While FDD transmissions require a large frequency separation between the transmitter and receiver frequencies, TDD schemes require a guard time or guard interval between transmission and reception. In other words, the downlink and uplink transmissions are multiplexed in time and are not concurrent.
The Time Division Duplex is a duplex scheme where uplink and downlink transmissions occur at different times but may share the same frequency. Since there is a frequency separation between the uplink and downlink directions, it is not typically possible to reallocate spectrum to change the balance between the capacity of the uplink and downlink directions, if the capacity requirements for each direction vary over time. The spectrum used for FDD systems is allocated by the regulatory bodies. However, two RF channels are required, which in some cases may not be the efficient use of the available spectrum. The use of an FDD system does enable simultaneous transmission and reception of signals. While implementation cost is not a significant constraint for the base stations, placing a filter in the user terminal involves higher complexity and cost. For cellular systems using FDD, filters are required in the base station and the user terminal to ensure sufficient isolation of the transmitter signal without desensitizing the receiver. The receiver blocking is an important issue in FDD schemes and often highly selective filters may be required. For the FDD scheme to properly operate, it is necessary that the frequency separation, that is, channel separation between the transmission and reception frequencies, to be sufficient in order to prevent the receiver blocking due to high-power transmitter signal. The downlink and uplink frequencies are separated by sufficiently large frequency offset. The Frequency Division Duplex is a duplex scheme in which uplink and downlink transmissions occur simultaneously using different frequencies. Sassan Ahmadi, in 5G NR, 2019 3.4.1 Frequency and Time Division Duplex Schemes More details on dynamic TDD can be found in Chapter 15. To exploit these benefits, LTE release 12 includes support for dynamic TDD, or eIMTA as it the official name for this feature in 3GPP. In dynamic TDD, the network can dynamically use resources for either uplink or downlink transmissions to match the instantaneous traffic situation, which leads to an improvement of the end-user performance compared to the conventional static split of resources between uplink and downlink. To better handle the high traffic dynamics in a local-area scenario, where the number of devices transmitting to/receiving from a local-area access node can be very small, dynamic TDD is beneficial. An existing wide-area FDD network could be complemented by a local-area layer using TDD, typically with low-output power per node. Another reason is that many problematic interference scenarios in wide-area TDD networks are not present with below-rooftop deployments of small nodes. One reason is unpaired spectrum allocations being more common in higher-frequency bands not suitable for wide-area coverage. However, with an increased interest in local-area deployments, TDD is expected to become more important compared to the situation for wide-area deployments to date. Having a static split is a reasonable assumption in larger macrocells as there are multiple users and the aggregated per-cell load in uplink and downlink is relatively stable.
The fundamental approach to this in LTE, as well as in many other TDD systems, is to statically split the resources in to uplink and downlink. In TDD, the same carrier frequency is shared in the time domain between uplink and downlink.
TETRAPOL FDMA MODULATION PRO
Johan Sköld, in 4G LTE-Advanced Pro and The Road to 5G (Third Edition), 2016 3.5.5 Dynamic TDD
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