LTE FDD, TDD, TD-LTE Duplex Schemes

LTE is able to utilise both frequency division and time division, FDD & TDD (TD-LTE) forms of duplex to accommodate the uplink and downlink.

4G LTE includes:
What is LTE     LTE OFDMA / SCFDMA     MIMO     LTE Duplex     LTE frame & subframe     LTE data channels     LTE frequency bands     LTE EARFCN     UE categories / classes     LTE-M (Machine to Machine)     LTE-LAA / LTE-U     VoLTE     SRVCC    

LTE Advanced topics:     LTE Advanced introduction     Carrier aggregation     Coordinated multipoint     LTE relay     Device to device, D2D    

LTE can utilise both FDD - frequency division duplex and TDD - time division duplex, often referred to as TD-LTE.

Both forms of duplex, FDD and TDD have their advantages and the areas where their deployment is advantageous.

For most normal LTE deployments the frequency division duplex, FDD is used, and paired spectrum with equal bandwidth in up and downlinks is utilised.

LTE FDD using the paired spectrum was considered to be the migration path for the UMTS 3G services which typically utilised paired spectrum.

However there was considerable development placed on the time divison duplex form of LTE: TDD LTE or TD-LTE which was seen as the upgrade path for TD-SCDMA that was developed and introduced by the Chinese as a 3G technology.

LTE Duplex schemes

It is essential that any cellular communications system must be able to transmit in both directions simultaneously. This enables conversations to be made, with either end being able to talk and listen as required. Additionally when exchanging data it is necessary to be able to undertake virtually simultaneous or completely simultaneous communications in both directions.

It is necessary to be able to specify the different direction of transmission so that it is possible to easily identify in which direction the transmission is being made. There are a variety of differences between the two links ranging from the amount of data carried to the transmission format, and the channels implemented. The two links are defined:

  • Uplink:   the transmission from the UE or user equipment to the eNodeB or base station.
  • Downlink   the transmission from the eNodeB or base station to the UE or user equipment.
Uplink and downlink transmissions in cellualr telecommunications system
Uplink and downlink directions

In order to be able to be able to transmit in both directions, a user equipment or base station must have a duplex scheme. There are two forms of duplex that are commonly used, namely FDD, frequency division duplex and TDD time division duplex..

Both FDD and TDD have their own advantages and disadvantages. Accordingly they may be used for different applications, or where the bias of the communications is different.

LTE TDD / LTE FDD comparison

There are a number of the advantages and disadvantages of TDD and FDD LTE that are of particular interest to mobile or cellular telecommunications operators. These are naturally reflected into LTE.

Comparison of TDD LTE and FDD LTE Duplex Formats

TDD & FDD LTE Advantages & Disadvantages
Paired spectrum Does not require paired spectrum as both transmit and receive occur on the same channel Requires paired spectrum with sufficient frequency separation to allow simultaneous transmission and reception
Hardware cost Lower cost as no diplexer is needed to isolate the transmitter and receiver. As cost of the UEs is of major importance because of the vast numbers that are produced, this is a key aspect. Diplexer is needed and cost is higher.
Channel reciprocity Channel propagation is the same in both directions which enables transmit and receive to use on set of parameters Channel characteristics different in both directions as a result of the use of different frequencies
UL / DL asymmetry It is possible to dynamically change the UL and DL capacity ratio to match demand UL / DL capacity determined by frequency allocation set out by the regulatory authorities. It is therefore not possible to make dynamic changes to match capacity. Regulatory changes would normally be required and capacity is normally allocated so that it is the same in either direction.
Guard period / guard band Guard period required to ensure uplink and downlink transmissions do not clash. Large guard period will limit capacity. Larger guard period normally required if distances are increased to accommodate larger propagation times. Guard band required to provide sufficient isolation between uplink and downlink. Large guard band does not impact capacity.
Discontinuous transmission Discontinuous transmission is required to allow both uplink and downlink transmissions. This can degrade the performance of the RF power amplifier in the transmitter. Continuous transmission is required.
Cross slot interference Base stations need to be synchronised with respect to the uplink and downlink transmission times. If neighbouring base stations use different uplink and downlink assignments and share the same channel, then interference may occur between cells. Not applicable


Apart from the technical reasons and advantages for using LTE TDD / TD-LTE, there are market drivers as well. With TD-SCDMA now well established in China, there needs to be a 3.9G and later a 4G successor to the technology. With unpaired spectrum allocated for TD-SCDMA as well as UMTS TDD, it is natural to see many operators wanting an upgrade path for their technologies to benefit from the vastly increased speeds and improved facilities of LTE. Accordingly there is a considerable interest in the development of LTE TDD, which is also known in China as TD-LTE.

With the considerable interest from the supporters of TD-SCDMA, a number of features to make the mode of operation of TD-LTE more of an upgrade path for TD-SCDMA have been incorporated. One example of this is the subframe structure that has been adopted within LTE TDD / TD-LTE.

Of the two versions of LTE, the frequency division duplex, FDD version is more widely used than the time division, TDD version also known as TD-LTE.

Although it is possible in TD-LTE to change the percentage of time that is occupied by the up and downlink, this is not as easy as it may appear at first sight. Nevertheless LTE TDD does have its place within the family of LTE technologies, FDD and TDD.

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