Browsing by Author "Mohammadi, A."

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Open Access
Analysis and optimization of spectral and energy efficiency in underlaid D2D multi-cell massive MISO over rician fading
(Institute of Electrical and Electronics Engineers Inc., 2023-02-15) Hashemi, R.; Kazemi, Mohammad; Mohammadi, A.
This paper investigates the uplink spectral efficiency (SE) characterization and energy efficiency (EE) optimization of device-to-device (D2D) communications underlying a multi-cell massive multiple-input single-output (m-MISO) network, assuming that the channels are modeled with Rician fading. First, an analytical expression for the lower-bound of the ergodic capacity of a typical cellular user (CU) is derived with imperfect channel state information in the presence of D2D links' interference. Then, a closed-form approximate achievable SE expression for a typical D2D pair is derived considering Rician fading between D2D pairs. The asymptotic SE behavior is analyzed in strong line-of-sight (LOS) conditions for CU and D2D pairs. Also, simplified expressions are obtained for the special case of Rayleigh fading. Next, to optimize the total EE, a transmit data power allocation based on the derived rate expressions is formulated. Since the optimization problem has a non-linear and non-convex objective function, it is intractable to be solved straightforwardly. Therefore, we resort to utilizing an iterative algorithm relying on fractional programming. The numerical results show that a stronger LOS component (i.e., larger Rician K-factor) between a typical CU and its serving BS leads to a significant improvement in the system performance, while it has less effect on the D2D network.
Open Access
Analysis of DF relay selection in massive MIMO systems with hardware ımpairments
(IEEE, 2020) Kazemi, M.; Mohammadi, A.; Duman, Tolga M.
We consider a massive multiple-input multiple-output (m-MIMO) system in which a source communicates with a destination with the help of multiple single-antenna decode-and-forward (DF) relays. Employing optimal relay selection, we analyze the system performance in presence of hardware impairments (HWI) for two m-MIMO scenarios: massive-antenna source and single-antenna destination (m-MIMO I), and massive-antenna source and destination (m-MIMO II). We obtain lower bounds on the average signal-to-noise plus distortion ratio (SNDR) of the system and show that in the m-MIMO II regime, the HWI levels at the relays become the only limiting factors. Employing extreme value theory, we demonstrate that as the number of relays increases the end-to-end SNDR of the system tends to Gumbel and Weibull distributions for the m-MIMO I and m-MIMO II systems, respectively. In addition, for both arbitrary numbers of source and destination antennas and m-MIMO scenarios, we provide closed form expressions for optimal power allocation between the source and the selected relay, and the effects of HWI level distributions between the receiving and the transmitting parts of the relay (which can be exploited for optimal system design under cost constraints).
Open Access
Crowdfunding as gambling: evidence from repeated natural experiments
(Elsevier, 2021-02-26) Demir, Tolga; Mohammadi, A.; Shafi, K.
Lenders in Prosper, one of the largest lending markets in the U.S., reduce their activity when playing multistate Powerball or Mega Millions lottery jackpot becomes attractive. This finding suggests that the desire for sensation seeking is an underlying motivation for participating in peer-to-peer crowdfunding markets; the thrill of winning a large lottery jackpot fulfills some lenders' desire for novelty and sensation seeking, thus decreasing their lending activity. We discuss our findings' implications for lenders, borrowers, platform organizers, and policymakers. © 2021 Elsevier B.V.
Open Access
In Press, Corrected Proof: Crowdfunding as gambling: Evidence from repeated natural experiments
(Elsevier, 2021-02-26) Demir, Tolga; Mohammadi, A.; Shafi, K.
Lenders in Prosper, one of the largest lending markets in the U.S., reduce their activity when playing multistate Powerball or Mega Millions lottery jackpot becomes attractive. This finding suggests that the desire for sensation seeking is an underlying motivation for participating in peer-to-peer crowdfunding markets; the thrill of winning a large lottery jackpot fulfills some lenders' desire for novelty and sensation seeking, thus decreasing their lending activity. We discuss our findings' implications for lenders, borrowers, platform organizers, and policymakers.