Listen-Before-Talk (LBT)

The physical mechanism of LBT, CCA uses energy detection to sample the wireless medium. A device must observe the channel for a specific duration, known as the CCA observation time, typically 20 µs or more. If the detected energy level exceeds a predefined energy detection threshold (e.g., -72 dBm for Wi-Fi), the channel is declared busy, and transmission is deferred. This process relies on signal correlation to distinguish actual Wi-Fi preambles from generic noise.

When a channel is found busy, LBT does not simply wait and retry. It enters an Extended CCA procedure. The device generates a random backoff counter N from a contention window. It then decrements N only when the channel is observed idle for a single CCA slot. Transmission occurs only when N reaches zero. This exponential backoff mechanism is critical for preventing multiple devices from colliding immediately after a long transmission ends.

Once a device gains access, it cannot transmit indefinitely. LBT enforces a maximum Channel Occupancy Time (COT). For example, in 5 GHz unlicensed bands, the maximum COT is typically between 8 ms and 10 ms. After this period, the device must cease transmission and perform a new CCA before it can contend for the medium again. This ensures fairness and prevents a single aggressive transmitter from hogging the channel.

To support differentiated Quality of Service (QoS), LBT defines multiple priority classes. Each class uses a different set of contention parameters:

• Higher priority: Shorter defer periods and smaller contention windows, enabling faster access for latency-sensitive traffic like voice.
• Lower priority: Longer defer periods and larger contention windows, suitable for best-effort data. This is a direct implementation of statistical priority-based access in shared spectrum.

Regulatory standards like ETSI EN 301 893 distinguish two LBT device types:

• Frame-Based Equipment (FBE): Transmits only at fixed, periodic time intervals. It performs a single CCA at the start of each frame. If the channel is idle, it transmits for the entire frame; if busy, it remains silent.
• Load-Based Equipment (LBE): Transmits on demand, using the full ECCA backoff procedure. This is the dominant mode for Wi-Fi and LTE-U/LAA, as it adapts dynamically to traffic load.

LBT is not a single universal standard. Its parameters are legally mandated and vary by region:

• Europe (ETSI): Mandates LBT with specific CCA thresholds and COT limits for 5 GHz and 6 GHz bands.
• Japan: Requires LBT for specific unlicensed bands, with strict carrier sense rules.
• USA (FCC): Historically did not mandate LBT for unlicensed Wi-Fi, relying instead on Duty Cycle limitations, though this is evolving for new shared bands like 6 GHz.

The physical-layer sensing mechanism that executes the LBT protocol. CCA measures the energy detected in the operating channel during a specific observation slot. Energy Detection (ED) thresholds define the boundary between a busy and idle channel. In Wi-Fi, CCA involves both preamble detection (carrier sense) and energy detection, with the latter requiring a higher threshold (typically -62 dBm for 20 MHz channels) to declare the medium busy. The accuracy of CCA directly determines collision probability.

The foundational medium access control (MAC) protocol that implements LBT in Wi-Fi (IEEE 802.11) networks. CSMA/CA uses a distributed coordination function (DCF) with binary exponential backoff. Key mechanisms include:

• DIFS (DCF Interframe Space): Mandatory idle period before initiating backoff.
• Contention Window (CW): Range from which a random backoff counter is drawn; doubles after collisions up to CWmax.
• RTS/CTS handshake: Optional virtual carrier sensing to mitigate the hidden node problem.
• Network Allocation Vector (NAV): A virtual timer set by duration fields in frames to reserve the medium.

Two distinct LBT channel access modes defined by ETSI EN 301 893 for 5 GHz operation in Europe:

• FBE: Uses fixed periodic frame structures. CCA is performed at the beginning of each fixed frame period. If the channel is clear, the equipment transmits for a fixed duration; if busy, it remains silent for the entire frame. Suited for deterministic traffic like LTE-LAA.
• LBE: Demand-driven access. CCA is performed when data arrives. Uses an Extended CCA process with a random backoff counter (N) drawn from a contention window. The counter decrements only during idle CCA slots. This mode closely mirrors Wi-Fi's CSMA/CA for fair coexistence.

3GPP standards (Release 13 for LAA, Release 16 for NR-U) that enable cellular operation in unlicensed 5 GHz and 6 GHz bands, mandating LBT for fair coexistence with Wi-Fi. Key LBT categories defined in 3GPP TR 36.889:

• Cat 1: No LBT (immediate transmission).
• Cat 2: LBT without random backoff (single 25 μs sensing interval).
• Cat 3: LBT with random backoff and a fixed contention window size.
• Cat 4: LBT with random backoff and a variable contention window (similar to Wi-Fi DCF). Cat 4 is the baseline for data transmission to ensure fairness.

The collision resolution mechanism integral to LBT protocols. Upon a failed transmission (no ACK received), the Contention Window (CW) size doubles exponentially: CW_new = min(2 * CW_old + 1, CW_max). This spreads retransmission attempts over a wider temporal range, reducing the probability of repeated collisions. Upon successful transmission, CW resets to CW_min. In Wi-Fi, CW_min = 15 and CW_max = 1023 slots. The backoff counter is uniformly selected from [0, CW]. This adaptive mechanism is critical for maintaining channel stability under high load.

A fundamental limitation of physical LBT where two transmitters (STA A and STA C) cannot hear each other but can both reach a central receiver (STA B), causing persistent collisions at the receiver. LBT alone fails here because CCA at the transmitters reports an idle channel. The solution is virtual carrier sensing using the RTS/CTS (Request-to-Send/Clear-to-Send) exchange:

• RTS from transmitter reserves the medium around itself.
• CTS from receiver reserves the medium around the receiver, alerting hidden nodes.
• The Network Allocation Vector (NAV) is updated by all stations overhearing these frames, creating a virtual busy state regardless of physical CCA results.