46 research outputs found
New Identification and Decoding Techniques for Low-Density Parity-Check Codes
Error-correction coding schemes are indispensable for high-capacity high data-rate communication systems nowadays. Among various channel coding schemes, low-density parity-check (LDPC) codes introduced by pioneer Robert G. Gallager are prominent due to the capacity-approaching and superior error-correcting properties. There is no hard constraint on the code rate of LDPC codes. Consequently, it is ideal to incorporate LDPC codes with various code rate and codeword length in the adaptive modulation and coding (AMC) systems which change the encoder and the modulator adaptively to improve the system throughput. In conventional AMC systems, a dedicated control channel is assigned to coordinate the encoder/decoder changes. A questions then rises: if the AMC system still works when such a control channel is absent. This work gives positive answer to this question by investigating various scenarios consisting of different modulation schemes, such as quadrature-amplitude modulation (QAM), frequency-shift keying (FSK), and different channels, such as additive white Gaussian noise (AWGN) channels and fading channels. On the other hand, LDPC decoding is usually carried out by iterative belief-propagation (BP) algorithms. As LDPC codes become prevalent in advanced communication and storage systems, low-complexity LDPC decoding algorithms are favored in practical applications. In the conventional BP decoding algorithm, the stopping criterion is to check if all the parities are satisfied. This single rule may not be able to identify the undecodable blocks, as a result, the decoding time and power consumption are wasted for executing unnecessary iterations. In this work, we propose a new stopping criterion to identify the undecodable blocks in the early stage of the iterative decoding process. Furthermore, in the conventional BP decoding algorithm, the variable (check) nodes are updated in parallel. It is known that the number of iterations can be reduced by the serial scheduling algorithm. The informed dynamic scheduling (IDS) algorithms were proposed in the existing literatures to further reduce the number of iterations. However, the computational complexity involved in finding the update node in the existing IDS algorithms would not be neglected. In this work, we propose a new efficient IDS scheme which can provide better performance-complexity trade-off compared to the existing IDS ones. In addition, the iterative decoding threshold, which is used for differentiating which LDPC code is better, is investigated in this work. A family of LDPC codes, called LDPC convolutional codes, has drawn a lot of attentions from researchers in recent years due to the threshold saturation phenomenon. The IDT for an LDPC convolutional code may be computationally demanding when the termination length goes to thousand or even approaches infinity, especially for AWGN channels. In this work, we propose a fast IDT estimation algorithm which can greatly reduce the complexity of the IDT calculation for LDPC convolutional codes with arbitrary large termination length (including infinity). By utilizing our new IDT estimation algorithm, the IDTs for LDPC convolutional codes with arbitrary large termination length (including infinity) can be quickly obtained
Spectrally and Energy Efficient Wireless Communications: Signal and System Design, Mathematical Modelling and Optimisation
This thesis explores engineering studies and designs aiming to meeting the requirements of enhancing capacity and energy efficiency for next generation communication networks. Challenges of spectrum scarcity and energy constraints are addressed and new technologies are proposed, analytically investigated and examined.
The thesis commences by reviewing studies on spectrally and energy-efficient techniques, with a special focus on non-orthogonal multicarrier modulation, particularly spectrally efficient frequency division multiplexing (SEFDM). Rigorous theoretical and mathematical modelling studies of SEFDM are presented. Moreover, to address the potential application of SEFDM under the 5th generation new radio (5G NR) heterogeneous numerologies, simulation-based studies of SEFDM coexisting with orthogonal frequency division multiplexing (OFDM) are conducted. New signal formats and corresponding transceiver structure are designed, using a Hilbert transform filter pair for shaping pulses. Detailed modelling and numerical investigations show that the proposed signal doubles spectral efficiency without performance degradation, with studies of two signal formats; uncoded narrow-band internet of things (NB-IoT) signals and unframed turbo coded multi-carrier signals. The thesis also considers using constellation shaping techniques and SEFDM for capacity enhancement in 5G system. Probabilistic shaping for SEFDM is proposed and modelled to show both transmission energy reduction and bandwidth saving with advantageous flexibility for data rate adaptation. Expanding on constellation shaping to improve performance further, a comparative study of multidimensional modulation techniques is carried out. A four-dimensional signal, with better noise immunity is investigated, for which metaheuristic optimisation algorithms are studied, developed, and conducted to optimise bit-to-symbol mapping. Finally, a specially designed machine learning technique for signal and system design in physical layer communications is proposed, utilising the application of autoencoder-based end-to-end learning. Multidimensional signal modulation with multidimensional constellation shaping is proposed and optimised by using machine learning techniques, demonstrating significant improvement in spectral and energy efficiencies
Optimizing LDPC codes for a mobile WiMAX system with a saturated transmission amplifier
In mobile communication, the user’s information is transmitted through a wireless communication link that is subjected to a range of deteriorating effects. The quality of the transmission can be presented by the rate of transfer and the reliability of the received stream. The capacity of the communication link can be reached through the use of channel coding. Channel coding is the method of adding redundant information to the user’s information to mitigate the deteriorating effects of the communication link. Mobile WiMAX is a technology that makes use of orthogonal frequency division multiplexing (OFDM) modulation to transmit information over a wireless communication channel. The OFDM physical layer has a high peak average to power ratio (PAPR) characteristic that saturates the transmitter’s amplifier quite easily when proper backoff is not made in the transmission power. In this dissertation an optimized graph code was used as an alternative solution to improve the system’s performance in the presence of a saturated transmission’s amplifier. The graph code was derived from a degree distribution given by the density evolution algorithm and provided no extra network overhead to implement. The performance analysis resulted in a factor of 10 improvement in the error floor and a coding gain of 1.5 dB. This was all accomplished with impairments provided by the mobile WiMAX standard in the construction of the graph code.Dissertation (MEng)--University of Pretoria, 2009.Electrical, Electronic and Computer Engineeringunrestricte
Advanced Statistical Signal Processing Methods in Sensing, Detection, and Estimation for Communication Applications
The applications of wireless communications and digital signal processing have dramatically changed the way we live, work, and learn over decades. The requirement of higher throughput and ubiquitous connectivity for wireless communication systems has become prevalent nowadays. Signal sensing, detection and estimation have been prevalent in signal processing and communications for many years. The relevant studies deal with the processing of information-bearing signals for the purpose of information extraction. Nevertheless, new robust and efficient signal sensing, detection and estimation techniques are still in demand since there emerge more and more practical applications which rely on them. In this dissertation work, we proposed several novel signal sensing, detection and estimation schemes for wireless communications applications, such as spectrum sensing, symbol-detection/channel-estimation, and encoder identification. The associated theories and practice in robustness, computational complexity, and overall system performance evaluation are also provided
Resource allocation software algorithms for AMC-OFDM systems
PhD ThesisIn recent years, adaptive modulation and coding (AMC) technologies,
resource allocation strategies and user scheduling for single-cell downlink
orthogonal frequency division multiplexing (OFDM) and orthogonal
frequency division multiple access (OFDMA) systems have been
widely researched in order to ensure that capacity and throughput are
maximised. In terms of AMC technologies, the correlation between the
channel coefficients corresponding to the transmitted sub-carriers has
not been considered yet. In the literature of resource allocation and user
scheduling, either channel coding is not considered or only a fixed code
rate is specified. Consequently, with a fixed number of data sub-carriers
for each user, all these criteria restrict the flexibility of exploiting the
available channel capacity, which reflects negatively on system throughput.
At the same time, the presented scheduling algorithms so far managed
the data of each user regardless the fair services of all users. The
philosophy of this thesis is to maximise the average system throughput
by proposing novel AMC, resource allocation and user scheduling
strategies for OFDM and OFDMA systems based on developed software
engineering life cycle models. These models have been designed to
guarantee the scalability, extendibility and portability of the proposed
strategies. This thesis presents an AMC strategy that divides the transmitted
frame into sub-channels with an equal number of sub-carriers and
selects different modulation and coding schemes (MCSs) amongst them
rather than considering the same MCS for the entire frame. This strategy
has been combined with a pilot adjustment scheme that reduces the
pilots used for channel estimation in each sub-channel depending on the
measured coherence bandwidth, signal to noise ratio (SNR), and SNR
fluctuation values. The reduced pilots are replaced with additional data
sub-carriers in order to improve the throughput. Additionally, a novel
resource allocation strategy has been introduced in order to maximise
the system throughput by distributing the users, transmission power and
information bit streams over the employed sub-channels. The introduced
method utilises the proposed AMC strategy in combination with pilot
adjustment scheme to tackle the problem of channel capacity exploiting
efficiently. It presents the throughput as a new cost function in terms
of spectral efficiency and bit-error rate (BER), in which both convolutional
coding rates and modulation order can be varied. The investigated
throughput maximisation problem has been solved by producing two approaches.
Firstly, optimised approach that solves the adopted problem
optimally using the well known Lagrange multipliers method. This approach
requires a huge search processes to achieve the optimal allocation
of the resources, which yields a high computational complexity. To overcome
the complexity issue, the second approach decouples the considered
maximisation problem into two sub-problems based on the decomposition
method on the cost of performance particularly for low SNR values.
The proposed resource allocation strategy has been developed to
work with multi-input-multi-output (MIMO) based AMC-OFDMA systems.
In this project, two MIMO transmission criteria are considered,
i.e. traditional and eigen-mode. In contrast, a user scheduling algorithm
combined with the proposed resource allocation and AMC strategies is
presented. The user scheduling algorithm aims to maximize the average
system throughput by arranging the users in distinct queues according
to their priorities and selecting the best user of each queue individually
in order to guarantee a fair user service amongst different priority levels.
When the involved users are scheduled, the scheduled users have been
passed to the resource allocation to implement the distribution of the
available resources. The proposed strategies have been tested over different
international telecommunication union (ITU) channel profiles. The
obtained simulation results show the superior performance of the introduced
approaches in comparison with the related conventional methods.
Furthermore, the gradually improvement in the throughput performance
of the AMC-OFDM/ODMA system throughout the combination of the
proposed strategies is clearly explained.Ministry of Higher Education and Scientific
Research/IRAQ
A Performance comparision of polar codes with convolutional turbo codes
Ankara : Department of Electrical and Electronic Engineering and the Institute of Engineering and Sciences of Bilkent University, 2009.Thesis (Master's) -- Bilkent University, 2009.Includes bibliographical references leaves 134-136.Polar codes introduced recently by Arıkan are the first low-complexity codes
achieving symmetric capacity for arbitrary binary-input discrete memoryless
channels (B-DMCs). Although being theoretically significant, their practical significance
is an issue that has not yet been fully explored. Previous studies have
compared polar codes with Reed-Muller codes, where it was found that polar
codes can outperform them. In this thesis, to investigate how polar codes perform
against state-of-the-art forward error correction (FEC) codes used in practice,
we implement a IEEE 802.16 based link-level Worldwide Interoperability
for Microwave Access (WiMAX) simulator which incorporates several WiMAX
FEC options, and polar codes. IEEE 802.16 standards family define standards
for current and next generation broadband wireless access, which will make high
data rate multimedia applications in mobile environments a reality. Next generation
broadband access standard, pursued by the IEEE 802.16 Task Group m is a
work in progress, and requires even more sophisticated error correction schemes
so that higher throughput, better QOS, higher mobilities, wider ranges and lower
latencies are supported. We perform performance comparison simulations with the convolutional turbo codes (CTC) configurations defined in IEEE 802.16e to
see how much of a performance gap exists between polar codes and CTCs. The
main findings of the thesis are that, although the polar codes achieve capacity
for specific conditions, as expected, for the code lengths and channel conditions
we have simulated, the performance of them cannot compete with that of the
CTCs with equivalent rates and lengths. It remains a task to see whether polar
codes can achieve similar performances with CTCs when used as component
codes in other configurations and aid in the advancement of new communication
technologies.Özgür, ÜstünM.S
Improved Nonlinear Transform Source-Channel Coding to Catalyze Semantic Communications
Recent deep learning methods have led to increased interest in solving
high-efficiency end-to-end transmission problems. These methods, we call
nonlinear transform source-channel coding (NTSCC), extract the semantic latent
features of source signal, and learn entropy model to guide the joint
source-channel coding with variable rate to transmit latent features over
wireless channels. In this paper, we propose a comprehensive framework for
improving NTSCC, thereby higher system coding gain, better model versatility,
and more flexible adaptation strategy aligned with semantic guidance are all
achieved. This new sophisticated NTSCC model is now ready to support large-size
data interaction in emerging XR, which catalyzes the application of semantic
communications. Specifically, we propose three useful improvement approaches.
First, we introduce a contextual entropy model to better capture the spatial
correlations among the semantic latent features, thereby more accurate rate
allocation and contextual joint source-channel coding are developed accordingly
to enable higher coding gain. On that basis, we further propose response
network architectures to formulate versatile NTSCC, i.e., once-trained model
supports various rates and channel states that benefits the practical
deployment. Following this, we propose an online latent feature editing method
to enable more flexible coding rate control aligned with some specific semantic
guidance. By comprehensively applying the above three improvement methods for
NTSCC, a deployment-friendly semantic coded transmission system stands out
finally. Our improved NTSCC system has been experimentally verified to achieve
considerable bandwidth saving versus the state-of-the-art engineered VTM + 5G
LDPC coded transmission system with lower processing latency
Fuzzy logic in handovers of Mobile WiMAX
Projecte final de carrera fet en col.laboració amb el centre Czech Technical Universit
Cellular, Wide-Area, and Non-Terrestrial IoT: A Survey on 5G Advances and the Road Towards 6G
The next wave of wireless technologies is proliferating in connecting things
among themselves as well as to humans. In the era of the Internet of things
(IoT), billions of sensors, machines, vehicles, drones, and robots will be
connected, making the world around us smarter. The IoT will encompass devices
that must wirelessly communicate a diverse set of data gathered from the
environment for myriad new applications. The ultimate goal is to extract
insights from this data and develop solutions that improve quality of life and
generate new revenue. Providing large-scale, long-lasting, reliable, and near
real-time connectivity is the major challenge in enabling a smart connected
world. This paper provides a comprehensive survey on existing and emerging
communication solutions for serving IoT applications in the context of
cellular, wide-area, as well as non-terrestrial networks. Specifically,
wireless technology enhancements for providing IoT access in fifth-generation
(5G) and beyond cellular networks, and communication networks over the
unlicensed spectrum are presented. Aligned with the main key performance
indicators of 5G and beyond 5G networks, we investigate solutions and standards
that enable energy efficiency, reliability, low latency, and scalability
(connection density) of current and future IoT networks. The solutions include
grant-free access and channel coding for short-packet communications,
non-orthogonal multiple access, and on-device intelligence. Further, a vision
of new paradigm shifts in communication networks in the 2030s is provided, and
the integration of the associated new technologies like artificial
intelligence, non-terrestrial networks, and new spectra is elaborated. Finally,
future research directions toward beyond 5G IoT networks are pointed out.Comment: Submitted for review to IEEE CS&