Telecom Articles by Samir Amberkar: wp-lte-allocations-explained-p16

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3GPP Modem
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LTE Allocations Explained

Requirements were:

1. system information messages (except MIB and SIB1)
2. RAR and DL RRC Connection Setup message
3. Layer 3 (NAS/RRC) signaling over SRB1/2
4. user data transmission and associated layer 2/1 signaling

Ref: [36.212:5.3.3], [36.213:7.1], [36.321:7.1]

Format	RA Type MCS	Details	CRC scrambled by	Requirements
0	RA Type 2 MCS index	PUSCH allocation	C-RNTI	UL allocations (req# 3, 4)
1	RA Type 0 or 1 MCS index	PDSCH allocation (one transport block)	C-RNTI	req# 2 (RRC msg³), 3, 4
1A	RA Type 2 MCS index	PDSCH allocation (one transport block)	SI-RNTI P-RNTI RA-RNTI C-RNTI	req# 2 (RAR, RRC msg³), 3, 4
1B	RA Type 2 MCS index	PDSCH allocation (one transport block)	C-RNTI	req# 3, 4
1C	RA Type 2 TBS index¹	PDSCH allocation (one transport block) QPSK modulation²	SI-RNTI P-RNTI RA-RNTI	req# 1, 2 (RAR), 3 (paging, SI change⁴)
1D	RA Type 2 MCS index	PDSCH allocation (one transport block)	C-RNTI	req# 3, 4
2	RA Type 0 or 1 MCS index	PDSCH allocation (two transport blocks i.e. for spatial multiplexing)	C-RNTI	req# 4
2A	RA Type 0 or 1 MCS index	PDSCH allocation (two transport blocks i.e. for spatial multiplexing)	C-RNTI	req# 4
3	-	PUCCH/PUSCH TPC commands	TPC-PUCCH-RNTI TPC-PUSCH-RNTI	power control
3A	-	PUCCH/PUSCH TPC commands	TPC-PUCCH-RNTI TPC-PUSCH-RNTI	power control

t
1: TBS index directly maps to TB size irrespective of RBs allocated [36.213:7.1.7.2.3].
2: Q_m = 2 if CRC is scrambled by SI-RNTI, P-RNTI, or RA-RNTI.
3: Temporary C-RNTI.
4: System Information change notification.

Even though there are number of formats, it is not very difficult to roughly see their relative sizes as RA Type 2 is of smaller size compared to RA TYPE 0 & 1. So we can roughly say 1C < 1A, 1B, 1C, 1D < 1 < 2, 2A.

Earlier we said that CRC is scrambled by RNTI. As there are number of RNTIs, how does PHY knows which RNTI to use ? This depend on RRC and MAC procedures as RNTI is relevant to type of information or procedure as required by RRC and MAC e.g. RA-RNTI relevant to MAC random access procedure, SI-RNTI is relevant to RRC system acquisition procedure. PHY acts as a slave to MAC/RRC.


Copyright © Samir Amberkar	Page 16 of 17