1. WF on RA-RNTI design 3GPP TSG RAN WG1 Meeting NR#3 R1-171xxxx
Nagoya, Japan, 18th – 21st, September 2017
Agenda item:
WF on RA-RNTI design
MediaTek

2. Background (1/5)
In LTE, the time and frequency information of a detected preamble is indicated by RA-RNTI which is defined as
RA-RNTI = 1 + t_id + 10*f_id
t_id: the subframe number in a radio frame where the Msg1 is started to be transmitted. t_id = 0, 1, …, 9
f_id: the index of RACH resources in the frequency dimension with f_id = 0, 1, …, 5

3. Background (2/5) Agreements from RAN1 NR AH#1:
For 4-step RACH procedure, a RACH transmission occasion is defined as the time-frequency resource on which a PRACH message 1 is transmitted using the configured PRACH preamble format with a single particular tx beam

4. Background (3/5) Agreements from RAN1 NR AH#2:
For L=127/139 with option 1, formats with 1,2,4,6, and 12 OFDM symbols are supported
Agreements from RAN2 #99
For multiple msg1 transmissions for contention free RACH
A single RAR window is applied for multiple msg1 transmission.
The RAR window is started after transmission of the first preamble after a “offset”.
The UE monitors multiple RA-RNTIs. The RA-RNTI is associated to the RACH transmission occasion in which the preamble was transmitted.
Once a RAR is received, the RAR reception is considered successful, as in LTE. The UE stops multiple preamble transmission.
Details of RA-RNTI calculations are FFS

5. Background (4/5)
In NR, preamble formats with L=127/139 can be as short as one to several OFDM symbols.
If an RA-RNTI formula similar to LTE is adopted in NR with t_id defined the symbol number in a radio frame, the number of required RA-RNTI values can be very large.
Assumptions: SCS = 120kHz, i.e. 80 slots in a frame, and f_id = 0, 1, …, 5 as same as LTE
The maximum number of RA-RNTI values required: 14 symbols/slot * 80 slots/frame * f_id = 14*80*6 > 212
NR needs to discuss how to indicate the (time and frequency) information of RACH transmission occasions where the detected preambles were transmitted.
by means of RA-RNTI and RAR

6. Background (5/5)
Comparison among alternatives for RA-RNTI and RAR design with the following assumptions
the time duration considered is a 10ms radio frame
the numerology is (a) 120 kHz and (b) 480kHz (i.e. 80 and 320 slots per frame, respectively)
RAN4 does not preclude 240kHz and 480kHz for data in later releases for forward compatibility [R ]
Six copies of RACH resources in the frequency dimension, i.e. f_id = 0,…,5
Variable(s) used in RA-RNTI besides f_id
Maximum number of RA-RNTI values with SCS=120 kHz and 𝒇 𝒊𝒅 =𝟎,…,𝟓
Impact on RAR design
Alt.1
The slot number within a frame
The starting symbol number within a slot
SCS=120kHz: 14∗80∗6=6720 < 2 13
SCS=480kHz: 14∗320∗6=26880 < 2 15
None
Alt.2
The RACH occasion (RO) index in a radio frame
=2∗64∗6=768 < 2 10
(assuming 2*64 ROs in the time dimension, i.e. periodicity of RACH resource is 5ms)
Alt.3
The slot number in a radio frame
SCS=120kHz: 80∗6=480 < 2 9
SCS=480kHz: 320∗6=1920 < 2 11
Indication of RO or symbol index within the slot
Alt.4
The slot number in a radio frame
And the index of SS blocks within a slot
(assuming RACH preambles/resources within a slot associated with 14 SS blocks)
RAPID should be re- defined accordingly

7. Proposal
In addition to f_id, NR down-selects one of the following alternatives for RA-RNTI calculation
Alt.1: The slot number within a frame and the starting symbol index within a slot
Alt.2: The RACH transmission occasion (RO) index in a radio frame
Alt.3: The slot number in a radio frame
Note: Besides RAPID, RAR indicates information of RO or symbol index within the slot when more than one RO is allocated within a slot
Alt.4: The slot number in a radio frame and the index of SS blocks within a slot
Note: RAPID in RAR should be redefined accordingly as the preamble index within the subset of RACH preamble indices and/or resources associated with one SS block
Note: f_id is the index of RACH resources in the frequency dimension