LTE : Why does MIB transmitted over PBCH and SIBs at PDSCH ?

Question

I mean to say, why does MIB transmit only on sub frame 0 not on other sub frames of same radio frame ?

Answer 1

MIB carries very limited and important information. PBCH physical channel occupies middle 6 RBs of deployed bandwidth and MIB information can be transmitted through these 6 RBs. Unlike MIB, system information blocks (i.e. SIBs) contains a lot of information which need more resource as compare to transmit master information block. Due to huge information presents inside SIBs, these are transmitted on PDSCH channel.

Hopefully, I answered your query.

Answer 2

Take a look at http://www.sharetechnote.com/html/MAC_LTE.html , it reproduces figure 4.5.3.2-1 from the MAC Spec, 36.321.

I think your question may lie in the fact that SIB information is considered broadcast information at higher layers, and it makes logical sense to consider it such.

From the diagram, we can see that the (logical) broadcast control channel (BCCH) at the MAC layer is sent partially through the physical broadcast channel (PBCH) and partially through the physical downlink shared channel (PDSCH). What ends up in PBSCH is the MIB and on PDSCH are the SIBs.

That doesn't make any less "broadcast" information though....

Hope that helps.

Jeff

Answer 3

SIB is the information which is more UE- specific sent to a particular UE or a set of UEs(such as PLMN, TAC, power control etc), therefore it has to be sent over PDSCH.
MIB is the info that is more cell-specific,it is the info that UE gets first after achieving DL synch(using PSS and SSS). It contains the system bandwidth,SFN for the timing reference,antenna info,PHICH info.

Most importantly, the UE does not know the carrier (Resource Blocks) space unless or until it knows the System Bandwidth to decode the PDCCH/PDSCH. This info is in the MIB. Therefore once the UE reads it, it figure out the PDCCH/PDSCH, resource block space and all other info.

Comments

Hi Chandra, I am bit confused with your statement "SIB is the information which is more UE- specific sent to a particular UE". What do you mean by this statement ?

---

Sorry for the confusion..got carried away with some data in the net. :)
I stand corrected. Basically, as UE wont be able to decode the PDCCH/PDSCH, MIB will be sent in PBCH. As PBCH occupies only 6 RBs(irrespective of system bw) and SIBs contain a lot of info(as you said)..it is carried over PDSCH.

Answer 4

The MIB carries the most essential system information:
System bandwidth,
System Frame Number (SFN),
Physical Hybrid Automatic Repeat Request (HARQ) Indicator Channel (PHICH) Configuration.
PBCH physical channel occupies middle 6 RBs of deployed bandwidth and MIB information can be transmitted through these 6 RBs same as PSS & SSS.Its a very limited space so on this we are sending most essential information on it.
After decoding PBCH UE will decode PCFICH and find the value of CFI and on the basis of that UE will know how many symbol are allocated for Control channel and after decoding PDCCH , UE will know the DCI , in DCI there is RIV value , from RIV , UE will find the RB START and Length .By this UE will decode PDCCH and then it will find SIB 1 ,other SIB and other information .

Comments

Hi Sandeep, as you told "After decoding PBCH UE will decode PCFICH and find the value of CFI". So are you sure about this statement because what i think is that MIB(PBCH) does not carry any information about the PCFICH, it carries only PHICH parameters information or are you talking about only N_rb_dl because this is used as constraint for CFI calculation which is decoded from MIB.
Or is there any other parameter used by PCFICH which is decoded from MIB.