Browsing by Author "Rahmati, M."

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Achievable Rates for Noisy Channels with Synchronization Errors
(IEEE, 2014-11) Rahmati, M.; Duman, T.
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.
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Achieving Delay Diversity in Asynchronous Underwater Acoustic (UWA) Cooperative Communication Systems
(IEEE, 2014-03) Rahmati, M.; Duman, T. M.
In cooperative UWA systems, due to the low speedof sound, a node can experience significant time delays amongthe signals received from geographically separated nodes. Oneway to combat the asynchronism issues is to employ orthogonalfrequency division multiplexing (OFDM)-based transmissions atthe source node by preceding every OFDM block with anextremely long cyclic prefix (CP) which reduces the transmissionrates dramatically. One may increase the OFDM block lengthaccordingly to compensate for the rate loss which also degradesthe performance due to the significantly time-varying nature ofUWA channels. In this paper, we develop a new OFDM-basedscheme to combat the asynchronism problem in cooperativeUWA systems without adding a long CP (in the order of thelong relative delays) at the transmitter. By adding a muchmore manageable (short) CP at the source, we obtain a delaydiversity structure at the destination for effective processing andexploitation of spatial diversity by utilizing a low complexityViterbi decoder at the destination, e.g., for a binary phase shiftkeying (BPSK) modulated system, we need a two-state Viterbidecoder. We provide pairwise error probability (PEP) analysisof the system for both time-invariant and block fading channelsshowing that the system achieves full spatial diversity. We findthrough extensive simulations that the proposed scheme offers asignificantly improved error rate performance for time-varyingchannels (typical in UWA communications) compared to theexisting approaches.
Open Access
Bounds on the capacity of random insertion and deletion-additive noise channels
(IEEE, 2013) Rahmati, M.; Duman, T. M.
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. © 1963-2012 IEEE.
Open Access
An improvement of the deletion channel capacity upper bound
(IEEE, 2013-10) Rahmati, M.; Duman, Tolga M.
In this paper, we offer an alternative look at channels with deletion errors by considering equivalent models for deletion channels 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. Considering a random fragmentation processes applied to binary deletion channels, we prove an inequality relation among the capacities of the original binary deletion channel and the introduced binary deletion subchannels. This inequality enables us to provide an improved upper bound on the capacity of the i.i.d. deletion channels, i.e., C(d) ≤ 0.4143(1 - d) for d ≥ 0.65. We also consider a deterministic fragmentation process suitable for the study of non-binary deletion channels which results in improved capacity upper bounds. © 2013 IEEE.
Open Access
Spectrally Effiecient Alamouti Code Structure in Asynchronous Cooperative Systems
(IEEE, 2014-05) Rahmati, M.; Duman, T.
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.
Open Access
An upper bound on the capacity of non-binary deletion channels
(IEEE, 2013) Rahmati, M.; Duman, Tolga M.
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 [1] 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. © 2013 IEEE.
Open Access
Upper bounds on the capacity of deletion channels using channel fragmentation
(Institute of Electrical and Electronics Engineers Inc., 2015) Rahmati, M.; Duman, T. M.
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. © 1963-2012 IEEE.