Accurate testing of MIMO transceivers presents significant challenges to existing test instrumentation architectures. New architectures require not only advanced signal processing algorithms to multiplex and demultiplex various spatial streams but also tight synchronization between each transmit and receive antenna. The following section addresses the challenges of synchronization and illustrates how a hybrid PXI/benchtop instrumentation approach can be used to provide baseband and RF synchronization.
Synchronization of baseband generation and acquisition instrumentation is the first step to synchronizing RF signals. In a typical test configuration, baseband signals are generated and acquired with standard arbitrary waveform generators (AWGs) and high-speed digital oscilloscopes. On the generation side, the significance of baseband synchronization is illustrated with a conventional direct conversion modulator block diagram where t corresponds to the clock distribution delay between the two digital-to-analog converters (DACs). The discrete time samples of the in-phase (I) and quadrature (Q) waveforms given by I[n] and Q[n], respectively, are converted to analog waveforms I(t) and Q(t-t). This is shown in Figure 16:
Figure 16. Modulator with I/Q Skew Induced by Clock Delay
As Figure 16 illustrates, the local oscillator (LO) is mixed with each of the baseband I(t) and Q(t-t) signals to produce the RF output. Thus, phase skew between I[n] and Q[n] results in quadrature skew at the RF output.
With National Instruments PXI signal generators, the NI-TClk API can be used to synchronize multiple AWGs with a common 10 MHz system reference clock. In addition, these modules can be synchronized with a typical slot-to-slot skew of 100 ps with the following three VIs.
Figure 17. Basic Functions of the NI-TClk Driver
In addition, baseband generators must also meet a bandwidth requirement. Because the IEEE 802.11n specification includes signals with a bandwidth of 40 MHz, multiple NI PXI-5422 AWGs (200 MS/s, 80 MHz bandwidth) can be synchronized to generate each of the baseband signals. Moreover, with high-bandwidth baseband signals, tight synchronization between instruments becomes critically essential.
Measuring the error vector magnitude (EVM) of a baseband signal is one way to characterize both the amplitude and phase accuracy of the generated signals. In addition, EVM measured from synchronized PXI-5422 modules is measured at -57 dB. This level is easily sufficient to meet the test requirements for 802.11n test systems.
This is illustrated in Figure 18, which shows the degradation in EVM as t increases from 100 to 1,000 ps for a 40 MHz bandwidth 802.11n signal.
Figure 18. EVM versus I/Q Skew for 40 MHz Bandwidth 802.11n Signal
Figure 18 shows that skew on the order of several hundred nanoseconds provides a hard-to-detect and significant EVM contribution into the measurement system. Skew on the order of milliseconds can render the measurement system useless for many testing applications.
The same skew requirements needed for baseband signal generation applies equally to baseband signal analysis. The multimodule synchronization capability is also built into the NI PXI-5124 digitizers and, therefore, provides an equally suitable solution for signal analysis of MIMO devices. These modules have 12-bit resolution and 200 MS/s sampling rate, and they also provide synchronization to within 20 ps. The EVM contribution from these modules was measured at -48 dB, which is well within the requirements for engineering grade test equipment. An added benefit of using both the PXI-5422 and PXI-5124 is that the signal generator and analyzer can be synchronized as well. This is useful for specific MIMO testing modes requiring simultaneous signal generation and signal analysis.
Local Oscillator Synchronization
Tight baseband synchronization is only one step to ensure that each transmitter of a MIMO system is properly synchronized. As previously mentioned, a MIMO system requires that the phase between each transmitter antenna remain constant. When synchronizing baseband I and Q signals, skew should be minimal to prevent distortion of the RF signal. When synchronizing multiple RF signals, phase skew between each of the RF signals is tolerable but should be minimized as much as possible.
Typical Test Architectures: The Hybrid Approach
One approach to ensure both baseband and RF synchronization is using a hybrid test architecture featuring both PXI and traditional instruments. With this architecture, customers can benefit both from the synchronization features available in PXI and the synchronization capabilities of benchtop RF instruments with a 10 MHz reference clock. A typical test architecture for a MIMO system is shown in Figure 19:
Figure 19. Hybrid PXI/Traditional Instrument MIMO Test Architecture
As Figure 19 illustrates, a 2x2 MIMO configuration requires four channels of synchronized baseband I and Q signals. The baseband generation occurs in the PXI platform to take advantage of the synchronization capabilities of PXI. In addition, two benchtop vector signal generators are able to share a common 10 MHz reference clock to ensure synchronization of the RF signals.
IEEE 802.11n Software Implementation
National Instruments recommends the Lyocom WLAN Test Suite software to provide a complete signal generation and signal analysis solution for the 802.11a/b/g/n standards. As described above, MIMO systems require significant signal processing to correlate each of the input streams from the RF antenna. Using digital signal processing (DSP) techniques in the NI LabVIEW programming environment, engineers can demultiplex and analyze each of the original transmitted streams.
The Lyocom WLAN Test Suite was developed for both stand-alone operation and automated testing environments. A menu-driven GUI combines intuitive hardware control with easy-to-navigate analysis controls and display options. The solution can also be seamlessly integrated into an automated testing environment with the extensive LabVIEW programming interface. The solution comes with more than 100 well-documented VIs providing a wide range of testing possibilities. Various examples and flow diagrams are also included to take the guesswork out of integration. A screenshot of the software suite is shown in Figure 20.
Figure 20. WLAN Test Suite Software
While MIMO communications systems and, specifically, the IEEE 802.11n standard present many new challenges for test engineers, these challenges can be met through the implementation of hybrid PXI/benchtop instrumentation. Fundamentally, this architecture is well-suited to address challenges of MIMO test systems including baseband synchronization, RF synchronization, and advanced signal processing routines.
To learn more about the products used to generate and acquire baseband signals, visit ni.com/modularinstruments.
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