65 research outputs found
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
Wave-like Decoding of Tail-biting Spatially Coupled LDPC Codes Through Iterative Demapping
For finite coupling lengths, terminated spatially coupled low-density
parity-check (SC-LDPC) codes show a non-negligible rate-loss. In this paper, we
investigate if this rate loss can be mitigated by tail-biting SC-LDPC codes in
conjunction with iterative demapping of higher order modulation formats.
Therefore, we examine the BP threshold of different coupled and uncoupled
ensembles. A comparison between the decoding thresholds approximated by EXIT
charts and the density evolution results of the coupled and uncoupled ensemble
is given. We investigate the effect and potential of different labelings for
such a set-up using per-bit EXIT curves, and exemplify the method for a 16-QAM
system, e.g., using set partitioning labelings. A hybrid mapping is proposed,
where different sub-blocks use different labelings in order to further optimize
the decoding thresholds of tail-biting codes, while the computational
complexity overhead through iterative demapping remains small.Comment: presentat at the International Symposium on Turbo Codes & Iterative
Information Processing (ISTC), Brest, Sept. 201
Low-Density Parity-Check Coded High-order Modulation Schemes
In this thesis, we investigate how to support reliable data transmissions at high speeds in future communication systems, such as 5G/6G, WiFi, satellite, and optical communications. One of the most fundamental problems in these communication systems is how to reliably transmit information with a limited number of resources, such as power and spectral.
To obtain high spectral efficiency, we use coded modulation (CM), such as bit-interleaved coded modulation (BICM) and delayed BICM (DBICM). To be specific, BICM is a pragmatic implementation of CM which has been largely adopted in both industry and academia. While BICM approaches CM capacity at high rates, the capacity gap between BICM and CM is still noticeable at lower code rates. To tackle this problem, DBICM, as a variation of BICM, introduces a delay module to create a dependency between multiple codewords, which enables us to exploit extrinsic information from the decoded delayed sub-blocks to improve the detection of the undelayed sub-blocks. Recent work shows that DBICM improves capacity over BICM. In addition, BICM and DBICM schemes protect each bit-channel differently, which is often referred to as the unequal error protection (UEP) property. Therefore, bit mapping designs are important for constructing pragmatic BICM and DBICM. To provide reliable communication, we have jointly designed bit mappings in DBICM and irregular low-density parity-check (LDPC) codes. For practical considerations, spatially coupled LDPC (SC-LDPC) codes have been considered as well. Specifically, we have investigated the joint design of the multi-chain SC-LDPC and the BICM bit mapper. In addition, the design of SC-LDPC codes with improved decoding threshold performance and reduced rate loss has been investigated in this thesis as well.
The main body of this thesis consists of three parts. In the first part, considering Gray-labeled square M-ary quadrature amplitude modulation (QAM) constellations, we investigate the optimal delay scheme with the largest spectrum efficiency of DBICM for a fixed maximum number of delayed time slots and a given signal-to-noise ratio. Furthermore, we jointly optimize degree distributions and channel assignments of LDPC codes using protograph-based extrinsic information transfer charts. In addition, we proposed a constrained progressive edge growth-like algorithm to jointly construct LDPC codes and bit mappings for DBICM, taking the capacity of each bit-channel into account. Simulation results demonstrate that the designed LDPC-coded DBICM systems significantly outperform LDPC-coded BICM systems. In the second part, we proposed a windowed decoding algorithm for DBICM, which uses the extrinsic information of both the decoded delayed and undelayed sub-blocks, to improve the detection for all sub-blocks. We show that the proposed windowed decoding significantly outperforms the original decoding, demonstrating the effectiveness of the proposed decoding algorithm. In the third part, we apply multi-chain SC-LDPC to BICM. We investigate various connections for multi-chain SC-LDPC codes and bit mapping designs and analyze the performance of the multi-chain SC-LDPC codes over the equivalent binary erasure channels via density evolution. Numerical results demonstrate the superiority of the proposed design over existing connected-chain ensembles and over single-chain ensembles with the existing bit mapping design
Wave-like Decoding of Tail-biting Spatially Coupled LDPC Codes Through Iterative Demapping
For finite coupling lengths, terminated spatially coupled low-density
parity-check (SC-LDPC) codes show a non-negligible rate-loss. In this paper, we
investigate if this rate loss can be mitigated by tail-biting SC-LDPC codes in
conjunction with iterative demapping of higher order modulation formats.
Therefore, we examine the BP threshold of different coupled and uncoupled
ensembles. A comparison between the decoding thresholds approximated by EXIT
charts and the density evolution results of the coupled and uncoupled ensemble
is given. We investigate the effect and potential of different labelings for
such a set-up using per-bit EXIT curves, and exemplify the method for a 16-QAM
system, e.g., using set partitioning labelings. A hybrid mapping is proposed,
where different sub-blocks use different labelings in order to further optimize
the decoding thresholds of tail-biting codes, while the computational
complexity overhead through iterative demapping remains small.Comment: presentat at the International Symposium on Turbo Codes & Iterative
Information Processing (ISTC), Brest, Sept. 201
High Order Modulation Protograph Codes
Digital communication coding methods for designing protograph-based bit-interleaved code modulation that is general and applies to any modulation. The general coding framework can support not only multiple rates but also adaptive modulation. The method is a two stage lifting approach. In the first stage, an original protograph is lifted to a slightly larger intermediate protograph. The intermediate protograph is then lifted via a circulant matrix to the expected codeword length to form a protograph-based low-density parity-check code
Low-Density Hybrid-Check Coded Superposition Mapping and its Application in OFDM and MIMO
Since Shannon’s landmark paper, many approaches have been proposed to achieve the channel capacity. In the low SNR regime, the problem has almost been solved by capacity achieving channel codes. The research on coded modulation in the high SNR regime is still under development. Among many methods in accomplishing this goal, superposition mapping is an elegant way as it does not require extra shaping to generate a Gaussian-like distributed signal. Superposition mapping has been shown to offer very close to capacity performance for the AWGN channel by combining with an irregular channel code. The aim of this thesis is to search for a code which provides stable performance for moderate sequence length and sufficient number of iterations, which is more suitable for implementation.
Concerning channel coding for superposition mapping, a generalized code design has recently been proposed. The so-called low-density hybrid-check (LDHC) coding intends to contrive coding and modulation in a joint way. The LDHC coding is constructed by integrating modulation into the Tanner graph. Thus, the complete code can be obtained by taking the effects of all the components into account. In this thesis, the LDHC code design is extended to OFDM and MIMO. For OFDM, the bit loading can be realized in the graph. In case of MIMO with spatial multiplexing, the code is extended to the spatial domain. In both cases, a suitable system structure will be proposed in this thesis. It will also be shown how this novel code design improves the system performance
LDPC code-based bandwidth efficient coding schemes for wireless communications
This dissertation deals with the design of bandwidth-efficient coding schemes
with Low-Density Parity-Check (LDPC) for reliable wireless communications. Code
design for wireless channels roughly falls into three categories: (1) when channel state
information (CSI) is known only to the receiver (2) more practical case of partial CSI
at the receiver when the channel has to be estimated (3) when CSI is known to the
receiver as well as the transmitter. We consider coding schemes for all the above
categories.
For the first scenario, we describe a bandwidth efficient scheme which uses highorder
constellations such as QAM over both AWGN as well as fading channels. We
propose a simple design with LDPC codes which combines the good properties of
Multi-level Coding (MLC) and bit-interleaved coded-modulation (BICM) schemes.
Through simulations, we show that the proposed scheme performs better than MLC
for short-medium lengths on AWGN and block-fading channels. For the first case,
we also characterize the rate-diversity tradeoff of MIMO-OFDM and SISO-OFDM
systems. We design optimal coding schemes which achieve this tradeoff when transmission
is from a constrained constellation. Through simulations, we show that with
a sub-optimal iterative decoder, the performance of this coding scheme is very close
to the optimal limit for MIMO (flat quasi-static fading), MIMO-OFDM and SISO OFDM systems.
For the second case, we design non-systematic Irregular Repeat Accumulate
(IRA) codes, which are a special class of LDPC codes, for Inter-Symbol Interference
(ISI) fading channels when CSI is estimated at the receiver. We use Orthogonal Frequency
Division Multiplexing (OFDM) to convert the ISI fading channel into parallel
flat fading subchannels. We use a simple receiver structure that performs iterative
channel estimation and decoding and use non-systematic IRA codes that are optimized
for this receiver. This combination is shown to perform very close to a receiver
with perfect CSI and is also shown to be robust to change in the number of channel
taps and Doppler.
For the third case, we look at bandwidth efficient schemes for fading channels
that perform close to capacity when the channel state information is known at the
transmitter as well as the receiver. Schemes that achieve capacity with a Gaussian
codebook for the above system are already known but not for constrained constellations.
We derive the near-optimum scheme to achieve capacity with constrained constellations
and then propose coding schemes which perform close to capacity. Through
linear transformations, a MIMO system can be converted into non-interfering parallel
subchannels and we further extend the proposed coding schemes to the MIMO case
too
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