s are frequency- multiplexed. 3.4.1 Frequency and Time Division Duplex Schemes The Frequency Division Duplex is a duplex scheme in which uplink and downlink transmis- sions occur simultaneously using different frequencies. The downlink and uplink frequen- cies are separated by sufficiently large frequency offset. For the FDD scheme to properly operate, it is necessary that the frequency separation, that is, channel separation between the transmission and reception frequencies, to be sufficient in order to prevent the receiver blocking due to high-power transmitter signal. The receiver blocking is an important issue in FDD schemes and often highly selective filters may be required. For cellular systems using FDD, filters are required in the base station and the user terminal to ensure suffi- cient isolation of the transmitter signal without desensitizing the receiver. While imple- mentation cost is not a significant constraint for the base stations, placing a filter in the user terminal involves higher complexity and cost. The use of an FDD system does enable simultaneous transmission and reception of signals. However, two RF channels are required, which in some cases may not be the efficient use of the available spectrum. The spectrum used for FDD systems is allocated by the regulatory bodies. Since there is a fre- quency separation between the uplink and downlink directions, it is not typically possible to reallocate spectrum to change the balance between the capacity of the uplink and down- link directions, if the capacity requirements for each direction vary over time. 370 Chapter 3 The Time Division Duplex is a duplex scheme where uplink and downlink transmissions occur at different times but may share the same frequency. In other words, the downlink and uplink transmissions are multiplexed in time and are not concurrent. While FDD transmissions require a large frequency separation between the transmitter and receiver frequencies, TDD schemes require a guard time or guard interval between transmission and rec