With any transition between old and new generations of technology, there are compatibility issues. The transition to 5G wireless communication is no different. One hurdle to be overcome in this transition involves incorporating, within a single phone, new antennas that can support the millimeter wavelengths of signals on 5G networks alongside existing antennas that support the longer wavelengths transmitted by LTE networks. In a new proof-of-concept study, Joni Kurvinen of Aalto University and his colleagues reveal a new design that incorporates, within a single device, both types of antenna.
Harnessing the high-frequency wavelengths of 5G networks will allow for much greater rates of data transfer, yet these wavelengths tend to attenuate very quickly over short distances. Therefore it’s desirable to have new phones that can harness not just 5G, whose signals are transmitted at frequencies around 28 gigahertz, but also support current 4G LTE networks, which transmit at frequencies of 700 megahertz and slightly higher.
“LTE and millimeter-wave frequencies are so far apart [on the spectrum] that both cannot be covered with one single antenna. Therefore, multiple antennas are required,” Kurvinen explains. “The main challenge is the placement of the antennas such that both fit inside the mobile device and do not deteriorate each other’s performance.”
Many LTE antennas are currently integrated within the metal rim of a phone, which acts as a coupling element, exciting resonating currents throughout the whole device. The design by Kurvinen and his colleagues maintains this approach, with an LTE antenna embedded within the metal rim along one side of the bottom of the phone. In this case, the antenna supports both low and high frequency bands of LTE networks.
The second antenna, which supports millimeter-waves, is designed so that all its metal parts are sufficiently far from the metal parts of the LTE antenna. How far is sufficient? As it turns out, putting it on the opposite side of the phone’s bottom edge is distance enough. The antenna is slipped into a hole drilled into the metal frame, which allows millimeter-wave radiation to pass through.
The device can pack a mm-wave antenna within the same volume as the LTE antenna.Illustration: Aalto University/IEEE
The hole is then filled with plastic, which supports the design in a number of ways. First, the plastic insulates the LTE and mm-wave antennas from one another so that neither disturbs the other’s operation. The plastic also decreases the wavelength of the signal being fed to the mm-wave antenna, which allows for a smaller and more compact antenna structure.
“This work is probably the first one to completely include both antennas in the same device, and moreover, to accommodate them in a shared volume without significantly sacrificing either’s performance,” says Kurvinen.
Simulations and direct measurements suggest that this dual antenna system yields 60 percent efficiency, which is fairly high for mobile phone systems. But that figure comes with a few caveats. Kurvinen and his colleagues acknowledge that this is a prototype phone lacking many other components that appear in commercialized phones. What’s more, the size of the printed circuit board for this prototype is larger than what’s used in typical mobile phones. Lastly, this design supports mm-wave communication coming only to and from the end of the device—whereas, for optimal mm-wave coverage, a handset should also be equipped with antennas producing broadside radiation so signals can be received through the phone’s display and/or back cover.