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:

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:

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.

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.

dev/ath_hal(4)/LnaDiversity (last edited 2013-06-11T09:14:21+0000 by AdrianChadd)