Why is NASA claiming 10-100x better BW performance from optical-based comm (DSOC)?

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The DSOC (deep-space optical communication) experiment on the Psyche spacecraft had a successful test (even though the test distance was mouse nuts, IMO):

https://www.nasa.gov/directorates/s...bout-nasas-deep-space-optical-communications/

But NASA seems to be suggesting that optical comm has a 10x to 100x better bandwidth for deep-space comms compared to traditional RF comms. I have not been able to figure out where this improvement is coming from given the articles by NASA and in the popular press. Does anybody have links to where this improvement is coming from? Is it because of a lower noise floor at those optical wavelengths (NIR), or maybe higher directivity or something else?

Thanks for any insights...
 
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I think it is the usual difference between an analog and a digital signal. I have found an example for phone frequencies: ~3kHz versus 100kBits/s = 100kHz for VoIP, which gives a factor of 30.
 
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DaveE said:
Higher carrier frequency means more modulation BW typically. Like how the internet runs better on fiber optics than cables.

https://www.ll.mit.edu/r-d/projects/terabyte-infrared-delivery-tbird
Good point, but the noise floor in optical fiber is very low. Is the noise floor in the NIR way lower than the RF bands they are currently using for satellite probe comms?
 
  • #5
Sorry, link to Quora didn't work, for whatever reason, so it has to be a picture:

1701128360097.png
Here are the data from the lunar test:
https://www.nasa.gov/mission/lunar-laser-communications-demonstration-llcd/
 
  • #6
It may be due to the size of the remote antenna, measured in wavelengths.
 
  • #7
berkeman said:
Good point, but the noise floor in optical fiber is very low. Is the noise floor in the NIR way lower than the RF bands they are currently using for satellite probe comms?
IDK. It's really about SNR of the recovered signal though. More noise can be fixed by more source power. Absorption is also a thing; what about clouds and such. It's also about the filtering and detection functions at the receiver front end. It's hard to find good comparative data since they each live in their own silos.

From mostly unrelated laser comm work I was a bit involved with; you might be surprised at how far a bright laser can travel through "stuff" and still be detected.
 
  • #8
It's frustrating to me that NASA didn't link to more technical artices...
 
  • #10
Baluncore said:
It may be due to the size of the remote antenna, measured in wavelengths.

Transmitting antenna large in terms of wavelengths --> Narrow transmit beam --> Less spread of energy --> More energy picked up by receiver.

Given a certain area, the receiving antenna intercepts the same amount of energy, irrespective of the wavelength. (The transmit antenna produces a certain flux density around the receiver, and that's it).

However, engineering formulas often contain a "receiving antenna gain" but this doesn't reflect anything physically going on, but rather is a side effect of the way gain was historically defined.
 
  • #12
Swamp Thing said:
Given a certain area, the receiving antenna intercepts the same amount of energy, irrespective of the wavelength.
That is misleading. It assumes the incident energy density is the same at RF as at optical wavelengths.

The optical wavelength employed is so much shorter than the RF wavelength, that the beam widths are much narrower for optical systems than for manageable RF antennas.
Optical transmission has a narrower beam, so produces a higher Effective Incident Radiated Power at the receive-antenna, which also has a narrower beam, so captures less noise through the unnecessary sides of the beam, noise that would otherwise enter the receiver and signal processor. Both those effects improve the signal-to-noise ratio, and both are due to the size of the antenna, measured in wavelengths.
Now is the decade of optical communications.
 
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fresh_42 said:
https://www.nasa.gov/wp-content/uploads/2015/03/tglavich_dsoc.pdf

Considering more details: there might be patents involved.
Good point. I didn't find any info on the modulation scheme, other than that they were "counting photons" at that distance. Does that mean OOK (on-off keying)? It did mention 1kbit/s, but I don't know if that is 100x what RF comms is at those distances, or just starting out at slow datarates as a proof of concept for the optical comms...
 
  • #14
They first tested it with the moon. I found dozens of papers about LLOC (Lunar Laser Optical Communication) by a Google (scholar) search, e.g. https://arc.aiaa.org/doi/pdf/10.2514/6.2014-1685.

Searching for DSOC also produces a lot of hits, e.g. https://www.spiedigitallibrary.org/...ons-DSOC-transceiver/10.1117/12.2256001.short.

However, as has been said already,
DaveE said:
Yes, but soooo much stuff on google scholar.
I cannot assess whether and where the information you're looking for is hidden.
 
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fresh_42 said:
e.g. https://arc.aiaa.org/doi/pdf/10.2514/6.2014-1685.
Thanks!
Prior to transmission, the data are encoded using a ½-rate serially concatenated turbo code8. The
encoded data are modulated using 16-ary pulse position modulation (PPM). The resulting symbols are interleaved with a ~1-second convolutional channel interleaver prior to being amplified to a nominal 0.5-W for transmission on the optical downlink. The combination of this powerful forward error correction code with channel interleaving
enables reliable error-free communication through the turbulent Earth atmosphere with >1 received bit per detected photon,
 

1. What is DSOC and how does it differ from current communication systems?

DSOC, or Deep Space Optical Communications, refers to the use of laser-based technology to enable data transmission over vast distances in space. Unlike current radio frequency (RF) systems that use radio waves, DSOC utilizes light (optical lasers) to send information. This shift to a higher frequency band of light allows for narrower beam widths and higher data rates, making communications more efficient and robust over interplanetary distances.

2. Why does NASA claim that DSOC can achieve 10-100x better bandwidth performance?

The claim for improved bandwidth performance with DSOC stems from the use of optical lasers, which operate at much higher frequencies than radio waves. Higher frequencies allow for faster data rates because they can carry more information per second. The narrow beam width of lasers also reduces the energy spread and interference, enhancing the signal quality and efficiency over long distances.

3. What are the practical benefits of improved bandwidth in space communications?

Improved bandwidth significantly enhances the speed and volume of data transmission between spacecraft and Earth. This allows for high-resolution images and videos from distant planets and moons to be sent back to Earth more quickly. It also supports more sophisticated scientific instruments aboard spacecraft, facilitating real-time data analysis and faster decision-making for mission control.

4. What challenges does DSOC face in its implementation?

DSOC technology faces several challenges, including the need for precise pointing and tracking systems due to the narrow laser beams. Any misalignment, due to spacecraft movement or mechanical vibrations, can disrupt the communication link. Additionally, atmospheric interference, such as clouds and turbulence, can affect the transmission of laser signals as they pass through Earth's atmosphere.

5. How is NASA planning to overcome these challenges?

NASA is developing advanced pointing, acquisition, and tracking technologies to maintain accurate alignment of the laser beams between spacecraft and Earth stations. Adaptive optics and other atmospheric compensation techniques are also being explored to mitigate the effects of atmospheric disturbances. Moreover, NASA is considering the deployment of relay satellites equipped with DSOC technology to ensure consistent and reliable communication links, even when direct line-of-sight is obstructed.

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