5G Update Series: 5G Antenna Trends

With the advent of the later 4G LTE releases (3GPP Release 13 and 14) and the latest 5G standard (3GPP Release 15), antennas for wireless telecommunications have begun to change substantially. Beyond the addition of sub-6 GHz frequencies for 5G, there are other technologies for enabling 5G use cases that are being developed that generate substantial changes for current and future antenna. Earlier standards released for 4G LTE allowed for multiple-input-multiple-output (MIMO) antenna structures to take advantage of spatial multiplexing and enhance throughput and capacity for cellular installations. With the latest release there are advances to MIMO technology that are transforming antennas from the traditional passive design to active devices highly integrated with both RF front-end (RFFE) hardware and configurable digital hardware designed to adapt to the latest complex MIMO and network requirements.

For several years, antennas with MIMO and carrier aggregation (CA) capabilities have been fielded providing, most commonly, 2×2 MIMO. This required two antenna elements for transmit and two for receive at the base station and two antenna pairs on the user equipment. Using spatial multiplexing, users are able to experience a boost in throughput from data transfers through two spatial streams. In this case, these 2×2 MIMO antennas typically just involved antennas with multiple coaxial feeds to the base transceiver station (BTS), or later to a digitized version of the BTS, the digital unit (DU), and a remote radio head (RRH). Typically, these types of antennas used cross-polarized elements. For later 4×4 MIMO installations, the DU was typically connected to two RRHs that fed two 2×2 MIMO antenna systems.

Continuing this trend would require 32 RRHs and complex interconnect to feed each RRH and 2×2 MIMO antenna to feature 64×64 MIMO established in 3GPP Release 15. Hence, a new antenna and MIMO approach has been under development with some early releases for the initial, largely experimental, 5G deployments. The result is antenna systems with integrated radios and other active elements, including digital control and signal processing technology. These so called active antenna systems (AAS) have been regarded as the most likely method of producing high order MIMO antennas economically and within the constraints of practical installation on already crowded antenna towers and platforms. In this latest generation of cellular antennas for infrastructure, the digital processing for MIMO, modulation, and demodulation are integrated into the antenna systems, along with the signal distribution and RFFE hardware. These components often include power combiners/dividers, mixers/upconverters/downconverters, analog-to-digital converters/digital-to-analog converters (ADCs/DACs), switches, filters, power amplifiers (PAs), low-noise amplifiers (LNAs), circulators/isolators, attenuators, tuning elements, and a myriad of RF and digital interconnect. These AAS also include a high density of antenna elements, up to 128 if separate antenna elements are used for transmit and receive. However, some modern AAS divide the elements into sub-arrays only requiring transmit/receive modules (TRMs) for each sub-array instead of each transmit/receive pair. To produce full 64×64 MIMO, some of these AAS may be split into several modular sub-systems, such as 8×8, 16×16, or 32×32 that can be assembled into a full 64×64 MIMO system.

The next generation of MIMO may be what is called massive MIMO. This could result in extremely complex and dense AAS. Massive MIMO systems may need to be integrated into a single complete module with all of the necessary RF, signal processing, and even networking hardware built-in. As this type of system is likely to be for dense urban and suburban areas, they may also need to be much more compact than typical AAS used today, which would allow for their placement on metropolitan infrastructure, such as street lighting, traffic light poles, and etc.

The result of high density AAS is the need for RF/microwave hardware that is much more compact and power efficient than previous cellular technology. Part of these requirements is derived from the need to maintain relatively similar cell tower footprints for AAS. Testing such systems is also an emerging challenge within the industry, which may be addressed in future systems that include extensive self-diagnostics and other built-in-self-test (BIST) features.