DESIGN AND SYSTEM IMPLEMENTATION OF PULSE POSITION MODULATION (PPM) BASED CODING SYSTEMS

The physical layer or modulation scheme plays a key role in a communication system, where performance features like Bit-Error Rate (BER), bandwidth efficiency and sensitivity are all dependent on the type of modulation scheme used. Currently, there are numerous modulation schemes for any given communication system, requiring the designer to decide which modulation scheme to apply. Many researchers propose different modulation schemes as the optimal for a given system with the aid of mathematical models and equations. However, there is minimal evidence on the practical implementation and testing of difference schemes to fully justify the selection. The scope of this research is to practically analyse, compare and validate the performance of different Pulse Position Modulation (PPM) schemes, where PPM is preferred for modulating and demodulating the signal in optical communications. This work presents, for the first time, practical analysis and comparison of different PPM techniques including Digital PPM (DPPM), Multipulse PPM (MPPM), Offset PPM (OPPM), Dicode (DiPPM) and Duobinary PPM (DuoPPM). The system implementation of these PPM techniques was carried under identical operational system conditions using Hardware-in-loop (HIL) approach, to validate the performance.

A visible light communication (VLC) system incorporating a high power commercial 20 W LED was used for the implementation of PPM schemes. An FPGA was used to encode the message into PPM formats and to transmit it over the LED. A comprehensive comparison was performed between several PPM schemes in terms of BER, power estimation and bandwidth utilisation. Additionally, a new modified form of MPPM, called modified MPPM (MPPM64), was proposed in this study, which improves the bandwidth utilisation of the communication system by 14.28%. Furthermore, a new error correction method for OPPM, called Priority Decoding, was proposed to improve the BER of OPPM. Experimentation revealed the improved performance of OPPM by achieving 10 times fewer errors with BER of less than 10-8.

A testbench was developed, which enables the user to apply any PPM from the given PPM schemes to a communication system. This testbench can be used to evaluate the performance of the communication system and to find the suitable PPM scheme which will deliver the best performance. Error correction techniques including Parity check and Cyclic Redundancy Check (CRC) were implemented to improve system performance. MPPM64 was implemented with Parity check and CRC achieving 0.4 m in transmission distance at identical transmission speeds when compared with the original scheme. Maximum Likelihood Detection (MLSD) was implemented with DiPPM and DuoPPM, showing 40% and 37.4% theoretical improved performance, respectively. In the practical implementation of MLSD, a 10 times achievement was recorded in BER at 1.5 m, outperforming original DiPPM and DuoPPM schemes.

Determining the most appropriate PPM scheme for a VLC or any given system is not straight forward as it depends on many system parameters. However, this work enables the user to identify the most appropriate scheme for any given VLC system. This work enables different system parameters such as BER, transmission distance, power estimation and bandwidth utilization to be taken into account when determining the most appropriate setup. A detailed comparison is shown to guide modulation scheme selection in optical applications based on different parameter limitations.