Why do high frequency signals reflect?

Impedance discrepancies can cause high-frequency signals, such as those utilized in 5G communication, to reflect. The shorter wavelengths of these signals can alter their impedance characteristics.

Upon encountering an impedance transition, a signal is partially absorbed and transmitted while the rest is reflected. The intensity of this reflection is determined by the degree of impedance mismatch.

The concept of signal reflection can be illustrated using a Smith chart. The key elements of a Smith chart include (Figure 1):

The chart is a polar representation of the complex reflection coefficient, Γ, where the distance from the center signifies the reflection strength and the angle denotes its phase.

All points located 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, often known as the “SWR circle,” signifying a constant voltage standing wave ratio (VSWR).

The center of the chart symbolizes a perfect match with no reflection (Γ = 0).

The periphery of the chart signifies complete reflection (Γ = 1), akin to a short circuit or open circuit.

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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, particularly in the mmWave spectrum, are highly prone to reflections due to their extremely short wavelengths, making them susceptible to even minor impedance fluctuations. Various obstacles, ranging from buildings to foliage, can reflect mmWave signals to varying extents.

Uncontrolled reflections can lead to signal distortion and interference, resulting in reduced signal quality and coverage issues. Fortunately, signal reflections can be managed through signal processing techniques such as multipath propagation.

Multipath propagation refers to a signal reaching a receiver through multiple paths, including reflections. This is especially relevant in complex urban environments where direct line-of-sight may be limited. By employing advanced signal processing methods, multipath propagation can utilize reflection, refraction, and scattering to generate multiple weaker signals arriving at the receiver at different times (Figure 2).

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Figure 2. Multipath propagation can be used to mitigate the negative impacts of signal reflection. (Image: ResearechGate)

When line-of-sight is achievable, beamforming can help alleviate the effects of mmWave reflections. Instead of utilizing multiple paths, beamforming employs multiple antennas to direct radio waves straight at the receiver, eliminating any reflections. Consequently, beamforming can enhance signal strength efficiency while consuming less power.

Deploying numerous small cells with shorter transmission distances can eradicate signal reflections and enhance signal reliability. While this approach can be effective for specific areas, it may be cost-prohibitive if deployed extensively. Alternatively, passive reflectors can be employed.

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Smart reflection surfaces

Various materials have been developed to create 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 comprise matrices of reflecting surfaces, some of which can be produced using various printing techniques.

Another approach involves using metamaterials that can be tailored to reflect mmWaves at a specific frequency in an asymmetric manner and disperse the beam over a wider area. Additionally, multiple reflectors can direct radio signals into hard-to-reach locations (Figure 3).

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Summary

High-frequency 5G signals, especially in the mmWave band, are sensitive to minor impedance variations and, consequently, are susceptible to reflections. A Smith chart can visualize and address complex impedance matching issues like mmWave reflections. Signal processing technologies such as multipath propagation and beamforming can alleviate the impact of mmWave reflection, while purpose-built reflectors have been designed to harness reflections and enhance mmWave coverage.