d receive at the same time. A major issue in limiting the capacity of non-cooperative cellular massive MIMO networks operating in TDD mode is the pilot contamination. The rise of asymmetric uplink/downlink traffic patterns necessitates that the ratio between downlink/uplink traffic changes over the time; thus the static paired spectrum for downlink/uplink is not efficient for supporting such dynamic asymmetric traffic, particularly in UDNs. Flexible duplex can better adapt to dynamic asymmetric traffic. With flexible duplex, the uplink spectrum defined in FDD sys- tems can be reallocated for downlink transmission with high flexibility. Considering the potential cross-link interference, that is, downlink to uplink and uplink to downlink, the transmission power for downlink transmission on the uplink spectrum should be constrained to a relatively low. Flexible duplex can be applicable to small cells with low transmission power and relay base stations. 3.4.2 Half-Duplex and Flexible-Duplex Schemes The LTE and NR support TDD and FDD schemes with a great extent of commonality in the baseband processing. In order to reduce the implementation complexity and cost of FDD terminals and further to increase the reuse of baseband functional elements, a half- duplex FDD (H-FDD) operation is supported where the downlink and uplink transmissions are not simultaneous, but occur in two different frequencies. A classic H-FDD operation does not efficiently utilize the radio resources on the downlink and uplink RF carriers. The complementary grouping and scheduling of users would allow efficient use of downlink and uplink resources in an H-FDD operation. For H-FDD operation, a guard period is virtually 372 Chapter 3 Table 3.7: Comparison of time division duplex (TDD) and frequency division duplex (FDD) attributes. Attribute Paired Does not require paired spectrum as both Requires paired spectrum with sufficient spectrum transmit and receive occur on the same frequency separation to allow simultaneous channel transmission and receptio