  What does millimeter-wave (mmW) seek assistance for from micro-wave ({\mu}W)
in a mmW overlaid 5G cellular network? This paper raises the question of
whether to complement downlink (DL) or uplink (UL) transmissions, and concludes
that {\mu}W should aid UL more. Such dedication to UL results from the low mmW
UL rate due to high peak-to-average power ratio (PAPR) at mobile users. The
DL/UL allocations are tractably provided based on a novel closed-form mm-{\mu}W
spectral efficiency (SE) derivation via stochastic geometry. The findings
explicitly indicate: (i) both DL/UL mmW (or {\mu}W) SEs coincidentally converge
on the same value in an ultra-dense cellular network (UDN) and (ii) such a mmW
(or {\mu}W) UDN SE is a logarithmic function of BS-to-user density ratio. The
corresponding mm-{\mu}W resource management is evaluated by utilizing a three
dimensional (3D) blockage model with real geography in Seoul, Korea.
