2
\$\begingroup\$

Having a WiFi emitter sending a frame, how much time does each symbol take? And therefore, what distance do they spread over?

This depends on the Modulation Coding Scheme. I would like to know which of the various 802.11 norms has the shortest symbols.

Here is an example with 802.11b:

  • at 11Mb/s using DQPSK it encodes one symbol per chip, each symbol carry 2 bits
  • this means roughly 5.5M symbol/s
  • 1.81818182e-7 s per symbol
  • 54.5 m per symbol

We could note that one symbol spreads over 436 periods, each one spreading over 12.5 cm at the speed of light.

Am I right? Are there any other 802.11 norms that have shorter symbols?

\$\endgroup\$
4
  • \$\begingroup\$ I'm not an expert but from first glance your calculations seem right. Just need to watch out that you have the phy symbol rate, and not the higher-level bitrate, as a lot of these protocols add a good bit of FEC and other protocol-related overhead. (eg, 25G ethernet is actually about a 27 gigabit/s signal) \$\endgroup\$
    – Joren Vaes
    May 28, 2020 at 8:04
  • 1
    \$\begingroup\$ "I would like to know which of the various 802.11 norm has the shortest symbols." - why? \$\endgroup\$ May 28, 2020 at 9:13
  • \$\begingroup\$ @BruceAbbott at first because I thought about using this for a positioning system, but it's impossible. I still want to know because I am very curious, and also I like having those kind of numbers in mind, it helps me scale things and grasp concepts more clearly. Most schematic about modulation show 1 period per symbol, which is misleading. \$\endgroup\$
    – bokan
    May 28, 2020 at 16:47
  • \$\begingroup\$ Considering that 802.11 is a LAN protocol, which version has the shortest symbol period is little more than a curiosity. \$\endgroup\$ May 28, 2020 at 23:46

1 Answer 1

1
\$\begingroup\$

You can download the 802.11-2016 standard for free from IEEE (it has been published more than 6 months ago, so IEEE allows it to be downloaded for free, see https://ieeexplore.ieee.org/browse/standards/get-program/page/series?id=68) and look through the numbers in there.

But you're looking for, I guess, 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac? No problem. All these amendments have now been approved and rolled back into the 802.11-2016 standard (the 2016 means that revision of the standard; every few years, a new version comes out, where some of the latest approved amendments are rolled back into the standard). But will the physical layers specs for each of these amendments all be mixed together and hard to separate? No, they are neatly separated into different sections, so the numbers are not hard to find.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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