# Practical Performance Analysis of Interference in DSS System
## 3 Dynamic Spectrum Sharing  
DSS supports LTE and NR to jointly use the uplink and downlink spectrum resources of the same band. When there is no independent NR spectrum resource, the spectrum can be shared with the existing LTE to obtain the ability to rapidly deploy 5G networks. When independent NR spectrum resources are available, they can be also shared with LTE to improve spectrum utilization. The DSS architecture is shown in Figure 1, where the evolved packet core (EPC) is the 4G core network, the 5G core (5 GC) is the 5G core network, the evolved node B (eNodeB) is the 4G base station, the next-generation node B (gNodeB) is the 5G base station, and UE is the user equipment. The eNodeB and the gNodeB are one base station physically, but two base stations logically.  
DSS adopts a flexible scheduling algorithm: LTE and NR service requests are judged with the accuracy of transmission time interval (TTI). The available frequency-domain resources are dynamically allocated based on the service application volume of LTE and NR according to the allocation algorithm so that spectrum utilization efficiency is maximized, as shown in Figure 2. The minimum scheduling precision in the frequency domain is the RB level, and the minimum scheduling precision in the time domain is the TTI level.  
Figure 1: DSS architecture.  
Figure 2: DSS dynamically adjusts spectrum resources.  
The basis for DSS between 4G and 5G is that the physical layers of both NR and LTE are designed based on orthogonal frequency division multiplexing (OFDM). Meanwhile, NR can choose the same parameter set as LTE, i.e., the same subcarrier spacing and time slot structure. The NR architecture is fully compatible with LTE and guarantees its independence. The LTE cell reference signal (CRS) and control channels occupy specific positions in continuous time- and frequency-domain resources, while NR reference signals, data channels, and control channels are highly flexible, allowing dynamic configuration. Therefore, using the dynamic flexibility of the NR physical layer to adapt to the relatively fixed LTE can avoid conflicts between the two technologies.  
For uplink physical channels, the physical uplink shared channel (PUSCH) of LTE and NR are dynamically shared according to uplink service requests; the physical uplink control channel (PUCCH)/physical random access channel (PRACH)/PUSCH of LTE and NR can avoid each other by means of frequency division multiplexing. The LTE PUCCH occupies 4-6 RBs, and NR PUCCH occupies 8-10 RBs. The PRACH of LTE and NR are periodically sent at fixed positions, each occupying 6 RBs. The LTE PRACH is close to the LTE PUCCH at low frequency; the NR PRACH is close to the LTE PUCCH at high frequency. The uplink channel map is shown in Figure 3.  
For downlink physical channels, the PDSCH of LTE and NR avoid each other in a frequency-division multiplexing (FDM) manner, where the LTE frequency is allocated from high to low, and NR frequency is allocated from low to high. LTE and NR PDCCH use time-division multiplexing (TDM) to avoid each other, occupy the full bandwidth in the frequency domain, and dynamically occupy the first three symbols of each TTI in the time domain. The resource conflict of single side band (SSB), system information block (SIB), Page, message2 (Msg2), message4 (Msg4), and LTE CRS in NR can be avoided by adopting NR rate matching. The downlink channel map is shown in Figure 4.  
Figure 3: Uplink channel map.