Summary: An Adaptive Detector and Channel Estimator
for Deep Space Optical Communications
R. Mukai, P. Arabshahi, T.-Y. Yan
Jet Propulsion Laboratory
4800 Oak Grove Drive, MS 238343
Pasadena, CA 91109 USA
Over the past years, NASA and JPL have continuously sought to reduce spacecraft size and mass while
increasing its information return capability. Laser communications provide a way of achieving this goal.
The highly collimated beam allows for significant reductions in the size and mass of the communications
terminal along with reduced power requirements. Optical communications also avoids problems involving
radio frequency resource and spectrum allocation, interference, and frequency and bandwidth regulation.
Since an increasing number of missions will operate at high downlink data rates, the avoidance of these
issues is a significant advantage.
The optical communication system under study at JPL uses pulse-position modulation (PPM) to transmit
data. Each PPM symbol consists of 256 signal slots of 20 ns each, and approximately 15 µs of "dead time"
(see Fig. 1). The "dead-time" after the 256 slots is present to allow the Q-switched laser sufficient charging
time between pulses. Within the slot, there is a small (2 ns) guard time on each side of the 16 ns duration
pulse to provide a safety margin against pulse jitter associated with Q-switched lasers [1, 2].