LTE (Long-Term Evolution) is a standard wireless communication of high-speed data for mobile phones and data terminals, which is a natural upgrade for carriers with both GSM/UMTS networks and CDMA200 networks.
A simplified architecture of a Long Term Evolution (LTE) system can be illustrated as shown in FIG. 1. In LTE the downlink is based on orthogonal frequency division multiplexing (OFDM) while the uplink is based on a single carrier modulation method known as discrete Fourier transform spread OFDM (DFT-S-OFDM).
The communications network 100 illustrated in FIG. 1, comprises an air interface applied for LTE, referred to as Evolved Universal Terrestrial Radio Access (E-UTRAN) and is here made up of three eNB nodes, 110a, 110b, 110c, which are connected to each other via the X2 interface (solid lines), and connected to two Mobility Management Entities (MME), or Serving Gateways (S-GW), 120a, 120b via the S1 interface (dashed lines). Both the S1 and the X2 interface can be divided into control plane and user plane parts.
LTE Broadcast is a point-to-multipoint service, which may be applicable in LTE systems, in which data is transmitted from a single source entity to multiple recipients. This service is based on enhanced Multimedia Broadcast Multicast Services (eMBMS), which was included in the 3GPP release 9 specifications and which is evolved from MBMS. For an overall description of E-UTRAN eMBMS functionality, see 3GPP TS 36.300.
On the eUTRAN-UE (Uu) interface, the MBMS Single Frequency Network (MBSFN) transmission mode applies. This is characterized by synchronous transmission of the same signal from all cells within a MBSFN area, which enables User Equipments (UEs) to connect to the network to combine eMBMS transmission from multiple cells. Transmitting the same data to multiple recipients allows network resources to be shared.
The eMBMS is realized in the 3GPP specifications by the addition of a number of new capabilities to existing functional entities of the 3GPP architecture, as well as the addition of a new functional entity. It is to be understood that throughout this document MBMS is to be referred to as both MBMS and eMBMS, in case not explicitly indicated otherwise.
eMBMS was introduced in LTE Rel-9 to allow for efficient transmission of the same content from a content provider to many UEs.
When broadcasting eMBMS via LTE Broadcast on one or multiple Carriers in a deployment scenario with overlapping carriers the following requirements were defined in 3GPP:
Network Related Behavior:
Support of mobility procedures such that an interested UE is moved to a correct frequency to start MBMS service reception kept on the correct frequency to start/continue MBMS service reception. An applicable service layer and network then provide the UE with LTE broadcast reception.
UE Related Behavior:
A UE in IDLE mode can perform autonomous frequency prioritization while a UE in CONNECTED mode informs the network about its interest (MBMS frequency).
In order to enable use of MBMS or eMBMS, MBMS assistance information is made available to UEs. A User Service Description (USD) applied in the service layer, provides UEs with MBMS frequency, session start and time as well as information about the usability of an available MBMS service, which is identified by the Temporary Mobile Group Identities (MBMS SAIs).
System Information Blocks (SIBs) comprise system information that is transmitted from UTRAN, here E-UTRAN, to UEs. SystemInformationBlockType15, from hereinafter referred to as SIB 15 or SIB type 15, is a new SIB, described in 3GPPTS 36.331 “Radio Resource Control (RRC) Protocol Specification” (Release 11), which is broadcasted by the E-UTRAN to the UEs where all cells provide MBMS Service Area Identities (SAIs) for a current frequency and neighboring frequencies from where MBMS is provided. Based on the USD and SIB15, a UE will know when its MBMS service can be provided and on which frequency it can be provided. An example of such information, which can be referred to as MBMS assistance information, is illustrated in FIG. 2.
In FIG. 2 a combination of service layer information, as mentioned above, and information from RAN, comprise a User Service Description (USD) 210 in the service layer. The USD 210 provides information on the availability of MBMS service, which is identified by a Temporary Mobile Group Identity (TMGI) and comprise MBMS Service Area Identities (MBMS SAIs) here represented by SAI41, SAI42 and SAI43), MBMS frequencies (f2, f3), as well as start time (Start) and end time (End) of a respective service. In addition, the new SIB, SIB15, which is broadcasted from all cells of the RAN comprise respective MBMS SAIs for the current frequencies (current SAIs), as well as for the neighboring frequencies (neighboring SAIs). Based on the relevant USD and the SIB15 the UE 220 will be made aware of when its MBMS service is provided and on which frequency. In the present example f1 is a frequency used for unicast, while f2 is used for MBMS transmission.
Summary of Functionality Requested by 3GPP:
Below, a summary of functionality requested by 3GPP, described in 3GPPTS 36.331 “Radio Resource Control (RRC) Protocol Specification” (Release 11) is given.                1. The access node, here represented by an eNB, shall support MBMS Service Acquisition by notifying all UEs about the availability of specific MBMS service areas per carrier. In the present example this is executed by providing this information in SIB15, but in an alternative system or network the corresponding information may be provided in any other type of message, capable of providing or carrying such information to UEs.        2. The eNB react upon the UE's indicated MBMS Service preferences by means of Inter-Frequency HO. The UE is capable of indicating the MBMS interest, via which the MBMS service preferences can be reached with the RRC MBMS INTEREST INDICATION.        3. This eNB shall also ensure MBMS Service continuity by preventing Inter-Frequency HO due to load balancing.        
Focusing on step 1) mentioned above, the specific SIB, denoted as the IE SIB Type15, contains the MBMS SAIs of both the current and/or neighboring carrier frequencies, as indicated in FIG. 3, where element “sai-IntraFreq” contains a list of MBMS SAIs broadcasted in the current frequency; “sai-InterFreqList’ contains a list of neighboring frequencies on which one or more MBMS services are broadcasted and the corresponding MBMS SAIs, while “sai-List” contains a list of MBMS SAIs for a specific frequency.