Raphael Rolny

Distributed gain matrix optimization in non-regenerative MIMO relay networks

We consider a half-duplex multiple-input multiple-output relay network with multiple source and relay nodes and a set of collocated destination antennas. Each relay is equipped with multiple antennas and is constrained to map its receive signal linearly to its transmit signal. We devise a gradient based scheme that enables each relay to locally optimize its gain matrix with respect to the achievable sum-rate without requiring explicit knowledge of all fading coefficients characterizing the channel state. In particular, we demonstrate by numerical evidence that the overhead due to exchange of channel state information required by the scheme does either not scale at all or, if so, very slowly with the number of relay nodes or antennas therein.

The relay carpet: Ubiquitous two-way relaying in cooperative cellular networks

We consider the up- as well as downlink of a cellular network with multi-user multiple-input multiple-output (MIMO) communication and propose to turn the entire network into a two-hop network where a large amount of relays assist the bidirectional communication. To this end, many two-way relays are spread over the entire area of the network. In addition to introducing the concept of ubiquitous relaying, different two-way relaying protocols are developed, for amplify-and-forward (AF) as well as decode-and-forward (DF) relays. The goal is to efficiently apply two-way relaying to interference limited cellular networks. To achieve this, relaying is combined with a simple form of base station (BS) cooperation that allows canceling some interference terms. The relays are used to reduce the complexity at the terminal nodes and to improve the overall performance by distributed interference management. Our results show that the proposed concept can offer significant gains compared to a conventional multi-user MIMO strategy without relays.

The Potential of Restricted PHY Cooperation for the Downlink of LTE-Advanced

We investigate the potential of restricted PHY co- operation for the downlink of 4G networks. The cooperation is restricted to a cluster of eNodeBs. We distinguish between low and high mobility users, and propose two appropriate cooperation methods. The goal is to guarantee high user rates even on cell edges. We present a simulation study to analyze the spectral efficiency achieved by cooperation methods for urban micro cells. To investigate their influence on coordinated multipoint (CoMP), we consider different frequency allocation strategies and different sector orientations in a cell. In addition, we compare CoMP transmission to Multiuser MIMO and investigate how cooperation can improve power allocation. Based on the results, we provide important insights into future cell planing aspects.

Power control for cellular networks with large antenna arrays and ubiquitous relaying

We consider a cellular network in which the base stations (BSs) are supported by a large amount of very low-complexity relays that are spread over the entire area, like a carpet. This carpet of relays enables massive antenna arrays and sophisticated multi-user MIMO transmission at the BSs, as they see only the static relays as the nodes they communicate with. On the other hand, the communication via the small relay cells allows to improve coverage and data rates by distributed signal processing. In order to control the residual interference caused by the massively deployed relay nodes, we apply power control to either minimize the transmit power at the BSs and relays required to achieve desired user rates, to maximize the minimum rate, or to minimize the outage probability. The proposed schemes are all of low complexity and show that the relay carpet is a promising concept for communication in future cellular networks.

MIMO relaying with compact antenna arrays: Coupling, noise correlation and superdirectivity

It is well known that compact arrays introduce spatial channel correlation, antenna coupling, superdirectivity and noise correlation. While these effects have been thoroughly investigated for point to point MIMO, only isolated results are available on the effect of such phenomena on performance of relaying systems. This paper tries to fill part of this void. Specifically we study the impact of lossless and lossy compact antenna arrays on multiuser amplify and forward (AF) relaying in the presence of different noise sources. We optimize the gain allocation for sum rate maximization under a dissipated power constraint. For compact arrays this constraint is more relevant than the commonly used radiated power constraint. We provide an extensive insight-oriented discussion of our results, some of which may appear counterintuitive at first glance. We show that for compact MIMO relaying the standard model, considering only channel correlation but ignoring antenna coupling and noise correlation, is insufficient and may lead to a wrong system characterization.

Relaying and base station cooperation: A comparative survey for future cellular networks

We develop a unified framework to investigate the performance of future cellular networks with relays and/or co-ordinated multipoint (CoMP) transmission. Based on this framework, we study the benefits of CoMP and relaying in a realistic setup. We consider imperfect channel knowledge, different power levels, and simple (thus practically relevant) cooperation schemes with different complexity: non-cooperative reference, decode-and-forward relaying with relay selection, base station cooperation with block zero-forcing (coherent joint transmission), and a combination of relaying and CoMP. Based on computer simulations, we compare the different schemes with respect to performance, robustness, complexity, and required transmit power.

The Cellular Relay Carpet: Distributed Cooperation with Ubiquitous Relaying

We consider the up- as well as downlink of a cellular network in which base stations (BSs) are supported by a large amount of relays spread over the entire area like a carpet. The BSs only see the static relays as the nodes they communicate with, which enables large antenna arrays at the BSs with sophisticated multi-user MIMO transmission. Together with a simple form of BS cooperation, the communication via the small relay cells allows to improve the data rates by distributed interference management and to reduce the complexity at the terminals. We investigate different types of relays as well as different relaying strategies for this relay carpet and compare them with respect to complexity, required channel state information (CSI), and performance in the interference-limited environment of dense cellular networks. The robustness of the different schemes with respect to channel estimation errors is studied and we conclude that especially relays of very low complexity are not sensitive to CSI imperfections. Relays can thus be applied in large numbers and enable massive MIMO at the BSs. The relay carpet proves thereby to be an efficient approach to enhance future generations of cellular networks significantly.

The Impact of Combined Physical Layer Cooperation and Scheduling for the Downlink of LTE-Advanced Networks

In this paper, five different locally restricted cooperation schemes for the downlink of 4G systems are compared. Interference among different sectors is mitigated and the downlink capacity is increased. The focus of our research is to investigate the potential of combined physical layer cooperation and scheduling by considering user-outage within the coordinated multipoint (CoMP) schemes to accomplish further power reduction of the eNodeBs, while maintaining a target data rate. Two methods to identify and discard users requiring large powers are proposed. One is based on a simple peak power constraint, whereas the second approach assumes a channel-aware scheduler which is able to defer data packets. Both outage criteria are implemented for all cooperation schemes and compared with each other. The power reduction achieved is determined by means of the CDF of the peak powers per sector for all considered cooperation schemes.

Impact of relays and supporting nodes on locally restricted cooperation in future cellular networks

In this paper, we compare different locally restricted cooperation schemes for the downlink of LTE-Advanced. The focus in our investigations is on schemes which are easy to implement and thus have a high level of practical relevance. Two different optimization goals are considered. We first investigate how a target data rate can be achieved with lowest transmit (Tx) power. Reduction of Tx power will lead to reduced electromagnetic radiation as well as lower total power consumption of the eNodeBs. Furthermore, it will result in reduced interference between adjacent cooperation sets. We secondly maximize the data rate that can be achieved by different schemes for fixed Tx power while considering fairness between the users. Additionally, we consider implementation issues and give an estimation of achievable data rates in case of limited backhaul capacity of practical 4G networks.

Universal computation with low-complexity wireless relay networks

We propose a method for enabling complex computations in a network of low-complexity wireless devices. By utilizing multihop relaying, such devices can form the wireless equivalent of an artificial neural network (ANN). We provide a method for programming the network functionality in a decentralized fashion and demonstrate the robustness of wireless ANNs against node failures and imperfections. Applications of this scheme exist in low-complexity sensor networks, where elaborate calculations can be carried out in a distributed fashion, or for creating powerful ANNs with very high degrees of interconnectivity realized by the wireless medium.