22-02-2015, 09:17 PM
the WiFi-based localization system has been intensely studied
and demonstrated, we will only focus on the optical wireless
transmission. The wavelength of the transmitted optical signal
is generated in a CO with a DFB laser at 1550.12 nm and it is
modulated using 12.5 Gb/s PRBS data in on-off-keying
(OOK) modulation format. The modulated signal is transmitted
to the access point via 5.6 km standard single mode fiber. Then
this signal passes through a lens to increase its divergence before
propagating in free space. At the subscriber end, the signal is
Fig. 2. Experimental setup.
captured by a CPCwith 45 field-of-view and detected by a PD
(PIN with 11.5 GHz electrical 3 dB bandwidth and 0.8 A/W responsivity
at 1550 nm). Here a small sensitive area PD (0.5 mm
diameter) and a coupling system consisting of multiple lenses
and a fiber collimator NA are used instead of a large
sensitive area PD due to our device limitation. A coupling loss
less than 2 dB can be achieved by carefully adjustments. Then
the received data is then amplified with an 18 dB gain 25 GHz
amplifier and measured using a bit error rate tester (BERT) and
a broadband digital communication analyzer (DCA). The measurement
in our experiment has also included the strong background
light from the overhead lamps.
In real scenarios the subscriber maybe constantly moving inside
the room, therefore the distance between the beam center
and the subscriber is always varying and the signal light will
enter the CPC with different incident angles. To quantify this we
investigate and characterize the light properties at different distances
from the beam center. Since the light coupling configuration
in the receiver is sensitive to any tiny movement, the characterization
and quantification are realized by tilting the transmitter
horizontally.
The received signals with respect to different distances from
beam center are shown in Fig. 3(a). In all the measurements we
fix the beam footprint at the receiver to be 1 m and the transmission
power to be 7 mW, which is the maximum allowable power
according to the laser eye and skin safety regulations [14]. We
have changed the distance between the fiber transmitter and the
receiver from 0.65 m to 2.5 m while keeping the beam footprint
constant at the receiver end and little difference in BER performance
has been observed. The bit rates in the experiment are
10 Gb/s and 12.5 Gb/s respectively. It is obvious that for both
cases when the distance from beam center increases the BER increases
as well. This is simply due to the larger signal power collected
by the CPC for smaller distance from beam center while
the noise power remains almost constant. At the beam boundaries,
a BER of for the 10 Gb/s system and for the
12.5 Gb/s system is achieved. If we define an error-free operation
as a BER , then error-free operation can be achieved
when the distance from beam center is smaller than 28.9 cm and
38.2 cm respectively.
To achieve error-free operation over the entire footprint of
the signal beam, we measured the BER at beam boundaries for
different beam footprints since the BER there is the highest
i want ppt for this topic .it should be around 10 slides
