Low latency RF transceiver with good wall penetration

Hi all,

I am a beginner to electronics and have a few quick question. I am interested in creating a device that can measure time differences remotely, with as low latency as possible. I was thinking about using an RF tranceiver for this.

The requirements are the following:

  1. It should be able to penetrate through lots of walls (or ceilings) and send a signal 15-20 meters.

  2. It does not have to have a lot of throughput.

  3. It must have very low latency. The transmitter must reliably be able to trigger the receiver at-least, say, 10 times faster than the speed of sound in air (~ 343m/s).

  4. Power consumption is not an issue.

The idea is that I have a vibration at location A propagating to location B (<20m apart), and I want to measure this propagation time. Unit 1 at location A detects a vibration and sends a signal to unit 2 at location B. Unit 2 starts a timer. When unit 2 detects the vibration, it will stop the timer and then know the time difference.

I was thinking about using RF to start the timer at Unit 2, as they travel with the speed on light, thus much faster than the speed I want to measure. I was thinking about using NRF24 for this, but I am not sure it can send signals through 3-4 floors. There seem to be lower frequency modules with better wall penetration, but I fear that they have too much latency.

Can I use a NRF24 for this purpose? If not, any better suggestion?

Thanks for the help.

Can you get a WiFi signal through those walls? Then RF24 is likely to work.

Otherwise look at lower frequencies. As a rule of thumb the lower the frequency the better the penetration. A pair of HC-12 modules (433 MHz) may do the job.

Everything of course depends on the walls/floors in question.

For minimal latency use a wired connection.

Do appreciate that walls (and ceilings) vary greatly in how transparent they are to RF.

And the forum has no idea how many is ‘lots’

Best to describe the actual project, where it is, why your doing it, etc.

Remember you might know the project, the forum does not.

4tnemele:
Hi all,

I am a beginner to electronics and have a few quick question. I am interested in creating a device that can measure time differences remotely, with as low latency as possible. I was thinking about using an RF tranceiver for this.

The requirements are the following:

  1. It should be able to penetrate through lots of walls (or ceilings) and send a signal 15-20 meters.

  2. It does not have to have a lot of throughput.

  3. It must have very low latency. The transmitter must reliably be able to trigger the receiver at-least, say, 10 times faster than the speed of sound in air (~ 343m/s).

  4. Power consumption is not an issue.

The idea is that I have a vibration at location A propagating to location B (<20m apart), and I want to measure this propagation time. Unit 1 at location A detects a vibration and sends a signal to unit 2 at location B. Unit 2 starts a timer. When unit 2 detects the vibration, it will stop the timer and then know the time difference.

I was thinking about using RF to start the timer at Unit 2, as they travel with the speed on light, thus much faster than the speed I want to measure. I was thinking about using NRF24 for this, but I am not sure it can send signals through 3-4 floors. There seem to be lower frequency modules with better wall penetration, but I fear that they have too much latency.

Can I use a NRF24 for this purpose? If not, any better suggestion?

Thanks for the help.

Before you get too far into your project, let’s clear up some possible confusion.

The RF does travel at the speed of light in air. BUT your data signal does not travel at the speed of light. It travels at the modulation rate of your transmitter. Also, you suggest a “signal” being sent. You will be sending some data in a message to the receiver, which must process it.

Your latency will have to include all the above and may be less than the speed of sound you are looking for.

Good project, however.

Paul

wvmarle: Can you get a WiFi signal through those walls? Then RF24 is likely to work.

Otherwise look at lower frequencies. As a rule of thumb the lower the frequency the better the penetration. A pair of HC-12 modules (433 MHz) may do the job.

Everything of course depends on the walls/floors in question.

For minimal latency use a wired connection.

WiFi signals do not work very well where I want to use this. I was hoping that RF24 at Max power might still work, but I guess not. I will have to look at lower frequency modules and see how much latency they have.

srnet: Do appreciate that walls (and ceilings) vary greatly in how transparent they are to RF.

And the forum has no idea how many is 'lots'

Best to describe the actual project, where it is, why your doing it, etc.

Remember you might know the project, the forum does not.

I was mainly looking for general advice and guidelines for this type of problems, I am not sure about all the details yet. I can elaborate on the project; it's just a very silly project that I'm using as an excuse to learn some basic electronics. In the building where my friends live whenever somebody enters the building, their apartment door (third or forth floor) rattles with quite a lot of energy. It's either due to sound waves propagating in air or (less likely) vibrations in the building itself (acoustic phonons). The latter propagates several times faster than the former. I want to measure the speed of this wave as accurate as possible (and ideally with the arduinos and NRF24's that I happen to have already).

Paul_KD7HB: Before you get too far into your project, let's clear up some possible confusion.

The RF does travel at the speed of light in air. BUT your data signal does not travel at the speed of light. It travels at the modulation rate of your transmitter. Also, you suggest a "signal" being sent. You will be sending some data in a message to the receiver, which must process it.

Your latency will have to include all the above and may be less than the speed of sound you are looking for.

Good project, however.

Paul

Yes, these are the exact worries that I have (and which I mean with 'latency'). I was hoping to send the minimal possible data to avoid much processing time, I just need it as a 'trigger'. But I think you just mentioned the key thing I need to read up on and understand better, 'modulation rate'. Hopefully I can cook up a good formula to roughly predict the effective speed, as a function of modulation type and frequency of the transmitter.

The frequency of the transmitter is of no consequence in your project, only the modulation/data rate.

Paul

ps: The RF travels at the same speed, no matter the frequency.

Paul_KD7HB: The frequency of the transmitter is of no consequence in your project, only the modulation/data rate.

Paul

ps: The RF travels at the same speed, no matter the frequency.

But doesn't the modulation/data rate depend on the base frequency? As in if you fix a modulation type/format, say GFSK as in NRF24, then the modulation rate can change as function of the base frequency? I need to study modulation more, I know very little about this.

Electromagnetic waves of course all travel at the same speed, no matter the frequency.

Actually, quickly reading up on this, it seems that I was wrong. Modulation works differently than I thought.

Frequency does, however, affect the range through obstacles. Generally, a lower modulation frequency at the same power means longer range. They make 900MHz UART radios if the NRFs don't cut it in terms of range...

I doubt the transmission speed of wireless is reliable enough for this. There are likely to be minor variations there, not noticeable normally but you’re looking at millisecond timing.

The most accurate timing and fastest transmission (latency based on the speed of light) you can get is by wire. In your case all you need is an on/off signal, so just set a voltage on one end and look for it on the other.