The 4G LTE frame structure accommodates both Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD) configurations, with key differences in how they manage downlink and uplink transmissions. These differences allow FDD to offer consistent and simultaneous downlink and uplink capabilities with potentially lower latency due to dedicated frequencies, while TDD provides flexibility in allocating more resources to downlink or uplink based on current demand, which can be more efficient in terms of spectrum usage.
FDD Configuration:
- Separate Channels for Downlink and Uplink: In FDD, the frame structure is symmetrical, with separate frequency bands allocated for downlink and uplink. This separation allows simultaneous downlink and uplink transmissions.
- Consistent Subframe Allocation: All subframes within a frame can be used for either downlink or uplink, based on fixed allocations, allowing for a consistent and predictable scheduling of resources.
- Dedicated Frequencies: Since FDD uses separate frequencies for downlink and uplink, there's no need for special subframes for switching between downlink and uplink modes.
TDD Configuration:
- Shared Frequency for Downlink and Uplink: TDD operates on a single frequency band for both downlink and uplink, with the allocation of downlink and uplink transmissions varying within the frame structure.
- Dynamic Subframe Allocation: The allocation of subframes for downlink and uplink can be dynamically adjusted based on the network's needs, allowing for flexibility in resource allocation to match demand.
- Special Subframes: TDD configurations include special subframes that consist of three parts: Downlink Pilot Time Slot (DwPTS) for initial downlink transmission, Guard Period (GP) to prevent overlap between downlink and uplink transmissions, and Uplink Pilot Time Slot (UpPTS) for initial uplink transmission. These special subframes facilitate the switching between downlink and uplink modes within the same frequency band.