**Author detail**

28 Will-be-set-by-IN-TECH

0 5 10 15

BPSK, Perfect Compensation

DPSK, Postdetection EGC

**6. Concluding remarks and future research directions**

some directions of future research on coherent FSO systems.

BPSK, σΔφ=20° BPSK, σΔφ=30°

Average SNR Per Branch (dB)

**Figure 12.** Performance comparison of two-branch EGC BPSK and postdetection DPSK FSO systems through a strong atmospheric turbulence channel with imperfect phase noise compensation.

To further illustrate the usefulness of the proposed postdetection DPSK FSO system in the presence of phase noise, we compare its error performance with that of the coherent EGC system with different standard deviations of phase noise compensation errors. As shown in Fig. 12, the proposed DPSK system is robust to the optical phase noise variations and can outperform the coherent PSK system where large phase noise compensation errors are

In this chapter, we performed case studies on coherent systems for terrestrial FSO applications. We demonstrated the effectiveness of spatial diversity techniques in mitigating atmospheric turbulence effects. Important diversity reception techniques, MRC and EGC, were studied, and we showed that the EGC systems can provide comparable error performance to the optimal MRC FSO systems. We compared coherent PSK using EGC to DPSK using postdetection EGC when phase noise compensation error is present. It was found that the coherent PSK based system outperforms the DPSK based system when phase noise compensation error is small. However, it was also demonstrated that the DPSK is an excellent alternative to coherent PSK with EGC in terrestrial coherent diversity FSO communication systems where large phase noise compensation errors exist. These results can be useful in coherent FSO system design and performance evaluation. The rest of this section outlines

For practical reasons, real-time estimation of the instantaneous SNR may be difficult or expensive. Therefore, a more practical selection based combining can be proposed for

10−5

present.

10−4

10−3

BER

10−2

10−1

100

Mingbo Niu, Julian Cheng and Jonathan F. Holzman *The University of British Columbia (UBC), School of Engineering, Faculty of Applied Science, Kelowna, British Columbia, Canada*

## **7. References**

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30 Will-be-set-by-IN-TECH

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