# A Review on 5G Sub-6 GHz Base Station Antenna Design Challenges
###### Abstract
Therefore, with every generation, the existing spectrum is reused, while additional bands are introduced. The radiating antenna elements used in BSAs, however, have limited bandwidth. Hence, a single element cannot operate in multiple bands. The most common approach in BSAs is to have two to three distinct radiating elements to cover these bands, i.e., one radiating element to cover lower bands 650-960 MHz [1] and another for 1695-2760 MHz [1; 2; 3] and a third element type to operate in 3.4-3.6 GHz [4]. As a result, each of these bands is provided by a separate antenna array. Although early 2G networks used separate antennas for each band, later generations used antennas with multiple bands in one housing. This technology is called multiband antennas. A multiband antenna has multiple arrays, each serving a different band within one enclosure. In the initial generations of multiband antennas, the arrays were physically separated as shown in Figure 3a. These multiband antennas provided space and weight savings compared to two separate antennas. Later generations resulted in multiple interspersed arrays, as shown in Figure 3b, which further improved the space and weight savings. However, interspersed arrays present significant challenges during antenna designs, such as inter-band coupling and pattern distortions, discussed in detail in Section 3.2 of this paper. The antenna arrays in higher frequency bands occupy less area since the vertical element spacing is much smaller in higher frequencies compared to lower frequency bands. Therefore, multiple higher-frequency band arrays can be accommodated with one low band array, as shown in Figure 4. The most common configurations were 1:2 or 1:4 array ratio between low band to high band. The antenna shown in Figure 4, has two ports for two polarizations used in low band array and two ports each for high band array, resulting in a total of 10 ports. This antenna architecture also allows multiple operators to share antenna arrays. Alternatively to such multiband arrays, antenna co-sharing was still practiced among operators using a diplexer to combine multiple bands into one BSA, even with single-band antennas. Some advanced BSAs in the 3G era comprised dual polar multiband and multibeam antennas. Notable changes compared to previous mobile network generations in the air interface are first introduced with LTE-A. A significant increase in data rates is achieved in LTE-A due to MIMO capability. Up to eight layers of MIMO are first introduced in 3 GPP Release 10. MIMO transmit data in parallel both in time and frequency in segregated streams. The BSAs are required to have spatially separated antenna arrays or polarization diversity to achieve the decorrelation in RF paths. The spatial separation has to be at least 0.7A or more [5]. Some of the multiband antennas used in 3G networks at the time already had 2-4 high band arrays already, and the operators could use them for MIMO operation without upgrading the BSA design. High band arrays are commonly used for MIMO operation, but some BSA designs can support up to 4X MIMO for low band arrays as well [6]. Figure 3: (**a**) Multiband antenna with physically separated arrays; (**b**) multiband antenna with interspersed arrays. The 3 GPP release 12 and 13 introduced active antenna systems (AAS) and massive MIMO (mMIMO) operation, which allowed real-time beamforming to provide increased capacity and reduced interference. These capabilities are adopted in 5G air interface and are commonly referred to as 5G antennas among the antenna community. The mMIMO with AASs are extensively used in mm-wave bands due to advancements in the Antenna in Package (AiP) and millimeter-wave integrated circuit (MMIC) technologies. The smaller wavelengths at mm-wave require only small antenna footprints, allowing them to be directly integrated with the transceivers. However, the peak power limitations, efficiencies at mm-wave transceivers and propagation and penetration losses at mm-wave frequencies limit the use of mm-wave BSAs to indoor use and small cells.