Idle Mode Procedure

I was lucky in terms of technology. I had experience in working with UE test equipment (Network Simulator) / looking mostly into eNB side log and working with UE modem development team looking mostly into UE log.

Why idle is the busy state

The word idle describes the network, and the paragraphs below explain why it does not describe the UE. Two logs of the same minute look completely different depending on which end you read, and that mismatch is the point.

When I first working in Network simulator side, the term 'Idle' made very clear sense to me. It is really as it sound. In Network side, 'idle' is real 'idle' meaning 'doing almost nothing'. The only thing that Network is doing during the idle mode is to keep transmitting MIB/SIBs based on predefined schedule and keep checking whether it is detecting any PRACH from any UE. If you see the network side log, you woudn't see any meaningful event there.

However, when I switched to UE development team, the first thing that surprised me was 'Idle' would not mean 'doing almost nothing' on UE side. To put a little bit of exaggeration, 'Idle' is the states in which UE gets the busiest'.

Have you thought of what's happending within your mobile phone when you turn on the power of a mobile phone or you just get off the plane and turn off 'Airplane mode'.

i) First, you would see a small text message usually at the top of the LCD screen saying "Searching for Network"

ii) And then you would see Antenna Bars start showing up

iii) And then you may see what kind of access technology the UE get connected to (e.g, 3G, HSDPA, GSM etc)

iv) Finally you would see the name of the network operator displayed on top of the LCD.

Unless you make any phone call or browsing or somebody call you, your mobile phone stays as it is seemingly "Being Idle".

In a sense, I would say the term 'Idle' in this title may be a little misleading because it may give you an impression of "doing nothing". But in reality, this period is the time during which your mobile phone is in the busiest state. In short, your device is doing all the procedure described in 3GPP TS 36.304 (TS 25.304 for UMTS). The power-on procedure listed above is only a small fraction of 'idle mode procedure' and in reality a mobile phone is doing much more jobs during the idle period.

36.304 gives the same answer from the other direction, by listing what camping on a cell is for. Clause 4.1 calls the purpose fivefold, and each of the five is a standing obligation rather than a one time action.

It lets the UE receive system information : the broadcast has to be read and re-read as it changes.
It gives an RRC connection somewhere to start : a registered UE accesses the network on the control channel of the cell it is camped on.
It lets the network page the UE : the PLMN knows which tracking areas the UE is camped in, and pages every cell in them.
It lets the UE receive ETWS and CMAS notifications : the emergency and commercial warning systems reach an idle UE this way.
It lets the UE receive MBMS services : broadcast and multicast reception does not need a connection.

Read that list as a work schedule and the anecdote above stops being a paradox. A UE that is camped has to keep the broadcast current, keep a cell worth camping on, and keep listening for its own paging message. All three continue while nothing is being sent.

36.304 also names the failure case in the same clause. A UE that cannot find a suitable cell, or whose location registration failed, camps on a cell regardless of its PLMN and enters a limited service state.

Clause 4.3 says what limited service leaves working. It is one of three levels the specification defines, alongside normal service on a suitable cell and operator service on a reserved one. Limited service covers emergency calls, ETWS and CMAS, on what 36.304 calls an acceptable cell. A UE in that state can still be warned, and still reach an emergency number, and can do nothing else.

Idle describes the traffic, not the UE : the air interface is quiet while the UE keeps working.
Camping carries five obligations : 36.304 clause 4.1 lists them, and none of them is a one time action.
Paging is why camping has to be maintained : the network can only reach the UE through the cell it is camped on.
Failing all of it has a name : a UE with no suitable cell enters the limited service state, which keeps emergency calls, ETWS and CMAS and nothing else.

How the specification divides idle mode

The list further down this page is a list of radio activities. 36.304 divides the same ground differently, into tasks rather than activities, and the two do not line up one for one. One task in particular has no counterpart on the radio list.

Clause 4.1 opens by saying the idle mode tasks can be subdivided into four processes. They are PLMN selection, cell selection and reselection, location registration, and support for manual CSG selection. Note the third one.

Location registration is not a radio procedure at all. It is the UE telling the core network where it is, by means of a NAS procedure. Clause 4.1 then makes a successful registration the step that turns the selected PLMN into the registered PLMN. A list of radio work alone leaves that out.

The split runs deeper than the list. Clause 4.2 divides every one of the four processes between the non-access stratum and the access stratum, and the table below is that division in short form.

Idle mode process	What NAS does	What AS does
PLMN selection	Keeps the PLMN list in priority order and selects one, then asks AS for a cell belonging to it	Searches for available PLMNs, measures, synchronises to a broadcast channel to identify them, and reports them to NAS
Cell selection	Names the RAT to search first, and supplies the forbidden and restricted lists	Searches for a suitable cell, and camps on it when one is found
Cell reselection	Maintains the forbidden registration areas, the equivalent PLMNs and the permitted CSG list	Measures, reads the broadcast, and changes cell when a more suitable one appears
Location registration	Registers the UE after power on, registers its presence in a registration area, and deregisters at shutdown	Reports registration area information to NAS
Support for manual CSG selection	Requests a search for available CSGs, evaluates the reports, and selects one	Searches for cells with a CSG ID, reads the HNB name from SIB9, and reports what it found

Table 4.2-1 of 36.304 v19.2.0, shortened to one line for each half, and limited to the four processes of clause 4.1

Reading it that way gives two results. The access stratum never decides anything about a PLMN, and the non-access stratum never measures anything. Every row is a request in one direction and a report in the other.

36.304 names four processes, not four procedures : PLMN selection, cell selection and reselection, location registration, and support for manual CSG selection.
Location registration is the one the radio list misses : it is a NAS procedure, and it is what makes the selected PLMN the registered PLMN.
Each process is split across two stratums : clause 4.2 gives NAS the decisions and lists, and AS the measurement and the camping.
The traffic between them is requests and reports : NAS asks for a cell, AS reports what it found, and neither does the other's job.

The procedures this page leads to

The four items below are the radio side of the table above. Each one has a page of its own, so what follows is a map rather than a treatment. The two questions after the list are the ones every one of those pages has to answer.

In short, Idle Mode Procedure is the collective name of all the procedure which are happening during power-on and 'after power-on'. Major procedures for the idle mode can be described as follows :

i) Cell Measurement (Measure the signal quality of Neigbour cells)

ii) Cell Search/PLMN Search (Detect PLMN of cells and determine which cell it would camp on to)

iii) Cell Selection (Determine a cell and perform registration)

iv) Cell Reselection (Change cell from existing cells to another cell)

Each of these procedures are very complicated one and will be described in a separate pages in sharetechnote. To understand these procedure, you have to have clear answers to the following questions.

a) What is the purpose of the procedure ?

b) What is the condition (trigger) to make the procedure happen ? (See Cell Selection Criteria)

You will get the answers to these questions for each procedure in separate pages which deals with the topics. The related pages are listed below and this list will get longer.

One ordering point is worth making before those links. The four run in sequence the first time and in a loop afterwards. A UE that has just powered on measures, searches, selects and registers once. After that it stays in the last step, reselecting whenever a better cell appears, and it only returns to the earlier ones when it loses coverage of the registered PLMN.

The first pass is a sequence : measure, search, select, then register, in that order.
Everything after it is a loop : reselection runs for as long as the UE stays camped.
Losing the PLMN restarts the sequence : 36.304 clause 4.1 has the UE select a new PLMN automatically, or offer the user the list.
Each page answers the same two questions : what the procedure is for, and what triggers it.

What the UE repeats while idle

One job on the camping list repeats on a fixed schedule, and it is the reason an idle UE cannot simply sleep. The UE has to be listening when the network pages it. 36.304 clause 7.1 says how rarely it needs to.

Discontinuous reception is the mechanism, and the clause states its purpose plainly: the UE may use DRX in idle mode in order to reduce power consumption. A Paging Occasion is one subframe in which the P-RNTI may appear on the control channel. A Paging Frame is one radio frame, and it may hold one or several occasions.

The saving comes from one sentence. When DRX is used the UE needs only to monitor one Paging Occasion per DRX cycle. Everything else in the cycle is time the receiver can spend switched off.

Which occasion belongs to which UE is arithmetic rather than signalling. The paging frame satisfies SFN mod T = (T div N) times (UE_ID mod N), and the occasion inside it follows from i_s = floor(UE_ID / N) mod N_s. The network computes the same two numbers and transmits there.

Three of those terms decide everything. T is the DRX cycle, and in RRC_IDLE it is the shorter of the UE specific value and the default the cell broadcasts. N is min(T, nB), where nB runs from 4T down to T/256. UE_ID is the subscriber identity modulo 1024 when the P-RNTI is monitored on PDCCH. For a UE with no IMSI it is 0, which is the emergency call made without a USIM.

That last value matters. The paging schedule is derived from the permanent subscriber identity, so it is fixed for a given UE in a given cell configuration. Two UEs whose identities agree modulo N share a paging frame, and a UE with no identity at all lands on the occasion that UE_ID 0 selects.

DRX exists to save power : 36.304 clause 7.1 says so in its opening sentence.
One occasion per cycle is the whole saving : the UE monitors a single subframe and sleeps through the rest.
The schedule is computed, not signalled : both ends derive the paging frame and occasion from the same formulae.
The subscriber identity drives it : UE_ID is the IMSI modulo 1024 on PDCCH, and 0 when there is no IMSI.
T is the shorter of two values : the UE specific DRX cycle and the default the cell broadcasts.

Reference

[1] 36.304 : 3GPP - E-UTRA; User Equipment procedures in idle mode and in RRC_INACTIVE state, v19.2.0. Clause 4.1 gives the four idle mode processes, the definition of camping and its fivefold purpose. Clause 4.2 gives the functional division between NAS and AS in Table 4.2-1, clause 4.3 the three service levels, and clause 7.1 the paging frame and paging occasion formulae.