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Two different modulation schemes have been employed in photonic sub-THz systems to transmit intensity modulated signals: the dual-carrier (D-C) and single-carrier (S-C) schemes. Although it has been stated in the literature that D-C modulation performs better than S-C modulation, no demonstration of such claim has been provided so far. In this letter, we show that the superiority of one scheme or the other depends on the normalization factor that is used for comparison. Through mathematical analysis we show that, when 2-pulse amplitude modulation (2-PAM) signals are considered and the average photocurrent is taken as the normalization factor, the D-C scheme exhibits a 3-dB gain over the S-C scheme in the peak voltage of the recovered signal. However, the analysis also reveals that equal performance should be obtained when these schemes are compared in terms of transmitted sub-THz energy and that the S-C scheme should achieve better results when the comparison is made in terms of the output power of the lasers. We also run simulations to examine the impact of the non-linear transfer curve of a Mach-Zehnder modulator (MZM) on higher-order IM formats such as 4- and 8-PAM. The results of these simulations show that the penalty of the S-C scheme under photocurrent normalization progressively vanishes as the modulation order increases, and for 8-PAM signals, the S-C achieve better results than the D-C scheme. We attribute the deterioration of the D-C scheme with higher modulation orders to the signal-signal beat interference (SSBI).
amplitude modulation; communication systems; microwave photonics; optoelectronic devices; sub-thz communication