Mühendislik Fakültesi / Faculty of Engineering

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Adaptive dual-mode OFDM with index modulation
(Elsevier Science Bv, Po Box 211, 1000 Ae Amsterdam, Netherlands, 2018-10) Aldırmaz Çolak, Sultan; Başar, Ertuğrul; ACAR, YUSUF; 41633; 237377; 149116
With the integration of the index modulation concept, the popular OFDM has gained an appealing new dimension. However, OFDM with index modulation (OFDM-IM) causes a decrease in data rate for higher order modulations due to its unused subcarriers. Recently, dual-mode OFDM with index modulation (DM-OFDM-IM) has been proposed to prevent this decrease in data rate. The spectral efficiency (SE) of both techniques depends on the selection of different parameters, such as modulation types and number of active subcarriers. However, the quality of the wireless channel has not been taken into account yet to increase the data rate. In this paper, we propose a new adaptive DM-OFDM-IM (A-DM-OFDM-IM) system to enhance the bit error probability (BER) performance considering the channel conditions and obtain a substantial increase in SE. Unlike the previous DM-OFDM-IM and the classical OFDM-IM schemes, our proposed system enables the use of different types of modulations for different subblocks in one OFDM signal. We demonstrate that the SE of the proposed system is comparable with that of its OFDM-IM and DM-OFDM-IM counterpart under frequency selective Rayleigh channels. Moreover, computer simulations corroborate that the derived theoretical results are considerably accurate for the SE of A-DM-OFDM-IM systems. (C) 2018 Elsevier B.V. All rights reserved.
Open Access
Channel estimation for OFDM-IM systems
(2019) Aldırmaz Çolak, sultan; Başar, Ertuğrul; ACAR, YUSUF
Orthogonal frequency division multiplexing with index modulation (OFDM-IM) has been recently proposed to increase the spectral efficiency and improve the error performance of multicarrier communication systems. However, all the OFDM-IM systems assume that the perfect channel state information is available at the receiver. Nevertheless, channel estimation is a challenging problem in practical wireless communication systems for coherent detection at the receiver. In this paper, a novel method based on the pilot symbol-aided channel estimation technique is proposed and evaluated for OFDM-IM systems. Pilot symbols, which are placed equidistantly, allow the regeneration of the response of the channel so that pilot symbol spacing can fulfill the sampling theorem criterion. Our results shows that the low-pass interpolation and SPLINE techniques perform the best among all the channel estimation algorithms in terms of bit error rate and mean square error performance.
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Channel Estimation for SM Systems over Time-Varying Rayleigh Fading Channels
(IEEE, 345 E 47Th St, New York, Ny 10017 Usa, 2014) Doğan, Hakan; Panayırcı, Erdal; ACAR, YUSUF; 237377; 41066; 10331
This paper is concerned with the challenging and timely problem of channel estimation for spatial modulated systems in the presence of time varying channels. Recently, estimation of channel state information for SM systems is investigated by the recursive least square (RLS) algorithm for slow fading channels. However, it is clear that the RLS based receiver will have a performance degradation for fast fading channels. Therefore, we developed iterative channel estimation based on detected symbols and curve fitting to track the channel variations for SM systems. Simulation results have demonstrated that the proposed iterative channel estimation offer substantial performance gains over the RLS channel estimation. In particular, a savings of about 4dB is obtained at BER = 10(-7), as compared with RLS based receiver at 150km/h for 2 x 4 single carrier SM systems.
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Channel estimation for spatial modulation orthogonal frequency division multiplexing systems
(IEEE, 345 E 47th St, New York, Ny 10017 USA, 2015-10-14) Doğan, Hakan; Panayırcı, Erdal; ACAR, YUSUF; 237377; 10331
A combination of spatial modulation and orthogonal frequency division multiplexing (SM-OFDM) has been recently proposed for multicarrier systems while assuming the perfect channel state information (P-CSI) available at the receiver. But, in practical scenarios, the estimation of CSI is crucial in order to detect the transmitted data coherently. In this work(1), a new frame structure and pilot symbol aided channel estimation (PSA-CE) technique with a piecewise linear interpolation is proposed for the SM-OFDM systems operating in the presence of frequency selective channel. Bit error rate (BER) performance of the proposed channel estimation technique is then investigated with different channel models such as Typical Urban ( TU) and Rural Urban (RU) for 4-quadrature amplitude modulation (QAM). In particular, it is shown that the performance with estimated channel came close to that with known channel within 2.3 dB in SNR for BER = 10(-4) for TU channels.
Embargo
Data detection based iterative channel estimation for coded SM-OFDM systems
(IEEE, 345 E 47Th St, New York, Ny 10017 USA, 2016) Leblebici, Mehmet Merih; Doğan, Hakan; Panayırcı, Erdal; ACAR, YUSUF; 237377; 249850; 10331
The combination of spatial modulation (SM) and orthogonal frequency division multiplexing (OFDM) as SM-OFDM is a recently proposed efficient modulation scheme with exploiting the index of transmit antenna to convey information bits for multiple-input multiple-output (MIMO) wireless communication systems due to its low complexity feature. The pioneering works assume that the perfect channel state information (P-CSI) or pilot symbols aided channel estimation (PSA-CE) is available at the receiver. The open question is whether the accuracy of pilot based channel estimation is satisfactory to accomplish high data rate transmission in SM-OFDM systems. This paper explores the iterative channel estimation (ICE) problem for the coded SM-OFDM systems. We showed that the quality of the channel estimation can be further enhanced by the detection of symbols at the receiver. Through comparative simulations, we show that the proposed ICE method has notable bit error rate (BER) performance compared with the PSA-CE method for the binary phase shift keying (BPSK) modulation over the typical urban (TU) channel model.
Open Access
Dual-mode index modulation aided OFDM with generalized prefix
(2019-06) Aldırmaz Çolak, Sultan; Cooklev, Todor; ACAR, YUSUF
Recently, a dual-mode OFDM relying on index modulation (DM-OFDM-IM) has been introduced to increase the data rate of the conventional IM based OFDM techniques. However, such a mechanism has bit error rate performance loss due to the deep fading channels such as the practical Rayleigh channels. In this work, to improve the BER performance of the DM-OFDM-IM technique, a new technique, that is called as DM-OFDM-IM with generalized prefix (DM-OFDM-IM-GP) is proposed. The simulation results illustrate that the proposed DM-OFDM-IM with GP achieves about 10 dB SNR gain over the existing DM-OFDM-IM at the spectral efficiency of 4, 2.22 and 1.33 bits/s/Hz in Rayleigh fading channel. Keywords-Index Modulation; Generalized Prefix; Orthogonal Frequency Division Multiplexing; Dual-Mode, Index Modulation, Spatial Modulation.
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Generalised prefix for space-time block-coded orthogonal frequency division multiplexing wireless systems over correlated multiple-input multiple-output channels
(Inst Engineering Technology-Iet, Michael Faraday House Sıx Hılls Way Stevenage, Hertford Sg1 2Ay, England, 2014-06-12) Yıldız, Hakan; Cooklev, Todor; Doğan, Hakan; ACAR, YUSUF; 253657; 237377; 41066
Space-time block coding (STBC) orthogonal frequency division multiplexing (OFDM) is known to improve the reliability of broadband communication over wireless links. However, this technique suffers from a performance loss when the multiple-input multiple-output (MIMO) channel is correlated. This study addresses this problem. A STBC-OFDM system is proposed that is based on convolution that is skew-circular, rather than circular. This skew-circular convolution leads to one additional degree of freedom. The additional parameter can be optimised leading to improved performance for MIMO channels that are correlated. The skew-circular convolution is achieved via the introduction of a generalised prefix. The simulation results are given, illustrating the performance improvement of the proposed STBC-OFDM system. Furthermore, the proposed system is shown to be robust when channel state information is known not precisely, but imperfectly.
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Generalized Prefix Approach for Alamouti Coded OFDM Systems over Gaussian Channels Correlated in Space
(IEEE, 345 E 47Th St, New York, Ny 10017 Usa, 2014) Yıldız, Hakan; Cooklev, Todor; Doğan, Hakan; ACAR, YUSUF; 237377; 253657; 41066
In this ivorkt, generalized frelix (GP) lbr space time block coded orthogonal frequency division multiplexing (STBC-OFDM) systems is considered in the presence of correlated channel. The 2x2 STBC case, in practice, lead to significant correlation among the spatial signatures and reduce the systems performance. There are several structures to reduce correlation. To solve this problem, we propose the generalized prefix that has low computational complexity. It is shown that the proposed generalized prefix thr 2x2 STBC-OFDNI outperforms the conventional 2:c2 STBC-OFDNI system.
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High performance OFDM with index modulation
(2019-02) Cooklev, Todor; ACAR, YUSUF
Orthogonal frequency division multiplexing (OFDM) with index modulation (OFDM-IM), which has been recently proposed as an alternative waveform to classical OFDM, generally achieves lower bit error rate compared to classical OFDM because it is more robust to bit errors by encoding the information in the location of the active subcarriers instead of only modulating the subcarriers. In this paper, we propose a higher-performance OFDM-IM scheme. The proposed system is more reliable than OFDM-IM, particularly for channels that have deep fading. The proposed system uses generalized prefix (GP) as a guard interval and this GP provides additional degrees of freedom, which can be optimized. Comprehensive computer simulation results are included to demonstrate the significant performance improvement for different channel models, such as deep Rayleigh fading and realistic channels with high mobility. (C) 2018 Published by Elsevier B.V.
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High-Performance Mimo Ofdm Wireless Systems With Generalized Prefix
(IEEE, 345 E 47Th St, New York, Ny 10017 Usa, 2013) Yıldız, Hakan; Cooklev, Todor; Doğan, Hakan; ACAR, YUSUF; 237377; 41066
Consumers require increasing data rates and therefore it is important to develop techniques that increase the spectrum efficiency. MIMO OFDM is a fundamental technology for high data rate wireless systems. Correlation among the different spatial streams is the main factor limiting the spectral efficiency. All practical MIMO OFDM wireless systems use cyclic prefix (CP). In this paper we propose a novel prefix for MIMO OFDM wireless systems, referred to as generalized prefix. This generalized prefix provides additional degree of freedom, which can be optimized and provides improved performance for correlated MIMO channels.
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Interpolation based pilot-aided channel estimation for STBC spatial modulation and performance analysis under imperfect CSI
(INST Engineering Technology-Iet, Michael Faraday House Six Hills Way Stevenage, Hertford Sg1 2Ay, England, 2016-09-20) Doğan, Hakan; Başar, Ertuğrul; Panayırcı, Erdal; ACAR, YUSUF; GÜNTEKİN, BAHAR; 237377; 149116; 10331
A combination of space-time block coding and spatial modulation (STBC-SM) has been recently proposed for multiple input-multiple output systems to obtain both spatial diversity gain and more capacity simultaneously while assuming the perfect channel state information (P-CSI) was available at the receiver. However, in practical scenarios the CSI is unknown to the receiver and should be estimated in order to detect the transmitted data in a reliable way. Therefore, channel estimation (CE) is a major challenge in designing the STBC-SM systems. In this study, the problem of CE is investigated and a new pilot-aided channel estimation (PA-CE) technique, coupled with an interpolation, is proposed for the STBC-SM systems operating in the presence of rapidly time-varying mobile channels. Several interpolation schemes such as the linear, nearest neighbour, piecewise cubic Hermite and the low-pass interpolations are applied and their performances are compared to determine the best suitable interpolation technique to be employed in STBC-SM systems. Bit error rate (BER) performance of the proposed CE technique is then investigated in time-varying channels with different modulation. Moreover, the pairwise error probability of the STBC-SM scheme is derived and its average bit error probability is evaluated analytically in the presence of CE errors.
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Iterative Channel Estimation For Spatial Modulation Systems Over Fast Fading Channels
(IEEE, 345 E 47Th St, New York, Ny 10017 Usa, 2014) Doğan, Hakan; Panayırcı, Erdal; ACAR, YUSUF; 237377; 41066; 10331
Spatial Modulation (SM) based on the use of antenna indices to transmit information in addition to the conventional signal constellations has been recently proposed to solve practical problems encountered in MIMO systems. In the literature, estimation of CSI for SM systems is simply investigated by RLS for slow fading channels. However, iterative receivers that offer significant performance gains over a non- iterative when the total number of pilot is not sufficient and the channel is fast fading have not been investigated for SM based systems. In this paper, it shown that the RLS based receiver has a performance degradation for fast fading channels and a new iterative channel estimation techniques is proposed with the necessary signal model to enhance receiver performance for fast fading channels. Computer simulation results indicate that proposed iterative receiver has a significant performance advantage over the RLS based receiver for time-varying Rayleigh channels.
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On channel estimation for spatial modulated systems over time-varying channels
(Academic Press inc Elsevier Science, 525 B St, Ste 1900, San Diego, Ca 92101-4495 USA, 2015-02) Doğan, Hakan; Panayırcı, Erdal; ACAR, YUSUF; 237377; 10331
Spatial Modulation (SM) has been proposed recently for multiple-input multiple-output (MIMO) systems to cope with the interchannel interference and to reduce the detection complexity as compared to the conventional MIMO systems. In SM system, the data symbols are transmitted by a randomly selected active antenna of a MIMO transmitter to the receiver through a wireless channel. The information is carried both by the data symbol from any signal constellation such as M-ary phase shift keying (M-PSK) or M-ary quadrature amplitude modulation (M-QAM), by the index of the selected antenna. The channel estimation is a critical process at the receiver during the coherent detection of the transmitted symbol and the antenna index, randomly selected. Recently, the channel estimation of channel for SM systems has been investigated by the recursive least square (RLS) algorithm for only quasi-static fading channels. In this paper, a novel channel estimation is proposed for SM systems in the presence of rapidly time-varying channels. The Bayesian mean square error (MSE) bound has been derived as a benchmark and the performance of the proposed approaches is studied in terms of MSE and bit-error rate (BER). Computer simulation results have confirmed that the proposed iterative channel estimation technique has significant BER/MSE performance advantages compared with existing channel estimation algorithm proposed earlier in the literature. (C) 2014 Elsevier Inc. All rights reserved.
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Performance Improvement for Correlated 4x4 MIMO-OFDM Systems by Generalized Prefix Approach
(IEEE, 345 E 47Th St, New York, Ny 10017 Usa, 2013) Cooklev, Todor; Doğan, Hakan; Yıldız, Hakan; ACAR, YUSUF; 237377; 41066; 253657
Antenna correlation limits the multiplexing gain achieved by practical MIMO-OFDM systems and degrades the performance significantly. Recently, investigations are focused to reduce the antenna correlation by the designing new antenna structures. Differently, in this paper, we propose the generalized prefix approach to solve problem for correlated MIMO-OFDM system. It is shown that the proposed generalized prefix for 4x4 MIMO-OFDM exhibits a gain of about 8 dB over the conventional 4x4 MIMO-OFDM system at BER = 10(-4)
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Pilot symbol aided channel estimation for spatial modulation-OFDM systems and its performance analysis with different types of interpolations
(Springer, 233 Spring St, New York, Ny 10013 USA, 2017-06) Doğan, Hakan; Panayırcı, Erdal; ACAR, YUSUF; 237377; 10331
Spatial modulation orthogonal frequency division multiplexing (SM-OFDM) system has been recently proposed as an alternative for multiple-input multiple-output (MIMO)-OFDM systems to increase spectral efficiency by keeping a low-complexity implementation. In the literature, SM-OFDM systems assume a perfect channel state information (P-CSI) available at the receiver for coherent detection and the channel estimation problem is not considered. It is clear that the channel estimation is a critical issue on the performance of SM-OFDM systems. In this paper, a frame structure where pilot symbols are typically interspersed with data symbols among the sub-carriers to aid the channel estimation is considered. Then the pilot symbol aided channel estimation (PSA-CE) technique with different interpolations is proposed for the SM-OFDM systems. It is shown that in the proposed PSA-CE, equidistantly spaced pilot symbols allow to reconstruct the channel response by means of interpolation if the spacing of the pilots is sufficiently close to satisfy the sampling theorem. Linear, nearest neighbor, piecewise cubic Hermite and the low-pass interpolations are investigated to explore trade-off between complexity and performances. We show that the low pass interpolator yields better performance than the other interpolation techniques for selected cases such as higher order modulations. The results validate the potential of the proposed PSA-CE estimation applying to SM-OFDM systems.
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Pilot symbol assisted channel estimation for 4x4 space time block coded spatial modulation systems
(IEEE, 345 E 47th St, New York, Ny 10017 USA, 2015) Doğan, Hakan; Panayırcı, Erdal; ACAR, YUSUF; 237377; 10331
Recently, spatial modulation (SM) and space-time block coding (STBC) are combined to take advantage of the benefits of both while avoiding their drawbacks for multiple-input and multiple-output (MIMO) systems. The pioneering works on STBC-SM assume that perfect knowledge of the channel fading coefficients is available at the receiver. This work(1) addresses the challenging and timely problem of channel estimation for 4 x 4 STBC-SM systems in the presence of time-varying channels. In this paper, the estimation of channel at pilot durations is done by least square (LS) method and then the channel interpolation is performed by linear interpolation or nearest neighbor interpolation algorithms. Simulation results have demonstrated that the proposed channel estimation based on the linear interpolation offer substantial performance gains over the channel estimation based on the nearest neighbor interpolation. In particular, a savings of about 5dB is obtained at BER = 10(-5), as compared with the nearest interpolation based receiver at 120km/h for 4 x 4 STBC-SM systems with the binary phase shift keying (BPSK) modulation.
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Receiver Design for Dual-Mode Index Modulation Aided OFDM
(2019-04-12) Aldırmaz Çolak, Sultan; ACAR, YUSUF; 237377; 41633
Nowadays, variants of Spatial Modulation (SM) are studied by a lot of researchers in order to increase Spectral Efficiency (SE) more. One of the important variants of SM is known as Orthogonal Frequency Division Multiplexing Index Modulation (OFDM-IM). In OFDM-IM, selection of active subcarriers is based on the idea of the selection of active antennas in the SM. Then, to take OFDM-IM a step further in terms of SE, Dual Mode OFDM- IM (DM-OFDM-IM) has been proposed that activates entire subcarriers unlike that of OFDM- IM. However, in all these studies same assumption is made, i.e., channel is perfectly known at the receiver. However, this assumption is not accurate for practical applications. For this reason, the presented simulation results and theoretical analysis without channel estimation cannot represent the real system performance. In this study, a pilot symbol aided channel estimation for DM-OFDM-IM scheme is considered. We investigate the effect on Bit Error Rate (BER) performance of five different interpolation techniques. The simulation results illustrate that, interpolation techniques perform similiar BER performance for low and moderate Signal to Noise Ratio (SNR) range. However, at the high SNR values (SNR> 30 dB), Low-Pass and SPLINE interpolation techniques give the highest performance among all considered techniques.
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Spline Based Channel Estimation for STBC-SM Systems over Fast-Varying Rician Fading Channels
(IEEE, 345 E 47Th St, New York, Ny 10017 USA, 2015-05-16) Doğan, Hakan; Panayırcı, Erdal; ACAR, YUSUF; 237377; 10331
In the STBC-SM systems, the information is carried both by STBC symbols and indices of the transmit antennas from which these symbols are transmitted. Channel estimation is a critical process at the receiver during the coherent detection of the transmitted STBC symbols and indices of the transmit antennas. In this paper, a spline interpolation and pilot symbols based channel estimation has been proposed for the space-time block coded spatial modulation (STBC-SM) systems to track the channel variations in the presence of Rician fading channels. The estimation of channel at pilot durations is done by least squares (LS) method and then the spline based channel interpolation is performed. It has been shown via computer simulations that the proposed spline based channel estimation channel estimation outperforms the piecewise linear interpolation (PLI) and the nearest neighbor interpolation (NNI) based channel estimations.
Open Access
Sub-Block Aided OFDM with Index Modulation
(Bajece (İstanbul Teknik Üniversitesi), 2019) ACAR, YUSUF
Recently, orthogonal frequency division multiplexing (OFDM) with index modulation (IM) has been appeared as a novel method for future wireless communication systems. However, such a mechanism has low spectral efficiency since some sub-carriers are not activated in order to implicitly convey information. In this paper, a subblock dependent approach, called sub-block aided OFDMIM (SA-OFDM-IM) technique, is proposed for spectral efficiency enhancement of the OFDM-IM method with lower complexity. The simulation results illustrate that the proposed SA-OFDM-IM and well known OFDM-IM have same bit error rate (BER) performance while SA-OFDMIM has 40% more spectral efficiency with low complexity.

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