MIMO RFIC Design Verification
RFIC devices are production tested for compliance in a cabled RF environment with only one transmission path per RF port. Although this is adequate for production test, verification of operation or design performance in a true MIMO mode requires the simulation of the multipath transmission of a highly-scattered open-air environment. This section discusses additional design verification techniques.
Other Transmitter Tests
In addition to the parallel test configuration specified by the IEEE 802.11 WLAN standards, there are two other MIMO transmitter test configurations that use a single VSA. Figure 8 shows the additional combined VSA and switched VSA MIMO transmitter test configurations. Both configurations reduce test equipment costs.
Figure 8) There are several MIMO transmitter test configurations
Combined VSA Tests
The combined VSA transmitter test configuration is a step closer to approximating an open-air environment where two transmitted signals are received by a single antenna. Note that the RF combiner must have very good isolation to prevent interaction between transmitters (which causes intermodulation distortion). The combined transmitter configuration offers a different method to measure composite EVM performance. For example, one transmitter may create an in-band spurious signal that degrades the EVM of all of the other MIMO transmitters.
Also, a combined VSA configuration can be used to test some MIMO operational modes, such as STBC, where time-shifted data streams are received at a single VSA. Note that SDM cannot be tested with the combined VSA configuration because the two signals cannot be spatially separated.
Switched VSA Tests
The switched VSA transmitter test configuration uses multiple sequential VSA captures on a repeating waveform, and processes the sequential data as if it was transmitted simultaneously. The switched transmitter test configuration is very flexible and can simulate operational modes that use a multipath environment, such as STBC demodulation and SDM demodulation. The device under test (DUT) must be capable of generating a sequential or repeating waveform that can be synchronized over multiple captures within the VSA. Results will be more susceptible to timing jitter and phase variations between captures. Also, due to the sequential captures, test time is longer than the parallel VSA or combined VSA configurations.
Interleaved Subcarrier Test
The interleaved subcarrier test provides a measure of transmitter-to-transmitter signal isolation and can be performed using a standard parallel VSA transmitter test configuration. This test creates an interleaved set of subcarriers on two transmitters by offsetting the center frequency of one transmitter by one-half of the OFDM subcarrier spacing. For WLAN, where subcarrier spacing is 312.5 kHz, the center frequency is offset by 156.25 kHz. In this test, each VSA captures a packet and separates out the long training sequence. Measuring the spectrum of the time-gated LTS results in both desired and interfering subcarriers tones.
Other Receiver Tests
In addition to the parallel VSG test configuration specified by the IEEE 802.11 WLAN standards, there is another MIMO receiver test configuration that uses a single VSG. Figure 9 shows the split MIMO receiver test. Similar to the single-VSA transmitter test configurations, the split receiver test configuration reduces the amount of required test equipment.
Figure 9) The split-MIMO receiver test configuration requires only one VSG.
The split VSG configuration offers fast test times because all setup and testing is performed simultaneously. In the split VSG configuration, an identical signal applied to all receivers provides an input sensitivity gain as compared to a single receiver. (Note that STBC and SDM cannot be tested with the split VSG configuration because the two signals are identical.)
A split VSG testing technique (based upon the emulation of the keyhole effect) can assist with MIMO system design verification  . Within this test, the split VSG configuration applies the identical signal to all receiver inputs and the MIMO channel matrix is estimated. An ideal MIMO receiver would result in a channel matrix of one dimension where all signal path coefficients are equal to either 0 or 1. A single dimension matrix indicates that the MIMO channel capacity is equal to that of a SISO system. In a test scenario, noise in the receiver or an imperfect channel estimate creates signal path coefficients not equal to the ideal coefficients of 0 or 1. The deviations from ideal provide a measure of receiver performance.
The VSG flexibility offers the ability to perform MIMO receiver testing in simulated multipath environments. The VSG uses an arbitrary waveform generator (AWG) to create any type of I/Q modulation waveforms. This allows the simulation of fading channels within the cabled RF connections. Other RF channel imperfections can also be simulated such as spurious signals, noise, and distortion. This type of simulation offers a flexible and powerful design verification and characterization tool.