Dynamic Spectrum Sharing (DSS): The 5G deployment x-factor
5G aims to provide gigabits of throughput, highly reliable low latency connections for industrial automation and other time sensitive applications. To deliver, however, 5G networks require ubiquitous and seamless 5G coverage. The big challenge is the availability and the type of RF spectrum. 5G standards, starting from release 15, support spectrums both in the sub-six GHz and mmWave bands. Yet to offer gigabits of throughput, large chunks of RF spectrum are needed, most of which are available in the mmWave. Challengingly, RF free space loss increases with the frequency of operation.
For example, roughly an 8dB additional loss occurs as we move from 800MHz service operation to 1900MHz service operation. This loss is more pronounced in the mmWave and RF free space loss can be as high or even higher than 30dB compared to lower bands. Additionally, RF coverage is more susceptible to environmental conditions in that frequency range, which means to offer a seamless coverage in an 800MHz service area, potentially 20 to 30 times the number of cell sites are needed in the mmWave.
This creates a 5G deployment challenge for service providers in two ways:
- Deployment Scale: Providers would have to deploy an unreasonable number of cell sites to achieve a comparable sub six GHz coverage area.
- Band Availability: Lower bands that are already limited in bandwidth may already be capacity constrained with LTE users.
How Can Service Providers Accelerate 5G Deployment?
The question service providers face is how to accelerate the velocity of 5G deployment under such conditions. Typically, mobile operators acquire new spectrum to deploy new wireless technology (2G, 3G, 4G and 5G) or re-farm existing spectrum; however, both methodologies are expensive and lengthy to implement. Further, re-farming may not be a good option unless a significant subscriber base upgrade to the latest technology. So, for deploying 5G, re-farming LTE low band spectrum when the number of 5G subscribers is not large, can potentially mean capacity bottleneck for LTE subscribers, higher interference, and performance degradation to existing LTE network.
A more streamlined and efficient way to introduce 5G quickly would be dynamically assigning RF resources between the 5G and the LTE subscribers in the existing low band LTE service area. This method can allow providers to maximize 5G coverage in a short amount of time and depending on the high throughput demand they can deploy hot spots of 5G in the higher band spectrum.
Dynamic Spectrum Sharing (DSS)
Dynamic spectrum sharing (DSS) technology is that x-factor. In most cases, DSS can be deployed as a software upgrade to the existing LTE radios, it allows service providers to accelerate 5G NR deployment and expand 5G NR coverage area without significant investment in re-farming existing low band spectrum. DSS technology takes advantage of non-stand-alone (NSA) mode by dynamically and intelligently sharing low band LTE spectrum with both LTE and 5G NR subscribers. Additionally, NSA configuration allows control plane communication and mobile network management to be performed through the LTE infrastructure whereas the new 5G spectrum, especially in the mid to mmWave band, can be used to boost capacity for the user plane traffic.
DSS is implemented by scheduling NR users in the LTE subframes while ensuring no respective impact on LTE users in terms of essential channels, such as reference signals used for synchronization and downlink measurements. The other aspect of DSS design is to fit the 5G NR reference signals within the subframes in a way to avoid affecting NR downlink measurements and synchronization.
For that, DSS considers the options shown in the figure below to ensure NR reference signals such as Synchronization Signal Block (SSB) or Demodulation Reference Signal (DMRS) are placed in time-frequencies away from any collision with LTE signals. Multi-Broadcast Single-Frequency Network (MBFSN) is used in LTE for point-to-multipoint transmission, such as Evolved Multimedia Broadcast Multicast Services (eMBMS). The general idea of MBSFN is that specific subframes within an LTE frame are reserved and free from other LTE channel transmission. These symbols are intended to be used for broadcast services and are not used for data transmission for other LTE UEs. In DSS these reserved symbols are used for NR signals instead of eMBMS.
Using MBSFN is completely transparent to legacy LTE-only devices. This method has disadvantages as well: primarily, if MBSFN subframes are used very frequently, it takes away resources from LTE users, reducing LTE user throughput. As we all know, nothing comes for free, hence, DSS implementation in an LTE service area also presents some challenges:
- Spectral efficiency impact that may result from the additional management “overhead” traffic required by DSS.
- Optimizing traffic management policies, basically managing capacity vs. user experience and overall LTE and 5G user experience.
- Isolating performance (interference, signal quality or network) issues for both LTE and 5G at the same time and the mutual effect they have on each other.
Understanding the RF Environment for DSS
But to overcome these challenges, knowledge of the RF environment is the key. With the help of VIAVI CellAdvisor™ 5G base station analyzer, operators can isolate spectral or signaling issues quickly with simple over-the-air measurements using a single field instrument. CellAdvisor 5G with DSS and NSA mode offers the following functionality and features to quickly validate both 5G as well as LTE performance:
- RF characterization to validate the performance requirements and 3GPP conformance tests of LTE and 5G NR radios
- 5G beam analysis and LTE reference signal power analysis
- NSA Scanner that delivers 5G channel/beam power and signal quality measurement along with LTE channel/reference signal power measurement
- NSA Route Map delivers both 5G and LTE coverage concurrently
With the right test solution, I believe service providers can easily accelerate their 5G deployments and can be sure that their DSS deployment doesn’t any noticeable user performance degradation.