how many nRF24L01+ RF tranceivers in a network

[Jasper]

A friend of mine asked me to help with thinking about a solution to create a RF communication between 8 robots and 1 access point. Not a lot of traffic, just to signal that the robot is (almost) at the end of his track and get some control information. once per few minutes. This is not very complicated and I was thinking about the nRF24L01+ chip with external antenna. We need a range of approx. 100 - 150 meter. LOS

the complex part is the following: it is used in a field with solar panels, and we need about 4000 robots and 4000/8 = 500 access points.

So each access point with 8 robots is actually a small network. But because all of these networks are in the same field (not all of them are discoverable) I need to have multiple network ID's and unique ID's for each RF tranceiver, right?

Is this feasible with this amount of tranceivers?

Best regards,

Jasper

[Øyvind Karlsen]

Hi Jasper,

Are you concerned about noise between the different centrals? If you have a configuration that is something like this:(* is a robot and C is an access point)

* * * * * *
* C * * C *
* * * * * *
* * * * * *
* C * * C *
* * * * * *

Then the distance between robots that are transmitting different network ids and the closest access point with a different network id would be 200-300m based on the way I am reading your post.

To get a range of 100-150m reliably with the nRF24L01+ you will need to use a power amplifier(PA) or a low noise amplifier(LNA). Since you are able to control the effect that you transmit you will be able to tune your system so that you can "hear" a robot/access point that is within your 100-150m range, but not within the 200-300m range, i.e. the loss is so high for ranges over 200m that the robots appear to be invisible.

I might be interpreting your question incorrectly, please elaborate if so.

Best regards,

Øyvind

[Jasper]

Hello Øyvind,

Thank you for your reply.

The configuration you have described is nearly what I would like to have. But instead of multiple robots per row, I just have only 1 robot in a row. The accesspoint in in the middle of a square of 200x200 meter, so the next AP is also 200m further. See below.

• = robot, ------ = robot track AP = accesspoint

  • • • • • • • • • | * - - - - - - - -
  • • • • • • • • • | - * - - - - - - -
  • • • • • • • • • | - - * - - - - - -
  • • • • • • • • • | - - - * - - - - - AP1 < 200m > AP2
  • • • • • • • • • | - - - - - - * - -
  • • • • • • • • • | - - - - - * - - -
  • • • • • • • • • | - - - - * - - - -
  • • • • • • • • • | - - - - - - - - *

So when a robot from AP1 is on the border of his grid, AP2 will also receive his signal. But in the worst case, an AP will receive max. 24 robots, right?

I also have to think about addressing of the units. In this case, I don't need a unique ID for all robots I think. And I can use different channels.

EDIT: When using dipswitches to set the network ID 0 to 3 for example, I can use address 0 for all AP's and 1 to 8 for all robots when using a different network ID?

Why using a PA or LNA. I have found some modules (sparkfun) which have a range of 100m with external antenna and no PA or LNA.

Best regards,

Jasper

[Øyvind Karlsen]

I'm not sure that it would be a max of 24 robots, potentially you could suddenly see a lot longer range than expected and be able to see all 8 adjacent squares, however this is not necessarily a problem as the robots in your square will always have a higher RSSI than the ones from adjacent squares.

While the modules have a range of 100m described, this is almost always best-case scenario. You want an application that is robust to changing atmospheric conditions and other things that might cause path loss. What is going to happen to your robots if the range suddenly drops to 80m?

Another option is to look at antennas that have higher gain than a regular dipole, for instance since your robots are moving along a track you can have some mechanical contraption that moves a yagi antenna in a way so that it points towards the correct access point.

[Jasper]

yes, you're right. It is possible to see more than 24 robots, but due to lower RSSI and also a different network ID, the AP should discard that message anyway, right? So with 4 dipswitches, I'm able to create 16 network IDs.

there are 500 APs in this configuration which make a really long solar panel field (+/- 100km)

The 100 meter is a must, so an amplifier should be a better option. I prefer to have a LOS range coverage of 150 meter. Not much further. Is there a reference design for the nRF24L01+ with PA or LNA. Or can I buy this as a module. I prefer the last option. RF PCB design is not my daily job.

A mechanical contraption is probably not an option. I can't move/rotate the antenna from the AP which is located in the middle.

[Øyvind Karlsen]

Yes, this the APs should be able to do this.

A quick google search gave this result, unfortunately I'm not very familiar with nRF24L01+ modules, so there might be more/better options.

You are right that rotating the antenna on the AP is probably not a great idea, but what about rotating the antenna on the robots?