Full-duplex wireless enables the operation of the transmitter and the receiver simultaneously in the same frequency band. This can potentially result in significant improvements of the wireless network performance, such as spectral efficiency, link capacity, and network latency when compared to conventional duplexing schemes such as time-domain duplex and frequency-domain duplex. Despite the advantages, full-duplex transceivers remain a significant challenge as they require >100dB of cancellation of high levels of self-interference, a recreation of large SI channel delay spreads, and real-time canceller adaptation.
SI cancelers based on frequency-domain equalization (FDE) demand multiple widely-tunable power-hungry high-Q filters, while those based on FIR-based time-domain equalization (TDE) require large delays with fine resolution. Additionally, supporting realistic antenna interface isolations of ~20dB requires a low-loss canceler and stresses canceler noise and linearity. In this work, we leveraged the large delays offered by the switched capacitor networks to realize full-duplex receivers with integrated RF and BB cancelers. The first generation implementation is equipped with a delay ranging from 0.2ns-1.1ns and 10ns-75ns respectively, and gain control of 5 bits and 7 bits, respectively in the RF and BB cancelers. These large tunable delays perform FIR-filtering-based cancellation and result in a total cancellation >50dB from antenna interface, RF, and BB domains.
In the second generation prototype we introduced additional circuit techniques such as (i) an N-path switched-capacitor (SC) delay-line with stacked-capacitor voltage gain while enabling nearly ten nanoseconds of RF true-time delay across a large BW (DC-1GHz), (ii) a new LNTA-canceler where the FIR weighting, summation, and output buffer of the canceler is absorbed into the LNTA, and (iii) a closed-loop adaptation algorithm leveraging analytical modeling of tap non-idealities that reduces the computational complexity and data storage. Leveraging a 16-tap RF canceler operating across DC – 1GHz with delays ranging from 0.25ps – 8ns (8× compared to first-generation and 40× compared to other works) and a complex-weighted 8-tap BB canceler with delays ranging from 10ns – 85ns. This prototype FD receiver achieves (i) tunable operation across 200MHz-1GHz, (ii) wideband SI suppression of up to 65dB (54dB) across 40MHz (80MHz), when operating at 800MHz (13dB higher than first-generation while achieving 2× cancellation BW), with (iii) modest NF degradation of 0.8dB (2.8dB) for the low power mode (high power mode), while (iv) handling TX power of up to +15dBm (6dB higher than first-generation) across initial circulator isolation of only 23dB. Check out the related publications:
- A. Nagulu*, S. Garikapati*, I. Kadota, M. Essawy, T. Chen, A. Natarajan, G. Zussman and H. Krishnaswamy, “Full-Duplex Receiver With Wideband Multi-Domain FIR Cancellation Based on Stacked-Capacitor, N-path Switched-Capacitor Delay Lines Achieving >+54dB SIC Across 80MHz BW and >+15dBm TX Power Handling,” accepted and to appear in IEEE International Solid-State Circuits Conference – (ISSCC), 2021. * – equal contributors.
- A. Nagulu, A. Mekkawy, M. Tymchenko, D. Sounas, A. Al u and H. Krishnaswamy. “Ultra-Wideband Switched-Capacitor Delays and Circulators – Theory and Implementation.” (invited paper) IEEE Journal of Solid-State Circuits, vol. 56, no. 5, pp. 1412-1424, May 2021.