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Mobile Initiated Connection Only (MICO) Mode and UE Capabilities in 5G Networks

Mobile Initiated Connection Only (MICO) Mode and UE Capabilities in 5G Networks
Mobile Initiated Connection Only (MICO) Mode and UE Capabilities in 5G Networks


Mobile Initiated Connection Only (MICO) mode is a specialized operation mode in 5G networks where only mobile-originated data transfers are allowed while the User Equipment (UE) is in CM-IDLE state. In this mode, the UE is considered unreachable and does not listen for paging messages from the network. Consequently, requests for downlink data transfer are rejected by the Access and Mobility Management Function (AMF), and the AMF does not proceed with downlink transport over the Non-Access Stratum (NAS), such as mobile-terminated SMS.

However, both mobile-originated and mobile-terminated data transfers are possible for a MICO mode device when it is in CM-CONNECTED state. The UE can indicate a preference for MICO mode during the initial registration procedure or a registration update via the NAS Registration Request message. The AMF then decides whether the UE is permitted to operate in MICO mode and communicates this decision through the 'MICO Indication' in the NAS Registration Accept message.

A UE in CM-IDLE and using MICO mode can be provided with a registration area equal to the entire Public Land Mobile Network (PLMN) if the AMF serves the whole PLMN. This means the UE does not trigger registration updates due to mobility, and it will not be paged, eliminating the need to broadcast paging messages across the entire PLMN. If a MICO mode UE is allocated a smaller registration area, it evaluates whether it has remained within the registration area when it has mobile-originated data or signaling to transfer. The UE does not update immediately after a cell reselection outside its registration area but uses the periodic registration timer (T3512) as a keep-alive to confirm it remains connected to the network.

UE Capability Enquiry in 5G

The Base Station (BS) uses the RRC: UE Capability Enquiry message to request capability information from the UE, specifying the Radio Access Technologies (RAT) of interest. For 5G, the set of RATs for which UE capability information can be requested includes NR, EUTRA-NR, and EUTRA, aligned with the strategy to allow inter-system handover from 5G to 4G. This enquiry is crucial for the Non-Standalone (NSA) Base Station architectures using Dual Connectivity.

The UE responds with the RRC: UE Capability Information message, which includes instances of the UE-CapabilityRAT-Container applicable to either 'NR', 'EUTRA-NR', or 'EUTRA'. The NR and EUTRA-NR instances are specified in 3GPP TS 38.331 with detailed descriptions in 3GPP TS 38.306, while the EUTRA instance is specified in 3GPP TS 36.331 with descriptions in 3GPP TS 36.306.

Mandatory UE Capabilities

According to 3GPP TS 38.306, certain UE capabilities are mandatory. However, the UE must signal support for these capabilities due to potential interoperability issues. A UE vendor may implement a mandatory capability before networks are available to test it, in which case the UE should report that the capability is not supported until tested with network equipment.

Unlike 4G, where UE Categories define a subset of capabilities, 5G does not specify UE Categories. Instead, each capability is explicitly signaled, such as support for specific MIMO layers and modulation schemes using information elements like MIMO-LayersDL and ModulationOrder. Maximum data rates are calculated using an equation specified in 3GPP TS 38.306, taking into account factors like bandwidth, numerology, and coding rate.

UE Capability Signaling to Core Network

The UE also provides capability information to the Core Network via the NAS Registration Request, including the 5G Mobility Management (5GMM) Capability, Security Capability, and S1 Network Capability. The NAS PDU Session Establishment Request sent to the Session Management Function (SMF) includes the 5G Session Management (5GSM) Capability, indicating support for features like Reflective QoS and IPv6 Multi-homing.

The Base Station forwards UE capability information to the AMF using the NGAP: UE Radio Capability Info Indication message. The AMF stores this information as part of the UE context and provides it to other Base Stations serving the UE, ensuring seamless mobility and connectivity across different network nodes.


The integration of 5G technology, AI, and cloud computing is set to revolutionize energy management and various other sectors by 2024. These technologies offer unparalleled capabilities in real-time monitoring, predictive maintenance, and efficient resource utilization, leading to improved efficiency, cost savings, and enhanced reliability. Institutions like Telecom Gurukul and Apeksha Telecom are playing a crucial role in this transformation by providing top-notch education and placement opportunities in these cutting-edge fields.


  1. Telecom Gurukul - 5G Technology Insights

  2. Apeksha Telecom - AI and Cloud Integration

  3. 3GPP TS 38.306 - UE Capabilities

  4. 3GPP TS 38.331 - RRC Procedures

  5. 3GPP TS 24.501 - NAS Procedures

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