Differences and Characteristics between FDD and TDD

　　1. Duplex mode

　　TDD: Time Division Duplex, where the transmitter and receiver share a common RF frequency point, and the uplink and downlink use different time slots for communication.

　　FDD: Frequency Division Duplex, which uses different radio frequency points for communication between transmission and reception.

　　2. Speed

　　In theory, under the same bandwidth conditions, such as FDD allocating 10M+10M and TDD allocating 20M, the speed of TDD will be lower than FDD. This is mainly because TDD has a frame structure called a special subframe, which wastes a part of these frames (such as the protection time slot) and does not transmit any data. FDD frames do not have this situation of complete waste.

　　3. Coverage

　　TD-LTE is suitable for hotspot area coverage, while FDD is suitable for wide area coverage.

　　A report from Qualcomm in the early years showed that under the same frequency and power conditions, FDD can provide better coverage than TDD, with TDD coverage being 80% smaller (DL/UL=2:1)/40% smaller (DL/UL=1:1) than FDD. The main reason is that the TDD uplink has a shorter transmission power time (in a 10ms frame) compared to FDD.

　　4. Mobile station movement speed

　　FDD is a continuous control system, while TDD is a time separated control system. When moving at high speeds, the Doppler effect can cause rapid fading, with higher speeds resulting in higher fading transformation frequencies and deeper fading depths.

　　On the basis of current chip processing speed and algorithms, when the data rate is 144kb/s, the maximum mobile speed of TDD can reach 250km/h, which is still a certain gap compared to FDD systems. Generally, the mobile speed of TDD mobile stations can only reach half or even lower than that of FDD mobile stations.

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