eption. This gap must be sufficient to allow the signals traveling from the remote transmitter to arrive before a transmission is started and the receiver is shut down. Although this delay is relatively short, switching between transmission and reception several times in a second, even a small guard time can reduce the spectral efficiency of the system since a percentage of time must be used for the guard interval. For small- sized cells, for example, up to one mile, the guard interval is typically small and accept- able. However, for large cell sizes, it may become an issue and may introduce signifi- cant overhead. FDD has been the dominating duplex scheme since the beginning of the mobile com- munication era. In the 5G era, FDD will remain the main duplex scheme for lower fre- quency bands; however, for higher frequency bands, especially above 10 GHz and targeting very dense deployments, TDD will play more important role. In very dense deployments with low-power nodes, the TDD-specific interference scenarios (direct base station-to-base-station and device-to-device interference) will be similar to the base-station-to-device and device-to-base-station interference that also occurs in FDD schemes. Furthermore, for the dynamic traffic variations expected in very dense deployments, the ability to dynamically assign transmission resources (e.g., time slots) to different transmission directions may allow more efficient utilization of the avail- able spectrum. To reach its full potential, 5G will allow for very flexible and dynamic assignment of TDD transmission resources. This contrasts with current TDD-based mobile technologies, including TD-LTE, for which there are restrictions on the down- link/uplink configurations; thus there typically exist assumptions about using the same configuration for neighboring cells and between neighboring operators. The guard interval required for TDD will comprise two main elements: (1) A time- allowance for the propagation delay for any transmission from a remote transmitter to arri