Stefan Berger

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.

Cooperative Distributed Multiuser MMSE Relaying in Wireless Ad-Hoc Networks

We consider a wireless ad-hoc network with single antenna nodes under a two-hop traffic pattern. Two system architectures are investigated in this paper: Either linear amplify-and-forward relays (LinRel) or a distributed antenna system with linear processing (LDAS) serve as repeaters. The gain factors of the repeaters are assigned such that the mean squared error (MSE) of the signal at the destinations is minimised (multiuser MMSE relaying). A scalar multiplier γm ∈ C at each destination m allows for received signals that are scaled and rotated versions of the transmitted symbols. We distinguish two cases: 1) The factors are equal for all destinations: γm = γ. 2) An individual factor γm is chosen for each destination m. Multiuser MMSE relaying essentially realizes a distributed spatial multiplexing gain with single antenna nodes as all source/destination pairs can communicate concurrently over the same physical channel. The main contribution of this paper is the derivation of the MMSE gain factors. We evaluate the relaying scheme in comparison to multiuser zero forcing (ZF) (1) and show that it can outperform the latter in terms of average sum rate and diversity gain. Keywords - cooperative relaying, ad-hoc networks, distributed spatial multiplexing, minimum mean squared error (MMSE)

Optimizing Zero-Forcing Based Gain Allocation for Wireless Multiuser Networks

We consider a wireless multiuser network where a number of source/destination pairs communicate concurrently over the same physical channel. A set of amplify-and-forward relay nodes assist the communication by multiplying their received signal with a complex gain factor before retransmission. We propose and compare different strategies for finding the gain factor for every relay terminal, motivated by the chosen figures of merit: average sum rate, diversity, and fairness. A new and practical definition of diversity is provided in the context of our system model. For each strategy we investigate the provided gains in terms of the figures of merit, and the trade-off between them. The existing schemes in the literature act as references to the proposed schemes.

Carrier phase synchronization ofOF multiple distributed nodes in a wireless network

In this work we present a method for achieving global carrier phase synchronization in a wireless network of distributed, autonomous nodes. Other authors who have investigated this problem mainly focus on a master/slave strategy based on one master broadcasting a sinusoidal reference signal to all slaves. We follow a slightly different approach where the slaves concurrently or sequentially transmit a broadband sequence to the master. Receiving the conjugate complex of the bounced-back signal, each slave can locally determine the phase difference between its own and the masters local oscillator (LO). This information suffices to allow for coherent cooperation. The broadband nature of the used sequences makes our approach robust to deep fades in a frequency-selective environment. We revisit the popular strategy for global carrier phase synchronization using the sinusoidal reference and compare it to our own approach. A potential application then stresses the benefits of our scheme.

Experimental performance evaluation of multiuser zero forcing relaying in indoor scenarios

We consider a wireless ad hoc network with single antenna nodes under a two-hop relay traffic pattern. All source-destination pairs communicate concurrently on the same physical channel with the assistance of relay nodes. The gains of the amplify-and-forward relays are assigned such that the interference between different source-destination links is nulled [A Wittneben and B. Rankov, Sept. 2004]. This essentially realizes a distributed spatial multiplexing gain with single antenna nodes. The main contribution of this paper is the performance evaluation of this approach on the basis of multihop/multinode channel measurements at 5.25 GHz. We provide comprehensive results on the sum rate supported by multiuser zero forcing (ZF) in relaying and linear distributed antenna systems in an indoor propagation scenario.

A coherent amplify-and-forward relaying demonstrator without global phase reference

In this work we present a demonstrator for coherent linear amplify-and-forward (AF) relaying. Two source/destination pairs communicate with the help of three half-duplex relays in a two-hop manner. We implemented a coherent gain allocation scheme that suppresses inter-user interference at each destination and does not require a global phase reference at the relays. In order to assess the performance of the scheme, we calculated the signal-to-interference ratio (SIR) at both destination nodes. Inter-user interference arises when the local oscillator phases of the relays change independently during one transmission cycle, e.g. due to phase noise. Consequently, the shorter the time between reception and retransmission at the relays, the more accurate the interference suppression. Measurement results show that our demonstrator is able to suppress inter-user interference at both destinations reasonably well.

Impact of Noisy Carrier Phase Synchronization on Linear Amplify-and-Forward Relaying

We consider a distributed wireless network where amplify-and-forward relays assist the communication between multiple source/destination pairs. To provide a global phase reference at the relays it suffices for each relay to know the phase difference between its own local oscillator (LO) and the LO of a global master node. Based on a scheme to achieve this, we characterize the phase synchronization inaccuracy caused by additive signal noise and LO phase noise. It turns out that relay phase noise only has a limited impact on the performance. Merely the phase uncertainty during the time between phase estimation and channel measurements at the relays affects the system. Finally, we quantify the impact of phase estimation inaccuracy on multiuser zero-forcing relaying.

Experimental performance evaluation of joint cooperative diversity and scheduling

We investigate the performance of adaptive schedul- ing schemes in a cooperative network based on a channel measurement campaign at 5.25 GHz in an usual indoor office scenario. We compare the performance of different scheduling algorithms. Furthermore, we show that the performance results based on the measurements match quite well with our results evaluated by means of computer simulations.

Impact of Local-Oscillator Imperfections on Nonregenerative TDD and FDD Relaying

In this work we investigate the impact of imperfect local oscillators (LOs) at the relays in nonregenerative two-hop networks. Due to the specific traffic pattern of multihop networks, hardware imperfections at the relays have a different impact on the system than hardware imperfections at the sources and destinations. Based on the assumption that one LO generates all oscillations in each node, we consider baseband sampling time as well as sampling frequency offsets, RF frequency and phase offsets, timing errors, and phase noise. In the presence of these imperfections, time division duplex (TDD) and frequency division duplex (FDD) relays introduce errors to their forwarded signals. We identify these errors for both types of relays and use Monte-Carlo simulations to compare the performance of a multiuser network employing either TDD or FDD relays with imperfect LOs.