Browsing by Author "Tutay, Mehmet Emin"
Now showing 1 - 6 of 6
Results Per Page
Sort Options
Item Open Access Maximization of correct decision probability via channel switching over Rayleigh fading channels(IEEE, 2016) Keskin, Musa Furkan; Kurt, Mehmet Necip; Tutay, Mehmet Emin; Gezici, Sinan; Arıkan, OrhanIn this study, optimal channel switching (time sharing) strategies are investigated under average power and cost constraints in order to maximize the average number of correctly received symbols between a transmitter and a receiver that are connected via multiple additive Gaussian noise channels. The optimal strategy is shown to perform channel switching either among at most three channels with full channel utilization (i.e., no idle periods), or between at most two channels with partial channel utilization. In addition, it is stated that the optimal solution must operate at the maximum average power and the maximum average cost, which facilitates low-complexity approaches for calculating the optimal strategy. For two-channel strategies, an upper bound in terms of the noise standard deviations of the employed channels is provided for the ratio between the optimal power levels. Furthermore, a simple condition depending solely on the systems parameters is derived, under which partial channel utilization cannot be optimal. Numerical examples are presented to demonstrate the validity of the theoretical results.Item Open Access Optimal channel switching in the presence of stochastic signaling(IEEE, 2013) Dulek, B.; Varshney P.K.; Tutay, Mehmet Emin; Gezici, SinanOptimal channel switching and detector design is studied for M-ary communication systems in the presence of stochastic signaling, which facilitates randomization of signal values transmitted for each information symbol. Considering the presence of multiple additive noise channels (which can have non-Gaussian distributions in general) between a transmitter and a receiver, the joint optimization of the channel switching (timesharing) strategy, stochastic signals, and detectors is performed in order to achieve the minimum average probability of error. It is proved that the optimal solution to this problem corresponds to either (i) switching between at most two channels with deterministic signaling over each channel, or (ii) time-sharing between at most two different signals over a single channel (i.e., stochastic signaling over a single channel). For both cases, the optimal solutions are shown to employ corresponding maximum a posteriori probability (MAP) detectors at the receiver. Numerical results are presented to investigate the proposed approach. © 2013 IEEE.Item Open Access Performance analysis of code-multiplexed transmitted-reference ultra-wideband systems(IEEE, 2011) Tutay, Mehmet Emin; Gezici, Sinan; Poor H.V.In code-multiplexed transmitted-reference (CM-TR) ultra-wideband (UWB) systems, data signals and reference signals are transmitted using two distinct orthogonal codes. In this way, performance improvements and/or implementation advantages are obtained compared to transmitted-reference (TR) and frequency-shifted reference (FSR) ultra-wideband (UWB) systems. In this study, performance of CM-TR systems is investigated, and probability of error expressions are obtained. For the single user case, a closed-form expression for the exact probability of error is derived, whereas a Gaussian approximation, the accuracy of which depends on the number of frames per symbol, is considered for the multiuser case. Also, the maximum likelihood detector is derived, and numerical examples are presented. © 2011 IEEE.Item Open Access Receiver design and performance analysis for code-multiplexed transmitted-reference ultra-wideband systems(Bilkent University, 2010) Tutay, Mehmet EminIn transmitted-reference (TR) and frequency-shifted reference (FSR) ultrawideband (UWB) systems, data and reference signals are shifted relative to each other in time and frequency domains, respectively. The main advantage of these systems is that they remove strict requirements of channel estimation. In order to implement TR UWB systems, an analog delay line, which is difficult to build in an integrated fashion, is needed. Although FSR systems require frequency conversion at the receiver, which is much simpler in practice, they have data rate limitations. Instead, a code-multiplexed transmitted-reference (CM-TR) UWB system that transmits data and reference signals using two distinct orthogonal codes can be considered. This system requires a simpler receiver and has better performance than TR and FSR. In the first part of the thesis, CM-TR systems are investigated and probability of error expressions are obtained. For the single user case, a closed-form expression for the exact probability of error is derived. For the multiuser case, a closed-form expression is derived based on the Gaussian approximation, and the results are compared in different scenarios. In the second part of the thesis, some optimal and suboptimal receivers are studied. First, low complexity receivers, such as the blinking receiver (BR) and the chip discriminator, are presented. The requirements for these types of receivers are explained, and the conditions under which their performance can be improved are discussed. Then, an analytical analysis of the linear minimum mean-squared error (MMSE) receiver and the requirements to implement this MMSE receiver are provided. Lastly, the optimal maximum-likelihood (ML) detector is derived, which has higher computational complexity and more strict requirements than the other receivers. Finally, simulation results are presented in order to verify the theoretical results and to compare the performance of the receivers.Item Open Access Signal and detector randomization for multiuser and multichannel communication systems(Bilkent University, 2013) Tutay, Mehmet EminRandomization can be considered as a possible approach to enhance error performance of communication systems subject to average power constraints. In the first part of this dissertation, we consider downlink of a multiuser communications system subject to an average power constraint, where randomization is employed at the transmitter and receiver sides by modeling signal levels as random variables (stochastic signals) and employing different sets of detectors via time-sharing (detector randomization), respectively. In the second part, we consider single-user systems, where we assume that there exist multiple channels between the transmitter and receiver with arbitrary noise distributions over each of them and only one of the channels can be employed for transmission at any given time. In this case, randomization is performed by choosing the channel in use according to some probability mass function and employing stochastic signaling at the transmitter. First, the jointly optimal power control with signal constellation randomization is proposed for the downlink of a multiuser communications system. Unlike a conventional system in which a fixed signal constellation is employed for all the bits of a user (for given channel conditions and noise power), power control with signal constellation randomization involves randomization/time-sharing among different signal constellations for each user. A formulation is obtained for the problem of optimal power control with signal constellation randomization, and it is shown that the optimal solution can be represented by a randomization of (K + 1) or fewer distinct signal constellations for each user, where K denotes the number of users. In addition to the original nonconvex formulation, an approximate solution based on convex relaxation is derived. Then, detailed performance analysis is presented when the receivers employ symmetric signaling and sign detectors. Specifically, the maximum asymptotical improvement ratio is shown to be equal to the number of users, and the conditions under which the maximum and minimum asymptotical improvement ratios are achieved are derived. In the literature, it is known that employing different detectors with corresponding deterministic signals via time-sharing may enhance error performance of communications systems subject to average power constraints. Motivated by this result, as a second approach, we study optimal detector randomization for the downlink of a multiuser communications system. A formulation is provided to obtain optimal signal amplitudes, detectors, and detector randomization factors. It is shown that the solution of this joint optimization problem can be calculated in two steps, resulting in significant reduction in computational complexity. It is proved that the optimal solution is achieved via randomization among at most min{K, Nd} detector sets, where K is the number of users and Nd is the number of detectors at each receiver. Lower and upper bounds are derived on the performance of optimal detector randomization, and it is proved that the optimal detector randomization approach can reduce the worst-case average probability of error of the optimal approach that employs a single detector for each user by up to K times. Various sufficient conditions are obtained for the improvability and nonimprovability via detector randomization. In the special case of equal crosscorrelations and noise powers, a simple solution is developed for the optimal detector randomization problem, and necessary and sufficient conditions are presented for the uniqueness of that solution. Next, a single-user M−ary communication system is considered in which the transmitter and the receiver are connected via multiple additive (possibly nonGaussian) noise channels, any one of which can be utilized for a given symbol transmission. Contrary to deterministic signaling (i.e., employing a fixed constellation), a stochastic signaling approach is adopted by treating the signal values transmitted for each information symbol over each channel as random variables. In particular, the joint optimization of the channel switching (i.e., time-sharing among different channels) strategy, stochastic signals, and decision rules at the receiver is performed in order to minimize the average probability of error under an average transmit power constraint. It is proved that the solution to this problem involves either one of the following: (i) deterministic signaling over a single channel, (ii) randomizing (time-sharing) between two different signal constellations over a single channel, or (iii) switching (time-sharing) between two channels with deterministic signaling over each channel. For all cases, the optimal strategies are shown to employ corresponding maximum a posteriori probability (MAP) decision rules at the receiver.Item Open Access Stochastic signal design on the downlink of a multiuser communications system(IEEE, 2012-06) Tutay, Mehmet Emin; Gezici, Sinan; Arıkan, OrhanStochastic signal design is studied for the downlink of a multiuser communications system. First, a formulation is proposed for the joint design of optimal stochastic signals. Then, an approximate formulation, which can get arbitrarily close to the optimal solution, is obtained based on convex relaxation. In addition, when the receivers employ symmetric signaling and sign detectors, it is shown that the maximum asymptotical improvement ratio is equal to the number of users, and the conditions under which that maximum asymptotical improvement ratio is achieved are presented. Numerical examples are provided to explain the theoretical results. © 2012 IEEE.