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  • 1.
    Carlson, Johan E.
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Andren, Erik
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Mohamad, Medhat
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Beek, Jaap van de
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Monitoring Changes in Mechanical Properties of Rock Bolts Using a Low-Power Coded-Excitation Scheme2018In: 2018 IEEE INTERNATIONAL ULTRASONICS SYMPOSIUM (IUS), IEEE, 2018, article id 8580038Conference paper (Refereed)
    Abstract [en]

    In-situ inspection and quality assessment of rock bolts and other load-bearing structural elements, has received significant attention over the years, but there are still no techniques available capable of monitoring changes over time in mechanical properties of already installed bolts. Since mechanical changes will also affect the propagation of mechanical waves, ultrasound is a strong candidate. In this paper we propose a technique based on Orthogonal Frequency Division Multiplexing (OFDM) for coding the transmitted ultrasound pulse in such a way that the propagation channel (transducer and rock bolt combined) can be estimated. We show that a transmit voltage of 1.1 V r.m.s. is sufficient to obtain reliable channel estimates even in bolts as long as one meter. The channel estimates are then used to predict changes in tensile stress on the bolt, from experiments conducted in a laboratory environment.

  • 2.
    Carlson, Johan
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    van de Beek, Jaap
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Mohamad, Medhat
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Mbit/second Communication through a Rock Bolt Using Ultrasound2017Conference paper (Refereed)
    Abstract [en]

    In most industrial processes transfer of data and software from or to sensors is an essential part of the monitoring and control systems. Many of the older, wired communication systems have been or are being replaced with wireless alternatives. A number of challenges are associated with this replacement: Radio receivers are subject to interference from other radio sources. Similarly, radio transmitters may cause undesired interference into other equipment and environments. Compared to wired solutions, security becomes an issue as radio communication links are more vulnerable to eavesdropping than wired schemes. Radio communication with sensors and sensor platforms embedded deep inside large metal structures or fluid tanks may be difficult or even impossible. The objective of this work is to develop and evaluate a high data-rate communications scheme based on ultrasound, which can be used to transmit wirelessly through solid structures. An example will be given using a one-meter segment of a rock bolt.

  • 3.
    Carlson, Johan
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    van de Beek, Jaap
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Mohamad, Medhat
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Mbit/second Communication through a Rock Bolt Using Ultrasound2017In: IEEE International Ultrasonics Symposium, IUS, Piscataway, NJ: IEEE Computer Society, 2017, article id 8092474Conference paper (Refereed)
    Abstract [en]

    This paper presents a digital communication system based on Orthogonal Frequency Division Multiplexing (OFDM) and MHz-range ultrasound, for transmission of data through solid materials and liquids. The system is tested on experiments using a off-the-shelf ultrasound transducers with a center frequency of 3.5 MHz as transmitter and receiver, respectively. The propagation medium in the experiment was a 1 m long section of an epoxy coated steel rock bolt. The results show that data rates in excess of 1 Mbit/second is attainable, using readily available hardware and software.

  • 4.
    Mohamad, Medhat
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Activity: An SDR-based prototype of spectrally precoded OFDM2016Conference paper (Other (popular science, discussion, etc.))
    Abstract [en]

    OFDM scheme possesses high OOB emission which degrades the spectral efficiency of OFDM systems. Poor spectral efficiency limits the validity of OFDM as a candidate for 5G. Spectrally precoded OFDM is a recent approach that treats the OFDM signal to suppress the out of band (OOB) emission. Although the theoretical results of spectral precoding show very efficient OOB emission suppression (more than 50 dB reduction compared to plain OFDM), practically, hardware impairments limit its performance.We, at Luleå University of Technology, examine the practicality of the suggested approach and show for the first time through this demonstration that despite the hardware impairments, implemented spectral precoded OFDM has OOB emission tens of decibels less than that of plain OFDM. We set up and examine a communication link through which we approve that communication systems are less vulnerable to interference coming from spectrally precoded OFDM signals than to interference coming from plain OFDM signals.

  • 5.
    Mohamad, Medhat
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering.
    Design, Analysis and Prototyping of Spectrally Precoded OFDM2016Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Despite shifting towards mm-wave bands, the sub 6-GHz band will continue tobe a fundamental spectral band in 5G. Yet, the severe crowdedness of this bandmakes a well constrained spectrum one of the critical 5G requirements. A wellconstrained spectrum means that the communications regimes should dwell politelywithin their dedicated spectral bands and not interfere with other systems workingon neighboring bands. Consequently, communications community seeks convenientmodulation schemes.Accordingly, high Out Of Band (OOB) emission phenomenon in Orthogonal Fre-quency Division Multiplexing (OFDM) is unfavorable for some regimes operating in5G. Therefore, to legitimize OFDM with all 5G regimes, we need to suppress OFDMOOB emission.Since the discontinuous nature of the OFDM signal is the main reason for the highOOB emission, one solution is to render the discontinuous OFDM signal continuous.Two factors control this discontinuity: the physical shape of the modulated signaland the correlation property of the data symbols that modulate the OFDM signal.While most of the traditional approaches focus on reshaping the OFDM signalto render it continuous, in this work we give our attention to the spectral precod-ing approaches. These approaches manipulate the correlation property of the datasymbols to control the high OOB emission in OFDM.On the other hand, tweaking the correlation property of the modulating datasymbols will violate their orthogonality. This violation will yield in-band interfer-ence within the OFDM signal which would degrade the bit error performance of thereceived data.The thesis explains the spectral precoding techniques from conceptual and math-ematical point of view. We discuss the OOB emission suppression capability of theprecoding techniques and study their drawbacks and limitations. We provide ana-lytical trade o study between precoding approaches and classical OFDM treatmentapproaches at the level of OOB emission suppression and in-band interference. Weshow that the in-band interference in precoding techniques is independent on thecommunications channel behavior contrary to that of classical techniques. More-over, we dene the optimal precoder that minimizes the in-band interference. Con-sequently, we design a novel practical precoder that approaches the performanceof the optimal precoder. Furthermore, we analyze the complexity of the precodingapproaches and study the implementation computational requirements.Finally, we test the real time performance of these precoding techniques usingSoftware Designed Radio (SDR) Universal Software Radio Peripherals (USRPs). Wespotlight the hardware limitations and show that despite these limitations, the spec-tral precoder is able to suppress the OOB emissions by tens of decibels. We check the reliability of spectral precoding in practical over air communications systems bysetting up the rst spectral precoding proof of concept prototype. The prototypeproves that precoded OFDM systems cause less OOB interference on neighboringcommunications systems.

  • 6.
    Mohamad, Medhat
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Spectrally Precoded OFDM-Design and Analysis2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Despite shifting towards mm-wave bands, the sub-6 GHz band will continue to be a fundamental spectral band in 5G. However, the severe congestion in this band makes a well-constrained spectrum a critical requirementof 5G. A well-constrained spectrum means that the communications regimes should dwell within their dedicated spectral bands and do not interfere with other systems working on neighboring bands. Consequently, the communi-cations community seeks convenient modulation schemes.

    The 5G standards have already defined cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) as the modulation scheme for most of the sub-6 GHz band. However, the high out-of-band (OOB) emission phe-nomenon in OFDM is unfavorable for some regimes operating in 5G. Therefore, to legitimize OFDM in all 5G regimes, we need to suppress the OFDM OOB emission.

    Since the discontinuous nature of the OFDM signal is the main reason forthe high OOB emission, one solution is to render the discontinuous OFDM signal continuous. Two factors control this discontinuity: the physical shape of the modulated signal and the correlation of the data symbols that modulate the OFDM signal.

    While most traditional approaches focus on reshaping the OFDM signal to render it continuous, in this work, we give our attention to the spectral precoding approaches. These approaches manipulate the correlation of the data symbols to control the high OOB emission in OFDM.

    After almost ten years of analytical and theoretical study of the spectralprecoding approaches in the literature, we believe it is time to investigate the practicality and ability of spectral precoding methods to compete with traditional approaches. Consequently, we check whether spectral precoding satisfies the spectral requirements of LTE measures. We analyze the precoders' implementation complexity and provide methodologies to reduce it. We design practical spectral precoders that match with the practical OFDM transmitters, and we study spectral precoding extensibility toward MIMO (and massive MIMO) systems.

    Moreover, in this work, we introduce the first proof-of-concept prototype of spectrally precoded OFDM. In the prototype, we use software-defined radios (SDRs) to perform a real-time examination of how spectral precoding improves the spectral containment of OFDM systems.

  • 7.
    Mohamad, Medhat
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Nilsson, Rickard
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Beek, Jaap van de
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    A Novel Transmitter Architecture for Spectrally-Precoded OFDM2018In: IEEE Transactions on Circuits and Systems Part 1: Regular Papers, ISSN 1549-8328, E-ISSN 1558-0806, Vol. 65, no 8, p. 2592-2605Article in journal (Refereed)
    Abstract [en]

    Frequency nulling spectral precoding is an approachthat suppresses the out-of-band emission in OFDM systems.In this paper, we discuss the transmitter architecture of thespectrally precoded OFDM systems. We design a novel precoderthat matches the practical implementation of the OFDM modulator.We show that spectral precoding can relax the analog lowpass filtering requirements of the OFDM system transmitter. Weexamine the effect of spectral precoding on the PAPR as wellas the effect of the PA on the spectral precoding suppressionperformance. We also study the compliance of the spectrallyprecoded OFDM transmitter with the 3GPP standardizationmeasures and analyze its computation complexity. At the receiverside, we analyze the in-band interference and BER performanceof the suggested precoding approach.

  • 8.
    Mohamad, Medhat
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Nilsson, Rickard
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Beek, Jaap van de
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    An analysis of out-of-band emission and in-band interference for precoded and classical OFDM systems2015In: European Wireless 2015: 21th European Wireless Conference, 20-22 May 2015, Budapest, Hungary, Piscataway, NJ: IEEE Communications Society, 2015, article id 7147703Conference paper (Refereed)
    Abstract [en]

    In this paper we present analytical expressions for the out-of-band (OOB) emission and in-band interference of two different OOB suppressed OFDM-systems; classical low-pass filtered and precoded. Then, we analytically compare their performance in terms of OOB emission suppression and introduced level of in-band interference. We analyse the fact that the in-band interference introduced by the filter depends on the length of the cyclic prefix as well as the behavior of the channel while that of the precoded OFDM does not. The analysis confirms that edged subcarriers suffer higher in-band interference than central subcarriers. Moreover, frequency precoders, for a specific choice of notching frequencies, can outperform time precoders.

  • 9.
    Mohamad, Medhat
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Nilsson, Rickard
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Beek, Jaap van de
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Minimum-EVM N-continuous OFDM2016In: 2016 IEEE International Conference on Communications (ICC): Kuala Lumpur, 22-27 May 2016, Piscataway, NJ: IEEE Communications Society, 2016, article id 7511164Conference paper (Refereed)
    Abstract [en]

    The N-continuous precoder for the suppression ofOOB emission in OFDM systems that minimizes the EVM isintroduced. We show that the minimum in-band interference dueto the introduced precoding approach is 3 dB less than the inbandinterference of existing N-continuous precoders. Moreover,we introduce a novel practical block precoder that approachesthe performance of the suggested optimal precoder. We analyzethe spectrum of the OFDM signal after block precoding, the inbandinterference due to block precoding and the effect of blockprecoding on the BER. The price to pay for block precoding isthe computational complexity that grows linearly with the blocksize.

  • 10.
    Mohamad, Medhat
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Nilsson, Rickard
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Beek, Jaap van de
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    N-smooth OFDMIn: Article in journal (Refereed)
    Abstract [en]

    This paper introduces a new multicarrier system - a practical alternative for N-continuous OFDM: N-smooth OFDM. N-smooth OFDM improves the spectral compactness of practical OFDM systems by suppressing OOB emission. The higher spectral compactness of N-smooth OFDM allows the system to relax requirements of the digital to analog converter (DAC) construction filter. Moreover, it decreases the signal's self interference that results from spectral precoding as well as precoder's computational complexity. 

  • 11.
    Mohamad, Medhat
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Nilsson, Rickard
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    van de Beek, Jaap
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    An SDR-based prototype of spectrally-precoded OFDM2017In: EuCNC 2017: European Conference on Networks and Communications, Piscataway, NJ: Institute of Electrical and Electronics Engineers (IEEE), 2017, article id 7980758Conference paper (Refereed)
    Abstract [en]

    Spectrally precoded OFDM is a recent approach that aims for the suppression of the OOB, as well as the in-band gap, emissions in OFDM systems. Theoretically, the technique shows High suppression of the OOB and the in-band gap emissions. Although, practically hardware impairments may limit the precoder's performance. Yet, this demonstration shows that despite the hardware impairments, implemented spectrally precoded OFDM generates emissions tens of decibels less than that of plain OFDM. Therefore, the demonstration proves that communications links are less vulnerable to interference coming from spectrally precoded OFDM signals than to interference coming from plain OFDM signals

  • 12.
    Mohamad, Medhat
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Nilsson, Rickard
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    van de Beek, Jaap
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Spectrally-Precoded OFDM for 5G Wideband Operation in Fragmented sub-6GHz SpectrumArticle in journal (Refereed)
    Abstract [en]

    We consider spectrally-precoded OFDM waveforms for 5G wideband transmission in sub-6GHz band. In this densely

    packed spectrum, a low out-of-band (OOB) waveform is a critical 5G component to achieve the promised high spectral

    efficiency. By precoding data symbols before OFDM modulation, it is possible to achieve extremely low out-of-band

    emission with very sharp spectrum transition enabling an efficient and flexible usage of frequency resources. Spectrallyprecoded

    OFDM shows promising results for reaching 5G targets in high-data rate enhanced mobile broadband and ultrareliable

    low-latency communications use cases. Spectral precoding is particularly efficient for wideband transmission

    enabling short-time transmission, which will often require flexible fragmented spectrum usage.

  • 13.
    Mohamad, Medhat
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Taheri, Tayebeh
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Nilsson, Rickard
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Beek, Jaap van de
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Subspace-precoded OFDM with low Out-of-Band Emissions:it will workManuscript (preprint) (Other academic)
    Abstract [en]

    Today’s multicarrier transmitters, deployed in essentially all communications radio equipment we use(phones, TV’s, computers, etc.), contain means to avoid undesired out-of-band power emissions. Standards and regulations typically restrict the levels of these emissions in order to allow for an efficient use of the radio spectrum. Radio vendors deploy well-established algorithms such as low-pass filtering or windowing to satisfy these regulations. A new approach, specifically applicable to OFDM systems, known as spectralprecoding, has been studied during the last decade. In particular, subspace precoding has attracted theinterest of academia, for its elegance, and despite a few adamant disadvantages. Recent advances now provide a breakthrough: subspace precoding is now a viable alternative to today’s algorithms.

  • 14.
    Taheri, Tayebeh
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Mohamad, Medhat
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Nilsson, Rickard
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Beek, Jaap van de
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Joint spectral-spatial precoders in massive-MIMO OFDM transmittersIn: Article in journal (Refereed)
    Abstract [en]

    In this paper we propose novel joint spectral-spatial precoders for large-antennaOFDM systems that suppress both spatial multi-user interference and spectral out-ofbandpower emissions. We analyse the performance of the proposed precoder in termsof spectral characteristics, system sum-rate and transmitter complexity. We show inparticular that, interestingly, the proposed joint maximum-ratio precoder has a simpletransmitter structure of two separate and consecutive banks of spectral and spatialprecoders.

1 - 14 of 14
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