Daniel Bielefeld

Optimization of cooperative spectrum sensing and implementation on software defined radios

Reliable detection of primary user activity by spectrum sensing is a crucial issue of cognitive radio systems. The objective of cooperative spectrum sensing is to combine the detection results of multiple cognitive radios in order to maximize the probability of detecting unused spectrum while meeting a required reliability of detecting primary user activity. In this paper, a Kullback-Leibler distance-based optimization approach for the local decision thresholds of cooperative spectrum sensing is proposed. It is both computationally efficient and scalable with the number of cognitive radios. To validate the concept, real spectrum sensing results are used. The employed practical setup is based on software defined radio and detects a WiMAX-like OFDM signal. The presented numerical results illustrate the feasibility and effectiveness of the approach.

Topology Generation and Power Assignment in IR-UWB Networks

In this paper, two different power assignment strategies for an ultra-wideband impulse radio (IR-UWB) multi-user system are presented and compared in terms of computational complexity. The algorithms aim at finding the lowest possible transmission power for each node in the network under the constraint that individual QoS demands of all nodes are met. The first direct approach results in a high computational complexity contradicting the intention of building low complexity IR-UWB transceivers. We show how the complexity of the power assignment problem can be substantially lowered by analytical means if the network topology is taken into consideration. Additionally, an algorithm creating the necessary network topology is discussed.

Power-aware distributed detection in IR-UWB sensor networks

The interplay between signal processing and wireless networking plays a crucial role in sensor networks deployed for detection and estimation applications. In this paper, an opportunistic power assignment strategy for IR-UWB sensor networks is presented which is designed to optimize detection performance in terms of the global probability of error. The opportunistic power assignment strategy utilizes both the detection error probabilities of individual sensors as well as network topology information, leading to significant performance gains compared to uniform power assignment.

Optimal rate and power allocation for OFDM in the presence of channel uncertainty

Adaptive modulation of subcarriers for orthogonal frequency division multiplexing (OFDM) has been shown to improve system performance significantly. As a prerequisite, accurate channel state information (CSI) must be available at the transmitter. For time varying, noisy channels, however, this is difficult to achieve in practical systems. In this paper, we consider optimal subcarrier assignment under incomplete CSI. Soft estimates and the Cramer-Rao bound are used to analyze the effect of CSI errors. As an important control parameter, the frame length is coming in. We investigate analytically its influence on the performance of adaptive modulation in OFDM systems for the rate and power optimization problems. Simulations demonstrate that the frame length may not be neglected as an important overall system performance parameter.

Power-Aware Sensor Selection for Distributed Detection in Wireless Sensor Networks

The limited energy of sensor nodes in wireless sensor networks strongly recommends power-aware design methodolo- gies. In this paper, a power-aware sensor selection strategy for wireless sensor networks is presented that is especially designed for distributed detection with soft decision fusion. The objective is to minimize the global probability of error at the fusion center under a total network power constraint. The cross-layer approach for the selection of a proper subset of sensors is based on a measure of individual sensor detection quality as well as location information. It corresponds to a low-complexity power allocation algorithm and enables significant performance gains in terms of reduction of the global probability of error compared to the inclusion of all sensors.

Power Allocation and Node Clustering for Distributed Detection in IR-UWB Sensor Networks

In wireless sensor networks, the limited energy of the nodes should be utilized in such a way that the performance measure of the sensing application is optimized. In this paper, power-aware design of IR-UWB sensor networks for distributed signal detection is discussed. The design approach consists of two parts which aim at minimizing the global probability of detection error. First, an application-specific node clustering algorithm is performed. Based on the generated topology a resource allocation scheme adapted to distributed detection is carried out. It is based on both, information from the network topology and individual sensor detection quality. Numerical results indicate significant performance gains for sensor networks with realistic resource constraints. I. INTRODUCTION The initial task of many applications of wireless sensor networks is the detection of targets in a region of interest. In distributed detection, the sensor nodes process their observa- tions locally and make preliminary decisions about the state of the monitored environment. The local decisions are transmitted to a fusion center where they are combined to obtain a final detection result with high reliability. In practice, the detection performance of wireless sensor networks is influenced by resource constraints like limited available energy or a restricted maximum transmission range. Hence, resource allocation and networking algorithms should be adapted to the detection application (1) in order to optimally design the sensor system. In the parallel fusion network, where all sensor nodes transmit their local decisions directly to a fusion center, the maximum area that can be covered is limited by the maximum transmission range of a node. The covered area can be extended by a clustering of the network into a tree structure resulting in hierarchical transmission of node decisions. This requires algorithms that perform the clustering. In the litera- ture, several algorithms with different optimization objectives and different complexity have been suggested. The authors of (2) consider a TDMA scheme. In (3) two algorithms are presented, which are combined with an impulse radio ultra- wideband (IR-UWB) specific multiple access scheme with non-orthogonal channels. Usually, clustering algorithms are not adapted to specific applications. The asymptotic detection behavior of a tree network for distributed detection has been analyzed in (4). However, for realistic numbers of sensor nodes, these asymptotic results provide only limited informa- tion. In this paper, we present an algorithm which considers the interdependency between energy consumption and the over- all detection performance by including the individual sensor detection performance in the process of cluster head elec- tion and cluster formation. Based on the generated topology, we furthermore suggest an application-specific assignment of transmission power levels that depends both on individual sensor qualities as well as the generated topology. It aims at minimizing the global probability of detection error given a budget of transmission power. As enabling technology for wireless sensor networks, we consider IR-UWB transceivers. Due to the possibility of power control, IR-UWB transceivers are well suited to adapt networking algorithms to specific applications. Compared to our preceding work (5), where the reduction of transmission energy for a given detection performance is analyzed, we ask the complementary ques- tion of how much the probability of detection error can be decreased given a fixed total power budget. Furthermore, we conduct a direct comparison of the detection performance of the parallel and the tree network with and without limitations of the transmission range, which reveals which topology is advantageous in which parameter range. Moreover, a trade- off between the power budget and the number of nodes is discussed.

Cross-layer design of cluster formation and power allocation in IR-UWB sensor networks

Wireless sensor networks are usually deployed for specific purposes. A cross-layer design of networking algorithms adapted to the considered application can significantly improve overall performance metrics. In this paper, distributed signal detection in IR-UWB sensor networks with severe resource constraints is considered. The presented algorithms for power assignment aim at minimizing the energy necessary to meet an overall detection performance by exploiting dependencies between application and physical layer. To account for a limited transmission range of the sensor nodes, the approach is furthermore combined with an application-specific formation of node clusters which allows for a hierarchical transmission of the sensor decisions to a fusion center. Numerical results validate that this cross-layer approach leads to substantial energy savings compared to uniform power assignment.

Distributed power and topology control for ir-uwb sensor networks

To extend the lifetime of wireless sensor networks a power-aware design of the communication units is required. This can be accomplished by employing power-controllable impulse radio ultra-wideband (IR-UWB) transceivers. In this paper, two distributed power control algorithms are suggested that are based on a variation of IR-UWB specific parameters. The iterative algorithms converge to the optimal transmit power levels for all nodes of a given network topology. The global transmission power can be further reduced by the construction of a network hierarchy. For this purpose a topology control algorithm is proposed that can be combined with the power control procedures. This combination realises an efficient application specific multiple access scheme for IR-UWB sensor networks.

Cross-Layer design of IR-UWB sensor networks for distributed detection applications

Cross-layer design for wireless networks aims at optimizing system-wide performance measures by exploiting dependencies between different network layers. In this paper, an opportunistic power assignment algorithms for IR-UWB sensor networks is presented that is especially designed for distributed signal detection under resource constraints. Specifically, the objective is to minimize the global probability of error of distributed detection systems, given a fixed level of total transmission power. The cross-layer approach for the allocation of transmission power is based on individual sensor detection quality as well as location information. It leads to significant performance gains compared to uniform power assignment for both the parallel and the serial sensor network topology.

Distributed Detection in UWB Sensor Networks under Non-Orthogonal Nakagami-m Fading

Several attractive features of ultra wideband (UWB) communications make it a good candidate for physical-layer of wireless sensor networks (WSN). These features include low power consumption, low complexity and low cost of implementation. In this paper, we present an opportunistic power assignment strategy for distributed detection in parallel fusion WSNs, considering a Nakagami-m fading model for the communication channel and time-hopping (TH) UWB for the transmitter circuit of the sensor nodes. In a parallel fusion WSN, local decisions are made by local sensors and transmitted through wireless channels to a fusion center. The fusion center processes the information and makes the final decision. Simulation results are provided for the global probability of detection error and relative performance gain to evaluate the efficiency of the proposed power assignment strategy in different fading environments.