Error vector magnitude (EVM) defined as the vector difference between the ideal error-free decision points in a signal constellation and that of a measured signal, is a direct measure of the modulation accuracy and overall signal quality of the transmitter. EVM captures both amplitude error and phase error and reduces the many parameters that characterize distortions of a transmitted RF signal into a single one.

Parameters such as transmit EVM, symbol constellation diagram, transmitter power, transmitter spectral mask, and crosstalk between transmitters are all important measurements of the transmitted signal quality. They provide valuable information about the root cause of impairments during the phases of MIMO chip design, product development and design validation. Accurately measuring these parameters becomes even more important to designers of MIMO-OFDM-based systems due the complexity involved when multiple radios operate in the same frequency.

To measure and analyze the above parameters, designers and test engineers require a test instrument that combines a vector signal analyzer (VSA), spectrum analyzer, and a power meter. Figure 4 shows the block diagram of the IQnxn MIMO test system from LitePoint Corporation. The system, as shown, consists of two VSAs and two vector signal generators (VSGs). This test system is scalable and can be expanded to include more VSAs and VSGs to enable concurrent measurement of MIMO devices.

Figure 5 shows the measurement results of a device under test (DUT) with MIMO radio. The data rate transmitted from the DUT measures 108 Mbps total with a 54 Mbps datastream coming from each transmitter. The RF signal was sent through coax cables with 20 dB attenuators. The IQnxn MIMO system from LitePoint measured the following key parameters simultaneously in a single capture. Those measurements provide detailed information about each individual transmitter.

The transmitter spectral mask for both channels is shown at the top left and center window of the display. The blue trace is the power spectrum density. The red trace defines the spectrum mask as defined by the 802.11 standard. For each Tx channel in the MIMO system, the power spectrum measurement needs to stay below the spectrum mask limit in order to reduce adjacent-channel interference.

The time domain waveforms for both transmitters are shown at the top right window.

The channel estimation results are shown at the middle right window of the display. Channel 1 [h₁₁] and channel 2 [h₂₂] display the channel flatness and transmitter power balance of the OFDM signals. Channel 3 [h₂₁] and channel 4 [h₁₂] display isolation or leakage between channels.

Average EVM over each subcarrier for both channels is shown in the middle left window of the display. Notice that both EVM measurements are reasonably good over the 52 subcarriers except at the edge of channel 2 where EVM degrades due to group delay. Group delay imbalance between I and Q signal paths can adversely affect modulation accuracy and cause constellation distortion. Such an imbalance usually relates to the different trace lengths and dielectric constant of the PCB layer for the baseband I and Q signals. The difference in the I and Q signal paths inside an RFIC can also contribute to the imbalance. Group delay imbalance is frequency dependent, thus, affects each OFDM subcarrier differently. Typically, the end subcarrier will be affected the most.

The symbol constellation diagram combines with the corresponding system EVM measurement to give a good indication of signal quality. Sharp, well-defined points in the symbol constellation diagram represent good signal quality while distorted or smeared symbol constellation points represent poor signal quality. Designers can use a symbol constellation diagram as an easy way of qualitatively assessing and diagnosing adverse impacts such as I/Q imbalance, phase noise, and amplitude compression.

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