5G NR in unlicensed spectrum (NR-U) is a new NR Release 16 mode of operation that provides the necessary technology for cellular operators to fully integrate the unlicensed spectrum into 5G networks. NR-U enables both uplink and downlink operation in unlicensed bands, supporting 5G new features, for example, wideband carriers, flexible numerologies, dynamic TDD, beamforming, and dynamic scheduling/HARQ timing.
Listen before Talk (LBT)
In NR-U, channel access in both downlink and uplink rely on the listen-before-talk (LBT) feature. A wireless device or a base station must first “sense” the communications channel to find out there is no communications prior to any transmission. When a communication channel is a wide bandwidth unlicensed carrier (e.g., several hundreds MHz), the “channel sensing” procedure relies on detecting the energy level on multiple sub-bands of the communications channel. The LBT parameters (such as type/duration, clear channel assessment parameters, etc.) are configured in a wireless device by the base station.
Frame Structure type 3
The new frame structure type 3 has been introduced by the 3rd Generation Partnership Project (3GPP) for operations in the unlicensed communications channels. Similar to the LTE time division duplex (TDD), the uplink and downlink operations using frame structure type 3 are on the same frequency channel but are separated in time. However, unlike in LTE TDD, a subframe is not configured as a downlink subframe or an uplink subframe and may be used by either the base station or the wireless device.
Channel Occupancy Time (COT) Structure
NR will support dynamic TDD, where the uplink-downlink allocation may change over time to adapt to traffic conditions. To enable dynamic TDD, a wireless device determines when and where to transmit and receive based on an indication of a channel occupancy time (COT) structure. The COT comprises multiple slots and each slot comprises downlink resources, uplink resources, or flexible resources. The COT structure reduces power consumption and channel access delay.
Interlace Resource Allocation
The resource allocation framework in the NR-U system has some major differences with the legacy LTE system. The NR-U system supports both contiguous and interlaced uplink resource allocations while complying with regulations. In the interlaced uplink resource allocation, the basic unit of resource allocation for the NR unlicensed channels is an interlace, which, for example, is composed of ten equally spaced resource blocks within a 20MHz frequency bandwidth for 15KHz sub-carrier spacing.
In the eLAA system, a base station can allocate resources in maximum of four multiple consecutive subframes to a wireless device using the multi-subframe grants. The eLAA system specifies two new downlink control information (DCI) formats (i.e., format 0B and 4B) that the base station can transmit on the physical downlink control channel (PDCCH) to schedule resources for a wireless device in up to four consecutive subframes.
In the legacy LTE, a wireless device needs four subframes to process an uplink grant. The eLAA system enables fast resource allocation by specifying the triggered grants. The base station may first transmit a resource allocation command to the wireless device and may later transmit a trigger, with a short processing time, to indicate to the wireless device the time for transmission. The timing between resource allocation command and the trigger can be flexible.