equalize power levels

Hi, Mike:
It was not mentioned, but I am assuming that Node1,2,3 are hidden transmitters to each other.
If you reduce power on Node1 and Node2 to have a MCS equal to Node3, then
Node1 and Node2 may transmit data consuming a longer time due to a reduction in MCS.
This may reduce channel availability.
Equal signal strength helps to ensure that, in the event of a collision, no data is transferred.

A solution:
It is not really a 'mesh' if there are 3 hidden nodes.
Use 'Charts' as an Access Point and Nodes1,2,3 as clients thus letting the AP poll the clients one at a time.

I hope this helps,
Chuck
 

Strong SNR neighbor

It should not be a problem if one neighbor has a stronger SNR than others, in most situations.  The way to determine if the stronger SNR neighbor is an issue, is to review the /tmp/rssi.log file on the tower node.    Where there has been a problem historically, it is due to the fact that some neighbors have in contrast a ~weak SNR.  What happened, is that neighbors with low SNR, sufficient to do some communications, can be filtered out, or the tower node becomes deaf to that neighbor.   This is in the logic of the receiver trying to filter out noise or interference for the strong SNR neighbor.    A program was written to detect if a neighbor node was being filtered out, and the radio was reset to start communicating with the node again.   By inspection of this /tmp/rssi.log file, you can see if this is happening.  If the node is not becoming deaf to some weak signal neighbors, then no reason to reduce the performance of the strong signal node.

[after comment]  There is cause to reduce power, and part 97 relevant rules mandating lower power, if the node will continue to maintain same performance or if lower performance is still adequate for the usage. 

Joe AE6XE

Some misc details, maybe more than wanted, about how these radios work.

Here's a typical description of how the radio receiver works (from the AR9331 data sheet example).  Most of the smarts is done digitally.  Note, you're probably always be using 802.11n with OFDM modulation (not a spread spectrum modulation technique).  CCK referenced below is an older modulation technique from 802.11b  (a spread spectrum modulation), generally obsolete and not out there today (unless you're still running an old linksys WTG door stop :) ). 

-------------
5.1 Receiver (Rx) Block

The receiver converts an RF signal (with 20 MHz or 40 MHz bandwidth) to baseband I and Q outputs. The receiver operates in the 2.4 GHz bands to support CCK and OFDM signals for 802.11b, 802.11g, and 802.11n.The 2.4 GHz receiver implements a direct conversion architecture.

The receiver consists of low noise amplifiers (LNA1 and LNA2), diversity combiner inphase (I) and quadrature (Q) radio frequency mixers, and a baseband programmable gain amplifier (PGA). The mixer converts the output of the on-chip LNA to baseband I and Q signals. The I and Q signals are low-pass filtered and amplified by a baseband programmable gain filter controlled by digital logic. The baseband signals are sent to the ADC.

The DC offset of the receive chain is reduced using multiple DACs controlled by the MAC/ Baseband processor. Additionally, the receive chain can be digitally powered down to conserve power.
------------

[after edit -- to correct that digital sampling occurs in the time domain and then it is converted to a frequency domain]
Here's some of the smarts that can be done, from the wireless driver code comments.  This is an example of how receiving the same signal is combined from the 2 vertical and horizontal antennas after converting the signal to digital format and from a time representation to a frequency representation.  For 802.11n, this is MCS0-MCS7  (MCS8-MCS15 sends different data on the 2 polarizations, so does not apply):

-------------------------------
/*
 * MRC (Maximal Ratio Combining) has always been used with multi-antenna ofdm.
 * With OFDM for single stream you just add up all antenna inputs, you're
 * only interested in what you get after FFT. Signal alignment is also not
 * required for OFDM because any phase difference adds up in the frequency
 * domain.
 *
 * MRC requires extra work for use with CCK. You need to align the antenna
 * signals from the different antenna before you can add the signals together.
 * You need alignment of signals as CCK is in time domain, so addition can cancel
 * your signal completely if phase is 180 degrees (think of adding sine waves).
 * You also need to remove noise before the addition and this is where ANI
 * MRC CCK comes into play. One of the antenna inputs may be stronger but
 * lower SNR, so just adding after alignment can be dangerous.
 *
 * Regardless of alignment in time, the antenna signals add constructively after
 * FFT and improve your reception. For more information:
 *
 * http://en.wikipedia.org/wiki/Maximal-ratio_combining
 */
------------------------------

The linux wireless ath9k 802.11n implementation in use on AREDN firmware supports RTS/CTS for hidden nodes.   This means that a node will send a Request to transmit to the tower node, which then in turn sends out the Clear to Send to everyone with the coordinated timeslot.  For a handful of nodes on the channel, this is negligible overhead and adequately handles the coordination.    if you install the tcpdump package on the tower node, you can capture this traffic to a data file, then see this handshaking on your desktop using wireshark.  I found it very interesting to see how this handshaking works in reality.

Note, exposed node situation is best to avoid -- don't put 2 radios at the tower site on the same channel, rather use 2 different channels and dtdlink the 2 nodes together.    Also, avoid this in the shack -- don't put a radio on the roof pointing to the tower, and another radio in the office-shack, both on the same channel.

Joe AE6XE