Multiple Transmitters for Gain Saturated Pre-Amplified FSO Communication Systems Limited by Strong Atmospheric Turbulence and Pointing Error

The use of multiple transmitters in a turbulent free-space optical (FSO) communication link can enhance the systems performance because greater received power can improve receiver sensitivity. Nevertheless, the effect of having more power at the receiver can be severe. This paper investigates the impact of using multiple transmitters in preamplified FSO communication links limited by strong atmospheric turbulence (AT), amplified spontaneous emission noise, fixed path loss and pointing errors (PEs) using different preamplifier operating modes and different decision thresholding schemes. Results obtained show that regardless of the number of transmitters used, the best bit error rate (BER) performances are obtained with normalised decision threshold levels of about 0.2 and 0.5 when the decision thresholding scheme at the receiver is non-adaptive and adaptive, respectively. Also, in the strong AT regime, an additional transmitted power of about 7dB is required for the FSO communication systems under minimal PE effects to have the same performance as FSO communication systems without PE. The results also show that when the effects of PE are absent or minimal, a larger receiver would require about 15dB less transmitted power to record the same performance as a smaller receiver. However, with a non-adaptive decision threshold, smaller receivers perform better than larger receivers when the PE effects are severe. Additionally, it is shown in this paper that when the effects of PE is severe, the BER performances consistently get better with more transmitters regardless of the decision thresholding scheme employed at the receiver. However, in the absence of PEs, increasing the number of transmitters indefinitely will not always guarantee improved BER performances when the receiver decision threshold is non-adaptive. The use of multiple transmitters is particularly advantageous for applications where it is either necessary or unavoidable to use lower transmitted power.