Wireless Infrastructure Addressing the demands of LTE
Kinana Hussain looks at how to optimise Doherty amplifier performance as the wireless industry has to address the rapid growth of LTE
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nvented in 1936, the Doherty amplifier was rarely used before it became popular for use in wireless base-station transmitters with the increase in WCDMA and LTE networks. Over the last few years, the Doherty amplifier has dominated the wireless infrastructure equipment market, largely due to the architecture's ability to accommodate an exceptionally high peak- to-average ratios (PAR) - something LTE demands.
As much as one hears of LTE, the industry has only explored the tip of the LTE iceberg. According to Ericsson's mobility report, LTE networks covered 20- percent of the world's population in 2013 with that number projected to grow to more than 65-percent by 2019. In order to accommodate the projected 10x growth in mobile data traffic during the same time period, the density of the LTE networks is having to increase exponentially. As the rapid growth of LTE continues and more networks implement LTE-A, it becomes increasingly critical to optimise the Doherty amplifier's performance and efficiency. Before any discussion on optimising
path. In the architecture, signals enter through a main RF input and are then split into zero- and 90-degree phases before they run through the carrier and peaking paths within the amplifier chain. At the end, the signals are combined to form the output signal. The carrier amplifier path consists of a class AB amplifier, which is well suited for maintaining amplifier linearity but fall shorts on peak efficiency. On the other hand, the peaking amplifier path has a class C amplifier, which has very high peak efficiency but not suited for carrier signals. The Doherty architecture combines these two amplifier types to simultaneously optimise the amplifier linearity and efficiency to provide an ideal architecture for modern wireless standards with high PAR versus traditional architectures. There are two main types of Doherty configurations in use today - symmetric and asymmetric. With a more simple design, symmetric Doherty amplifiers use two identical amplifiers. In an asymmetric amplifier, the size of the peaking amplifier
there are several clear limitations. As the architecture is very complex, it requires careful design and optimisation. If the carrier and peaking amplifier are not in sync, for example, then the final output will not reach the PAE needed in the transmit chain. Today, most macrocell transceiver RF engineers manage this
can quickly contribute to higher costs and degradation of the system's overall performance. These problems are only compounded in asymmetric Doherty configurations. With a smaller carrier-path power amplifier (PA) and a larger peaking- path PA, it is more complex to align the two paths. Path misalignment contributes
Figure 2: The first product in the MPAC family is the PE46120. Currently being sampled to select customers, the PE46120 covers a frequency range of 1.8 to 2.2 GHz
Figure 1: UltraCMOS MPAC is a monolithic RF controller that optimizes Doherty amplifier performance
performance, it is critical to understand the Doherty's architecture. Named after its inventor, William H. Doherty of Bell Telephone Technologies, the amplifier has a dual-path architecture that consists of a carrier amplifier and a peaking amplifier
24 September 2014
is increased to achieve a higher power added efficiency (PAE), and the size of the carrier amplifier is decreased to lower costs. Asymmetric Doherty configuration is in mainstream use today. Despite the Doherty's PAR advantages,
Components in Electronics
complexity by using discrete components to tune the phase and amplitude for each one of the carrier and peaking paths. It is a proven methodology, and it keeps the bill of materials (BOM) low because the discrete components are cheap. On the other hand, this methodology requires substantial engineering time and expertise because optimisation is both manual and laborious. Engineers must determine what the discrete component values are and how they need to be put on the board. Furthermore, once the discrete components are on the board, there is minimal flexibility to make changes for unexpected power-transistor variances. The RF engineer is left with very few options to optimise the phase and amplitude. Any mismatch or misalignment in phase and amplitude between the Doherty architecture's carrier and peaking paths
to a reduction in the overall PAE, a reduction in the effectiveness of the digital pre-distortion (DPD) loop, a reduction in Doherty PA yield and a reduction in uniformity between transmitters. Aware of the industry's challenges in optimising Doherty amplifier performance, Peregrine Semiconductor has introduced a solution, a monolithic RF controller (Figure 1). Named UltraCMOS MPAC (monolithic phase & amplitude controller), the product enables alignment of the phase and amplitude between the Doherty amplifier's carrier and peaking paths through a digital interface.
Built on Peregrine's UltraCMOS
technology, MPAC independently adjusts the phase and the amplitude on the carrier and peaking paths. The single-chip system integrates a digital serial peripheral
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