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Increasing the bandwidth until the physical limit of the optical medium itself (fiber optics) has always been the striving goal in designing an optical fiber communication system. Wavelength division multiplexing (WDM) and its dense version (DWDM) are widely accepted as the best approaches thus far. In this paper, a study on the sub-carrier multiplexing at radio frequency (RF) bands is proposed as a complementary technique to increase the transmission bandwidth. Two channels (each at 155 Mbps rate) and eight channels (each at 622 Mbps rate) of pseudo-random bit sequence of the NRZ-encoded data were tested. The channels were sub-carried by two RF signals at 500 MHz and 1 GHz respectively, while the eight channels system were sub-carried by 1 GHz, 2 GHz, 3 GHz and 4 GHz over each of the two wavelengths. The simulations were carried out over a fiber distance of 70 km. The effect of transmission native rate on the selection of the sub-carrier frequency is also presented.