Major features of 802.11n
PHYdata rates in 802.11n are significantly improved over 802.11a and 802.11g primarily
through the use of spatial multiplexing using MIMO and 40MHz operation. To take
advantage of the much higher data rates provided by these techniques, MAC efficiency
is also improved through the use of frame aggregation and enhancements to the block
acknowledgment protocol. These features together provide the bulk of the throughput
enhancement over that achievable with 802.11a and 802.11g.
Robustness is improved inherently through the increased spatial diversity provided
by the use of multiple antennas. Space-time block coding (STBC) as an option in
the PHY further improves robustness, as does fast link adaptation, a mechanism for
rapidly tracking changing channel conditions. More robust channel codes are adopted
in the form of low density parity check (LDPC) codes. The standard amendment also
introduces transmit beamforming, with both PHY and MAC enhancements to further
improve robustness.
A number of other enhancements provide further gains. In the PHY, these include
a shorter guard interval, which may be used under certain channel conditions. The
PHY also includes a Greenfield preamble, which is shorter than the mandatory mixed
format preamble. However, unlike the mixed format, it is not backward compatible
with existing 802.11a and 802.11g devices without MAC protection. In the MAC,
the reverse direction protocol provides a performance improvement for certain traffic
patterns, by allowing a station to sublease the otherwise unused portion of its
allocated transmit opportunity to its remote peer and thus reducing overall channel
access overhead. A reduced interframe space (RIFS) used when transmitting a
burst of frames reduces overhead in comparison to the existing short interframe space
(SIFS).
An overview of the mandatory and optional features of the 802.11n PHY is given in
Figure 1.8. At the time this book went to press, two generations of so called pre-n or
draft 2.0 products have been released. The first generation of products typically operate
in the 2.4 GHz band only, with up to two spatial streams and 40 MHz channel width.
In this book the term spatial streams is used to refer to one or more independent data
streams transmitted from the antennas. A device requires at least as many antennas as
spatial streams. When using the short guard interval, these initial products are able to
achieve a PHY data rate of 300 Mbps. With second generation products, we begin to
see dual-band 2.4 GHz and 5 GHz products. These products also achieve 300 Mbps, but
several incorporate an extra receive antenna chain for additional receive diversity. Some
products also support the Greenfield preamble format. We expect that third generation
devices will add another transmit antenna chain to support three spatial streams and
450 Mbps. For robustness, these devices may begin employing STBC and transmit
beamforming (TxBF).
An overview of the features added to the MAC in 802.11n is given in Figure 1.9.
In addition to the throughput and robustness enhancing features already mentioned, the
MAC is extended in a number of other areas.
The numerous optional features in 802.11n mean that extensive signaling of device
capability is required to ensure coexistence and interoperability. For example, whether
a device supports certain PHY features such as the Greenfield format preamble or MAC
features such as the ability to participate in a reverse direction protocol exchange.
The existence of 40 MHz operation also creates a number of coexistence issues. The
AP needs to manage the 40 MHz BSS so that 40 MHz and 20 MHz devices, both
legacy and high throughput, are able to associate with the BSS and operate. Because
40 MHz operation uses two 20 MHz channels, mechanisms are needed to mitigate the
effect this might have on neighboring 20 MHz BSSs operating independently on those
two channels. Coexistence is primarily achieved through careful channel selection, i.e.
choosing a pair of channels that have little or no active neighborhood traffic. To this end,
the amendment adds scanning requirements to detect the presence of active neighborhood
BSSs as well as the ability to actively move the BSS to another pair of channels should a
neighboring 20 MHz BSS become active. If neighboring BSSs cannot be avoided then a
fallback technique called phased coexistence operation (PCO) may be used. This allows
the BSS to alternate between 20 MHz and 40 MHz phases of operation, with the 40
MHz phase entered after a frame exchange on the two 20 MHz channels has silenced
devices operating there.
Finally, in recognition of the growing importance of handheld devices, a channel
access scheduling technique called power-save multi-poll (PSMP) has been added to
efficiently support a large number of stations.
发表于 2011-6-23 16:09:26
