While in the LTE era, not many MNOs were operating in LTE-TDD, the critical band for 5G NR introduction is the 3.5 GHz band, where it is aimed to be used in TDD mode. In typical 3G & 4G commercial networks, the use of Frequency Division Duplex (FDD) was the norm. In FDD, one frequency is used for the uplink and a different for the downlink. The fundamental idea of Time Division Duplex (TDD), however, is that the uplink and downlink use the same channel but transmit and receive at different time intervals. TDD can offer advantages such as efficient spectrum management with specific use of allocated blocks of frequencies and is beneficial, especially for higher frequencies and beamforming. However, the use of guard bands will result in an unavoidable waste of some spectrum.
Time Division Duplex (TDD)
In TDD, isolated cell clusters at higher frequencies can be used, where path loss reduces inter-cell interference. In 5G NR, there is the concept of flexible TDD uplink and downlink sub-frame allocation. This is a cluster-based frame structure alignment, where the instantaneous uplink and downlink traffic situation is matched. Additional time, frequency, and spatial separation is needed to avoid interference. This means that advanced sensing and reservation might be required, and centralized coordination within C-RAN can be beneficial. Compared to LTE technology, NR defines Flexible Slot Formats. In this way, a very dynamic adaptation is possible, since the UL/DL allocation happens on symbol level and not per slot. This offers a variation on all kinds of communication possible, including downlink only, uplink only, or a flexible symbol with UL/DL hybrid operation. Many configurations are now feasible with UL/DL switching on a subframe, slot, or even symbol level to support regulatory requirements, reserve resources for other use, and override periodic transmissions. Flexible symbols are used either as TDD guard time for switching or for fast UL/DL traffic adaptations. The gNB scheduler can allocate UEs using symbols without changing the format since the UEs only measure downlink symbols. Further reduction of UE power consumption is also possible.
TDD characteristics
In TDD, there is a switch between UL / DL and use measurements for link adaptation, while in the case of no uplink data, the gNB can configure uplink sounding signals. The benefit is a detailed channel knowledge without explicit feedback, less delay (no encoding, reporting, decoding) for adaptation, and more accurate measurements since there is no quantization. Nevertheless, regular calibration of different UL/DL RF chains is required, and interference issues may arise. Within a network coverage area, NBs are not allowed to transmit while others receive at the same time. This is the case for adjacent UEs as well. Synchronized operation, e.g., based on the exact time of day with equal frame structures, is needed. Even if the other NB operates on the adjacent frequency by another Operator, the interference would still be stronger than the wanted in-band UE signal and degrade the service quality. Interference avoidance can also happen by synchronized and aligned frame structures. Another alternative is the use of guard bands but comes with a higher waste of spectrum and might not always help due to user blocking.
Interference may be present not only between NBs on the same frequency but also via external, out-of block interference from neighboring frequencies to the outer parts of the signal. Within each coverage area, MNOs should select compatible frame structures and synchronize them between the networks so that no NB transmits while a neighboring one expects to receive.
TDD guard time
While FDD operation wastes spectrum for guard bands, TDD requires network synchronization and introduces a guard time. In FDD operation to separate the DL from the UL, the system runs uninterrupted but requires a duplex gap with a guard band being equal to the spectrum between the DL and UL. TDD uses only one channel but requires a guard period, which is defined as the time gap between DL and UL. The TDD guard period between DL and UL limits the maximum cell range but avoids interference from other, synchronized NBs. The TDD guard period must be long enough for the farthest NB of which the signal still could be seen. 3GPP defined seven frame structures for LTE-TDD, where only a limited number of configurations allocate more transmission time to the DL than the UL and have a 5 ms periodicity, which is necessary for compatibility with NR for quicker response and shorter RTT.
5G TDD challenges
TDD brings new challenges to Mobile Operators since there is more coordination needed to avoid interference. The complexity rises with NR due to higher flexibility in configurations. Mobile Operators need to prepare their networks to support synchronization to the Base Stations. Furthermore, they need to select the frame structures which best support the foreseen use cases. This means requesting selected frame structures with flexibility for configuration from all vendors. Finally, synchronized frame structures with the competing operators must be agreed together with the regulatory authorities to avoid inter-system interference and alternatively define the rules for providing specific guard bands.
TDD Synch
By acquiring and using the 3.5 GHz band, Mobile Carriers will have to operate TDD technology, either LTE-TDD or 5G NR. Operating TDD on outdoor sites requires intra-network synchronization of the sites within a coverage area with regards to Downlink and UpLink timing. Equipment of other Operators running on adjacent frequencies in the same coverage area will interfere and degrade the network performance. This means that a significant number of guard bands are introduced, resulting in a waste of spectrum, or their equipment becomes part of inter-network frame synchronization. Feasible means to achieve frame synchronization between network nodes exist to provide the exact time of day via GPS or IEEE 1588, with up to +/- 1.5 μs accuracy. UL/DL frame synchronization between operators includes the selection of compatible frame structures. For LTE-TDD such a selection is relatively easy only in 1 or 2 out of the 7 possibilities, while NR has various extra options.
Industry focus
The strongest focus in the industry is currently on 3 NR frame structures with 30 kHz SCS, the DDSU, DDDSU, and DDDSUUDDDD. Performance analysis is still ongoing, but it seems that the DDSU (Downlink – Downlink – Special – Uplink) option is more suitable for early eMBB deployments. All three above proposals are a compromise and not optimized for cases of ultra-low latency, thus an additional frame structure for URLLC cases is needed. A pending issue is the length of the guard period, ranging from 2 up to 6 symbols and the split between DL and UL symbols in the special/switching S-slot. MNOs should be free in their technology choice and allowed to agree among them on frame structures or guard bands. Only for the case, that agreements could not be reached the NRAs should pre-define the rules on who has to provide guard bands. Until a final agreement is reached, the 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. Finally, the position on the agreed frame structures should be revised on a regular basis once more NR features are developed.