Antenna efficiency plays a critical role in overall smartphone RF performance. However, current broad trends in RF requirements – especially the forthcoming transition to 5G – and in smartphone industrial design, mean that smartphones must fit more antennas into less space. As a result, antennas are shrinking in size, which reduces antenna efficiency. Without compensating for this problem, the reduced efficiency can impact Tx and Rx performance, causing shorter battery life, lower data rates and connectivity problems.
5G: More Antennas to Deliver Faster Data Rates
The transition to 5G, which continues the quest to deliver higher data rates, will drive a significant increase in the typical number of antennas in each handset.
Two primary techniques for delivering higher data rates, CA and multiple input multiple output (MIMO), both require multiple antennas that operate simultaneously. 5G will further drive this trend because it mandates support for four independent downlink channels for most bands, requiring handsets to include at least four antennas operating simultaneously for cellular communications.
At the same time, handset antennas will need to support a wider range of frequency bands, largely due to the introduction of new 5G bands (Example – n78 & n79). 5G handsets may need to support frequencies ranging from 600 MHz at the low end to as high as 6 GHz.
To support these requirements as well as Wi-Fi, GPS, and Bluetooth, the typical number of antennas will increase from four to six in today’s LTE handsets to six to 10 in 5G smartphones. It is becoming increasingly difficult to fit all these antennas into the limited space available.
Reduced Antenna Area
Exacerbating the problem, the space available for antennas is shrinking as manufacturers make industrial design changes and add new features. One key change is the shift to full-screen handsets, in which the display occupies nearly the entire face of the phone; as a result, there is less space available outside the screen for cellular antennas. Manufacturers are also adding more cameras, further reducing the space available within the handset.
The need to fit more antennas into less space means antennas are becoming smaller, and the reduced antenna size results in lower antenna efficiency.
The higher number of antennas, combined with their reduced size, also means the handset is more sensitive to transient effects caused by changes in its environment, such as phone handling. These transient effects may include reduced efficiency and shifts in frequency response.
The Antenna Performance Tradeoff Triangle
A reduced antenna size impacts efficiency and bandwidth. If antenna size remains constant, efficiency can be traded in exchange for greater bandwidth. In older-generation phones with larger antennas, this tradeoff may be acceptable because the antenna may still be able to meet performance requirements while supporting a wider range of bands. But as the antenna size decreases, this tradeoff is no longer possible; with new full-screen designs, the antenna can only achieve the required level of efficiency over a narrow frequency range. Therefore, to support the wide range of frequencies supported in current handset designs, the antenna must be tuned to operate efficiently on each frequency.
Tuning Techniques Addressing Antenna Challenges
Today, aperture tuning is the primary method used in handsets to overcome the problems caused by reduced antenna area and efficiency. It is essential to enable smartphones to support the ever-broadening range of frequency bands, especially with the transition to 5G.
Aperture tuning can make a large impact on antenna efficiency for both transmit and receive communications, improving total radiated power (TRP) and total isotropic sensitivity (TIS) by 3 dB or even more depending on the application.
An antenna has multiple natural resonant frequencies. These are harmonically related: for example, an antenna may have resonant frequencies at 900 MHz, at 1800 MHz (2nd order harmonic), at 2700 MHz (3rd order harmonic) and so on. By using aperture tuning switches to tune each of these frequencies, a single antenna can support many bands distributed across a very wide spectrum range.
Each of these resonant frequencies can be tuned independently by placing an aperture tuning switch at the point where it has the greatest effect, which is generally close to the apex of the voltage distribution for that frequency.
Aperture Tuning for CA
LTE operators worldwide are using CA to provide higher data rates. CA combines two or more LTE carriers, often in different frequency bands, to deliver increased bandwidth. Due to the limited total number of antennas in handsets, this often means that a single antenna must communicate on two bands simultaneously.
Aperture Tuning Key to Enabling Smartphone Support of Additional Frequency Bands
Aperture tuning is essential in enabling today’s smartphones to support the ever-growing range of frequency bands. It significantly increases Tx and Rx performance, overcoming the challenges caused by handset industrial design changes and making it possible to meet increasingly complex RF requirements. The effective implementation of aperture tuning requires considerable knowledge of how to apply the technology to optimize each application. The increasing number of antennas also means that aperture tuning solutions must be small to fit into the shrinking available space.