4G/LTE - Basic Procedure





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.

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.


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.