6.4 Radio Resource Management Procedures


6.4.1  RRC Connection Setup Procedure


In the idle mode, when the non-access layer of the UE requests to establish a signaling connection, the UE will initiate the RRC connection procedure. Each UE has up to one RRC connection only.
When the SRNC receives the RRC CONNECTION REQUEST message from the UE, its RRM (Radio Resource Management) module decides whether to accept or reject the RRC connection request based on specific algorithm. If it accepts the request, it will then decide whether to establish a dedicated channel or a common channel. The RRC connection setup procedure differs with the channel used for the RRC connection establishment.

1. RRC connection established on a dedicated channel

Description:
1)      The UE sends an RRC Connection Request message via the uplink CCCH to request to establish an RRC connection.
2)      Based on the RRC connection request cause and the system resource state, the SRNC decides to establish the connection on the dedicated channel, and allocates the RNTI and L1 and L2 resources.
3)      The SRNC sends a Radio Link Setup Request message to Node B, requesting the Node B to allocate specific radio link resources required by the RRC connection.
4)      After successfully preparing the resources, the Node B responds to the SRNC with the Radio Link Setup Response message.
5)      The SRNC initiates the establishment of Iub user plane transport bearer with the ALCAP protocol and completes the synchronization between the RNC and the Node B.
6)      The SRNC sends an RRC Connection Setup message to the UE in the downlink CCCH.
7)      The UE sends an RRC Connection Setup Complete message to the SRNC in the uplink DCCH.
By now, the RRC connection setup procedure ends.

2. RRC connection established on a common channel

When the RRC connection is set up on a common channel, since the cell common resources already established are used, it is unnecessary to set up radio link and user plane data transport bearer. The rest procedure is similar to that when the RRC connection is set up in a dedicated channel.

6.4.2  Signaling Setup Procedure


After the RRC connection between the UE and the UTRAN is successfully set up, the UE sets up a signaling connection with the CN via the RNC for NAS information exchange between the UE and the CN, such as authentication, service request and connection setup. This is also called the NAS signaling setup procedure.
For the RNC, the signaling exchanged between the UE and the CN is a direct transfer message. After receiving the first direct transfer message, that is, the Initial Direct Transfer message, the RNC sets up a signaling connection with the CN on the SCCP. 

The specific procedure is given as follows:
1)      After the RRC connection is established, the UE sends the Initial Direct Transfer message to the RNC via the RRC connection. This message carries the NAS information content sent to the CN by the UE.
2)      After receiving the Initial Direct Transfer message from the UE, the RNC sends the SCCP Connection Request (CR) message to the CN via the Iu interface. The message content is the Initial UE Message sent from the RNC to the CN, and carries the message content sent from the UE to the CN.
3)      If the CN is ready to accept the connection request, then it returns the SCCP Connection Confirm (CC) message to the RNC. The SCCP connection is successfully set up. The RNC receives the message and confirms the signaling connection setup success.
4)      If the CN cannot accept the connection request, then it returns the SCCP Connection Reject (CJ) message to the RNC. The SCCP connection setup fails. The RNC receives the message and confirms the signaling connection setup failure. Then it initiates the RRC release procedure.
After the signaling connection is successfully set up, the message sent by the UE to the CN is forwarded to the RNC via the Uplink Direct Transfer message, and the RNC converts it into the Direct Transfer message to send to the CN. The message sent by the CN to the UE is forwarded to the RNC via the Direct Transfer message, and the RNC converts it into the Downlink Direct Transfer to send to the UE.

6.4.3  RAB Establishment Procedure


The RAB refers to the user plane bearer that is used to transfer voice, data and multimedia services between the UE and the CN. The UE needs to complete the RRC connection establishment before setting up the RAB.
The RAB setup is initiated by the CN and executed by the UTRAN. The basic procedure is as follows:
l   First the CN sends the RAB assignment request message to the UTRAN, requesting the UTRAN to establish the RAB.
l   The SRNC in the UTRAN initiates the establishment of the data transport bearer between the Iu interface and the Iub interface (Iur interface).
l   The SRNC sends the RB setup request to the UE.
l   After completing the RB establishment, the UE responds to the SRNC with the RB setup complete message.
l   The SRNC responds to the CN with the RAB assignment response message and the RAB setup procedure ends.
When the RAB is successfully established, a basic call is set up and the UE enters the conversation process.
The RAB setup procedure falls into the following three cases based on the radio resource utilization (the radio resource states when the RRC connection is set up and when the RAB is set up):
1)      DCH-DCH: The RRC uses the DCH, while the RAB prepares to use the DCH.
2)      RACH/FACH-RACH/FACH: The RRC uses the CCH, while the RAB prepares to use the CCH.
3)      RACH/FACH-DCH: The RRC uses the CCH, while the RAB prepares to use the DCH.
Detailed description of the RAB setup procedure is given as follows:

1. DCH-DCH

When the current RRC state of the UE is the DCH (Dedicated CHannel), the assigned RAB can only be set up on the DCH. According to the RL (radio link) reconfiguration, the RAB setup procedure falls into two cases: Synchronous RL (DCH-DCH) reconfiguration and asynchronous RL (DCH-DCH) reconfiguration. Their difference lies in whether the Node B and the UE can use the new configuration parameters immediately after receiving the configuration message from the SRNC.
l   In the synchronous case, after receiving the configuration message from the SRNC, the Node B and the UE cannot use the new configuration parameters immediately. Instead, they obtain the synchronization time specified by the SRNC from the message, and use the new configuration parameters at the synchronization time.
l   In the asynchronous case, after receiving the configuration message from the SRNC, the Node B and the UE immediately use the new configuration parameters.
1)      Synchronous RL reconfiguration
In the DCH-DCH synchronization case, synchronous reconfiguration of RL is required among the SRNC, the Node B and the UE.
l   After receiving the RL reconfiguration message from the SRNC, the Node B cannot use the new configuration parameters immediately. Instead, they prepare the corresponding radio resources, and wait for the reconfiguration commit message from the SRNC so as to get the synchronization time specified by the SRNC from the message.
l   After receiving the configuration message from the SRNC, the UE cannot use the new configuration parameters immediately. Instead, it gets the synchronization time specified by the SRNC from the message.
l   The Node B and the UE use the new configuration parameters at the same time in the synchronization time specified by the SRNC.
The following is the DCH-DCH synchronous RL reconfiguration in the RAB setup procedure.

Description:
Ø     The CN sends the RANAP RAB (Radio Access Bearer) Assignment Request message to the UTRAN to initiate the RAB setup request.
Ø     After receiving the RAB setup request, the SRNC maps the QoS parameter of the RAB as the AAL2 link feature parameter and the radio resource feature parameter. The ALCAP of the Iu interface initiates the user plane transport bearer setup procedure of the Iu interface based on the AAL2 link feature parameter (only for setuping RAB of Iu-CS interface user plane).
Ø     The SRNC sends the NBAP Radio Link Reconfiguration Prepare message to its subordinate Node B, requesting it to prepare to add one (or more) DCH for RAB bearer on the existing radio link.
Ø     The Node B allocates the corresponding resources, and sends the Radio Link Reconfiguration Ready message to its home SRNC, notifying it that the radio link reconfiguration is ready.
Ø     The ALCAP of the Iub interface in the SRNC initiates the user plane transport bearer setup procedure of the Iub interface. The Node B and the SRNC establish synchronization with each other by exchanging the uplink and downlink synchronization frames of the DCH frame protocol.
Ø     The SRNC sends the Radio Link Reconfiguration Commit message to its subordinate Node B.
Ø     The SRNC sends the RRC Radio Bearer Setup message to the UE.
Ø     After executing the RB setup, the UE sends the Radio Bearer Setup Complete message to the SRNC.
Ø     After receiving the Radio Bearer Setup Complete message, the SRNC responds to the CN with the Radio Access Bearer Assignment Response message. The RAB setup procedure ends.
2)      Asynchronous RL reconfiguration
In the DCH-DCH asynchronous case, synchronous RL reconfiguration is not required among the SRNC, the Node B and the UE: After receiving the configuration message from the SRNC, the Node B and the UE immediately use the new configuration parameters.
The following is the DCH-DCH asynchronous RL reconfiguration in the RAB setup procedure.

Description:
Ø     The CN sends the RANAP RAB (Radio Access Bearer) Assignment Request message to the UTRAN to initiate the RAB setup request.
Ø     After receiving the RAB setup request, the SRNC maps the QoS parameter of the RAB as the AAL2 link feature parameter and the radio resource feature parameter. The ALCAP of the Iu interface initiates the user plane transport bearer setup procedure of the Iu interface based on the AAL2 link feature parameter.
Ø     In the asynchronous case, synchronization is not required for the radio reconfiguration. The SRNC sends the NBAP Radio Link Reconfiguration Request message to its subordinate Node B, requesting it to set up a new DCH on the existing radio link.
Ø     After receiving the Radio Link Reconfiguration Request message, the Node B allocates the corresponding resources, and sends the Radio Link Reconfiguration Response message to its home SRNC, notifying it that the radio link reconfiguration is complete.
Ø     The ALCAP of the Iub interface in the SRNC initiates the user plane transport bearer setup procedure of the Iub interface. The Node B and the SRNC establish synchronization with each other by exchanging the uplink and downlink synchronization frames of the DCH frame protocol.
Ø     The SRNC sends the RRC Radio Bearer Setup message to the UE.
Ø     After executing the RB setup, the UE sends the Radio Bearer Setup Complete message to the SRNC.
Ø     After receiving the Radio Bearer Setup Complete message, the SRNC responds to the CN with the Radio Access Bearer Assignment Response message. The RAB setup procedure ends.

6.4.4  Call Release Procedure


The call release procedure is the RRC connection release procedure. The RRC connection release procedure falls into two types: UE-initiated release and CN-initiated release. The difference between the two release types lies in who sends the call release request message of the upper layer first, though the final resource release is initiated by the CN.
After deciding to release a call, the CN sends the IU RELEASE COMMAND message to the SRNC. The SRNC takes the following steps after receiving the release command:
1)      Returns the IU RELEASE COMPLETE message to the CN.
2)      Initiates the release of the user plane transport bearer of the Iu interface.
3)      Releases the RRC connection.
The RRC release is to release the signaling link and all the radio bearers between the UE and the UTRAN. Based on the resources seized by the RRC connection, the RRC release can be further divided into two types: Release of the RRC connection established on a dedicated channel, and release of the RRC connection established on a common channel.

1. Release of the RRC connection established on a dedicated channel

Description:
1)      The RNC sends the RRC Connection Release message to the UE.
2)      The UE returns the RRC Connection Release Complete message to the RNC.
3)      The RNC sends the Radio Link Deletion message to the Node B to delete the radio link resources in the Node B.
4)      After releasing its related resources, the Node B returns the Radio Link Deletion Response message to the RNC.
5)      The RNC initiates the ALCAP release of the user plane transport bearer of the Iu interface.
In the end, the RNC initiates the local L2 resource release again. By now, the RRC connection release procedure ends.

2. Release of the RRC connection established on a common channel

During the release of the RRC connection established on a common channel, since the cell common resources are used, it is only necessary to release the UE directly instead of releasing the Node B resources and the data transport bearer.

6.4.5  Handover Procedure


The handover procedure is a characteristic distinguishing the mobile communication from the fixed communication. When the cell or the mode (FDD or TDD) used by the UE changes, we say the UE performs the handover. The WCDMA system supports the following types of handover: Soft handover, hard handover, forward handover, and inter-system handover. The soft handover and the hard handover are initiated by the network side, the forward handover is initiated by the UE, and the inter-system handover can be initiated by both the network side and the UE. The causes of the handover include UE movement, resource optimization configuration, and manual interference, etc.

1. Soft handover

In the WCDMA system, since the intra-frequency exists among neighboring cells, the UE can communicate with the network via multiple radio links, and can select one with good signal quality by comparison when these radio links are merged, thus optimizing the communication quality. The soft handover can be conducted only in the FDD mode. The soft handover falls into the following cases according to the locations of the cells. The first case is the soft handover among difference cells of the Node B. In this case, the radio links can be merged within the Node B or the SRNC. If they are merged within the Node B, it is called softer handover. The second case is the soft handover among different Node Bs within the same RNC and among different RNCs.
An important issue during the soft handover is the merge of multiple radio links. In the WCDMA system, the MACRO DIVERSITY technology is adopted for the merge of the radio links, that is, the system compares the data from different radio links based on certain standards (such as BER), and selects the data with better quality to send to the upper layer.
The following are some key concepts about the neighboring cell in the soft handover:
1)      Active set: The set of cells currently used by the UE. The execution result of the soft handover indicates the increase or decrease of the cells in the active set.
2)      Observation set: The set of cells that are not in the active set but are being observed by the UE based on the neighboring cell information from the UTRAN. The UE measures the cells in the observation set. When the measurement results satisfy certain conditions, the cells may be added to the active set. Therefore, the observation set sometimes is also called the candidate set.
3)      Detected set: The set of cells that have been detected by the UE but do not belong to the active set or the observation set. The UTRAN can request the UE to report the measurement result of the detected set. Since the cells in the detected set are not listed in the neighboring cell list, this set is also called the unlisted set.
The soft handover procedure comprises the following steps:
1)      Based on the measurement control information from the RNC, the UE measures the intra-frequency neighboring cells, and reports the measurement result to the RNC after processing.
2)      The RNC compares the reported measurement result with the set threshold to decide the cells to be added and deleted.
3)      If some cells are to be added, the RNC notifies the Node B to get ready.
4)      The RNC notifies the UE to add and/or delete cells via the active set update message.
5)      After the UE successfully update the active set, if the cells are deleted, the Node B will be notified to release the corresponding resources.
The original communication is not affected during the soft handover procedure so that smooth handover from a cell to another can be successfully completed.

2. Hard handover

In the case the neighboring cells are inter-frequency cells, hard handover instead of soft handover can be conducted. In the hard handover procedure, the communication with the previous cells is interrupted first, and then is connected with new cells. Therefore, the soft handover performance is not as good as soft handover. Generally, the hard handover is considered only when the soft handover cannot be conducted.
The target cell of hard handover may not be measured. It is applicable for the hard handover in emergency, but it has a high failure ratio. In the common hard handover, it is necessary to measure the target cell. However, the UE is generally configured with only one decoder, it cannot decode signals of two frequencies simultaneously. Therefore, to enable the UE to conduct inter-frequency measurement, the compressed mode technology is introduced into the WCDMA system.

The basic operating mechanism of the compressed mode technology is: When sending some frames (the data sent per 10ms is a frame), the Node B speeds up to send the data that are previously sent in 10ms in less than 10ms, so that the UE can use the time saved to conduct inter-frequency measurement. The mode and time for increasing the transmit rate is controlled by the RNC.
Similar to soft handover, the hard handover falls into the following types based on the location relationship between the source cell and the target cell.
1)      Hard handover between FDD and TDD in the same cell.
2)      Hard handover among cells in a Node B.
3)      Hard handover among cells in different Node Bs in the same RNC.
4)      Hard handover among cells in different RNCs.
Generally, when hard handover occurs between different RNCs, both RNCs have the Iur interface. Otherwise, the hand handover should be completed via relocation.
There are 5 signaling procedures at the Uu interface to complete the hard handover:
1)      Physical channel reconfiguration.
2)      Transport channel reconfiguration.
3)      Radio bear setup.
4)      Radio bear release.
5)      Radio bear reconfiguration.
The following figure gives the hard handover signaling procedure of cells in different Node Bs by taking the physical channel reconfiguration as an example

Description:
1)      The SRNC sends the Radio Link Setup Request message to the Node B where the target cell is, requesting the Node B to establish a radio link.
2)      The Node B where the target cell is sends the Radio Link Setup Response message to the SRNC, indicating the radio link is successfully established.
3)      The SRNC adopts the ALCAP protocol to set up the Iub interface transport bearer between the SRNC and the target Node B, and synchronizes the FP.
4)      The SRNC sends the Physical Channel Reconfiguration message carrying the target cell information to the UE via the downlink DCCH.
5)      After the UE hands over from the source cell to the target cell, the Node B of the source cell detects the radio link communication failure and then sends the Radio Link Failure Indication message to the SRNC, indicating the radio link failure.
6)      After successfully handing over to the target cell, the UE sends the Physical Channel Reconfiguration Complete message to the SRNC via the DCCH, notifying the SRNC that the physical cannel reconfiguration is complete.
7)      The SRNC sends the Radio Link Deletion Request message to the Node B where the source cell is, requesting the Node B to delete the radio link of the source cell.
8)      The Node B where the source cell is deletes the radio link resources, and then responds to the SRNC with the Radio Link Deletion Response message.
9)      The SRNC adopts the ALCAP protocol to release the Iub interface transport bearer of the SRNC and the Node B where the source cell is.

3. Forward handover

Forward handover is a part of the RRC connection mobility management. The forward handover consists of cell update and URA update. It is used to timely update the UE-related information at the UTRAN side when the UE location changes. It can also be used to monitor the RRC connection, switch the RRC connection state, and report errors and forward information. For both the cell update and the URA update, the update procedure is initiated by the UE.
1)      Cell update
The UE in the CELL_FACH, CELL_PCH or URA_PCH can initiate the cell update procedure. There are different cell update causes and cell update procedures for different connection states.
l   In the case the cell update cause is periodical cell update, and the UTRAN side does not allocate CRNTI or URNTI to the UE, the procedure is as follows:

The specific procedure is given as follows:
Ø     The UE sends the CELL UPDATE message to the UTRAN via the CCCH.
Ø     After processing the CELL UPDATE message received from the UE, the UTRAN sends the CELL UPDATE CONFIRM message to the UE. The cell update ends at the UTRAN side, and it ends at the UE side after the UE receives the CELL UPDATE CONFIRM message.
l   Physical channel reconfiguration will accompany the procedure in the following two cases: 1) The cell update cause is there is uplink data transport or response to paging, the UTRAN side does not allocate CRNTI or URNTI to the UE or indicate the related physical channel information, and the TFS/TFCS saved in the UE is the same as that in the PRACH/SCCPCH broadcasted by the system message; 2) The cell update cause is there are uplink data, or response to paging, or cell reselection, the UTRAN side allocates CRNTI or URNTI to the UE but does not indicate the related physical channel information, and the TFS/TFCS saved in the UE is the same as that in the PRACH/SCCPCH broadcasted by the system message.
l   Transport channel reconfiguration will accompany the procedure in the following two cases: 1) The cell update cause is there is uplink data transport or response to paging, the UTRAN side does not allocate CRNTI or URNTI to the UE or indicate the related physical channel information, and the TFS/TFCS saved in the UE differs from that in the PRACH/SCCPCH broadcasted by the system message; 2) The cell update cause is there are uplink data, or response to paging, or cell reselection, the UTRAN side allocates CRNTI or URNTI to the UE but does not indicate the related physical channel information, and the TFS/TFCS saved in the UE differs from that in the PRACH/SCCPCH broadcasted by the system message.
l   In the case the cell update cause is periodical update, the UTRAN side allocate CRNTI or URNTI to the UE but does not indicate the related physical channel information, the UE will update its identification, that is, the RNTI reallocation will accompany the procedure.
2)      URA update
The purpose of the URA update procedure is that the UE in the URA_PCH state uses the current URA to update the UTRAN after the URA reselection. This procedure can also be used to monitor the RRC connection when there is no URA reselection. Several different URA IDs can be broadcast in a cell, and different UEs in a cell can belong to different URAs. There is one and only one valid URA when the UE is in the URA_PCH state. When the UE is in the URA_PCH state, if the URA assigned to the UE is not in the URA ID list broadcast in the cell, the UE will initiate the URA update procedure. Or, if the UE is in the service area but T306 expires, the UE will also initiate the URA update procedure.
l   In the case the UTRAN does not allocate a new CRNTI or URNTI to the UE during the URA update procedure.

The specific procedure is given as follows:
Ø     The UE sends the URA UPDATE message to the UTRAN via the CCCH.
Ø     After processing the URA UPDATE message received from the UE, the UTRAN sends the URA UPDATE CONFIRM message to the UE and ends the URA update at its end. The URA update procedure ends at the UE side after the UE receives the URA UPDATE CONFIRM message.
l   During the URA update procedure, if the UTRAN allocates a new CRNTI or URNTI to the UE, then there is the RNTI REALLOCATION COMPLETE message sent by the UE to the UTRAN in the procedure.

4. Inter-system handover

The WCDMA system supports UE handover between the UTRAN and the existing systems (such as the GSM/GPRS). The inter-system handover falls into two cases: Handover under network control (such as the GSM) and cell reselection of the UE (such as GPRS), both of which are further divided into handover-to UTRAN and handover-from UTRAN cases. The following details the inter-system handover procedure by taking the handover-to UTRAN under the network control as an example only. It only covers the signaling in the UTRAN.
l   Handover to UTRAN

The specific procedure is given as follows:
1)      The CN notifies the UTRAN that a UE is to be handed over to it via the Relocation Request message.
2)      After getting the resources ready, the UTRAN sends the Relocation Request Acknowledge message to the CN. This message carries the Handover To UTRAN Command message that is to be forwarded to the UE by the peer system.
3)      After successfully handed over to the UTRAN, the UE sends the Handover To UTRAN Complete message to the UTRAN.

6.4.6  SRNS Relocation


The RNC relocation refers to that the SRNC of the UE changes from one RNC to another RNC. It is divided into two cases based on the UE location at the time of relocation: Static relocation and associated relocation, or in other words, UE Not Involved and UE Involved.

1. Static relocation

The precondition for the static relocation is that the UE accesses the network from one and only one DRNC. Since the relocation procedure does not require the UE’s participation, it is also called the UE Not Involved relocation. The following is an example of two radio links. After the relocation, the original DRNC becomes the SRNC, the Iur interface connection is released, and the Iu interface migrates.

There are two CN domains in the WCDMA system. When the relocation occurs, if the UE has connections with both of the CN domains, then the domains must be migrated at the same time.

2. Associated relocation

Associated relocation refers to that the UE accesses the target RNC from the SRNC via hard handover, and the Iu interface changes at the same time. Since the relocation procedure requires the UE’s participation, it is also called the UE Involved relocation. The connection change is shown in the following figure:

The 5 signaling procedures that can complete the hard handover can be used to complete the associated relocation. 

Комментариев нет:

Отправка комментария