The ABCs of 5G LTE Router-DSS
To garner the benefit of Dynamic Spectrum Sharing (DSS), not only the 5G networks, but also the terminals such as smart phone and 5G LTE routers should support this feature. Now, the question is: what is DSS, and why is DSS needed?
What is DSS or dynamic spectrum sharing?
Dynamic Spectrum Sharing (DSS) allows 4G LTE and 5G NR to share the same spectrum and dynamically allocate time-frequency resources to 4G and 5G users.
Figure 1 Static & dynamic spectrum sharing
Spectrum sharing can be achieved in both static and dynamic ways.
Static spectrum sharing refers to the provision of separate carrier within the same frequency band for different technologies (such as 4G and 5G). This method is "simple and transparent", but the spectrum utilization is low.
Dynamic spectrum sharing refers to the dynamic and flexible allocation of spectrum resources within the same frequency band for technologies of different standards. This method can improve spectrum efficiency and facilitate the smooth evolution between 4G and 5G.
Why is dynamic spectrum sharing needed?
- To achieve extended 5G coverage by using lower frequency band
In 5G NR, services that require high-speed and high-capacity utilize frequency bands that are largely divided into three categories. The first is a low frequency band that sits below 3 GHz, which is primarily occupied by LTE services. This portion of the frequency band is mostly operated in a frequency division duplex (FDD) method. Next, there is the mid frequency band (mid-band) from 3 GHz to 5 GHz. Last, the mmWave frequency band (mmWave-band) is located between 24-40 GHz. Both the mid-band and mmWave-bands are operated by time division duplex (TDD).
In general, NR is co-deployed in LTE sites and reuses existing LTE infrastructure. In this scenario, if the NR were to only utilize the mid-band TDD carriers (since the low bands would be reserved for LTE use), it would have larger propagation and penetration losses, compared to when using low-band FDD carriers. This physical limitation of the mid-band frequency inevitably reduces its coverage, especially in the uplink (UL) transmission, resulting in coverage holes. Figure 2 depicts the coverage holes that would exist in the hypothetical situation mentioned previously, where NR is operated on mid-band only. The indoor coverage hole is a direct result of penetration loss found in mid-band frequency. It is also important to note that coverage reduction is much more increased in mmWave-band TDD carriers.
Dynamic spectrum sharing technology can dynamically share 4G low-frequency resources between 4G and 5G networks, and thus easily achieve further and in-depth 5G coverage.
Figure 2 Coverage hole of mid-band NR
- To facilitate the smooth transition from 4G to 5G, and reduce the initial investment in 5G
DSS provides flexible resource management that corresponds accordingly to NR UE penetration and NR traffic demand, resulting in high spectrum utilization. Figure 3 shows the changes in LTE and NR traffic as the demand for NR gradually increases, and more importantly, how the spectrum would be utilized under re-farming and DSS scenarios.
Figure 3 DSS and spectrum re-farming as NR demand increase
In early NR market, traffic demand of NR may not be explosive enough to require all of the available resources from the re-farmed band. Therefore, this can lead to underutilization of resources in the re-farmed band that could otherwise be used for LTE traffic. This is to say that so long as LTE traffic dominates the market, some resources will be left unused. On the other hand, when demand for NR surpasses that of LTE, in a re-farming scenario, there will be insufficient NR resources to handle all the NR traffic demand while some resources allocated to LTE will be left idle as LTE traffic demand subsides.
DSS overcomes this setback by dynamically allocating resources according to traffic demands between LTE and NR across the entire band. To enable this feature, LTE and NR schedulers must coordinate with each other in order to interchange traffic status or resource sharing status, as well as dynamically assign available resources in a synchronized manner. Through sophisticated coordination between schedulers, LTE resource allocation increases and NR resource allocation decreases when LTE traffic peaks; and vice versa when NR traffic peaks.
- To facilitate SA networking
As we all know, there are two 5G networking modes: NSA and SA. The NSA mode uses the existing 4G network as anchor and introduces 5G NR, which helps operators to quickly roll out 5G services. However, NSA networking mode is still a continuation of 4G ecology, mainly targeting eMBB scenarios and consumer market.
SA mode is the final networking architecture of 5G, enabling diversified vertical industry applications and increasing revenue sources for operators. To this end, the world's leading operators are actively preparing for 5G SA.
Figure 2 ENDC and Carrier Aggregation
However, since SA network no longer depends on the legacy 4G network, a complete 5G network with wide coverage needs to be deployed from the beginning. Considering the higher frequency band of 5G, the coverage range of a single station is smaller, which means greater network investment.
With the adoption of dynamic spectrum sharing technology, wide coverage of 5G SA can be quickly achieved by using lower 4G frequency band.
- Easy to support 5G carrier aggregation
NSA network introduces 5G services and data rates in a predominantly 4G network by EN-DC, i.e., LTE & 5G Dual Connectivity. In terms of performance, EN-DC is lower than Carrier Aggregation.
By adopting dynamic spectrum sharing, lower frequency band used by 4G is dynamically allocated to 5G, which can realize 5G carrier aggregation between FDD low band and TDD mid-band to maximize performance.
