References of "Gharanjik, Ahmad 50001845"
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See detailLearning-based rainfall estimation via communication satellite links
Gharanjik, Ahmad UL; Mishra, Kumar Vijay; Shankar, Bhavani UL et al

in 2018 IEEE Statistical Signal Processing Workshop (SSP) (2018)

We present a method for estimating rainfall by opportunistic use of Ka-band satellite communication network. Our approach is based on the attenuation of the satellite link signal in the rain medium and ... [more ▼]

We present a method for estimating rainfall by opportunistic use of Ka-band satellite communication network. Our approach is based on the attenuation of the satellite link signal in the rain medium and exploits the nearly linear relation between the rain rate and the specific attenuation at Ka-band frequencies. Although our experimental setup is not intended to achieve high resolutions as millimeter wavelength weather radars, it is instructive because of easy availability of millions of satellite ground terminals throughout the world. The received signal is obtained over a passive link. Therefore, traditional weather radar signal processing to derive parameters for rainfall estimation algorithms is not feasible here. We overcome this disadvantage by employing neural network learning algorithms to extract relevant information. Initial results reveal that rainfall accumulations obtained through our method are 85% closer to the in situ rain gauge estimates than the nearest C-band German weather service Deutscher Wetterdienst (DWD) radar. [less ▲]

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See detailCoverage Extension via Side-Lobe Transmission in Multibeam Satellite System
Gharanjik, Ahmad UL; Kmieciak, Jarek; Shankar, Bhavani UL et al

in 23rd Ka and Broadband Communications Conference (2017, October 16)

In this paper, we study feasibility of coverage extension of a multibeam satellite network by providing low-rate communications to terminals located outside the coverage of main beams. Focusing on the MEO ... [more ▼]

In this paper, we study feasibility of coverage extension of a multibeam satellite network by providing low-rate communications to terminals located outside the coverage of main beams. Focusing on the MEO satellite network, and using realistic link budgets from O3b networks, we investigate the performance of both forward and return-links for terminals stationed in the side lobes of the main beams. Particularly, multi-carrier transmission for forward-link and single carrier transmission for return-link are examined and the resulting coverage and data rate for different setups are evaluated. Simulation results verifies that side-lobe transmission can extend the coverage area and provide considerable data rate, thereby providing a solution for enhancing capacity of existing networks. [less ▲]

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See detailCentralized Rainfall Estimation using Carrier-to-Noise of Satellite Communication Links
Gharanjik, Ahmad UL; Shankar, Bhavani UL; Zimmer, Frank et al

in IEEE Journal on Selected Areas In Communications (2017)

In this paper, we present a centralized method for real-time rainfall estimation using carrier-to-noise power ratio (C/N) measurements from broadband satellite communication networks. The C/N data of both ... [more ▼]

In this paper, we present a centralized method for real-time rainfall estimation using carrier-to-noise power ratio (C/N) measurements from broadband satellite communication networks. The C/N data of both forward link and return link are collected by the gateway station from the user terminals in the broadband satellite communication network and stored in a database. The C/N for such Ka-band scenarios is impaired mainly by the rainfall. Using signal processing and machine learning techniques, we develop an algorithm for real-time rainfall estimation. Extracting relevant features from C/N, we use artificial neural network in order to distinguish the rain events from dry events. We then determine the signal attenuation corresponding to the rain events and examine an empirical relationship between rainfall rate and signal attenuation. Experimental results are promising and prove the high potential of satellite communication links for real environment monitoring, particularly rainfall estimation. [less ▲]

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See detailMax-min transmit beamforming via iterative regularization
Gharanjik, Ahmad UL; Shankar, Bhavani UL; Soltanalian, Mojtaba et al

in 50th Asilomar Conference on Signals, Systems and Computers, November 2016 (2016, November)

This paper introduces an iterative optimization framework to tackle the multi-group multicast Max-Min transmit beamforming problem. In each iteration, the optimization problem is decomposed into four sub ... [more ▼]

This paper introduces an iterative optimization framework to tackle the multi-group multicast Max-Min transmit beamforming problem. In each iteration, the optimization problem is decomposed into four sub-problems, all of which can be solved using computationally efficient algorithms. The advantage of proposed method lies in its ability to handle different types of signal constraints like total power and unimodularity; a feature not exhibited by other techniques. The proposed technique outperforms the well-known semidefinite relaxation method in terms of quality of solutions [less ▲]

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See detailAn Iterative Approach to Nonconvex QCQP with Applications in Signal Processing
Gharanjik, Ahmad UL; Shankar, Bhavani UL; Soltanalian, Mojtaba et al

in 2016 IEEE Sensor Array and Multichannel Signal Processing Workshop (SAM) (2016, July 10)

This paper introduces a new iterative approach to solve or to approximate the solutions of the nonconvex quadratically constrained quadratic programs (QCQP). First, this constrained problem is transformed ... [more ▼]

This paper introduces a new iterative approach to solve or to approximate the solutions of the nonconvex quadratically constrained quadratic programs (QCQP). First, this constrained problem is transformed to an unconstrained problem using a specialized penalty-based method. A tight upper-bound for the alternative unconstrained objective is introduced. Then an efficient minimization approach to the alternative unconstrained objective is proposed and further studied. The proposed approach involves power iterations and minimization of a convex scalar function in each iteration, which are computationally fast. The important design problem of multigroup multicast beamforming is formulated as a nonconvex QCQP and solved using the proposed method. [less ▲]

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See detailTRANSMISSION OPTIMIZATION FOR HIGH THROUGHPUT SATELLITE SYSTEMS
Gharanjik, Ahmad UL

Doctoral thesis (2016)

Demands on broadband data service are increasing dramatically each year. Following terrestrial trends, satellite communication systems have moved from the traditional TV broadcasting to provide ... [more ▼]

Demands on broadband data service are increasing dramatically each year. Following terrestrial trends, satellite communication systems have moved from the traditional TV broadcasting to provide interactive broadband services even to urban users. While cellular and land-line networks are mainly designed to deliver broadband services to metropolitan and large urban centers, satellite based solutions have the advantage of covering these demands over a wide geography including rural and remote users. However, to stay competitive with economical terrestrial solutions, it is necessary to reduce the cost per transmitted bit by increasing the capacity of the satellite systems. The objective of this thesis is to design and develop techniques capable of enhancing the capacity of next generation high throughput satellite systems. Specifically, the thesis focuses on three main topics: 1) Q/V band feeder link design, 2) robust precoding design for multibeam satellite systems, and 3) developing techniques for tackling related optimization problems. Design of high bandwidth and reliable feeder links is central towards provisioning new services on the user link of a multibeam SatCom system. Towards this, utilization of the Q/V band and an exploitation of multiple gateway as a transmit diversity measure for overcoming severe propagation effects are being considered. In this context, the thesis deals with the design of a feeder link comprising N + P gateways (N active and P redundant gateways). Towards satisfying the desired availability, a novel switching scheme is analyzed and practical aspects such as prediction based switching and switching rate are discussed. Building on this result, an analysis for the N + P scenario leading to a quantification of the end-to-end performance is provided. On the other hand, frequency reuse in multibeam satellite systems along with precoding techniques can increase the capacity at the user link. Similar to terrestrial communication channels, satellite based communication channels are time-varying and for typical precoding applications, the transmitter needs to know the channel state information (CSI) of the downlink channel. Due to fluctuations of the phase components, the channel is time-varying resulting in outdated CSI at the transmitter because of the long round trip delay. This thesis studies a robust precoder design framework considering requirements on availability and average signal to interference and noise ratio (SINR). Probabilistic and expectation based approaches are used to formulate the design criteria which are solved using convex optimization tools. The performance of the resulting precoder is evaluated through extensive simulations. Although a satellite channel is considered, the presented analysis is valid for any vector channel with phase uncertainty. In general, the precoder design problem can be cast as power minimization problem or max-min fairness problem depending on the objectives and requirements of design. The power minimization problem can typically be formulated as a non-convex quadratically constrained quadratic programming (QCQP) problem and the max-min fairness problem as a fractional quadratic program. These problems are known to be NP-hard in general. In this thesis, the original design problem is transformed to an unconstrained optimizationproblem using the specialized penalty terms. The efficient iterative optimization frameworks are proposed based on a separate optimization of the penalized objective function over its partition of variables at each iteration. Various aspects of the proposed approach including performance of the algorithm and its implementation complexity are studied. This thesis is made under joint supervision agreement between KTH Royal Institute of Technology, School of Electrical Engineering, Stockholm, Sweden and University of Luxembourg, Luxembourg. [less ▲]

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See detailGrab-n-Pull: An Optimization Framework for Fairness-Achieving Networks
Soltanalian, Mojtaba UL; Gharanjik, Ahmad UL; Shankar, Bhavani UL et al

in IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP) 2016 (2016, March 20)

In this paper, we present an optimization framework for designing precoding (a.k.a. beamforming) signals that are instrumental in achieving a fair user performance through the networks. The precoding ... [more ▼]

In this paper, we present an optimization framework for designing precoding (a.k.a. beamforming) signals that are instrumental in achieving a fair user performance through the networks. The precoding design problem in such scenarios can typically be formulated as a non-convex max-min fractional quadratic program. Using a penalized version of the original design problem, we derive a simplified quadratic reformulation of the problem in terms of the signal (to be designed). Each iteration of the proposed design framework consists of a combination of power method-like iterations and the Gram-Schmidt process, and as a result, enjoys a low computational cost. Moreover, the suggested approach can handle various types of signal constraints such as total-power, per-antenna power, unimodularity, or discrete-phase requirements—an advantage which is not shared by other existing approaches in the literature. [less ▲]

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See detailPrecoding Design and User Selection for Multibeam Satellite Channels
Gharanjik, Ahmad UL; Shankar, Bhavani UL; Arapoglou, Pantelis-Daniel UL et al

in 16th IEEE International Workshop on Signal Processing Advances in Wireless Communications, SPAWC 2015 (2015, June 28)

Precoding for the downlink of a multibeam satellite system has been recently shown, under ideal conditions, to be promising technique towards employing aggressive frequency reuse gainfully. However, time ... [more ▼]

Precoding for the downlink of a multibeam satellite system has been recently shown, under ideal conditions, to be promising technique towards employing aggressive frequency reuse gainfully. However, time varying phase uncertainties imposed by the components and the channel, combined with delayed feedback perturbs the channel state information at the transmitter (CSIT). In this paper, we consider a power constrained robust formulation of the downlink precoding problem to counter the phase uncertainties. In particular it considers imposing conditions on the average signal to interference plus noise ratio (SINR), to deal with imperfect CSIT. In addition to the robust formulation, the primacy of user selection is highlighted and a new approach exploiting the satellite system design is proposed. Performance of the derived robust precoder in conjunction with the proposed location based user selection is then evaluated and the gains are tabulated. [less ▲]

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See detailRobust Precoding Design for Multibeam Downlink Satellite Channel with Phase Uncertainty
Gharanjik, Ahmad UL; Shankar, Bhavani UL; Arapoglou, Pantelis-Daniel UL et al

in 40th IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP) 2015 (2015, April 19)

In this work, we study the design of a precoder on the user downlink of a multibeam satellite channel. The variations in channel due to phase noise introduced by on-board oscillators and the long round ... [more ▼]

In this work, we study the design of a precoder on the user downlink of a multibeam satellite channel. The variations in channel due to phase noise introduced by on-board oscillators and the long round trip delay result in outdated channel information at the transmitter. The phase uncertainty is modelled and a robust design framework is formulated based on availability and power constraints. The optimization problem is cast into the convex paradigm after approximations and the benefits of the resulting precoder are highlighted. [less ▲]

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See detailMultiple Gateway Transmit Diversity in Q/V Band Feeder Links
Gharanjik, Ahmad UL; Shankar, Bhavani UL; Arapoglou, Pantelis-Daniel et al

in IEEE Transactions on Communications (2015)

Design of high bandwidth and reliable feeder links is central towards provisioning new services on the user link of a multibeam satellite communication (SatCom) system. Towards this, utilization of the Q ... [more ▼]

Design of high bandwidth and reliable feeder links is central towards provisioning new services on the user link of a multibeam satellite communication (SatCom) system. Towards this, utilization of the Q/V band and an exploitation of multiple gateways (GW) as a transmit diversity measure for overcoming severe propagation effects are being considered. In this context, this contribution deals with the design of a feeder link comprising N + P GWs (N active and P redundant GWs). Towards provisioning the desired availability, a novel switching scheme is analysed and practical aspects such as prediction based switching and switching rate are discussed. Unlike most relevant works, a dynamic rain attenuation model is used to derive analytically average outage probability in the fundamental 1 + 1 gateway case. Building on this result, an analysis for the N +P scenario leading to a quantification of the end􀀀to􀀀end performance is provided. This analysis aids system sizing by illustrating the interplay between the number of active and redundant gateways on the chosen metrics : average outage and average switching rate. [less ▲]

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See detailSpatial Multiplexing in Optical Feeder Links for High Throughput Satellites
Gharanjik, Ahmad UL; Liolis, Konstantinos; Shankar, Bhavani UL et al

in Spatial Multiplexing in Optical Feeder Links for High Throughput Satellites (2014, December 02)

Optical feeder links are an attractive alternative to the RF feeder links in satellite communications (SatCom). In this paper, we present initial results from an optical feeder link study. We discuss the ... [more ▼]

Optical feeder links are an attractive alternative to the RF feeder links in satellite communications (SatCom). In this paper, we present initial results from an optical feeder link study. We discuss the architecture of a geostationary earth orbit (GEO) satellite system based on optical feeder links. To mitigate the effects of cloud coverage, which is the main availability-limiting factor, Optical Ground Station (OGS) diversity is employed. Moreover, a spatial multiplexing scheme is considered. Assuming an ON-OFF channel model, the number of required OGSs to ensure availability and throughput requirements is analyzed [less ▲]

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See detailLarge Scale Transmit Diversity in Q/V Band Feeder Link with Multiple Gateways
Gharanjik, Ahmad UL; Shankar, Bhavani UL; Arapoglou, Pantelis-Daniel UL

in Proceeding of IEEE 24th International Symposium on Personal, Indoor, and Mobile Radio Communication(PIMRC) (2013, September 08)

Exploiting transmit diversity amid a high number of multiple gateways (GW) is a new research challenge in Q/V band satellite communication providing data rates of hundreds of Gbit/s. In this paper, we ... [more ▼]

Exploiting transmit diversity amid a high number of multiple gateways (GW) is a new research challenge in Q/V band satellite communication providing data rates of hundreds of Gbit/s. In this paper, we propose a practical switching strategy in a scenario with N +P GWs (N active and P redundant GWs) towards achieving GW transmit diversity. Differently from other works, the treatment in this paper is analytical and explores two key factors: outage performance and switching rate in detail. Further, the interplay between the number of redundant and active GWs on the availability is illustrated highlighting the contribution of the work towards system sizing. [less ▲]

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See detailGateway Switching in Q/V Band Satellite Feeder Links
Gharanjik, Ahmad UL; Shankar, Bhavani UL; Arapoglou, Pantelis-Daniel UL et al

in IEEE Communications Letters (2013), 17(7), 1384-1387

A main challenge towards realizing the next generation Terabit/s broadband satellite communications (SatCom) is the limited spectrum available in the Ka band. An attractive solution is to move the feeder ... [more ▼]

A main challenge towards realizing the next generation Terabit/s broadband satellite communications (SatCom) is the limited spectrum available in the Ka band. An attractive solution is to move the feeder link to the higher Q/V band, where more spectrum is available. When utilizing the Q/V band, due to heavy rain attenuation, gateway diversity is considered a necessity to ensure the required feeder link availability. Although receive site diversity has been studied in the past for SatCom, there is much less maturity in terms of transmit diversity techniques. In this paper, a modified switch and stay combining scheme is proposed for a Q/V band feeder link, but its performance is also evaluated over an end-to-end satellite link. The proposed scheme is pragmatic and has close to optimal performance with notably lower complexity. [less ▲]

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