Tuesday, July 17, 2012

Digging into LTE MIB

MIB will be read after UE does the Primay and Secondary synchronization.

MIB transmits for every 10 ms in subframe number ‘0’ and slot 1’s  First 4 OFDM Symbols. 
But new Retransmissions happen where SFN%4 == 0 and in the rest of places it is retransmission.
i.e in given SFN n where (n%4 == 0), same MIB content will be sent in n, n+1, n+2 and n+3 System Frames.

What MIB has  w.r.to RRC Spec :  SystemFrameNumber (8 Bits), PHICH Configuration and DL Bandwidth

PHICH Configuration is required in MIB, this helps UE to caliculate PDCCH symbols to find locate CCE of that UE (Because next step after MIB reading is reading SIB1 reading, to read SIB1 UE should be knowing PDCCH info).

SystemFrameNumber (Except 2 LSB Bits i.e 8 MSB Bits) to find SFN. Being it is only 8 bits worst case UE can read 5 radio frames to get exact SFN  number.

DLBandWidth : Based on this , UE will know the number of RBs and number of CCEs , with which Common  Search Space will be derived. This helps in cornering the CCE related to SIB1 (which is next step).

But as RRC Says, that’s all the information does MIB conveys. 
No, it also gives NumberOfTXAntennas also. Number of DL antenna Ports (not physical antennae). Each antenna port uses one cell specific reference signal, that is used by UE for channel estimation.

MIB Transmission happens as below:
MIB content given by MAC is 24 Bits .
With the 24 Bits + 16 CRC bits are added = 40 Bits
Code Rate applicable = 1/48 which means 40 * 48  = 1920
As the same MIB content is transmissted in 4 Subsequence SystemFrames 1920/4 = 480 bits will be transmitted in each RadioFrame. 
How this 480 ?
MIB Transmission is : In 6 ResourceBlocks , 4 OFDM Symbols of Slot1 of Subframe 0
i.e Each RB 12*4 = 48 Res – 8 Res (towards Referece Signals) = 40 REs are left for MIB content per RB
for 6 RBs = 40 * 6 = 240 Res
Being MIB is transmitted with QPSK , 1 RE can carry 2 bits of Data
i.e 240 * 2 = 480
 Note: 6 RBs correspond to the minimum DL BW supported by LTE, which UE assumes when it first receives BCH. After decoding MIB it comes to know of the actual DL BW used by the cell and will adjust its receiver BW accordingly                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
Next How UE understand TX-Antennas Ports (as that is not part of MIB content) :
MIB has CRC which gets scrambled with one of 3 sequences which represents (One, Two OR Four Anttenas). UE when it received it tries to decode with all three based on CRC check UE understands what is TXAntennas.

PBCH can use single antenna or transmit diversity transmission modes. UE blindly detects number of antenna ports used for PBCH using CRC scrambling sequence. From this UE determines number of cell specific reference signals configured in the cell. If cell is configured with 2 antenna ports, SFBC transmit diversity scheme is used for BCH, PCH and L1/L2 Control signalling. If 4 antenna ports are detected, SFBC/FSTD transmit diversity is used for BCH, PCH and L1/L2 control signalling.


  1. Can you please explain the difference between the DL anteena port and Physical Anteena port. As you written in the blog. (Number of DL antenna Ports (not physical antennae))?

  2. Hi, For your above question:

    Each Antenna port is referring to specific Reference Signal. But Physical Antenna Ports are 1,2 or 4. (untill R9).

    But all Antenna Ports gets mapped to Physical Antennas

  3. The MIB is transmitted every 40ms and not 10 ms. Let me know if i am wrong.

    1. Example: UE reads MIB at SFN = 257, and MIB contains following:
      3 bits for system bandwidth
      3 bits for PHICH information,
      1 bit to indicate normal or extended PHICH
      2 bit to indicate the PHICH Ng value
      8 bits for system frame number (8 most significant bits)
      10 bits are reserved for future use
      This very same information is repeated in SFN=258, and SFN=259. Meanwhile, if there is any change in the MIB contents, then it is transmitted in the SFN mod 4 = 0.
      The SFN in LTE is 10 bits long. SFN = 257 in binary =
      SFN 257 = 0100 0000 01
      SFN 258 = 0100 0000 10
      SFN 259 = 0100 0000 11
      The only 8 MSB (most significant bits) are sent in MIB, and these remain same in the MIB transmitted in SFN =257,258,259.
      SFN = 260 = 0100 0001 00, and 260 (SFN) mod 4 = 0
      Now, when UE reads MIB in SFN 260, it notices different SFN bits, and synchronizes to 40ms, and derives the correct SFN.
      Hope this helps...