Efficiency enhancement technique using doherty-like over-the-air spatial combining in a 28 GHz CMOS phased-array transmitter

This paper presents an efficiency enhancement technique for high bandwidths and high PAPR signals using over-the-air spatial beamforming implemented in a 4-element phased-array transmitter at 28 GHz. The baseband signal is decomposed in the time domain into two streams. One is the signal with PAPR reduction, and the other consisting only the signal peaks, which are of low occurrence. These two streams are up-converted and transmitted through different RF chains, each optimized for the corresponding output power, and recombined over-the-air to reconstruct the original signal. An LO-phase-shifting architecture using sub-sampling PLL is employed per array element, where the PLL serves also as a phase shifter with sub-1° phase resolution. The transmitter was fabricated in 65 nm CMOS process. The measurements show efficiency enhancement of 75% relative to a 3-element linear phased array with similar output power for a 320 MHz signal with 9 dB PAPR and EVM of -31 dB at 21 dBm EIRP.