Performance evaluation of digital pulse position modulation for wavelength division multiplexing FSO systems impaired by interchannel crosstalk

Abstract

Wavelength division multiplexing (WDM) has been proposed for fibre, intersatellite, free space and indoor optical communication systems. Digital pulse position modulation (DPPM) is a more power efficient modulation format than on-off keying (OOK) and a strong contender for the modulation of free space systems. While DPPM obtains this advantage in exchange for a bandwidth expansion WDM systems using it are still potentially attractive, particularly for moderate coding levels. However WDM systems are susceptible to interchannel crosstalk and modelling this in a WDM DPPM system is necessary. Models of varying complexity, based on simplifying assumptions, are presented and evaluated for the case of a single crosstalk wavelength. For a single crosstalk, results can be straightforwardly obtained by artificially imposing the computationally convenient constraint that frames (and thus slots also) align (FA). Multiple crosstalk effects are additionally investigated, for the most practically relevant 2 cases of modest coding level, and using both simulation and analytical methods. In general, DPPM maintains its sensitivity advantage over OOK even in the presence of crosstalk while predicting lower power penalty at low coding level in WDM systems.

Performance evaluation of turbulence‐accentuated interchannel crosstalk for hybrid fibre and free‐space optical wavelength‐division‐multiplexing systems using digital pulse‐position modulation

“…The required optical power for the DPPM system is seen to be lower than that of the OOK system for all turbulent regimes considered. This result is consistent with the findings in the non-turbulent model which show that DPPM requires less power compared to OOK in a WDM free space system [25]. In Fig.…”

“…Demux adjacent channel rejection values typically ≤ -15 dB and ≥ -45 dB have been demonstrated experimentally in [36][37][38] and are considered. As seen in [25], the impact of a single high power crosstalk is worse than that of many crosstalk of equivalent power. Thus in this work, a single crosstalk source from a dominant interferer is studied.…”

“…And, although there is only one pulse in the interfering signal frame, the desired signal may be impaired by zero ( ) crosstalk pulses depending on the position of the pulse in the interfering signal frame and the pattern of (mis)alignment with the desired signal frame. This condition which neglects partial slot misalignment was referred to as only slots aligned (OSA) in [25], and leads to a quick approach which predicts sensible results for both single and multiple crosstalk sources. …”

“…Following [11,18,25], the general equations for the upstream transmission are derived below in (16) - (20), the equations for the downstream are recovered by replacing the attenuation due to atmospheric turbulence for the desired signal/interferer (…”

Effects of carbon dioxide on different PSK modulation formats of Optical Wireless Communications using USRP

“…Other widely used modulation formats are Phase Shift Keying (PSK), Pulse Width Modulation (PWM), and Pulse Position Modulation (PPM) [11], [12], and [13]. Here we focus on comparison of different PSK modulation formats generated by the Universal Software Radio Peripheral (USRP) unit [15].…”