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Difference between LTE FDD vs TDD LTE



This tutorial section on LTE basics covers following sub topics:
Main page  features  terminologies  Frame  TDD FDD  Channel types  PHY  stack  throughput  VoLTE  CA   cell search  network entry  Timers  PSS vs SSS  Security   LTE Bands  EARFCN  Hotspot  router 



TDD and FDD are two topologies by which critical resources time and frequency are shared among mobile subscribers or terminals. LTE uses both of these flavors to provide facility for the mobile subscribers or UEs to utilize the scarse resource efficiently based on the need. To know more about TDD vs FDD refer our article on Difference between TDD and FDD topologies.

Let us understand LTE FDD and TDD LTE versions with figures and band example below. LTE has radio frame of duration 10ms consisting of 10 subframes. Each subframe has two slots. The slot is of 0.5ms duration.Hence there are total 20 slots in a radio frame.

LTE FDD

LTE FDD topology

In LTE base station is referred as eNodeB and mobile subscriber is referred as UE. The figure-1 describes LTE FDD scenario. As shown in the figure f1 and f2 are one pair of frequencies allocated separately for both the uplink and downlink direction.

Figure mentions LTE band-13 with uplink frequency of range 777 to 787MHz and downlink frequency of range 746 to 756 MHz. Hence f1 is allocated from uplink band and f2 is allocated from downlink frequency band. The entire radio frame of 10ms is used simultaneously over downlink and uplink directions.

Pls. note that downlink always refers to transmission from LTE eNodeB to UEs and uplink refers to transmission from UEs to eNodeB. Both uplink and downlink will have 10MHz bandwidth each on which entire frame will be used.

TDD LTE

TDD LTE topology

The figure-2 describes TDD LTE scenario.As shown in the figure both uplink and downlink has been allocated same frequency f1 and but both uses different time slots for mapping their information data.

Figure mentions LTE band-33 which is from frequency 1900 to 1920MHz. Entire bandwidth of 20MHz is used for both eNodeB and UEs. Figure mentions configuration of radio frame time slots 0 to 9 for UL/DL configuration of zero and 5ms DL/UL switch point periodicity. It is D,S,U,U,U,D,S,U,U,U. Here D stands for downlink and U stands for uplink.Hence the subframes of the entire radio frame is divided and used for both the uplink and downlink direction.

Leading chipset manufacturers such as Ericsson,Altair semiconductor,Qualcomm support both TDD and FDD versions of LTE on a single chip.

Following table summarizes LTE FDD and TDD versions.It compares both with respect to application,frame structure, Guard period,frequency band,interference,data rate and interoperability with other RATs.


Feature LTE FDD TDD LTE
Application FDD version is used where both uplink and downlink data rates are symmetrical. TDD version is used where both uplink and downlink data rates are asymmetrical.
Frame structure Uses FDD frame structure Uses TDD frame structure, READ MORE
Guard periods Not provided,every downlink subframe can be associated with an uplink subframe. Provided in the center of special subframes and used for the advance of the uplink transmission timing. The no. of downlink and uplink subframes is different
Frequency bands REFER LTE Frequency Bands for FDD frequency ranges REFER LTE Frequency Bands for TDD frequency ranges
Interference Interference between neighbouring base stations less as transmission and reception is done on seperate frequencies. Interference between neighboring base stations more, as transmission and reception is done on the same frequency.
Peak Downlink data rate for FDD/TDD LTE Minimum: 1.728 Mbps with 1.4MHz BW,6 RBs, QPSK modulation,
Maximum: 345.6 Mbps with 20MHz,100 RBs, 64QAM,4X4 MIMO
Peak Uplink data rate for TDD/LTE FDD Minimum: 1.8 Mbps with 1.4MHz BW, 6 RBs, QPSK modulation,
Maximum: 86.4 Mbps with 20MHz BW, 100 RBs, 64QAM modulation
Working with other RAT Interference will be higher than TD version TDD LTE works well with minimum interference along with TD-SCDMA RAT

Both LTE FDD and TDD versions have their own applications and the same can be exploited by telecom operators based on traffic and other requirements.

RELATED LINKS

3GPP references
LTE tutorial
LTE System Features

RF and Wireless Terminologies


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