Discontinuous Reception (DRX) is a power-saving mechanism in LTE networks designed to extend the battery life of user equipment (UE), such as smartphones.
This FAQ will begin by explaining the Physical Downlink Control Channel (PDCCH) role in LTE from the DRX perspective. Then, it will illustrate the complete DRX process with short/long DRX cycles and a state diagram.
How does PDCCH contribute to power consumption in the DRX mechanism?
PDCCH is responsible for transmitting essential control information to UEs, including:
- Resource Allocation: instructions on how UEs should utilize available resources.
- Scheduling: timing details for when UEs can send or receive data.
- Power Control Commands: guidance on the power levels UEs should use to optimize network performance.
UEs must continuously monitor the PDCCH for control information, which can lead to increased power consumption. The frequency and duration of monitoring directly impact battery life, especially in scenarios where multiple serving cells are configured. Power demands can be substantially elevated if a UE is required to monitor several PDCCHs simultaneously.
To counter the excessive power consumption during PDCCH monitoring, a monitoring occasion is determined during a PDCCH search space set. What is the determination? Remove the passive voice so the reader knows how this works. This leaves me confused. The monitoring occasion is given a set of guidelines on when to monitor the PDCCH. Such an implementation is part of the DRX process in LTE applications. The DRX function consists of time slots called occasions, which refer to periods of activity or inactivity.
Figure 1 illustrates the PDCCH monitoring occasion concept, where the parameters such as periodicity, offset, duration, and monitoring pattern determine the PDCCH search space set.
Figure 1. A PDCCH monitoring occasion implementation as part of DRX in LTE. (Image: IEEE)
The System Frame Numbers (SFN), shown in Figure 1, carry the PDCCH monitoring guidelines. The monitoring is carried out during two slots, which is determined by the “Duration” parameter. It is delayed by one slot, and the pattern is repeated every four slots, which are determined by the “Offset” and “Periodicity” parameters.
What is the complete DRX cycle in LTE applications?
Figure 2 illustrates the DRX mechanism in LTE/5G networks, showing how a device alternates between active and sleep states to conserve power while maintaining connectivity.
In the first stage, the device wakes up to check for control information from the network, which is represented as PDCCH reception. At this point, the DRX timers, drx-InactivityTimer, and drx-onDurationTimer, are active. The drx-InactivityTimer keeps the device active for a certain period after data reception. The drx-onDurationTimer defines how long the device stays awake during a DRX cycle to monitor downlink transmissions.
Figure 2. DRX mechanism in LTE using short and long DRX cycles. (Image: How LTE Stuff Works)
After the inactivity timer expires, the device enters a short DRX cycle with more frequent wake-up intervals. The Short DRX Cycle Occasions indicate when the device briefly wakes up to check for data. The short cycle duration is defined (e.g., 80 ms) and occurs for a set number of cycles (drx-ShortCycleTimer = 2).
If no data is received during a short DRX, the device transitions to a Long DRX Cycle, where wake-up intervals are longer (e.g., 320 ms). This further reduces power consumption.
State diagram representation of DRX cycles in radio resource control
Figure 3 represents a state diagram of a cellular network’s Radio Resource Control (RRC) states, showing how a device transitions between different power consumption states. The key elements of the state diagram can be categorized into RRC Idle, RRC Connected, and the arrows indicating transitions.
Figure 3. RRC State Diagram: Transitions Between IDLE, DRX, and Active Modes. (Image: Kevin Sookocheff)
RRC IDLE (left section):
- The device is disconnected from the network and consumes the least power.\
- When data transfer occurs, it transitions to an Active state (red arrow).
- If no activity occurs for a certain period, it remains in the IDLE state.
RRC CONNECTED (right section):
- The device is connected to the network.
- It can be in:
- Active state: high power consumption, used for data transfer.
- Short DRX: lower power usage with short sleep cycles.
- Long DRX: even lower power consumption with longer sleep cycles.
- The transition between these states happens based on network activity and timeouts.
Arrows indicating transitions:
- Red arrows (Data Transfer): Move from lower to higher power states.
- Blue arrows (Timeout): Move from higher to lower power states.
Summary
Efficient use of the PDCCH ensures that control information is effectively communicated, allowing for optimal scheduling and resource allocation within the network. The combination of short and long DRX cycles allows for a balance between power savings and latency. Short DRX cycles enable quick responses to incoming data, while long DRX cycles provide greater power savings during prolonged inactivity.
References
Power Consumption Modeling of Discontinuous Reception for Cellular Machine Type Communications, MDPI
Power Saving Techniques for 5G and Beyond, IEEE Access
5G NR: Connected Mode DRX, How LTE Stuff Works
How Does LTE Work?, Kevin Sookocheff
Related EE World Online Content
Power consumption for IoT modules: Protocols matter, 5G Technology World
Power sequencing — options and tradeoffs: part 2, Power Electronics Tips
Power sequencing — options and tradeoffs: part 1, Power Electronics Tips
IoT: How 5G differs from LTE, 5G Technology World
