Dynamic Spectrum Sharing (DSS)

5G & LTE Dynamic Spectrum Sharing (DSS) is a technology that allows to deploy both 4G LTE and 5G NR in the same frequency band. It allows a dynamic allocation of spectrum resources between LTE & 5G based on user demand.Thanks to DSS, the same spectrum could be shared by the two "4G LTE" and "5G NR" technologies and avoid splitting it into two mid-band spectrum or have a dedicated spectrum each technology.

NOTE : For a tutorial for DSS, click here.

eNB gNB DSS configuration

For LTE NR DSS testing, we deliver an example of gnb DSS configuration file "gnb-dss.cfg" available under /root/enb/config/ folder. Since NR and LTE share the same physical resources, this adds NR LTE time/freq dependencies and could lead to LTE/NR physical channel collision (in case of bad configuration). Thus, it is highly recommended to start from "gnb-dss.cfg" file and adapt it the specific usecase.

Bellow the list of LTE NR dependencies and corresponding configuration parameters:

1) configure LTE and NR cells with same:

rf port and frequency center

cell_list: [ { rf_port: 0, dl_earfcn: 2525, /* 881.5 MHz */ ... }, ], /* cell_list */ nr_cell_list: [ { rf_port: 0, dl_nr_arfcn: 176300, /* 881.5 MHz */ ... },

bandwidth & subcarrier spacing

cell_default: { n_rb_dl: 50, /* Bandwidth: 25: 5 MHz, 50: 10 MHz, 75: 15 MHz, 100: 20 MHz */ ... }, nr_cell_default: { subcarrier_spacing: 15, /* kHz */ bandwidth: 10, /* MHz */ ... },

ul & dl antenna ports:

cell_default: { n_antenna_dl: N_ANTENNA_DL, n_antenna_ul: 1, ... }, nr_cell_default: { n_antenna_dl: N_ANTENNA_DL, n_antenna_ul: 1, ... },

2) insert MBSFN slots to carry 5G signal

/* reserved MBMS subframes for the NR SSB */ reserved_mbms_subframes: { sf_alloc: [ { radio_frame_allocation_period: 2, /* 20 ms */ radio_frame_allocation_offset: 0, subframe_allocation: "100000", /* subframe 1 */ } ],

3) align 5G SSB with LTE MBMS slots

ssb_pos_bitmap: "0001", ssb_period: 20, /* in ms */

4) configure NR PRACH on the same LTE PRACH occasion

// LTE prach config prach_config_index: 3, /* subframe 1 every 10 ms */ // NR prach config prach: { prach_config_index: 16, /* subframe 1 every frame, LTE PRACH at the same position */ ... },

5) configure NR PDCCH symbol on different LTE PDCCH ofdm symbol to avoid collision.

use ofdm symbol 0 for LTE and ofdm symbol 1 for NR

n_symb_cch: 0, css: { n_candidates: [ 0, 0, 4, 0, 0 ], start_symb: 1, }, ... uss: { n_candidates: [ 0, 4, 0, 0, 0 ], dci_0_1_and_1_1: true, start_symb: 1, },

6) disable LTE SRS

sib_sched_list: [ { filename: "sib2_3_nosrs.asn", si_periodicity: 16, /* frames */ }, ],

7) lte crs can be configured to auto.

lte_crs: "auto",

In this case the SW configures the lte CRS to avoid collision with NR CRS.

You can also configure manually the lte crs postions by configuring the whole lte_crs object:

lte_crs: { carrier_freq_dl: 50 * 6, /* center of the NR bandwidth */ carrier_bandwidth_dl: 50, nr_of_crs_ports: N_ANTENNA_DL, v_shift: 1, mbsfn_subframe_config_list: [ { radio_frame_allocation_period: 2, radio_frame_allocation_offset: 0, subframe_allocation1: "100000", }, ], },

In this case, v_shift should be set to n_id_cell(LTE) % 6.

8) align NR UL/DL shift with LTE UL/DL shift

n_timing_advance_offset: 0, /* to align with LTE */

9) align NR and LTE UL frequency

ul_frequency_shift_7p5_khz: true, /* to align with LTE */

10) add LTE CRS configuration to NR cell to avoid LTE NR CRS overlapping

11) adjust NR PDSCH rate matching:

pdsch: { k1: 4, mcs_table: "qam256", #if N_ANTENNA_DL >= 2 x_overhead: 12, /* 12 REs reserved for LTE CRS */ #else x_overhead: 6, /* 6 REs reserved for LTE CRS */ #endif ... },

Known limitations

LTE and NR scheduling on the same UL/DL slot is not supported. This means that, for a given UL/DL slot, gnb schedule at a time either LTE or NR UEs.
collision between LTE and NR PUCCH is possible.
collision between LTE and NR PRACH is possible.
TDD mode is not supported.