Investigation of the pulse time modulation techniques for transmission of wideband signals
The choice of the modulation format is the principle factor in realising a highperformance bandwidth efficient communication system at an acceptable cost and complexity. Pulse time modulation technique represents an attractive alternative to purely digital or purely analogue modulation schemes which has received considerable attention over the years. But very little work has been reported on pulse slope modulation. In this work a pulse slope modulation introduced and full wavefonn characteristics is given. A simple novel receiver has been proposed, which is based on converting the PSM waveform into a PAM waveform by sampling the received PSM signal at the rise time interval. This design eliminates the use of a differentiator and a voltage slicer adopted in the classical demodulation technique, and it offers simplicity and improved noise performance. A new signal-t-noise formula has been presented for the first time thus enabling users to predict the system noise performance. Experimental results have shown excellent agreement to within + 1 dB with theoretical predictions using the new formula. Results obtained show the potential of PSM in tenns of simplicity and better noise perfonnance compared to its counter part pulse amplitude modulation. Although PTM has many advantages over analogue and digital schemes when employed as a single channel system, it becomes a challenge when multiplexing is involved. Isochronous PTM schemes are suitable while the anisochronous schemes are not. One solution to overcome this problem is to adopt a hybrid or compound modulation technique, where both isochronous and anisochronous schemes can be combined. Compound frequency and width modulation (CPFWM) is one such a scheme which offers bandwidth efficiency, simplicity and low cost over the more commonly used multiplexed techniques. Detailed investigation of CPFWM has been given and in an expression for its spectrum has been developed. The results obtained have been clarified practically and by means of computer simulation to within + 1 dB. In CPFWM, cross talk in the PFM and pWM channels are due to width modulation and frequency modulation, respectively. The main cause of cross talk is the carryover energy from one edge to the next edge. This effect is studied and he results obtained for the cross talk are compared with the predicted data showing an agreement to within + 1 dB. Results were also compared with time division multiplexed pulse position modulation, showing comparable performance and under certain conditions CPFWM offering better performance. Noise performance of PFM and PWM channels has been theoretically and experimentally investigated. It has been shown that PFM is superior to PWM over a wide range of modulation indices and pulse characteristics. When both channels are identical in bandwidth and modulation conditions, PFM offers a 7 dB improvement compared to PWM channel. The noise perfonnance of CPFWM system is also compared with digital and analogue modulation schemes. For 25 dB CNR CPFWM offers a 40 dB SNR which is 20 dB higher than amplitude modulation, 1 7 dB lower than PCM system. At CNR > 35 dB its performance approaches that of PCM system. Thus showing the potential of the scheme.