Evolution of antenna technology in mobile communications: from analog voice to intelligent beamforming

　　Evolution of antenna technology in mobile communications: from analog voice to intelligent beamforming

　　As the core transducer device of wireless communication systems, the technological evolution of antennas has directly driven the leapfrog development of mobile communications from 1G to 5G and beyond. This paper systematically sorts out the core features, design breakthroughs and performance improvements of antenna technology in various generations of mobile communications, analyzes the constraint mechanism of antenna design under different frequency band characteristics, and discusses the academic principles and engineering implementation of key technologies such as MIMO and beamforming. Studies have shown that the evolution of antenna technology has always revolved around the three core propositions of spectrum efficiency improvement, equipment miniaturization and environmental adaptability. In the future, with the integration of technologies such as AI/ML and reconfigurable smart surfaces, it will lay a key hardware foundation for scenarios such as 6G holographic communication.

　　1. Introduction: Basic functions and technical challenges of antennas

　　Antenna is essentially an electromagnetic transducer that realizes the bidirectional conversion of guided electromagnetic waves and free space electromagnetic waves, and is the "electromagnetic interface" of wireless devices. Its technological development has always faced three physical constraints: first, the size-frequency relationship, that is, the ratio of the antenna electrical size to the working wavelength determines the radiation efficiency, and the low frequency band (long wavelength) places strict requirements on the physical size of the antenna; second, multi-band compatibility, the mobile communication band has expanded from 800MHz to millimeter waves, and wideband coverage must be achieved in a limited space; third, interference suppression, and electromagnetic compatibility issues in dense user environments are becoming increasingly prominent.

　　From 1G to 5G, the performance leap of each generation of mobile communication technology is based on breakthroughs in antenna technology. Based on the evolution of 3GPP standards, this article systematically analyzes the design paradigms and technical characteristics of antennas in each generation of technology.

　　2. 1G era: analog voice and external monopole antenna (1980s-1990s)

　　2.1 Technical background and standard system

　　1G is represented by North American AMPS (Advanced Mobile Phone System), which uses analog frequency modulation (FM) technology to achieve voice communication and works in the 800MHz frequency band. The system has no encryption mechanism and has inherent defects such as easy eavesdropping and poor anti-interference.

　　2.2 Antenna design and performance characteristics

　　Limited by the wavelength of the 800MHz frequency band (about 37.5cm), a quarter-wavelength monopole antenna is used with a physical length of about 9-10cm. Its core features include:

　　Structural form: external retractable metal rod, mechanical adjustment to ensure electrical size matching

　　Radiation characteristics: omnidirectional radiation mode, gain of about 2dBi, and directional pattern similar to spherical wave diffusion

　　Engineering limitations: efficiency depends on the mechanical unfolding state, and human body occlusion causes 20-30dB signal attenuation

　　The antenna design during this period aimed to "achieve basic communication connection" and did not involve complex signal processing technology.

　　3. 2G/3G era: digital communication and built-in design (1990s-2010)

　　3.1 Breakthroughs in 2G technology and PIFA antennas

　　2G introduced digital standards such as GSM (TDMA/FDMA) and CDMA, and the operating frequency band was expanded to 850/900/1800/1900MHz. Planar inverted F antenna (PIFA) has become a landmark design:

　　Structural innovation: A resonant cavity is formed by a folded metal patch and a ground plane, and the size is reduced to 40×20×5mm³

　　Multi-band implementation: Dual-band (900/1800MHz) coverage is achieved by loading parasitic elements and slotting technology

　　Performance indicators: Standing wave ratio (VSWR) <2, gain 3-5dBi, meeting the space constraints of pocket portable devices

　　3.2 3G technology and antenna diversity technology

　　3G UMTS/HSPA system introduces 2100MHz frequency band to support packet data transmission. To solve the problem of multipath fading, antenna diversity technology has become standard:

　　Spatial diversity architecture: The terminal has 2 independent antennas built in, with a spacing of ≥λ/4 (about 3.5cm@2100MHz)

　　Switching mechanism: Real-time selection of the optimal receiving path through RSSI (received signal strength indication) to improve link reliability 10-15dB

　　Engineering challenge: The mutual coupling problem between the two antennas needs to be solved (isolation > 15dB)

　　4. 4G LTE era: MIMO technology and multi-antenna system (2010-2020)

　　4.1 LTE standard and spectrum expansion

　　4G is based on OFDMA technology, with an operating frequency band covering 700MHz-2.6GHz and a peak rate requirement of 100Mbps (downlink)/50Mbps (uplink). MIMO (multiple input multiple output) has become a core technology.

　　4.2 Multi-antenna design and spatial multiplexing

　　Terminal implementation: 2×2 or 4×4 MIMO configuration, reducing antenna spacing (<10mm) through polarization diversity (±45° orthogonal polarization)

　　Channel capacity: Based on information theory, N×N MIMO can achieve N-fold spectrum efficiency improvement (Shannon limit)

　　Engineering optimization: Use notch filters to suppress inter-band interference, and reduce antenna mutual coupling to below -12dB through decoupling networks

　　Fiberglass antennas have unique advantages in 4G military applications: low RCS (radar cross-section) characteristics improve concealment, multi-band compatibility (700MHz-2.6GHz) meets tactical communication needs, and high-gain directional design achieves 10km line-of-sight transmission.

　　5. 5G and 5G-Advanced: Smart Beamforming and Phased Array (2020-)

　　5.1 Dual-band architecture and technical challenges

　　5G NR adopts a dual-band strategy of FR1 (Sub-6GHz, 450MHz-6GHz) and FR2 (millimeter wave, 24GHz-52GHz):

　　Sub-6GHz: Continue MIMO technology and support massive MIMO (64T64R)

　　Millimeter wave: Faced with 200dB/km level path loss, rely on beamforming technology to compensate

　　5.2 Phased array antenna and beam management

　　Array design: Millimeter wave terminal integrates 4×4 or 8×8 phased array, unit spacing λ/2 (such as about 5.3mm in 28GHz band)

　　Beamforming: Control the phase of each unit through the phase shifter to achieve ±60° beam scanning, half-power beam width < 15°

　　Dynamic tracking: Predict user movement trajectory based on AI, beam switching delay < 1ms

　　5.3 5G-Advanced Technology Evolution (3GPP R18+)

　　AI/ML Fusion: Beam optimization introduces reinforcement learning algorithm, and prediction accuracy is improved to more than 85%

　　RIS (Reconfigurable Intelligent Surface): Compensate for occlusion loss by adjusting reflection coefficient, and expand coverage by 30%

　　XR Adaptation: End-to-end delay is reduced to less than 5ms, supporting 120fps AR/VR data stream transmission

　　6. Conclusion and Outlook

　　The evolution of antenna technology shows three major trends: miniaturization from external to internal, broadband from single frequency to multi-frequency, and intelligence from passive to active. In the future, 6G will face the challenge of communication in the terahertz band (100GHz-1THz), and it is necessary to break through disruptive technologies such as quantum antennas and intelligent metasurfaces.

　　In the field of military communications, 5G smart antennas have achieved beam stabilization in motion (±0.5° pointing accuracy), and will develop in the direction of anti-interference (frequency hopping + beam agility) and low interception (ultra-low sidelobe < -40dB) in the future. Antennas are "electromagnetic nerve endings", and their technological breakthroughs will continue to promote the deep integration of mobile communications from connecting people to connecting everything, and from the physical world to digital twins.