High sensitivity optical digital pulse position modulation systems.
Digital pulse position modulation (PPM) is a transmission format that can be used
to exchange excess channel bandwidth for improved receiver sensitivity. This thesis
is concerned with the performance of digital PPM for use in optical fibre systems
such that increased single-span transmission distances can be achieved. It includes
a literature survey and a thorough theoretical investigation into uncoded directly
detected and coherently detected digital PPM, along with an evaluation of forward
error correction coded PPM.
Original contributions are described for digital PPM systems employing PIN-BJT
transimpedance and PIN -FET high impedance pre-amplifiers. In the PIN -BJT case,
a new analysis is presented that accounts for the band-limiting effect of the preamplifier
and the non-white noise power spectral density, resulting from equalisation.
For the PIN-FET, a causal noise-whitening filter is considered and the factors that
limit the maximum bit-rate determined. By using an upper Chernoff Bound to the
average binary error probability, results are calculated for APD-BJT and APD-FET
The practical implementation of the optimal PPM filter is considered and two novel
techniques are described for automating the realisation process.
Coherent PPM is investigated and new work is presented for Reed-Solomon coded
homodyne PPM. The influence of the Reed-Solomon code rate on receiver sensitivity
is studied and it is shown that a 3/4 code rate leads to optimum sensitivity.
Digital PPM employing soliton pulses is examined and it is shown that, under ideal
conditions, a higher bit-rate than PCM can be achieved. The influence of timing
jitter, associated with soliton pulse energy fluctuations and the Gordon-Haus effect,
on the performance of digital PPM is investigated.
The simplification of the PPM pre-detection filter is studied and a range of suboptimum
filters investigated. The calculations demonstrate that the optimal predetection
filter can be replaced by a simple 3rd order filter without incurring a severe
The sensitivity results of the various digital PPM systems investigated in this thesis
are compared to those of equivalent PCM systems. It is illustrated that digital PPM
offers an improvement of between 5-10 dB depending upon which coding format/
detection technique is employed. This represents an increase in transmission distance
of 25-50 km and demonstrates that digital PPM is a promising modulation format
and has potential for future high performance telecommunication routes.