# LTE : Which sub frames of a radio frames can't be used for multicast and broadcast services ?

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Can someone please explain which all sub frames of a radio frame (FDD mode) can't be used for multicast and broadcast services and what are the reasons for that ?

posted Aug 1, 2015

In LTE FDD, one radio frame consists of ten sub frames and each sub frame is 1ms long. Sub frames 0 and 5 are used to transmit Primary Synchronization Signal and Secondary Synchronization Signal. Apart from PSS and SSS, Sub frame 0 is also used to transmit broadcast information.

RRC Paging procedure is used to page RRC-IDLE mode UEs. UEs in rrc - idle mode, use DRX. After each drx cycle, UE wakes up and look for paging message. There are two terms (paging radio frame and paging occasion) related to RRC paging. Paging radio frame tells which specific radio frames will carry paging messages and paging occasions will inform what are all specific sub frames of that radio frame carries paging message.

No. of paging occasions are depend on value of nB which is transmitted within the SIB2. Formula is # of paging occasions = max (1 , nb/T) . The possible values of nb are (fourT, twoT, oneT, halfT, quarterT, oneEighthT, oneSixteenthT, oneThirtySecondT). By using the formula possible sub frames come out (1 , 2 or 4).

If # of paging occasion = 1 then sub frame 9 of paging frame will carry RRC paging message.
If # of paging occasion = 2 then sub frames 4 and 9 shall be used for RRC paging message.
if # of paging occasion = 4 then sub frames 0, 4, 5 and 9 shall be used for RRC paging message.

Now coming to your question, which sub frames can not be used for MBMS services ?
Answer : Sub frames 0, 4 , 5 and 9.

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I came to know total six subframes of a radio frame can be used for MBSFN transmission. How UE come to know about exact number of sub frames that are being used in a radio frame ? Does the eNodeB send this information to UE or UE itself figure out ?