I. Altunbas

Istanbul Technical University, İstanbul, Istanbul, Turkey

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Publications (41)13.05 Total impact

  • A.S. Demiroğlu, I. Altunbaş
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    ABSTRACT: Continuous phase modulation (CPM) is a non-linear power/bandwidth efficient modulation method. Linear modulation methods such as phase shift keying or quadrature amplitude modulation produce large power drain at the power amplifier of the transmitter. Therefore CPM may be an alternative to these modulations in relay/cooperative systems because of its constant envelope property, which enables us to use inexpensive and energy-efficient non-linear power amplifiers. In this study, the authors investigate optimum distributed space-time CPM trellis codes for amplify and forward relaying systems with one to four relays and specifically consider minimum shift keying and quaternary continuous phase frequency shift keying with modulation indices of 1/2 and 1/4. Novel distributed space-time CPM trellis codes are obtained. Error performances of the novel codes and their classical counterparts are compared over frequency non-selective quasi-static Rayleigh-fading channels using the Viterbi algorithm at the destination terminal and superiority of the novel codes are presented.
    IET Communications 01/2013; 7(2):110-118. · 0.72 Impact Factor
  • Ahmet F. Coşkun, Oğuz Kucur, İbrahim Altunbaş
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    ABSTRACT: In this paper, error performances of multiple‐input multiple‐output systems that employ Alamouti‐coded transmission with transmit antenna selection are examined for binary phase‐shift keying, binary frequency‐shift keying, M‐ary phase‐shift keying, and M‐ary quadrature amplitude‐modulation signals in independent but non‐identically distributed flat Nakagami‐m fading channels. Exact symbol error rate expressions are derived by using the moment‐generating function‐based analysis method. Upper bound expressions have been obtained in order to examine the asymptotic diversity order of transmit antenna selection/Alamouti scheme. Also, outage probability analysis of investigated systems has been given in order to examine the system capacity. Monte Carlo simulations have validated the analytical symbol error rate performance results. Copyright © 2011 John Wiley & Sons, Ltd.
    Wireless Communications and Mobile Computing 01/2013; 13(7). · 0.86 Impact Factor
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    ABSTRACT: Error performance of a cooperative system can be enhanced by using transmit and receive diversity techniques at transmission links. The number of transmit/receive RF chain pairs required to achieve full diversity can be decreased to one for each link by using transmit antenna selection (TAS) method at the transmitter and selection combining (SC) method at the receiver. Thus, hardware complexity of a multiple input multiple output (MIMO) cooperative scheme can be significantly reduced when compared to systems that use TAS and maximum ratio combining (MRC). In this paper, we investigate the performance of an amplify-and-forward cooperative system where TAS/SC is utilized. We derive the probability density function (pdf) of end-to-end SNR of the system for Rayleigh fading channels. By using this pdf, we obtain the exact symbol error rate expressions for M-PSK and M-QAM modulations and the exact outage probability expression. We also obtain the asymptotical diversity order using upper and lower bounds of the outage probability expression and show that our system provides the same diversity order as the cooperative system where TAS/MRC is utilized. We verify our results via computer simulations. Copyright © 2010 John Wiley & Sons, Ltd.
    Wireless Communications and Mobile Computing 10/2012; 12(14). · 0.86 Impact Factor
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    ABSTRACT: In this paper, we present outage probability and symbol error rate (SER) performance analyses of a dual-hop transmission using fixed-gain amplify-and-forward relaying in flat Nakagami-m fading channels. The system under consideration is equipped with multiple antennas at source and destination adopting orthogonal space-time block coding to provide transmit diversity and maximum ratio combining to provide receive diversity, respectively. For integer and half-integer m values, closed forms of exact outage probability and moment generating function (MGF) expressions are derived through cumulative distribution function (CDF) of the overall system signal-to-noise ratio. Closed-form exact SER expressions based on the overall CDF are obtained for binary phase shift keying, binary frequency shift keying and M-ary pulse amplitude modulation. Exact SER expressions based on the MGF method are also obtained for binary differential phase shift keying, M-ary phase shift keying and M-ary quadrature amplitude modulation. Moreover, the asymptotic diversity order analysis is performed through derivations of asymptotic outage probability and SER. Theoretical analyses are validated by Monte Carlo simulations showing perfect match between each other.
    AEU - International Journal of Electronics and Communications 10/2012; 66(10):841–846. · 0.55 Impact Factor
  • Source
    H. İlhan, I. Altunbaş, M. Uysal
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    ABSTRACT: In this study, the authors investigate the error rate performance of amplify-and-forward relaying over N*Nakagami fading channels. This is a recently introduced channel model that involves the product of N Nakagami- m -distributed random variables. Employing the moment generating function approach, the authors derive symbol error rate expressions for a single-relay system under instantaneous power scaling (IPS) and average power scaling (APS) factors at the relay node, that is, variable and fixed gains. The results achieved by the authors demonstrate that the achievable diversity order is a function of Nakagami fading parameter (m) and degree of cascading ( N ). An identical diversity order is obtained under both scaling factors when the relay is close to the destination. When the relay is close to the source, IPS becomes advantageous over APS. Monte-Carlo simulations are further provided to confirm the analytical results.
    IET Communications 03/2011; · 0.72 Impact Factor
  • Haci Ilhan, Ibrahim Altunbas
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    ABSTRACT: In this paper, error performances of distributed parallel con- catenated (turbo) coded relay/cooperative systems, whose com- ponent codes are recursive space-time trellis codes are inves- tigated over cascaded fading channels. Two seperate systems with one and two relays are considered. In the first system, the relay uses "decode and forward" method while in the second one, the relays use "amplify and forward" and decode and for- ward methods. Computer simulations are performed under the double Rayleigh fading channel assumption and the superiority of the distributed turbo coded systems is presented. 1. Giris¸
    01/2011;
  • Source
    H. İlhan, I. Altunbaş, M. Uysal
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    ABSTRACT: In this letter, we consider the deployment of distributed space-time trellis codes (STTCs) for user cooperation. We derive a pairwise error probability (PEP) expression for distributed STTCs over cascaded Rayleigh fading channels. Using the derived PEP expression, we determine a code design criterion and propose novel 4-/8-/16-state 4-PSK and 8-/16-state 8-PSK distributed STTCs through a systematic code search. We confirm the superiority of the proposed codes through extensive Monte-Carlo simulations.
    IEEE Communications Letters 01/2011; · 1.16 Impact Factor
  • O. Özdemir, I. Altunbaş, M. Bayrak
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    ABSTRACT: Super-orthogonal space-time trellis (SOSTT) codes provide a significant coding gain with respect to the conventional space-time trellis (STT) codes without increasing the decoding complexity at the receiver. Transmit antenna selection (TAS) is an important technique to solve the implementation complexity problems of multiple-antenna systems that arise from employing a separate RF chain for each antenna. This study treats a scheme combining SOSTT coding with TAS in order to exploit performance improvements provided by SOSTT codes while realising the promises made by the TAS technique. In the proposed scheme, transmit antennas that maximise the signal-to-noise ratio at the receiver are chosen and activated out of all available transmit antennas for transmission of the baseline SOSTT code, whereas all other transmit antennas are silent. The error performance of the proposed structure is investigated by deriving moment generating function (MGF)-based upper bound expressions on the pairwise error probability in quasi-static flat Rayleigh fading channels for both perfect and imperfect antenna subset selection. The performance analysis demonstrates that the proposed scheme provides full diversity order as if all transmit antennas were used when antennas are perfectly selected. Moreover, it is shown that this structure provides better error performance almost at the same decoding complexity compared to the previous STT coding structures with TAS in the literature.
    IET Communications 12/2010; · 0.72 Impact Factor
  • I. Levent Karaevli, İbrahim Altunbas, Gunes Karabulut
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    ABSTRACT: Performance of cooperative communication systems can be improved effectively by using multiple input multiple output (MIMO) techniques at transmission links. We make use of amplify and forward cooperative relaying techniques with transmit antenna selection (TAS) and selection combining (SC) methods to present an effective transmission scheme. Applying TAS method at the transmitter and SC method at the receiver reduces hardware complexity at both sides as the required number of radio frequency chains is decreased. We assume that Rayleigh fading takes place at all links. We derive the probability density function of end-to-end SNR. Using this, we obtain the exact symbol error rate expression analytically for M-PSK modulation and verify our results via computer simulations for BPSK. We also derive a bound for error probability, assuming the link between relay and destination has sufficiently larger SNR than the other links. Using this bound, we analyze asymptotic diversity orders for several system configurations.
    Computers and Communications (ISCC), 2010 IEEE Symposium on; 07/2010
  • Deniz Çelik, Ibrahim Altunbas, Aygölü
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    ABSTRACT: In this paper, coded user cooperation systems are integrated with combined frequency and phase modulation. Frequency/phase shift keying modulation defined over a signal set varying in both frequency and phase is a nonlinear constant envelope multidimensional modulation technique. Error performance of the proposed scheme is evaluated by computer simulations in quasi-static Rayleigh fading channels, for both full and selective cooperation cases. The selective cooperation is performed first by cyclic redundant check code control and then by threshold signal-to-noise control. Simulation results show that in all cases, frequency/phase shift keying modulation provides much better error performance than the conventional phase shift keying modulation in coded cooperation systems.
    2010 Third International Conference on Communication Theory, Reliability, and Quality of Service. 06/2010;
  • D.Çelik, İ.Altunbaş, Ü.Aygölü
    Third International Conference on Communication Theory, Reliability, and Quality of Service, CTRQ 2010, Athens/Glyfada, Greece; 06/2010
  • A.F. Coskun, O. Kucur, I. Altunbas
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    ABSTRACT: In this work, bit/symbol error rate (BER/SER) performances of multiple-input multiple-output (MIMO) systems that employ Alamouti coded transmission with transmit antenna selection (TAS) are examined for independent but non-identical flat Nakagami-m fading channels. Exact BER/SER expressions are derived by using the moment generating function (MGF)-based analysis method for binary phase shift keying (BPSK), binary frequency shift keying (BFSK), M-ary phase shift keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM) signals. Also, upper bound expressions have been obtained in order to examine the asymptotic diversity order of TAS/Alamouti scheme. Monte Carlo simulations have validated the theoretical SER performance results derived for different numbers of transmit and receive antennas.
    Signal Processing and Communications Applications Conference (SIU), 2010 IEEE 18th; 05/2010
  • Ahmet F. Coşkun, Oğuz Kucur, İbrahim Altunbaş
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    ABSTRACT: In this paper, multiple-input multiple-output systems employing space-time block codes (STBCs) with transmit antenna selection (TAS) are examined for flat Nakagami-m fading channels. Exact symbol error rate (SER) expressions for M-ary modulation techniques are derived by using the moment generating function based analysis method. In the SER analysis, the receiver is assumed to use maximal ratio combining whereas a subset of transmit antennas that maximizes the instantaneous received signal-to-noise ratio (SNR) is selected for STBC transmission. The analytical SER results are validated by Monte Carlo simulations. By deriving upper and lower bounds for SER expressions, it is shown that TAS/STBC schemes achieve full diversity orders at high SNRs. KeywordsSpace-time block codes (STBCs)–Transmit antenna selection (TAS)–Nakagami-m fading
    Wireless Personal Communications 05/2010; 67(3):1-15. · 0.43 Impact Factor
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    ABSTRACT: Continuous phase frequency shift keying is a power/bandwidth efficient modulation technique, hence, attractive for use in relay/cooperative systems employing inexpensive nonlinear power amplifiers. We investigate optimum distributed space-time 4-ary continuous phase frequency shift keying trellis codes for amplify & forward relaying method and propose novel codes for relay transmission systems. The error performances of the classical and novel codes are investigated for one and two-relay systems using various transmission protocols.
    Communication Theory, Reliability, and Quality of Service (CTRQ), 2010 Third International Conference on; 01/2010
  • I. Altunbaş, M. Uysal
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    ABSTRACT: In this paper, error performance of a coded cooperative relay system is investigated when channels have cascaded Nakagami distribution. By using general pairwise error probability upper bound of the system, code design criterion for the distributed space-time code is derived and using this criterion, 8-state distributed space-time trellis codes for 4-PSK and 8-PSK modulations are designed. The error performances of the new codes are evaluated by computer simulations and compared those of the corresponding codes.
    01/2010;
  • Source
    H. Ilhan, M. Uysal, I. Altunbas
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    ABSTRACT: Although there has been a growing literature on cooperative diversity, the current literature is mainly limited to the Rayleigh fading channel model, which typically assumes a wireless communication scenario with a stationary base station antenna above rooftop level and a mobile station at street level. In this paper, we investigate cooperative diversity for intervehicular communication based on cascaded Nakagami fading. This channel model provides a realistic description of an intervehicular channel where two or more independent Nakagami fading processes are assumed to be generated by independent groups of scatterers around the two mobile terminals. We investigate the performance of amplify-and-forward relaying for an intervehicular cooperative scheme assisted by either a roadside access point or another vehicle that acts as a relay. Our diversity analysis reveals that the cooperative scheme is able to extract the full distributed spatial diversity. We further formulate a power-allocation problem for the considered scheme to optimize the power allocated to the broadcasting and relaying phases. Performance gains up to 3 dB are obtained through optimum power allocation, depending on the relay location.
    IEEE Transactions on Vehicular Technology 10/2009; · 2.06 Impact Factor
  • H. Ilhan, I. Altunbas, M. Uysal
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    ABSTRACT: In this paper, error performance of a cooperative relay system using amplify and forward method is investigated when channels between source, relay and destination have cascaded Nakagami distribution. General pairwise error probability of the system is derived and effective and asymptotic diversity orders are investigated for some special cases. Bit error probability curves are obtained by computer simulations.
    Signal Processing and Communications Applications Conference, 2009. SIU 2009. IEEE 17th; 05/2009
  • Source
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    ABSTRACT: In this paper, we consider using minimum shift keying, a special form of continuous phase modulation in relay sys-tems. In this way, we aim at taking advantage of constant envelope, usage compatibility with low cost nonlinear am-plifiers, and spectral efficiency of continuous phase modula-tion on relay/cooperative systems. We investigate optimum distributed space-time trellis codes for amplify & forward relaying method and propose novel codes which outperform their classical counterparts.
    01/2009;
  • Deniz Çelik, I. Altunbas, U. Aygolu
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    ABSTRACT: In this paper, coded user cooperation systems are integrated with combined frequency and phase modulation FSK/PSK. Bit error performance of the proposed scheme is evaluated by computer simulations in quasistatic Rayleigh fading channels. Simulation results show that the FSK/PSK modulation provides much better error performance than the classical MPSK modulation in coded cooperation systems.
    01/2009;
  • H. Ilhan, I. Altunbas, M. Uysal
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    ABSTRACT: Cooperative communication techniques promise the advantages of MIMO (multi-input multi-output) communications for wireless scenarios with single-antenna terminals. In this paper, we investigate the performance of a vehicle-to-vehicle cooperative scheme where another vehicle in the vicinity of the source vehicle acts as a relay. The underlying source-to-relay, relay-to-destination, and source-to-destination links are modeled as cascaded (double) Rayleigh fading. This statistical model provides a realistic description of inter-vehicular channel where two or more independent Rayleigh fading processes are assumed to be generated by independent groups of scatterers around the two mobile terminals. We derive a pairwise error probability (PEP) expression for the inter-vehicular cooperative scheme under consideration and show that the full distributed spatial diversity is extracted. Based on the derived PEP expressions, we obtain union bounds on the bit error rate performance which are then minimized to optimally allocate power between broadcasting and relaying phases. Optimum power allocation brings performance gains up to 3dB depending on the relay location and deployed modulation scheme.
    Vehicular Technology Conference, 2008. VTC 2008-Fall. IEEE 68th; 10/2008

Publication Stats

95 Citations
13.05 Total Impact Points

Institutions

  • 1994–2012
    • Istanbul Technical University
      • • Department of Electronics and Communications Engineering
      • • Department of Electronics and Communication Engineering
      • • Department of Electrical Engineering
      İstanbul, Istanbul, Turkey
  • 2006
    • Yeditepe University
      • Department of Electrical and Electronics Engineering
      İstanbul, Istanbul, Turkey
  • 2005
    • Gebze Institute of Technology
      • Department of Electronics Engineering
      Gebze, Kocaeli, Turkey