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.
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