BER counts individual bit errors. BLER counts entire transport block failures (a block fails if any bit is wrong after FEC decoding). In LTE/5G NR, a 10% BLER target means 1 in 10 transport blocks fails initial decoding and requires HARQ retransmission. After HARQ, residual BLER drops to less than 0.1%.

Operating at 10% initial BLER maximizes spectral efficiency. The MCS is set aggressively so that most blocks decode correctly but some fail. HARQ retransmission recovers failures with minimal delay (8 ms round-trip in LTE). Operating at lower BLER (e.g., 1%) would require more conservative MCS, wasting capacity. For URLLC, the target drops to 0.001% (10^-5) to meet 1 ms latency.

How is BLER measured?

The UE reports CQI (Channel Quality Indicator) based on estimated SINR. The eNB/gNB selects MCS to target the BLER operating point. BLER is measured by counting HARQ NACK/ACK ratios. CQI reports every 1-5 ms enable rapid link adaptation.

Digital Communications

BLER

BLER (Block Error Rate) is the ratio of errored transport blocks to total transmitted transport blocks, the primary link quality metric for LTE and 5G NR. A transport block fails if any bit remains incorrect after FEC (LDPC/Turbo) decoding. The network targets ~10% initial BLER for eMBB (maximizing spectral efficiency with HARQ recovery) and 10⁻⁵ for URLLC (ultra-reliable low-latency communication).

Category: Digital Communications

eMBB Target: 10% initial

URLLC Target: 10⁻⁵

Understanding BLER

In LTE/5G NR, data is transmitted in transport blocks. Each block is encoded with CRC and LDPC (5G) or Turbo (LTE) FEC. The receiver decodes the block and checks CRC. If CRC fails, the block is declared in error (NACK sent via HARQ). The transmitter retransmits, and the receiver combines both attempts (incremental redundancy or chase combining). After HARQ, residual BLER drops to <0.1%.

The eNB/gNB uses CQI feedback from the UE to select the MCS (Modulation and Coding Scheme) that achieves the target BLER. Too aggressive MCS causes high BLER and retransmission overhead; too conservative wastes spectral efficiency.

BLER Definition

BLER = N_{errored_blocks} / N_{total_blocks}

After HARQ (max 4 retx in LTE):
Residual BLER < BLER_initial^N_retx+1
10% initial, 3 retx: <10⁻⁴

BLER Targets by Service

Service	Initial BLER	After HARQ	Latency
eMBB	10%	<0.1%	~4 ms
VoNR	1%	<0.01%	~10 ms
URLLC	10⁻⁵	10⁻⁶	<1 ms

Common Questions

Frequently Asked Questions

BLER vs BER?

BER counts bit errors. BLER counts block failures. 10% BLER = 1 in 10 blocks fails initial decode. HARQ retransmission drops residual to <0.1%.

Why 10%?

Maximizes spectral efficiency. Aggressive MCS + HARQ recovery is more efficient than conservative MCS at 1% BLER. URLLC uses 10⁻⁵ for reliability.

How measured?

UE reports CQI based on SINR. Network selects MCS for target BLER. Measured via HARQ ACK/NACK ratios. CQI updated every 1-5 ms.

Related Terms

← BLE Throughput Blind Estimation →

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