Armin Wittneben

Spectral efficient protocols for half-duplex fading relay channels

We study two-hop communication protocols where one or several relay terminals assist in the communication between two or more terminals. All terminals operate in half-duplex mode, hence the transmission of one information symbol from the source terminal to the destination terminal occupies two channel uses. This leads to a loss in spectral efficiency due to the pre-log factor one-half in corresponding capacity expressions. We propose two new half-duplex relaying protocols that avoid the pre-log factor one-half. Firstly, we consider a relaying protocol where a bidirectional connection between two terminals is established via one amplify-and-forward (AF) or decode-and-forward (DF) relay (two-way relaying). We also extend this protocol to a multi-user scenario, where multiple terminals communicate with multiple partner terminals via several orthogonalize-and-forward (OF) relay terminals, i.e., the relays orthogonalize the different two-way transmissions by a distributed zero-forcing algorithm. Secondly, we propose a relaying protocol where two relays, either AF or DF, alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that both protocols recover a significant portion of the half-duplex loss

Achievable Rate Regions for the Two-way Relay Channel

We study the two-way communication problem for the relay channel. Hereby, two terminals communicate simultaneously in both directions with the help of one relay. We consider the restricted two-way problem, i.e., the encoders at both terminals do not cooperate. We provide achievable rate regions for different cooperation strategies, such as decode-and-forward based on block Markov superposition coding. We also evaluate the regions for the special case of additive white Gaussian noise channels. We show that a combined strategy of block Markov superposition coding and Wyner-Ziv coding achieves the cut-set upper bound on the sum-rate of the two-way relay channel when the relay is in the proximity of one of the terminals

Power Allocation Schemes for Amplify-and-Forward MIMO-OFDM Relay Links

We consider a two-hop MIMO-OFDM communication scheme with a source, an amplify-and-forward relay, and a destination. We examine the possibilities of power allocation (PA) over the subchannels in frequency and space domains to maximize the instantaneous rate of this link if channel state information at the transmitter (CSIT) is available. We consider two approaches: (i) separate optimization of the source or the relay PA with individual per node transmit power constraints and (ii) joint optimization of the source and the relay PA with joint transmit power constraint. We provide the optimal PA at the source (or the relay) with a node transmit power constraint that maximizes the instantaneous rate for a given relay (or source) PA. Furthermore, we show that repeating this separate optimization of the source and the relay PA alternately converges and improves the achievable rate of the considered link. Since the joint optimization of the source and the relay PA is analytically not tractable we use a high SNR approximation of the SNR at the destination. This approximation leads to rates which are quite tight to the optimum.

Spectral Efficient Signaling for Half-duplex Relay Channels

We study two-hop communication protocols where one or two relay terminals assist in the communication between two transceiver terminals. All terminals operate in half-duplex mode, i.e., may not receive and transmit simultaneously at the same time and frequency. This leads to a loss in spectral efficiency due to the pre-log factor 1/2 in corresponding expressions for the achievable rate (capacity). We propose and analyze two relaying protocols that avoid the pre-log factor 1/2 but still work with halfduplex relays. Firstly, we consider a relaying protocol where two half-duplex relays, either amplify-and-forward (AF) or decodeand-forward (DF), alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that the protocol can recover a significant portion of the halfduplex loss. Secondly, we propose a relaying protocol where a bidirectional connection between two transceiver terminals is established via one half-duplex AF or DF relay (two-way relaying). It is shown that the sum rate of the two-way half-duplex AF relay channel achieves the rate of the one-way full-duplex AF relay channel, whereas the sum rate of the two-way half-duplex DF relay channel achieves the rate of the one-way full-duplex DF relay channel only in certain cases.

UWB for noninvasive wireless body area networks: channel measurements and results

The paper presents UWB channel measurements from 3 to 6 GHz for a body area network (BAN) in an anechoic chamber and an office room. Both, transmit and receive antenna were placed directly on the body. Channel parameters as delay spread and path loss are extracted from the measurements and the influence of the body is highlighted. We show that in some situations there are significant echoes from the body (e.g. from the arms) and we observed deterministic echoes from the floor that could help to simplify a RAKE receiver structure. Finally, we consider the overall energy consumption of the BAN and give decision regions for singlehop and multihop links in relation to signal processing energy.

On the Optimal Power Allocation for Nonregenerative OFDM Relay Links

We consider a two-hop communication scheme using OFDM modulation. The relay is assumed to be nonregenerative (or amplify-and-forward). We examine the possibilities of power allocation (PA) over the frequency subchannels at source and relay, with respect to separate transmit power constraints, if channel state information at the transmitter (CSIT) is given. We provide the optimal PA at the relay (source) that maximizes the instantaneous rate for a given source (relay) PA. Furthermore, we show that alternate, separate optimization of source and relay PA converges to the solution of the joint optimization of source and relay PA. To further enhance the rate of the considered scheme, the OFDM subchannels of the source to relay and relay to destination channel can be paired according to their actual magnitude. In the case of a joint sum power constraint, we propose to allocate the transmit power between source and relay with respect to average channel attenuation of first and second hop. It is shown that this leads to a information rate that is near to the optimal rate achieved by a joint optimization of source and relay PA with joint transmit power constraint.

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.