COL 12(9), 090602(2014) CHINESE OPTICS LETTERS September 10, 2014
1671-7694/2014/090602(5) 090602-1 © 2014 Chinese Optics Letters
Recently, the increase in use of portable computing
devices has created an intense demand for wireless
data access. Spectral allocations and regulations limit
our ability to increase the capacity of existing chan-
nels within the radio frequency (RF) spectrum. Ad-
vances made in the solid-state lighting industry are
driving signicant deployments of energy-eicient light-
emitting-diode based luminaries. This has created an
opportunity to use such luminaries to establish high
capacity indoor visible light communication (VLC)
links and reduce the bottleneck on existing RF wireless
channels. Under this model, luminaries simultaneously
support illumination and wireless data transmission
[1]
.
Optical spatial modulation (OSM) and optical orthogo-
nal frequency division multiplexing (O-OFDM) are two
techniques that have been proposed to implement such
a dual-use VLC channel.
OSM is a multiple-transmitter technique in which in-
formation is encoded over a) index of luminaires that
are spatially separated and b) modulation scheme over-
layed on indexed luminaire
[2]
. Within a symbol period,
only one luminaire emits a radiant ux while all other
luminaires are idle. This minimizes the inter-channel in-
terference (ICI) thus simplifying the detection process
and the overall system complexity as compared with
spatial multiplexing (SMP). In OSM, the bit-stream
to be transmitted is divided into contiguous sections
of k = log
2
(N
tx
) spatial bit-stream and m = log
2
(M)
modulation bit-stream where N
tx
is the number of lu-
minaires and M is the modulation order. The k bits
Sample indexed spatial orthogonal frequency
division multiplexing
Pankil Butala
*
, Hany Elgala, and Thomas D. C. Little
Department of Electrical and Computer Engineering, Boston University, Boston, MA
02215, USA
*
Corresponding author: pbutala@bu.edu
Received March 4, 2014; accepted June 3, 2014; posted online August 28, 2014
Optical spatial modulation (OSM) is a multiple-transmitter technique that can provide higher data rates with
low system complexity as compared with single-input single-output systems. Orthogonal frequency division
multiplexing (OFDM) is widely implemented to achieve better spectral eiciency in wireless channels. Asym-
metrically clipped optical OFDM (O-OFDM) and DC-biased O-OFDM are two well-known O-OFDM tech-
niques suitable for intensity-modulation direct-detection optical systems. In this work, sample indexed spatial
OFDM (SIS-OFDM) is proposed to combine OSM and O-OFDM in a novel way and achieve signicant per-
formance gain. By assigning time-domain samples of the O-OFDM transmit symbol to dierent transmitters,
SIS-OFDM achieves much better spectral eiciency and reduces computational complexity at the transmit-
ter as compared with previous work that combines OSM with O-OFDM in the frequency domain. We also
consider the impact of optical source biasing on overall performance, and the relative performance of imaging
receiver (ImR) versus non-imaging receiver (NImR) design for our proposed SIS-OFDM technique. Results
indicate that for an N
tx
× N
rx
multiple-input multiple-output conguration where N
tx
= N
rx
= 4, SIS-OFDM
using ImR can achieve up to 135 dB of signal-to-noise ratio gain over comparable system using a NImR. Also,
using N
sc
number of O-OFDM subcarriers provides up to N
sc
× log
2
(N
tx
) additional bits per symbol of spectral
eiciency over techniques that combine OSM and O-OFDM in the frequency domain.
OCIS codes: 060.4080, 060.4230, 060.4510, 060.2605.
doi: 10.3788/COL201412.090602.
select the luminaire to be activated while the m bits
select the M-ary modulation symbol to be transmitted.
Thus, OSM system provides log
2
(MN
tx
) bits per sym-
bol. Fath et al.
[3]
proposed an OSM system with pulse
amplitude modulation (PAM) as the overlayed modula-
tion scheme. Popoola et al.
[4]
proposed a scheme that
combines OSM with pulse position modulation (PPM)
to benet from the energy eiciency of PPM as com-
pared with PAM. Butala et al.
[5]
showed that imaging
receiver (ImR) can provide signicant signal-to-noise
ratio (SNR) gains for OSM and SMP as compared with
non-imaging receiver (NImR).
Mesleh et al.
[6]
showed implementation and perfor-
mance comparisons of asymmetrically clipped optical
OFDM (ACO-OFDM) and DC-biased optical OFDM
(DCO-OFDM). In ACO-OFDM, data are assigned only
on odd subcarriers while in DCO-OFDM all odd and
even subcarriers are assigned data. Hermetian symme-
try is enforced across the frequency-domain O-OFDM
symbol. An inverse fast Fourier transform (IFFT) pro-
cess then results in a real-valued time-domain signal
that multiplexes the streams before transmission over
the channel. In intensity-modulation direct-detection
(IM/DD) systems, the signal is transmitted by varying
the output ux from the transmitter. Thus, the trans-
mitted signal must be non-negative and real valued.
The ACO-OFDM signal can be clipped at values be-
low zero because the resulting clipping noise is shown
to be orthogonal to the signal
[7]
. Conversely, in DCO-
OFDM an oset must be added to the multiplexed
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