Nonlinear effect of direct-sequence CDMA in optical transmission

Abstract

The intermodulation distortion (IMD) due to laser diode (LD) nonlinearity of an asynchronous direct-sequence code division multiple access (DS/CDMA) system in optical transmission is analyzed. A third-order polynomial without memory is used to present LD nonlinearity. In DS/CDMA systems, only one harmonic of the third-order intermodulation term falls on the signal frequency band and influences the system performance. The average distortion is derived with only the information of auto-correlation functions. The results are useful for CDMA system design and performance analysis. With LD nonlinearity it is necessary to select an optimal modulation index that provides a maximum carrier-to-noise ratio (CIN). The effect of the LD nonlinearity on CDMA and frequency-division multiple access (FDMA) is also compared. The analytical method is applicable to other general nonlinearities in CDMA systems.<>