Abstract: In order to improve the relatively poor orthogonality and capacity for more users in OCDMA system, which uses the single polarity code, the differential check scheme on the receiver side of the spectrum encoding OCDMA system with the bipolarity codes is studied. The condition, which should be met by the address code used for the differential spectrum amplitude encoding scheme, is discussed. The bit error rate of the OOK and the bipolarity encoding scheme is analyzed with chirp and JohnsonNyquist noise taken into account. Through the numerical calculation, the performance comparison between one system (that both “one” and “zero”represent the signals) and another system (that only “one” represents the signals) was made. The results show that the scheme of the bipolarity address code and the higher laser power can improve the system performance.