FAQ on RRC (Radio Resource Control Layer)

By Zahid Ghadialy (zahidtg@yahoo.com)

Last Updated: 01/10/2005

For SIBs to be transmitted on physical channels, segmentation of SIB blocks may or may not be needed (because of the fixed size), So where is this segmentation actually done ?? .. at Node B or RNC?

The segmentation is done in RNC (CRNC). 25.430 says

Section 5.2.4 System Information Management

System Information is sent by the CRNC to a Node B. CRNC can also request the Node B to autonomously create and update certain Node B related system information. Scheduling of system broadcast information is carried out in the CRNC. Scheduling information is always sent by the CRNC to the Node B. The Node B is responsible for transmitting the received system information according to the scheduling parameters provided. If requested by the CRNC, the Node B is also responsible for autonomously creating and updating the Node B related system information according to the scheduling parameters provided.

There are 18 types of SIBs in all, Is there any SIB that is independently generated at Node B, and is sent to UE, without any RNCs involvement.?

Sib 7 is generated by Node B independent of RNC

Between Node B and RNC, NBAP Protocol is used, is all the Information Elements (IE) transmitted from RNC to Node B are ASN.1 Encoded? At Node B, is Decoding of these IEs is not required and can be sent directly to UE?

The elements sent from RNC to Node B are already encoded. They are encoded by the RRC layer in RNC. They are sent directly by Node B without need for decoding. Since Sib 7 is generated by Node B, it has to call a method in RNC to encode Sib 7 and then it is transmitted by Node B.

Annex D in 25.433 shows how data is first split into segments and then RRC encoding is performed on those segments. These F and V (Fixed and Variable size) segments are sent to Node B over Iub in IB_SG_DATA IE in SYSTEM INFORMATION UPDATE REQUEST message.

When does the UE send "Physical Channel Reconfiguration Failure" message with the cause set to "compressed mode runtime error"? What is the UTRAN expected to do afterwards?

The UE sends "Physical Channel Reconfiguration Failure" message with the cause set to "compressed mode runtime error" in case when more than one compressed mode patterns are setup. If the compressed mode patterns create an illegal overlap than this error should be reported to the UTRAN.

The following is extract from 25.331
*************** Runtime error due to overlapping compressed mode configurations
When the UE has received from the UTRAN the configurations of several compressed mode transmission gap pattern sequences, and if several of these patterns are to be simultaneously active, the UE shall check to see if these simultaneously active transmission gap pattern sequences create transmission gaps in the same frame. An illegal overlap is created if two or more transmission gap pattern sequences create transmission gaps in the same frame, irrespective of the gaps are created in uplink or downlink.
If the parallel transmission gap pattern sequences create an illegal overlap, the UE shall:
1> delete the overlapping transmission gap pattern sequence configuration stored in the variable TGPS_IDENTITY, which is associated with the highest value of IE "TGPSI";
1> transmit a PHYSICAL CHANNEL RECONFIGURATION FAILURE message on the DCCH using AM RLC, setting the information elements as specified below:
2> not include the IE "RRC transaction identifier";
2> set the cause value in IE "failure cause" to value "compressed mode runtime error".
1> terminate the inter-frequency and/or inter-RAT measurements corresponding to the deleted transmission gap pattern sequence;
1> when the PHYSICAL CHANNEL RECONFIGURATION FAILURE message has been submitted to lower layers for transmission:
2> the procedure ends.

UTRAN has O&M logging facility for logging everything. It will also log all these kind of errors and the engineers that analyse the logs will fix this problem in the network. In a stable network this problem cannot occur.

How does the initial UE Cell Selection takes place?

The initial cell-selection procedure is used in case there is no information on the current environment stored in the UE. However, normally the UE starts the cell selection with a stored information cell-selection procedure. The UE may have stored the necessary information of the cell it was previously camped on, such as frequency and scrambling code. The UE may first try to 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 carrier frequencies is quite small. One operator typically operates only on two or three frequency carriers. 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 frequencies of 5-MHz bandwidth each. These carriers are then divided between up to six operators. Each carrier will only support one operator. This obviously forces the operators to coordinate their networkplanning activities near national borders because the same frequency can be used by different operators in adjacent countries.

The specifications do not accurately dictate how the initial cellselection procedure should be implemented; 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 should yield a set of P-SCHs in the area. Because the P-SCH is only transmitted during the first 256 chips of each time slot, the beginning of its transmission also indicates the start of a time slot in the corresponding cell.

In the second phase of the process, the received signal is correlated with all possible secondary synchronization code (S-SCH) words on the S-SCH. There are 16 different SSCs, and these can be combined into 64 different code words, each with a length of 15 SSCs. Once the right code word is found, this gives the UE the frame synchronization and the code group identity, which indicates eight possible primary scrambling codes for the control channels.

The third phase of the procedure consists of finding the right primary scrambling code for this cell. Each candidate cell’s primary scrambling code (there are eight of them as shown in the second phase) is applied, in turn, to the common pilot channel (CPICH) of that cell. Because the CPICH carries a predefined bit/symbol sequence, the UE knows when it has found the correct primary scrambling code. The resolved primary scrambling code can then be used to detect the CCPCH, 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 five actually contains another major procedure, PLMN (i.e., the operator) selection. PLMN is identified 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 should first scan through all UTRAN frequencies until a good PLMN is found, and then start an initial cell-selection process on that frequency.

Note that one frequency can only be used by one operator (except in areas near country borders). However, while looking for the right PLMN code, the UE has already obtained all the necessary information for camping on a suitable cell, and no new scanning procedure is necessary once the correct PLMN is found. The situation is different if the UE is roaming abroad, and the home PLMN is not found. In that case RRC has to report all available PLMNs to NAS and wait for its selection decision, which can be either automatic or manual (user selection). This is time consuming, and many readers may have noticed this phenomenon when arriving at an airport in a new country and switching their GSM phones on. It may take a very long time before the phone registers to a network, especially if the phone is a multimode model with several frequency bands to scan.

The initial cell-selection process is repeated as many times as necessary until the first suitable cell 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 UE find the best cell to camp onto. Note that the initial cell-selection procedure only found a cell to camp on (the first possible cell). It is possible that this cell will not be the best possible cell. For example, there could have been other frequencies including better cells for this particular UE that had not yet been scanned.

The neighbor cell list immediately tells the UE which frequencies and neighbor cells should be checked while the best possible cell is being searched for. The list includes additional information that can be used to optimize the cell-synchronization procedure, information such as the 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 neighbor cell.

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 this information, the UE can determine whether there are better cells available. From a possible candidate cell, the UE must decode the system information to check that it is not barred for access.

If the neighbor cell list contains cells from another RAT—for example, GSM cells—and the serving cell quality level is worse than the Ssearch parameter, then the GSM cells must be taken into consideration in the cell reselection procedure.

Introduction to 3G Mobile Communications - Juha Korhonen
3GPP TS 25.304: UE Procedures in Idle Mode and Procedures for Cell Reselection in Connected Mode.
3GPP TS 23.122: NAS Functions Related to Mobile Station (MS) In Idle Mode.

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