Cell Broadcast Service

1 Introduction

This document describes Cell Broadcast Service (CBS) within the WCDMA Radio Access Network (RAN).

1.1 Scope

The document describes the CBS features including operator parameters, engineering guidelines and information about the activation and deactivation of the features.
There are two optional features for CBS controlled by separate licenses in the RNC:

• Cell Broadcast Service, FAJ 121 1326
• Cell Broadcast Service - UE battery consumption improvements, FAJ 121 1406

1.2 Target Groups

The target groups for this document are the following:

• System operators who need a general understanding of the CBS feature in WCDMA RAN
• Personnel working on Ericsson products or systems

1.3 Revision Information

Apart from editorial changes this document has been revised as listed in Table 1.

Table 1 Revision History

Revision	Reason for Revision
A	This is a new document based on 126/1553-HSD 10102/7. The document is updated with information about the feature CBS - UE battery consumption improvements (FAJ 121 1406).
B	Information about counter pmNoDiscardedBmcCbsMsgs is added in section 4.2. The engineering guidelines for Level 2 Scheduling in section 6.2 is updated.
C	Revised Section 6.2 about recommended parameter setting for Level 2 Scheduling.
D	Incorrect information describing the mapping of CBS Schedule periods to the SFN cycle is removed in Section 4.4.2. Also Figure 5 in Section 4.4.2 is revised.

1.4 Concepts

The following concepts are addressed in this document:

Cell Broadcast (CB) message		User data as transmitted from the CBC to the UE.
Cell Broadcast Service Area (CBSA)		A CBSA represents the smallest geographical area where CB messages can be broadcast. One CBSA contains one UTRAN cell.
BMC CBS Message		BMC CBS Messages are sent from WCDMA RAN to the UE using the BMC protocol. A BMC CBS Message contains one CB message.
CTCH occasion		A radio frame on the Secondary CCPCH where CTCH data can be sent.
CBS schedule period		A CBS schedule period consists of a number of consecutive CTCH occasions scheduled on S-CCPCH.
BMC Schedule Message		BMC Schedule Messages are sent from WCDMA RAN to the UE using the BMC protocol. A BMC Schedule message contains information about the BMC messages (BMC Schedule Messages and BMC CBS Messages) to be broadcast in the next CBS schedule period.

2 Overview

This section provides an overview of the CBS features in WCDMA RAN.
The CBS features enable broadcast of short text messages to defined geographical areas known as Cell Broadcast Service Areas (CBSAs). The messages are repeated over the air interface at a frequency and for a duration agreed by the content provider.
CB messages are generated by the Cell Broadcast Center (CBC) and sent to WCDMA RAN via the Iu-BC interface, see Figure 1. The CBC is an external node provided by the operator, that is, the functionality is not integrated in the Ericsson Core Network.

Figure 1 CBS Overview

The Iu-BC interface is standardized by 3GPP and the protocol stack is illustrated in Figure 2 below.

Figure 2 Iu-BC protocol stack

The Service Area Broadcast Protocol (SABP), specified in Reference [2], is used to distribute CB messages from CBC to a number of CBSAs in RNC. Each CBSA contains only one UTRAN cell. The lower layers of the Iu-BC interface are specified in Reference [3]. Both IP and ATM transport is supported in WCDMA RAN.
The CB messages received from CBC are stored in WCDMA RAN and the Broadcast Multicast Control (BMC) protocol, specified in Reference [4], is used on the Uu interface to schedule and broadcast corresponding BMC CBS messages in the cell. Each BMC CBS message contains one CB message. The BMC CBS messages are sent on the logical channel CTCH using RLC unacknowledged mode. The CTCH is multiplexed on a FACH transport channel carried by a Secondary CCPCH.
CB messages are received by UEs in Idle mode and in state URA_PCH. The UE will monitor the CTCH during time intervals which do not conflict with the UE specific paging occasions on PICH.

2.1 Cell Broadcast Service (FAJ 121 1326)

This feature provides the basic functionality for CBS including support for the Iu-Bc interface and broadcast of BMC CBS Messages on CTCH using the Level 1 Scheduling as described in Section 4.4.1.

2.2 Cell Broadcast Service - UE battery consumption improvements (FAJ 121 1406)

This feature is an enhancement of the basic CBS functionality introduced by FAJ 121 1326. It makes it possible to improve UE standby time in idle mode and state URA_PCH by introducing support for Level 2 Scheduling, that is, definition of CBS schedule periods and broadcast of BMC Schedule Messages as described in Section 4.4.2.

3 Capabilities

A maximum of 20 CB messages, or a total payload of 2.5 kbyte, can be stored per CBSA in the RNC. When this limit is reached new CB messages addressed to the CBSA are discarded. If a CB message is discarded the cell counter pmNoDiscardedCbsMsgOrders is stepped, see Reference [6].
The maximum number of CBSAs is 65535 within a PLMN and 2304 within an RNC.
The size of incoming SABP messages is limited to 18000 octets. This should be large enough to cover a SABP WRITE-REPLACE message containing 2304 CBSAs. If the size exceeds 18000 octets the message is not decoded and WCDMA RAN responds by sending an SABP ERROR INDICATION to CBC.
RNC hardware: SPB21, SPB3 or later HW is required in RNC to activate the feature Cell Broadcast Service - UE battery consumption improvements (FAJ 121 1406).

4 Technical Description

4.1 Cell Broadcast Service Area

The CBS feature includes the area concept Cell Broadcast Service Area (CBSA) which represents the smallest geographical area where a CB message can be broadcast. The CBSA is only used towards the CBC node and it contains one UTRAN cell.
The service area identity (SAI) for a CBSA is defined as:
PLMN id + Location Area Code (LAC) + CB Service Area Code
The CB Service Area Code is configured in the RNC and the value is set by the cell parameter cellBroadcastSac. For PLMN id and LAC the already configured values for the cell are used.
In order to avoid configuration mismatch OSS provides a consistency check rule for verifying that the CBSAs are unique in all cells in the network. Warnings will be issued in case on non-unique SAI as same SAI in different cells may cause broadcasting of CB messages in the wrong area.
If the cellBroadcastSac parameter is set to “undefined” no CB messages can be broadcast in the cell.

4.2 Reception of new CB messages

Broadcast of a new CB message, or replacement of an old CB message, is initiated by CBC with SABP message WRITE-REPLACE. In addition to the actual CB payload the SABP WRITE-REPLACE message contains the following information

Message Identity and Serial Number		The Message Identity and the 12 left-most bits of the Serial Number uniquely identifies a CB message within a CBSA.
Old Serial Number		This information is optional and only included in the SABP WRITE-REPLACE message if there is a need to replace an already stored CB message for a given Message Identity.
Data Coding Scheme		This information is transferred transparently to the UE.
Service Areas List		List of CBSAs where the CB message is to be broadcast.
Repetition Period		Information about how often the CB message is to be broadcast.
Number of Broadcasts Requested		Information about how many times the CB message is to be broadcast. The broadcast can be either finite or infinite.
Category		This information indicates the priority of the CB message.

Figure 3 illustrates the steps performed in WCDMA RAN at reception of a SABP WRITE-REPLACE message.

Figure 3 WRITE-REPLACE

1. At reception of the SABP WRITE-REPLACE message RNC checks that all service areas included in the message are defined as CBSAs and responds with a SABP WRITE-REPLACE COMPLETE message.
2. For each defined CBSA listed in the SABP WRITE-REPLACE message the following steps are performed:
   • If the cell contained in the CBSA is enabled, the new CB message is stored. If the SABP WRITE-REPLACE message contains an Old Serial Number, the previously stored CB message is replaced with the new information. Broadcast of the old CB message is stopped.
   • The new CB message is broadcast on CTCH using the BMC protocol. The BMC CBS messages are broadcast with the repetition period requested by CBC. If the amount of data generated by the BMC layer is higher than the available CTCH capacity a congestion situation may occur where BMC CBS messages are discarded in RNC. If a BMC CBS message is discarded due to congestion the cell counter pmNoDiscardedBmcCbsMsgs is stepped.
3. For finite broadcast, the CB message is cleared when it has been repeated the number of times requested by CBC. For infinite broadcast, the CB message is repeated until it is deleted by CBC, see Section 4.3.

4.3 Deletion of CB messages

An active CB message can be deleted by request from CBC using one of the SABP procedures KILL or RESET. The KILL procedure is used to stop broadcast of one CB message in one or several CBSAs whereas the RESET procedure is used to stop broadcast of all CB messages stored in one or several CBSAs. Both procedures are supported in WCDMA RAN.
If a cell with a defined CBSA becomes disabled the broadcast is stopped in that cell and all stored CB messages are cleared.

4.4 Scheduling of BMC messages over the Uu interface

Broadcast of BMC data over the Uu interface is controlled by the following two scheduling schemes:

• CBS Level 1 Scheduling
• CBS Level 2 Scheduling (DRX mode)

Level 1 Scheduling is the basic scheduling scheme provided by FAJ 121 1326. This scheduling can be used as a stand alone scheduling scheme or it can be combined with the Level 2 Scheduling provided by FAJ 121 1406. The different scheduling schemes are further described below.

4.4.1 CBS Level 1 Scheduling

CBS Level 1 Scheduling introduces support for CTCH occasions on S-CCPCH and makes it possible to broadcast BMC CBS Messages over the Uu interface.
The logical channel CTCH is multiplexed on FACH1, carried by the existing S-CCPCH used for 32 kbps Interactive PS RB and SRBs. The CTCH is scheduled on S-CCPCH during predefined CTCH occasions. The CTCH occasions are fixed on the SFN cycle 0...4095 and thus repeated cyclically.
CTCH occasions are defined as the radio frames in which the following equation is true:
SFN = K + m*N, m = 0, 1, 2, ... M (Reference [5])
K is the offset to the first CTCH occasion on the SFN cycle. The value is hard coded to 0.
N is the period of CTCH occasions on S-CCPCH. The value is set by the parameter ctchOccasionPeriod.
M is the maximum value that fulfills the equation K + M*N 4095 (Max SFN)

Figure 4 Scheduling of CTCH occasions on Secondary CCPCH

Note that the TTI for FACH1 is 10 ms, that is, each CTCH occasion corresponds to one TTI on FACH1.
The parameters K and N are signalled to the UE in system information (SIB5).

4.4.2 CBS Level 2 Scheduling

CBS Level 2 Scheduling introduces support for CBS schedule periods and transmission of BMC Schedule Messages on CTCH.
A CBS schedule period consists of P consecutive CTCH occasions scheduled on S-CCPCH. P is set by parameter cbsSchedulePeriodLength.
The CTCH occasion period N, that is, the time between two consecutive CTCH occasions within a CBS schedule period, is controlled by the Level 1 Scheduling described in Section 4.4.1. Thus the length of a CBS Schedule period can be defined as (P *N /100) seconds.

Figure 5 Example of CBS Level 2 Scheduling using P=8.

The length of a CBS schedule period is fix and the periods are repeated on S-CCPCH every P CTCH occasion. A BMC Schedule Message is sent in the beginning of each CBS schedule period. It contains information about the messages to be scheduled in the next coming period, that is, the BMC Schedule message sent in period x lists the BMC messages to be scheduled on CTCH in period x+1. Based on this information the UE knows beforehand when there is a need to listen to S-CCPCH. Only if a CTCH occasion contains a BMC Schedule Message or a new BMC CBS Message the UE is required to listen to S-CCPCH. For all other CTCH occasions the UE can remain idle.

4.5 Iu-BC Transport

4.5.1 Network Aspects

4.5.1.1 Overview

Iu-BC and O&M traffic will share the same IP host in RNC.
For ATM case Iu-BC traffic is carried to O&M router using ATM, see Figure 5. The CBC node must be connected using IP/Ethernet to O&M Router.

 Figure 6 Iu-BC using ATM transport
For IP case Iu-BC can be carried over IP/Ethernet straight to the CBC node from RNC, see Figure 6.

Figure 7 Iu-BC using IP transport

For IP over ATM case, all IP Routing configuration in RNC for Mur can be reused for Iu-BC, as the Iu-BC interface is connected to the same O&M IP network as Mur. There is no need to add or modify routes as all traffic is sent to the O&M Router.
The fact that the Iu-BC interface will be connected to the same O&M IP network as Mur implies the following:

• QoS must be considered as the Iu-BC traffic will compete for bandwidth with the O&M traffic
• Iu-BC interface and traffic will experience the same level of network security as O&M traffic and as provided by the O&M IP network
• The CBC node must be connected to the O&M IP network
• The O&M IP network must allow the TCP port for SABP 3452 and will compete for bandwidth with the O&M traffic

4.5.1.2 QoS

From RNC to CBC node the Iu-BC traffic will get the same QoS characteristics as O&M traffic. For the traffic from the CBC to the RNC, QoS separation in the IP network using different settings of the DSCP (DiffServ Code Point) can be done if the CBC node supports it. The QoS characteristics currently recommended for O&M traffic is assumed to be adequate for Iu-BC traffic. An operator may choose to implement an O&M IP network with a higher QoS level.

4.5.1.3 Dimensioning

The dimensioning of the O&M IP network should take into account Iu-BC traffic. The Iu-BC traffic volume is expected to be low and the impact on the dimensioning should be small.
The Iu-BC capacity demand can be calculated using the formula below:
C_Iu-BC = Msglength * numMsgs/second
Msglength is the length of the SABP message.
numMsgs/second is the number of SABP messages sent over the Iu-BC interface per second.

4.5.1.4 Security

IP network security must be considered when adding Iu-BC traffic to the O&M IP network. This network has many access points compared to the networks for user traffic and is potentially exposed to greater security threats.
To improve security it is recommended to turn on the Filter for IP source address validation in the RNC by setting parameter sourceIpAddressValidation to TRUE. This filter will avoid the establishment of connections from other IP source addresses than the configured IP address for the CBC.
It is possible to turn this filter off if there is a need to establish connections from more than one IP address.

Note:

If the filter is not used the security level may need to be increased for the entire O&M IP network.

4.5.2 Configuration

In order to start CBS operation the MO IuBcLink has to be created in RNC. The recommended configuration of the IuBcLink can be found in Table 2 below.

Table 2 Iu-BC configuration

MO: =	=	Comment
IuBcLink=1	administrativeState = UNLOCKED	UNLOCKED will activate CBS
	cbcIpAddress [0..15]	IP address of the CBC according to IP address plan
	sourceIpAddressValidation = TRUE	It is recommended to set the parameter to TRUE

4.6 Iub Transport

The new logical channel CTCH is carried over the existing transport bearer established for FACH1.

5 Activation and Deactivation

This section describes activation and deactivation of the CBS features.

5.1 FAJ 121 1326, Cell Broadcast Service

Cell Broadcast Service (FAJ 121 1326) is an optional feature.

5.1.1 Preconditions for activation

• Order and install the feature Cell Broadcast Service (FAJ 121 1326).

5.1.2 Activation

In order to activate CBS operation the following steps must be performed:

1. Activate and install the license key for Cell Broadcast Service (CXC 403 0043) in RNC. The feature activates by RNS when the featureState attribute, for designation cellBroadcastService, is set to ACTIVATED in the RncFeature MO.
2. Configure the Iu-BC link as described in Section 4.5.2.
3. Make sure that the cells in which CB messages are to be broadcast are enabled and that a CBSA is defined for each cell, see Section 4.1.

Note that the actions described in 1-3 above can be performed in any order.
As soon as step 1 and 3 are performed, the CTCH configuration is included in system information, SIB5, and the UE will start monitoring the CTCH occasions on S-CCPCH in cells where a CBSA has been defined. However, no CB messages can be broadcast in the cells as long as the Iu-BC link is not configured.
Once all steps 1, 2 and 3 are performed, RNC informs CBC about the service areas that are operable by sending a SABP RESTART message. The SABP RESTART message contains a list of all defined CBSAs within the RNC.
If a cell with a defined CBSA becomes disabled the broadcast is stopped in that cell and all stored CB messages are cleared. As soon as the cell is up again a restart indication is triggered for the cell. To avoid that SABP RESTART messages are sent too often over the Iu-BC interface a periodic timer of 30 seconds is implemented in the RNC to monitor the restart indications. Whenever the timer expires RNC will send a SABP RESTART message to CBC containing a list all CBSAs that have triggered a restart indication during the last 30 seconds. If there were no restart indications when the timer expires, no SABP RESTART message is sent for that period.

5.1.3 Preconditions for Deactivation

None.

5.1.4 Deactivation

The feature deactivates when the featureState attribute, for designation cellBroadcastService, is set to DEACTIVATED in the RncFeature MO.

5.2 FAJ 121 1406, Cell Broadcast Service - UE battery consumption improvements

Cell Broadcast Service - UE battery consumption improvements (FAJ 121 1406) is an optional feature.

5.2.1 Preconditions for activation

• Order and install the feature Cell Broadcast Service (FAJ 121 1326).
• Order and install the feature Cell Broadcast Service - UE battery consumption improvements (FAJ 121 1406).

5.2.2 Activation

In order to activate the feature the following steps must be performed:

1. Activate and install the license key for Cell Broadcast Service - UE battery consumption improvements (CXC 403 0070) in RNC. The feature activates by RNS when the featureState attribute, for designation cbsBatteryImprovements, is set to ACTIVATED in the RncFeature MO.
2. Activate feature Cell Broadcast Service (FAJ 121 1326) as described in Section 5.1.2.

Note that the steps 1 - 2 can be performed in any order.

5.2.3 Preconditions for Deactivation

None.

5.2.4 Deactivation

The feature deactivates when the featureState attribute, for designation cbsBatteryImprovements, is set to DEACTIVATED in the RncFeature MO.

6 Engineering guideline

6.1 CBS Level 1 Scheduling

The CTCH will share existing S-CCPCH resources used for SRB signalling and PS Interactive data on FACH. However, transmission of BMC messages will have highest priority in the RNC during a CTCH occasion. Thus the parameter ctchOccasionPeriod can be used to control the maximum bandwidth of the CTCH channel that can vary from 0.12 kbps (ctchOccasionPeriod = 256) up to 30.4 kbps (ctchOccasionPeriod = 1). If there are no BMC messages to transmit in a CTCH occasion, the TTI can be used by RNC to send other FACH1 or FACH2 data. Note that if the CTCH occasions are scheduled frequently, and the CBS load is high in the cell, the transmission of SRB signalling and PS Interactive data on FACH1 and FACH2 might get delayed.
When the Level 2 Scheduling is not applied a UE in idle mode and state URA_PCH must listen to every CTCH occasion scheduled on S-CCPCH even if it does not contain any CTCH data. If the CTCH occasions are scheduled frequently on S-CCPCH it will impact the UE battery consumption in idle mode and URA_PCH state. Thus the setting of the parameter ctchOccasionPeriod is a trade off between maximum CTCH bandwidth and UE standby time.
If a CTCH occasion conflicts with a paging occasion, the UE will prioritize paging before reading CTCH. The paging occasions are calculated based on the IMSI and the DRX cycle length coefficient. Different DRX cycle length coefficients can be configured for UEs in idle mode and in state URA_PCH, see Idle Mode and Common Channel Behavior.

Note:

In order to avoid that the same UE will miss all CTCH occasions it is important to make sure that the ctchOccasionPeriod is not set to a multiple of the DRX cycle length.

Note:

Changing the attribute ctchOccasionPeriod will affect any ongoing traffic in the cell. Whenever its value is set, the cell is disabled automatically and then re-enabled.

6.2 CBS Level 2 Scheduling

The Level 2 Scheduling introduces a DRX mode for CTCH reception that makes it possible to reduce the power consumption in the UE when CBS is activated in WCDMA RAN. Based on the schedule information the UE knows beforehand when there is a need to read CTCH. Only if a CTCH occasion contains a BMC Schedule Message or a new BMC CBS Message the UE is required to listen to S-CCPCH. For all other CTCH occasions the UE can remain idle.
Since a BMC Schedule Message is transmitted once every CBS schedule period the length of the period, defined as (cbsSchedulePeriodLength * ctchOccasionPeriod / 100) seconds, controls how often the UE must read S-CCPCH. It should be noted that the size of a BMC Schedule Message is variable and the message can be transmitted in several CTCH occasions. The size depends on the setting of parameter cbsSchedulePeriodLength and the amount of user data scheduled in the period. When the schedule period contains no messages (or only a few messages) the BMC Schedule message will typically occupy {1, 1, 2, 3, 4} CTCH occasions for cbsSchedulePeriodLength = {8, 16, 32, 64, 128}.
The Level 2 Scheduling will increase the time to delivery (TTD) for new cell broadcast messages, that is, there will be a delay from reception of SABP WRITE-REPLACE in RNC until the BMC CBS Message can be transmitted on CTCH the first time. This is because the schedule information associated with the BMC CBS Message must be sent in a CBS schedule period prior to the one containing the actual user data. The TTD is typically between one and two CBS schedule periods.
To get an optimal setting of the parameters ctchOccasionPeriod and cbsSchedulePeriodLength the TTD requirement as well as the UE battery consumption must be considered. The recommendation is to use the highest possible value of ctchOccasionPeriod and then adjust the parameter cbsSchedulePeriodLength to get an acceptable delay for the first transmission of the BMC CBS Message. For example, if the TTD requirement is 90 seconds a setting of ctchOccasionPeriod = 255 and cbsSchedulePeriodLength = 16 is recommended. With this setting the CBS contribution to UE battery consumption can theoretically be reduced by 16 times compared to the scenario where only Level 1 Scheduling is applied. A TTD requirement of 11 minutes can theoretically reduce the UE battery consumption by 32 times using a setting of ctchOccasionPeriod = 255 and cbsSchedulePeriodLength = 128.
It should be noted that all segments of a BMC CBS Message must be transmitted within the same schedule period, that is, the lower values of parameter cbsSchedulePeriodLength should not be used for broadcast of long BMC CBS messages. If the size of the message is larger than 3-pages the cbsSchedulePeriodLength parameter should use a setting of 16 (or higher), if the message size is larger than 6-pages a setting of 32 (or higher) should be used, and for messages larger than 13-pages a setting of 64 (or higher) is recommended. If the schedule period is too short the BMC CBS message is discarded in RNC and counter pmNoDiscardedBmcCbsMsgs is stepped.

Note:

cbsSchedulePeriodLength 256 is not a valid setting according to 3GPP and should not be used.

Note:

Changing the attributes cbsSchedulePeriodLength and ctchOccasionPeriod will affect any ongoing traffic in the cell. Whenever their values are set, the cell is disabled automatically and then re-enabled.

7 Parameters

This section describes all parameters that the operator can configure to control the CBS features in WCDMA RAN. Further parameter information can be found in Radio Network Parameters.

7.1 Descriptions

cellBroadcastSac		Cell Broadcast Service Area Code within a location area. The parameter can be set per cell.
ctchOccasionPeriod		Period of CTCH allocations on S-CCPCH. The parameter can be set per cell.
cbsSchedulePeriodLength		Number of consecutive CTCH occasions on S-CCPCH that, together with the CTCH occasion period (configured through UtranCell.ctchOccasionPeriod), define a CBS schedule period. The parameter can be set per cell.
cbcIpAddress		IP address of the CBC. The input format used by the operator is four fields of digits, separated by dots. Each field may consist of three digits. The value of each field shall be in the range 0..255.
sourceIpAddressValidation		Indicates if the source IP address shall be validated when the CBC establishes a connection. When set to TRUE, only a source IP address equal to cbcIpAddress is accepted.

7.2 Range and Default Values

Table 3 WCDMA RAN Cell Broadcast Parameters

Parameter Name	Default Value	Value range	Unit of Measurement
cellBroadcastSac	-1 [undefined]	-1..65535 [undefined, 0..65535]	-
ctchOccasionPeriod	256	1..256	10 ms
cbsSchedulePeriodLength	64	[8, 16, 32, 64, 128, 256] Note 1	Number of consecutive CTCH occasions on S-CCPCH per CBS schedule period
cbcIpAddress	-	Four fields of digits, separated by dots. Each field may consist of three digits. The value of each field shall be in the range 0..255.	-
sourceIpAddressValidation	TRUE	FALSE, TRUE	-

Note 1: 256 is not a valid setting according to 3GPP and should not be used.