CECS 572
Wednesday, 12th Dec. 2012
Dr. Tracy Bradley Maples
)
Wireless LANs going Gigabit – Overview of IEEE 802.11ac
To meet the everyone's growing need for high speed networks, the IEEE has been working for
Quite some time now on a new set of enhancements to achieve very high throughput speeds for
the operations which lie inside of the bands below 6 GHz, and IEEE has given these protocols
names as the 802.11ac and 802.11ad Wi-Fi standards. These new IEEE standards for the wireless
protocols promises with the additional bandwidth gains they are going to support. The
networking industry is looking this as the next big thing to happen to the wireless standards and
speculating this is going to result in increase of current bandwidth standards; raising the current
possible bandwidth levels for a wireless connection to the wired connection levels.
We all know that today's wired connections already run at 1 Gb/sec levels, now its the
turn of the wireless connections to give the end user similar experience. Both the proposed new
standards will make amendments to the existing wireless networking protocols and will push the
network throughput beyond the gigabit rates. The 802.11ac standard will add a multi-user access
techniQues which is in the form of "downlink multi-user (DL MU) multiple input multiple output
(MIMO)"[1] and the 80MHz as well as the 160MHz channels in the band of 5GHz which is going
to be helpful as well as compatible with the applications which rely on heavy bandwidth along
with simultaneous content streaming such as multiple video playback inside a single home. And
the 802.11ad will be taking the advantages of using the large unused portion of the 60GHz
spectrum and it will be defining the protocols which are going to enable or help the application
which are very much throughput intensive for example the wireless input/output or the
uncompressed videos. The new waveforms for the freQuency band of 60Ghz will include the
single carrier as well as the "orthogonal freQuency division multiplex (OFDM)"[1]. The
enhancements apart from the new 60GHz PHY include the Personal Basic Service Set (PBSS)
operations, then we also have the directional medium access and the beamforming[1].
The 802.11n has lot of new features such as the MIMO, 40MHZ as well as the
aggregation of the packets which in turn significantly increase the data rates over the other
existing protocol standards such as the 802.11b, 802.11g and the 802.11a. The 802.11ac is
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considered to be an evolution of the 802.11n, and 802.11ac adds the 80MHz, 160MHz as well as
the non-contiguous 160MHz (80 + 80MHz) channels. There is another important enhancement in
which the modulation constellation size has been considerably increased from 64 QAM to 256
QAM. And also the number of stream which are placed spatially has been increased to 8 in order
to support the feature DL MU-MIMO in a more compatible manner. There is a increase in the
size limits for the packet aggregation which are prevailing in the current wireless networking
standards so as to achieve a considerably higher data rates. More enhancements are there which
are modifying the mechanisms for co-existence helping to gain wider channel bandwidths. Few
of the existing features in the 802.11n are being further improved in the 802.11ac standard so that
the devices with this new standard can simplify the implementation of the future standards in a
streamlined manner. Currently, an important interoperability issue between the different
companies which are fabricating the devices exists because of the "transmit beamforming" in the
present 802.11n standard. To improve on this, in the 802.11ac, the "transmit beamforming" is
being restricted only to the explicit feedback mechanism and is being skipped for the implicit
feedback which causes the interoperability. Also, in the 802.11ac the feedback type is
"compressed-V feedback" only and no uncompressed-V or no CSI type feedbacks will be
supported. Another enhancement to improve on the "transmit beamforming" in 802.11ac is that
now the channel sounding is being limited to no "staggered sounding (NDP)"[1][2]. There is a
change in the "modulation/coding schemes (MCS)", now they are restricted only to the same
MCS on each of the streams available.
For channelization, 802.11n has two channels, that is 20 and 40MHz bands. In this the
40MHz band comprises of two contiguous bands of 20MHz. Also, the 802.11n does not have
any 20MHz as well as the 40MHz channels which have incomplete convergence, as the
overlapping convergence in the channels can introduce considerable amount of in-band
interference, now for this there is a further need of defining complex coexistence schemes to
sustain during such interoperability issues. In order to prevent these scenarios resulting in
overlapping band interference issues and also to make it simpler for protocol setup, 802.11ac
also uses the same ideology of the 802.11n, by which it will not allow allocation of overlapping
channels. So, in similar lines, there will be no adjacent 40MHz channel to another channel inside
of the 80MHz channel. Also, there is a new channel 144 which is being added to the 802.11ac
protocol standard, and this was not present in case of the older 802.11n standard. By adding this
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new channel, now there is a possibility of more number of 80MHz channels which can be
allocated next to each other and maximum of 6 consecutive channels allocation is possible
considering the governing body authorization on this. In this standard there are only 2 of
available 160MHz channels; because of this there can be only one non-breaking operation
possible. This non-breaking 160MHz channel can have two 80MHz channels which are adjacent
to each other. Further, if the 160MHz channels are non-contiguous, then there can be an
allocation of 2 80MHz channels which are not next to each other. So, with this there can be many
non-breaking operations over the 160MHz channels which can be further split into two 80MHz
channels. The 802.11ac PHY implementation is almost similar to 802.11n. And similar to the
802.11n, all of the fields in the packet will consist of 20MHz waveform and same pattern is used
in all of the consecutive sub-channel which are used up for providing the large sized channel
bandwidths. After the preamble the next field will be used for sending the actual data. In
802.11ac, the field which holds section SERVICE in the starting of the data field is altered to
have additional information of CRC for the VHT-SIG-B segment of the packets. Also, for the
PHY implementation part, the padding is added before the data section which comes after the
preamble section. This is in contrast to the 802.11n, in which the pad bits are added before the
tail bits are added; this is done so as to add MU which further results in the increase of the
number of bytes which are sent for each packet which further results in the need to communicate
the packet length separately. Additionally, the data which sent inside the packet is scrambled and
is further encrypted for adding more security to the transmitted data. This is further extended by
the "constellation mapper" and the "spatial mapper". For the 802.11ac the 80Mhz section
follows the logic of "256 point FFT". There are many sub-carriers which are termed as data,
pilot, null and of these sub-carriers 3 are at DC. It also stated that the number of the sub-carriers
in the 80MHz waveform will be two times as compared to the sub-carriers in case of the 40MHz
waveform, because of which the data rate for 80MHz is also more than two times than 40MHz.
Also, the implementation of the 160MHz sub-carrier section is almost similar to two 80MHz
segments. This way it allows designing of the sub-carrier section in a similar manner for a
non-breaking 160MHz section and for the breaking 160MHz waveform. Further, the period of
the local oscillator is also unnecessary in order for it to be in co-relation with the upper and the
lower sections of the transmitted waveform in case of the non-breaking 160MHz and the
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breaking 160MHz spectrum. This enables the 802.11ac standard to have more in common with
the non-breaking and breaking 160MHz spectrum.
802.11ac primarily changes the 802.11n MAC to its corresponding address structure and
provides the medium access with the wider channels[1][2]. Additionally, there are many small
changes implemented from the 802.11n techniQues which helps in doing the accumulation of the
data packets in order to make the channel more efficient when the data will be transmitted at the
Gbps speeds. Further, with the large number of channels which are available of 20MHz, 40MHz
inside the 5GHz spectrum in the 802.11n, it in turns overlaps the waveforms which lie between
BSS's which is made easier to avoid now by using any of the available channels differing in the
freQuency. In the worst scenario, there is a chance of overlapping among the adjacent waveforms
when 40MHz is being used as well as cannot be avoided., then the another 20MHz sub-channel
can be chosen in order to match to have more inter-operable capabilities. And by using these
channels which are having a wide spectrum in the 802.11n standard it is found to be difficult for
the channel to avoid any possible overlapping among the adjacent BSS's. Overall, it is difficult
now to find a primary channel which can overlap all of the other possible networks. To
circumvent these problems in the 802.11n, in the 802.11ac standard, there is drastic improvement
in the channel which are operating adjacent by implementing some of the enhancements, such as
using the "enhanced secondary channel Clear Channel Assessment (CCA)"[1], improving the
overall operation inside the entire channel's spectrum dynamically, and further helping in
creating a new mode of operation for the notifying the frames. Also, in the 802.11 the CCA
techniQues are used which helps in identifying the signals which are also present in the spectrum
or adjacent spectrum and in a way can further reduce the possibility of having in overlapping by
deferring the transmit signal.
Another important reQuirement for a device which is working on the basis of the OFDM,
is to check whether there is a correct level in the -82dBm signal. Also, it needs to check if there
is different signal present of -62dBm level, which is named as the "Energy Detect (ED)" signal.
802.11n also included the 40MHz channel later on, which can be further split into two 20MHz
channels of them one can be the main and the other can act as the second level of the 20MHz
waveform, then in this case there is only the need of the ED because this already adds a lot of
complexity to detect a valid 802.11 signaling message which could be present on the second
level channel of 20MHz. This further means that if there are some more devices which have
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taken up the second level channel of lets say another 40MHz BSS, then there is going to be a loss
of 20 db. In 802.11ac, this has been improved by detecting a valid signal over the second level
channels which are added at a completely different signal levels of -72dBm or -69dBm as per the
corresponding bandwidths in order to gain more CCA efficiency on the second level channels[1].
Additionally, it is further needed for all of the eQuipments to look for a valid packet on the
second level channel and should simply not rely onto the packet's preamble, and it should also
look inside of the packet.
Another important change made to the 802.11n while implementing the 802.11ac is that
the "ReQuest-to-send (RTS)" as well as the "Clear-to-Send (CTS)" techniQues of the basic 802.11
protocol standard is changed in order to gain the benefits of the wider spectrum operations by
dynamically using the spectrum completely[1].
To conclude, it is evident that the products using the new 802.11 technologies will be
flocking the retail market soon and will be focusing on addressing the areas which demand
higher bandwidth as well as the higher throughput rates. 802.11ac is going to evolve more in the
spectrum of 5GHz as new wider channels are being supported by these wireless network
standards. These will give more room to the multiple user environment we are moving towards
and will help in easing the areas of higher bandwidth reQuirements such high definition video
streaming in a home. The 802.11ad protocol will be helping easing the pressure currently
prevailing in the personal area networks of 802.11 domain by enabling techniQues such as
wireless docking with couple of Gbps network linking catering to the large amount of data
reQuired by these, and this will be achieved by utilizing the currently unlicensed 60MHz
spectrum available across the world. It is also expected that by late 2014 or early 2015, products
aligned with the final 11ac standard will nudge the Wi-Fi speed limit closer to 7 Gbps. For IT,
11ac promises room for growth by focusing on less-cluttered 5-GHz channels, doubling or
Quadrupling channel widths, using more efficient encoding, doubling maximum spatial streams
and eventually letting wireless access points service multiple clients simultaneously. And further
there is a hope that the Wi-Fi certified products which are going to be based on a draft of these
new standards 802.11ad as well as the 802.11ac will most likely emerge by mid-2013 (or by the
start of 2014), which is going to give a boost to the maximum data rates prevailing in today's
wireless networks and this is going to be above the 1 Gbps level.
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