Performance Evaluation and Comparison of Coherent and Incoherent Receivers under Atmospheric Turbulence

Sammanfattning: Optical free space communication system faces the major challenge because of the atmospheric condition. Signals receive in the ground station using two different types of receivers (Coherent detection and Intensity Modulation and Direct Detection (IM/DD)). Coherent detection uses PIN photo detector in the receiver end to attain the more sensitivity of the receiver. It receives the input data as a carrier signal and the local oscillator signal is mixed with the received signal and down convert the carrier signal to an intermediate frequency signal. The Intensity Modulation direct detection uses the Avalanche photo detector in the receiver end to attain the more sensitivity. This detection receives the input signal as a carrier signal and it is directly demodulated at the receiver back into the original signal. Signals receive in the ground station from the aircraft will be affected by the various types of noise like shot noise, thermal noise, etc. The occurrence of noises in the coherent detection is not exactly same as the IM/DD. Some noise get varies according to the electrical circuit noise produced in the receiver side. By deriving the signal-to-noise ratio, the background noise occur in the desired signal can be calculated. One of the main goals would be to derive a Probability Density Function (PDF) of the Signal-to-Noise Ratio (SNR) of the each type of receiver to check the efficiency of the receivers. Transmitting the optical signal from aircraft will face some data loss problem due to atmospheric turbulence disturbances, to identify the loss arises in the transmitting signal will be done by using the probability error method. Bit Error Rate (BER) derivation will take place to calculate and to identify the data loss occurs in the received signal. The project deals with measuring the efficiency and sensitivity among those two optical receivers and to check the robustness between those receivers against scintillations (power fades and surges) effects. In this work performance of the coherent receiver and IM/DD receiver using APD is compared with the different system characteristics. Sensitivity and performance of both the receivers are calculated with the same fading vector. Signal to noise ratio and bit error rate are theoretically derived and numerically analyzed in the case of atmospheric turbulence. Numerical results predict the performance of both the receivers.