Localized LO Phase Shifting for Phased Array Systems

Multiple-input-multiple-output is a promising technology to enable spatial multiplexing and improve throughput in wireless communication systems. The phase shifter needed in each element of the phased array to perform the electronic steering of the beam typically introduces a non-ideal transfer function in the signal path and consumes significant area and power, making it a major source of cost and dissipation. To address those issues, this chapter describes a novel technique referred to as localized LO phase-shifting, where the array of phase shifters in the receiver and in the transmitter is replaced by an array of synchronized PLLs, providing the local oscillation to each path, with fine and inherently linear phase regulation. This approach not only helps reduce power consumption and area occupation, in modern CMOS nodes, but also improves phase noise since the beam is formed by the combination of uncorrelated noise sources. To demonstrate the concept, a dual-element LO phase-shifting system, based on fractional-N digital PLLs in the 8.5-to-10.0-GHz range, is implemented in a standard 28-nm CMOS process. Each element occupies 0.23 mm2 of area and dissipates 20 mW of power. An arbitrary phase shift between the LO outputs can be set over the full 360∘ range with a resolution of 0.7 millidegrees. The measured rms phase accuracy is 0.76∘, and the peak-to-peak phase error is 2.1∘, without requiring any linearity or gain calibration. Combining the outputs of the two elements, the measured integrated random jitter scales down from 58.5 to 44.6 fs rms.