5G networks are expected to support applications which demand uRLLC (ultra-reliable and low latency communications), mMTC (massive Machine type communications), and mIoT (massive Internet of things). To support these kinds of applications, 5G-NR introduced a grant-free uplink transmission feature: Transmission without grant (TWG) i.e data transmission without resource request. With grant-free scheduling, the BS reserves resources for UL transmissions and informs the UEs of the reserved resources. When a UE wants to initiate a UL transmission, it directly utilizes the reserved resources, without sending an SR (Scheduling Request) and waiting for the subsequent grant message from the BS.
The 3GPP standards introduce the possibility for grant-free scheduling to reserve resources to the dedicated UE’s or a group of UE’s. each resource is reserved for a specific UE, and only this UE can utilize the resource at any time. This approach is completely adequate for periodic traffic since the resource allocations can be planned, and resources can then be utilized efficiently. 5G NR introduces the possibility to transmit K replicas of the same packet in consecutive slots to combat potential collisions. The 5G NR standard introduces the use of grant-free scheduling. With grant-free scheduling, the network pre-configures the radio resources and assigns them to UEs without waiting for UEs to request resources. UEs can utilize the pre-assigned resources as soon as they have data to transmit. This is completely in contrast to grant-free scheduling, where UE’s must request access to radio resources through the transmission of scheduling requests.
The BS assigns the radio resources to the UEs and notifies them using grant messages. UEs must wait to receive these grant messages before transmitting any data. Grant-free scheduling eliminates all delays introduced by the handshaking present in grant-based scheduling. It also improves the energy consumptions of the UEs, reduces their complexity, and decreases the signaling overhead compared with grant-based scheduling. Grant-free scheduling can assign dedicated or shared resources to the UEs. The BS decides whether resources are dedicated to specific UE’s, or are shared by a group of UEs. reserving resources to dedicated UEs is a very good approach when we can plan for what is the demand for the resources.
Grant-free scheduling with K-repetitions and shared resources to evaluate the reliability and latency levels that can be achieved in the presence of aperiodic traffic. Reliability for URLLC services is defined as the percentage of data packets that are successfully delivered before the latency deadline. The reliability at the medium access level that can be achieved with grant-free scheduling with K-repetitions and shared resources depends on the two main factors which are the possibility that a packet is not correctly received due to the collisions and the second factor is the effect of the self-collisions.
5G NR grant-free scheduling can support URLLC (ultra-reliable low latency communications) services with strict reliability and latency levels such as those demanded by Industry 4.0. It has a focus on aperiodic and sporadic traffic and implementation of 5G NR grant-free scheduling with K-repetitions and shared radio resources. Grant-based scheduling introduces additional latency due to the exchange of messages between the UE’s and the BS for assigning the radio resources. semi-persistent scheduling with dedicated resources inefficiently utilizes the available resources when considering dedicated resources and aperiodic traffic. Innovative grant-free scheduling solutions are necessary to meet the URLLC requirements identified for 3GPP Release-16 and beyond.
- Downlink: Semi-persistent Scheduling (SPS) is configured by the RRC (Radio Resource Control) per serving cell and per BWP (Bandwidth Part). Activation and deactivation of the DL SPS are independent among the serving cells. For the DL SPS, a DL assignment is provided by PDCCH, and stored and cleared based on L1 signaling indicating SPS activation or deactivation. RRC configures parameters when SPS is configured. These parameters are cs-RNTI, nrofHARQ, and periodicity. When SPS is released by upper layers, all the corresponding configurations shall be released.
- Uplink: there are two types of transmission without dynamic grants. Configured type 1: where an uplink grant is provided by RRC, and stored as configured uplink grant and COnfigured type 2: where an uplink grant is provided by PDCCH and stored or cleared as a configured uplink grant based on L1 signaling indicating configured uplink grant activation or deactivation. Type 1 and type 2 are configured by RRC per serving cell and per BWP (Bandwidth part).
For the same serving cell, the NR MAC entity is configured with either Type 1 or Type 2.