FDD vs. TDD operation. A bi-directional communication device typically contains a transmitter and a receiver in close proximity. The requirement from the transmitter and receiver are conflicting in the sense that the transmitter is typically required to produce as much RF power as possible, while the receiver is required to detect a very weak RF signal, close to the thermal noise floor. In such a scenario, the transmitter is likely to degrade the performance of the receiver. The typical remedy for this problem is to separate the transmitter and the receiver either in the time domain or in the frequency domain (or sometimes in both). For the purpose of defining the frequencies scheme, the time-division-duplex (TDD) mode of operation poses no constrains on the frequencies scheme, and typically, due to the consideration of efficient use of spectrum, the transmit and receive frequency will be set to be the same and all the isolation between transmitter and receiver is achieved by having them operate at different, non-overlapping periods. The frequency-division- duplex (FDD) mode of operation does pose constraints on the frequency scheme. The two main considerations are the amount of in-band power and amount of out-of-band power reaching the receiver, as illustrated in Figure 8. Power RX signal filter mask TX signal power mask RX band TX band Frequency In-band transmitter signal reaching the receiver Out-of-band transmitter signal reaching the receiver Figure 8 In-band and out-of-band power coupled from transmitter to receiver The in-band power (depicted in orange) is a residue of the transmitter power (typically due to non- linearity) that falls under the receiver filter mask (and hence not rejected by the receiver) while out- of-band power (depicted in green) falls outside of the receiver filter mask (and hence rejected to a certain degree), but typically include the major component of the transmitter power. The consideration in FDD operation is therefore to keep sufficient separation between the transmit and receive band such that the suppression available by the receiver and transmitter filters is enough to prevent any degradation of the receiver operation due the transmitter. This consideration certainly applies to the designated RF band where the two signals couple via the antenna, but is often also relevant to any intermediate frequency band where coupling may exist due to leakages inside the radio hardware.
