DIRECT DETECTION IN OPTICAL COMMUNICATION USING INTENSITY MODULATION V.Vadivu #1 , M.Sathya #2 , S.Anjugam #3 ,R.Ramya #4 , #1, #2, #3 PG students [Communication system], Dept.of ECE, As-salam engineering and technology, Aduthurai, Tamilnadu, India. #4 Assistant Professor [O.G], Dept.of ECE, As-salam engineering and technology, Aduthurai , Tamilnadu, India. [email protected]#1 [email protected]#2 [email protected]#3 [email protected]#4 Abstract— Optical wireless communication (OWC) refers to communication through an unguided medium using modulated light. In this paper, the performance of OWC systems employing intensity modulation and direct detection (IM-DD), established by modulating light intensity at the source and using an intensity detector at the destination is proposed. The capacity of the intensity- modulation direct-detection optical broadcast channel (OBC) is investigated, under both average and peak intensity constraints. An outer bound on the capacity region is derived by adapting Bergmans’ approach to the OBC. Inner bounds are derived by using superposition coding with either truncated-Gaussian (TG) distributions or discrete distributions. At high signal-to-noise ratio (SNR), it is shown that the TG distribution is nearly optimal. Keywords— Intensity-modulation; optical broadcast; capacity region; truncated Gaussian; discrete inputs. I. INTRODUCTION The Optical Wireless Communications is a type of communications system that uses the atmosphere as a communications channel. The OWC systems are attractive to provide broadband services due to their inherent wide bandwidth, easy deployment and no license requirement. The idea to employ the atmosphere as transmission media arises from the invention of the laser. However, the early experiments on this field did not have any baggage of technological development derived from the fiber optical communications systems, because like this, the interest on them decreased. At the beginning of the last century, the OWC systems have attracted some interest due to the advantages mentioned above. However, the interaction of the electromagnetic waves with the atmosphere at optical frequencies is stronger than that corresponding at microwave. The intensity of a laser beam propagating through the atmosphere is reduced due to phenomena such as scattering and molecular absorption, among other. The changes in the refractive index of the atmosphere due to optical turbulence affect the quality of laser beam through distortion of its phase front and random modulation of its optical power. Also the presence of fog may completely prevent the passage of the optical beam that leads to a no operational communications link. The information signal analog or digital is applied to the optical transmitter to be sent through the atmosphere using an optical antenna. At the receiver end the optical beam is concentrated using an optical antenna to the photo-detector sensitive area which output is electrically processed in order to receiver the information signal. II. EXISTING SYSTEM In the existing systems the direct detection is used in optical communication. By using direct detection the signal from input side is directly given to the receiver side with an unwanted noise. Orthogonalizing users this way allows serving multiple users in the OBC without interference. Hence, the channel from the transmitter to each receiver reduces to an IM-DD P2P, and capacity results on IM-DD P2P channels can be applied. For a Gaussian broadcast channel which is physically degraded by nature, superposition coding (SC) is optimal and orthogonalizing users is not efficient. The performance of SC in the OBC in terms of bit- error rate and throughput III. PROPOSED SYSTEM The goal of this paper is to study the capacity of the N-user OBC, which models the downlink in VLC. The main focus is finding simple closed-form statements on the channel capacity. This requires developing outer and inner bounds on the capacity region of the channel. To this end, we modify Bergmans’ outer bound to obtain outer bounds on the capacity
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