ZigBee is a wireless standard for personal area network (PAN) sensor monitoring and control. National Instruments' Alliance Partner SeaSolve has developed a test suite including transmit (Tx), receive (Rx) and compliance testing for ZigBee. In this two-part article, we will describe test methodologies and techniques for each type of testing.
Intro to ZigBee
ZigBee, also known as IEEE 802.15.4 is a communications standard designed for low-power short-range communications between wireless devices. It is classified as a Wireless Personal Area Network (WiPAN), a term which includes the Bluetooth (IEEE 802.15.3) standard as well.
The ZigBee standard has seen increasing interest from both commercial and military markets for applications such as wireless sensor networks, home automation, and industrial control. One interesting facet of the ZigBee standard is that it is designed such that devices can form a self-forming and self-healing ad hoc or mesh networks. In this scenario, a central 'PAN coordinator' device oversees the health of the network configuration. In recent years, sensor networks have been the subject of much research in military / battlefield applications as well. Thus, there is significant interest in using the ZigBee standard to define the communications links in ad-hoc battlefield intelligence scenarios.
One design decision of the ZigBee specification that makes it ideal for remote wireless sensors is the implementation of a low-power physical layer (PHY). As an overview, the PHY specifications allow ZigBee devices to operate at one of three bands: 868 MHz (Europe), 915 MHz (North America), and 2.4 GHz (worldwide). The 2.4-GHz band, in which ZigBee transceivers are most commonly deployed, uses the offset quadrature phase-shift-keyed (OQPSK) modulation stream.
This scheme is a derivation of traditional QPSK and is used because it requires less power than similar schemes, while achieving the same or better throughput. OQPSK uses a maximum phase transition of 90 degrees from one symbol to the next. This prevents symbol overshoot and requires slightly less transmission power than the traditional QPSK modulations scheme. This design decision, combined with the use of a 5-MHz channel bandwidth enables devices to achieve a data rate of up to 250 kb/sec in a reasonably power-efficient manner.
Because ZigBee transceivers are designed for low-power applications, the PHY is relatively tolerant to significant error. In fact, devices are able to tolerate an error vector magnitude (EVM) of up to 35% while maintaining reasonable bit-error-rate (BER) performance. Thus, design validation and product request requires a variety of test methodologies. In the following sections, we will explain why specific tests much be conducted and provide tips to enable the most accurate testing methodologies.
As an overview, we will divide our discussion into three parts. These include:
- Transmitter Testing with a vector signal analyzer (VSG)
- Receiver Testing with a vector signal generator (VSA)
- Automated Compliance Testing (ACT) with both VSA and VSG
ZigBee Transmitter Testing
When testing a ZigBee transceiver's Tx signal quality, a VSA must be used in order to characterize both spectrum information and modulated signal quality. With the SeaSolve's WiPAN LVSA Signal Analysis toolset along with a PXI-5660 VSA, we were able to perform both spectrum and modulation measurements on IEEE 802.15.4-compliant signals.
It is important to remember that both measurement types are a requirement for both design validation and production test. As an overview, the spectral emissions of a ZigBee transmitter will dictate its interoperability with other devices in the industrial, scientific, and medical (ISM) band. In addition, the modulation quality of the Tx signal, combined with the antenna performance, dictates the range of distance over which the device can reliably perform. A typical test configuration is shown in Figure 1.
1. Typical transmitter is tested through either direct connection or air interface
The most common spectral measurements performed include: power spectral density, occupied bandwidth, power in upper/lower bands, and total power in band. In addition, typical modulation analysis tools include the: constellation plot, eye diagram, complementary cumulative distribution function curve (CCDF), and returned bitstream. Typical modulation measurements are: EVM, frequency offset, and BER.
Note that various stages of product development will require different measurements and/or analysis. For example, the design validation and verification stage of development requires more intensive analysis tools such as a constellation plot to debug various issues in product design. On the other hand, production test requires more definitive measurements such as EVM and frequency offset such that performance can be compared to test limits.
ZigBee Tx Spectrum Analysis
Below, we describe each of the basic frequency domain measurements and explain their importance. Note that each of the following measurements can be made with either a spectrum analyzer or VSA. In general, a VSA is the recommended instrument because it can be used for modulation measurements (next section) as well.
Power Spectral Density
Power spectral density (PSD) is a measurement that describes how the power of a given packet of data is spread over a broad frequency range. This measurement is used to ensure that the transmitter operates within the spectral mask requirements of the IEEE 802.15.4 standard. As Figure 2 illustrates, a frequency mask is compared with the output power. The frequency mask, shown as the white line, represents the limit of power that transmitter is allowed to emit into adjacent bands. When troubleshooting a device, factors such as poor filter design or images resulting from amplifier compression can contribute to unwanted power in adjacent frequency bands.
2. Plot of Power Spectral Density
Power in Band
The power in band measurement calculates the integrated power (dBm) in the specified channel or band. This measurement is used to ensure that the transmitter does not exceed power specifications of the IEEE 802.15.2 standard.
Occupied bandwidth returns the bandwidth of the specified frequency band that contains 99% percent of the total power of the span.
Adjacent Channel Power
Adjacent channel power measurement comprises of power in the upper and lower bands. According to IEEE 802.15.4, upper band is 5MHz towards the right of the operating frequency and the lower band is 5MHz towards the left of the operating frequency.
Baseband parametric measurements are used to ensure that the ZigBee transmit packets will be able to be successfully decoded by the receiver. Because ZigBee transceivers are designed to operate at low-power and do not require high data throughput, modulation quality is often sacrificed to reduce power consumption. Overall, the purpose of measuring quality is to evaluate the likelihood of bit errors. As an example, we estimate BER as a function of EVM (%), shown in Figure 3.
3. BER vs. EVM for a QPSK Modulated Transmission
As the graph shows, BER increased dramatically when the EVM of a QPSK transceiver increases from 15% to 30%. By contrast, most ZigBee devices are required to operate at an EVM that is below 35%. Thus, it is important to measure modulation accuracy to validate that a transceiver will operate effectively in its deployment environment. This can be done with several plots and measurements, shown below.