High datarate current consumption change a lot when changing distance

[Ziyao Zhou]

Hi,

I am using two nRF52833 DK and both are connected with nRF21540.I notice that when I have a very high throughput, never stop sending 244 bytes, I will notice that average current reach up to 60mA(1.8V) when Tx and RX are close to each other. However, when I move away for 5m, the current drop to 20mA (1.8V). I guess it's because some connection has falied so some interval are not sending data. Here is a video:

My com paramters are: 

.interval_min = 320,
	.interval_max = 320,
	.latency = 0,
	.timeout = 400, /* supervision timeout, 单位 10 ms */

They are the same with Bluetooth: Throughput defualt setting.

Does that means a lot packet loss? I have test datarate vs RSSI at this ticket and notice two different result. One shows RSSI has no ralationship with data rate. Even low RSSI can have high data rate given in this ticket. Another result shows lower RSSI lower data rate, which is compatible with this current change.

Which one is correct?Thank you so much for your help!

[Edvin]

Hello,

The reason you are seeing a lower current consumption when your distance increases is because of packet loss. The way that a BLE connection works when you are sending a lot of packets is that every connection interval, it starts by sending one packet, with a flag saying "I have more data" set to true. Then, when the packet has been transmitted, and received with the correct CRC, then it will start the transmission of the next packet. This next packet will be sent directly after the previous one, before the next connection interval.

However, if the CRC is incorrect, meaning that at least one bit was flipped (due to noise on air), then by the specification, the peripheral needs to retransmit the previous packet, and it will have to wait for the next connection event (meaning one connection interval later) to do so.

Therefore, it will loose all that time in between the connection events, because it can no longer send packets directly after the previous one. Therefore, it will also use the radio less (because it needs to wait), and therefore you will see a lower current consumption, when the reception is bad.

So if you are struggling with the throughput, and the range is expected to be high, it may be beneficial to use 1MBPS, and not 2MBPS. You may also want to shorten the connection interval, because it will more often be able to restart the retransmission sequence. 

Shortening the connection interval in itself leads to a lower throughput, but it will be more robust for places with a noisy environment, if you struggle with packetloss. There is no good answer with two lines under it saying exactly what you should use. You should experiment with the throughput sample, and different connection parameters, in an environment that resembles the environment where the device will be operating.

Best regards,

Edvin

[Ziyao Zhou]

Hi,

I have changed the connection interval and found change it will not affect the data throughput and power consumption.

	320 units power(mW)	320 units data rate (kbps)	24 units (mW)	24 units data rate(kbbs)
-10	1.842	126.02	1.836	127.57
-7	2.394	164.16	2.178	180.18
-4	3.066	231.34	2.568	179.53
-1	3.144	219.26	3.33	241.1
2	3.684	250.02	3.6	226.17
5	4.638	275.76	5.898	260.43
8	6.78	302.52	8.208	297.36