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Two-stage MIMO over-the-air testing

Posted: 09 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:over-the-air? Multiple-Input Multiple-Output? LTE?

Cost-effectively validating performance is still a significant concern for engineers involved with LTE device development. One way to accurately predict the real-world performance of a mobile device is through the use of over-the-air (OTA) testing, or in the case of LTE, Multiple-Input Multiple-Output (MIMO) OTA testing.

The move to OTA testing for LTE devices with MIMO is being driven by cellular operators, who observed differences between the field performance and conducted test performance of their Single-Input Single-Output (SISO) cellular devices. This led to a desire to do more field-like test in a lab environment to ensure field performance would be satisfactory to end users. OTA test is a good fit because it evaluates handset connectivity and allows device manufacturers and operators to evaluate simulated field performance. As a result, MIMO OTA test will soon be mandatory for LTE devices as part of 3GPP conformance test requirements in order to evaluate handset radiation performance. CTIA is also working to develop its own set of requirements around MIMO OTA for LTE devices. The goal in creating these standards is to be able to distinguish a good LTE MIMO device from a bad one.

Currently, three main methods for MIMO OTA testing are being considered the multiprobe method, the reverberation chamber method and the two-stage method. The multiprobe method utilizes an anechoic chamber that includes a set of probe antennas placed within the chamber and positioned around the MIMO device-under-test (DUT). The reverberation chamber method makes use of a metal cavity sized to support numerous cavity modes (reverberation chamber). Several challenges face each of these techniques. In contrast, the two-stage method provides an accurate and cost-effective MIMO OTA measurement system by combining the benefits of traditional anechoic chamber radiated measurements with the flexibility of digital channel emulation. Because it is fast, efficient and flexible, the two-stage approach now offers engineers a viable solution for design validation test until 3GPP and CTIA decide on which test method or methods will be supported for MIMO OTA testing.

With both 3GPP and CTIA each working to establish requirements for MIMO OTA testing of LTE devices, finding agreement on required measurement metrics and test conditions will be critical. One of the key test conditions is the choice of channel model. MIMO performance is strongly influenced by spatial correlation introduced by spatial spreading and antenna characteristics. As a result, it is extremely important to measure MIMO devices using an OTA test system that accurately emulates realistic channels. A simplified model of the multipath environment that needs to be modeled in a test environment is shown in figure 1.

Figure 1: A simplified model of a 2x2 multipath environment.

One solution touted to meet this challenge is Agilent Technologies' N5106A PXB baseband generator and channel emulator (figure 2). By delivering fully parameterized, real-time channel emulation capabilities for the latest LTE standards, the PXB replicates real-world MIMO conditions and channels, and generates realistic fading scenarios, including path and channel correlations. Such capabilities are critical to maximizing device performance, minimizing design uncertainty and reducing development cycle.

Figure 2: The N5106A PXB provides up to 4 baseband generators (BBGs), 8 fading channels, custom MIMO correlation settings (e.g., predefined channel models, antenna pattern and correlation matrix), and supports testing and troubleshooting of 2x2, 2x4, 2x6, and 4x2 MIMO.

The PXB emulates both predefined and custom channel models. Predefined LTE channel models used for MIMO OTA test include: modified SCME urban micro-cell, SCME urban micro-cell, SCME urban macro-cell, WINNER II, single cluster EPA, single cluster SCME, and 2D Uniform.

Two-stage MIMO OTA test
The two-stage technique is designed to provide a fast, accurate, cost-effective, and efficient means of MIMO OTA test. The flexibility of the PXB's channel emulation capabilities makes it ideal for engineers looking to use the two-stage method for MIMO OTA test.

MIMO OTA test with the PXB is comprised of two stages. In the first stage, antennas are characterized in an anechoic chamber by stimulating the device with an LTE base station signal, emulated here by Agilent's PXT wireless communications test set in figure 3. Representing a significant breakthrough in UE testing, the PXT incorporates flexible base station/network emulation and RF parametric tests into one integrated unit. It can be configured with a number of advanced test and analysis software options.

Figure 3: The stage 1 steps of the two-stage technique are graphically illustrated here. Note that the device or chipset must have the ability to measure the relative power and phase of the antenna elements to calculate the antenna pattern without using intrusive means.

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