Why Does NASA Claim 10-100x Better Bandwidth from Optical Communications?

  • Context: Undergrad 
  • Thread starter Thread starter berkeman
  • Start date Start date
  • Tags Tags
    Nasa
Click For Summary

Discussion Overview

The discussion revolves around NASA's claims regarding the bandwidth improvements of deep-space optical communications compared to traditional radio frequency (RF) communications. Participants explore the underlying reasons for the purported 10x to 100x better bandwidth, considering various factors such as noise levels, modulation techniques, and antenna sizes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question the basis for NASA's bandwidth claims, seeking clarification on whether the improvement is due to lower noise floors at optical wavelengths or other factors.
  • One participant suggests that the difference in bandwidth may stem from the transition from analog to digital signals, drawing parallels to voice over IP technology.
  • Another participant notes that higher carrier frequencies typically allow for greater modulation bandwidth, referencing the performance of fiber optics compared to traditional cables.
  • Concerns are raised regarding the noise floor in near-infrared (NIR) communications compared to RF bands, with discussions on the signal-to-noise ratio (SNR) and the impact of environmental factors like clouds.
  • Some participants propose that the size of the transmitting antenna, measured in wavelengths, contributes to a narrower beam and improved energy capture at the receiver.
  • There is mention of the potential for patents affecting the availability of detailed technical information regarding the modulation schemes used in optical communications.
  • One participant highlights that the effective incident radiated power is higher for optical systems due to narrower beam widths, which could enhance SNR.
  • Another participant shares links to various papers and resources related to lunar laser communications and deep-space optical communications, indicating a wealth of information available through academic channels.
  • Discussion includes technical details about modulation techniques, such as pulse position modulation and error correction methods used in optical communication systems.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement regarding the factors contributing to the claimed bandwidth improvements. While some points are acknowledged, the discussion remains unresolved with multiple competing views on the underlying mechanisms.

Contextual Notes

Participants note limitations in available technical resources from NASA and express frustration over the lack of detailed explanations regarding the modulation schemes and noise characteristics in optical communications.

Who May Find This Useful

This discussion may be of interest to those involved in communications technology, aerospace engineering, and optical physics, particularly in the context of deep-space missions and advancements in communication methods.

berkeman
Admin
Messages
69,384
Reaction score
24,743
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...
 
Astronomy news on Phys.org
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.
 
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?
 
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/
 
It may be due to the size of the remote antenna, measured in wavelengths.
 
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.
 
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.
 
Reply
  • Like
Likes   Reactions: berkeman
  • #13
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.
 
Reply
  • Informative
Likes   Reactions: berkeman
  • #15
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,