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These days, worldwide interoperability for microwave access (WiMAX) is garnering increasing attention. In part, its popularity is driven by developments in worldwide spectrum allocation and standardization, product announcements and ongoing research and development. While this attention has created opportunity for an increasing number of consumer devices to incorporate wireless capability, it has led to additional regulatory/compliance constraints — especially given that how a device is built is left up to the implementer as opposed to defined by the standards. Standards-compliant devices can vary from company to company. As a result, the ability to take advantage of emerging market opportunities is tied to the ability to test products for regulatory and standards compliance.

Let's take a look at some of the test issues surrounding WiMAX and the solutions available to address them.

With mobile WiMAX networks being deployed, it has become increasingly important to optimize WiMAX testing, especially given the complexity of WiMAX design cycles and its rapid deployment schedule. Among other things, test instrumentation must demonstrate cost effectiveness and exceptional flexibility.

• Cost-effective solutions

  Keeping WiMAX devices affordable requires that cost be an overriding concern, from the beginning of the development cycle through the manufacturing process. The cost of test is not immune to this requirement. In the past, engineers might have opted to perform less sophisticated tests using lower-cost sources in place of more comprehensive testing to minimize cost. But because of WiMAX signal complexity, these lower-cost sources can lead to field failures in devices that seem to pass performance tests in the factory, which in turn, can increase manufacturing costs.

  What's needed for testing WiMAX products is equipment that is inexpensive and does not force engineers to trade-off key features such as modulated signal generation. The use of software-defined radio (SDR) architectures in test instrumentation may offer one option to this need.

• Flexible test instrumentation

  The IEEE 802.16 standards were created with much flexibility (e.g., in terms of how they can be implemented). Test instrumentation must be able to accommodate this flexibility. Consider, for example, that 802.16d-2004 supports many modulation schemes, including OFDM with BPSK, QPSK, 16 QAM or 64 QAM, with channelization of 1.25 MHz or 20 MHz implemented at any frequency range allowed in the regional deployment below 11 GHz. The ideal test instrument must be able to perform basic end-to-end testing on these different configurations, as well as the range of transmitter (e.g., adjacent-channel power measurement, power level and spectral flatness), and receiver tests (e.g., maximum power signal, receiver image rejection and sensitivity).

Some of the other key issues that OEMs and ODMs face with regard to WiMAX measurement include:

1. Protocol monitoring. Bringing WiMAX out of the lab and into live networks requires special attention to the Access Service Network (ASN). Signaling and payload, for example, which are physically transported on the air interface, need to be traced and analyzed. Protocol monitoring is required.

   One solution, Tektronix K1297-G35, is a protocol- monitoring platform designed for the functional testing of WiMAX products based on the IEEE 802.16e-2005 standard (Figure 1). It provides the user with a flexible, modular high-performance hardware/software solution to address the required protocol testing applications.

   Another solution — a WiMAX test system designed to test the ASN Gateway and Connectivity Service Network (CSN) known as the Torrent 7100 WiMAX test system (WTS) — comes from Mobile Metrics. Exercising the traffic and conformance capabilities of these nodes, it helps accelerate development of central nodes in the WiMAX network and, in turn, WiMAX rollouts.

2. Executable specification. The IEEE 802.16 standard document is about 900 pages and provides a comprehensive description of WiMAX's physical layer (PHY) and media access control layer (MAC). Its scope and complexity creates challenges for designers of standard-compliant components as they try to capture all the requirements for implementation and test. If the document changes, the problem is made worse, mandating a new translation of the requirements. This can introduce errors or omissions that may not be discovered until compliance testing.

   One possible solution is to use an execution-specification model that can act as an interactive test harness throughout the development process. It can also enable continuous test and verification, making errors of translation easier to identify and correct.

   The MathWorks' MATLAB and Simulink solutions are examples of tools that can help create such a model. Once the model is complete — in this case, of the WiMAX transmitter — it can be used to design a standard-compliant receiver. The model not only provides continuity and prevents misinterpretation of the standard; it is a natural test harness for the receiver design. Rather than relying on the handful of test vectors supplied in the printed standard, the model can be used to generate an inexhaustible supply of test cases.

3. Accurate, consistent test results. Because WiMAX is an evolving global technology, different countries may have different criteria for testing products. Test solutions, therefore, must provide accurate and consistent results for all critical fixed and mobile WiMAX physical layer parameters across product development, volume manufacturing and quality assurance. Solutions that are software upgradable can more easily address the evolving 802.16 standard's need, and are well positioned to support future WiMAX implementations like beamforming or MIMO.

   LitePoint's one-box IQmax test system with its integrated vector signal analyzer (VSA) and vector signal generator (VSG) resources is such a solution. With support for IEEE 802.16e-2005 WirelessMAN OFDMA (mobile WiMAX) and 802.16-2004 WirelessMAN OFDM (fixed WiMAX) physical layer standards, including all bandwidth and data rate options, this family of test solutions can be used to test mobile WiMAX physical layer parameters at the development, manufacturing and quality assurance stages. The newest family member, IQmax-100, is optimized to address the needs of WiMAX volume manufacturing. It accommodates a fast non-link-based test methodology that enables low-cost manufacturing with extensive test coverage that can reduce test times.

   Another solution, Agilent Technologies' E6651A mobile WiMAX test set, incorporates a suite of RF measurements for characterization, calibration and verification purposes. It can be extended to allow engineers to test, stress and debug IP data-handling capabilities of their designs in a controlled laboratory environment. As a result, WiMAX subscriber station designers and manufacturers can rapidly move from development to volume production, while at the same time improving the integrity and quality of their products and reducing cost.

   Agilent has also partnered with Beceem Communications to develop high-speed manufacturing test solutions for mobile WiMAX devices. Based on the company's MXA signal analyzer, MXG signal generator and test executive and control software, the test system will be optimized to control Beceem's MS120 baseband/RF chipset and the WiMAX modem software.

4. Thorough MAC evaluation. While many in the industry are counting on WiMAX to meet consumer demand for high-speed, mobile data services and devices, service providers remain wary, unwilling to adopt WiMAX without rigorous evidence of end-to-end performance, reliability and quality of service. Without thorough evaluation of MAC conformance, base station scalability and device performance, these metrics can't be measured.

   Spirent Communications offers solutions that support key fixed and mobile WiMAX testing areas: performance, functional, protocol analysis, and conformance. Its SR5500 wireless channel emulator, for example, accurately emulates complex wideband radio channel characteristics such as time-varying multipath delay spread, fast fading, slow shadow fading and channel loss. It replicates real-world deployment conditions using powerful digital signal-processing techniques, making it possible to isolate performance issues early in the development and design verification cycle. This accelerates time to market and minimizes post-deployment issues.

5. Increasingly complex spectrum and modulation measurements. With packet-oriented network technologies like WiMAX, packet collisions and spurious signal detection becomes more complex. Further complicating matters is the fact that WiMAX services may share spectrum with other services operating asynchronously. This makes interference detection difficult.

In response to this complexity, LitePoint has developed an application-specific test tool for characterizing and troubleshooting WiMAX device designs using a Tektronix RSA3408A real-time spectrum analyzer. The RSA-IQWIMAX demodulation and analysis software provides spectrum and modulation measurements on OFDM and OFDMA signals in accordance with fixed and mobile WiMAX standards. The combination of RSA3408A with RSA-IQWIMAX is a powerful WiMAX research and development test solution for performing time-, frequency- and modulation-domain measurements to decompose signals and uncover anomalies, and generate in-depth analysis for troubleshooting WiMAX devices at the design stage. Test results from RSA3408A and RSA-IQWIMAX can be integrated with LitePoint's IQmax to form an integrated end-to-end test solution from research and development through manufacturing test.

Optimized test and measurement solutions are critical to successfully take WiMAX out of the lab and into the marketplace. While the challenges and solutions detailed are not a comprehensive listing, they provide a glimpse of the emerging challenges in WiMAX test and the tools that vendors are creating to overcome them.