4G/LTE - MBSFN
MBSFN (Multicast Broadcase Single Frequency Network)
With the introduction of mobile device and mobile network, one thing a lot of mobile users wanted to have was "I want to see TV (Movies etc on my mobile phone.". A set of first solutions to this requirement was DVB-H/DVB-T, DMB, ISDB-T, MediaFLO etc. These technlogies are still very widely used in some contries. There are many mobile device supporting both normal mobile phone capability and the mobile TV reception functionality. So for the users point of view, it was very good since they can have both mobile phone and TV on a single device with a small extra cost. But for the service provider's point of view, it is not that simple story. Mobile phone network and mobile TV network is totally different and separate. So it would be pretty big investment to deploy the network for mobile TV.
Then many people start having another idea saying "Why don't we prvide this kind of mobile TV (Broadcasting/Multicasting) service through the existing mobile phone network/technology ?".
The intial implementation of this idea was MBMS (Multimedia Broadcast Multicast Services) in UMTS and its LTE counter part is MBSFN (Multimedia Broadcast Single Frequency network or Multicase Broadcast Single Frequency Network).
Overall concept is as follows. A eNodeB can transmit the same data (idential data) to multiple UE simulteneously. In some case, multiple eNodeB can transmit the identical data simultaneously so that UE can receive the same data from multiple eNodeBs.
eMBMS Implementation Overview
To implement eMBMS, we need to tweak (or implement new feature) across almost all layers from PHY to Core Network. Following shows some of key feature on each layer that need to be implemented on each layer. I would not explan the details on each of these items. The details will be described in following sections.
Network Components for MBSFN(eMBMS)
For implementing eMBMS, a couple of components are added in the core network side as shown below. MCE, MBSFN Gateway and BM-SC are those components. (Refer to 36.300 15.1.1 E-MBMS Logical Architecture for details).
MCE (Multi-call/multicast Coordination Entity) : This is a logical entity and, physically it can be integrated into another network element. It has following functionality,
MBMS Gateway(GW) : This is a logical entity and, physically it can be integrated into another network element. It has following functionality,
M3 interface : Interface between MME-MCE
M2 interface : Interface between MCE-eNB
M1 interface : Interface between MBMS GW-eNB
MBSFN Access Network Structure/Definition
Followings are common terminologies you need to understand in MBSFN/MBMS. (Based on 3GPP TS 36.300 Chapter 15)
MBSFN Synchronization Area : This refers to an area of the network where all eNodeBs can be synchronized and perform MBSFN transmissions.
MBSFN Transmission/Transmission in MBSFN mode : This refers to a simulcast transmission technique realized by transmission of identical waveforms at the same time from multiple cells. An MBSFN Transmission from multiple cells within the MBSFN Area is seen as a single transmission by a UE.
MBSFN Area : This is an area which consists of a group of cells within an MBSFN Synchronization Area, which are co-ordinated to achieve an MBSFN Transmission.
MBSFN Area Reserved Cell : This refers to a cell within a MBSFN Area which does not contribute to the MBSFN Transmission. These cells may transmit data for other services but it should not transmit it in too high power that may interfere other MBSFN cell. So it should control its transmission power very carefully.
Synchronization Sequence : This refer to a sequence of MBSFN service which has the same duration and start time which is configured in the BM-SC and the MCE. Each SYNC PDU contains a time stamp which indicates the start time of the synchronisation sequence.
Syncronizsation Period : This refer to a duration which multiple synchronized sequence maintain the synchronization. The synchronization period provies the time reference for the indication of the start time of each synchronisation sequence.
Radio Channels for MBSFN(eMBMS)
LTE uses totally separate channel (logical cand transport channel) for MBSFN. As you may guess, it uses MCCH for control information and MTCH for data transmission. The information carried by MCCH includes subframe allocation and MCS(Modulation Coding Scheme).
RLC mode for MBSFN is UM (It is understandable since MBSFN is for broadcasting and there is no feedback from the reciever).
One or Several MTCH and one MCCH are multiplexed onto MCH by MAC layer.
MIMO is not defined for PMCH.
MCCH Implementation Structure/ Information in MCCH
Following is the overall principles for MCCH implementation listed 36.300 15.3.5.
< MCCH and MBSFN Area association >
< MCCH Transmission >
< Notification Mechanism >
When MCCH gets modified due to either Session Start or presense of an MBMS counting request message, Network needs to inform UE of the modification. For this, network is using special notification mechanism summarized below.
< UE side procedure >
Following summary comes from 36.300 15.3.6 and further details follows the summary.
< MBSFN Subframe Configuration >
Since the MBSFN data is carried by the same physical channel which is used for mobile comunication, we have to use carefull scheduling for MBSFN so that it would not interfer normal mobile communication. This physical layer scheduling is specified in SIB2 as shown below.
Radio Frame meeting the following equation is allocated for MBSFN.
SFN mod radioframeAllocationPeriod = radioframeAllocationOffset
Subframes that is allocated for MBSFN within the MBSFN Frame is determined by a bitmap as shown below.
Following is an example of MBSFN SFN, Subframe allocation. Try with following parameters and See if you come out with the same result as mine.
< MBSFN Neighbour Cell Configuration >
< MBSFN Control Channel Information >
As I explained above, MBSFN is using two different channel : MCCH and MTCH. As you may easily guess, MTCH carry the MBMS traffic data and MCCH is for conveying the control message. You learned that the MBSFN physical frame and broadcasting cycle is defined in SIB2, but there is no MBSFN control channel information in SIB2. To carry the MBSFN control channel information, 3GPP defined a separate SIB : SIB13 and major component of SIB13 is as shown below.
MBSFN control channel information and MBSFN Area specification is specified by SIB13 as shown below.
There are several triggers that let UE aquire MCCH message which is summarized as below. (Refer to 36.331 126.96.36.199 for further information)
Once any one of the trigger is set, UE have to aquire MCCH and decode following informations. (Refer to 36.331 6.2.2, 6.3.7)
commonSF-Alloc : It indicates the subframes allocated to the MBSFN area
commonSF-AllocPeriod : It indicates the period during which resources corresponding with field commonSF-Alloc are divided between the (P)MCH that are configured for this MBSFN area. Subfram allocation patterns defined by commonSF-Alloc repeat continously during this period.
New LCID is specified for MCH as shown below.
MAC PDU structure for MCH is defined as shown below.
MBSFN subframe has different structure from the normal (non-MBSFN) subframe as shown below. The first one or two OFDM Symbol in MBSFN subframe is allocated for control region as in normal subframe. But the number of symbols for the control region may or may not be same as non-MBSFN subframe. Location of Reference Signal for MBSFN is different from Non-MBSFN Reference Signal as shown below.
The number and locations of MBSFN subframes within a specific radio frame is determined by Network and is broadcasted to UE via SIB.
This is based on 36.300 15.3.3.
Muli-Cell Transmission refers to Synchronous transmission of MBMS within a MBSFN Area and it has following characteristics.
Reference - 3GPP
Reference - 3GPP Implementation
Reference - 3GPP Test Specificaiton
Following is from 36.521-1 (V11.2.0 (2013-10))
Following is from 36.523-1 (V11.4.0 (2013-10))