yer Aspects (Part 2) 573 the extreme case one-bit resolution data converters would be very important for practical implementation of massive MIMO systems [16]. In summary, there are three types of beamforming architectures used for antenna arrays [56]: Analog beamforming: The traditional way to form beams is to use attenuators and phase shifters as part of the analog RF circuit where a single data stream is 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: It assumes there is a separate RF chain for each antenna element. The beam is then formed by matrix-type operations in the baseband where the ampli- tude and phase weighting are applied. For frequencies lower than 6 GHz, this is the pre- ferred method since the RF chain components are comparatively inexpensive and can combine MIMO and beamforming into a single array. For frequencies of 28 GHz and above, the PAs and ADC/DACs are very lossy for standard CMOS components. Gallium arsenide and gallium nitrate can be used in high frequencies to decrease losses at the expense of higher cost. Hybrid beamforming: It combines digital and analog beamforming in order to allow the flexibility of MIMO and beamforming while reducing the cost and losses of the beam- forming unit. Each data stream has its own separate analog beamforming unit with a set of Nantennas antennas. If there are Nstreams data streams, then there are Nstreams Nantennas antennas. The analog beamforming unit loss due to phase shifters can be mitigated by replacing the adaptive phase shifters with a selective beamformer such as a Butler matrix.21 Some architectures use the digital beamforming unit to steer the direction of the main beam while the analog beamforming unit steers the beam within the digital envelop. 4.1.9.4 Full-Dimension MIMO Beamforming is a signal processing method that generates directional antenna beam patte