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Power Savings for Cellular IoT Devices

A Look at the Essential Measures Recommended by 3GPP

In the past decades, the Internet has been booming and connecting people around the world. Current technology trends tell us that not only the people but also machine-type devices, e.g., household appliances, street facilities such as traffic lights, and connected vehicles, are going to be connected and communicate with each other. The Internet of Things (IoT) is a buzzword crossing all vertical industries. We are now, so to speak, in the dawn of the IoT era. With IoT technology, the world is getting increasingly smart. New concepts such as smart cities, smart homes, and smart agriculture are becoming a part of daily life and gradually changing our lifestyles. 

Cellular IoT (CIoT) is considered one of the most attractive contributions to the IoT industry. The technologies referred to as licensed spectrum-based low-power wide-area (LPWA) access technologies are deployed in the GSM, LTE, or 5G new radio (NR) network and provide benefits with respect to quality of service, reliability, latency, and coverage range. Yet they also have the characteristics of low complexity, low cost, and low power consumption. CIoT technology provides the opportunity for enterprises to increase efficiency and improve value for the customer.

Figure 1: Cellular IoT connections by segment and technology (billion) (source: Ericsson Mobility Report, June 2021)

Standardized by the 3rd Generation Partnership Project (3GPP), CIoT is the general term for radio technologies known as LTE-M (for long-term evolution for machines) and NB-IoT (for narrow band-internet of things). As shown in Figure 2, the first machine type communication (MTC) standard based on LTE network (also called LTE-M) was specified in 3GPP Release 12. Starting with Release 13, 3GPP included NB-IoT technology under the scope of the standard. By the first NR Release 15, CIoT was already an integral part of the whole NR standardization effort. 

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Figure 2: 3GPP cellular IoT (CIoT) evolution path

Two future focuses for the 3GPP include ensuring a smooth integration from LTE-based CIoT to 5G core network and developing NR-based CIoT to serve IIOT applications where mobile broadband (MBB) communication and ultra-reliable low latency communication (URLLC) are required. The feature sets of both CIoT technologies are enhanced steadily along the entire evolution path, and power saving has always been a perpetual topic of the standard. In this article, we will shed light on some of the major power-saving measures for LTE-based CIoT implementations that are recommended by 3GPP.

Power Saving Mechanisms

Discontinuous Reception (DRX)

Discontinuous reception (DRX) is a generic mechanism in mobile communication where the user equipment (UE) is allowed to stop monitoring the radio channel, e.g., physical downlink control channel (PDCCH), and enters the low power consumption mode or sleep mode for a certain period of time.

There are two types of DRX deployments, namely, idle DRX (i-DRX) and connected DRX (c-DRX) that correspond to the UE’s radio resource control (RRC) idle and connected mode, respectively. 

The UE operated in i-DRX mode monitors the PDCCH at defined time intervals. The UE will enter sleep mode between two consecutive PDCCH monitoring (see Figure 3).

Figure 3: DRX in RRC idle mode (i-DRX)

In the c-DRX mode, the UE is allowed to monitor the PDCCH discontinuously to check if the scheduling messages can be detected by its cell radio network temporary identifier (C-RNTI) on PDCCH. Figure 4 illustrates the concept of a c-DRX process. Short DRX and long DRX cycles can be included in the c-DRX mode. The UE monitors the PDCCH during the On time and sleeps during the Off time in each DRX cycle. The DRX cycle starts when the DRX inactivity timer expires. The UE enters into a short DRX cycle(s) first, followed by a long DRX cycle. The adoption of a short DRX cycle is optional for LTE-M UE. However, NB-IoT UEs support only a long DRX cycle.

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Figure 4: DRX in RRC connected mode (c-DRX)

Extended Discontinuous Reception (eDRX)

Extended discontinuous reception (eDRX) is a power-saving optimization feature introduced in 3GPP Release 13. As the name implies, eDRX supports a longer DRX or paging cycle compared to the legacy DRX power-saving features described in the previous section.

Figure 5 shows the basics of eDRX in comparison to the legacy DRX where the DRX cycle is extended from 2.56 seconds to minutes or even hours. In an RRC idle state, a UE can be configured for up to approximately 44 minutes for LTE-M and approximately 3 hours for NB-IoT. 

Figure 5: Comparison of DRX and eDRX

Of course, the extension of DRX or paging cycle has the side effect where the UE becomes less responsive, i.e., mobile terminated connections will show a much longer call setup behavior. However, for some particular CIoT use cases, this drawback is acceptable. Typically, those CIoT UEs send a small amount of data and the data that has been sent or received is not time-critical, leading to more delay tolerance. Furthermore, there is an assumption that LTE-M and NB-IoT UEs have more uplink-oriented data traffic than the downlink ones. Therefore, the tradeoff between UE reachability and power consumption in these CIoT applications is now more in favor of reducing energy consumption. Long battery lifetime, for example, a minimum of 10 years, is required for LTE-M and NB-IoT UEs. To achieve this, an eDRX approach is highly recommended.

Power Saving Mode (PSM)

Power saving mode (PSM) is a feature designed for LTE-M/NB-IoT UEs to help conserve more battery power with its deep sleep characteristic. This feature was first introduced in 3GPP Release 12.

To update the network about its availability, the UE performs periodic tracking area updates (TAU) after a configurable TAU timer (T3412 timer) has expired. After that, the UE stays reachable for paging in the idle state for a period of time (T3324 timer). Once the T3324 expires, the UE enters the deep sleep mode, also called power-saving mode (PSM), becomes dormant, and is therefore unreachable until the next periodic TAU occurs. 

During the PSM, the UE turns off its circuitry but is still registered in the network, meaning that the UE still keeps the non-access stratum (NAS) status while closing the access stratum (AS) connection. The advantage of such an approach is that the UE can wake up immediately from the PSM without having to reattach or re-establish the packet data network (PDN) connections. This avoids extra power consumption due to the transmission of additional signaling messages required for establishing a higher layer connection. 

Figure 6 indicates the principle of the PSM and its message flow. The UE can exit PSM either after the expiration of the T3412 timer, i.e., renewed TAU, or when the UE initiates a mobile originated (MO) service or detach. With the latter, the UE can proactively exit PSM and enter the RRC idle state and connected state later to ask for the service.

Figure 6: Principle of power-saving mode

The utilization of PSM is particularly interesting for use cases requiring infrequent mobile-terminated or mobile-originated events which allows the certain latency of the services, for example, a water meter sends the counter once a month. With a PSM mechanism, a 10-year battery lifetime, as recommended for the LTE-M and NB-IoT UEs, becomes possible.

Release Assistance Indication (RAI)

Release assistance indication (RAI) for access stratum (AS), a 3GPP Release 14 feature, allows the LTE-M/NB-IoT UE to trigger a buffer status report (BSR) with zero-byte size to indicate to the eNB that it has no more uplink data, and that the UE does not anticipate receiving further downlink data. This makes it possible for the RRC connection to be released. As a result, the early transition from the RRC connected state to RRC idle state to save power is enabled. Without having the RAI introduced, the UE would have to wait for the eNB to release the connection via explicit signaling or when the RRC inactivity timer expires. 

Mobile Originated Early Data Transmission (MO‑EDT)

In 3GPP Release 15, CIoT data transfer in the control plane (CP) and the user plane (UP) modes is enhanced to the so-called CP-EDT and UP-EDT, respectively. In contrast to the legacy data transfer using CP and UP CIoT EPS optimization defined in Release 13, the main benefit of the EDT lies in the fact that the uplink and downlink data can be transmitted early during the contention-based random access (CBRA) procedure. 

Uplink and downlink data can already be sent together with message 3 (Msg3) and message 4 (Msg4), either piggybacked (CP-EDT) or multiplexed (UP-EDT) with the RRC message. The procedure could actually terminate after Msg4 if no more data has to be received or transmitted by the UE. The approach reduces the signaling overhead as well as message latency and therefore lowers the UE’s power consumption. Specifically, battery life can be improved by almost three years and the message latency is reduced by more than three seconds under poor radio conditions compared to performance under Release 13.

Certain maximum transport block size (TBS) is expected by EDT. For an LTE-M UE, TBS given in Msg3 is dependent on the coverage enhancement (CE) level. For CE0 and CE1, the UE can utilize the maximum 1000 bits TBS to transmit data, whereas for CE2 and CE3, the UE is only allowed to apply maximum 456 bits TBS. For an NB-IoT UE, the maximum TBS is about 1000 bits. 

RRC message in Msg4 implicitly communicates whether more data has to be exchanged. By receiving the message “RRCEarlyDataComplete/RRCConnectionRelease,” the UE understands that the eNB has no more data to transmit and can go to RRC idle mode. Otherwise, by receiving the message “RRCConnectionSetup/RRCConnectionResume,” the UE will expect more data from eNB and fall back to legacy mode (Release 13) by establishing/resuming the connection.

 Figure 7 shows the signaling flow of the CP-EDT and UP-EDT. The message flow plotted in the dotted line indicates the Release 13 CIoT UP and CP which serves the fallback procedure of the data transmission. This happens when more UE data is expected to be sent. 

Figure 7: Signaling flow of mobile-originated CP-EDT and UP-EDT including data transmission fallback

Mobile Terminated Early Data Transmission (MT‑EDT) [5]

3GPP Release 16 extends the EDT feature to mobile-terminated EDT (MT-EDT) as well, intended for a single downlink data transmission. It allows the downlink data transmission during the random access (RA) procedure triggered in response to a paging message. Depending on the received data size from the core network, eNB can decide whether MT-EDT should be initiated or not. If MT-EDT occurs, the UE is informed by the MT-EDT indication added by eNB in the paging message. Subsequently, the UE triggers the EDT procedure which can be based on CP-EDT or UP-EDT, as presented in Figure 8. The difference to MO-EDT is that it does not contain the UL data in Msg3 of the RA procedure here. 

Figure 8: Signaling flow of mobile terminated EDT

Preconfigured Uplink Resources (PUR)

Preconfigured uplink resources (PUR) represent an additional aspect of the power saving mechanism defined in 3GPP Release 16. As depicted in Figure 9, the UE requests PUR configuration (number of occurrences, periodicity, time offset, TBS, etc.) in RRC connected state towards eNB. The eNB then decides to provide the PUR resource to the UE and at the same time sends the UE to RRC idle state with the message “RRCConnectionRelease.” By the subsequent uplink data transmission, instead of acquiring resources through the RA procedure, the UE transmits UL data by utilizing MO-EDT (“RRCEarlyDataRequest/RRCConnectionResumeRequest” in Msg3) on the agreed PUR resource. In this case, Msg 1 and 2 of the RA procedure can be waived, and the uplink transmission power efficiency is increased. 

Figure 9: PUR configuration request

Wake-Up Signal (WUS) 

A wake-up signal (WUS) is a 3GPP Release 15 feature which is similar to the UMTS paging indicator channel. It is a physical signal in conjunction with DRX operation that can be decoded or detected before the UE monitors the paging on PDCCH. The benefit of introducing WUS is to reduce unnecessary power consumption related to the PDCCH monitoring. By having the WUS approach, the UE only needs to decode the PDCCH when WUS is detected; otherwise, the UE will stay in the sleep mode. This represents an efficiency improvement, especially when considering low activity periods on the control channels within a cell, e.g., at nighttime. Figure 10 illustrates the WUS principle in comparison to the conventional i-DRX operation.

Figure 10: I-DRX vs. DRX with WUS detection

The timing of the WUS with respect to the associated PO is shown in Figure 11. WUS duration is the maximum time duration that is configured by the network for the UE to detect the WUS. After the WUS is detected, the network leaves gap time to allow the UE to re-synchronize to the network and eventually switch over from the low-power wake-up receiver to main baseband circuitry in order to be ready to decode the PDCCH. 

Figure 11: Timing of WUS

To ensure that the UE does not miss any paging message, the WUS adopts a robust Zadoff-Chu sequence of length 131 to keep the missing detection rate below 1%. 

The latest development of WUS is addressed in 3GPP Release 16. A so-called UE-group WUS (GWUS) was introduced [5]. With this evolution, eNB instructs the UEs in the defined group to monitor the paging on PDCCH. The intention of this is to reduce the false alarm rate. In principle, several UEs may be mapped to the same PO. With the Release 15 WUS, some UEs may be unnecessarily awakened by the WUS when the intention of the eNB was actually to page the other UEs associated to the same PO.

Reduce System Acquisition Time

Several features to reduce system acquisition time are specified in 3GPP Release 15. Their handling differs somewhat for LTE-M and NB-IoT UE. In the LTE-M case, several measures are included in the 3GPP Release 15 specification as explained in the following section.

After the UE is awake from a PSM or a DRX sleep mode, it usually needs to resynchronize with the network to acquire the time and frequency synchronization. This is typically due to the clock drift in the UE. In order to enable the UE to carry out fast time and frequency synchronization to save power, a newly designed resynchronization signal (RSS) is introduced. It is denser in time and frequency than the legacy PSS/SSS (still required for initial synchronization to a new cell). The potential power saving by using RSS can reach up to 98% in comparison to legacy PSS/SSS. Furthermore, RSS is also capable of indicating whether or not there are any changes in the MIB. Based on this, the UE may even skip decoding the MIB. 

Furthermore, the UE may re-acquire SIB1 less often. This can be achieved by setting a flag bit in MIB indicating the change of SIB1 during the system information validity time. The UE shall read MIB after DRX or after cell reselection. If there is no indication of a change in SIB1, then the previous SIB1 stored in UE should be considered. 

Improving the MIB and SIB demodulation performance is also addressed. The reduced acquisition time is enabled by enhanced cell global identity (CGI) reading delay requirements based on an accumulation of transmissions within two 40 ms periods for MIB and one modification period for SIB1/SIB2.

For NB-IoT UE operated in FDD mode, reduced system acquisition time is achieved during the cell access. This happens when eNB transmits SIB1‑NB message (maximum 16 repetitions of SIB1-NB) in additional subframes on anchor carriers and non‑anchor carriers. This approach enables the UE to decode the SIB1-NB faster, thus contributing to the power saving.

Relaxed Monitoring for Cell Reselection

This feature intends to reduce the radio resource management (RRM) monitoring during the cell reselection procedure when the NB-IoT UE has low mobility or is not at the cell edge. Network signals the UE with an NB-IoT reference signal received power (NRSRP) delta threshold. When the changes in RSRP in the current cell do not exceed the given threshold, then the UE does not need to monitor the neighbor cells for 24 hours. With this approach, the UE power consumption is reduced.

Increase the Peak Data Rate

Several feature proposals regarding 3GPP Release 17 standardization are currently being discussed. One of the main objectives is to increase the peak data rates for NB-IoT and LTE-M. This is intended to support the broadening use cases for cellular LPWA IoT, addressing the lessons drawn from deployment and trials, and supporting the long-term lifecycle of NB-IoT and LTE-M. This throughput increase will indirectly achieve power saving by allowing the UE to terminate the connection earlier, or by entering into a lower power mode sooner. For example, supporting 16-QAM for unicast in UL and DL for NB-IoT UE. For LTE-M UE, 14-HARQ processes in DL for half-duplex FDD mode, maximum DL TBS of 1736 bits for half-duplex FDD in CE Mode A.

Conclusion

CIoT (LTE-M and NB-IoT) technology is gaining more and more momentum and its evolution continues in the 5G era. At the same time, the massive scale of CIoT poses a big challenge to network efficiency, user experience, and field service efforts. Thus, introducing power-saving mechanisms for CIoT technology are essential.

In this article, we touched upon a few of the common power-saving measures specified by 3GPP. They cover the feature optimization on the physical layer right up to the RRC layer. Different aspects such as reducing downlink monitoring (enter sleep mode to save power), faster release of the RRC connection, minimizing signaling overhead, and enabling the fast system acquisition can contribute to the overall power saving of the CIoT UE.

Power saving feature 3GPP Release
DRX 121
Power saving mode (PSM) 12
eDRX 13
Release assistance indication (RAI) 14
Mobile originated early data transmission (MO-EDT) 15
Reduced system acquisition time 15
Relaxed monitoring for cell reselection 15
Wake-up signal (WUS) / UE-group wake-up signal (GWUS) 15/16
Mobile terminated early data transmission (MT-EDT) 16
Preconfigured uplink resource (PUR) 16
Increase peak data rate 17

Table 1: Summary of power-saving mechanisms in 3GPP releases for CIoT

  1.   Legacy technology from GSM era. It was included in the very first MTC specification in 3GPP Release 12. 

The evolution of the power saving techniques is an ongoing process and further enhancements are foreseen in the future IoT application based on non‑terrestrial networks. Stay tuned and be excited. 

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