Wednesday 8 April 2015

Master Information Block

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.

Monday 10 November 2014

LTE Frame Structure

The frame structures for LTE differ between the Time Division Duplex, TDD and the Frequency Division Duplex, FDD modes as there are different requirements on segregating the transmitted data.
There are two types of LTE frame structure:
  1. Type 1:   used for the LTE FDD mode systems.
  2. Type 2:   used for the LTE TDD systems.
Type 1 LTE Frame Structure
The basic type 1 LTE frame has an overall length of 10 ms. This is then divided into a total of 20 individual slots. LTE Subframes then consist of two slots - in other words there are ten LTE subframes within a frame.

Type 1 LTE Frame Structure

The duration of one LTE radio frame is 10 ms. One frame is divided into 10 subframes of 1 ms each, and each subframe is divided into two slots of 0.5 ms each. Each slot contains either six or seven OFDM symbols, depending on the Cyclic Prefix (CP) length. The useful symbol time is 1/15 kHz= 66.6 mircosec. Since normal CP is about 4.69 microsec long, seven OFDM symbols can be placed in the 0.5-ms slot as each symbol occupies (66.6 + 4.69) = 71.29 microseconds. When extended CP (=16.67 microsec) is used the total OFDM symbol time is (66.6 + 16.67) = 83.27 microseconds. Six OFDM symbols can then be placed in the 0.5-ms slot. Frames are useful to send system information. Subframes facilitate resource allocation and slots are useful for synchronization. Frequency hopping is possible at the subframe and slot levels.
Type 2 LTE Frame Structure

Type 2 LTE Frame Structure
(shown for 5ms switch point periodicity).
The subframes may be divided into standard subframes of special subframes. The special subframes consist of three fields;
  • DwPTS - Downlink Pilot Time Slot
  • GP - Guard Period
  • UpPTS - Uplink Pilot Time Stot.
These three fields are also used within TD-SCDMA and they have been carried over into LTE TDD (TD-LTE) and thereby help the upgrade path. The fields are individually configurable in terms of length, although the total length of all three together must be 1ms.
Frame structure Type 2 is applicable to TDD is as shown in the figure. Each radio frame of 10 ms in length consists of two half-frames of 5 ms in length. Each half-frame consists of eight slots of the length Ts=5 ms and three special fields DwPTS, GP, and UpPTS of 1 ms in length.
Different configurations, numbered zero to six, are defined in the standard for the subframe number allocated for the uplink and downlink transmission. Subframe 1 in all configurations and subframe 6 in configurations 0, 1, 2 and 6 consist of DwPTS, GP and UpPTS. All other subframes are defined as two slots.
Switch-point periodicities of 5 ms and 10 ms are supported. The standard defines the table for the uplink and downlink allocations for switch-point periodicity. In the case of a 5-ms switch-point periodicity, UpPTS and subframes 2 and 7 are reserved for uplink transmission.
In the case of a 10-ms switch-point periodicity, UpPTS and subframe 2 are reserved for uplink transmission and subframes 7 to 9 are reserved for downlink transmission.
Subframe 0 and 5 are always for the DL. The subframe following the special SF is always for the UL. The DwPTS field carries synchronization and user data as well as the downlink control channel for transmitting scheduling and control information. The UpPTS field is used for transmitting the PRACH and the Sounding Reference Signal (SRS).
LTE TDD / TD-LTE subframe allocations
One of the advantages of using LTE TDD is that it is possible to dynamically change the up and downlink balance and characteristics to meet the load conditions. In order that this can be achieved in an ordered fashion, a number of standard configurations have been set within the LTE standards.
A total of seven up / downlink configurations have been set, and these use either 5 ms or 10 ms switch periodicities. In the case of the 5ms switch point periodicity, a special subframe exists in both half frames. In the case of the 10 ms periodicity, the special subframe exists in the first half frame only. It can be seen from the table below that the subframes 0 and 5 as well as DwPTS are always reserved for the downlink. It can also be seen that UpPTS and the subframe immediately following the special subframe are always reserved for the uplink transmission.
UPLINK-DOWNLINK CONFIGURATION
DOWNLINK TO UPLINK SWITCH PERIODICITY
SUBFRAME NUMBER


0
1
2
3
4
5
6
7
8
9
0
5 ms
D
S
U
U
U
D
S
U
U
U
1
5 ms
D
S
U
U
D
D
S
U
U
D
2
5 ms
D
S
U
D
D
D
S
U
D
D
3
10 ms
D
S
U
U
U
D
D
D
D
D
4
10 ms
D
S
U
U
D
D
D
D
D
D
5
10 ms
D
S
U
D
D
D
D
D
D
D
6
5 ms
D
S
U
U
U
D
S
U
U
D
Where:
    D is a subframe for downlink transmission
    S is a "special" subframe used for a guard time
    U is a subframe for uplink transmission
Uplink / Downlink subframe configurations for LTE TDD (TD-LTE)