Fundamental Difference
| Characteristic | FDD (Frequency Division Duplex) | TDD (Time Division Duplex) |
|---|---|---|
| TX/RX separation | Different frequencies (duplex gap) | Same frequency, different time slots |
| Frontend switch | Duplexer (passive filter) | RF switch (T/R switch) |
| Simultaneous TX+RX? | Yes (different frequencies) | No (alternating) |
| LTE bands | Band 1,3,7,8,20,28… (FDD) | Band 38,39,40,41… (TDD) |
| 5G NR | n1,n3,n5,n8,n28… (FDD) | n77,n78,n79,n38,n41… (TDD) |
FDD Frontend: Duplexer Requirements
Key requirement: TX leakage into RX must be <−50 dBm At 33 dBm TX power: need 83 dB TX→RX isolation total Duplexer provides 50–55 dB LNA protection: LNA P1dB typically >−10 dBm → 33−50=−17 dBm < P1dB ✓ FDD duplexer S-parameter analysis (3-port): S21: TX→ANT passband (1710–1785 MHz, IL <2 dB for Band 3) S31: TX→RX isolation (>52 dB in 1805–1880 MHz for Band 3) S32: ANT→RX passband (1805–1880 MHz, IL <2 dB)
TDD Frontend: RF Switch Requirements
RF switch (T/R switch) alternates between TX and RX modes No duplex filtering needed — same frequency, time division Switch S-parameter specs: ON state: S21 < −0.5 dB insertion loss (TX or RX path) OFF state: S21 < −40 dB isolation (TX→RX isolation when RX is active) S11, S22: < −15 dB (matched ports) TDD filter: only needs to reject out-of-band signals (no TX/RX isolation) Simpler filter spec → lower IL possible 5G NR n78 (TDD): only one BPF needed (both TX and RX use same filter)
S-Parameter Analysis Strategy
For FDD: analyze duplexer .s3p — check TX passband, isolation, and RX passband independently. For TDD: analyze RF switch .s2p in both ON and OFF states — check IL and isolation. Analyze BPF .s2p — one file covers both TX and RX paths.
RF View FDD/TDD Analysis: Load duplexer .s3p for FDD analysis or switch/filter .s2p for TDD analysis. Extract port pairs, compare ON/OFF states, and verify all specs. Free on Android.