6 research outputs found
Frames in Outdoor 802.11 WLANs Provide a Hybrid Binary-Symmetric/Packet-Erasure Channel
Corrupted frames with CRC errors potentially provide
a useful channel through which information can be transmitted.
Using measurements taken in an outdoor environment, it is
demonstrated that for 802.11 wireless links the channel provided
by corrupted frames alone (i.e. ignoring frames with PHY errors
and frames received correctly) can be accurately modelled as a
binary symmetric channel (BSC) provided appropriate pre- and
post- processing is carried out. Also, the channel provided by
corrupted frames and other frames combined can be accurately
modelled as a hybrid binary-symmetric/packet-erasure channel.
Importantly, it is found that this hybrid channel offers capacity
increases of more than 100% compared to a conventional packet
erasure channel over a wide range of RSSIs. This indicates that
the potential exists for significant network throughput gains if the
information contained in 802.11 corrupted packets is exploited
Frames in Outdoor 802.11 WLANs Provide a Hybrid Binary-Symmetric/Packet-Erasure Channel
Corrupted frames with CRC errors potentially provide
a useful channel through which information can be transmitted.
Using measurements taken in an outdoor environment, it is
demonstrated that for 802.11 wireless links the channel provided
by corrupted frames alone (i.e. ignoring frames with PHY errors
and frames received correctly) can be accurately modelled as a
binary symmetric channel (BSC) provided appropriate pre- and
post- processing is carried out. Also, the channel provided by
corrupted frames and other frames combined can be accurately
modelled as a hybrid binary-symmetric/packet-erasure channel.
Importantly, it is found that this hybrid channel offers capacity
increases of more than 100% compared to a conventional packet
erasure channel over a wide range of RSSIs. This indicates that
the potential exists for significant network throughput gains if the
information contained in 802.11 corrupted packets is exploited
All bits are not equal – A study of IEEE 802.11 communication bit errors
techniques such as acknowledgement, retransmission, and transmission rate adaptation, are frame-level mechanisms designed for combating transmission errors. Recently sub-frame level mechanisms such as frame combining have been proposed by the research community. In this paper, we present results obtained from our bit error study for identifying sub-frame error patterns because we believe that identifiable bit error patterns can potentially introduce new opportunities in channel coding, network coding, forward error correction (FEC), and frame combining mechanisms. We have constructed a number of IEEE 802.11 wireless LAN testbeds and conducted extensive experiments to study the characteristics of bit errors and their location distribution. Conventional wisdom dictates that bit error probability is the result of channel condition and ought to follow corresponding distribution. However our measurement results identify three repeatable bit error patterns that are not induced by channel conditions. We have verified that such error patterns are present in WLAN transmissions in different physical environments and across different wireless LAN hardware platforms. We also discuss our current hypotheses for the reasons behind these bit error probability patterns and how identifying these patterns may help improving WLAN transmission robustness. Index Terms—Sub-frame bit errors; bit error patterns; measurement study; calibration; IEEE 802.11. I
Coding in 802.11 WLANs
Forward error correction (FEC) coding is widely used in communication systems to correct transmis-
sion errors. In IEEE 802.11a/g transmitters, convolutional codes are used for FEC at the physical
(PHY) layer. As is typical in wireless systems, only a limited choice of pre-speci¯ed coding rates is
supported. These are implemented in hardware and thus di±cult to change, and the coding rates are
selected with point to point operation in mind.
This thesis is concerned with using FEC coding in 802.11 WLANs in more interesting ways that are
better aligned with application requirements. For example, coding to support multicast tra±c rather
than simple point to point tra±c; coding that is cognisant of the multiuser nature of the wireless
channel; and coding which takes account of delay requirements as well as losses. We consider layering
additional coding on top of the existing 802.11 PHY layer coding, and investigate the tradeo® between
higher layer coding and PHY layer modulation and FEC coding as well as MAC layer scheduling.
Firstly we consider the joint multicast performance of higher-layer fountain coding concatenated
with 802.11a/g OFDM PHY modulation/coding. A study on the optimal choice of PHY rates with and
without fountain coding is carried out for standard 802.11 WLANs. We ¯nd that, in contrast to studies
in cellular networks, in 802.11a/g WLANs the PHY rate that optimizes uncoded multicast performance
is also close to optimal for fountain-coded multicast tra±c. This indicates that in 802.11a/g WLANs
cross-layer rate control for higher-layer fountain coding concatenated with physical layer modulation
and FEC would bring few bene¯ts.
Secondly, using experimental measurements taken in an outdoor environment, we model the chan-
nel provided by outdoor 802.11 links as a hybrid binary symmetric/packet erasure channel. This
hybrid channel o®ers capacity increases of more than 100% compared to a conventional packet erasure
channel (PEC) over a wide range of RSSIs. Based upon the established channel model, we further
consider the potential performance gains of adopting a binary symmetric channel (BSC) paradigm for
multi-destination aggregations in 802.11 WLANs. We consider two BSC-based higher-layer coding
approaches, i.e. superposition coding and a simpler time-sharing coding, for multi-destination aggre-
gated packets. The performance results for both unicast and multicast tra±c, taking account of MAC
layer overheads, demonstrate that increases in network throughput of more than 100% are possible
over a wide range of channel conditions, and that the simpler time-sharing approach yields most of
these gains and have minor loss of performance.
Finally, we consider the proportional fair allocation of high-layer coding rates and airtimes in 802.11
WLANs, taking link losses and delay constraints into account. We ¯nd that a layered approach of
separating MAC scheduling and higher-layer coding rate selection is optimal. The proportional fair
coding rate and airtime allocation (i) assigns equal total airtime (i.e. airtime including both successful
and failed transmissions) to every station in a WLAN, (ii) the station airtimes sum to unity (ensuring
operation at the rate region boundary), and (iii) the optimal coding rate is selected to maximise
goodput (treating packets decoded after the delay deadline as losses)