PERFORMANCE IMPROVEMENT OF GROUND TO DEEP SPACE OPTICAL LINK RECEIVER USING SPATIAL DIVERSITY TECHNIQUE Hemani Kaushal 1 , Ankita Singh 2 , V.K.Jain 3 , Subrat Kar 1 1 Department of Electrical Engineering, Indian Institute of Technology, Delhi-100 016, India. 2 Vellore Institute of Technology, VIT University, Tamil Nadu-632 014, India. 3 Department of Electr. & Electronics Engn., Universiti Teknologi PETRONAS (UTP), Malaysia. ( 1 himaniz@yahoo.com, 2 ankita_vit@yahoo.co.in, 3 drvirander_jain@petronas.com.my, 1 subrat@ee.iitd.ac.in) Abstract: Spatial diversity technique is one of the mitigation techniques to combat the effect of atmospheric turbulence. In this paper, a spatial diversity technique with equal gain combining (EGC) is employed to analyze the performance of ground to deep space optical link receiver for various modulation schemes viz., OOK, Subcarrier Intensity Modulation (SIM) and M-ary PPM. It is observed that BPPM continue to perform better than the M-ary PPM (M=8, 16, 64 and 256) even with the spatial diversity. SIM outperforms the other schemes in terms of lesser SNR requirement for a given BER. 1. INTRODUCTION Free space optical (FSO) communication has shown promising advantages among other wireless schemes. It provides a very high data rate, freedom from licensing, low cost of deployment and enjoys low power and less mass requirement. Unfortunately FSO links face several challenges such as scattering, absorption and turbulence induced scintillation. Out of these, turbulence induced scintillation is the major cause of impairment which results in amplitude and phase fluctuations of the received signal. Hence some mitigation techniques need to be deployed for FSO links particularly for long distance communication. Various mitigation techniques exist to combat the effect of atmospheric induced turbulence like aperture averaging, spatial diversity and adaptive optics. It is not always possible to implement aperture averaging beyond certain detector size and further increase in the size of detector in the receiver will not help in the reduction of scintillation [1]. Spatial diversity involves deployment of multiple transmitters and multiple receivers so that multiple copies of the same signal are sent through uncorrelated path to the receiver. If the beams are spatially separated and they pass through the uncorrelated path, then the likelihood that all the beams simultaneously being faded will be substantially reduced relative to the likelihood of the same result for a single beam. Adaptive optics techniques have been used in past and are becoming common in many astronomical observations [2, 3]. But the overall objective is different in optical communication relative to astronomical observations. In astronomy, the objective is to increase the sharpness of the images and any loss of signal energy can be made by longer observation time. But in optical communication, the signal energy for a data bit is fixed and must be conserved for efficient communication. Also adaptive optics needs complicated design which results in increases system cost. In this paper, a spatial diversity technique involving multiple receiver antennas that are placed at a separation greater than the correlation length of the fading state leading to spatially independent and uncorrelated channels has been employed. For this diversity technique with equal gain combining (EGC), performance of ground to deep space link receiver is analysed for different modulation schemes viz., OOK, SIM and M-ary PPM. Subsequently, a comparative study of the performance for the above modulation schemes is made. The rest of the paper is organized as follows: Section 2 describes the system and the channel models. Section 3 gives the BER expressions for OOK, SIM and M-ary PPM modulation schemes. The computed results in graphical and tabular forms and their comparative study are given in Section 4. Finally, conclusions of the study are presented in Section 5. 2. SYSTEM AND CHANNEL MODELS The system consists of one transmitting laser and multiple receive apertures where the received signals are combined using EGC technique. The performance of EGC is midway between the performance levels of selection combining and maximal ratio combining, the latter providing the best performance though at the cost of increased system complexity. Hence EGC has been chosen as a combining technique which creates a new signal by linear combination of all the multiple signals with weights set to unity. In this study, IM/DD is adopted and the laser beam intensity fluctuations in weak turbulence are modelled as lognormal distribution. Various studies have been carried out to model the atmospheric channel. A widely used ICOP 2009-International Conference on Optics and Photonics CSIO, Chandigarh, India, 30 Oct.-1 Nov. 2009