1,651 research outputs found
Spectrally Effiecient Alamouti Code Structure in Asynchronous Cooperative Systems
Cataloged from PDF version of article.A relay communication system with two amplify and
forward (AF) relays under flat fading channel conditions is considered
where the signals received from the relays are not necessarily
time aligned. We propose a new time-reversal (TR)-based scheme
providing an Alamouti code structure which needs a smaller overhead
in transmitting every pair of data blocks in comparison with
the existing schemes and, as a result, increases the transmission
rate significantly (as much as 20%) in exchange for a small performance
loss. The scheme is particularly useful when the delay
between the two relay signals is large, e.g., in typical underwater
acoustic (UWA) channels
Upper bounds on the capacity of deletion channels using channel fragmentation
Cataloged from PDF version of article.We study memoryless channels with synchronization
errors as defined by a stochastic channel matrix allowing for
symbol drop-outs or symbol insertions with particular emphasis
on the binary and non-binary deletion channels. We offer
a different look at these channels by considering equivalent
models by fragmenting the input sequence where different
subsequences travel through different channels. The resulting
output symbols are combined appropriately to come up with an
equivalent input–output representation of the original channel
which allows for derivation of new upper bounds on the channel
capacity. We consider both random and deterministic types
of fragmentation processes applied to binary and nonbinary
deletion channels. With two specific applications of this idea,
a random fragmentation applied to a binary deletion channel
and a deterministic fragmentation process applied to a nonbinary
deletion channel, we prove certain inequality relations among the
capacities of the original channels and those of the introduced
subchannels. The resulting inequalities prove useful in deriving
tighter capacity upper bounds for: 1) independent identically
distributed (i.i.d.) deletion channels when the deletion probability
exceeds 0.65 and 2) nonbinary deletion channels. Some extensions
of these results, for instance, to the case of deletion/substitution
channels are also explored
Achievable Rates for Noisy Channels with Synchronization Errors
Cataloged from PDF version of article.We develop several lower bounds on the capacity of binary input symmetric output channels with synchronization errors, which also suffer from other types of impairments such as substitutions, erasures, additive white Gaussian noise (AWGN), etc. More precisely, we show that if a channel suffering from synchronization errors as well as other type of impairments can be decomposed into a cascade of two component channels where the first one is another channel with synchronization errors and the second one is a memoryless channel (with no synchronization errors), a lower bound on the capacity of the original channel in terms of the capacity of the component synchronization error channel can be derived. A primary application of our results is that we can employ any lower bound derived on the capacity of the component synchronization error channel to find lower bounds on the capacity of the (original) noisy channel with synchronization errors. We apply the general ideas to several specific classes of channels such as synchronization error channels with erasures and substitutions, with symmetric q-ary outputs and with AWGN explicitly, and obtain easy-to-compute bounds. We illustrate that, with our approach, it is possible to derive tighter capacity lower bounds compared to the currently available bounds in the literature for certain classes of channels, e.g., deletion/substitution channels and deletion/AWGN channels (for certain signal-to-noise ratio (SNR) ranges). © 2014 IEEE
Decoding Strategies at the Relay with Physical-Layer Network Coding
Cataloged from PDF version of article.A two-way relay channel is considered where two
users exchange information via a common relay in two transmission
phases using physical-layer network coding (PNC). We consider
an optimal decoding strategy at the relay to decode the network
coded sequence during the first transmission phase, which is
approximately implemented using a list decoding (LD) algorithm.
The algorithm jointly decodes the codewords transmitted by
the two users and sorts the L most likely pair of sequences
in the order of decreasing a-posteriori probabilities, based on
which, estimates of the most likely network coded sequences and
the decoding results are obtained. Using several examples, it is
observed that a lower complexity alternative, that jointly decodes
the two transmitted codewords, has a performance similar to the
LD based decoding and offers a near-optimal performance in
terms of the error rates corresponding to the XOR of the two
decoded sequences. To analyze the error rate at the relay, an
analytical approximation of the word-error rate using the joint
decoding (JD) scheme is evaluated over an AWGN channel using
an approach that remains valid for the general case of two users
adopting different codebooks and using different power levels.
We further extend our study to frequency selective channels
where two decoding approaches at the relay are investigated,
namely; a trellis based joint channel detector/physical-layer
network coded sequence decoder (JCD/PNCD) which is shown
to offer a near-optimal performance, and a reduced complexity
channel detection based on a linear receiver with minimum mean
squared error (MMSE) criterion which is particularly useful
where the number of channel taps is large
Multi-Input Multi-Output Deletion Channel
Cataloged from PDF version of article.We describe a new channel model suitable in certain
applications, namely the multi-input multi-output (MIMO) deletion
channel. This channel models the scenarios where multiple
transmitters and receivers suffering from synchronization errors
are employed. We then consider a coding scheme over such
channels based on a serial concatenation of a low-density parity
check (LDPC) code, a marker code and a layered space-time
code. We design two detectors operating at the bit level which
jointly achieve synchronization for the deletion channel (with the
help of the marker code) and detection for the MIMO channel.
Utilizing the proposed detector together with an LDPC code
with powerful error-correction capabilities, we demonstrate that
reliable transmission over a MIMO deletion channel is feasible
Achieving Delay Diversity in Asynchronous Underwater Acoustic (UWA) Cooperative Communication Systems
Cataloged from PDF version of article.In cooperative UWA systems, due to the low speed
of sound, a node can experience significant time delays among
the signals received from geographically separated nodes. One
way to combat the asynchronism issues is to employ orthogonal
frequency division multiplexing (OFDM)-based transmissions at
the source node by preceding every OFDM block with an
extremely long cyclic prefix (CP) which reduces the transmission
rates dramatically. One may increase the OFDM block length
accordingly to compensate for the rate loss which also degrades
the performance due to the significantly time-varying nature of
UWA channels. In this paper, we develop a new OFDM-based
scheme to combat the asynchronism problem in cooperative
UWA systems without adding a long CP (in the order of the
long relative delays) at the transmitter. By adding a much
more manageable (short) CP at the source, we obtain a delay
diversity structure at the destination for effective processing and
exploitation of spatial diversity by utilizing a low complexity
Viterbi decoder at the destination, e.g., for a binary phase shift
keying (BPSK) modulated system, we need a two-state Viterbi
decoder. We provide pairwise error probability (PEP) analysis
of the system for both time-invariant and block fading channels
showing that the system achieves full spatial diversity. We find
through extensive simulations that the proposed scheme offers a
significantly improved error rate performance for time-varying
channels (typical in UWA communications) compared to the
existing approaches
Capacity Bounds and Concatenated Codes Over Segmented Deletion Channels
Cataloged from PDF version of article.We develop an information theoretic characterization
and a practical coding approach for segmented deletion
channels. Compared to channels with independent and identically
distributed (i.i.d.) deletions, where each bit is independently
deleted with an equal probability, the segmentation assumption
imposes certain constraints, i.e., in a block of bits of a certain
length, only a limited number of deletions are allowed to occur.
This channel model has recently been proposed and motivated
by the fact that for practical systems, when a deletion error
occurs, it is more likely that the next one will not appear
very soon. We first argue that such channels are information
stable, hence their channel capacity exists. Then, we introduce
several upper and lower bounds with two different methods in an
attempt to understand the channel capacity behavior. The first
scheme utilizes certain information provided to the transmitter
and/or receiver while the second one explores the asymptotic
behavior of the bounds when the average bit deletion rate is
small. In the second part of the paper, we consider a practical
channel coding approach over a segmented deletion channel.
Specifically, we utilize outer LDPC codes concatenated with inner
marker codes, and develop suitable channel detection algorithms
for this scenario. Different maximum-a-posteriori (MAP) based
channel synchronization algorithms operating at the bit and
symbol levels are introduced, and specific LDPC code designs are
explored. Simulation results clearly indicate the advantages of the
proposed approach. In particular, for the entire range of deletion
probabilities less than unity, our scheme offers a significantly
larger transmission rate compared to the other existing solutions
in the literature
Історія створення музею археології Волинського державного університету імені Лесі Українки
Purpose: This study investigated the comprehension of counterfactual conditionals in monolingual Turkish children with specific language impairment (SLI) and typically developing (TD) children. Comprehending counterfactuals requires a well-developed cognitive system (Beck, Riggs, & Gorniak, 2009). Children with SLI have impaired cognitive functioning (Im Bolter, Johnston, & Pascaul-Leone, 2006) and this impacts on their ability to comprehend counterfactuals. Method: The sample consisted of 13 children with SLI who were matched on age and nonverbal IQ with 13 TD children (mean age 6;9 [years; months] for both groups). Each group completed a sentence comprehension and repetition task with three sentence conditions: nonconditional, factual and counterfactual. Nonconditionals do not have if embedding whereas factual and counterfactual conditionals are morphosyntactically equivalent if-clauses, but only the latter is cognitively complex. Results: Conditionals were more difficult to comprehend than nonconditionals for both groups. Counterfactuals were more difficult to comprehend than the morphosyntactically equivalent factual counterparts for the SLI group. There was no discrepancy between the groups for repetition of counterfactuals and factuals. Conclusions: Children with SLI have difficulty processing counterfactuals due to morphosyntactic complexity (if-embedding) and the cognitive processes involved in comprehending counterfactuals. This indicates that cognitive complexity adds to sentence comprehension deficits in SLI
Landslide susceptibility mapping of Cekmece area (Istanbul, Turkey) by conditional probability
International audienceAs a result of industrialization, throughout the world, the cities have been growing rapidly for the last century. One typical example of these growing cities is Istanbul. Today, the population of Istanbul is over 10 millions. Depending on this rapid urbanization, new suitable areas for settlements and engineering structures are necessary. For this reason, the Cekmece area, west of the Istanbul metropolitan area, is selected as the study area, because the landslides are frequent in this area. The purpose of the present study is to produce landslide susceptibility map of the selected area by conditional probability approach. For this purpose, a landslide database was constructed by both air ? photography and field studies. 19.2% of the selected study area is covered by landslides. Mainly, the landslides described in the area are generally located in the lithologies including the permeable sandstone layers and impermeable layers such as claystone, siltstone and mudstone layers. When considering this finding, it is possible to say that one of the main conditioning factors of the landslides in the study area is lithology. In addition to lithology, many landslide conditioning factors are considered during the landslide susceptibility analyses. As a result of the analyses, the class of 5?10° of slope, the class of 180?225 of aspect, the class of 25?50 of altitude, Danisment formation of the lithological units, the slope units of geomorphology, the class of 800?1000 m of distance from faults (DFF), the class of 75?100 m of distance from drainage (DFD) pattern, the class of 0?10m of distance from roads (DFR) and the class of low or impermeable unit of relative permeability map have the higher probability values than the other classes. When compared with the produced landslide susceptibility map, most of the landslides identified in the study area are found to be located in the most (54%) and moderate (40%) susceptible zones. This assessment is also supported by the performance analysis applied at end of the study. As a consequence, the landslide susceptibility map produced herein has a valuable tool for the planning purposes
Design of LDPC Codes for Two-Way Relay Systems with Physical-Layer Network Coding
Cataloged from PDF version of article.This letter presents low-density parity-check (LDPC) code design for two-way relay (TWR) systems employing physical-layer network coding (PLNC). We focus on relay decoding, and propose an empirical density evolution method for estimating the decoding threshold of the LDPC code ensemble. We utilize the proposed method in conjunction with a random walk optimization procedure to obtain good LDPC code degree distributions. Numerical results demonstrate that the specifically designed LDPC codes can attain improvements of about 0.3 dB over off-the-shelf LDPC codes (designed for point-to-point additive white Gaussian noise channels), i.e., it is new code designs are essential to optimize the performance of TWR systems
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