A battery-powered NTN device may send only a few reports each day. Keeping its receiver active between reports can consume more energy than transmitting the reports themselves. But if UE sleeps for a long time, how can the network deliver a command to it?

Paging and power saving answer different parts of this question. Paging lets the network contact a UE that does not have an active radio connection. DRX and eDRX limit when UE listens. PSM and MICO can allow longer periods without paging reachability, under their respective EPS and 5GS procedures.

For NTN, usable coverage is another condition. A paging occasion is useful only if service is available and UE can receive it. This page uses the Release 18 specifications and the baseline EPS NAS reference listed in References, with separate NR/5GC and IoT NTN/EPC treatment. All timing and power examples are illustrative.

• What Does Reachable Mean ?
• How Paging Reaches UE
  • NR: Core-Network Paging and RAN Paging
  • IoT NTN: MME and E-UTRAN Paging
• DRX: Listen Periodically
  • Connected-Mode DRX and Active Time
  • Paging DRX and Waiting Time
• eDRX and the Paging Time Window
  • Who Configures the Cycle and PTW ?
  • Worked Example: An 81.92 Second Cycle
• PSM for IoT NTN in EPS
  • Active Time and Periodic TAU
  • Worked Example: 60 Seconds Active, One Hour to Update
• MICO for NR NTN in 5GS
  • MICO with Active Time
  • MICO with Extended Connected Time
• What Changes When Satellite Coverage Disappears ?
• Battery Life Versus Delivery Delay
  • A Simple Monitoring-Energy Calculation
  • Choose from the Application's Reachability Requirement
• What to Check in a Trace
• References

What Does Reachable Mean ?

Registration does not mean UE listens continuously. The network may retain UE registration while UE monitors only selected paging occasions, or while UE cannot receive paging at all. Application delivery has to respect that distinction.

For a mobile-terminated command, three conditions need to coincide: service must be available, UE must monitor the applicable paging resources, and the network must retain and route the pending request. Even then, UE needs to decode paging and complete the applicable access procedure.

The following comparison separates mechanisms that are often grouped together as "sleep mode". Actual support depends on UE capability, network configuration and the radio access technology.

PSM and MICO are not interchangeable names for one radio state. Likewise, RRC_INACTIVE is not PSM. An inactive NR UE retains RAN context and can be subject to paging and notification-area procedures.

How Paging Reaches UE

When downlink activity arrives for an idle UE, the core network can request paging through the radio network. UE listens at its calculated paging occasion, checks the paging information and performs access if it is addressed.

NR: Core-Network Paging and RAN Paging

For NR, RRC_IDLE and RRC_INACTIVE have different context ownership. In idle, the core network initiates the normal paging path. In inactive, NG-RAN retains UE context and can initiate RAN paging within the applicable notification area.

AMF sends paging requests toward NG-RAN for a CM-IDLE UE. On the radio interface, UE monitors the paging PDCCH occasions and receives the scheduled paging message. The P-RNTI identifies paging-related scheduling; the paging record identifies the UE.

An inactive UE normally initiates RRC resume when addressed by RAN paging. If it receives core-network paging, it follows the corresponding idle/NAS handling instead. RRC state, CM state and the type of paging therefore matter when interpreting a trace.

TS 38.304 clause 7.1 defines paging frames and paging occasions using UE identity, the applicable DRX cycle and broadcast/configured parameters. A Paging Occasion can contain multiple PDCCH monitoring occasions, including the beam-related arrangement. It is not necessarily a single instant or one fixed-duration receiver pulse.

IoT NTN: MME and E-UTRAN Paging

For NB-IoT and eMTC connected to EPC, MME coordinates core-network paging through E-UTRAN. UE follows the radio paging procedures for its RAT. The message channels and timing are not copied directly from NR.

NB-IoT uses its narrowband paging control/data channels, including NPDCCH and NPDSCH. Coverage-enhancement repetitions can increase reception time and energy. An example that assumes a short listening burst should not be treated as a measured NB-IoT wake-up duration.

For either access family, NTN propagation delay and access time come after the wait for a usable paging opportunity. Paging is the notification that starts recovery of communication; it does not itself deliver arbitrary application data.

DRX: Listen Periodically

DRX saves energy by avoiding continuous control-channel monitoring. The procedure differs between connected operation and paging. Before using a DRX cycle value in a calculation, identify the UE state and the type of monitoring it controls.

Connected-Mode DRX and Active Time

In NR RRC_CONNECTED, RRC configures the MAC DRX behavior. UE monitors scheduling PDCCH during the applicable Active Time. A configured on-duration is only one contributor to that Active Time.

For example, drx-onDurationTimer controls the periodic on-duration, while drx-InactivityTimer can keep monitoring active after new traffic. Retransmission timers and other MAC conditions can also require monitoring. So on-duration divided by cycle length is not generally the actual receiver duty factor.

Long NTN round-trip timing affects HARQ-related activity. A UE need not listen continuously throughout every propagation interval, but the configured timers and pending transmissions determine its real activity. See HARQ Timing and DRX and Scheduling and Timing.

Paging DRX and Waiting Time

For regular paging DRX, UE returns to its paging occasions at the applicable cycle. A longer cycle reduces the number of monitoring events, but a downlink request may wait longer before UE can receive paging.

As a simple model, assume one usable paging opportunity every 1.28 seconds, continuous coverage and uniformly distributed packet arrivals. The wait to the next opportunity is less than 1.28 seconds and averages 0.64 seconds. These values exclude core-network scheduling, missed pages, radio propagation and access delay.

This cycle is not the same as a 1.28-second satellite round-trip delay. It is a listening schedule. UE also needs synchronization and other required idle-mode work, so sleeping between paging occasions does not imply zero energy consumption.

eDRX and the Paging Time Window

eDRX extends the time between paging opportunities. For the longer cycles, UE monitors paging within a periodic Paging Time Window, or PTW. Outside that window, the network treats UE as unavailable for paging under the applicable procedure.

Who Configures the Cycle and PTW ?

UE can request eDRX through NAS signalling, but the network decides whether to accept it and which parameters to provide. The accepted values, rather than the requested values alone, determine UE behavior.

For NR/5GC, the negotiation uses Registration Request and Registration Accept with AMF. TS 23.501 clause 5.31.7.2 covers NR idle eDRX and supported inactive-state eDRX. RRC_INACTIVE also needs the applicable RAN configuration and capability support; its long-cycle handling can involve core-network buffering coordination.

In NR, PTW applies when the eDRX cycle is longer than 10.24 seconds. Shorter NR eDRX cycles follow the applicable paging-cycle rules without that long-cycle PTW structure. Do not assume the EPS boundary is identical: the relevant E-UTRA procedures use PTW for cycles of 10.24 seconds and longer.

For IoT NTN/EPC, UE negotiates idle eDRX with MME during Attach or TAU. MME supplies the accepted cycle and RAT-specific PTW. The PTW does not mean the receiver remains continuously on for the entire window; UE still follows its paging-occasion schedule within it.

Worked Example: An 81.92 Second Cycle

Take an illustrative NB-IoT idle eDRX configuration: cycle 81.92 seconds, PTW 10.24 seconds and a paging interval of 2.56 seconds within PTW. Use t = 0 for the first shown paging opportunity; this timing illustration omits intra-frame offsets and repetition duration.

Figure 1 shows opportunities at 0, 2.56, 5.12 and 7.68 seconds in the first PTW. The next PTW begins at 81.92 seconds. The narrow marks represent paging opportunities; their drawn width is not an assumed decode or repetition duration.

Figure 1. PTW contains periodic paging opportunities; it is not continuous receiver activity.

A command arriving at t = 11 seconds has missed the first PTW. If the request remains pending and the network pages at the next shown opportunity, the paging wait is 81.92 - 11 = 70.92 seconds. Propagation, decoding and access add more time.

The fraction PTW/eDRX cycle is 10.24/81.92 = 12.5%. This is the fraction of time inside the paging window, not a measured 12.5% receiver duty cycle. Synchronization, repetitions and the actual paging schedule determine energy consumption.

PSM for IoT NTN in EPS

PSM allows a registered EPS UE to deactivate Access Stratum functions and stop idle-mode radio activity. While in PSM, UE is not available for paging. It can resume activity for mobile-originated communication or the applicable periodic update procedure.

Active Time and Periodic TAU

UE requests PSM during Attach or TAU, and the network supplies the accepted Active Time when it allows the feature. Active Time provides a period of idle-mode paging reachability after the connection is released.

The EPS Active Time timer is T3324. The periodic TAU timer is T3412, including the extended value when applicable. At Active Time expiry, UE enters PSM. Periodic TAU timing continues under its NAS rules; UE resumes AS activity in time to perform the update.

UE does not have to wait for the periodic timer if it has mobile-originated traffic. Conversely, the network cannot wake a UE in PSM simply by transmitting more paging messages. High-latency communication features may retain pending downlink work, subject to support, buffering limits and application deadlines.

eDRX and PSM can be used together where negotiated. During Active Time, UE can follow the applicable eDRX paging schedule. After it enters PSM, those paging windows no longer make it reachable until it resumes activity.

Worked Example: 60 Seconds Active, One Hour to Update

Assume UE enters ECM-IDLE at t = 0 with 60 seconds of Active Time and 3600 seconds remaining until its periodic TAU. Assume continuous coverage and no intervening UE activity. This time origin makes the two intervals easy to compare.

Figure 2 shows the reachable Active Time first, followed by the PSM interval. UE must allow preparation and acquisition time before the periodic update is due. This preparation is omitted from the simple interval arithmetic.

Figure 2. PSM removes paging reachability after Active Time; the periodic update is not the only possible reason to wake.

The nominal PSM interval is 3600 - 60 = 3540 seconds, before allowing for preparation. It is not an additional hour after the 60-second Active Time.

A command arriving at t = 120 seconds cannot page UE during PSM. If it is retained until the next update, the nominal wait to that update is 3480 seconds. If UE instead generates an uplink alarm at t = 600 seconds, it can resume activity earlier. Neither path guarantees that pending downlink data has survived in every buffer.

MICO for NR NTN in 5GS

MICO means Mobile Initiated Connection Only. UE requests a preference during registration, and AMF decides whether to allow it. In basic MICO operation, AMF treats UE as unreachable in CM-IDLE and UE need not listen to paging.

UE can initiate communication when it has uplink data or signalling, when periodic registration is due, or when its registration needs updating. It can receive mobile-terminated communication while connected. MICO therefore does not mean that UE can only ever transmit.

MICO with Active Time

The optional Active Time enhancement temporarily keeps UE reachable for paging after CM-CONNECTED transitions to CM-IDLE. UE and AMF start the negotiated timer at that transition and stop/reset it if UE becomes connected again.

When Active Time expires, the UE enters the no-paging MICO behavior. Without negotiated Active Time, it does so directly upon entering CM-IDLE. This resembles the reachability pattern of EPS PSM, but uses the 5GS MICO and registration procedures.

For a device that uploads a report and expects a server response, this interval can preserve a chance to receive the response. The response deadline, server processing time and NTN access delay must fit the chosen reachability strategy.

MICO with Extended Connected Time

AMF can also provide Extended Connected Time when it knows that mobile-terminated traffic is pending or expected after UE initiates communication. This keeps the connection available instead of relying on a later idle-mode page.

Extended Connected Time increases the chance of receiving the response during the same interaction, but keeping resources and radio activity available has an energy cost. It does not imply continuous PDCCH monitoring: applicable connected-mode DRX can still matter.

These are optional features, not defaults for every NR NTN terminal. Emergency-service rules also constrain power-saving operation. For example, the applicable specifications disallow extended idle DRX during an emergency session and specify disabling or avoiding MICO/PSM in the corresponding emergency cases.

What Changes When Satellite Coverage Disappears ?

A paging schedule cannot create a usable satellite link. Even if UE wakes at exactly the right time, it cannot decode paging while outside service coverage. Coverage prediction and power-saving configuration need to be considered together.

Reuse the eDRX example, but assume no service from t = 70 to 100 seconds. The PTW from 81.92 to 92.16 seconds falls entirely inside the gap. Service returns at 100 seconds, but the next scheduled PTW in this example starts at 163.84 seconds.

If the schedule remains unchanged and no UE-initiated procedure intervenes, the command arriving at 11 seconds may wait until 163.84 seconds for the next usable shown opportunity: 152.84 seconds. This is an illustrative scheduling result, not a mandatory post-gap delay. Actual return procedures may change UE state and reachability earlier.

NR unavailability reporting to AMF and IoT NTN satellite assistance can help manage expected gaps. They do not make PSM, eDRX and coverage gaps equivalent. See Discontinuous Coverage for the before/during/after procedures.

Coverage can also return on a different satellite, beam or carrier. UE may need time and energy for synchronization, GNSS, current assistance, cell selection and access. A calculation based only on the nominal sleep interval misses these costs.

Battery Life Versus Delivery Delay

A longer sleep interval can save energy, but only if the saved monitoring energy exceeds the extra wake-up and reacquisition cost. Application traffic, coverage quality and repetitions often dominate the result for a real NTN device.

A Simple Monitoring-Energy Calculation

Assume, only for this calculation, that each monitoring event consumes 1 mJ and background sleep power is 10 microwatts. Ignore access, GNSS, measurements, processing and data transfer. This model compares monitoring schedules, not complete battery life.

Average monitoring power = event energy x events per cycle / cycle duration

One event every 1.28 seconds gives 1 mJ / 1.28 s = 0.78125 mW. Adding background power gives 0.79125 mW. Four events per 81.92-second eDRX cycle give 4 mJ / 81.92 s = 0.04883 mW, or 0.05883 mW with background power.

That is about a 13.45-fold reduction for this limited model, assuming equal event energy in both cases. A real long sleep can require more expensive reacquisition. PTW duration alone is therefore insufficient to estimate battery life.

Choose from the Application's Reachability Requirement

Start with the maximum acceptable downlink delay and the frequency of uplink reports. Then check which supported configuration can meet that requirement during the expected service windows. The longest available timer is not automatically the best choice.

Measure the full cycle at the device supply: sleep, wake-up, acquisition, signalling, receive and transmit. Include retries and failed access attempts. Energy per successfully delivered report is often more useful than the lowest advertised sleep current.

What to Check in a Trace

When a sleeping device misses a command, first establish whether it was supposed to be reachable. Check the accepted configuration, UE state and actual coverage before treating a missing paging response as a radio failure.

Optional mechanisms such as NR Paging Early Indication can further reduce unnecessary paging monitoring when supported and configured. They complement the listening schedule; they do not make UE reachable through a coverage gap or while its applicable no-paging mode is active.

References

The relevant clauses of these specifications define the state, paging and power-saving behavior discussed here. The timing diagrams, arrival times and energy values are explanatory examples rather than measurements or guaranteed device performance.

• 3GPP TS 23.501 V18.12.0 : Clauses 5.4.1.3, 5.4.13 and 5.31.7; MICO, NTN unavailability and eDRX/power-saving enhancements.
• 3GPP TS 38.304 V18.6.0 : Clauses 7.1, 7.2 and 7.4; NR paging, Paging Early Indication and eDRX.
• 3GPP TS 38.321 V18.5.0 : Clause 5.7; connected-mode DRX.
• 3GPP TS 23.401 V18.5.0 : Clauses 4.3.22 and 5.13a; EPS PSM and extended idle DRX.
• 3GPP TS 36.304 V18.3.0 : Idle-mode and paging procedures for E-UTRA, including NB-IoT.
• 3GPP TS 24.301 V17.12.0 : Clause 5.3.11; baseline EPS PSM timer naming and handling.