Abstract
Satellites in low earth orbit (LEO) are currently being deployed for numerous communication, positioning, space and Earth-imaging missions. To provide higher data rates in direct-to-user links and earth observation downlinks, the free-space optics technology can be employed for LEO-to-ground downlinks. Moreover, the hybrid automatic repeat request (HARQ) can be adopted since the propagation latency is low for LEO satellites. In this work, a power allocation methodology is proposed for optical LEO-to-ground downlinks under weak turbulence employing HARQ retransmission schemes. Specifically, the average power consumption is minimized given a maximum transmitted power constraint and a target outage probability threshold to ensure energy efficiency and reliability, respectively. The optimization problem is formulated as a constrained nonlinear programming problem and solved for Type I HARQ, chase combining (CC) and incremental redundancy (IR) schemes. The solutions are derived numerically via iterative algorithms, namely interior-point (IP) and sequential quadratic programming (SQP), and validated through an exhaustive (brute-force) search. The numerical simulations provide insight into the performance of the retransmission schemes regarding average power. More specifically, Type I HARQ has the worst output, CC has a moderate one, and IR exhibits the best performance. Finally, the IP algorithm is a slower but more accurate solver, and SQP is faster but slightly less accurate.
Highlights
As early as the 1960s, the development of optical and laser pumping brought freespace optical (FSO) communication to life [1,2]
The average power is minimized for Type I, chase combining (CC), incremental redundancy (IR) hybrid automatic repeat request (HARQ) schemes, and the sensitivity to the scintillation index, average path loss and target outage probability is examined
Optical HARQ-based low earth orbit (LEO)-to-ground downlinks under weak turbulence conditions are studied, and a power allocation methodology is proposed to ensure reliable and energy-efficient transmission
Summary
As early as the 1960s, the development of optical and laser pumping brought freespace optical (FSO) communication to life [1,2]. For optical satellite downlinks with HARQ schemes under weak turbulence, there has not been a power allocation investigation in the literature In this contribution, a power allocation methodology is proposed for optical LEO-to-ground downlinks under weak scintillation conditions employing HARQ retransmission schemes. Simulations are executed for various channel conditions and system settings by simulating a LEO passing over various turbulence intensities and ground weather conditions to investigate the sensitivity of the three HARQ schemes to weak scintillation, path loss and target outage probability.
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