[nRF52] ESB: one PTX, many PRX + frequency hopping. How to increase probability to of recived data?

[Mateusz Gancarczyk]

In traditional approach to ESB communication we have many PTX device and one PRX device:

In my application, situation is inverted: I have only one PTX device and many PRX devices:

Each PRX device have implemented Bluetooth and timeslot ESB.

Each PTX device has implemented only ESB protocol, and only transmit data – so the communication is only in one direction. So I did it, and it works very well, but i don't thing is the best way to do it.

PTX device sends payload with no ack:

payload->noack  = 1;

And later, send packed on 5 different channels: 4, 25, 42, 63 and 77 channel. For increase the probability of a chance of reaching the frame data to the PRX device I send each frame:

for ( i = 0 ; i < 10 ; i++ )
{	
  sendEsbDataOnChannelNr1();
  wait();
  sendEsbDataOnChannelNr2();
  wait();
  sendEsbDataOnChannelNr3();
  wait();
  sendEsbDataOnChannelNr4();
  wait();
  sendEsbDataOnChannelNr5();
  wait();
}

In the each PRX device I jumping through RX channel in interval 10ms:

nrf_esb_stop_rx();
nrf_esb_set_rf_channel( frequencyHopingTable[ channelIndex ] );
nrf_esb_start_rx();
channelIndex++;
if ( channelIndex > 4 ) channelIndex = 0;

So my question is:

1. How to choose the best channel for frequency hopping?

2. How to maximize probability to recive frame by PRX device? We have a few variable:

• a total number of frequency hopping channel
• number of burst transfer. Less is better - more energy efficiency. (one burst = transfer data ion each frequency hopping channel )
• time and algorithm between how to jump between RX channel in PRX device? Random? What time between hop?

Mayby there are some books or article about this problems?

[Mateusz Gancarczyk]

Firstly thanks for the answer! Secondly, let me summarize:

PTX DEVICE:

for ( i = 0 ; i < 10 ; i ++ ) // for example - do this 10 times - less is better
{
	for ( j = 0; j < 5 ; j ++ ) // total time send in one channel is ~5ms
	{
		sendEsbDataOnChannelNr1();
		wait1ms();
	}
	for ( j = 0; j < 5 ; j ++ )
	{
		sendEsbDataOnChannelNr2();
		wait1ms();
	}
	for ( j = 0; j < 5 ; j ++ )
	{
		sendEsbDataOnChannelNr3();
		wait1ms();
	}
	for ( j = 0; j < 5 ; j ++ )
	{
		sendEsbDataOnChannelNr4();
		wait1ms();
	}
	for ( j = 0; j < 5 ; j ++ )
	{
		sendEsbDataOnChannelNr5();
		wait1ms();
	}
}

PRX DEVICE:

in every 1ms:

channelIndex++;
if ( channelIndex > 4 ) channelIndex = 0;
nrf_esb_stop_rx();
nrf_esb_set_rf_channel( frequencyHopingTable[ channelIndex ] );
nrf_esb_start_rx();

so all frequency hop have 5ms period.

Please, let me know is that ok?

I can't use ACK because I have many PRX device which have the same address and I want to trasmit data to all of them. So I can't synchronize frequency between PRX and PTX - so I think the only way to do this - is "stupid" burst data throught all the channels in PTX device and hope that all PRX devices will recive data :)

[ovrebekk]

Hi Mateusz

Sending a packet every 1ms when the PRX stays on each channel for 1ms might not be quick enough, because you might send the packet at the exact moment the PRX is switching to a different channel (causing it to drop the data).
It would be safer to send a packet every 500us. Then you know that if one packet is sent during the channel switch, the next packet will not be.

In other words I would recommend sending the same packet every 500us, for a total of 5ms. If you want to try on all channels you should go through the list and do the same for each one.

Best regards

[Mateusz Gancarczyk]

Of course You right. In PTX device i send 20 times in each channel:

for ( j = 0; j < 20 ; j ++ ) {
	sendFrameOnChannel1();
	wait500us();
}
for ( j = 0; j < 20 ; j ++ ) {
	sendFrameOnChannel2();
	wait500us();
}
for ( j = 0; j < 20 ; j ++ ) {
	sendFrameOnChannel3();
	wait500us();
}
for ( j = 0; j < 20 ; j ++ ) {
	sendFrameOnChannel4();
	wait500us();
}
for ( j = 0; j < 20 ; j ++ ) {
	sendFrameOnChannel5();
	wait500us();
}

Send data on one channel takes ~10ms.

In PRX device I switch channel in every 2ms.

And it works very well :) Thanks for help.

[ovrebekk]

Hi Mateusz

It's good to hear that you got it working :)

In general I would recommend sending the packets in a non-blocking way, but that shouldn't affect packet loss ;)

Best regards

[Mateusz Gancarczyk]

"In general I would recommend sending the packets in a non-blocking way, but that shouldn't affect packet loss ;)" Can You get more detail? Blocking = wait500us(); ? ;)

[ovrebekk]

I was referring to the wait500us() function, yes ;)