Adaptive multiple-input multiple-output mode switching
for indoor visible light communication system with
orthogonal frequency division multiplexing modulation
Xinyue Guo (郭心悦)*, Xin Li (李 鑫), and Ruyi Huang (黄如意)
Shanghai Key Lab of Modern Optical System, School of Optical-Electrical and Computing Engineering,
University of Shanghai for Science and Technology, Shanghai 200093, China
*Corresponding author: guoxinyue21@163.com
Received August 31, 2017; accepted September 8, 2017; posted online October 13, 2017
We propose and experimentally demonstrate an adaptive multiple-input multiple-output (MIMO) mode
switching scheme for an indoor visible light communication system combined with orthogonal frequency division
multiplexing modulation. Only requiring 1 bit feedback from the receiver, the MIMO mode at the transmitter
switches between spatial multiplexing and transmit diversity adapting to the channel correlation. In such a way,
we can take advantage of both spatial multiplexing and transmit diversity, where the spatial multiplexing
benefits for its multiplexing gain in the low channel correlation environment, and the transmit diversity is robust
to high channel correlation. Experimental results validate the performance improvement over the pure spatial
multiplexing or transmit diversity system. With the increasing of the channel correlation, the measured bit error
rates of the proposed system are below the 7% pre-forward error correction (pre-FEC) limit of 3.8 × 10
−3
when
the transmitted data rate is 50 Mb/s, and below the 20% pre-FEC threshold of 5.0 × 10
−2
when the transmitted
data rate is raised up to 100 Mb/s.
OCIS codes: 060.2605, 060.4080.
doi: 10.3788/COL201715.110604.
Visible light communication (VLC) based on white
light emitting diodes (LEDs) has recently emerged as a
compelling wireless communication technology beyond
traditional radio frequency (RF) communications. Com-
pared to traditional RF communications, VLC offers sev-
eral advantages, including cost effectiveness, licensing
free, immunity to electromagnetic interference, and high
security
[1–3]
. However, the modulation bandwidth of LEDs
is very limited, where signals modulated at high frequen-
cies are severely attenuated, resulting in serious inter-
symbol interference (ISI) for high-speed transmissions
[2]
.
Therefore, it is vital to employ spectral-efficient tech-
niques to achieve high data rate. Among existing works,
the orthogonal frequency division multiplexing (OFDM)
scheme has been proved particularly useful for the VLC
system, because the channel is decomposed into multiple
frequency-flat channels, and the ISI can be eliminated
[4,5]
.
Besides, the multiple-input multiple-output (MIMO)
technique has also been considered a promising approach
to combine with OFDM to further increase the data rate
without the additional need of frequency resources
[6]
.
Usually there are multiple LEDs in a room to provide suf-
ficient illumination; as a result, it is natural to implement
the MIMO technique to realize parallel data transmission.
In VLC, MIMO schemes have already been proved to be
able to provide gains even under line-of-sight (LOS) con-
ditions in contrast to the use of MIMO in the RF domain
[7]
.
Spatial multiplexing, as one of the mainstream MIMO
schemes, is able to transmit multiple data streams simul-
taneously on different LEDs with high data rates
[6–9]
.
However, it only enjoys the multiplexing gain when the
channel correlation is sufficiently low
[7,10]
. Once the chan-
nel is highly correlated, i.e., channel gains between trans-
mitters and receivers are similar to each other, it would be
difficult for the receiver to separate the single data
streams.
However, the channel of the typical indoor VLC MIMO
environment tends to be highly correlated as Lambertian
surfaces of multiple LEDs render similar coefficients
among MIMO channels
[10]
. Several methods have been
proposed to attack the high channel correlation problem.
For instance, power imbalance is used to reduce the
channel correlation by allocating transmission power un-
equally
[7]
. However, the transmission power of some chan-
nels may be too low to provide a sufficient sig nal-to-noise
ratio (SNR) for decoding. A link-blocked receiver is an-
other way to decrease channel correlation by placing an
opaque boundary to block a particular link from an
LED to a photodiode (PD)
[7]
. Such a simple way obviously
restricts the receiver moving around the room freely. Im-
aging MIMO, which creates an approximate image of the
source array on a detector array by using lenses, forms an
uncorrelated channel matrix
[8]
. However, precise align-
ment is crucial to focusing an LED image onto a dedicated
detector, which needs extra efforts to calibrate in practice.
Different from spatial multiplexing, another MIMO
scheme based on transmit diversity has also been applied
in VLC, where same information-bearing signals are sent
from multiple LEDs simultaneously
[11–13]
. As a result,
transmit diversity benefits for its high reliability and in-
sensitivity to channel correlation, but lack of multiplexing
gains
[7]
. In order to maintain the same data rate as spatial
COL 15(11), 110604(2017) CHINESE OPTICS LETTERS November 10, 2017
1671-7694/2017/110604(6) 110604-1 © 2017 Chinese Optics Letters
资源评论
