Spectrum reframing from 2G to 3G and then to LTE was a slow and comparatively complex process and also expensive. It was not possible to share the spectrum between different technologies. Spectrum reframing from LTE to 5G will be simpler with DSS.
Dynamic Spectrum Sharing (DSS) is a technology that will allow both 4G and 5G to simultaneously operate within the same spectrum. In other words, we can say that 5G & LTE occupy one spectrum band from the control channel viewpoint. The resources are allocated dynamically between the two technologies based on device distribution & capacity requirements. That includes the set of resource elements shared among different technologies. DSS is applicable to frequency range 1 (FR1) as existing 4G bands operate below 6GHz.
It will allow the spectrum to be reframed without any changes in its utilization, facilitating a smooth evolution from one technology to another. It provides better 5G coverage to the end-user. It also provides faster download speeds with high quality & low latency to consumers. The application of DSS on lower bands would be significant in enabling low latency and deeper in-building coverage.
For example, without DSS an operator with 20MHz of mid-band spectrum would have to split the spectrum into 2. In other words, operators would have to allocate 10 MHz of spectrum to 4G LTE and include all their LTE users into that 10MHz of spectrum. The remaining 10 MHz of AWS (advanced wireless spectrum) could be used for 5G users. With DSS, an operator doesn’t have to split the mid-band spectrum or have a dedicated spectrum for either 4G or 5G. Instead, they can share the 20 MHz of the spectrum.
Fig: Spectrum split between 4G and 5G
DSS requires coordination of the resources used by each system. It requires the 5G and 4G systems to be synchronized with respect to time and frequency domains. In DL direction, it is relatively simple to achieve radio frame synchronization between two technologies. Both sets of transmissions can share the same DL timing reference at the base station, for instance, derived from GPS or timing over the packet. Whereas, in the UL direction it is necessary to align the timing advance applied by the population of 4G and 5G UE.
Fig: Deployment of DSS
What are the implications for NR Devices?
NR devices can support DSS either at the RB level or at the RE level. NR UE can support RRC based or DCI based RB-level rate matching operation through Rate Match Pattern signalling in order to avoid the NR data mapping to the RBs overlapped with LTE CRS. In the case of NR, OFDM symbols overlapped with LTE OFDM symbols, the LTE spectrum cannot be shared with NR. In the NR OFDM symbols not overlapped with LTE OFDM symbols, the LTE spectrum and NR spectrum can be shared dynamically through data scheduling. In this scenario, the NR network and LTE network should exchange the scheduling information.
What Are the implications for an operator or a base station manufacturer?
DSS is clearly ideal for low band rollout, as it will allow operators to continue using the valuable spectrum for 4G while adding 5G capacity as demand grows. But whether it will be enough to address the looming issue of massive data demand on 5G networks remains to be seen. At the base station side, it will allow the operators to dynamically allocate some of their existing 4G LTE spectrum to 5G and use existing radios to deliver 5G services by deploying a software upgrade.
DSS will help a mobile operator to flexibly allocate existing spectrum across low-, mid- and high- frequency bands, by dynamically switching between 4G LTE and 5G NR coverage based on traffic demand. Mobile operators can hold DSS to deliver the best possible performance and coverage for a mix of 4G and 5G devices.
5G capacity is impacted by LTE Cell Reference Signals (CRSs), PDCCH, PSS, SSS, PBCH, and MBSFN subframes overheads as well as NR PDSCH DMRS and NR corset. There is less impact from the 5G control channels on LTE as 5G does not use any CRS. UE can support DSS at either RB level and RE level. Using the RATE matching technique network can avoid LTE overhead.
Beyond the NR operation above, on the LTE side, one of the most promising ways is configured with the MBSFN subframe which can potentially avoid the CRS impact.
Thus, DSS for 5G will be used as part of a strategy that takes into consideration the existing LTE network and the customer experience and balances it with the knowledge that the customer’s expectation from 5G might go beyond the capabilities offered by this technology alone.
We at Simnovus, offer solutions to overcome these challenges and enable the transition to 5G using DSS. Our Network Emulator solution can be configured to simulate eNB and gNB in the DSS mode and check how the devices operate under that configuration. Similarly, our UE Simulator solution can simulate UEs operating in both 4G and 5G and verify the capability of eNB and gNBs supporting DSS mode. This helps Network Equipment Manufacturers and Mobile Network Operators to identify the issues in the lab before going out to the field.