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).
- 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.
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
- 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.
- 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 |
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 |
- 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.
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