The wake-up signal was first introduced in 3GPP Release 15
In 3GPP Release 15, a paging signal sent over the physical downlink shared channel (PDSCH) that “wakes up” a user device from an idle state so that can prepare to receive data was first introduced. Known as the wake-up signal (WUS), this feature reduces power consumption and improves battery life by dramatically reducing a device’s resource draw. This has resulted in a much better user experience.
As conversations regularly turned to environmental responsibility, power consumption — and how to limit it — it has become even more central to the future of cellular technology and those who are responsible for enabling it. Release 16 (Rel-16), therefore, contained several enhancements to the 5G standard designed specifically to save power, all of which culminated in a feature called low-power WUS.
Enhanced cross-slot scheduling
One of the updates in Rel-16 is enhanced cross-slot scheduling, which informs a device in advance if the time between the uplink or downlink control information slot and data slots is sufficient to enable microsleep. Microsleep is defined as an intermediate low-power state that reduces power usage without compromising performance.
In other words, the network can tell a device when a guaranteed minimal time interval exists between packet downlink transmissions, cutting down significantly on unnecessary RF operation.
Adaptive MIMO layer reduction
Massive multiple-input/multiple-output (MIMO) technology is used in 5G New Radio to increase throughput. However, MIMO requires more antennas, and therefore, more power.
Rel-16 introduced adaptive MIMO layer, which enables a more efficient use of MIMO configurations. This leads to power savings because it allows for the adaptive and dynamic reduction of the number of downlink MIMO layers in a transmission. Fewer downlink MIMO layers means fewer antennas are being used during a transmission.
Relaxed radio resource management measurement
While measuring radio resource management (RRM) is necessary to understand the network resource usage, it typically requires a lot of power. An enhancement in Rel-16 introduced a feature designed to reduce the number and frequency of RRM measurements for signals from the cell it is connected to and neighboring cells by “relaxing” this networking activity.
The network on which a device is operating determines the criteria for relaxed RRM measurement, but can include maintaining a relative proximity to the cell, maintaining reference signal received power (RSRP) and reference signal received quality (RSRQ), as well as the signal-to-interference-plus-noise ratio (SINR).
Beyond Rel-16: Low-power WUS in Rel-17 & 18
Release-17 offered several low-power enhancements in addition to those outlined above, one of which is the increased support for “reduced capability” (or RedCap) devices. These include things like sensors or wearables, which typically do not need (and/or cannot handle) the wide, 100-megahertz channels that are supported in 5G.
These devices, according to Juan Montojo, Qualcomm Technologies’ VP of technical standards, “efficiently support lower complexity IoT devices.”
“Release 17 scales down wideband 5G NR design (i.e., 100 MHz bandwidth) to 20 MHz/100 MHz in sub-7/mmWave,” he continued. “It also reduces the number of receive antennas required at the device for regular 5G NR from four in most of the new sub-7 bands (i.e., n77/n78/n79, n41, etc.) to one or two in NR-Light.”
When it comes to the latest 3GPP standard, Ericsson suggested in a blog post that further changes to RedCap devices aimed at power savings is expected: “Rel-18 RedCap solutions will further reduce device cost and power consumption. Solutions enabling energy harvesting, such as energy-efficient wake-up radios, will be investigated,” the vendor commented.
