LnaDiversity [FreeBSD WIKI]

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====== Receive Diversity using LNA mixing ======

The AR9285 and AR9485 are single radio (1x1), non-MIMO 802.11n devices. This means that any diversity needs to occur external to the baseband - ie, it can't rely on signal processing techniques to achieve diversity.

The AR9285/AR9485 have two receive inputs - LNA1 and LNA2. These lead to two separate LNAs (Low Noise Amplifiers) which amplify the incoming signal before processing.

So, as well as supporting an external antenna switch for TX and RX diversity, they also support LNA diversity. This involves three main components:

  * The ability to select up to two receive LNA configurations - "main" and "alt(ernate)";
  * The ability to configure the LNAs to select one LNA (LNA1 or LNA2), as well as a mix of the two (LNA1+LNA2, or LNA1-LNA2);
  * If there's time, trying the alt configuration after the main configuration and if the RSSI is higher, choosing that to receive the frame.

===== What's LNA mixing? =====

Put simply, there's a configurable mixer/switch between the two receive LNA paths. This allows for something resembling classic external switch based diversity (ie, selecting one or the other) but it also allows for a mix of both paths. Ie:

  * LNA1 - only enables LNA1
  * LNA2 - only enables LNA2
  * LNA1+LNA2 - mixes them both together and uses the resulting output
  * LNA1-LNA2 - subtracts LNA2 from LNA1 and uses the resulting output

Now, depending upon the orientation of the antennas in relation to the transmitter and the surrounding environment, this may provide a couple of extra dB improvement in signal level.

===== What are the downsides? =====

Yes, there are a couple of downsides.

  * With the AR9285, the LNA1 path has ~ 3dB loss in receive sensitivity versus LNA2. Thus some NICs just statically configure LNA2 to be the only receive interface and leave LNA1 just for transmit. The AR9485 doesn't have this problem.
  * It's all done as mixing rather than (much more) complicated signal correlation/processing.  Thus the results aren't always going to be as good as the MIMO receivers. However, that said, it is better than nothing.

===== Ok, so what's this "slow diversity" that the AR9285/AR9485 HAL supports? =====

If the AR9285/AR9485 NIC EEPROM is configured to enable diversity, the HAL can start trying to probe various LNA configurations in order to determine the best "main" and "alternate" configurations.

The radio defaults to main=LNA1, alt=LNA2.

It then will test different combinations to determine which two give the highest RSSI for received frames.