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In this paper, we propose a novel multiple-access interference (MAI) cancellation technique using a reference signal, which contains no data components (data-free) of the desired signal, and then simplifies the receiver configuration in synchronous M-ary frequency-shift keying optical code-division multiple-access (FSK-OCDMA) network. In doing so, we have taken advantage of a recently introduced energy-saving unipolar prime-code family, referred to as double-padded modified prime-code (DPMPC) as the spreading codes. In the theoretical analysis, the system upper bounded bit error rate (BER) is derived taking into account the Poisson effect on the I/O characteristics of the photodetectors. We have found that when the bit rate is constant, the capacity of this system increases by employing the proposed interference canceller and spreading code as compared with the synchronous M-ary pulse-position modulation OCDMA system with an existing interference canceller. In contrast to wavelength-division multiple access, a fewer set of wavelengths is needed as a result of only M-ary source coding.