A Performance Comparison of Optical Heterodyne OOK and FSK Communications Systems

Abstract

An error probability analysis is performed for two types of optical communications systems employing spread-spectrum code-division multiple-access (CDMA) techniques. Spread-spectrum is specifically employed to overcome additive noise and increase system user capacity. The two types of systems considered are a coherent optical heterodyne on-off keying (OOK) system using optical orthogonal spreading code sequences (OOCs), and a coherent optical heterodyne frequency shift keying (FSK) system using randon spreading codes. The OOK system spreads the signal optically, and the FSK system spreads the signal electromagnetically before converting it to an optical signal for transmission. System performance is degraded by laser phase noise, additive Gaussian multiuser interference, and additive Gaussian receiver noise. Single user OOK system performance for different linewidth-to-bit rate ratios is analyzed over a range of both signal-to-noise-ratios (SNR) and normalized decision thresholds. This analysis is used to determine which noise source dominates system performance and the maximum acceptable linewidth-to-bit rate ratio. The limiting linewidth-to-bit rate ratio is then used in previously derived equations to obtain the performance of the coherent optical heterodyne OOK-CDMA system for various spreading code length and numbers of users. An error probability analysis is then performed for the coherent optical heterodyne FSK-CDMA system for various spreading code length and numbers of users. Finally, the performance of the two systems is compared, and it is shown that optical FSK-CDMA systems perform better than optical OOK-CDMA systems