05-07-2013, 04:49 PM
Coding in 802.11 WLANs
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Abstract
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.
Introduction
Forward error correction (FEC) coding [15, 41] is widely used in communication systems to correct
transmission errors. The idea is that the sender protects the information message by adding redun-
dancy. The redundancy allows the receiver to detect, and often correct, a limited number of errors.
In IEEE 802.11a/g transmitters, convolutional codes [41] are used for FEC at the physical 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.
However, in a network the potential exists to use coding 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. This is the focus of the
present thesis. Rather than adopting a clean slate approach, we consider 802.11 WLANs, which are
now ubiquitous, and investigate layering additional coding on top of the existing 802.11 physical layer
coding. Of course this causes a performance cost compared to a clean slate design, but it has the
compelling advantage of potentially being useful to the large number of users of the existing 802.11
devices.
IEEE 802.11 WLAN
A Wireless Local Area Network (WLAN) is a computer network that uses a wireless communi-
cation method to connect computers and devices in a limited geographical area, such as home, school,
computer laboratory or o±ce building. It typically extends an existing wired Local Area Network
(LAN) by attaching a device, called the access point (AP), to the edge of the wired network. Users
communicate with the AP using a wireless network adapter similar in function to a traditional Eth-
ernet adapter. This gives users the mobility to move around within a local coverage area and still be
connected to the network. Therefore, WLAN often provides the last mile wireless access to the wired
network [19].
The 802.11 MAC
The 802.11 MAC de¯nes two di®erent access mechanisms, the mandatory Distributed Coordination
Function (DCF) which provides distributed channel access based on Carrier Sense Multiple Access
with Collision Avoidance (CSMA/CA), and the optional Point Coordination Function (PCF) which
provides centrally controlled channel access through polling.
Distributed Coordination Function
The DCF is the basic access mechanism of IEEE 802.11. In the DCF, all stations contend for access
to the medium in a distributed manner, based on the CSMA/CA access mechanism. DCF is hence
referred to as contention-based channel access.
CSMA works in a listen-before-talk fashion. Before transmitting, a station ¯rst listens (by carrier
sensing) whether the radio link is clear. If the medium is sensed idle for at least a period of DCF
Inter-Frame Space (DIFS), the station starts transmitting; meanwhile, all other stations which intend
to transmit during this period have to wait until the medium becomes idle again for a DIFS period. If
the destination station successfully receives a frame, it acknowledges by sending back an ACK frame
after a period of Short Inter-Frame Space (SIFS). Fig. 1.1 illustrates the procedure.
Contributions
This thesis is concerned with the application of coding in 802.11 WLANs. We consider the joint
performance of higher-layer coding concatenated with 802.11 PHY layer modulation/coding in two
channel paradigms i.e. packet erasure channel (PEC) and binary symmetric channel (BSC). In the
BSC paradigm, we further derive the proportional fair allocation of higher-layer coding rates and
airtimes in an 802.11 WLAN.
In Chapter 2 we consider the joint multicast performance of higher-layer fountain coding concate-
nated with 802.11a/g OFDM PHY modulation/coding. We are interested in the cross-layer trade-o®s
between fountain coding and PHY layer modulation and coding rate selection. A detailed study on
the optimal choice of PHY modulation/coding rates with and without higher layer fountain coding
is carried out for standard 802.11 WLANs. Optimality is considered both in terms of maximising
goodput and minimising energy. 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 and PHY layer rate control can be carried out without regard to the use of fountain coding at
higher layers.