Performance analysis of a coherent communication system employing quadrature PSK and FM feedback using multirate sampling simulation.

摘要

This dissertation introduces a multirate sampling scheme for simulating a coherent communication system. This system employs quadrature phase shift keying (QPSK) and frequency-modulated feedback (FMFB) to achieve improvements in the signal-to-noise ratio (SNR).; This work has been done with computer simulation utilizing DSP-based communications software known as Signal Processing WorkSystem. Multirate approach facilitated the simulations to run at a speed twelve times faster compared to a uniformly sampled system. This allowed extensive experimentation on the coherent communication system deploying digitally-modulated subcarriers. A pattern for optimum response was observed for the specifications of the components of the FMFB section. This pattern differed in several ways compared to what one finds in the literature.; Our simulation results show improved performance by increasing the number of subcarriers up to ten. With ten subcarriers, it was possible to maintain an error probability of 10{dollar}sp{lcub}-9{rcub}{dollar} with only 8.7 photons per bit. Further work suggested that additional improvements would be easier to obtain by a higher order modulation scheme, like 8-PSK, than by adding more subcarriers. Even with ten subcarriers, it was difficult to obtain optimum performance from each of them. An important constraint was to stay within the frequency response (40 GHz) of efficient photodetectors.; Finally, we have also estimated the extent of performance improvement possible with PSK-based trellis-coded modulation (TCM). The 4-state 8-PSK and 8-state 16-PSK TCM should permit 6.97 and 5.8 photons/bit respectively for an error probability better than 10{dollar}sp{lcub}-9{rcub}{dollar}. Such performance means that low-power lasers could be used for wavelength-division multiplexing, avoiding non-linear effects in the optical fiber. At the same time, frequency modulation should help counter the effects of relative intensity noise.