A wireless Ad Hoc network consisting of a source and multiple receiving nodes is considered. The source wants to transmit a common message throughout the whole network. The message has to be spread in a multi-hop fashion, as the transmit powers at the source and the nodes are limited. The goal of this paper is to find the multi-hop broadcast tree with a minimum energy consumption in the network. To reach this goal, a new decentralized game theoretic approach is proposed which considers the following two aspects jointly for the first time: Firstly, it optimizes the transmit powers at the source and at the individual intermediate nodes. Secondly, it employs maximum ratio combining at the receiving nodes following the fact that a node can receive several copies of the message from different sources in different time slots. The game is modeled such that the nodes are incentivized to forward the message to their neighbors. In terms of the total transmit energy, the results show that the proposed algorithm outperforms other conventional algorithms.

Corridor-based Routing enables advanced physical layer schemes in Wireless Multihop Networks (WMNs). It widens paths in order to span multiple nodes per hop. As a result, groups of nodes cooperate locally at each hop to forward packets. Recent theoretical work suggests using Orthogonal Frequency-Division Multiple Access (OFDMA) in combination with Corridor-based Routing to improve throughput in WMNs. However, results focus on achievable capacity and do not consider practical issues such as modulation and coding schemes. In this paper, we study OFDMA for corridors in practice and implement it on software-defined radios. We show that OFDMA corridors provide significantly larger throughput gains when considering realistic modulation and coding, achieving up to 2x throughput gain compared to traditional routing not based on corridors.

Corridor-based Routing widens traditional hop-by-hop paths to enable advanced physical layer mechanisms in Wireless Multihop Networks. The key concept is to let groups of nodes cooperate to jointly forward data. Instead of establishing a path formed by a fixed sequence of nodes, the network layer builds a "corridor" from source to destination to allow for such an approach. The corridor is divided into stages, which consist of the aforementioned groups of nodes. Existing mechanisms for Corridor-based Routing focus on the physical layer and assume a routing protocol that builds the corridor. In this paper, we design a corridor construction algorithm and a practical protocol that implements it. In particular, we address in detail the overhead introduced by our protocol, since this is crucial for performance and an open issue of Corridor-based Routing. We implement our approach both in simulation and practice. We obtain the turning point at which corridors become profitable and show that our protocol builds corridors, which enable throughput gains up to 74%.

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.

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.

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.

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.

State-of-the-art physical layers such as OFDMA are widely used in infrastructure-based networks to enhance efficiency in one-to-many transmissions. Application to wireless mesh networks is highly promising, as the diversity increases in many-to-many scenarios. While theoretical work on OFDMA for this scenario exists, it has not yet been implemented in practice. In this demonstration, we show practical measurements of OFDMA in a topology with multiple transmitters and receivers, which represents a fully-connected segment of a mesh network. Moreover, we model our system analytically and in simulation. Our measurements show the validity of these models. We demonstrate over 90% reduction of the symbol error rate and a 29% channel capacity increase.

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.

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.

The knowledge of the impact of imperfect channel state information (CSI) on data transmission techniques is helpful to improve the performance of wireless communication systems. In this paper, based on the proposed cooperative downlink (DL) transmission scheme applying adaptive scheduling of mobile stations (MSs), significant channel selection, and joint transmission (JT) with partial CSI, i.e., JT considering only part of the useful channels and the interference channels, a novel performance assessment is performed taking imperfectness of the CSI into consideration. The performance degradation caused by the imperfectness of the CSI which is used in the adaptive scheduling, the significant channel selection and the JT with partial CSI is investigated. The question about how much CSI should be taken into consideration as a function of the extent of imperfectness of the CSI in order to achieve optimum system performance is answered based on numerical results.

In this paper, the throughput of an adaptive multiuser SISO-OFDMA/FDD system with channel quality information (CQI) signalled digitized over a feedback channel to the transmitter is investigated, where the instantaneous signal-to-noise ratio (SNR) of the different subcarriers is used as CQI to exploit multi-user diversity using adaptive subcarrier allocation. The CQI available at the transmitter is assumed to be imperfect due to estimation errors and quantization at the receiver side, time delay and feedback errors. In this paper, a closed form expression of the average throughput of an adaptive multi-user OFDMA system using imperfect CQI and uncoded M-QAM modulation is derived. Furthermore, a closed form expression of the average throughput of an OFDMA system exploiting frequency diversity, which does not require CQI at the transmitter, is presented. Both throughput performances are compared in order to identify the optimal transmission strategy depending on the grade of CQI imperfectness.

In this paper, the performance of an adaptive multi-user OFDM system with imperfect channel knowledge at the transmitter is investigated, where the ergodic capacity is taken as performance criterion. This performance is then compared to the performance achievable with the use of diversity, where no channel knowledge at the transmitter is required. For that reason, different models of imperfect channel knowledge are introduced: noisy, outdated and quantised channel knowledge, where we provide an analytical derivation of the ergodic capacity for the combinations of the different models. By means of the ergodic capacity it is shown to which grade of channel knowledge imperfectness the use of adaptive subcarrier allocation with imperfect channel knowledge is still better than the use of diversity.