135 results found
Piovano E, Joudeh H, Clerckx B, 2019, Generalized degrees of freedom of the symmetric cache-aided MISO broadcast channel with partial CSIT, IEEE Transactions on Information Theory, ISSN: 0018-9448
We consider the cache-aided MISO broadcast channel (BC) in which a multi-antenna transmitter serves K singleantenna receivers, each equipped with a cache memory. The transmitter has access to partial knowledge of the channel state information. For a symmetric setting, in terms of channel strength levels, partial channel knowledge levels and cache sizes, we characterize the generalized degrees of freedom (GDoF) up to a constant multiplicative factor. The achievability scheme exploits the interplay between spatial multiplexing gains and codedmulticasting gain. On the other hand, a cut-set-based argument in conjunction with a GDoF outer bound for a parallel MISO BC under channel uncertainty are used for the converse. We further show that the characterized order-optimal GDoF is also attained in a decentralized setting, where no coordination is required for content placement in the caches.
Clerckx B, Zhang R, Schober R, et al., 2019, Fundamentals of wireless information and power transfer: From RF energy harvester models to signal and system designs, IEEE Journal on Selected Areas in Communications, Vol: 37, Pages: 4-33, ISSN: 0733-8716
Radio waves carry both energy and information simultaneously. Nevertheless, radio-frequency (RF) transmissions of these quantities have traditionally been treated separately. Currently, the community is experiencing a paradigm shift in wireless network design, namely, unifying wireless transmission of information and power so as to make the best use of the RF spectrum and radiation as well as the network infrastructure for the dual purpose of communicating and energizing. In this paper, we review and discuss recent progress in laying the foundations of the envisioned dual purpose networks by establishing a signal theory and design for wireless information and power transmission (WIPT) and identifying the fundamental tradeoff between conveying information and power wirelessly. We start with an overview of WIPT challenges and technologies, namely, simultaneous WIPT (SWIPT), wirelessly powered communication networks (WPCNs), and wirelessly powered backscatter communication (WPBC). We then characterize energy harvesters and show how WIPT signal and system designs crucially revolve around the underlying energy harvester model. To that end, we highlight three different energy harvester models, namely, one linear model and two nonlinear models, and show how WIPT designs differ for each of them in single-user and multi-user deployments. Topics discussed include rate-energy region characterization, transmitter and receiver architectures, waveform design, modulation, beamforming and input distribution optimizations, resource allocation, and RF spectrum use. We discuss and check the validity of the different energy harvester models and the resulting signal theory and design based on circuit simulations, prototyping, and experimentation. We also point out numerous directions that are promising for future research.
Clerckx B, Kim J, 2018, On the beneficial roles of fading and transmit diversity in wireless power transfer with nonlinear energy harvesting, IEEE Transactions on Wireless Communications, Vol: 17, Pages: 7731-7743, ISSN: 1536-1276
We study the effect of channel fading in WirelessPower Transfer (WPT) and show that fading enhances the RF-to-DC conversion efficiency of nonlinear RF energy harvesters.We then develop a new form of signal design for WPT, denoted asTransmit Diversity, that relies on multiple dumb antennas at thetransmitter to induce fast fluctuations of the wireless channel.Those fluctuations boost the RF-to-DC conversion efficiencythanks to the energy harvester nonlinearity. In contrast with(energy) beamforming, Transmit Diversity does not rely onChannel State Information at the Transmitter (CSIT) and doesnot increase the average power at the energy harvester input,though it still enhances the overall end-to-end power transferefficiency. Transmit Diversity is also combined with recentlydeveloped (energy) waveform and modulation to provide furtherenhancements. The efficacy of the scheme is analyzed usingphysics-based and curve fitting-based nonlinear models of the en-ergy harvester and demonstrated using circuit simulations, pro-totyping and experimentation. Measurements with two transmitantennas reveal gains of 50% in harvested DC power over a singletransmit antenna setup. The work (again) highlights the crucialrole played by the harvester nonlinearity and demonstrates thatmultiple transmit antennas can be beneficial to WPT even in theabsence of CSIT.
Mao Y, Clerckx B, Li VOK, 2018, Rate-splitting multiple access for downlink communication systems: bridging, generalizing and outperforming SDMA and NOMA, EURASIP Journal on Wireless Communications and Networking, Vol: 2018, ISSN: 1687-1472
Space-division multiple access (SDMA) utilizes linear precoding to separate users in the spatial domain and relies on fully treating any residual multi-user interference as noise. Non-orthogonal multiple access (NOMA) uses linearly precoded superposition coding with successive interference cancellation (SIC) to superpose users in the power domain and relies on user grouping and ordering to enforce some users to fully decode and cancel interference created by other users.In this paper, we argue that to efficiently cope with the high throughput, heterogeneity of quality of service (QoS), and massive connectivity requirements of future multi-antenna wireless networks, multiple access design needs to depart from those two extreme interference management strategies, namely fully treat interference as noise (as in SDMA) and fully decode interference (as in NOMA).Considering a multiple-input single-output broadcast channel, we develop a novel multiple access framework, called rate-splitting multiple access (RSMA). RSMA is a more general and more powerful multiple access for downlink multi-antenna systems that contains SDMA and NOMA as special cases. RSMA relies on linearly precoded rate-splitting with SIC to decode part of the interference and treat the remaining part of the interference as noise. This capability of RSMA to partially decode interference and partially treat interference as noise enables to softly bridge the two extremes of fully decoding interference and treating interference as noise and provides room for rate and QoS enhancements and complexity reduction.The three multiple access schemes are compared, and extensive numerical results show that RSMA provides a smooth transition between SDMA and NOMA and outperforms them both in a wide range of network loads (underloaded and overloaded regimes) and user deployments (with a diversity of channel directions, channel strengths, and qualities of channel state information at the transmitter). Moreover, RSMA provid
Park J, Clerckx B, Song C, et al., 2018, An Analysis of the Optimum Node Density for Simultaneous Wireless Information and Power Transfer in Ad Hoc Networks, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, Vol: 67, Pages: 2713-2726, ISSN: 0018-9545
Varasteh M, Rassouli B, Clerckx B, 2018, Wireless Information and Power Transfer over an AWGN channel: Nonlinearity and Asymmetric Gaussian Signaling, 2017 IEEE Information Theory Workshop (ITW), Publisher: IEEE, Pages: 181-183, ISSN: 2475-420X
Clerckx B, 2017, Wireless information and power transfer: nonlinearity, waveform design and rate-energy tradeoff, IEEE Transactions on Signal Processing, Vol: 66, Pages: 847-862, ISSN: 1053-587X
The design of wireless information and power transfer (WIPT) has so far relied on an oversimplified and inaccurate linear model of the energy harvester. In this paper, we depart from this linear model and design WIPT considering the rectifier nonlinearity. We develop a tractable model of the rectifier nonlinearity that is flexible enough to cope with general multicarrier modulated input waveforms. Leveraging that model, we motivate and introduce a novel WIPT architecture relying on the superposition of multicarrier unmodulated and modulated waveforms at the transmitter. The superposed WIPT waveforms are optimized as a function of the channel state information so as to characterize the rate-energy region of the whole system. Analysis and numerical results illustrate the performance of the derived waveforms and WIPT architecture and highlight that nonlinearity radically changes the design of WIPT. We make key and refreshing observations. First, analysis (confirmed by circuit simulations) shows that modulated and unmodulated waveforms are not equally suitable for wireless power delivery, namely, modulation being beneficial in single-carrier transmissions but detrimental in multicarrier transmissions. Second, a multicarrier unmodulated waveform (superposed to a multicarrier modulated waveform) is useful to enlarge the rate-energy region of WIPT. Third, a combination of power splitting and time sharing is in general the best strategy. Fourth, a nonzero mean Gaussian input distribution outperforms the conventional capacity-achieving zero-mean Gaussian input distribution in multicarrier transmissions. Fifth, the rectifier nonlinearity is beneficial to system performance and is essential to efficient WIPT design.
Huang Y, Clerckx B, 2017, Waveform Design for Wireless Power Transfer with Limited Feedback, IEEE Transactions on Wireless Communications, Vol: 17, Pages: 415-429, ISSN: 1536-1276
Waveform design is a key technique to jointly exploit a beamforming gain, the channel frequency selectivity, and the rectifier nonlinearity, so as to enhance the end-to-end power transfer efficiency of wireless power transfer (WPT). Those waveforms have been designed, assuming perfect channel state information at the transmitter. This paper proposes two waveform strategies relying on limited feedback for multi-antenna multi-sine WPT over frequency-selective channels. In the waveform selection strategy, the energy transmitter (ET) transmits over multiple timeslots with every time a different waveform precoder within a codebook, and the energy receiver (ER) reports the index of the precoder in the codebook that leads to the largest harvested energy. In the waveform refinement strategy, the ET sequentially transmits two waveforms in each stage, and the ER reports one feedback bit indicating an increase/decrease in the harvested energy during this stage. Based on multiple one-bit feedback, the ET successively refines waveform precoders in a tree-structured codebook over multiple stages. By employing the framework of the generalized Lloyd’s algorithm, novel algorithms are proposed for both strategies to optimize the codebooks in both space and frequency domains. The proposed limited feedback-based waveform strategies are shown to outperform a set of baselines, achieving higher harvested energy.
Joudeh H, Clerckx B, 2017, Rate-Splitting for Max-Min Fair Multigroup Multicast Beamforming in Overloaded Systems, IEEE Transactions on Wireless Communications, Vol: 16, Pages: 7276-7289, ISSN: 1536-1276
In this paper, we consider the problem of achieving max-min fairness amongst multiple co-channel multicast groups through transmit beamforming. We explicitly focus on overloaded scenarios in which the number of transmitting antennas is insufficient to neutralize all inter-group interference. Such scenarios are becoming increasingly relevant in the light of growing low-latency content delivery demands, and also commonly appear in multibeam satellite systems. We derive performance limits of classical beamforming strategies using DoF analysis unveiling their limitations; for example, rates saturate in overloaded scenarios due to inter-group interference. To tackle interference, we propose a strategy based on degraded beamforming and successive interference cancellation. While the degraded strategy resolves the rate-saturation issue, this comes at a price of sacrificing all spatial multiplexing gains. This motivates the development of a unifying strategy that combines the benefits of the two previous strategies. We propose a beamforming strategy based on rate-splitting (RS) which divides the messages intended to each group into a degraded part and a designated part, and transmits a superposition of both degraded and designated beamformed streams. The superiority of the proposed strategy is demonstrated through DoF analysis. Finally, we solve the RS beamforming design problem and demonstrate significant performance gains through simulations.
Huang Y, Clerckx B, 2017, Large-Scale Multi-Antenna Multi-Sine Wireless Power Transfer, IEEE Transactions on Signal Processing, Vol: 65, Pages: 5812-5827, ISSN: 1053-587X
Wireless Power Transfer (WPT) is expected to be a technology reshaping the landscape of low-power applications such as the Internet of Things, RF identification (RFID) networks, etc. To that end, multi-antenna multi-sine waveforms adaptive to the Channel State Information (CSI) have been shown to be a promising building block of WPT. However, the current design is computationally too complex to be applied to large-scale WPT, where the transmit signal is sent across a large number (tens) of antennas and frequencies. In this paper, we derive efficient singleuser and multi-user algorithms based on a generalizable optimization framework, in order to design transmit waveforms that maximize the weighted-sum/minimum rectenna DC output voltage. The study highlights the significant effect of the nonlinearity introduced by the rectification process on the design of waveforms in single/multi-user systems. Interestingly, in the single-user case, the optimal spatial domain beamforming, obtained prior to the frequency domain power allocation optimization, turns out to be Maximum Ratio Transmission (MRT). On the contrary, in the general multi-user weighted sum criterion maximization problem, the spatial domain beamforming optimization and the frequency domain power allocation optimization are coupled. Assuming channel hardening, low-complexity algorithms are proposed based on asymptotic analysis, to maximize the two criteria. The structure of the asymptotically optimal spatial domain precoder can be found prior to the optimization. The performance of the proposed algorithms is evaluated. Numerical results confirm the inefficiency of the linear model-based design for the single and multi-user scenarios. It is also shown that as nonlinear modelbased designs, the proposed algorithms can benefit from an increasing number of sinewaves at a computational cost much lower than the existing method. Simulation results highlight the significant benefits of the large-scale WPT architecture to
Clerckx B, Dai M, 2017, Multiuser Millimeter Wave Beamforming Strategieswith Quantized and Statistical CSIT, IEEE Transactions on Wireless Communications, Vol: 16, Pages: 7025-7038, ISSN: 1536-1276
To alleviate the high cost of hardware in mmWave systems, hybrid analog/digital precoding is typically employed. In the conventional two-stage feedback scheme, the analog beamformer is determined by beam search and feedback to maximize the desired signal power of each user. The digital precoder is designed based on quantization and feedback of effective channel to mitigate multiuser interference. Alternatively, we propose a one-stage feedback scheme which effectively reduces the complexity of the signalling and feedback procedure. Specifically, the second-order channel statistics are leveraged to design digital precoder for interference mitigation while all feedback overhead is reserved for precise analog beamforming. Under a fixed total feedback constraint, we investigate the conditions under which the one-stage feedback scheme outperforms the conventional twostage counterpart. Moreover, a rate splitting (RS) transmission strategy is introduced to further tackle the multiuser interference and enhance the rate performance. Consider (1) RS precoded by the one-stage feedback scheme and (2) conventional transmission strategy precoded by the two-stage scheme with the same firststage feedback as (1) and also certain amount of extra secondstage feedback. We show that (1) can achieve a sum rate comparable to that of (2). Hence, RS enables remarkable saving in the second-stage training and feedback overhead.
Hao C, Rassoul B, Clerckx B, 2017, Achievable DoF regions of MIMO networks with imperfect CSIT, IEEE Transactions on Information Theory, Vol: 63, Pages: 6587-6606, ISSN: 1557-9654
We focus on a two-receiver Multiple-Input-Multiple-Output (MIMO) Broadcast Channel (BC) and InterferenceChannel (IC) with an arbitrary number of antennas at eachnode. We assume an imperfect knowledge of local Channel StateInformation at the Transmitters, whose error decays with theSignal-to-Noise-Ratio. With such configuration, we characterizethe achievable Degrees-of-Freedom (DoF) regions in both BC andIC, by proposing a Rate-Splitting (RS) approach, which divideseach receiver’s message into a common part and a private part.Compared to the RS scheme designed for the symmetric MIMOcase, the novelties of the proposed block lie in 1) deliveringadditional non-ZF-precoded private symbols to the receiver withthe greater number of antennas, and 2) a Space-Time implemen-tation. These features provide more flexibilities in balancing thecommon-message-decodabilities at the two receivers, and fullyexploit asymmetric antenna arrays. Besides, in IC, we modifythe power allocation designed for the asymmetric BC based onthe signal space where the two transmitted signals interfere witheach other. We also derive an outer-bound for the DoF regionsand show that the proposed achievable DoF regions are optimalunder some antenna configurations and CSIT qualities.
Clerckx B, Zawawi ZB, Huang K, 2017, Wirelessly powered backscatter communications: waveform design and SNR-energy tradeoff, IEEE Communications Letters, Vol: 21, Pages: 2234-2237, ISSN: 1558-2558
This paper shows that wirelessly powered backscatter communications is subject to a fundamental tradeoff between the harvested energy at the tag and the reliability of the backscatter communication, measured in terms of SNR at the reader. Assuming the RF transmit signal is a multisine waveform adaptive to the channel state information, we derive a systematic approach to optimize the transmit waveform weights (amplitudes and phases) in order to enlarge as much as possible the SNR-energy region. Performance evaluations confirm the significant benefits of using multiple frequency components in the adaptive transmit multisine waveform to exploit the nonlinearity of the rectifier and a frequency diversity gain.
Clerckx B, Bayguzina E, 2017, A low-complexity adaptive multisine waveform design for wireless power transfer, IEEE Antennas and Wireless Propagation Letters, Vol: 16, Pages: 2207-2210, ISSN: 1548-5757
Channel-adaptive waveforms for Wireless Power Transfer significantly boost the DC power level at the rectifier output. However the design of those waveforms is computationally complex and does not lend itself easily to practical implementation. We here propose a low-complexity channel adaptive waveform design whose performance is very close to that of the optimal design. Performance evaluations confirm the new design’s benefits in various rectifier topologies, with gains in DC output power of 100% over conventional waveforms.
Clerckx B, 2017, Downlink and Uplink Decoupling in Two-Tier Heterogeneous Networks with Multi-Antenna Base Stations, IEEE Transactions on Wireless Communications, Vol: 16, Pages: 2760-2775, ISSN: 1558-2248
In order to improve the uplink performance offuture cellular networks, the idea to decouple the downlink (DL)and uplink (UL) association has recently been shown to providesignificant gain in terms of both coverage and rate performance.However, all the work is limited to SISO network. Therefore,to study the gain provided by the DL and UL decoupling inmulti-antenna base stations (BSs) setup, we study a two tierheterogeneous network consisting of multi-antenna BSs, andsingle antenna user equipments (UEs). We use maximal ratiocombining (MRC) as a linear receiver at the BSs and using toolsfrom stochastic geometry, we derive tractable expressions forboth signal to interference ratio (SIR) coverage probability andrate coverage probability. We observe that as the disparity inthe beamforming gain of both tiers increases, the gain in term ofSIR coverage probability provided by the decoupled associationover non-decoupled association decreases. We further observethat when there is asymmetry in the number of antennas of bothtier, then we need further biasing towards femto-tier on the topof decoupled association to balance the load and get optimal ratecoverage probability.
Piovano E, Joudeh H, Clerckx B, 2017, Overloaded multiuser MISO transmission with imperfect CSIT, 50th Asilomar Conference on Signals, Systems and Computers, Publisher: IEEE
A required feature for the next generation of wireless communication networks will be the capability to serve simultaneously a large number of devices with heterogeneous CSIT qualities and demands. In this paper, we consider the overloaded MISO BC with two groups of CSIT qualities. We propose a transmission scheme where degraded symbols are superimposed on top of spatially-multiplexed symbols. The developed strategy allows to serve all users in a non-orthogonal manner and the analysis shows an enhanced performance compared to existing schemes. Moreover, optimality in a DoF sense is shown.
Clerckx B, 2017, Communications and Signals Design for Wireless Power Transmission, IEEE Transactions on Communications, Vol: 65, Pages: 2264-2290, ISSN: 1558-0857
Radiative wireless power transfer (WPT) is a promising technology to provide cost-effective and real-time power supplies to wireless devices. Although radiative WPT shares many similar characteristics with the extensively studied wireless information transfer or communication, they also differ significantly in terms of design objectives, transmitter/receiver architectures and hardware constraints, and so on. In this paper, we first give an overview on the various WPT technologies, the historical development of the radiative WPT technology and the main challenges in designing contemporary radiative WPT systems. Then, we focus on the state-of-the-art communication and signal processing techniques that can be applied to tackle these challenges. Topics discussed include energy harvester modeling, energy beamforming for WPT, channel acquisition, power region characterization in multi-user WPT, waveform design with linear and non-linear energy receiver model, safety and health issues of WPT, massive multiple-input multiple-output and millimeter wave enabled WPT, wireless charging control, and wireless power and communication systems co-design. We also point out directions that are promising for future research.
Clerckx B, Hao C, 2017, MISO networks with imperfect CSIT: a topological rate-splitting approach, IEEE Transactions on Communications, Vol: 65, Pages: 2164-2179, ISSN: 1558-0857
Recently, the Degrees-of-Freedom (DoF) region ofmultiple-input-single-output (MISO) networks with imperfectchannel state information at the transmitter (CSIT) has at-tracted significant attention. An achievable scheme, knownas Rate-Splitting (RS), integrates common-message-multicastingand private-message-unicasting. In this paper, focusing on thegeneralK-cell MISO IC with an arbitrary CSIT quality of eachinterfering link, we firstly identify the DoF region achieved byRS. Secondly, we introduce a novel scheme, so called TopologicalRS (TRS), whose novelties compared to RS lie in a multi-layerstructure and in transmitting multiple common messages to bedecoded by groups of users rather than all users. The designof TRS is motivated by a novel interpretation of theK-cell ICwith imperfect CSIT as a weighted sum of a series of partiallyconnected networks. We show that the DoF region achievedby TRS yields the best known result so far, and we find themaximal sum DoF via hypergraph fractional packing. Lastly,for a realistic scenario where each user is connected to threedominant transmitters, we identify the sufficient condition whereTRS strictly outperforms conventional schemes, and show thatTRS is optimal for some CSIT qualities.
Clerckx B, MISO Networks with Imperfect CSIT: A Topological Rate-Splitting Approach, IEEE Transactions on Communications, ISSN: 1558-0857
Artuso M, Boviz D, Checko A, et al., 2016, Enhancing LTE with Cloud-RAN and Load- Controlled Parasitic Antenna Arrays, IEEE COMMUNICATIONS MAGAZINE, Vol: 54, Pages: 183-191, ISSN: 0163-6804
Rassouli B, Clerckx B, 2016, On the Capacity of Vector Gaussian Channels With Bounded Inputs, IEEE Transactions on Information Theory, Vol: 62, Pages: 6884-6903, ISSN: 0018-9448
The capacity of a deterministic multiple-inputmultiple-output (MIMO) channel under the peak and averagepower constraints is investigated. For the identity channel matrix,the approach of Shamai et al. is generalized to the higher dimensionsettings to derive the necessary and sufficient conditions forthe optimal input probability density function. This approachprevents the usage of the identity theorem of the holomorphicfunctions of several complex variables which seems to fail inthe multi-dimensional scenarios. It is proved that the support ofthe capacity-achieving distribution is a finite set of hyper-sphereswith mutual independent phases and amplitude in the sphericaldomain. Subsequently, it is shown that when the average powerconstraint is relaxed, if the number of antennas is large enough,the capacity has a closed form solution and constant amplitudesignaling at the peak power achieves it. Moreover, it will beobserved that in a discrete-time memoryless Gaussian channel,the average power constrained capacity, which results from aGaussian input distribution, can be closely obtained by an inputwhere the support of its magnitude is a discrete finite set. Finally,we investigate some upper and lower bounds for the capacity ofthe non-identity channel matrix and evaluate their performanceas a function of the condition number of the channel.
Song C, Park J, Clerckx B, et al., 2016, Generalized Precoder Designs Based on Weighted MMSE Criterion for Energy Harvesting Constrained MIMO and Multi-User MIMO Channels, IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, Vol: 15, Pages: 7941-7954, ISSN: 1536-1276
Joudeh H, Clerckx B, 2016, Sum-Rate Maximization for Linearly Precoded Downlink Multiuser MISO Systems with Partial CSIT: A Rate-Splitting Approach, IEEE Transactions on Communications, Vol: 64, Pages: 4847-4861, ISSN: 0090-6778
This paper considers the Sum-Rate (SR) maximizationproblem in downlink MU-MISO systems under imperfectChannel State Information at the Transmitter (CSIT). Contraryto existing works, we consider a rather unorthodox transmissionscheme. In particular, the message intended to one of the users issplit into two parts: a common part which can be recovered byall users, and a private part recovered by the corresponding user.On the other hand, the rest of users receive their informationthrough private messages. This Rate-Splitting (RS) approachwas shown to boost the achievable Degrees of Freedom (DoF)when CSIT errors decay with increased SNR. In this work,the RS strategy is married with linear precoder design andoptimization techniques to achieve a maximized Ergodic SR(ESR) performance over the entire range of SNRs. Precoders aredesigned based on partial CSIT knowledge by solving a stochasticrate optimization problem using means of Sample Average Approximation(SAA) coupled with the Weighted Minimum MeanSquare Error (WMMSE) approach. Numerical results show thatin addition to the ESR gains, the benefits of RS also includerelaxed CSIT quality requirements and enhanced achievable rateregions compared to conventional transmission with NoRS.
Cui Y, Wu Y, Jiang D, et al., 2016, User-centric interference nulling in downlinkmulti-antenna heterogeneous networks, IEEE Transactions on Wireless Communications, Vol: 15, Pages: 7484-7500, ISSN: 1558-2248
In heterogeneous networks (HetNets), strong interference due to spectrum reuse affects each user's signal-to-interference ratio (SIR), and hence is one limiting factor of network performance. In this paper, we propose a user-centric interference nulling (IN) scheme in a downlink large-scale HetNet to improve coverage/outage probability by improving each user's SIR. This IN scheme utilizes at most maximum IN degree of freedom (DoF) at each macro-base station to avoid interference to uniformly selected macro (pico) users with signal-to-individual-interference ratio below a macro (pico) IN threshold, where the maximum IN DoF and the two IN thresholds are three design parameters. Using tools from stochastic geometry, we first obtain a tractable expression of the coverage (equivalently outage) probability. Then, we obtain the asymptotic expressions of the coverage/outage probability in the low and high SIR threshold regimes. The analytical results indicate that the maximum IN DoF can affect the order gain of the outage probability in the low SIR threshold regime, but cannot affect the order gain of the coverage probability in the high SIR threshold regime. Moreover, we characterize the optimal maximum IN DoF, which optimizes the asymptotic coverage/outage probability. Finally, numerical results show that the proposed scheme can achieve good gains in coverage/outage probability over some baseline schemes.
Clerckx B, 2016, Waveform Design for Wireless Power Transfer, IEEE Transactions on Signal Processing, Vol: 64, Pages: 6313-6328, ISSN: 1941-0476
Far-field Wireless Power Transfer (WPT) has attractedsignificant attention in recent years. Despite the rapidprogress, the emphasis of the research community in the lastdecade has remained largely concentrated on improving thedesign of energy harvester (so-called rectenna) and has leftaside the effect of transmitter design. In this paper, we studythe design of transmit waveform so as to enhance the DCpower at the output of the rectenna. We derive a tractablemodel of the non-linearity of the rectenna and compare witha linear model conventionally used in the literature. We thenuse those models to design novel multisine waveforms that areadaptive to the channel state information (CSI). Interestingly,while the linear model favours narrowband transmission withall the power allocated to a single frequency, the non-linearmodel favours a power allocation over multiple frequencies.Through realistic simulations, waveforms designed based on thenon-linear model are shown to provide significant gains (interms of harvested DC power) over those designed based onthe linear model and over non-adaptive waveforms. We alsocompute analytically the theoretical scaling laws of the harvestedenergy for various waveforms as a function of the number ofsinewaves and transmit antennas. Those scaling laws highlightthe benefits of CSI knowledge at the transmitter in WPT and of aWPT design based on a non-linear rectenna model over a linearmodel. Results also motivate the study of a promising architecturerelying on large-scale multisine multi-antenna waveforms forWPT. As a final note, results stress the importance of modelingand accounting for the non-linearity of the rectenna in any systemdesign involving wireless power.
Joudeh H, Clerckx B, 2016, A Rate-Splitting Strategy for Max-Min Fair Multigroup Multicasting, IEEE 17th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), Pages: 1-5
We consider the problem of transmit beamforming to multiple cochannel multicast groups. The conventional approach is to beamform a designated data stream to each group, while treating potential inter-group interference as noise at the receivers. In overloaded systems where the number of transmit antennas is insufficient to perform interference nulling, we show that inter-group interference dominates at high SNRs, leading to a saturating max-min fair performance. We propose a rather unconventional approach to cope with this issue based on the concept of Rate-Splitting (RS). In particular, part of the interference is broadcasted to all groups such that it is decoded and canceled before the designated beams are decoded. We show that the RS strategy achieves significant performance gains over the conventional multigroup multicast beamforming strategy.
Joudeh H, Clerckx B, 2016, Robust Transmission in Downlink Multiuser MISO Systems: A Rate-Splitting Approach, IEEE Transactions on Signal Processing, Vol: 64, Pages: 6227-6242, ISSN: 1053-587X
We consider a downlink multiuser MISO systemwith bounded errors in the Channel State Information at theTransmitter (CSIT). We first look at the robust design problemof achieving max-min fairness amongst users (in the worstcasesense). Contrary to the conventional approach adopted inliterature, we propose a rather unorthodox design based on aRate-Splitting (RS) strategy. Each user’s message is split intotwo parts, a common part and a private part. All commonparts are packed into one super common message encodedusing a public codebook, while private parts are independentlyencoded. The resulting symbol streams are linearly precodedand simultaneously transmitted, and each receiver retrieves itsintended message by decoding both the common stream andits corresponding private stream. For CSIT uncertainty regionsthat scale with SNR (e.g. by scaling the number of feedbackbits), we prove that a RS-based design achieves higher max-min(symmetric) Degrees of Freedom (DoF) compared to conventionaldesigns (NoRS). For the special case of non-scaling CSIT (e.g.fixed number of feedback bits), and contrary to NoRS, RS canachieve a non-saturating max-min rate. We propose a robustalgorithm based on the cutting-set method coupled with theWeighted Minimum Mean Square Error (WMMSE) approach,and we demonstrate its performance gains over state-of-the artdesigns. Finally, we extend the RS strategy to address the Qualityof Service (QoS) constrained power minimization problem, andwe demonstrate significant gains over NoRS-based designs.
Hao C, Clerckx B, 2016, Achievable sum DoF of the K-user MIMO interference channel with delayed CSIT, IEEE Transactions on Communications, Vol: 64, Pages: 4165-4180, ISSN: 1558-0857
This paper considers a K-user multiple-inputmultiple-output(MIMO) interference channel (IC) where 1) thechannel state information obtained by the transmitters (CSIT) iscompletely outdated, and 2) the number of transmit antennasat each transmitter, i.e., M, is greater than the number ofreceive antennas at each user, i.e., N. The usefulness of thedelayed CSIT was firstly identified in a K-phase RetrospectiveInterference Alignment (RIA) scheme proposed by Maddah-Aliet al for the Multiple-Input-Single-Output Broadcast Channel,but the extension to the MIMO IC is a non-trivial step as eachtransmitter only has the message intended for the correspondinguser. Recently, Abdoli et al focused on a Single-Input-SingleOutputIC and solved such bottleneck by inventing a K-phaseRIA with distributed overheard interference retransmission. Inthis paper, we propose two K-phase RIA schemes suitable forthe MIMO IC by generalizing and integrating some key featuresof both Abdoli’s and Maddah-Ali’s works. The two schemesjointly yield the best known sum Degrees-of-Freedom (DoF)performance so far. For the case MN ≥K, the achieved sum DoFis asymptotically given by 6415N when K→∞.
Huang Y, Clerckx B, 2016, Relaying Strategies for Wireless-Powered MIMO Relay Networks, IEEE Transactions on Wireless Communications, Vol: 15, Pages: 6033-6047, ISSN: 1558-2248
This paper investigates relaying schemes in an amplify-and-forward multiple-input multiple-output relay network, where an energy-constrained relay harvests wireless power from the source information flow and can be further aided by an energy flow (EF) in the form of a wireless power transfer at the destination. However, the joint optimization of the relay matrix and the source precoder for the energy-flow-assisted (EFA) and the non-EFA (NEFA) schemes is intractable. The original rate maximization problem is transformed into an equivalent weighted mean square error minimization problem and optimized iteratively, where the global optimum of the nonconvex source precoder subproblem is achieved by semidefinite relaxation and rank reduction. The iterative algorithm finally converges. Then, the simplified EFA and NEFA schemes are proposed based on channel diagonalization, such that the matrices optimizations can be simplified to power optimizations. Closed-form solutions can be achieved. Simulation results reveal that the EFA schemes can outperform the NEFA schemes. Additionally, deploying more antennas at the relay increases the dimension of the signal space at the relay. Exploiting the additional dimension, the EF leakage in the information detecting block can be nearly separated from the information signal, such that the EF leakage can be amplified with a small coefficient.
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