Unstable RSSI signal in cc2540

Hi Yaniv,

I could not reproduce the problem on my 2540 keyfob right now looking at the SmartRF Packet Sniffer log, getting only two different RSSI values for around 20 trials. How are you measuring the RSSI value?

Is something interfering? Do you not see this variation on the advertisement packets' RSSI?

Aslak

Maybe this is caused due to advertizing in 3 channels while there is an interferer in one of them which causes different RSS levels...?

Yanniv,

That would be my guess. Try using the packet sniffer, which only listens on one advertisement channel and see what happens.

http://processors.wiki.ti.com/index.php/BLE_sniffer_guide

BR,
Aslak

Hi Yaniv... Did you ever solve this issue with RSSI levels?  I'm seeing the same thing...  I have a 5 X 6 panel of BLE advertisers based upon the CC2541 chip, they are advertising non-connection broadcasts once a second on all three channels.  The panel is placed a std. distance away from a BLE gateway that's also based upon the CC2541 chip.   The RSSI is monitored with an internal .net application.  Most of the advertisers consistently display RSSI values in the -29 to -32 range on all three channels.  A few, however, will display in that range for two of the channels, but in the -48 to -52 for the other channel... and it's not any one specific channel that's always the "bad" channel.  And if I "spin" the panel in 90 degree increments, the same advertisers are inconsistent, so it's not related to position on the panel or orientation.  I also see "good" advertisers go "bad" for awhile, then go back to "good"... this testing is taking place in an isolated test bed so interference is minimal.

Hi,

The problem was bypassed not solved.

I changed the advertise to one channel only and I increased the output power to 4 in the transmitting device(I think you can't get to 4 db in CC2541, this is possible in CC2540)

The advertising channel was not chosen randomly, I check which channel has the most stable RSSI and used it only.

If you are getting around -30dBm in your RSSI it means the devices are too close to each other and the AGC(part of the HW) may not function right.

You should try to work with the BLE roughly between -45 to -90(this is not official data).

I found that when advertising(Undirected, same as you) with all 3 channel the RSSI is received differently.

This could be some hardware fault ti isn't reporting, such as different RX sensitivity levels between channels, I have witnessed issues such as these in the past in my previous role developing BT chip(I can't write which company I worked for,,,,) or some other HW issues....

But more likely the problem is mainly caused by interfering WLAN channels.

Yaniv Reibenbacg said:

If you are getting around -30dBm in your RSSI it means the devices are too close to each other and the AGC(part of the HW) may not function right.

Can you please elaborate on that. I have never heard of such handicap.

Thanks!

Yaniv Reibenbacg said:

If you are getting around -30dBm in your RSSI it means the devices are too close to each other and the AGC(part of the HW) may not function right.

There is no AGC for BLE on CC254x.

Yaniv Reibenbacg said:

This could be some hardware fault ti isn't reporting, such as different RX sensitivity levels between channels, I have witnessed issues such as these in the past in my previous role developing BT chip(I can't write which company I worked for,,,,) or some other HW issues....

A plot of sensitivity level vs. frequency can be seen on page 22 of the CC2541 datasheet.

...and I definately see that in my emperical data... but what I'm also seeing that isn't explained is the variability between advertisers with the same hardware and firmware AND variabilty within the same advertiser over time.

What you are describing looks like a multipath fading effect. In some locations and on some frequencies, you will have destructive interference. On a given frequency, there is not really much to do about it except to use antenna diversity. This is one of the main reasons for using frequency hopping or frequency agility. In BLE you have that, so even if one channel is not performing well, you can get your data through on another channel.