5G/NR - SUL

SUL

SUL stands for Supplimentary Uplink. Basic idea (or Use Case) on SUL is described/illustrated as shown below in 38.300.

< 38.300-Figure B.1-1: Example of Supplementary Uplink >

The basic idea behind this is as follows.

i) Usually cell coverage in UL direction is lower than DL direction because UE Tx Power(UL Power) is not as strong as gNB Tx Power(DL Power).

ii) The performance degradation on UL direction due to this difference gets very serious as the UE approaches to cell edge.

iii) As a possible solution, an idea to use very low frequency than the original UL frequency is proposed. As you know, cell coverage gets larger as frequency gets lower.

iv) this (to use a secondary UL at much lower frequency) is the idea behind SUL. When the channel condition is good, Network tell UE to use the original UL frequency and when channel condition gets poor than a certain criteria, Network directs UE to use the secondary(supplementary) UL frequency.

There are several NR bands dedicated for SUL which are n80, n81, n82, n83, n84, n86. As you see in this table, these are all under 2Ghz which is lower than commonly used NR frequency(e.g, over 3Ghz). However, one possible drawback of this approach is that these SUL bands are colliding with existing LTE bands. So it would cause NR-LTE Co Existance issues. You would need some researches on SUL and NR/LTE Coexistance issue before you make decision to deploy any SUL. See TR 37.872 and TR 37.716 for the details on SUL and NR/LTE CoExistance.

Would SUL be useful in real practice and widely deployed ? See this blog by Martin.

Overall Procedures

Overall procedure for SUL operation can be summarized in illustration shown below.

Step 1 : SUL configuration is broadcast by SIB1 of Normal Cell
Step 2 : UE trigger RACH to Normal Cell or SUL cell depending on signal condition it percieves and Completes the initial attach
Step 3 : Once in Connection Mode. gNB can dynamically switching between Normal cell and SUL cell via DCI 0_1.

Example 01 > Following example is from the test with Amarisoft Callbox and UEsim

[1] PRACH : RACH to SUL Cell

Message: sequence_index=0 ta=5 prb=3:3sul_index=0 snr=29.0

[2] PDCCH : Scheduling PUSCH on Normal Cell

Message: ss_id=2 cce_index=6 al=2 dci=0_1 k2=4

Data:

rb_alloc=0x12f

mcs=14

ndi=0

rv_idx=1

harq_process=3

dai=3

tpc_command=1

antenna_ports=0

srs_request=0

dmrs_seq_init=0

ul_sch_indicator=1

sul_ind=0

[2a] PUSCH : on Normal Cell Nacked

Message: harq=3 prb=2:47 symb=0:14 CW0: tb_len=1985 mod=4 rv_idx=1 cr=0.54 retx=1 crc=KO snr=3.0 epre=-43.4 ta=0.6

[3] PDCCH : Scheduling PUSCH on SUL Cell

Message: ss_id=2 cce_index=6 al=2 dci=0_1 k2=4

Data:

rb_alloc=0x2

mcs=7

ndi=1

rv_idx=0

harq_process=4

dai=3

tpc_command=1

antenna_ports=0

srs_request=0

dmrs_seq_init=0

ul_sch_indicator=1

sul_ind=1

[3a] PUSCH : on SUL Cell : Acked

Message: harq=4 prb=2 symb=0:14 CW0: tb_len=20 mod=2 rv_idx=0 cr=0.56 retx=0 crc=OK snr=5.2 epre=-24.6 ta=5.3sul=1

Operation Highlights

What I mentioned above is based on my intuitive understanding on SUL operation. In this section, I will quote some formal description from 3GPP specification for your (my) reference.

38.300 - 5.4.2 state as follows :

In conjunction with a UL/DL carrier pair (FDD band) or a bidirectional carrier (TDD band), a UE may be configured with additional, Supplementary Uplink (SUL). SUL differs from the aggregated uplink in that the UE may be scheduled to transmit either on the supplementary uplink or on the uplink of the carrier being supplemented, but not on both at the same time.

38.300 - 6.9 state as follows :

In case of Supplementary Uplink, the UE is configured with 2 ULs for one DL of the same cell, and uplink transmissions on those two ULs are controlled by the network to avoid overlapping PUSCH/PUCCH transmissions in time. Overlapping transmissions on PUSCH are avoided through scheduling while overlapping transmissions on PUCCH are avoided through configuration (PUCCH can only be configured for only one of the 2 ULs of the cell).

38.300 - 9.2.6 state as follows :

For initial access in a cell configured with SUL,the UE selects the SUL carrier if and only if the measured quality of the DL is lower than a broadcast threshold. Once started, all uplink transmissions of the random access procedure remain on the selected carrier.

38.300 - 10.3 state as follows :

When CA is configured, at most one configured uplink grant can be signalled per serving cell.When BA(Bandwidth Adaptation) is configured,at most one configured uplink grant can be signalled per BWP. On each serving cell, there can be only one configured uplink grant active at a time. A configured uplink grant for one serving cell can either be of Type 1 or Type 2. For Type 2, activation and deactivation of configured uplink grants are independent amongthe serving cells.When SUL is configured, a configured uplink grant can only be signalled for one of the 2 ULs of the cell.,

38.213 - 8 states as follows :

If a UE is configured with two UL carriers for a serving cell and the UE detects a ,PDCCH order, the UE uses the UL/SUL indicator field value from the detected PDCCH order to determine the UL carrier for the corresponding PRACH transmission.

38.321 - 5.1.1 states as follows :

if the RSRP of the downlink pathloss reference is less than rsrp-ThresholdSSB-SUL:

select the SUL carrier for performing Random Access procedure;

else:

select the NUL carrier for performing Random Access procedure;

Band Combinations for SUL

Following is the frequency band assigned to SUL in 3GPP.

< 38.101-1 v17.5 - Table 5.2-1: NR operating bands in FR1 >

Followings are the table that I consolidated multiple tables from TR 37.872. NOTE that SUL Uplink Frequency is much lower than the uplink of Normal Cell.

SUL Band combination	NR Band	Uplink (UL) band		Downlink (DL) band		Duplex mode
		BS receive / UE transmit		BS transmit / UE receive
		F_{UL_low} F_{UL_high}		F_{DL_low} F_{DL_high}
SUL_n78-n80	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
SUL_n78-n80	n80	1710 MHz	1785 MHz			SUL
SUL_n79-n80	n79	4400 MHz	5000 MHz	4400 MHz	5000 MHz	TDD
SUL_n79-n80	n80	1710 MHz	1785 MHz			SUL
SUL_n78-n84	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
SUL_n78-n84	n84	1920 MHz	1980 MHz	N/A		SUL
SUL_n78-n82	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
SUL_n78-n82	n82	832 MHz	862 MHz	N/A		SUL
SUL_n78-n81	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
SUL_n78-n81	n81	880 MHz	915 MHz	N/A		SUL
SUL_n78-n83	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
SUL_n78-n83	n83	703 MHz	748 MHz	N/A		SUL
SUL_n78-n86	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
SUL_n78-n86	n86	1710 MHz	1780 MHz	N/A		SUL
SUL_n79-n81	n79	4400 MHz	5000 MHz	4400 MHz	5000 MHz	TDD
SUL_n79-n81	n81	880 MHz	915 MHz	N/A		SUL

Following is the table for SUL with ENDC. You would notice that all LTE is FDD and all NR is TDD, and NR SUL is in same frequency range in LTE UL range implying that NR UL and LTE UL would share the frequency range. NOTE that SUL Uplink Frequency is much lower than the uplink of Normal Cell.

E-UTRA and NR DC Band	E-UTRA and NR Band	Uplink (UL) band		Downlink (DL) band		Duplex mode
		BS receive / UE transmit		BS transmit / UE receive
		F_{UL_low} F_{UL_high}		F_{DL_low} F_{DL_high}
DC_3-SUL_n79-n80	3	1710 MHz	1785 MHz	1805 MHz	1880 MHz	FDD
	n79	4400 MHz	5000 MHz	4400 MHz	5000 MHz	TDD
	n80	1710 MHz	1785 MHz	NA		SUL
DC_1-SUL_n78-n84	1	1920 MHz	1980 MHz	2110 MHz	2170 MHz	FDD
	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
	n84	1920 MHz	1980 MHz	NA		SUL
DC_20-SUL_n78-n82	20	832 MHz	862 MHz	791MHz	821 MHz	FDD
	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
	n82	832 MHz	862 MHz	NA		SUL
DC_8-SUL_n78-n81	8	880 MHz	915 MHz	925 MHz	960 MHz	FDD
	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
	n81	880 MHz	915 MHz	N/A		SUL
DC_28-SUL_n78-n83	28	703 MHz	748 MHz	758 MHz	803 MHz	FDD
	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
	n83	703 MHz	748 MHz	N/A		SUL
DC_66-SUL_n78-n86	66	1710 MHz	1780 MHz	2110 MHz	2200 MHz	FDD
	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
	n86	1710 MHz	1780 MHz	N/A		SUL
DC_8-SUL_n79-n81	8	880 MHz	915 MHz	925 MHz	960 MHz	FDD
	n79	4400 MHz	5000 MHz	4400 MHz	5000 MHz	TDD
	n81	880 MHz	915 MHz	N/A		SUL
DC_3-SUL_n78-n82	3	1710 MHz	1785 MHz	1805 MHz	1880 MHz	FDD
	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
	n82	832 MHz	862 MHz	N/A		SUL
DC_20-SUL_n78-n83	20	832 MHz	862 MHz	791 MHz	821 MHz	FDD
	n78	3300 MHz	3800 MHz	3300 MHz	3800 MHz	TDD
	n83	703 MHz	748 MHz	N/A		SUL

Reference

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