Dynamic Spectrum Sharing (DSS) – Coexistence of 5G NR with 4G LTE

5G is the first ever mobile radio system that is designed to use any spectrum from below 1 GHz up to millimetre waves. 5G is also designed to be deployed in licensed, shared and unlicensed spectrum bands.

5G can use Frequency Division Duplex (FDD) technology for paired spectrum and Time Division Duplex (TDD) technology for unpaired spectrum.

Dynamic spectrum sharing (DSS) technology allows spectrum resources to be shared dynamically between 4G (Long Term Evolution (LTE)) and 5G New Radio (NR) radios. DSS functionality was defined in 3GPP standardization by adding enabling flexibility in the relevant specifications.

As operators have deployed 4G LTE on low / mid band spectrum as on date and re-farming the spectrum may not be possible. Such operators can choose to use DSS to launch 5G services. DSS allows dynamic allocation of frequency resources in time and frequency domain as per traffic and other factors.

DSS with 5G carrier aggregation (CA) have the full potential of the technology, especially when combined with standalone (SA) architecture. CA provides the highest data rates while SA maximizes low-band coverage and access to 5G services.

DSS is supported by large number of 5G devices launched in 2020. The devices coming this year in 2021 will support DSS, SA and CA.

As the waveform used by both 4G and 5G is OFDM and frame structure is similar, implementation of DSS becomes easier. The Sub frame duration of both 4G and 5G is 1 millisecond, sharing resources on one millisecond basis becomes available leading to good spectral efficiency. Use of general LTE sub frame for 5G NR is possible using Cell Reference Signal ( CRS) rate matching. 3GPP specifications have enabled shifting DMRS position from symbol 11 to symbol 12 to avoid collision with 4G CRS. There may still be limitation in transmitting 5G SSB on 4G LTE general sub frame.

The option of using LTE MBSFN type sub frame for 5G NR is straight forward. In this type of sub frame, barring symbol 0 and 1, balance 12 symbols are meant for broadcast services which LTE devices ignores. These 12 symbols can be used to carry 5G NR traffic. Operator can even decide to use MBSFN for SSB and other control channel on periodic basis and use non MBSFN subframe to carry data traffic. The recommendation of the industry is to use mix of MBSFN and non MBSFN sub frames for DSS as explained above.

Please go through the video to understand all these concept in super simple way.

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