Duplex transmission is the simultaneous transmission of two information signals that allows simultaneous (or near simultaneous) 2-way communication. The WiMAX system can use frequency division duplex (FDD), time division duplex (TDD) or half frequency division duplex (H-FDD).
FDD is the process of simultaneously allowing the transmission of information in both directions via separate frequency bands. When using FDD, each device transmits on one frequency while listening on a different one.
TDD refers to the process of allowing two way communications between two devices by time-sharing. When using TDD, device 1 transmits while device 2 listens for a short period of time. After the transmission is complete, the devices reverse their role so device 1 becomes a receiver and device 2 becomes a transmitter. The process continually repeats itself so data appears to flow in both directions simultaneously.
H-FDD is a process that allows for two-way communications between two devices through the combination of frequency division and time sharing. When using H-FDD, device 1 transmits on one frequency while the device 2 listens for a short period of time on that frequency. After the transmission is complete, the devices reverse their roles and device 2 transmits on a different frequency and the other device listens for a short period of time on that frequency. The process continually repeats itself so data appears to flow in both directions simultaneously. The use of H-FDD systems allows for the simplification of radio design as the transmitters and receivers in the same unit are separated in both frequency and time so a duplex filter is not required.
As shown in Figure 1, the WiMAX system may use three types of duplexing: frequency division duplex (FDD), time division duplex (TDD) and half frequency division duplex (H-FDD). FDD allows the transmission of information in both directions at the same time by using separate frequency bands ( frequency division). Half frequency division duplex uses different frequencies for transmission but does not allow transmission and reception at the same time.
When operating in time division duplex mode, WiMAX devices require reserved time periods to allow for transmission time (guard time) and to allow the device to transition between receive and transmit modes (transition gap).
Guard time is an amount of time that is allocated within a single time slot period in a communication system to help ensure variable amounts of transit times (e.g. from close and distant transmitters) do not cause overlap ( collisions) between adjacent time slots. Transmission of information does not occur within the guard period.
Receive-transmit transition gap is the amount of time that is allocated between the reception of a packet and transmission of a packet in a time division duplex (TDD) system. Transmit-receive transition gap is the amount of time that is allocated between the transmission of a packet and the reception of a packet in a time division duplex (TDD) system.
WiMAX systems have the capability to dynamically change the amount of bandwidth that is transmitted in either direction through a process called adaptive time division duplex (ATDD). ATDD is a process of allowing two way communications between two devices by time sharing on the same communication channel (e.g. the same frequency) and the amount of transmission rate or time that is used by each device can dynamically change.
Figure 1 illustrates how the WiMAX system can use adaptive time division duplex transmission to vary the amount of bandwidth that is transferred in either direction. A base station initially sends data at a high rate. However, after the user has received the data, the base station begins to send a response at a high rate. The time periods allocated for transmission on the downlink and uplink continually vary to allow for variable data transmission rates.
