D. Modulation Techniques Used in LI-FI
Since LI-FI uses visible light for sending data, it is necessary to modulate the data into a signal which can be transmitted.
These signals consist of light pulses. Some of the common modulation techniques used in LI-FI are discussed below:
(a) OFDM: Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on
multiple carrier frequencies. OFDM is a frequency-division multiplexing (FDM) scheme used as a
digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier
signals are used to carry data on several parallel data streams or channels. Each sub-carrier is
modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phaseshift
keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier
modulation schemes in the same bandwidth.
(b) OOK: On-off keying (OOK) denotes the simplest form of amplitude-shift keying (ASK) modulation
that represents digital data as the presence or absence of a carrier wave. In its simplest form, the
presence of a carrier for a specific duration represents a binary one, while its absence for the same
duration represents a binary zero. Some more sophisticated schemes vary these durations to convey
additional information. It is analogous to unipolar encoding line code. It is very easy to generate and
decode but is not very optimal in terms of illumination control and data throughput.
(c) PWM: Pulse-width modulation (PWM) is a technique used to encode a message into a pulsing signal.
Although this modulation technique can be used to encode information for transmission, its main use is
to allow the control of the power supplied to electrical devices, especially to inertial loads such as
motors. Pulse Width Modulation transmits the data by encoding the data into the duration of the pulses.
More than one bit of data can be conveyed within each pulse.
(d) PPM: Pulse-position modulation (PPM) is a form of signal modulation in which M message bits are
encoded by transmitting a single pulse in one of possible required time-shifts. This is repeated every T