Thread

Various Projects Find Hidden SDR Capabilities in ESP32 Microcontrollers

288 points · 67 comments · nkw

mallets · 1790879919 · focus · HN ↗
So many $1 wireless ICs have a powerful "SDR", but will never be documented due to certification/compiliance/export-control reasons. The projects listed here rightly limited the scope to RX-only but Espressif might be forced to "patch" this away if arbitrary TX too is possible (or starts making headlines). Hope it doesn't come to that, more cheap options for RF-to-bits is always welcome.
On a technical note, not much I can find about the signal quality. Phase noise, images, DC Offset, etc. Have to get my hands on one, ESP32-S31 seems promising.
1. ErroneousBosh · 1790880388 · focus · HN ↗
  > The projects listed here rightly limited the scope to RX-only but Espressif might be forced to "patch" this away if arbitrary TX too is possible
  Why would they? It's perfectly legal to operate radio transmitters on any frequency you like, if you hold a suitable licence.
  1. ssl-3 · 1790883444 · focus · HN ↗
    It is legal for a person with an appropriate license, yes. But laws (in the US) about RF transmission differ from how many other laws are structured: We're only allowed to transmit what we have permission to transmit -- and by default, we're forbidden.
    But right now, I've got an ESP32 WROOM module on a dev board sitting in my windowsill reporting some sensor values over wifi.
    The WROOM itself is registered with the FCC[1]. If I stick to using the documented operation, then I have permission to use it in those documented ways. As a practical concept, I don't need a unique license to use it in the way that I am; I'm granted the ability to use it by Part 15 of the FCC rules.
    And that's pretty great: The hardware was inexpensive to buy and very easy to find. I was able to spin this sensor up very quickly and get moving on the software side that deals with those readings. It has been a very rewarding little project with a fantastic cost:benefit ratio.
    The fear being expressed here (which I don't necessarily subscribe to) is that things like this WROOM module will become less-available when/if methods to use it in undocumented ways are published and start becoming used.
    [1]: <a href="https://fccid.io/2AC7Z-ESPWROOM32" rel="nofollow">https://fccid.io/2AC7Z-ESPWROOM32
    wewewedxfgdf · 1790888604 · focus · HN ↗
    >>"We're only allowed to transmit what we have permission to transmit -- and by default, we're forbidden."
    It is not true that by default you are forbidden to transmit. In fact there are quite open rules in some ways for transmissions. Whenever these discussions about transmission come up there's always people who, without really undrstanding, freak out about transmitting anything and declare that all tranmissions are illegal. Not so.
    USA hobbyists can legally transmit without a licence under FCC Part 15, but the limits are pretty low and depend on the frequency. AM, up to 100 mW into the final RF stage, with the antenna, feedline and ground limited to a total of 3 metres. FM maximum field strength of 250 µV/m measured 3 metres away. There are special cases, such as 49.82–49.90 MHz, where home-built transmitters can go to 100 mW on a 1 metre antenna. One-off home-built transmitters don't need FCC certification if for personal use not being sold.
    It's pretty interesting to read if you are a radio head: <a href="https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15" rel="nofollow">https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A...
    <a href="https://www.ecfr.gov/current/title-47/section-15.209" rel="nofollow">https://www.ecfr.gov/current/title-47/section-15.209
    "operation within these frequency bands is permitted under other sections of this part, e.g., §§ 15.231 and 15.241."
    -->
    0.009-0.490Mhz 0.490-1.705Mhz 1.705-30.0Mhz 30-88Mhz 88-216Mhz 216-960Mhz Above 960Mhz
    ssl-3 · 1790890117 · focus · HN ↗
    We're granted permission to make the transmissions you speak of; it is otherwise deny-by-default. This is described right there at the top of your first link, in the first three paragraphs.
    It's broadly possible to sort any rule (or superset or subset of rules) into one of two different buckets:
    One bucket contains the rules about the things things we're not allowed to do. Like a firewall with a blacklist, whereby we can do whatever we want that explicitly isn't blacklisted. It is allow-by-default.
    The other bucket contains the rules about things that we are permitted to do. This is like a firewall with a whitelist, where we can do whatever the whitelist explicitly allows us to do and nothing more. It is deny-by-default.
    Drug laws in the USA are in the first bucket (blacklisting). Radio transmission rules in the USA are in the second bucket (whitelisting).
    (There is no third bucket.)
    Borealid · 1790930196 · focus · HN ↗
    I believe the default state, in the absence of any law whatsoever, is that you can do whatever you want. Nothing is forbidden by default. You could assault another person, and it would be legal absent a law forbidding it.
    Then there are laws that prohibit things like broadcasting radio signals without a license.
    Then within those very same laws there are exceptions allowing certain transmissions WITHOUT a license, in specific circumstances.
    The "default" is that things are legal unless they are made illegal, and I think it's pointless wordplay to try to argue the default state is somehow the law prohibiting unlicensed operation, and not the part of those same laws that allow unlicensed operation in particular bands with particular transmission power limits.
    I understand the point you're trying to make that some parts of U.S. law say that all not-explicitly-mentioned radio transmissions are unlawful, but that's not meaningfully different from drug policies that say that all "harmful substances" are unlawful and so forth. They all just boil down to what the law defines as being within its own scope.
    ssl-3 · 1790930361 · focus · HN ↗
    Yes.
    And in the United States of America, at least: Radio transmissions are illegal by law, except where permission is granted by law.
    (Try not to overthink it too much. A previous responder provided a citation that did not serve them well.
    This situation can seem ugly or fantastic or jubilant or something else -- and it can be changed! -- but it still is just is how it is.)

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