Clearly, the continued network densification
and increased heterogeneity poses challenges for the
support of mobility. Although a hefty share of data is served
to stationary indoor users, the support of mobility and alwayson
connectivity is arguably the single most important feature
of cellular networks relative to WiFi. Because modeling and
analyzing the effect of mobility on network performance is
difficult, we expect to see somewhat ad hoc solutions such as in
LTE Rel-11 [51] where user-specific virtual cells are defined to
distinguish the physical cell from a broader area where the user can roam without the need for handoff, communicating with
any BS or subset of BSs in that area. Or in mmWave, restricting
highly mobile users to macrocells and microwave frequencies,
thereby forcing them to tolerate lower rates. Handoffs
will be particularly challenging at mmWave frequencies since
transmit and receive beams must be aligned to communicate.
Indeed, the entire paradigm of a handoff initiated and managed
at layer 3 by the core network will likely not exist in 5G;
instead, handoffs may be opportunistic, based on mmWave
beam alignments, or indistinguishable from PHY/MAC interference
management techniques whereby users communicate
with multiple coordinated BSs, as exemplified by [52] in this
special issue.