SuperDSS: An Effective Scheme for Integrating FDD NR Spectrum Resources in 5G Era

With the deep integration of new-generation information technologies represented by mobile internet, IoT and cloud computing with traditional industries, the broadband, ubiquitous, mobile and multi-standard characteristics of radio technologies have become more and more obvious. Various network standards such as GSM, UMTS, LTE, NB-IoT, eMTC and 5G coexist in the wireless network. Radio spectrum resources are crucial to mobile communication, and the frequency bands below 3G with the best coverage are non-renewable treasure in the radio bands. But in fact, there are some problems such as the coexistence of multiple modes and serious fragmentation in these frequency bands, which limit the use of them by 3G, 4G and 5G networks. Facing the challenges, ZTE has proposed SuperDSS, a unique scheme for tri-RAT dynamic spectrum sharing (DSS). 

The Trouble of FDD Spectrum Refarming
The lower the frequency band, the longer the wavelength, and the farther the coverage distance. Therefore, in the early stage of wireless network development, 2G/3G/4G networks using the FDD mode basically occupy all low frequency bands.
With the increasing demand for user data and the development of wireless networks, countries around the world have begun to build 5G networks. However, the coverage of high frequency bands is weak, so it is necessary to build more sites and invest more to achieve continuous coverage of 5G networks. The urgent problem for operators is how to deploy 5G carriers in the lower frequency bands already occupied by 2G/3G/4G, improve 5G basic coverage and reduce construction costs.
At present, the solutions in the industry generally involve the following three aspects:
—Spectrum refarming: Consider disabling 2G/3G networks directly and refarming the spectrum to the 5G network. Although spectrum refarming is the most direct way, according to an operator's current network traffic statistics, 70% of existing voice service is still carried over 2G/3G networks. Therefore, it is difficult to withdraw most 2G/3G spectrum for refarming.
—Static spectrum allocation：2G/3G/4G/5G exclusively occupies a part of the spectrum, and the spectrum bandwidth obtained by each standard is limited. This way of division cannot leverage fully the advantage of 5G wide bandwidth, so it is of little significance.
—Spectrum sharing: At present, Magic Radio Pro, Cloud Air and DSS used in the industry solve the dynamic spectrum sharing between the two systems, but can not cope with the scenario where the three systems coexist at the same time.

SuperDSS: Improving 5G Spectrum Utilization and Cell Throughput
Considering 5G spectrum and 2G/3G features, ZTE has proposed the SuperDSS scheme. Under the premise of ensuring the service quality of traditional voice users under the limited spectrum resources, SuperDSS aims to dynamically realize the sharing of spectrum resources among different systems according to the service load, avoid spectrum fragmentation, and ensure that 5G network occupies as much bandwidth as possible to maximize its value. The scheme includes 3G/4G/5G dynamic spectrum sharing (Fig. 1) and 2G/4G/5G dynamic spectrum sharing (Fig. 2).

To implement the tri-RAT dynamic spectrum sharing, it is necessary to consider the following issues among multiple systems.
—Principle of spectrum allocation: 5G NR bandwidth is greater than or equal to 4G LTE bandwidth, and the spectrum bandwidth occupied by 2G/3G is less than that of 4G LTE. The NR/LTE bandwidth is greater than or equal to 15 MHz, and the total spectrum occupied by 2G/3G is not greater than 5 MHz.
—Service priority: As voice services have a high requirement for real-timeliness, it is necessary to give priority to 2G/3G users to occupy spectrum resources. LTE/NR users carry more data traffic (VoLTE/VoNR users have a higher priority than data users), so their priority is secondary. The priority scheduling between LTE and NR should be judged according to traffic load of the network itself. The higher the load, the higher the priority.
—Interference avoidance: Spectrum sharing is actually resource multiplexing. The resource multiplexing is divided into frequency division multiplexing, time division multiplexing, code division multiplexing, and space division multiplexing. Code division multiplexing and space division multiplexing are generally only used within the systems, while frequency division multiplexing and time division multiplexing can be applied within and between the systems. The dynamic spectrum sharing technology is a typical inter-system resource multiplexing. According to GSM/UMTS/LTE/NR technical features, the interference between 3G/4G/5G or 2G/4G/5G channels can be addressed by intelligent time-division and frequency-division scheduling algorithm, interference suppression and cancellation, as well as baseband and RF filtering.
—Dynamic bandwidth adjustment: The gain of dynamic spectrum sharing is mainly reflected in the change of dynamic bandwidth that enables LTE/NR to obtain more spectrum resources and thus improve spectrum utilization and cell throughput. There are broadcast carriers and service carriers in GSM cells. GSM BSC dynamically activates and deactivates GSM carriers according to BTS traffic conditions. UMTS cells generally have single carriers with a bandwidth of 5 MHz. It is impossible to directly deactivate UMTS carriers through RNC like GSM, otherwise UMTS users will lose network services. UMTS can dynamically compress the bandwidth from 5 MHz to 2.6 MHz through the baseband and RF filtering while ensuring the voice quality of UMTS users remains unchanged. The spectrum resources released by GSM/UMTS are used by LTE/NR to obtain capacity gain.
On the condition of effective spectrum resources, 4G/5G DSS can rapidly deploy 5G network while satisfying the current LTE capacity. However, SuperDSS can not only rapidly deploy 5G network, but also address the issue of FDD spectrum fragmentation and waste caused by 2G/3G/4G/5G coexistence. It can also bring operators the improvement in 5G network coverage and cell capacity as well as lower O&M costs.

SuperDSS Market Application
ZTE released its SuperDSS, the industry's first tri-RAT dynamic spectrum sharing scheme in February 2020. The scheme has attracted the attention of all parties in the industry. ZTE cooperated with Henan branch of China Unicom to complete commercial verification of the world's first 3G/4G/5G tri-RAT dynamic spectrum sharing solution in April 2020.
Field test results show that provided that the total spectrum bandwidth remains unchanged, SuperDSS gives priority to ensuring 5G user experience while taking into account 3G and 4G users, so that spectrum resources can be flexibly scheduled and allocated among 3G/4G/5G users. Compared with the statically allocated spectrum scheme, SuperDSS increases  the total throughput by 35%.
SuperDSS is the core innovation of ZTE Magic Radio Pro for 5G evolution. This indicates that ZTE has reached a new level in the field of spectrum sharing, leading the development of the industry.