Timo Unger

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

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.

Duplex schemes in multiple antenna 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.

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). 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.

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

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.

Applying Relay Stations with Multiple Antennas in the One- and Two-Way Relay Channel

This paper considers the two-hop relaying case with bidirectional communication of two nodes 51 and 52 via an intermediate relay station (RS). The RS is equipped with multiple antennas and channel state information is available at the RS. Either the multiple antennas can be used to achieve spatial diversity by applying receive and transmit maximum ratio combining (MRC) at the RS in a one-way relaying approach where up- and downlink are transmitted on orthogonal channel resources, or they can be used to apply the recently introduced multiple input multiple output (MHVIO) two-way relaying. In MIMO two-way relaying, the number of required channel resources is reduced since up- and downlink are transmitted on the same channel resources. In this paper, it is investigated which approach provides a better average and outage performance, respectively. Concerning the average performance, MIMO two-way relaying always outperforms MRC one-way relaying. However, the outage performance in MIMO two-way relaying significantly depends on the choice of the linear filter at the RS. MIMO two-way relaying with a linear zero forcing filter provides a worse outage performance than MRC one-way relaying while MIMO two-way relaying with a linear minimum mean square error filter outperforms MRC one-way relaying.

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

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

Cooperative MIMO Relaying with Distributed Space-Time Block Codes

In this paper, space-time block codes (STBCs), which gain from spatial transmit diversity, are applied in a distributed fashion at several cooperating relay stations (RSs) with multiple transmit antennas. It is well known that non-distributed STBCs exhibit a degraded bit error rate (BER) performance in spatially correlated MIMO channels. Applying distributed STBCs in cooperative relay networks reduces the probability of correlated channel coefficients as the RSs are spatially separated. In this paper, the Chernoff bound of the BER in Rayleigh fading channels is extended to the case of correlated channel coefficients at the same relay station and different receive powers from different cooperating RSs. It is shown that the BER performance has a higher sensitivity to spatial correlation in MIMO channels than to different receive powers at the receiver from several cooperating RSs for distributed space-time coding. The theoretical results are confirmed by means of simulations

Self-interference aware MMSE filter design for a cellular multi-antenna two-way relaying scenario

A single cell scenario with different multi-antenna nodes, i.e., a base station and several mobile stations, is considered. An intermediate multi-antenna relay station is used to support the bidirectional communications. In such a scenario, the required data rates for up - and downlink are typically different which is considered by introducing asymmetric rate requirements. Non-regenerative two-way relaying is applied and the nodes can subtract the back-propagated self-interference. To fully utilize this capability of the nodes for the transceive filter design at the relay station, an MMSE based transceive filter is derived which does not suppress self-interference. Thus, the required number of antennas at the relay station is reduced. Additionally, the transceive filter is optimized for asymmetric rate requirements by introducing a weighting parameter. Furthermore, an alternating optimization between the transceive filter at the relay station and the transmit and receive filters at the nodes is introduced to fulfill the asymmetric rate requirements and to increase the achievable sum rates. Simulation results show that the proposed filter design at the nodes and at the relay station requires less antennas and achieves higher sum rates compared to conventional approaches.

Maximum sum rate of non-regenerative two-way relaying in systems with different complexities

This paper considers the two-hop relaying case with bi-directional communication of two multiple-antenna nodes S1 and S2 via an intermediate multiple-antenna node, namely the relay station (RS). A general framework for the sum rate maximization in linear, non-regenerative two-way relaying with multiple-antenna nodes is proposed assuming all nodes have perfect channel state information (CSI) in order to perform linear beamforming (BF). There may be application cases in which the system cannot provide perfect CSI to all nodes or the nodes cannot be such complex. Hence, three different cases of system complexity are investigated which are shown to be special cases of the introduced framework. Firstly, all nodes can perform BF, secondly only S1 and S2 can perform BF, and thirdly only the RS can perform BF. The optimum performances of all three cases are compared to each other by means of numeric optimizations, and approaches to solve the resulting optimization problems are proposed. It is shown that applying BF exclusively at the RS provides the same performance as applying BF at all nodes.