Aerospace and Electronic Systems Magazine April 2018 - 16

Feature Article:

DOI. No. 10.1109/MAES.2018.170179

Fully Optical Spacecraft Communications: Implementing an
Omnidirectional PV-Cell Receiver and 8 Mb/s LED Visible
Light Downlink With Deep Learning Error Correction
Sihao Huang, Haowen Lin, Aphelion Orbitals, Inc., Titusville, FL, USA

INTRODUCTION
The large surface area of the photovoltaic (PV) cells on a spacecraft provides a significant sensor area, which can be used as an
omnidirectional receiver on many solar panel layouts. Previous
researchers have proposed the use of this unique asset as a backup
communication system [1], realizing that a low-cost, low-power
consumption uplink could be created by using the photovoltaics
as a laser receiver. Such proposed systems have data rates of less
than 10 Kb/s, allowing them to serve as effective emergency alternative-band communication systems, though not as an operational
uplink.
PV cells are intrinsically difficult to work with as high-bandwidth components due to their parasitic capacitance and high photocarrier drift and diffusion times. Building on the work of Shin et
al. [2], who demonstrated 17.05 Mbps of discrete multitone transmission on a solar panel receiver, we propose an implementation
for overcoming these limitations through reverse biasing with little
impact on the overall efficiency of the power system.
One of the most significant advantages of the use of a PV-cell
receiver is the low mass and power penalty to a small spacecraft.
This becomes more apparent on CubeSats and even femtosatellites, negating the need for an uplink receiver or antenna and providing mass, cost, and power budget savings in a design environment in which all three are critical to mission success.
Successful commercial implementation of a fully optical communication system depends on high reliability and a low requirement for pointing accuracy. The second part of the link uses a
noncoherent, light-emitting diode (LED)-based transmitter, which
enables a stable downlink without a high-accuracy attitude determination and control system (ADCS). The use of LED beacons
is not new; the idea has been tested on FITSAT-1, which successfully recovered a 5-kHz modulation on the signal [3]. ShindaiSat
[4], flown in 2014, used an array of 36 LEDs on its high-gain arAuthors' current address: S. Huang, H. Lin, Aphelion Orbitals, Inc., 3433 Joe Murell Dr., Titusville, FL 32780, USA, E-mail:
(haowen.lin@aphelionorbitals.com).
Manuscript received September 10, 2017; ready for publication
December 11, 2017.
Review handled by M. De Sanctis.
0885/8985/18/$26.00 © 2018 IEEE
16

ray, with a total output of 86.4 W. This allowed for a data rate of
9.6 Kb/s while using basic frequency-shift keying modulation and
white light, which negated the possibility of spectrum filtering.
Moreover, light fidelity and similar technologies have matured the
field of visible light communication (VLC) considerably, and the
characteristics of LEDs are well known [5].
Despite lower directionality, the significant efficiency improvement of LED transmitters (around 70% for LEDs and 30%
for laser diodes) enables a power-per-bit value that is much lower
than expected. The data rate ceiling for a transmitter bound by
the power and pointing limits of a 3U CubeSat is predicted to be
around 50 Mb/s. This sparked significant internal interest for further research, because it indicates a large potential in nanosatellite
applications to fill the gap in communication bandwidth. Future
advancements such as larger ground stations, improved focusing
systems, and optimized modulation and recovery techniques will
enable the envelope to be pushed even farther.

Figure 1.

Implementation of the proposed VLC transmitter and PV-cell receiver
on the Calypso spacecraft

IEEE A&E SYSTEMS MAGAZINE

APRIL 2018

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