Ioannis Dagres

Flexible-Radio: A General Framework with PHY-layer algorithm-design insights

This is a tutorial paper on the concept of flexible/reconfigurable radio systems and, especially, on the elements of flexibility residing in the PHYsical layer (PHY). It reviews past research efforts towards flexible radios and introduces the various ways in which a reconfigurable transceiver can be used to provide multi-standard capabilities, link adaptation and user/service personalization. It overviews specific tools developed within two IST projects aiming at such flexible transceiver architectures and presents a specific example of a mode-selection algorithmic architecture which incorporates all the proposed tools. Thus, it serves to illustrate a baseband version of flexibility mechanisms

Algorithms and bounds for energy-based multi-source localization in log-normal fading

The problem of multi-source localization in lognormal fading is addressed. A Gaussian-Mixture Model (GMM) is proposed as an approximation of the measured signal strengths from various sensors in space, parameterized by the positions and the transmission (Tx) powers of the sources. The iterative Expectation-Maximization algorithm is invoked for the estimation of the relevant GMM parameter set. The complexity of the proposed algorithm grows only linearly with the number of sources, a major advantage in comparison with the direct Maximum Likelihood approach. When the number of sources is unknown, both the Akaike Information Criterion and the Minimum-Description-Length method are employed to estimate the proper number of sources. In addition, approximate Cramer-Rao lower bounds are derived for the estimation of the source positions and Tx powers. Simulations plus laboratory experiments are presented, which show fine agreement with theory.

Flexible radio: a framework for optimized multimodal operation via dynamic signal design

The increasing need for multimodal terminals that adjust their configuration on the fly in order to meet the required quality of service (QoS), under various channel/system scenarios, creates the need for flexible architectures that are capable of performing such actions. The paper focuses on the concept of flexible/reconfigurable radio systems and especially on the elements of flexibility residing in the PHYsical layer (PHY). It introduces the various ways in which a reconfigurable transceiver can be used to provide multistandard capabilities, channel adaptivity, and user/service personalization. It describes specific tools developed within two IST projects aiming at such flexible transceiver architectures. Finally, a specific example of a mode-selection algorithmic architecture is presented which incorporates all the proposed tools and, therefore, illustrates a baseband flexibility mechanism.

Decision-directed least-squares phase perturbation compensation in OFDM systems

A low-complexity decision-directed iterative scheme is proposed for the estimation and mitigation of strong phase noise plus frequency offset in OFDM-modulated signals, transmitted over frequency-selective or frequency-flat channels. It is based on a time-domain, windowed least-squares estimation algorithm of the total phase perturbation vector, developed through proper system modeling and optimized so as to minimize the residual inter-carrier interference. An extension of the diagonal-loading technique is also introduced in order to enhance the convergence of the algorithm in high-SNR regimes. The resulting scheme outperforms previously proposed approaches in terms of achievable error rate in the data-detection stage while, in the channel-estimation stage, it attains performance near the Cramer-Rao bound with comparatively smaller computational complexity.

Robust Phase Noise Compensation in OFDM via a Dual-Model Approach

A low-complexity, decision-directed, iterative scheme is proposed for severe phase noise mitigation in OFDM modulation over frequency-selective as well as flat channels. The proposed approach relies on two modeling steps: first, an alternative model formulation in the time domain; second, a maximum likelihood (ML)-inspired estimator which incorporates SNR-maximizing concepts. The resulting scheme is shown to outperform already proposed approaches in terms of achievable error rate and to do so with comparatively reduced complexity.

Power-based localization in correlated log-normal fading aided by conditioning measurements

The paper addresses the performance evaluation via the Cramer-Rao Lower Bound (CRLB) of power-based localization of a source in spatially-correlated log-normal propagation. The novel element is the inclusion and assessment of the impact of conditioning measurements on such performance. The proposed model parameterizes performance by both the sensor topology (density, positioning) producing the current measurements as well as by conditioning measurements (essentially, prior or training data) which reduce the statistical uncertainty in the model. Experimental results for indoor and outdoor environments are presented which quantify the expected localization accuracy, as well as identify practical issues to be further addressed. One main concern is the quantification of scaling on required sensor-network size for achieving a pre-specified localization accuracy.

Heterogeneous and opportunistic wireless networks

Recent years have witnessed the evolution of a large plethora of wireless technologies with different characteristics, as a response of the operators’ and users’ needs in terms of an efficient and ubiquitous delivery of advanced multimedia services. The wireless segment of network infrastructure has penetrated in our lives, and wireless connectivity has now reached a state where it is considered to be an indispensable service as electricity or water supply. Wireless data networks grow increasingly complex as a multiplicity of wireless information terminals with sophisticated capabilities get embedded in the infrastructure.

Bit-power loading algorithms based on Effective SINR Mapping techniques

Bit-Power Loading (BPL) algorithms are proposed based on Effective SINR Mapping (ESM) techniques. The ESM techniques used are the Exponential ESM (EESM) as well as the Mean Mutual Information per Bit (MMIB). The algorithms are based on proper usage and modifications of algorithms already proposed in the literature, applied to ESM compatible link-level performance models. The performance gain when using the proposed algorithms is assessed for an IEEE 802.16 OFDM system.