5G TDD: Synchronization Across Networks Required?

I recently came across this paper of Huawei on 5G Spectrum matters and one paragraph in particular caught my attention:

“[…] inter-operator synchronisation and alignment of uplink/downlink transmissions (slot and frame synchronization) is […] necessary for efficient deployment of 5G NR networks in unpaired assignments.”

In other words this sentence says that network operators using adjacent spectrum have to synchronize their radio networks to avoid interference. So why is this necessary and what exactly does that mean?

Today, most 4G LTE networks in Europe use Frequency Division Multiplexing (FDD) which means uplink and downlink traffic uses different frequencies ranges. In other words, downlink and uplink traffic does not get in each others way as they are separated in frequency. With 5G things will be different as the spectrum in the 3.5 GHz band in Europe will be used in TDD mode, i.e. downlink and uplink use the same frequency and are separated by using different time slots. This is a challenge when two network operators start using two directly adjacent channels as uplink transmissions at the edge of one channel can interfere with downlink transmissions at the edge of the other channel if they happen at the same time. The solution to this is to synchronize the networks with each other and ensure both use the same times for downlink and uplink transmissions.

But there is a catch, and from my point of view a pretty big one: Synchronizing uplink and downlink transmissions in a TDD system with one or more network operators means that they all have to use the same downlink to uplink ratio. That means that they loose the flexibility to decide on their own which ratio is best for their customers. Qualcomm also recently discussed this topic in a public paper (which contains some technical details such as BEM, EIRP Emission mask limits and other things) and raises the additional issue that in 5G, future ultra low latency features use special timeslot arrangements that are difficult to synchronize across network operators. They therefore propose to think about semi-synchronized networks that leave it to network operators what they want to do in some timeslots.

Apart from the later issue it seems that inter-operator network synchronization is not a new thing. Huawei mentions in their paper that some network operators in China already use TDD for LTE today and have their radio networks synchronized to avoid interference.