Browsing by Author "Çoğalan, Tezcan"

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Femtocell Design for Data Communications in Mobile Networks
(Kadir Has Üniversitesi, 2013) Çoğalan, Tezcan; Panayırcı, Erdal; Panayırcı, Erdal
in this thesis game theoretic utility-based adaptive power control algorithm for uplink of femtocell networks is performed for mitigation of interference in two different ways. The first one is allocation of femtocell interference threshold in randomly activated femtocell base stations in a known macrocell coverage area and the second one is adaptation of the power of the femtocell users for mitigating total interference from femtocell users at the macrocell base station. Accordingly we consider two different pricing schemes which are centralized pricing and de-centralized pricing for updating the power level at mobile station. in addition we examined two different technologies which are frequency division multiple access (FDMA) and code DMA (CDMA). The results show that each active user at network could reach the given signal to interference plus noiseratio (SiNR) threshold value with iterative power control algorithm and macrocell base station encountered with given interference threshold value with power control and user removal algorithms. -- Abstract'tan.
Citation - WoS: 0
A Novel Transmit Array Structure for Optical Spatial Modulation
(IEEE, 2019) Çoğalan, Tezcan; Panayırcı, Erdal; Haas, Harald; Panayırcı, Erdal
The performance of multiple input single output (MISO) and multiple input multiple output (MIMO) systems is limited by spatial channel correlation. This limitation becomes particularly severe in light lidelity (LiFi) systems, which use intensity modulation/direct detection (IM/DD) for data transmission, because light emitting diodes (LEDs) can only emit incoherent light. This paper proposes a novel transmitter structure that provides spatially separated channels and enables optical MISO/MIMO spatial modulation (SM) transmission without the need for a power allocation algorithm or a transmit precoding technique. A single LED array, which consists of multiple LEDs with different characteristics, is considered in an indoor environment. Different numbers of transmit LEDs are chosen for SM transmission based on the relation between channel correlation and bit error probability. It is shown that the proposed structure provides reliable SM transmission when the modulation order for M-ary pulse amplitude modulation (PAM) is considered as 2 and 4. Computer simulations also show that higher modulation order, e.g., 8-PAM, can be supported but at a slightly degraded bit error ratio (BER) performance.
Citation - WoS: 22
Citation - Scopus: 30
Physical-Layer Security in Visible Light Communications
(Institute of Electrical and Electronics Engineers Inc., 2020) Yeşilkaya, Anıl; Şadi, Yalçın; Çoğalan, Tezcan; Erküçük, Serhat; Erküçük, Serhat; Panayırcı, Erdal; Şadi, Yalçın; Panayırcı, Erdal; Haas, Harald; Poor, Harold Vincent
Optical wireless communications (OWC) and its potential to solve physical layer security (PLS) issues are becoming important research areas in 6G communications systems. In this paper, an overview of PLS in visible light communications (VLC), is presented. Then, two new PLS techniques based on generalized space shift keying (GSSK) modulation with spatial constellation design (SCD) and non-orthogonal multiple access (NOMA) cooperative relaying are introduced. In the first technique, the PLS of the system is enhanced by the appropriate selection of a precoding matrix for randomly activated light emitting diodes (LEDs). With the aid of a legitimate user's (Bob's) channel state information (CSI) at the transmitter (CSIT), the bit error ratio (BER) of Bob is minimized while the BER performance of the potential eavesdroppers (Eves) is significantly degraded. In the second technique, superposition coding with uniform signaling is used at the transmitter and relays. The design of secure beamforming vectors at the relay nodes along with NOMA techniques is used to enhance PLS in a VLC system. Insights gained from the improved security levels of the proposed techniques are used to discuss how PLS can be further improved in future generation communication systems by using VLC.
Citation - WoS: 35
Citation - Scopus: 41
Physical-Layer Security With Optical Generalized Space Shift Keying
(Ieee-Inst Electrıcal Electronıcs Engıneers Inc, 2020) Panayırcı, Erdal; Yeşilkaya, Anıl; Çoğalan, Tezcan; Poor, H. Vincent; Haas, Harald
Spatial modulation (SM) is a promising technique that reduces inter-channel interference while providing high power efficiency and detection simplicity. In order to ensure the secrecy of SM, precoding and friendly jamming are widely adopted in the literature. However, neither of those methods can take advantage of SM. In this paper, a novel spatial constellation design (SCD) technique is proposed to enhance the physical layer security (PLS) of optical generalized space shift keying (GSSK), which can retain some benefits of SM. Due to the lack of small-scale fading, the quasi-static characteristics of the optical channel is used to tailor the received signal at the legitimate user's (Bob's) side. The PLS of the system is guaranteed by the appropriate selection of the power allocation coefficients for randomly activated light emitting diodes (LEDs). With the aid of Bob's channel state information at the transmitter, the bit error ratio (BER) of Bob is minimized while the BER performance of the potential eavesdroppers (Eves) is significantly degraded. Monte-Carlo simulation results show that the proposed SCD-zero forcing precoding (ZFP) forces Eve to experience a BER of around 0.5 by outperforming both the conventional and ZFP based GSSK for all practical signal-to-noise-ratio regimes and Bob-Eve separations.
Citation - WoS: 8
Power Control-Based Multi-User Li-Fi Using a Compound Eye Transmitter
(IEEE, 2015) Çoğalan, Tezcan; Panayırcı, Erdal; Haas, Harald; Panayırcı, Erdal
In visible light communications (VLC) the transmitted signal is non-negative and real valued and these characteristics constrain the signal space and provide new challenges for waveform design in a multi-user system. In Light Fidelity (Li-Fi) a VLC technology and similar to radio frequency (RF) systems precoder designs can be used to serve multiple users. However a signal shaping algorithm is required to address the non-negativity of the transmitted signal in Li-Fi. In this study a power allocation algorithm is proposed to achieve a multi-user Li-Fi system. A compound eye transmitter consisting of several LEDs pointing in different directions is used. This means that the process of the obtaining the inverse of the channel matrix in precoder design is not required. Results show that when a 30 degrees semi-angle is used at the transmitter and the system has 4 users the proposed transmission model can achieve 10(-5) average bit error rate (BER) performance at 1 dBm average transmission power.