NB-IoT over NTN
NB-IoT Template is implemented by the NTN simulator. It provides a single PRB NB-IoT scenario giving option to use various generic TR38.812 scenarios or NB-IoT specific ones. It can show how different repetitions configurations (static or dynamic) impact on throughputs, delays or link budget in general on those different scenarios.
Essential pre-conditions
Stationary satellite and UEs, LOS channel
1 PRB (12 subcarriers) system bandwidth
NB-IoT channel coding with MCS values of 2-12 available (QPSK)
Fixed 1 PRB allocation width per UE
Default resource unit length of 5 subframes
NB-IoT Round Robin Scheduler in time domain
Full buffer traffic model
H-FDD UEs, most relevant transmission gaps modelled:
Npdcch K0 = 5. Minimum delay between DL DCI and NPDSCH rece reception
Npucch N0 = 8. Minimum delay between UL DCI and NPUSCHÂ tranmission
NpdschAcDelay = 13. Fixed delay between end of NPDSCH reception and start of HARQ ACK/NACK TX on NPUSCH
NpdschAcSlots = 2. Number of ACK/NACK subframes reserved
Overhead from control messaging modelled but no errors can occur
Configurable options
Simulation time
The number of the UEs simulated
Scenario
TR38.821 scenarios
Satellite set1
4, 9, 14 (GEO, LEO600, LEO1200)
Satellite set 2
19, 24 and 29 (GEO, LEO600, LEO1200)
Eutelsat satellite set 3 (LEO 600 and LEO 1200)
Sateliot set 4 (LEO 600)
Scenario configuration names: TR38.821_4, TR38.821_9, TR38.821_14, TR38.821_19,TR38.821_24, TR38.821_29, EutSet3_LEO600, EutSet3_LEO1200, SateliotSet4Â
HARQ:
Enabling/disabling
Max retransmission count
Process count
Default number of DL data channel repetitions
Default number of UL data channel repetitions
Dynamic DL data channel repetition:
Enabling/disabling
Coupling loss threshold
Dynamic UL data channel repetition:
Enabling/disabling
Coupling loss threshold
Static and Dynamic Repetitions
If dynamic repetitions are disabled then NPDSCH and NPUSCH use the configured default DL/UL repetition values for all transmissions and UEs. Since subframe lenght is 5 all tranmission lenghts are multiple of 5.
If dynamic repetitions are enabled, initially all transmissions use the configured default DL/UL repetition values. Once the first NPUSCH transmission is received the gNB measures the coupling loss value for the UE uplink channel based on the received signal power, power headroom report, allocation size and power control configuration. This UL coupling loss measurement is then used as a base for both NPDSCH and NPUSCH repetition count selection, along with selected DL or UL MCS, respectively. For example in DL direction for a single UE:
if: UL coupling loss measurement is unavailable
if: DL MCS is larger than smallest available MCS, then no repetitions are configured since AMC is able to pick a suitable MCS
else: Utilize the default DL repetition count
else (UL CL measruement available available):Â
if: Uplink coupling loss is below the DL CL threshold, then no repetitions are utilized
else: Add repetitions dynamically with formula:
repetitionCount = 10^(dBdiffToThreshold/10) -1
E.g. if coupling loss is 158dB, and the threshold is 148dB then 9 repetitions will be configured which leads to estimated 10 dB increase in received signal level
A good value for dynamic coupling loss is a value which approximately leads to usage of smallest MCS available. The default values should be suitable for SateliotSet4 configuration. A suitable uplink coupling loss threshold value is 6-7 dB smaller at least in SateliotSet4 configuration due to smaller uplinlink TX power etc.