  The limited bandwidth of optical wireless communication (OWC) front-end
devices motivates the use of multiple-input-multiple-output (MIMO) techniques
to enhance data rates. It is known that very high multiplexing gains could be
achieved by spatial multiplexing (SMX) in exchange for exhaustive detection
complexity. Alternatively, in spatial modulation (SM), a single light emitting
diode (LED) is activated per time instance where information is carried by both
the signal and the LED index. Since only an LED is active, both transmitter
(TX) and receiver (RX) complexity reduces significantly while retaining the
information transmission in the spatial domain. However, significant spectral
efficiency losses occur in SM compared to SMX. In this paper, we propose a
technique which adopts the advantages of both systems. Accordingly, the
proposed flexible LED index modulation (FLIM) technique harnesses the inactive
state of the LEDs as a transmit symbol. Therefore, the number of active LEDs
changes in each transmission, unlike conventional techniques. Moreover, the
system complexity is reduced by employing a linear minimum mean squared error
(MMSE) equalizer and an angle perturbed receiver at the RX. Numerical results
show that FLIM outperforms the reference systems by at least 6 dB in the low
and medium/high spectral efficiency regions.
