RRC States
Legacy GSM and UMTS air interfaces used a variety of terms to describe the states an MS (Mobile Station) or UE could be in over time: GSM used one set of descriptions, GPRS a slightly different set and R99 a different set again. E-UTRA also uses a different, but very simple, set of RRC state descriptions.
The RRC idle state refers to terminals that are powered on and have performed network access, but are currently not supporting any connections.
RRC idle terminals will monitor the paging channel in the camped-on cell and will perform cell reselection as required. Idle UEs have no RRC content with any eNB and therefore have no C-RNTI assigned. The only transitory identity they have will be the TAI used for paging purposes by the MME.
A connected UE will have an active RRC context in place with one or more eNBs. Its location will therefore be known down to the serving-cell level and it will have at least one C-RNTI assigned.
As part of the RRC context establishment process the eNB will have contacted the HSS (via the MME) and received security and authentication vectors for the UE. Ciphering and integrity keys will therefore also be in place. RRC-connected does not necessarily imply that the UE has any active EPS bearers, only that it has made contact with an eNB.
Further Reading: 3GPP TS 36.331
LTE/SAE Engineering Overview
Radio Resource Management Functions
As well as the RRC layer, the eNB has also inherited the RRM (Radio Resource Management) that previously resided in the RNC. The basic set of RRM functions performed by the eNB are described below.
The RBC (Radio Bearer Control) functions in the E-UTRAN share more in common with those in HSPA than in R99 due to the lack of a dedicated channel structure. RBC functions are closely allied to those of the scheduler in that they are both concerned with making the best possible use of the radio resources available in a cell.
The task of RAC (Radio Admission Control) is to admit or reject the establishment requests for new radio bearers. Each new request made in response to a user request, a paging event or a handover, is evaluated in terms of the current cell load and the effect the new bearer may have on existing connections.
CMC (Connection Mobility Control) is concerned with mobility management functions such as the creation and control of broadcast channel parameters that aid cell selection and reselection for idle-mode UEs. CMC also manages handover events for active-idle-mode UEs and the collation and analysis of measurements associated with handover.
The task of DRA (Dynamic Resource Allocation) or PS (Packet Scheduling) is to allocate and deallocate eNB and air interface resources for the transmission of user and control plane packets. At the physical layer this includes the selection of resource blocks and at higher layers these functions take aspects such as priority and QoS into account. ICIC (Inter-Cell Interference Coordination) has to manage radio resources so that inter-cell interference is kept under control. This may involve
communication over the X2 interface to ensure that neighbouring eNBs co-ordinate their ICIC activities.
LB (Load Balancing) seeks to provide even distribution of the air interface traffic load over the cells controlled by the eNB so that no individual cell is overloaded. Inter-RAT RRM ensures that the eNB co-ordinates its activities with base stations supporting other air interface variants such as GSM or WCDMA.
Further Reading: 3GPP TS 36.300, 36.331
Radio Access Protocols
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Network Nodes and Areas
MME-1 MME-1
SG-W
MME Pool Area 1 MME Pool Area 2
TA1
TA2
TA3
TA4
TA5
TA6
Network Nodes and Areas
Individual network nodes, such as eNBs, MMEs and gateways, will continue to require unique
identifiers in EPS just as in legacy networks. The MME ID and eNB ID will therefore perform the same functions (and may have the same format) as the VLR and base station IDs employed currently in GSM and R99 UMTS.
For paging purposes, the E-UTRAN is subdivided into one or more TAs (Tracking Areas), each of which is assigned a unique TAI (Tracking Area ID). The use and functions of the TA are analogous to the location areas employed by GSM.
A TA is, in theory, the responsibility of one MME, but to ensure resilience in the event of device failure, MMEs can be logically grouped into MME pools with some or all TAs being multihomed to more than one MME.
Further Reading: 3GPP TS 23.401, 29.803
LTE/SAE Engineering Overview
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PBCH and P/S-SCH
3 MHz channel
5 MHz channel
10 MHz channel
20 MHz channel UE scans for DC
carrier
System Acquisition
Cell search is a prerequisite to network entry and must be performed each time a UE in the detached state finds itself with no valid assigned or stored channel details.
Compared to systems such as GSM and UMTS R99/R4, which employ fixed channel bandwidths, E-UTRA’s support for a variety of channel bandwidths – from 1.4 MHz to 20 MHz – represents a slightly more complex challenge in relation to cell search. There are, however, a few basic system constants designed to ensure that cell search operates in much the same way across all channel bandwidths.
Firstly, although the overall channels have a variety of bandwidths, they are all constructed from resource blocks, which are always 180 kHz wide. Additionally, irrespective of the bandwidth, all the main control channel functionality is contained in 72 subcarriers in the centre of the channel.
The UE continues to employ the cell search conventions used by GSM and UMTS. Where possible the UE will store details of the last used channel on the SIM on power-off and will attempt to reuse that channel when powered back on. If the stored channel is not available the UE will attempt to use the operator-defined channel list stored on the SIM, if such a list exists. If not then the UE will be forced to perform a full cell search over the range of channel bandwidths and frequency bands that it supports.
Neighbour-cell search for handover and reselection purposes is based on the same downlink signals as initial cell search.
Further Reading: 3GPP TS 23.401, 36.300, 36304
Radio Access Protocols
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MME A
HSS MME B
MME C eNB selects an MME to
serve newly connected UE
MME A requests authentication vectors from HSS
UE answers MME authentication challenge using security details stored
on USIM
Attach Procedures
Once acquisition has taken place, the process of synchronizing with a channel to read its broadcast control information must be achieved. Again, the basic architecture of the E-UTRA channel is designed to ensure that the process is the same for any channel bandwidth.
The P-SCH and S-SCH are transmitted every 5 ms and use the six RBs clustered around the DC carrier. Six RBs, at 180 kHz per block, occupy a little over 1 MHz of bandwidth. As the narrowest channels currently being considered for E-UTRA operation are 1.4 MHz wide it means that the P/S-SCH (Primary/Secondary Synchronization Channel) will be able to occupy the same position in the frequency structure of any E-UTRA variant.
The MIB (Master Information Block) of the BCCH also occupies the same six RBs at the centre of each E-UTRA downlink channel, allowing UEs to discover information such as PLMN identity, Cell ID (or eNB identity) and random access parameters in a standardized way. In response to a random access preamble transmitted by a UE, the eNB will grant a series of uplink and downlink capacity blocks via which it and the UE can perform the network attach procedure. If the S1-flex facility is available and the serving eNB has access to multiple MMEs, it will select, possibly at random or following a ‘round robin’ process, an MME with which to initiate the attach process. Not all UEs camped-on the same cell will necessarily have contexts established with the same MME, allowing a greater degree of resilience in the event of an MME failing.
In principle, the E-UTRAN attach process operates in the same way as in GSM, only the names of some of the nodes and the location of some of the events has changed; the UE provides an IMSI (International Mobile Subscriber Identity) to the MME; the MME interrogates the HSS and receives AVs (Authentication Vectors); the UE is challenged using these vectors and if it provides the correct response it is deemed to be authentic; the eNB is provided with security vectors that allow ciphering and integrity protection to be established over the air interface; the MME assigns an M-TMSI (MME Temporary Mobile Subscriber Identity) to the UE and commences location tracking. A default EPS bearer will be created to allow the UE to register with the IMS.
In order to perform the attach, the UE will move to the RRC connected state for the duration of the attach process before moving back down to the RRC Idle state. Once attached, the UE will be logged as in the EMM (EPS Mobility Management) attached, ECM (EPS Connection Management) idle states by the MME.
Further Reading: 3GPP TS 23.401, 36.300, 36.304
LTE/SAE Engineering Overview
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Current best cell
Optional NCL
Reselection made if candidate achieves reselection criteria
Idle Mode Procedures
RRC idle mode procedures for a UE are mainly concerned with cell reselection functions.
Reselection is intended to ensure that a UE is camped-on to the best available cell for the purposes of monitoring the paging channel and for handling any outgoing connection requests from the user interface.
Reselection is triggered if measurements taken of the current camped-on cell fall below thresholds defined on the BCCH. Neighbour cell measurements are only required if the signal received from the current camped-on cell drops below an advertised threshold.
The reselection process undertaken by the UE can be simplified if the eNB publishes an idle mode neighbour cell list that gives details of the cells available for reselection. This facility is optional in E-UTRA and if a neighbour cell list is not available the UE will be required to perform a full cell search each time reselection is triggered.
Further Reading: 3GPP TS 23.401, 36.300, 36.304
Radio Access Protocols
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Current best cell
Optional NCL
Reselection made New best
cell
MME
TA ID=10 TA ID=11
Location Update on MME as TA changes
Tracking Areas and Paging
UEs in the idle state will be performing cell reselections as required to ensure that the best available cell is camped on (even if it means changing air interface RAT).
After reselection, the UE will monitor the BCH in the new cell. If the Tracking Area ID transmitted in the new cell differs from that transmitted in the old cell then the UE moves to the RRC connected state and performs a location update. The MME will track the location of each attached UE down to the TA level and will update its stored information on receipt of a TAU (Tracking Area Update).
In legacy GSM networks, the location tracking function performed by the MME was the responsibility of the VLR in the serving MSC (Mobile-services Switching Centre).
Each VLR had sole responsibility for a specific set of base stations and if the VLR failed then paging and locations updates within that set of cells also failed. In LTE, the S1-flex facility ensures that each eNB has access to a number of MMEs and S-GWs. This flexibility ensures more resilience than was previously available and mirrors the Iu-flex facility introduced into UMTS in Release 5.
Each eNB is responsible for compiling its own paging channel, based on a paging request sent from the MMEs. In an S1-flex environment each eNB will be receiving paging information from multiple MMEs, which will be combined and propagated through its cells.
Further Reading: 3GPP TS 23.401, 36.300, 36.304
LTE/SAE Engineering Overview
(non-delay tolerant, errortolerant) (delay tolerant, non-error
tolerant)
LTE Handover Types
LTE offers two mechanisms for maintaining data flow to a UE as it moves from one cell coverage area to another. Once a handover decision has been made based on measurement information the system may transfer the data flow using either a ‘seamless’ handover or a ‘lossless’ handover.
A seamless handover is intended for delay tolerant services such as VoIP. In general seamless handover is applied for radio bearers using the UM mode of RLC. In this procedure, untransmitted packets can be transferred from the source eNB to the target eNB, but no acknowledgment status is supplied. Thus in the example shown the packet transmitted with PDCP sequence number 3, which has not been acknowledged will be lost. Only PDCP packet 4 is transferred since it has not yet been transmitted. When transmission is resumed on the target cell, PDCP and other counters are reset.
This results in a faster handover, but may result in packet loss.
For data services that can tolerate some delay but for which packet loss is not acceptable, a lossless handover is used. In general, lossless handover is applied for radio bearers using the AM mode of RLC. In this procedure PDCP context is transferred from the source eNB to the target eNB. PDCP and other counters continue the existing sequence. Thus in the example, PDCP packet 3 is retransmitted on the target eNB since although it had already been transmitted, it had not yet been acknowledged.
Further Reading: 3GPP TS 23.401, 36.300
LTE/SAE Engineering Overview
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