Xilinx, TI, and Skyworks Tackle PA Efficiency Problems in 5G Small Cell Radios

November 18, 2020 by Adrian Gibbons

As modulation scheme efficiency goes up, power amplifier efficiency goes down. Xilinx, TI, and Skyworks are bringing their expertise to the table to reconcile the trade-off.

Today, Xilinx announced its collaboration with Texas Instruments and Skyworks to addresses the issues of power amplifier (PA) efficiency in 5G radio solutions. In this endeavor, the three companies plan to develop a scalable and adaptable digital front-end (DFE) for lower antenna count radios (nominally, 2T/2R & 4T/4R). 

Before delving into the implications of this collaboration, it's first important to discuss the design and market challenges of LTE and 5G small cells.


How CAPEX and OPEX Hinges on Design Efficiency

One competing factor for low-channel radio operators is cost. Capital expenditures or "CAPEX" and operating expenses, "OPEX," can be the difference between business viability or being left out of the market.

The key to reducing the CAPEX and OPEX of such systems, Xilinx says, is design efficiency—particularly the efficiency of power amplifiers


Depiction of the efficiency problems inherent with power amplifiers

Depiction of the efficiency problems inherent with power amplifiers. Image used courtesy of Xilinx

According to Liam Madden, Executive VP and General Manager of Xilinx's Wired and Wireless Group, the RF power amplifier's efficiency and performance is critical for radio platforms to succeed. "Even for low-power small cell applications, the PA consumes over 50% of the power of a typical next-generation radio and is, therefore, key to driving the OPEX and CAPEX.”


The Challenge of Competing Efficiencies: Modulation Schemes vs. PAs

But designers of RF radio solutions are faced with an efficiency conundrum: the more efficient a modulation scheme, the less efficient a power amplifier will be.

The advent of more advanced modulation schemes like quadrature amplitude modulation in 5G (QAM-256) has dramatically increased the spectral efficiency of an RF channel in terms of bits/MHz. This efficiency consequently decreases efficiency at the power amplifier stage due to the poor peak-to-average-power ratios (PAPR).


Performance specifications of wireless radio technology generations

Performance specifications of wireless radio technology generations, demonstrating how improved specifications (bit rate) can disadvantage other parts of the system (like PAPR). Image used courtesy of Xilinx

In other words, efficiency in the power amplifier (PA) stage is lost because of the excess peaks inherent in the more advanced modulation schemes. PAs must back-off the max operating point, and stay around a lower nearly-linear region in order to produce coherent waveforms on the output. 

High PAPR means that the linear region must be constrained to prevent clipping the high-peak amplitudes of the OFDMA (orthogonal frequency division multiple access) signals. This results in significantly increased power dissipation in the power amplifier. 


Crest Reduction Factor and Digital Pre-Distortion

In this collaboration amongst Xilinx, TI, and Skyworks, two techniques are combined to mitigate the high-PAPR in 5G systems: crest factor reduction (CFR) and digital pre-distortion (DPD). Together, these two algorithmic techniques improve the operating point for the power amplifier stage. 


Crest reduction factor algorithms

Crest factor reduction algorithms reduce the peak excursions of a signal and maintain the signal shape below a defined amplitude specification. Image used courtesy of Intel

Digital pre-distortion algorithms counteract the inherent nonlinearities in the power amplifier by mixing adaptive anti-phase components to the RF signal prior to amplification. The result is a mostly-linear representation of the RF signal, which improves PA efficiency and adjacent channel leakage ratios (ACLR). 


Bench test of an LTE signal with (red) and without (blue) DPD

Bench test of an LTE signal with (red) and without (blue) DPD. Non-linearities in the amplification of the RF signal cause signal leakage into the adjacent spectrum. Image used courtesy of Keysight Technologies

Xilinx claims that their digital RF IP cores for CFR and DPD on the Zynq UltraScale+ MPSoC are the sole market solutions to support features for wide bandwidths and variable radio configurations. When combined with Texas Instruments' AFE7769 wide-band transceiver and Skyworks' SKY66318-21 PA, the MPSoC offers a power-efficient solution for low-channel radios.


The Industry Expertise Xilinx and TI Bring to the Collab

In a conversation with Anthony Collins, Principal Systems Architect RF at Xilinx, All About Circuits discussed the benefits of the Zynq UltraScale+ MPSoC in this collaboration—considering this device has been around now for several years.

Collins explains, “You can pick one of thirty different types of MPSoCs to attach to an RFIC for these low-channel radios . . . and that gives them so much more flexibility in that part of the market.”

Karthik Vasanth, TI's VP and Business Unit Manager of Data Converters, emphasizes the place of TI's AFE7769 wide-band transceiver in this design: “PA linearity for spectral efficiency and RF power delivery is key to achieving the performance benefits of 5G new radio systems."

He continues, "Wide-bandwidth transceivers like the AFE7769 help to address higher-order PA nonlinearities and enable more efficient power delivery."


Block diagram of the Texas Instruments AFE7769

Block diagram of the Texas Instruments AFE7769 4-channel transceiver designed for small cell base stations and distributed antennas systems. Image used courtesy of Texas Instruments 

“With this implementation, designers can address the market needs for higher instantaneous bandwidth and antenna counts to support MIMO (multiple-input, multiple-output) applications while also offering scalability to meet system cost targets.”


PA Efficiency Translates to Reduced CAPEX and OPEX 

In the past few months, Xilinx's FPGA technology has spread throughout the 5G ecosphere, enabling carrier aggregation and spectrum-sharing applications of large macrocells in remote radio units. Xilinx's FPGA influence is also extending to hardware-accelerated functions in baseband units

Now, Xilinx is addressing the requirements of low-channel operators with the Zynq UltraScale+ MPSoC families. Texas Instruments and Skyworks bring expertise in RF to market in the form of RFIC and PA technology respectively, providing operators with the technology to compete in a market with tight CAPEX and OPEX requirements.