Parent Category: 2020 HFE
By Mark Miller
Introduction
It is largely believed by market research groups, industry consortiums/standards bodies, original equipment manufacturers (OEMs), academic researchers, and even consumers, that 5G technologies will offer seamless voice, data, and control services magnitudes beyond the services commonly experienced in today’s metropolitan and suburban regions [1,2,3,4,5,6,7].
It is only a common opinion that advanced antenna systems (AAS) or smart antennas (SAs), will be necessary in delivering enhanced mobile broadband (eMBB), ultra-reliable low-latency communications (URLLC), and massive machine-type communications (mMTC), which are considered three pillars of early 5G use cases. Upgraded antenna technologies, such as massive multi-input multi-output (mMIMO), sub-6 GHz and millimeter-wave (mmW) carrier aggregation (CA), and full-dimensional (FD) beamforming/MIMO (FD-MIMO) are also considered essential antenna technologies that enable 5G use cases. In order to reach these lofty performance goals, antennas need to evolve past the passive and limited MIMO technology currently deployed, and become actively driven dense antenna arrays more akin to the phased array antennas used with military active antenna array (AESA) technology.
This article aims to provide readers into insights of the current generation of 4G antennas, and how 5G standards and use cases are leading to antenna designs that rely on active, highly integrated, and extremely dense MIMO systems. Moreover, this article will also discuss some of the impacts of this new antenna technology for telecommunications suppliers and installers.
Figure 1 • The progression of antenna technology as antennas and radio systems have adopted more digital techniques and RF integration. Source: [3]
Traditional Cellular Antenna Technology & Early MIMO
In the recent past, telecommunications installers have mainly used sectorized (3-sector) antennas that offer frequency performance adequate to provide specified service for a specific carrier’s frequency bands. With the advent of 4G, this also included passive MIMO technologies and CA, which allowed for devices to connect over multiple spatial multiplexed paths and over several frequency bands.
For the most part, 2x2 MIMO and limited CA (UE limitations) is deployed throughout most countries, which means that at most only two spatial paths are used from/to devices and base station antenna for most users, if MIMO is even available [3]. Until recently, there were a select few UE and recent base station/antenna deployments that offer higher order MIMO (such as 8x8) and CA for up to 7 component carriers. In order to prepare their networks for 5G-like services and early 5G rollouts, there has been a way of antenna system and infrastructure upgrades to enable higher level MIMO and CA, which is mostly outlined in 4G-LTE Advanced Pro (3GPP Release 14).
Currently, 4G MIMO antennas are 2x2 MIMO cross polarized antennas connected to a remote radio head/radio unit (RRU) via front-haul fiber-optic cable to a digital unit/baseband (DU). This is an advancement over previous cell tower antenna systems, which used a base transceiver station/base station (BTS) that transmitted RF through coaxial assemblies to a single antenna unit. In the case of 4x4 and 8x8 MIMO, typically, multiple RRUs are connected to multiple cross polarized MIMO antennas, making an antenna system that scales linearly with the increase of MIMO complexity. As this is infeasible due to cell tower clutter and otherwise unattractive from a cost and complexity standpoint, OEMs have developed, and are continuing to develop, more compact antenna systems based on active technology. The result of this is 64x64 MIMO antennas with integrated transceivers, MIMO, and beamforming hardware in the same assembly as the antenna.
In order to avoid cell-edge interference, carriers have mainly deployed 2-sector cell strategies, which will likely not be adequate to meet the expectations of the final 4G rollout or early 5G rollouts. There is movement toward developing advanced vector sectorization using 6 sectors per cell, with technologies, such as proactive cell shaping, that help to mitigate cell-edge interference. Currently, there is a shift in antenna purchases toward multi-band and MIMO antennas, namely 64x64 MIMO antennas, which are also referred to as massive MIMO (mMIMO) antennas.
5G Beamforming & MIMO
With 3GPP Release 15 in 2019 a Non-Stand Alone (NSA) 5G option was born, which enables dual connectivity for LTE and 5G New Radio (NR) communication links. This involves the use of LTE services for the control plane and redundancy, and millimeter-wave 5G links for high-speed data communications. Hence, a variety of network options now exist, each with their own antenna requirements and technologies. Beamforming and MIMO are seen as essential for millimeter-wave 5G services to function given the heightened atmospheric attenuation and proportionally narrow antenna patterns. More support is gaining for mMIMO to enabling high numbers of concurrent users and machine-type communications within a cell, which yields yet a different requirement for antenna systems and base station infrastructure.
Figure 2 • Beam adjustments enabling azimuthal, elevation, and antenna pattern shaping are advantages of active antenna arrays.
Network Deployment Cases
Due to the ongoing deployment of 4G technology and initial deployments of 5G NR technology, many of which are experimental, there will likely be a variety of network deployment configurations and strategies. Of the many, 5G Americas identifies three main strategies, N+1, 1+1, and all-in-one, which accounts for both augmented 4G sites and new 5G sites.
N+1
This deployment strategy consists of deploying 5G NR high bands using separate active or passive antennas to an existing 4G site. No changes would occur to the existing antennas, and no additional bands would be added to the existing service. This method uses 4G LTE as a control plane to manage 5G NR communications, possibly offering fixed-wireless internet (FWI) services.
1+1
With replacement of existing antennas and additional of 5G NR antennas, the 1+1 strategy would enable the support of legacy bands/technologies. Moreover, this strategy would also enable 5G NR low-bands and mid-bands to be deployed, though on separate active or passive antenna systems. This configuration would likely support 4T4R for high/low bands and new LTE bands, with the 5G NR low-bands located within the LTE low-band radome.
All-in-One
The all-in-one approach would require full consolidation of the legacy bands/technologies as well as the inclusion of new 5G NR bands into the same radome. This would likely result in the placement of two different antenna systems within the same radome: a fully passive antenna with modern technology that enables 4T4R and 8T8R (4T4R 700/850/1900/AWS/1800/2100/2600 + 8T8R 3500), and either passive or active antenna for the mid-term use with higher level MIMO (4T4R700/850/1900/AWS/1800/2100/2600 + 16T16R/32T32R/64T64R 3500).
Active & Passive Antennas
Active antennas differ from passive antennas in that active control of the phase and amplitude to the antenna elements or subarrays allows for beamforming. Beamforming is control of RF energy that drives an antenna array in such a way that the antenna pattern of the array is controlled and shaped as desired. Beamforming offers additional freedom and granular control over electrical tilting, in that advanced enough beamforming can generate multiple antenna lobes (beams) directed toward target communications devices.
Further advancements of beamforming can include both elevation, or vertical beamforming, and azimuthal, or horizontal beamforming. Both azimuthal and elevation control of beams combined with MIMO capability is referred to as full dimension MIMO (FD-MIMO), which would allow for UE and fixed-wireless devices used as customer premise equipment (CPE) to receive optimal signal strength and quality be limiting interference and more efficiently directing signal energy.
Passive antenna only require electronic components that need to be adjustment either manually or electrically periodically, or just during installation. On the other hand, active antennas require continuous electronic (active) adjustment of the phase and amplitude of the signals sent to each antenna element, which is why there has been an increasing trend toward integrating RF and digital technology to enable smaller footprint and lower cost antenna systems.
Integration of RF and Antennas (AAS)
Advanced antenna systems, or active antenna systems (AAS) are widely considered as key technology for optimizing BST efficiency, capacity, and coverage and enabling the high user expectations of 5G services. However, leveraging AAS also means changes to cell structure, infrastructure, and antenna/RF technology to enable new 5G networks that also accommodate legacy services. In essence, an AAS is a remote radio head (RRU) combined with an antenna array, which is fed data and control from a digital unit (DU)/digital baseband unit (BBU) in place of a base-transceiver station. The networking information for 5G AAS base stations is most likely to be fiber optic communications and microwave/millimeter-wave backhaul for last-mile connection. This has been the trend in the tail-end of 4G deployments and is commonly proposed for 5G deployments.
Beamforming/MIMO Architectures Impact on Antenna Systems
In order to enable AAS that meet design requirements in roughly the same footprint as existing passive antenna, AAS OEMs must tackle several design challenges. Integrating RF, digital, and antenna technology into a single encapsulation, while including configurability enough to avoid early obsolescence, leads to additional design considerations: thermal management, footprint minimization, power management, interconnect routing, and minimizing the added weight of all of the additional RF hardware. Hence, there are several design approaches to AAS which leverage analog/RF, digital. Or hybrid approaches to antenna transmission, reception, beamforming, and MIMO functions.
Analog
Analog beamforming involves the use of discrete RF hardware that handles the combining/splitting, amplitude adjustment, phase adjustment, transmission, reception, and filtering necessary for beamforming and communications. With an analog approach, each antenna element, or sub-array, would require distinct RF Front-end (RFFE) hardware, of which some components could be integrated in an RF integrated circuit (RFIC), or microwave monolithic integrated circuit (MMIC), to save space and power. Moreover, to limit the size and weight of analog beamforming AAS, the use of sub-arrays that minimize some of the flexibility of single-element control is common.
However, other components, such as the power amplifier (PA), mixers, oscillators, modulators, power combiner/divider, filters, and other components may pose challenging to integrate. Do to availability of technology, flexibility, and timing, analog beamforming is currently used for the final 4G rollouts and early 5G rollouts. High-band 5G (millimeter-wave 5G) may be deployed with analog RFFE hardware for a longer period of time, is less digital technology available that operates in the 20, 30, and 50 GHz spectrum. Typically, for millimeter-wave applications, oscillators, mixers, and frequency synthesizer technology is used to generate signals in the millimeter-wave region, with recent exceptions of newer early 28 GHz digital transceivers demonstrated is the past couple of years.
Digital
All-digital beamforming technology, while still in development, is designed to fully replace RFFE components all the way to the transceiver and receiver, which would likely then be integrated or assembled in a compact transmit/receive module (TRM). Using direct digital synthesis (DDS) and direct digital conversion (DDC)/direct RF sampling (DRF), the baseband unit is entirely digitized along with any frequency conversion, beamforming configuration, modulation/demodulation, and filtering (outside of receiver and transmitter filtering to mitigate interference and enable compliance).
Such units, sometimes called antenna processing units (AAUs), would be able to feed a digital baseband unit with a digitized feed of the RF signals, which would then enable extreme bandwidth applications that could account for low-band, mid-band, and high-band 5G signals in a single AAS, as long as the AAU was capable of reaching the highest frequencies of the desired 5G millimeter-wave bands. Currently, this technology is extremely new and has likely not been deployed yet, or is undergoing field trials.
With a fully capable fully-digital AAS would allow for single-element FD-MIMO, which could possibly enhance performance compared to MIMO systems where the elements are divided into sub-arrays to reduce cost, complexity, power requirements, size, or weight.
Hybrid
Hybrid beamforming is a combination of digital and analog beamforming techniques that helps to eliminate some of the RF hardware with digital synthesis and sampling/conversion technology. In this case, high element MIMO and beamforming antennas may still be divided into sub-arrays, but lower complexity MIMO/beamforming AAS (4T4R, 8T8R, 16T16R) may be implementing with single-element control. Hybrid AAS are likely heavier than comparable digital AAS, but are considered cost and complexity trade-offs in order to deliver late 4G technology and early 5G technology. This is likely the most common form of MIMO antenna, and also mMIMO AAS, until volume production of viable all-digital beamforming technology is available. There are currently prototypes and products being prepared for product in 2020.
Conclusion
Cellular antennas have continually evolved since the inception of cellular services. The MIMO, beamforming, and CA technologies introduced in the latest 3GPP releases for 4G have already greatly impacted cellular antenna design. With the introduction of additional low-band, mid-band, and high-band frequencies to cellular services, as well as the addition of mMIMO, cellular antennas are morphing yet again. These new features and changes to cellular antenna function is also greatly impacting cellular antenna design, to the point where modern cellular antennas must now contained integrated RF and digital hardware to meet the latest standards and stay competitive.
About the Author
Mark Miller serves as Product Manager at L-com.
References
5G Solutions and Market Opportunities: Technologies, Infrastructure, Capabilities, Leading Apps and Services 2019 – 2024
https://www.5gamericas.org/wp-content/uploads/2019/08/5G-Americas_Advanced-Antenna-Systems-for-5G-White-Paper.pdf
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