ECOC 1975 - ECOC 2013

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Conclusion: 170 To summarize, FO offers very attractive solutions to problems in many areas of -military information transfer. A good start has been made toward translating this important new communications technology into military applications.
171 RANDOM CODING FOR DIGITAL OPTICAL SYSTEMS C. Game & A. Jessop Introduction The choice of a transmission code is a compromise between producing a signal which is easily regenerable against one which maximizes repeater spacing. The code should also provide an independent means of measuring the binary bit error rate. Over the years wire line systems have evolved that employ line codes which produce a signal having adequate timing information for wide-band (low Q) timing extraction circuits and which can be transmitted by a system with low-frequency limitations. Optical fibre systems, while retaining the same need for simple and cheap timing extraction circuits, do not have the same low-frequency limitations. Wire line systems avoid low frequencies to obviate the need to equalize cable down to low frequencies, to facilitate the protection of equipment from surges and to facilitate power feeding and supervision over the same cable as the signal. These restrictions do not apply to optical fibre systems, the low frequency limitation being one of a.c. coupling within the repeater. The implications of this difference and those listed below should be investigated before choosing a line code for optical systems Factors Affecting Choice of Code for Optical Systems Choosing a code involves consideration of the following: (a) means of limiting the laser mean pulse density (b) spectrum manipulation (e.g. elimination of d.c. content) (c) repeater timing information (d) in-traffic error monitoring at terminals (e) location of faulty repeaters (f) data transparency (g) code re-frame time if applicable (h) efficient use of information capacity (i) general simplicity of repeater (j) non-linearity of source and detector (k) dispersion in the fibre (pulse broadening) This paper shows that all the requirements above can be met by the use of scrambled binary as line code except for point (d) which can be met by a little added redundancy. Whether or not point (e) needs to be met depends on whether in-traffic location of a failing repeater is a requirement. For an optical fibre system with negligible pulse broadening the maximum repeater spacing is obtained with a binary system whether the source is peak or mean power limited. This is because the increase in signal-to-noise ratio due to reduced bandwidth by increasing the code radix is more than offset by transmitting less power per level. If pulse broadening is significant t.hen a higher radix code could maximise repeater spacings. However. progressive improvements in optical fibre attenuation have usually been accompanied by reduction in dispersion; with currently available multimode fibres transmission is mainly attenuation limited for speeds up to 140 Mbit/s. C. Game & A. Jessop are with STL Ltd., Harlow, Essex, UK.
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