What is the difference between TD-LTE and FDD-LTE?

Lets understand what is TDD and FDD first:

In communication systems, a user needs to exchange data with one or more parties through a shared resource – a common channel. Depending
on whether the data is transmitted/received simultaneously, the following
transmission techniques exist:

  • Simplex – One party transmits data and the other party receives data. No simultaneous transmission is possible – the communication is one-way and only one frequency (channel) is used. Examples of simplex communication are traditional radio and TV (non interactive).

  • Half Duplex - Each party can receive and transmit data, but not at same time. The communication is two-way and only one frequency (channel) is used. Walike talkie is a typical example.

  • Full Duplex - Each party can transmit and receive data simultaneously. The communication is two-way and two frequencies are used. one for transmitting and one for receiving.

  • In the case of cellular networks, a limited shared resource (spectrum) needs to be shared with all users so full duplex communication is possible. The two main methods used are:
    1. Time Division Duplexing (TDD) – The communication is done
    using one frequency, but the time for transmitting and receiving is
    different. This method emulates full duplex communication using a
    half duplex link.
    2. Frequency Division Duplexing (FDD) – The communication is
    done using two frequencies and the transmitting and receiving of
    data is simultaneous.

  • FDD LTE (FD-LTE) vs TDD LTE (TD-LTE) Networks: FDD LTE and TDD LTE are two different standards of LTE 4G Technology. LTE is a high speed wireless technology from 3GPP standard. 3G growth ends up at HSPA+ and mobile operators have already started deploying 4G networks to provide much more bandwidth for mobile users. 4G speed will give us a virtual LAN reality to mobile handset by offering very high speed access to Internet to experience the real triple play services such as data, voice and video from a mobile network. LTE is defined to support both paired spectrum for Frequency Division Duplex (FDD) and unpaired spectrum for Time Division Duplex (TDD). LTE FDD uses paired spectrum that comes from a migration path of 3G network whereas TDD LTE uses unpaired spectrum that evolved from TD-SCDMA.

Difference Between FD LTE and TD LTE:
(1) TD LTE does not require paired spectrum since transmit and receive occurs in the same channel whereas in FD LTE , it requires paired spectrum with different frequencies with guard band.

(2) TD LTE is cheaper than FD LTE since in TD LTE no need of duplexer to isolate transmission and receptions.

(3) In TD LTE, its possible to change the uplink and downlink capacity ratio dynamically according to the need whereas in FD LTE capacity is determined by frequency allocation by the regulatory authorities. So it is difficult to make dynamic change.

(4) In TD LTE larger guard period is necessary to maintain the uplink and downlink separation which will affect the capacity whereas in FD LTE the same concept is referred as guard band for isolation of uplink and downlink which will not affect the capacity.

(5) Cross slot interference is there in TD LTE which is not applicable to FD LTE.

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