Pulse time modulation for subcarrier multiplexed systems.

Subcarrier multiplexed (SCM) systems are an attractive alternative to the evolving digital technology for transmitting broadband services, at an affordable price. However, the majority of existing systems are based on analogue signal transmission and therefore, the strict noise and nonlinear requirements undermine the system performance. The work carried out in this thesis presents the feasibility of pulse time modulation (PTM), as a second stage modulator, in SCM systems.PTM techniques offer simplicity and low cost, and with the additional bandwidth available on optical fibres can trade bandwidth to significantly higher signal-to-noise ratio (SNR) levels, compared to analogue systems. Three different PTM techniques, square wave frequency modulation (SWFM), pulse frequency modulation (PFM) and pulse position modulation (PPM) has been investigated. A prototype system capable of transmitting a video channel, two audio channels and a data channel is implemented for each technique in order to evaluate the performance potential of PTM as a second stage modulator in SCM systems.The SNR expressions for all three schemes are derived from the first principles and the obtained results were verified experimentally. The optimum SNR performance is delivered by a raised cosine shaped pulse and the PPM technique delivers 5 dB SNR improvement over PFM. For SWFM systems a 3 dB SNR advantage is gained over single-edge detection technique and PFM systems by employing double-edge detection at the receiver. PPM spectrum contains a clock component which could be employed at the receiver for signal recovery. Demodulation technique, based on clock recovery using a phase locked loop (PLL) is proposed and implemented. This technique is cost effective and less complex compared to the existing demodulation schemes.The PFM implementation shows a 6 dB improvement in the receiver sensitivity compared to conventional SCM systems, while the PPM system offers an extra 2.5 dB improvement. The improved receiver sensitivity of the SCM-PTM technique, results in an increased optical power budget, where the transmission distance, number of subscribers and the number of channels in a network can be optimized. The nonlinear performance of the overall system is also shown to be within the specified performance levels.