LTE FDD using the paired spectrum is anticipated to form the migration path for the current 3G services being used around the globe, most of which use FDD pared spectrum. However there has been as additional emphasis on including TDD LTE using unpaired spectrum. TDD LTE which is also known as TD-LTE is seen as providing the evolution or upgrade path for TD-SCDMA.
In view of the increased level of importance being placed upon LTE TDD or TD-LTE, it is planned that user equipments will be designed to accommodate both FDD and TDD mode. With TDD having an increased level of importance placed upon it, it means that TDD operations will be able to benefit from the economies of scale that were previously only open to FDD operations.
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.
In order to be able to be able to transmit in both directions, a user equipment or base station must have a duple 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.
| PARAMETER | LTE-TDD | LTE-FDD |
|---|---|---|
| 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 |
Advantages / disadvantages of LTE TDD and LTE FDD for cellular communications.
There are a number of the advantages and disadvantages of TDD and FDD that are of particular interest to mobile or cellular telecommunications operators. There are naturally reflected into LTE.
LTE TDD/TD-LTE and TD-SCDMA
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 for 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 supporter 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.
Summary
While both LTE TDD (TD-LTE) and LTE-FDD will be widely used, it is anticipated that LTE FDD will be them more widespread, although LTE TDD has a number of significant advantages, especially in terms of higher spectrum efficiency that can be used by many operators. It is also anticipated that phones will be able to operate using either the LTE FDD or LTE-TDD (TD-LTE) modes. In this way the LTE USs or user equipment will be dual standard phones, and able to operate in countries regardless of the flavour of LTE that is used - the main problem will then be the frequency bands that the phone can cover
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