Browsing by Author "Al-Nahhal, Mohamed"

Now showing 1 - 5 of 5

Metadata only
Adaptive multiple-input multiple-output (MIMO) techniques for visible light communications
Al-Nahhal, Mohamed; Uysal, Murat; Uysal, Murat; Durak, Kadir; Başar, E.; Department of Electrical and Electronics Engineering; Al-Nahhal, Mohamed
Visible Light Communication (VLC) technology is a promising solution of wireless communication systems in beyond 5G communication networks, which provides low implementation cost, high energy efficiency, and high-speed data transmission. The VLC systems can be considered as a complementary or alternative for the existing Radio Frequency (RF) based wireless communication systems to satisfy the demands of increasing high capacity. In the VLC systems, Light-Emitting Diodes (LEDs) are used as transmitters, and Photodetectors (PDs) are used as receivers. Multiple LEDs are installed to offer sufficient illumination for indoor environments. This feature can be utilized to implement Multiple-Input Multiple-Output (MIMO) communications systems in order to achieve high data rates by improving spectral efficiency. However, LEDs can only convey unipolar signals. Furthermore, the nature of the VLC channel is frequency-selective, which causes significant degradation in the performance of the VLC system due to critical Inter-Symbol Interference (ISI). Several schemes based on pulse modulation and Orthogonal Frequency Division Multiplexing (OFDM) have been proposed to overcome the ISI of the VLC system. A new Optical Orthogonal Frequency Division Multiplexing (O-OFDM), referred to as a Unipolar Orthogonal Frequency Division Multiplexing (U-OFDM), appears as an attractive solution for emerging VLC systems. In the future of 5G and beyond 5G communication networks, the spectral efficiency needs further improvement through MIMO with adaptive transmission technique. Motivated by these, we focus on improving the spectral efficiency and Bit Error Rate (BER) performance for the MIMO VLC systems in both the single-carrier and multi-carrier. Firstly, we propose a novel Adaptive Spatial Modulation (ASM) scheme, referred to as Flexible Generalized Spatial Modulation (FGSM), for single-carrier MIMO VLC systems to achieve better average Symbol Error Rate (SER) and higher spectral efficiency with a fixed overall number of LEDs compared to existing ASM and Generalized Spatial Modulation (GSM) schemes. The proposed system varies the modulation sizes over the available LEDs as well as the number of active LEDs. The selection of the modulation sizes is based on an optimization problem for the average SER under a predefined value of the spectral efficiency. We derive a closed-form expression of approximate average SER for the proposed system and evaluate its decoding complexity. We show that the FGSM scheme can be a potential candidate for future MIMO-VLC systems. Secondly, we firstly propose adaptive transmission technique for the MIMO VLC system in conjunction with U-OFDM to exploit the U-OFDM benefits. The proposed adaptive MIMO U-OFDM VLC system is implemented to support three different MIMO modes that enable a set of different modulation sizes. The considered MIMO modes are Repetition Coding (RC), Spatial Multiplexing (SMUX) and Spatial Modulation (SM). In the RC, the same signal is transmitted simultaneously from all LEDs, while independent signals are transmitted simultaneously from all LEDs in the SMUX. In the SM technique, a single LED is activated and the index of the activated LED carries information in addition to modulated signals. Depending on the received Signal-to-Noise Ratio (SNR) and target BER, the proposed adaptive transmission system switches between the available MIMO modes and adjusts its modulation size to achieve higher spectral efficiency. The proposed U-OFDM system is applied over different VLC MIMO setups with realistic channel models for 8×8, 4×4 and 2×2 MIMO systems. Recently, Generalized LED Index Modulation (GLIM) appears as an attractive MIMO OFDM scheme for emerging VLC systems. Therefore, in the last part of this thesis, we propose a Magnitude and Wrap-Phase OFDM (MW-OFDM) scheme for the MIMO VLC systems. The proposed MW-OFDM scheme relies on the conversion of complex signals into polar form with the magnitudes and wrap-phases to decrease the restriction on the number of LEDs compared to the conventional GLIM-OFDM. Moreover, the Maximum Likelihood (ML) estimator for the proposed scheme is derived. The proposed MW-OFDM scheme improves the average bit error rate and provides a significant reduction in the decoding complexity, compared to the conventional GLIM-OFDM. Moreover, a half number of LEDs are required for the proposed scheme to deliver the same spectral efficiency in a comparison with the conventional GLIM-OFDM.
Metadata only
Adaptive unipolar MIMO-OFDM for visible light communications
(IEEE, 2019) Al-Nahhal, Mohamed; Basar, E.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Al-Nahhal, Mohamed
Unipolar orthogonal frequency division multiplexing (U-OFDM) appears as an attractive optical OFDM solution for emerging visible light communication (VLC) systems. This paper proposes spectral efficiency improvement for U-OFDM systems by applying adaptive transmission over realistic VLC links. This adaptive transmission includes switching among a number of multiple-input multiple-output (MIMO) modes combined with appropriate modulation size selection. The considered MIMO modes are repetition coding, spatial modulation, and spatial multiplexing, where each mode supports different modulation sizes. The selection of the corresponding MIMO mode and its modulation size is based on the received signal-to-noise ratio and target bit error rate. The proposed U-OFDM system is applied over different VLC MIMO setups with realistic channel models for 8 x 8, 4 x 4 and 2 x 2 MIMO systems. Our simulation results show that the proposed adaptive system provides a significant spectral efficiency improvement over stand-alone U-OFDM MIMO modes/setups.
Metadata only
Flexible generalized spatial modulation for visible light communications
(IEEE, 2021-01) Al-Nahhal, Mohamed; Başar, E.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Al-Nahhal, Mohamed
Adaptive spatial modulation (ASM) varies the modulation size across the transmit light-emitting diodes (LEDs) to improve the overall spectral efficiency of visible light communication (VLC) systems. This article proposes a novel ASM scheme, referred to as flexible generalized spatial modulation (FGSM), for VLC systems. The proposed FGSM system changes the modulation sizes over the LEDs and the number of active LEDs, to improve the average symbol error rate (SER) and spectral efficiency compared to ASM with a fixed overall number of LEDs. The modulation sizes are selected in order to optimize the average SER under a predefined spectral efficiency value. A closed-form expression of approximate SER is derived along with decoding complexity calculations for the proposed system. Numerical results are provided to confirm the superiority of the proposed system and to support theoretical derivations.
Metadata only
Magnitude and wrap-phase OFDM for MIMO visible light communication systems
(IEEE, 2021-07) Al-Nahhal, Mohamed; Başar, E.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat
Generalized light emitting diode (LED) index modulation (GLIM) has been proposed as an effective multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) scheme for visible light communications (VLC) systems. In this letter, we propose a magnitude and wrap-phase OFDM (MW-OFDM) scheme for MIMO VLC systems. The proposed MW-OFDM system relies on the conversion of complex signals into polar form with the magnitudes and wrap-phases to decrease the restriction on the number of LEDs compared to the conventional GLIM-OFDM. Furthermore, maximum likelihood and best linear unbiased estimators of the proposed scheme are derived. The proposed MW-OFDM scheme improves the average bit error rate and provides a significant reduction in the decoding complexity, compared to the GLIM-OFDM. Moreover, a half number of LEDs are required for the proposed scheme to deliver the same spectral efficiency in a comparison with the GLIM-OFDM. Numerical results are presented over MIMO VLC channels to elaborate the superiority of our proposed scheme.
Metadata only
Transmit laser selection for underwater visible light communication systems
(IEEE, 2019) Elamassie, Mohammed; Al-Nahhal, Mohamed; Kızılırmak, R. Ç.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Elamassie, Mohammed; Al-Nahhal, Mohamed
Visible light communication (VLC) has the potential to serve as a high-speed underwater wireless connectivity solution to support real-time image and video transmission. Underwater propagation medium imposes additional challenges on the design of VLC systems which were originally proposed for indoor applications. Temperature and salinity fluctuations result in fluctuations of the refractive index of seawater and eventually introduce turbulence-induced fading. In this paper, to mitigate the effects of fading, we propose transmit laser selection for a diver-to-diver underwater VLC link where the transmitter has multiple laser sources and the receiver has one photodetector. The source with the highest received instantaneous signal-to-noise ratio is selected for transmission while the remaining sources remain idle. We derive a closed-form expression for asymptotical bit error rate over log-normal distributed underwater turbulence channels and derive the achievable diversity order. We further present simulation results to confirm our derivations and diversity gain analysis.