Browsing by Subject "Channel capacity"
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Item Open Access 45 GHz bandwidth-efficiency resonant cavity enhanced ITO-Schottky photodiodes(OSA, 2001) Bıyıklı, Necmi; Kimukin, İbrahim; Aytür, Orhan; Özbay, Ekmel; Gökkavas, M.; Ünlü, M. S.We demonstrated high-performance resonant cavity enhanced ITO-Schottky photodiodes. We achieved a peak efficiency of 75% around 820 nm with a 3-dB bandwidth of 60 GHz resulting in a bandwidth-efficiency product of 45 GHz.Item Open Access 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.Item 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.Item Open Access Capacity bounds and concatenated codes over segmented deletion channels(IEEE, 2013) Wang, F.; Duman, T. M.; Aktas, D.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.Item Open Access Capacity bounds for an ultra-wideband channel model(IEEE, 2004-10) Arıkan, ErdalThere is an ongoing effort by the IEEE 802.15.3a subcommittee to reach a UWB personal area network standard. We estimate the achievable rates for such networks using a channel model specified by the same group. The analysis of this channel model is of interest in light of recent information-theoretic work on multipath fading channels which show that in order to take full advantage of such channels' capacity the transmitted signals have to be "peaky" in a certain sense. The immense bandwidth of the UWB channel also suggests at first that peaky signals should be used. However, unlike the many other wireless systems where the transmitter energy is limited, in the UWB channel only the power spectral density of the transmitted signal is constrained. As a result, the signal power can grow in proportion to the utilized bandwidth and peaky signals are not needed. © 2004 IEEE.Item Open Access Capacity of noisy, discrete memoryless channels under input constraints(1990) Karaşan, EzhanIn this thesis work, we examine the capacity of discrete memoryless channels under input constraints. We consider a certain class of input-restricted channels for which constrained sequences can be modeled as outputs of a finite-state machine(FSM). No efficient algorithm is known for computing the capacity of such a channel. For the noiseless case, i.e., when the channel input letter and the corresponding output letter are identical, it is shown that [1] the channel capacity is the logarithm of the largest eigenvalue of the adjacency matrix of the state-transition diagram of the FSM generating the allowed channel input sequences. Furthermore, the probability distribution on the input sequences achieving the channel capacity is first-order markovian. Here, we discuss the noisy case. For a specific input-restricted channel, we show that unlike the noiseless case, the capacity is no longer achieved by a first-order distribution. We derive upper and lower bounds on the maximum rate achievable by a K-th order markovian distribution on the allowed input sequences. The computational results show that the second-order distribution does strictly better than the first-order distribution for this particular channel. A sequence of upper bounds on the capacity of an input-restricted channel is also given. We show that this sequence converges to the channel capacity. The numerical results clarify that markovian distribution may achieve rates close to the capacity for the channel considered in this work.Item Open Access Channel polarization: a method for constructing capacity-achieving codes for symmetric binary-input memoryless channels(IEEE, 2009) Arikan, E.A method is proposed, called channel polarization, to construct code sequences that achieve the symmetric capacity I(W) of any given binary-input discrete memoryless channel (B-DMC) W. The symmetric capacity is the highest rate achievable subject to using the input letters of the channel with equal probability. Channel polarization refers to the fact that it is possible to synthesize, out of N independent copies of a given B-DMC W, a second set of N binary-input channels {WN (i): 1 ≤ i ≤ N} becomes large, the fraction of indices i for which I(WN (i) is near 1 approaches I(W) and the fraction for which I(WN (i) is near 0 approaches 1 - I(W). The polarized channels WN (i) are well-conditioned for channel coding: one need only send data at rate 1 through those with capacity near 1 and at rate 0 through the remaining. Codes constructed on the basis of this idea are called polar codes. The paper proves that, given any B-DMC W with I(W) and any target rate R < I(W), there exists a sequence of polar codes {Cn;n ≥ 1 such that Cn has block-length N = 2n, rate ≥ R, and probability of block error under successive cancellation decoding bounded as Pe (N, R) ≤ O(N-1/4 independently of the code rate. This performance is achievable by encoders and decoders with complexity O(N\log N) for each.Item Open Access Computer aided frequency planning for the radio and TV broadcasts(Institute of Electrical and Electronics Engineers, 1996-06) Altıntaş, Ayhan; Ocalı, O.; Topçu, Satılmış; Tanyer, S. G.; Köymen, HayrettinThe frequency planning of the VHF and UHF broadcasts in Turkey is described. This planning is done with the aid of computer databases and digital terrain map. The frequency offset is applied whenever applicable to increase the channel capacity. The offset assignment is done through simulated annealing algorithm. The international rules and regulations concerning Turkey are also considered.Item Open Access Enhancements to linear least squares localization through reference selection and ML estimation(IEEE, 2008-03-04) Güvenç, İsmail; Gezici, Sinan; Watanabe F.; Inamura, H.Linear least squares (LLS) estimation is a low complexity but sub-optimum method for estimating the location of a mobile terminal (MT) from some distance measurements. It requires selecting one of the fixed terminals (FTs) as a reference FT for obtaining a linear set of expressions. However, selection of the reference FT is commonly performed arbitrarily in the literature. In this paper, a method for selection of the reference FT is proposed, which improves the location accuracy compared to a fixed selection of the reference FT. Moreover, a covariancematrix based LLS estimator is proposed in line of sight (LOS) and non-LOS (NLOS) environments which further improves accuracy since the correlations between the observations are exploited. Simulation results prove the effectiveness of the proposed techniques. © 2008 IEEE.Item 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.Item Open Access Investigation of planar and conformal printed arrays for MIMO performance analysis(IEEE, 2006) Tunç, Celal Alp; Ircı, Erdinç; Bakır, Onur; Aktaş, Defne; Ertürk, Vakur B.; Altıntaş, AyhanMIMO channel capacity of printed arrays with dipole elements is analyzed. A MIMO channel model based on electric fields is used. The effects of mutual interactions among the array elements through space and surface waves are included into the channel matrix using a full-wave hybrid Method of Moments (MoM)/Green's function technique in the spatial domain. MIMO capacity of printed arrays is then compared with that of free standing thin wire dipole arrays. Results show better performance of printed arrays.Item Open Access On the capacity of fading channels with amplitude-limited inputs(IEEE, 2016) Elmoslimany, A.; Duman, Tolga M.We address the problem of finding the capacity of fading channels under the assumption of amplitude-limited inputs. Specifically, we show that if the fading coefficients have a finite support and the channel state information is only available at the receiver side, there is a unique input distribution that achieves the channel capacity and this input distribution is discrete with a finite number of mass points.Item Open Access On the capacity of multiple-antenna systems and parallel Gaussian channels with amplitude-limited inputs(Institute of Electrical and Electronics Engineers Inc., 2016) Elmoslimany A.; Duman, T. M.We propose upper and lower bounds on the capacity of multiple-input multiple-output (MIMO) systems with amplitude-limited inputs. The results are derived by considering an equivalent channel via singular value decomposition, and by enlarging and reducing the corresponding feasible region of the channel input vector, for the upper and lower bounds, respectively. We analytically characterize the asymptotic behavior of the derived bounds for high and low noise levels, and study the gap between them. We also consider parallel Gaussian channels with peak and average power-constrained inputs. For such channels, the capacity-achieving distribution has been reported in the literature to be discrete, which can be computed using numerical optimization techniques. However, there is no closed-form expression and finding the capacity-achieving distribution is computationally tedious. With this motivation, we derive approximate expressions for the capacity at low and high noise variance levels. We illustrate our findings on both MIMO channels and parallel Gaussian channels via several numerical examples. © 1972-2012 IEEE.Item Open Access On the discreteness of capacity-achieving distributions for fading and signal-dependent noise channels with amplitude-limited inputs(Institute of Electrical and Electronics Engineers, 2018) Elmoslimany A.; Duman, TolgaWe address the problem of finding the capacity of two classes of channels with amplitude-limited inputs. The first class is frequency flat fading channels with an arbitrary (but finite support) channel gain with the channel state information available only at the receiver side; while the second one we consider is the class of additive noise channels with signal-dependent Gaussian noise. We show that for both channel models and under some regularity conditions, the capacity-achieving distribution is discrete with a finite number of mass points. Furthermore, finding the capacity-achieving distribution turns out to be a finite-dimensional optimization problem, and efficient numerical algorithms can be developed using standard optimization techniques to compute the channel capacity. We demonstrate our findings via several examples. In particular, we present an example for a block fading channel where the channel gain follows a truncated Rayleigh distribution, and two instances of signal-dependent noise that are used in the literature of magnetic recording and optical communication channels.Item Open Access On the reliability exponent of the exponential timing channel(IEEE, 2002) Arikan, E.We determine the reliability exponent E(R) of the Anantharam-Verdú exponential server timing channel with service rate μ for all rates R between a critical rate R c = (μ/4) log 2 and the channel capacity C = e -1μ. For rates between 0 and R c, we provide a random-coding lower bound E r(R) and a sphere-packing upper bound E sp(R) on E(R). We also determine that the cutoff rate R o for this channel equals μ/4, thus answering a question posed by Sundaresan and Verdú. An interesting aspect of our results is that the lower bound E r (R) for the reliability exponent of the timing channel coincides with Wyner's reliability exponent for the photon-counting channel with no dark current and with peak power constraint μ. Whether the reliability exponents of the two channels are actually equal everywhere remains open. This shows that the exponential server timing channel is at least as reliable as this type of a photon-counting channel for all rates.Item Open Access Optimization of linear wire antenna arrays to increase MIMO capacity using swarm intelligence(Institution of Engineering and Technology, 2007) Olgun, Uğur; Tunç, Celal Alp; Aktaş, Defne; Ertürk, Vakur B.; Altıntaş, AyhanFree standing linear arrays (FSLA) are analyzed and optimized to increase MIMO capacity. A MIMO channel model based on electric fields is used. The effects of mutual interactions among the array elements are included into the channel matrix using method of moments (MoM) based full-wave solvers. A tool to design an antenna array of superior MIMO capacity for any specified volume is developed. Particle swarm optimization is used as the main engine for the optimization tasks of the tool. Uniform linear arrays, uniform circular arrays and non-uniform arrays are analyzed and compared in terms of their channel capacity.Item Open Access Quantum transport through one-dimensional aluminum wires(American Vacuum Society, 2002) Batra, I. P.; Sen, P.; Çıracı, SalimQuantum conductance in narrow channels has been well understood by using the two-dimensional electron gas, a model system which has been realized in semiconductor heterojunctions. An essential property of this electron gas is its ability to support a constriction of width comparable to the Fermi wavelength, a property not shared by even thin metal films. The advent of scanning tunneling microscope has made possible the fabrication of metallic wires of atomic widths. We investigate one-dimensional wires consisting of aluminum atoms, to be specific. Using the first-principles density functional calculations, we obtain the optimal structures and report the bonding as deduced from the charge density analysis. With the calculated electronic structure in hand, we discussed the quantum ballistic transport using channel capacity arguments motivated by the Heisenberg’s uncertainty principle. By comparing our results with the detailed pioneering calculations by Lang, we inferred an average value for channel transmitivity and touched upon material specific contact resistance. Finally, the validity of the Wiedemann–Franz law in the quantum domain is established by studying thermal conductance in nanowires.Item Open Access Random access over wireless links: optimal rate and activity probability selection(2017-07) Karakoç, NurullahDue to the rapidly increasing number of devices in wireless networks with the proliferation of applications based on new technologies such as machine to machine communications and Internet of Things, there is a growing interest in the random access schemes as they provide a simple means of channel access. To this end, various schemes have been proposed based on the ALOHA protocol to increase the e ciency of the medium access control layer over the last decade. On the other hand, physical layer aspects of random access networks have received relatively limited attention, and there is a need to consider optimal use of the underlying physical layer properties especially for transmission over wireless channels. In this thesis, we study uncoordinated random access schemes over wireless fading channels where each user independently decides whether to send a packet or not to a common receiver at any given time slot. To characterize the system throughput, i.e., the expected sum-rate, an information theoretic formulation is developed. We consider two scenarios: classical slotted ALOHA, where no multiuser detection (MUD) capability is available and slotted ALOHA with MUD. Our main contribution is that the optimal rates and the channel activity probabilities can be characterized as a function of the user distances to the receiver to maximize the system throughput in each case (more precisely, as a function of the average signal to noise ratios of the users). We use Rayleigh fading as our main channel model, however, we also study the cases where log-normal shadowing is observed along with small scale fading. Our proposed optimal rate selection schemes o er signi cant increase in expected system throughput compared to the same rate approach commonly used in the literature. In addition to the overall throughput optimization, the issue of fairness among users is also investigated and solutions which guarantee a minimum amount of individual throughput are developed. We also design systems with limited individual outage probabilities of the users for increased energy e ciency and reduced delay. All of these analytical works are supported with detailed numerical examples, and the performance of the proposed methods are evaluated.Item 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.