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Full-Duplex Mobile Device - Pushing the Limits

Dani Korpi, Joose Tamminen, Matias Turunen, Timo Huusari, Yang-Seok Choi, Lauri Anttila, Shilpa Talwar, Mikko Valkama

arXiv:1410.3191v3cs.IT

TL;DR

Mobile full-duplex operation is difficult because residual self-interference cannot be reproduced and cancelled accurately enough. The paper presents shared-antenna architecture with adaptive RF cancellation and nonlinear digital cancellation, achieving beyond 40 dB RF cancellation over 80 MHz and reducing residual self-interference to the receiver noise floor.

  • Problem

    Practical performance gains are difficult to realize because residual self-interference cannot be reproduced and cancelled accurately enough.

  • Method

    The paper demonstrates a mobile shared-antenna full-duplex prototype combining automatically adaptive RF cancellation with a wideband nonlinear digital canceller.

  • Results

    Beyond 40 dB RF cancellation was achieved over 80 MHz with a highly nonlinear low-cost power amplifier, while residual self-interference reached the receiver noise floor.

  • Takeaways & Limitations

    The measured results indicate that full-duplex operation can be feasible in mobile devices.

Abstract

from arXiv · show

In this article, we address the challenges of transmitter-receiver isolation in \emph{mobile full-duplex devices}, building on shared-antenna based transceiver architecture. Firstly, self-adaptive analog RF cancellation circuitry is required, since the capability to track time-varying self-interference coupling characteristics is of utmost importance in mobile devices. In addition, novel adaptive nonlinear DSP methods are also required for final self-interference suppression at digital baseband, since mobile-scale devices typically operate under highly nonlinear low-cost RF components. In addition to describing above kind of advanced circuit and signal processing solutions, comprehensive RF measurement results from a complete demonstrator implementation are also provided, evidencing beyond 40~dB of active RF cancellation over an 80 MHz waveform bandwidth with a highly nonlinear transmitter power amplifier. Measured examples also demonstrate the good self-healing characteristics of the developed control loop against fast changes in the coupling channel. Furthermore, when complemented with nonlinear digital cancellation processing, the residual self-interference level is pushed down to the noise floor of the demonstration system, despite the harsh nonlinear nature of the self-interference. These findings indicate that deploying the full-duplex principle can indeed be feasible also in mobile devices, and thus be one potential technology in, e.g., 5G and beyond radio systems.

I. INTRODUCTION

The article examines whether mobile devices can implement inband full-duplex operation despite stringent isolation, size, cost, power, bandwidth, and RF-component constraints. It presents adaptive RF and nonlinear digital cancellation approaches and evaluates them in a prototype.

  • Motivation: Inband full-duplex aims to improve spectral efficiency and data rates by transmitting and receiving simultaneously at the same center frequency.The principle uses available temporal and spectral resources more fully without requiring additional bandwidth.
  • Challenges: Self-interference cancellation is the central research problem because the own transmit signal overlaps the received signal and is distorted linearly and nonlinearly.These distortions make it difficult to reproduce a sufficiently accurate cancellation signal.
  • Motivation: Mobile full-duplex operation could significantly increase cell data rates, whereas BS-only full-duplex avoids implementing challenging mobile transceivers.The BS typically has higher-quality components and greater spatial isolation between transmitter and receiver.
  • Challenges: Mobile-scale transceivers generally require a shared antenna because restricted dimensions leave little space for separate transmit and receive antennas.The shared antenna must still provide a reasonable amount of transmitter-receiver isolation.
  • Approach: Wideband RF cancellation must model and track frequency- and time-dependent self-interference, while low-cost analog components require nonlinear digital processing.Linear digital processing alone cannot accurately reproduce and cancel the residual self-interference waveform.

II. SHARED-ANTENNA MOBILE FULL-DUPLEX DEVICE ARCHITECTURE

The mobile full-duplex architecture shares one antenna between transmitter and receiver, using a circulator and active cancellation to manage substantial self-interference. Practical operation requires adaptive, wideband suppression because passive isolation is limited and coupling varies with the device environment.

  • Shared-antenna architecture: A circulator connects the shared antenna to the transceiver while providing directional transmitter-receiver isolation.Its practical isolation is typically 20–60 dB, with desired-direction attenuation usually below 0.5 dB.
  • Shared-antenna architecture: Circulators trade wideband operation against worse overall isolation and have size constrained by operating wavelength.Electrical balance duplexers are more compact but introduce insertion loss and require active tuning for time-varying antenna impedance.
  • Self-interference components: Self-interference combines circulator leakage with antenna-reflected power and weaker environmental multipath components.Near-field changes, such as a hand around the antenna, alter matching and reflected power.
  • Cancellation requirements: Active RF and digital cancellation are required because passive attenuation may not protect the receiver LNA or preserve ADC dynamic range for the desired signal.Active RF cancellation subtracts a modified transmit-signal copy before the receiver chain.
  • Cancellation requirements: A multi-tap RF canceller uses delayed transmit references with tunable amplitude and phase to model wideband coupling.Using the transmitter output as the reference also incorporates transmitter-induced impairments into the cancellation signal.
  • Adaptivity: Closed-loop self-adaptation monitors canceller-output power and automatically tracks sudden changes caused by nearby motion or device movement.This tracking is described as crucial for practical mobile inband full-duplex operation.

III. ADVANCED SELF-ADAPTIVE RF CANCELLATION PRINCIPLE

The RF cancellation principle combines a wideband multi-tap analog canceller with closed-loop weight adaptation. Complex per-tap control and transmitter-output referencing improve robustness to frequency dependence, time variation, and transmitter impairments.

  • Cancellation principle: The proposed RF canceller reconstructs the composite self-interference using predefined delays and tunable amplitudes and phases.It acts as an interpolator because the actual self-interference component delays are unknown and time varying.
  • Wideband filtering: Complex per-tap phase shifting reduces cancellation performance dependence on frequency, tap delays, and true self-interference delays.This reduces the number of taps required, which is important for mobile cost, size, and power consumption.
  • Adaptive control: Closed-loop adaptation directly minimizes self-interference power at the canceller output and tracks time-varying channels under strict delay requirements.The implementation uses a negative-feedback structure with real-time weight adjustment; the three-tap structure is controlled using an LMS-based algorithm.
  • Adaptive control: The proposed structure is reported to provide strong cancellation under wide bandwidth and highly varying channel conditions while remaining robust to circuit imperfections.These properties are presented as measurements-based characteristics of the implemented approach.
  • Transmitter impairments: Using the power-amplifier output as the RF reference includes transmit-chain impairments in the cancellation signal.The approach provides immunity to nonlinear distortion, phase noise, and transmitter noise, reducing demands on baseband suppression and receiver ADC dynamic range.

IV. ADAPTIVE NONLINEAR DIGITAL CANCELLATION FOR FINAL SI SUPPRESSION

Mobile-scale full-duplex radios require nonlinear digital cancellation because low-cost transmitter power amplifiers create model mismatch for linear self-interference models. The paper uses adaptive nonlinear channel modeling after analog cancellation to suppress the residual self-interference.

  • Motivation: Linear digital cancellation is inadequate for mobile-scale radios because heavily nonlinear, low-cost transmitter power amplifiers distort the residual self-interference.The transmitter PA’s nonlinearity must therefore be included in the digital self-interference channel model.
  • Nonlinear modeling: The nonlinear canceller models residual self-interference as weighted nonlinear transformations of transmit data with delayed memory components.The transceiver chain is represented as a nonlinear PA followed by a linear filter containing PA memory, multipath, and RF-canceller effects.
  • Nonlinear modeling: A parallel Hammerstein model provides the nonlinear residual self-interference channel representation.Its parameters are estimated and tracked for the different nonlinear transformations rather than only for the original transmit signal.
  • Adaptive estimation: The estimation procedure transforms transmit data with nonlinear basis functions, orthogonalizes them, and adaptively estimates filters from the observed self-interference.Block least squares or LMS can be used depending on application and computational resources.
  • Limitations: The nonlinear digital canceller’s performance depends on model validity, since the parallel Hammerstein model cannot perfectly represent arbitrary power amplifiers.Frequency selectivity before the PA and phase noise are identified as sources of mismatch or coefficient-estimation error.
  • Integrated design: Combining adaptive nonlinear digital cancellation with the multi-tap adaptive RF canceller yields a design that adapts in both analog and digital domains.The prototype integrates these cancellation stages with a low-cost PA driven into nonlinear operation.

V. DEMONSTRATOR IMPLEMENTATION AND MEASURED RESULTS

The demonstrator evaluates a shared-antenna mobile full-duplex transceiver using LTE waveforms and a nonlinear low-cost power amplifier. Measurements capture RF cancellation and offline linear or nonlinear digital baseband cancellation.

  • The measurement setup evaluates the complete mobile full-duplex transceiver architecture using a vector signal transceiver as transmitter and receiver with an external power amplifier.The received I/Q samples are captured for offline digital cancellation processing.
  • The experiments use 20, 40, and 80 MHz LTE waveforms centered at 2.46 GHz with a commercial 24 dB-gain low-cost power amplifier.The amplifier produces significant nonlinear distortion in the self-interference waveform at the measurement input powers.
  • The shared-antenna architecture provides only about 20 dB transmitter-receiver isolation before cancellation, mainly because of reflection from the deployed circulator and antenna.The approximate transmit power at the antenna is +6...+8 dBm, depending on bandwidth.
  • The RF-cancelled signal is routed to the receiver and control block, while captured digital I/Q samples support linear and nonlinear baseband cancellation with LMS parameter learning.The nonlinear digital canceller uses a highest nonlinearity order of 11 and computationally efficient parameter learning.
  • The digital cancellation results are measured after the adaptive algorithm converges to steady state, ensuring that the reported performance reflects the learned digital canceller coefficients.The resulting measurements are described as showing the true performance of the digital canceller.

A. Self-Adaptive RF Canceller Implementation and Measured Performance

The self-adaptive RF canceller uses digitally controlled amplitude and phase adjustments to track changing coupling conditions. Measurements show strong wideband RF suppression and nonlinear digital cancellation to the receiver noise floor.

  • Self-Adaptive RF Canceller Implementation and Measured Performance: The RF canceller uses three taps whose amplitudes and phases are controlled through digital baseband signals and LMS adaptation.Analog vector modulators receive I and Q control voltages from the self-adaptive control block.
  • Self-Adaptive RF Canceller Implementation and Measured Performance: The control block downconverts and digitizes RF signals for LMS learning, then drives vector modulators to form and subtract the cancellation signal.The cancelled signal is also returned to the control block for the next LMS iteration.
  • Self-Adaptive RF Canceller Implementation and Measured Performance: The reported RF cancellation values are described as the highest reported active RF cancellation values, especially for wideband signals.The passage frames the comparison as applying to the authors’ knowledge.
  • Self-Adaptive RF Canceller Implementation and Measured Performance: Increasing the number of canceller taps can potentially support even wider bandwidths.The statement presents wider-band operation as a potential extension rather than a demonstrated result.
  • Self-Adaptive RF Canceller Implementation and Measured Performance: The canceller rapidly self-heals after deliberate changes to antenna reflections by automatically retuning the amplitudes and phases of its RF cancellation paths.Different reflecting materials are brought close to the antenna to create a time-varying reflection scenario.

B. Total Integrated System Performance Including Nonlinear Digital Cancellation

The integrated system combines circulator, RF, and adaptive nonlinear digital cancellation to address residual self-interference from a nonlinear mobile-scale power amplifier. Across 20–80 MHz bandwidths, the architecture reaches the receiver noise floor despite shared-antenna operation.

  • Total Integrated System Performance Including Nonlinear Digital Cancellation: The combined circulator and RF canceller provide 63–68 dB attenuation for 20, 40, and 80 MHz signals before nonlinear digital cancellation.
  • Total Integrated System Performance Including Nonlinear Digital Cancellation: Adaptive nonlinear digital cancellation further attenuates the self-interference by another 25 dB.
  • Total Integrated System Performance Including Nonlinear Digital Cancellation: Classical linear digital cancellation provides roughly 10 dB less total self-interference attenuation than nonlinear digital cancellation.The difference illustrates the impact of transmitter power-amplifier-induced nonlinear distortion.
  • Total Integrated System Performance Including Nonlinear Digital Cancellation: The complete architecture attenuates self-interference practically to the receiver noise floor even with 80 MHz bandwidth, a nonlinear transmitter PA, and a shared antenna.The reported performance is described as practical wideband operation under these conditions.
  • Total Integrated System Performance Including Nonlinear Digital Cancellation: The reported architecture is capable of coping with circuit imperfections, wideband operation, and time-varying conditions in the mobile device environment.

VI. CONCLUSION

The article identifies shared-antenna operation, environmental adaptation, low-cost component impairments, and wideband signals as central challenges for mobile full-duplex devices. Its prototype combines adaptive RF and nonlinear digital cancellation, achieving strong suppression and supporting potential use in future 5G and beyond systems.

  • Challenges and requirements: Mobile full-duplex devices must support shared-antenna operation while adapting to changes in the channel environment.These requirements are presented as central implementation challenges.
  • Challenges and requirements: Low-cost components introduce circuit impairments that directly affect self-interference cancellation, while wideband operation is needed for high data rates.The conclusion links mobile-scale hardware constraints with the need to handle very wideband signals.
  • Prototype solution: The demonstrated prototype integrates a shared transmit/receive antenna, an adaptive wideband multi-tap RF canceller, and an adaptive wideband nonlinear digital canceller.The architecture addresses both RF-domain and digital-baseband cancellation requirements.
  • Measured performance: Beyond 40 dB of RF cancellation was achieved over waveform bandwidths around 80 MHz despite a highly nonlinear low-cost power amplifier.The reported RF result includes automatic tracking of self-interference channel changes.
  • Measured performance: Measured examples showed good self-adaptation against fast environmental changes around the antenna.This demonstrates the control solution’s ability to respond to changing coupling conditions.
  • Measured performance: The nonlinear digital canceller pushed residual self-interference to the receiver noise floor under a heavily nonlinear transmitter power amplifier.The findings indicate potential deployment of full-duplex capability in mobile devices for 5G and beyond radio systems.
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