Transforming MIMO Test With Fast, Accurate Signal Creation and Channel Emulation

Multiple-Input Multiple-Output (MIMO) is one of several forms of smart antenna technology available today designed to significantly improve communication performance. The promise of higher data rates with increased spectral efficiency makes MIMO especially attractive in wireless communications where systems operate in high multipath environments. For this reason, wireless standards like 3GPP Long Term Evolution (LTE), IEEE 802.16 (adopted by the WiMAX™ Forum) and 802.11n have recently adopted or are considering its use. All upcoming 4G wireless communication systems are expected to employ MIMO technology.

While spatial diversity and MIMO offers increased signal robustness or capacity improvements when operating in rich multipath environments (with suitable signal to noise and interference conditions), those benefits come at the cost of increased complexity. For the R&D engineer developing and integrating MIMO receivers, that translates in a key challenge: how to quickly and accurately test the receivers under real-world conditions and early enough in the design cycle to easily find and fix any problems. This capability is critical to maximizing receiver performance, minimizing design uncertainty and reducing development cycle time.

In wireless communications systems, multiple antenna systems like MIMO take advantage of the spatial diversity obtained by placing separate antennas in a dense multipath scattering environment. These systems can be implemented in a number of different ways to obtain either a diversity gain to combat signal fading or a capacity improvement. Generally, there are three categories of multiple antenna techniques, including spatial diversity, spatial multiplexing and beamforming. MIMO systems utilize spatial multiplexing. Under rich scattering environments, independent data streams are simultaneously transmitted over different antennas to increase the effective data rate. MIMO spatial multiplexing requires at least 2 transmitters and 2 receivers, and the receivers must be in the same place (i.e., same device). Because the transmitters are not required to be in the same device, two mobiles can be used together for MIMO in the uplink. In this case, the transmissions need to be synchronized (i.e., the power level and timing of the mobiles need to be aligned)—a process done by the base station as a part of normal cellular operation.

MIMO uses multiple transceivers at both the transmitter and receiver to operate. Because MIMO allows more bits/sec/hertz to be transmitted in a given bandwidth, it improves spectral efficiency and allows operators to simultaneously support more users with high data-rate requirements. Increased spectral efficiency, higher data rates and the ability to increase data throughput without additional bandwidth or transmit power, makes MIMO especially attractive for use in wireless communication systems.

In MIMO terminology, the “Input” and “Output” are referenced to the wireless channel, which includes the antennas. Performance gains are achieved as multiple transmitters simultaneously input their signal into the wireless channel and then combinations of these signals simultaneously output from the wireless channel into multiple receivers. For downlink communication, a single base station (BS) would contain multiple transmitters connected to multiple antennas and a single Mobile Station (MS) would contain multiple antennas connected to multiple receivers.

Several basic configurations for connecting the transmitters and receivers in a wireless system using multiple antennas are shown in Figure 1. Each individual arrow represents all signal paths between two antennas (including the direct Line of Sight (LOS) path if one exists), and the numerous multipath signals created from reflection, scattering and diffraction from the surrounding environment. The Single-Input Single-Output (SISO) configuration—traditionally used for radio and television broadcast and early first-generation cellular—includes the LOS path and all multipaths present over the wireless link. The Single-Input Multiple-Output (SIMO) and Multiple-Input Single-Output (MISO) configurations require the use of a single antenna at either the transmitter or the receiver. SIMO is particularly useful when transmitting uplink data from a mobile device with its single antenna, to a cellular BS or WLAN access point containing two or more antennas. MISO is used for the downlink transmission of data with transmit diversity.

The 2x2 MIMO configuration is also shown in Figure 1. Two antennas are placed at the transmitter which has two separate transmit channels and two antennas are placed at the receiver which has two separate receive channels. Numerous other MIMO configurations using combinations of multiple antenna pairs, such as 3x3 and 4x4, are also possible. A MIMO system can even be configured with an unequal number of antennas at the transmitter and the receiver, such as an MxN case where M transmit antennas does not equal N receive antennas.

In wireless communication systems, the wireless channel is a key factor in determining system performance, making understanding and dealing with channel correlation effects (e.g., path loss and multipath fading) a crucial part of receiver design. Ensuring optimal MIMO operation therefore, requires the engineer to accurately test the MIMO receiver—a challenging task given the large combination of variables that must be tested in a given MIMO configuration.

Various approaches can be used to accomplish this task. In a typical 2x2 MIMO configuration, for example, two separate SIMO channel emulators can be used to model the four separate channels that exist between the pairs of transmit and receive antennas. But SIMO channel emulators do not provide the correct correlation between MIMO channels—an important characteristic when testing system performance since real-world channels are correlated to some degree. The engineer might opt to test directly in a “real” wireless environment, but the channel is very sensitive, not controllable and not repeatable. This approach also is not practical in test situations where different environments are required or when mobility testing is necessary. Another option is to use software-based tools to create realistic MIMO channels, a time-consuming proposition that does not produce real-time results, although it does provide some indication of the correct operation of the RF and baseband functions. Despite these available options, today’s R&D engineers need a better alternative to MIMO receiver test—one that is specifically designed to handle MIMO complexity.

Agilent Technologies’ PXB MIMO Receiver Tester effectively addresses the challenge of testing MIMO receivers in realistic wireless channels and conditions (Figure 2). Delivering the most up-to-date, versatile signal creation and channel emulation capabilities for the latest LTE and WiMAX standards, it quickly replicates real-world MIMO conditions and channels, and generates realistic fading scenarios including path and channel correlations. The PXB MIMO Receiver Tester provides up to 4 baseband generators (BBGs), 8 faders, the industry’s widest bandwidth of 120 MHz, custom MIMO correlation settings (e.g., predefined channel models, antenna pattern and correlation matrix), and supports testing and troubleshooting of 2x2, 2x4, and 4x2 MIMO.

Using PXB, R&D engineers can simulate real-world conditions in the lab that more quickly test corner cases and stress devices beyond standards requirements. They can also test co-existence to ensure design robustness earlier in the design process, a critical capability since interference testing is typically difficult and not always thoroughly tested in simulation due of the complexity of the test combinations. Using Agilent’s Signal Studio, Advanced Design System, or even the engineer’s own waveform creation tool to create waveforms, up to four high-performance BBGs can be summed for multi-format co-existence testing. Each BBG supports 120-MHz modulation bandwidth with 512 Msa of playback memory for long, complex signal simulation.

Featuring a scaleable platform based on a field-upgradeable architecture, the PXB MIMO Receiver Tester ensures a simple and cost-effective upgrade in an hour to support the test needs of future technologies. This maximizes the user’s equipment investment and investment longevity.

Some of the PXB MIMO Receiver Tester’s key benefits include:

• Minimized design uncertainty

Because the PXB MIMO Receiver Tester allows design problems to be identified earlier in the lifecycle, engineers can design with a higher level of confidence, thereby reducing design uncertainties, minimizing rework and shortening time-to-market.

• Minimized equipment and lab setup time

Seamless signal routing and automated power calibration eliminate the tedious, time-consuming system setup required for fading and multi-format co-existence signal summing, and minimizes test time. Further test time reductions come from pre-defined test configurations which allow the engineer to quickly define complex instrument settings for BBG and fading. An intuitive graphical user interface with easy-to-use drop-down menus provides quick and easy setup of test settings (e.g., MIMO channel models and editable path configuration table) and also shortens the user’s learning curve.

Despite the array of performance improvements offered by MIMO technology, its complexity makes testing MIMO receivers challenging. While there are solutions available to address this task, Agilent’s PXB MIMO Receiver Tester provides a greater benefit. Specifically designed to replicate real-world MIMO conditions in the lab, it allows R&D engineers to quickly and accurately isolate issues early in the lifecycle. Its ability to minimize design uncertainty and equipment and lab setup time, while maximizing the performance and scalability needed to meet future test needs, is literally transforming MIMO test and quickly making it a critical tool for any engineer developing and integrating MIMO receivers.

About Agilent
Agilent Technologies Inc. (NYSE: A) is the world’s premier measurement company and a technology leader in communications, electronics, life sciences and chemical analysis. The company’s 20,000 employees serve customers in more than 110 countries. Agilent had net revenues of $5.4 billion in fiscal 2007. Information about Agilent is available on the Web at www.agilent.com.

###

“WiMAX,” “Fixed WiMAX,” “Mobile WiMAX,” “WiMAX Forum,” the WiMAX Forum logo, “WiMAX Forum Certified,” and the WiMAX Forum Certified logo are trademarks of the WiMAX Forum. All other trademarks are the properties of their respective owners.