URLLC and
conventional enhanced Mobile BroadBand (eMBB) through superposition/puncturing
naturally arises as a promising option due to the tolerance of the latter in
terms of latency and reliability. The idea here is to transmit URLLC packets
over resources occupied by ongoing eMBB transmissions while minimizing the
impact on the eMBB transmissions. In this paper, we propose a novel downlink
URLLC-eMBB multiplexing technique that exploits possible similarities among
URLLC and eMBB symbols, with the objective of reducing the size of the
punctured eMBB symbols. We propose that the base station scans the eMBB
traffic' symbol sequences and punctures those that have the highest symbol
similarity with that of the URLLC users to be served. As the eMBB and URLLC may
use different constellation sizes, we introduce the concept of symbol region
similarity to accommodate the different constellations. We assess the
performance of the proposed scheme analytically, where we derive closed-form
expressions for the symbol error rate (SER) of the eMBB and URLLC services. {We
also derive an expression for the eMBB loss function due to puncturing in terms
of the eMBB SER}. We demonstrate through numerical and simulation results the
efficacy of the proposed scheme where we show that 1) the eMBB spectral
efficiency is improved by puncturing fewer symbols, 2) the SER and reliability
performance of eMBB are improved, and 3) the URLLC data is accommodated within
the specified delay constraint while maintaining its reliability.
