FAQ on UMTS Idle Mode Procedures By Zahid Ghadialy (firstname.lastname@example.org) Last Updated: 25/06/2006 How does PLMN Selection take place in UMTS?The UE normally operates on its home PLMN (HPLMN) or equivalent home PLMN(EHPLMN). However a visited PLMN (VPLMN) may be selected, e.g., if the MS losescoverage. There are two modes for PLMN selection: 1. Automatic mode: This mode utilizes a list of PLMNs in priority order. The highest priority PLMN which is available and allowable is selected. 2. Manual mode: Here the MS indicates to the user which PLMNs are available. Only when the user makes a manual selection does the MS try to obtain normal service on the VPLMN.There are two cases: y International Roaming: This is where the MS receives service on a PLMN of a different country than that of the HPLMN. y National Roaming: This is where the MS receives service from a PLMN of the same country as that of the HPLMN, either anywhere or on a regional basis. The MS makes a periodic search for the HPLMN while national roaming.To prevent repeated attempts to have roaming service on a not allowed LA, when the MS isinformed that an LA is forbidden, the LA is added to a list of "forbidden LAs for roaming"which is stored in the MS. This list is deleted when the MS is switched off or when the SIM isremoved. Such area restrictions are always valid for complete location areas independent ofpossible subdivision into GPRS routing areas. The structure of the routing area identifier (3GPPTS 23.003) supports area restriction on LA basis.If a "No Suitable Cells In Location Area" message is received by an MS, that location area isadded to the list of "forbidden LAs for roaming" which is stored in the MS. The MS shall thensearch for a suitable cell in the same PLMN but belonging to an LA which is not in the"forbidden LAs for roaming" list.If a "PLMN not allowed" message is received by an MS in response to an LR request from aVPLMN, that VPLMN is added to a list of "forbidden PLMNs" in the SIM and thereafter thatVPLMN will not be accessed by the MS when in automatic mode. A PLMN is removed fromthe "forbidden PLMNs" list if, after a subsequent manual selection of that PLMN, there is asuccessful LR. This list is retained when the MS is switched off or the SIM is removed. TheHPLMN (if the EHPLMN list is not present or is empty) or an EHPLMN (if the EHPLMN list is
present) shall not be stored on the list of "forbidden PLMNs".In A/Gb mode, an ME not supporting SoLSA may consider a cell with the escape PLMN code(see 3GPP TS 23.073) to be a part of a PLMN belonging to the list of "forbidden PLMNs".Optionally the ME may store in its memory an extension of the "forbidden PLMNs" list. Thecontents of the extension of the list shall be deleted when the MS is switched off or the SIM isremoved.If a "GPRS services not allowed in this PLMN" message is received by an MS in response to anGPRS attach, GPRS detach or routing area update request from a VPLMN, that VPLMN isadded to a list of "forbidden PLMNs for GPRS service" which is stored in the MS and thereafterthat VPLMN will not be accessed by the MS for GPRS service when in automatic mode. Thislist is deleted when the MS is switched off or when the SIM is removed. A PLMN is removedfrom the list of "forbidden PLMNs for GPRS service" if, after a subsequent manual selection ofthat PLMN, there is a successful GPRS attach. The maximum number of possible entries in thislist is implementation dependant, but must be at least one entry. The HPLMN (if the EHPLMNlist is not present or is empty) or an EHPLMN (if the EHPLMN list is present) shall not bestored on the list of "forbidden PLMNs for GPRS service".In the UE, the AS shall report available PLMNs to the NAS on request from the NAS orautonomously.UE shall maintain a list of allowed PLMN types. The allowed PLMN type can be GSM-MAPonly, ANSI-41 only or both. During PLMN selection, based on the list of allowed PLMN typesand a list of PLMN identities in priority order, the particular PLMN may be selected eitherautomatically or manually. Each PLMN in the list of PLMN identities can be identified by eitherPLMN identity (GSM-MAP) or SID. In the system information on the broadcast channel, theUE can receive a PLMN identity (GSM-MAP) or a SID or a PLMN identity (GSM-MAP)and a SID, in a given cell. For a given cell, the UE might receive several PLMN identitiesfrom the system information on the broadcast channel. The result of the PLMN selection is anidentifier of the selected PLMN, the choice being based on the allowed PLMN types, UEcapability or other factors. This identifier is one of either PLMN identity for GSM-MAP typeof PLMNs or SID for ANSI-41 type of PLMNs.On request of the NAS the AS should perform a search for available PLMNs and report them toNAS.The UE shall scan all RF channels in the UTRA bands according to its capabilities to findavailable PLMNs. On each carrier, the UE shall search for the strongest cell and read its systeminformation, in order to find out which PLMN the cell belongs to. If the UE can read one orseveral PLMN identities in the strongest cell, each found PLMN shall be reported to the NAS asa high quality PLMN (but without the RSCP value), provided that the following high qualitycriterion is fulfilled: 1. For an FDD cell, the measured primary CPICH RSCP value shall be greater than
or equal to -95 dBm. 2. For a TDD cell, the measured P-CCPCH RSCP shall be greater than or equal to - 84 dBm.Found PLMNs that do not satisfy the high quality criterion, but for which the UE has been ableto read the PLMN identities are reported to the NAS together with the CPICH RSCP value forUTRA FDD cells and P-CCPCH RSCP for UTRA TDD cells. The quality measure reported bythe UE to NAS shall be the same for each PLMN found in one cell.The search for PLMNs on the rest of the carriers may be stopped on request of the NAS. TheUE may optimise this search by using stored information of carrier frequencies and optionallyalso information on cell parameters, e.g. scrambling codes, from previously receivedmeasurement control information elements.Once the UE has selected a PLMN, the cell selection procedure shall be performed in order toselect a suitable cell of that PLMN to camp on.References:3GPP TS 23.122: Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idlemode3GPP TS 25.304: UE Procedures in Idle Mode and Procedures for Cell Reselection inConnected Mode.Introduction to 3G Mobile Communications - Juha Korhonen How does the initial UE Cell Selection takes place?The initial cell-selection procedure is used in case there is no information on the currentenvironment stored in the UE. However, normally the UE starts the cell selection with a storedinformation cell-selection procedure. The UE may have stored the necessary information of thecell it was previously camped on, such as frequency and scrambling code. The UE may first tryto synchronize into that cell, and if it fails, it may trigger the initial cell selection.The purpose of the initial cell-selection procedure is to find a cell, not necessarily the best cell,but a usable cell, for the UE to camp on after power-on. In the UTRAN, the number of carrierfrequencies is quite small. One operator typically operates only on two or three frequencycarriers. In the first phase of UMTS in Europe, the frequency allocation for UMTS-FDD is 2 ×60 MHz (uplink/downlink), which means that there can be, at most, only 12 carrier frequenciesof 5-MHz bandwidth each. These carriers are then divided between up to six operators. Eachcarrier will only support one operator. This obviously forces the operators to coordinate theirnetworkplanning activities near national borders because the same frequency can be used bydifferent operators in adjacent countries.The specifications do not accurately dictate how the initial cellselection procedure should beimplemented; it is left for the UE manufacturers to decide. Most of the functionality, however,has to be in the physical layer, and the RRC layer has only a management role. The initial cell-selection procedure is performed on one carrier frequency at a time until a suitable cell is found.
In principle the process includes the following: 1. Search for primary synchronization channels (P-SCHs); 2. Once such a channel is found, acquire time-slot synchronization from it; 3. Acquire frame synchronization from the corresponding S-SCH; 4. Acquire the primary scrambling code from the corresponding CPICH; 5. Decode system information from the cell to check whether it is a suitable cell for camping (i.e., it contains the right PLMN code and access to it is allowed).All P-SCHs have the same fixed primary synchronization code. The search procedure shouldyield a set of P-SCHs in the area. Because the P-SCH is only transmitted during the first 256chips of each time slot, the beginning of its transmission also indicates the start of a time slot inthe corresponding cell.In the second phase of the process, the received signal is correlated with all possible secondarysynchronization code (S-SCH) words on the S-SCH. There are 16 different SSCs, and these canbe combined into 64 different code words, each with a length of 15 SSCs. Once the right codeword is found, this gives the UE the frame synchronization and the code group identity, whichindicates eight possible primary scrambling codes for the control channels.The third phase of the procedure consists of finding the right primary scrambling code for thiscell. Each candidate cell¶s primary scrambling code (there are eight of them as shown in thesecond phase) is applied, in turn, to the common pilot channel (CPICH) of that cell. Because theCPICH carries a predefined bit/symbol sequence, the UE knows when it has found the correctprimary scrambling code. The resolved primary scrambling code can then be used to detect theCCPCH, which carries the BCH, which contains the system information the UE is seeking.There are various ways to optimize this procedure to make it quicker. Note that phase fiveactually contains another major procedure, PLMN (i.e., the operator) selection. PLMN isidentified by a PLMN code, a number that is transmitted on the BCCH channel of that network.A UE tries to find its home PLMN, the operator it has a contract with. In principle, a UE shouldfirst scan through all UTRAN frequencies until a good PLMN is found, and then start an initialcell-selection process on that frequency.Note that one frequency can only be used by one operator (except in areas near countryborders). However, while looking for the right PLMN code, the UE has already obtained all thenecessary information for camping on a suitable cell, and no new scanning procedure isnecessary once the correct PLMN is found. The situation is different if the UE is roamingabroad, and the home PLMN is not found. In that case RRC has to report all available PLMNsto NAS and wait for its selection decision, which can be either automatic or manual (userselection). This is time consuming, and many readers may have noticed this phenomenon whenarriving at an airport in a new country and switching their GSM phones on. It may take a verylong time before the phone registers to a network, especially if the phone is a multimode modelwith several frequency bands to scan.The initial cell-selection process is repeated as many times as necessary until the first suitablecell is found for camping. Once the UE has managed to camp on a cell, it decodes the system
information from it, including the neighbor cell list. This information can be used to help the UEfind the best cell to camp onto. Note that the initial cell-selection procedure only found a cell tocamp on (the first possible cell). It is possible that this cell will not be the best possible cell. Forexample, there could have been other frequencies including better cells for this particular UEthat had not yet been scanned.The neighbor cell list immediately tells the UE which frequencies and neighbor cells should bechecked while the best possible cell is being searched for. The list includes additionalinformation that can be used to optimize the cell-synchronization procedure, information such asthe primary scrambling codes and timing information (optional, relative to the serving cell).With this information it should be possible to quickly descramble the CPICH from a neighborcell.From the CPICH it is possible to calculate the received chip energy to- noise ratio (Rx Ec/No)for this cell. This measurement is acquired for each neighbor cell in the list. Based on thisinformation, the UE can determine whether there are better cells available. From a possiblecandidate cell, the UE must decode the system information to check that it is not barred foraccess.If the neighbor cell list contains cells from another RAT²for example, GSM cells²and theserving cell quality level is worse than the Ssearch parameter, then the GSM cells must be takeninto consideration in the cell reselection procedure.Reference:Introduction to 3G Mobile Communications - Juha Korhonen3GPP TS 25.304: UE Procedures in Idle Mode and Procedures for Cell Reselection inConnected Mode.3GPP TS 23.122: NAS Functions Related to Mobile Station (MS) In Idle Mode.BackHOME About Us Careers Contribute Advertise