Active antenna systems reshaping RF front-ends
Article By : Jean-Jaques (JJ) DeLisle
The transition from passive antennas systems to active antenna systems (AAS) is a salient RF technology highlight in 5G and 6G applications.
There is a clear transition toward advanced or active antenna systems (AAS) from passive antenna systems (PAS). This transition is not only in terrestrial communications, such as 5G/6G and Wi-Fi, but also in satellite, aerospace and sensing applications.
For some use cases, especially military/defense, aerospace, and space-borne communications and sensing, the desired transmit power levels have exceeded the capabilities of most semiconductor technologies. At the same time, for many newer use cases, developments with AAS and enhanced integration of some semiconductor technologies have opened the door for new antenna designs previously considered too low power or otherwise unsuitable.
The main AAS trends are likely being driven by 5G/6G technology development, where highly complex AAS with tens or even over 100 antenna elements are being deployed. This level of complexity and parts count places a greater emphasis on compactness, efficiency, and integrability compared to less complex and more traditional communications and sensing use cases, such as with heterogeneous cellular communications with sparsely and widely separated base stations.
Figure 1 The AAS signal chains encompass PAs, LNAs and several other RF components. Source: Techplayon
Modern AAS designs are commonly multi-input multi-output (MIMO) and/or beam-steering capable, which requires RF front-end (RFFE) hardware to be at least a subset of antenna elements, and in some cases, there may be a distinct RFFE signal chain for each antenna element. This means that with modern AAS solutions, there are at least several, if not tens of RFFE signal chains per antenna. Each of these signal chains may be composed of power amplifiers (PAs), low noise amplifiers (LNAs), phase shifters, variable attenuators, switches, circulators/isolators, filters, combiners/dividers, and possibly also mixers and oscillators.
For a typical AAS, this could mean RFFE hardware part counts in the hundreds, depending on the number of MIMO channels, antenna elements, and design of the AAS. If implemented with discrete hardware, routing for these several signal chains would itself be large, expensive, and inefficient due to RF losses through connectors and transmission lines. That makes it extremely difficult to troubleshoot. The next logical step would be to implement modern AAS with board-level hardware. Board-level AAS solutions are currently common, but these solutions usually don’t achieve a desired efficiency, form factor, or cost level.
The next AAS design quest is for more integrated solutions built on largescale semiconductor manufacturing. Though these highly integrated solutions are still built into AAS using standard circuit board technologies, employing more highly integrated components can dramatically reduce the footprint of RFFE hardware. Additionally, more integrated solutions may also result in overall cost savings and become an enabling factor for millimeter-wave (mmWave) communications and sensing solutions.
Figure 2 The mmWave RFFE solutions for 5G infrastructure are simplifying AAS design configurations by adopting a modular approach that facilitates on-chip calibration and digital correction. Source: pSemi
Surveying semiconductor options
A good example of these trends and shifts in the semiconductor landscape is the realization of 5G New Radio (NR) mmWave systems and base stations. The latest 3GPP release for 5G NR, in addition to the sub-6 GHz FR1 frequency bands, also specifies mmWave frequency bands from 24.250 GHz to 52.600 GHz (FR2-1) and 52.600 GHz to 71.000 GHz. In order to achieve the necessary performance for mmWave mMIMO base stations, there needs to be far more RF lines, possibly up to 256 for both TX and RX.
To maintain a practical size, weight and power efficiency, it is commonly thought that the only way to achieve mmWave mMIMO base stations is to rely on greater levels of integration than was necessary for sub-6 GHz MIMO technologies. These technologies need to be highly integrable with relatively high power PAs in a compact and efficient form factor. They also call for high isolation between RF lines and good switching performance between TX/RX with low insertion loss (IL) and high isolation.
Silicon-on-insulator (SOI) and gallium arsenide (GaAs) technologies are suitable for many of the components within a mmWave mMIMO base station, where they would otherwise be too low power, expensive, or outside of the typical supply chain for traditional cellular base stations. GaAs and GaN are excellent choices for high output PAs, with silicon germanium (SiGe) and SOI as good candidates for the output power levels needed for mmWave mMIMO applications at around 20 dBm.
CMOS is an inferior choice for this application due to poorer frequency and output power performance. However, CMOS does exhibit the highest integrability to the digital baseband. SOI technology is the second most integrable in the list, followed by SiGe. SOI shines in isolation performance compared to the other semiconductor technologies, and SOI switches are also known to exhibit superior IL and power handling compared to GaN and GaAs. Hence, SOI may be one of the best candidates for realizing high integrated mmWave mMIMO blocks.
As both GaAs and GaN are made from class III-V semiconductor technology without an established path for integration with CMOS technologies or other silicon-based technologies, they are considered the least integrable. However, there are substantial research and development efforts to enhance the integrability of GaN and GaAs technology, so high-frequency RF technology and high-speed digital capability can be built into the same chip.
The next-generation AAS
It’s likely that the trend toward more complex AAS and higher frequency communication and sensing technologies will result in a shift in what semiconductor solutions are used in RFFE for these applications. This shift will see some applications move toward higher element count AAS designs using lower power semiconductors that are more integrable, such as SOI. Moreover, there will be further development of more integrable GaAs and GaN technology.
This article was originally published on Planet Analog.
Jean-Jaques (JJ) DeLisle, an electrical engineering graduate (MS) from Rochester Institute of Technology, has a diverse background in analog and RF R&D, as well as technical writing/editing for design engineering publications. He writes about analog and RF for Planet Analog.