Why is analog beamforming dominant at mmWave frequencies?

At mmWave (24 GHz and above), the free-space path loss is severe, requiring high-gain arrays with 64 to 256 elements to close the link budget. Providing a dedicated ADC and full digital chain for each element would consume 10 to 50 watts per element in current silicon technology, making a 256-element fully digital array draw 2.5 to 12 kW, which is impractical. Analog phase shifters in silicon (CMOS or SiGe) consume only 5 to 20 mW per element, reducing total beamforming power by 100x. The one-beam-at-a-time limitation is addressed by rapid beam sweeping during the SSB burst in 5G NR, cycling through candidate beams in milliseconds.

Signal Processing

Analog Beamforming

Q: What is the difference between analog and digital beamforming?

In analog beamforming, phase shifts are applied in the RF domain before the signals are combined into a single RF chain and digitized by one ADC. This means the array can form only one beam direction per RF chain per time instant. In digital beamforming, every antenna element has its own dedicated ADC (or DAC), and beam steering is performed in the digital baseband processor. Digital beamforming can form multiple simultaneous beams in any direction, supports MIMO spatial multiplexing, and enables per-element adaptive nulling. The trade-off: digital beamforming for a 256-element array at 28 GHz would require 256 high-speed ADCs consuming enormous power, which is why 5G NR FR2 base stations use hybrid beamforming combining a small number of digital chains with analog subarrays.

Q: What is hybrid beamforming?

Hybrid beamforming combines a small number of digital baseband chains (typically 2 to 8) with analog subarrays (each containing 16 to 64 elements). Each digital chain feeds one analog subarray. The digital layer provides multi-stream MIMO capability (spatial multiplexing, MU-MIMO), while the analog layer provides the high gain needed to overcome mmWave path loss. A typical 5G NR FR2 base station uses 2 digital chains with two 128-element analog panels, achieving 2-layer MIMO while keeping power consumption under 100 watts.

An antenna array beam steering technique that applies variable phase shifts (and optionally amplitude weights) to each element in the RF domain using analog phase shifters before combining the signals into a single RF chain. Analog beamforming forms one beam per RF chain per time instant, trading flexibility for dramatically lower power consumption and simpler data converter requirements compared to fully digital beamforming. It is the dominant architecture for 5G NR FR2 (mmWave) user equipment and the analog subarrays in hybrid base station designs.

Category: Signal Processing

Key component: RF Phase Shifter

Beams per chain: 1

Understanding Analog Beamforming

In a phased array, beam steering is achieved by applying a progressive phase shift across the elements. The beam points in the direction where the element contributions add constructively. In analog beamforming, this phase shift is applied by RF-domain phase shifters, one per element, before the element signals are summed (receive) or after a common signal is split (transmit).

The combined signal feeds a single RF chain: one mixer, one filter, one ADC (receive) or one DAC, one mixer, one filter (transmit). This means the entire array produces one beam at a time. To scan different directions, the phase shifter settings are updated, typically every few microseconds in 5G NR beam sweeping. The one-beam limitation is the fundamental trade-off: analog beamforming cannot simultaneously serve users in different directions or perform spatial multiplexing within a single time-frequency resource.

Analog Beamforming Equations

Array Factor (uniform linear array):
AF(θ) = Σ_n=0^N−1 w_n × e^{jn(kd sinθ + φ_n)}

Phase Shift for Beam at θ₀:
φ_n = −nkd sin(θ₀), where k = 2π/λ, d = element spacing

Array Gain:
G_array = 10 × log₁₀(N) + G_element (dBi)

Example: 64-element array at 28 GHz with 5 dBi element gain → G_array = 18 + 5 = 23 dBi.

Beamforming Architecture Comparison

Architecture	ADCs Required	Beams/Chain	MIMO Layers	Power (256 elem)	Use
Analog	1	1	1	~5 W	UE, small cell
Digital	N (256)	Many	Up to N	~2.5 kW	Sub-6 GHz massive MIMO
Hybrid	K (2-8)	K	K	~50-100 W	5G FR2 gNB
Lens-based	K (beamspace)	K	K	~30-80 W	Research, FWA

Common Questions

Frequently Asked Questions

What is the difference between analog and digital beamforming?

Analog applies phase shifts in the RF domain before a single ADC. It forms one beam per chain. Digital gives every element its own ADC and steers beams in baseband, enabling multiple simultaneous beams and spatial multiplexing. Digital is impractical at mmWave with hundreds of elements due to power consumption (10-50 W per ADC), which is why 5G FR2 uses hybrid (a few digital chains + analog subarrays).

Why is analog beamforming dominant at mmWave?

mmWave path loss demands 64-256 element arrays. Fully digital arrays at 28 GHz would consume 2.5-12 kW. Analog phase shifters in CMOS consume only 5-20 mW per element, reducing power 100×. The one-beam-at-a-time limitation is mitigated by rapid beam sweeping in 5G NR SSB bursts, cycling beams in milliseconds.

What is hybrid beamforming?

Hybrid combines 2-8 digital chains with analog subarrays of 16-64 elements each. The digital layer provides multi-stream MIMO; the analog layer provides high gain. A typical 5G FR2 base station uses 2 digital chains with two 128-element panels, achieving 2-layer MIMO under 100 W.

Related Terms

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