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Antenna Design Considerations for 5G Applications


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A 5G mobile device’s antenna design needs specific consideration to maintain performance. The entire performance is impacted by the ground plane, the antenna positioning on the board, and other associated components. The reliability needed by wireless devices is made possible by analyzing and making corrections from the very beginning of design.

In order for smartphones to function well over a wider variety of RF frequency bands and support the transition to 5G and other technologies, tuning the aperture of an antenna is crucial. To accommodate expanding RF needs such as the usage of multiple input, multiple output (MIMO), and carrier aggregation (CA) methods, smartphones require an increasing number of antennas.

But as smartphones get smaller and smaller, there is less and less surface area for these antennas. More antennas must fit in less area due to current developments in RF demand. As it mostly depends on the final device’s form factor and OEM preferences, antenna design is by far the most perplexing step in this process.

5G vs. 4G

The cellular industry continues in its evolution of higher data rates, lower latency, and maximum performance.

5G evolves from 4G by implementing some improvements in its architecture, such that it increases channel capacity expressed in bits/second, according to the Shannon–Hartley formula: C = M × B log2(1 + S/N). The formula’s parameters are affected by CA, MIMO designs, the designation of additional frequency bands, adaptive adoption of higher–order modulation techniques, and other factors to boost channel capacity.

CA is a method of merging numerous data streams to increase performance. MIMO systems contain several antennas for both receiving and sending, in contrast with SISO systems, which have only one antenna for each.

In comparison with 4G, 5G pushes design to the next level of complexity and capacity. As a result, antenna design must advance to satisfy the ever–rising demands for more bandwidth, more frequency bands, and improved interference immunity.

With 5G, each receiver’s normal number of antennae will rise significantly. Multiple antennas must be active concurrently to use the two major methods for generating larger data rates: CA and MIMO. Because of the requirement to cram more antennas into a smaller area, antenna size must be reduced, which lowers antenna efficiency. For all devices that want to transmit more data to more people in more demanding use cases, RF circuit design is a bottleneck.

Figure 1: Antenna efficiency reduction when migrating from a SISO to a MIMO system (Source: Digi International Inc.)

Antenna challenges

Modern wireless devices generally employ active tuners to decrease size due to severe size limits. According to changes in the operating environment, frequency band, and bandwidth coverage, the system can autonomously tune the antenna. Multiple tuning states and a greater frequency spectrum per tuning state must be supported by the antenna–tuning system.

Two fundamental frequency bands, FR1 and FR2, will be utilized for 5G, according to 3GPP Release 15 (FR2 [mmWave]: 24.25 to 52.6 GHz; FR1: 410 MHz to 7.125 GHz).

In addition to the current sub–3–GHz frequencies used in 4G LTE, 5G uses the 3.3– to 3.8–, 3.8– to 4.2–, and 4.4– to 4.9–GHz bands in FR1. As a result, cellular antennas must now meet revised specifications for increased sub–6–GHz frequency coverage.

Designing antennas presents a physical problem. The wavelength of a 1–GHz signal is approximately 30 cm. The wavelength of a signal at 28 GHz is 1.07 mm. The same antenna will not function for these two signals, necessitating at least two sets of antennas for 5G devices running in both the FR1 and FR2 bands.

Scalable orthogonal frequency–division multiplexing waveforms are used in 5G to handle varying subcarrier signal spacings and the variety of channel widths available across different frequency bands. Greater subcarrier spacing and broader channels are available at higher frequencies. Channel widths and subcarrier spacing are smaller for lower frequencies.

Antenna designs for FR2–exploiting devices or systems must be substantially different. Millimeter–wave (mmWave) transmissions experience significant route loss because the propagation loss of a signal is inversely proportionate to its wavelength. Increasing antenna gain via phased–array antenna design becomes a dependable, industry–recognized remedy that we will examine in the next section to make up for the path loss.

As previously mentioned, MIMO functioning requires several antennas. 4G networks have utilized single–user MIMO (SU–MIMO) and multiple–user (MU-MIMO), two forms of similar technology.

MIMO comes in a variety of forms. One is massive MIMO (mMIMO), a type of antenna that crams many more antenna components into a small amount of space than earlier MIMO versions. Because millimeter wavelengths can operate with considerably smaller antennas, it is possible to construct compact mMIMO arrays. Manufacturers are creating 128–element mMIMO antennas. Data rate and connection dependability increase thanks to mMIMO’s use of multiple data streams, which boosts signal capacity without using more spectrum.

mMIMO will be a crucial component in advancing cell capacity and data download rate. Additionally, maintaining the connection range will require resolving new issues brought on by the consistency of Bluetooth/WLAN communications.

Design

Sub–6–GHz and mmWave 5G antennas fall into these two groups based on their operating frequency. The sole difference between 4G and 5G sub–6–GHz is that the same set of system–side components will be employed, and the antenna will still be an independent omnidirectional antenna (instead of an array).

The dipole antenna, monopole antenna, PIFA, IFA, loop antenna, and so on will continue to play a dominant role in 2G/3G/4G. Antenna form factors can vary from a simple printed track antenna to an intricate laser–directed–structure antenna.

Active antennas can be used to meet size requirements. Active impedance matching and antenna aperture tuning make up the two main types of active antenna systems. Active aperture tuning directly alters the inherent properties of the antenna, whereas active impedance matching allows the antenna system to select between several impedance–matching networks depending on changes in operating parameters. OEMs must thus modify designs using unique matching networks.

In order to compensate for signal route loss on mmWave frequencies, phased–array antennas are required because of their capacity to generate extremely high gain (dBi). A phased–array antenna must be able to orient and adjust the radiation beam to maximize the peak EIRP (dBm) to a receiving mobile device inside its cell sector. This design differs from earlier ones. It will help to overcome the signal losses. Some of the key factors to consider include:

  • Dual polarization
  • Array size
  • Side–lobe level
  • Beam–steering angle range
  • Beam–steering angle resolution
  • System noise

The side–lobe level must be reduced, the beam direction angle’s range and resolution must be increased, system noise must be suppressed, and energy efficiency must be increased with a phased–array antenna.

Because of the requirement to cram more antennas into a smaller area, antenna size must be reduced, which lowers antenna efficiency. Antenna effectiveness in full–screen phones diminishes as the distance between the radiating element at the top of the device and the ground (placed at the edge of the screen) gets smaller, according to simulation models of an ideal antenna.

The receiver is more susceptible to transitory effects brought on by changes in its surroundings, such as holding the phone, due to the greater number of antennas and their smaller size. Reduced effectiveness and modifications to the frequency response are two examples of these transitory impacts.

Figure 2: A 5G mobile device prototype (Source: Qualcomm)

MIMO design

The signals that the two or more antennas separately receive must be as unconnected to one another as feasible for them to be considered independent. Three criteria — diversity of space, variety of polarization, and diversity of beam, or, most frequently, a combination of them — are used to attain this essential property that antennas must possess. By positioning the antennas at specific intervals (defined in terms of wavelengths) from one another, decorrelation between the signals picked up is made possible in space diversity.

When applying polarization diversity, antennas with mutually orthogonal characteristic polarizations are used to achieve decorrelation between received signals. In beam diversity, decorrelation between received signals is achieved using radiation patterns that are potentially complementary to one another and that are mutually dissimilar.

The correlation coefficient and isolation are used to indicate the level of independence between the ports in a multi–antenna system. The correlation coefficient measures how similar the two antennas’ emission patterns are to one another or how effectively they can perform a type of spatial filtering of electromagnetic rays coming at the receiver from various directions and with various polarizations. On the other hand, the level of decoupling between the radiating components is determined by the isolation between the two antennas.

Embedded considerations

RF energy is absorbed by the human body. It may be necessary to position the antenna on the side of the wearable/mobile device that faces away from the body if it will be worn on or close to the body. This is one of the reasons that RF design companies run experiments in anechoic rooms using phantom heads, bodies, and hands.

The performance of an antenna can also be impacted by metal objects nearby. The performance of an antenna might also be impacted by the device’s housing. It can also reduce the amount of energy the antenna radiates if the case is constructed of metal or plastic with glass filling. Think about using plastic instead of glass to create the casing. RF performance may vary depending on where the antenna is placed on the circuit board. Antennas should radiate in six directions.

Figure 3: For best performance, place the antenna on the end of the PCB, not along the side edge. (Source: 5G Technology World)

Antennas are often designed to work at an angle, but some antennas work best on the long or short edge of a board. Batteries, LCDs, motors, and other metal objects can create noise or reflections that interfere with antenna performance.

PCB considerations

The fifth generation of this technology will provide 10× to 20× faster transmission rates (up to 1 Gbps), 1,000× greater traffic density, and 10× more connections per square kilometer than the 4G network. While operating across far larger frequency ranges than 4G, 5G aspires to deliver latency of 1 ms, which is 10× quicker than 4G.

Antenna–equipped PCBs will have to accommodate both larger data speeds and frequencies than they do now, straining mixed–signal design to its limit. The frequencies used by 4G networks range from 600 MHz to 5.925 GHz, whereas the frequencies used by 5G networks will go down to the mmWave, with an average bandwidth of 26 GHz, 30 GHz, and 77 GHz.

In order to emit energy, surface–mount–device antennas often need a ground plane. The ground plane is a flat surface that functions somewhat like a mirror to balance the antenna for reciprocity. In most cases, the ground plane is longer than the antenna. The minimum operating frequency determines the length.

A PCB’s whole design is built on handling combined high–speed and high–frequency signals for 5G applications. To comply with FCC EMC rules, electromagnetic interference, which can also happen between the components of the board that handle analog signals and those that process digital data, must be avoided. Thermal conductivity and the thermal coefficient of dielectric constant, which measures variations in the dielectric constant (often in ppm/˚C), are the two factors that influence the choice of material.

When describing layer thickness and transmission line properties, PCB shape is also crucial. When it comes to the first point, it is required to select a laminate thickness that is typically between 1/4 and 1/8 of the wavelength of the highest working frequency. The laminate may begin to vibrate and possibly spread waves on the conductors if it is too thin.

Choosing between microstrips, stripline, or GCPW as the conductor type for transmission lines is required. Designers should adhere to standard guidelines for high–frequency circuit board design after choosing the substrate material. These guidelines include using the shortest tracks feasible and controlling their width and distance from one another to preserve impedance along all interconnections.





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