Dynamic Spectrum Sharing: DSS

The introduction of dynamic spectrum sharing (DSS) will permit Coexistence of 5G NR with 4G LTE, allowing network operators a smooth transition from LTE to 5G NR.
The Dynamic Spectrum Sharing (DSS) solution allows mobile network operators to use LTE and NR services using the same frequency bands, although in an interleaved mode. This allows NR services without the need of acquiring new and dedicated frequency spectrum, antenna, or radio frequency units.
Initial 5G Deployment:

The first 5G NR networks are on-air using NSA mode. (Combining a low band LTE carrier with a high band NR carrier)
Majority of frequency bands worldwide are frequency-division duplex (FDD) and are used by 4G LTE, so the first 5G NR network deployments used underutilized time-division duplex (TDD) frequency bands. [TDD FR1 bands at higher frequencies (e.g. 3.5 GHz, 2.6 GHz or 4.8 GHz).]
The next phase of 5G NR deployments will be based on FDD in the paired spectrum.

Why Spectrum sharing?

5G NR can operate on Low, Mid and High Band frequencies for different coverage / Capacity/ and usage scenario.
5G for High Capacity & Coverage
Low and mid band frequencies are already allocated for 4g and may not be available.
Cost of new spectrum may be very High
Re-farming of new spectrum may be very high
Spectrum re-farming is done by draining all previous generation users from a frequency band and reutilizing the same frequency band for next generation user
Get early advantage of 5G Radio which is 3X
spectral efficiency then 4G
Opportunity to expand 5G coverage without having to permanently re-farm LTE spectrum or buy 5g spectrum.
Can be deploy in both SA and NSA mode.

Dynamic Spectrum Sharing

Dynamic Spectrum Sharing allows an existing LTE carrier to operate 5G New Radio (NR) and LTE simultaneously – with a simple software upgrade [Rollout is possible through a software upgrade on existing base station hardware.].
The enabling mechanism is DSS, which allows 5G NR and 4G LTE to coexist while using the same spectrum(band) and dynamically allocates spectrum resources between 4G and 5G based on user demand.
Spectrum is dynamically shared between 5G and LTE Users.
Depending on Load and traffic demand from both technologies, the base station dynamically change the spectrum allocation to use more of the spectrum for 5G or LTE.