# Design of True Time Delay Millimeter Wave Beamformers for 5G Multibeam Phased Arrays
## 1 Introduction  
The evolution of fifth generation (5G) wireless systems toward high microwave and millimeter wave (mm-Wave) frequencies provides significant advantages because of the wide available spectrum (i.e., high throughput, high capacity, and low latency) and improved features resulting from the smaller wavelength [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. Of course, higher frequencies are characterized by challenges of higher propagation losses, higher phase noise in local oscillators, and higher insertion loss in the RF-front-end. This is accompanied by possible shadowing even by trees, limited propagation range due to rain attenuation, atmospheric and molecular absorption, and lower or impossible penetration to buildings [13; 14; 15; 16]. The latter can be compensated by ensuring line-of-sight links, thereby requiring dense base stations even down to distances of 150-200 m [17]. Therefore, mm-Wave communications are mainly used for indoor environments and small cell access. This densification is anyways needed to ensure the excessively high communication rates of 5G. Of course, densely deployed small cells demand high cost to connect 5G base stations (BSs) to other BSs and to the network by fiber-based backhaul [18]. In contrast, wireless backhaul in mm-Wave bands that offers high speed, wide bandwidth, several \(Gbps\) data rates is by far more cost-effective, flexible and much easier to deploy. As shown in Figure 1, a wireless mm-Wave communication system is an extremely promising backhaul solution for small cells that can support the desired high speed transmission between the small cell base stations (BSs)or between BSs and the gateway. However, high antenna gain (or narrow beamwidth) is needed to compensate for high propagation losses; therefore, beamforming is required. Besides beamforming, antenna diversity (two or more antennas at different positions) has been used at low microwave frequencies (i.e., below 6 GHz) to compensate the effects of multipath propagation. Even though multipath effects are negligible at millimeter waves or above 7 GHz (\(FR3\) band) as they require line-of-sight paths, antenna diversity is still beneficial at these frequencies. Specifically, it has been shown that the capacity of communication systems at these frequencies can be increased by employing multiple antennas (i.e., MIMO) to receive signals. A MIMO configuration can be deployed in single-user systems, where it offers the best throughput, or for multi-user systems to increase the overall cell capacity. On the other hand, beamforming needs antenna elements with the same polarization. A beam can be made directive even with two antenna elements, but with very poor results. Instead, four antennas offer acceptable directivity, while eight antennas provide the best compromise (e.g., especially for the 7-24 GHz (\(FR3\) range)]. Additionally, it has been shown that beamforming is more efficient than MIMO in hard-to-cover areas (cell edges or through buildings) where the SNR is low [19]. On the contrary, MIMO is more efficient than beamforming in areas with strong \(SNR\) (best for \(SNR>17\) dB), i.e., near the center of a cell, where multiple layers of MIMO are used. Therefore, the most practically feasible configuration of a passive antenna at frequencies below 6 GHz, is one that implements both beamforming and MIMO operations. The established configuration, which incorporates both beamforming and MIMO, is known as \"Hybrid Beamforming\" and it is generally accepted as the best option for the upper microwave (7-33 GHz) and millimeter wave bands [20; 21; 22; 23]. Hybrid beamforming has been adopted since 1970. In addition, various hybrid beamforming systems have been presented in recent papers (e.g., [22; 24]).