The Dicode PPM (DiPPM) - Essay Example

CONCLUSION AND FURTHER WORK The following section provides a conclusion on the work done in this thesis, along with which there certain plausible suggestions have also been included. Conclusion The Dicode PPM (DiPPM) was found to be a rather easy technique to be implemented more so because it involved the use of two slots for the transmission process that allowed the passage of one bit of PCM. In addition, the technique also provided greater sensitivity and the slot rate were found to be twice as high compared to that of the original PCM.

Such functional advantages in the DiPPM technique have resulted in its increased usage in the more recent optical communications systems. Even as DiPPM is being widely recognised as being highly effective in optical communications, it does have its share of problems in the form of three major types of errors which have affected the functioning of the technique. These errors include the wrong-slot errors, the erasure errors, and the false alarm errors. The major aim of this research was to develop and investigate the new DiPPM connection system with the Reed Solomon (RS) Code to reduce or prevent the occurrence of errors encountered in the DiPPM technique.

The RS decoder used will help in the correction of symbol errors found within its boundary without taking into account the type of damage caused to the symbol. For example, while decoding a damaged byte the RS Code simply replaces the incorrect byte with a correct byte without considering whether the original errors was caused by the corruption of a single bit or all the eight bits. The results from the simulation tests have revealed that when the RS decoder is used it increases the transmission efficiency of the DiPPM to a large extent by decreasing the number of photons.

In addition the system using the RS code has also been shown to provide an improvement of 5.12 dB as compared to the systems which do not employ the RS code. Such an improvement is observed when the code functions at the optimum rate of (3/4) and at a code length of (25). Further the results have also shown that at this optimum code rate, the DiPPM system achieves maximum transmission efficiency. However, when the system is operated below this optimum level, there is an increase in the number of redundant symbols which in turn negatively affects the performance of the system.

It is only above the optimum coding rate that the redundant symbols are found to decrease which implies that the amount of correcting symbols also decrease thereby reducing the transmission efficiency. From the results it is also evident that the DiPPM system while using the RS code required only about 14.3 x 103 photons per pulse when it is operated at a bandwidth equal to or above 0.9 times the PCM data rate. On a comparative basis when the DiPPM system uses the MLSD system it achieves a reduction in number of photons per pulse when it is operated at a bandwidth of less than 1 normalisation.

From this it is evident that the DiPPM system when using the RS code outperforms that of the MLSD system when it is operated at a high bandwidth. This is essentially due to the expansion of the operating bandwidth for the system based on the RS code rate. The DiPPM system using the RS decoder has been designed using the Matlab software. The DiPPM system has been able to achieve what has been theorized in the DiPPM scheme. With the further addition of the RS decoder, the DiPPM scheme has been able to overcome the errors that had initially caused damage to the transmitted message.

This could however be achieved only when the RS decoder works within its optimum parameters. The VHDL has been used for designing and synthesis of digital systems simulation, due to its reliable design process, minimum cost involved, lesser time requirement, low design errors and due to its synthesizable code. The source code has been described using VHDL for every part of the designed system. The optimum RS code parameters have been used incorporated in the system design and the results of the simulation have shown that all the parts of the system are working correctly and are in tandem with the system theory.

There is a test bench environment comprising of erasure only, error only, and erasure and error, which are used to examine the designed system. In addition the system has also shown that it has the ability to detect and correct the erasure and error symbols when it is not overcome by their limitations. A practical implementation of the designed system using the Altera Quartus II software, and Cyclone III Field Programmable Gate Array (FPGA) based DSP development board has been achieved. The implementation of the optical system transceiver has also been carried out and the output results are in agreement with the obtained simulation results.

Further Work The following is a proposal for future research to be carried out in this area: The Mathcad simulation of the system can be extended by computing the sensitivity for different bit error rate, and code rate. The system can be further analysed using other filter types like the tuneable filter. The results obtained could be compared with those of the coded Digital PPM system. The Altera DSP builder under the Matlab software can be used in the contruction of the RS code Simulink system (figure 9.1) along with an optical fibre package which could help in determining the bit error rate performance of the RS decoder in a noisy communication channel.

The DiPPM technique is to be used for this simulation evaluation of the coded communication system. Communication System Model with RS Encoder/ RS Decoder over AWGN Channel Further the Matlab program in chapter 5 can be upgraded in order to send and receive an audio video data, and to measure the optical spectrum of the system. The RS code can be upgraded by using a rate-adaptive transmission scheme with variable-rate forward error correction codes along with a fixed signal constellation and a fixed symbol rate.

This would help to quantify the variation of the bit rates with distance in a long-haul fiber system. The FEC scheme should use the serially concatenated RS codes. The practical implementation of the system can be improved by the addition of a timing extraction circuit. Further if high frequencies need to be reached a double clock frequency can be used for the purpose. This will allow the system to function at a positive end even while emulating working for the system on both edges of a single clock.