The rollout of Dynamic Spectrum Sharing (DSS) solutions during 2020 marked the emergence of this technology in supporting the 5G transition. DSS will continue gaining momentum in the next few years as it is expected that the worldwide investments in LTE and 5G NR RAN infrastructure operating in a shared spectrum will surpass $1.3 billion by the end of 2021; nearly $4 billion by 2024.
What’s the fuss about DSS?
Globally, most frequency-division duplex (FDD) bands are occupied by 4G LTE, so operators lack low and mid-band spectrum for its 5G NR deployments. These bands are required for the overall transition from 4G to 5G and from 5G non-standalone to standalone.
Dynamic Spectrum Sharing (DSS) plays an essential role in facing spectrum availability and coverage issues. It allows for the adoption of the existing 4G LTE deployments to coexist with 5G NR and share the same low-band spectrum used exclusively by LTE today. This is sometimes preferably done rather than implementing costly solutions, such as acquiring new spectrum assets through long auction processes or rolling trucks to re-farm spectrum that are already in use on a site-by-site basis.
How does DSS work?
Dynamic Spectrum Sharing technology was defined in 3GPP Rel-15 and was further enhanced in Rel-16. It introduces techniques for the waveforms to allow for the deployment of both 4G LTE and 5G NR to simultaneously operate in the same frequency band/channel. The base stations and the network can then allocate spectrum resources to dynamically shift between the 4G and 5G users at each cell site. This DSS capability can be enabled as a software upgrade for the existing LTE radios.
The spectrum resource allocation is based on traffic load levels, device capability, the amount of data in the buffer, and the 4G – 5G radio signal quality. The network then determines the amount of Physical Resource Blocks (PRB) to be given to 4G and 5G users in every Transmission Time Interval (TTI). DSS mechanism does such dynamic spectrum re-allocation in 1-millisecond increments, which means it measures the radio channel conditions and allocates a different mix of PRB radio resources instantly, every 1ms.
Dynamic Spectrum Sharing’s value proposition is crystal clear when it comes to ramping-up 5G New Radio rollouts. This 5G-4G co-existence provides a smoother transition to 5G by allowing network operators to cost-efficiently establish a 5G coverage layer in low-frequency bands that are more extensive than that offered by millimeter-wave.
DSS can significantly increase spectral efficiency by introducing 5G NR into the 4G spectrum band. Moreover, DSS combined with carrier aggregation allows for higher peak rates across a larger coverage area. This combination provides higher flexibility in deploying and establishing a 5G network that will be developed and enhanced over time.
DSS market adoption
Some network operators already had taken advantage of DSS in 2020, and large-scale network rollouts are expected during the 2021 year. It facilitates network operators, such as Vodafone Ziggo, to be first-to-market with nationwide 5G coverage. Through utilizing DSS, they were the first operator to launch 5G in the Netherlands in April 2020. There was no need to wait until the country’s 5G multi-band auction to be completed in July 2020 to start the deployment.
Likewise, Verizon has been one of the greatest advocates of DSS. The operator has stressed that DSS is the key to its 5G strategy. Its 5G Nationwide service covers more than 200 million customers and runs in low-band using its existing spectrum. It permits that when Verizon’s customers move outside the Verizon 5G Ultra-Wideband coverage area, their 5G-enabled devices remain on 5G technology.
The next in entering the 5G market using Dynamic Spectrum Sharing is TIM Brazil. The operator has recently announced that their 5G service will be available in twelve Brazilian cities by March 2021.
According to the current deployments, this technology is being deployed in low sub-6GHZ frequencies, typically for the 10, 15, and 20 MHz carrier bandwidth, providing much better 5G coverage than can be achieved from higher new 5G bands.
Does DSS eat away the net capacity of the shared radio?
Despite Dynamic Spectrum Sharing (DSS) bringing an excellent advantage for accelerating 5G network rollouts, some operators consider that splitting resources could impact the LTE performance experience. This is because DSS reduces the base capacity on the LTE side. Nonetheless, this is a marginal impact. Here is why:
In order for a 5G device to access the network, the 5G NR transmits its synchronization signal blocks (SSB) and tracking reference signals (TRS). The SSB permits the 5G device to initially synchronize to the downlink signal and perform the initial access procedure. Meanwhile, the Tracking Reference Signals (TRS) are required to maintain synchronization in time and frequency.
SSBs must be sent periodically, with a gap defined to transmit the SSB on an already occupied frequency channel used by LTE. Multimedia Broadcast Single Frequency Network (MBSFN) subframes are enabled to allow this gap to have a continuous LTE transmission. In typical configurations, only three out of 40 subframes are MBSFN subframes. Those subframes would not be available for LTE, but it allows for a 5G NR device using the same frequency band. With just three subframes available for 5G NR, the technology operates under its potential, a fair trade-off considering it allows the 4G-5G co-existence.
Extensive monitoring is required to ensure minimal impact on LTE performance when the 5G NR is present. This monitoring comprises LTE and 5G user equipment behavior analysis and network performance measurements to evaluate the coverage, end-to-end (E2E) throughput, and performance.
CovMo provides a holistic view of the actual user traffic demand and network performance. It harnesses the DSS features such as own cell identity and frequency band (according to the operator deployment strategy) to identify DSS traffic, allowing for fine-grained DSS-specific KPIs monitoring. Moreover, leveraging the most advanced 5G Planning module on the CovMo platform, DSS utilization can be analyzed and forecasted. This allows operators to activate the DSS in the right sites and ensure the most optimized Dynamic Spectrum Sharing (DSS) implementation.
For more information, contact us at https://www.ghtinc.com/alliance-request/.
 SNS Telecom & IT’s latest research report: Shared & Unlicensed Spectrum LTE/5G Network Ecosystem: 2021 – 2030.
 Within 4G and 5G, radio resources are organized into physical resource blocks (PRBs), which are transmitted within a specific transmission Time Interval (TTI).