Publications
282 results found
Joudeh H, Clerckx B, 2017, On the DoF of Parallel MISO BCs with Partial CSIT: Total Order and Separability, IEEE Global Communications Conference (GLOBECOM)
We study the degrees of freedom (DoF) of a $K$-user parallel MISO broadcastchannel with arbitrary levels of partial CSIT over each subchannel. We derive asum-DoF upperbound which depends on the average CSIT quality of each user. Thisupperbound is shown to be tight under total order, i.e. when the order of userswith respect to their CSIT qualities is preserved over all subchannels. In thiscase, it is shown that separate coding over each subchannel is optimum in asum-DoF sense.
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.
Piovano E, Clerckx B, 2017, Optimal DoF Region of the <i>K</i>-User MISO BC With Partial CSIT, IEEE COMMUNICATIONS LETTERS, Vol: 21, Pages: 2368-2371, ISSN: 1089-7798
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- Citations: 40
Papazafeiropoulos A, Clerckx B, Ratnarajah T, 2017, Rate-Splitting to Mitigate Residual Transceiver Hardware Impairments in Massive MIMO Systems, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, Vol: 66, Pages: 8196-8211, ISSN: 0018-9545
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- Citations: 64
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.
Piovano E, Joudeh H, Clerckx B, 2017, On Coded Caching in the Overloaded MISO Broadcast Channel, IEEE International Symposium on Information Theory (ISIT), ISSN: 2157-8117
This work investigates the interplay of coded caching and spatialmultiplexing in an overloaded Multiple-Input-Single-Output (MISO) BroadcastChannel (BC), i.e. a system where the number of users is greater than thenumber of transmitting antennas. On one hand, coded caching uses the aggregateglobal cache memory of the users to create multicasting opportunities. On theother hand, multiple antennas at the transmitter leverage the available CSIT totransmit multiple streams simultaneously. In this paper, we introduce a novelscheme which combines both the gain derived from coded-caching and spatialmultiplexing and outperforms existing schemes in terms of delivery time andCSIT requirement.
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.
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
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.
Kim J, Clerckx B, Mitcheson PD, 2017, Prototyping and Experimentation of a Closed-Loop Wireless Power Transmission with Channel Acquisition and Waveform Optimization, IEEE Wireless Power Transfer Conference (WPTC), Publisher: IEEE, ISSN: 2474-0225
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.
Papazafeiropoulos A, Clerckx B, Ratnarajah T, 2017, Mitigation of Phase Noise in Massive MIMO Systems: A Rate-Splitting Approach, IEEE International Conference on Communications (ICC), Publisher: IEEE, ISSN: 1550-3607
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- Citations: 1
Clerckx B, 2016, MISO Networks with Imperfect CSIT: A Topological Rate-Splitting Approach, IEEE Transactions on Communications, ISSN: 1558-0857
Clerckx B, Bayguzina E, 2016, Waveform design for Wireless Power Transfer, IEEE Transactions on Signal Processing, Vol: 64, Pages: 6313-6328, ISSN: 1053-587X
Far-field Wireless Power Transfer (WPT) has attracted significant attention in recent years. Despite the rapid progress, the emphasis of the research community in the last decade has remained largely concentrated on improving the design of energy harvester (so-called rectenna) and has left aside the effect of transmitter design. In this paper, we study the design of transmit waveform so as to enhance the dc power at the output of the rectenna. We derive a tractable model of the nonlinearity of the rectenna and compare with a linear model conventionally used in the literature. We then use those models to design novel multisine waveforms that are adaptive to the channel state information (CSI). Interestingly, while the linear model favours narrowband transmission with all the power allocated to a single frequency, the nonlinear model favours a power allocation over multiple frequencies. Through realistic simulations, waveforms designed based on the nonlinear model are shown to provide significant gains (in terms of harvested dc power) over those designed based on the linear model and over nonadaptive waveforms. We also compute analytically the theoretical scaling laws of the harvested energy for various waveforms as a function of the number of sinewaves and transmit antennas. Those scaling laws highlight the benefits of CSI knowledge at the transmitter in WPT and of a WPT design based on a nonlinear rectenna model over a linear model. Results also motivate the study of a promising architecture relying on large-scale multisine multiantenna waveforms for WPT. As a final note, results stress the importance of modeling and accounting for the nonlinearity of the rectenna in any system design involving wireless power.
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-input multiple-output channel under the peak and average power constraints is investigated. For the identity channel matrix, the approach of Shamai et al. is generalized to the higher dimension settings to derive the necessary and sufficient conditions for the optimal input probability density function. This approach prevents the usage of the identity theorem of the holomorphic functions of several complex variables which seems to fail in the multi-dimensional scenarios. It is proved that the support of the capacity-achieving distribution is a finite set of hyper-spheres with mutual independent phases and amplitude in the spherical domain. Subsequently, it is shown that when the average power constraint is relaxed, if the number of antennas is large enough, the capacity has a closed-form solution and constant amplitude signaling at the peak power achieves it. Moreover, it will be observed that in a discrete-time memoryless Gaussian channel, the average power constrained capacity, which results from a Gaussian input distribution, can be closely obtained by an input where the support of its magnitude is a discrete finite set. Finally, we investigate some upper and lower bounds for the capacity of the non-identity channel matrix and evaluate their performance as a function of the condition number of the channel.
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
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) maximization problem in downlink multi-user multiple input simgle output (MU-MISO) systems under imperfect channel state information at the transmitter (CSIT). Contrary to existing works, we consider a rather unorthodox transmission scheme. In particular, the message intended to one of the users is split into two parts: a common part which can be recovered by all users, and a private part recovered by the corresponding user. On the other hand, the rest of users receive their information through private messages. This rate-splitting (RS) approach was shown to boost the achievable degrees of freedom when CSIT errors decay with increased SNR. In this paper, the RS strategy is married with linear precoder design and optimization techniques to achieve a maximized ergodic SR (ESR) performance over the entire range of SNRs. Precoders are designed based on partial CSIT knowledge by solving a stochastic rate optimization problem using means of sample average approximation coupled with the weighted minimum mean square error approach. Numerical results show that in addition to the ESR gains, the benefits of RS also include relaxed CSIT quality requirements and enhanced achievable rate regions compared with conventional transmission with no rate-splitting.
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
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.
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.
Clerckx B, Joudeh H, Hao C, et al., 2016, Rate Splitting for MIMO Wireless Networks: A Promising PHY-Layer Strategy for LTE Evolution, IEEE Communications Magazine, Vol: 54, Pages: 98-105, ISSN: 1558-1896
MIMO processing plays a central part towards the recent increase in spectraland energy efficiencies of wireless networks. MIMO has grown beyond theoriginal point-to-point channel and nowadays refers to a diverse range ofcentralized and distributed deployments. The fundamental bottleneck towardsenormous spectral and energy efficiency benefits in multiuser MIMO networkslies in a huge demand for accurate channel state information at the transmitter(CSIT). This has become increasingly difficult to satisfy due to the increasingnumber of antennas and access points in next generation wireless networksrelying on dense heterogeneous networks and transmitters equipped with a largenumber of antennas. CSIT inaccuracy results in a multi-user interferenceproblem that is the primary bottleneck of MIMO wireless networks. Lookingbackward, the problem has been to strive to apply techniques designed forperfect CSIT to scenarios with imperfect CSIT. In this paper, we depart fromthis conventional approach and introduce the readers to a promising strategybased on rate-splitting. Rate-splitting relies on the transmission of commonand private messages and is shown to provide significant benefits in terms ofspectral and energy efficiencies, reliability and CSI feedback overheadreduction over conventional strategies used in LTE-A and exclusively relying onprivate message transmissions. Open problems, impact on standard specificationsand operational challenges are also discussed.
Rassouli B, Clerckx B, 2016, An Upper Bound for the Capacity of Amplitude-Constrained Scalar AWGN Channel, IEEE Communications Letters, Vol: 20, Pages: 1924-1926, ISSN: 1558-2558
This paper slightly improves the upper bound in Thangaraj et al. for thecapacity of the amplitude-constrained scalar AWGN channel. This improvementmakes the upper bound within 0.002 bits of the capacity for$\frac{E_b}{N_0}\leq 2.5$ dB.
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