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MIMO measurements require multiple signal analyzers. The number depends on the number of data streams you have to capture.

For valid measurement results, the frequencies of the analyzers in the test setup have to be synchronized. It is also necessary to configure the trigger system properly to cap- ture the data simultaneously.

Synchronizing the frequency

The frequency of the analyzers in the test setup have to be synchronized. Thus, one of the analyzers (master) controls the other analyzers (slaves) in the test setup. The mas- ter analyzer has to be equipped with the LTE MIMO application and provides the refer- ence oscillator source for the slave analyzers.

► Connect the REF OUT of the master to the REF IN connector of the slaves. Make sure to configure the slaves to use an external reference (➙ General Setup menu). If you are using a measurement setup with several R&S signal generators (for example R&S SMW), the situation is similar. One of the generators controls the other via the external reference.

► Connect the REF OUT of the master to the REF IN of the slaves. Make sure to configure the slaves to use an external reference (➙ Reference Oscillator settings). Triggering MIMO measurements

For valid MIMO measurements, it is crucial to capture all data streams simultaneously. To do so, you need a trigger signal provided by the DUT or the signal generator. The trigger signal has to be connected to all analyzers. If you have several signal genera- tors in the setup, the master generator has to trigger the slave as well.

The 8-2 shows a MIMO setup with two (or optional four) analyzers and one (or optional two) signal generators with two channels.

Fig. 8-2: MIMO Hardware Setup

You can use several trigger configurations, with or without additional hardware.

Measurements with a delayed trigger signal

Simultaneous capture of the I/Q data requires the trigger inputs of all instruments in the setup to be armed.

Arming a trigger does not happen immediately when you start a measurement, but is delayed slightly for a number of reasons, for example:

● Connecting several instruments with a LAN or GPIB connection usually causes a certain network delay.

● Tasks like the auto leveling function require some time to finish.

Because of these factors, you have to make sure that the trigger event does not occur during this time frame. You can do so, for example, by configuring an appropriate delay time on the DUT.

The exact delay depends on the GPIB or network condition and the input settings. You can estimate the delay by performing a single measurement on one analyzer. Measure the time it takes until the "DSP" indicator starts flashing.

Note that this estimation also includes the time it takes to transfer the I/Q samples from the analyzer to the software.

A typical delay to arm the trigger is 2 seconds per instrument.

The minimum delay of the trigger signal must now be greater than the measured time multiplied with the number of measured antennas (the number of analyzers), because the spectrum analyzers are initialized sequentially.

The usage of an LTE frame trigger is not possible for this measurement setup.

Measurements with a frame trigger signal

You can use a frame trigger if all transmitted LTE frames use the same frame configu- ration and contain the same data. In this case, the analyzers in the test setup capture data from different LTE frames but with the same content.

This method to analyze data, however, raises one issue. The phase variations of the reference oscillators of the different signals that are transmitted are not the same, because the data is not captured simultaneously.

The result is a phase error which degrades the EVM (see the figures below).

An application for this measurement method is, for example, the test of the MIMO pre- coding implementation. Because of the bad EVM values, it is not recommended to use this test setup to measure hardware performance.

Fig. 8-3: Constellation diagram

Fig. 8-4: EVM vs OFDM symbol number

Measurements with the R&S FS-Z11 trigger unit

The trigger unit R&S FS-Z11 is a device that makes sure that the measurement starts on all analyzers (master and slaves) at the same time.

Connecting the trigger unit

► Connect the NOISE SOURCE output of the master analyzer to the NOISE SOURCE CONTROL input of the trigger unit.

► Connect the EXT TRIG inputs of all analyzers (master and slaves) to the TRIG OUT 1 to 4 (or 1 and 2 in case of measurements on two antennas) of the trigger unit. The order is irrelevant, that means it would be no problem if you connect the master analyzer to the TRIG OUT 2 of the trigger unit.

With this setup, all analyzers (including the master analyzer) are triggered by the trigger unit.

The trigger unit also has a TRIG INPUT connector that you can connect an external trigger to. If you are using an external trigger, the external trigger supplies the trigger event. If not, the analyzer noise source control supplies the trigger event. Note that if you do not use an external trigger, the TRIG INPUT must remain open.

To use the R&S FS-Z11 as the trigger source, you have to select it as the trigger source in the "General Settings" dialog box of the LTE measurement application. For more information see "Configuring the Trigger" on page 81.

RF INPUT TRIGGER INPUT Master Analyzer RF INPUT TRIGGER INPUT Slave Analyzer 1 RF INPUT TRIGGER INPUT RF INPUT TRIGGER INPUT RF OUTPUT 1 TRIGGER OUTPUT DUT Cable Trigger

Cable Trigger Optional (DUT with TRIGGER OUTPUT) Cable RF TRIG INPUT TRIG OUT 2 TRIG MANUAL TRIG OUT 3 NOISE SOURCE FS-Z11 Trigger Unit NOISE SOURCE Slave Analyzer 1 Slave Analyzer 1 RF OUTPUT 2 RF OUTPUT 3 RF OUTPUT 4 TRIG OUT 1 TRIG OUT 4