Electronic Design

Complex Wireless Standards Put Instruments To The Test

Hardware and software advances promise faster and simpler WiMAX and LTE testing.

Once upon a time, RF testing was relatively simple. You would measure power output in a transmitter and look for spurious signals with a spectrum analyzer. At the receiver, you measured noise and sensitivity. Unfortunately, those halcyon days are gone forever.

Radio complexity has intensified dramatically with advanced digital modulation schemes, softwaredefined radio architectures featuring digital signal processing, I/Q signal chains, multiple-input/multiple-output (MIMO), and other advanced techniques. And let’s not forget the complex protocols that must be tested for regulatory compliance and interoperability requirements. On top of that, the frequencies of operation continue to skyrocket, stretching the limits of current equipment as well as budgets.

Thankfully, test instrument manufacturers aren’t sitting on their hands. The leading vendors all offer instruments with higher frequency capability. Moreover, their software and hardware automates many of the tests, whether you’re testing MIMO on a new Wi-Fi 802.11n router or WiMAX basestation or evaluating your new chip for compliance with the new 4G Long Term Evolution (LTE) cell-phone standard. Most of the latest announcements feature products for testing LTE and the newer HSDPA 3G technology, as well as WiMAX with MIMO.

NEW STANDARDS PUSH THE BOUNDARIES
Nowadays, everybody seems to be focusing on LTE and WiMAX. LTE is the 4G cell-phone technology being developed by the Third Generation Partnership Project (3GPP). Though not completed (final approval is expected later this year), it’s far enough along that both chip and equipment manufacturers are hard at work testing and evaluating products.

The LTE standard is the next step beyond the 3G WCDMA and HSPA technologies used by AT&T and T-Mobile, as well as most European carriers. Also, Verizon adopted LTE as its 4G path rather than the ultra-mobile broadband (UMB) solution developed by Qualcomm.

WiMAX, of course, is the broadband wireless technology standardized by the IEEE. Fixed (802.16d-2004) and mobile (802.16e-2005) versions are available. Its primary application is broadband wireless service to compete with cable TV and DSL for Internet access. But other applications such as cellular and other back-haul systems are becoming popular.

Some experts say WiMAX mobile is also a great contender for 4G cell-phone service. With Voice over Internet Protocol (VoIP), it could compete with LTE. The jury is still out, though. Most insiders say LTE will dominate, with WiMAX filling other niches. In any case, both technologies are complex.

Each standard uses orthogonal frequency-division multiplexing (OFDM) and orthogonal frequency-division multiple access (OFDMA). Also, each offers MIMO as an option for increasing range, data speed, and reliability. MIMO is a multiradio multi-antenna technology that transmits coded parallel data streams on the bandwidth to boost data rate and help mitigate the problems of multipath interference. Testing these technologies has become a major challenge.

INSTRUMENTS TACKLE TEST TRIBULATIONS
Instrument powerhouse Agilent introduced a whole slew of wireless test products at the Mobile World Congress show in Barcelona in February and at the CTIA show in Las Vegas in April. For example, its latest software for WiMAX Wave 2 testing works with Agilent’s Infiniium scopes and its MXG signal generators.

Also, Agilent’s one-box, fully automated E6651A WiMAX Wave 2 MIMO tester performs conformance testing, radiated performance test, end-to-end data transfer, and network entry/data connection (Fig. 1). The N8300A wireless networking test set provides fast and accurate measurements for WiMAX Wave 2 manufacturing test.

Then, there’s the J7910 A signaling analyzer for WiMAX troubleshooting. Agilent is also cooperating with WiMAX chipset manufacturers Beceem and Sequans to provide test solutions that work for conformance and interoperability verification.

LTE efforts include some LTE protocol development solutions based on the Agilent E6620A wireless communications test set (Fig. 2). It features Anite’s SAT LTE protocol development toolset. This solution targets engineers working in the early protocol design stage of LTE handsets.

Another LTE product, the J7910A real-time signaling analyzer platform, is the only integrated high-density solution for Gigabit Ethernet analysis. Agilent also offers LTE vector signalanalysis software for its MXA signal analyzer and MXG vector signal generator.

Finally, the company has a line of receivers for wireless testing and evaluation. The six models in the W1314A family cover all relevant RF bands and all wireless technologies, such as GSM, WCDMA, cdma2000, EV-DO, iDEN, and WiMAX, including the mobile version.

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These receivers enable network equipment manufacturers and carriers to effectively deploy, optimize, and troubleshoot all technologies in their own networks by quickly identifying coverage and interference problems. Spectrum analysis can be used on any of the RF bands supported. The receivers also have a highsensitivity GPS receiver to ensure accurate reporting of position and measurement data.

Anritsu simplifies things with three software packages for LTE that run on its MS269XA Signal Analyzer (Fig. 3). The MX269020A TE Downlink Measurement Software, the MX269021A LTE Uplink Measurement Software, and the MX269908A LTE IQ producer all help ensure the compliance of LTE devices.

The MX269020A and MX269021A measure the transmit characteristics of 3GPP LTE in the frequency-division duplex (FDD) mode. Both packages include special features, such as a sliding fast Fourier transform (FFT) analysis window to provide measurement flexibility and a user-defined reference signal.

The MX269908A generates 3GPP LTE-compliant waveform pattern files. These can be output as RF signals from the MS269XA’s optional signal generator to test RF receiver characteristics and to perform transmitter and receiver evaluations. Userdefined reference signals can be created and incorporated into the waveform files for transmission.

All of these products go a long way toward boosting measurement efficiencies in LTE basestations, mobile terminals, and components. The software installs directly on the MS269XA analyzer, eliminating the need for an external PC in the test setup. The MS269XA comes in several models with frequency ranges that begin at 50 Hz and go to 6, 13.5, and 26.5 GHz. U.S. prices begin at $34,000. The MX269020A and the MX269021A cost $17,212 each. The MX269908A goes for $5164.

Anritsu’s MS271xB economy spectrum analyzers include models available for maximum frequencies of 7.1, 13, and 20 GHz. Their special demodulation hardware and pre-written test routines suit popular wireless technologies like fixed and mobile WiMAX, WCDMA and HSDPA, cdmaOne, cdma2000, EVDO, and GSM/GPRS/EDGE.

All of the models have a 10-MHz bandwidth and a 100-dB dynamic range. They support 13 wireless test options that reduce production test cost. The zero span function’s digital time markers let users measure RF power versus time with improved accuracy. The marker function is now scrollable, and measurement speeds, remote I/O data transfer, and button response times all clock in much faster.

For test automation creation, MS271xB programmable test functions are now available on National Instruments’ LabVIEW, permitting easy test writing with high-level languages. All models have 256-Mbyte and 2-Gbyte USB flash drives, Ethernet, and USB 2.0 connectivity. U.S. prices range from $12,950 for the 7.1-GHz model up to $19,959 for the 20-GHz model.

Anritsu’s MG37020A microwave signal generator targets automated test systems in defense signal simulation and manufacturing automatic test equipment (ATE) where, minimum test time and maximum throughput are critical. Applications include antenna test, satellite payload test, and terrestrial microwave link testing.

Its key spec is its fast frequency switching speed of 100 µs. This YIG-based (yttrium-iron-garnet) generator also has a frequency range of 10 MHz to 20 GHz in 0.001-Hz steps with a typical low phase noise of –101 dBc/Hz at 10 kHz offset from 10 GHz. Furthermore, it has a color touchscreen and is based on a Windows XP platform. Connectivity includes USB 2.0, Ethernet, IEEE-488 GPIB, and RS-232.

Azimuth Systems was one of the first companies to develop a hardwired RF channel emulator to test wireless systems. The company’s first efforts addressed Wi-Fi testing, while its later developments are now critical to 802.11n Wi-Fi products. Now, Azimuth’s ACE MX channel emulator tests LTE, WiMAX (including the forthcoming 802.16m standard), and UMB (Fig. 4).

This emulator meets complex 4G smart antenna requirements, including MIMO and beamforming methods. The ACE MX also covers the range from 400 MHz to 6 GHz. It has scalable MIMO configurations and accommodates unidirectional or bidirectional operation in either frequency-division duplex (FDD) or timedivision duplex (TDD) formats. It’s ready to test almost any of the forthcoming products that operate in the new 700-MHz band.

Keithley addresses the MIMO testing problem with its 2920 vector signal generator (VSG) to create the OFDM signal used as the test input. By employing additional VSGs, users can generate two, three, or four additional signals on the same frequency with different data to produce the final MIMO signal. Keithley’s 2895 MIMO Synchronization Unit is needed to coordinate the inputs and produce the final MIMO output (Fig. 5).

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On the receive end of the chain, the Keithley 2820 vector signal analyzer (VSA) handles receipt and recovery of signals. It uses the company’s MIMO Signal Analysis software for signal recovery, measurement, and display. As many as four VSGs and VSAs may be used together. In addition, any MIMO configuration up to 4x4 (e.g., 2x3, 4x3, 2x1) can be accommodated.

Keithley’s 2920 VSG can generate signals from 10 MHz to 6 GHz. An optional arbitrary waveform generator (AWG) lets users produce modulating signals for GSM, EDGE, WCDMA, cdma2000, single-input/single-output (SISO) wireless local-area network (WLAN), the very demanding 40-MHz wide 802.11n Wi-Fi signal, and WiMAX. The 2920 is also available as a standalone test unit.

Meanwhile, the 2820 VSA receives signals up to 6 GHz. Bandwidth is 40 MHz. It can receive, analyze, and display most popular wireless standards such as those mentioned for the 2920, both SISO and MIMO.

To use these instruments in a MIMO test system, you need the 2895 MIMO Synchronization Unit, which synchronizes all of the VSGs and VSAs. It provides a common local-oscillator (LO) output to all units, as well as a 100-MHz digital clock and trigger signals to sync all of the units for the selected MIMO configuration. The PC-based MIMO Signal Analysis software lets you test and measure all standards with configurations to 4x4 MIMO.

Pricing begins at $17,500 for the 2920 VSG and at $22,500 for the 2820 VSA. The 2895 MIMO Synchronization Unit costs $9900. MIMO signal analysis software is $9500.

Rohde & Schwarz offers several new LTE, WiMAX, and MIMO products, too. One complete 2x2 MIMO test setup that works with 3GPP LTE, WiMAX, 802.11 Wi-FI, and HSPA+ consists of the SMU200A RF signal generator and the AMU200A baseband generator along with a fading simulator in one instrument (Fig. 6). The instrument permits real-time fading measurement on all four propagation paths.

A typical MIMO system employs the 2x2 format with two transmitters and two receivers. The test setup requires two signal generators and one fading simulator. R&S offers an option that allows measurements on 2x2 MIMO receivers using a single instrument. The generator can be equipped with an internal fading simulator as well as two RF sources and two baseband sources. When equipped with this option, the instrument can simulate the four fading channels required.

R&S offers optional firmware for the SMU200A/AMU200A combo. It adds channel coding and MIMO pre-coding for up to four transmit antennas for 3GPP LTE. With it, users no longer need external fading hardware or control software for testing 3GPP LTE mobile devices.

Since the signal generators provide 3GPP TS 36.211 standardcompliant signals, the full scope of RF performance can be tested. It requires additional instruments or an external PC for signal calculation. Another option for the R&S SMU200A and some other R&S generator models, the SMx-K55 software, offers downlink and uplink testing functionalities.

R&S’s CMW500 LTE protocol tester makes testing basestations and handsets easy despite the complexity of the LTE protocol (Fig. 7). It has a frequency range up to 6 GHz with a bandwidth of 40 MHz, and it greatly facilitates conformance, performance, and interoperability tests. A full suite of tested software tools and test sequences greatly reduces LTE development efforts.

As for a WiMAX test solution, R&S’s CMW270 communications tester emulates a basestation. It’s a great testing solution for chip sets and mobile stations, combining signal generation and signal analysis in one box. It supports the 802.16e mobile WiMAX standard and covers from 100 MHz to 6 GHz. The instrument also supports all RF profiles defined by the WiMAX Forum.

Tektronix Communications recently announced some LTE and WiMAX test platforms. For instance, its G35 comprehensive network management and diagnostics solution covers all protocol layers and physical interfaces, including air and fixed line interfaces (Fig. 8). It includes the G35-LTE functional and load test platform and the NSALTE scalable platform for monitoring, troubleshooting, and optimization.

Tektronix also has a WiMAX version of the G35. The WiMAX Forum recently selected this version as the test solution to verify the interoperability of WiMAX network nodes in mobile WiMAX certified test labs around the world.

Although not an LTE or WiMAX test product, the Tektronix H600 RFHawk handheld digital RF signal hunter is designed for the surveillance and security market (Fig. 9). Its receiver contains a high-performance spectrum analyzer with an intuitive set of user controls, allowing for the quick and simple classification and location of both analog and digital RF transmissions.

Finding and physically locating RF emitters that are misusing the radio spectrum can be a challenge, especially when risk mitigation and time to response are critical. Many covert signals are designed to avoid detection by hiding among legitimate transmissions. The RFHawk was developed based on customer needs to quickly spot and locate illegitimate analog and digital RF transmission sources

Its spectrogram mode lets customers see the true signal shape through FFT-based spectrum analysis. The RFHawk uses a DSP technique called spectral correlation analysis to look at internal frequencies within the signal to discover a digital signal’s symbol rate and other repetitive internal rates to identify valid signals.

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