5 g RA process

2022-05-11 0 By

From the point of view of the physical layer, the RACH process of NR includes RACH Lead code (Msg1), random access response (Msg2), Msg3 and Msg4.While 5G system is beam-based system, multi-beam and single-beam conditions need to be considered for initial access.Initial access based on multi-beam systems and RACH high frequency (e.g.6GHz NR system is expected to become a multi-beam based system.Due to the large path loss and penetration loss at high frequency, the NR system will cover a cell with multiple beams.The beam can be an analog beam formed by a phase shifter in the RF chain or a composite beam formed by analog beam and digital beam.The initial access signal is assumed to be received by any UE in the cell.When the initial access signal is sent, there is no beam alignment between TRP and UE.Therefore, the initial access signal is transmitted through multiple Tx beams, and these Tx beams sweep multiple OFDM symbols.UE expects to receive sufficient initial access signal strength from one or more of these Tx beams (that is, one or more of these OFDM symbols).By detecting the initial access signal, for example, the initial sync signal, PBCH, or some reference signal, the UE is able to select the appropriate TRP Tx beam for the downlink from those used to send the initial access signal.For random access, the TRP should expect to receive random access leads from any UE in the cell.Similar to downlink transmission, TRP can also cover a cell with multiple TRP Rx beams.TRP should provide multiple RACH resources, where the UE can send RACH leads, and TRP applies an Rx beam to each RACH resource.If a UE is well covered by the TRP Rx beam I, the TRP will reliably receive the RACH leads sent on the RACH resource applied with the TRP Rx beam I.TRP may have different Tx/Rx beam exchanges.In a TRP with Tx/Rx beam reciprocity, the UE can know the appropriate Rx beam for the uplink based on detection of the downlink initial access signal.But in TRP without Tx/Rx beam reciprocity, UE may not be able to select the appropriate Rx beam for uplink based on downlink signal measurements.Beam reciprocity RACH flow of TRP For TRP with Tx/Rx beam reciprocity, TRP can configure mappings between downstream initial access signal transmissions and upstream RACH resources.This configuration can be sent in the PBCH and some SIBs detected by the UE prior to the RACH process.This configuration can be a mapping between the OFDM symbol index of the initial access signal and the RACH resource index.A high-level 4-step RACH program for TRP with Tx/Rx beam reciprocity is shown below: 1) UE first selects the RACH resource based on mapping configuration and downlink signal measurement.2) UE sends the RACH lead sequence on the selected RACH resource.3) TRP sends a random access response (RAR) in response to the detected lead sequence.RAR can be sent in PDSCH using the selected Tx beam.Based on the index of the RACH resource that sent the lead sequence, TRP knows which Rx beam is the appropriate beam for the UE that sent the lead.Through beam reciprocity, TRP can select the appropriate Tx beam for downlink transmission to UE.After this, TRP and UE can establish coarse-beam alignment, which can be used for the following RACH message transmission, and also for system information transmission after the RACH process.4) After receiving RAR, UE sends RACH MSG3 according to RAR scheduling.Because MSG3 is scheduled by TRP, TRP knows where to detect MSG3 with which Rx beam.5) Finally, TRP sends RACH MSG4 to UE in PDSCH, competing for resolution message.The Tx beam used here is the TRP Tx beam of the rough beam-alignment implemented in the previous step.RACH process for beam-free TRP without Tx/Rx beam interoperability For TRP without Tx/Rx beam interoperability, UE may not be able to select the appropriate TRP Rx beam based on downlink signal transmission measurements.Therefore, you need to select the RACH resource index for the UE to be used for RACH precursor transport.The lack of TRP beam reciprocity required special design for the transmission of other RACH messages.About RACH lead transmission: One solution is that UE can send RACH lead sequences across all RACH resources in an RACH situation to cover all TRP Rx beams.However, this approach may waste UE transmitting power and also increase collision probability.Another solution is that UE can first send the RACH lead sequence in an RACH resource.If the lead transfer fails, the UE switches to another RACH resource and retransmits the lead sequence.However, in this approach, RACH process latency may increase due to retransmission.There are trade-offs between the two approaches and research is needed to determine the problem.About RAR transmissions: Because there is no TRP beam reciprocity, TRP may not be able to select the appropriate Tx beam for RAR transmissions based on RACH resources that detect lead sequences.One solution is to transmit RAR across multiple Tx beams via Tx beam scanning.Another solution is to require UE to report a Tx beam ID in the RACH lead sequence.For example, the lead sequence ID can be used to carry information about a Tx beam ID.TRP sends RAR using the Tx beam carried in the corresponding detected lead sequence.About RACH MSG3: After receiving the RAR, the UE can send the RACH MSG3 (first uplink transmission) according to the RAR’s schedule.TRP can receive an RACH MSG3 with an Rx beam corresponding to the RACH resource, where TRP detects lead sequences corresponding to the RACH MSG3 and RAR.The MSG3 can also carry a Tx beam ID for TRP to use for the following downlink transmissions.About RACH MSG4: TRP sends RACH MSG4 (Competing resolution message) in PDSCH by using the Tx beam ID indicated by UE.