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Chapter 17:
System Link Budget Calculation and Impairment Aspects
The previous chapters provided a detailed review of an optical analog link at both ends. In the central downlink, there is a predistorted multichannel community access television (CATV) transmitter. On the user's end, an analog receiver employs simple linearization techniques to combat internally created distortions. However, when considering the entire system, there are several parameters that should be taken into account to measure the quality of service. Quality of service is measured either by carrier-to-noise ratio (CNR) or bit error rate (BER) performance. Quality-of-service parameters such as BER and CNR are affected by optical-link building blocks and their inherent limitations. Those limitations include reflections, dispersion, and fiber-related nonlinearities. Additionally, the CATV channel is affected by external modulation and clipping effects of the laser used in the transmitter in case of DML. Moreover the whole ling is affected system wise by the transmitter quality, RF circuits and laser as well as the receiver's photo detector and the receivers RF circuit performance. An analog receiver produces NLD (nonlinear distortions) such as composite second orders (CSO) and composite triple beats (CTB), which can be countered with proper electrical designing. A photodetector (PD) produces NLDs such as CSO and CTB that are minimized by defocusing the spot and the PD active area as well as using a high reverse bias. The inherent receiver noise floor is determined by the PD shot noise as well as its noise figure performance. The CATV laser transmitter is optimized and predistortion techniques are utilized to combat NLD products.
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