 are very lossy for standard CMOS components. If alternative semiconductor technologies, such as GaAs and gallium nitrate, are used, the losses decrease at the expense of higher cost. Hybrid beamforming (HBF): HBF combines DBF with ABF in order to allow the flexi- bility of MIMO plus beamforming while reducing the cost and losses of the beamform- ing unit (BFU). Each data stream has its own [separate] analog BFU with a set of M antennas. If there are N data streams, then there are NXM antennas. The analog BFU loss due to phase shifters can be mitigated by replacing the adaptive phase shifters with a selective beamformer such as a Butler matrix. One proposed architecture uses the dig- ital BFU to steer the direction of the main beam while the analog BFU steers the beam within the digital envelop. A 32-element linear antenna array (with X/2 antenna spacing) is simulated and reported in [19] and the array factor is illustrated in Fig. 5.30, where the transceiver number is 4, and the antenna number per transceiver is 8. The maximum array gain is achieved at azimuth 90 degrees. The first curve in Fig. 5.30 depicts the array factor of ABF consisting of eight ABF (1TX) Envelope (ABF+DBF) ABF+DBF (84 degrees) ABF+DBF (86 degrees) ABF+DBF (88 degrees) ABF+DBF (90 degrees) ABF+DBF (92 degrees) ABF+DBF (94 degrees) ABF+DBF (96 degrees) Azimuth angle (degree) Figure 5.30 Comparison between analog, digital, and hybrid beamforming [18,19]. 740 Chapter 5 with the main beam direction at azimuth 90 degrees. The beamforming. second curve in It antenna elements the factor of the 32-element hybrid analog and digital of that of the first the figure is that the array amplitude of the second curve is exactly four times with the main beam can be seen ABF transceiver, seven DBF designs are shown, shown in curve. With of same azimuth 84, per 86, 88, 90, 92, 94, and 96 degrees, respectively. Some As observations Fig. direction 5.30, the main beam direction can be controlled with HBF design. are summarized as follows [19]: 1. further the ma