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Compact Antenna Systems for Future Smartphone Multi-Frequency Wireless Standards

May 11, 2023

Future smartphones will contain a mix of wireless technologies. Hardware operating in different wireless spectra will co-exist. To realize these multiple hardware transceivers, compact antenna systems must be designed without significant compromise in individual antenna electrical performance and be compliant with respective standards. They must be operational in both the sub-6 GHz and mmWave bands. These systems must also be physically compact with minimal form factors constrained by the height of commercial smartphones. Several design approaches are described.

Most of the smartphones available in the market today have numerous sub-6 GHz antennas integrated within them. These radiators are electrically small with poor radiation efficiency. Antenna gain is not a critical parameter to be considered during design; thus, they radiate omnidirectionally with low gain.

Antennas operating in the mmWave band, however, must deliver high gain without compromise in their form factors. Furthermore, mmWave antennas must be impedance matched for wide bandwidth to cover existing 5G bands in their respective geographical locations.

Figure 1

Figure 1 Typical radiation characteristics of mmWave and microwave antennas in a smartphone.

Figure 2

Figure 2 Electrical real estate available within a typical smartphone.

While antennas with multi-frequency functionalities are integrated together, the performance integrity of the sub-6 GHz and mmWave antennas must be maintained. The challenge, with reference to both bands, is to accomplish this in an electrically small form factor, with reference to both the bands. Mutual coupling between the sub-6 GHz and the mmWave antennas must be minimal. The forward gain of the mmWave 5G antenna must be high to support the radio link.

In this article, several design strategies to achieve the aforementioned requirements are illustrated with experimental results.

DESIGN CONSTRAINTS

The reason for multi-octave carrier systems is to enhance the bandwidth allotted per user. Sub-6 GHz antenna systems must be co-designed with the mmWave antenna systems for optimal functionality of smartphone transceivers.1 Typically, the microwave antennas operate at sub-6 GHz frequencies while mmWave antennas function in the 28 GHz bands; specific frequencies depend on the geography and the licensing authorities. The carrier frequency of the mmWave band (28 GHz) is the eighth harmonic of the sub-6 GHz band (3.5 GHz). Hence, co-design of antennas operating in these bands is challenging.2-4

Figure 1 shows the typical use case for microwave and mmWave antennas integrated within the panel of a commercial smartphone. The sub-6 GHz antenna radiates omnidirectionally, in contrast to the unidirectional beam of the mmWave antenna. The beam radiated by the mmWave antenna must be pointed away from the user to facilitate a reliable high data rate link with the nearest base station.

The available real estate on a commercial smartphone is shown in Figure 2. The physical form factor of a sleek smartphone is approximately 8 × 6 × 0.7 cm, which translates to 0.93 × 0.7 × 0.08 λ for the sub-6 GHz band at 3.5 GHz. The same physical form factor translates to 7.44 × 5.6 × 0.65 λ for the mmWave band at 28 GHz.5 The co-designed, or integrated, antenna system must fit within the dimensional constraints of the mobile device without performance deterioration in either band.

The simplest solution for this design problem is a single antenna tuned to both carrier frequencies. The problem is that the antenna will operate quite well in the sub-6 GHz band but its gain and pattern integrity in the mmWave band will be significantly deteriorated. Hence, various multi-port antenna systems are explored in this article.

MULTI-PORT ANTENNA SYSTEM DESIGN EXAMPLES

Example 1

The first antenna system is shown in Figure 3. Port 1 is connected to the mmWave antenna, and Port 2 is connected to the microwave antenna. The same nomenclature is followed for all the designs presented in this article. The mmWave antenna is fabricated on a 20-mil thick Rogers 5880 substrate. It is a corporate inset-fed patch antenna array with a standard half-wavelength separation between elements.6 The radiators are in line with the panel of the smartphone, while the feed network is in the orthogonal plane. This corner-bent antenna exhibits a 10 dB impedance bandwidth of 10 percent (see Figure 4).

Figure 3

Figure 3 Fabricated prototype of co-design Example 1.

Figure 4

Figure 4 Input reflection coefficient of Port 1 for Example 1.

The electrically close placement of the sub-6 GHz antenna does not detune the mmWave antenna. Unidirectional radiation patterns of the mmWave antenna are shown in Figure 5. The antenna system offers high gain of close to 8 dBi at 28 GHz with minimal back radiation toward the user. The narrow beamwidth is due to the corporate-fed array action of the inset-fed patch antennas.

Figure 5

Figure 5 Radiation patterns of Port 1 for Example 1.

Figure 6

Figure 6 Radiation patterns of Port 2 for Example 1.

Radiation patterns of the microwave antenna within the co-designed ecosystem are shown in Figure 6. These patterns offer omnidirectionality. The slight tilt in the patterns is due to the presence of the system ground of the mmWave antenna. The printed dipole antenna operating at 3.5 GHz detunes slightly due to the electrical proximity of the ground plane.



The microwave antenna was redesigned for this topology to preserve its input impedance characteristics. The actual placement is shown in Figure 7a. It occupies minimal space in the smartphone. The mmWave antenna radiates away from the user in contrast to the microwave antenna, which is connected to Port 2 (see Figure 7b).

Figure 7

Figure 7 Antenna placement within a smartphone for Example 1: orientation (a) and radiation patterns (b).

Figure 8

Figure 8 Fabricated prototype of co-design Example 2.

Example 2

The second design approach mounts the mmWave and microwave antennas orthogonally for enhanced isolation between the ports. The fabricated antenna assembly is shown in Figure 8. The corner-bent mmWave antenna array is truncated at the non-radiating edge without hampering its radiating characteristics. The microwave antenna is a simple electrically small, printed monopole resonating at 3.5 GHz. This concept could be generalized with any mmWave antenna with a unidirectional beam and a printed monopole operating in the sub-6 GHz bands.

Example 3

The third design approach employs a shared overlapped ground for both antennas (see Figure 9). A high gain broadside radiator operating in the 28 GHz band is placed on top of its microwave counterpart. The ground plane of the mmWave antenna is common with the printed microwave monopole. The entire ground plane for Port 1 ensures a unidirectional beam. On the other hand, the partial ground plane associated with Port 2 delivers an omnidirectional beam throughout the operating band. Isolation between the ports is greater than 20 dB in both the microwave and mmWave frequency bands. Mutual coupling and the input reflection coefficient for the mmWave band are shown in Figure 10.

Figure 9

Figure 9 Schematic of co-design Example 3.

Figure 10

Figure 10 Mutual coupling and input reflection coefficient of co-design Example 3.

Panel mounting is shown in Figure 11. Only one smartphone panel is needed, saving integration space. The 3D-printed smartphone mockup matches the dimensions of a commercial smartphone with a panel height of 7 mm.

Figure 11

Figure 11 Panel mounting of co-design Example 3.

Example 4

Another uniplanar design technique is to integrate both antennas on a single substrate (see Figure 12). The broadside high gain mmWave antenna is microstrip-fed at one of the substrate edges. The microwave monopole is fed with a microstrip as well, on another edge of the substrate. A small gap of 0.1 mm is introduced in the ground plane to enhance isolation to 25 dB in both frequency bands. The input reflection coefficient and mutual coupling in the mmWave band is shown in Figure 13.

Figure 12

Figure 12 Fabricated prototype of co-design Example 4.

Figure 13

Figure 13 Mutual coupling and input reflection coefficient of co-design Example 4.




Neither the impedance nor the radiation characteristics of the constituent antennas change when co-designed with this technique. The panel mockup and its corresponding 3D-radiation patterns are shown in Figure 14. The design meets the radiation specifications for both the microwave and mmWave frequency bands.

Figure 14

Figure 14 Antenna placement within a smartphone for Example 4: orientation (a) and radiation patterns (b).

Example 5

The last approach is to integrate a high gain end-fire mmWave antenna with a microwave-printed monopole (see Figure 15). Here, the high gain compact printed Yagi-Uda antenna delivers a unidirectional beam in the mmWave frequency band. The printed monopole with a truncated ground is tuned along with the presence of the printed Yagi antenna. The panel mount configuration for commercial phones and corresponding 3D radiation patterns are shown in Figure 16.

Figure 15

Figure 15 Schematic of co-design Example 5.

Figure 16

Figure 16 Antenna placement within a smartphone for Example 5: orientation (a) and radiation patterns (b).

CONCLUSION

Multi-frequency antenna systems are essential for compatibility with hardware transceivers in upcoming smartphones. In addition, the impedance and radiation integrity of the individual antennas operating in the mmWave and microwave bands must be preserved. Several design approaches to realize these requirements are demonstrated in this article as potential candidates for future smartphones.

References

  1. T. S. Rappaport, S. Sun, R. Mayzus, H. Zhao, Y. Azar, K. Wang, G. N. Wong, J. K. Schulz, M. Samimi and F. Gutierrez, “Millimeter Wave Mobile Communications for 5G Cellular: It Will Work!,” IEEE Access, Vol. 1, May 2013, pp. 335–349.
  2. S. K. Koul and G. S. Karthikeya, Antenna Architectures for Future Wireless Devices, Springer Nature, Singapore, 2021.
  3. S. K. Koul and G. S. Karthikeya, Millimeter Wave Antennas for 5G Mobile Terminals and Base Stations, CRC Press, 2020.
  4. M. I. Magray, G. S. Karthikeya, J. H. Tarng and S. K. Koul, “Co-Design of 4G LTE and Millimeter-Wave 5G Antennas for Future Mobile Devices,” Wideband, Multiband, and Smart Antenna Systems. Signals and Communication Technology, Springer Nature, Switzerland, 2021.
  5. Y. Huo, X. Dong and W. Xu, “5G Cellular User Equipment: From Theory to Practical Hardware Design,” IEEE Access, Vol. 5, July 2017, pp. 13992–14010.
  6. G. S. Karthikeya, M. P. Abegaonkar and S. K. Koul, “CPW Fed Wideband Corner Bent Antenna for 5G Mobile Terminals,” IEEE Access, Vol. 7, January 2019, pp. 10967-10975.