Radiated Immunity Certification problems/Unclear documentation about CCA

RE: Radiated Immunity Certification problems/Unclear documentation about CCA

DalelvenGabriel — Tue, 15 Jan 2019 07:44:44 GMT

I've talked with a collegue, who explained that this is nothing new to us, even if it's new to me. We'll take your insight into consideration, thanks for your help!

RE: Radiated Immunity Certification problems/Unclear documentation about CCA

AmbystomaLabs — Mon, 14 Jan 2019 19:57:21 GMT

I suggest you get someone on your team familiar with RF terms. Baseband is anything not on a carrier. Normally this implies your broadcast bandwidth centered around DC, but some designs do direct sampling at IF and RF frequencies so the meaning of baseband can change a little depending on the design.

12dB amplification is meaningless to talk about unless you know the noise figure at baseband. The compression points and the intermod intercept points are all highly influenced by the receiver gain. You need someone on your team who understands these terms otherwise you are likely to create a design that will continue to fail immunity and will have serious issues in the field.

As I noted you can likely have the benefit of the FEM LNA while still having a receiver that works well and passes immunity. But you need someone to do the receiver modeling and the link budget calculations. You may find that while 12dB gain fails, 4dB passes and gives nearly the same performance in the field.

RE: Radiated Immunity Certification problems/Unclear documentation about CCA

DalelvenGabriel — Mon, 14 Jan 2019 10:17:41 GMT

While I'm not completely sure I understand you, my interpretation is that the signal at the nRF is too strong which distorts the signal, causing us to lose packets when we're also receiving the certification noise. Is that accurate? If not, please clarify what you mean with compression, as well as baseband processes.

The problem is that a 12 dB amplification helps our range a lot, and we're hesitant to remove that advantage, but we will take your suggestion into account if we can't pass certification even after the other problems we found.

EDIT: And yes, the FEM is in LNA mode during RX.

RE: Radiated Immunity Certification problems/Unclear documentation about CCA

DalelvenGabriel — Mon, 14 Jan 2019 08:32:36 GMT

Hello, Bjørn.

After doing some double-checking on my own, I'm afraid I've arrived at the conclusion that both your answers are factually incorrect. I'd like to show my findings for any others with the same problems:

1. I used our send/receive software for the certification and only changed the cca settings between reflashing. When I set the mode to ED only, and the limit to -94 (which is the minimum), the TX side stops sending due to background noise, as expected. My results of the remaining modes are presented in this table:

ED -94 - No packets arrive
ED -74 - Packets arrive
CARRIER - Packets arrive
CARRIER_AND_ED -94 - Packets arrive
CARRIER_AND_ED -74 - Packets arrive
CARRIER_OR_ED -94 - No packets arrive
CARRIER_OR_ED -74 - Packets arrive

Due to those results, the only conclusion I can find is that you're supposed to read the documentation as "the medium is busy if ed AND carrier is reported busy". If you want to send when one of the CCA methods are okay, you should use AND. I'd like to request an update of the documentation, as there clearly is ambiguity.

2. I found some release notes for your 802.15.4 library:

https://github.com/NordicSemiconductor/nRF-IEEE-802.15.4-radio-driver/blob/master/release_notes/nrf_802154_1.1.0.md

which speaks of the following function being added:

https://github.com/NordicSemiconductor/nRF-IEEE-802.15.4-radio-driver/blob/master/src/nrf_802154.h#L225

It's a conversion function from dBm to CCAEDTHRESH value that in essence subtracts -94 from the input number. This means writing 0 to the register in question will result in an ED threshold at -94 dBm. This means setting that number to -80 will result in an ed threshold at (255 - 80 - 94) = +81 dBm. Instead, you should use the above mentioned function like the following, or just add 94 to the requested dBm value before setting it:

    nrf_802154_cca_cfg_t cca_cfg = {
        .mode = NRF_RADIO_CCA_MODE_ED,
        .ed_threshold = nrf_802154_ccaedthres_from_dbm_calculate(-74),
        .corr_threshold = 0,
        .corr_limit = 0
    };

This is confirmed by my tests of setting the CCAEDTHRESH 0, which gives no received packets, and then to -1, which loops around to 255 and always sends, letting packets get through. This would not be the case if the provided value would be interpreted as dBm.

I'd like to request another update of the documentation here, preferably in the table which is under 6.20.14.38 CCACTRL in the nRF52840_PS_v1.0.pdf version of the documentation. That's where I went to find out.

Thank you for your help either way!

Gabriel

RE: Radiated Immunity Certification problems/Unclear documentation about CCA

DalelvenGabriel — Fri, 21 Dec 2018 12:49:27 GMT

I'll return after the new year with my findings.

RE: Radiated Immunity Certification problems/Unclear documentation about CCA

AmbystomaLabs — Thu, 20 Dec 2018 16:37:32 GMT

You don't mention what state you have the FEM in during RX. The fem has an LNA and you can easily be losing your packets due to compression even though the CCA is working. Although FSK is highly immune to IMD, the baseband processes on the nRF are not.

Since the nRF already has a good receiver you should operate the FEM in bypass mode during RX. Or, you could have the LNA on but pad down a bit after the FEM to normalize the gain and keep the system noise figure intact.

RE: Radiated Immunity Certification problems/Unclear documentation about CCA

bjorn-spockeli — Thu, 20 Dec 2018 15:43:52 GMT

Hi Gabriel,

Q1: The following is stated in the nRF52840 PS

IEEE 802.15.4 implements a listen-before-talk channel access method to avoid collisions when transmitting
- namely carrier sense multiple access with collision avoidance (CSMA-CA). The key part of this is
measuring if the wireless medium is busy or not.
At least three methods must be supported:
4413_417 v1.0 298
Peripherals
• Mode 1 (energy above threshold): The medium is reported busy upon detecting any energy above theED threshold
• Mode 2 (carrier sense only): The medium is reported busy upon detection of a signal compliant with the IEEE 802.15.4 standard with the same modulation and spreading characteristics
• Mode 3 (carrier sense and threshold): The medium is reported busy by logically ANDing or ORing the
results from mode 1 and mode 2.

Hence, both the ED and Correlator threshold must be met for a channel to be reported clear when using NRF_RADIO_CCA_MODE_CARRIER_AND_ED. If only one should be enough, then you should use NRF_RADIO_CCA_MODE_CARRIER_OR_ED .

Q2: Yes, if the Energy Detect Threshold is set to -80, then the channel will be reported as clear if the measured energy is below -80dBm.

BEst regards

Bjørn