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1.  Introduction

Visible light communications VLC is an evolving technology that uses the visible range (370-780nm) for both lighting and transmitting data. It is becoming a popular technology for its several advantages, the VLC spectrum is unlicensed and therefore it can be used as a substitute of radio frequency (RF) spectrum and thereby reduce the clogging of the 2.4 GHz ISM (industrial medical, scientific) band. It also provides a higher level of security and high transmission rates than is obtained with radio frequencies because the different VLC signals of neighbouring rooms will not interfere with each other (Jovicic, Junyi& Richardson 2013). Today white Light-emitting diodes (LEDs) can be seen almost everywhere starting from the displays of electronic devices to the advertisements on billboards. For having increasing luminous efficacy (lm/W) high power and lifetime LEDs have started to work as an alternative to other illumination sources. Moreover, high rate modulation can be applied to them over other illumination sources and therefore LEDs are widely used for the implementation of VLC with lighting (Borogovac et al. 2011).  By modulating the amplitude of the LEDs increasing data rate is obtained in VLC and this technique is known as intensity modulation/direct detection (IM/DD).In IM/DD systems the two main components are VLC transmitter and VLC receiver wherein the transmitter a DC bias is applied to manage the lighting and a silicon photodiode is used as a receiver for transforming the received optical signalinto an electric signal. The electrical signal is further amplified using a transimpedance amplifier whose output signal is finally fed into a digital baseband processor (Jovieic, Junyi& Richardson 2013). VLC has eminent benefits in wide bandwidth, high transmission rate, adjustable coverage as well as it is environmentally friendly, cost effective and energy efficient. LEDs are also used for VLC implementation in an indoor environment, as a result, it is becoming a popular means of high speed communication in an indoor environment or for short range communications (Hua et al. 2014). Although implementation of VLC in indoor environments have numerous advantages there are some significant problems associated with this VLC implementation that need to be addressed (Elgal, Mesleh& Haas 2011). Among these problems the following are identified as imposing several constraints on the systems: low modulation bandwidth, multipath fading via Inter Symbol Interference (ISI), co-channel interference for multiple transmitters, uplink bandwidth and high precision in indoor positioning systems (Hussein &Elmirghani 2015). In addition, there are also minor challenges such as maintaining communications when the lights arc off (Borogovac et al. 2011). Finally, the effect of shot noise on VLC receivers when they are revealed to the sunlight  therefore reducing transmission rate (Jovicic,Junyi& Richardson 2013). This literature review mainly focuses on elaborating and juxtaposing the proposed solutions by researchers for a number of major challenges associated with the implementation in indoor environment and thereby enabling to find future research scope that is not appropriately addressed by those solutions.


2.  Literature Review

The limited modulation bandwidth of LEDs which is confined to several MHz, limits the data rate of VLC. As a result increasing the data rate of VLC is completed by increasing the modulation bandwidth.  Different solutions to this approach have been proposed by various researchers. One of the prime solutions is using MIMO (multiple input and multiple output) technologies where the use of multiple LEDs would be able to provide adequate lighting in the indoor environment. But in a real-life indoor system dispersed user devices create VLC systems with the multi-user along with the multiple LEDs and thereby causing multi-user interference (MUI). Yang, Jian &Changyuan(2014) claimed that if data is processed at the transmitter before transmitting then the complication of user devices can be largely decreased combined with reduction of power consumption. Yang, Jian &Changyuan(2014)further suggested that a receiver or user device requires multiple field of views (FOVs) in order to reduce the complicacy of it and consequently removing MUI in the MIMO system. Yang, Jian &Changyuan(2014) proposed pre-coded Multi-user MIMO system and the upgraded SNR and BER(J3it error rate) performance graphs achieved by the system using simulations and mathematicalcalculations. From the details it is evident that their suggested system successfully employed MIMO in indoor environment to increase the modulation bandwidth of LEOs by eliminating MUI. However, they did not show practical implementation of their system which in my point of view is necessary for finding actual effectiveness of the system in real life. Mobile communication using VLC in indoor systems is gaining increasing popularity. Limited modulation bandwidth is also affecting this area. To solve this issue Vijay & Green (2012) described the idea of fragmenting and translating frequency including a frequency mapping table. They have further demonstrated an indoor system which utilizes VLC in GSM technology by solving the limited modulation bandwidth problem of LEDs.  Although it was a unique system, it was not explicitly stated why they have used Infrared (IR) instead ofVLC for uplink and did not leverage the fact VLC is more suitable for downlink as stressed by Sihua et al.(2014).


Moreover, a VLC system using Laser Diodes instead of LEDs have also been put forward by Hussein &Elmirghani(2015), to solve the limited modulation bandwidth problem as well as another major Issus of VLC in indoor environments namely Inter Symbol Interference (ISI). ISI is caused by multi path dispersion in an indoor environments that causes signals to reach the receiver at different times as elaborated by Ihalcrao&Sonavanc(2014). Hussein &Elmirghani(2015) also demonstrated the use of an imaging receiver to circumvent the ISI issues as well as applying delay adaptation technique to remove co-channel interference. Their proposed (OAT imaging LD_VLC) system showed significant improvement in the modulation bandwidth (exceeding 16 Mbps) as well as reducing ISI. These advancements enabled VLC to achieve a very high data rate of 10Gbps that is by far the highest data rate achieved in the indoor environments. Nevertheless, the main drawback of the system is that it causes a reduction in SNR in order to reduce the ISI which is a concerningissue. Moreover, ISI can be also removed by using OFDM (Orthogonal Frequency division multiplexing) which uses guard intervals justified by Sahaet al. (2012) using MATLAB simulations. It was not explicitly stated why QPSK only was employed, whereas other modulation techniques such as 8-PSK could have also been used. Another prime solution for the mitigation of ISI is the use of equalizers in a receiver as theonceillustrated by Xiu-Xiu et al. (2011) called MAP (maximum a-posteriori probability) equalization. However, the performance of this equalizer has been compared with only one equalization method called (MMSE-LE) where there existed many.


VLC is mainly suitable for downlink channel as the mobile receivers are not well suited for the use of VLC in uplink because of several factors such as different alignments of the device and energy restrictions as elaborated by Sihua et al. (2014). Sihua et al. (2014)further argued that this uplink problem can be solved by the implementation of a composite system using VLC in downlink and RF in uplink in a heterogeneous network. Therefore,Sihua et al. (2014) proposed a VLC system based on software called (S DVLC) with the practical implementation where Wi-Fi has been used as uplink and VLC in the downlink. SDVLC system has been able to achieve much higher throughput in multi-user indoor environment than typical Wi-Fi. Sihuaet al. (2014) also added that all theother hybrid systemspreviouslyproposed by theresearchers such as Room division multiplexing(RDM) work not practically implemented for solving this uplink challenge. However, the limitation of this system is that high throughput compared to typical Wi-Fi system is only achievablewhen the number of users is equal or greater than 5 as shown in the throughput comparison graph between only Wi-Fi and Hybrid VLC system. Theimportance of heterogeneous network or Hybrid VLC system has also been highlighted by Hua et al. (2014) for combating the uplink challenge. Therefore, it clarifies that the combination of VLC with other wireless communications work such as Wi-Fi. Bluetooth or cellularnetwork is the single way to ensure reliability of high-speed services offered to clients and consequently broadening the scope of VLC use in indoor environment.


3.  Evaluation

VLC is gaining increasing popularity in indoor positioning systems. Even so, those systems face a significant challenge in terms of the most important parameter that is high precision. There are also some other parameters for example simplicity, security, cost, FOV (field of view), location and horizontal requirement of receiver and characteristicsof lightsources that arc needed to be satisfied for being a good indoor positioning system (Yasir,Siu-Wai&Vcllambi 2014).


Significantresearchhas been completed for solving the identified problems, Yasir,Siu-Wai&Vcllambi(2014)suggesteda low cost indoor positioning system which satisfied precision as well as other criteria without assuming receiver height and calibration, unlike other existing indoor positioning systems. However, they made an assumption that all the light sources are at the same level above ground which is difficult to maintain in all situations. Moreover,Phal,Jaejin&Myungsik(2015) showed the use of image sensor (IS) in their system instead of photodiode for gaining high accuracy with the logic that photodiodes are impractical in reality. Although their system shows practicality in MATLAB simulations results and has not been validated in a practical implementation. Yamaguchi et al. (2014) illustrated how high precision can be achieved by separating transmitted signals with optical orthogonal codes (OCC) using mathematical and simulation results. Although their calculated results in three different scenarios were able to get high confidence results, the selection of this method adds significant complexity.


Discussing major challenges and some crucial solutions provided by different researchers enabled me to get multiple research scope for implementing VLC in an indoor environment Firstly,the (OAT imaging LD VLC) system caused a reduction in SNR level for reducing SI and improving modulation bandwidth. This concerning issue stimulated my enthusiasm for finding techniques in order to increase the SNR level and get data rate exceeding 10Gbis while reducing IS (Hussein &Elmirghani 2015). Mitigating the solution of using OFDM with QPSK it wouldappear the use of OFDM with different modulation techniques other than QPSK (Saha et al. 2012) is appropriate. Secondly, for increasing modulation bandwidth the idea of using MIMO for overcoming the MU by closing some of the links as highlighted by Yang. Jian &Changyuan(2014)appears worthwhile further investigating.Thus, the idea of closing link for overcoming MUI opened another small research scope. Thirdly, for combating the uplink challenge of VLC in an indoor environment the proposed SDVLC system has some throughput related issues for the number of efficient multiple devices (Sihua et al. 2014). Moreover. it has been also stated that many advantages and problems of the heterogeneous network with VLC are still not resolved (Hua et al. 2014). In addition to the major challenges, maintaining communications while the light is off is aIso a concerning issue in VLC and current research aimed at solving this problem. For example,Bogovacet al. (2011) claimed that data transmission is possible when light is off by characterization of the channel stressed by Chvojka et al. (2015). As a result, these unsolved issues would provide me ample opportunity to conduct research in this area. Finally and most importantly, the indoor positioning systems using VLC would provide me with the biggest scope of research in future.


From these research avenues, the pivotal assumptions of almost all the indoor positioning systems make then prohibitive to work with different indoor environments, tied to satisfying all above constraints required from a positioning system. Therefore, future research on an indoor positioning system using VLC that would not make any assumptions on indoor environment configuration, could satisfy all the light variations, and maintain efficiency of the system implemented as noted by Phat, Laejin&Myungsik(2015)is the research objective. In order to reach this research objective my envisaged research question at this stage are A) whether OFDM with QPSK can be augmented with other modulation techniques without detrimental effect to throughput B) if SDVLC system can be effectively combined with OFDM/QPSK variants.


4.  Conclusions

The recent research completed in solving the major challenges of VLC in indoor environment enabled me to get a deep insight into different solutions provided by various researchers in overcoming these issues. As a result several research gaps were identified that need further investigation and almost all of the challenges and their solutions have still some further analysis required. Among the challenges ISIS to SNR level modulation technique remain the largest challenge to be solved. The limited modulation bandwidth is another more constrained challenge for which a solution needs to be provided. Moreover, the challenge of throughput in un-configured heterogeneous networks remains inconclusive and tied to indoor positioning systems challenges of assumptions of system configuration. This implies that all these unsolved issues hold high research scope. As a result, these gaps led to different research opportunities for me where fulfilling the gaps of VLC systems for indoor positioning.In particular modulation techniques evaluations on SDVLC systems with respect to throughput have been identified as opportunities for direct contributions to the existing body of knowledge. Providing clarity with a robust analysis of these conditions will present another view on the main challenges and recent solutions given by different researchers for implementing VLC in an indoor environment.

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