High-frequency signals, like those used in 5G communications, are reflected by impedance discontinuities. Their shorter wavelengths can change their impedance.
When a signal arrives at an impedance transition, part of it is absorbed and transmitted, and part is reflected. The strength of the reflection depends on the magnitude of the impedance mismatch.
The phenomenon of signal reflection can be depicted using a Smith chart. The elements of a smith chart include (Figure 1):
- The chart is a polar plot of the complex reflection coefficient, Γ, where the distance from the center indicates the strength of the reflection and the angle represents its phase.
- All points lying on a circle correspond to the same level of impedance mismatch and reflection along a transmission line.
- Each circle represents the same magnitude of reflection coefficient, which is often referred to as the “SWR circle” and represents a constant voltage standing wave ratio (VSWR).
- The center of the chart represents a perfect match with no reflection (Γ = 0).
- The edge of the chart represents a complete reflection (Γ = 1), like a short circuit or open circuit.
Figure 1. Smith chart where Γ = 0 at the intersection of the red lines and Γ = 1 at the dashed black line. (Image: Analog Devices)
High-frequency 5G signals, especially in the mmWave band, are quite susceptible to reflections due to their very short wavelengths, which make them sensitive to even small variations in impedance. Many obstacles, from buildings to foliage, reflect mmWave signals to varying degrees.
Uncontrolled reflections can cause signal distortion and interference, reducing signal quality and coverage issues. Fortunately, signal reflections can be harnessed using signal processing techniques like multipath propagation.
Multipath propagation refers to a signal reaching a receiver via multiple paths, like reflections. This is especially true in complex urban areas where line-of-sight can be limited. By applying advanced signal processing techniques, multipath propagation can use reflection, refraction, and scattering, effectively creating numerous weaker signals arriving at different times at the receiver (Figure 2).
Figure 2. Multipath propagation can be used to mitigate the negative impacts of signal reflection. (Image: ResearechGate)
When line-of-sight is available, beamforming can alleviate the impact of mmWave reflections. Instead of leveraging multiple paths, beamforming uses multiple antennas to aim radio waves directly at the receiver, eliminating any reflections. Thus, beamforming can increase the effective signal strength while using less power.
Deploying many small cells with shorter transmission distances can eliminate signal reflections and improve signal integrity. This strategy can be effective for limited areas but can be cost-prohibitive if used too widely. Instead, passive reflectors can be deployed.
Smart reflection surfaces
Various materials have been developed to fabricate mmWave reflection surfaces. These passive structures can be designed to support specific reflection angles.
Examples include ferroelectric ceramic plates, metal-backed dielectric cuboids, and electromagnetic surface technology on glass or printed circuit board substrates. These structures consist of matrices of reflecting surfaces, some of which can be fabricated using various printing technologies.
Another approach is to use a metamaterial that can be fabricated to reflect mmWaves with a specific frequency in an asymmetric pattern and even spread the beam over a wider area. In addition, multiple reflectors can direct radio signals into difficult-to-reach locations (Figure 3).
Summary
High-frequency 5G signals, especially in the mmWave band, are sensitive to even small variations in impedance and, as a result, are quite susceptible to reflections. A Smith chart can visualize and solve complex impedance matching problems like mmWave reflections. Signal processing technologies like multipath propagation and beamforming can mitigate the impact of mmWave reflection, and purpose-built reflectors have been developed to harness reflections and improve mmWave coverage.
References
5G Bands Explained: How They Work & Why They Matter, Celona
Direct Optimisation of a Five-State Reconfigurable Reflectarray for 5G Applications, TICRA
Impedance Matching and Smith Chart Impedance, Analog Devices
Outdoor to Indoor Wireless Propagation Simulation Model for 5G Band Frequencies, IOP Conf. Series: Materials Science and Engineering
Reflect Array for 5G, Dai Nippon Printing Co., Ltd.
Reflection of electromagnetic radiation, McGraw Hill Access Science
Understanding RF Reflection, Cadence
Understanding Signal Reflections for High-Speed Design, Altium
EE World Online related links
Basics of mmWave and its applications
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Six key considerations when selecting and integrating GHz connectors into 5G applications
Experiments bring hope for 6G above 100 GHz
Choosing PCB materials to optimize applications, Part I
