The principles and benefits of soft decisions are well known and widely applied. The advantage of knowing that a decision is reliable or not is obvious. Belief propagation within a factor graph enables a unified use of soft information for both channel estimation and data detection. However, if soft information does not reflect the true reliability of a decision, the achievable performance may degrade. In this paper, the calculation of reliability information is refined to consider the event of unreliable soft decisions. The proposed solution is based on the mean bit error probability calculated after each iteration and integrates nicely within the existing factor graph. Simulation results are provided to illustrate the performance gains.

A graph-based receiver is presented that iteratively performs soft channel estimation and soft data detection. Reliability information of data symbols is utilized to improve channel estimation, and in turn, soft channel estimates refine data symbol estimates. The proposed multi-dimensional factor graph introduces transfer nodes that exploit correlation of adjacent channel coefficients in an arbitrary number of dimensions (e.g. time, frequency, and spatial domain). This establishes a simple and flexible receiver structure that facilitate soft channel estimation and data detection in multi-dimensional dispersive channels, and supports arbitrary modulation and channel coding schemes. Simulation results demonstrate that the proposed multi-dimensional graph-based receiver outperforms iterative and non-iterative state-of-the-art receivers.

Iterative receiver structures that jointly perform channel estimation and decoding promise substantial performance gains. However, these gains only materialize with sufficiently accurate initial channel estimates. In this paper, initialization by multi-objective particle swarm optimization (MOPSO) is investigated. MOPSO supports low-complexity initial channel estimation with superimposed training symbols. Furthermore, it is shown that MOPSO works well in rank-deficient scenarios with arbitrary training sequences. Numerical results validate the performance enhancement of MOPSO initialization integrated within a graph-based iterative receiver.

Femtocells were originally conceived to be deployed in private households and to solve the problem of poor indoor radio coverage as well as to offload traffic from the macrocellular network. The BeFEMTO project investigates and develops technologies that not only vastly improve the radio access capacity of femtocells, but also enables their use in novel scenarios like cooperative femtocell networks, outdoor relay femtocells and mobile femtocells. This paper presents the novel use cases and femtocell services considered in BeFEMTO as well as their enabling technologies and technical requirements.

Decentralized synchronization requires cooperation among network participants so that all nodes agree on a common reference timing. What happens if one node does not follow local synchronization rules and randomly transmits? This paper studies the resilience of two classes of decentralized slot synchronization against random disturbances, and quantifies the impact of the random behavior. It is shown that the coupling strength is a key factor for resilience, and that the synchronization approach based on the theory of coupled oscillators generally behaves better and is more robust than the approach that updates clocks based on the average neighboring timings.

This paper presents a biologically inspired approach for distributed slot synchronization in wireless networks. This is facilitated by modifying and extending a synchronization model based on the theory of pulse-coupled oscillators. The proposed Meshed Emergent Firefly Synchronization (MEMFIS) multiplexes synchronization words with data packets and adapts local clocks upon the reception of synchronization words from neighboring nodes. In this way, a dedicated synchronization phase is mitigated, as a network-wide slot structure emerges seamlessly over time as nodes exchange data packets. Simulation results demonstrate that synchronization is accomplished regardless of the arbitrary initial situation. There is no need for the selection of master nodes, as all nodes cooperate in a completely self-organized manner to achieve slot synchrony. Moreover, the algorithm is shown to scale with the number of nodes, works in meshed networks, and is robust against interference and collisions in dense networks.

In this paper decentralized power control in ad hoc networks is addressed, so to maintain a certain carrier-to- interference ratio (C/I) at the receiver. The objective is to extend the Pareto optimal C/I power control algorithm to time division duplex (TDD) links, where each node in the network transmits and receives data to/from other nodes. By exploiting the channel reciprocity of TDD, we demonstrate that Pareto optimal C/I power control can be implemented without any explicit feedback of the achieved link quality.

Decentralized slot synchronization protocols inspired from nature have recently gained considerable interest. By adjusting the internal time reference of a node in response to the detected timing of a received synchronization word, and by following simple rules, synchronization emerges from an initially completely uncoordinated situation. However these protocols typically assume that synchronization words are detected error free. In this paper the performance of biologically inspired slot synchronization is investigated when a realistic timing synchronization scheme is employed. We consider an ad hoc network where nodes communicate over an orthogonal frequency-division multiplexing (OFDM) air interface. The transmission is organized in frames, and each frame is preceded by a synchronization preamble with known repetitive parts. Specifically one common synchronization preamble is employed for all transmitting terminals, as, e.g., for the wireless LAN standard 802.11. Computer simulations verify that provided a sufficiently long synchronization word, reliable slot synchronization is maintained using a practical synchronization unit.

The present article develops a decentralized inter-base station slot synchronization algorithm suitable for cellular mobile communication systems. The proposed algorithm is derived from the theory of pulse-coupled oscillators, commonly used to describe synchronization phenomena in biological systems, e.g. spontaneous synchronization of fireflies. In order to maintain synchronized base stations (BSs), even when there is no direct link between them, some selected user terminals (UTs) participate in the network synchronization process. Synchronization emerges thanks to two synchronization words, one transmitted by BSs and the other by UTs, and each group helping the other synchronize. It is demonstrated through simulations that in a typical indoor environment, synchronization is always achieved within 10 super-frames when coupling parameters are appropriately chosen, without any a priori assumption on the initial timing misalignments of BSs and UTs.

This paper presents a new cross-layer approach to achieve active link adaptation and QoS support in ad hoc networks. Upon data reception in a transmission slot, receivers broadcast a feedback signal in a time-multiplexed mini-slot. The level of this feedback or busy signal is made dependent on the received signal from the intended transmitter and the required SINR (signal-to-interference-plus-noise ratio). Potential new interfering transmitters adjust their data transmit power based on the received busy signal power. It is demonstrated that due to the channel reciprocity in TDD (time division duplex) the received busy signal power is sufficient information to be able to maintain the SINR at the already existing links. Since multiple links are coupled via the busy signal, convergence and stability issues are analyzed.

This paper presents a fully decentralized link adaptation algorithm to manage interference in an ad hoc network operating in TDD mode. The algorithm, called busy tone interference tolerance signaling, is of low complexity and easy to implement. Two functions are explored to set the busy tone signal power dependent on the level of tolerable interference, and their performance is compared with a system with fixed transmit powers. Results show that an appropriately chosen function for setting the busy-tone exhibits significantly enhanced power efficiency, implying that battery life can be extended, while providing a similar capacity than a fixed power system.

In this paper a reference timing is imposed onto a network that synchronizes in a completely distributed manner. Synchronization is not always obtained because normal nodes are deaf while transmitting and cannot always receive synchronization messages from reference nodes. To counter this unwanted effect the transmission scheme of reference nodes is modified so that synchrony can be ensured and all network members agree on the imposed reference timing

A two-dimensional physical model of the human vocal tract is described. Such a system promises increased realism and control in the synthesis of both speech and singing. However, the parameters describing the shape of the vocal tract while in use are not easily obtained, even using medical imaging techniques, so instead a genetic algorithm (GA) is applied to the model to find an appropriate configuration. Realistic sounds are produced by this method. Analysis of these, and the reliability of the technique (convergence properties) is provided.