UE working states in LTE and 5G

1
UE working states in LTE
In LTE, we have two specified states for the User Equipment (UE) : Idle mode and Connected mode. Let
us have some explanation on each mode.
Idle mode : This state is named as idle as in this state user is not using UE for communication. In the IDLE
mode UE is not known by the eNodeB but is known by the network and can communicate with the
network when it notices any incoming call and has an IP address. In this mode the mobility management
is done by the UE by cell selection and re-selection whenever handover is required or a better connection
is available. Network does not control UE’s movement in this state, UE automatically selects new cell as it
moves. If UE enters a new location area, based on hearing information from base station, the UE informs
the network of the new tracking area it has entered. The UE does not transmit or receive any data in this
state. It merely monitors the paging and broadcast channel so as to maintain the connectivity. UE moves
from idle mode to the connected mode when RRC signalling connection is established. The idle mode
saves the overall power of both the UE and the network as this period avoids unnecessary signalling and
communication. As the UE has limited power, this feature of LTE increases the user equipment life time.
Connected mode : In this state the UE is known by both the eNodeB and the network. Moreover the radio
is in the active state as the data is being communicated. The mobility management in this state is done by
the network and it is based on handover. In the “Connected Mode” UE keeps its transmitter and receiver
always ON i.e. the UE’s radio is in the ON state. In the Connected Mode the UE constantly communicates
with the network and so it knows its location at the cell level. Depending upon this cell level, it can make
decision that when the handover is required and when it is not necessary. The network here controls the
mobility decision based on UE measurement reports from the cells and other RAT reachable by the UE.
UE moves from connected mode to the idle mode when RRC connection is released. By releasing the RRC
connection it means that there is no incoming call or data to send by the UE. When the network is not
communicating for a particular period of time, called the inactivity period, the network moves the UE’s
state to idle-state. During the period which the data is being transmitted the UE remains in the connected
mode. If the data is not being transmitted by the UE for a particular period i.e. for the inactivity period
then the UE moves from connected state to idle mode. When the UE have data to send then it again starts
transmitting the data, at this time the UE moves to connected state from idle state. This state of UE keeps
on changing according to the measurement reports and data availability for communication

2
UE working states in 5G
In 5G, we have three specified states for the User Equipment (UE) : RRC Idle, RRC Inactive and RRC
Connected. Let us have some explanation on each mode.
RRC Idle state : In RRC_IDLE state, the UE is not registered to a particular cell, hence the UE doesn’t have
an AS context and any other information received from the network. The network initiates the RRC
connection release procedure to transit a UE in RRC_CONNECTED to RRC_IDLE state. The UE is in (kind of)
sleep mode and wakes up periodically (according to a configured DRX cycle) and monitors for paging
messages from the network. The network can reach UEs in RRC_IDLE state through Paging messages, and
to notify UEs in RRC_IDLE change of system information. While in RRC_IDLE, the UE monitors the paging
channels for CN-initiated paging. In this state, the UE itself manages mobility based on the network
configurations via cell (re-) selections. The UE performs the required neighbouring cell measurements
which are required for cell (re-) selections. On transition from RRC_CONNECTED or RRC_INACTIVE to
RRC_IDLE, a UE should camp on a cell as result of cell selection according to the frequency be assigned by
RRC in the state transition message if any. In RRC_IDLE state, the UE cannot transmit anything in the uplink
except for PRACH as part of RA procedure initiated when UE desires to transit to RRC_CONNECTED state
or to request for On-demand system information.
RRC Inactive state : RRC_INACTIVE state is used to reduce network signaling load as well as to reduce
latency involved in transitioning to RRC_CONNECTED state. Moreover, the UE in RRC_INACTIVE state is
allowed to behave similar way as in RRC_IDLE state to save power. In RRC_INACTIVE state, the AS context
is stored by both UE and gNB, as a result of which the state transition from inactive state to connected
state is faster. Moreover, the core network connection is also maintained (UE remains in CM-
CONNECTED). Similar to RRC_IDLE, the UE is in (kind of) sleep mode and wakes up periodically (according
to configured DRX cycle) and monitors for paging messages from the network. The network can reach UEs
in RRC_INACTIVE state through Paging messages, and to notify UEs in RRC_INACTIVE, change of system
information. In RRC_INACTIVE state, the UE cannot transmit anything in the uplink except for PRACH as
part of RA procedure initiated when UE desires to transit to RRC_CONNECTED state (to
transmit RRCResumeRequest) or to request for On-demand system information.

3
RRC Connected state : In RRC_CONNECTED state, an RRC connection is established and the network have
configured the UE with all the required parameters for communication between them. In this state, the
cell to which the UE belongs to is known and a cell level temporary identity for the device (C-RNTI) is used
to identify the UE in NG-RAN. The network can notify UEs in RRC_CONNECTED state of system information
change and ETWS/CMAS indications through Short Messages. Both Paging messages and Short
Messages are addressed with P-RNTI on PDCCH, but while the former is sent on PCCH, the latter is sent
over PDCCH directly.
Resources Links
How LTE Stuff Works?: 5G NR: UE RRC States and State Transitions