Duplexing Scheme in WiMAX: TDD or FDD

Duplexing refers to the way downlink and uplink data is arranged in a two-way wireless transmission. The downlink carries information from a Base Station (BS) to Subscriber Stations (SSs). Downlink is also known as forward link. The uplink carries information from a SS to a BS. It is also called reverse link. There are two types of duplexing scheme, i.e. FDD and TDD.

Picture. Downlink and Uplink
Downlink and uplink traffic in a 2-way communication.

FDD (Frequency Division Duplex) requires two distinct channels for transmitting downlink sub-frame and uplink sub-frame at the same time slot. FDD is suitable for bi-directional voice service since it occupies a symmetric downlink and uplink channel pair. FDD is commonly used in cellular networks (2G and 3G). Meanwhile, WiMAX supports full-duplex FDD and half-duplex FDD (HFDD or HD-FDD). The difference is in full-duplex FDD a user device can transmit and receive simultaneously, while in half-duplex FDD a user device can only transmit or receive at any given moment.

Picture. Frequency Division Duplex (FDD) - full duplex mode
Downlink and uplink sub-frames are transmitted at the same time in two adjacent channels.

FDD is inefficient for handling asymmetric data services since data traffic may only occupy a small portion of a channel bandwidth at any given time. TDD (Time Division Duplex) is another duplexing scheme that requires only one channel for transmitting downlink and uplink sub-frames at two distinct time slots. TDD therefore has higher spectral efficiency than FDD. Moreover, using TDD downlink to uplink (DL/UL) ratio can be adjusted dynamically. TDD can flexibly handle both symmetric and asymmetric broadband traffic.

Picture. Time Division Duplex (TDD)
Downlink and uplink sub-frames are transmitted at different time slots in one channel.

Most WiMAX implementations either on licensed or license-exempt bands will most likely use TDD. The reasons are TDD uses half of FDD spectrum hence saving the bandwidth, TDD system is less complex and thus cheaper, and WiMAX traffic will be dominated by asymmetric data. The first release of Fixed WiMAX profiles support both TDD and FDD, while Mobile WiMAX profiles only include TDD.