Ingmar Hammerström

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.

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.

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.

Joint Power Allocation for Nonregenerative MIMO-OFDM Relay Links

We consider a two-hop MIMO-OFDM communication scheme with a source, a relay, and a destination. The relay is assumed to be nonregenerative (or amplify-and-forward (AF)). We assume channel state information at each transmitter (CSIT), source and relay. We present a jointly optimized power allocation (PA) over the subchannels in space and frequency domain at source and relay with a joint transmit power constraint. The PA is based on a high SNR approximation of the SNR expression at the destination. In the low SNR regime, the performance is still very tight to the optimal rate. The presented PA can be computed with low computational complexity and achieves a considerable performance gain compared to a uniform PA at source and relay. To further enhance the performance of the considered scheme, all subchannels, in space and frequency domain, of the source to relay and relay to destination channel are paired according to their actual magnitude

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.

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.

Multiuser zero forcing relaying with noisy channel state information [wireless ad hoc network applications]

We consider a wireless ad hoc network with single antenna nodes under a two-hop relay traffic pattern. Some of the nodes in the network form source/destination pairs, while the other nodes serve as amplify and forward relays. The relay gains are assigned such that the interference between different source/destination links is nulled (multiuser zero forcing relaying). This essentially realizes a spatial multiplexing gain with distributed single antenna nodes. We introduce a specific zero forcing gain allocation and give comprehensive performance results, which include the impact of noisy channel state information and time variant channel coefficients. One of our most intriguing results shows that in a typical indoor scenario (delay spread<500 ns) multiuser zero forcing relaying at 5 GHz and pedestrian speed achieves a six fold increase in the sum rate of the network, even if we take the overhead for the measurement and the dissemination of the channel matrices into account.