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  • 1. Fei, Zesong
    et al.
    Cao, Congzhe
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Hussain, Iqbal
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Kuang, Jingming
    Improved Luby transform codes in low overhead regions for binary erasure channels2014In: Transaction on Emerging Telecommunications Technologies, ISSN 2161-3915Article in journal (Refereed)
    Abstract [en]

    We study improved degree distribution for Luby transform (LT) codes, which exhibits improved bit error rates particularly in low overhead regions. We construct the degree distribution by modifying robust soliton distribution. The performance of our proposed LT codes is evaluated and compared with the conventional LT codes via And-Or tree analysis. Then we propose a transmission scheme based on our proposed degree distribution to improve the frame error rate in full recovery regions. Furthermore, the improved degree distribution is applied to distributed multi-source relay networks and unequal error protection. It is shown that our schemes achieve better performance and reduced complexity especially in low overhead regions, compared with conventional schemes.

  • 2.
    Hussain, Iqbal
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Analysis and Design of Rateless Codes2014Doctoral thesis, monograph (Other academic)
    Abstract [en]

    The invention of turbo codes and the re-discovery of sparse graph codes constitute a milestone in error-correction codes designed for communication and storage systems. Sparse graph codes such as low-density parity-check codes can offer a performance that approaches the previously elusive Shannon capacity with reasonable practical computational complexity. Fountain codes have emerged in the realm of sparse graph codes, and  have shown excellent performance for multicast and broadcast transmission without channel state information at the transmitter. A fountain code is inherently rateless, and as a consequence, such codes may potentially generate an unlimited number of encoded symbols on the fly. Thus due to the rateless property, these codes are suitable for transmission over time varying channels. The results presented in this thesis aim at providing insight into the fundamental design of rateless codes, which could serve as a guideline for the optimal design of rateless codes in real-world applications.

    The thesis is divided into two parts. The first part considers the analysis and design of rateless codes for point-to-point communication. To this end, we commence by considering the concatenation of Luby transform (LT) codes, which were the first practical realization of rateless codes, with differential modulators to exploit the inherent coding gain of differential modulations. An algorithm is developed based on the extrinsic information transfer (EXIT) chart to obtain optimized degree distributions of LT coded differential modulator systems in terms of convergence performance. Then, we delve deeper into the characteristics of LT codes with the objective of improving the error floor performance over noisy channels. An encoding scheme is proposed, which is subsequently used to reduce the error floor. To observe the consequences of the modified encoding scheme, the convergence behavior of the proposed LT code is analyzed using EXIT charts, and shown to be similar to the convergence performance of conventional LT codes. This idea is then extended to LT codes for transmission over erasure channels and a design framework is developed to jointly improve the transmission efficiency and erasure floor performance. For complexity-constrained applications, we construct low-complexity LT codes and devise a reduced-complexity LT decoder for transmission over noisy channels.

    The second part of the thesis deals with the analysis and design of rateless codes for multi-point communication. To address the shortcomings of existing distributed LT (DLT) codes, we introduce buffer-based DLT codes for a multi-source and multi-relay network to virtually convert lossy source-relay links to corresponding lossless links. We optimize the proposed DLT codes in terms of transmission efficiency; thus exhibiting better performance as compared to their conventional counterparts at the expense of increased computational complexity. The idea is then extended to a multi-way relay network where a linear-programming design framework is outlined for optimizing degree distributions in terms of transmission efficiency. Finally, a design framework is provided for DLT coding schemes, to jointly improve the transmission efficiency and erasure floor performance.

  • 3.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Land, I.
    Chan, T.H.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Rasmussen, Lars K.
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    A new design framework for LT codes over noisy channels2014In: 2014 IEEE International Symposium on Information Theory (ISIT), IEEE , 2014, p. 2162-2166Conference paper (Refereed)
    Abstract [en]

    Luby transform (LT) codes are a class of rateless codes that automatically adapt their rate to the quality of the communication channel. In the original LT codes, fixed check-node degree distributions are used to combine variable nodes uniformly at random to extend the code graph and produce code bits. Here we propose a different approach: we design a sequence of rate-compatible degree distributions, and develop an algorithm that produces code bits in a manner such that the resulting degree distributions follow the designed sequence. Using this new design framework, we develop low-complexity LT codes suitable for time-varying noisy channels. Performance and complexity of the proposed LT codes are measured in terms of bit error rate and average number of edges per information and coded bit, respectively. Numerical examples illustrate the resulting trade-off between performance and complexity of the designed LT codes.

  • 4.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Rasmussen, Lars
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Design of LT Codes with Equal and Unequal Erasure Protection over Binary Erasure Channels2013In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 17, no 2, p. 261-264Article in journal (Refereed)
    Abstract [en]

    The erasure floor performance of Luby Transform (LT) codes is mainly determined by the minimum variable-node degree. Thus we propose a modified encoding scheme that maximizes the minimum variable-node degree for transmission over binary erasure channels. The proposed scheme leads to an almost-regular variable-node degree distribution. The encoding process is generalized to accommodate arbitrary variable-node degree distributions for additional improved performance. The asymptotic performance is investigated using density evolution and compared with a conventional LT code. The scheme is further extended to enable a higher level of unequal erasure protection.

  • 5.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Rasmussen, Lars
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Design of Spatially-Coupled Rateless Codes2012In: 2012 IEEE 23rd International Symposium on Personal, Indoor and Mobile Radio Communications - (PIMRC), IEEE , 2012, p. 1913-1918Conference paper (Refereed)
    Abstract [en]

    We investigate the design and performance of spatially-coupled rateless codes. A modified encoding process is introduced for spatially-coupled Luby Transform (SCLT) codes which leads to an almost regular variable-node degree distribution at the encoding graph. The proposed SCLT codes outperform its counterparts significantly over binary erasure channels, particularly in the erasure floor region. To further improve the erasure floor performance, the approach of spatial coupling is then extended to Raptor codes by concatenating a high-rate pre-coder to the SCLT codes. It is shown that the spatial coupling improves the convergence threshold of Raptor codes. Different ensembles of spatially-coupled Raptor codes are constructed depending on whether pre-coders and/or LT codes are spatially-coupled. The performance of different ensembles of spatially-coupled Raptor codes is then evaluated and compared based on density evolution, leading to an improved spatially-coupled Raptor code in terms of convergence threshold and lower complexity.

  • 6.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Communication Theory.
    Rasmussen, Lars K.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Buffer-Based Distributed LT Codes2014In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 62, no 11, p. 3725-3739Article in journal (Refereed)
    Abstract [en]

    We focus on the design of distributed Luby transform (DLT) codes for erasure networks with multiple sources and multiple relays, communicating to a single destination. The erasure-floor performance of DLT codes improves with the maximum degree of the relay-degree distribution. However, for conventional DLT codes, the maximum degree is upper-bounded by the number of sources. An additional constraint is that the sources are required to have the same information block length. We introduce a D-bit buffer for each source-relay link, which allows the relay to select multiple encoded bits from the same source for the relay-encoding process; thus, the number of sources no longer limits the maximum degree at the relay. Furthermore, the introduction of buffers facilitates the use of different information block sizes across sources. Based on density evolution we develop an asymptotic analytical framework for optimization of the relay-degree distribution. We further integrate techniques for unequal erasure protection into the optimization framework. The proposed codes are considered for both lossless and lossy source-relay links. Numerical examples show that there is no loss in erasure rate performance for transmission over lossy source-relay links as compared to lossless links. Additional delays, however, may occur. The design framework and our contributions are demonstrated by a number of illustrative examples, showing the improvements obtained by the proposed buffer-based DLT codes.

  • 7.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Rasmussen, Lars K.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Rateless Codes for the Multiway Relay Channel2014In: IEEE Wireless Communications Letters, ISSN 2162-2337, E-ISSN 2162-2345, Vol. 3, no 5, p. 457-460Article in journal (Refereed)
    Abstract [en]

    We consider distributed Luby transform (DLT) codes for efficient packet transmission in a multi-way relay network, where the links are modeled as erasure channels. Density evolution is applied for asymptotic performance analysis, and subsequently used in a linear-programming design framework for optimizing the degree distribution at the relay in terms of overhead. Moreover a buffer is introduced at the relay to enable efficient downlink transmission even if packets are lost during uplink transmission. Performance losses in terms of delay and/or erasure rates caused by link erasures during uplink transmission are thus alleviated. The proposed DLT codes provide significant improvements in overhead and decoded erasure rates. Numerical results for finite-length codes follow closely the asymptotic analysis. Our results demonstrate that the proposed buffer-based DLT codes outperform its counterparts for lossy uplink transmission.

  • 8.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Rasmussen, Lars K.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Reduced-complexity decoding of LT codes over noisy channels2013In: 2013 IEEE Wireless Communications and Networking Conference (WCNC), IEEE Communications Society, 2013, p. 3856-3860Conference paper (Refereed)
    Abstract [en]

    We propose an adaptive decoding scheme for Luby Transform (LT) codes over noisy channels which exhibits lower complexity as compared to the conventional LT decoder. The corresponding modified degree distributions have been derived for the low-complexity LT decoder. The complexity and performance comparison demonstrate that the decoding complexity can be reduced with negligible degradation in bit error rate performance.

  • 9.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Rasmussen, Lars K.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Regularized Variable-Node LT Codes with Improved Erasure Floor Performance2013Conference paper (Refereed)
    Abstract [en]

    We consider the use of extrinsic information transfercharts for the design of Luby Transform (LT) codes over thebinary erasure channel (BEC). In particular, we formulate anoptimization problem to determine asymptotically good check-node degree distributions in terms of decoder overhead. Wefurther propose a modified encoding scheme that maximizes theminimum variable-node degree, thus optimizing the erasure-floorperformance at the expense of decoder overhead, and resultingin a regularized variable-node degree distribution. The two ap-proaches are combined to jointly improve decoder overhead anddecoder erasure floor by incorporating the proposed encodingstrategy into the convex optimization problem. The performanceof the proposed schemes is investigated for transmission over theBEC through density evolution and numerical simulations. Theoptimized codes compare favorably to conventional LT codes,and are further extended to enable improved performance forunequal erasure protection.

  • 10.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Rasmussen, Lars K.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Unequal error protection of LT codes over noisy channels2012In: 2012 Swedish Communication Technologies Workshop, Swe-CTW 2012, IEEE , 2012, p. 19-24Conference paper (Refereed)
    Abstract [en]

    We propose a new unequal error protection (UEP) scheme for Luby Transform (LT) codes over additive white Gaussian noise (AWGN) channels. In contrast to the state-of-the-art UEP scheme for LT codes where the effective code rates are exploited for UEP, we characterize the variable-node degree distribution to achieve UEP for LT codes. For performance analysis, we compare the lower bounds on bit error rate for our proposed scheme and a conventional UEP-based LT codes over AWGN channels for various parameters. Moreover, it is demonstrated through numerical examples that our proposed UEP scheme has better performance than the conventional UEP-based LT codes over a wide range of code rates and channel conditions.

  • 11.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Rasmussen, Lars Kildehoj
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. University of South Australia, Australia.
    Erasure Floor Analysis of Distributed LT Codes2015In: IEEE Transactions on Communications, ISSN 0090-6778, E-ISSN 1558-0857, Vol. 63, no 8, p. 2788-2796Article in journal (Refereed)
    Abstract [en]

    We investigate the erasure floor performance of distributed Luby transform (DLT) codes for transmission within a multi-source, single-relay, and single-destination erasure-link network. In general, Luby transform (LT) codes exhibit a high erasure floor due to poor minimum-distance properties, which can be improved by maximizing the minimum variable-node degree. The same behavior is observed for DLT codes, and therefore a new combining scheme at the relay is proposed to maximize the minimum variable-node degree in the decoding graph. Furthermore, the encoding process at the sources and the combining scheme at the relay are coordinated to improve the transmission overhead. To characterize the asymptotic performance of the proposed DLT codes, we derive closed-form density-evolution expressions, considering both lossless and lossy source-relay channels, respectively. To support the asymptotic analysis, we evaluate the performance of the proposed DLT codes by numerical examples and demonstrate that the numerical results correspond closely to the analysis. Significant improvements in both the erasure floor and transmission overhead are obtained for the proposed DLT codes, as compared to conventional DLT codes.

  • 12.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Rasmussen, Lars Kildehöj
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Error floor Analysis of LT Codes over the Additive White Gaussian Noise Channel2011In: 2011 IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE (GLOBECOM 2011) , IEEE , 2011, p. 6133650-Conference paper (Refereed)
    Abstract [en]

    We investigate the error floor performance of Luby Transform (LT) codes over the additive white Gaussian noise channel. We first derive a lower bound on the bit error rate for an LT code, which we subsequently use to show that the corresponding error floor is predominantly caused by low-degree variable nodes. Based on this observation, we propose a modified encoding scheme for LT codes that provides a lower error floor with no increase in encoding and decoding complexities. The convergence behavior of the proposed scheme is analyzed using extrinsic information transfer charts, and shown to be similar to the original LT code. Numerical examples demonstrate the improvements of the modified LT code as a stand-alone code and as a component code of a Raptor code.

  • 13.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Rasmussen, Lars Kildehöj
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    LT coded MSK over AWGN channels2010In: 6th International Symposium onTurbo Codes and Iterative Information Processing (ISTC) 2010, 2010, p. 289-293Conference paper (Refereed)
    Abstract [en]

    We investigate the design of Luby Transform (LT) codes with minimum-shift-keying (MSK) modulation over additive white Gaussian noise (AWGN) channels. Both systematic and nonsystematic LT codes are considered from the perspectives of decoding threshold and bit error rate. Using systematic LT codes, coding complexity can be reduced by eliminating the need for decoder-doping, and less parity bits are required as compared to the nonsystematic LT codes. In addition, systematic LT codes exhibit better performance than their nonsystematic counterparts for low fixed code rates. To evaluate the performance of the LT coded MSK system, we consider a family of rate-compatible fixed-rate codes, which allows us to use extrinsic information transfer (EXIT) charts for analysis and design. We observe that systematic LT codes have a better decoding threshold than their corresponding nonsystematic counterparts for all tested rates up to R = 0.59 bits per symbol. Furthermore, with the use of EXIT charts, we propose a new degree distribution function for the LT code, which leads to improved decoding thresholds for all examined code rates, as compared to codes using existing degree distributions optimized for binary erasure and AWGN channels. Numerical results confirm our EXIT chart analysis and conclusions.

  • 14.
    Hussain, Iqbal
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Xiao, Ming
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Rasmussen, Lars Kildehöj
    KTH, School of Electrical Engineering (EES), Communication Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Serially concatenated LT code with DQPSK modulation2011In: Wireless Communications and Networking Conference (WCNC), 2011 IEEE, 2011, p. 1811-1816Conference paper (Refereed)
    Abstract [en]

    We consider serial concatenation of a Luby Transform (LT) code with a differential quadrature phase-shift-keying (DQPSK) modulator for transmission over an additive white Gaussian noise (AWGN) Channel. Assuming a target average rate for the operation of the rateless LT DQPSK scheme, the degree distribution of the LT code is optimized in terms of convergence threshold using extrinsic information transfer (EXIT) charts. From the EXIT chart analysis, we show that the proposed LT DQPSK scheme has a similar convergence performance, but lower complexity, as compared to a Raptor code with differential modulation, and a LDPC code optimized for DQPSK. The EXIT chart analysis framework is also applied to evaluate the throughput performance for the three schemes in terms of the average code rate as a function of the signal-to-noise ratio. The comparison demonstrates that the proposed structure is well-suited for adaptive-rate transmission over a wide range of rates.

  • 15.
    Malik, Naveed ur Rehman
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Husain, Iqbal
    KTH, School of Electrical Engineering (EES), Communication Theory. NCSU, FREEDM Systems Center, Raliegh, USA.
    Dynamic and Steady-State 3-D Thermal Design and Investigation of the Rotating Power Electronic IGBT ConverterManuscript (preprint) (Other academic)
    Abstract [en]

    This paper deals with a 3-dimensional developmentof a thermal model of a power electronic converter mountedon the generator shaft and rotating with it. The dimensions ofthe heat sink are determined and the temperature gradients ofthe converter, its heat sink, and shaft during natural and forcedconvection are analyzed for variable rotor speeds. It is shown thatthe chosen sizes of the IGBT and heat sink offers compact designof the rotating converter, which is sufficient for its mounting inthe limited space, offered by the generator shaft. Furthermore,transient temperature profile is also presented. Additionally,transient thermal profile of the converter and dimensions of thecooling fan are also calculated. Besides analysis of the coolingrequirements of the converter during over-currents due to gridfaults is also investigated.

  • 16.
    Malik, Naveed ur Rehman
    et al.
    KTH, School of Electrical Engineering (EES), Electrical Energy Conversion.
    Husain, Iqbal
    KTH, School of Electrical Engineering (EES), Communication Theory. NCSU, FREEDM Systems Center, Raliegh, USA.
    Transient and Steady-State 3-D Electro-Thermal Design and Analysis of the Rotating Power Electronic IGBT Converter2015In: IEEE InternationalElectric Machines and Drives Conference (IEMDC), 10-13 May 2015., Institute of Electrical and Electronics Engineers (IEEE), 2015Conference paper (Refereed)
    Abstract [en]

    This paper deals with a 3-dimensional developmentof a thermal model of a power electronic converter mountedon the generator shaft and rotating with it. The dimensions ofthe heat sink are determined and the temperature gradients ofthe converter, its heat sink, and shaft during natural and forcedconvection are analyzed for variable rotor speeds. It is shown thatthe chosen sizes of the IGBT and heat sink offers compact designof the rotating converter, which is sufficient for its mounting inthe limited space, offered by the generator shaft. Furthermore,transient temperature profile is also presented.

  • 17.
    Malik, Naveed-ur-Rehman
    et al.
    KTH, School of Electrical Engineering (EES).
    Husain, Iqbal
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Dynamic and Steady-State 3-D Thermal Design and Investigation of the Rotating Power Electronic IGBT Converter2016In: IEEE Journal of Emerging and Selected Topics in Power Electronics, ISSN 2168-6777, E-ISSN 2168-6785, Vol. 4, no 2, p. 679-688Article in journal (Refereed)
    Abstract [en]

    This paper deals with a 3-D development of a thermal model of a power electronic converter mounted on the generator shaft and rotating with it. The dimensions of the heat sink are determined, and the temperature gradients of the converter, its heat sink, and shaft during natural and forced convection are analyzed for variable rotor speeds. It is shown that the chosen sizes of the insulated gate bipolar transistor (IGBT) and the heat sink offer a compact design of the rotating converter, which is sufficient for its mounting in limited space offered by the generator shaft. Furthermore, a transient temperature profile is also presented. In addition, a transient thermal profile of the converter and dimensions of the cooling fan are also calculated. Besides, the analysis of the cooling requirements of the converter during over-currents due to grid faults is also investigated.

1 - 17 of 17
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