TDD is a duplex method of communication systems that is used in mobile communication systems to separate receiving and transmitting channels.
China submitted the draft third-generation mobile communication standard (TD-SCDMA) in June 1997, and its TDD model and new technology of smart antennas were highly evaluated and became one of the three main candidate standards. Considering that TD-SCDMA has been retired from the network, the focus of this article is based on TDD-LTE and FDD-LTE (abbr. TDD & FDD respectively).
In the first and second generation of mobile communication systems, FDD mode dominates the world, TDD mode has not attracted attention so much. However, due to the need for new business and the development of new technologies, as well as the many advantages of the TDD mode, the TDD mode will gain increasing attention.
Part 1: What do TDD and FDD stand for?
TDD(Time Division Duplexing), one of the duplex technologies used in mobile communication technology. It is a technology that distinguishes wireless channels in time during the downlink operation of the frame period, and continues uplink operation.
FDD is a kind of full-duplex communication technology used in mobile communication systems. FDD uses the technology of two independent channels for transmitting information downward and upward, respectively. To prevent mutual interference between neighboring transmitters and receivers, there is a protection frequency between the two channels.
1 slot = 0.5 ms
1 subframe = 1 ms
1 frame = 10ms
1 half-frame = 5 ms （TDD only）
TDD Special subframe
DwPTS : Downlink Pilot Time Slot
GP : Guard Period
UpPTS : Uplink Pilot Time Slot
Part 2: What are the differences?
1.Duplex mode
TDD (Time Division Duplexing): Transceivers share the same radio frequencies; the Uplinks and Downlinks use different time slots for communication.
FDD (Frequency Division Duplexing): Different radio frequencies are used for sending and receiving as communication.
2.Speed rate
Theoretically, under the same bandwidth conditions, such as FDD allocating 10M+10M, TDD allocating 20M, TDD’s rate will be lower than FDD. The main reason for this is that there is a frame called a special subframe in the frame structure of TDD. These frames are wasted (such as the protective time slots) and no data is transmitted. While FDD frames don’t have such situation.
3.Coverage
TD-LTE is suitable for hotspot area coverage, FDD is suitable for wide area coverage.
4.The movement speed of mobile station
FDD is a system of continuous control, and TDD is a system of time division control.
At high speeds, the Doppler effect causes fast fading, and the higher the velocity, the higher the fading transformation frequency and the deeper the fading depth.
Based on current chip processing speed and algorithm, when the data rate is 144kb/s, the maximum movement speed of TDD can reach 250km/h, which is still a certain gap compared with FDD system.
In general, the movement speed of the TDD mobile station can only reach half or even lower than that of the FDD mobile station.
Part 3: How to choose？
1.TDD pros:
- flexibility to configure frequencies, and able to use scattered frequency bands that are not easily used by FDD；
- Asymmetric services can be flexibly supported by adjusting the up-and-down time slot transition points；
- With uplink and downlink channel consistency, the receiving and transmitting of the base station may share part of the RF unit, which reducing equipment cost；
- When receiving uplink and downlink data, there is no need to add transceiver isolators, only a switch should be enough. It helps to reducing the complexity of the device.
2.TDD cons:
- The coverage of TDD base stations is significantly smaller than that of FDD base stations;
- TDD system transmits and receives at the same frequency, so there is interference within and between systems.
- In order to avoid interference with other wireless systems, TDD needs to reserve a certain protective band, which affects the overall spectrum utilization efficiency;
- Because the channel changes rapidly under high-speed, the TDD time-sharing system causes the delay of channel message reported by the mobile phone, so the TDD system is inferior to FDD in high-speed scenarios.
To summarize, TDD and FDD have advantages that cannot be ignored in a variety of application scenarios. The combined networking of TDD and FDD are also good choices. The gap between TDD LTE and FDD LTE now is much smaller than TD-SCDMA and WCDMA used to be in the 3G era.