Principle and practice of optical code-division multiple access communication

摘要

This research examines the theoretical and experimental aspects of code-division multiple access (CDMA) communication in the incoherent, or direct detection, optical domain. We develop new modulation and detection principles that permit all-optical implementation of the bipolar, +1/$-$1, code and correlation detection available in the radio frequency (RF) CDMA systems. This is possible in spite of the non-negative, or unipolar, +1/0, nature of the incoherent optical system that only detects and processes the signal intensity. The unipolar optical system using the new principles is equivalent to the bipolar RF system in that the correlation properties of the codes are completely preserved.;The optical CDMA system can be realized either in time or frequency domain encoding with all-optical components. All-optical implementation is extremely important in practice because the symbol rates of the individual users are far less than the bandwidth of the optical fiber. Frequency domain or spectral amplitude encoding significantly increases the number of available codewords that can be assigned to the subscribers, and is the focus of this work.;The spectral amplitude encoding uses incoherent, broad bandwidth superfluorescent fiber source (SFS) that has the limiting signal-to-noise ratio characteristics associated with thermal light. Measurements of the photodetector illuminated by an erbium-doped SFS confirm that the noise distribution follows a negative binomial probability mass function as predicted for the photoelectron counting statistics of thermal light. The analysis based on this statistical model results in improved performance over the Gaussian assumption, which predicts a performance floor. The analysis also shows that optical CDMA system, even with the use of orthogonal codes, is interference limited.;We implement an experimental prototype system that demonstrates the first use of bipolar codes for spectral amplitude CDMA. The experimental results validate the modulation and detection principles and demonstrate the feasibility of optical CDMA systems based on those principles.