How does FDD compare to TDD?

FDD uses paired spectrum: separate frequency bands for UL and DL, always active in both directions. TDD uses unpaired spectrum: single frequency band, time-shared between UL and DL. FDD advantages: constant low latency (no guard period), simpler timing, works better for symmetric traffic. FDD disadvantages: requires paired spectrum (expensive), duplexer adds cost/loss/size, cannot dynamically adjust UL/DL ratio. TDD advantages: flexible UL/DL ratio, no duplexer needed, unpaired spectrum (more available above 3 GHz), channel reciprocity enables beamforming. TDD disadvantages: guard period overhead (3-5%), requires network synchronization, latency varies with DL/UL pattern. 5G NR uses both: FDD in sub-3 GHz bands, TDD in mid-band (3.5 GHz) and mmWave.

What is duplex spacing and why does it matter?

Duplex spacing is the frequency gap between the UL and DL bands. LTE Band 1: UL 1920-1980 MHz, DL 2110-2170 MHz, duplex spacing = 190 MHz. LTE Band 3: UL 1710-1785 MHz, DL 1805-1880 MHz, duplex spacing = 95 MHz. Wider duplex spacing makes the duplexer filter design easier because the UL and DL bands are farther apart, allowing lower-order filters with less insertion loss. Narrow duplex spacing (e.g., Band 13: 31 MHz) requires very sharp-skirt filters with high Q resonators, increasing cost and insertion loss. The duplex spacing also determines the image frequency separation in the receiver, affecting image rejection requirements.

Why is FDD dominant in sub-3 GHz cellular?

Historical spectrum allocation: sub-3 GHz bands were allocated as paired spectrum in the 1990s-2000s when FDD was the only practical duplexing method for cellular. Regulatory inertia: changing existing allocations is extremely difficult. Performance at low frequency: FDD provides better coverage at low frequencies because both UL and DL are always active (no TDD guard period overhead), and the symmetric spectrum allocation ensures equal UL and DL performance. Voice traffic: VoLTE requires low, consistent latency, which FDD provides naturally. However, new spectrum above 3 GHz is increasingly allocated as unpaired (TDD), especially for 5G NR n77 (3.3-4.2 GHz) and n78 (3.3-3.8 GHz), where TDD's flexibility and beamforming advantages are dominant.

Duplexing Methods

FDD (Frequency Division Duplexing)

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FDD assigns separate paired frequency bands to uplink and downlink for simultaneous full-duplex operation. A duplexer filter isolates Tx/Rx. Constant low latency (no guard period). Requires paired spectrum and a duplexer (adds cost/loss). Dominant in sub-3 GHz cellular (LTE bands 1-32). 5G uses FDD below 3 GHz and TDD above.

Category: Duplexing Methods

Spectrum: Paired

Latency: Constant, low

Understanding FDD

FDD is the original duplexing method for cellular networks. By dedicating separate frequency bands to uplink (mobile to base station) and downlink (base station to mobile), FDD enables truly simultaneous two-way communication. The tradeoff is the need for paired spectrum and duplexer filters in every device. As unpaired spectrum becomes more available above 3 GHz, TDD is gaining ground, but FDD remains essential for sub-3 GHz coverage and voice services.

FDD Spectrum Allocation

FDD channel separation:
Δf = f_UL,center − f_DL,center

Duplexer requirements:
TX→RX isolation ≥ 50 dB
Guard band: Δf_guard ≥ 5–20 MHz

Spectral efficiency:
η_FDD = 2η_simplex (full duplex)

FDD vs. TDD Comparison

Feature	FDD	TDD
Spectrum	Paired	Unpaired
Latency	Constant, low	Variable (guard period)
UL/DL ratio	Fixed (symmetric)	Flexible (adjustable)
Duplexer	Required	Not required
Beamforming	Needs CSI feedback	Channel reciprocity
5G bands	Sub-3 GHz (n1,n3,n5)	Mid/mmWave (n77,n78,n257)

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

LTE Band	DL (MHz)	UL (MHz)	Duplex gap	BW
B1	2110–2170	1920–1980	130 MHz	60 MHz
B3	1805–1880	1710–1785	20 MHz	75 MHz
B7	2620–2690	2500–2570	50 MHz	70 MHz
B20	791–821	832–862	11 MHz	30 MHz
B28	758–803	703–748	−10 MHz	45 MHz

Common Questions

Frequently Asked Questions

FDD vs. TDD?

FDD: paired spectrum, always-on, constant latency, needs duplexer. TDD: unpaired, time-shared, flexible ratio, no duplexer, enables beamforming via reciprocity. 5G uses both: FDD sub-3 GHz, TDD above.

Duplex spacing?

Gap between UL and DL bands. Band 1: 190 MHz (easy filter). Band 13: 31 MHz (hard filter). Wider spacing = easier duplexer, less IL. Narrow spacing needs high-Q resonators.

Why sub-3 GHz?

Historical paired allocation from 1990s-2000s. Regulatory inertia. Better coverage (always-on). Good for voice (VoLTE needs constant low latency). New spectrum above 3 GHz is unpaired (TDD).

Related Terms

← EVM FDTD →

Cellular Technology

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