Low Power Modes of Cellular LPWA
The optimization of battery power and its longevity have been a challenge in IoT deployments in the early stages. This changed significantly with the advent of LPWAN technologies, which addressed critical inefficiencies, notably in battery power management.
IoT modules employing LPWAN technology such as NB-IoT can now offer extended battery life, theoretically up to 10 years under ideal conditions. It implements different power optimization techniques, such as reduced data transmission rates and efficient sleep modes, enhancing the life span of the IoT devices. These advancements not only enhance device lifespan but also reduce the costs associated with frequent replacements.
In this blog, we mainly focus on covering the major low power modes used in cellular LPWAN modules for battery management.
We will cover the following sections in this bog
- What is Battery Management?
- Different battery management techniques
- Low-Power Modes used in LPWA and NB-IoT Modules
- Power Saving Mode (PSM)
- Discontinuous Reception (DRX )
- What is eDRX?
What is Battery Management ?
As the IoT realm is swiftly growing, the energy efficiency of these devices are directly inclined to the battery performance. It is here, battery management comes into play, ensuring all these power sources function efficiently and safely for a longer period of time.
Battery management is the collective term for practices and technologies employed to optimize battery health, performance and lifespan. This encompasses monitoring various factors like State of Charge (SOC) or the percentage of remaining battery capacity, State of Health (SOH) indicating a battery’s overall health and degradation over time. Monitoring of voltage, current, and temperature is also pertinent as these factors indirectly influence the choice and effectiveness of power-saving modes.
Battery Management Techniques in IoT
Sleep Modes and Energy-Efficient Design
This involves optimizing both hardware and software to reduce energy consumption. In hardware, this could mean using low-power components. In software, techniques such as efficient coding practices and utilizing sleep modes can significantly reduce energy consumption.
Energy Harvesting
Energy Harvesting involves using external sources such as solar, thermal, kinetic, or wind energy to charge the device’s battery or directly power the device and thereby reducing the dependency on the internal battery.This can be particularly beneficial for devices deployed in remote locations or those requiring ultra-long lifespans.
Duty Cycling
Duty cycling refers to the practice of periodically switching a device or component between active and sleep modes. In the active mode, the device performs its intended functions, while in the sleep mode, it enters a low-power state to conserve energy.
Adaptive Data Rate (ADR)
ADR is especially used in LPWAN (Low Power Wide Area Network) technologies like LoRaWAN. ADR optimizes data transmission rates and RF parameters to ensure minimal energy consumption during data transmission. Lower data rates translate to lower power consumption.
Battery-efficient Data Transmission
It involves optimizing the amount and frequency of data transmission. Techniques include data compression, batching (sending data in bulk rather than continuously), and using efficient communication protocols.
Low Power Communication Protocols
Wireless Communication protocols like Bluetooth Low Energy (BLE), Zigbee, and others are designed for short-range communication with minimal energy consumption, suitable for many IoT applications.
Energy-efficient Microcontrollers
Selecting microcontrollers with built-in features like low-power modes and efficient clock management can considerably reduce overall power consumption in IoT devices.
Dynamic Power Management (DPM)
Dynamic power management involves dynamically adjusting the power consumption of various components or subsystems within a device based on their workload or usage patterns. DPM techniques monitor the device’s activity and adapt the power supply or clock frequencies accordingly.
Battery Chemistry
Selecting batteries with chemistry suitable for the device’s operating conditions and usage patterns can also impact longevity and efficiency. For instance, lithium-thionyl chloride batteries are often used for long-term deployments due to their high energy density and low self-discharge rate.
Low-Power Modes used in NB-IoT Modules
Low Power Operation Modes
Low-power operation modes in IoT devices, or specifically NB-IoT modules refer to the defined techniques that minimize energy consumption to extend the battery life of wireless sensors and other connected devices. These techniques are crucial for IoT devices and applications deployed in remote areas that are inaccessible on a regular basis for maintenance or replacements.
The two widely used low power operation techniques for better battery life are PSM ( Power Saving Mode) and eDRX ( Extended Discontinuous Reception). These two features work in association with each other to optimize the battery power consumption and increase the longevity of the battery. Before we get into the introduction of modes, you need to have an idea about the sleep and idle modes in the working of an IoT module.
Idle Mode
Idle mode, a step above sleep mode. The device reduces its activity but keeps essential functions like the operating system and basic connectivity (e.g Bluetooth) operational. It uses more power than sleep mode.
Sleep Mode
In the sleep mode, the device turns off non-essential functions like processors, displays, and network interfaces and enters a low-power state. The key advantage of sleep mode is the ability of the device to wake quickly. It does not require a complete reboot as in full shutdown as the sleep mode preserves the current state in the device’s memory.
Deep Sleep Mode
This is a more extreme version of sleep mode where the device shuts down almost all of its operations and enters a power-saving state with a trade-off in wake-up speed. including the main processor and most peripherals. Only the essential components needed to wake the device up remain active.
- Deep Sleep Mode
Power Saving Mode (PSM)
The Power Saving Mode (PSM) was introduced in the 3GPP Release 12. From then all the LTE device categories could use this feature. While it is available in modern cellular technologies derived from LTE (like LTE-M, NB-IoT), it may not be supported by all cellular IoT modules.
In a device, the main conservation method of battery power is by turning off the radio module. But when turned back on, it consumes a small amount of energy to reattach to the network. This is a cumulative process and there can be a significant energy loss in this process. To avoid this procedure and enhance the lifetime of the battery, PSM is introduced. PSM helps IoT devices to conserve the battery power and achieve a life span of maximum 10 years.
How does PSM Save Energy in LPWA IoT?
When PSM is initiated with the network, it provides two timers- Timer T3324 and Timer T3412.
This timer defines the duration a device stays active after going idle following an Attach or Tracking Area Update (TAU) procedure. Once this timer expires, the device enters a deeper sleep state.
- T3324 (Active Timer):
This timer is an optional extension to T3324. It defines a longer period during which the device can remain inactive and still periodically signal the network (through Periodic Tracking Area Updates, pTAU) that it’s available. This allows the network to locate the device efficiently when it needs to be contacted. It allows a deeper sleep mode while maintaining network connectivity.
- T3412 (Extended Timer):
This timer is an optional extension to T3324. It defines a longer period during which the device can remain inactive and still periodically signal the network (through Periodic Tracking Area Updates, pTAU) that it’s available. This allows the network to locate the device efficiently when it needs to be contacted. It allows a deeper sleep mode while maintaining network connectivity.
Once the device wakes up, the device sends its TAU (Tracking Area Update), informing the network of its current status. The periodic TAU (pTAU) is the period between two TAUs. It contains the Active Time (known as T3324) in which the device can receive incoming data, and the PSM Cycle, the duration of the device in sleep mode.
The active time has to be at least 16 seconds. Once the active time expires, the device starts its PSM cycle, also known as the Hibernate state. When the device wakes up it sends data within the expiration of the PSM time interval agreed with the network eliminating the reattaching process.
PSM is a device side or UE (User Equipment) mechanism optimizing the energy used by the device. PSM mode is similar to power-off, but the device remains registered with the network. When the device becomes active again, there is no need to re-establish network connections.
The maximum sleep time of a device is approximately 413 days (set by 3GPP Release 13 for T3412). The maximum time a device stays reachable is 186 minutes (an equivalent of the maximum value of the Active timer T3324).
The one disadvantage that PSM possesses is that when the radio module is powered off, the PSM cannot contact the network. This is the reason why it is not used in some applications. While asleep, the operator can store the data packets (up to 100 bytes) and forward them once the device awakes.
PSM AT Commands
An AT command is a type of instruction used to control modems.The PSM AT command is used to set or query the PSM parameters of a cellular IoT module. The PSM AT command specifically is used to manage power saving settings, enabling devices to enter a low-power state when they are not transmitting data. Setting PSM can be done using the AT Command AT+CPSMS
Discontinuous Reception (DRX )
It is a power-saving mechanism used in cellular networks, specifically for user equipment (UE) like smartphones and other mobile devices. It was released in the 3GPP Release 8 specifically for user equipment (UE) like smartphones and other mobile devices. DRX enables the device to switch between active and sleep modes.
During the active mode, the device listens for control signals from the network. In sleep mode, the device periodically wakes up for short durations to check for any incoming signals from the network. The duration of sleep mode and the frequency of wake-ups are determined by a parameter called the DRX cycle.
The DRX cycle length is configurable. It is negotiated between the UE and the network, allowing flexibility based on the device’s needs. For instance, an IoT device with infrequent data exchange might have a longer DRX cycle (waking up less often) to maximize battery life. Conversely, a device requiring more frequent communication might have a shorter DRX cycle.
Here RRC or Radio Resource Control refers to the signaling protocol that manages the connection between the User Equipment (UE) and the network even when the UE is in a sleep state. During DRX mode, the UE doesn’t maintain a continuous connection with the network. Instead, it alternates between active and sleep periods.
While active, the UE can communicate with the network using the RRC connection, similar to how it functions in normal operation.
However, during sleep periods, the UE powers down the RRC connection to conserve battery.
Paging Mechanism
To wake the UE up for communication, the network sends a special signal called a paging message. Paging messages are special signals from the network to wake the UE up for further communication (outside the DRX cycle).
RRC protocol uses PDCCH (Physical Downlink Control Channel) to send paging messages. It is a dedicated channel on the downlink (network to UE) side of the LTE communication. It carries control information from the network to the UE, essentially acting as a notification system.
Once the UE receives a paging message, it wakes up, re-establishes the RRC connection, and can then exchange data with the network.
The UE configures a specific timer (eDRX cycle) that determines how often it wakes up to listen for paging messages. This allows the network to reach the UE even when it’s asleep.
What is eDRX?
eDRX, Extended Discontinuous Reception is the advanced version of Discontinuous Reception (DRX). eDRX was introduced in the 3GPP Release 13 which enabled the IoT devices to reduce power consumption with significantly longer sleep periods compared to traditional DRX.
How does eDRX work in Cellular LPWA and NB-IoT?
The Extended DRX feature allows the time interval during which a device is not listening to the network to be increased. IoT devices perform the DRX procedure during the idle mode, called Paging Time Windows (PTW). While DRX defines the overall DRX cycle (sleep/wake pattern), PTW specifies fixed time windows within the DRX cycle when the network is allowed to send paging messages to wake the IoT device.
TDRX (DRX Period Duration) is set by the mobile network operator and defines the overall length of the DRX cycle (active + sleep periods). TPTW (Paging Time Window Duration) is determined by the IoT application developer and specifies the duration of the fixed windows within the DRX cycle when the network can send paging messages.
eDRX provides a less amount of power savings when compared to PSM, but it offers better device reachability than PSM. Based on the particular requirement, we can choose from PSM or eDRX or employ both in tandem.
To improve battery power optimization furthermore, the SIM (UICC) or eSIM (eUICC) can be turned off during the eDRX process.
Closing Notes
Battery management in IoT plays a pivotal role in ensuring the successful deployment and operation of IoT devices. By implementing these strategies and techniques it is possible to significantly extend battery life, device reliability, and minimize maintenance costs. As battery technology continues to evolve, there are new advancements coming through and the future of IoT looks bright with efficient and sustainable energy management solutions.
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