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Testing WLAN Devices
According to IEEE 802.11
Standards
Application Note
Table of Contents
The Evolution of 802.11 ........................................................................... 4
Frequency Channels and Frame Structures ......................................... 5
Frame structure: 802.11a...................................................................... 5
Frame structure: 802.11b...................................................................... 6
Frame structure: 802.11g...................................................................... 7
Frame structure: 802.11n...................................................................... 8
Frame structure: 802.11ac.................................................................... 9
Transmitter Test ...................................................................................... 10
Receiver Test ........................................................................................... 13
Conclusion................................................................................................ 14
Related Information ................................................................................ 14
2
Introduction
Wireless local area networking (WLAN) capabilities are being integrated into an
increasing number of products: smartphones, digital cameras, printers, tablets,
HDTVs, Blu-ray players and more. This trend is leading to a greater number of
simultaneous connections to any given network in a home, school or business
environment. Even though each individual connection may not require a high
data rate, the cumulative demand results in higher requirements for total data
throughput.
To keep pace with these trends, every new generation of the various IEEE 802.11
standards--a, b, g, n, and ac--supports increased data capacity by providing
greater throughput and wider bandwidths. As the standards evolve, backward
compatibility remains a fundamental requirement. For example, modulation tech-
niques used in older standards, such as DSSS and FHSS, have carried forward
into newer standards of OFDM and MIMO.
The continuing evolution of 802.11 standards and products has important
implications for those who test WLAN or Wi-Fi transmitters, receivers, and
transceivers from R&D, through design verification and manufacturing. The
testing of receivers and transceivers requires the use of vector signal generators
capable of producing the complex modulated signals used by the 802.11 wire-
less connectivity standards. Software such as Agilent Signal Studio can be used
to create test signals--with or without impairments--and then download the
waveforms to a vector signal generator.
Testing transmitters and transceivers requires a signal analyzer configured
with sufficient frequency coverage and analysis bandwidth. Standard-specific
measurement and analysis capabilities are available in the Agilent N/W9077A
WLAN measurement applications that can run inside the Agilent X-Series signal
analyzers. These measurement applications provide one-button measurements
with pass/fail indicators.
This application note provides a broad survey of transmitter and receiver test
requirements with a focus on 802.11a, b, g, n, and ac. It also presents an over-
view of Agilent's test equipment, software, and measurement applications for
WLAN testing.
3
The Evolution of IEEE 802.11
Since the release of the initial version of 802.11 in 1997, the standard has
evolved to satisfy new applications and meet the need for ever-faster data rates.
A closer look: Along the way, a series of amendments have defined changes to the physical or
PHY layer (Table 1).
Modulation techniques
Table 1. Overview of the evolution of the 802.11 standards
The following modulation
techniques are used in the various IEEE Year Notes
standard released
802.11 standards.
802.11 1997 Provides 1- or 2-Mbps transmission in the 2.4 GHz band
Direct-sequence spread spectrum using either frequency-hopping, spread spectrum (FHSS),
(DSSS): This spreads a single carrier or direct-sequence spread spectrum (DSSS)
over a wider spectrum by multiplying the 802.11a 1999 Uses orthogonal frequency-division multiplexing (OFDM)
data bits with a special bit pattern called in the 5 GHz band and provides connections as fast as
a Barker key. Although this is typically 54 Mbps
an 11-bit pattern, 802.11b uses an 8-bit 802.11b 1999 Uses DSSS in the 2.4 GHz band and provides connections
key, at an 11 MHz chip rate. The net as fast as 11 Mbps, with fallback to 5.5, 2 and 1 Mbps,
result is a reduction in the interference depending on signal strength
caused by narrowband sources.
802.11g 2003 Uses OFDM in the 2.4 GHz band and provides 54 Mbps
Complementary code keying (CCK): connections
Used as a supplement to the Barker 802.11n 2009 Includes many enhancements to extend WLAN range,
Code, CCK enables a 2-Mbps data rate reliability and throughput. PHY-layer examples include
but reduces the transmission range due multiple-input/multiple-output (MIMO) and 20 or 40 MHz
to greater susceptibility to narrowband bandwidth. Operates in the 2.4 and 5 GHz bands and
interference. provides data rates of up to 600 Mbps. Also called High
Throughput or HT LAN.
Packet binary convolution coding 802.11ac 2012 (draft) Expected to provide very high throughput (VHT) data rates
(PBCC): Uses forward error correction of 1 Gbps in the 5 GHz band. Uses RF bandwidth of up to
to improve link performance in the 160 MHz, higher-order modulation such as 256QAM and
presence of excess noise. Scrambled up to eight MIMO spatial streams.
data is fed into a convolutional encoder,
which has a six-stage memory and
taps that are combined to produce When 802.11g was introduced, it became the preferred standard over
two outputs. The four possible output both 802.11a and 802.11b and was widely adopted by both consumers and
states (00, 01, 10 and 11) are mapped businesses. While 802.11a and 802.11g use the same OFDM modulation
onto a pair of QPSK states (11 Mbps). A scheme, they operate on different frequency bands- 2.4 GHz and 5 GHz,
codeword controls how the chosen state respectively- however, backward compatibility is still required. Later, when
alternates over time.
the 802.11n standard was introduced, offering breakthrough benefits including
enabling Wi-Fi networks to do more, faster, and over a larger area, IEEE
Orthogonal frequency-division multiplex
defined three modes in the physical layer and required backward compatibility
(OFDM): This splits the transmission
into a high-rate data stream and several with 802.11 a/b/g gear, even though they were later designated as legacy
lower-rate streams that are transmitted technologies.
simultaneously over several subcarriers.
Lower data rates in the parallel Three upcoming versions of the standard are also worth noting:
subcarriers result in longer symbol
duration, which decreases the relative