Spatial degrees-of-freedom in large-array full-duplex: the impact of backscattering

Evan Everett,Ashutosh Sabharwal

doi:10.1186/s13638-016-0781-3

Evan Everett, Ashutosh Sabharwal

Open Access

https://doi.org/10.1186/s13638-016-0781-3

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Abstract

The key challenge for in-band full-duplex wireless communication is managing self-interference. Many designs have employed spatial isolation mechanisms, such as shielding or multi-antenna beamforming, to isolate the self-interference waveform from the receiver. Because such spatial isolation methods confine the transmit and receive signals to a subset of the available space, the full spatial resources of the channel may be under-utilized, expending a cost that may nullify the net benefit of operating in full-duplex mode. In this paper, we leverage an antenna-theory-based channel model to analyze the spatial degrees of freedom available to a full-duplex capable base station. We observe that whether or not spatial isolation out-performs time-division (i.e., half-duplex) depends heavily on the geometric distribution of scatterers. Unless the angular spread of the objects that scatter to the intended users is overlapped by the spread of objects that backscatter to the base station, then spatial isolation outperforms time division, otherwise time division may be optimal.

Highlights

Deployed wireless communications equipment operates in half-duplex mode, meaning that transmission and reception are orthogonalized either in time or frequency
That we have constructed X1, the uplink wavevector signal transmitted on the the uplink user, and X2, the wavevector signal transmitted on the dowlink by the base station, we show how the base station receiver, R1 and the downlink user R2 process their received signals to detect the original information-bearing symbols
By “overlapped” we mean that the directions of departure from the base station transmitter, T2, that scatter to the intended downlink receiver, R2, are identical to the directions of departure that backscatter to the base station receiver, R1, as self-interference, so that T11 = T12

Summary

Introduction

Deployed wireless communications equipment operates in half-duplex mode, meaning that transmission and reception are orthogonalized either in time (time-division-duplex) or frequency (frequency-divisionduplex). In the full-duplex design of [21] which demonstrated full-duplex feasibility at WiFi ranges, of the 95 dB of self-interference suppression achieved, 70 dB is due to spatial isolation, while only 25 dB is due to cancellation. If multiple antennas are used and if the self-interference channel response can be estimated, the radiation pattern can be shaped adaptively to mitigate both direct-path and backscattered self-interference This pattern shaping (i.e., beamforming) will consume spatial degrees-of-freedom that could have otherwise been leveraged for spatial multiplexing. Given a constraint on the size of the antenna arrays at the base station and at the user devices, and given a characterization of the spatial distribution of the scatterers in the environment, what is the uplink/downlink degree-offreedom region when the only self-interference mitigation strategy is spatial isolation?.

Overview of the PBT model

Extension of PBT model to three-node full-duplex

Achievability Overview of achievability proof

Overlapped scattering case

Symmetric spread

Conclusions