Optical Orthogonal Frequency Domain Multiplexing (oofdm): Prof K. Surendra Nath

OPTICAL ORTHOGONAL FREQUENCY DOMAIN MULTIPLEXING (OOFDM) PROF K. SURENDRA NATH

 OFDM is a multi carrier transmission (MCM) technique  Key distinction of OFDM from general MCM is the use of Orthogonality between the sub carriers  Due to Orthogonality of the signals, they won’t overlap and the guard bands are not required.  Harmonically related sub carriers  Multiple modulated carriers are squeezed with reduced band width as shown in the figure.  In FDM, 10 sub carriers signals are separated by guard bands.  In OFDM , the peak of one peak coincides with the trough of the other peak.  Sub carriers are separated by 1/Ts

 Uses inverse fast Fourier transforms (IFFT) for modulation and FFT for de modulation.  OFDM exhibits a nearly white frequency spectrum and tolerant to signal dispersion, thus high bit transmission is possible  Bloch diagram of OOFDM is shown in the figure

 Data from N- input channels are modulated using quadrature amplitude modulation (QAM) onto N equally spaced electronical sub carriers  Orthogonal electrical sub carriers differ by 900 phase shift even though they have the same frequency and can be transmitted with in the same frequency band  The data from QAM is sent for computation of the IFFT  Parallel to serial and D/A converters , produces a complex superimposed electrical subcarrier wave form.  In – phase and quadrature phase ( I-Q) modulator produces OFDM by modulating sub carriers with RF frequency.  A Dc component is added to recover QAM symbols at the receiver end directly.

 OOFDM signal can be produced by using a DFB laser source to produces a intensity modulation by using MZM modulator  Filters are used to remove the unwanted frequencies to produce a single side band signal to be coupled to fiber.