# Design of True Time Delay Millimeter Wave Beamformers for 5G Multibeam Phased Arrays
## 4 Discussion  
Wireless backhaul in mm-Wave bands that offers high speed, wide bandwidth, and several \(Gbps\) data rates will be a key component of 5G and early 6G communication systems. As discussed  
Figure 31: Calculated array factor of the tree topology. Figure 30: Simulated results of the 3D tree beamformer: (**a,c,e**) amplitude of the s-parameters for the bottom top and middle network respectively; and (**d,e,f**) the phase by the s-parameters for the bottom, top and middle network, respectively. in the Introduction, depending on the channel properties, different communication systems (e.g., beamforming and MIMO) can be accordingly utilized to meet the system requirements, offering the optimal performance. In this work, we develop and show three different beamforming topologies, each with its own advantages and disadvantages, that can be appropriately used on switched beam phased arrays as hybrid beamforming stages. These are designed mainly as proofs of concept with small number of input- and output- ports. Our future work will aim to develop designs that fulfill the practical needs of 5G systems. First, a novel semi-circular multilayer Blass matrix is proposed (Figure 8). By appropriately choosing the paths of the Blass network, we design a true-time-delay beamforming network aiming to a uniform amplitude distribution. To avoid the high number of crossovers that the typical couplers introduce at its original configuration (e.g., [28]), a double-layer directional coupler [49], is utilized, showing excellent performance throughout the desired band of operation. Notably, a size reduction is achieved compared to a regular Blass design (Figure 3). As shown in Section 3, both the amplitudes and the group delays observed at the output ports are fairly constant for the entire desired frequency band. This response offers an acceptable array factor radiation pattern with a side lobe of at most \(-10\) dB fulfilling the purpose of the current work. For our second true-time-delay beamforming network, a \(3\times 3\) Rotman lens design is utilized with four dummy ports. To improve the typical poor performance of Rotman lenses with a small number of dummy ports, an innovative technique of field absorbers at the position of its dummy ports is introduced. Specifically, the dielectric substrate that surrounds the area of the dummy ports is doped with a conductive material. By engineering the substrate in this fashion, the dielectric constant remains the same, but its loss tangent increases approximately 10 times, which eliminates in turn the reflections that deteriorate the lens' performance. Following this design approach, we were able to build a pivotal implementation of a Rotman lens with only four dummy ports, a number that is significantly smaller compared to the typical number of dummy ports used in related works found in the literature [46]. As our last true-time-delay beamforming network, a multi-layer tree topology is introduced for the first time herein (Figure 13). The primary advantage of this topology is the extremely small number of delay lines used along with the corresponding power dividers. Aiming at a Chebyshev amplitude distribution at the excited elements, power dividers of unequal power division are utilized as a proof of concept. As shown in our results for all three beams, there is only a slight phase deviation of \(\pm 3^{\circ}\) at the center frequency, while the amplitude responses of the outer ports do not deviate more than \(\pm 0.2\) dB in respect to the amplitude responses of the center ports, thereby producing a radiation pattern of 19 dB SLL. Notably, even though the tree network is inherently ultra- wideband, since its transmission lines are true time delay elements, the unequal power dividers we use in the proposed design are narrowband. Specifically, the unequal power dividers are limited to operate in a narrow frequency band around the center frequency of 29 GHz thereby making the response of the proposed tree network narrowband as well.