Compact Base Station Antennas

The Enhanced Mobile Broadband (eMBB) scenario demands that 5G peak data rates increase by at least 10 times compared to 4G (throughput at 10 Gbit/s). To achieve this speed enhancement, available measures include the application of at least 4T4R MIMO for downlink, introducing new frequency bands and/or applying CA (carrier aggregation), and improving the wireless environment to enhance SINR. Meeting these requirements inevitably leads to the addition of more antennas, wireless transmission units, and mounting structures for antennas. However, the current footprint of most mobile communications infrastructure is already quite congested.

In the practical 5G deployment project, completing the Site Acquisition, Environmental, and Design (SAED) milestone for these sites poses a significant challenge. The introduction of 5G typically requires upgrades on existing site civil structures (brownfield site upgrade). Therefore, constructing integrated, compact 4G/5G antennas that cover both high and low frequencies emerges as the most cost-effective solution. Additionally, achieving small cell coverage by placing antennas on utility poles or building corners necessitates a more compact design.

To enhance capacity within limited antenna apertures, the following two approaches are promising solutions:

1. Multi-beam antenna

This approach involves implementing multiple independent radiating beams. This technique enables the simultaneous transmission of data through different beams, providing spatial multiplexing/multi-user MIMO and therefore boosting overall network capacity. Advanced beamforming technologies play a crucial role in ensuring that these beams operate independently with very low interference between those beams.

Here, the multi-beam antenna refers to an antenna can form more than one main beams across the azimuthal direction and over several sub-frequency bands (such as from 1800MHz to 2600MHz). These main beams can be formed independently by a dedicated antenna cluster or coherently by several clusters. Each main beam direction is not the same as the antenna physical facing direction (or mechanical boresight). The transceiver unit and the antenna cluster are connected via a RDU (radio distribution unit) [1] and each main beam can correspond to a dedicated transceiver (or RF sector).

As per the NGMN recommendation [2], there are three types defined as follows:

Multi-beam Type-I: Each main beam is physically due to a single antenna cluster (each cluster consists of a pair of ports for dual-polarization). As per the author’s understanding, the Commscope RR2VV-6533D-R6 antenna falls into this category. There are 8 higher-band ports forming 4 antenna clusters. Two clusters are placed at the top part of the entire antenna aperture (Y3[+27օ] and Y4 [-27օ]) and another two are placed at the bottom (Y1[+27օ] and Y2[-27օ]). It is worth to mention that the beamwidth of each beam is around 30օ such that it forms a 60օ coverage area. This is effectively divided the full 360օ area into 6 sectors.

Multi-beam Type-II: Those whose set of beams can be formed by properly feeding network and combining each contribution through the RDU. Therefore, there is no 1-to-1 corresponding between the beam and certain antenna cluster. Each beam is formed coherently involving all the antenna clusters together with RDN when certain port pairs are fed. The Butler Matrix can be the promising solution to achieve such RDU from the author’s understanding [3]. Some of the penta-beam antennas from Rosenberger has the beamwidth around 8 to 11օ for each beam. This can be a good solution to meet extreme high-capacity requirement in a special event for instance.

Multi-beam Type-III: A single cluster radiates more than one beam in the azimuth plane. In this case, the beam split is realized for a single sector.

In the aforementioned multi-beam antennas, the resulting individual beam normally has narrower beamwidth in the azimuth plane. This is expected to enhance the SINR or coverage. It is also important to minimize the interference among those beams through the design idea of orthogonal beams [3].

2. Integrated Passive and Active Antenna.

The integration of passive and active antenna components offers a promising solution to the spatial constraints of 5G deployment. By combining both types of antennas into a common aperture, it is possible to meet the requirement of aperture size for low-band technology, integrate additional NR band TU while the higher band technology performance is not significantly impacted. This integrated approach minimizes the need for additional physical infrastructure. This is particularly true when the building infrastructure is close to its loading capacity and adding more mounting pole is not practically feasible in crowded urban environments.

Ericsson and Nokia both have their IPAAs available. The 5G NR35 AAU is interleaved on top part of antenna aperture with some low-band elements to form an entire aperture for other bands. From the author’s experience, they are applied in some extreme step-back deployment scenarios due to structural limitation or EME concerns.

Due to the varying conditions at each site, the utilization of a multi-beam antenna or IPAA would undoubtedly prove highly advantageous. The compact antenna size with rounded antenna radome edge contribute to the reduction of wind load or ESA (Equivalent Sail Area). As antenna design becomes more intricate, with closer spacing between antenna elements/clusters, the mutual coupling between ports requires meticulous measurement and mitigation to minimize its impact. In addition, achieving the desired front-to-back ratio for the antenna also poses a challenge. Another antenna design consideration is regarding the EMF compliance. It is beneficial when modelling EMF radiation from each antenna clusters with their accurate phase centres or their offset locations across the antenna aperture. Meanwhile, visualizing the antenna cluster location can be helpful to understand antenna cluster sections defined associated with certain physical ports and set the correct power based on RF design inputs.

References:

[1] NGMN Alliance, Recommendation on Base Station Active Antenna System Standards. V3.0, 2023.

[2] NGMN Alliance, Recommendation on Standards for Passive Base Station Antennas, V12.0, 2021. 

[3] Arun K. Bhattacharyya, Phased Array Antennas, Floquet Analysis, Synthesis, BFNs, and Active Array Systems, John Wiley & Sons, 2006.