Test IEEE 802.15.4 HRP UWB Modulation Accuracy
2021-07-28 | 7 min read
The IEEE 802.15.4 standard defines the physical layer (PHY) and medium access control (MAC) sublayer for low-rate wireless networks. It focuses on low-cost, low-power consumption, and low-speed ubiquitous communication between devices. Many IoT wireless alliances and consortiums leverage the PHY and MAC sublayer defined in the IEEE 802.15.4 standard, such as ZigBee, WiSUN, and FiRa. In addition, the standard provides modes that allow for precision ranging capability.
Signal fidelity is critical to ensure robust communication links in such a complex environment.
To address different usability scenarios – low-cost, reasonable battery life, and ease of installation, the standard defines multiple radio formats for the PHY layer, such as offset quadrature phase-shift keying (O-QPSK), binary phase-shift keying (BPSK), chirp spread spectrum (CSS), Gaussian frequency-shift keying (GFSK), minimum shift keying (MSK), pulsed ultra-wideband (UWB), frequency-shift keying (FSK), and orthogonal frequency division multiplexing (OFDM). These different radio formats operate in a crowded wireless environment and some of them are on the same shared spectrum. Therefore, signal fidelity is critical to ensure robust communication links in such a complex environment.
Modulation accuracy measurements evaluate how close either the constellation states or the signal trajectory is relative to a reference signal trajectory. Error Vector Magnitude (EVM) measurements provide a simple and quantitative figure-of-merit for general digital modulation signals, such as QPSK and OFDM. For FSK and MSK, modulation quality shall be measured by observing the frequency deviation tolerance and the zero-crossing tolerance of the eye diagram.
HRP UWB modulation
Different from the others, the IEEE 802.15.4 high rate pulse frequency (HRP) UWB PHY uses a combination of burst position modulation (BPM) and binary phase-shift keying (BPSK) to modulate symbols as shown in Figure 1. Each symbol is composed of an active burst of UWB pulses, and the variable-length bursts support various data rates. IEEE 802.15.4 standard defines a reference UWB pulse as a root-raised-cosine pulse with a roll-off factor of β = 0.5. A transmitted pulse shape refers to the reference UWB pulse.
Baseband Impulse Response
Section 15.4.4 specifies pulse shape in the time domain that ensures interoperability and performance. The transmitted pulse cross-correlates with the reference pulse. Figure 2 illustrates impulse response measurements. For the main lobe width (Tw), the magnitude of the cross-correlation function ³ 0.8, is less than 0.5 ns for a channel bandwidth of 499.2 MHz, 0.2 ns for wider bandwidths. The maximum magnitude of any side lobe (marker 1) is < 0.3.
Figure 2. The HRP UWB impulse response measurement using Keysight PathWave vector signal analysis (VSA) software solution
Chip rate clock and chip carrier alignment
The error between the standard chip clock (499.2 MHz) and the signal chip clock is the average for all repetitions of the symbol in the SYNC portion of SHR except for the first and last repetition.
Section 15.4.6 specifies chip rate clock and chip carrier alignment. The chip clock error has an accuracy of ± 20 × 10-6. The PathWave VSA software reports the chip clock error in parts per million (ppm) in the summary table, as shown on the left side of Figure 1.
In addition, the center frequency of transmitted energy has an accuracy of ± 20 × 10-6. The setups for this measurement should set the resolution bandwidth to 1 MHz and the video bandwidth to 1 kHz on the signal analyzer.
Accelerate HRP UWB Design and Testing
With high precision and ranging capabilities, HRP UWB uses ultra-wide bandwidth and shaped pulses. HRP UWB devices need to meet RF test requirements specified in IEEE 802.15.4 and regulatory compliance tests to ensure interoperability and performance. To learn more about IEEE 802.15.4 HRP UWB RF tests, download the white paper An Overview of IEEE 802.15.4 HRP UWB Test Requirements.
Keysight offers a flexible suite of signal generation tools that reduce the time you spend on signal simulation and powerful analysis tools to understand the structure and quality of the transmitted HRP UWB signal.
|Signal analyzer||Signal generator||Vector transceiver1|
|Model||N9042B UXA||N9040B UXA||N9032B UXA||N9030B PXA||M9384B VXG||M9415A VXT|
|Frequency (GHz)||26.5, 44, 50||13.6, 26.5, 44, 50||13.6, 26.5||13.6, 26.5, 44, 50||14, 20, 31.8, 44||12|
|RF bandwidth (GHz)||1, 1.5, 2, 4||0.51, 1||1, 1.5, 2||0.51||0.5, 1, 2||0.8, 1.2|
|Software||PathWave signal generation for IoT waveform signal creation (N7610C)|
1 Vector transceiver integrates a vector signal generator and a vector signal analyzer in a three-slot PXIe module.