WiMAX TDD FDD Comparison

- an overview, summary or tutorial comparing WiMAX FDD and TDD duplex modes and giving the advantages and disadvantages of each for of WiMAX 802.16.

Today WiMAX is being deployed in many areas of the globe for a variety of applications. With these WiMAX deployments being aimed at different markets with different applications, the two flavours of duplex scheme available WiMAX, namely TDD and FDD WiMAX need to be considered to ensure that the correct option is taken. Accordingly it is necessary to undertake a WiMAX TDD FDD comparison and assess which option is best for each application within the regulatory limitations. This is in addition to assessing other WiMAX options including the fixed and mobile variants that are available

With WiMAX offering a high level of service and considered by many as a 4G cellular technology, it is a certain option for deployment in many areas of the globe.

Duplex modes

The WiMAX, 802.16 standard offers two forms of duplex transmission to separate the uplink and downlink messages. Both WiMAX TDD (time division duplex) and WiMAX FDD (frequency division duplex) are available. Each method offers its own advantages and disadvantages.

Note on TDD and FDD duplex schemes:

In order for radio communications systems to be able to communicate in both directions it is necessary to have what is termed a duplex scheme. A duplex scheme provides a way of organizing the transmitter and receiver so that they can transmit and receive. There are several methods that can be adopted. For applications including wireless and cellular telecommunications, where it is required that the transmitter and receiver are able to operate simultaneously, two schemes are in use. One known as FDD or frequency division duplex uses two channels, one for transmit and the other for receiver. Another scheme known as TDD, time division duplex uses one frequency, but allocates different time slots for transmission and reception.

Click on the link for more information on TDD FDD duplex schemes

Each of the two schemes is applicable for different types of service. Accordingly it is necessary to opt for the most effective form of duplex for the given application.

WiMAX duplex requirements

It is possible to use both FDD and TDD for WiMAX. However FDD transmissions require the use of two channels, one for the uplink and one for the downlink. These need to be separated sufficiently to enable the receiver to operate without being desensitised by the transmitter which needs to operate simultaneously. This not only requires there to be a separate frequency separation between uplink and downlink, but it is normally also necessary to incorporate additional filtering to remove the transmitter frequency from the receive band. This can add additional cost into consumer items that are normally very cost sensitive.

Any paired spectrum allocated, is normally split such that there is equal bandwidth available in both directions. This is not ideal where the traffic may be asymmetric and the balance may be variable.

Using WiMAX TDD it is possible to accommodate the asymmetry in the traffic balance. By altering the number of frames for traffic in each direction it is possible to make far more efficient use of the available spectrum. The balance of frames for each direction, and hence the capacity can be altered dynamically, enabling the system to respond the actual traffic travelling in each direction. Therefore TDD can handle both symmetric and asymmetric broadband traffic and therefore TDD therefore has higher spectral efficiency than FDD for these applications.

With both types of WiMAX duplex scheme being available for use, it is necessary to choose the correct form of duplex scheme for the particular type of deployment. To achieve the optimum form, it is necessary to undertake a WiMAX TDD FDD comparison to ascertain the optimum version subject to any regulatory constraints. It is useful to note that the first release of fixed WiMAX support both TDD and FDD duplex modes, although Mobile WiMAX only includes TDD mode.

By Ian Poole

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