  A novel modulation scheme termed orthogonal frequency-division multiplexing
with subcarrier number modulation (OFDM-SNM) has been proposed and regarded as
one of the promising candidate modulation schemes for next generation networks.
Although OFDM-SNM is capable of having a higher spectral efficiency (SE) than
OFDM with index modulation (OFDM-IM) and plain OFDM under certain conditions,
its reliability is relatively inferior to these existing schemes, because the
number of active subcarriers varies. In this regard, we propose an enhanced
OFDM-SNM scheme in this paper, which utilizes the flexibility of placing
subcarriers to harvest a coding gain in the high signal-to-noise ratio (SNR)
region. In particular, we stipulate a methodology that optimizes the subcarrier
activation pattern (SAP) by subcarrier assignment using instantaneous channel
state information (CSI) and therefore the subcarriers with higher channel power
gains will be granted the priority to be activated, given the number of
subcarriers is fixed. We also analyze the proposed enhanced OFDM-SNM system in
terms of outage and error performance. The average outage probability and block
error rate (BLER) are derived and approximated in closed-form expressions,
which are further verified by numerical results generated by Monte Carlo
simulations. The high-reliability nature of the enhanced OFDM-SNM makes it a
promising candidate for implementing in the Internet of Things (IoT) with
stationary machine-type devices (MTDs), which are subject to slow fading and
supported by proper power supply.
