New Radio Unlicensed (NR-U)

The foundational coexistence mechanism mandated by global regulations. Before transmitting, an NR-U gNB or UE performs a Clear Channel Assessment (CCA) to sense the channel's energy level. If the channel is busy, the device defers transmission using a random backoff counter. This ensures NR-U does not monopolize the medium and shares fairly with Wi-Fi and other technologies. 3GPP specifies multiple LBT categories, from no LBT (Cat 1) for short control transmissions to full random backoff with variable contention windows (Cat 4) for data.

NR-U supports carrier bandwidths up to 100 MHz in the 5 GHz band and wider in the 6 GHz band, far exceeding Wi-Fi's typical 20/40/80 MHz channels. To coexist with narrower-band incumbents, NR-U employs bandwidth part (BWP) switching and LBT sub-bands. A wideband carrier is divided into 20 MHz sub-bands, each performing independent LBT. Transmission occurs only on sub-bands sensed as free, maximizing throughput while respecting occupied channels.

NR-U operates in two deployment architectures:

• Non-Standalone (NSA): NR-U acts as a secondary cell anchored to a licensed LTE or NR primary cell. The licensed anchor carries control signaling and provides a reliable fallback.
• Standalone (SA): NR-U operates entirely in unlicensed spectrum with no licensed anchor. This requires NR-U to handle all synchronization, paging, and random access procedures over unlicensed channels, enabling private networks in venues without owned spectrum.

NR-U defines four Channel Access Priority Classes (CAPCs) to differentiate quality of service. Each CAPC maps to specific LBT parameters—defer duration and contention window size—that determine how aggressively a device contends for the channel.

• CAPC 1: Lowest priority, longest backoff (best-effort data)
• CAPC 4: Highest priority, shortest backoff (URLLC, control signaling) This allows NR-U to prioritize latency-sensitive traffic like industrial automation commands over background file transfers.

Once a device acquires the channel via LBT, it can transmit for a maximum Channel Occupancy Time (COT)—typically 8-10 ms. NR-U introduces COT sharing, where the initiating device (gNB or UE) can share its acquired COT with the responding device for uplink or downlink transmission without requiring a new LBT cycle. This reduces overhead and improves spectral efficiency. A COT structure indicator in the control channel signals the remaining transmission duration to all devices.

To reduce latency and signaling overhead in unlicensed spectrum, NR-U supports Configured Grant transmissions. The gNB pre-allocates periodic time-frequency resources to UEs, which can transmit uplink data immediately upon acquiring the channel via LBT—without waiting for a dynamic scheduling grant. This is critical for URLLC use cases in factory automation where every millisecond counts. The gNB blindly decodes these transmissions using the pre-configured parameters.

The mandatory channel access mechanism that defines NR-U's coexistence etiquette. Before transmitting, a gNB or UE must perform a Clear Channel Assessment (CCA) to detect energy above a threshold. If the channel is busy, the device defers transmission for a random backoff period. NR-U uses Category 4 LBT—the same contention-based protocol as Wi-Fi—with a variable contention window that doubles upon collision, ensuring fair airtime sharing in the 5 GHz and 6 GHz bands.

A QoS differentiation framework that maps NR-U traffic to four Channel Access Priority Classes (CAPCs) , each with distinct LBT parameters. Higher-priority classes use shorter defer periods and smaller contention windows:

• CAPC 1: Background data, largest contention window
• CAPC 2: Best-effort traffic
• CAPC 3: Video and streaming
• CAPC 4: URLLC and real-time control, smallest contention window This allows NR-U to prioritize latency-sensitive services while maintaining coexistence fairness.

A grant-free transmission mode that reduces latency by allowing UEs to transmit on pre-configured uplink resources without waiting for a dynamic scheduling grant. In NR-U, AUL is combined with Configured Grant operation and must still respect LBT before each transmission. The gNB provides a bitmap of valid time-frequency resources, and the UE autonomously selects one after a successful CCA. This eliminates the quadruple LBT problem that would otherwise plague scheduled uplink in unlicensed spectrum.

A set of periodic signals—including SSB, RMSI, and CSI-RS—transmitted within a Discovery Burst Transmission Window (DBTW) . Unlike always-on licensed carriers, NR-U gNBs must perform LBT before each discovery burst. If LBT fails, the burst is dropped for that window. To improve transmission probability, NR-U allows multiple candidate SSB positions within the DBTW and permits short control signaling without LBT for very brief transmissions (<10% of duty cycle).

NR-U supports carrier bandwidths up to 100 MHz in the 5 GHz band and 160 MHz in the 6 GHz band. For wideband operation exceeding the 20 MHz LBT bandwidth unit, NR-U employs carrier aggregation with per-LBT-bandwidth-part channel access. The gNB performs independent LBT on each 20 MHz sub-band and transmits only on sub-bands that pass CCA. This frequency-selective transmission maximizes throughput while respecting Wi-Fi's 20 MHz primary channel structure.

The 6 GHz band (5925-7125 MHz) is the primary new frontier for both NR-U and Wi-Fi 6E/7. Both technologies use identical LBT mechanisms, creating a level coexistence playing field. Key differentiators:

• NR-U offers scheduled access with centralized QoS control
• Wi-Fi 6E provides distributed OFDMA with trigger-based uplink
• Both support 160 MHz channels and 1024-QAM Regulatory frameworks like ETSI EN 301 893 in Europe mandate identical channel access rules, ensuring neither technology has an unfair advantage.