3GPP Release 16 brings new features to smartphones and IoT devices that reduce power consumption. It’s a start.
Smartphone power consumption and battery life rank high in the user experience. Indeed, a 2018 survey of about 1,000 American smartphone buyers conducted by Morning Consulting found that battery life was the most commonly cited important factor in their buying, with 95% of respondents including it. In a “survey conducted earlier this year by Android Authority of nearly 1,000 readers, 58% of respondents listed daily battery life as the most important battery feature for their smartphone.
In addition to its impact on the consumer user experience, smartphone power consumption and battery life have come under scrutiny as part of a broader societal movement toward energy efficiency in consumer electronics and beyond. With the transition to 5G, power consumption is also a much more relevant factor for non-smartphone UEs such as IoT devices and sensors.
Smartphone makers, IoT device makers, and the wireless communications industry have, of course, taken note. Power-saving techniques have become a critical part of integrated circuit and handset designs. Reducing power consumption in the operation of the user equipment (UE) and its interactions with the network has become a significant area of focus. To that end, the 3rd Generation Partnership Project (3GPP)’s Release 16 — finalized earlier this year — contains several power-saving enhancements to the 5G standards. The enhancements include:
- A wakeup signal to wake UE devices when transmissions are incoming, enabling them to remain in low-power mode otherwise.
- Enhanced cross-slot scheduling, which allows the network to inform UEs when a guaranteed minimal time interval exists between packet downlink transmissions, enabling the UE to reduce unnecessary RF operation and use more efficient receiver configurations.
- Adaptive MIMO layer reduction, which enables the UE to more efficiently use massive multiple-input/multiple-output configurations for data transmission and reception.
- Relaxed radio resource management measurement, which enables the UE to use less power when managing radio resources.
The wakeup signal was initially introduced in 3GPP Release 15. A paging signal sent over the physical downlink shared channel (PDSCH) literally wakes the UE from an idle state informing it to prepare to receive data. The wakeup signal enables the UE’s main receiver and most of its resources to remain in an idle state until required, dramatically reducing current draw.
Most UEs can be configured for discontinuous reception (DRX) — remaining in an idle state for a certain period, waking up periodically to check for traffic on the PDSCH. Currently, when configured for a long DRX period, the UE will wake up at the scheduled time and stay awake for the entire duration of the configurated “on” period. Release 16 introduces a new downlink control information (DCI) format that can be read by the UE before the long DRX wakeup time. This short DCI can inform the UE if there is no relevant downlink traffic on the PDSCH, enabling it to return immediately to an idle state through the next “on” duration.
The wakeup signal may have only minimal impact on the power a phone’s consumption. It will, however, have a much more significant effect on the power consumption of non-smartphone UEs such as IoT devices and sensors, many of which by design remain in an idle state for long periods — weeks, months, or even years — waking up only to transmit or receive information only when an event occurs (Figure 1).
Enhanced cross-slot scheduling
3GPP Release 15 introduced the concept of flexibility and scalability in the transmission frame structure to help support the many new and diverse use cases expected in 5G, including ultra-reliable low-latency communications (URLLC), massive machine-type communications (mMTC), and enhanced mobile broadband (eMBB). A standard slot can be broken down into mini-slots that are 2-, 4-, or 7- orthogonal frequency division multiple access (OFDM) symbols long. Using a mechanism known as dynamic-time division duplex (TDD), 5G New Radio networks dynamically balance uplink and downlink traffic requirements and include control and acknowledgment messages within the same subframe.
Release 16 adds the concept of enhanced cross-slot scheduling, which enables a UE to go into a microsleep state, rather than performing some non-essential decoding tasks, if applicable. A new bit field in some DCI formats informs the UE in advance if the time between the uplink or downlink control information slot and data slots is sufficient to enable microsleep, an intermediate low-power state that reduces current draw without impacting performance (Figure 2).
Adaptive MIMO layer reduction
5G NR makes use of massive multiple-input/multiple-output (MIMO) technology to dramatically increase throughput. By utilizing dozens or even hundreds of antennas, massive MIMO employees dozens or even hundreds of base station antenna elements to stream data to multiple users, offering approximately 50 times the spectral efficiency of a single-input, single-output system. The use of MIMO requires more resources and thus, greater power consumption than the single input, single-output system.
Adaptive MIMO layer reduction in Release 16 creates the ability to adaptively reduce the number of downlink MIMO layers in a transmission, saving the UE a significant amount of power by allowing the UE to reduce the number of antennas in use. For example, the initial bandwidth part — set of contiguous common physical resource blocks (PRBs) — can be configured for a single MIMO layer, while other bandwidth parts could use a higher number of MIMO layers. The adaption of the maximum number of downlink MIMO layers is done on a per-bandwidth part basis.
Similar gains can also be obtained in the uplink by restricting the maximum number of MIMO layers, enabling the UE to turn off certain transmitters and reduce power consumption.
Relaxed radio resource management measurement
Measuring radio resource management (RRM) is critical to ensuring the efficient use of available network resources. It is generally a very power-intensive activity. In 5G, cell signal measurements use synchronization signal block (SSB) beams to measure things like channel quality, signal strength, and signal power.
Release 16 enables the UE to reduce the number and frequency of RRM measurements for signals from the cell it is connected to and neighboring cells. The criteria for relaxing these measurements include maintaining a relative proximity to the cell the UE is connected to and maintaining reference signal received power (RSRP), reference signal received quality (RSRQ), and signal-to-interference-plus-noise ratio (SINR) within a given threshold. Specific criteria for relaxed RRM measurement are determined by the network that the UE is operating on.
While the power saving potential of any of these individual enhancements may seem trivial, taken together, the effect can be quite significant. The actual power savings available through these measures is dependent on many factors, including the specific device, network, and other factors. For small IoT devices and sensors, the power savings enhancements added to Release 16 could literally add months or even years to some low-power UEs’ battery life, particularly those that make use of energy harvesting mechanisms.
The cumulative effect of these power-saving enhancements is likely to be far less dramatic in smartphones. But it will certainly extend the battery life of a smartphone and could well make the difference between your trusty sidekick saving the day or leaving you in a jam.