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Conquer Testing Of IEEE 802.11ac MIMO

Dec. 21, 2012
By combining simulation with signal generators, signal analyzers, and oscilloscopes, engineers can quickly evaluate MIMO transmitter hardware EVM performance.

The IEEE 802.11ac wireless-local-area-network (WLAN) standard features multiple-input multiple-output (MIMO) support with up to eight spatial streams, despite the fact that only one is mandatory. In a five-page application note titled “Solutions for Design and Test of 802.11ac MIMO,” Agilent Technologies delves into the design and test challenges posed by MIMO and how they can be handled. MIMO’s complexity creates challenges for the IEEE 802.11ac system engineer—especially when it comes to RF transmitter design. An IEEE 802.11ac system’s MIMO performance can be degraded by MIMO spatial multiplexing algorithms, multiple transmit/receive RF chains, multiple antennas, and system-level impairments. Among other issues is the need to establish RF transmitter performance required for 256QAM operation as well as the local-oscillator (LO) phase-noise performance needed to meet an error-vector-magnitude (EVM) specification for this modulation.

Such challenges make it essential to understand any design issues prior to hardware testing. In the research and development (R&D) testing phase, however, system engineers must create and analyze MIMO test signals with enough flexibility to address the large number of spatial streams, bandwidths, and modulation formats supported by IEEE 802.11ac. When measuring the transmitter’s output EVM, any problems must be isolated to either the in-phase/quadrature (I/Q) or intermediate-frequency (IF) stages. Alternatively, there could be a crosstalk or timing issue. 

To overcome IEEE 802.11ac MIMO system design and test challenges, the note suggests the use of software design simulation to evaluate design-performance tradeoffs. For example, the designer can change the LO phase noise being modeled in dBc/Hz to determine the phase-noise performance required for 256QAM versus 64QAM. Power-amplifier (PA) linearity, I/Q modulator gain/phase impairments, and mixer/filter impairments also can be modeled. In the R&D testing phase, MIMO test signals can be generated by combining the simulation with test equipment. The different spatial streams, bandwidths, and modulation orders supported by IEEE 802.11ac can be addressed. With the addition of wideband oscilloscopes and modular test instruments, MIMO EVM testing can be performed.

Agilent Technologies, Inc., 5301 Stevens Creek Blvd., Santa Clara, CA 95051; (408) 345-8886, www.agilent.com.

About the Author

Nancy Friedrich | RF Product Marketing Manager for Aerospace Defense, Keysight Technologies

Nancy Friedrich is RF Product Marketing Manager for Aerospace Defense at Keysight Technologies. Nancy Friedrich started a career in engineering media about two decades ago with a stint editing copy and writing news for Electronic Design. A few years later, she began writing full time as technology editor at Wireless Systems Design. In 2005, Nancy was named editor-in-chief of Microwaves & RF, a position she held (along with other positions as group content head) until 2018. Nancy then moved to a position at UBM, where she was editor-in-chief of Design News and content director for tradeshows including DesignCon, ESC, and the Smart Manufacturing shows.

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