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Testing New-Generation WLAN 802.11ac
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Introduction
The first popular standards for wireless LAN (IEEE 802.11a and b) were designed primarily
to serve the needs of a laptop PC in the home and office, and later to allow connectivity “on
the road” in airports, hotels, Internet cafes, and shopping malls. Their main function was
to provide a link to a wired broadband connection for Web browsing and email. Since the
speed of the broadband connection was the limiting factor, a relatively low-speed wireless
connection was sufficient—802.11a provided up to 54 Mb/s at 5 GHz, and 802.11b up to
11 Mb/s at 2.4 GHz, both in unlicensed spectrum bands. To minimize interference from other
equipment, both used forms of spread-spectrum transmission and were heavily encoded.
A later revision of the standard, 802.11g in 2003, consolidated use in the 2.4 GHz band but
maintained the maximum data rate at 54 Mb/s. However, by the same time, new usage
models with the need for higher throughput had been recognized: data sharing amongst
connected devices in the home or small office and wireless printing as examples. A study
project was set up which produced 802.11n in 2009. As well as improving the maximum
single-channel data rate to over 100 Mb/s, this new standard introduced MIMO (multiple
input, multiple output) sometimes referred to as spatial streaming, where up to 4 separate
physical transmit and receive antennas carry independent data that is aggregated in the
modulation/demodulation process.
Technical Differences From 802.11n
The 802.11ac physical layer is an extension of the existing 802.11n standard,
and as already discussed, maintains backward compatibility with it. The follow
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ing discussion highlights the changes. Table 2 shows the physical layer features
of 802.11n, and Table 3 shows how this is extended for 802.11ac. The theoreti
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cal maximum data rate for 802.11n is 600 Mb/s using 40 MHz bandwidth with
4 spatial streams, though most consumer devices are limited to 2 streams. The
theoretical 802.11ac maximum data rate is 6.93 Gb/s, using 160 MHz bandwidth,
8 spatial streams, MCS9 with 256QAM modulation, and short guard interval.
A more practical maximum data rate for consumer devices might be 1.56 Gb/s
which would require an 80 MHz channel with 4 spatial streams, MCS9, and nor
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mal guard interval
Technical Differences From 802.11n
The new wider mandatory channel bandwidths are shown in Figure 2 for the
U.S. region, along with possible placements of the non-contiguous 80+80 MHz
channels specified in the standard. Note that due to the need to avoid operation
in channels that may interfere with weather radars, in certain locations there may
only be one available 160 MHz channel. While 160 MHz and 80+80 MHz modes
are both included as optional features in the 802.11ac standard, the first devices
available – wireless routers and dongles – have settled on 3x3 and 2x2 spatial
streams respectively
Test Requirements
The high volumes for WLAN devices call for strict attention to manufacturing
costs, and the use of innovative design techniques to maximize repeatability
and minimize cost of test. This leads to the need for exhaustive testing during
the design and pre-production stages of development, and optimized produc
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tion test for both components and complete devices.
The 802.11ac standard includes the transmitter and receiver tests shown in
Table 4. These are similar to the tests for 802.11n, with some new definitions
and specification limits added to cover the new features in 802.11ac. To get
the latest test specifications, download the current version of 802.11ac from
www.ieee802.org
, and see section 20.3.20 for transmitter specifications and
section 22.3.18 for receiver specifications. In addition to these tests, designs will
need to pass conformance tests and additional functional tests to verify perfor
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mance and prove interoperability
Design and Test Challenges
EVM is critical
Some of the new features in the 802.11ac standard result in new challenges in
design and test. One of these is the use of 256QAM modulation, which requires
better error vector magnitude (EVM) or constellation error in the transmitter
and receiver. EVM problems may be caused by imperfections in the IQ modula
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tor, phase noise or error in the LO, or amplifier nonlinearity. Vector signal analy
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sis is a valuable tool for measuring and identifying causes of poor EVM, and the
Keysight 89600 VSA software provides detailed analysis of 802.11ac signals,
with support for all bandwidths and modulation types and up to 8x8 MIMO.
Power amplifier needs to be designed for linearity and efficiency
Improving amplifier linearity and power efficiency are other challenges in multi-
format handsets. New techniques have been introduced to improve the linear
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ity and decrease the power consumption of the power amplifier, Linearity is
improved using digital pre-distortion (DPD), where the input is adjusted to give a
flat output, and envelope tracking (ET) improves the power efficiency of the am
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plifier by allowing the amplifier’s drain bias to track the magnitude of the input
signal envelope. ET offers significant advantages in terms of improved battery
life and RF PA performance, along with reduced heat dissipation.
Designing an ET PA is challenging since it has to be treated as a 3-terminal ac
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tive device. It requires a low noise, high bandwidth power supply, and designing
and optimizing a shaping curve or table (which determines characteristics of the
ET system).
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