nRF21540 antenna diversity for BLE

[akif]

Hi,

We are working with nRF21540DK and developing a BLE application. nRF21540DK with nRF Connect SDK 2.5.0 . I have couple of questions.

1. In nRF21540DB product page says "The two antenna ports on the nRF21540 DK are useful for antenna diversity scenarios in 802.15.4 based protocols (i.e. Thread or Zigbee) to reduce multipath propagation effects.". I couldn't see any explanation on the datasheet regarding two antenna and BLE.
   
   Does BLE protocol use the both antennas?
2. If BLE doesn't use both antennas, which antenna it use, POUTA or POUTB?
3. POUTA set to 20dB and POUTB set to 10dB as default. Is there any reason you have selected these values as default? For example why it is not 21db which is the maximum value.
4. Why you haven't selected 20dB for both antenna? Why the POUTB is exactly the half value of the POUTA ?
5. In datasheet page 25, CONFREG0 TX_GAIN TX gain control (0: minimum, 31: maximum). What is the unit of this value, obviously it is not dB, because maximum is 21dB. 
6. What is the minimum and maximum dB value we can set for POUTA and POUTAB individually?
   

Thanks.

[Ders]

It's kind of a IoT system where the HUB/gateway is providing internet to the the battery powered BLE devices.  So the hub sees 5-20 devices.

• All installations are different and I'm trying to maximize the number of devices the HUB can connect to.
• The hub/gateway is fixed attached to a wall outlet
• The battery devices are also fixed and installed on a "walls". 
• Due to multiple devices I can't use directional antennas and also size will not allow for such antenna.
• The wireless enviroment at 2.4GHz is typically quite noisy due to residential scenario

I will attempt to ensure the antennas on the HUB and devices are in the polarized plane but due to nature of multipath/fading what have you, one never know what the real scenario is. Seeing the Nordic nRF52 + FEM devkit with 2 antennas made me think I can take advantage of the to improve the case when a wireless link is compromised due to polarization mismatch. 

Here is a quote from an article on the subject

"Polarization mismatch between antennas is characterized by polarization loss factor (PLF). The parameter is expressed in decibels (dB) and is a function of the angular difference in polarization between the transmit and receive antennas. In theory, PLF can range from 0 dB (no loss) for perfectly aligned antennas to infinite dB (infinite loss) for perfectly orthogonal antennas."

If you don't have any real nature test data on BLE and antenna diversity than that's fine and I'll just use the LNA and not the second antenna port.  In theory it seams like "free power" in a scenario where loss is caused by polarization mismatch.

[Ders]

Got some information from Nordic through email which explains the complication with BLE and antenna diversity.   

We don't have a similar example for other protocols. The reason is really because the 802.15.4 have a very long pre-amble. This allows you to sample the pre-amble with both antennas and decide during the pre-amble which has the best signal strength and used this. This can be done for every packet received. Bluetooth has a very short pre-amble so you can't change antenna during the packet reception so you can only switch antenna in between packets -> moving the antenna diversity out of the controller and onto the application space. You would then have to run something to a QoS service on the antennas and then switch antennas based on the QoS of each. This is not something we have.

Another complicating factor for Bluetooth (and other short pre-amble protocols): if you receive from multiple transmitters you will need to run a separate QoS algorithm for each transmitter. This is not the case if you have long pre-amble as you can do it per packet.

Also have in mind, for Bluetooth you are also frequency hopping, so the deep "nulls" will be different on each frequency. Antenna diversity is primarily useful if you have two stationary devices communicating on a single frequency; if you are moving or frequency-hopping, you might as well retransmit as change antennas.