The 5G system supports a wide range of services such as enhanced mobile broadband (eMBB), ultra-reliable low-latency communications (URLLC), and massive machine type communications (mMTC). Compared to its predecessor, 5G technology provides its users with enhanced experiences through faster data speeds, higher capacity, lower latency, and higher reliability. To arrive at such benefits without any disruption, service providers must find a seamless transition path from 4G long-term evolution (LTE) to 5G new radio (NR). However, newly released frequency bands for use in 5G deployments – such as the C-band (3-5 GHz) and millimeter wave (mmWave) band (24-40 GHz) – are higher than the current 4G frequency bands that sit below 3 GHz. This implies that while deploying 5G on the high frequencies with wider bandwidth may yield higher data rates, doing so will be inherently disadvantageous in terms of coverage due to the large amount of signal loss via propagation and penetration. A lower frequency spectrum, on the other hand, is favorable for 5G deployment, in that it provides a wide-area coverage. Therefore, for smooth transition, it is pivotal to deploy 5G in the lower frequency bands, which are mostly occupied by 4G frequencies

To this end, spectrum re-farming is the most logical and straightforward approach. As such, prior to 4G, spectrum re-farming was the conventional choice when transitioning from one communication generation to the next. Spectrum re-farming is done by draining all previous generation users from a frequency band and re-utilizing the same frequency band for next generation users. Re-farming is usually carried out at carrier levels, where a gradual reduction is made in the number of previous generation users, followed by a subsequent increase in the number of next generation users. For example, in Figure 1, in the second diagram from the left, a single LTE carrier is replaced with an NR spectrum to provide services to a few new NR users. As the number of NR users increases, more LTE carriers are replaced by NR carriers until the entirety of the frequency band is occupied by NR carriers and used for NR purposes.

Today, 4G services dominate most of the communications market and this trend is expected to continue for several years to come. At the early stages of 5G commercial deployment, LTE users outnumber NR users in a given network; therefore, allocating LTE carrier spectrums only for a small number of NR users significantly hinders LTE users.

Against this backdrop, LTE-NR spectrum sharing emerges as a technology that allows service providers to deploy LTE and NR in the same carriers and bands. That is to say, spectrum sharing enables both LTE and NR to be simultaneously deployed and share resources in the carrier, as shown in Figure 2. The time-frequency resources in the carrier are dynamically assigned to either LTE or NR according to their respective traffic demands. This dynamic allocation is known as dynamic spectrum sharing (DSS). In an early NR market, DSS is advantageous in that it allocates only the required amount of time-frequency resources to the few NR users, and reserves the remaining resources for LTE services. Over time, as the number of NR users increases, DSS accordingly allocates the required resources for NR purposes. In turn, this flexible spectrum sharing solution allows for a smooth 5G migration. 

Considering the aforementioned reasons, DSS appears to be a promising technology that enables the coexistence of multiple radio access technologies and the utilization of low frequency band for NR – all the while eliminating the need for new spectrum allocation for 5G. However, DSS comes with its share of concerns that raises the question of whether or not it is worth incorporating the dynamic solution as a migration tool from LTE to NR.  
In this paper, the overall benefits of DSS are presented. In addition, 3GPP standard functions that deal with the coexistence of LTE and NR – as well as the implications, design principles, and side effects that they dictate – are introduced. Lastly, the considerations, performance degrading factors, and concerns associated with DSS are discussed.  

Prepared by: Samsung

Download: /en/leon/a/upfiles/file/202104/2021042315190794794.pdf