Duplexing Techniques

The technologies of today enable speech and data transmission in both directions. Duplexing is the name for this specific capability to permit simultaneous bidirectional data transport. Duplexing is possible in both the time and frequency domains.

• Frequency Division Duplexing (FDD)
• Time Division Duplexing (TDD)

Frequency Division Duplexing (FDD)

The simultaneous transfer is achieved by using two different frequency bands, one for uplink transmission and the other for downlink transmission. These uplink and downlink frequencies might be fixed and are separate from one another. Hence, devices that enable FDD often have two antennas—one for each link. FDD allows for simultaneous signal transmission and reception since a frequency band divides the uplink and downlink communication channels. Both cellular networks like 3G and 4G and satellite communication technologies typically use this technique. FDD can be used for applications that require an even distribution of capacity between the uplink and the downlink.

Time Division Duplexing (TDD)

This duplexing method employs the full frequency range for the period of the one-time slot for transmission and the following slot for the reception. The time frame is kept brief enough to give the user the impression that both uplink and downlink are occurring at the same time in order to prevent interference and time delays. Time-division duplexing (TDD), a duplexing technique, employs a single frequency band for both signal transmission and reception, but the transmission and reception happen at different times. Since the uplink and downlink communication channels use the same frequency range but are separated by time intervals, TDD enables consecutive signal transmission and reception.

Comparison of FDD and TDD Duplexing Techniques

Criteria	Frequency Division Duplexing (FDD)	Time Division Duplexing (TDD)
Frequency Band Usage	Two separate frequency bands are used for uplink and downlink transmission	A single frequency band is used for both uplink and downlink transmission
Time Interval Usage	NA	Time is divided into time slots for both uplink and downlink transmission
Antenna Requirement	Two antennas are required, one for uplink and one for downlink transmission	One antenna is sufficient for both uplink and downlink transmission
Spectral Efficiency	Lower than TDD	Higher than FDD
Flexibility	Limited	More flexible
Interference and Delay	Less susceptible to interference and delay than TDD	More susceptible to interference and delay than FDD
Capacity Allocation	FDD is suitable for applications that require even distribution of capacity between uplink and downlink	TDD is suitable for applications with variable capacity allocation between uplink and downlink
Examples of Usage	3G, 4G cellular networks, satellite communication technologies	WiMAX, TD-SCDMA, LTE TDD, Wi-Fi