Spatial Heterodyning Optical Code Division Multiple Access Technique for Near-Field Free-Space Optical Communication Systems

We present a novel optical code division multiple access (OCDMA) scheme based on spatial optical heterodyning for free-space optical communication systems. In this technique, in particular, the decoding process is established by means of a spatial optical heterodyne receiver. The spatial heterodyning OCDMA introduced can be considered to be in the class of spread-space techniques, which implies that there is no structural limitation on the shape of time domain signals, e.g., digital or analog modulation. For the sake of simplicity and practicality we considered on-off keying modulation. However, there is no limitation on using more advanced modulation schemes such as pulse position modulation. In this scheme a set of independent orthogonal spatial channels at both transmitting and receiving apertures is needed. Thus this class of OCDMA is applicable only to near-field free-space systems. The structures of the encoder and decoder are presented, and the wave propagation is completely analyzed by means of the Fresnel approximation for the free-space kernel. Only the effect of multiuser interference is considered. The effects of background noise and atmospheric turbulence on the performance of the system are avoided in order to highlight the main capabilities and features of spatial optical heterodyning OCDMA. A thorough statistical analysis is carried out, and the characteristic function of the sampled output is computed. The bit error rate is obtained by means of Gaussian and saddle-point approximations. Results show that the signal-to-interference ratio is approximately a function of processing gain and the number of users, as expected. The optical spatial heterodyning OCDMA technique can prove to be of importance in optical switching and free-space optical communication systems.