  We show that digital subcarrier multiplexing (DSM) systems require much
greater complexity for Nyquist pulse shaping than single-carrier (SC) systems,
and it is a misconception that both systems use the same bandwidth when using
the same pulse shaping. Through back-to-back (B2B) experiments with realistic
transmitter (TX) modules and amplified spontaneous emission (ASE) noise
loading, we show that even with optimized waterfilling and entropy loading, DSM
does not achieve a larger net data rate (NDR) compared to SC when only ASE
noise exists in the channel in long-haul transmission scenarios.
