Know The Ins And Outs Of Testing IEEE 802.11ac

Currently, wireless-local-area-networking (WLAN) standards are evolving to satisfy an increasing number of usage models. These efforts have spawned two IEEE project groups. Working Group TGac wants to specify IEEE 802.11ac as an extension of 802.11n. Running in the 5-GHz band, this standard will provide a minimum of 500 Mb/s single link and 1 Gb/s overall throughput. In partnership with the Wireless Gigabit Alliance (WiGig), Working Group TGad has proposed IEEE 802.11ad. It provides short-range throughput ranging to 7 Gb/s using about 2 GHz of spectrum at 60 GHz. In an 11-page application note titled, "Testing New-Generation Wireless LAN," Agilent Technologies [4] looks at the testing needs of IEEE 802.11ac in particular.

As an extension of the IEEE 802.11n standard, the IEEE 802.11ac PHY is already backward compatible with 802.11n. The theoretical maximum data rate for IEEE 802.11ac is 6.93 Gb/s using 160-MHz bandwidth, eight spatial streams, MCS9 with 256QAM, and a short guard interval. In contrast, the theoretical maximum data rate for IEEE 802.11n is 600 Mb/s using 40-MHz bandwidth with four spatial streams.

Some of IEEE 802.11ac's new features add complexity to both design and test. 256QAM demands better error-vector-magnitude (EVM) or constellation-error performance in the transmitter and receiver. EVM issues may be derived from imperfections in the in-phase/quadrature (I/Q) modulator, phase noise or error in the local oscillator (LO), or amplifier nonlinearity. To measure and identify causes of poor EVM, vector signal analysis may be used.

When it comes to 80-MHz-bandwidth signals, a lot of RF signal generators do not have a sufficiently high sampling rate to support the typical, minimum 2X oversampling ratio. The result may be images in the signal due to aliasing. With proper filtering and oversampling of the waveform file, however, 80-MHz signals can be generated with good spectral characteristics and EVM.

To generate 160-MHz-bandwidth signals, a wideband arbitrary waveform generator (AWG) can be used to create the analog I/Q signals. Those signals, in turn, can be applied to the external I/Q inputs in a vector signal generator for upconversion to RF frequencies. Alternatively, a 160-MHz-bandwidth signal can be created using 80+80-MHz models to create two 80-MHz segments in separate signal generators. The RF signals would then be combined.

By going from an overview of the standard to test needs and how they can be satisfied, this application note succeeds in providing essential information about IEEE 802.11ac. The note underscores the need for system simulation tools and the generation and analysis of 80- and 160-MHz-bandwidth signals and 256QAM for 802.11ac. With IEEE 802.11ac slated to be finalized at the end of 2013, such information is much in demand.

Agilent Technologies, Inc. [4]
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