How does phased array affect LNA requirements?

In a phased array with N elements, the signals add coherently while noise adds incoherently, providing N-fold improvement in system SNR. The effective system noise figure is approximately NF_element - 10*log10(N). For a 64-element array with per-element NF of 3.5 dB: effective system NF = 3.5 - 18 = -14.5 dB (referenced to array input). This relaxes individual LNA NF requirements. However, the LNA still dominates the receiver noise budget (Friis equation), so each dB of LNA NF improvement translates to 1 dB better receiver sensitivity. mmWave 5G systems rely heavily on array gain to compensate for the inherently higher NF at these frequencies.

What semiconductor technologies are used?

FR1 (sub-6 GHz): SOI CMOS dominates for cost and integration. GaAs pHEMT used for premium NF. Typical: SOI CMOS 0.8 dB NF at 3.5 GHz. FR2 (24-40 GHz): SiGe BiCMOS is the mainstream choice, offering 2.0-2.5 dB NF with excellent integration into beamforming ICs. GaAs mHEMT achieves 1.5 dB but requires separate die and is used in infrastructure. CMOS 22nm FD-SOI is emerging for cost-sensitive UE applications at 28 GHz (2.5-3.0 dB NF). FR2 (40-71 GHz): InP HEMT achieves the lowest NF (1.5-2.0 dB at 60 GHz) but is expensive. SiGe BiCMOS achieves 3.0-4.0 dB.

mmWave & 5G

5G LNA

Q: What NF do 5G LNAs achieve?

FR1 (sub-6 GHz): 0.5-1.5 dB NF using GaAs pHEMT or SOI CMOS. At 3.5 GHz (n78), state-of-art is 0.6 dB NF with 18 dB gain. FR2 (mmWave): 2.0-3.5 dB NF at 28 GHz, 2.5-4.0 dB at 39 GHz. SiGe BiCMOS achieves 2.0 dB at 28 GHz. GaAs mHEMT achieves 1.5 dB but at higher cost. At 60+ GHz: 3.5-5.0 dB NF. The array gain of a phased array (10*log10(N) dB) effectively reduces the system NF, so individual element NF requirements are relaxed compared to single-antenna receivers.

A low-noise amplifier optimized for 5G NR receiver chains across sub-6 GHz (FR1) and mmWave (FR2) bands. At FR1, 5G LNAs achieve 0.5 to 1.5 dB noise figure using GaAs or SOI CMOS. At FR2 (24-71 GHz), the design challenge intensifies: NF rises to 2.0 to 4.0 dB, but phased array gain (10 log N dB) compensates, making individual element NF less critical than in single-antenna receivers. Integration into compact antenna modules is the defining design constraint at mmWave.

Category: mmWave & 5G

FR1 NF: 0.5-1.5 dB

FR2 NF: 2.0-4.0 dB

Understanding 5G LNA Design

The LNA is the first active element in the receive chain, and by Friis' equation, its noise figure dominates the system noise figure. At sub-6 GHz, mature semiconductor processes (GaAs pHEMT, SOI CMOS) deliver sub-1 dB NF with 18+ dB gain, making the LNA design straightforward. The challenge is wideband operation: 5G FR1 bands span 600 MHz to 7.125 GHz.

At mmWave, transistor f_max and NF_min degrade, interconnect losses increase, and the LNA must integrate into a beamforming IC with 32-256 parallel channels. Per-element power consumption must be minimized (5-15 mW per LNA) because the array has many elements. SiGe BiCMOS has emerged as the mainstream technology, balancing NF, gain, integration density, and cost.

5G LNA Performance

Friis equation (receiver chain):
NF_sys = NF_LNA + (NF₂−1)/G_LNA + ...
High G_LNA suppresses downstream noise contributions

Array effect on system NF:
NF_array ≈ NF_element − 10 log₁₀(N)
64 elements, NF_elem=3.5 dB → NF_sys = −14.5 dB (ref. array)

Typical specifications:
FR1 n78 (3.5 GHz): NF 0.6 dB, Gain 18 dB, IIP3 −5 dBm
FR2 n257 (28 GHz): NF 2.2 dB, Gain 20 dB, P_DC 10 mW
FR2 n260 (39 GHz): NF 2.8 dB, Gain 18 dB, P_DC 12 mW

5G LNA Technology Comparison

Technology	NF @ 28 GHz	Gain	Integration	Cost
SiGe BiCMOS	2.0-2.5 dB	18-22 dB	Excellent (BFIC)	Low
CMOS 22nm FD-SOI	2.5-3.0 dB	15-18 dB	Excellent	Very low
GaAs mHEMT	1.5-2.0 dB	20-25 dB	Separate die	Medium
InP HEMT	1.0-1.5 dB	22-28 dB	Separate die	High

Common Questions

Frequently Asked Questions

What NF do 5G LNAs achieve?

FR1: 0.5-1.5 dB (GaAs/SOI CMOS). FR2 28 GHz: 2.0-2.5 dB (SiGe). FR2 39 GHz: 2.5-4.0 dB. FR2 60+ GHz: 3.5-5.0 dB. Array gain compensates: 64 elements provide 18 dB effective NF improvement.

How does phased array affect LNA specs?

Array gain (10 log N dB) effectively reduces system NF. But each dB of LNA NF improvement still translates to 1 dB better sensitivity. mmWave systems rely heavily on array gain to compensate for inherently higher NF.

Which semiconductor technology?

SiGe BiCMOS is mainstream for FR2 (2.0-2.5 dB NF, excellent integration into beamforming ICs). GaAs mHEMT for premium NF (1.5 dB) in infrastructure. CMOS FD-SOI emerging for cost-sensitive UE at 28 GHz.

Related Terms

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