# Frequency Division Duplexing Transmit and receive at different frequencies concurrently.  Convention: Uplink in a lower frequency. Rationale: power budget of the [base station](Base%20station.md) is higher than the mobile user, so we put mobile users (uplink) in a lower frequency band to save transmit power. Exception: Indoor [communications](Communications.md) (uplink/downlink doesn't really matter) A filter can separate [Tx](Transmitter.md) and [Rx](receiver.md) signals Frequency separation shuold be large enough so we can filter out the received [Tx](Transmitter.md) signal #ROT Frequency separation for FDD is 5% of the nominal frequency ## Practical Considerations - Isolation often needs 100dB - Needs a good filter and accurate impedance matching - Notch filters are a common choice ### Characteristics of filters   ## Benefits No delay if needing to send an urgent message. - Preferred ## Drawbacks - Use half the [bandwidth](Bandwidth.md) on uplink and half on downlink. - Hard to reconfigure the proportions (e.g. how to dynamically give more bandwidth to each signal? non trivial since you get ripple) [^1] # References [^1]: