Performance enhancement of the power penalty in DWDM FSO communication using DPPM and OOK modulation

Abstract

This paper presents the design and performance enhancement of the power penalty (PP) in a dense wavelength division multiplexing based on free space optical communication (FSOC) link using digital pulse position modulation (DPPM) and on–off keying (OOK) modulation. Such a system has a high performance, low cost, robust and power efficient, reliable, excessive flexibility, and higher data rate for access networks. The system performance is evaluated for an 8-channel wavelength-division-multiplexing for hybrid fiber FSOC system at 2.5 Gbps on widely accepted modulation schemes under various atmospheric turbulence (AT) regimes conditions. The performance of system is introduced in terms of PP, bit-error rate (BER), transmission distance and the average received optical power. The numerical results shows that the improvement of the PP using DPPM modulation of 0.2–3.0 dB for weak turbulence (WT) regimes for BER of 10⁻⁶ and above 20, 25 dB for strong turbulence (ST) regimes are reported for BER of 10⁻⁶ and 10⁻⁹, as respectively (depending on the AT level). Further, we develop of improvement the PP caused by multiple-access interference about 6.686 dB which is predicted for target BER of 10⁻⁹ in WT and 1 dB at target BER of 10⁻⁶ in ST when the 8 user are active on the system of optical network units. Additionally, the optical power budget and margin losses of a system are calculated with different link length. The proposed approach of DPPM merges superiority with higher enhancement of PP about 0.8 dB for BER equal 10⁻⁹ at FSO link length l_fso= 2000 m compared to OOK at 1 dB for WT. An improvement of 2 dB is observed using the DPPM scheme over an OOK due to capability of detect pulses under background noise conditions with increased receiver sensitivity.

Appendix

1.1 This turbulence accentuated interchannel crosstalk effect is considered for the cases are written as Aladeloba et al. (2013)

1.1.1 The signal with turbulence but the interferer does not

To do so Eqs. (6–13) are used in an optically preamplified case [gain G, ASE PSD at the amplifier output], the ASE power spectral density (PSD) is $N_{ \circ \, } \, = 0.5 \, \left( {NFG - 1} \right){\text{ E}}$ where $G$ and $NF$ are the OA gain and noise figure, respectively, with no amplifier (G = 1). Then $P_{R, \, sig} \left( {h_{sig} } \right) = GP_{inst.sig} \left( {h_{sig} } \right)$ where $P_{inst.sig} \left( {h_{sig} } \right)$ is the instantaneous received signal power and thus $P_{{\text{int} ,{\text{sig}}}} \left( 1 \right)$ is also the turbulence-free average received power $\left( {TrblncFrp_{sig} } \right)$ of the input signal at the optical preamplifier. $P_{{R, \, \text{int} }} = GP_{int}$, is fixed by setting a signal-to-crosstalk ratio $C_{XT} = {{P_{R,sig} \left( 1 \right)} \mathord{\left/ {\vphantom {{P_{R,sig} \left( 1 \right)} {P_{ int} }}} \right. \kern-0pt} {P_{ int} }}$ where $P_{ int}$ is also the crosstalk optical signal power.

1.1.2 The interferer experiences turbulence, but not the signal

$P_{R, \, int} \left( {h_{int} } \right) = GP_{inst,int} \left( {h_{int} } \right)$, where $P_{inst, \, int} \left( {h_{int} } \right)$ is the instantaneous received interferer power and thus $P_{R, \, sig} = GP_{sig}$ is fixed by setting a signal-to-crosstalk ratio $C_{XT} = {{P_{sig} } \mathord{\left/ {\vphantom {{P_{sig} } {P_{R, \, int} \left( 1 \right)}}} \right. \kern-0pt} {P_{R, \, int} \left( 1 \right)}}$, where $P_{R, \, int} \left( 1 \right)$ is also the turbulence-free average received interferer power $\left( {TrblncFrp_{\text{int}} } \right)$ and $P_{sig}$ is the (non-turbulent) optical signal power.

1.2 Algorithms

See Table 7.

Table 7 MATLAB code to calculate the value of (Trb ln cFrP)

1.3 BER comparison of the impact of two crosstalk sources and a single crosstalk source of equivalent power

See Fig. 16.

Fig. 16

(Reproduced with permission from Mbah et al. 2014)

BER comparison of the impacts of two crosstalk sources and a single crosstalk source of equivalent power in a turbulent FSO DWDM DPPM system with ${\text{coding level M}} = 2,l_{fso,int} = 2 0 0 0\, {\text{m}}$ and signal-to-crosstalk ratio ${\text{L}}_{\text{demux,XT}}$ (eprints.nottingham.ac.uk).