Resolving ZigBee IEEE 802.15.4 PHY Layer Test Challenges Using a WPAN Test Suite Based on NI LabVIEW and RF PXI Instruments

Author(s):
Mahammad Rasul Mastan - Seasolve Software, Inc.
Siddharth Bhatnagar - Seasolve Software, Inc.
Narayanan P I - Seasolve Software, Inc.

Industry:
Electronics

Products:
RF, LabVIEW

The Challenge:
Designing and developing an efficient, cost-effective, and time-saving test bench with a good platform for testing the PHY layer receiver and transmitter functionality of a Zigbee device, for conformance to the IEEE 802.15.4 – 2003 (2450 MHz) standard, to replace expensive, inflexible, and time-consuming conventional test procedures.

The Solution:
Developing an automated PXI-based Zigbee WPAN compliance test suite using National Instruments PXI-5660 RF signal analyzer and PXI-5670 RF signal generator in NI LabVIEW.

Zigbee WPAN Device Test

Zigbee WPAN supports reliable, cost-effective, low-power, wireless, monitoring, and control products based on an open global standard. The IEEE-802.15.4 standard specifies the physical (PHY) and media access control (MAC) layers at the 868 MHz, 915 MHz, and 2450 MHz ISM bands for Zigbee devices.

Device manufacturers are following the emerging trend of using Zigbee devices for control applications, leading to increased competition for market share. This presents significant new challenges for design engineers and device manufacturers to get their products tested and validated and released as quickly as possible with a relatively competitive pricing. Many times this results in failure to meet performance targets per the standard requirements. The demand for interoperability, together with the opportunity costs related with time-to-market drives the need for precompliance and compliance testing, making time-efficient, reliable, and cost-effective test solution a necessity.

The application we developed addresses and overcomes the challenges faced by a WPAN designer or an engineer during the testing phase of the PHY of Zigbee device for compliance with the IEEE 802.15.4 standard.

Test Suite Description

The WPAN test suite is comprised of the following:

• WiPAN LVCT – PHY compliance testing tool for Zigbee devices in the 2450 MHz band
• WiPAN LVSG – PHY receiver precompliance testing tool for Zigbee devices
• WiPAN LVSA – PHY transmitter precompliance testing tool for Zigbee devices

This complete test suite is for testing of the PHY of Zigbee devices at all phases of development. We developed the test suite in LabVIEW, which integrates seamlessly with the other NI products (NI PXI-RFSG 5670 and NI PXI-RFSA 5660).

The plug and play, cost-effective devices from NI and their easy integration with LabVIEW made our development work easier and faster. This helped us optimize testing of the Zigbee devices, addressing both the time-to-market and the cost factors.

The development of wireless technology is taking place at a rapid pace. With new techniques quickly replacing the older ones, the profit window is open for a short period of time and only to early technology providers. Thus, the need for reliable testing and certifying of the products in a rather short span of time arises. At the same time, we need to effectively address the cost and complexity of the test system, which otherwise has an adverse effect on the overall cost of the product.

Zigbee Devices and Need for Compliance Testing

With the emergence of many applications for Zigbee devices, multiple vendors are creating Zigbee devices. It is important that the devices from different vendors work with each other. This means that standardization and interoperability are crucial, making a testing solution that is cost-effective, reliable, and time-efficient a necessity. There is a need for a reliable compliance test system that is capable of testing the performances of the transmitter and receiver of a device, for conformance with the specified standard.

Our WPAN test suite effectively uses the virtues of virtual instrumentation in LabVIEW with advanced PXI technology-driven instruments to meet these requirements. Thus, the WPAN test suite is an automated, single-click and observe flexible solution that perfectly addresses the challenges faced by design engineers, testers, and vendors of Zigbee devices.

The Compliance Tests and the WPAN Test Solution

The IEEE 802.15.4-2003 specifies a number of tests, such as power spectral density (PSD), error vector magnitude (EVM), transmit power, center frequency tolerance, maximum input power, receiver sensitivity, adjacent and alternate channel jamming resistance, turnaround times, and symbol rate, to check the performance and interoperatability of a Zigbee device for conformance to the standard.

Among the list of tests specified in IEEE 802.15.4, tests such as EVM, turnaround times, and receiver tests such as receiver sensitivity, and jamming resistance tests require special considerations to obtain reliable test results. Performing these tests with conventional test procedures would be cumbersome and require different hardware/software for different tests, increasing cost as well as testing time. The SeaSolve WPAN test suite addresses all the above challenges by delivering a single reliable, cost-effective, and time-efficient solution for all the major tests.

Challenges – Case Study

Below are some of the compliance test challenges facing test technologists today, and how the WPAN test solution addresses the challenges.

Case 1 - EVM

EVM is a measure of the transmitter’s modulation accuracy representing any deviation from the ideal bit location. In the real world of radio frequency integrated circuit (RFIC) test, the measured EVM is a composite measurement of the actual device performance, the performance of the equipment used to make the measurement, and all of the interconnections between the two (such as load boards, contactors, and cables). The challenge is to carefully select the tester, and design a baseband receiver that guarantees the best performance with the tester.

We address this challenge using NI modular instruments, which guarantee high accuracy and robust front-end algorithms, built into the software, that work in real time to guarantee fast testing times. We used the triggering facilities provided by the NI PXI-5660 (supported by NI-Scope 2.7) for a reliable real-time solution. The solution would now behave as an ideal receiver (sensing power in the channel, for start of data acquisition), guaranteeing an accurate and reliable measurement, as below.

Case 2 - Receiver to Transmitter Turnaround Time (Rx-Tx Turnaround Time)

The Rx-Tx turnaround time is the time taken by a device to switch from the receiver state to the transmitter state. Under the conventional method of testing, one would require an additional instrument (namely an oscilloscope), along with the signal generator/analyzer to perform this test. The conventional method also requires the device under test (DUT) to provide a CCA pin/signal for this purpose. Thus, the conventional method would add the burden of cost and probing/interfacing with the scope. The challenge was to conduct this test using the same PXI modular instruments (NI-RFSA and NI-RFSG) only, to cut down on the cost, and to automate the process to save time and make the solution more reliable.

The solution provided by SeaSolve uses the same set of instruments (NI-RFSA and NI-RFSG) as the other tests. This in effect addresses the cost factor. In addition the solution uses advanced triggering features provided by the NI-Scope 2.7 (in conjunction with PXI 5620), to address the burden of probing the CCA signal. This is a unique test provided by SeaSolve.

Case 3 – Receiver Sensitivity

An important parameter in receiver testing is the proper synchronization between the signal source and the receiver, which is a challenge when conducting these tests. This is because the validity of these tests depends on the number of packets/frames in error. Improper synchronization would lead to improper packet error information leading to inaccurate test results.

The solution addresses this issue by using advanced features provided by NI-RFSG 1.1 such as script triggering, which allows for proper synchronization between the signal generator used for receiver testing and the receiver of the DUT.

Reliable, Cost-Effective Solution

We can use the WPAN test suite, developed in LabVIEW, for fast real-time testing of Zigbee devices. Users can conduct tests to verify the compliance of a device to the IEEE 802.15.4 standard using the WiPAN LVCT module of the suite. Precompliance tests and performance verification can be performed using the WiPAN LVSG for testing the receiver of the DUT and the WiPAN LVSA for testing the transmitter of DUT.

WiPAN LVSG also provides additional features like MAC configuration and addition of impairments, proving to be the best available solution for receiver testing. WiPAN LVSA, with its exhaustive measurements and features, is a handy tool for transmitter testing. Thus, this test suite proves to be a reliable, cost-effective, and time-efficient test application that addresses all major concerns faced by design and test engineers. In all, it is an innovative, cost-effective, and click-and-observe solution.

For more information, contact:

Mahammad Rasul Mastan

RF Design Engineer

Seasolve Software, Inc.

501 Canal Blvd. Ste. E

Richmond, CA 94804

Tel: (501) 231-4754

Fax: (501) 231-4789

E-mail: rasul@seasolve.com

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