Multiple-element Direct Digital Beamforming for Next Generation Wireless Communication System

Emerging wireless networking standards such as Wi-Fi 6E (an extension of 802.11 ax), optimize capacity using Orthogonal Frequency-Division Multiple Access (OFDMA), and multi-user multiple-input multiple-output (MU-MIMO). Digital direct RF (DDRF) systems and phased array systems are studied and designed for the next generation wireless system.

The first prototype is a digital 8-element beamforming RF modulator which allows accurate steering of multiple independent beams. Key to the efficiency is the pairing of area-efficient bandpass ∆Σ modulation with N-path filtering to suppress quantization noise. A 40-nm CMOS prototype generates two 1.2 GHz beams, with 40MHz RF bandwidth.

The second part of the work extends the bandwidth and frequency range of digital-phase-shifting DDRF TX, paving the way for use in MU-MIMO wireless networking applications at 6 GHz. A sigma-delta modulation chain enables an inherently linear 1b RF DAC. Low-loss FIR filtering suppresses sigma-delta DAC noise. An H-bridge combines current-DAC, FIR-filtering, and RF up-conversion for efficiency in the 28 nm CMOS.

Noise-shaping is essential for DDRF but filtering of out-of-band (OOB) quantization noise is difficult at 6 GHz for Wi-Fi 6E. A filter-free DDRF architecture in the third prototype realizes targeted noise-shaping (TaNS) to meet the spectral mask. The work utilizes TaNS to attenuate noise at an offset of 2-10xBW, moving it far out-of-band. The prototype is fabricated in 28nm CMOS and shows the feasibility of 64-QAM modulation up to 281Mbps.

Chair: Michael P. Flynn