Christos G. Tsinos

Distributed Blind Hyperspectral Unmixing via Joint Sparsity and Low-Rank Constrained Non-Negative Matrix Factorization

Hyperspectral unmixing is a crucial processing step in remote sensing image analysis. Its aim is the decomposition of each pixel in a hyperspectral image into a number of materials, the so-called endmembers, and their corresponding abundance fractions. Among the various unmixing approaches that have been suggested in the literature, we are interested here in unsupervised techniques that rely on some form of non-negative Matrix factorization (NMF). NMF-based techniques provide an easy way to simultaneously estimate the endmembers and their corresponding abundances, though they suffer from mediocre performance and high computational complexity due to the nonconvexity of the involved cost function. Improvements in performance have been recently achieved by imposing additional constraints to the NMF optimization problem related to the sparsity of the abundances. Another feature of hyperspectral images that can be exploited is their high spatial correlation, which is translated into the low rank of the involved abundance matrices. Motivated by this, in this paper we propose a novel unmixing method that is based on a simultaneously sparse and low-rank constrained NMF. In addition, prompted by the rapid evolution of multicore processors and graphics processing units, we devise a distributed unmixing scheme that processes in parallel different parts of the image. The proposed distributed unmixing algorithm achieves improved performance and faster convergence than existing state-of-the-art techniques as it is verified by extensive simulations on synthetic and real hyperspectral data.

Hybrid analog-digital transceiver designs for cognitive radio millimiter wave systems

Recent advances in Milimeter wave (mmWave) band mobile communications may provide solutions to the increasing traffic demand in modern wireless systems. Even though mmWave bands are scarcely occupied, the design of a prospect transceiver should guarantee the efficient coexistence with the incumbent services in these bands. To that end, in this paper, two underlay cognitive transceiver designs are proposed based on a hybrid Analog/Digital transceiver architecture that enables the mmWave spectrum access while controlling the interference to the incumbent users with low hardware complexity and power consumption. The first cognitive solution designs a codebook free cognitive hybrid pre-coder by maximizing the mutual information between its two ends subject to interference, power and hardware constraints related to the analog counterpart. The second solution is codebook based and exhibits less complexity than the first one at the cost of inferior spectral efficiency. A novel codebook free solution for the post-coder at the cognitive receiver part is further proposed, based on a hardware constrained Minimum Mean Square Error criterion. Simulations study the performance of both the proposed hybrid approaches and compare it to the one of a fully digital solution for typical wireless environments.

On the Energy-Efficiency of Hybrid Analog–Digital Transceivers for Single- and Multi-Carrier Large Antenna Array Systems

Hybrid analog-digital transceivers are employed with the view to reduce the hardware complexity and the energy consumption in millimeter wave/large antenna array systems by reducing the number of their radio frequency (RF) chains. However, the analog processing network requires power for its operation and it further introduces power losses, dependent on the number of the transceiver antennas and RF chains that have to be compensated. Thus, the reduction in the power consumption is usually much less than it is expected and given that the hybrid solutions present in general inferior spectral efficiency than a fully digital one, it is possible for the former to be less energy efficient than the latter in several cases. Existing approaches propose hybrid solutions that maximize the spectral efficiency of the system without providing any insight on their energy requirements/efficiency. To that end, in this paper, a novel algorithmic framework is developed based on which energy efficient hybrid transceiver designs are derived and their performance is examined with respect to the number of RF chains and antennas. Solutions are proposed for fully and partially connected hybrid architectures and for both single- and multi-carrier systems under the orthogonal frequency division multiplexing modulation. Simulations and theoretical results provide insight on the cases, where a hybrid transceiver is the most energy efficient solution or not.

Simultaneous Sensing and Transmission for Cognitive Radios With Imperfect Signal Cancellation

In conventional cognitive radio systems, the secondary user employs a “listen-before-talk” paradigm, where it senses if the primary user is active or idle, before it decides to access the licensed spectrum. However, this method faces challenges, with the most important one being the reduction of the secondary user’s throughput, as no data transmission takes place during the sensing period. In this context, the idea of simultaneous spectrum sensing and data transmission is proposed. This paper studies a system model where this concept is obtained through the collaboration of the secondary transmitter with the secondary receiver. First, the secondary receiver decodes the signal from the secondary transmitter, removes it from the total received signal, and then carries out spectrum sensing in the remaining signal in order to determine the presence/absence of the primary user. Different from the existing literature, this paper considers the imperfect signal cancellation, evaluating how the decoding errors affect the sensing reliability, and derives the analytical expressions for the probability of false alarm. Finally, numerical results are presented illustrating the accuracy of the proposed analysis.

On-board the satellite interference detection with imperfect signal cancellation

Interference issues have been identified as a threat for satellite communication systems and services, resulting in throughput degradation and revenue loss to the satellite operators. In this context, an on-board spectrum monitoring unit (SMU) can be used to detect interference reliably. Current satellite SMUs are deployed on the ground and the introduction of an in-orbit SMU can bring several benefits, e.g. simplifying the ground based station in multibeam systems. This paper proposes a two-step algorithm for on-board interference detection, exploiting the frame structure of DVB-S2X standard, which employs pilot symbols for data transmission. Assuming that the pilot signal is known at the receiver, it can be removed from the total received signal. Then, an Energy Detection (ED) technique can be applied on the remaining signal in order to decide the presence or absence of interference. The simulation results show that the proposed technique outperforms the conventional ED in low interference-to-signal and noise ratios (ISNRs).

Carrier allocation for Hybrid Satellite-Terrestrial Backhaul networks

In this paper, we consider the problem of carrier allocation in Hybrid Satellite-Terrestrial Backhaul (HSTB) networks, where the satellite segment and the terrestrial backhaul network are integrated in a seamless manner. To enhance the overall spectral efficiency of the backhaul network, we consider that both terrestrial and satellite segments operate in the 17.7–19.7 GHz band, where the sharing between Fixed-Service (FS) microwave links and satellite communications is allowed. Due to sharing the same spectrum, both systems are subject to interference constraints which should be properly taken into account in the carrier allocation algorithm design. Focusing on sum-rate as the key performance indicator, we formulate the underlying optimization problem which tends to be NP-hard. To overcome this hurdle, we propose to tackle the satellite and the terrestrial carrier allocation in a sequential manner. The proposed algorithm is compared and validated using numerical results considering a realistic topology and system parameters.

Resource Allocation for Licensed/Unlicensed Carrier Aggregation MIMO Systems

The extension of long term evolution (LTE) networks in unlicensed spectrum areas under the licensed assisted access concept aims at achieving higher transmission rates via the aggregation of the aforementioned bands along with the licensed ones within the 3G Partnership Project framework. A prospect carrier aggregation (CA) scheme should handle efficiently the coexistence of the LTE systems that compete for the same unlicensed spectrum areas along with their incumbent users (i.e., Wi-Fi). In this paper, a novel CA scheme is proposed for licensed/unlicensed MIMO LTE systems that allocates optimally the resources (power and resource blocks) of an evolved Node B to user equipments. Furthermore, the proposed approach handles the coexistence matters within the unlicensed bands with an efficient decentralized way. The new scheme involves the solution to a mixed integer nonlinear programming problem and thus, an optimal low complexity method is proposed based on the Lagrange dual decomposition. Furthermore, the proposed technique is extended to the imperfect channel state information (CSI) case. To that end, a novel listen-before-talk scheme is developed via which the required unlicensed bands CSI are estimated in a blind manner. The performance of all of the proposed techniques is verified via indicative simulations.

On the energy-efficiency of hybrid analog-digital transceivers for large antenna array systems

Hybrid Analog-Digital transceivers are employed with the view to reduce the hardware complexity and the energy consumption in millimeter wave/large antenna array systems by reducing the number of their Radio Frequency (RF) chains. However, the analog processing network requires power for its operation and it further introduces power losses, dependent on the number of the transceiver antennas and RF chains, that have to be compensated. Thus, the reduction in the power consumption is usually much less than it is expected and given that the hybrid solutions present in general inferior spectral efficiency than a fully digital one, it is possible for the former to be less energy efficient than the latter in several cases. Existing approaches propose hybrid solutions that maximize the spectral efficiency of the system without providing any insight on their actual energy requirements/efficiency. To that end, in this paper, a novel algorithmic framework is developed based on which energy efficient hybrid transceiver designs are developed and their performance is examined with respect to the employed number of RF chains. Solutions are proposed for fully and partially connected hybrid architectures. Numerical results provide insight on when a hybrid transceiver is the most energy efficient solution or not.

Weak interference detection with signal cancellation in satellite communications

Interference is identified as a critical issue for satellite communication (SATCOM) systems and services. There is a growing concern in the satellite industry to manage and mitigate interference efficiently. While there are efficient techniques to monitor strong interference in SATCOM, weak interference is not so easily detected because of its low interference to signal and noise ratio (ISNR). To address this issue, this paper proposes and develops a technique which takes place on-board the satellite by decoding the desired signal, removing it from the total received signal and applying an Energy Detector (ED) in the remaining signal for the detection of interference. Different from the existing literature, this paper considers imperfect signal cancellation, examining how the decoding errors affect the sensing performance, derives the expressions for the probability of false alarm and provides a set of simulations results, verifying the efficiency of the technique.

Spectrum sharing in hybrid terrestrial-satellite backhaul networks in the Ka band

This work evaluates interference mitigation mechanisms to enable the operation of self-organizing hybrid terrestrial-satellite networks with aggressive frequency reuse between terrestrial and also satellite links. The objective is to take advantage of the improved capacity and resilience to congestion of such networks while assuring and efficient use of the spectrum. In particular, single- and multi-user hybrid analog-digital beamforming (HADB) techniques are considered as well as hybrid carrier allocation. The simulation results show network spectral efficiency improvements, with respect to conventional terrestrial backhaul systems, up to 2× when applying carrier allocation, and between 3.5× to 9× applying HADB in different environments.