rach faqs in lte

Download RACH FAQs in LTE

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1. FAQs on RACH 1) Why RACH procedure is required? In order to be synchronized with the network, RACH procedure is used.Suppose a UE wants to access the network, so first it will try to attach or synchronize with the network. In LTE a separate channel PRACH (Physical Random Access Channel) is provided for initial access to the network. 2) When does UE need to perform RACH procedure As per specifications, RACH can be performed in below mentioned scenarios:- Initial Access - UE is trying to access the network in RRC idle state. During RRC connection re-establishment procedure. Handover When uplink synchronization is lost: - It means from a certain duration network has not received anything from UE in uplink. When UE does not have any PUCCH resources available for SR (Scheduling Request) . When timing advance is needed for positioning purpose in RRC connected state for UE. To initiate the procedure UE lower layers will send a Random Access Request (RACH Request) after receiving request from UE RRC. 3) Difference between Contention based and Contention based RACH Contention based RACH Message1: Random access Preamble on RACH 6bit: 5 bit preamble ID and 1 bit to indicate size of message3 Message2: RAR RACH Response by MAC on DL-SCH No HARQ Addressed to RA-RNTI Contains RAPID (Preamble ID) , Timing Alignment, Initial UL grant, T-CRNTI Message3: Schedule transmission on UL-SCH HARQ used For initial Access: NAS UE ID but no NAS message RRC Connection Req over CCCH RLC TM After Radio Link Failure: RRC Connection Reestablishment via CCCH RLC TM 2. No NAS message After HO in target cell: RRC connection Reconfiguration Complete i.e., the confirmation for the RRC connection Reconfiguration over DCCH. Allocated CRNTI. Includes Uplink Buffer Status Report when required. Message4: Not synchronized with Message HARQ supported T-CRNTI for Initial Access and after Radio Link failure CRNTI for UE in RRC_CONNECTED Non -Contention based RACH Msg 1: RA preamble assignment on DL dedicated signaling HO: HO Command generated by Target eNB sent via source eNB DL data arrival: MAC signaling in case of DL data arrival Message2: RA preamble on Rach Use preamble received from message1. Message3: No HARQ Addressed to RA-CRNTI. Contains at least Timing Alignment, Initial UL grant for HO case, Timing alignment for DL data arrival case, RA-preamble identifier. 4) Steps followed by UE for sending a RACH request 3. 3) How UE decides about when and where it needs to send RACH request It is decided on the basis of parameters received in SIB2 PRACH configurations. UE can use 6 Resource blocks for sending the RACH Request. From PRACH Config Index - There is a mapping in spec 36.211 table 5.7.1-2 from which UE can find the available sub frames where it can send a RACH. Example : Prach Config index value = 7, From table it is clear that UE can send a RACH Req in any system frame but sub frame should be 2 and 7. From PRACH Frequency Offset - This value is also received in SIB2 by UE. It governs which frequency resource UE can use for the RACH Req. Example: If the value of PRACH Freq Offset is 10. UE can use the 6 PRB's starting from PRB 10 for RACH Request. 4) Exactly when and where Network transmit RACH Response The following is what 3GPP 36.321 (section 5.1.4) describes. 4. Once the Random Access Preamble is transmitted and regardless of the possible occurrence of a measurement gap, the UE shall monitor the PDCCH for Random Access Response(s) identified by the RA-RNTI defined below, in the RA Response window which starts at the sub frame that contains the end of the preamble transmission [7] plus three sub frames and has length ra-ResponseWindowSize sub frames. It means the earliest time when the network can transmit the RACH response is 3 sub frames later from the end of RACH Preamble. Then what is the latest time when the network can transmit it? It is determined by ra-ResponseWindowSize. This window size can be the number between 0 and 10 in the unit of sub frames. This means that the maximum time difference between the end of RACH preamble and RACH Response is only 12 sub frames (12 ms) which is pretty tight timing requirement. 5) How many preamble sequences are available and how they are grouped? Total 64 preamble sequences are available; they can be divided into group A and group B. UE select a sequence from any of the groups based on a) The size of uplink packet to be sent b) The radio conditions This allows the enodeb to decide upon the quantity of PUSCH resources to allocate to UE for its uplink data transfer; Group A are intended for small packets, or large packets in poor radio conditions Group B are intended for large packets in good radio conditions Once the UE as selected either group A or group B, a preamble is selected at random from within that group . 8 preamble sequences are reserved by enodeb out of 64 for the purpose of non-contention based RACH during intra handover scenarios 6) What exactly RACH request Contains 1. Preamble Index: - There are total of 64 preambles available which are divided into two groups Group A and Group B. UE decides the preamble index from a group on the basis of parameters received in SIB2 NumberofRaPreambles:- eNodeB sends this value in SIB2 which denotes the total number of 5. preambles available for UE to send a Rach Request.Range of this parameter is 4 to 64 SizeOfRaPreamblesGroupA: -Total number of random access preambles sequences available within Group A. Preambles are divided into Group A and Group B. Group A preambles are intended for sending small packets and Group B preambles are intended for sending large packets. Range of this parameter is 4 to 60 MessageSizeGroupA: Message size threshold for selecting preamble Group A in term of bits. Its value is in bits(56, 144, 208 or 256 bits) Now UE needs to decide the group from which it needs the preamble. Group is decided on the basis of size of MSG3 (RRC connection request). If Msg3 size > messageSizeGroupA, Preamble will be selected from GroupB Else Preamble will be selected from Group A the actual formula for selecting a preamble is given in spec 36321 sec 5.1.2. That contains factors other than MSG3 size as well. But mainly this is how UE decides the Group. From the selected group, randomly UE selects a preamble index. 7) How can we get RA RNTI? 5.1.4 Random Access Response reception" in "TS36.321 says how to calculate RA_RNTI as follows. The RA-RNTI associated with the PRACH in which the Random Access Preamble is transmitted, is computed as: RA-RNTI = 1 + t_id + 10 * f_id Where t_id is the index of the first subframe of the specified PRACH (0 < t_id