Evan Everett

Passive Self-Interference Suppression for Full-Duplex Infrastructure Nodes

Recent research results have demonstrated the feasibility of full-duplex wireless communication for short-range links. Although the focus of the previous works has been active cancellation of the self-interference signal, a majority of the overall self-interference suppression is often due to passive suppression, i.e., isolation of the transmit and receive antennas. We present a measurement-based study of the capabilities and limitations of three key mechanisms for passive self-interference suppression: directional isolation, absorptive shielding, and cross-polarization. The study demonstrates that more than 70 dB of passive suppression can be achieved in certain environments, but also establishes two results on the limitations of passive suppression: (1) environmental reflections limit the amount of passive suppression that can be achieved, and (2) passive suppression, in general, increases the frequency selectivity of the residual self-interference signal. These results suggest two design implications: (1) deployments of full-duplex infrastructure nodes should minimize near-antenna reflectors, and (2) active cancellation in concatenation with passive suppression should employ higher-order filters or per-subcarrier cancellation.

Empowering full-duplex wireless communication by exploiting directional diversity

The use of directional antennas in wireless networks has been widely studied with two main motivations: 1) decreasing interference between devices and 2) improving power efficiency. We identify a third motivation for utilizing directional antennas: pushing the range limitations of full-duplex wireless communication. A characterization of full-duplex performance in the context of a base station transmitting to one device while receiving from another is presented. In this scenario, the base station can exploit “directional diversity” by using directional antennas to achieve additional passive suppression of the self-interference. The characterization shows that at 10 m distance and with 12 dBm transmit power the gains over half-duplex are as high as 90% and no lower than 60% as long as the directional antennas at the base station are separated by 45° or more. At 15 m distance the gains are no lower than 40% for separations of 90° and larger. Passive suppression via directional antennas also allows full-duplex to achieve significant gains over half-duplex even without resorting to the use of extra hardware for performing RF cancellation as has been required in the previous work.

Crowdsourced estimation of cognitive decline and resilience in Alzheimer's disease

Identifying accurate biomarkers of cognitive decline is essential for advancing early diagnosis and prevention therapies in Alzheimers disease. The Alzheimers disease DREAM Challenge was designed as a computational crowdsourced project to benchmark the current state‐of‐the‐art in predicting cognitive outcomes in Alzheimers disease based on high dimensional, publicly available genetic and structural imaging data. This meta‐analysis failed to identify a meaningful predictor developed from either data modality, suggesting that alternate approaches should be considered for prediction of cognitive performance.

Self-interference cancellation in multi-hop full-duplex networks via structured signaling

This paper discusses transmission strategies for dealing with the problem of self-interference in multi-hop wireless networks in which the nodes communicate in a full-duplex mode. An information theoretic study of the simplest such multi-hop network: the two-hop source-relay-destination network, leads to a novel transmission strategy called structured self-interference cancellation (or just “structured cancellation” for short). In the structured cancellation strategy the source restrains from transmitting on certain signal levels, and the relay structures its transmit signal such that it can learn the residual self-interference channel, and undo the self-interference, by observing the portion of its own transmit signal that appears at the signal levels left empty by the source. It is shown that in certain nontrivial regimes, the structured cancellation strategy outperforms not only half-duplex but also full-duplex schemes in which time-orthogonal training is used for estimating the residual self-interference channel.

SoftNull: Many-Antenna Full-Duplex Wireless via Digital Beamforming

In this paper, we present and study a digital-controlled method, called SoftNull, to enable full-duplex in many-antenna systems. Unlike most designs that rely on analog cancelers to suppress self-interference, SoftNull relies on digital transmit beamforming to reduce self-interference. SoftNull does not attempt to perfectly null self-interference, but instead seeks to reduce self-interference sufficiently to prevent swamping the receivers dynamic range. Residual self-interference is then cancelled digitally by the receiver. We evaluate the performance of SoftNull using measurements from a 72-element antenna array in both indoor and outdoor environments. We find that SoftNull can significantly outperform half-duplex for small cells operating in the many-antenna regime, where the number of antennas is many more than the number of users served simultaneously.

Angle-of-arrival based beamforming for FDD massive MIMO

A key challenge in FDD massive MIMO is the large overhead in CSI acquisition for closed-loop MIMO transmission. In this paper, we propose two novel types of angle-of-arrival (AoA) based beamforming schemes that harness the reciprocity of dominant AoA. Both schemes require CSI acquisition overhead that only scales with the number of served mobiles, not the number of base-station antennas. We analyze the performance of the proposed schemes both analytically and numerically. We show that both our proposed schemes lead to sum throughput that scales with the number of base-station antennas, and hybrid beamforming performs close to ideal zero-forcing beamforming.

A signal-space analysis of spatial self-interference isolation for full-duplex wireless

The challenge of full-duplex wireless communication is self-interference received directly from the transmit antennas and backscattered from nearby objects. Spatial isolation of the receive antennas from the transmit antennas can mitigate self-interference, but may cause the spatial resources of the channel to be under-utilized, sacrificing spatial multiplexing performance. We present an analysis of spatial isolation of self-interference for full-duplex base stations, which leverages the antenna-theory based channel model of Poon et. al.We characterize the scattering conditions under which spatial isolation can enable a degrees-of-freedom gain over half-duplex, and show that the gain is inversely proportional to the overlap between the backscattering intervals (set angles of departure/arrival of backscattered self-interference), and the forward scattering intervals (set of angles of departure/arrival to/from the intended users).

Poster: SoftNull: All-digital Massive MIMO Full-duplex Wireless

Today’s wireless base stations are half-duplex, meaning that transmission and reception are relegated to separate time slots or separate frequency bands. Data rates would be multiplicatively increased if base stations were full-duplex, meaning they could both transmit and receive at the same time and in the same frequency band. The challenge to fullduplex operation is self-interference: the base station generates high-powered interference to its own receiver, swamping the receiver electronics and preventing the base station from receiving the much weaker uplink signal. Research over the last ten years [1, 2, 3, 4], has shown that full-duplex operation is feasible for small cells. The key enabler of full-duplex has been a combination analog cancellation and digital cancellation of the self-interference [3, 5]. Another promising wireless innovation is massive multipleinput multiple output (“Massive MIMO”), in which the base station uses very large antenna arrays (on the order of hundreds) to communicate with many users simultaneously. The benefit of Massive MIMO is that the beam to each user is very focused, which enables the base station to leverage simple signal processing and mitigates interference between cells [6]. The grand vision for next-generation wireless communication is to combine Massive MIMO and full-duplex in a single system. Full-duplex Massive MIMO brings both new challenges and new opportunities. Challenge: Analog cancellation has been considered necessary to prevent the self-interference from overwhelming the dynamic range of the receiver electronics. However, as the number of antennas grows, the complexity of the hardware required for analog cancellation grows superlinearly. Therefore solutions are needed to suppress self-interference prior to the receiver front end whose analog complexity does not scale with the number of antennas. Opportunity: Massive MIMO also presents a new opportunity for full-duplex. Many antennas provides transmit spatial degrees of freedom that can be leveraged for transmit beamforming to suppress self-interference. However, supPermission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for third-party components of this work must be honored. For all other uses, contact the Owner/Author(s). Copyright is held by the owner/author(s). S3’15, September 11, 2015, Paris, France. ACM ISBN 978-1-4503-3701-4/15/09. DOI: http://dx.doi.org/10.1145/2801694.2801716. pressing self-interference via transmit beamforming requires sacrificing transmit dimensions that could have been leveraged for reaffirming to the downlink users. In particular, for receive antenna that is nulled, a transmit dimension (i.e. a virtual transmit antenna) must be sacrificed. Proposed Solution: We consider removing the analog cancellation stage altogether, and relying on SoftNull, an all-digital-architecture for self-interference suppression, which uses transmit beamforming and digital cancellation to suppress self-interference. SoftNull can be implemented on existing base stations with existing radios; no special-purpose analog components would be needed. SoftNull leverages the observation that that the self-interference need not be zero-forced, but only suppressed to a level commensurate to the desired uplink signal, so that the selfinterference no longer overloads the receiver and can then be cancelled digitally. Given a required number of “virtual antennas” for the downlink, SoftNull choses the transmit beamweights which best suppress self-interference, which turn out to have a closed-form expression. Initial experiments on a 72-element array have shown that SoftNull can sufficiently suppress self-interference while maintaining strong links to the users, for moderate numbers of users (4-12), and moderate path loss (< 90 dB). Much more research needs to be performed in terms of both algorithm development and experimental evaluations.