  Far-field wireless power transfer (WPT) is a promising technique to resolve
the painstaking power-charging problem inherent in various wireless terminals.
This paper investigates the power transfer efficiency of the WPT segment in
future communication systems in support of simultaneous power and data
transfer, by means of analytically computing the time-average output direct
current (DC) power at user equipments (UEs). In order to investigate the effect
of channel variety among UEs on the average output DC power, different policies
for the scheduling of the power transfer among the users are implemented and
compared in two scenarios: homogeneous, whereby users are symmetric and
experience similar path loss, and heterogeneous, whereby users are asymmetric
and exhibit different path losses. Specifically, if opportunistic scheduling is
performed among $N$ symmetric/asymmetric UEs, the power scaling laws are
attained by using extreme value theory, and reveal that the gain in power
transfer efficiency is $\ln{N}$ if UEs are symmetric whereas the gain is $N$ if
UEs are asymmetric, compared with that of conventional round-robin scheduling.
Thus, the channel variety among UEs inherent to the wireless environment can be
exploited by opportunistic scheduling to significantly improve the power
transfer efficiency when designing future wireless communication systems in
support of simultaneous power and data transfer.
