Transforming MIMO Test With Fast, Accurate Signal Creation, Signal Analysis, and Protocol Development and Conformance
August 26, 2009
Multiple-Input Multiple-Output (MIMO) is one of several forms of multiple antenna techniques 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 WLAN 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 and increased demands on processors. For the R&D engineer developing and integrating MIMO receivers, that translates into 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.
Figure 1: This graphic depicts antenna and channel configurations for SISO, SIMO, MISO, and MIMO (2x2) systems.
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. As a result, 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.
Delivering a Fast, Accurate Solution
Since Agilent is a world leader in test and measurement solutions and at the forefront of emerging markets like MIMO, its broad range of comprehensive test solutions, spanning R&D through integration and manufacturing, address many of the key challenges facing engineers developing and integrating MIMO devices, whether for LTE-, WiMAX- or WLAN-based applications. In particular, Agilent's solutions address:
In order to make multi-channel measurements of MIMO signals, the engineer must first generate an up-to-date standard-compliant waveform. This can be accomplished using the following Agilent solutions:
The N5182A MXG vector signal generator offers industry-best adjacent channel power ratio (ACPR) performance and switching speeds (1.2 ms in SCPI mode), coupled with simplified self-maintenance to maximize uptime. It provides higher output power and improved distortion performance than its predecessor, thereby enabling increased measurement accuracy for power amplifiers (PAs) and multi-carrier power amplifiers (MCPAs) tested with high-PAR signals such as WiMAX, LTE and W-CDMA. To assist engineers designing MIMO systems, the MXG offers baseband synchronization and an RF phase coherency capability between multiple MXGs. As a result, the MXG can provide a simple, complete solution for WiMAX, LTE, and WLAN multi-transmitter test set-ups.
Agilent Signal Studio runs on the MXG and aides in signal generation. As a powerful, PC-based software application, it can be used to create standards-based MIMO signals for WiMAX (N7615B), WLAN (N7617B) and LTE (N7624B). Signal Studio features advanced receiver test capabilities that support 4x4 MIMO pre-coding and static fading.
- The PXB MIMO Receiver Tester is Agilent's baseband solution to signal generation. Generally, the signals it creates are upcoverted through at least two stacked and phase-locked MXG signal generators. The PXB delivers 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 (Figure 2). It 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.
Figure 2:The PXB MIMO Receiver Tester is designed for R&D engineers testing MIMO receivers and addresses the challenge of testing MIMO receivers in realistic wireless channels and conditions. Agilent's Signal Studio signal creation software runs in the instrument and provides the engineer with up-to-date standards-compliant signal creation.
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 scalable platform based on a field-upgradeable architecture, the PXB ensures a simple and cost-effective upgrade to support the test needs of future technologies. This maximizes the user's equipment investment and investment longevity. Because the PXB 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. Additionally, its 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.
Agilent offers a range of solutions for superior signal analysis that support both single and multi-channel measurements of MIMO signals. These solutions include:
- Agilent's industry-leading 89601A Vector Signal Analysis (VSA) software provides superior general-purpose and standards-specific signal evaluation and troubleshooting tools. It can run on a PC or within a variety of Agilent signal analyzers, oscilloscopes and logic analyzers. The VSA software offers industry-leading performance with EVM of less than -50 dB (hardware dependent) and bandwidths of 1.4 MHz to 20 MHz. The VSA supports 2x2 MIMO measurements and analysis in conjunction with the MXA signal analyzer and Infiniium 90000 Series oscilloscopes. MIMO testing of specific standard-based devices and modules is available in the VSA using one of a number of options, including: Option B7Y for WiMAX, Option B7Z for WLAN and Option BHD for LTE.
- The N9020A MXA signal analyzer seamlessly integrates a broad range of standards-based measurements, including WiMAX and Wave 2 2x2 MIMO, with Agilent's 89601A VSA software (Figure 3). MIMO signals are captured by multiple, stacked Agilent MXAs and analyzed using the VSA software. The MXA offers engineers the highest performance in a mid-range analyzer and, in addition to supporting MIMO signals, allows flexible analysis measurements of wireless communication devices to a range of current and emerging standards. Furthermore, it features the industry's fastest speed--up to 300% faster than all other spectrum and signal analyzers, 0.23-db absolute amplitude accuracy and supports multiple frequency range options from 20 Hz to 26.5 GHz.
Figure 3:The Agilent MXA signal analyzer can be utilized with the 89601A VSA software to capture, evaluate and troubleshoot MIMO systems.
Agilent's SystemVue system design solution provides an integrated design environment for baseband and RF design in next-generation physical layer (PHY) communications systems, such as those employing MIMO technology. Its algorithm development capability and integrated mixed-signal design environment help accelerate design activities, while minimizing baseband/RF system integration risks.
During the design process, SystemVue's algorithm references are used to generate reference vectors to facilitate baseband development when hand writing HDL code. The algorithm references provide an independent check of standards interpretation relative to the baseband design and implementation--a feature which is especially useful given the complex and evolving nature of standards like Mobile WiMAX, WLAN and LTE. They can also be used as a starting point, with the Baseband Exploration Library (BEL) option, to modify an algorithm for a particular version of a standard. The baseband HDL can then be co-simulated in SystemVue, together with the RF design. Integrated instrumentation links with the MXA and with the VSA software enable RF/baseband testing (demodulation) at every step of the design phase.
- Agilent's Infiniium 90000 Series oscilloscopes can be linked with the 89601A VSA software for MIMO development. They offer the broadest measurement capability with true analog bandwidths up to 4 GHz, impressive scope performance (e.g., the world's fastest integrated digital channels running at 2 GSa/s), and built-in logic and protocol analysis.
Protocol Development and Conformance
To accelerate protocol development and conformance, Agilent provides engineers with real-time LTE and WiMAX base station emulation capabilities for mobile development. Its solutions for protocol development and conformance test include:
- The E6651A Mobile WiMAX Test Set is the industry's first integrated test set for testing mobile WiMAX devices and is the foundation on which all of Agilent's protocol and network conformance test solutions are built (Figure 4). It provides unrivaled test coverage for Mobile WiMAX developers performing R&D, system integration and verification, or conformance test in RF, protocol and functional application test.
Figure 4:In addition to protocol conformance test, the Agilent E6651A Mobile WiMAX Test Set platform supports multiple R&D testing needs--including base-station emulation, RF parametric measurements and Radiated Performance Testing.
The E6651A offers a unique combination of flexible base-station emulation and RF parametric tests for subscriber station tests in one integrated unit, with support for IEEE 802.16e 2005 protocol conformance test. It provides instrument-grade RF signal generation and signal analysis capability up to 6 GHz and is capable of addressing current and planned WiMAX profiles. Dual analog IQ hardware inputs added to the E6651A's uplink path (baseband analyzer) support baseband chipset/module tests during both development and integration. A full suite of RF measurements can be used for characterization, calibration and verification of WiMAX transmitter and receiver performance.
- Agilent's E6620A Wireless Communications Test Set is a powerful, scalable platform that can be used across the entire R&D lifecycle, from early protocol development through RF conformance and interoperability test. Designed for LTE UE designers, it uses a common 3GPP-compliant LTE protocol stack for real-time, system-rate network emulation for L1/L2/L3 uplink and downlink via RF or digital baseband. It also provides a path to MIMO 2 cell and RF measurement capabilities.
- Agilent's multi-channel N4010A Wireless Connectivity Test Set and N4011A MIMO/Multi-port Adapter solution are specifically designed for 802.11n MIMO WLAN device and module testing. Used during manufacturing to facilitate high data throughput and wider range, they provide full draft-802.11n test coverage, plus the fast switching hardware and software to allow sequential capture and analysis of WLAN bursts on multiple channels.
Despite the array of performance improvements offered by MIMO technology, its complexity makes testing MIMO receivers challenging--and that's just one of the many challenges facing engineers developing and integrating MIMO systems. Agilent's PXB MIMO Receiver Tester provides an optimal solution for MIMO receiver test. 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. In addition to the PXB, any one of Agilent's comprehensive offering of MIMO-specific solutions, spanning R&D through integration and manufacturing, can be used to effectively address other MIMO-related challenges. These solutions are quickly becoming critical tools for any engineer developing and integrating MIMO components and systems.
About Agilent Technologies
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 18,000 employees serve customers in more than 110 countries. Agilent had net revenues of $5.8 billion in fiscal 2008. Information about Agilent is available on the Web at www.agilent.com.
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