Discussion on Several Parallel Test Architectures for Wireless Test

One technology that has developed rapidly in recent years is wireless local area network (WLAN) technology. Although this technology has been rapidly developed due to the popularity of personal devices such as laptops, smart phones and tablets, market surveys show that this development momentum will continue to be rapid, because WLAN technology is more and more widely used in more In consumer devices, this ecosystem has also been extended to the "Internet of Things".

    Not only does WLAN service need to be more widely used, new standards such as 802.11ac will also provide devices with the necessary bandwidth to meet more applications such as video streaming. For equipment manufacturers, this means that test methods also need to keep pace with the times to cope with the rapid growth in manufacturing demand, and they need to reduce costs while maintaining the same level of quality. If a multi-device-under-test (DUT) test architecture is adopted, companies will be able to significantly reduce the time required to achieve these goals.

    I. Production test of WLAN equipment

    In the past, WLAN production test methods usually measure data throughput and verify signal levels by connecting a well-functioning device (also called a "golden sample") and a power meter. In recent years, companies have increasingly adopted more advanced transmit and receive tests to measure error vector magnitude (EVM), spectrum mask, transmit power, packet error rate (PER), and receiver sensitivity. The realization of this new test method is based on the WLAN chipset supplier providing customers with the required software to control the chips embedded in the device. By directly controlling the device under test without wireless communication with the device, test manufacturers and end users can develop applications more quickly while broadening the test coverage.

    A. Signaling

    In traditional signaling testing, for WLAN testing, the test system is usually used to simulate wireless access points, while for cellular testing, the test system is used to simulate base stations. The advantage of signaling testing is that it can test both the standard physical (PHY) layer and the media access control (MAC) protocol layer. Testing the MAC layer by simulating wireless access points is very useful for the design and verification process, but this function is usually unnecessary in production testing. In addition, signaling testing needs to implement a complete protocol stack on the test system, and the speed is much slower than non-signaling testing, because signaling testing is used for real network operation, not for extremely fast production testing . On the contrary, non-signaling testing optimizes the speed for production applications, so that the device under test can quickly complete parameter tests such as power level, bandwidth, channel or frequency, and modulation scheme [1], thereby helping equipment manufacturers to broaden the test coverage, and Will not extend the test time. Of course there are advantages and disadvantages-the main disadvantage of non-signaling testing is that the chip supplier's DUT control requires additional pre-development.

    B. WLAN non-signaling test framework

    A typical non-signaling WLAN test system is controlled by a host. The test execution program run by the host can read the test vector and measurement settings of the file, run the required tests, and write the results to the log file or database. The test executive calls the application programming interface (API) to control the test instruments, which usually include one or more vector signal analyzers, vector signal generators, or vector signal transceivers. In addition, the test execution program also measures the collected data by calling the WLAN measurement algorithm.

    The non-signaling test also requires the host to have the chipset control library provided by the chipset supplier to control the DUT in non-signaling mode. Figure 1 shows how these components constitute a typical WLAN non-signaling test framework.



    II. Multiple DUT testing can improve test throughput

    Although equipment manufacturers have generally adopted non-signaling tests in recent years to significantly shorten test time, the emergence of more complex new wireless technologies and the continuous shortening of product cycles have further increased the pressure to reduce test time and cost. For example, 802.11ac extends 802.11n by adding new data rates, bandwidth, and spatial streams. As equipment manufacturers apply 802.11ac to their products, they not only need to test this new standard, but in order to maintain backward compatibility, they also need to continue testing the previous standard.

    Such factors significantly increase the test time, which in turn increases the test cost. The new solution is that chipset suppliers, test vendors, and end users test multiple devices in parallel to maximize efficiency. This method is also called "multi-DUT" testing. By utilizing the multi-DUT test software and hardware architecture, equipment manufacturers can significantly increase their production test throughput without increasing test costs. Here to study and compare various multi-DUT test schemes. The scheme shown in Figure 2 uses a VSA+VSG or VST to test 4 DUTs through a switch matrix.



    Figure 2: Multi-DUT hardware configuration example

    Different methods are discussed here based on this test configuration. In order to compare these methods, a typical WLAN test is divided into the following common steps, and their relative time units are specified, as shown in Table 1.



    Table 1. Common elements of WLAN test

    The time-consuming part of these steps is the time required to communicate with the device under test to achieve the correct operating mode. Depending on the DUT and test plan, DUT control time can account for 45% to 90% of the total test time. Therefore, if a test system with a total test cost is to be developed, the system must have a low test equipment cost while reducing the standby time of the test equipment.

    C. Serial test

    In the traditional test plan, the equipment is tested serially through a test station supported by fixtures and radio frequency instruments. The test sequence is similar to the time frame shown in Figure 3. The main advantage of this type of test application is that it is very easy to implement. However, this method does not utilize any type of software or hardware parallel mechanism, which results in all RF instruments in a standby state during DUT startup.



    Figure 3: Time block diagram of serial test

    D. Pipeline

    Utilizing the multi-threaded parallel software architecture and the external switch circuit, the parallel software thread can controllably drive the next DUT startup process when the DUT that has completed the startup performs TX and RX tests, thereby realizing the pipeline execution of the test. This method reduces the downtime of the instrument and shortens the test time by up to 35%, as shown in Figure 4.



    Figure 4: Time block diagram of pipeline test

    E. Parallel RX test

    Another way to reduce test time is to perform multiple RX tests at the same time by sending the same waveform to multiple DUTs. By assigning each DUT to the corresponding software thread, all DUTs can be started at the same time. Using the technology shown in Figure 5, the test time of multiple DUTs can be reduced by 25% compared to pipeline testing and more than 50% compared with serial test equipment.



    Figure 5: Parallel RX test time block diagram

    F. Other multi-DUT test methods

    Pipeline and parallel RX testing are just two examples to illustrate how to improve the throughput of WLAN manufacturing testing without adding additional test equipment. As test software becomes increasingly complex and device control drivers continue to be optimized for testing, new test methods will continue to emerge to help device manufacturers continue to increase test throughput, reduce test costs, and keep up with the pace of future wireless technology development.

    III. Multi-DUT manufacturing test method

    When selecting test equipment for parallel multi-DUT WLAN production testing, it is necessary to ensure that the selected equipment uses commercial off-the-shelf (COTS) technology to increase current throughput while obtaining scalability for future needs, including From multi-core CPU processors and data buses for data analysis and data transmission, to multi-threaded software architectures for managing parallel testing.

    A. Multi-core processor and instrument data bus

    Usually the expensive components in an RF test system are the circuits used to generate and receive RF signals. Many other components, such as memory, hard drives, and CPU processors are commonly found in personal computers, so the product life cycle is short and very cheap. When choosing a test system, you also need to ensure that the system is easy to upgrade to the new COTS that appears on the market.

    PXI is a PC-based rugged test platform that provides high-performance, low-cost deployment solutions for measurement and automation systems, including wireless product production testing. PXI combines the electrical bus characteristics of PCI Express with rugged mechanical packaging, and adds a dedicated synchronous bus and major software features for manufacturing testing. This powerful combination provides equipment manufacturers with one of the solutions currently on the market with throughput and latency data buses, which significantly reduces the test time. Another advantage of PXI is the embedded computer, usually also called PXI embedded controller, which provides a high-tech CPU processor in a compact form factor. This modular feature allows users to upgrade the entire wireless test system at a relatively low cost when new technologies appear.



    Figure 6: The NI PXI Express chassis contains RF analyzers and generators, vector network analyzers, switches, power supplies, and embedded computers

    B. Multithreaded software architecture

    Just as the test method mentioned in the previous example of this article, when selecting a test system for large-scale multi-DUT wireless manufacturing, the software architecture of the system must be multi-threaded and capable of parallel testing. NI LabVIEW is a graphical programming language with an inherent parallel mechanism, which is very suitable for multi-DUT manufacturing testing. Compared with sequential languages, the LabVIEW graphical data flow program itself contains information about which parts can be run in parallel.



    Figure 7: As an inherently multithreaded programming language, NI LabVIEW is ideal for large-scale multi-DUT manufacturing testing

    C. Ideal multi-DUT WLAN test system

    The flexibility and scalability of the PXI platform and the inherent parallel mechanism of NI LabVIEW provide a revolutionary new tool for multi-DUT WLAN manufacturing test. The NI PXIe-5646R vector signal transceiver (VST) has a frequency coverage of up to 6GHz and an RF bandwidth of up to 200MHz, making it not only an ideal instrument for testing WLAN standards such as 802.11ac, but also very suitable for a very challenging future Standard (such as LTE-Advanced carrier aggregation, etc.) measurement.

    When paired with the multiple switch functions of the PXI platform and NI LabVIEW-based NI WLAN measurement suite software, the NI vector signal transceiver becomes an ideal instrument for large-scale multi-DUT manufacturing and testing. In addition, the NI vector signal transceiver can also test many other wireless standards, including Bluetooth, NFC, GPS/GNSS, 2G/3G cellular and LTE, etc., often with industry performance and test time [2].



    Figure 8: A complete WLAN test system consists of an NI vector signal transceiver and NI WLAN measurement suite software