cific direction in order to mitigate interference in a MU-MIMO system. Therefore, in addition to phase shifting, weighting of the signal amplitude is applied to reduce the side lobes. For example, a symmetric linear tapering of the signal amplitudes in a linear antenna array results in sidelobes that are 10-15 dB lower, but it increases the beam-width of the main lobe by approximately 5 degrees. As we discussed in Chapter 4, there are three types of beamforming architectures used for antenna arrays (see Fig. 5.29): Analog beamforming (ABF): The traditional way to form beams is to use attenuators and phase shifters as part of the analog RF circuitry where a single data stream divided into separate paths. The advantage of this method is that only one RF chain (PA, LNA, filters, switch/circulator) is required. The disadvantage is the loss from the cascaded phase shifters at high power. Digital beamforming (DBF): DBF assumes that there is a separate RF chain for each antenna element. The beam is formed by matrix-based operations in the baseband where Digital Analog domain domain Digital Analog domain domain Antenna 1 Antenna 1 Antenna 2 Analog beamforming Digital Antenna 2 phase shifters beamforming baseband Antenna M N: number of DACs Antenna M M: number of antennas N = 1, N< M N: number of DACs/PAs M: number of antennas N = M Analog beamforming Digital beamforming Digital Analog domain domain Analog Antenna 1 beamforming phase shifters Antenna p Digital beamforming baseband Analog Antenna beamforming phase shifters Antenna M N: number of DACs/PAs/analog beamformers M: number of antennas Hybrid beamforming Figure 5.29 Beamforming architectures for active antenna systems [18]. New Radio Access RF and Transceiver Design Considerations digital amplitude and phase weighting is performed. For frequencies lower than 6 GHz, this is the preferred method since the RF chain components are comparatively inexpen- sive and can combine MIMO and beamforming into a single array. For frequencies of 28 GHz and above, the PAs and ADCs