Introduced as part of the GSMA’s 3GPP release 16 specifications, 5G New Radio Unlicensed (NR-U) is an evolution of the 4G LTE License Assisted Access (LAA) standards, first introduced in 3GPP release 13. NR-U supports three deployment modes:
- Carrier Aggregation, which is based on LTE-LAA where the unlicensed spectrum is used only to augment downstream user plane capacity. Control plane data is transported over licensed spectrum only.
- Dual Connectivity, which supports both upstream and downstream user plane traffic over the unlicensed spectrum. This is built on extended LAA (eLAA), introduced in 3GPP release 14. Again, this is designed for traffic offload and not coverage so control plane traffic is transported only over the licensed spectrum.
- Standalone, which represents the first time the 3GPP has defined a mode of operation that relies solely on unlicensed spectrum for control and user plane traffic. This implementation was built on LTE technology developed by the MulteFire Alliance.
With NR-U’s standalone mode eliminating any dependency on licensed network operators, it is open for implementation by to private enterprises, managed service providers or network systems integrators. This enables private 5G deployments supporting new consumer and Industry 4.0 applications demanding secure, low-latency, reliable, high bandwidth connectivity to densely populated endpoints. While the carrier aggregation and dual connectivity modes are designed to operate in the 5Ghz spectrum, the NR-U standalone mode has been built for all sub-7 GHz mid-band ranges plus high-band frequencies in the 57 to 71 GHz ranges that are opening up around the globe. Support for these mmWave bands is being defined within 3GPP release 17 specifications.
The frequency ranges in which NR-U is deployed - in any mode of operation - dictates how it is implemented. As with LAA, the primary concern of operating NR-U in the highly utilized 5 GHz bands is ensuring harmonious coexistence with other mobile technologies, like Wi-Fi. The introduction of a 5G NR radio access network (RAN) should cause no more impact to an existing Wi-Fi system than adding another Wi-Fi access point. While NR-U employs the 5G NR physical (PHY) layer and therefore enjoys the benefits of evolutions such as multi-numerology and mini-slots, the medium access control (MAC) layer protocols for processes such as channel access have been modified to align with Wi-Fi. Once again, this follows the principles of NR-U’s LAA predecessor and employs the same contention-based listen before talk (LBT) protocol employed in 802.11 to ensure equal access to available channels. This is referred to asynchronous NR-U operation and does limit some of the benefits of 5G NR, such as the ability to support ultra-reliable low-latency communication (URLLC)
Characteristics of 5G NR-U in the sub-7 GHz mid-band frequency ranges.
Opportunities for better spectral efficiencies and the implementation of URLLC arise when new, less populated, frequency ranges become available to NR-U. In the US, EU and South Korea, the 5.925GHz to 7.125GHz (or 6GHz) frequency range is now being opened-up for unlicensed devices. In the US, this covers the Federal Communications Commission (FCC) Unlicensed National Information Infrastructure (U-NII) sub-bands 5 through 8. The 2020 FCC report and order outlining the unlicensed use of the 6 GHz band allowed for universal access for low power (30dBm) indoor applications. The U-NII-5 and U-NII-7 sub-bands can operate outdoors at the same levels currently employed by 5 GHz systems (36dBm) but must use an automated frequency control (AFC) system, like the spectrum access system (SAS) employed in the citizens broadband radio service (CBRS). This is to prevent interference with existing earth-to-space satellite and point-to-point microwave links also operating on these wavelengths.
While 802.11ax (Wi-Fi 6) will also use the 6 GHz band, it is considered greenfield spectrum when operating within a building location. The abundance of available spectrum together with the absence of any incumbency, coupled with the natural isolation of signals owing to their low propagation rates though air and physical structures, mean interference will be negligible. Outdoors, the addition of an access controller (AFC) ensures that there will be no interference from alternative radio access technologies.
This enables NR-U operating in or above the 6 GHz frequency range to be unencumbered by the strict LBT protocols adopted for harmonious coexistence in the 5 GHz bands. Where category 4 (CAT4) LBT is the baseline for 5 GHz operation, forcing strict back off in accordance with carrier sense multiple access / collision avoidance (CSMA/CA) procedures, NR-U can employ CAT1 LBT in and above the 6 GHZ range, where channel access is immediate and requires no LBT. Instead, channel availability can be determined by simple energy detection, thereby permitting rapid access to transmission channels in support of low latency automation and IoT applications. Future iterations of the standard will include preamble detection, should a common signal be agreed upon across all relevant radio access technologies.