Browsing by Author "Heidarpour, Ali Reza"

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Diversity-multiplexing tradeoff for network coded cooperative OFDMA systems
(2016-08) Heidarpour, Ali Reza; Uysal, Murat; Uysal, Murat; Demiroğlu, Cenk; Kurt, G. K.; Department of Electrical and Electronics Engineering; Heidarpour, Ali Reza
Cooperative communication (CC) exploits the broadcasting nature of wireless transmission and creates a distributed multiple-input multiple-output (MIMO) system among the wireless nodes which are physically separated from each other. Earlier works on cooperative communication typically build on time division multiple access (TDMA). However, it is well known that conventional CC systems experience throughput loss due to additional time slots required for relaying phase which particularly become prohibitive with the increasing number of relay nodes. Network coded cooperative (NCC) systems have been proposed as a powerful alternative with significantly higher throughput efficiency. In an NCC system, the relay nodes perform either XOR operation or form a linear combination of the messages received from distinct sources. Through proper combining, network coding improves the spectral efficiency over conventional unidirectional cooperative communication systems. In the first chapter of this thesis, we provide an overview of NCC systems and discuss their advantages over conventional CC systems in terms of diversity multiplexing tradeoff (DMT). All existing works on the DMT analysis of NCC systems assume perfect channel state information (CSI) at relay and destination nodes and focus on asymptotical analysis. It is known that asymptotical DMT analysis overestimates the performance since most communication systems operate in low and moderate signal-to-noise ratio (SNR) regime. To characterize DMT within more practical SNR regime, finite-SNR DMT is a more meaningful performance measure. In the second chapter, we investigate the outage probability and finite-SNR DMT performance for NCC wireless networks in the presence of imperfect CSI. We show that our results are generalized versions of those earlier presented in the literature and coincide with them in high SNR regime. Our analysis further reveals that the DMT is highly dependent of channel estimation quality particularly at finite-SNR values. In an effort to have further gains over the initial works, which build upon the assumption of TDMA, the combined use of orthogonal frequency division multiplexing (OFDM) with NCC has been proposed in the literature. In the third chapter, we consider orthogonal frequency division multiple access (OFDMA), an extension of the OFDM to a multiuser system where subsets of carriers are assigned to different users. We derive a closed-form expression for the outage probability of the system over Rayleigh fading channels and present the DMT analysis. Our results demonstrate that NCC-OFDMA system is able to fully exploit both frequency and spatial diversity. In the third chapter, we investigate only asymptotic DMT analysis of NCC-OFDMA systems over Rayleigh fading channels. In the fourth chapter, we investigate both finite-SNR DMT and asymptotic DMT performance over Rician fading channels. Such an analysis helps us see the difference of performances over Rayleigh and Rician fading channels. Otherwise, DMT results for Rayleigh and Rician channels coincide each other for asypmtotical SNR values.
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Diversity-multiplexing tradeoff for network coded cooperative OFDMA systems
(IEEE, 2015) Heidarpour, Ali Reza; Kurt, G. K.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Heidarpour, Ali Reza
Network coded cooperation (NCC) is an effective method to improve the throughput efficiency in cooperative wireless networks. In an effort to have further gains over the initial works, which build upon the assumption of time division multiple access, the combined use of orthogonal frequency division multiplexing (OFDM) with NCC has been proposed in the literature. In this paper, we consider orthogonal frequency division multiple access (OFDMA), an extension of the OFDM to a multiuser system where subsets of carriers are assigned to different users. We derive a closed-form expression for the outage probability of the system under consideration and present the diversity-multiplexing tradeoff (DMT) analysis. Our results demonstrate that NCC-OFDMA system is able to fully exploit both frequency and spatial diversity. Simulation results are presented to verify our theoretical analysis.
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Finite-SNR diversity-multiplexing tradeoff for network coded cooperative OFDMA systems
(IEEE, 2017-03) Heidarpour, Ali Reza; Karabulut Kurt, G.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Heidarpour, Ali Reza
Network-coded cooperation (NCC) is an effective method to improve the throughput efficiency in cooperative wireless networks. The combined use of orthogonal frequency division multiplexing (OFDM) with NCC has been further studied in the literature to exploit the multipath diversity gains. In this paper, we consider orthogonal frequency division multiple access (OFDMA), an extension of OFDM to a multiuser system where subsets of carriers are assigned to different users. We first derive a closed-form expression for the outage probability of NCC-OFDMA over Rician fading channels and then present the asymptotical and finite-SNR DMT expressions. Our results provide insight into the performance mechanisms under practical SNR regime of NCC-OFDMA systems and demonstrate that NCC-OFDMA is able to fully exploit both frequency and spatial diversity. We also show that the derived finite-SNR DMT converges to an asymptotical one as expected. Furthermore, special cases for our derived analytical expressions are presented, to show that the derived expression is a generalized case of the related the state of the art results. Simulation results are further presented to verify our theoretical analyses.
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Finite-SNR DMT analysis for multisource multirelay NCC systems with imperfect CSI
(IEEE, 2016) Heidarpour, Ali Reza; Karabulut Kurt, G.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Heidarpour, Ali Reza
Network coded cooperative (NCC) systems have recently attracted attention with their high spectral and power efficiency. In this paper, we investigate the information theoretical limits of NCC systems in the presence of imperfect channel state information (CSI). Specifically, we derive exact outage probability and finite-SNR diversity- multiplexing tradeoff (DMT) for general multisource multirelay NCC wireless networks over Rayleigh fading channels. Numerical results are presented to verify our analytical derivations and provide insight into the impact of imperfect CSI on the NCC system performance.
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Non-line-of-sight ultraviolet communications over atmospheric turbulence channels
(IEEE, 2015) Ardakani, Maryam Haghighi; Heidarpour, Ali Reza; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Ardakani, Maryam Haghighi; Heidarpour, Ali Reza
Ultraviolet (UV) communication enables non-line-of sight (NLOS) outdoor wireless connectivity and is particularly desirable to relax or eliminate pointing and tracking requirements of infrared links. In the current literature, atmospheric turbulence effects are typically ignored under the assumption of short link distances and clear weather conditions. In this paper, we address the statistical modeling of turbulence experienced in NLOS UV links. We assume that the NLOS link consists of two line-of-sight (LOS) paths each of which experiences log-normal distributed fading. We obtain a closed-form expression for the probability density function of turbulence induced fading. Based on the derived expression, we investigate bit error rate performance for different system configurations. Numerical simulations are further presented to confirm the accuracy of our derivation.
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On the performance of NCC-OFDMA systems in the presence of carrier frequency offset
(IEEE, 2017) Heidarpour, Ali Reza; Kurt, G. K.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Heidarpour, Ali Reza
Network coded cooperative (NCC) systems have recently gained increasing attention due to their high power and spectral efficiency. For high speed data transmission over frequency-selective channels, NCC is further combined with orthogonal frequency division multiple access (OFDMA). In this paper, we derive outage probability and diversity gain of NCC-OFDMA systems in the presence of carrier frequency offset (CFO). Our results demonstrate that NCC-OFDMA system is capable to achieve full spatial and frequency diversity in noise limited system where the CFO variance σ2ε → 0. However, in interference limited case where the CFO variance σ2ε > 0, error floor occurs leading zero diversity gain. Numerical results are further presented to validate our analytical findings.
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Performance analysis of MIMO NLOS UV communications over atmospheric turbulence channels
(IEEE, 2016) Ardakani, Maryam Haghighi; Heidarpour, Ali Reza; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Ardakani, Maryam Haghighi; Heidarpour, Ali Reza
Strong molecular and aerosol scattering in ultraviolet (UV) wavelengths enable the non-line-of-sight (NLOS) outdoor wireless connectivity which is attractive in many applications due to covertness and security. In the current literature, atmospheric turbulence effects are typically ignored under the assumption of short link distances and clear weather conditions. In this paper, we investigate the performance of multiple-input multiple-output (MIMO) UV systems over turbulence channels. Specifically, we derive bit error rate (BER) expressions over an NLOS UV channel which is modeled as an overlap of two log-normal LOS links. Our results demonstrate that fading variance is scaled by the number of transmitters and receivers and MIMO deployment with proper configurations results in significant BER improvements. Simulation results are further provided to confirm the analytical findings.
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Performance analysis of MIMO NLOS UV communications over atmospheric turbulence channels
(IEEE, 2016) Ardakani, Maryam Haghighi; Heidarpour, Ali Reza; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Ardakani, Maryam Haghighi; Heidarpour, Ali Reza
Strong molecular and aerosol scattering in ultraviolet (UV) wavelengths enable the non-line-of-sight (NLOS) outdoor wireless connectivity which is attractive in many applications due to covertness and security. In the current literature, atmospheric turbulence effects are typically ignored under the assumption of short link distances and clear weather conditions. In this paper, we investigate the performance of multiple-input multiple-output (MIMO) UV systems over turbulence channels. Specifically, we derive bit error rate (BER) expressions over an NLOS UV channel which is modeled as an overlap of two log-normal LOS links. Our results demonstrate that fading variance is scaled by the number of transmitters and receivers and MIMO deployment with proper configurations results in significant BER improvements. Simulation results are further provided to confirm the analytical findings.
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Performance investigation of the modified costas carrier recovery loop for a QPSK modulated signal in the presence of co-channel interference
(IEEE, 2016) Heidarpour, Ali Reza; Montazerolghaem, M. A.; Saberali, S. M.; Heidarpour, Ali Reza
One of the important challenges in communication systems is drift in the carrier frequency of received signal due to the clock skew and Doppler effect. Every communication receiver requires an automatic frequency control (AFC) loop to estimate the carrier drift in the received signal. One of the well-known AFC loops in communication systems is the Costas loop. In this paper a modified version of Costas loop for binary phase-shift keying (BPSK) modulation is presented which is capable to estimate the carrier frequency of the quadrature phase-shift keying (QPSK) modulation, and therefore significantly simplifies practical implementation. The performance of the modified Costas loop is investigated for the carrier recovery of a QPSK signal in the presence of a co-channel interference. Both simulations and analytical results illustrate that the modified Costas loop may estimate the carrier frequency drift of received signal when the desired signal is stronger than interference.