UWB O-QPSK PSD Mask Test: Significance & Limits

Introduction : With the introduction of Narrowband (NB) Assistance in IEEE 802.15.4ab, UWB devices now operate in the crowded 5 GHz and 6 GHz bands alongside Wi-Fi. To prevent interference, the standard enforces a rigorous O-QPSK Transmit Power Spectral Density (PSD) Mask Test.

While traditional UWB uses ultra-wide pulses (500 MHz bandwidth), the 802.15.4ab standard introduces a companion “Narrowband” radio (operating in 5800 MHz or 6200 MHz bands) to handle control data and synchronization. This test ensures that this specific Narrowband radio “stays in its lane” and does not cause interference.

What is the O-QPSK PSD Mask Test?

This test measures the Power Spectral Density (PSD) of the Narrowband O-QPSK transmitter to verify that its energy is contained within a specific frequency limit and drops off sharply outside that limit.

The Test Setup:

• Resolution Bandwidth (RBW): 100 kHz.
• Video Bandwidth (VBW): 1 kHz (for CF/Symbol rate alignment tests).
• Frequency Bands: 5800 MHz (UNII-3) and 6200 MHz (UNII-5).

The Mask Limits:

The standard defines a “Relative Limit” mask:

• Reference Level (0 dB): The highest average spectral power measured within +/- 1 MHz of the carrier frequency (fc).
• The Drop-off Point: At frequency offsets greater than 3.5 MHz (|f-fc| > 3.5 MHz), the signal power must drop by at least 20 dB.

Image Courtesy : Rohde & Schwarz

Significance in UWB

Following are some of the benefits which are critical for the PSD mask test.

• Protecting the channel grid (i.e. interference mitigation)
• Regulatory compliance (5 GHz/ 6 GHz Bands)
• Ensuring efficient offloading
• Signal Quality Verification

Summary: Strict adherence to this mask protects the integrity of the control data that manages the UWB link, ensuring that the power saving benefits of Narrowband Assistance are not lost to interference and retransmissions.