sine wave, i.e. change the driven current of the
LED and its dimming [5]. Through I2
C and
SPI serial interface, digital dimming utilizes a
chip microcomputer or other microcontroller
to send digital signal to the LED driver and
adjusts the brightness of LED [6]. PWM dimming
linearly changes the brightness of LED
with the duty cycle of a constant DC voltage at
certain frequency [7-9]. In these schemes, the
data modulation and the dimming controlling
are realized by a single driven circuit. The
data transmission is affected by the dimming
controlling, especially when the dimming
controlling is realized by tuning the driven
current. For a visible light communications
system, it is desired that the data transmission
is not affected by the lighting dimming controlling.
In order to realize this, we propose a
scheme of separate dimming controlling and
data transmission, i.e. to partitioning LEDs for
different functions with some LEDs for the diming
controlling and the others for data transmission.
The dimming controlling is realized
by an analog dc driver, and the data transmission
is realized by NRZ-OOK modulation circuit.
In this way, the diming control and data
transmission can be realized simultaneously,
and the diming control function will not affect
the data transmission. The rest of the paper is
organized as the following. The dimming controlling
in the proposed scheme is described
in section II, and the data transmission is described
in section III. The experimental results
are demonstrated in section IV. In section V,
the conclusions are drawn.