5G Testing: What Is Dynamic Spectrum Sharing?
2020-05-28 | 5 min read
Dynamic spectrum sharing (DSS) is a hot topic these days in 5G circles and for good reason. It provides several advantages to mobile network operators (MNOs), the main one being the opportunity to expand 5G coverage without having to permanently re-farm Long Term Evolution (LTE) spectrum or buy 5G spectrum.
DSS is a technique where LTE spectrum allocation is dynamically shared between 5G and LTE users. Depending on the load and traffic demand from both technologies, the base station dynamically changes the spectrum allocation to use more of the spectrum for 5G or LTE (Figure 1). As a result, the split between LTE and 5G New Radio (NR) in the spectrum changes over time.
Figure 1. DSS concept
DSS is especially appealing to MNOs because its rollout is possible through a software upgrade on existing base station hardware. The 3rd Generation Partnership Project (3GPP) only provides guidance on how to configure systems to enable efficient spectrum sharing for standalone (SA) and nonstandalone (NSA) deployments. Network equipment vendors (NEMs) can therefore implement DSS in various ways. Each vendor has its own proprietary DSS implementation with some using multimedia broadcast multicast service over single frequency network (MBSFN).
LTE MBSFN subframes can play a key role in downlink DSS implementation. Introduced in the 3GPP LTE specifications long ago to provide broadcast services over a mobile connection, these subframes can now be used to carry 5G traffic in the LTE channel. Figure 2 shows the control channels in blue and red color at the beginning of the subframe. The blank region is the part of the subframe originally meant for the broadcast service payload that can now carry 5G data. The figure also shows the combined subframe transmitted on the DSS channel with the MBSFN region filled with 5G data. Using MBSFN subframes allows any 5G numerology for the 5G transfer. It does not have to match the subcarrier spacing of LTE.
Figure 2. LTE MBSFN subframes with and without 5G data
Control channel resources in DSS are another important aspect for network operators considering using the technology. The scheduler must manage always-on periodic channel or channel/signal resources efficiently to limit the overhead impact on LTE and NR capacity. Both LTE and NR have control channels to keep the network operational. They include:
- Primary synchronization signal (PSS)/Secondary synchronization signal (SSS)/Physical broadcast channel (PBCH), periodic: 5 ms
- Cell specific reference signal (CRS), always-on
- Physical downlink control channel (PDCCH), at least 1 symbol due to 2/4 transmitter support
- Physical random access channel (PRACH) and corresponding msg 2/3/4 related resources
- Physical uplink control channel (PUCCH), channel state information (CSI) feedback needed to guarantee quick service restart
- Synchronization signal block (SSB), periodic
- PRACH and corresponding msg 2/3/4 related resources
- Paging, SA
- PUCCH, CSI feedback needed to guarantee quick service restart
- Sounding reference signal (SRS), if needed
Even though they are not carrying any user payload, control channels use capacity that could otherwise be used for user payload. In DSS, this issue is compounded by the fact that both LTE and NR generate overhead. While the impact on capacity varies depending on the DSS implementation technique, it could range anywhere between 10 and 40%.
Although it has an impact on network capacity, DSS is a powerful feature for MNOs allowing NR deployment using available spectrum while limiting the impact of NR services on existing LTE users. For more information on this topic, listen to the following on-demand webinars:
- 5G NR Dynamic Spectrum Sharing (DSS): Overview and Test Challenges
- Dynamic Spectrum Sharing (DSS) in 5G – Rollout Faster with at the Expense of Capacity?