Frequency domain phase encoding and decoding OCDMA system
The OCDMA communication system based on frequency domain phase encoding and decoding is shown in Figure 1 [3]. The frequency domain phase encoding and decoding OCDMA communication system, whether in the experimental or theoretical aspects, compared with other OCDMA communication systems, the research is not deep enough. Because the coherent system itself is difficult to implement, it is now basically in the laboratory research stage, and there are some breakthroughs in the United States, Japan and other countries [1 ~ 4].
The super-fluorescent fiber light source (SFS) passes through a dichroic mirror (DM), EDFA, filter and isolator, and then passes through a polarizing beam splitter (PBS) into two identical beams, which enter the encoders E (x) and E ' (x) (belongs to complementary encoder). In order to enable the decoder to accurately recover the desired user information, the system uses a pseudo-random phase code (PN) to encode the transmitted data to form the encoded optical pulse sequence, which is then coupled by the beam splitter to the mirror (RM) In the optical transmission network, the optical pulses are reversely processed at the receiving end to realize optical pulse decoding. After passing through EDFA, filter, equalization detector and threshold deciding device, finally get the expected user information.
In order to improve the system performance, an intensity discriminator composed of a dispersion-shifted fiber and a long-wavelength band-pass filter can be set; because in coherent optical communication, the duration of the optical pulse is required to be very short, the dispersion and nonlinear effects will cause a relatively high performance of the system For large impact, techniques such as dispersion compensation and optical power control can be used in the system.
Figure 1 Block diagram of frequency domain phase encoding and decoding OCDMA system
Codec structure and principle The codec is a key device to realize B-OCDMA system technology and coherent detection. The encoder of the system shown in Figure 1 consists of a dispersion grating (DG), a confocal lens (FL) and a liquid crystal modulator (LCM) ) Structure, structure shown in Figure 2. After the light pulse passes through the dispersion grating and the lens 1, the signal light pulse spectrum is distributed on the LCM (LCM is a complex conjugate template, also known as a phase mask), and the phase code is flexibly controlled by the voltage of the upper electrode of the LCM. The optical pulse maps a pseudo-random phase code ({0, π} sequence), that is, the Fourier transform from the time domain to the frequency domain is completed, and the first-order spectrum of the signal is extracted by LCM, and the frequency domain coding is performed. The optical pulses encoded in the frequency domain are recombined through the lens 2 and the dispersion grating and then coupled to the optical network to complete the encoding of the optical pulses. The principle and structure of the decoder and the encoder are the same.
Figure 2 Schematic diagram of encoder based on grating and LCM
Research on Performance of Frequency-domain Phase Optical Code Division Multiple Access System
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