BER Performance of DPSK Subcarrier Modulated Free Space Optics in Fully Developed Speckle W.O. Popoola, Z. Ghassemlooy, Fellow IET, Senior Member IEEE and E. Leitgeb* (emails: wasiu.popoola@unn.ac.uk ; fary.ghassemlooy@unn.ac.uk and erich.leitgeb@tugraz.at ) Optical Communications Research Group, NCRLab Northumbria University, Newcastle upon Tyne, NE1 8 ST UK. *Institute of Broadband Communications, TU, Graz, Austria Abstract--In this paper a DPSK subcarrier intensity modulated (SIM) free space optical (FSO) link is considered in negative exponential atmospheric turbulence environment. To mitigate the scintillation effect the selection combining spatial diversity scheme (SelC) is employed at the receiver. Bit error rate (BER) analysis is presented with and without SelC spatial diversity, and it is shown that at BER of 10 -6 using four independent PIN-photodetectors, a diversity gain of not less than 38 dB is achievable. Key words—Free-space optics, DPSK, turbulence, spatial diversity, Negative exponential distribution. I. INTRODUCTION In today’s access network, the FSO technology is playing an increasing complementary role to the RF based techniques. This is attributable to its fundamental feature of huge bandwidth that is comparable to that obtainable from optical fibre but with an added advantage of lower deployment cost and time [1]. In recent years we have seen a steadily growing research interest in FSO links, supported by the successful field trials that are now culminating into commercial deployments. [1-4]. The earlier scepticism about FSO’s efficacy, its dwindling acceptability by service providers cum slow market penetration are now rapidly fading away judging by the number of service providers, organisation, government and private establishments that now incorporate FSO into their network infrastructure [5, 6]. Terrestrial FSO is not free of challenges though; the atmospheric constituents (gases, aerosol, rain, fog, and smoke) extinguish and scatter photons traversing the atmosphere. The most deleterious being the thick fog, which could result in up to 270 dB/km attenuation coefficient [1]. This potentially limits the achievable link range to less than 500 m during such condition. In clear atmospheric conditions, longer link ranges are palpable. However, due to atmospheric turbulence effects small but random changes occur in the atmospheric temperature. This by extension results in random changes in the atmospheric index of refraction along the path of the optical radiation. This metamorphoses into random phase and irradiance fluctuations (scintillation) of the optical radiation at the photodetector. Detailed study of the atmospheric turbulence can be found in [7-9]. The scintillation effect can be likened to the random fading effect on radio communication though not caused by the multipath propagation and can cause severe degradation in FSO link performance if unmitigated. Moreover, in terrestrial FSO link; the simple and widely used on-off keying (OOK) [10] signalling technique requires adaptive threshold to perform optimally because of turbulence. This poses a serious design difficulty, which can be overcome by employing SIM. The SIM FSO also outperforms OOK with adaptive threshold [11, 12] and for the full and seamless integration of FSO into existing networks the study of SIM becomes compelling because existing networks already contain subcarrier signals. In weak turbulence modelled using the tractable log normal distribution, the spatial diversity has been studied to mitigate turbulence induced irradiance fading [10, 13]. Likewise, FSO employing various forward error control techniques have been reported in literature [14-16] with varying degrees of gain and complexity. In this paper differential phase shift keying (DPSK) pre-modulated SIM terrestrial FSO is presented with the selection combining spatial diversity adopted to ameliorate scintillation effect. In addition to mitigating scintillation without introducing additional latency into the system, spatial diversity also helps to prevent temporary outage/blocking due to birds or other small flying object cross the propagation path; and it’s by far simpler/cost effective than using adaptive optics. The SIM is described in section II, the system performance metric analysis with and without selection combining spatial diversity is detailed in section III while the conclusion is presented in section IV. II. DPSK SUBCARRIER INTENSITY MODULATION A. System description To achieve SIM, the data to be transported is pre- modulated on to a radio frequency (RF) signal of frequency ω c . The modulated subcarrier RF signal is then used to directly modulate the irradiance of an optical carrier which is either an LED or a laser diode. Since the subcarrier signal is sinusoidal with both negative and positive values and since the intensity of an optical carrier can never be negative, a DC bias is usually applied to the subcarrier prior to modulating the optical source to ensure that the driving current of the laser diode is not less than its threshold current. After traversing the atmospheric channel, the transmit 978-1-4244-1876-3/08/$25.00 ©2008 IEEE Proceedings - 273 - CSNDSP08