  Wireless powered communication networks (WPCNs), where multiple
energy-limited devices first harvest energy in the downlink and then transmit
information in the uplink, have been envisioned as a promising solution for the
future Internet-of-Things (IoT). Meanwhile, non-orthogonal multiple access
(NOMA) has been proposed to improve the system spectral efficiency (SE) of the
fifth-generation (5G) networks by allowing concurrent transmissions of multiple
users in the same spectrum. As such, NOMA has been recently considered for the
uplink of WPCNs based IoT networks with a massive number of devices. However,
simultaneous transmissions in NOMA may also incur more transmit energy
consumption as well as circuit energy consumption in practice which is critical
for energy constrained IoT devices. As a result, compared to orthogonal
multiple access schemes such as time-division multiple access (TDMA), whether
the SE can be improved and/or the total energy consumption can be reduced with
NOMA in such a scenario still remains unknown. To answer this question, we
first derive the optimal time allocations for maximizing the SE of a TDMA-based
WPCN (T-WPCN) and a NOMA-based WPCN (N-WPCN), respectively. Subsequently, we
analyze the total energy consumption as well as the maximum SE achieved by
these two networks. Surprisingly, it is found that N-WPCN not only consumes
more energy, but also is less spectral efficient than T-WPCN. Simulation
results verify our theoretical findings and unveil the fundamental performance
bottleneck, i.e., "worst user bottleneck problem", in multiuser NOMA systems.
