With 5G, rolling out faster than the rate at which legacy technologies like LTE rolled out. Countries are in a race to provide nationwide 5G coverage to gain from the socio-economic benefits the 5G technology has to offer. For the network operators, it is also about retaining the subscriber base by offering the best of technology in time.
At the same time, we have to be aware of the regulated and restricted availability of spectrum, which is a finite resource at hand. Dynamic spectrum sharing is a key feature that enables network operators to offer 5G NR in the same spectrum as LTE, enabling a faster adoption of 5G NR services. Even though 5G exhibits a faster uptake of subscribers compared to the legacy LTE, it is worth noting that the LTE will still be the dominant serving technology for a few more years to come. LTE subscription shall peak to 5.1 billion by 2025 and projections are that 5G subscriptions shall reach 2.8 billion by 2025 as per Ericsson Mobility report. Which means LTE deployments are important and are here to stay and any new feature introduction like the DSS shall not adversely affect the existing LTE user base.
What is DSS?
Dynamic Spectrum Sharing is a 3GPP Rel.15 feature, which allows co-existence of LTE and 5G NR on the same frequency carrier. Within a given time instance, the resources are dynamically shared between LTE and NR based on instantaneous traffic load. The introduction of DSS is backward compatible i.e. the existing LTE devices should continue to be able to decode the signaling messages and operate normally even after the introduction of 5G NR. In most cases, infrastructure vendors are offering DSS as a software upgrade making the feature even more lucrative for deployment.
DSS ensures 5G NR deployments on the same LTE low band carriers, providing the wider coverage as required. DSS involves the smart algorithm that balances the resources allotted for LTE and 5G NR enabling a smoother transition from LTE to 5G NR. This is a more viable option than having to refarm the spectrum or acquire new license for 5G NR deployments.
DSS Deployment Options
DSS is about scheduling of 5G NR users on the existing LTE sub frames without affecting and interfering the existing LTE channels and signals. Cell specific Reference signals (CRS) in LTE are the most essential signals with pre-defined time-frequency resource pattern, which is transmitted always in a cell for the LTE users to do cell search, initial acquisition and provide channel estimation feedback. Therefore, 5G NR scheduling should take care of not impairing or overlapping the LTE CRS signals which is a very important criterion considered in DSS deployments. There are a few options to do deploy DSS:-
MBSFN means Multicast Broadcast Single Frequency Network, a point to multipoint transmission mechanism supposed to serve broadcast data through cellular technology like LTE. In Rel.9, a feature called eMBMS was introduced, where in within the LTE frame, certain subframes are marked for MBSFN. In these MBSFN subframes, the last 12 symbols could be used for broadcast data. Although this feature did not fly and was hardly deployed by the network operators. Now, in DSS these MBSFN subframes are used to introduce 5G NR users instead of eMBMS, without affecting the LTE signals. The MBSFN subframes are anyway known to the LTE devices as subframes carrying broadcast data and would ignore. The more MBSFN subframes scheduled for NR, the lesser the capacity available for LTE. This is the most commonly deployed option in the field today due to the simplicity of implementation.
Rate Matching based:
In this method, the non-MBSFN or the normal LTE subframes are used for 5G NR scheduling. As highlighted previously, the LTE CRS are transmitted across the resource time plan at pre-defined resource elements. The 5G NR scheduling should avoid these resource elements meant for LTE CRS and therefore not schedule NR PDSCH in those RE’s. To achieve this, the scheduler uses rate matching/puncturing on the 5G NR PDSCH allocation such that the specific RE’s meant for LTE CRS are not affected. As the position of RE’s carrying LTE CRS within the resource time plan differs based on the bandwidth, number of MIMO layers, number of carriers – the scheduler algorithm responsible for rate matching is complex. Although, rate matching based DSS implementation offers higher spectral efficiency than the other methods
This method is based on the already available provision within NR to use mini-slots. Here, while using minislots for 5G NR scheduling within the LTE subframe, again the RE’s meant for LTE CRS is omitted from NR scheduling. As the mini-slots are ideal for low latency applications, might not serve the eMBB use case well due to limited mini-slots available for NR.
From a test and measurement perspective
DSS is a key enabler with lots of flexibility and many ways to implement. With flexibility comes complexity and calls for extensive testing on the devices and networks for RF and protocol behaviors. Test arena for DSS involves lab based testing during the development phases of devices and base station, followed with field test using scanners, and specialized test devices to test for coverage and end-to-end user experience after DSS deployment.
On the device side, it is important that the various configuration permutations with the introduction of DSS be followed properly on the NR side. Improper configurations could adversely affect the performance of LTE and 5G NR in terms of KPI’s like throughput achieved, sensitivity of the device etc. Typically, test cases to check DSS functionality would include criterions as below:-
- 3gpp specification compliance on both RF and Protocol front.
- Ensure that Impacts of MBSFN based DSS implementation on LTE performance is minimal.
- To verify NR and LTE devices can synchronize, attach and register to the network after DSS implementation
- 5G NR SSB signals induced into MBSFN subframes are not affecting the LTE device receiver sensitivity.
- It is also important that UE capabilities and network configurations are optimized for performance and higher spectral efficiency.
The R&S CMX500 mobile radio tester platform together with the CMW500 offers the complete flexibility in parameterization of DSS required for extensive testing and benchmarking on LTE and 5G NR devices. 3GPP lays the fundamental guidelines to facilitate DSS implementation. However, the DSS feature implementation itself is infrastructure vendor specific and mostly proprietary algorithms are in use. Hence testing the DSS feature in field is equally important.
Similarly, the signal generator and spectrum analyzer combo of R&S®SMW200A and R&S®FSW are ideal toolsets to verify DSS configuration during the RF development process on both the devices and gNBs.
Besides testing the device, field testing is needed also to ensure the performance of the DSS feature. Here, the R&S®5G Site Testing Solution (STS) brings you automatic detection and characterization of 5G signals together with functional and over-the air tests. making site acceptance testing and troubleshooting easy. As soon as the operating bands are selected, 5G STS instantly detects the 5G and LTE signals on air and delivers detailed information about each of them such as signal quality, power level, beam analysis, MIB and SIB content, spectral measurements functional tests such as uplink throughput, downlink throughput and latency. The 5G STS is an important tool for verifying 5G sites by simultaneously measuring 5G and LTE In combination with the QualiPoc Android, quality tests such as voice, data and video make it possible to verify the service availability of the site.
Spectrum is a scarce resource, and has to be efficiently used. Most NetOps around the globe would use DSS as an important 5G-network roll out strategy. In Indian context it is still to be seen, which operator would win how much of 5G NR spectrum and in which bands. Nevertheless, for sure given the large geography to cover, DSS would be a powerful and handy feature for nationwide 5G coverage. Besides, it allows network operators to plan for a smoother and cost effective transition from LTE to 5G NR.
Further with the advancements in the device ecosystem and support for features like 5G NR FDD + TDD CA, Cross carrier scheduling and higher order carrier aggregation, DSS indeed becomes more sophisticated and requires extensive testing in the lab on both devices and gNB, in addition to coverage and QoE in the field.
Rohde & Schwarz is ideally positioned to address the end-to-end test requirements for DSS with its wide offerings in terms of Signal generator and Analyzer for RF developments, Mobile Radio Testers for Functional tests and Network scanners to measure coverage.