Wireless LANs (WLANs), based on the IEEE 802.11 standard, have become
the standard means for indoor wireless connectivity. At the same time, the rising
number of smart mobile devices, broadband access speeds, and bandwidth hungry
applications (e.g., high definition video streaming) have led to an increase not only
of usage but also of demand for higher data-rates. This demand for higher rates
is being met with newer IEEE 802.11 standards (e.g., 802.11n/ac) that introduce
new features and also increase the different possible settings for each feature.
Inherent channel variations and the possible interference conditions when operating
in unlicensed spectrum necessitate adaptation of the various medium access
control (MAC) and physical (PHY) layer features to ensure high performance.
Selecting the values of those features to optimise a criterion such as
throughput is the link adaptation problem. Link adaptation, the focus of this thesis,
can play a key role in improving the performance of 802.11 WLANs. Increasing
number of features and feature setting combinations with newer 802.11 standards
is not only making link adaptation even more important but also more challenging.
The contributions made in this thesis significantly advance the state of the art
on link adaptation for 802.11 WLANs along three dimensions. First, we show
that not knowing the exact cause of loss is not an impediment to effective link
adaptation. Nevertheless, actions taken in response to losses are more crucial and
they ought to be holistic and not solely dependent on the exact cause of loss. Second,
we make significant methodological contributions for analysing the impact
of multiple parameters on a given criterion, based on comprehensive experimental
measurements. The application of this methodology on 802.11n measurements,
examining the interaction of the protocols various parameters on performance under
varying conditions, has lead to several valuable findings on how to perform
efficient link adaptation in a complex WLAN scenario like 802.11n and future
802.11 standards. Adaptation should be holistic, based on the channel quality instead
of the interference scenario, and independent of loss differentiation. Based
on these insights, lastly and most importantly, we propose two novel holistic link
adaptation schemes for legacy 802.11a/b/g and 802.11n WLANs, termed Themis
and SampleLite, respectively. Both Themis and SampleLite take a hybrid approach
relying on easily accessed channel quality information at the sender side
to perform holistic adaptation. The hypothesis that adaptation should be holistic
is validated by our results, with both Themis and SampleLite outperforming the
current state of the art
