Radio Resource Management in 4G – Idle mode

How is the cellular radio managed, e.g. mobile (UE) connection to base stations (eNB), maintaining the best radio connection service according to the subscribed network (MNO) service?

This article is a quick explainer for the 3GPP spec, TS 36.304 Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC). I shall refer to sections of this below as “spec.”

Most of the 3GPP specs. such as this define the behaviour of the module (UE – user entity). There are 2 main layers of protocol for this functionality:

  • MM – Mobility management, also known as: EMM (enhanced mobility management), EPS (evolved packet system), connects to the network peer entity MME (mobility management entity), EPC (evolved packet core) in the core network. All these terms are used interchangeably in 3GPP to refer to the NAS (non-access stratum) layer of end-to-end protocol. This is the layer that sits above the AS (access stratum) layer in next point, and is non-access in that it does not directly control the radio access aspects, but is independent of it. It’s main function is controlling the network service provision such as authentication, enabling disabling services such as circuit/packet switched data, voice, roaming onto foreign networks etc. Much of this information is read from the USIM. The NAS companion to the above AS spec, is 3GPP TS 23.122.
  • RRC – Radio Resource Control. This is the top-level control of the AS (access stratum) i.e. everything concerned with the radio access such as which radio base station (eNB) to connect to, reading/receiving radio config data, moving between different operating modes and top level control of the lower protocol layers (Layer 2 – RLC, MAC, layer 1 physical radio). This protocol is the main focus of this article and the 3GPP TS 36.304 spec. linked above. See also Spec: section 4.2.

Main states of RRC:

  • No Cell – in this state there is no connection to any network cell e.g after power-on, service loss, so it is searching for “suitable cell”. This task is called Cell Selection, i.e. there is no service, and the main task is to quickly find a likely cell to establish service. Note that this may not be the strongest cell available as that may take longer time to establish. If there is a much better cell then later cell reselection may change to it.
    There is a quick scan of the available cells in the band, then the UE shall read the SIB (system information blocks) on broadcast channels. If this is suitable PLMN and fulfils criterion in section spec: 4.3 then RRC shall move to the next state to “camp on” to this cell. Spec: section shows “S” criterion for suitable cell. Note that parameters such as Qrxlevmin are read from the SIBs – can be different for different network operators/locations.
    After the initial camp onto the cell, then MM layer is notified. MM shall then start the attach process, and move into RRC connected state where a successful 2-way authentication security challenge (USIM information exchange with network MME) should result in an attached state, and RRC release to Idle mode.
  • Idle Mode – also known as DRX or discontinuous receive mode. Tasks in this state are
    * save power – RF TX is off, and RX is also off most of the time. This is maximised for CAT.M1 or NB IoT where eDRX (extended discontinuous receive) extends this radio off time. Even more extreme is PSM mode where everything is turned off and only sipping uAmperes of current.
    * Paging monitoring – check periodically for network paging; briefly turn on RX paging channels. If received a page then network attention is required, move into RRC connected state.
    * Neighbour cell monitoring – periodically, repetitively measure signal strength/quality on all the relevant neighbour cells, perform Cell Reselection Evaluation procedure (Spec: 5.2.4) i.e. check if there is a better cell to change to. Note that the UE may stay on the camped cell even though there is a stronger neighbour cell, since target cell often has to pass a higher strength/quality threshold. This hysteresis prevents unnecessary cell change that is service interruptions.
    For this neighbour cell monitoring task, the RX is turned on at check of paging channels and this is same time for reading neighbour cells. See 3GPP TS 36.133 for deep dive into the requirements of this task, and how long it should take for cell reselection to better neighbour cell. Start at section 4 for description of requirements.

Idle Mode Mobility
Note that for idle mode (or no cell mode), the UE id disconnected i.e. the network has has no real-time information of where the UE is located, which particular cell-ID the UE is camped on. Network divides the service area up into “tracking” areas which contain a number of cells. The network only knows the UE is somewhere within this group of cells within the tracking area (routing area in 3G, location area in 2G). The only visibility the network has is periodically the UE checks into the network by signalling, known as TAU procedure (tracking area update) in 4G (RAU in 3G, LAU in 2G). UE should do this once every few hours or so just to confirm with network – “hello, I’m still here on this cell-ID”. The UE should do this TAU procedure:
* when the TAU timer expires (set by network)
* if there is cell change, reselection outside the network tracking area
* PSM wake-up

Therefore in idle mode, the UE performs neighbour cell measurements and is in control of the mobility, when to reselect to a better cell. In RRC connected mode (2 way RX TX data), the neighbour cell measurements are set to network, and network is in control of mobility. The network sends handover commands to the UE for cell change.

Mobility for LPWA applications
For CAT.M1 and NB IoT battery powered applications that use eDRX or PSM mode, power-saving is the priority. Generally these devices are stationary e.g. utility meters. They wake up, send their data and then PSM as fast as possible, so there is very little time awake to do any cell reselection evaluation. This usually is not a problem since they are not moving, there may only be one or 2 cells that are the best at any time. It’s quite possible that cell1 is chosen as a “suitable cell” and there is another cell2 that has higher strength/quality, but the UE has no chance to reselect to it. Especially NB IoT where cell reselection evaluation is slower for power-saving reasons. Cell1 should still provide basic network connectivity. If due to changing atmospheric/environment conditions that cell1 is no longer fulfils “suitable cell” criterion then service shall be lost and cell selection procedure would likely find cell2 to camp on.

Note that CAT.M1 is basically a cut-down LTE and performs cell reselection and RRC connected mode handovers the same as LTE. However NB IoT is quite different, it has it’s own separate physical channels and in 3GPP specifications there is usually separate paragraphs that specify NB IoT behaviour that is modified from LTE.
NB IoT cannot support real-time data streaming applications that move, such as voice: it has no handover in RRC connected mode, and the cell reselection takes longer than LTE. This is not a problem for water meters that do not move and send small data occasionally. NB IoT cell reselection can support slow moving devices such as bicycle or scooter trackers, but not faster-moving car-speed tracker applications.