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.
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))?
ReplyDeleteHi, For your above question:
ReplyDeleteEach 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
The MIB is transmitted every 40ms and not 10 ms. Let me know if i am wrong.
ReplyDeleteExample: UE reads MIB at SFN = 257, and MIB contains following:
Delete3 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...