FDD

Frequency-division duplex is a method for running simultaneous two-way communication on a single radio channel by splitting the available spectrum into two separate frequency bands, one for transmit and one for receive. A mobile device or base station transmits on one frequency while listening on another, allowing both ends to speak at the same time without collision or crosstalk. The two frequencies are held apart by a fixed guard band, typically measured in megahertz, to prevent the transmitter's own signal from drowning out the receiver.

FDD is the dominant duplex method in cellular networks. Global System for Mobile Communications (GSM), Long-Term Evolution (LTE), and many 3G and 4G standards use FDD. In LTE, for example, uplink and downlink bands may be separated by 50 to 90 megahertz depending on the frequency band in use. The method requires careful filter design and power management at the handset to prevent self-interference, and it demands paired spectrum, meaning regulatory bodies must allocate frequencies in matched pairs rather than single blocks.

The alternative is time-division duplex (TDD), where both directions share the same frequency but take turns in time, switching between transmit and receive in rapid alternating slots. FDD works better when uplink and downlink traffic is balanced and when latency must be low; TDD adapts better to asymmetric traffic (much more data one way) and can use unpaired spectrum. Many networks deploy both: FDD for smartphones and standard coverage, TDD for capacity hotspots or satellite use.

FDD's main disadvantage is its spectrum hunger. Because frequencies cannot be reused for the opposite direction, a network needs twice as much spectrum width as a TDD system might. This makes FDD licensing expensive and inflexible; a carrier cannot easily shift capacity from uplink to downlink on a bad day. Conversely, FDD offers faster switching and cleaner separation, reducing the need for complex digital signal processing to suppress interference.

In practice, FDD devices require a duplexer, a filter network that isolates the transmitter from the receiver on the same antenna. Duplexers use surface-acoustic-wave (SAW) or bulk-acoustic-wave (BAW) technology to achieve the steep rejection slopes needed at microwave and millimeter-wave frequencies. Without good duplexing, the strong outbound signal leaks into the sensitive receiver front end and blocks incoming signals entirely.