In the first of a three-part blog series on the radio frequency (RF) front-end designs of mobile phones, we discussed the rising complexities in the transition from 4G to 5G (see Advances in RF Front-Ends Made 5G Phones Possible). The RF front-end has had to scale up to cope with new demands on radio connectivity: more radios, wider spectrum bandwidths, and the overall mix of 4G and 5G frequencies and frequency combinations.
All these design complications affect other aspects of mobile phones, particularly power consumption and energy efficiency. In this second article, we turn our attention to how the RF front-end is evolving to support additional radio connectivity without adding further burdens on the limited battery power of devices.
It's all about power efficiency
For all the great things our smartphones can do, they often fall short of the main expectation users have: a battery life that lasts long enough to allow them to get their day's activities done. It's easy to be frustrated by a short usable life of a phone and the constant need for recharging. Therefore, power efficiency is an operative term when discussing essential functionalities like mobile RF communications. What good is a smartphone if its battery drains too quickly, leaving you without vital connectivity?
Doing more with less
Much of the discussion about 5G has focused on data speeds, especially downlink capabilities. Yet it's really the uplink performance that's most reliant on battery power. Wireless communication between the phone and the signal source is dictated by output power and signal propagation. At the network infrastructure end, there's no concern about power limitations because all network equipment is fixed to a constant power source. But at the device end, the capacity for power in mobile phones is limited by the size of their battery and the rate that power is consumed.
As 5G creates faster pathways to wireless connectivity, the rate and volume of data transfer in the uplink increases alongside download speeds. So, as the RF front-end becomes more complex with the evolution from 4G to 5G, it's imperative that the added uplink radio paths draw power more efficiently. That way, 5G will allow mobile communication to remain practical and usable.
So how does the industry address this power concern in devices? The answer, as it turns out, is by implementing updated power management controls over the uplink portion of the RF front-end. Since the days of LTE, companies have adopted an advanced power management technique called envelope tracking.
Most mobile phone users have experienced their devices getting warm after extended use. For instance, a mobile phone can heat up when taking a long call or doing network-intensive activities. This excess heat can best be explained as a by-product of inefficient power in the RF chain.
The RF front-end has two primary paths for each wireless signal: a downlink and an uplink. It's the uplink path that primarily draws power from the mobile device; the signal is generated by the modem and transceiver and then passed onto the power amplifier. The latter, as its name suggests, amplifies this signal to impart enough energy for broadcast back to the network. The power amplifier has a specific power profile and can amplify input signals to a set maximum output.
In modern wireless protocols of LTE and 5G New Radio (NR), the signal is modulated in amplitude to maximize bit rate and spectral efficiencies by using higher-order quadrature amplitude modulation and fast Fourier transform operations. These techniques result in a high ratio of peak to average power, which introduces large differences in output power of the uplink signal.
Without an envelope-tracking circuit designed into the RF front-end, 5G phones would see a tremendous amount of wasted energy in the form of dissipated heat from the uplink communication (see figure below). But with the addition of the mechanism, RF front-end designs are made more efficient by constantly monitoring the uplink signal modulation, shaping the power envelope and output intensity of the power amplifier and minimizing wasted energy.
Challenges in 5G envelope-tracking technology
The concept of envelope tracking is relatively simple. By creating a feedback control loop, the power amplifier can inject the right amount of supply voltage at the right time to avoid spending more power than is needed. The technology has been around since the days of LTE. Adding the necessary circuitry adds cost to RF front-end designs, so in cost-sensitive 4G smartphone designs, manufacturers often forgo the technology. Unfortunately, with 5G that option is simply not viable if they want to deliver a good user experience.
One of the biggest differences in 5G NR is the use of much wider bandwidths: the maximum uplink bandwidth is between 100 MHz and 400 MHz — that's five to 20 times wider than that of LTE, which is fixed at 20 MHz. This added bandwidth places demands on the envelope-tracking circuitry — not only to handle and process a much wider signal bandwidth, but to do so without incurring power inefficiencies in the tracking circuitry itself.
Qorvo, a supplier of envelope-tracking integrated chips, touts the tracking accuracy of its 100 MHz bandwidth 5G solutions. As with most new technologies, Qorvo had to invest significant research and development in envelope-tracking capabilities over the past several years, as it transitioned from older average power tracking solutions to modern ones enabled with envelope tracking. However, currently some handset-makers, especially low-cost 5G smartphone brands, have chosen to keep using average power tracking techniques of power management to keep costs low.
The current leader in envelope-tracking technology is Qualcomm. Its QET6100 series envelope trackers are found in 5G models like the Xiaomi Mi 11 (pictured below), along with its Snapdragon 888 and RF system designs.
In February 2021, Qualcomm announced its seventh-generation envelope tracker, QET7100, which will complement its upcoming Snapdragon X62 and X65 fourth-generation 5G modem systems. The new multimode, multi-output power amplifier solution uses Qualcomm's signal boost technology, enhanced with artificial intelligence, and adaptive antenna-tuning solution. The latter can detect a user's hand position and tune the antenna and power output to improve the 5G experience and power efficiency.
Overall, the QET7100 boasts power savings of 30% over its predecessors. Being a multimode solution, it can be used for both LTE and 5G, making it a more integrated option than current envelope-tracking designs found in flagship 5G smartphones. Integration brings design benefits, including smaller board size, cost savings and improved power efficiencies for the uplink.
Qualcomm's solution is part of a holistic approach to an efficient RF front-end transmission path. As part of this approach, the company is promoting its Smart Transmit 2.0 suite of RF front-end technologies. This aims to optimize uplink speeds and improve power efficiency, with technologies like spatial averaging of transmit power for the uplink throughout the many antennas in a device.
RF performance and power consumption — two sides of the same coin
So far in this blog series, we've reviewed how the RF front-end design in phones has evolved to keep up with the complex needs of 5G communications, and how these designs are further challenged by requirements for power efficiency, forcing modern smartphones to do more with less.
For all that it does, the RF front-end of 5G smartphones hasn't garnered the appreciation it deserves for keeping up with the needs of an evolving wireless connectivity technology. This aspect of phone design is a marvel of engineering, enabling technologies that enhance the usefulness of these devices and the overall user experience. The wireless industry has made huge gains in reining in power consumption while achieving more, all the while containing cost and complexity.
In the third and final article of this series, we'll investigate how developments in RF integration and development of multifunction RF modules will help write the next chapter of the RF front-end evolution.
Wayne Lam is senior director, Research, Americas at CCS Insight. He has more than 12 years of experience covering mobile devices, wireless technology and associated supply chains. He joined CCS Insight after starting with smartphone pioneers such as Symbian and progressing to market intelligence firms including iSuppli.
"Industry Voices" are opinion columns written by outside contributors—often industry experts or analysts—who are invited to the conversation by Fierce staff. They do not represent the opinions of Fierce.