Marc Kuhn

MIMO two-way relaying with transmit CSI at the relay

Conventional half-duplex relaying schemes suffer from the loss in spectral efficiency due to the two channel uses required for the transmission from the source to the destination. Two-way relaying is an efficient means to reduce this loss in spectral efficiency by bidirectional simultaneous transmission of data between the two nodes. In this paper we study the impact of transmit channel state information at the relay in a MIMO two-way decode-and-forward relaying scheme. We investigate and compare two different re-encoding schemes at the relay. The first is based on superposition coding, whereas the second one is based on the bitwise XOR operation.

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.

Power line enhanced cooperative wireless communications

In this paper, we investigate the use of power line communication (PLC) to assist cooperative wireless relaying. We consider a communication scheme that uses the power line to initialize and synchronize wireless amplify-and-forward relays and to broadcast information between the relays. Starting from an analysis of transfer functions and noise measurements of PLC channels in office and residential environments, we propose a power line transmission scheme for the inter-relay-communication and assess the influence of this scheme on wireless relaying. This scheme is based on linear precoded orthogonal frequency-division multiplexing; it is designed to optimally exploit the frequency diversity available on PLC channels. The use of PLC leads to a very flexible way of enhancing wireless communications by plugging in additional relays where they are needed-without additional wiring.

Impact of relay gain allocation on the performance of cooperative diversity networks

We consider a wireless network with one source/destination pair and several linear amplify-and-forward relays. The influence of the gain allocation at the relays on the performance in cooperative relay communication links is analyzed. We present an optimal gain allocation which results in a coherent combining of all signal contributions at the destination and maximizes the instantaneous throughput of the link. If the channel state information (CSI) at the relays is limited, cooperative diversity schemes can be used. We show that the right choice of the amplification gains is crucial to achieve high outage throughput.

Cooperative diversity by relay phase rotations in block fading environments

We consider a wireless network with one source/destination pair and several linear amplify-and-forward relays. All nodes are equipped with one single antenna. To achieve cooperative diversity we propose time-variant and relay specific phase rotations induced at each relay to make the effective channel time-variant. This transformation of spatial diversity into temporal diversity can be exploited by an appropriate outer code. Furthermore we show that the allocation of the amplification gains at the relays has great influence on the diversity performance and we give a low complexity extension to existing gain allocations.

Space-time processing for cooperative relay networks

Future wireless communication systems have to support high data rates. The capacity of these systems can be increased dramatically by using multiple antennas at the transmitter and the receiver working in a rich scattering environment. However, in a line-of-sight environment, the MIMO channel matrix is rank deficient, and therefore the capacity increase diminishes. Using cooperative amplify and forward relay nodes, it is possible to overcome this problem by increasing the rank of the MIMO channel. A special signaling scheme is necessary to achieve this increase of channel rank. Therefore the use of existing space-time algorithms is not straightforward. We show that a recently proposed class of linear space-time block codes exploits the offered capacity and achieves good performance results. The codes are able to use transmit diversity in combination with spatial multiplexing with reasonable and scalable computational complexity.

Channel adaptive scheduling for cooperative relay networks

We consider a wireless network consisting of several active source/destination pairs and several idle nodes. All nodes are equipped with one antenna. The communication between source/destination pairs takes place over two-hop links. Idle nodes are used as amplify-and-forward relays assisting the communication. We investigate the benefits and potential of joint cooperative diversity and channel adaptive scheduling in such a network. We show that this scheme is able to exploit multiuser diversity as well as cooperative diversity. It even makes fair scheduling possible. Performance analysis is done analytically and by means of simulation.

Joint cooperative diversity and scheduling in low mobility wireless networks

We consider a wireless network, which consists of several active source/destination pairs and a number of idle nodes. Multiple idle nodes volunteer as relays to facilitate the communication. All nodes have low mobility and the fading channel is essentially constant over the latency time scale of interest (block fading). Conventional adaptive scheduling techniques are either unfair or inefficient in this context. We introduce a new approach to jointly utilize adaptive scheduling and cooperative diversity in the low mobility regime. In contrast to known approaches the new scheme does not compromise the quality of service of individual source/destination links (fairness) if the number of relay nodes is sufficiently high. In a typical setup the 1%-outage aggregate throughput is improved by a factor of nine, if six source/destination pairs are considered jointly. We conclude that the joint cooperative diversity and scheduling scheme extends the benefits of adaptive scheduling to the low mobility regime.

UWB impulse radio based distance bounding

Today-s communication systems are often vulnerable to wormhole or relaying attacks, leading to severe security problems. Distance Bounding (DB) protocols are authentication protocols designed to protect against these attacks. They determine an upper bound on the physical distance between two communication parties — the verifier V (e.g. a door requiring an access key) and the prover P (e.g. a wireless key device). UWB technology promises an innovative wireless implementation of DB protocols, using low cost components. A crucial aspect for DB algorithms, besides a high temporal resolution, is the processing delay of P between receiving a challenge from V and transmitting the answer to V. Even current UWB transceivers may add a considerable processing delay, which decreases the provided security. We propose and analyze a novel analog UWB transceiver architecture which is able to both detect incoming UWB pulses and transmit answers with minimal delay.

Cooperative Transmission Schemes for Decode-and-Forward Relaying

We consider a low mobility cellular relaying system downlink where two mobile users are served by two neighboring decode-and-forward relays concurrently using the same frequency channel. We propose two cooperative relaying transmission schemes where each relay can choose proper precoding vectors based on its local channel knowledge to transmit data in the second hop. Each user can receive its own data without interference, which simplifies the user receiver design. We show that the diversity of each users received data signal can be improved by receiving data from multiple relays. In addition, higher array gain can be achieved by the first scheme at the cost of higher synchronization accuracy requirements. Furthermore, we show that the two transmission schemes achieve higher transmission rate than serving different users in separate channels.