Theoretical Analyses and Practical Implementation of Duobinary Pulse Position Modulation Using Mathcad, VHDL, FPGA and Purpose-built Transceiver

Duobinary pulse-position modulation (PPM), a novel channel coding scheme, has been proposed in this thesis as an alternative method of improving bandwidth utilisation efficiency and sensitivity over existing coding schemes such as digital PPM, dicode PPM, multiple PPM and offset PPM while operating over slightly or highly dispersive graded-index (GI) plastic optical fibre (POF) channels of limited bandwidth. Theoretical investigation based on simulations of mathematical models with maximum likelihood sequence detection (MLSD) at 1 Gbps on-off keying (OOK) data shows that duobinary PPM significantly outperforms optimised digital PPM at low fibre bandwidths by 8.7 dB while only operating at twice the original pulse code modulation (PCM) data rate. It has also been shown at high fibre bandwidth duobinary PPM gives a sensitivity of -42.2 dBm which is favourably comparable to digital PPM seven-level coding sensitivity of -44.1 dBm. Results presented in the thesis also demonstrate that at very low normalised fibre bandwidths (below 1 and down to 0.43) duobinary PPM outperforms dicode PPM by 1.2 dB requiring 27 x 103 photons per pulse compared to 40.3 x 103 required by Dicode PPM. Due to the use of MLSD at low bandwidths, wrong-slot errors are completely eliminated, and the effect of erasure and false-alarm errors are significantly reduced thus resulting in significantly improved sensitivity. Successful VHSIC hardware description language (VHDL) and field programmable gate array (FPGA) implementation of duobinary PPM coder, decoder and MLSD as a single system has been presented in the thesis. An FPGA embedded bit error rate (BER) test device has also been implemented for sensitivity measurements purposes and all the designs have been tested successfully with back-to-back testing. A purpose-built VCSEL 850 nm wavelength based transceiver system has been designed and successful functional tests have been carried out. Maximum operational data rate of the transceiver is currently 622 Mbps to match the maximum operating frequency of the FPGA, however, it has the capability to operate up to 3.2 Gbps. Further work on receiver characterisation and slot and frame synchronisation of duobinary PPM is required. All the results and analyses indicate that duobinary PPM is an ideal alternative to be considered for highly dispersive optical channels, and performance evaluation for higher bandwidths also favourably compares to existing coding schemes with only twice the expansion of original PCM data rate

Simulation and timing analyses of VHDL models of coder and decoder of duobinary pulse position modulation

Duobinary Pulse Position Modulation (DuoPPM) has been proposed as an alternative novel coding scheme which manifests many advantages over currently existing PPM formats according to the preliminary results obtained from theory and simulations run by one of the authors, Sibley. It combines the bandwidth reduction of the three-level duobinary code with the dispersion characteristics of conventional PPM. This coding is intended to be implemented using VHDL (VHSIC hardware description language) and an FPGA (Field-programmable gate array) over Plastic Optical Fibre (POF). Nevertheless if the preliminary promises of this new coding scheme are fulfilled upon successful implementation, it may be adapted to high-speed optical communications links. For the first time, the original VHDL designs, simulation results and timing analyses of DuoPPM coder and decoder are presented in this paper. These design developments are the fundamental circuits required for further investigation of this proposed coding scheme

Duobinary pulse position modulation – a novel coding scheme for the dispersive optical channel

Pulse position modulation (PPM) coding schemes have been proposed and investigated widely as a technique of utilising the very high bandwidth available in optical fibres, achieving a significant improvement in sensitivity of 5-11 dB compared with an equivalent non-return-to-zero on-off keying scheme. However, this improvement has some tradeoffs. When using eight-level digital PPM, the final data rate can be as high as 32 times that of the original data, thus implementation becomes extremely difficult to the point where commercial viability becomes doubtful. In this study, the authors describe a novel coding scheme that combines the duobinary scheme with PPM to form DuoPPM. It is shown that DuoPPM gives a sensitivity greater than digital PPM while operating at two times the original data rate. Original results presented in this study predict that a high fibre bandwidth DuoPPM system can give a sensitivity of -42.2 dBm when operating with 1 Gbit/s on-off keying (OOK) data and a fibre bandwidth of 100 GHz. In addition, it is shown that DuoPPM outperforms optimised digital PPM at low fibre bandwidths (1 GHz) by 8.7 dB