By allocating more or less time for DL, the cell capacity can be adapted to the needs of the cell. 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. GP 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.
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 GP also needs 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.
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. In NR, 3 different signaling mechanisms provide information to the device on whether the resources are used for UL or DL transmission:.
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. 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. 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.
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. In particular, it offers the possibility for the network to overrule periodic transmissions of uplink sounding 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.
The 2 nd pattern is concatenated onto the 1 st pattern. It contains a set of slot configuration where each slot in the configuration set is assigned an index. Diplexer is needed and cost is higher. It is therefore not possible to make dynamic changes to match capacity. Regulatory changes would normally be required and capacity is normally allocated so that it is the same in either direction.
Large guard period will limit capacity. Larger guard period normally required if distances are increased to accommodate larger propagation times. Guard band required to provide sufficient isolation between uplink and downlink. Large guard band does not impact capacity. Discontinuous transmission Discontinuous transmission is required to allow both uplink and downlink transmissions.
This can degrade the performance of the RF power amplifier in the transmitter. Continuous transmission is required. Cross slot interference Base stations need to be synchronised with respect to the uplink and downlink transmission times. If neighbouring base stations use different uplink and downlink assignments and share the same channel, then interference may occur between cells. Shopping on Electronics Notes Electronics Notes offers a host of products are very good prices from our shopping pages in association with Amazon.
Half of the subframes are reserved for uplink and half for downlink in both full-duplex and half-duplex FDD. The uplink and downlink bands are separated in the frequency domain using a guard band. In TDD, each radio frame consists of two half-frames of 5 subframes each.
Subframes can be either uplink or downlink or special subframes. Special subframes are used when switching from downlink transmission to uplink transmission. The fifth generation of mobile networks, 5G, use a technology called New Radio for the air interface. Even though 5G NR networks have two modes of deployment including standalone and non-standalone , they are expected to co-exist with 4G LTE networks for a long time.
The basic radio frame structure of 5G NR is designed to support both half-duplex and full-duplex communication. Since 5G NR networks can operate in the considerably higher frequency bands both licensed and unlicensed compared to earlier technologies, TDD can be very effective for some of the futuristic use cases of 5G. It is also more pragmatic to use TDD for higher frequency bands because those bands are mainly beneficial for deployments in smaller areas such as factories or shopping malls etc.
That way, frequency interference is less of an issue because there are fewer base stations and devices to plan for. Look at this post if you want to find out which frequency bands are used by 5G NR. But the reason why mobile operators ended up with either TDD or FDD in the first place in the 3G era can be justified through the advantages and disadvantages of each of these duplex schemes.
FDD is ideal for systems where the uplink and downlink for upload and download requirements are symmetric. As FDD offers a continuous flow of data in both uplink and downlink directions, it has a higher overall capacity to offer higher data throughput.
On the downside, it uses more spectrum as it requires two dedicated data streams continuously. So, whenever the data requirements are not symmetric, one of the communication links uplink or downlink can be under-utilised. As both communication links, uplink and downlink, require a portion of the frequency spectrum, it does not seem like the most efficient use of an expensive resource like frequency spectrum.
Mobile devices that use FDD-based cellular technologies require a duplexer when using the uplink and downlink signals on the same antenna simultaneously. Duplexer can increase the noise level as well as the cost of the receiver.
TDD is ideal for systems where the uplink and downlink requirements vary considerably. TDD utilises the available spectrum more efficiently and offers higher flexibility when the data demand changes, i.
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