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

/pdf/spectral-efficient-protocols-for-half-duplex-fading-relay-4nr22j3vyn.pdf

Spectral efficient protocols for half-duplex fading relay channels

[IEEE 2006 IEEE International Symposium on Information Theory - Seattle, WA (2006.7.9-2006.7.9)] 2006 IEEE International Symposium on Information Theory - Improving Amplify-and-Forward Relay Networks: Optimal Power Allocation versus Selection

In cognitive radio networks, cognitive (unlicensed) users need to continuously monitor spectrum for the presence of primary (licensed) users. In this paper, we illustrate the benefits of cooperation in cognitive radio. We show that by allowing the cognitive users operating in the same band to cooperate we can reduce the detection time and thus increase the overall agility. We first consider a two-user cognitive radio network and show how the inherent asymmetry in the network can be exploited to increase the agility. We show that our cooperation scheme increases the agility of the cognitive users by as much as 35%. We then extend our cooperation scheme to multicarrier networks with two users per carrier and analyze asymptotic agility gain. In Part II of our paper [1], we investigate multiuser single carrier networks. We develop a decentralized cooperation protocol which ensures agility gain for arbitrarily large cognitive network population.

Cooperative Spectrum Sensing in Cognitive Radio, Part II: Multiuser Networks

-We consider an Amplify-and-Forward cooperative diversity system where a source node communicates with a destination node with the help of one or more relay nodes The conventional system model assumes all relay nodes participate, with the available channel and power resources equally distributed over all nodes This approach being clearly sub-optimal, we first present two power allocation schemes to minimize the system outage probability, based on complete channel state information and channel statistics, respectively We further show that the proposed optimal power allocation methods minimize system symbol error rate as well Next, we propose a selection scheme where only one "best" relay node is chosen to assist in the transmission We show that the selection-AF scheme maintains full diversity order, and at reasonable power levels has significantly better outage behavior and average throughput than the conventional all-participate scheme or that with optimal power allocation Finally we combine power allocation and selection to further improve performance

/pdf/improving-amplify-and-forward-relay-networks-optimal-power-50mduvwndy.pdf

Improving amplify-and-forward relay networks: optimal power allocation versus selection

This paper is on relay selection schemes for wireless relay networks. First, we derive the diversity of many single-relay selection schemes in the literature. Then, we generalize the idea of relay selection by allowing more than one relay to cooperate. The SNR-optimal multiple relay selection scheme can be achieved by exhaustive search, whose complexity increases exponentially in the network size. To reduce the complexity, several SNR-suboptimal multiple relay selection schemes are proposed, whose complexity is linear in the number of relays. They are proved to achieve full diversity. Simulation shows that they perform much better than the corresponding single relay selection methods and very close to the SNR-optimal multiple relay selection scheme. In addition, for large networks, these multiple relay selection schemes require the same amount of feedback bits from the receiver as single relay selection schemes.

Single and multiple relay selection schemes and their achievable diversity orders

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.

/pdf/power-allocation-schemes-for-amplify-and-forward-mimo-ofdm-2doa5irbqw.pdf

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

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.

/pdf/power-line-enhanced-cooperative-wireless-communications-4dll0pnryf.pdf

Power line enhanced cooperative wireless communications

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.

/pdf/on-the-optimal-power-allocation-for-nonregenerative-ofdm-263s9lgire.pdf

On the Optimal Power Allocation for Nonregenerative OFDM Relay Links

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.

MIMO two-way relaying with transmit CSI at the relay

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.

/pdf/impact-of-relay-gain-allocation-on-the-performance-of-5635kngziw.pdf

I. Hammerstrom

Papers

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

Power line enhanced cooperative wireless communications

On the Optimal Power Allocation for Nonregenerative OFDM Relay Links

MIMO two-way relaying with transmit CSI at the relay

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