Ratecontrol 1 Mbps

[Matteo]

Hi all,
I created an IBSS network with 5 nodes.
A couple of nodes are Android tablet Nexus 7 (2012) with kernel version 3.2.x where I plugged a TPlink-722N so ath9k_htc driver. The remaining nodes have ath5k driver.
I modified the firmware in a couple of points to report the bitrate into ath9k_htc/tgt_tx_status file.
Sometimes I noticed  that during the data transmission the bitrate stay fixed to1 Mbps and I don't know why. I thought that the common behavior was to try different bitrate. 
Instead sometimes I can look different bitrate during the data transmission.
Do I need update the driver? Is it common with IBSS mode?

Thanks a lot for the help


Matteo