  Backward compatibility is an essential ingredient for the success of new
technologies. In the context of in-band full-duplex (FD) communication, FD base
stations (BSs) should support half-duplex (HD) users' equipment (UEs) without
sacrificing the foreseen FD gains. This paper presents flexible and tractable
modeling framework for multi-tier cellular networks with FD BSs and FD/HD UEs.
The presented model is based on stochastic geometry and accounts for the
intrinsic vulnerability of uplink transmissions. The results show that FD UEs
are not necessarily required to harvest rate gains from FD BSs. In particular,
the results show that adding FD UEs to FD BSs offers a maximum of $5\%$ rate
gain over FD BSs and HD UEs case if multi-user diversity is exploited, which is
a marginal gain compared to the burden required to implement FD transceivers at
the UEs' side. To this end, we shed light on practical scenarios where HD UEs
operation with FD BSs outperforms the operation when both the BSs and UEs are
FD and we find a closed form expression for the critical value of the
self-interference attenuation power required for the FD UEs to outperform HD
UEs.
