MBMS continued [Under LTE > 3G till
Rel 6]
» MBMS stage 1 - 22.146, Rel 6 «
» MBMS architecture - 23.246, Rel 6 «
» MBMS in RAN - 25.346, Rel 6 «
» MBMS protocols and codecs - 26.346, Rel 6 «
To implement MBMS, we need changes in UTRAN and CN.
Core network changes
Rel 6 MBMS introduced an additional entity BM-SC (Broadcast/Multicast Service Centre) connected to GGSN. On other side BM-SC is connected to content provider. The content provider could be internal to operator network or external (internet). Authorisations, Scheduling of MBMS sessions & transmissions, Charging etc. are functions of BM-SC.
Note that over air interface (RAN), transmission is one way i.e. there is no retransmission (HARQ) loop. As air interface in more error prone, in addition to coding, little higher up (close to application level) error correction mechanism is needed to ensure good quality to end user. TS 26.346 specify Raptor codes for this purpose. This coding is done by BM-SC (to be confirmed).
UTRAN changes
MBMS added three new "logical channels" - MTCH, MCCH, and MSCH - and a new physical channel, MICH. MBMS logical channels goes over existing FACH and S-CCPCH. MCCH contain control information about services offered and configuration for MTCH. MSCH, sent along with S-CCPCH carrying MCCH, contain service transmission schedule (in terms of TTIs). Note that mobile need not be monitoring MCCH all the time; MICH provide ways for mobile to sleep and wake up to check MICH if there are any changes in MCCH (similar to what PICH-paging does).
Find below updated channel mapping (click on image to view enlarged version) and L2 stack.
Another interesting point to be noted for MBMS UTRAN changes is: MBMS can make use of macro diversity i.e. same transmission can be made in neighboring cells (note that MCCH will contain required information about neighboring cell MBMS transmission). TS 25.346 specify selective and soft combining methods to achieve the diversity. Soft combining combines Rake receiver outputs (before decoding) from neighboring cells whereas Selective combining selectively takes best Rake receiver output (after decoding) among received from neighboring cells. It is apparent that Soft combining will happen at MAC-L1 interface whereas Selective will happen higher up at RLC-MAC interface.
This complete brief on MBMS; please refer release 6 document for complete list of MBMS affected and dedicated specifications.
References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming, release 6 document at 3GPP.
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