399 research outputs found
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
Distributed Space-Time Message Relaying for Uncoded/Coded Wireless Cooperative Communications
During wireless communications, nodes can overhear other transmissions through the wireless medium, suggested by the broadcast nature of plane wave propagation, and may help to provide extra observations of the source signals to the destination. Modern research in wireless communications pays more attention to these extra observations which were formerly neglected within networks. Cooperative communication processes this abundant information existing at the surrounding nodes and retransmits towards the destination in various forms to create spatial and/or coding diversity, thereby to obtain higher throughput and reliability. The aim of this work is to design cooperative communication systems with distributed space-time block codes (DSTBC) in different relaying protocols and theoretically derive the BER performance for each scenario. The amplify-and-forward (AF) protocol is one of the most commonly used protocols at the relays. It has a low implementation complexity but with a drawback of amplifying the noise as well. We establish the derivation of the exact one-integral expression of the average BER performance of this system, folloby a novel approximation method based on the series expansion. An emerging technology, soft decode-and-forward (SDF), has been presented to combine the desired features of AF and DF: soft signal representation in AF and channel coding gain in DF. In the SDF protocol, after decoding, relays transmit the soft-information, which represents the reliability of symbols passed by the decoder, to the destination. Instead of keeping the source node idling when the relays transmit as in the traditional SDF system, we let the source transmit hard information and cooperate with the relays using DSTBC. By theoretically deriving the detection performance at the destination by either using or not using the DSTBC, we make comparisons among three SDF systems. Interesting results have been shown, together with Monte-Carlo simulations, to illustrate that our proposed one-relay and two-relay SDF & DSTBC systems outperform traditional soft relaying for most of the cases. Finally, these analytic results also provide a way to implement the optimal power allocation between the source and the relay or between relays, which is illustrated in the line model
Iterative receiver in multiuser relaying systems with fast frequency-hopping modulation
In this thesis, a novel iterative receiver and its improved version are proposed for
relay-assisted multiuser communications, in which multiple users transmit to a destination
with the help of a relay and using fast frequency-hopping modulation. Each
user employs a channel encoder to protect its information and facilitate interference
cancellation at the receiver. The signal received at the relay is either amplified, or
partially decoded with a simple energy detector, before being forwarded to the destination.
Under flat Rayleigh fading channels, the receiver at the destination can
be implemented non-coherently, i.e., it does not require the instantaneous channel
information to demodulate the usersâ transmitted signals. The proposed iterative
algorithm at the destination exploits the soft outputs of the channel decoders to
successively extract the maximum likelihood symbols of the users and perform interference
cancellation. The iterative method is successfully applied for both cases of
amplify-and-forward and partial decode-and-forward relaying. The error performance
of the proposed iterative receiver is investigated by computer simulation. Under the
same spectral efficiency, simulation results demonstrate the excellent performance of
the proposed receiver when compared to the performance of decoding without interference
cancellation as well as the performance of the maximum likelihood multiuser
detection previously developed for uncoded transmission. Simulation results also suggest
that a proper selection of channel coding schemes can help to support significant
more users without consuming extra system resources.
In addition, to further enhance the receiverâs performance in terms of the bit error
rate, an improved version of the iterative receiver is presented. Such an improved receiver
invokes inner-loop iterations between the channel decoders and the demappers
in such a way that the soft outputs of the channel decoders are also used to refine the
outputs of the demappers for every outer-loop iteration. Simulation results indicate
a performance gain of about 2.5dB by using the two-loop receiver when compared to
the performance of the first proposed receiver
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 usersâ 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
Collaborative HARQ Schemes for Cooperative Diversity Communications in Wireless Networks
Wireless technology is experiencing spectacular developments, due to the emergence of interactive and digital multimedia applications as well as rapid advances in the highly integrated systems. For the next-generation mobile communication systems, one can expect wireless connectivity between any devices at any time and anywhere with a range of multimedia contents. A key requirement in such systems is the availability of high-speed and robust communication links. Unfortunately, communications over wireless channels inherently suffer from a number of fundamental physical limitations, such as multipath fading, scarce radio spectrum, and limited battery power supply for mobile devices. Cooperative diversity (CD) technology is a promising solution for future wireless communication systems to achieve broader coverage and to mitigate wireless channelsâ impairments without the need to use high power at the transmitter. In general, cooperative relaying systems have a source node multicasting a message to a number of cooperative relays, which in turn resend a processed version message to an intended destination node. The destination node combines the signal received from the relays, and takes into account the sourceâs original signal to decode the message. The CD communication systems exploit two fundamental features of the wireless medium: its broadcast nature and its ability to achieve diversity through independent channels. A variety of relaying protocols have been considered and utilized in cooperative wireless networks. Amplify and forward (AAF) and decode and forward (DAF) are two popular protocols, frequently used in the cooperative systems. In the AAF mode, the relay amplifies the received signal prior to retransmission. In the DAF mode, the relay fully decodes the received signal, re-encodes and forwards it to the destination. Due to the retransmission without decoding, AAF has the shortcoming that noise accumulated in the received signal is amplified at the transmission. DAF suffers from decoding errors that can lead to severe error propagation. To further enhance the quality of service (QoS) of CD communication systems, hybrid Automatic Repeat-reQuest (HARQ) protocols have been proposed. Thus, if the destination requires an ARQ retransmission, it could come from one of relays rather than the source node. This thesis proposes an improved HARQ scheme with an adaptive relaying protocol (ARP). Focusing on the HARQ as a central theme, we start by introducing the concept of ARP. Then we use it as the basis for designing three types of HARQ schemes, denoted by HARQ I-ARP, HARQ II-ARP and HARQ III-ARP. We describe the relaying protocols, (both AAF and DAF), and their operations, including channel access between the source and relay, the feedback scheme, and the combining methods at the receivers. To investigate the benefits of the proposed HARQ scheme, we analyze its frame error rate (FER) and throughput performance over a quasi-static fading channel. We can compare these with the reference methods, HARQ with AAF (HARQ-AAF) and HARQ with perfect distributed turbo codes (DTC), for which correct decoding is always assumed at the relay (HARQ-perfect DTC). It is shown that the proposed HARQ-ARP scheme can always performs better than the HARQ-AAF scheme. As the signal-to-noise ratio (SNR) of the channel between the source and relay increases, the performance of the proposed HARQ-ARP scheme approaches that of the HARQ-perfect DTC scheme
Efficient Transmission Techniques in Cooperative Networks: Forwarding Strategies and Distributed Coding Schemes
This dissertation focuses on transmission and estimation schemes in wireless relay network, which involves a set of source nodes, a set of destination nodes, and a set of nodes helps communication between source nodes and destination nodes, called relay nodes. It is noted that the overall performance of the wireless relay systems would be impacted by the relay methods adopted by relay nodes. In this dissertation, efficient forwarding strategies and channel coding involved relaying schemes in various relay network topology are studied.First we study a simple structure of relay systems, with one source, one destination and one relay node. By exploiting âanalog codesâ -- a special class of error correction codes that can directly encode and protect real-valued data, a soft forwarding strategy ââanalog-encode-forward (AEF)âscheme is proposed. The relay node first soft-decodes the packet from the source, then re-encodes this soft decoder output (Log Likelihood Ratio) using an appropriate analog code, and forwards it to the destination. At the receiver, both a maximum-likelihood (ML) decoder and a maximum a posterior (MAP) decoder are specially designed for the AEF scheme.The work is then extended to parallel relay networks, which is consisted of one source, one destination and multiple relay nodes. The first question confronted with us is which kind of soft information to be relayed at the relay nodes. We analyze a set of prevailing soft information for relaying considered by researchers in this field. A truncated LLR is proved to be the best choice, we thus derive another soft forwarding strategy â âZâ forwarding strategy. The main parameter effecting the overall performance in this scheme is the threshold selected to cut the LLR information. We analyze the threshold selection at the relay nodes, and derive the exact ML estimation at the destination node. To circumvent the catastrophic error propagation in digital distributed coding scheme, a distributed soft coding scheme is proposed for the parallel relay networks. The key idea is the exploitation of a rate-1 soft convolutional encoder at each of the parallel relays, to collaboratively form a simple but powerful distributed analog coding scheme. Because of the linearity of the truncated LLR information, a nearly optimal ML decoder is derived for the distributed coding scheme. In the last part, a cooperative transmission scheme for a multi-source single-destination system through superposition modulation is investigated. The source nodes take turns to transmit, and each time, a source âoverlaysâ its new data together with (some or all of) what it overhears from its partner(s), in a way similar to French-braiding the hair. We introduce two subclasses of braid coding, the nonregenerative and the regenerative cases, and, using the pairwise error probability (PEP) as a figure of merit, derive the optimal weight parameters for each one. By exploiting the structure relevance of braid codes with trellis codes, we propose a Viterbi maximum-likelihood (ML) decoding method of linear-complexity for the regenerative case. We also present a soft-iterative joint channel-network decoding. The overall decoding process is divided into the forward message passing and the backward message passing, which makes effective use of the available reliability information from all the received signals. We show that the proposed âbraid codingâ cooperative scheme benefits not only from the cooperative diversity but also from the bit error rate (BER) performance gain
Initial synchronisation of wideband and UWB direct sequence systems: single- and multiple-antenna aided solutions
This survey guides the reader through the open literature on the principle of initial synchronisation in single-antenna-assisted single- and multi-carrier Code Division Multiple Access (CDMA) as well as Direct Sequence-Ultra WideBand (DS-UWB) systems, with special emphasis on the DownLink (DL). There is a paucity of up-to-date surveys and review articles on initial synchronization solutions for MIMO-aided and cooperative systems - even though there is a plethora of papers on both MIMOs and on cooperative systems, which assume perfect synchronization. Hence this paper aims to ?ll the related gap in the literature
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 channelsâ 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 Channelsâs (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 schemeâs performance to a direct transmission link between the two sources having the same throughput
- âŚ