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.
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