How is it that NASA receives broadcasts from 300 million miles away but Sprint needs to install transmitters every 500ft for 5G wireless?

  • 8
    $\begingroup$ Have you compared the transmission bandwidths/speeds and the energy used in both cases? $\endgroup$
    – DarkDust
    Feb 19 '19 at 9:25
  • 6
    $\begingroup$ You could talk to deep space too if you put a 20 kilowatt battery in your cell phone and sat at the focus of this parabola, and still were willing to put up with text messages at one letter per second ;-) planetary.org/blogs/emily-lakdawalla/2011/3253.html The trick to deep space communication lies in a combination of having a large dish at one or both ends of the connection and using one or both high power and low data rates compared to cell phones. $\endgroup$
    – uhoh
    Feb 19 '19 at 10:15
  • 1
    $\begingroup$ 5G differs from 4G in granularity by beam-forming and directing different signals preferentially to different individual users; the architecture requires a higher density and smaller spacing, with lower power per user. Actually answer to Could “live” video be transmitted from Mars? show that video is possible from Mars with current-level technology, but it's not easy. $\endgroup$
    – uhoh
    Feb 19 '19 at 10:18
  • $\begingroup$ Related Can I text on my cell phone from the ISS to Earth? $\endgroup$ Feb 19 '19 at 13:26

Let's compare with 4G, for which I could find some numbers:

Your cell phone is transmitting with speeds of up to 50MBps with a maximum of 1 Watt (can be as low as a few µW!), using a tiny antenna that is often obscured by water bags (human flesh and organs), rocks, buildings, trees, etc.

Let's compare this with Mars Express, for example: its maximum bandwidth back to earth is "just" 230kBps (AFAIK in the same order of magnitude as EDGE) at 5 Watt (S-Band) or 65 Watt (X-Band) using a large 1.6 Meter parabolic antenna. Its signal is received by huge and very expensive antenna dishes here on earth.

So these are very different systems with very different requirements. We don't want to transmit with too much power from our cell phones, yet also want to transmit with high speeds through various obstacles. This requires receivers in closer proximity than with lower speeds and/or higher energy.

  • 1
    $\begingroup$ In addition, the space between the spacecraft and the Earth is vacuum for the most part. $\endgroup$ Feb 19 '19 at 19:54
  • 6
    $\begingroup$ @mgarciaisaia, the space between your cell phone and the tower is also vacuum for the most part, at least as far as radio waves are concerned. $\endgroup$
    – Mark
    Feb 20 '19 at 0:08
  • 4
    $\begingroup$ The huge advantage Mars <-> Earth communication has is that it can use directional antennas and that there are no obstacles in the way. With directional antennas even cheap Wifi equipment with a few dozen mW transmit power can communicate over >100km through dense atmosphere. $\endgroup$
    – Michael
    Feb 20 '19 at 8:27
  • $\begingroup$ I would put more emphasis to directional antennas compared to the send in all directions of your average wireless router. $\endgroup$
    – yunzen
    Feb 20 '19 at 10:07
  • $\begingroup$ It fact, assuming a linear density drop (close, but not quite), there is more air between the surface of Mars and the surface of Earth than there is between you and a cell phone tower 50 km away. $\endgroup$
    – dotancohen
    Feb 20 '19 at 15:27

In addition to DarkDust's answer:

In radio, each two-way radio link needs its own channel, to avoid interference with other links. There is a limited number of channels available (don't know for 5G specifically, but Wifi only has 3 channels that don't interfere with each other).

So in order to support millions of simultaneous radio links, you need to reuse those channels. You can do that by reducing the range of each transmission: if a user on channel 1 only broadcasts in a 1-km radius, another user 2 km away can use channel 1 without interfering with the first user. Reduction in power also means the battery in your cell phone lasts much longer.

This is why Sprint installs so many transmitters.

For the Mars link, NASA uses a dedicated dish antenna 34 m in diameter, and a high-powered transmitter. It also supports communications with just one spacecraft at a time, so there's no need to share the radio link.

  • 3
    $\begingroup$ +1 5G (specified in the question) also uses MIMO and Beamforming $\endgroup$
    – uhoh
    Feb 19 '19 at 12:53
  • $\begingroup$ If we are talking about 5G wouldn't it also be because the wave length is so short that it doesn't have the strength to penetrate walls/buildings or travel very far so an extensive antenna network must be established? $\endgroup$ Feb 20 '19 at 6:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.