RACH Multiplexing in Support to Diverse Access Requirements Sample Clauses

RACH Multiplexing in Support to Diverse Access Requirements. In LTE, during initial access, the UE randomly selects one of the random access preambles (64 preambles) informed via broadcasted system information. The current design may create problems e.g. for mMTC applications, where a large number of devices may simultaneously attempt to access the system (Figure 4-9). According to [ZN13] the collision probability for the RACH procedure will be almost certain (99.97%) for a cell with 1000 users and 30 ms packet arrival interval. These potential collisions will lead to additional access delays which may impact services differently. Up to now, several schemes have been proposed in the literature for handling the random access procedure. These schemes may be classified into two large groups, namely pull-based and push- based [CKS+15]. In the first set of the solutions, the RACH preambles are being split in prioritization groups and the more delay sensitive devices compete against fewer devices for accessing the system. Additionally, the time to have a second attempt to access the system could be fine-tuned according to the collision rate. In the second set of solutions, push based procedure is used to achieve Small Data Transmission (SDT) from the MTC devices. However, these schemes, are designed mainly for prioritizing access based on the transmission requirements and are not, on the one hand, aiming at solving the collision rate problem, and on the other hand, are not focusing on 5G use cases (such as V2X, smart grid, mMTC) but rather focus on traditional xMBB use cases. Even in the cases where the solutions are applied for MTC scenarios, the number of the considered devices is rather small, thus making their applicability in scenarios where large number of devices is considered (e.g., [MET13-D11], [NGM15]) highly questionable. Furthermore, the sparse RACH resources may increase the latency to unacceptable levels, for ultra-reliability scenarios. One possible solution for the devices with strict latency requirements could be to reserve a set of dedicated preambles for the use of devices with high priority. This solution however is not efficient, since the number of RACH preambles is very small (i.e., 64 preambles) which has to be used both for random access and for handover purposes. In order to provide an efficient prioritization mechanism for delay-sensitive services (not relying on the assignment of dedicated preambles) METIS-II is currently investigating random access solutions to provide some level of access dif...