Average BER performance of FSO SIM-QAM systems in the presence of atmospheric turbulence and pointing errors

Abstract

This paper presents the exact average bit error rate (BER) analysis of the free-space optical system employing subcarrier intensity modulation (SIM) with Gray-coded quadrature amplitude modulation (QAM). The intensity fluctuations of the received optical signal are caused by the path loss, atmospheric turbulence and pointing errors. The exact closed-form analytical expressions for the average BER are derived assuming the SIM-QAM with arbitrary constellation size in the presence of the Gamma–Gamma scintillation. The simple approximate average BER expressions are also provided, considering only the dominant term in the finite summations of obtained expressions. Derived expressions are reduced to the special case when optical signal transmission is affected only by the atmospheric turbulence. Numerical results are presented in order to illustrate usefulness of the derived expressions and also to give insights into the effects of different modulation, channel and receiver parameters on the average BER performance. The results show that the misalignment between the transmitter laser and receiver detector has the strong effect on the average BER value, especially in the range of the high values of the average electrical signal-to-noise ratio.

Keywords:

• atmospheric turbulence
• bit error rate (BER)
• free-space optical systems
• quadrature amplitude modulation (QAM)
• pointing errors
• subcarrier intensity modulation (SIM)

Notes

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by European Science Foundation under COST IC1101 OPTICWISE Action “Optical Wireless Communications – An Emerging Technology”, Ministry of Education, Science and Technology Development of Republic of Serbia under [grant number TR-32028] and Ministry of Foreign Affairs of Norway within the NORBAS project.