Ali Reza Heidarpour

Diversity-multiplexing tradeoff for network coded cooperative OFDMA systems

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.

Finite-SNR Diversity-Multiplexing Tradeoff for Network Coded Cooperative OFDMA Systems

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.

Performance analysis of relay-assisted NLOS ultraviolet communications over turbulence channels

Ultraviolet (UV) communication enables non-line-of-sight (NLOS) outdoor wireless connectivity and is particularly desirable for relaxing or eliminating pointing and tracking requirements of infrared links. In the current literature, atmospheric turbulence effects are typically ignored under the assumption of short link distances. In this paper, we consider a multi-hop UV system with decode-and-forward relaying and analyze its performance in the presence of log-normal (LN) turbulence. Specifically, we assume that each NLOS link consists of two line-of-sight (LOS) paths, each of which experience LN distributed fading.We first obtain a closed-form expression for the statistical distribution of turbulence-induced fading in NLOS UV links. Then we obtain the outage probability of the multi-hop UV system and quantify diversity gain as a function of system and channel parameters. Our results show that, with proper choice of these parameters, the turbulence variance for the NLOS link can be lower than that of the LOS link. This in turn yields higher-diversity gains. We also present numerical results to confirm the accuracy of our derivations and discuss the effect of several system parameters on the outage probability and diversity gain.

Performance analysis of MIMO NLOS UV communications over atmospheric turbulence channels

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.

Non-line-of-sight ultraviolet communications over atmospheric turbulence channels

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.

Implementation of Network Coding in Wireless Systems

In this chapter, we target to give extensive performance analyses about application of network coding (NC) in wireless systems, referred to as network coded cooperation (NCC), brings both diversity and multiplexing gains. We use the diversity-multiplexing trade-off (DMT) to determine performance bounds of NCC systems. Within the scope of this study, NCC is integrated with orthogonal frequency division multiple access (OFDMA) and the corresponding system model is characterized by specifically focusing on frequency diversity gain. DMT expressions of the NCC-OFDMA system is given. A real-time implementation of the NCC-OFDMA system is presented by creating a testbed NI USRP-2921, NI PXIe-5644R, NI PXI-6683H software defined radio (SDR) modules and LabVIEW software. Obtained real-time performance measurements are essential to demonstrate the practical advantages or disadvantages of the usage of the NCC-OFDMA system. Overall, we aim to present a detailed overview of the fundamental performance bounds of NCC and its extension to the practical applicability of NCC in wireless networks.

On the performance of NCC-OFDMA systems in the presence of carrier frequency offset

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.

Finite-SNR DMT Analysis for Multisource Multirelay NCC Systems with Imperfect CSI

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.