  3GPP release 15 focusing on 5G general outline has been published in December
2017. The major difference with respect to currently deployed LTE is the
support of various physical layer numerologies. Making the physical layer
scalable allows to properly address new services such as low latency or
millimeter communications. However it poses the problem of numerology
coexistence. Indeed the orthogonality of the OFDM waveform is broken by the use
of different subcarrier spacings and therefore multiplexed communications may
interfere with each others. A first and simple solution to limit the distortion
is to consider guard bands. In this paper, the authors develop analytical
metrics to quantify the level of distortion induced by 5G multi-service
multiplexing. Besides, general comments and guard band dimensioning are carried
out. It is shown that high interference rejection needs to be associated with
side lobe reduction techniques to favor an efficient bandwidth use.
