What’s the difference between BLER for Radio Link Management and BLER in Data throughput?
A Block Error Ratio(BLER) is defined as the ratio of the number of erroneous blocks received to the total number of blocks sent.
BLER = Number of erroneous blocks / the Total number of blocks received.
The BLER calculation is based on evaluating the CRC on each transport block. An erroneous block is a Transport Block, of which the cyclic redundancy check (CRC) is wrong.
Suppose a UE is receiving 1000 Transport Blocks per second and 200 are Erroneous Block then the BLER is 20%.
- When there are CRC failures on TB, UE sends feedback for that TB by sending a HARQ NACK.
- The first CRC Failure of Transport block CRC failure is considered as Initial BLER.
- Network re-transmits the TB with a lower MCS, if UE fails to decode it then it will send a HARQ NACK again this continues until the network configured max HARQ retransmissions occur and that TB is discarded by the network, This is known as Residual BLER.
- The ratio of BLER is controlled by the network and is part of link adaptation.
- The BLER target is maintained by the IBLER so this means that the network tries to maintain an IBLER of ~10% for each UE.
What is Residual BLER?
Example Transmission :
1000 TB,s transmitted by the gNB over a second
200 TB’s CRC Errors detected at the UE, This constitutes towards 20% Initial BLER
10 TB’s CRC Error detected at the UE from Retransmission of the 200 TB’s which constitutes towards 0.1% of Residual BLER.
So the BLER for RLM is based on reference signal quality monitoring whereas BLER for data throughput is calculated with the ratio of erroneous blocks / the Total number of blocks received.
Further reading