In the LTE era, only a fraction of the MNOs had adopted LTE operation in TDD mode. By acquiring and using the 3.x GHz band for 5G deployments, Operators will have to operate TDD technology for 5G NR.
TDD Challenges
TDD brings new challenges to the network operators since there is more coordination needed to avoid interference. Furthermore, the higher flexibility in 5G NR configurations also increases the complexity of the network. Mobile Operators have, therefore, to adopt and adequately prepare their networks accordingly. This can be achieved by bringing synchronization signals to the Base Stations. Furthermore, selecting frame structures that best support the foreseen use cases and agreeing on those frame structures with the other Operators. Finally, regulation is vital to avoid intersystem interference or the need for guard bands.
Operating TDD on outdoor sites requires intra-network synchronization of the sites within a coverage area regarding Downlink or Uplink timing. Equipment of other MNOs running on adjacent frequencies in the same coverage area will interfere and degrade the network performance unless significant guard bands will be introduced or their equipment becomes part of inter-network frame synchronization.
Feasible means to achieve frame synchronization between networks exist, such as exact time of day via GPS or IEEE1588v2/ITU-T G.8275.1 offering +/- 1.5 μs accuracy. Bringing the synchronization signals to the Base Stations would mean deciding on whether to bear the cost of deploying a GPS device in each and every gNodeB or transport synchronization protocols from the network. Moreover, Operators need to select the frame structures which best support their use cases and carefully select frame structures with flexibility for configuration from all vendors. Finally, the synchronized frame structures should be agreed upon with the competing operators and/or with the regulators to avoid inter-system interference and alternatively define the rules for providing guard bands. In this latter case, some waste of spectrum is unavoidable.
TDD duplex gap
TDD operation requires a duplex gap in time. To separate the Downlink from the Uplink, FDD runs uninterrupted but requires a duplex gap that is defined as the guard band, an extra spectrum between DL and UL. While FDD wastes spectrum for guard bands, TDD requires network synchronization and introduces a guard time. TDD uses only one channel but requires a guard period, which is the time gap between the DL and the UL. The TDD guard period between DL and UL limits the maximum cell range. Moreover, this TDD guard band avoids interference from other, synchronized NBs and must be long enough for the farthest NB of which the signal still could be received.
LTE-TDD Frame Structures
3GPP has defined 7 frame structures for LTE-TDD. Each Operator can select the Frame Structure (1 out of 7 options), defining the UL/DL ratio. Special Sub-Frames and their configuration determine the switching, i.e., the latency, and impact the capacity. LTE-TDD configurations 2 (and maybe 1) are compatible with NR since they allocate more transmission time to the DL than the UL and have a 5 ms periodicity, which means quicker response and shorter RTT.
NR Frame Structures
More options and flexibility are available for frame structures in the 5G NR standard. Namely, there are more parameters to choose from, such as numerology (Sub-Carrier-Spacing), which can be considered for Inter-MNO Agreements. The focus should be on a basic/limited set of frame structures. There is also an extra challenge if the MNO on the adjacent frequency runs on another technology. That is because LTE should have further restrictions for switching points and latency, while for WiMax, there is no compatible frame structure. TDD Frame Structure needs to include cross-border coordination since there is a potential need for cross-border synchronization or guard zones
A Special Slot Structure is also needed to define the consequences of non-agreement. Restrictive emission limits lead to guard bands and are deployment-dependent. The question is who has to provide them, and of course, an agreement must also be reached with the NRA.
For small-cell deployments, a semi-synchronization approach could be adopted. For the 3.x GHz band, spectrum allocations and respective carrier bandwidths up to 100 MHz are expected (> 30 kHz SCS). In general, an MNO can select each slot structure for each of the slots, which provides the highest flexibility with NR.
Industry focus
The strongest focus in the industry is currently on 3 NR frame structures. DDSU, DDDSU, and DDDSUUDDDD for LTE compatibility. Performance analysis is still ongoing, but the preference for eMBB deployments is currently for the DDSU option. All three proposals are a compromise and not optimized for ULL. Thus an additional FS for ULL cases is needed. An open parameter is the length of the guard period (2 to 6 symbols) and the split between DL and UL symbols in the special/switching S-slot.
Operators should lobby for all spectrum acquisitions so that MNOs will be free in their technology choice and agree among them on frame structures or guard bands. Only if agreements could not be reached NRAs should pre-define the rules on who has to provide the guard bands. Operators should enter into negotiations with competition and NRAs so that the final agreement per region/country is achieved. Until an agreement is reached, Operators should expect the vendors to be prepared to support all the above frame structures while also requesting flexibility for freely configuring the guard period in the S-slot. The agreement shall now hold only for the years of NR introduction with the choice of a revision once further NR features are developed. This will provide the extra flexibility needed rather than fixing them now on a permanent regulatory basis across the globe.
5G Spectrum Regulation
This maximum technical flexibility should be kept for all spectrum areas to ensure efficient spectrum usage. For all current spectrum acquisitions, Operators should be free in their technology choice in order to achieve technology neutrality. For bands where TDD is used, the MNOs within a country should be allowed to agree among them on compatible, synchronized frame structures or guard bands – ideally on a regional basis.
For the case that agreements between the MNOs could not be reached, NRAs should pre-define – ideally before the spectrum acquisitions – the rules to provide guard bands (e.g. by defining out-of-block and maybe in-block emission limits). It shall become a joint position that after a few years of operation, the above agreements shall be revised considering the latest technological developments and operational experience.
In cases where TDD deployments affect x-border I/F, all involved MNOs should be encouraged to find common agreements. In case of non-agreements between the MNOs, the NRAs shall negotiate rules for a synchronized frame structure and guard zones. Furthermore, NRAs should set the respective emission limits for not operating in line with the synchronization rules or apply the results of currently ongoing CEPT work and related decisions. Those rules shall only be applied to coverage areas at borders. Of course, these rules and agreements shall be subject to later revisions.
