The best monomode optical fiber links have bandwidths orders of magnitude greater than that of the information currently transmitted over them. This excess bandwidth can be exploited using digital PPM to improve receiver sensitivity. This paper analyzes the receiver sensitivity of an optical PPM system over a slightly dispersive channel, i.e., where both "wrong slot" and "false alarm" errors are important. It is shown that receiver sensitivity of better than 100 photons per binary bit-time is theoretically possible using direct detection and uncoded PPM. Ideal heterodyne detection should reduce this to below 5 photons per binary bit-time. Timing extraction and a digital modulation method are discussed.

Pulse-Position Modulation for Transmission Over Optical Fibers with Direct or Heterodyne Detection

The abundance in bandwidth available in the best monomode fibres may be exchanged for improved receiver sensitivity by employing digital PPM. The paper presents a performance and optimisation analysis for a digital PPM coding scheme operating over a fibre channel employing a PIN-BJT receiver and assuming a Gaussian received pulse shape. The authors present original results for a 50 Mbit/s, 1.3 mu m wavelength digital PPM system and conclude that, provided the fibre bandwidth is several times that of the data rate, digital PPM can outperform commercially available PIN-BJT binary PCM systems.

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Optical fibre digital pulse-position-modulation assuming a Gaussian received pulse shape

This work is concerned with the development and investigation of a Dicode PPM (DiPPM) system


A DiPPM coder was developed to code any input PCM signal into DiPPM format A further investigation took place on the DiPPM spectrum and associated output Software simulation and mathematical analysis of this PPM code was considered and comparison with previous theoretical results presented Results show that DiPPM is an advantageous PPM code for optic communication; DiPPM spectrum is not concentrated near to DC and
it is possible to extract the DiPPM framerate
component directly from the pulse stream


A timing extraction circuit that recovers the clock from a DiPPM sequence and synchronises the slots within the frames, was constructed successfully This enabled transmission through fibre optics and Free Space Optics (FSO) The technique used for the timing extraction circuit of the DiPPM scheme gives an advantage over many of the PPM formats


An optical transmitter/receiver system was developed and the DiPPM scheme was investigated through optical channels Results show that the DiPPM sequence transferred through the optic system was not changed and the clock had been recovered A DiPPM decoder was constructed and the received DiPPM signal returned to its original PCM form without errors


Both DiPPM coder and decoder were developed in VHDL and measurements were taken The timing extraction was programmed in VHDLAMS with the use of digital, analogue and mathematical equations


DiPPM MLSD was also constructed in VHDL Simulation results proved the theoretical
expectations

E. V. Borisov

Papers

Noise immunity of reception of coded messages in optical communication lines