Listen-Before-Talk (LBT)

The physical layer sensing process that determines if the channel is idle or busy. LBT requires the transmitter to sample the energy in the channel for a specific duration (the CCA slot).

• Energy Detect (ED) Threshold: Typically set at -62 dBm to -82 dBm depending on regulatory domain and transmit power.
• Busy Determination: If detected energy exceeds the threshold, the channel is declared occupied.
• Precision: CCA must be fast enough to detect gaps between other transmissions, often requiring detection within a 9 µs slot in Wi-Fi.

When the channel is found busy, LBT defers transmission using an Extended CCA (ECCA) process to avoid immediate collisions after the channel clears.

• Contention Window (CW): A variable size window from which a random backoff counter is drawn.
• Exponential Backoff: The CW doubles after a collision or busy assessment, up to a maximum limit (CWmax), reducing the probability of repeated collisions.
• Deferral: The counter only decrements when the channel is sensed idle, ensuring fairness among competing nodes.

Regulatory standards like ETSI EN 301 893 distinguish between two types of LBT devices:

• Frame-Based Equipment (FBE): Transmits only at fixed, periodic frame boundaries. Performs a single CCA at the start of each frame. If busy, it must remain silent for the entire fixed frame period.
• Load-Based Equipment (LBE): Transmits on-demand. Must perform an ECCA procedure with a random backoff, making it more efficient for bursty, asynchronous data traffic typical of Wi-Fi and LTE-U/LAA.

To prevent a single device from monopolizing the channel, LBT enforces a strict Maximum Channel Occupancy Time.

• Regulatory Limits: ETSI specifies MCOT limits (e.g., 8 ms or 10 ms depending on priority class).
• Transmission Gap: After reaching the MCOT, the device must cease transmission and perform a new CCA/ECCA before resuming.
• Coexistence Guarantee: This hard limit ensures that other LBT-compliant devices, including different radio access technologies (e.g., Wi-Fi vs. NR-U), get a fair chance to access the medium.

To support differentiated Quality of Service (QoS), LBT defines Channel Access Priority Classes with distinct contention parameters.

• Higher Priority: Uses smaller Contention Windows and shorter deferral periods, granting faster channel access for latency-sensitive traffic (e.g., voice, control signaling).
• Lower Priority: Uses larger CWs and longer defer periods, suitable for best-effort data.
• Mapping: In 3GPP standards (LAA/NR-U), four priority classes map directly to specific QoS Class Identifiers (QCIs).

While both 5G NR-U and Wi-Fi use LBT, implementation nuances affect coexistence:

• Wi-Fi (CSMA/CA): Uses a strict exponential backoff with DCF Interframe Space (DIFS) and Short Interframe Space (SIFS) timing.
• 5G NR-U: Employs a flexible LBT framework that can mimic Wi-Fi timing but also supports a Cat-4 LBT (full ECCA) and a Cat-2 LBT (one-shot 25 µs CCA) for rapid channel re-acquisition within a COT.
• Gap Handling: NR-U can initiate a transmission within a 16 µs gap, potentially grabbing the channel faster than Wi-Fi if not carefully calibrated.