WWireless systems must often contend With limited range and interference. For that reason, multiple-input, multiple-output (MIMO) technology is being applied to many emerging wireless standards, including 802.11n WLAN, Mobile WiMAX Wave 2 and 3GPP Long-Term Evolution (LTE) systems. The improved performance of MIMO approaches brings with it increased complexity, and the challenge of testing the hardware using MIMO methods. Fortunately, the N5106A PXB MIMO Receiver Tester from Agilent Technologies (Santa Clara, CA) offers flexible signal generation and channel emulation capabilities for exercising equipment based on MIMO technology, including the latest LTE and WiMAX standards.

MIMO technology is attractive for many wireless system operators since it helps deliver higher data rates with increased spectral efficiency. It does, however, also result in more complex receivers. In a wireless system, the wireless channel must be considered a system component, since it plays a role in determining the overall system performance. To achieve optimal MIMO operation, an engineer must be able to accurately test a MIMO receiver under real-world conditions. But testing receivers under realistic channel environments remains a critical challenge due to the large combination of variables that must be tested in a given MIMO configuration. The N5106A PXB MIMO Receiver Tester is specifically designed for the complexities of MIMO and provides a fast, accurate and scalable way for engineers to replicate real-world MIMO conditions and channels (Fig. 1).

The N5106A PXB MIMO Receiver Tester is a multipurpose solution offering signal creation and channel emulation capabilities, as well as real-time fading of MIMO signals. It can be used to quickly and accurately test and troubleshoot MIMO receivers with 2 x 2, 2 x 4 and 4 x 2 configurations. Agilent’s Signal Studio signal-creation software runs in the instrument and provides access to a library of the most up-to-date standardscompliant and specialized waveforms. The PXB features as many as four baseband generators (BBGs), eight faders, a wide modulation/fading bandwidth of 120 MHz, and custom MIMO correlation settings (e.g., predefined channel models, antenna pattern and custom correlation matrix).

DUAL VSGS
Figure 2 shows a simplified configuration with the PXB for testing a 2 x 2 MIMO receiver. For signal upconversion (signal generation), the PXB is connected with two RF vector signal generators (VSGs) such as the Agilent N5182A MXG VSGs. The PXB’s internal BBGs enable the playback of custom waveforms created by signal creation software tools such as Agilent’s Advanced Design System (ADS) or Matlab, and can also playback LTE and WiMAX waveforms created by Agilent Signal Studio. For baseband receiver designs, the PXB is connected to the Agilent N5102A Digital Signal Interface Module in support of digital baseband outputs.

The BBGs are easily connected to the faders through a software graphical user interface (GUI). Each fader can be independently configured with a standards-compliant channel model such as the WiMAX International Telecommunication Union (ITU) Pedestrian B, or custom-configured model using a variety of path and fading conditions.

The PXB can be configured to fade RF signals using Agilent’s MXA signal analyzers as RF inputs. The MXA downconverts the RF signal and sends it to the PXB for real-time fading. As an example, the block diagram in Fig. 3 shows a 2 x 2 MIMO RF fading system using MXA signal analyzers. In this configuration, the PXB connects the faded signals to MXG VSGs for upconversion back to RF signals.

Using the PXB MIMO Receiver Tester, R & D engineers can simulate real-world conditions in the laboratory that more quickly test corner cases and stress devices beyond standards requirements. They can also test coexistence with other wireless devices to ensure design robustness earlier in the design process. This is a critical capability since interference testing is typically difficult and not always thoroughly tested in simulation due of the complexity of the test combinations. In a 2 x 2 MIMO configuration, for example, using two separate single-input, single-output (SISO) channel emulators is simply not adequate to model the four separate channels that exist between the pairs of transmit and receive antennas. Using Agilent’s Signal Studio, ADS, or even an engineer’s own waveform creation tool to create waveforms, as many as four high-performance BBGs can be summed for multiformat coexistence testing. Each BBG supports 120-MHz modulation bandwidth with 512 MSamples of playback memory for long, complex signal simulation.

The N5106A PXB MIMO Receiver Tester specifically addresses the complexity of MIMO on three fronts: speed, accuracy, and scalability. In terms of speed, the PXB replicates real-world MIMO conditions using powerful digital signal processing (DSP) technology—as many as 12 DSP blocks can be configured from the PXB hardware. Each DSP block can be configured as a BBG or as a real-time fader with as many as 24 paths. The flexibility in configuring the DSP blocks allows as many as 4 BBGs to be used with as many as eight faders. The DSP blocks deliver modulation and fading bandwidths to 120 MHz.

The PXB’s digital signal processing capability makes it possible to rapidly isolate performance issues early in the design, development and verification cycle, and provides the quickest path for troubleshooting advanced radio components and systems. In addition, a faster and automated power calibration process and intuitive GUI speeds the time-to-measurement.

SCALABLE PLATFORM
At the heart of the PXB is a highperformance, scalable platform based on a field-upgradable architecture. The flexibility of this architecture ensures a simple and cost-effective upgrade of the PXB in an hour using field-installable cards. This capability enables the instrument to support the test needs of future technologies like 4G and higher-order MIMO implementations.

In terms of accuracy, the PXB provides a comprehensive set of selectable fading-power spectrum shapes for accurate modeling of various multipath channels, including the classical 6-dB, classical 3-dB, flat, and rounded models. The classical 6-dB model is the most commonly used model and adheres to the spectral requirements detailed in numerous mobile communications standards for Rayleigh fading conditions. Other models supported include the bell-shape, Jakes-classical and Jakesrounded models.

When testing MIMO receiver performance, information regarding any correlation between channels is crucial for accurate measurements. The Power Azimuth Spectrum (PAS) is an example of one spatial characteristic that may introduce correlations between the various MIMO channels. With its support for three widely used PAS distribution models—the Laplacian, Gaussian and Uniform—the PXB has the ability to accurately examine spatial effects introduced by such things as angular spread and antenna positions within the multipath environment (Fig. 4).

With its ability to support fast and accurate signal generation and real-time fading of MIMO signals, the PXB delivers a host of critical capabilities to ease the burden of R&D engineers developing and integrating MIMO receivers. To deal with the lengthy process of configuring custom correlation matrices targeted for a specific application, the PXB provides a set of pre-defined MIMO channel models based on the specifications of several wireless standards including Mobile WiMAX and LTE. The instrument is configured using a simple menu structure for selecting the channel model (Fig. 5).

Custom correlation matrices and path definitions may also be created and saved using the instrument’s intuitive GUI. This is a critical capability since an R & D engineer is continually looking to improve the process of entering the correlation matrices into a wireless channel emulator. At the same time, they want to minimize the mathematical complexity and be able to model realistic wireless channels. With the PXB, the engineer enters physical antenna characteristics directly into the instrument, greatly improving the MIMO channel emulation process.

Figure 6 shows the user interface for entering the receive (Rx) channel spatial parameters including antenna radiation pattern and spacing. A similar table is used to enter the transmit antenna parameters using the same menu. This spatial information, along with the signal’s angle of arrival and angle of departure (entered in the PXB’s fading paths table), is used to automatically calculate the correlation matrix. The resulting entry table eliminates the burden of calculating the correlation matrices and manually entering the coefficients into the emulator.

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