Simulation
and Analysis of Telescope Array based Receiver for Deep-Space Optical
Communication Links
Ali Hashmi ( Photonics Research Group)
Introduction and Background
The
international scientific community and NASA continue to send
discovery missions in space to unravel the mysteries of deep-space
planets in the solar system, galaxies and the universe. These great
voyages of exploration have evolved from the early reconnaissance
flybys and orbiters to the new data-intensive paradigm of
constellation of planetary landers and spacecraft probes. The
NASA’s deep-space ventures so far have relied upon a global
RF-based deep-space network (DSN) to capture the signal returns from
the planets. The DSN uses X-band and more recently Ka-band
capabilities have been successfully tested. The current RF technology
has reached an extremely high limit of performance, since the
spacecraft antennas sizes and transmitter powers are already at
maximum feasible limit. On Earth, DSN receiving antennas are enormous
(34-m and 70-m diameters), and the receiving systems are already
operating at but a few degrees above absolute zero. In spite of these
technological improvements, the current data rates are limited to
just tens to hundreds of kilobits per second (Kbps) range from Mars
distances , and considerably lower data rates for the outer planets.
This data return capability is insufficient and orders-of-magnitude
lower than that required for the current and future celestial
exploration missions.
Optical communication technology has the
startling benefits of increased bandwidth, ability to concentrate
power in narrow beams, and significant reduction in component sizes
(antennas etc.). Hence, it is considered the appropriate technology
for deep-space communication links.
The need for deep-space optical communications has been articulated
in the NASA 2003 Strategic Plan as a “New Effort Building Block”
under the “Communications Technological Barrier” for “providing
efficient data transfer across the solar system.”
Our
Research
Here, our main objective in this research is to design and evaluate
optical array-based receiver architecture for a large bandwidth and
efficient deep-space optical communication link. The array receiver
is a new concept in the optical domain, hence, several design
challenges need to be addressed. A typical deep-space optical link
suffers from several limiting factors. We will carry out an in-depth
statistical analysis and modeling of such deleterious factors, e.g.,
background optical noise, atmospheric attenuation, optical
turbulence, stochastic synchronization and tracking errors. We will
quantify the impact of these factors on the overall receiver
performance. Then, we will design and incorporate the robust,
adaptive filters based synchronization and tracking subsystems in the
array receiver to mitigate the synchronization and tracking losses.
In the next step, various approaches to mitigate the coupled effects
of background noise and atmospheric turbulence will be investigated.
We will propose the use of focal-plane arrays employing a novel
Space-Time Adaptive Processor (STAP) for adaptive processing and
reception of the turbulence-degraded signal fields. We will also
investigate the use of adaptive optics (AO) systems for mitigation of
the turbulence effects for the deep-space and terrestrial free-space
optical links.
Finally, we will simulate an end-to-end optical communication link
between a spacecraft in Mars orbit and the proposed Earth-based
optical array receiver. We will evaluate and compare a single large
telescope-based receiver with our proposed array architecture and
algorithms to find the performance bounds in terms of bit-error-rates
(BER) and achievable data rates, by using MLCD mission specifications
as a baseline platform. We believe that the optical communication
technology based upon the telescope arrays receiver architecture is a
cost-effective and efficient alternative to the RF technology, and
can fulfill the future data needs of exploration missions in
deep-space and further into the universe. Our initial analysis has
shown that the data rates in the range 1-30 M bits/sec can be
achieved with the planet Mars, as the channel conditions vary from
best to worst. This data return capability is orders of magnitude
greater than the current RF-based bandwidth. The conceptual design
of an inter-planetary link between Earth and Mars is shown in Figure
1 and block diagram of a telescope array-based receiver is depicted
in Figure 2.
Figure 1. Conceptual design of laser communication link between
Earth and Mars.
Figure
2. Conceptual design of a telescope array-based optical receiver.