5G and 4G TDD synchronization

In TDD, the transmission is divided into time domain, means at one moment of time either « D » downlink subframe is transmitted or « U » for uplink, and then we have « S » Special subframe which comes when there is a transition from downlink subframe to uplink subframe

– For higher throughput – the frame structure should contain a high number of consecutive Downlinks (D)

– For higher uplink data – the frame structure should contain a high number of consecutive Uplinks (U)

– For lower latency and more accurate coverage (and higher speed mobile) – the frame structure should have a lower number of consecutive Downlinks (D) and Uplinks and more frequent switching. More frequent switching decreases throughput.

Most of frame structure used  in 5G and supported by the ecosystem  are :

• DDDSU is the frame structure most suitable in 5G
• DDDDDDDSUU is the frame structure used in case of coexistence for synchronisation between 4G TDD and 5G TDD

Both frames are equivalent in terms of throughput and coverage, however the main drawback of using  DDDDDDDSUU is having an impact on latency which will be higher by around 8ms in comparison with DDDSU 5G Frame structure.

In case of coexistence between 4G TDD and 5G TDD it is important to adopt this second  frame structure and synchronisation need to be made between all operators in the country and operators of surrounding countries using the same TDD band, otherwise huge interference will be experienced.