... efficiency to be the number of modulated subcarriers over the total2 EURASIP Journal on Advances in Signal ProcessingElectricalmodulatorElectrical to opticalconverter(LED)Optical to electricalconverter(photodiode)SymboletectorNoise ... of the N symbols of S to 2N Hermitian symmetricsymbols and add zeroes to form the 4N× 1vectorX =[0, S0,0,S1, ,0,SN−1,0,S∗N−1,0,S∗N−2, ,0,S∗0]T.Due to the structure of X (zeros ... 4N-point IFFT is used to obtain the time domain signal denotedby x= [x0, x1, , x4N−1]T. A CP is then added to x to yieldx and the negative values are clipped to zero as in ACO-OFDM....