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I'm relatively new to electronics, so sorry for my awkward understanding of topics. I'm just trying to wrap my head around some basic concepts here.

From my understanding, in OFDM, the information that we want to send (with a high data rate) gets split unto multiple partial datastreams with lower data rate. Therefore, my question is if there's a minimum data rate that enables OFDM? Like, if the rate is too low, it's probably hard/impossible to split the data unto datastreams with an even lower rate.

I'm asking because I'm trying to trigger OFDM with my dev boards which send raw 802.11n packets at 1mb/s, but it's not working. So I'm thinking that the rate's too low for OFDM.

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  • \$\begingroup\$ my dev boards which send raw 802.11n packets at 1mb/s The datarate and modulation needs to be negotiated after a link is established so just sending "something" is never going to work in my view. You really need to intricately understand how WiFi works before you can successfully start to do things "your own way" as chances are it simply doesn't work, which is what you see now. \$\endgroup\$
    – Bimpelrekkie
    Jun 19, 2020 at 20:53
  • \$\begingroup\$ Oh, I got a bit confused with the title. Actually, I want to send packets without any WiFi connections - just raw 802.11n broadcasts. \$\endgroup\$
    – Noel93
    Jun 19, 2020 at 21:03
  • \$\begingroup\$ and the point of doing that is...? My guess is that most WiFi modules / chips don't even allow for this as the "raw" part is behind a software layer that will only work when you establish a link. For sending "raw packets" my guess is that you need special equipment like an RF signal generator with the required options installed. If you have to ask what such a device costs you cannot afford it. \$\endgroup\$
    – Bimpelrekkie
    Jun 19, 2020 at 21:08
  • \$\begingroup\$ I'm using the ESP32 boards and they provide an API to send arbitrary packets. Here's an example to spam beacon frames: github.com/Jeija/esp32-80211-tx/blob/master/main/main.c My goal is to extract channel state information from the channel between two ESP32s. This is working right now when both boards share a WiFi and send UDP packets to each other. However when not in the same WiFi, the raw packets (e.g. the beacon ones) don't trigger the callback function to extract CSI. So I was thinking that the connection is needed for data rate reasons (you can only change rate when connected to a WiFi) \$\endgroup\$
    – Noel93
    Jun 19, 2020 at 21:20

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Therefore, my question is if there's a minimum data rate that enables OFDM?

No, there's other limiting factors.

Like, if the rate is too low, it's probably hard/impossible to split the data unto datastreams with an even lower rate.

No, that's easier the slower you go. It's not more of a problem at low rates.

You're forgetting why we're doing that splitting: A very high-bandwidth signal sees a wide channel – and a wide channel experiences frequency-selective fading, due to multipath propagation. If you have for example a direct path and a path reflected off some structure, then for one frequency, these waves would interfere constructively, for a different frequency, destructively. If you have both frequencies in your wideband signal, then you're in trouble because you can't revert that effect with a simple multiplication.

So, you split your wideband channel into frequency-flat subchannels. Problem solved.

At some point, it doesn't make sense to split any further. If your channel is flat already, splitting it doesn't make it "flatter". On the contrary, you're incurring the problem that channels change over time (that's due to movement of the transmitter, receiver, or obstacles and reflectors, as well as due to oscillator imperfections), and if you increase your signal duration enough, you will hit a wall where your nice flat channel sadly becomes a channel that's not nice at all but changes faster than you can follow.

I'm asking because I'm trying to trigger OFDM with my dev boards which send raw 802.11n packets at 1mb/s, but it's not working.

OK, you don't "trigger" OFDM. OFDM is a method of transporting data. IEEE 802.11n is indeed an OFDM standard. It's multiple MHz wide, in the microwave region: of course you'd use OFDM there. These channels very much are multipath.

So I'm thinking that the rate's too low for OFDM.

Nah. No such thing. There's digital voice transmission schemes that do OFDM at around 700 bits per second. There's probably a few HF (or below!) modems that do even slower OFDM for long-reach comms across half the planet.

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  • \$\begingroup\$ Thanks, I think I found where I took a wrong turn. The API I'm using to send arbitrary frames seems to indeed use 1mb/s with 802.11b, which uses DSSS as a modulation scheme. \$\endgroup\$
    – Noel93
    Jun 19, 2020 at 22:20
  • \$\begingroup\$ well, there's no reason why a specific standard would support arbitrary rates, indeed. It's not about OFDM, however. Glad you've got it sorted out! \$\endgroup\$
    – Marcus Müller
    Jun 19, 2020 at 22:22
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OFDM is just a variant of the classic FM-FM telemetry data links of pre-PCM scientific experiments.

Numerous slow sensor voltages are used to frequency-modulate an equal number of oscillators; with each oscillator center frequency placed so as to not interfere with any other oscillator.

The combination of these numerous oscillators becomes the modern OFDM.

However, the telemetry people then used that combination of numerous sensors to MODULATE a transmitted carrier, using FM modulation.

Is there any minimum to the sensor data rate? No.

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