IEEE 802.11AC what does it mean for test?

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Introduction to 802.11ac
Late in 2008, a new task group (TG) was formed within the IEEE 802 Standards Committee with the goal of creating a new amendment
to the 802.11-2007 standard. The new amendment, known as 802.11ac, includes mechanisms to improve the data throughput of the
existing Wireless Local Area Networks (WLAN), enabling wireless networks to offer wired network performance.
Since its formation, the TGac has made significant progress in the definition of the technology: in January 2011, the framework
specifications for the technology moved into draft form, which then underwent subsequent revisions and is currently available in
version D1.1. The draft is expected to be completed by the end of 2012, and final approval should occur by the end of 2013 (as
shown in Figure 1).
The TGac
is formed
Draft D 1.0
is released
First 802.11ac
end-user products
Framework moves
into draft D 0.1
Final Working
Group approval
First 802.11ac
Si shipments
2008 2011 2012 2013
Figure 1: History and Future Dates of the 802.11ac Amendment
Even though the 802.11ac amendment will not be published until late in 2013, the draft availability means that the silicon requirements
are generally finalized, and chipset companies can start developing and marketing their 802.11ac devices. The first 802.11ac silicon
shipments are anticipated at the end of 2011, or early in 2012.
802.11ac is expected to make an impact in the marketplace, with over one billion ICs forecasted to be shipped worldwide by 2015.
There are multiple reasons for the very high expectations around 802.11ac. First, not only does the technology promise to deliver
for the first time data rates over 1 Gbps, but also includes advanced features to improve the user experience. Like LTE Advanced,
it employs more spatial streams through 8 x 8 Multiple-Input Multiple-Output (MIMO), offers wider channel bandwidths (up to 80
MHz channels) and even makes use of channel aggregation, for up to 160 MHz of total bandwidth. Furthermore, key to the success
of 802.11ac will be that it is an evolutionary technology: it achieves its goals, and breaks a few important paradigms, while building
on the existing 802.11n amendment. This is a great advantage since it enables manufacturers and users a relatively easy transition
from existing WLAN networks and applications (which use 802.11n or previous amendments), to those that will use 802.11ac.
In this document we will describe the motivations, key features, and market forecast of 802.11ac. We will then review the current 802.11ac
physical layer (PHY), and the several applications enabled by the new 802.11ac capabilities. Finally, we will discuss how new 802.11ac
specifications impact the requirements for test equipment necessary to validate the technology in both R&D and manufacturing
environments.
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IEEE 802.11ac: What Does it Mean for Test? 3
802.11ac: Overview
Motivations
The purpose of the 802.11ac amendment is to improve the WiFi user experience by providing significantly higher throughput for
existing application areas, and to enable new market segments for operation below 6 GHz including distribution of multiple data
streams. With a data rate over 1 Gbps and several new features, throughput and application-specific performance of 802.11ac
promises to be comparable to that of existing wired networks.
Also known as Very High Throughput(VHT), 802.11ac achieves this purpose by building on the existing 802.11n technology. In
doing so, it continues the long-existing trend towards higher data rates (Figure 2), to meet the growing application demand for
WiFi network capacity and enable WiFi to remain the technology of choice at the edge.
To increase data rates, the TG for 802.11ac has defined an ample set of optional parameters in addition to some that are
mandatory. The flexibility built in the technology is typical of the latest wireless technologies (see LTE), and enables chipset and
device manufacturers to make the best use of the available resources and tailor their products to the specific need of the targeted
application. Specifically, the TGac has defined optional parameters for:
1995
10,000
1,000
100
10
1
.1
Max Data Rate (Mbps)
2000 2005
Year
802.11
802.11b
802.11 a(g)
802.11n
(4x4, 40 MHz)
802.11ac
(4x4, 160 MHz)
2010 2015
•  Channel Bandwidth
•  Modulation
•  Number of Spatial Streams
An 802.11ac device making use of only
the mandatory parameters (80 MHz
bandwidth, 1 spatial stream, 64 QAM
5/6 with long guard interval) will be
capable of a data rate of about 293
Mbps. A device that implements all
optional parameters (160 MHz bandwidth,
8 spatial streams, 256QAM 5/6 with
short guard interval) will be able to
achieve over 6 Gbps.
Figure 2. 802.11ac continues the trend of WiFi
technologies towards higher data rates
Key Characteristics
802.11ac adopts several features and unique mechanisms to increase throughput, improve the user experience, and more.
The key characteristics of this new technology are:
•  5 GHz Frequency Band
•  Wide Channel Bandwidth
•  New Modulation and Coding Scheme (MCS)
•  Backwards Compatibility
•  Coexistence
•  Multiple Spatial Streams
•  Beamforming and Multi-User MIMO
•  Energy Efficiency
IEEE 802.11ac: What Does it Mean for Test? 4
5 GHz Frequency Band
Contrary to 802.11n, which operates in both the 2.4 GHz and 5 GHz RF bands, 802.11ac devices will operate only in the 5 GHz RF
band. The choice to restrict usage in this band is mainly driven by the wider channel bandwidth requirements for 802.11ac. As
the bandwidth increases, channel layout becomes a challenge, especially in the crowded and fragmented 2.4 GHz band. Even
in the relatively expansive 5 GHz band, manufacturers will need to adapt automatic radio tuning capabilities to use the available
resources wisely and conserve spectrum.
Wide Channel Bandwidth
802.11ac includes both manditory and optional bandwidth enhancements over 802.11n.
In addition to the 20 MHz and 40 MHz channel bandwidths, supported by most 802.11n devices today, the 802.11ac draft
specifications include a mandatory, contiguous 80 MHz channel bandwidth. The key benefit of this wider bandwidth is that
it effectively doubles the PHY rate over that of 802.11n at negligible cost increase for the chipset manufacturer. With 80 MHz
contiguous bandwidth mode, not only is the data rate/throughput higher, but also the efficiency of the system increases, and data
transfers can be made faster, thus enabling new applications not supported by the current 802.11n specifications.
In addition, the 802.11ac specifications include an optional 160 MHz channel bandwidth, which can be either contiguous or
non-contiguous (80+80 MHz). In the non-contiguous case, the frequency spectrum consists of two segments; each segment is
transmitted using any two 802.11ac 80 MHz channels, possibly non-adjacent in frequency. Compared with 40/80 MHz transmissions,
160 MHz PHY transmission has the advantages of reducing the complexity of the requirements (e.g. MIMO order, MCS, etc) that
allow devices achieve Gbps wireless throughput, and opening the door to more applications. However, 160 MHz bandwidth in
the 5 GHz band is not available worldwide, and implementations to support this feature will likely be higher in cost – hence, the
decision to make this feature optional in 802.11ac devices.
New Modulation and Coding Scheme (MCS)
802.11ac uses 802.11n OFDM (Orthogonal Frequency Division Multiplexing) modulation, interleaving, and coding architecture.
Specifically, both 802.11ac and 802.11n require device support for BPSK, QPSK, 16QAM and 64QAM modulation. However, there
are two key differences with respect to the 802.11n specifications.
First of all, 802.11ac includes an approved constellation mapping enhancement, specifically, optional 256QAM (3/4 and 5/6 coding
rates) that can be used for both 802.11ac 80 MHz and 160 MHz transmissions. The benefit of 256QAM is that it offers 33% greater
throughput than a 64QAM transmission. This increase comes, however, at the cost of less tolerance of bit errors in lossy signal
environments. The 256QAM modulation was added as an optional mode, as opposed to a mandatory mode, for the following
reasons:
•  Allow design flexibility
•  Lower implementations cost for applications that do not need the higher modulation
•  Ease the adoption of 802.11ac in devices that cannot meet the stringent requirements of the 256QAM mode in terms of:
-  EVM (Error Vector Magnitude)
-  SNR (Signal-to-Noise Ratio)
-  PAPR (Peak-To-Average-Power Ratio)
The second difference to 802.11n is that the number of defined MCS indices is greatly reduced. Only ten single user MCS (0 to 9)
are defined in 802.11ac, significantly fewer than the 77 MCS indices specified in 802.11n.
•  802.11n required 77 MCS indices to support “unequal” modulations, e.g. a single user might receive a BPSK-modulated signal
on one stream and a 16QAM-modulated signal on another.
•  802.11ac supports only equal modulations. The TGac decided to drop support of unequal modulations because this feature proved
not to be successful in the marketplace (very few 802.11n devices actually supported it). Also, given the additional channel bandwidth
and modulation options in 802.11ac, the number of possibilities (hence, the number of MCS indices) would be impractical.
IEEE 802.11ac: What Does it Mean for Test? 5
Backwards Compatibility
802.11ac provides backwards compatibility with 802.11a and 802.11n devices operating in the 5 GHz band. This means that:
•  802.11ac interworks with devices supporting 802.11a and 802.11n technologies
•  802.11ac frame structures can accommodate transmission with 802.11a and 802.11n devices
The backward compatibility of 802.11ac is a definite advantage of 802.11ac over alternative revolutionary technologies (such as
802.11ad) that also promise to increase data rate over 802.11n, but do not operate with existing WLAN devices. Backward compatibility
will ease adoption into the marketplace and ensure 802.11ac devices can seamlessly “plug into” existing WLAN networks.
Coexistence
An important component of the work in TGac is to design mechanisms to coexist with existing networks using 802.11a and 802.11n
in the 5 GHz band. Examples of these mechanisms are Clear Channel Assessment (CCA), channel access fairness, and scanning
and channel selection mechanisms. Coexistence mechanisms are also being defined to ensure 802.11ac with different channel
bandwidths (20/40/80, and up to 160 MHz) interoperate.
Multiple Spatial Streams
802.11ac includes support for up to eight spatial streams, versus four in 802.11n. As in 802.11n, spatial multiplexing of multiple
streams of data over the same frequencies takes advantage of the extra degrees of freedom provided by the independent spatial
paths to effectively multiply channel capacity. The streams become combined as they pass across the channel, and the task at the
receiver is to separate and decode them. Despite the complexity of this technique, manufacturers of 802.11n devices have learned
to use the independent paths between multiple antennas to great effect, and can now effectively transpose this knowledge to the
making of 802.11ac devices. It is likely that the first 802.11ac silicon will use multiple spatial streams.
Beamforming and Multi-User MIMO
With 802.11n, manufacturers of WiFi devices learned how to use transmit beamforming, that is, the ability to focus RF energy in a
given direction to improve delivery to individual stations. 802.11ac builds on this knowledge and includes enhancements such as
single sounding and feedback format (vs. multiple in 802.11n).
More importantly with 802.11ac, the TGac has built on the beamforming capabilities of 802.11n new mechanisms that enable
an access point (AP) to communicate with multiple client devices in different directions simultaneously using the same channel,
multiple antennas, and spatial multiplexing. For example, an eight-antenna AP might be able to use 4x4 MIMO to two physically
separated stations at once. To contrast, MIMO devices today only considers point-to-point access to the multiple antennas
connected to each individual terminal; hence, the AP must time multiplex to serve multiple clients.
This set of advanced mechanisms takes the name of Multi-User MIMO (MU-MIMO), and it is one of the most interesting
enhancements currently on the drawing board of the TGac to increase the efficiency (number of megabits transmitted per
megahertz of spectrum, Mbps/MHz) of the newest 802.11 standard.
To use an analogy, MU-MIMO leverages the fundamental of Ethernet switching, by reducing contention: it extends transmit
beamforming technology to allow the AP to provide “switched” WiFi with dedicated bandwidth to stations, similar to the way the
typical wired Ethernet network works today.
While the promised advantages of MU-MIMO are many and attractive, correctly using the technology requires that chipset
designers and manufacturers develop spatial awareness of clients and sophisticated queuing systems that can take advantage
of opportunities to transmit to multiple clients when conditions are right. In other words, the increased system capacity comes at
the cost of significantly more expensive signal processing and increased complexity. For this reason, MU-MIMO (one transmitting
device, multiple receiving devices) is included in the 802.11ac draft specifications only as an optional mode.
IEEE 802.11ac: What Does it Mean for Test? 6
Energy Efficiency
A little known fact is that the energy efficiency, described in bits per microjoule, of the 802.11 standards has been increasing since
the first amendment of the technology was ratified. Specifically, 802.11ac promises a twofold increase in energy efficiency over the
existing 802.11n, as shown in Figure 3. This improvement is an effect of the several enhancements introduced by each amendment
to increase the data rate of a transmission, with all other parameters constant (RF frequency, power, and bandwidth).
While the trend is little known, the increasing energy efficiency certainly has numerous benefits for the growing number of portable
devices that integrate WiFi, which must work with small batteries and limited power consumption available to support the wireless
communications link.
0
1,000
1,200
1,400
1,600
800
600
400
200
0
Energy Efficiency (bits per microjoule)
100 200
Data Rate (bps)
802.11 a/g
802.11n
(1x1)
802.11ac
(1x1)
300
Figure 3. 802.11ac offers improved energy
efficiency over previous 802.11 standards
Market Forecast
The number of wireless devices that support 802.11n has been growing steadily in the past years, and this growth is expected to
continue in the future. Not only devices that have traditionally used previous amendments (802.11a/b/g) are adopting the newer
technology, but also wireless is moving into a growing number of devices that previously did not have this capability.
Over 50 million consumer electronics devices are estimated to allow for connectivity to WLAN networks in 2010 alone, and by the
end of 2011, the number could rise by up to about 40%. Overall these devices are estimated to account for 1 billion 802.11 Silicon
shipments by the end of this year 2011. That estimate is expected to grow to over 1.2 billion shipments in 2012 and nearly 2.2
billion shipments in 2015, supported by both the growth of existing markets of wireless devices, and the penetration of wireless in
an increasing number of new devices.
IEEE 802.11ac: What Does it Mean for Test? 7
The 802.11ac implementation will definitely play an important role in this growth, by both replacing 802.11n in current devices and
opening the door to new applications - similarly to what 802.11n has done in the past years. The first 802.11ac silicon shipments
are expected sometime by the first quarter of 2012, and end-user products could appear in the marketplace as early as in the third
quarter of the same year. The real impact of 802.11ac, however, will probably be felt in 2014 and beyond. In 2015, shipments of
802.11ac-equipped mobile devices are forecast to approach 1 billion (Figure 4), accounting for nearly half of the entire WLAN market
in that year.
2010
1000.0
800.0
600.0
400.0
200.0
0.0
2011 2012
Year
Total SISO/MIMO 802.11ac Market
Shipments (Millions)
2013 2014 2015
Figure 4. World market forecast of 802.11ac
unit shipments.
Specifications
As mentioned, 802.11ac PHY is based on the well known OFDM PHY used for 802.11n, with some important modifications
necessary to meet the 802.11ac’s goals. Some of the key technical specifications that distinguish 802.11ac from 802.11n are
summarized in Table 1, and discussed below.
As will be discussed, some differences have a significant impact on the requirements for the test equipment needed to verify the
functionality of 802.11ac-enabled devices. This topic will be the subject of a later part of this document.
Table 1. Comparison of 802.11ac and 802.11n Technical Specifications
Technical Specification 802.11n 802.11ac
Frequency  2.4, 4.9, 5 GHz  5 GHz
Modulation Scheme OFDM OFDM
Channel Bandwidth  20, 40 MHz
20, 40, 80 MHz
(160 MHz optional)
Nominal Data Rate, Single Stream Up to 150 Mbps (1x1, 40 MHz)
Up to 433 Mbps (1x1, 80 MHz)
Up to 867 Mbps (1x1, 160 MHz)
Aggregate Nominal Data Rate,
Multiple Streams
Up to 600 Mbps (4x4, 40 MHz)
Up to 1.73 Gbps (4x4, 80 MHz)
Up to 3.47 Gbps (4x4, 160 MHz)
Time to Stream 1.5hr HD  ~ 30 min (4x4, 40 MHz)  ~ 15 min (4x4, 80 MHz)
Spectral Efficiency per Gbps  400 bps/Hz (4x4, 40 MHz)  200 bps/Hz (4x4, 80 MHz)
EIRP  22-36 dBm  22-29 dBm
Range  12-70 m indoor  12-35 m indoor
Through Walls  Y  Y
Non-Line-of-Sight  Y  Y
World-Wide Availability  Y
Y
limited in China
IEEE 802.11ac: What Does it Mean for Test? 8
Channels
Figure 5 describes the spectral mask specifications for 802.11ac devices to operate with 20, 40, 80 and contiguous 160 MHz
channel bandwidth. Importantly, the widest channel occupies a wide range of frequencies of 240 MHz: as will be discussed later,
this requires manufactures to have proper care in choosing the test equipment for their devices, to ensure it can transmit (capture)
the 802.11ac signals to (from) their devices.
Given the limited spectrum availability, and significant design and test challenges, the 160 MHz channel is specified “optional” in
the currently available 802.11ac draft (D1.1).
ƒ
1
ƒ
2
ƒ
3
ƒ
4
0 dBr
-20 dBr
-28 dBr
-40 dBr
Channel Size ƒ
1
ƒ
2
ƒ
3
ƒ
4
20 MHz 9 MHz 11 MHz 20 MHz 30 MHz
40 MHz 19 MHz 21 MHz 40 MHz 60 MHz
80 MHz 39 MHz 41 MHz 80 MHz 120 MHz
160 MHz 79 MHz 81 MHz 160 MHz 240 MHz
Figure 5. Spectral Mask for 20, 40, 80, and contiguous 160 MHz Channels.
In addition to a contiguous 160 MHz channel, the TGac has also specified an optional non-contiguous 160 MHz channel, which
uses two nonadjacent 80 MHz channels. Creating the proper spectral mask for the channel requires the following steps:
1.  The 80 MHz spectral mask is placed on each of the two 80 MHz segments
2.  Where both masks of the two 80 MHz channels have values between -20 dBr and -40 dBr:
  •  The resulting mask value is the sum of the two mask values in the linear domain
3.  Where neither mask has value between 0 dBr and -20 dBr:
  •  The resulting mask value is the highest of the two masks
4.  For any other frequency region,
  •  The resulting mask value is a linear interpolation in the dB domain between the two nearest frequency points with defined mask values
IEEE 802.11ac: What Does it Mean for Test? 9
0 dBr
PSD
Freq [MHz]
-200 -160
-121 -119
-80
-41 -39 39 41
80
119 121
160 200
-20 dBr
-25 dBr
-28 dBr
-40 dBr
Figure 6 shows an example of a transmit
spectral mask for a non-contiguous
transmission using two 80 MHz channels
where the center frequencies of the
two 80 MHz channels are separated by
160 MHz.
Figure 6. Example of 802.11ac 160 MHz NonContiguous Channel.
US
Europe
& Japan
India
China
5170
MHz
IEEE channel #
20 MHz
40 MHz
80 MHz
160 MHz
36
40
44
48
52
56
60
64
100
104
108
112
116
120
124
128
132
136
140
149
153
157
161
165
5330
MHz
5490
MHz
5710
MHz
5735
MHz
5835
MHz
5170
MHz
IEEE channel #
20 MHz
40 MHz
80 MHz*
160 MHz*
36
40
44
48
52
56
60
64
100
104
108
112
116
120
124
128
132
136
140
5330
MHz
5490
MHz
5710
MHz
5170
MHz
IEEE channel #
20 MHz
40 MHz
80 MHz
160 MHz
IEEE channel #
20 MHz
40 MHz
80 MHz
36
40
44
48
52
56
60
64
149
153
157
161
165
5330
MHz
5735
MHz
5835
MHz
149
153
157
161
165
5735
MHz
5835
MHz
Channelization
As mentioned, the choice to restrict
usage of 802.11ac in the 5 GHz RF band
only was dictated by the wider channel
bandwidth requirements for 802.11ac,
which makes channel layout challenging
in the crowded 2.4 GHz band. Even
in the 5 GHz spectrum, however, the
availability of 80 MHz and 160 MHz
channel is somewhat limited, especially
in some regions.
Figure 7 shows the current spectrum
availability for 802.11ac operation, by
geography. Availability is greatest in the
US, with five 80 MHz channels and two
160 MHz channels. Europe and Japan
follow closely, lacking only the highest
frequency 80 MHz channel. In India, the
availability is almost halved and in China,
only one 80 MHz channel is available for
devices to operate.
Figure 7. Current spectrum availability for
802.11ac operation in select geographic
markets