IEEE 802.11ac -- BRIEF INTRO

IEEE 802.11ac
CHAPTER 1

INTRODUCTION
1.1 WLANs in Transition
The appetite for wireless bandwidth is seemingly insatiable. Fifteen years ago, the first standard
wireless LANs emerged at 1Mbps and 2Mbps to serve niche applications such as warehouse
picking, inventory scanning, in office buildings where mobility wasn’t a requirement and
cordless PC connections aimed at lowering cabling costs. Fast forward through several WLAN
generations to today, and the story has completely changed.
IEEE 802.11ac is the fifth generation in Wi-Fi networking standards and will bring fast, high
quality video streaming and nearly instantaneous data syncing and backup to the notebooks,
tablets, and mobile phones that have become our everyday companions. Improvements in
transmission speeds will be dramatic. 802.11ac is the emerging standard from the IEEE. It takes
something great and makes it even better. 802.11ac is a faster and more scalable version of
802.11n. 802.11ac couples the freedom of wireless with the capabilities of Gigabit Ethernet.
Wireless LAN sites will see significant improvements in the number of clients supported by an
access point (AP), a better experience for each client, and more available bandwidth for a higher
number of parallel video streams. Even when the network is not fully loaded, users see a benefit:
their file downloads and email sync happen at low lag gigabit speeds. Also, device battery life is
extended, since the device’s Wi-Fi interface can wake up, exchange data with its AP, then revert
to dozing that much more quickly.
802.11ac achieves its raw speed increase by pushing on three different dimensions:
 More channel bonding, increased from the maximum of 40 MHz in 802.11n, and now up
to 80 or even 160 MHz (for 117% or 333% speed-ups, respectively)
 Denser modulation, now using 256 quadrature amplitude modulation (QAM), up from
802.11n’s 64QAM (for a 33% speed burst at shorter, yet still usable, ranges)

DEPT OF TCE, T. JOHN INSTITUTE OF TECHNOLOGY

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IEEE 802.11ac
 More multiple input, multiple output (MIMO). Whereas 802.11n stopped at four spatial
streams, 802.11ac goes all the way to eight (for another 100% speed-up).
The design constraints and economics that kept 802.11n products at one, two, or three spatial
streams haven’t changed much for 802.11ac, so we can expect the same kind of product
availability, with first-wave 802.11ac products built around 80 MHz and delivering up to 433
Mbps (low end), 867 Mbps (mid tier), or 1300 Mbps (high end) at the physical layer. Secondgeneration products promise still more channel bonding and spatial streams, with plausible
product configurations operating at up to 3.47 Gbps.
802.11ac is a 5 GHz-only technology, so dual-band APs and clients will continue to use 802.11n
at 2.4 GHz. However, 802.11ac clients operate in the less crowded 5 GHz band.
Second-generation products should also come with a new technology, multiuser MIMO (MUMIMO). Whereas 802.11n is like an Ethernet hub that can only transfer a single frame at a time
to all its ports, MU-MIMO allows an AP to send multiple frames to multiple clients at the same
time over the same frequency spectrum. That’s right: with multiple antennas and smarts, an AP
can behave like a wireless switch. There are technical constraints, and so MU-MIMO is
particularly well suited to bring-your-own-device (BYOD) situations where the devices such as
smart phones and tablets might only have a single antenna.
802.11ac-enabled products are the culmination of efforts at the IEEE and Wi-Fi Alliance
pipelines. IEEE 802.11ac delivered an approved Draft 2.0 amendment in January 2012 and a
refined Draft 3.0 in May 2012, with final ratification planned for the end of 2013. In parallel, the
Wi-Fi Alliance is expected to take an early IEEE draft, most likely Draft 3.0, and use that as the
baseline for an interoperability certification of first-wave products in 2013. Later, and more in
line with the ratification date of 802.11ac (that is, after December 2013), the Wi-Fi Alliance is
expected to refresh its 802.11ac certification to include testing of the more advanced 802.11ac
features. This second-wave certification should include features such as channel bonding up to
160 MHz, four spatial streams, and MU-MIMO. Overall, this arrangement closely follows how
802.11n was rolled out.


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IEEE 802.11ac
To achieve its goals, 802.11ac relies on a number of improvements in both the MAC and
Physical Layer (PHY).
The PHY improvements include:
 Increased bandwidth per channel
 Increased number of spatial streams
 Higher‐order modulation ‐‐ 256 Quadrature Amplitude Modulation (QAM)
 Multi‐User Multiple Input Multiple Output (MU‐MIMO)
In addition to these new PHY features, 802.11ac also supports a number of advanced digital
communication concepts that were first introduced in 802.11n, such as space division
multiplexing, Low‐Density Parity Check (LDPC) coding, shortened guard interval (short GI),
Space‐Time Block Coding (STBC), and explicit feedback transmit beamforming (Tx BF).
The Media Access Control (MAC) layer includes many of the improvements that were first
introduced with 802.11n. One notable enhancement is the larger maximum size of aggregate
MAC Protocol Data Units (MPDUs). Also, the Request to Send/Clear to Send (RTS/CTS)
mechanism has been enhanced to allow more efficient implementation of dynamic bandwidth
operation.

1.2 What is 802.11ac?
802.11ac is an evolutionary improvement to 802.11n. One of the goals of 802.11ac is to deliver
higher levels of performance that are commensurate with Gigabit Ethernet networking:
 Seemingly “instantaneous” data transfer experience.
 A pipe fat enough that delivering high quality of experience (QoE) is straightforward.
In the consumer space, the target is multiple channels of high-definition content delivered to all
areas of the house. The enterprise has different challenges:
 Delivering network with enterprise-class speeds and latencies.


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IEEE 802.11ac
 High-density environments with scores of clients per AP which are exacerbated by the
BYOD trend such that one employee might carry two or even three 802.11 devices and
have them consuming network resources all at once.
 The increased adoption of video streaming.
802.11ac is about delivering an outstanding experience to each and every client served by an AP,
even under demanding loads.
Meanwhile 802.11 is integral to hugely broad range of devices, and some of them are highly
cost, power, or volume constrained. One antenna is routine for these devices, yet 802.11ac must
still deliver peak efficiency. The one thing that 802.11ac has in its favor is the evolutionary
improvement to silicon technology over the past half-dozen years: channel bandwidths can be
wider, constellations can be denser, and APs can integrate more functionality.

Figure 1.1: How 802.11ac Accelerates 802.11n


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IEEE 802.11ac
Table 1: Feature Enhancement Comparison: 802.11n & 802.11ac
802.11n

802.11ac

Frequency Band

2.4 GHz & 5 GHz

5 GHz

Channel Widths

20, 40 MHz

Spatial Streams

1 to 4

1 to 8, total up to 4 per client

No

Yes

150 Mbps

450 Mbps

450 Mbps

1.3 Gbps

Multi User MIMO
Single Stream [1x1]
Maximum Client Data Rate
Three Stream [3x3]
Maximum Client Data Rate

1.3

20, 40, 80 MHz
160 MHz optional

What's in a Name?

Sometimes, nothing at all. The “ac” in “IEEE 802.11ac” doesn't really stand for anything. In fact,
the standard got its name just by standing in line. Wi-Fi standards are developed by scores of
electronics companies working together under the auspices of the Institute of Electrical and
Electronics Engineers (IEEE) in an ongoing project called IEEE 802.11. (Insiders pronounce that
“eight-oh-two-dot-eleven.”) Each technical paper released by the group is given a letter suffix;
the one setting forth the specifications for IEEE 802.11g came out in 2003, followed by IEEE
802.11n in 2007. After reaching “z,” the papers started over with “aa.” Most of the papers
between IEEE 802.11n and IEEE 802.11ac involved intricate technical matters, rather than a new
networking standard meant for widespread use.


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IEEE 802.11ac
CHAPTER 2

OVERVIEW OF 802.11ac FEATURES
2.1 Introduction
802.11ac shares many features with 802.11n. Advanced coding (LDPC) is used for increased
coding gain. STBC technology can be used for transmitter diversity. A number of other features
provide various levels of performance improvement, such as explicit feedback Tx BF, and short
GI. These features are essentially identical in 802.11ac and 802.11n. Below we will discuss and
evaluate features that truly distinguish 802.11ac from 802.11n.

2.2 Modulation enhancements
Like most recent wireless specification, 802.11ac uses Orthogonal Frequency‐Division
Multiplexing (OFDM) to modulate bits for transmission over the wireless medium. While the
modulation approach is identical to that used in 802.11n, 802.11ac optionally allows the use of
256 QAM in addition to the mandatory Quadrature Phase Shift Keying (QPSK), Binary PSK
(BPSK), 16 QAM and 64 QAM modulations. 256 QAM increases the number of bits per
sub‐carrier from 6 to 8, resulting in a 33% increase in PHY rate under the right conditions. It
should be noted however that 256 QAM can only be used in high signal‐to‐noise ratio (SNR)
scenarios (across the used spectrum and desired streams); i.e. for very favorable channel
conditions. The support of 256 QAM will increase the maximum PHY rate that can be supported
by the system, but will have no effect in typical scenarios and will not lead to any reach increase
for the service. Also, supporting 256 QAM requires transmitter and receiver to be designed such
that the inherent SNR (transmit and receive Error Vector Magnitude, or EVM) of the system is
able to accommodate the higher constellation.
This will make the RF design of a system that supports 256 QAM more challenging. Unlike
802.11n, 802.11ac does not support the use of unequal modulation (UEQM). This means that all
streams in a multi‐stream transmission have to be modulated with the same constellation size.
UEQM, by contrast, enables the system to modulate weaker streams with lower modulations,


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IEEE 802.11ac
which allows for more fine‐grained optimization of the data rate to a particular channel
environment. This may be important for higher numbers of streams, especially in combination
with beam forming.

2.3 Increased bandwidth
The most notable feature of 802.11ac is the extended bandwidth of the wireless channels.
802.11ac mandates support of 20, 40 and 80 MHz channels (versus 20 and 40 MHz in 802.11n).
Optionally, the use of contiguous 160 MHz channels or non‐contiguous 80+80 MHz channels is
also allowed. The doubling of the channel bandwidth (from 40 to 80 MHz) is a very efficient
way to increase performance in a cost efficient way. Alternatively, an 80 MHz system can use a
lower number of antennas to provide the same performance as a 40 MHz system. However, this
approach should be weighed against other spectrally efficient techniques that provide
performance increase. In addition, in most realistic scenarios the performance is not only a
function of the PHY rate, but will also be affected by interference from other networks in close
proximity. Different bandwidth levels will be affected differently in an interference scenario.
Also, reducing the number of antennas eliminates diversity and reduces the robustness of the
transmission

2.4 Cost of increased‐bandwidth solution
By doubling the bandwidth (from 40 to 80 MHz), each spatial stream can roughly support twice
the number of bits per symbol. As such, an 80 MHz single‐stream transmission can provide the
same performance as a two‐stream 40 MHz transmission. To support two streams, both
transmitter and receiver should have at least two antennas, while the single stream transmission
can be sent (or received) with a single antenna. This means that an 80 MHz systems designed
with only a single RF and baseband transmit/receive chain requires less hardware for the same
performance than a 40 MHz system designed with two RF and baseband transmit/receive chains.
It is true that the 80 MHz RF and baseband design will be more demanding than the 40 MHz
design, but it is likely that some cost advantage remains for the wider‐bandwidth system.


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IEEE 802.11ac
However, single‐antenna systems will not provide the reliability that is required for certain
Quality of Service (QoS) scenarios. Even 80 MHz systems will need multiple chains to reliably
transport video. Any remaining cost advantage is expected to become marginal over time as
silicon costs decrease.

2.5 Power consumption of increased‐bandwidth solution
When enhanced bandwidth is used to deliver the same data rate with fewer RF chains, the power
consumption of the device will be lower by virtue of the lower number of RF components. This
gives an advantage to the 80 MHz system over a 40 MHz system with two streams from this
perspective. However, one has to consider the fact that a single antenna will not suffice for
certain services.

2.6 Required antenna diversity
Increasing the bandwidth enhances the performance of a single stream. If the target is to improve
the PHY rate or the maximum throughput of a system regardless of QoS considerations, this may
be all that is needed. One has to recognize, however, that transmission of high‐quality content
such as video has more requirements than just increasing the maximum ideal PHY rate. To
ensure stable delivery of video, the number of antennas should be higher than the number of
spatial streams. Diversity is a critical part of stable data delivery with QoS. Therefore, even 80
MHz systems will have to be built using multiple antennas if they are going to be used in
applications that require stable and reliable transmission of data (such as video). This narrows
the cost and power advantage between a (single‐stream) 80 MHz bandwidth system and a
(two‐stream) 40 MHz system.

2.7 Range and Coverage Area
Wi-Fi transmission rates slow down the further away you are from a transmitter. Absolute top
speeds are usually available only within a few dozen yards, with performance gradually tapering
off as you move further away. This relationship, which is determined by the laws of physics, is
true no matter what network standard is being used, IEEE 802.11ac being no exception. But

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IEEE 802.11ac
because IEEE 802.11ac transmissions start out so much faster than those from earlier networks,
you can be, say, 30 feet away from an IEEE 802.11ac access point and get the same data
throughput that you would if you were 10 away feet from an IEEE 802.11n transmitter.
Many factors affect the coverage area of a network—most notably, the way a structure is built.
Concrete walls, ceramic bathroom tile, and metal appliances are more difficult for Wi-Fi signals
to penetrate, in contrast to wooden walls with gypsum board, which are easier to penetrate. But
signals from IEEE 802.11ac networks, with beam forming and other innovations, do a much
better job in penetrating all forms of building materials than do the signals from its predecessor
networks. In fact, the ability of IEEE 802.11ac signals to transmit through some concrete walls is
expected to help homes in India and China, where concrete is used extensively as a construction
material.

2.8 Performance in interference environment
The use of the 5 GHz band has significantly increased the amount of bandwidth available for
wireless transmission. However, even this band is ultimately a limited resource, and
ever‐increasing competition for bandwidth share will be a reality for any 802.11 system
operating in this band.

Figure 2.1: US channel allocations for 20/40/80/160 MHz


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IEEE 802.11ac
IEEE 802.11ac specifies that 80 MHz channels consist of two adjacent 40 MHz channels,
without any overlap between the 80 MHz channels. This results in channel allocations as
illustrated in Figure 2.1 and Figure 2.1 for the U.S. and Europe, respectively. The number of 80
MHz channels for the U.S. is 5, while in Europe and Japan the number is 4.

Figure 2.2: European channel allocations for 20/40/80/160 MHz
For 20 and 40 MHz systems, one of the primary mechanisms for interference mitigation is a
judicious channel selection algorithm. This allows neighboring networks to essentially avoid
each other by using different channels when they are within each other’s range. Channels can be
reused by networks that are sufficiently far removed. With only four or five available 80 MHz
channels, it becomes much harder for an 80 MHz system to avoid interference from neighboring
networks (which could be either 80 MHz networks or 20/40 MHz networks).
Overlapping Base Station Subsystem (OBSS) problems will be more prevalent for 80 MHz
systems. This means that neighboring 80 MHz networks will have to share the same channel
with a neighboring BSS, with each getting access only part of the time. In addition, an 80 MHz
system is disadvantaged by the fact that all four 20 MHz (or two 40 MHz) channels with which it
overlaps have to be clear before the 80 MHz transmission is allowed to start. Even a 20 or 40
MHz transmission in part of the 80 MHz channel will preempt the complete 80 MHz channel
from sending. This reduces the probability that an 80 MHz system will gain access to the channel
in a dense environment. It should be noted that 802.11 ac provides a mechanism for 80 MHz to
fall back to lower bandwidth modes, but only under some conditions (for instance, 40 MHz

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IEEE 802.11ac
transmission on the secondary channel, only, is not allowed). Also, this mechanism is optional
and need not be implemented to be 802.11ac‐compliant. Figure 2.3 illustrates a possible
difference in interference scenarios between an 80 MHz system and a 40 MHz system. Figure
2.3(a) shows an 80 MHz system occupying four 20 MHz channels, while one of the 20 MHz
channels is also used by a legacy 20 MHz system. In this scenario, the 80 MHz system has no
way to avoid the occupied channel and has to share access to the medium with the 20 MHz
system. If access is equally shared between the two systems, the capacity of the 80 MHz system
is cut in half. Note that the 80 MHz system cannot fall back to 40 MHz transmission in this case,
since the overlap happens in the primary 40 MHz channel. In comparison, Figure 2.3(b) shows
how a 40 MHz system can avoid the occupied 20 MHz channel by channel selection. In this
scenario, the 40 MHz system has full unshared access to the medium. A single‐stream 40 MHz
system would have the same capacity as the single‐stream 80 MHz system shown in scenario
2.3(a). If the same 40 MHz system were to support two streams, however, its capacity would be
double that of the single‐stream 80 MHz system shown in scenario 2.3(a).

Figure 2.3: Two interference scenarios
There are many different interference scenarios, and the above example only aims to illustrate
the importance of fully understanding interference dynamics and implications when it comes to
assessing the true capacity of a system. It is also important to realize that one is unlikely to
encounter “greenfield” scenarios without legacy or other 802.11ac systems, where no
pre‐existing interference‐related constraints exist. Interference will virtually always be an issue,
to one degree or another. In the appendix, we include simulation results that compare 80 MHz
and 40 MHz performance in a specific simulation scenario. This further emphasizes the fact that
performance is, to a large extent, dependent on the interference environment. The interference
issue is very important to service providers, where it is essential that the wireless infrastructure


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IEEE 802.11ac
be future‐proofed. To be competitive in an interference environment, it is important for an 80
MHz system to have a fallback mode to lower bandwidth levels that provide similar
performance. This means the equipment has to support a sufficient number of antennas to fully
and effectively exploit the benefits of channel diversity.

2.9 Increased number of streams
802.11ac allows support for up to 8 spatial streams – up from a maximum of 4 streams in
802.11n. Support for more than one spatial stream is optional, however. It is not clear whether a
real‐world, single‐user MIMO channel can realistically support that many streams. The increased
number of streams may be most useful in combination with MU‐MIMO.

2.10 Multi User - MIMO
MU‐MIMO was added to 802.11ac to address the multi‐STA throughput requirement. In
MU‐MIMO, the Access Point (AP) – or possibly another STA – transmits independent data
streams to several STAs at the same time. Through preprocessing of the data streams at the
transmitter (similar to what happens in beam forming), the interference from streams that are not
intended for a particular STA is eliminated at the receiver of each STA. Therefore, in theory,
each STA receives its data free of interference from the transmissions that are simultaneously
directed towards other STAs. In MU‐MIMO, the spatial degrees of freedom are used to create
independent transmissions to different STAs, while in single‐user MIMO, these spatial degrees
of freedom are used to increase the throughput from AP to STA. The complexity of MU‐MIMO
falls mostly on the AP (or transmitting STA), where the preprocessing happens. The receiving
STAs only need the capability to report channel information to the AP so it can calculate the
preprocessing matrices. The required channel information from the receiving STA is very similar
to what is required for explicit feedback beam forming. As such, the complexity for the STA is
no more than the complexity already involved in supporting explicit feedback beam forming as a
receiver. One drawback of MU‐MIMO is that the amount of time that the medium is occupied is
determined by the slowest link among all AP‐STA pairs (or, more generally, the link that
requires the most time to finalize its transmission). No new data can be sent to any of the STAs


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IEEE 802.11ac
until all transmissions to STAs in the MU‐group have ended. If there is too much difference in
either the amount of data or throughput going to various STAs, this may lead to inefficient use of
the wireless medium. At this point, MU‐MIMO is a well‐studied concept, but practical
considerations will likely defer implementation of this feature to later generations of 802.11ac
products. Additional work may be needed to guarantee the efficient use of MU‐MIMO.

2.11 Beamforming
Beamforming is the ability of a Wi-Fi transmitter to “learn” to avoid inefficient pathways
between it and the device it is transmitting to. Beamforming is analogous to a car being able to
automatically avoid a highway lane that is full of pot holes. Beamforming is possible in the
current generation of IEEE 802.11n products, but many of them did not take advantage of it.
With IEEE 802.11ac, beamforming is a standard feature, and all products that implement it will
be interoperable and thereby able to operate at maximum range and coverage for the IEEE
802.11ac network.

2.12 Multiple antennas
An IEEE 802.11ac Wi-Fi device can contain between one and eight antennas. Transmission
speeds increase in direct proportion to the number of antennas. Companies selling computers,
mobile phones, networking gear, and other Wi-Fi equipment can choose how many antennas to
include, depending on considerations such as their price and performance targets for each
product. (This is a lot like car companies offering a model with a choice of four-, six-, or eightcylinder engines.) Entry-level, price-sensitive networking products can be built with a single
antenna, whereas high performance devices, especially for the enterprise, can be equipped with
more antennas.
Form factors are another consideration. A device that must, of necessity, be extremely compact,
such as a mobile phone, will simply have less room for extra antennas than a home networking
device. Regardless of the number of internal antennas they have, all IEEE 802.11ac devices will
work with all other IEEE 802.11ac devices, though speeds will be capped at those of the slower


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IEEE 802.11ac
device. (And again, all IEEE 802.11ac products will work with all earlier generations of Wi-Fi
products, but with the same speed limitation.)

2.13 Bigger frames
As we’ve seen thus far, 802.11ac techniques increase the physical data rate, improving spectral
efficiency. However, as we’re able to transfer more data in shorter periods of time, our realworld throughput will always be limited by protocol overhead, of which there are several
sources: random backoff, interframe spaces, acknowledgments, errors and retries, and frame
headers. Protocol overhead always reduces real-world throughput to some fraction of the
signaling rate, often near 50% without enhancements like frame aggregation. (See Table 2)

Table 2: Maximum data rate and throughput of different protocols.
Protocol

Max Data Rate

Throughput

802.11b

11 Mbps

5 - 6 Mbps

802.11a/g

54 Mbps

20 - 25 Mbps

802.11n

450 Mbps

~300+ Mbps

802.11ac

1.3 Gbps

Up to 800Mbps

Inter
Frame
Space

Random
Back off

Preamble
/ PHY
Header

MAC
Header

Overhead

Inter
Payload

Frame

ACK

Space
Effective

Overhead

Figure 2.4: 802.11 Protocol efficiency overview


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IEEE 802.11ac
If wireless channels are a highway, frame aggregation is analogous to carpooling. When each
frame (vehicle) has only one data packet (person), we need more cars, which creates more
overhead. However, if we transport multiple data packets or frames within the same data unit, we
decrease total overhead and increase airtime efficiency. As wireless efficiency improves, larger
payloads can be used without an increase in frame corruption caused by interference (Wi-Fi or
non-Wi-Fi), which is sporadic and bursty by nature (see Figure 2.4). PHY enhancements from
802.11n increased data rates enough to justify the use of frame aggregation without significant
interference and retry penalties. 802.11ac continues that trend by significantly increasing PHY
rates again, which permits an increase in the maximum frame size.
Real-world 802.11ac products will use frame aggregation commensurate with the connection
speed and quality, and the bandwidth demands of the application. As data rates increase or
interference decreases, frame size can increase, improving overall throughput. Support for frame
aggregation is a critical requirement for high throughput, so all 802.11ac frames must be AMPDUs.

2.13 MAC improvements
2.13.1 Increased Aggregated MPDU (A‐MPDU) size
The maximum size of an A‐MPDU can optionally be increased to a maximum of 1,048,575
octets (compared to a maximum of 65,535 octets in 802.11n).

2.13.2 RTS/CTS operation for wider bandwidth
Because of the wider bandwidth used in 802.11ac and the limited number of 80 MHz channels,
hidden nodes on the secondary channels are an important problem to address. The RTS/CTS
mechanism has been updated to better detect whether any of the non‐primary channels are
occupied by a different transmission. To this end, both RTS and CTS (optionally) support a
“dynamic bandwidth” mode. In this mode, CTS may be sent only on the primary channels that
are available in case part of the bandwidth is occupied. The STA that sent the RTS can than fall


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IEEE 802.11ac
back to a lower bandwidth mode. This helps to mitigate the effect of a hidden node. Note
however that the final transmission bandwidth always has to include the primary channel.

2.13.3 Other
Reduced Inter‐Frame Spacing (RIFS) is a deprecated feature of the 802.11n specification whose
purpose was to increase MAC efficiency by reducing the gap between successive transmissions.
RIFS can be applied between transmissions within the same burst. This mechanism was removed
from 802.11ac, except for what is needed to maintain backward compatibility with 802.11n. It
was felt that aggregation provided a more efficient way to increase MAC efficiency, and that the
complexity of RIFS implementation did not outweigh its gains as a stand‐alone mechanism.


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IEEE 802.11ac
CHAPTER 3

802.11ac - GIGABIT PERFORMANCE
802.11ac uses a variety of advancements in order to achieve the targeted performance. The new
specification addresses the need for performance improvement through three primary initiatives:
increasing the raw bandwidth, enabling multiple flows to use the medium concurrently, and
optimizing performance to specific clients.
Table 3: Calculating the Speed of 802.11n and 802.11ac
Bandwidth (as

11n
or
11ac

11ac
only

no. of Data

Spatial

Subcarriers)

PHY

No. of

Streams

Time per

Data Bits per

OFDM

Subcarrier

Symbol

56 (20 MHz)

108 (40 MHz)

3.6 µs
x

1 to 4

Up to
x

5/6xlog2(64) = 5

(short
÷

guard
interval)

234 (80 MHz)
2x234 (160

SPEED

5 to 8

Up to
5/6xlog2(256)=6.67

MHz)

4 µs (long
guard

=

PHY
Data
Rate
(bps)

interval)

3.1 Boosting raw bandwidth
To increase the physical-layer transport rate, 802.11ac makes use of a higher rate encoding
scheme known as 256-QAM – which transmits 33% more data as the 64-QAM used in the
802.11n standard. Signal-to-noise ratios that worked for 802.11n are no longer sufficient for the
higher speeds in 802.11ac because the difference in detectable signal level is now significantly
smaller.
To further increase the amount of data transported per second, channel bonding approaches made
popular in 802.11n have been taken further to provide 80 MHz-, and ultimately, 160 MHz-wide


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IEEE 802.11ac
channels. Increasing channel bandwidth allows for more data to be transmitted simultaneously
out of the same antenna. Legacy versions of the 802.11 devices commonly used 20 MHz
channels. When using 802.11n, users could select between 20 MHz or 40 MHz channel
operation. These wider channel bandwidths – and the need for proper channel separation – mean
that 802.11ac can only be used in the 5.0 GHz band (where more non-interfering bandwidth is
available). Note that dual-band APs will still be produced, but the 2.4 GHz band will be limited
to 802.11bgn and will not be able to be configured for 802.11ac. The use of multiple spatial
streams is also a key factor in the 802.11ac equation. The 802.11n standard accommodates for up
to four spatial streams to achieve a maximum of 600 Mbps of performance, although most access
points today use only three spatial streams for a maximum PHY rate of 450 Mbps. The 802.11ac
standard allows for up eight spatial streams. Early products will use three or four spatial streams,
but increased numbers of spatial streams are expected in future solutions.
Finally, packet aggregation is present in 802.11n as well, but is worth a mention in this
discussion because it is often the single biggest performance multiplier on a pertransmission
basis. To understand the importance of aggregation, it is critical to realize that the amount of
overhead required to obtain a chance to transmit a frame and acknowledge its transmission is
often much longer than the time required to actually transmit the useful data. With aggregation,
once a high performance device obtains its transmit opportunity, the transmitter strings multiple
frames together, and transmits them in succession without having to reacquire the medium.
Through this process, the overhead cost of the time to obtain and acknowledge each additional
frame is saved.

3.2 Real information superhighway
To support more users, 802.11ac has moved away from only allowing one 802.11 device to
transmit at a time. Multiuser multiple-input, multiple-output technology, or MU-MIMO for
short, allows an access point to transmit data to multiple client devices on the same channel at
the same time. In previous versions of 802.11 whenever the access point transmitted data, all of
the traffic at any given instance in time was being directed to a single client. As a consequence, if
a set of devices included a mix of fast and slow client devices the fast traffic was often delayed


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IEEE 802.11ac
substantially by the transmissions to the slower clients – much like traffic being delayed behind a
slow driver on a single-lane road.
In contrast, MU-MIMO allows a single AP to transmit to multiple client devices at the same
time. MU-MIMO works by directing some of the spatial streams to one client and other spatial
streams to a second client. In the previous analogy, this is much like driving down a four lane
freeway. Traffic is only significantly delayed when all of the lanes are blocked, so overall
throughput can be much higher. There are a number of permutations to this basic principle, but
MU-MIMO is critical to performance improvements in environments with high client counts.

3.3 Individual client channel optimization
The final major performance boost comes from technologies that optimize the communications
when speaking to a specific client. The first major enhancement is a concept known as transmit
beam forming, (TxBF for short). The reflections and attenuations, common during the
transmission of 802.11 signals, have a significant performance impact on overall network
performance. With TxBF, the access point communicates with the client devices to determine the
types of impairment that are present in the environment. Then the access point “precodes” the
transmitted frame with the inverse of the impairment such that when the next frame is
transmitted and transformed by the medium, it is received as a clean frame by the client. Since
no two clients are in the same location, TxBF needs to be applied on a client-by-client basis and
constantly updated to reflect the changing environment. A critical component of this feature is
the fact that the Access Point and client device are participating in a controlled handshake
between themselves, each sharing information to the other about the propagation channel that
exists between them.


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IEEE 802.11ac
CHAPTER 4

QUADRATURE AMPLITUDE MODULATION
(QAM)
4.1 Introduction
In its mission to improve data rates, 802.11ac also introduces more efficient modulation. Like
802.11a/g/n, 802.11ac will continue to rely on OFDM, but VHT devices will have the option of
higher-order 256-QAM (quadrature amplitude modulation). Compared with 64-QAM and
equivalent coding rates, 256-QAM provides 1.33x (33%) efficiency gains. Of course, higherorder modulation techniques increase efficiency, but they also require greater signal quality.
Low-order modulation like BPSK (binary phase shift keying) is very simple and reliable at very
long ranges. As modulation complexity increases, signal quality (and radio capabilities) must
increase with it, so range must generally decrease. Visually, we can see how modulation
complexity changes by looking at a constellation diagram, where each point on the diagram
represents a specific bit pattern—four bits for 16-QAM and six bits for 64-QAM. Constellation
density is proportional to signal quality requirements: more dense = better SNR required. 16QAM and 64-QAM are currently used with 802.11a/g

4.2 Constellation diagrams for QAM
The constellation diagrams show the different positions for the states within different forms of
QAM, quadrature amplitude modulation. As the order of the modulation increases, so does the
number of points on the QAM constellation diagram.


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IEEE 802.11ac
The diagrams below show constellation diagrams for a variety of formats of modulation:

Figure 4.1(a): BPSK Constellation Diagram

Figure 4.1(b): 16 QAM Constellation Diagram

Figure 4.1(c):32-QAM Constellation Diagram

Figure 4.1(d): 64-QAM Constellation Diagram


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IEEE 802.11ac

Figure 4.1(e): 256-QAM Constellation Diagram
As shown in above Figure 4.1(e), 256-QAM modulation with 802.11ac increases modulation
complexity by another order of magnitude, signifying 8 bits with each constellation point.
In Figure 4.1(e), as we evaluate the usefulness of 256-QAM, we must remember that it is not a
feature that we simply enable or disable. Such complex and efficient modulation techniques
require the right environmental conditions and high quality wireless receivers. With today’s
products, 256-QAM will only work at very short range (20-30m). So once again, the data rate
gains offered by 802.11ac are dependent on the right scenario. 256-QAM is also optional for
802.11ac; first generation devices would support it.

4.3 QAM bits per symbol
The advantage of using QAM is that it is a higher order form of modulation and as a result it is
able to carry more bits of information per symbol. By selecting a higher order format of QAM,
the data rate of a link can be increased.

Table 4: Bit rates of different forms of QAM and PSK.

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IEEE 802.11ac
Modulation

Bits per symbol

Symbol Rate

BPSK

1

1 x bit rate

QPSK

2

1/2 bit rate

8PSK

3

1/3 bit rate

16QAM

4

1/4 bit rate

32QAM

5

1/5 bit rate

64QAM

6

1/6 bit rate

4.4 QAM noise margin
While higher order modulation rates are able to offer much faster data rates and higher levels of
spectral efficiency for the radio communications system, this comes at a price. The higher order
modulation schemes are considerably less resilient to noise and interference.
As a result of this, many radio communications systems now use dynamic adaptive modulation
techniques. They sense the channel conditions and adapt the modulation scheme to obtain the
highest data rate for the given conditions. As signal to noise ratios decrease errors will increase
along with re-sends of the data, thereby slowing throughput. By reverting to a lower order
modulation scheme the link can be made more reliable with fewer data errors and re-sends.


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IEEE 802.11ac
CHAPTER 5

SMART ANTENNA SYSTEMS
5.1 Introduction
Smart antenna is one of the most promising technologies that will enable a higher capacity in
wireless networks by effectively reducing multipath and co-channel interference. This is
achieved by focusing the radiation only in the desired direction and adjusting itself to changing
traffic conditions or signal environments. Smart antennas employ a set of radiating elements
arranged in the form of an array. The signals from these elements are combined to form a
movable or switchable beam pattern that follows the desired user. In a Smart antenna system the
arrays by themselves are not smart, it is the digital signal processing that makes them smart. The
process of combining the signals and then focusing the radiation in a particular direction is often
referred to as digital beam forming. This term will be extensively used in the following sections.
There are basically two approaches to implement antennas that dynamically change their antenna
pattern to mitigate interference and multipath affects while increasing coverage and range. They
are
 Switched beam
 Adaptive Arrays

5.2 Switched beam
The Switched beam approach is simpler compared to the fully adaptive approach. It provides a
considerable increase in network capacity when compared to traditional omnidirectional antenna
systems or sector-based systems. In this approach, an antenna array generates overlapping beams
that cover the surrounding area as shown in figure 5.1. When an incoming signal is detected, the
base station determines the beam that is best aligned in the signal-of-interest direction and then
switches to that beam to communicate with the user.


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IEEE 802.11ac

Figure 5.1: Beam formation for
switched beam antenna system

adaptive array antenna system

5.3 Adaptive arrays
The Adaptive array system is the “smarter” of the two approaches. Switched beam systems offer
limited performance enhancement when compared to conventional antenna systems in wireless
communication. However, greater performance improvements can be achieved by implementing
advanced signal processing techniques to process the information obtained by the antenna arrays.
Unlike switched beam systems, the adaptive array systems are really smart because they are able
to dynamically react to the changing RF environment.
This system tracks the mobile user continuously by steering the main beam towards the user and
at the same time forming nulls in the directions of the interfering signal as shown in figure 5.2.
Like switched beam systems, they also incorporate arrays. Typically, the received signal from
each of the spatially distributed antenna elements is multiplied by a weight. The weights are
complex in nature and adjust the amplitude and phase. These signals are combined to yield the
array output. These complex weights are computed by a complicated adaptive algorithm, which

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IEEE 802.11ac
is pre-programmed into the digital signal-processing unit that manages the signal radiated by the
base station.
This signal processing steers the radiation beam towards a desired mobile user, follows the user
as he moves, and at the same time minimizes interference arising from other users by introducing
nulls in their directions. This is illustrated in a simple diagram shown below in figure 5.3.

adaptive array antenna system

The adaptive array systems are really intelligent in the true sense and can actually be referred to
as smart antennas. The smartness in these systems comes from the intelligent digital processor
that is incorporated in the system. The processing is mainly governed by complex
computationally intensive algorithms.

5.4

Basic working mechanism

A smart antenna system can perform the following functions: first the direction of arrival of all
the incoming signals including the interfering signals and the multipath signals are estimated
using the Direction of Arrival algorithms. Secondly, the desired user signal is identified and
separated from the rest of the unwanted incoming signals. Lastly a beam is steered in the
direction of the desired signal and the user is tracked as he moves while placing nulls at
interfering signal directions by constantly updating the complex weights.
The direction of radiation of the main beam in an array depends upon the phase difference
between the elements of the array. Therefore it is possible to continuously steer the main beam in
any direction by adjusting the progressive phase difference β between the elements. The same


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IEEE 802.11ac
concept forms the basis in adaptive array systems in which the phase is adjusted to achieve
maximum radiation in the desired direction. To have a better understanding of how an adaptive
array system works, let us consider a typical adaptive digital beam forming network shown
below in figure 5.4.

D/C

ADC

W1

D/C

ADC

W2

∑
Demodulator

D/C

ADC

Wn

ADC = Analog Digital Converter
D/C = Down Converter

Adaptive
Algorithm

W = Complex Weight

Figure 5.4: Block diagram of Adaptive array systems
In a beam forming network typically the signals incident at the individual elements are combined
intelligently to form a single desired beamformed output. Before the incoming signals are
weighted they are brought down to baseband or intermediate frequencies (IF’s). The receivers
provided at the output of each element perform the necessary frequency down conversion.
Adaptive antenna array systems use digital signal processors (DSP’s) to weight the incoming
signal. Therefore it is required that the down-converted signal be converted into digital format
before they are processed by the DSP. Analog-to-digital converters (ADC’s) are provided for this
purpose. For accurate performance, they are required to provide accurate translation of the RF
signal from the analog to the digital domain. The digital signal processor forms the heart of the
system, which accepts the IF signal in digital format and the processing of the digital data is


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IEEE 802.11ac
driven by software. The processor interprets the incoming data information, determines the
complex weights (amplification and phase information) and multiplies the weights to each
element output to optimize the array pattern. The optimization is based on a particular criterion,
which minimizes the contribution from noise and interference while producing maximum beam
gain at the desired direction. There are several algorithms based on different criteria for updating
and computing the optimum weights.


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IEEE 802.11ac
CHAPTER 6

NEED FOR FASTER Wi-Fi
6.1 Why do we need faster Wi-Fi?
A number of factors are fueling industry progress with 802.11ac standards. Among them:
• More users. Sheer traffic volume is exploding. Wi-Fi has more or less succeeded at displacing
Ethernet in the access portion of the corporate network, so there are simply more Wi-Fi users
creating traffic. In addition, guest traffic in certain verticals is adding to the loads. For example,
retail customers often want to use their Wi-Fi-enabled devices in stores to comparison shop; in
turn, retailers take advantage of customers’ wireless connectivity by pushing in-store advertising
to them over the airwaves.
• More devices per user/BYOD. In addition, users now tend to carry at least two devices; most
carry a mobile phone and a laptop, and some carry a tablet computer, as well. This has created a
dense population of devices with varying transmit power levels, generating more traffic and
creating new Wi-Fi design considerations for the enterprise.
• “Big” apps. Users are running bandwidth-hungry apps such as Apple iCloud and Google Drive
over- the-air synchronization services, high-def video, Web conferencing, social networking
apps, Apple FaceTime videoconferencing and Pandora radio streaming, to name just a few.
These consume far more capacity than the low-speed data transfers of yesterday.
• Cellular offload. A number of 3G/4G cellular carriers are growing anxious to offload mobile
WAN traffic onto Wi-Fi wherever possible to prevent cellular traffic jams. This works because
most popular mobile devices support both cellular and Wi-Fi connections, so cellular subscribers
can hop onto Wi-Fi when they are in range.
802.11ac will help address and facilitate all of these situations. Final IEEE 802.11ac Working
Group approval is expected in late 2013, though, as with most WLAN standards, the industry’s
Wi-Fi Alliance expects to certify “pre-standard” products six to 12 months earlier than that, most
likely for the home/consumer market. 802.11ac products earning a pre-standard Wi-Fi

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IEEE 802.11ac
certification by the alliance have been tested for interoperability by the alliance’s labs with a
number of other early products built to the 802.11ac standard in its near-final state.
Note that to be considered “standards-compliant,” ratification of the final standard must take
place, and products must be tested for conformity to all the mandatory components of that
standard. Standards conformance/compliance still is not an assurance of product interoperability,
as standard features might be interpreted and executed slightly differently. So interoperability
testing and certification remains a good idea, whether products are “pre-standard” or built to the
final, ratified standard.

6.2 Other uses for IEEE 802.11ac
More reliable video delivery and faster syncing between phones and computers will be two of
the most important applications of IEEE 802.11ac networks. But, they are expected to find a
home in many other applications as well.

6.2.1 The Enterprise
Wi-Fi is becoming as important at work as it is in the home. Some offices already have nearly as
many Wi-Fi access points as printers or copiers. With IEEE 802.11ac, coverage can be
accomplished with fewer devices, even while transmission rates increase. Among those
benefiting from this more efficient Wi-Fi networking technology will be the many office workers
using mobile devices, either their own or ones that have been supplied by the IT department.
These devices have caused a spike in enterprise demand for Wi-Fi, an increase that IEEE
802.11ac can easily accommodate. The new IEEE 802.11ac standard will also be useful for
companies experimenting with new seating arrangements, such as “virtual teams,” in which
workers don't use the same desk every day but assemble themselves into ad hoc groups that are
determined by the job that needs doing. Traditional wired Ethernet networks don't always give
enterprises the flexibility they need to support these constantly-evolving workplace layouts.

6.2.2 Set-top Boxes

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IEEE 802.11ac
Right now, cable providers have to run a cable not only from the street to the house for each
subscriber, they also have to run an interior cable to each room with a TV set. The process of
drilling holes and pulling wires is expensive and time-consuming for the companies involved and
also extremely inconvenient for customers. But the high data throughput and wide coverage
range of IEEE 802.11ac networks makes possible the installation of “satellite” set-top boxes that
receive their signals over Wi-Fi from a central wired device. Customers wanting to add a second
or third TV in another room wouldn't need to make an appointment with an installer. Instead,
they'll simply plug in a Wi-Fi device and hook it up to the new TV set.

6.2.3 Wi-Fi Direct
This exciting new capability for wireless devices is not technically part of the IEEE 802.11ac
standard, but it is expected to grow in popularity along with it. Wi-Fi Direct allows two Wi-Fi
devices to communicate with each other directly, without the need for a Wi-Fi access point in
between. For example, suppose you and a seatmate on an airplane want to swap files from your
notebooks or mobile phones. Right now, you need Wi-Fi access points to do so. But with Wi-Fi
Direct, the two devices could communicate back and forth directly, even without wireless
coverage being provided. Wi-Fi Direct is supported in the current IEEE 802.11n standard, but
has not been widely used. That is changing rapidly, however. Microsoft has built native support
for Wi-Fi Direct into Windows 8, which released in 2012. In addition, the latest version of
Google's Android mobile operating system supports Wi-Fi Direct connections.

6.2.4 3G and 4G Offloading
While most mobile carriers are building out their 3G and 4G networks as fast as they can, they
are facing challenges as they attempt to satisfy the ever-growing download and streaming
demands of their users, especially for mobile video. As a result, both mobile carriers and mobile
users are becoming excited about using IEEE 802.11ac Wi-Fi networks to offload 3G and 4G
traffic. There are many ways these hybrid systems might work. One of the most commonly
discussed methods involves using a Near Field Communications (NFC) link to identify nearby
Wi-Fi networks, and then automatically setting up a connection with a network within range.
After that, Wi-Fi would take over and do the actual transmission.


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IEEE 802.11ac
NFC is an entirely separate wireless system being built into a growing number of mobile devices.
It works only over very short ranges—a few feet—because it was designed with commerce
applications in mind, such as paying for a purchase by tapping an NFC-equipped mobile phone
at the cash register, rather than swiping a credit card. Although NFC networks are not by
themselves fast enough to transmit high-data applications such as video, they can easily handle
the intradevice negotiations and communications necessary to set up an IEEE 802.11ac
connection, which would then step in and do the heavy lifting.


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IEEE 802.11ac
CHAPTER 7

INTEROPERABILITY AND MIGRATION
CONSIDERATIONS
7.1 Interoperability
802.11ac will be backward-compatible with 802.11n but only with 802.11n clients operating in
the matching 5GHz range. The 2.4GHz range, which has been home to 802.11b, 802.11g, much
of 802.11n and a variety of non-Wi-Fi devices, has been deemed impractical for enhancing
WLAN throughput going forward.
This is a potential sticking point for some of today’s mobile devices. Though a number of
handsets and tablets built specifically for enterprise applications operate in the 5GHz band, most
consumer-class handsets growing popular within the enterprise operate in the 2.4GHz band only.
As such, they will not be compatible with 802.11ac per se. Two things will likely happen here:
1) Many organizations will continue to support 802.11n as they introduce 802.11ac, and the
2.4GHz radios in their 802.11n infrastructures will continue to serve the 2.4GHz clients.
2) Newer devices will have multiple radios, including both 2.4GHz and 5GHz Wi-Fi radios. For
example, the previous version of the Apple iPhone (the iPhone 4S) supported 2.4GHz Wi-Fi
only; however, the latest version of the Apple iPad tablet computer, the iPad 3 and iphone 5
supports both Wi-Fi frequencies.
While 802.11ac will not be available in many such devices for a while, support for 5GHz
suffices for backward compatibility. And air time fairness algorithms, described earlier, will
make sure that each device transmits only for the length of time allotted by the standard it
supports so as not to degrade performance of 802.11ac clients running on 802.11ac backbones.
As mentioned, many of the technology concepts that were created for and used by 802.11n were
expanded upon and applied to 802.11ac, which bodes well for interoperability testing, as well as
initial price points.


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IEEE 802.11ac
7.2 Migration
The most difficult decision in migrating, perhaps, will be in determining channel plans that take
advantage of 802.11ac’s fatter channels while maintaining performance for single-stream mobile
devices. Otherwise, depending what the range and throughput requirement is for the new
installation, it is likely that an 802.11ac AP can be dropped directly into a legacy AP location,
one for one.
While 802.11n is specified to support up to four spatial streams, most products on the market
today currently support two or three. Early 802.11ac products are likely to support just two or
three, as well. Determining whether to invest in early 802.11ac products or mature 802.11n
products in the near-term, then, is likely to depend on how product costs fall out and each
organization’s philosophy over whether to continue investments in “legacy” technology if it
continues to do the job needed. 802.11n was so long in coming and brought such dramatic
advances to the Wi-Fi industry, it is hard to imagine it as a “legacy” technology. However, in
about a two- to three-year time frame, that’s exactly what it will be.


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IEEE 802.11ac
CHAPTER 8

WHAT'S NEXT IN WI-FI?
With the introduction of IEEE 802.11ac, the wireless industry is accommodating the increased
importance of streaming video to mobile devices. Similarly, the industry is already planning for
the wireless transmission needs of tomorrow. Two projects deserve mentioning.

8.1 Ultralow power
There has been much recent discussion of the “Internet of things,” a network that connects not
only people and computers, but also household appliances, security systems, door locks, light
switches, garage door openers, and the scores of other devices we depend on every day. With
these devices online, we would be able to control them from our computers or mobile phones.
We might, for example, use a mobile phone app to turn on the home heating system should we
find ourselves coming home earlier on a winter day than the time programmed into the home
thermostat.
These types of devices transmit much less data than do traditional PCs. And, because many of
these devices are powered by AA and AAA batteries, their Wi-Fi chips must use an absolute
minimum amount of power. A group of leading companies is working together on a standard for
this form of low-power, low-throughput networking, called IEEE 802.11ah. The standard is
expected to be finalized in the near future.

8.2 Super High-Speed Networking
The other ongoing development effort in Wi-Fi is at the opposite end of the performance
spectrum called 60 GHz Communications, it is designed for super high-speed connectivity
several gigabits per second, which is much faster than even the broadband Internet connections
most people have coming in to their homes. At these speeds, a high-definition movie could be
transferred in just a few seconds. Most of these speed gains come from moving to an entirely
different part of the spectrum from either 802.11n or 802.11ac. Because of the laws of physics,


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IEEE 802.11ac
transmissions in this part of the spectrum can travel only relatively short distances and can't
easily penetrate walls or furniture. Thus, 60 GHz communications will be on a “line of sight”
basis within a single room. Even with those limitations, however, this form of high-speed
wireless networking will find its way into many applications.


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IEEE 802.11ac
CHAPTER 9

CONCLUSION
802.11ac has the potential to provide the next generation in high‐throughput wireless systems.
To fully realize this potential, 802.11ac systems will have to go beyond a minimal
implementation that simply exploits the wider bandwidth channels available to this technology.
Any new system will be measured against currently available 802.11n systems that already
implement MIMO processing with space‐division multiplexing, LDPC, STBC, beam forming,
multiple streams and a variety of other PHY, MAC and coexistence enhancements.
First‐generation 802.11ac systems must be evaluated in light of this comparison. As a minimum,
such systems would have to match the feature set that is already provided by current‐generation
802.11n. Preferably, any next‐generation system would include some truly next‐generation
features (such as MU‐MIMO) in addition to the channel bandwidth increases that are readily
available in this new technology. The bandwidth increase of 802.11ac is currently a concern in
situations with limited bandwidth resources. Frequency is a scarce resource that needs to be used
as efficiently as possible. Exploiting channel diversity by using a higher number of spatial
streams allows more efficient spectrum use than simply doubling the bandwidth of the
transmission. Channel and antenna diversity, therefore, remain important requirements, even for
systems that are capable of wider bandwidth. It is believed that a 4x4 system with a maximum
number of spatial streams and MU‐MIMO will be required, at a minimum, in order for 802.11ac
to fully realize its potential. Such a system would provide higher bandwidth in sparsely
populated networks, while providing QoS, good performance and coexistence in denser network
environments.


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IEEE 802.11ac -- BRIEF INTRO

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