Difference between BLER for Radio Link Management and BLER in Data throughput?

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?

Residual BLER is when Maxiumum re-transmissions have happened and UE is not able to decode those Transport Blocks then those TB’s are discarded and they will constitute towards Residual BLER. RBLER is usually very low and it is supposed to be less than 0.5%. When the retransmissions happen the network lowers the MCS for the TB’s. Continuing with the above example if the maximum HARQ Retransmissions are set as 4 and after four attempts of re-sending certain TB’s fails CRC decoding at the UE, Then that ratio of erroneous blocks are considered as 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

Radio Link Monitoring in 5G-NR

BLER Target in 5G-NR