FDD
Understanding FDD
FDD has been the foundation of cellular communications since the first analog systems. By assigning different frequencies to uplink and downlink with a guard band between them, the base station and mobile can communicate in both directions simultaneously.
FDD vs TDD
| Parameter | FDD | TDD |
|---|---|---|
| Spectrum | Paired bands | Single band |
| TX/RX | Simultaneous | Alternating |
| Latency | Lower | Higher (switching) |
| Flexibility | Fixed UL/DL ratio | Adjustable UL/DL |
| Hardware | Duplexer needed | Switch only |
FDD Bands (examples)
- Band 1: DL 2110-2170 MHz, UL 1920-1980 MHz (190 MHz spacing).
- Band 3: DL 1805-1880 MHz, UL 1710-1785 MHz (95 MHz spacing).
- Band 7: DL 2620-2690 MHz, UL 2500-2570 MHz (120 MHz spacing).
Frequently Asked Questions
What is FDD?
FDD uses different frequencies for uplink and downlink, enabling simultaneous two-way communication. A duplexer filter separates the bands at the antenna port. FDD is the dominant scheme for cellular below 3 GHz.
What is the advantage of FDD over TDD?
FDD provides true full-duplex communication (TX and RX simultaneously) with no switching latency. It provides consistent, symmetric performance. FDD is preferred for voice and low-latency applications.
Why is TDD gaining over FDD in 5G?
TDD is dominant in 5G above 2.5 GHz because: unpaired spectrum is cheaper and more available, TDD allows flexible UL/DL ratio, and TDD enables channel reciprocity for massive MIMO beamforming. FDD remains for lower bands.