# A Review on 5G Sub-6 GHz Base Station Antenna Design Challenges
## 3 Challenges in BSA Design
### Port Measurements: Port to Port Isolation  
The increased use of multiband, multibeam, and dual-polarized base station antennas for cost and space savings leads to challenges in achieving isolation requirements. The port-to-port isolation indicates how well any two RF signals on a multiport or MIMO antenna are decoupled from each other. In general, isolation can be categorized into three main types: intra-band isolation, inter-band isolation, and beam-to-beam isolation. Intra-band isolation is the coupling between the polarizations of the same/multiple antenna arrays within the same frequency band. This is often referred to as cross-polar isolation when measured within the same array. Figure 8a shows a two-port BSA where each port represents two orthogonal polarizations of the same array, while Figure 8b shows a four-port BSA with two antenna arrays of same band. Ports 1 and 2 feed orthogonal polarizations of array 1, while ports 3 and 4 feed the two polarizations of array 2. The isolation between any of the ports 1 to 4 represents intra-band isolation. Generally, the intra-band isolations are required to be greater than 25 dB or 30 dB [34], depending on the frequency range of operation and operator requirements. Inter-band isolation denotes the coupling between the arrays of different bands. In Figure 9, a dual-band antenna example is given, in which ports 1 and 2 represent feeds for Band 1, and ports 3 and 4 represent feeds for Band 2. The \(\ket{S131}\), \(\ket{S141}\), \(\ket{S231}\), and \(\ket{S241}\) represent the inter-band isolation between ports of high and low band elements. In multiband antennas, the isolation is measured in all frequency bands of operations, and typically these levels need to be above 30 dB or higher. The higher the number of arrays in the antenna, the higher the port-to-port isolation combinations. Table 3 shows all possible port-to-port isolation combinations for a single-beam multiband antenna shown in Figure 4. The row entries in Table 3 are depicted as transmitting arrays, while the column entries are depicted as receiving arrays to make it simpler to distinguish the coupled band of interest. For example, \(\mathrm{HB1_{bx}}\)-\(\mathrm{LB_{rx}}\) indicate the HB array 1 coupling to LB array at High Band frequencies, while \(\mathrm{LB_{tx}}\)-\(\mathrm{HB1_{rx}}\) indicate the LB array to HB array 1 coupling at Low Band frequencies. All diagonal entries in the table represent cross-polar isolations within the same band. All non-diagonal entries represent coupling between different arrays of the same band or a different band. It should also be noted that all non-diagonal entries can be further divided into four entries since the arrays are dual polarized. For example, \(\mathrm{LB_{tx}}\)-\(\mathrm{HB1_{rx}}\) comprises \(\mathrm{LB_{+45}}\)-\(\mathrm{HB_{+45}}\), \(\mathrm{LB_{+45}}\)-\(\mathrm{HB_{-45}}\)-\(\mathrm{HB_{+45}}\), and \(\mathrm{LB_{-45}}\)-\(\mathrm{HB_{-45}}\), where subscripts \(+45\) and \(-45\) represent two slant polarizations. Usually, the isolation between orthogonal polarizations is always better compared to the same polarization. Beam-to-beam isolation is a special case applied for multibeam antennas representing the coupling between each beam in the array. Typically, the beam-to-beam isolation needs to be 20 dB or higher. The techniques used to improve isolation performance include differential feeding and decoupling network/structures. Differential feed structure, in theory, can provide infinite isolation in a dual-polarized symmetrical feed since an excitation of one port does not induce common mode or different voltage currents in the other port. In practice, intra-band isolation greater than 36 dB [21] and 26 dB [16] were achieved through a differential fed scheme. In [22], folded feeding lines based on differential feeding techniques are used to achieve intra-band isolation larger than 43 dB. Decoupling networks are also another technique used to improve port-to-port isolations.