There are an increasing number of communications and RF/millimeter-wave applications that involve tighter link budgets in tough environments for signals to propagate with transmitter/receivers in smaller and smaller form factors. This is a trend that can be seen across an array of industry verticals. For instance, there are cellular applications where cellular base stations for 5G (gNB) are either decreasing in size (small cells) or increasing in complexity (mMIMO). There is also satellite communications that traditionally involve a singular, massive GEO satellite, which has shifted to either a High Throughput Satellite (HTS) to the smallsats found in the up-and-coming LEO constellations. Then there are millimeter-wave applications which have seen a significant rise in usage, cluttering up a once sparse and vast spectrum space. These high frequency applications require smaller components as the wavelength at high frequencies grows smaller.
Beyond millimeter-wave cellular applications for 5G, there are also automotive radar applications, imaging use cases, and even potentially medical applicability. This article explores the challenge of maximizing the output power of the transmitter while maintaining a small system package with a main focus on Power Amplifier (PA) design considerations. Additionally, the challenges in PA design for various modern communications systems is touched upon with the goal of offering a more in-depth perspective on the unique system requirements found in these various applications and how this, in turn, affects PA design parameters.
Maximizing the Link Budget
The link budget equation can grow complex rapidly given there are a variety of environments for signal transmission, each with its own type of obstacles that cause multipath fading as well as a number of other losses (e.g, destructive interference, body loss, etc). Moreover, there are a variety of antenna feeds and culprits for loss within the transmitter. The simple link budget equation can be seen in Equation 1. Where PRX/PTX is the power received/transmitted, GRX/GTX is the gain of the receive/transmit antennas, LTX/LRX is the loss of the transmitter/receiver, LFS is the free space loss, and LM accounts for the miscellaneous losses.
Equation 1
As made apparent by the equation, transmitter output power and transmitter/receiver antenna gain are the major contributors to received power. Of the typical components found within a transmitter, the high power amplifier (HPA) is primarily responsible for the transmitter power output, or the actual power produced at the output of the transmitter. Maximizing the transmit power within a RF system very quickly translates to optimizing the HPA and gain of the transmit and receive antennas.
There is much progress being made in the realm of antenna design with Active Electronically Scanned Arrays (AESAs) and phased array antennas, offering highly directive, high gain beams to precise locations. However, the remainder of this article will focus primarily on PA application considerations and design techniques for the sake of brevity.
Choice of PA
The choice PA varies per application. For cellular base stations (macro-cells), this was typically the Si LDMOS and has recently shifted towards GaN HPAs. Radar applications with both high transmit and high frequency powers would typically involve the use of vacuum-tube technologies such as klystrons or TWTA, however some iterations involve the use of Solid State Power Amplifiers (SSPAs); utilizing a variety of III-IV semiconductors such as Gallium Arsenide (GaAs), Gallium Nitride (GaN), or Si-based PAs.
For smaller, lower power components such as radio handsets, mobile devices, and wearables, GaAs-based or Si-based power amplifiers may be used for cost and accessibility. Much of the above mentioned applications have already begun a switch to utilizing GaN-based SSPAs for the superior power handling performance of this semiconductor. This, in turn, loosens design restraints on link budgets, thermal design, and linearity/efficiency amplifier constraints.
Understanding the Balance Between Output Power, Linearity, and Efficiency for PA Design
In its most basic form, power amplifiers are weak, non-linear, active components where non-linearities are necessarily minimized. This differs from other active, non-linear components such as frequency multipliers and mixers, where the non-linearities are actively exploited to produce the desired result while the effects of linear phenomena are minimized. The output power and linearity requirements are set by the communications system; however, factors such as gain and efficiency tend to have more design flexibility (unless the system is energy-constrained). At small signal levels the ratio between output power and input power is constant with drive level and equal to the small signal gain, however, as the input power increases there is a plateau in gain (the compression region) and eventually saturation where no there is additional output power despite the increasing input power.
As the required output power of amplifiers increases, gain becomes a less pertinent parameter, rather, power added efficiency (PAE) becomes a more useful measurement for evaluation of the component; the ratio between the difference between the output and input powers and the supply power (Equation 2).
Equation 2
This difference between input and output power with regards to DC bias allows for an assessment of the distribution of energy within the amplifier ― the energy that does not go towards output power must be dissipated as either power delivered to the load at other frequency components or as heat1. For this reason, efficiency optimization relaxes DC power consumption constraints while also simplifying the thermal management system. Figure 1, for instance, shows a sample of the breakdown of power consumption for the various components within the BS architecture. Saving on PA power consumption ― the largest contributor of power consumption within a BS ― can drastically improve on energy saving. For amplifiers however, there is a balance to be maintained between efficiency and linearity. Making an amplifier more efficient often involves driving it to a point near saturation where modulated waveforms can quickly become distorted. However, keeping the amplifier at significant backoff in order to avoid signal distortion damages the efficiency of the component. This design conundrum can be readily seen in wireless systems utilizing amplitude/phase modulation schemes with a high peak-to-average power ratio (PAPR), requiring the amplifier to stay well within its linear region to satisfy alternate-channel power ratio (ACPR) requirements.
Figure 1: Estimated power consumption of the typical macro-cell BS2
Size, Weight, Output Power, Linearity, and Efficiency Considerations per Application
5G Macro-cell
The next generation of cellular networks has called for a wide array of technologies, protocols, and processing techniques to best service the growing number of global connected users, seamlessly. There are a number of radio techniques standardized in 3GPP’s New Radio (NR) in both its standalone (SA) and non-standalone (NSA) iterations. The various spectrum and throughput enhancing techniques ― such as Carrier Aggregation (CA), Coordinated Multi-Point (CoMP), network Multiple-Input Multiple-Output (MIMO), and interference coordination ― can cause the native HPA to require both high linearity and efficiency.
The CA feature involves the transmission of multiple carriers from the same terminal to achieve higher data rates. This can either be in a contiguous form or non-contiguous form utilizing either continuous blocks of spectrum or separate sections. The non-contiguous form of CA can require the PA to be at additional backoff to avoid interference between the two non-contiguous carriers transmitting simultaneously to a singular user terminal as this interference can cause strong intermodulation components, ultimately leading the transmitter to fail to meet strict emission requirements. This, in turn, affects efficiency of said amplifier as there is always a design trade-off between efficiency and linearity ― the more the amplifier operates within its linear region, the less efficient it becomes.
Newer generation cellular networks in 4G-LTE and beyond also utilize the OFDM modulation schemes as opposed to the previously used W-CDMA for 3G. The high throughputs that OFDM allows does have the downside of a high Peak-to-Average Power Ratio (PAPR), where the PA requires a high dynamic range to avoid any clipping and therefore also needs to operate at backoff, resulting in a lower power efficiency.
mMIMO
Massive MIMO is a highly researched and developed technology for 5G, where base stations carry hundreds of antennas that coordinate communications to achieve high spectral efficiency ― optimizing the number of bits transmitted per second within a given bandwidth (b/s/Hz). This technique permits more aggressive frequency reuse of scarce spectrum and more uniform Quality of Service (QoS) across all terminals by multiplexing and demultiplexing of uplink and downlink signals with multiple antennas structures and Channel State Information (CSI), or the instantaneous channel gains that guide the receiver and/or transmitter to update the transmission/reception methodology. Each base station can be equipped with a massive number of antennas (up to thousands), each with its own respective RF front-end. And with each discrete transceiver chain, so come all the losses, noise, imbalances, and nonlinearities within the system. The PA itself adds nonlinearities when operating close to saturation. However, there is a balance between efficiency and linearity for these PA’s as Energy-Efficiency (EE) is a critical requirement as it directly correlates to the operational costs for the Mobile Network Operator (MNO). In this regard, there are also design trade-offs between the optimum number of transmit antennas, the spectral efficiency ― which directly correlates to the number of antennas for spatial multiplexing ― and the efficiency at which each PA operates within.
Radar and Satellite
Traditional radar supports high transmit powers at frequencies ranging from the L-band up to the millimeter-wave spectrum. Typically, vacuum electronic technologies such as Traveling Wave Tube Amplifiers (TWTA) and klystrons have been leveraged within these applications for their ability to both operate at high peak powers and high frequencies reliably. This also applies to satellite applications in both the space and ground segments operating up to the ka-band. From marine radar to space-based weather radar, the backbone for the HPA technology has remained relatively unchanged. More recently, solid-state amplifiers have been utilized for their size and weight savings ― parameters that are particularly relevant for aerospace applications where the cost of launches (that are based on payload weight) demand a large capital expenditure (CAPEX) and are often quite space-constrained for electronics. Typically, the SSPAs didn't meet a key performance benefit of TWTA: operation at high efficiencies (~70%). However, with the emergence of GaN-based SSPAs, the efficiency of SSPAs have been able to climb steadily while maintaining high junction temperatures. This directly translates to high power handling capabilities over a longer lifetime with a longer Mean Time-To-Failure (MTTF). This, however, does not entirely remove the feasibility of vacuum electronics components from high power, high frequency systems as there is always a balance between efficiency, cost, power delivery, and weight/size that implies no one-size-fits-all solution.
Apart from large HPA considerations in radar there are also PA considerations for radar implementing AESAs. Similar to mMIMO, these systems are outfitted with an array of antennas, each with its own transmit/receive (T/R) module. These modules often have discrete phase shifters for customizing construction/destructive interference such that a high gain, directional beam can be pointed in a direction of choice. However, with these systems comes intense efficiency and linearity requirements on the PA. High output power of the PA would lead to a higher gain beam, however, this comes with thermal management considerations where higher mounting temperatures can cause hardware failures. In AESA particularly, linearity is also required to realize complex beamforming techniques with predictable amplitude and phase consistency.
Millimeter-wave technologies
Much of the above stated considerations apply to millimeter-wave components as AESAs and mMIMO can be realized within the millimeter-wave spectrum. In addition to these technologies, much of the small cell technology for 5G is set to operate within the millimeter-wave spectrum with high size, weight and power constraints. While size becomes less of a consideration at high frequencies, power management becomes a much more significant concern. Typically, GaN RF transistors dominate sub-6GHz systems for its excellent power density and thermal performance. However, other wide bandgap, high electron mobility substrates such as GaAs become serious contenders for higher frequencies. As shown in Figure 2, this is due to the fact that their operational efficiency becomes comparable at high saturated output power (PSAT).
Figure 2: Narrowband and broadband PA efficiency vs saturation power3
Solutions: Choice of Substrate and Linearity and Efficiency Enhancing Techniques
The goal of maximizing transmit power in smaller, less weighty form factors is a balancing act. And, depending upon the application, the relative amplifier requirements (output power, linearity, gain and efficiency) are subject to change, causing an additional layer of complexity to this particular design challenge. However, the utilization of wide bandgap semiconductor materials with a high power density ― namely GaN ― allows for highly efficient, high output power amplifiers. Depending upon the use case, this can be of high utility. Less power dense substrate materials must function at lower junction temperatures for less time, causing complications in heat dissipation, component reliability, and available transmit power.
In systems where amplifiers must operate well within backoff, efficiency enhancing techniques such as Envelope Tracking (ET) and the Doherty amplifier topology can be employed. Moreover, there are linearization methodologies such as Digital Predistortion (DPD) that are implemented to allow amplifiers to operate close to the saturation region without compromising on minimal spectral leakage.
Conclusion
Amplifiers are among the most critical components for maximizing transmit power within a wireless system. The ideal choice of amplifier class, substrate, topology, and packaging depends upon a number of parameters around a given application. This can include the propagation environment, the spectrum utilized, and modulation scheme, leading to a balance in basic amplifier design parameters.
- Intermodulation Distortion in Microwave and Wireless Circuits Jose Carlos Pedro
- L. M. Correia et al., "Challenges and enabling technologies for energy aware mobile radio networks," in IEEE Communications Magazine, vol. 48, no. 11, pp. 66-72, November 2010, doi: 10.1109/MCOM.2010.5621969.
- Dielacher F., Papananos Y., Singerl P., Tiebout M., Maistro D.D., Thomos C., “Overview about RF and PA Requirements for 5G NR and Challenges for Hardware Implementation,” Infineon Technologies, International Microwave Symposium Presentation. 2019.
