[Show abstract][Hide abstract] ABSTRACT: In this paper, we focus on the two-user Gaussian interference channel (GIC),
and study the Han-Kobayashi (HK) coding/decoding strategy with the objective of
designing low-density parity-check (LDPC) codes. A code optimization algorithm
is proposed which adopts a random perturbation technique via tracking the
average mutual information. The degree distribution optimization and
convergence threshold computation are carried out for strong and weak
interference channels, employing binary phase-shift keying (BPSK). Under strong
interference, it is observed that optimized codes operate close to the capacity
boundary. For the case of weak interference, it is shown that via the newly
designed codes, a nontrivial rate pair is achievable, which is not attainable
by single user codes with time-sharing. Performance of the designed LDPC codes
are also studied for finite block lengths through simulations of specific codes
picked from the optimized degree distributions.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we explore design aspects of adaptive modulation based on orthogonal frequency-division multiplexing (OFDM) for underwater acoustic (UWA) communications, and study its performance using real-time at-sea experiments. Our design criterion is to maximize the system throughput under a target average bit error rate (BER). We consider two different schemes based on the level of adaptivity: in the first scheme, only the modulation levels are adjusted while the power is allocated uniformly across the subcarriers, whereas in the second scheme, both the modulation levels and the power are adjusted adaptively. For both schemes we linearly predict the channel one travel time ahead so as to improve the performance in the presence of a long propagation delay. The system design assumes a feedback link from the receiver that is exploited in two forms: one that conveys the modulation alphabet and quantized power levels to be used for each subcarrier, and the other that conveys a quantized estimate of the sparse channel impulse response. The second approach is shown to be advantageous, as it requires significantly fewer feedback bits for the same system throughput. The effectiveness of the proposed adaptive schemes is demonstrated using computer simulations, real channel measurements recorded in shallow water off the western coast of Kauai, HI, USA, in June 2008, and real-time at-sea experiments conducted at the same location in July 2011. We note that this is the first paper that presents adaptive modulation results for UWA links with real-time at-sea experiments.
[Show abstract][Hide abstract] ABSTRACT: 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.
IEEE Signal Processing Letters 01/2014; 21(5):545-549. · 1.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: In this paper, two-user Gaussian interference channel(GIC) is revisited with
the objective of developing implementable (explicit) channel codes.
Specifically, low density parity check (LDPC) codes are adopted for use over
these channels, and their benefits are studied. Different scenarios on the
level of interference are considered. In particular, for strong interference
channel examples with binary phase shift keying (BPSK), it is demonstrated that
rates better than those offered by single user codes with time sharing are
achievable. Promising results are also observed with quadrature-shift-keying
(QPSK). Under general interference a Han-Kobayashi coding based scheme is
employed splitting the information into public and private parts, and utilizing
appropriate iterative decoders at the receivers. Using QPSK modulation at the
two transmitters, it is shown that rate points higher than those achievable by
time sharing are obtained.
[Show abstract][Hide abstract] ABSTRACT: We derive an upper bound on the capacity of non-binary deletion channels. Although binary deletion channels have received significant attention over the years, and many upper and lower bounds on their capacity have been derived, such studies for the non-binary case are largely missing. The state of the art is the following: as a trivial upper bound, capacity of an erasure channel with the same input alphabet as the deletion channel can be used, and as a lower bound the results by Diggavi and Grossglauser in  are available. In this paper, we derive the first non-trivial non-binary deletion channel capacity upper bound and reduce the gap with the existing achievable rates. To derive the results we first prove an inequality between the capacity of a 2K-ary deletion channel with deletion probability d, denoted by C2K(d), and the capacity of the binary deletion channel with the same deletion probability, C2(d), that is, C2K(d) ≤ C2(d)+(1-d) log(K). Then by employing some existing upper bounds on the capacity of the binary deletion channel, we obtain upper bounds on the capacity of the 2K-ary deletion channel. We illustrate via examples the use of the new bounds and discuss their asymptotic behavior as d → 0.
Information Theory Proceedings (ISIT), 2013 IEEE International Symposium on; 01/2013
[Show abstract][Hide abstract] ABSTRACT: We develop several analytical lower bounds on the capacity of binary insertion and deletion channels by considering independent uniformly distributed (i.u.d.) inputs and computing lower bounds on the mutual information between the input and output sequences. For the deletion channel, we consider two different models: i.i.d. deletion-substitution channel and i.i.d. deletion channel with additive white Gaussian noise (AWGN). These two models are considered to incorporate effects of the channel noise along with the synchronization errors. For the insertion channel case, we consider Gallager's model in which the transmitted bits are replaced with two random bits and uniform over the four possibilities independently of any other insertion events. The general approach taken is similar in all cases, however the specific computations differ. Furthermore, the approach yields a useful lower bound on the capacity for a wide range of deletion probabilities of the deletion channels, while it provides a beneficial bound only for small insertion probabilities (less than 0.25) of the insertion model adopted. We emphasize the importance of these results by noting that: 1) our results are the first analytical bounds on the capacity of deletion-AWGN channels, 2) the results developed are the best available analytical lower bounds on the deletion-substitution case, 3) for the Gallager insertion channel model, the new lower bound improves the existing results for small insertion probabilities.
IEEE Transactions on Information Theory 01/2013; 59(9):5534-5546. · 2.62 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we focus on orthogonal frequency-division multiplexing (OFDM) receiver designs for underwater acoustic (UWA) channels with user- and/or path-specific Doppler scaling distortions. The scenario is motivated by the cooperative communications framework, where distributed transmitter/receiver pairs may experience significantly different Doppler distortions, as well as by the single-user scenarios, where distinct Doppler scaling factors may exist among different propagation paths. The conventional approach of front-end resampling that corrects for common Doppler scaling may not be appropriate in such scenarios, rendering a post-fast-Fourier-transform (FFT) signal that is contaminated by user- and/or path-specific intercarrier interference. To counteract this problem, we propose a family of front-end receiver structures that utilize multiple-resampling (MR) branches, each matched to the Doppler scaling factor of a particular user and/or path. Following resampling, FFT modules transform the Doppler-compensated signals into the frequency domain for further processing through linear or nonlinear detection schemes. As part of the overall receiver structure, a gradient-descent approach is also proposed to refine the channel estimates obtained by standard sparse channel estimators. The effectiveness and robustness of the proposed receivers are demonstrated via simulations, as well as emulations based on real data collected during the 2010 Mobile Acoustic Communications Experiment (MACE10, Martha's Vineyard, MA) and the 2008 Kauai Acomms MURI (KAM08, Kauai, HI) experiment.
[Show abstract][Hide abstract] ABSTRACT: This article presents a contemporary overview of underwater acoustic communication (UWAC) and investigates physical layer aspects on cooperative transmission techniques for future UWAC systems. Taking advantage of the broadcast nature of wireless transmission, cooperative communication realizes spatial diversity advantages in a distributed manner. The current literature on cooperative communication focuses on terrestrial wireless systems at radio frequencies with sporadic results on cooperative UWAC. In this article, we summarize initial results on cooperative UWAC and investigate the performance of a multicarrier cooperative UWAC considering the inherent unique characteristics of the underwater channel. Our simulation results demonstrate the superiority of cooperative UWAC systems over their point-to-point counterparts.
[Show abstract][Hide abstract] ABSTRACT: 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.
IEEE Transactions on Communications 01/2013; 61(3):852-864. · 1.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We consider a multi-carrier cooperative underwater acoustic communication (UWAC) system and investigate the Doppler scaling problem arising due to the motion of different nodes. Specifically, we assume an orthogonal frequency division multiplexing (OFDM) system with amplify and forward (AF) relaying. Our channel model is built on large-scale path loss along with the short-term frequency-selective fading. For Doppler scaling compensation, we use multi-resampling (MR) receiver designs both at the relay and destination nodes. We present an extensive Monte Carlo simulation study to evaluate the error rate performance of the proposed UWAC system. In simulations, we use the publicly available VirTEX software in conjunction with the ray-tracing based BELLHOP software to precisely reflect the characteristics of an underwater geographical location and the movement of the nodes.
[Show abstract][Hide abstract] ABSTRACT: Memoryless channels with deletion errors as defined by a stochastic channel
matrix allowing for bit drop outs are considered in which transmitted bits are
either independently deleted with probability $d$ or unchanged with probability
$1-d$. Such channels are information stable, hence their Shannon capacity
exists. However, computation of the channel capacity is formidable, and only
some upper and lower bounds on the capacity exist. In this paper, we first show
a simple result that the parallel concatenation of two different independent
deletion channels with deletion probabilities $d_1$ and $d_2$, in which every
input bit is either transmitted over the first channel with probability of
$\lambda$ or over the second one with probability of $1-\lambda$, is nothing
but another deletion channel with deletion probability of $d=\lambda
d_1+(1-\lambda)d_2$. We then provide an upper bound on the concatenated
deletion channel capacity $C(d)$ in terms of the weighted average of $C(d_1)$,
$C(d_2)$ and the parameters of the three channels. An interesting consequence
of this bound is that $C(\lambda d_1+(1-\lambda))\leq \lambda C(d_1)$ which
enables us to provide an improved upper bound on the capacity of the i.i.d.
deletion channels, i.e., $C(d)\leq 0.4143(1-d)$ for $d\geq 0.65$. This
generalizes the asymptotic result by Dalai as it remains valid for all $d\geq
0.65$. Using the same approach we are also able to improve upon existing upper
bounds on the capacity of the deletion/substitution channel.
[Show abstract][Hide abstract] ABSTRACT: In this work we explore the feasibility of a cognitive acoustic communication system that exploits a dynamic closed loop between the transmitter and receiver with the goal of maximizing the information throughput. Specifically, we design a power control mechanism and couple it with an adaptive modulation method based on orthogonal frequency division multiplexing (OFDM). We propose two methods: the first method is adaptive overall power adjustment in which the transmitter modifies the power gain to provide a target SNR at the receiver by exploiting a feedback link in a time-varying channel. The second method adaptively adjusts the modulation level on each individual sub-carrier to achieve a target bit error rate (BER) at the receiver. Crucial to both of these methods is the ability to predict the acoustic channel and the signal-to-noise ratio (SNR) one travel time ahead, which enables adaptive adjustment of the transmit power and modulation level. The performance of the proposed algorithms is demonstrated using an in-air test bed and further verified with real-time at-sea experiments conducted off the coast of Kauai, HI, in June 2011. Experimental results obtained using real-time at-sea experiments show significant savings in power, as well as improvement in the overall information throughput (bit rate) as compared to conventional, non-adaptive modulation with the same power consumption and target BER.
The Journal of the Acoustical Society of America 09/2012; 132(3):1970. · 1.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 the channel with synchronization
errors can be decomposed into a cascade of two channels where only the first
one suffers from synchronization errors and the second one is a memoryless
channel, a lower bound on the capacity of the original channel in terms of the
capacity of the synchronization error-only channel can be derived. To
accomplish this, we derive lower bounds on the mutual information rate between
the transmitted and received sequences (for the original channel) for an
arbitrary input distribution, and then relate this result to the channel
capacity. The results apply without the knowledge of the exact capacity
achieving input distributions. A primary application of our results is that we
can employ any lower bound derived on the capacity of the first channel
(synchronization error channel in the decomposition) 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).
[Show abstract][Hide abstract] ABSTRACT: In this paper, we consider a multi-carrier and multi-relay underwater acoustic communication (UWAC) system and investigate the relay selection problem. Our channel model is built on an aggregation of both large-scale path loss and small-scale fading. For path loss modeling, we use the publicly available ray-tracing algorithm Bellhop software to precisely reflect the characteristics of an underwater geographical location (such as the sound speed profile, sound frequency, bathymetry, type of bottom sediments, depths of nodes, etc). For relay selection, we consider different selection criteria which rely either on the maximization of signal-to-noise ratio (SNR) or the minimization of probability of error (PoE). These are used in conjunction with so-called per-subcarrier, all-subcarriers, or subcarrier grouping approaches in which one or more relays are selected. We present an extensive Monte Carlo simulation study to evaluate the error rate performance of the UWAC system with different relay selection schemes under consideration and point out differences with terrestrial radio-frequency (RF) systems.
[Show abstract][Hide abstract] ABSTRACT: We consider coding schemes over an independent and identically distributed (i.i.d.) insertion/deletion channel with inter-symbol interference (ISI). The idea is based on a serial concatenation of a low-density parity check (LDPC) code with a marker code. First, we design a maximum-a-posteriori (MAP) detector operating at the bit level which jointly achieves synchronization for the insertion/deletion channel (with the help of the marker code) and equalization for the ISI channel. Utilizing this MAP detector together with an LDPC code with powerful error-correction capabilities, we demonstrate that reliable transmission over this channel is feasible. Then, we consider low-complexity channel detection algorithms needed for proper synchronization/equalization. Specifically, we use separate synchronization and equalization algorithms instead of joint detection and also explore the performance of M- and T-algorithms implemented as low complexity soft output channel detectors. Such schemes greatly reduce the amount of computations needed at the cost of some performance loss as illustrated via a set of simulation results.
Communications (ICC), 2012 IEEE International Conference on; 01/2012
[Show abstract][Hide abstract] ABSTRACT: We investigate different decoding strategies at the relay over a frequency selective two-way relay channel with physical-layer network coding (PNC). The incorporation of the PNC scheme enables two users exchange information via a relay in two transmission phases. We study two approaches at the relay to decode the XOR of the transmitted codewords namely; a) an approximately optimal decoding scheme which is implemented using a list decoding algorithm, and b) a minimum mean square error (MMSE) based detector followed by a PNC decoder. The list decoding scheme selects L most likely pairs of sequences corresponding to the transmitted codewords and sorts them in the order of decreasing a-posteriori probabilities. From this list, estimates of the highly likely network coded sequences are obtained. Numerical examples show that a joint detector/physical-layer network coded sequence decoder (JD/PNCD) has a performance similar to the list decoding scheme and can be implemented with a lower complexity. The detection scheme based on the MMSE criterion is a suboptimal approach that generates soft information corresponding to the linear sum of the received symbols and is particularly useful in scenarios where the number of channel taps is large.
Wireless Communications and Networking Conference (WCNC), 2012 IEEE; 01/2012
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we develop several lower bounds on the capacity of binary input symmetric q-ary output channels with synchronization errors, e.g., substitution/erasure channels with synchronization errors. More precisely, we show that if a channel with synchronization errors can be decomposed into a cascade of two independent channels where only the first one suffers from synchronization errors, a lower bound on its capacity related to the capacity of the one with only synchronization errors can be given. We present several examples with the new approach and demonstrate that for certain channels, e.g., deletion/substitution channel, it is possible to derive tighter capacity lower bounds than the existing ones.
Information Theory Proceedings (ISIT), 2012 IEEE International Symposium on; 01/2012