Published in IET Communications Received on 24th October 2011 Revised on 11th January 2012 doi: 10.1049/iet-com.2011.0780 ISSN 1751-8628 Experimental investigation of polarisation modulated free space optical communication with direct detection in a turbulence channel Z. Ghassemlooy X. Tang S. Rajbhandari Optical Communications Research Group, NCRLab, School of Computing, Engineering and Information Sciences, Northumbria University, Newcastle Upon Tyne, UK E-mail: sujan.rajbhandari@northumbria.ac.uk Abstract: Binary polarisation shift keying (BPOLSK) has been proposed to mitigate the atmospheric turbulence-induced fading in free space optical (FSO) communication systems. In this study, the Q-factors obtained for the BPOLSK-FSO system are verified in conjunction with theoretical results to confirm the validity of the proposed scheme. The analytical bit error rate (BER) for the BPOLSK and non-return-to-zero on– off keying (NRZ-OOK) schemes are presented. The authors show that the BPOLSK scheme with direct detection offers improved BER performances compared to NRZ-OOK in the presence of weak turbulence, which is inferred from the experimental Q-factor and theoretical BER. For a turbulence variance s 2 l of 0.003 and the transmitted optical power of 216.8 dBm, values for Q-factor are ≏11 and ≏8.5 for BPOLSK and NRZ-OOK schemes, respectively. The authors show that the predicted signal-to-noise ratio (SNR) for BPOLSK and NRZ-OOK schemes are ≏13.5 and ≏15 dB, respectively, for a BER of 10 26 and s 2 l of 0.01. When s 2 l increases to 0.1, ≏8 dB lower values of SNR is required for BPOLSK compared with NRZ-OOK. 1 Introduction The free space optical (FSO) technology is being proposed as an attractive complementary solution to the radio frequency (RF) schemes for the last mile access network application to overcome the bandwidth bottleneck and offer high- quality services such as a high-definition TV, online videos etc. [1, 2]. The full duplex transmission link can be set up between two distant points from several meters (local area network) to a few kilometers (wide area networks). Compared to the optical fibre and RF communication systems, FSO links are relatively low cost, highly secure with dense spatial reuse and low power usage per transmitted bit capabilities [1]. However, the performance of FSO links in outdoor environment can be degraded by the atmospheric conditions such as fog, rain, dust, snow, smoke and other aerosol particulate matters, which attenuate the information-bearing laser beam to a certain extent. The most deleterious factor in a terrestrial FSO link is the dense maritime fog (advection fog). In [3], it is shown that the advection fog can result in severe signal fading which is up to ≏480 dB/km. The impacts of other atmospheric phenomena, such as snow, clouds and rain on the terrestrial FSO links, have been further investigated. The results showed that the optical attenuations are not non-negligible even for these effects. For instance, the attenuation caused by rain at a rate of 150 mm/h is 25 dB/km [4], by the falling dry snow and cloud are 68 and 50 dB/km, respectively [3]. The achievable FSO link is limited to 500 m under such channel conditions [5]. However, the attenuation owing to fog, rain can be compensated by increasing the transmitting optical power provided it is within the eye safety limit [3]. Additionally, the FSO link performance is subjected to the degree of atmospheric turbulence [6, 7]. Atmospheric turbulence refers to in-homogeneities in the temperature and pressure of the atmosphere that leads to variations of the refractive index along the propagation path especially over 1 km or longer links [8]. In a turbulent channel, the travelling optical beam will also suffer from both the phase and irradiance fluctuations as well as the deterioration of image formed at the focal point, thus to the increased BER [1, 8]. The optical losses induced by the turbulence increase with the distance between the transmitter and receiver. Increasing the signal gain does not necessarily improve the link susceptibility to turbulence nor does operating at different optical wavelength transmission windows [9]. In intensity modulation/direct detection (IM/DD) systems, turbulence effects could result in deep irradiance fades that could last up to ≏1 2 100 ms [10]. For a link operating at say 1 Gbps, this could results in a loss of up to 10 5 consecutive bits (a burst error). Therefore to exploit the full potentials of FSO links under all weather conditions, a number of schemes have been proposed to combat the turbulence-induced effects. Multiple transmitters and receivers are used to combat the turbulence-induced fading and to compensate for pulse attenuation and broadening caused by scattering [11]. IET Commun., pp. 1–6 1 doi: 10.1049/iet-com.2011.0780 & The Institution of Engineering and Technology 2012 www.ietdl.org