  In this paper, we investigate the performance of simultaneous wireless
information and power transfer (SWIPT) in a point-to-point system, adopting
practical $M$-ary modulation. We take into account the fact that the receiver's
radio-frequency (RF) energy harvesting circuit can only harvest energy when the
received signal power is greater than a certain sensitivity level. For both
power-splitting (PS) and time-switching (TS) schemes, we derive the energy
harvesting performance as well as the information decoding performance for the
Nakagami-$m$ fading channel. We also analyze the performance tradeoff between
energy harvesting and information decoding by studying an optimization problem,
which maximizes the information decoding performance and satisfies a constraint
on the minimum harvested energy. Our analysis shows that (i) for the PS scheme,
modulations with high peak-to-average power ratio achieve better energy
harvesting performance, (ii) for the TS scheme, it is desirable to concentrate
the power for wireless power transfer in order to minimize the non-harvested
energy caused by the RF energy harvesting sensitivity level, and (iii) channel
fading is beneficial for energy harvesting in both PS and TS schemes.
