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

Abstract

Emerging wireless networking standards such as WiFi-6E optimize capacity using Orthogonal Frequency-Division Multiple Access (OFDMA), and multi-user multiple-input multiple-output (MU-MIMO). This work explores direct-digital-to-RF transmitters and phased array for next-generation wireless systems. Prototypes devices demonstrate the new techniques. Contributions include: 1) using delta-sigma modulation (DSM) to generate a low bit-width digital signal so that small and high-linearity DAC can be used, 2) different filtering strategies to eliminate the DSM quantization noise, including N-path filtering and FIR filtering, 3) developing a complete flow for the DSM design and implementation, prototyping a filter-free DSM NTF to directly meet the spectral mask, 4) circuit techniques to enhance the efficiency of the DDRF system, including B2T encoding, H-bridge DAC, and current dumping, 5) a frequency-plan methodology for the DDRF system that considers image duplication during up-sampling, noise folding during up-mixing, and digital automation bottlenecks. The first prototype is an 8-element digital beamforming RF modulator that allows accurate steering of multiple independent beams. The key to its efficiency is the pairing of area-efficient bandpass ∆Σ modulation with N-path filtering to suppress quantization noise. Beamforming Digital Signal Processing (DSP) is enabled by careful frequency management, facilitating efficient digital phase shifting, up-sampling, and up-conversion. A 40-nm CMOS prototype generates two 1.2 GHz beams, with 40 MHz RF bandwidth. The prototype consumes 128 mW and occupies an active area of only 0.19 mm2, which is only 16 mW and 0.02 mm2 per element. This represents an order-of-magnitude improvement in the area and power consumption per element compared to state-of-the-art digital to RF modulators. The second work extends the bandwidth and frequency range of digital-phase-shifting direct-digital-to-RF TX, paving the way for use in MU-MIMO wireless networking applications. A sigma-delta modulation chain enables an inherently linear 1b RF DAC. Low-loss FIR filtering suppresses delta-sigma DAC noise. An H-bridge combines current-DAC, FIR-filtering, and RF up-conversion for efficiency. A 28 nm CMOS 8-element 6 GHz beamforming TX has a per-element area of 0.01 mm2 and a per-element power of 47.5 mW. Noise-shaping is essential for DDRF, but filtering out-of-band (OOB) quantization noise is difficult at 6 GHz for WiFi 6E. We present a filter-free DDRF architecture in the third prototype that realizes targeted noise-shaping (TaNS) to meet the spectral mask. We apply TaNS to attenuate noise at an offset of 2-10xBW, moving it far out-of-band. The prototype is fabricated in 28nm CMOS with an active area of 0.022 mm2, achieves 80 MHz RF bandwidth, and shows the feasibility of 64-QAM modulation up to 281 Mbps.