Channel Emulator Exercises MIMO

April 11, 2011
This single instrument supports a wide range of MIMO configurations over a continuous frequency range of 400 MHz to 6 GHz, and can even synchronize multiple units for larger challenges.

Wireless communications systems must deliver for their users, whether the "package" is voice, data, or video information. Because of the need to maintain high-quality service under constantly changing channel conditions, newer technologies have adopted the use of multiple antennas in multiple-input, multiple-output (MIMO) arrangements to capture as much of a transmitted signal as multipath, fading, and other obstacles may allow. Unfortunately, the added complexity makes testing more complicated.

But Spirent Communications, a company with experience in building compact channel emulators, has created a test solution for large mimo systems and receivers: the VR5 HD spatial channel emulator. It simplifies receiver testing for the latest wireless technologies, such as Long-Term Evolution (LTE) and LTE-Advanced, that use MIMO techniques.

The VR5 HD spatial channel emulator (see figure) is a "one-box" solution to a complex test problem. Obviously, with more antennas, a wireless communications system can grow in complexity. Trying to represent signal paths in a multiple-antenna setup can challenge a test operator. The VR5 HD Spatial Channel Emulator makes it simple with a touchscreen graphical user interface (GUI).

With the touch of a finger, a test operator can make connections from input to output and rather quickly define unidirectional and bidirectional MIMO arrangements as large as 4 x 4 in bidirectional emulations and 8 x 4 in unidirectional configurations. These on-screen arrangements can represent test scenarios with literally dozens of radio communications channels.

"This unprecedented level of test integration demands a new way to control it, so the VR5's touchscreen Gui was built on a completely user-oriented design philosophy," says Rob VanBrunt, Vice-President of Spirent's Wireless Group. The GUI also provides step-by-step graphical feedback during setup and execution to help operators avoid mistakes and speed setup time.

In spite of its single-box architecture, the VR5 packs an enormous amount of signal-processing power and measurement capability into its small frame. It supports channel bandwidths as wide as 100 MHz at carrier frequencies from 400 mHz to 6 GHz. It can create channel emulations based on input signal levels from -50 to +10 dBm with 0.1-dB resolution. Its frequency stability is tied to an internal 10-mHz reference oscillator with 1 ppm stability, but it can also be locked to an external frequency reference.

It provides RF output levels from -110 to -20 dBm (to +5 dBm on peaks) with 0.1-dB resolution and 1-dB accuracy. Its spectral purity is impressive, with residual noise of better than -170 dBc/Hz for an output level at -40 dBm. The residual error vector magnitude (EVM) is typically better than -40 dB per subcarrier, measured at a -50-dBm output level for a 20-MHz LTE channel.

The VR5 controls as many as 24 paths per digital channel with relative path delays from 0 to 2000 s (and 0.1-ns resolution). Relative path losses can be set from 0 to 50 dB. For ease of use, the emulator provides many different standards-based models, including lte, WiMAX, UMTS, CDMA2000, Wlan, WiFi, and GSM channel models.

To study the effects of signal impairments on a communications system and its devices, the VR5 HD spatial channel emulator is also available with an additive white Gaussian noise (AWGN) source which can equip the emulator with an adjustable carrier-to-noise (C/N) ratio range of -40 to +32 dB with 0.1 dB accuracy. Noise impairments can be added over channel bandwidths as wide as 100 MHz.

In addition to evaluating MIMO receiver performance, the VR5 HD spatial Channel Emulator can be used for various other measurement applications, including virtual drive testing, dynamic environment emulation, and direct data playback to record actual outside radio environments for in-lab use.

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