Low-Power Wake-Up Signal Power Boost

Description

The Low-Power Wake-Up Signal (LP-WUS) power boost is a parameter defined in 3GPP Release 19 specifications (TS 38.101 and TS 38.521) that quantifies the power difference between the LP-WUS resource elements (REs) and the average power of all REs in the NR carrier. LP-WUS is a signal designed to wake up low-power devices, such as those in reduced capability (RedCap) or ambient IoT (Internet of Things) scenarios, from a deep sleep state. These devices need to monitor a very low-power wake-up signal while consuming minimal energy. The power boost ensures that the LP-WUS can be detected reliably even when the device is in a low-power mode with a simple receiver.

The LP-WUS occupies specific REs within an NR transmission bandwidth configuration. These REs are typically sparse in frequency and time to allow the device to detect the signal with minimal processing. The power boost is defined as the ratio (in dB) of the average power of the LP-WUS REs to the average power over all REs (including both LP-WUS REs and the regular NR carrier REs). This boost is necessary because the LP-WUS is transmitted at a higher power per RE compared to the average power of the carrier, which may include data and control channels. By boosting the LP-WUS power, the signal can be detected by a low-complexity receiver that may not have the capability to perform sophisticated channel estimation or equalization.

The power boost value is configured by the network and signaled to the UE. It must comply with regulatory requirements and ensure that the total transmit power does not exceed the maximum allowed. The boost is applied to the LP-WUS REs while the remaining REs carry the normal NR signals. The UE can then use a simple energy detector to sense the presence of the LP-WUS. The power boost is a critical parameter for link budget calculations and ensures that the wake-up signal can reach devices even in challenging coverage conditions.

In Rel-19, the LP-WUS power boost is specified in the RF requirements for NR. TS 38.101 defines the UE radio transmission and reception requirements, while TS 38.521 specifies the conformance testing. The power boost is part of the overall design to enable ultra-low-power wake-up receivers, which are expected to be used in devices that need to operate for years on a single battery. The LP-WUS is a key enabler for ambient IoT and other low-power applications in 5G-Advanced and beyond.

The concept of a wake-up signal is not new; it has been used in other wireless standards like LTE-M and NB-IoT. However, the LP-WUS in NR is designed to be even more power-efficient, allowing the main receiver to be completely turned off while a separate low-power wake-up receiver monitors the signal. The power boost ensures that this low-power receiver can detect the signal without needing a high-sensitivity front-end. This is a significant advancement in enabling massive IoT deployments with long battery life.

Purpose & Motivation

The LP-WUS power boost exists to solve the problem of reliably waking up low-power devices in a 5G NR network while minimizing their energy consumption. Traditional wake-up signals require the device to periodically wake up and monitor the paging channel, which consumes significant power. The LP-WUS allows the device to remain in a deep sleep state with only a simple, ultra-low-power receiver active. However, this simple receiver has limited sensitivity and cannot detect weak signals. The power boost compensates for this by increasing the transmit power of the LP-WUS REs relative to the average carrier power, ensuring that the signal can be detected even in poor coverage conditions.

Before the introduction of LP-WUS, low-power devices had to balance between wake-up latency and battery life. They could either wake up frequently to check for paging (consuming more power) or sleep for long periods (increasing latency). The LP-WUS with power boost enables a new paradigm where the device can sleep for extended periods and be woken up quickly when needed, without sacrificing battery life. This is particularly important for devices that are expected to operate for 10 years or more on a single battery, such as sensors in smart cities or industrial IoT.

The power boost also addresses the challenge of coexistence with regular NR transmissions. Since the LP-WUS is transmitted on specific REs within the NR carrier, it must be powerful enough to be detected by the low-power receiver but not so powerful that it causes excessive interference to other UEs. The definition of the power boost as the difference between the average power of LP-WUS REs and the average power over all REs provides a clear metric for network planning and ensures that the signal is both detectable and compliant with regulatory limits. This parameter is a key enabler for the next generation of low-power IoT devices in 5G-Advanced networks.