Block Error Rate (BLER)

Block error rate (BLER) is defined as the ratio of the number of blocks received with errors to the total number of blocks transmitted. An erroneous block is defined as a carrier block with an incorrect cyclic redundancy check (CRC).

This calculation is based on a CRC (Cyclic Redundancy Check) evaluation, which is used to check carrier blocks at the receiving end. A CRC is added to each carrier block and sent by the transmitter. At the destination, the carrier blocks are cross-checked by the receiver. If the attached CRC matches the CRC calculated by the receiver, the carrier block is successfully decoded.

CRC is a technique used to detect carrier block errors.

BLER = Number of received blocks with errors / Total number of received blocks

Or, BLER = EB / TB

Where, BLER = Block Error Rate, EB = Erroneous Blocks, TB = Total Blocks

If the computation yields an undesirable result, the receiver returns a HARQ NACK message, requesting retransmission. On the radio side, a typical BLER goal is 10%. This means that the receiver should get at least 90% successful transmissions. If the target is less than 10%, another retransmission may be required, potentially consuming radio resources.

There are two types of BLER found in many wireless radio measurements: iBLERs and rBLERs.

Initial BLER (iBLER): 

If the eNB sends data to the UE and the UE cannot decode it, it will send a HARQ NACK to the eNB. A NACK means the eNB should resend the data; this NACK is considered an iBLER, or initial block error. 

Residual BLER (rBLER): 

If the UE still fails to decode the data after retransmission, the UE should send another NACK and the eNB should retransmit again. However, there are limits to these retransmissions, which are usually configurable. Normally, these retransmissions are set to 4, and after 4 retransmissions, the eNB does not retransmit at the HARQ level and considers this as a rBLER, or residual block error.

BLOCK Error Rate Measurement