30-05-2013, 12:12 PM
Optical code-division multiple-access (O-CDMA)
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Introduction
Optical code-division multiple-access (O-CDMA) is an attractive technology since it potentially provides flexible, robust, and asynchronous communications in access networks. CDMA schemes are categorized as implementing the code through the optical field and relying on coherent detection, or through time slots and wavelengths with reliance on incoherent detection. Coherent schemes are susceptible to coherent beat noise that occurs when the correctly decoded signal temporally overlaps with the multiple access interference (MAI) from other users . So recent implementations of coherent O-CDMA resort to timing coordination between users, ranging from complete bit-level synchronization to time slot assignment within the bit . Another approach uses very long spreading codes to minimize the amplitude of the MAI while keeping users asynchronous; however, only 10 of 512 possible codes deliver adequate bit-error-rate performance (BER < 10−9) . Incoherent schemes are less susceptible to coherent interference , but due to time slot allocation, are difficult to implement and less spectrally efficient with increasing data rates and time slots. A single mode fiber is used for high-bit-rate transmission in low-loss-transmission windows but dispersion is an important impairment that degrades overall system performance of an optical communication system .
What is Optical CDMA?
Code Division Multiple Access, (CDMA), has been recognized to provide efficiency, security, and multi-access benefits in wireless communications. This has triggered interest in providing similar advantages for optical communication systems. Our encryption system takes the basic ideas of CDMA and implements it on top of the traditional Wave Division Multiplexing (WDM) system used in optics.
• LIMITATION
With the progress of phase coding technologies, beat noise becomes the main limitation of an OCDMA system. Accurate saddle-point approximation method is introduced to evaluate the moment generation functions of beat noise and multi-access interference simultaneously.
• ADVANTAGE
Optical signal processing is advantageous since it can potentially be much faster than electrical signal processing and the need for photon-electron-photon conversion would be eliminated. OCDMA is one such technology to realise multiple access by coding in the optical domain. It supports multiple simultaneous transmissions in the same time slot and the same frequency.
• SECURITY
In traditional WDM systems, if each wavelength represents the data of an individual user, an attacker need only look at this one wavelength to understand the information. In our new secure coding scheme, we will make a pseudo-random permutation of each of the output time frames. This means both the order of the bits in a stream and the wavelength in which it is transmitted may change.
Optical Multiplexer (Nx1 MUX)
This model represents an optical WDM multiplexer (see also the General Multiport Optical Device described below). It accepts multiple optical signals at its input ports and produces a WDM optical signal at its output port which includes all the input WDM optical signals. There are two signal representations that may be used at the output, depending on whether four wave mixing is desired for use in the fiber model. The multiple-band mode will put each optical signal band in its own signal representation, thereby decreasing the overall memory load of the simulation by not including the unused frequency bands between the bands. This approximation can only be made when the bands do not overlap significantly. If there is significant overlap, or it is desired to include the effects of four wave mixing in the fiber simulation, the single-band mode must be used. To use the single-band mode, the samples per bit (pointsPerBit) in the signal generator for all the bands must be set high enough to include all the frequency components of each of the bands in the frequency domain representation of the signal. A good rule of thumb is that the simulation bandwidth be chosen to be about 3 times the total signal bandwidth (number of bands times band spacing).
Optical DeMultiplexer (1xN DEMUX)
This model represents an optical WDM demultiplexer (see also the General Multiport Optical Device described below). It accepts a WDM optical signal at its input port and produces N single channel optical signals at its output ports, one channel per port. This is accomplished by applying the specified filter to the input signal for each of the output ports. For details on the optical filter types supported, see the documentation on the Optical Filter model.
Electroabsorption Modulator
This model represents an electroabsorption modulator. It allows the user to specify the extinction ratio of the output optical signal explicitly, and it scales the input modulating voltage signal as required to obtain the specified extinction ratio at the output. Nonlinear modulation response is also supported with either file-based data interpolation or a 7th
order polynomial in terms of the scaled input modulating voltage signal. The model also allows the chirp factor to be specified as a data file or as a 7th order polynomial in terms of the input modulating voltage signal and supports specification of dc chirp with another data file or 7th order polynomial in terms of the input modulating voltage signal. In
addition, the user may specify whether or not the output optical signal should be modulated as the inverse of the input electrical signal, and whether the alpha parameter should be considered positive or negative (to accommodate differing conventions in the literature).The model has two input ports and one output port. The first input port accepts an optical signal that is modulated to produce the output optical signal, and the second input port accepts an electrical signal that is used to modulate the input optical signal to produce the output optical signal. The input optical signal may be any type of optical signal, but it is recommended to use the CW laser or mode-locked laser sources to provide the input optical signal. The user must ensure that the total number of samples contained within the input electrical and optical signals are the same, as are the
sampling rates. An exception to this is when using the CW laser model to produce the PowerValue signal type.
Results and discussion
Using the simulation setup, the values of input signals, wavelength spectrum, eye diagrams and received signals are measured. The received signal is measured at the receiver end. The measurement components used are multiplot for the wavelength spectrum, eye diagram analyzer for an eye diagram and signal plotter for the input signal and the received signal. Eye diagram is measured at receiver end for different lengths of fiber. The measured wavelength spectrum is shown in Fig. 4 which consists of 32 wavelengths. The bold lines show the 16 spectral components which implement the code and the dashed lines show the remaining 16 spectral components which implement the complementary code, according to the modified PN code. The wavelengths corresponding to one in the modified implement the code and wavelengths corresponding to zero in the same code are transmitted to implement the complementary code.
Fig. 5(a) shows the data and (b) shows the data bar. The code is used to modulate the data and code-bar is used to modulate the data-bar in order to implement the code switching scheme in which a code sent for high bit of data and complementary code is sent for low bit of data (Figs. 6–PN code are transmitted to Figs. 10 and 12 show the eye diagrams for the receiver side of the OCDMA system with 75 Km and 95 Km fiber, there is considerable improvement in the signal and even the BER is in the order of 10−9.
Conclusion
The OCDMA system designed for this architecture, results in
improved performance compared to previous researches in terms
of quality factor and bit error rate. Moreover these results are more
realistic as practical impairments have also been considered.