Time, range and moving speed limit to pair and comunicate nRF52840 modules

[DevKam]

Hi,

I would like to use the nRF52840 modules for moving nodes. However, I'm concerned that the modules will not be able to cope with high moving velocities to communicate or pair among them. If there is a speed limit to work with moving devices, I would be glad to know, thank you.

I'm also wondering about the relation between the transmission power and the range, since there is no information about the range of this module in both urban and outdoor (line of sight) areas.

Thank you in advance,

Iñigo

[ketil]

Hi Iñigo,

We do not have any numbers on transferring data for devices moving at high velocities. However it will be possible to do some calculations if we have some more information. Could you please write some more details about your use case and specify at least:

1. What is the range requirement?
2. Are the devices in line-of-sight or in urban environment?
3. What type of devices are you planning on using? Mobile phones?
4. Are both devices moving?
5. How much data do you need to transfer?
6. What is the highest expected velocity?
7. Do you need pairing or are advertisements enough?

Ketil

[DevKam]

Hi Ketil,

The use case is composed by a fixed device and another installed in a flying object, which flies over the fixed device at a height of 100 meters and at a speed of 50 km / h. The problem is if they have to be paired to send data and how long it takes the transmission of 50 kB data.

 

1. What is the range requirement?

100m

2. Are the devices in line-of-sight or in urban environment?

Line-of-sight

3. What type of devices are you planning on using? Mobile phones?

Two nRF52840 devices

4. Are both devices moving?

Only one is moving

5. How much data do you need to transfer?

50KB

6. What is the highest expected velocity?

50 km/h

7. Do you need pairing or are advertisements enough?

I will be pleased if you could give me more information about pairing these devices. I suppose that the pairing is needed for the data transfer, isn’t it? In addition, what is the minimum time required in this type of use case for pairing?

 

Thank you in advance,

Iñigo

[ketil]

Hi Iñigo,

Thank you for providing the details, sounds like a fun project.

In the following calculations I will assume that the plane flies over the ground station in a straight path and that the environmental conditions are ideal.

We have tested nRF52840 in Bluetooth 5 Long Range Mode and +8dBm output power at distances over 1km. For simplicity I'll use 1000m as the value for maximum distance between plane and ground station where they can communicate. In this video we test at 750m+, but it can go much further.

Doing some hand calculations (using an online trigonometry calculator) we can see that we have a 995m distance where we can maintain a connection. Let's round it up to 1000m and double it since we have the same distance as the plane is flying away from the base station. 

Since the plane is moving at 50km/h ≈ 14m/s we have a total of (2000m) / (14m/s) ≈ 140 seconds available to transfer the data.

The theoretical data rate of BT5 Long Range is 125kbps. In practice we are closer to a throughput of 25kbps application data.

In 140 seconds we can transfer 25kb * 140 = 3500kb = 437kB of data. This is 8 times more than the specified 50kB requirement.

In conclusion, you should be able to solve your use case with two nRF52840 devices.

Please note that there are a range of environmental factors that could impact your results: rain, snow, humidity, salty air (over ocean), EM interference from motors etc. At least theory is on your side to create a real-life test. 

As my colleague pointed out, another option is to use BT5 2MB Mode and a power amplifier. This will give you long range and high throughput at the cost of increased power consumption. Adding external antennas will also improve your link budget.

Regarding pairing, this is only necessary if you want to establish an encrypted link between the master and slave devices. Otherwise a regular connection will work fine.

Ketil