  This paper presents a 4-channel beamforming TX implemented in 65nm CMOS. Each
beamforming TX is comprised of a C-2C split-array multiphase switched-capacitor
power amplifier (SAMP-SCPA). This is the first use of multiphase interpolation
(MPI) for beam-steering. This technique is ideal for low-frequency beamforming
and MIMO, as it does not require passive or LO based phase shifters. The SCPA
is ideal to use as the core element since it can perform frequency translation,
data conversion and drive an output at high power and efficiency in a compact
die area. A prototype 4-element beamforming TX, occupying 2 mm X 2.5 mm, can
achieve peak output power of 24.4 dBm with a peak system efficiency (SE) of
24%, while achieving < 1{\deg} phase resolution and <1 dB gain error. When
transmitting a 15 MHz, 64 QAM long-term evolution (LTE) signal it outputs 18.4
dBm at 14% SE with a measured adjacent channel leakage ratio (ACLR) < -30 dBc
and error vector magnitude (EVM) of 3.27 %-rms at 1.75 GHz. A synthesized beam
pattern based on measured results from a single die achieves <0.32{\deg}-rms
beam angle error and <0.15 dB rms beam amplitude error.
