Alexander Kuehne

Pair-Aware Interference Alignment in Multi-User Two-Way Relay Networks

In this paper, K bidirectionally communicating node pairs with each node having N antennas and one amplify and forward relay having R antennas are considered. Each node wants to transmit d data streams to its communication partner. Taking into account that each node can perform self interference cancellation, a new scheme called Pair-Aware Interference Alignment is proposed. In this scheme, the transmit precoding matrices and the relay processing matrix are chosen in such a way that at any given receiver all the interfering signals except the self interference are within the interference subspace and the useful signal is in a subspace linearly independent of the interference subspace. If the number of variables is larger than or equal to the number of constraints in the system, the system is classified as proper, else as improper. Through simulations it is shown that for a proper system (2Kd≤2N+R-d), interferences can be perfectly aligned and the useful signals can be decoded interference-free at the receivers. An iterative algorithm to achieve the interference alignment solution is proposed. Also for the proper system fulfilling a certain additional condition, which will be derived in this paper, a closed form solution is proposed.

Corridor-based routing using opportunistic forwarding in OFDMA multihop networks

In multi-hop networks, conventional unipath routing approaches force the data transmission to follow a fixed sequence of nodes. In this paper, we widen this path to create a corridor of forwarding nodes. Within this corridor, data can be split and joined at different nodes as the data travels through the corridor towards the destination node. To split data, decode-and-forward OFDMA is used since with OFDMA, one can exploit the benefits of opportunistically allocating different subcarriers to different nodes according to their channel conditions. To avoid interference, each subcarrier is only allocated once per hop. For the presented scheme, the problem of optimizing the network throughput by means of resource and power allocation is formulated and two suboptimal algorithms are proposed to solve this problem with feasible effort. Simulations show that in multi-hop networks corridor-based routing using opportunistic forwarding outperforms conventional unipath routing approaches in terms of achievable throughput.

Reducing aggregation Bias and time in gossiping-based wireless sensor networks

Wireless sensor networks are able to perform an aggregation of the data generated by sensors. In networks where no gateway or no central sensor is specified, gossiping algorithms are used such that sensors in the whole network can aggregate messages from all other sensors. In the gossiping algorithm, the bias problem limits the quality of the aggregation results and the lack of message identification results in large aggregation time. In this paper, we reveal the possibility of eliminating or reducing the bias at the sensors by using the concept of the divisible functions that are generally applied in a sensor network and by using the memory of the sensors. Furthermore, we show how the aggregation time can be reduced by using different communication strategies for sensors communicating with their neighbors. Simulation results show the reduction of the aggregation bias at sensors as well as a higher speed of the aggregation in the network.

Node selection for corridor-based routing in OFDMA multihop networks

In multi-hop networks, conventional forwarding along a unicast route forces the data transmission to follow a fixed sequence of nodes. In previous works, it has been shown that widening this path to create a corridor of forwarding nodes and applying OFDMA to split and merge the data as it travels through the corridor towards the destination node leads to considerable gains in achievable throughput compared to the case forwarding data along a unicast route. However, the problem of selecting potential nodes to act as forwarding nodes within the corridor has not been addressed in the literature, as in general a rather homogeneous network topology with equally spaced relay clusters per hop between source and destination node has been assumed. In this paper, a more realistic heterogeneous network is considered where the nodes in the area between source and destination are randomly distributed instead of being clustered with equal distance. A node selection scheme is presented which selects the forwarding nodes within the corridor based on a given unicast route between source and destination node. In simulations, it is shown that with the proposed node selection scheme, considerable throughput gains of up to 50 % compared to forwarding along unicast route can be achieved applying corridor-based routing in heterogeneous networks especially in sparse networks.

Comparison of different multicast strategies in wireless identically distributed channels

This paper analyzes multicast (MC) as an efficient approach in transmitting the same information to multiple receivers (RXs). In each transmission time slot (TS), based on the channel realizations and on the specific MC strategy, a subset of RXs to be served is selected. The members of the served subset may change in the next TS. We assume that the channels from the transmitter (TX) to the RXs are independent and identically distributed (i.i.d.). This makes it possible for each RX to get, on average, the same amount of information. Herein, we find a closed form solution regarding the throughput of the Opportunistic Multicast strategy with fixed group size (OppMC-FS) [1] and present a new variant of this strategy called OppMC with optimal group size (OppMC-OS), providing an analytical solution regarding its throughput. Both variants, OppMC-FS and OppMC-OS, require instantaneous channel state information (CSI) at the TX. In addition, we also propose a new MC strategy, named MC based on statistical channel knowledge (StCSI-MC), and find the throughput of this strategy in a closed form. Results show that our strategies outperform broadcast or unicast and also that a good MC strategy can be found without the need of instantaneous CSI.

Two-way relaying for multiple applications in wireless sensor networks

Recent work in wireless sensor networks implies possibilities of concurrent support of multiple applications. In this paper, we discuss a novel scheme called hybrid computation in two-way relaying, which introduces cooperation of three sensor nodes to support bi-directional communications of two applications. Applications in wireless sensor networks require different computations and forms of aggregation. In the proposed scheme, different computations at the intermediate node are integrated in a two-way relaying scheme. For computations and transmissions in the proposed scheme, data from all three nodes are considered.We propose a superposition coding protocol and a time division protocol to handle the transmission of the messages from the intermediate node. The problem of maximizing the sum rate is discussed. The results show that the superposition coding protocol outperforms the time division protocol.

Corridor-based routing: Opening doors to PHY-layer advances for Wireless Multihop Networks

Today, the performance of routing mechanisms in Wireless Multihop Networks (WMNs) is still limited by the lower layers. While recent cross-layer approaches take advantage of the characteristics of the medium, they are often based on traditional physical layers such as OFDM. State-of-the-art techniques used in one-hop scenarios, such as OFDMA or MIMO, pose a significant challenge in practical multihop networks, since typically Channel State Information (CSI) at the transmitter is required. Due to its volatile nature, disseminating timely CSI in the network is often infeasible. We generalize Corridor-based Routing to enable advanced physical layers in WMNs. Instead of routing packets from node to node, we forward them along fully-connected groups of nodes. As a result (1) CSI only needs to be exchanged locally to enable cooperation in a group, and (2) groups can adaptively choose the best physical layer technique according to CSI. We investigate the benefits of Corridor-based Routing and present a protocol design that enables operation of corridors in WMNs.

Node virtualization and network coding: Optimizing data rate in wireless multicast

This paper investigates the achievable sum rate for a wireless multihop network (WMN) with unequal link capacities. We focus on a multi-source multicast scenario, where the nodes are operating in half-duplex mode. For this scenario, we propose a framework which fully utilizes the broadcast (BC) gain of the wireless medium through extended virtualization. Further, we show that our framework can switch between the routing mechanisms plain routing and network coding at the network layer (NET) and can also switch between the communication types unicast (UC), multicast (MC) and BC communications at the physical layer (PHY). We show that our framework outperforms isolated layer solutions and also currently available cross-layer approaches in the literature.

Practical Interference alignment in the frequency domain for OFDM-based wireless access networks

Interference alignment (IA) is often considered in the spatial domain in combination with MIMO systems. In contrast, aligning interference in the frequency domain among multiple subcarriers can also benefit single-antenna OFDM-based access networks. It allows for flexible operation on a per-subcarrier basis. We investigate the gains achievable by frequency IA in practice for a scenario with multiple access points and clients. Previous work is predominantly theoretical and focuses on idealized cases where all nodes have the same average signal-to-noise ratio (SNR). On the contrary, in practical networks, nodes typically have heterogeneous SNRs depending on channel conditions, which might have a significant impact on IA performance. We tackle this problem by designing mechanisms that adaptively choose which nodes shall perform IA on which subcarriers depending on current channel conditions. We implement and validate our approach on software-defined radios. To the best of our knowledge, this is the first practical implementation of IA in the frequency domain. Our measurements show that (1) frequency IA is feasible in practice, and (2) choosing appropriate nodes and subcarriers overcomes the main limitations due to heterogeneous SNRs. Our mechanisms enable IA in scenarios where it would be infeasible otherwise, achieving throughput gains close to the 33% theoretical maximum.

Delay constraints for multiple applications in wireless sensor networks

Sensors are capable of supporting multiple applications concurrently. Recent works reveal several possibilities of running multiple applications in a wireless sensor network. In this work, we introduce the compression factor to quantify the change of the message length due to the aggregations of application messages. Delay constraints are introduced for two communication paradigms in wireless sensor networks, 1) the routing-based paradigm where sinks are specified and the routing trees are built for the aggregation; 2) the random-gossiping paradigm where aggregations are done by sensor nodes randomly communicating with the neighbour sensor nodes without specifying the sinks. Optimization problems which minimize the total energy consumption in the network are proposed for the two communication paradigms with the corresponding delay constraints. An example of a wireless sensor network with three sensor nodes and two concurrently running applications is used to demonstrate the optimization problems. Simulation results shows how compression factors affect the energy consumption while the message length of the application is increasing.