NR SA D2C (Direct To Cell)

 

This tutorial is to show you how to configure and test Direct to Cell in NR. Direct to Cell represents 'From Satellite directly to Cellular Phone'. The 'Cellular Phone' in this context is just an ordinary commercial mobile hone that you have and it does not need to support any special feature for satellite communication as long as it works with ordinary NR SA in terrestrial network.

Actually Direct To Cell is not the 3GPP based feature like NTN (NB NTN or NR NTN). So the UE is not expected to support any specific features to accommodate this type of connection. So all the functionality to handle such a long delay or doppler shift etc which are present for satellite communication should be implemented by gNB that is sitting on the Satellite. This gNB implementation is mostly up to gNB vendor since the functionality is not specified in 3GPP.

 

 

Table of Contents

• NR SA D2C (Direct To Cell)
• Test Setup
• Key Configuration Parameters
• Test 1 : D2C with Simulated LEO and UEsim
• Configuration
• Perform the Test
• Log Analysis

 

 

Test Setup

Test setup for this tutorial is as shown below.  This is just for low layer testing, you may not need any complicated IP layer setup.

• SIM Card used in this tutorial is the one delivered with the system as it is.
• If you want to change the configuration, The tutorial Configuration Guide would help

With Amarisoft eNB and UEsim, we can try with following two different setups.

 

 

 

Key Configuration Parameters

Followings are important configuration parameters for this tutorial. You may click on the items for the descriptions from Amarisoft documents.

• ntn : In this link, you can find the descriptions for all the parameters below.
• sv_filename
• sv_filetype
• tle_filename
• default_ephemeris
• default_sma
• default_elevation_offset
• ephemeris
• eccentricity
• inclination
• semi_major_axis
• longitude
• periapsis
• anomaly
• epoch
• use_state_vectors
• eci_reference
• ground_position
• same_as_ap_position
• latitude
• longitude
• altitude
• n_ta_common
• n_ta_drift
• n_ta_drift_var
• n_ta_common_offset
• feeder_doppler_compensation
• feeder_dl_freq
• feeder_ul_freq
• large_freq_shift
• prach_range_sc
• prach_n_acc
• ta_tolerance
• average_mode
• direct_to_cell
• cell_center
• latitude
• longitude
• altitude
• channel_sim_control
• type
• ue_position
• latitude
• longitude
• altitude
• ue_doppler_shift
• ue_dl_freq
• ue_ul_freq
• feeder_doppler_shift
• ue_dl_attenuation
• ue_dl_gain_offset
• ul_sync_validity
• k_offset
• k_mac
• dynamic_k_offset
• reference_location
• latitude
• longitude
• t_service
• neighbour_cells
• rat_type
• t318

 

 

Test 1 : D2C with Simulated LEO and UEsim

This is to configure and test Direct to Cell for Simulated LEO using Amarisoft UEsim as DUT.

 

Configuration

The configuration shown here is common configuration for all the subtests belonging to Test 1

I have used gnb-sa-direct-to-cell.cfg for gNB.

I am using mme-ims.cfg and ue_db-ims.cfg  as they are.

On UEsim, I used ue-nr-sa-fdd-10Mhz.cfg which is copied and modified from ue-nr-sa.cfg

 

gnb-sa-direct-to-cell.cfg  is configured as follows.

In gNB configruation, specify satellite orbit first with the test parameter NTN_MODE. You can select the type among GEO, MEO, LEO. In this test, GEO is selected.  Once this parameter (NTN_MODE) is specified, various other test parameters are configured based on the selected NTN_MODE. (NOTE : This is not for NTN, but Direct to Cell is implemented as a subset of NTN in Amarisoft, so some of NTN parameter is used for Direct To Cell case as well).

If CHANNEL_SIM is enabled, you can apply various channel profile to simulate the radio link between ground station and satellite. In this test, only awgn is applied.

An NR FDD band is configured for this tutorial. And note that ssb_period is set to very small number (short period) to help UE get better timing estimation. In addition, just to help visualization of the test result, I set force_full_bsr, force_dl_schedule to true.

This may not be a mandatory but you may need to tweak prach_config_index to accommodate the long distance situation.

Now as MAC configuration, configure retx_bsr_timer, sr_prohibit_timer, prohibit_phr_timer, sr_trans_max differently based on the type of orbit. The basic idea is to configure these parameters differently based on RTT between the satellite and ground station.

Then adjust RRC timers t300, t310  appropriately for each orbit type. And then adjust t_PollRetransmit based on the type of the orbit.

Now for the configurations specific for ntn. The configuration parameter name itself is ntn. In this parameter, you can specify the location of the satellite (i.e, the ephemeris), the location of ground station (i.e, the location of gNB) and ue location. The location of the ground station is specified by the parameter ground_position, the location of UE is specified by the parameter ue_position and the position of the satellite is specified by the parameter default_ephemeris. The parameter direct_to_cell indicates this is for Direct to Cell, not for NTN.

 

ue-nr-sa-fdd-10Mhz.cfg  is configured as shown below. (NOTE : If you are using the commercial UE as a DUT, you don't need this step. Just make it sure that your UE support UE-assistance-information for release-preference).

The key point here is that you don't need to configure any specific settings for satellite communication since Direct to Cell is designed to make Satellite communication possible with ordinary UE without any special modification. So in this tutorial, you will see all the configuration on UEsim remain same as in default configuration (ue-nr-sa.cfg) except band and channel bandwidth just to make it match gNB frequency settings.

Only band and channel (dl_nr_arfcn, ssb_nr_arfcn) is changed to match gNB frequency and band settings.

ue_list configuration is also set to be same as in default.

 

Perform the Test

Run lte service on callbox and then check 'cell phy' and 'cell' command. Make it sure that cell is configured as you intended. In this test, n7(FDD) is used

This is not the mandatory process.. but you may do this to collect SIB message in the log for a few seconds at the beginning. I did bcch=1 and after a few seconds did bcch=0.

Now start trace logging.

Power on UE on UEsim (If your DUT is a commercial phone, turn on the phone)

Wait until the DUT complete the initial attach. (NOTE : If you are using a commercial UE and/or real satellite, you may need to do some additional troubleshooting for this step to work through)

Try some data traffic (e.g, ping) to verify the data pipe setup.

 

Log Analysis

Following is the log snapshot that are involved in communication with NTN.

Sample Log

While the test is running, you can check out the position of eNB and UE in real time with WebGUI.  While running, select [ENB] tab and hit [NTN] button and you see the view as shown below.

[eNB] pannel shows the sky view of eNB (sky view from eNB) and [UE] pannel shows the sky view of UE (sky view from UE). When the following value goes negative, satellite link gets disconnected since the satellite is not obserbable from UE.

• Elevation
• DtC value in Distance on UE pannel

You may check out overall signaling sequence and details, in Direct to Cell there is no differences in signaling side from what you see regular communication (i.e, terrestrial communication)

Check out throughput and see if you are getting expected throughput. In this test, I set force_full_bsr, force_dl_schedule to true. So you would get relatively high throughput at PHY,MAC.

Then check SNR to check the radio link quality over the course of the test.

You can check out the signal quality percieved by UE from DL CQI plot

In [NTN] tab, you can check out Satellite position (Elevation and Azimuth) and Timing Advance command value.

If you magnify TA command value in [Time/Freq] tab, you would see a pattern as shown below. When TA dropes below -0.5, we send a command to do TA +1 and it goes to +0.5 and then slowly decreases as the satellite moves

If you want the check all the NTN status in values, you may apply the filter 'NTN' in the info combo box. (NOTE : in order to get this log, you should enable 'NTN' log collection option when you are running the simulation)