To maintain performance, antenna design for 5G mobile devices requires special consideration. The overall performance is affected by the ground plane, the positioning of the antenna on the board, and other related components. The reliability required for wireless devices is made possible by analyzing and correcting from the beginning of design.
Adjusting the aperture of the antenna is critical for smartphones to function properly over wider radio frequency bands and to support the transition to 5G and other technologies. To accommodate expanding RF requirements, such as using multiple-input multiple-output (MIMO) and carrier aggregation (CA) methods, smartphones need to employ more and more antennas.
5G VS 4G
The cellular industry continues to move towards higher data rates, lower latency and maximum performance.
5G has evolved from the 4G architecture by implementing some improvements in it, whereby the channel capacity expressed in bps is increased according to the formula C=M·B·log2(1+S/N).
The parameters of this equation are affected by CA, MIMO design, assignment of additional frequency bands, adaptive adoption of higher order modulation techniques, and other factors that increase channel capacity.
CA is a method of combining large streams of data to improve performance. MIMO systems contain multiple antennas for reception and transmission, while SISO systems have only one antenna each.
Compared to 4G, 5G pushes design complexity and capacity to new heights. Therefore, antenna design must continue to advance to meet the ever-increasing demands for larger bandwidths, more frequency bands, and higher immunity to interference.
With 5G, the normal number of antennas per receiver will increase significantly. Multiple antennas must be active simultaneously to generate higher data rates using two main methods: CA and MIMO.
Since more antennas need to be packed into a smaller area, the antenna size must be reduced, which reduces the antenna’s efficiency. RF circuit design is a bottleneck for all devices that want to transmit more data to more people in more demanding use cases.
Challenges on antenna design
Due to strict size constraints, modern wireless devices often employ active tuners to reduce size. According to changes in the working environment, frequency band and bandwidth coverage, the system can automatically tune the antenna. An antenna tuning system must support multiple tuning states, and a larger frequency spectrum for each tuning state.
Scalable OFDM waveforms are used in 5G to handle different subcarrier signal spacing and various channel widths available on different frequency bands. Greater subcarrier spacing and wider channels are available at higher frequencies. The lower the frequency, the smaller the channel width and subcarrier spacing.
The signals received separately by two or more antennas must be as disconnected as possible from each other so that they can be considered independent. Three criteria—space diversity, polarization diversity, and beam diversity, or a combination thereof, which is also the most common—can be used to achieve this fundamental property that an antenna must have. By placing antennas at specific intervals (defined in terms of wavelength) from each other, decorrelation between received signals can be achieved with spatial diversity.
When employing polarization diversity, antennas with mutually orthogonal characteristic polarizations can be used to achieve decorrelation between received signals. In beam diversity, decorrelation between received signals is achieved using radiation patterns that may be complementary to each other but are dissimilar to each other.
Correlation coefficient and isolation are used to express the degree of independence between ports in a multi-antenna system. The correlation coefficient is a measure of how similar the emission patterns of two antennas are to each other, or how efficiently they spatially filter electromagnetic rays entering the receiver from different directions and with different polarizations. On the other hand, the degree of decoupling between radiating elements is determined by the isolation between the two antennas.
RF energy is absorbed by the human body. If the wearable/mobile device is to be worn on or close to the body, it may be desirable to place the antenna on the side of the device facing away from the body. This is one of the reasons why RF Design conducts experiments in anechoic chambers using models of human bodies and hands.
Antenna performance can also be affected by nearby metal objects. Antenna performance may also be affected by the device enclosure. If the enclosure is made of metal or plastic and filled with glass, it may also reduce the amount of energy radiated by the antenna. That’s why plastic is used instead of glass for the enclosure. RF performance may vary depending on where the antenna is placed on the circuit board. The antenna radiates in six directions.
Antennas are usually designed to work at an angle, but some work best on the long or short sides of the board. Batteries, LCDs, motors, and other metal objects can create noise or reflections that interfere with antenna performance.
Inspirations for PCB design
5G networks will provide 10 to 20 times faster transmission rates (up to 1Gbps), 1,000 times higher traffic density, and 10 times more connections per square kilometer than 4G networks. While operating over a much larger frequency range than 4G, 5G aspires to deliver 1ms latency, which is 10 times faster than 4G.
PCBs equipped with antennas will have to accommodate much greater data rates and frequencies than today, pushing mixed-signal designs to their limits. The frequency range used by the 4G network is from 600MHz to 5.925GHz, while the frequency used by the 5G network will extend to millimeter wave, and its average bandwidth will be 26GHz, 30GHz and 77GHz.
In order to radiate energy, patch antennas usually require a ground plane. The ground plane functions somewhat like a mirror to balance the reciprocity of the antenna. In most cases the ground plane is longer than the antenna. The minimum operating frequency determines its length.
The overall design of the PCB is based on processing high-speed and high-frequency combined signals for 5G applications. To comply with FCC EMC rules, electromagnetic interference must be avoided, which can also occur between circuit board components that process analog signals and circuit board components that process digital data. Thermal conductivity and the thermal coefficient of permittivity (a measure of change in permittivity, usually in ppm/°C) are two factors that influence material selection.
The shape of the PCB is also important when it comes to layer thicknesses and transmission line characteristics. Referring to the first point, the thickness of the laminate that needs to be selected is usually between 1/4 and 1/8 of the wavelength of the highest operating frequency. If the laminate is too thin, it may start to vibrate and possibly propagate waves across the conductors.
Need to choose microstrip line, stripline or GCPW as the conductor type of the transmission line. After selecting the substrate material, the designer should follow standard guidelines for high-frequency circuit board design. These guidelines include using the shortest feasible tracks and controlling their width and distance from each other to maintain impedance along all interconnects.