All about Buffer Status Reporting (BSR)

Buffer Status Reporting (BSR) :

- The Buffer Status reporting procedure is used to provide the serving eNB with information about the amount of data available for transmission in the UL buffers of the UE.

Type Of BSR:

- UL data, for a logical channel which belongs to a LCG, becomes available for transmission in the RLC entity or in the PDCP entity and either the data belongs to a logical channel with higher priority than the priorities of the logical channels which belong to any LCG and for which data is already available for transmission, or there is no data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR".

- UL resources are allocated and number of padding bits is equal to or larger than the size of the Buffer Status Report MAC control element plus its subheader, in which case the BSR is referred below to as "Padding BSR".

- retxBSR-Timer expires and the UE has data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as "Regular BSR"

- periodicBSR-Timer expires, in which case the BSR is referred below to as "Periodic BSR".

When UE will Report Which Type:

- periodicBSR-Timer expires, "Periodic BSR".

For Regular and Periodic BSR:

 If (More than one LCG has data available for transmission in the TTI where the BSR is transmitted)

      Report Long BSR.

 else,

      Report Short BSR.

For Padding BSR:

If (Number of padding bits => size of the Short BSR plus its subheader) && If (Number of padding bits < size of the Long BSR plus its subheader)

Then Short BSR or Truncated BSR Can be transmitted instead of Padding.

If (More than one LCG has data available for transmission in the TTI where the BSR is transmitted): 
Then : 

     Report Truncated BSR of the LCG with the highest priority logical channel with data available for transmission;
Else

     Report Short BSR.

Elseif (Number of padding bits => Size of the Long BSR plus its subheader)         

     Report Long BSR.

All About HARQ

HARQ :

- It is a re-transmission technique used by LTE for re-transmitting of UL & DL Data.
- HARQ (Hybrid ARQ) = ARQ (Automatic Repeat Request) +  FEC (Forward Error Correction).

- The HARQ makes use of ARQ along with an Error Correction technique called 'Soft Combining', which no longer discards the received corrupted data.

- Using 'Soft Combining' data packets that are not decoded are not discarded anymore. The received signal is stored in a 'buffer', and combined with next re transmission.

- Hybrid ARQ (HARQ) leads to higher efficiency in transmission and error correction.
- There is one HARQ entity per UE with 8/16 stop-and-wait processes for each HARQ entity.
- It means Sender will not send new data or re-transmitted data until he will not get ACK or NACK from receiver respectively.
- As Sender is waiting for ACK/NACK from receiver, hence it decreases the through put. To overcome this issue, LTE uses multiple parallel HARQ Process with different process ID.
- FDD-LTE uses 8 HARQ Parallel Process having unique process ID 0, 1, 2...7 (3 bits reserve for HARQ Process ID in DCI Messages).
- In TD-LTE, it uses 16 HARQ Parallel Process ID having unique process ID 0, 1…15(4 bits reserve for HARQ Process ID in DCI Messages).
- Both Incremental redundancy(IR) and Chase combining(CC) are supported.
- The number of HARQ re transmissions targeted by the HARQ protocol depends on the network provided configuration.

FEC (Forward Error Correction) :

FEC or Channel Coding is a technique used for controlling and correcting error in LTE Data transmission.

Channel coding supported for LTE Data Transmission

HARQ with Soft Combining :

In practice, incorrectly received coded data blocks are often stored at the receiver rather than discarded, and when the re-transmitted block is received, the two blocks are combined. This is called Hybrid ARQ with soft combining

Soft Combining Techniques

- IR requires larger receiver buffer than CC but can achieve better performance than CC.
- CC is simple HARQ and requires small receiver buffer.

Chase Combining :

- Every re-transmission = The same information (data and parity bits).
- Receiver uses maximum-ratio combining to combine the received bits with the same bits 
from previous transmissions.
- All transmissions are identical So Chase combining seen as additional repetition coding.
- This scheme achieves gain with small buffer size in a receiver. 
- The buffer size becomes the number of coded symbols of one coded packet

Incremental Redundancy :

- To transmit additional redundant information in each re-transmission and receiver decode on each re-transmission. 
- Every retransmission contains different information than the previous one.
- IR requires larger size of buffer in a receiver than Chase Combining. The buffer size becomes the number of coded bits of total transmitted coded packets.

Redundancy Versions (RV) :

- Different combinations of systematic data bits + FEC bits.
- LTE HARQ has 4 RVs typically of a packet (0,1,2,3).

Difference between LTE HARQ used in UL and DL:

• UL: A synchronous HARQ mode is used.
• DL: An adaptive, asynchronous HARQ.