62 research outputs found
LDPC Code Design for Noncoherent Physical Layer Network Coding
This work considers optimizing LDPC codes in the physical-layer network coded
two-way relay channel using noncoherent FSK modulation. The error-rate
performance of channel decoding at the relay node during the multiple-access
phase was improved through EXIT-based optimization of Tanner graph variable
node degree distributions. Codes drawn from the DVB-S2 and WiMAX standards were
used as a basis for design and performance comparison. The computational
complexity characteristics of the standard codes were preserved in the
optimized codes by maintaining the extended irregular repeat-accumulate (eIRA).
The relay receiver performance was optimized considering two modulation orders
M = {4, 8} using iterative decoding in which the decoder and demodulator refine
channel estimates by exchanging information. The code optimization procedure
yielded unique optimized codes for each case of modulation order and available
channel state information. Performance of the standard and optimized codes were
measured using Monte Carlo simulation in the flat Rayleigh fading channel, and
error rate improvements up to 1.2 dB are demonstrated depending on system
parameters.Comment: Six pages, submitted to 2015 IEEE International Conference on
Communication
Iterative decoding and detection for physical layer network coding
PhD ThesisWireless networks comprising multiple relays are very common and it is
important that all users are able to exchange messages via relays in the
shortest possible time. A promising technique to achieve this is physical
layer network coding (PNC), where the time taken to exchange messages
between users is achieved by exploiting the interference at the relay due
to the multiple incoming signals from the users. At the relay, the interference
is demapped to a binary sequence representing the exclusive-OR of
both users’ messages. The time to exchange messages is reduced because
the relay broadcasts the network coded message to both users, who can
then acquire the desired message by applying the exclusive-OR of their
original message with the network coded message. However, although
PNC can increase throughput it is at the expense of performance degradation
due to errors resulting from the demapping of the interference to
bits.
A number of papers in the literature have investigated PNC with an iterative
channel coding scheme in order to improve performance. However,
in this thesis the performance of PNC is investigated for end-to-end
(E2E) the three most common iterative coding schemes: turbo codes,
low-density parity-check (LDPC) codes and trellis bit-interleaved coded
modulation with iterative decoding (BICM-ID). It is well known that in
most scenarios turbo and LDPC codes perform similarly and can achieve
near-Shannon limit performance, whereas BICM-ID does not perform
quite as well but has a lower complexity. However, the results in this
thesis show that on a two-way relay channel (TWRC) employing PNC,
LDPC codes do not perform well and BICM-ID actually outperforms
them while also performing comparably with turbo codes. Also presented
in this thesis is an extrinsic information transfer (ExIT) chart
analysis of the iterative decoders for each coding scheme, which is used
to explain this surprising result. Another problem arising from the use
of PNC is the transfer of reliable information from the received signal at
the relay to the destination nodes. The demapping of the interference to
binary bits means that reliability information about the received signal
is lost and this results in a significant degradation in performance when
applying soft-decision decoding at the destination nodes. This thesis
proposes the use of traditional angle modulation (frequency modulation
(FM) and phase modulation (PM)) when broadcasting from the relay,
where the real and imaginary parts of the complex received symbols
at the relay modulate the frequency or phase of a carrier signal, while
maintaining a constant envelope. This is important since the complex
received values at the relay are more likely to be centred around zero and
it undesirable to transmit long sequences of low values due to potential
synchronisation problems at the destination nodes. Furthermore, the
complex received values, obtained after angle demodulation, are used to
derive more reliable log-likelihood ratios (LLRs) of the received symbols
at the destination nodes and consequently improve the performance of
the iterative decoders for each coding scheme compared with conventionally
coded PNC.
This thesis makes several important contributions: investigating the performance
of different iterative channel coding schemes combined with
PNC, presenting an analysis of the behaviour of different iterative decoding
algorithms when PNC is employed using ExIT charts, and proposing
the use of angle modulation at the relay to transfer reliable information
to the destination nodes to improve the performance of the iterative decoding
algorithms. The results from this thesis will also be useful for
future research projects in the areas of PNC that are currently being
addressed, such as synchronisation techniques and receiver design.Iraqi Ministry of Higher
Education and Scientific Research
Self-concatenated code design and its application in power-efficient cooperative communications
In this tutorial, we have focused on the design of binary self-concatenated coding schemes with the help of EXtrinsic Information Transfer (EXIT) charts and Union bound analysis. The design methodology of future iteratively decoded self-concatenated aided cooperative communication schemes is presented. In doing so, we will identify the most important milestones in the area of channel coding, concatenated coding schemes and cooperative communication systems till date and suggest future research directions
Iterative decoding combined with physical-layer network coding on impulsive noise channels
PhD ThesisThis thesis investigates the performance of a two-way wireless relay channel (TWRC)
employing physical layer network coding (PNC) combined with binary and non-binary
error-correcting codes on additive impulsive noise channels. This is a research topic that
has received little attention in the research community, but promises to offer very
interesting results as well as improved performance over other schemes. The binary
channel coding schemes include convolutional codes, turbo codes and trellis bitinterleaved
coded modulation with iterative decoding (BICM-ID). Convolutional codes
and turbo codes defined in finite fields are also covered due to non-binary channel
coding schemes, which is a sparse research area. The impulsive noise channel is based on
the well-known Gaussian Mixture Model, which has a mixture constant denoted by α.
The performance of PNC combined with the different coding schemes are evaluated with
simulation results and verified through the derivation of union bounds for the theoretical
bit-error rate (BER). The analyses of the binary iterative codes are presented in the form
of extrinsic information transfer (ExIT) charts, which show the behaviour of the iterative
decoding algorithms at the relay of a TWRC employing PNC and also the signal-to-noise
ratios (SNRs) when the performance converges. It is observed that the non-binary coding
schemes outperform the binary coding schemes at low SNRs and then converge at higher
SNRs. The coding gain at low SNRs become more significant as the level of
impulsiveness increases. It is also observed that the error floor due to the impulsive noise
is consistently lower for non-binary codes. There is still great scope for further research
into non-binary codes and PNC on different channels, but the results in this thesis have
shown that these codes can achieve significant coding gains over binary codes for
wireless networks employing PNC, particularly when the channels are harsh
Cross-layer hybrid automatic repeat request error control with turbo processing for wireless system
The increasing demand for wireless communication system requires an efficient design in wireless communication system. One of the main challenges is to design error control mechanism in noisy wireless channel. Forward Error Correction (FEC) and Automatic Repeat reQuest (ARQ) are two main error control mechanisms. Hybrid ARQ allows the use of either FEC or ARQ when required. The issues with existing Hybrid ARQ are reliability, complexity and inefficient design. Therefore, the design of Hybrid ARQ needs to be further improved in order to achieve performance close to the Shannon capacity. The objective of this research is to develop a Cross-Layer Design Hybrid ARQ defined as CLD_ARQ to further minimize error in wireless communication system. CLD_ARQ comprises of three main stages. First, a low complexity FEC defined as IRC_FEC for error detection and correction has been developed by using Irregular Repetition Code (IRC) with Turbo processing. The second stage is the enhancement of IRC_FEC defined as EM_IRC_FEC to improve the reliability of error detection by adopting extended mapping. The last stage is the development of efficient CLD_ARQ to include retransmission for error correction that exploits EM_IRC_FEC and ARQ. In the proposed design, serial iterative decoding and parallel iterative decoding are deployed in the error detection and correction. The performance of the CLD_ARQ is evaluated in the Additive White Gaussian Noise (AWGN) channel using EXtrinsic Information Transfer (EXIT) chart, bit error rate (BER) and throughput analysis. The results show significant Signal-to-Noise Ratio (SNR) gain from the theoretical limit at BER of 10-5. IRC_FEC outperforms Recursive Systematic Convolutional Code (RSCC) by SNR gain up to 7% due to the use of IRC as a simple channel coding code. The usage of CLD_ARQ enhances the SNR gain by 53% compared to without ARQ due to feedback for retransmission. The adoption of extended mapping in the CLD_ARQ improves the SNR gain up to 50% due to error detection enhancement. In general, the proposed CLD_ARQ can achieve low BER and close to the Shannon‘s capacity even in worse channel condition
Self-concatenated coding for wireless communication systems
In this thesis, we have explored self-concatenated coding schemes that are designed for transmission over Additive White Gaussian Noise (AWGN) and uncorrelated Rayleigh fading channels. We designed both the symbol-based Self-ConcatenatedCodes considered using Trellis Coded Modulation (SECTCM) and bit-based Self- Concatenated Convolutional Codes (SECCC) using a Recursive Systematic Convolutional (RSC) encoder as constituent codes, respectively. The design of these codes was carried out with the aid of Extrinsic Information Transfer (EXIT) charts. The EXIT chart based design has been found an efficient tool in finding the decoding convergence threshold of the constituent codes. Additionally, in order to recover the information loss imposed by employing binary rather than non-binary schemes, a soft decision demapper was introduced in order to exchange extrinsic information withthe SECCC decoder. To analyse this information exchange 3D-EXIT chart analysis was invoked for visualizing the extrinsic information exchange between the proposed Iteratively Decoding aided SECCC and soft-decision demapper (SECCC-ID). Some of the proposed SECTCM, SECCC and SECCC-ID schemes perform within about 1 dB from the AWGN and Rayleigh fading channels’ capacity. A union bound analysis of SECCC codes was carried out to find the corresponding Bit Error Ratio (BER) floors. The union bound of SECCCs was derived for communications over both AWGN and uncorrelated Rayleigh fading channels, based on a novel interleaver concept.Application of SECCCs in both UltraWideBand (UWB) and state-of-the-art video-telephone schemes demonstrated its practical benefits.In order to further exploit the benefits of the low complexity design offered by SECCCs we explored their application in a distributed coding scheme designed for cooperative communications, where iterative detection is employed by exchanging extrinsic information between the decoders of SECCC and RSC at the destination. In the first transmission period of cooperation, the relay receives the potentially erroneous data and attempts to recover the information. The recovered information is then re-encoded at the relay using an RSC encoder. In the second transmission period this information is then retransmitted to the destination. The resultant symbols transmitted from the source and relay nodes can be viewed as the coded symbols of a three-component parallel-concatenated encoder. At the destination a Distributed Binary Self-Concatenated Coding scheme using Iterative Decoding (DSECCC-ID) was employed, where the two decoders (SECCC and RSC) exchange their extrinsic information. It was shown that the DSECCC-ID is a low-complexity scheme, yet capable of approaching the Discrete-input Continuous-output Memoryless Channels’s (DCMC) capacity.Finally, we considered coding schemes designed for two nodes communicating with each other with the aid of a relay node, where the relay receives information from the two nodes in the first transmission period. At the relay node we combine a powerful Superposition Coding (SPC) scheme with SECCC. It is assumed that decoding errors may be encountered at the relay node. The relay node then broadcasts this information in the second transmission period after re-encoding it, again, using a SECCC encoder. At the destination, the amalgamated block of Successive Interference Cancellation (SIC) scheme combined with SECCC then detects and decodes the signal either with or without the aid of a priori information. Our simulation results demonstrate that the proposed scheme is capable of reliably operating at a low BER for transmission over both AWGN and uncorrelated Rayleigh fading channels. We compare the proposed scheme’s performance to a direct transmission link between the two sources having the same throughput
Doubly-Irregular Repeat-Accumulate Codes over Integer Rings for Multi-user Communications
Structured codes based on lattices were shown to provide enlarged capacity
for multi-user communication networks. In this paper, we study
capacity-approaching irregular repeat accumulate (IRA) codes over integer rings
for -PAM signaling, . Such codes
feature the property that the integer sum of codewords belongs to the
extended codebook (or lattice) w.r.t. the base code. With it, \emph{%
structured binning} can be utilized and the gains promised in lattice based
network information theory can be materialized in practice. In designing IRA
ring codes, we first analyze the effect of zero-divisors of integer ring on the
iterative belief-propagation (BP) decoding, and show the invalidity of
symmetric Gaussian approximation. Then we propose a doubly IRA (D-IRA) ring
code structure, consisting of \emph{irregular multiplier distribution} and
\emph{irregular node-degree distribution}, that can restore the symmetry and
optimize the BP decoding threshold. For point-to-point AWGN channel with -PAM inputs, D-IRA ring codes perform as low as 0.29 dB to the capacity
limits, outperforming existing bit-interleaved coded-modulation (BICM) and IRA
modulation codes over GF(). We then proceed to design D-IRA ring codes for
two important multi-user communication setups, namely compute-forward (CF) and
dirty paper coding (DPC), with -PAM signaling. With it, a physical-layer
network coding scheme yields a gap to the CF limit by 0.24 dB, and a simple
linear DPC scheme exhibits a gap to the capacity by 0.91 dB.Comment: 30 pages, 13 figures, submitted to IEEE Trans. Signal Processin
Optimization of a Coded-Modulation System with Shaped Constellation
Conventional communication systems transmit signals that are selected from a signal constellation with uniform probability. However, information-theoretic results suggest that performance may be improved by shaping the constellation such that lower-energy signals are selected more frequently than higher-energy signals. This dissertation presents an energy efficient approach for shaping the constellations used by coded-modulation systems. The focus is on designing shaping techniques for systems that use a combination of amplitude phase shift keying (APSK) and low-density parity check (LDPC) coding. Such a combination is typical of modern satellite communications, such as the system used by the DVB-S2 standard.;The system implementation requires that a subset of the bits at the output of the LDPC encoder are passed through a nonlinear shaping encoder whose output bits are more likely to be a zero than a one. The constellation is partitioned into a plurality of sub-constellations, each with a different average signal energy, and the shaping bits are used to select the sub-constellation. An iterative receiver exchanges soft information among the demodulator, LDPC decoder, and shaping decoder. Parameters associated with the modulation and shaping code are optimized with respect to information rate, while the design of the LDPC code is optimized for the shaped modulation with the assistance of extrinsic-information transfer (EXIT) charts. The rule for labeling the constellation with bits is optimized using a novel hybrid cost function and a binary switching algorithm.;Simulation results show that the combination of constellation shaping, LDPC code optimization, and optimized bit labeling can achieve a gain in excess of 1 dB in an additive white Gaussian noise (AWGN) channel at a rate of 3 bits/symbol compared with a system that adheres directly to the DVB-S2 standard
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