BLER (Block Error Rate) in LTE

Block Error Rate (BLER) is a critical performance metric used in LTE to evaluate the reliability of data transmission over the radio interface. It is defined as the ratio between the number of transport blocks (TBs) that fail error checking (usually due to failed CRC) and the total number of transport blocks transmitted:

A transport block (TB) is the basic data unit exchanged between the physical layer (PHY) and MAC layer, and it typically contains a group of bits that are encoded, modulated, and transmitted together.

LTE uses Hybrid Automatic Repeat Request (HARQ) and adaptive modulation and coding (AMC) schemes, which operate at the transport block level, not at the bit level.

• Each TB undergoes channel coding (e.g., Turbo coding), rate matching, modulation, and CRC attachment before being transmitted over the air.
• At the receiver, the CRC is used to verify if the block was received without error.
• If the CRC check fails, the entire block is considered erroneous—even if only one bit is incorrect.

Thus, BLER reflects the real-world decoding success of these transport blocks and is more meaningful than BER for evaluating LTE performance.

BLER is monitored before HARQ—meaning, if a TB fails CRC, it contributes to the BLER metric. HARQ retransmissions are then triggered to recover the block. After a few retransmissions, if the block is successfully decoded, it still counts as one initial BLER error, even if eventually corrected.

Importance of BLER in LTE Performance

• Indicates Radio Link Quality

BLER reflects how reliably transport blocks are received. A high BLER means poor signal conditions, interference, or aggressive MCS, signalling degraded link quality.

• Guides MCS Selection

LTE dynamically adjusts the modulation and coding scheme (MCS) to maintain a target BLER of ~10%. High BLER triggers a lower MCS; low BLER allows a higher MCS to boost throughput.

• Impacts Throughput and QoS

Each block error leads to retransmissions, increasing delay and reducing throughput. Persistent high BLER affects latency-sensitive services like VoLTE and video streaming.

• Drives HARQ Retransmissions

When CRC fails, HARQ kicks in. High BLER leads to more HARQ attempts, consuming resources and reducing system efficiency.

BER vs FER vs BLER with SINR

In digital communication systems, three commonly used error metrics are BER, FER, and BLER, each representing errors at different data levels.

• Bit Error Rate (BER) refers to the probability that a single bit is received incorrectly, making it a fine-grained measure often used in laboratory or physical-layer simulations to assess the impact of noise or fading on individual bits.
• Frame Error Rate (FER) extends this concept to larger data units, representing the probability that an entire frame—which contains multiple bits—is received with at least one error. It’s particularly useful in scenarios involving packet-based transmissions.
• Block Error Rate (BLER) is the most relevant metric in LTE systems, as it denotes the probability that a transport block (TB), which is the basic unit of data exchanged between the PHY and MAC layers, fails the CRC check. BLER is directly tied to HARQ retransmissions and is a key performance indicator for assessing link quality, link adaptation, and throughput in real-world LTE deployments.

Points to consider

• As SNR increases, all error rates drop—but at different rates.
• BER drops fastest (bit-level error more sensitive to noise).
• FER and BLER drop slower because they depend on accumulated bit errors across larger data units.
• BLER is typically higher than BER for the same SNR, because even one bit error can cause the whole block to fail CRC.

Factors Affecting BLER in LTE

• SINR Degradation-When Signal-to-Interference-plus-Noise Ratio drops, the receiver struggles to differentiate signal from noise, leading to higher BLER even at moderate SNR levels.

• Interference (Co-channel/Adjacent Channel)-Interference from neighbouring cells or overlapping frequencies causes corruption of transport blocks, pushing BLER upward despite sufficient signal strength.

• Modulation Scheme (e.g., 64QAM vs QPSK)-Higher-order modulations like 64QAM are-more sensitive to noise and errors—BLER rises sharply if the radio conditions aren’t good enough to support them.

• Poor Coverage or Fading-At the edge of the cell or in shadow zones (buildings, terrain), weak or fluctuating signals lead to frequent CRC failures, raising BLER.

• Incorrect MCS Assignment-If the selected MCS is too aggressive for the actual channel quality, decoding fails often—BLER shoots up due to poor adaptation.

BLER Thresholds in LTE

• CQI-based MCS Selection (10%)-LTE targets a 10% BLER when selecting the MCS to ensure efficient use of radio resources with acceptable decoding reliability.

• Target DL BLER (≤10%)-The network aims to keep downlink BLER below 10% to balance throughput and retransmission overhead before HARQ is applied.

• After HARQ (≤1%)-post-HARQ, the effective BLER should ideally fall below 1%, ensuring nearly all transport blocks are correctly decoded after retries.

Troubleshooting BLER Issues

Check DL Power Allocation
Ensure that the downlink power per user is sufficient. Low power can result in weak signal strength, leading to high BLER.

Example:

• Total cell DL power = 40 dBm
• 10 UEs scheduled
• Power per UE ≈ 40 dBm – 10 dB = 30 dBm
• If a cell edge UE is assigned only 25 dBm → lower SNR → BLER increases.

How to fix:

• Prioritize power boosts to low-SINR UEs.
• Use power headroom reports and adjust PDSCH power allocation via link adaptation.

Validate MIMO Gain
Check if MIMO (e.g., 2×2, 4×4) is providing real spatial gain. Improper precoding, low correlation, or bad antenna calibration can nullify the benefit, leading to high BLER.

Example:

• UE capable of 4×4 MIMO
• Reported RI (Rank Indicator) = 1
• CQI = 7 (expected CQI ≥ 10 with RI = 2 or 3)→ Indicates MIMO gain not realized.

How to fix:

• Check physical antenna paths for isolation.
• Re-tune precoding matrix via PMI optimization.
• Investigate beamforming settings (especially for TM4/9).

Analyse SINR Trend
Low SINR causes decoding difficulty, which leads to increased BLER. Track SINR over time to identify issues like fast fading, noise rise, or interference.

Example:

• Normal SINR = 12 dB → BLER ~5%
• Current SINR = 4 dB → BLER observed = 20%

With a target BLER of 10%, a CQI mapped for 12 dB might choose 64QAM.
At 4 dB, 64QAM will result in high TB failure → HARQ → throughput drops.

How to fix:

• Adjust MCS via better CQI calibration.
• Improve RSRQ by managing interference.
• Re-assign PRBs or apply TPC (Transmit Power Control).

Check HARQ Stats (Failure Ratio)
High HARQ failure rates indicate that even after multiple retransmissions, blocks are failing—signifying either persistent poor link or wrong RV/MCS selection.

Example:

• Total HARQ attempts = 1000
• Failures = 150→ HARQ Failure Rate = 150 / 1000 = 15% (too high)

Expected Range:

• HARQ failure rate should be < 5% under healthy conditions.

How to fix:

• Tune RV usage (ensure RV = 0 → 2 → 3 → 1).
• Re-calibrate CQI-to-MCS mapping.
• Extend HARQ timing window if possible.

Monitor Neighbour Interference and PCI Planning
PCI collisions/confusions or poor neighbour cell planning can introduce co-channel interference, elevating BLER.

Example:

• Serving PCI = 10
• Strong neighbour PCI = 10 (collision)
• UE cannot differentiate → signal degradation → BLER

How to fix:

• Perform PCI audit: no neighbour PCI should match the serving cell within interference range.
• Use eICIC/ICIC to manage co-channel interference (especially in small cell deployments).
• Adjust neighbor relation tables (ANR).

BLER vs CQI in LTE

CQI reflects the UE’s estimate of downlink channel conditions. It is used by the eNodeB to select a suitable MCS that balances throughput and reliability.

As CQI increases, it indicates better radio conditions, allowing higher MCS levels and resulting in a lower BLER.

LTE is designed to select MCS that achieves a target BLER of ~10%, using CQI as the key input.

• Low CQI (1–4) → High BLER (QPSK, weak signal)
• Mid CQI (5–9) → BLER approaches 10% (16QAM possible)
• High CQI (10–15) → Low BLER (<10%, supports 64QAM)

BLER Improvement with HARQ Retransmissions in LTE

BLER decreases with each HARQ retransmission, demonstrating LTE’s built-in mechanism for ensuring reliable communication.

LTE’s HARQ strategy ensures that even if the first attempt fails, subsequent transmissions add new, meaningful data—allowing the UE to eventually decode the block successfully. This approach balances robustness and efficiency without excessively burdening the system.

• Initial Transmission-BLER starts high (e.g., 18%) due to poor radio conditions or use of a high MCS that’s too aggressive for the current channel quality.
• 1st HARQ Retransmission-New redundancy bits (RV = 2) are sent, and BLER drops significantly, showing improved decoding capability.
• 2nd and 3rd Retransmissions-More unique redundancy is added (RV = 3, then RV = 1), allowing the receiver to piece together the correct data—BLER continues to decrease.
• Final Outcome-Ideally, BLER drops below 1%, demonstrating the success of LTE’s incremental redundancy HARQ process.

References

• 3GPP TS 36.213 – Physical Layer Procedures
• 3GPP TS 36.321 – MAC Layer Specifications
• 3GPP TS 36.331 – RRC Protocol Specification
• Erik Dahlman et al. – 4G: LTE/LTE-Advanced for Mobile Broadband
• Keysight Technologies – LTE Test and Measurement
• NPTEL Course (IIT Roorkee) – 4G/5G Cellular Communication