Focusing on two-antenna infrastructure relays employed for coverage extension, we develop hybrid techniques that switch opportunistically between full-duplex and half-duplex relaying modes. To rationalize the system design, the classic three-node full-duplex relay link is first amended by explicitly modeling residual relay self-interference, i.e., a loopback signal from the transmit antenna to the receive antenna remaining after cancellation. The motivation for opportunistic mode selection stems then from the fundamental trade-off determining the spectral efficiency: The half-duplex mode avoids inherently the self-interference at the cost of halving the end-to-end symbol rate while the full-duplex mode achieves full symbol rate but, in practice, suffers from residual interference even after cancellation. We propose the combination of opportunistic mode selection and transmit power adaptation for maximizing instantaneous and average spectral efficiency after noting that the trade-off favors alternately the modes during operation. The analysis covers both common relaying protocols (amplify-and-forward and decode-and-forward) as well as reflects the difference of downlink and uplink systems. The results show that opportunistic mode selection offers significant performance gain over system design that is confined to either mode without rationalization.

Hybrid Full-Duplex/Half-Duplex Relaying with Transmit Power Adaptation

This paper studies the wireless two-way relay channel (TWRC), where two source nodes, S1 and S2, exchange information through an assisting relay node, R. It is assumed that R receives the sum signal from S1 and S2 in one timeslot, and then amplifies and forwards the received signal to both S1 and S2 in the next time-slot. By applying the principle of analogue network coding (ANC), each of S1 and S2 cancels the so-called "self-interference" in the received signal from R and then decodes the desired message. Assuming that S1 and S2 are each equipped with a single antenna and R with multi-antennas, this paper analyzes the capacity region of the ANC-based TWRC with linear processing (beamforming) at R. The capacity region contains all the achievable bidirectional rate-pairs of S1 and S2 under the given transmit power constraints at S1, S2, and R. We present the optimal relay beamforming structure as well as an efficient algorithm to compute the optimal beamforming matrix based on convex optimization techniques. Low-complexity suboptimal relay beamforming schemes are also presented, and their achievable rates are compared against the capacity with the optimal scheme.

Optimal beamforming for two-way multi-antenna relay channel with analogue network coding

Interference alignment is a radical idea that has recently emerged out of the capacity analysis of interference networks. In a relatively short time, this concept has challenged much of the conventional wisdom about the throughput limits of both wired and wireless networks. The technique is based on the idea that if interference or undesired signals are collectively engineered to occupy as low a dimensional space as possible, the desired signals can be more efficiently conveyed and recovered. Interference Alignment: A New Look at Signal Dimensions in a Communication Network introduces the concept via a series of simple linear algebraic examples that very effectively convey the central idea of interference alignment. The ideas behind the various communication theoretic instantiations and applications of the technique are illustrated through simple examples that come across as neat little puzzles which enables the reader to quickly understand the topic. Interference Alignment: A New Look at Signal Dimensions in a Communication Network provides both a tutorial and a survey of the state-of-art on the topic. The numerous concretely stated open problems sprinkled throughout tee up issues for further research for the reader. The presentation style of this tutorial is informal, favoring broad intuition over mathematical rigor and is written to be accessible to graduate students, researchers and professionals working in communication, signal processing, networking or information theory.

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Interference Alignment: A New Look at Signal Dimensions in a Communication Network

As a complement to high-layer encryption techniques, physical layer security has been widely recognized as a promising way to enhance wireless security by exploiting the characteristics of wireless channels, including fading, noise, and interference. In order to enhance the received signal power at legitimate receivers and impair the received signal quality at eavesdroppers simultaneously, multiple-antenna techniques have been proposed for physical layer security to improve secrecy performance via exploiting spatial degrees of freedom. This paper provides a comprehensive survey on various multiple-antenna techniques in physical layer security, with an emphasis on transmit beamforming designs for multiple-antenna nodes. Specifically, we provide a detailed investigation on multiple-antenna techniques for guaranteeing secure communications in point-to-point systems, dual-hop relaying systems, multiuser systems, and heterogeneous networks. Finally, future research directions and challenges are identified.

A Survey on Multiple-Antenna Techniques for Physical Layer Security

In this paper, multiple-input multiple-output (MIMO) relay transceiver processing is proposed for multiuser two-way relay communications. The relay processing is optimized based on both zero-forcing (ZF) and minimum mean-square-error (MMSE) criteria under relay power constraints. Various transmit and receive beamforming methods are compared including eigen beamforming, antenna selection, random beamforming, and modified equal gain beamforming. Local and global power control methods are designed to achieve fairness among all users and to maximize the system signal-to-noise ratio (SNR). Numerical results show that the proposed multiuser two-way relay processing can efficiently eliminate both co-channel interference (CCI) and self-interference (SI).

Multiuser Two-Way Amplify-and-Forward Relay Processing and Power Control Methods for Beamforming Systems

In this article we review an important class of wireless cooperation protocols known as amplify-and-forward relaying. One or more low-complexity relay nodes assist the communication between sources and destinations without having to decode the signal. This makes AF relaying transparent to modulation and coding of the source/destination communication protocol. It is therefore a highly flexible technology that also qualifies for application in heterogeneous networks comprising many nodes of different complexity or even standards. Recently, two-way relaying was introduced, which is readily combined with AF relaying. It is a spectrally efficient protocol that allows for bidirectional communication between sources and destinations. In order to investigate the potential of wireless AF relaying, we introduce three distributed network scenarios that differ in the amount of cooperation between nodes. New challenges arise in those networks, and we discuss approaches to overcome them. For the most general case of a completely distributed system, we present coherent relaying solutions that offer a distributed spatial multiplexing gain even for single-antenna nodes. Based on real-world experiments, we validate the feasibility of all schemes in our laboratory.

Recent advances in amplify-and-forward two-hop relaying

A novel scheme for two-hop relaying defined as space division duplex (SDD) relaying is proposed. In SDD relaying, multiple antenna beamforming techniques are applied at the intermediate relay station (RS) in order to separate downlink and uplink signals of a bi-directional two-hop communication between two nodes, namely, S1 and S2. For conventional amplify-and-forward two-hop relaying, there appears a loss in spectral efficiency due to the fact that the RS cannot receive and transmit simultaneously on the same channel resource. In SDD relaying, this loss in spectral efficiency is circumvented by giving up the strict separation of downlink and uplink signals by either time division duplex or frequency division duplex. Two novel concepts for the derivation of the linear beamforming filters at the RS are proposed; they can be designed either by a three-step or a one-step concept. In SDD relaying, receive signals at S1 are interfered by transmit signals of S1, and receive signals at S2 are interfered by transmit signals of S2. An efficient method in order to combat this kind of interference is proposed in this paper. Furthermore, it is shown how the overall spectral efficiency of SDD relaying can be improved if the channels from S1 and S2 to the RS have different qualities.

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Duplex schemes in multiple antenna two-hop relaying

This paper considers the two-hop relaying case where two nodes S1 and S2 in a wireless network can communicate with each other via an intermediate relay station (RS). Assuming that an RS can only receive and transmit on orthogonal channel resources, the required resources in oneway amplify-and-forward relaying are doubled compared to a direct communication between two nodes. Recently, two-way relaying has been introduced as a promising protocol in order to compensate for this drawback. In this paper, the two-way relaying protocol is extended to the case of nodes and RSs with multiple antennas where both nodes S1 and S2 have the same number of antennas and the number of antennas at the RS is at least twice as much. This multiple input multiple output (MIMO) two-way relaying protocol only requires channel state information (CSI) at the RS which reduces the CSI signaling overhead in the two-way relay network compared to previous work. Comparing with other relaying protocols, the performance gain in terms of sum rate of the proposed MIMO two-way relaying protocol is verified. It is shown that the sum rate may be further increased for the case of different channel qualities on the two channels to the RS.

On the Performance of Two-Way Relaying with Multiple-Antenna Relay Stations

This paper considers the two-hop relaying case where two nodes S1 and S2 in a wireless network can communicate with each other via an intermediate relay station (RS) which is equipped with multiple antennas and cannot transmit and receive simultaneously on the same channel resources. In oneway relaying, four orthogonal channel resources are required for the transmissions from S1 to S2 and from S2 to S1. MIMO two-way relaying has been introduced as an approach which requires only half the channel resources compared to one-way relaying due to the simultaneous transmission from S1 to S2 and vice versa. For MIMO two-way relaying, a spatial filter matrix is required at the RS which applies both, transmit and receive processing. The design of linear spatial filter matrices, termed transceive filter matrices, is given in this paper. In particular, linear transceive filters are derived which fulfill the zero forcing (ZF) and minimum mean square error (MMSE) criterion, respectively. It is shown that the linear MMSE transceive filter outperforms the linear ZF transceive filter in terms of overall bit error rate (BER). However, for different channel qualities on the two channels to the RS, the choice of the transceive filter influences which direction of communication has a better BER performance.

Linear Transceive Filters for Relay Stations with Multiple Antennas in the Two-Way Relay Channel

In this paper, space-time block codes (STBCs) are applied in a distributed fashion in a scenario with multiple cooperating relay stations (RSs) having multiple antennas. By applying the Chernoff bound to the theoretical bit error rate (BER) in Rayleigh fading channels, it turns out that the BER performance has a higher sensitivity to spatial correlation in multiple input multiple output channels than to different receive powers at the receiver from different cooperating RSs. If the number of overall available antennas exceeds the number of required antennas for the considered STBC, based on the theoretical analysis a criterion for the selection of the antennas which should cooperate in order to achieve the best BER performance is given

Timo Unger

Papers

Recent advances in amplify-and-forward two-hop relaying

Duplex schemes in multiple antenna two-hop relaying

On the Performance of Two-Way Relaying with Multiple-Antenna Relay Stations

Linear Transceive Filters for Relay Stations with Multiple Antennas in the Two-Way Relay Channel

On the Performance of Distributed Space-Time Block Codes in Cooperative Relay Networks