As discussed in the previous section, there is
some potential for higher link capacity in both
the optical subsystem and the baseband. In the
latter case, development depends strongly on the
availability of adaptive OFDM chips with an
extended baseband bandwidth. Depending on
the optical link setup, the choice of suitable optical
lenses can significantly increase the attainable
link margin. Until now, adaptive
transmission has been realized using DMT with
bit and power loading on individual subcarriers.
For enhanced robustness, advanced waveforms
with reduced clipping probability, such as blockwise
pulse amplitude modulation (PAM) with
frequency domain equalization [13] may bepromising because the non-directed LOS channel
is essentially flat, apart from the low-pass
character of the optical transmitters and
receivers. Single-carrier transmission combined
with frequency domain equalization and modifications
thereof [14], and extended to support
multiple users, as in Long Term Evolution (LTE)
mobile radio, also has high potential for VLC.