Fast-sequenced nonsignaling cellular test

Focused, nonsignaling test equipment will pave the way for the adoption of new nonsignaling chipsets and ultimately provide faster test techniques.

By Robin Irwin, Agilent Technologies -- Test & Measurement World, 10/1/2010 12:00:00 AM

Sidebar Signaling test still has a place Read more from the October 2010 issue.

Nonsignaling test methods make it possible for manufacturers to reduce both test times and test costs across a range of wireless technologies. Chipset vendors are investigating ways in which they can provide nonsignaling test capabilities to cellphone manufacturers, and their efforts are leading to the development of proprietary, chipset-specific, test modes, particularly for cellular verification test.

The evolution in technology has led to test modes with varying degrees of nonsignaling capability. Two terms describe the spectrum of this capability: nonsignaling and fast-sequenced nonsignaling.

Nonsignaling test modes often evolve from existing signaling test modes. Initial chipsets that have made this transition still need to receive appropriate downlink traffic as well as broadcast or pilot channels from a base station in order to synchronize. Because of this, the nonsignaling test modes offer less potential for reducing test times when compared to fast-sequenced nonsignaling test modes.

A fast-sequenced nonsignaling test mode is expected to evolve from existing nonsignaling modes by progressive development of chipset firmware. A fast-sequenced test mode is considered by many cellphone test engineers as the pinnacle of nonsignaling capability, because it helps simplify the development of RF tests and yields the most optimum test times.

For chipsets that integrate fast-sequenced nonsignaling test modes, particularly with the use of predefined test sequences for verification, engineers working for cellphone manufacturers must adopt next-generation nonsignaling test equipment to test the RF parameters of their products, both in R&D and in manufacturing. Yet, during the evolution to a nonsignaling test environment, signaling test equipment will continue to play a role and will be important to the success of any broad adoption of nonsignaling test modes in the test life cycle. The role that each type of test equipment plays will depend on both the technical merits of the test mode (Ref. 1) and on the nonsignaling test techniques featured in the equipment.

Emergence of nonsignaling
If you have been exposed to wireless technologies, you might already recognize and understand the challenge of using appropriate chipset control. For example, WLAN (IEEE 802.11) and WiMAX (IEEE 802.16) products are tested sufficiently throughout their life cycle without the use of signaling or OTA (over-the-air) protocol test at every stage of test, including manufacturing test. Bluetooth manufacturing test is also moving away from requirements for signaling test.

The test modes for such chipsets are designed to provide the test engineer with the freedom to have the device transmit and receive in a predefined manner on the required channels. This feature applies not only to chipsets being tested in the calibration stage of manufacturing test, but also to chipsets being tested during verification test.

Chipset control also offers engineers the potential to shorten the development and testing of cellular devices in which multiple bands and formats are combined into single chipsets or products. As these new chipsets take hold, test vendors are looking for new test techniques within test equipment. Similar to how they worked together to address specific test-mode requirements of wireless technologies, chipset designers and test vendors must once again cooperate in the development of nonsignaling test modes and the corresponding next-generation test equipment. By leveraging existing silicon-vendor relationships, test-equipment manufacturers can provide instruments that fulfill the potential of nonsignaling-oriented test.

Test vendors already provide off-the-shelf, chipset-specific test equipment that provides manufacturers with a simple route to integrating device and test equipment for signaling test.Today, the drive is toward integrating predefined nonsignaling test modes where chipsets can output a variety of signals across power and frequency ranges. Many cellphones are already capable of outputting predefined transmissions (typically power and frequency as a minimum), which has streamlined calibration test and has reduced device setup time as well as device test time. Now, chipset-specific nonsignaling test modes and techniques aimed at reducing cellular verification test times are emerging.

Adoption of next-generation test techniques

Where nonsignaling test modes are recognized as a means for reducing test time, major shifts are occurring in requirements for next-generation manufacturing-test techniques. When chipset vendors add nonsignaling test capabilities to their test modes, cellphone manufacturers must subsequently find a way to integrate nonsignaling test into their manufacturing-test processes. As a result, cellphone manufacturers are in transition from a signaling environment to a nonsignaling environment, and many of their test processes use a combination of signaling and nonsignaling test equipment (Figure 1).

Figure 1. The proprietary nature of test modes and their subsequent integration into manufacturing lines makes the transition to nonsignaling test unique for any given example within the cellular food chain.

The proprietary nature of the test modes and subsequent integration into manufacturing lines makes the transition unique for any given example within the cellular food chain. It is fair to say that fast-sequenced nonsignaling is seen as the goal for all cellphone manufacturers, as it should yield the largest cost reductions for them. This requires the correct choice of test equipment to match the technical needs and overcome new challenges.

The technical capabilities of a device's nonsignaling test modes drive the requirements for the test equipment. Test vendors must provide tools that simplify the adoption of nonsignaling test, particularly in cases where a cellphone manufacturer is using a variety of chipsets from a number of chipset vendors. Manufacturers need test equipment that can be used across each manufacturing line, whether this involves reusing existing signaling and nonsignaling equipment, introducing next-generation nonsignaling test equipment, or both.

Accompanying the requirement for suitable test hardware is the need for signal-design packages. In next-generation nonsignaling test equipment, arbitrary waveform files are used to transmit signals to devices. This is a significant change in how test equipment provides a downlink, and both R&D and manufacturing-test engineers, therefore, need comprehensive signal-creation tools that can handle the chipset test-mode requirements. For the R&D engineer, the tools must provide wide coverage. For the manufacturing-test engineer, the tools should offer an interface that makes it easy to select the key signal parameters for a test. Test vendors can offer these types of focused software packages by again collaborating with chipset vendors and cellphone manufacturers to capture key requirements.

Nonsignaling test in R&D
The success of nonsignaling manufacturing test can be facilitated by solid design and test-mode development early in the test life cycle. Nonsignaling test equipment is needed during all parts of the process, not just in manufacturing. Yet, because manufacturers need to build stability and confidence into their processes when moving to nonsignaling test, they are likely to continue to use signaling test equipment throughout the adoption of nonsignaling techniques (see "Signaling test still has a place").

One major reason for this is to allow correlation of RF parametrics for validation of the success of any newly added, nonsignaling-featured test modes. Despite the continued need for signaling test equipment, manufacturers should consider employing a comprehensive, nonsignaling one-box tester in R&D as nonsignaling development begins. Such a tester can offer R&D tools and the ability to debug problems on the lab bench, and the same tester can then be used throughout the development process and into manufacturing test for consistency. Generally, one-box testers

• provide a front panel with a user interface that simplifies the selection of signal parameters,
• include ready-to-use applications that can return format-specific measurement results including RF metrics with no software development required,
• support format-specific spectrum analyzer applications for spectral analysis,
• provide generic source and  generator capabilities that replicate the capabilities available on a separate signal generator, and
• offer spectral-analysis tools that make it possible to debug failing test sequences on the manufacturing line.

To make the adoption of nonsignaling test smooth, test equipment vendors need to provide customers with test solutions that address the need for signaling, nonsignaling, and fast-sequenced nonsignaling.

Nonsignaling test in manufacturing

In most manufacturing lines, the current signaling and nonsignaling test equipment will coexist with next-generation nonsignaling test equipment as manufacturers make the transition to fast-sequenced technologies. But how do you decide which tester is suitable for the two categories of nonsignaling test modes within a device?

Figure 2. Both signaling and next-generation nonsignaling test equipment can handle standard nonsignaling test modes during manufacturing verification test, but only next-generation equipment can handle the fast-sequenced nonsignaling test modes that are currently being developed.

For devices with today's nonsignaling test modes, either existing or next-generation test equipment can provide the necessary tests (Figure 2). If you want to use signaling test equipment for these tests, however, you must consider the precise technical requirements to ensure complete compatibility. The point at which signaling equipment becomes less attractive and you must switch to next-generation nonsignaling test equipment depends on the chipset's capability and on the test-speed benefits. Another consideration is the test equipment's ability to address the test-mode requirements. It is sometimes more economical to upgrade existing signaling equipment with nonsignaling support where test modes and test capability permit.

For devices with fast-sequenced nonsignaling test modes, you must use test equipment that offers flexible, sequence-based test during verification in order to provide the optimum test coverage. Signaling test equipment does not offer sequence-based test and cannot maximize the potential of  the fast-sequenced nonsignaling test modes. Next-generation nonsignaling testers, however, can remove any band, cellular format, channel, power range, and time slot limitations that may have been enforced as a rule within signaling.

Focused nonsignaling test equipment, therefore, can pave the way for adoption of new nonsignaling chipsets and ultimately provide new and faster test techniques. In R&D, one-box testers can provide test and comprehensive debug opportunities using format-specific tools. They are also capable of debugging nonsignaling sequences being designed for manufacturing. Similarly, a well-featured source in a one-box tester can provide test coverage like a separate signal generator. For sequencing purposes, specific tools can be provided to drive the source across a sequence simultaneously with the analyzer.

Existing signaling test equipment does provide test coverage for devices that integrate nonsignaling in manufacturing, but nonsignaling test modes benefit most from test-speed improvements by using next-generation nonsignaling test equipment. For chipsets that integrate fast-sequenced nonsignaling test modes, particularly with the use of predefined test sequences for verification, next-generation nonsignaling test equipment must be adopted as a means of testing RF parameters.

REFERENCE
1. Irwin, Robin, "Nonsignaling in Next-Generation Test," Communications Test Report, Evaluation Engineering, June 2010. www2.evaluationengineering.com/enews/Comm-TestReport_June10/features/F2-nonsignaling.aspx.

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Robin Irwin works as a wireless application engineer for the Mobile Communications Division in the Electronics Measurement Group at Agilent. He joined Agilent Technologies as a graduate from Queens University Belfast after achieving a first class master's degree in electrical and electronic engineering. He started his career working on power meters, noise figure analysis, and one-box testers. He is currently investigating nonsignaling test techniques, working with customers on requirements and technical evaluations.

Signaling test still has a place Here are some reasons why cellphone chipset test will continue to require signaling test equipment during the transition to nonsignaling test: Signaling test can serve as a traceable reference (measurement correlation) of device RF performance during design (between signaling and nonsignaling test modes) and on through the transition to manufacturing test. Signaling test provides a starting point or benchmark for test-time comparisons. Signaling test can serve as a standard for the design of arbitrary waveforms for the purposes of receiver test and downlink synchronization within new next-generation equipment. Arbitrary-waveform generation capability is important for nonsignaling devices that require a specific downlink signal. Major chipset providers and cellphone manufacturers have collaborated with test vendors using signaling equipment as a target platform for many years. Many test modes and tools are developed around such equipment. The significant installed base of signaling test equipment in manufacturing has proven to be trustworthy, robust, and reliable, and wholesale changes would require extensive code rewrites and evaluation time. Robin Irwin