Digital Signal Processing Reference
In-Depth Information
between 20 and 40 MHz, which can be considered to be another new flexibility
option. To increase the spectral efficiency even further, also at MAC level, several
optimization features have been added, such as frame aggregation or block acknowl-
edgments. Also, to improve the efficiency of the physical layer, a short guard inter-
val option is included, as well as advanced coding such as code rates up to
5
6
or the
option to use low density parity check (LDPC) coding.
2.2.1.3 The IEEE 802.11ac Physical Layer
The IEEE802.11ac Task Group has recently been formed within the IEEE802.11
Working Group. Its goal is to draft a standard for “Very High Throughput” WLANs,
with performances significantly higher than those of the IEEE802.11n standard. Be-
cause the task group TGac has just started its activity, the standard is not yet avail-
able. TGac is currently drafting the detailed functional requirements. The trend of
802.11n towards more flexibility in the MIMO options, channel bonding and more
spectrally efficient physical layer options will however persist in the newest stan-
dard. More specifically, bandwidths up to 80 or 160 MHz will be supported. Also,
advanced multi-stream operation supporting up to 8 streams that can be destined for
multiple users is included in the specs. Finally, constellations up to 256 QAM are
being discussed.
2.2.1.4 Multiple Access Through Collision Avoidance with Carrier Sensing
We introduce the terminology and communication principles that are standardized
in the 802.11 Medium Access Control (MAC) baseline protocol description. Ac-
cording to the 802.11 standard, wireless networks can be arbitrarily complex, and
consist of a hierarchy of basic communication networks. A basic wireless com-
munication network is referred to as a
Basic Service Set
(BSS) which is a set of
stations controlled by a single
Coordination Function
(CF). The CF manages the ac-
cess to the radio channel. The
Distributed Coordination Function
(DCF) is manda-
tory for each BSS and is described first. It standardizes a technique for distributed
contention-based channel access. The
Point Coordination Function
(PCF) for cen-
tralized contention-free channel access is optional. The PCF operation relies on the
DCF. To improve the QoS that can be achieved on top of 802.11 WLAN systems, the
Hybrid Coordination Function
(HCF) has been introduced to improve the baseline
DCF and PCF operations.
Further, a BSS can operate in Infrastructure mode (also referred to as regular
BSS), with an Access Point (AP) and multiple Stations (STAs). In this scenario,
the AP works as a bridge and every transmission is between AP and STA(s). Next,
the STAs can operate in Ad-Hoc mode, in which case the BSS is referred to as
Independent BSS (IBSS). In this scenario, there are multiple STAs, and no AP. Each
STA can be a router to connect the ad-hoc network to the wireline network.
The mandatory 802.11 DCF relies on a Carrier Sense Multiple Access with Col-
lision Avoidance (CSMA/CA) mechanism. When a packet (i.e., MAC frame) is to
