MIB or Master Information Block is
a very important message or information that is broadcasted by the LTE eNodeB
irrespective of any users presence. The MIB is first among the other system
information blocks or SIB, which are also broadcasted by the eNodeB.
MIB is special signal that carries the following
information. As you see, you can get the System Bandwidth and SFN by decoding
MIB.
i) DL Bandwidth, Number of Transmit Antenna, Reference
Signal Transmit Power
ii) System Frame Number (SFN)
iii) PHICH Configuration
iv) Transmit every 40 ms , repeat every 10 ms
MasterInformationBlock ::= SEQUENCE {
dl-Bandwidth ENUMERATED { n6, n15, n25, n50, n75, n100},
phich-Config PHICH-Config,
systemFrameNumber BIT STRING (SIZE (8)),
spare BIT STRING (SIZE (10))
}
The MIB is transmitted
using a physical layer channel called PBCH or Physical Broadcast Channel on
downlink. The MIB is a 24 bit information with following information within,
§
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
§
10 bits are reserved for future use
§
Apart from the information in the payload, the MIB CRC also
conveys the number of transmit antennas used by the eNodeB. The MIB CRC is
scrambled or XORed with a antenna specific mask.
The MIB has two perspectives,
1. Generation periodicity – The generation
periodicity is the duration between two consecutive MIB information generated
by the higher layers. This periodicity is 40 milliseconds and the physical
layer receives a new MIB for encoding every 40 milliseconds. The system frame
number within each MIB will be a rolling number between 0-255 and keeps
changing every MIB but other contents may or may not change.
2. Transmission periodicity – The transmission
periodicity is the duration between two consecutive PBCH transmission by the
physical layer. The physical layer transmits the PBCH for every 10
milliseconds. Although the contents within 4 consecutive PBCH will remain same
as the PBCH carries MIB and the MIB can change only after 40 milliseconds since
the first PBCH transmission.
PBCH Transmission
The MIB payload worth 24 bit is
received from higher layer to physical layer and the physical layer will follow
below processes,
1.
CRC Generation – where a 16 bit CRC is
generated by the CRC module and it is scrambled with a antenna specific mask
2.
CRC attachment to MIB – The generated CRC is
attached to the MIB payload after which the size of the payload will be 40 bits
(24 bit of MIB + 16 bit of CRC)
3.
Convolution encoding – A tail bit convolution
encoding is performed over the 40 bits and the output is 3 streams of 40 bits
each
4.
Rate matching – The rate matching here
is nothing but a repetition coding, where the 3 streams of size 120 bits (40x3bits)
is just repeated 16 times to get 1920 bits. The repetition rate is very high
since the MIB is a very vital information and the UE cannot afford to lose it.
5.
Scrambling – These 1920 bits are scrambled with a scrambling sequence
as long as 1920 bits
6.
Modulation (QPSK) – A QPSK modulation is performed over
these 1920 bits to obtain 960 complex QPSK symbols
This is the basic operation for
PBCH encoding, but since the PBCH has to be transmitted every 10 milliseconds
on subframe 0 of all radio frames, the PBCH modulation buffer is divided into 4
sub-buffers each as long as 240 complex symbols. Each sub-buffer is
transmitted in PBCH and by the time the last sub buffer is transmitted on PBCH,
a new MIB is arrived from higher layer which is again encoded using the above
method and the process continues.
MIB Transmission on PBCH
As you see from the
above diagram, a new MIB is generated by higher layers whenever the Radio frame
number satisfies the condition (Radioframe Number Modulo 4 is zero)
RF%4=0. The grey boxes in the diagram are the first subframe of each radio
frame or subframe-0. Also you know that PBCH output is 960 complex symbols long
and that is divided into 4 parts and each parts are transmitted in consecutive
radioframes, which can also be seen the diagram, where the first 240 bits goes
in RF-0, the next 240 bits in RF-1 and so on. Just to note, each individual
PBCH are independently decodable, what I mean here is, if the UE finds the PBCH
in RF-0 he can still decode the all contents of MIB without waiting for the
parts of the PBCH in coming in next radioframes.
Significance of MIB for
UE
MIB is a very crucial
information that a UE need to acquire before it can start any transaction with
the eNodeB because it know all the vital information in the MIB as explained
above and also it will come to know the 40 millisecond boundary from the
decoded MIB. The System frame number transmitted within the PBCH will remain
same for 4 radio frames, because of the same fact that 4 consecutive radio
frames are transmitted the same MIB information. Now for a UE to know the 40
millisecond boundary, all it has to find is 4 consecutive system frame
numbers that are same. Now from the same diagram, RF-0 to RF-3 will have same
system frame number within.
So for the best case, if the UE
starts decoding the MIB from RF-0 it will see same system frame number for 4
consecutive radio frames and it will understand that RF-0 is the 40 millisecond
boundary. For the worst case, if UE decodes RF-1 first then he will see same
system frame numbers in RF-1, RF-2, RF-3 and a different one in RF-4 so he
continues decoding the PBCH till RF-5, RF-6 & RF-7 and he sees a same
system frame number from RF-4 to RF-7 and hence decides that RF-4 is the 40
millisecond boundary.
So from system point of view, a
UE can decode the MIB & 40 millisecond boundary in 40 milliseconds for the
best case and 70 milliseconds for the worst case, considering the latency start
count from the time RF-0 was transmitted.
