## 3 Beamforming Antenna and Backhaul Line of Millimeter
### Highly Directional Steerable Millimeter Wave Antenna  
A phased antenna array represents a perfect and mature electronically controlled antenna technology. However, highly directional arrays with larger apertures may encounter various problems in implementation. These problems have a greater impact on small millimeter wave antennas. Two solutions are proposed in literature [23]: modular antenna array (MAA) architecture and lens array antenna (LAA). The first is to create a large-aperture, high-gain antenna array from a small phased antenna array (PAA) module. Hybrid beamforming can be implemented in MAA by coarse analog phase shifting in the RFIC (Radio Frequency Integrated Circuit) circuit and fine digital beamforming in the baseband (BB) (Figure 3(a)). Since the feed line is short in each sub-array and the feed is typically implemented at intermediate frequency (IF), this design can significantly reduce feeder power loss.  
Figure 4: Highly directional steerable millimeter wave antenna.  
The LAA architecture forms a highly directional phased antenna array using different methods. The array aperture is increased by using a dielectric lens in a particular geometrical shape. In order to efficiently perform azimuth plane beam steering, the literature [23] proposed an elliptical toroidal lens array antenna, including a transmitting phased antenna array module placed on the back (Figure 4b). In this antenna design, the antenna aperture is determined by: (a) vertical dimension of the lens; and (b) horizontal dimension of the phased array antenna.  
In order to prove the feasibility of the proposed LAA in access and backhaul applications, this paper performs real packet transmission with different MCS under field conditions. This paper examines three experimental scenarios: device-to-device (D2D) links (transmission of a single PAA module to a single PAA module); access links (LAA to a single PAA module) and long-range backhaul (LAA to LAA).  
The use of a single lensless module allows transmission at the highest data rate up to a 15-m distance in the SC mode (MCS#12: 4.7 Gbps physical layer rate, 16-QAM (Quadrature Amplitude Modulation), and 3/4 encoding rate). The application of the lens at the transmitting end makes a maximum data rate (4.7 Gbps) possible, so that 16-QAM modulation is practical in outdoor communication within a distance of 30 to 35 m, which is suitable for millimeter wave access links.  
The second experiment investigated the backhaul link based on two LAA antennas. Figure 5 presents the constellation diagram obtained in MCS #12 transmission at 100 m, 150 m, and 200 m, where the backhaul link with two LAAs achieves a maximum data transmission rate of 4.7 Gbps at a distance of up to 200 m, and the packet error rate is negligible.  
In this paper, a 2\(\times\)8 element antenna array module is used in LAA, which includes an RFIC chip and a BB module for digital signal processing. The total power consumption is about 4 to 6 Watts, depending on the working pattern. For the MAA prototype, eight antenna array modules as well as one BB module for digital signal processing were used. Accordingly, the total power consumption of the eight-module MAA is estimated to be 20 to 25 Watts. Therefore, both LAA and MAA devices can be powered by PoE (Power Over Ethernet).