Why Use One Radio When Four Will Do?

Which multiple-input multiple-output (MIMO) test methods are most appropriate depends on the purpose of the testing. Manufacturing test needs to determine that an assembly has been correctly built because satisfactory operation is guaranteed by design. On the other hand, design test and verification are much more detailed and actually set out to prove that the design conforms to the specification. They take into account various possible impairments as well as tolerances and all possible combinations of coding, modulation, and interference.

MIMO is a form of spatial diversity that uses multiple receive and multiple transmit antennas. Each receive antenna is affected simultaneously by all of the transmitted signals that have undergone multipath delays. The performance gain associated with MIMO depends on different delays in the various paths. Very intelligent algorithms extract the actual information.

One way to simulate the RF environment is to use several synchronized signal sources. This is the best approach during the development stage when detailed performance must be verified. In production, it may only be necessary to confirm the correct operation of each receiver and transmitter separately. Here, a switched source and analyzer could be appropriate at a much lower cost. An Agilent Technologies application note describes the trade-offs for these test situations.¹

The proposed 802.11n WLAN specification and today’s Wave 2 WiMAX test requirements don’t envision systems more complex than 4×4; that is, four receivers and four transmitters. WiMAX, however, operates differently than WLAN, and the 2×2 MIMO operation is a downlink (DL)-only feature. Several manufacturers have developed specialized MIMO test equipment with a range of capabilities.

MIMO improves performance by taking advantage of multipath reflections, but the detailed interaction of the multiple transmitted signals within a given air-interface environment is virtually unpredictable. Testing must be completely reproducible in every aspect, so this is one reason that most test setups use coaxial connections between the DUT and the analyzers or generators.

Synchronized vector signal generators (VSGs) can test multiple receivers for many critical performance parameters. If the generators include sufficient channel emulation capabilities, the receivers will operate in a good approximation of a multipath environment.

Multiple vector signal analyzers (VSAs) can measure the performance of a number of transmitters operating simultaneously. For example, they may determine how much RF leakage is occurring from one transmitter to another or if the parallel baseband-to-RF upconversion processes corrupt the transmitted data. Transmitter testing ensures that the protocol and power are correct as determined by the receiving signal analyzer.

As the number of signal sources or analyzers is reduced, generally to save test equipment cost, the types of tests that can be performed also are reduced or at least altered. For example, with a single source and switch, the receivers are tested sequentially. The packet error rate (PER) still can be determined and problems assigned to the appropriate receiver. MIMO operation is simulated by transmitting two to four data streams sequentially although this mode of operation is not as representative as simultaneous parallel testing.

In some cases, MIMO operation is only confirmed on a sample of the total production. In this case, most products are tested on a sequential basis to confirm individual transmitter and receiver performance. Test costs are reduced in several ways:

• Only one complex test setup is required to test a few production samples.
• The multiple test stations used to test the bulk of production consist of lower-cost equipment.
• Most of the products have only a limited number of tests performed, which saves considerable time.

Test Equipment Examples

Channel Emulators
If you use a VSG to produce a modulated RF test signal, you can vary its output amplitude to determine receiver sensitivity. However, MIMO relies on the much more complex interaction of the signal with its environment to provide multiple, delayed versions that are seen by the separate receivers. These multipath fading effects account for increased throughput when multiple receivers are used.

Because there are a number of different test purposes and several means by which they can be accomplished, it’s important to be clear about what your specific test equipment can and cannot do with regard to MIMO testing. If the RF signal already has been produced from a real device or a base-station emulator, a good place to start is the Spirent Communications SR5500 Wireless Channel Emulator (Figure 1).

According to the datasheet, “The SR5500 emulates wideband radio channel characteristics such as time-varying multipath delay spread, fast fading, shadow fading, and channel loss for advanced receiver implementations using diversity, beamforming, and MIMO. It replicates real-world spatial channel conditions, making it possible to isolate performance issues early in development and design verification. Additive white Gaussian noise (AWGN) further enhances the emulated real-world conditions.”

Further, MIMO receivers continuously estimate the channel characteristics so that they know how to interpret the multipath information. This functionality is particularly important in mobile applications where the relative positions of the transmitter(s) and receiver(s) are changing quickly. The SR5500’s dynamic environment emulation feature allows you to create a controlled dynamic environment for this type of testing.

The SR5500 has been redesigned to be an extensible building-block system. The basic instrument has 2×2 MIMO capability, and each transmitted signal can be subjected to 24-path fading. In addition, the channel model has very long repeat times, which means that statistically the real-time fading engine more accurately and realistically generates fading coefficients.

Two SR5500 instruments can be combined to test bidirectional 2×2 MIMO or 4×4 MIMO in one direction. The building-block construction supports using the instruments separately in lower-complexity MIMO applications.

Another instrument in this class is the Azimuth Systems ACE 400WB Mobile WiMAX MIMO Channel Emulator. According to Graham Celine, the company’s senior director of marketing, “The ACE 400WB is a single-box channel modeling solution used in validating MIMO operation and advanced antenna and protocol capabilities unique to WiMAX Wave 2.

“For MIMO to operate properly, a real-world testing environment is required to create the conditions in which the receiving device can decode the multiple signals it receives,” he explained. “The instrument provides fading and multipath to ensure that testing is conducted in a repeatable manner that can be used for certification, interoperability, and performance testing.”

The ACE 400WB has built-in industry-accepted channel models including the ITU M.225 Pedestrian B and Vehicular A profiles that allow users to recreate a variety of typical WiMAX scenarios for performance and interoperability testing. Mr. Celine elaborated, “A high-power DSP/FPGA design performs real-time calculation and implementation of the channel conditions according to the ITU models, the IEEE 802.11n models, and the 3GPP spatial channel models. The channel emulator captures, digitizes, and distorts incoming signals in real time.”

The proposed IEEE 802.11n channel models are the following:

• Model A—typical home/small office environment
• Model B—typical medium office environment
• Model C—typical large office environment
• Model D—typical open-space environment
• Model E—typical large open-space environment
• Model F—complex environment with many scatterers
• Bypass Mode—does not impose any channel conditions

Channel emulation also is provided in VeriWave’s 802.11n WaveDynamix™ Test Solution. A white paper from the company asks if a system under test really can be thoroughly tested in a cabled RF environment without channel emulation. Clearly, detailed performance testing under simulated air-interface conditions requires channel emulation.

The problems then become the availability and the cost of an external channel emulator if you have only a basic test system. And, using a separate emulator brings with it challenges of automation and test-system integration and result correlation.²

Rohde & Schwarz provides wireless test solutions on several levels. The most sophisticated platform-based test solutions are highly automated and comprehensive.

Brian Dobson, product marketing manager, said, “WiMAX with MIMO requires multiple signal sources, so our instruments integrate multiple sources into a single enclosure. For example, the SMU200A is an RF VSG that combines two signal generators as well as up to four fading channels correlated for WiMAX channel simulation.

“For baseband signal generation and fading simulation, the AMU200A can be similarly configured,” he continued. “For signal analysis, the Model FSQ combines spectrum and signal analyzers in a single enclosure and has 120-MHz demodulation bandwidth. And, for baseband signal analysis, the FMU36 integrates vector signal analysis and spectrum analysis.”

Aeroflex also provides a high-level test system for WiMAX. Together with AT4 wireless, Aeroflex co-developed the MiNT T2230 Protocol Conformance Test System. It supports testing of advanced Wave 1 features such as hybrid automatic request (HARQ) and sleep and idle mode. Handover testing is performed using multiple signaling units and a signal control PC. An inbuilt fading simulator and two RF interfaces are used to test MIMO service flows.

Arbitrary Waveform Generators
A highly repeatable and low-cost way of implementing channel effects is to make them part of an arbitrary waveform. Anritsu’s MG3700A VSG, when used with the company’s MX370105A Mobile WiMAX IQproducer Application Software, can generate any mobile WiMAX waveform pattern recommended by IEEE 802.16e. The MX370105A software also supports Mobile Certification Wave 2 and MIMO waveform patterns, which are critical for evaluating downlink functions of mobile WiMAX equipment.

“The MG3700A has a built-in 160-MHz arbitrary waveform baseband generator to output digital modulation signals by selecting the relevant waveform pattern file. Additionally, two built-in memories support output of both the wanted waveform and the modulation interference waveform or AWGN, using one memory for testing receiver characteristics. This eliminates the need for two expensive signal generators and increases the efficiency of performance tests for multisystem mobile terminals, modules, and devices.”

Lynne Patterson, a product marketing engineer, explained the operation of IQproducer in more detail. “IQproducer provides functions for setting parameters in custom-programmed waveforms, simulating the function and appearance of a created waveform, generating files, and transmitting data. The software can work with ASCII format files to create waveform patterns for the MG3700A or MS2690/MS2691A instruments. It also combines waveforms, such as the desired signal and a noise signal, into a composite signal-with-noise signal for evaluating receivers in WiMAX equipment.”

Agilent’s Peter Cain, WiMAX solutions planner, described some of the many test options available using the company’s WiMAX test equipment. “The 89601A VSA and N7615B Signal Studio measurement solutions address Wave 2 MIMO via a range of test capabilities that supports physical layer (PHY) signal generation and analysis for Wave 2 system profiles. Specific capabilities include matrix A and matrix B signals for DL, uplink (UL) collaborative MIMO, and creation of HARQ bursts and uplink sounding zones. The VSA software operates in both simulation and hardware environments, ensuring consistency of results between the two.

“N7615B Signal Studio also provides MIMO fading embedded in a waveform,” he said, “This capability enables testing of MIMO receivers without the added expense of fading hardware or software. It supports a far more realistic test environment than one with no fading.”

Another Agilent instrument, the E6651A Mobile WiMAX Test Set, incorporates flexible base-station emulation, IP traffic support, and on-board RF parametric measurement capabilities. Wave 2 enhancements include support for space-time coding (STC), spatial multiplexing (SM) in the DL, and UL collaborative MIMO measurements.

Keithley Instruments also supports MIMO testing through multiple VSGs and VSAs. Ron Rausch, senior marketing manager, said, “The new MIMO RF Test System provides a high-performance solution for 802.11n WLAN and WiMAX Wave 2 testing needs for RF R&D, product verification test, and production test. It consists of the Model 2920 RF VSG, the Model 2820 RF VSA, the Model 2895 MIMO Synchronization Unit, the System 2800 Multi-Input RF Signal Analyzer, the System 2900 Multi-Output RF Signal Generator, and the Model 280111 WLAN 802.11n MIMO Signal Analysis Software running on a separate PC (Figure 2).

“Both the VSG and VSA have continuous frequency coverage to 6 GHz and more than 40-MHz flat signal bandwidth.” He continued, “The test system can be configured with multiple instruments to address any combination of two-, three-, or four-channel multi-input or multi-output systems. Synchronization among the instruments is held to within ±1-ns pk-pk signal sampler jitter and <1° pk-pk RF carrier phase jitter between any two channels."

The Model 2920 has arbitrary waveform options with 100 MS of waveform memory and up to 80-MHz bandwidth to generate WLAN, WiMAX, and virtually any other test signal required. Many different signal waveforms can be simultaneously resident in the Model 2920’s memory, and switching between them takes less than 3 ms.

One-Box Solutions

One box can mean a signal source and analyzer with real-time base-station emulation like the Agilent Model E6651A Mobile WiMAX Test Set. The term also can indicate the combination of VSG and VSA and an associated switching unit.

Onno Harms, a LitePoint product manager, commented on the types of problems his company’s products address: “We provide one-box instruments that are complete test solutions supporting multiple wireless standards. The LitePoint test solutions focus on RF and baseband PHY testing. For WiMAX, this is equivalent to the test domain defined in the WiMAX Forum radio conformance test specification.

“Signal analysis with the LitePoint IQmax systems, including channel estimation, is done on digitized data, post-acquisition. Our IQmax systems target PHY functionality and performance verification, not real-time system-level performance analysis (Figure 3).”

This last point is a very important distinction and underscores the need to be clear about your testing needs when specifying test equipment. The IQmax can generate signals that include noise and other impairments as well as channel effects. As a result, the capability is claimed to create a worst-case channel environment for the highest quality discrimination.

Yet another approach to WiMAX testing is represented in the base-station and mobile-station emulators from Anite. These instruments replace a real mobile WiMAX station and connect via a coaxial cable to the DUT.

The company’s product manager, Ian Swan, described the products. “Both emulators have two operating modes: emulator mode and protocol conformance toolset (PCT) mode. In emulator mode, you can run interoperability tests, test data transfer, view statistics, and test throughput by generating traffic. The emulator automatically exchanges protocol messages with the mobile DUT, allowing you to monitor media access control (MAC) and PHY protocol exchange. However, unlike real WiMAX stations, the emulators provide a great deal of configurability, logging, and diagnostic analysis tools so that the behavior of the DUT can be truly measured.

“In PCT mode,” he continued, “conformance tests can be run against the WiMAX device. You can define protocol messages exchanged with the DUT, and the emulator supports test cases written in tree and tabular combined notation (TTCN-3). Test cases are selected from an extensive suite that includes WiMAX Forum-approved certification tests, or you can develop your own. The PCT product is used by WiMAX Forum-designated certification laboratories as part of the WiMAX Forum certification program.”

Summary

It’s not possible to include all models of test equipment suitable for WiMAX testing. Those discussed in this article directly relate to the MIMO test requirement resulting from the new Wave 2 WiMAX capabilities. Because of the increasing system complexity in both the 802.11n WLAN and 802.16e WiMAX areas, it’s interesting to note the modular architectures adopted by the major test and measurement companies.

For example, Anite’s Mr. Swan explained that the base-station emulator uses an FPGA-based software-defined radio architecture to allow easy firmware updates as the rollout of advanced PHY and MAC features progresses. A scalable backplane allows additional RF capability to be plugged in as testing needs increase from single-antenna single-input single-output through larger MIMO systems.

Similar considerations apply to many instruments, even to the point of becoming potentially confusing. For example, Anritsu’s MS2690A is called a signal analyzer, but the Model MS2690A-020 is a signal generator. It’s a matter of how much optional functionality can reside in the same instrument.

There are many sophisticated WiMAX and WLAN test solutions available today. You can deal with very complex systems including several levels of MIMO operation. On the other hand, the various systems are not just higher- or lower-accuracy versions of basic test sets. Each has distinct capabilities.

The message for anyone testing the latest wireless systems is to thoroughly understand your needs before venturing into the realm of competing instruments and specifications. The systems are complex, and so are the instruments used to test them.

References
1. Agilent MIMO Manufacturing Solution, Application Note, Agilent Technologies, 2007.
2. Asking the Right Questions When Considering an 802.11n Test Solution, VeriWave, 2007.

January 2008