In LTE,
MIB, SIB1, SIB2 is mandated to be transmitted for any cells. Since many of the
SIB are transmitted, it should be transmitted in such a way that the location
(subframe) where a SIB is transmitted should not be the same subframe where
another SIB is transmitted.
Overall SIB Scheduling concept is as follows. As you see
i) MIB is
transmitted at a fixed cycles (every 4 frames starting from SFN 0)
ii) SIB1
is also transmitted at the fixed cycles (every 8 frames starting from SFN 0).
iii) All
other SIB are being transmitted at the cycles specified by SIB scheduling
information elements in SIB1
You may
notice that LTE SIB1 is very similar to WCDMA MIB.
Especially at initial test case development, you have to be very
careful about item v). If you set this value incorrectly, all the other SIBs
will not be decoded by UE. And as a result, UE would not recognize the cell and
show "No Service" message.
According to 36.331 section 5.2.1.2, the MIB scheduling is as
follows :
The MIB
uses a fixed schedule with a periodicity of 40 ms and repetitions made within
40 ms. The first transmission of the MIB is scheduled in subframe #0 of radio
frames for which the SFN mod 4 = 0, and repetitions are scheduled in subframe
#0 of all other radio frames.
According to 36.331 section 6.2.2 Message definitions -
MasterInformationBlock field descriptions, the System Frame Number in MIB is
specified as follows :
Defines
the 8 most significant bits of the SFN. As indicated in TS 36.211 [21, 6.6.1],
the 2 least significant bits of the SFN are acquired implicitly in the P-BCH
decoding, i.e. timing of 40ms P-BCH TTI indicates 2 least significant
bits(within 40ms P-BCH TTI, the first radio frame: 00, the second radio frame:
01, the third radio frame: 10, the last radio frame: 11). One value applies for
all serving cells (the associated functionality is common i.e. not performed
independently for each cell).
According to 36.331 section 5.2.1.2, the SIB1 scheduling is as
follows :
The
SystemInformationBlockType1 uses a fixed schedule with a periodicity of 80 ms
and repetitions made within 80 ms.The first transmission of
SystemInformationBlockType1 is scheduled in subframe #5 of radio frames for
which the SFNmod 8 = 0, and repetitions are scheduled in subframe #5 of all
other radio frames for which SFN mod 2 = 0.
This means that even though SIB1 periodicity is 80 ms, different
copies (Redudancy version : RV) of the SIB1 is transmitted every 20ms. Meaning
that at L3 you will see the SIB1 every 80 ms, but at PHY layer you will see it
every 20ms. For the detailed RV assignment for each transmission, refer to
36.321 section 5.3.1 (the last part of the section)
The transmission cycles for other SIBs are determined by
schedulingInfoList in SIB1 as shown in the following example (This example is
the case where SIB2 and 3 are being transmitted).
+-schedulingInfoList
::= SEQUENCE OF SIZE(1..maxSI-Message[32]) [2]
| +-SchedulingInfo
::= SEQUENCE
| |
+-si-Periodicity ::= ENUMERATED [rf16]
| |
+-sib-MappingInfo ::= SEQUENCE OF SIZE(0..maxSIB-1[31]) [0]
| +-SchedulingInfo
::= SEQUENCE
|
+-si-Periodicity ::= ENUMERATED [rf32]
|
+-sib-MappingInfo ::= SEQUENCE OF SIZE(0..maxSIB-1[31]) [1]
|
+-SIB-Type ::= ENUMERATED [sibType3]
+-tdd-Config ::=
SEQUENCE OPTIONAL:Omit
+-si-WindowLength
::= ENUMERATED [ms20]
One thing you would notice that sib-MappingInfo IE in the first
node is not specified, but the first entity of schedulingInfoList should always
be for SIB2 as specified in the 36.331 as follows (See 36.331
SystemInformationBlockType1 field description).
List of the SIBs mapped to this SystemInformation message.There is
no mapping information of SIB2; it is always
present in the first SystemInformation message listed in the
schedulingInfoList list.
Understanding overall cycle in the unit of Subframe number is
pretty straightforward to understand. But understanding exactly at which
subframe a SIB should be transmitted is not that straightforward as you might
think. It is related to 'si-WindowLength'. si-WindowLength tells that a SIB
should be transmitted somewhere within the window length starting at the SFN
specified by si-Periodicity. But this parameter does not specify the exact
subframe number for the transmission.
The subframe for a specific SIB transmission is determined by a
algorithm defined in 36.331 5.2.3 Acquisition of an SI message as follows.
When acquiring an SI message, the UE shall:
1> determine the start of the SI-window for the concerned SI
message as follows:
2> for the concerned SI message,
determine the number n which corresponds to the order of entry
in the list
of SI messages configured by schedulingInfoList in SystemInformationBlockType1;
2> determine the integer value x = (n –
1)*w, where w is the si-WindowLength;
2> the SI-window starts at the subframe
#a, where a = x mod 10, in the radio frame for which SFN mod T =
FLOOR(x/10), where T is the si-Periodicity of the concerned SI
message;
NOTE: E-UTRAN should configure an SI-window of 1 ms only if all
SIs are scheduled before subframe #5 in
radio frames for which SFN mod 2 = 0.
1> receive DL-SCH using the SI-RNTI from the start of the
SI-window and continue until the end of the SI-window
whose absolute length in time is given by si-WindowLength, or
until the SI message was received, excluding the
following subframes:
2> subframe #5 in radio frames
for which SFN mod 2 = 0;
2> any MBSFN subframes;
2> any uplink subframes in TDD;
1> if the SI message was not received by the end of the
SI-window, repeat reception at the next SI-window occasion
for the concerned SI message;
