In conducting RF receiver sensitivity testing, engineers typically generate and send signals to a receiver, systematically dropping the power of the signal while monitoring the frame error rate (FER) of the IC or device being tested.  To verify performance, a representative mix of every possible frame type and combinations of frame sequences are sent to receivers under controlled conditions. 

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Even before 802.11n MIMO, testing every scenario was deemed inefficient. As a result, engineers typically limit test cases to particular scenarios and representative signal sequences that fit within the limitations of the test equipment, but are far from comprehensive coverage of what a device will experience in real networks. With 802.11n, this approach becomes a riskier proposition and a physical impossibility.

In traditional 802.11 a/b/g testing, expensive, complex software packages convert frame descriptions into files loaded onto general-purpose signal generators with RF upconverters.  These files contain “in-phase” and “quadrature” (I/Q) samples, enabling the I/Q signal generator to produce a single stream of digitally modulated frames. Changing from one frame definition to the next, such as moving from a 24-Mbit test case to one that’s 54 Mbits, means building the next I/Q file in software, downloading the file to the signal generator, and configuring the signal generator to produce the frame or loop on the loaded file. 

The real-time signal-generation method reduces test time by one-third versus traditional RF testing.
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Thorough testing entails building and managing separate I/Q files for each modulation case, in addition to combining frame types to see how receivers behave in the presence of multiple frame types. With 802.11n introducing multiple spatial streams, this approach becomes extremely expensive and unwieldy because general-purpose I/Q signal generators have only one output. 

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