Mutual Coupling Reduced Innovative Millimeter-Wave MIMO Antenna Designs for IoT and WBAN Applications

Abstract

The fifth-generation (5G) technology in Internet of Things (IoT) systems is enhancing modern developments owing to the faster communication and with minimal latency. In practice, two frequency bands are specifically allocated for 5G and futuristic communications. These include: FR1 sub-6 GHz band and the FR2 millimeter-wave (mmWave) band. The sub-6 GHz band has the capacity of wider coverage and better signal penetration, thus, it is more suitable for large-scale communication deployments. However, its limited bandwidth will not meet the rapidly increasing high-speed data transmission demand in emerging technologies. Alternatively, the mmWave band has significantly higher bandwidth, can support multi-gigabit per second data rates and growing IoT demands. However, owing to the smaller wavelength, the mmWave frequencies faces critical challenges. These include high path loss, blockages by obstacles, and atmospheric attenuation therefore restrict the mmWave communication range and its reliability. In general, only one antenna is mostly utilized for both transmission and reception in a conventional communication system. This approach is efficient but has limited channel capacity. Alternatively, the implementations of multiinput-multi-output (MIMO) can enhance the channel capacity, mitigate the mmWave blockages challenges and signal fading by incorporating multiple antennas at transmitter and receiver end. Multipath fading and blockages occurs when the transmitted signals from antennas reach to the receiver through reflecting off buildings, walls, and other obstacles. These multiple signals can interfere with one another, resulting in signal degradation. MIMO technology can mitigate this through spatial diversity, wherein multiple antennas transmit the same signal via different paths. This ensures that at least some portion of the signals will reach the receiver and with minimal distortion or fading. This way the communication in mmWave band will improves particularly in environments where there is significant non line of sight conditions. The design and implementation of mmWave MIMO antennas present challenges. In mmWave MIMO systems, antenna elements are typically placed at half-wavelength intervals. However, the shorter wavelengths of mmWave frequencies, ranging from 10 to 1 millimeter, require closely spaced antenna elements. This close spacing increases electromagnetic interaction, known as mutual coupling, degrades system performance by affecting impedance bandwidth, radiation patterns, gain, and efficiency. Minimizing mutual coupling is crucial for achieving optimal performance in mmWave MIMO communication systems. One method to reduce mutual coupling is to increase the inter-element spacing beyond half wavelength. Although this technique can decrease the mutual coupling, it will also increase the dimensions of a MIMO antenna, which is undesirable, particularly considering the futuristic device size constraints imposed by consumer device requirements. Thus, alternative mutual coupling reductions techniques are important. Therefore, this work aim to reduce mutual coupling by introducing advanced and novel techniques in MIMO antenna systems. These include defective ground structure (DGS), frequency selective surface (FSS), metasurface and metallic vias. An IoT application particularly wireless body area network (WBAN) was considered as application perspective of the designs. At first, a compact Vivaldi antenna was designed, comprising four-element Vivaldi patterns operating at 28 GHz and 30 GHz with a 36.44% fractional bandwidth. The design integrates a Vivaldi antenna with a FSS to reduce mutual coupling, achieving inter-element isolation below -20 dB within a compact 16×20 mm² size. Performance was assessed through simulations on Gustav’s model and practical tests on the human body, evaluating parameters such as impedance bandwidth, gain, efficiency, and radiation patterns. The MIMO antenna demonstrated excellent diversity characteristics: low envelope correlation coefficient (ECC) < 0.24, diversity gain > 9.95 dB, and total active reflection coefficient below -10 dB. A hybrid approach combining DGS and FSS to minimize mutual coupling was also investigated. A dual-band antenna with a modified elliptical patch and rotating arms was ist designed. and then converted into a six-element dual-band MIMO antenna covering ISM and 5G NR bands (23.63–32.90 GHz and 36.68–40 GHz). This hybrid coupling achieved reduction of -33 dB between MIMO elements. The antenna demonstrated broadside radiation patterns. In this case, MIMO diversity performance was: TARC below -10 dB, ECC under 0.04, and DG exceeding 9.91 dB. Another novel hybrid technique for mutual coupling reduction in a compact dual-band mmWave MIMO antenna, incorporating metallic vias and a metasurface, was investigated. This work particularly addressed surface wave coupling and near-field interference. A dual-band antenna element was designed with modified K-shaped patch arms, achieving resonances at 27 GHz and 28 GHz. The MIMO antenna design process included multiple configurations. The baseline 2×2 MIMO array exhibited high mutual coupling (-13 dB). To mitigate this, metallic vias were introduced, reducing surface wave coupling and improving isolation to -22 dB. Further enhancement was achieved by integrating a metasurface above the MIMO2 antenna which reduced coupling to below -30 dB across both operating bands. Fabricated prototypes were evaluated and measurements were performed in an anechoic chamber. This technique of combining metallic vias and metasurface in compact spacing has not been previously reported in literature, and represents the first implementation of such a technique. These reduction techniques demonstrated excellent results and can be implemented further in mmWave MIMO antenna designs.