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- 저자 : GABRIELA ALEJANDRA ROMERO MUNOZ (검색결과 1 건)
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GABRIELA ALEJANDRA ROMERO MUNOZ 경희대학교 대학원 2019 국내박사
In this dissertation, efficient mitigation schemes for co-channel interference (CCI) are proposed to improve the bit error rate (BER) performance of indoor visible light communication (VLC) systems. Due to multiple light emitting diodes (LEDs) are installed in indoor environments for lighting purposes, multiple-input single-output (MISO) schemes can be implemented in VLC systems to increase the transmission capacity and likelihood of the signal reception. However, the interference between adjacent LEDs known as CCI must be considered when VLC uses LEDs, which act as optical transmitters, to transmit unique data over the same frequency resources and the photodiode (PD), which acts as an optical receiver, obtains the different signals over its physical area; the signals can seriously interfere with each other to the point that the receiver cannot recover the information, obtaining useless data. Because the quality of communication link degrades as CCI increases, VLC must overcome CCI to provide a reliable communication throughout the room. VLC is considered as a promising complementary technology for existing wireless communications, which have a shortage of spectrum resources, because of its multiple advantages like it uses the lighting infrastructure for transmitting data using white LEDs, does not interfere with radio frequency (RF) communications and can be installed in electromagnetic interference sensitive areas, such as hospitals, airplanes, and so on. Since VLC can alleviate the increasing data traffic, increase the capacity transmission and prevent loss of connectivity, it can provide service in internet-of-things (IoT) networks, which are applied in healthcare, smart indoor environments, etc., to improve the daily life; for instance, IoT devices can communicate with each other or personal devices, e.g. smartphones, tablets and laptops, to control, monitor, track or locate a particular object. In order to deal with CCI, the proposed schemes are based on transmit diversity and inter-lighting interference cancellation (ILIC) to avoid or mitigate CCI, depending on the data transmission. Employing transmit diversity techniques, the LEDs cooperate to transmit data, which is encoded by space frequency block code-frequency switched transmit diversity (SFBC-FSTD) encoder. At the receiver side, there are no interference signals because each LEDs transmit an encoded data; it means that the signals transmitted by the cooperative LEDs are useful to decode the desired information. Therefore, CCI is avoided by converting the interfering signals into desired signals. By contrast, the signals interfere with each other at the receiver side when the LEDs transmit different data to increase the transmission capacity. While the PD receives the signals with different powers, the signals are detected by ILIC, which decodes the high received power signal and reduce CCI in the low received power signal before decoding. This dissertation presents efficient mitigation schemes for CCI in indoor VLC systems to provide reliable communication throughout the room; the proposed schemes avoid or mitigate CCI, depending on the data transmission. It is organized as follows. Chapter 1 gives a briefly overview of VLC system including advantages, challenges and applications, and depicts the motivation of the research and the contributions obtained from the proposed schemes through computer simulations. Chapter 2 introduces the most important parts of a basic VLC system such as optical channel and optical modulation, and describes an indoor MISO-VLC system, which is composed of multiple LEDs and a PD, that is used to evaluate the BER performance of the proposed schemes in presence of high CCI, i.e. the PD position is in middle of the LEDs, determining the signal-to-noise ratio (SNR) or the received optical power required to the target BER = 10^-3, which is the minimum BER for basic communication. Chapter 3 describes the decentralized and centralized cooperative transmission, which are proposed to avoid CCI. The proposed schemes are based on space frequency block code-frequency switched transmit diversity (SFBC-FSTD) to encode the data in spatial and frequency domain. Employing transmit diversity techniques, the LEDs cooperate to transmit the SFBC-FSTD vectors in centralized and decentralized way and the PD requires them to decode the data. In centralized scheme, the LEDs have to communicate with each other to perform code allocation because each cooperative LED transmits a SFBC-FSTD vector. By contrast, the LEDs do not need to communicate with each other before transmission when the decentralized scheme is used because the LEDs send its own random data, which contains the SFBC-FSTD vectors. The proposed schemes overcome CCI because the interfering signals are converted into desired signals and thereby improve the BER performance, providing spatial diversity gain. Chapter 4 describes two improved versions of inter-lighting interference cancellation (ILIC), enhanced ILIC (E-ILIC) and cooperative ILIC (C-ILIC), which are proposed to mitigate CCI when the LEDs transmit unique data to increase the transmission capacity. The proposed schemes are based on ILIC, which improves the performance of a MISO-VLC system when the signals are received with different power because there are high and low received power signal, to reduce CCI in low received power signal. Owing to the average BER performance of ILIC changes with the PD position due to the received power ratio between two signals is different at each position, E-ILIC and C-ILIC are used to receiver the signals with different power whose power ratio complies with the threshold by applying power allocation so that the LEDs transmit unique data with different powers and cooperative transmission so that the LEDs transmit a composite data, which contains two data, with equal powers, respectively. Therefore, E-ILIC and C-ILIC achieve the best average BER performance of ILIC, regardless of the PD position because the received power ratio between the signals is two, while increasing the transmission capacity. Finally, Chapter 5 provides a summary of the proposed schemes, which avoid or mitigate CCI, depending on the data transmission, and overcome link blockages when the decentralized cooperative transmission and cooperative-ILIC are used because the receiver can be recovered the data by receiving at least one received link, and concludes this dissertation.
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