Introduction to Duplexing Schemes in 5G NR
Duplex scheme implies the separation of transmission and reception or in other words, Uplink and Downlink data transmission. It is important that any cellular communications system must be able to transmit in both directions simultaneously. This enables conversations to be made, with either end being able to talk and listen as required. In order to be able to transmit in both directions, a device (UE) or base station must have a duplex scheme. To provide highest possible flexibility, 5G NR supports various duplexing schemes such as Frequency Division Duplex (FDD), Time Division Duplex (TDD), Semi-static TDD and Dynamic TDD.
Below is the basic description of above-mentioned duplexing schemes:
- Frequency Division Duplex (FDD): It means that a carrier is designated as paired spectrum having an Uplink and Downlink carrier. Data transfer is possible in both the directions simultaneously at the same time because of different carrier frequencies for different directions. Also, allocation of resources can be managed dynamically and assigned independently in either the UL or the DL direction. Paired Bands are used for FDD.
There is full set of slots in both UL and DL during each frame and transmissions can occur simultaneously within a cell. Duplex filters (transmission/reception filters) are used to isolate between UL and DL transmissions. There are 2 possibilities even with this duplex mode:
- Half Duplex Mode: For a certain frequency band, it is not possible to have simultaneous transmission and reception in both UL and DL within a cell. It allows for a simplified device implementation due to relaxed or no-duplex filters.
- Full Duplex Mode: For a certain frequency band, it is possible to have simultaneous transmission and reception in both UL and DL within a cell.
Note: This full/half capability is a property of the device and not the Base station as it can anyways operate in full duplex mode irrespective of the device capability.
One of the drawbacks of this scheme is that the band definition requires a guard band between UL and DL, and the receiver must be equipped with a duplex filter to suppress interference from the transmitter.
- Time Division Duplex (TDD): In TDD, only one carrier frequency is used. Transmission/Reception in UL and DL occurs on same frequency but at different time slots. Time slots can be allocated either to the UL or the DL. Unpaired bands are used for TDD, where UL and DL transmissions are non-overlapping in time, both for a device and cell’s perspective.
Receiver complexity is reduced in this case as the duplex filter is not needed. Also, the channel is reciprocal, thereby allowing improved implementation of channel estimation and link adaptation mechanisms such as precoding and AMC as well as directive antennas, which is a major advantage specifically for Beamforming methods.
Typically, a time interval such as frame structure or slot structure is divided into UL and DL time intervals. In 5G NR, one slot consists of 14 OFDMA symbols considering normal CP length and the slot configuration indicates the type of slot: UL or DL. In LTE, UL/DL changes were only possible at subframe level. So, switching between UL and DL at OFDMA symbol level in 5G NR allows much greater flexibility but causes challenges in implementation due to shorter time intervals and faster switching times. Also, in this case allocation of UL and DL is still static, within the cell.
One of the drawbacks of this scheme is related to the type of synchronization due to inter-cell interference and the need for a guard time between the transition from the RX to TX to compensate for propagation delay.
- Semi-Static TDD: It introduces more flexibility than the static TDD. In TD-LTE, 7 possible UL/DL configurations were defined on 1 frame corresponding to 10ms frame but here higher layer configuration parameters can be used in 5G NR to achieve cell-specific or even UE-specific UL/DL allocation parameterization. So, the slot configuration is flexible and can be changed from time to time while maintaining the focus on inter-cell interference aspects.
- Dynamic TDD: It is the most flexible concept for UL/DL configuration. It is the possibility of dynamic assignment and reassignment of time domain resources between the UL and DL transmission directions. It is fully dynamic and could be a use case for small cells or even for standalone or isolated indoor cells with overlapping coverage to neighbor cells and therefore less influence due to inter-cell interference.
It enables rapid traffic variations specifically in dense deployments with a relatively small number of users per cell. Consider a situation where a user (almost alone) in a cell wants to download a major object, so for that most of the resources should be utilized in the DL direction and only a small fraction in the UL direction. A major difference with LTE is that in LTE, UL and DL allocations doesn’t change over time.
Time Division Duplex in 5G NR
TDD operation will be the main duplex arrangement for higher frequencies in 5G. Lower frequencies will still be using FDD as the interference problems with large cells is reduced by having different frequencies in UL and DL.
If UL and DL data transmission takes places at the same time, then there could be interference problems between devices or between base stations.
In case of Base station to Base station, a weak signal from the UE sending a UL signal is disturbed by another base station sending a strong signal while in case of Device to Device communication, two UEs close to each other disturb each other. One UE receives a weak signal from the base station at the same time as a UE transmits a strong signal to its base station. The interference is worse in larger cells as the power is high from both base stations and UEs. This is the reason that TDD is easier to use in smaller cells having lower power. Small indoor cells are also rather isolated from each other which makes them quite suitable for TDD operation.
The advantage with TDD in high frequency bands is that the UL/DL capacity can be adapted to the traffic pattern in the cell. By allocating more or less time for DL, the cell capacity can be adapted to the needs of the cell. In FDD, this is not possible as the frequency allocation in UL/DL is static. With the flexible TDD system in 5G NR, each cell can be configured independently of others to adapt to traffic patterns in the cell.
An essential aspect of any half-duplex system in general, is the possibility to provide a sufficiently large guard period (GP) or guard time, where neither DL nor UL transmissions occur. TDD is also a type of Half Duplex system. Guard Period is necessary for switching from DL to UL transmission and vice versa and is obtained by using slot formats where the DL ends sufficiently early prior to the start of the UL. The GP should also ensure that UL and DL transmissions do not interfere at the base station. This is handled by advancing the UL timing at the devices such that, at the base station, the last uplink subframe before the UL-to-DL switch, ends before the start of the first DL subframe. The UL timing of each device can be controlled by the base station by using the timing advance mechanism. The GP must be large enough to allow the device to receive the DL transmission and switch from reception to transmission before it starts the (timing-advanced) UL transmission. As the timing advance is proportional to the distance to the base station, a larger GP is required when operating in large cells compared to small cells.
Also, the selection of Guard Period need to take interference between base stations into consideration. In a multicell network, intercell interference from DL transmissions in neighboring cells must decay to a sufficiently low level before the base station can start to receive UL transmissions. Hence a larger GP may be required as the last part of DL transmissions from distant base stations, otherwise it might interfere with UL transmission.
Dynamic TDD (in detail)
Dynamic TDD is supported, where the UL and DL transmissions are dynamically scheduled to adapt to actual traffic mix and load but requires coordination to avoid interference between cells, so that there is no fixed UL/DL allocation. The basic approach to dynamic TDD is for the device to monitor for DL control signaling and follow the scheduling decisions. If the UE is instructed to transmit, it transmits in the UL direction otherwise it will attempt to receive any DL transmissions. The UL-DL allocation is thus completely under the control of the scheduler and any traffic variations can be dynamically tracked. Since a half-duplex device can’t transmit and receive simultaneously, there a need to split the resources between 2 directions. In NR, 3 different signaling mechanisms provide information to the device on whether the resources are used for UL or DL transmission:
- Dynamic signaling for the scheduled device
The basic principle here is for the device to monitor for control signaling in the DL and transmit/receive according to the received scheduling grants/assignments. It is up to the scheduler to ensure that a half-duplex device is not requested to simultaneously receive and transmit. It is simple and provides a flexible framework.
- Semi static signaling through RRC
Consider a situation where network already have some prior information related to a certain UL/DL allocation. For Example: if it is known to a device that a certain set of OFDM symbols is assigned to UL transmissions, there is no need for the device to monitor for DL control signaling in the part of the DL slots overlapping with these symbols. This can help reducing the device power consumption. NR therefore provides the possibility to optionally signal the UL_DL allocation through RRC signaling.
The RRC-signaled pattern is expressed as a concatenation of up to two sequences of DL-flexible-UL, together spanning a configurable period from 0.5ms up to 10ms. Furthermore, 2 patterns can be configured, one cell-specific provided as part of system information and one signaled in a device-specific manner. The resulting pattern is obtained by combining these two where the dedicated pattern can further restrict the flexible symbols signaled in the cell-specific pattern to be either DL or UL. Only if both the cell-specific pattern and the device-specific pattern indicate flexible should the symbols be for flexible use.
- Dynamic slot-format indication shared by a group of devices
The concept here is to dynamically signal the current UL_DL allocation to a group of devices, monitoring a special downlink control message known as the slot-format indicator (SFI). Here also, the slot format can indicate the number of OFDM symbols that are DL, flexible or UL, and the message is valid for one or more slots. The SFI message will be received by a configured group of one or more devices and can be viewed as a pointer into an RRC-configured table where each row in the table is constructed from a set of predefined DL/flexible/UL patterns, with one slot duration. Upon receiving the SFI, the value is used as an index into the SFI table to obtain the UL_DL pattern for one or more slots.
Since a dynamically scheduled device will know whether the carrier is currently used for UL or DL transmission from its scheduling assignment/grant, the group common SFI signaling is primarily intended for nonscheduled devices. In particular, it offers the possibility for the network to overrule periodic transmissions of uplink sounding reference signals (SRS) or downlink measurements on channel-state information reference signals (CSI-RS), where both type of signals are used for assessing the channel quality.
Different TDD Configurations
5G NR supports 3 different TDD configurations:
- Static TDD Configuration on a common basis per cell
- Semi-static configuration on a UE with dedicated higher layer signaling
- Dynamic allocation based on possibility to indicate a TDD slot via DCI Scheduling
Let’s discuss each, one by one in detail below:
Static TDD Configuration
Higher Layer signaling uses, for example, system information block SIB1 or ServingCell-ConfigCommon to provide an IE TDD-UL-DL-ConfigCommon that contains configuration information on a cell-specific level. In this, the slots and symbols are defined over a period of time that are dedicated to either the UL or DL or can be declared ‘Flexible’, thereby enabling the overwriting with dynamic TDD configuration information. The IE TDD-UL-DL-ConfigCommon contains the following parameters:
- referenceSubcarrierSpacing: Used for dynamic TDD configuration. SFI-RNTI scheduled by DCI will use the reference SubCarrierSpacing to calculate the duration of scheduled slots.
- dl-UL-TransmissionPeriodicity: This is the slot configuration period in milliseconds to which the TDD Configuration applies. This time results in even number of slots depending on the SCS. Some of the possible values are: 0.5ms, 0.625ms, 1ms, 1.25ms, 2ms, 2.5ms, 5ms and 10ms.
- nrofDownlinkSlots: This is the number of slots with only Downlink symbols. The dl-UL-TransmissionPeriodicity starts with this number of DL slots that only contains DL symbols. The maximum number depends on the SubCarrierSpacing and transmission periodicity.
- nrofDownlinkSymbols: This is the number of downlink symbols, which is followed by DL-only slots within the transmission periodicity. Maximum value can be 14 symbols.
- nrofUplinkSlots: This is the number of slots with only UL symbols. This number describes the last number of slots within the total number of slots given by transmission periodicity.
- nrofUplinkSymbols: This is the possible number of UL symbols that precede the number of UL slots at the end of transmission periodicity.
Note: It is possible to configure a second TDD Config pattern TDD-UL-DL-ConfigCommon2 with the same parameters. The 2nd pattern is concatenated onto the 1st pattern. Using this configuration, 2 different TDD Patterns with different UL/DL configuration values can be aligned.
Semi-Static TDD Configuration
As we discussed above, the IE TDD-UL-DL-ConfigCommon configures certain number of UL and DL slots within a time period. So, the remaining slots from total number of slots per time period, which are neither UL nor DL, can be considered ‘Flexible’.
With the help of another IE TDD-UL-DL-ConfigDedicated, the network may configure these flexible slots in a UE-specific manner. It contains a set of slot configuration where each slot in the configuration set is assigned an index. The configuration of such a slot can be all DL or UL or mixed (or ‘explicit’). If it is all DL or UL, then all the 14 OFDM symbols within this slot are unidirectional. If the configuration is explicit, there is nrofDownlinkSymbols parameter to indicate the number of initial downlink symbols in this slot and nrofUplinkSymbols parameter to indicate the number of final symbols in this slot in the UL direction.
Note: Semi-static TDD configuration is a method that allows certain degree of flexibility in UL/DL TDD allocation, with the benefit that configuration can be UE Specific, thereby allowing a sort of traffic adaptation.
Dynamic TDD Configuration
This is valid for the flexible slot within the common configuration only. However, if the slot is not configured, it will be considered flexible anyways and a full dynamic TDD system will be possible. Due to inter-cell interference issues, such a fully dynamic TDD system is likely deployed or present only in isolated or very small cells. Using DCI format2_0 scheduling, a very dynamic TDD Configuration can be achieved on short notice. This DCI message is attached with a CRC, scrambled with SFI-RNTI and may be sent to a group of UEs to notify about the slot format for TDD operations. The length of DCI format is configurable by upper layers and this overall method is Dynamic TDD configuration.
Slot Configurations in 5G NR
NR defines a wide range of slot formats defining which parts of a slot are used for UL or DL. Each slot format represents a combination of OFDM symbols denoting DL, UL and flexible. Slot configurations can be pre-indicated and assigned to a UE via higher layer.
Using DCI format2_0 scheduling, a very dynamic TDD configuration can be achieved on short notice. As per TS 38.213, Slot configurations for Normal Cyclic Prefix is mentioned below in the table. In this table, a particular slot format index specifies the type of symbol within that slot. The type can be downlink ‘D’, uplink ‘U’ or flexible ‘F’, where flexible can be UL or DL or Guard Period.
“5G NR – The next generation wireless access technology” – By Erik Dahlman, Stefan Parkvall, Johan Sköld